Characterization of Kallikrein-related Peptidase-8 in …...KLK8 levels correlated with psoriasis...
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Characterization of Kallikrein-related Peptidase-8 in Normal Human Epidermis and Psoriasis by Azza Eissa A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Laboratory Medicine and Pathobiology University of Toronto © Copyright by Azza Eissa 2013
Transcript of Characterization of Kallikrein-related Peptidase-8 in …...KLK8 levels correlated with psoriasis...
Characterization of Kallikrein-related Peptidase-8 in Normal Human Epidermis and Psoriasis
by
Azza Eissa
A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy
Laboratory Medicine and Pathobiology University of Toronto
© Copyright by Azza Eissa 2013
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Characterization of Kallikrein-related Paptidase-8 in Normal
Human Skin Epidermis and Psoriasis
Azza Eissa
Doctor of Philosophy
Department of Laboratory Medicine and Pathobiology
University of Toronto
2013
Abstract
Kallikrein-8 (KLK8) is a relatively-uncharacterized epidermal protease. Although proposed to
regulate wound-healing and barrier repair in KLK8-deficient mouse skin, KLK8-catalytic
activity was never demonstrated in human epidermis and its regulators and targets remain largely
unknown. KLK8 overexpression was reported in inflammatory skin diseases, but the underlying
mechanisms are poorly understood. In this thesis, we elucidated for the first time KLK8-specific
activity in normal human non-palmoplantar stratum corneum and sweat, and identified epidermal
regulators and targets that augment its involvement in a skin-barrier proteolytic cascade. Given
that inflammatory skin diseases have interlinked immune and epidermal roots, we hypothesized
that epidermal KLK8 expression is distinctly regulated by the aberrant T-cell immunity
implicated in the two common skin diseases, psoriasis and atopic dermatitis, independent of skin-
barrier insults. We profiled secretion of KLK8 by normal human keratinocytes post-treatment
with T-helper (Th1, Th17 and Th2) cell-derived cytokines, and investigated the effect of KLK8
overexpression on terminal keratinocyte differentiation and innate immunity gene expression.
Our results show that TNFα and IL-17A synergistically induce potent KLK8 hyper-secretion,
while IL4 and IL13 reduce its expression. TNFα and IL-17A overexpression and KLK8
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hyperactivity resulted in hyperkeratosis and upregulation of keratinocyte innate defense genes’
expression mimicking psoriatic lesions. Consistently, KLK8 expression was reduced in lesional
skin of atopic dermatitis patients and significantly elevated in lesional skin and sera of psoriatic
patients. KLK8 levels correlated with psoriasis skin severity and were significantly reduced by
effective treatment with biologic TNFα-blockers, correlating positively with psoriasis clearance.
Thus, KLK8 is a new epidermal psoriasis therapeutic target. We performed high throughput
screens of small molecule compound libraries to identify KLK8-specific inhibitors and
discovered promising KLK8 small molecule inhibitors with IC50s in the nanomolar range. This
thesis provides original findings corroborating KLK8 as an active serine protease in normal
human skin and a down-stream epidermal respondent to TNFα and IL17A overexpression in
psoriatic skin. Our novel KLK8-specific inhibitors may have future potential as topical barrier-
enhancing agents in psoriasis.
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Acknowledgments
Though my name appears on the cover of this dissertation, it would not have come into existence
without the following remarkable individuals:
My supervisor, Dr. Eleftherios P. Diamandis – Thank you for providing me with exceptional
professional opportunities and teachable moments. I joined the lab as a naïve and enthusiastic
young student and will be graduating as a mature professional. Thank you for pushing me to
work independently and supporting me continuously. I learned valuable skills from you and I
feel fortunate to have you as my supervisor and mentor.
Our LMP graduate department coordinator, Dr. Harry Elsholtz – Thank you for being so
approachable and resourceful. I appreciate your guidance and open door policy over the years.
My thesis advisory committee, Dr. David Irwin, Dr. Sylvia Asa, Dr. Nades Palaniyar and
Dr.Herman Yeger – I appreciate all your valued contributions! Thank you for your insightful
feedback and advice. Your supervision and thoughtful review of my thesis was most helpful. My
sincere appreciation to my external thesis advisor, Dr. Alain Hovnanian, for taking the time to
contribute to my thesis and traveling long distance to be present during my thesis defense.
Last but not least, the ACDC lab members of the near past and present, my work family – Thank
you all for your valuable friendships. Your presence and support in the lab was a blessing.
Special thanks to Ferzeen Sammy, Rama Ponda, Denitza Roudeva, Yiannis Prassas, Yijin Yu,
Connie Zao, Vanessa Amodeo, Daniela Cretu, Antoninus Scoospialli, Dr. Gennady Poda, Dr.
Martin Steinhoff, Dr. Ulf Meyer-Hoffert, Dr. Vinod Chandran, and Dr. Morely Hollenberg for
helping me maneuver around some unexpected bumps in my research path and for being great
friends, mentors and collaborators.
Also, a big thank you to the Natural Science and Engineering Research Council (NSERC),
Canada Graduate Scholarship, Ontario Graduate Scholarship (OGS), Helen Marion Walker-
Soroptimist Women’s Health Research scholarship and LMP department for funding my
research.
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Dedication
To My Wonderful Family!
To Mama & Baba – for all the sacrifices you made and your endless love and support. Thank
you for encouraging me to be fearless in seeking insight and facing challenges. You taught me
how to cultivate the right attitude and learn from every single life lesson. You had blind faith in
me and supported my aspirations even when you were not sure where they were taking me.
Thank you! I am who I am today because of you.
To Omer ‘amoory’ – I couldn’t have asked for a better brother & best friend! I am very proud of
you. You’ve been our rock. Thank you for being you!
To ‘bit al siroor al azama’ Fatma, my extraordinary grandmother who raised six children on her
own and faced tough challenges with unequivocal strength, and to our family guru, my dear
uncle, ‘amo’ Abdel Salam– If you ask me about superheroes: “Superwoman” and “Superman”, I
would say: I know them! Despite the oceans and lands separating us, your superpowers reach
and move me. Your leadership, intellect, empathy and wisdom are legendary. They transcend
time and place to inspire generations to care and do more. I plan to share your life stories with
the world one day! For now, this thesis dedication will serve as a small token of my love and
admiration. Thank you for being my inspiring role models!
To ‘Denitza’ & ‘Puneet’ – my amazing girlfriends, my sisters! Living together and sharing over
10 years of friendships and laughs was a blast! Our incredible journey as young best friends from
completely different backgrounds made me a better person. Thank you for always being there
and for cheering me on through my entire undergrad and graduate programs. I couldn’t have
done it without you. I love you both.
Thank you all for allowing me to live “like a river flows, carried by the surprise of its own
unfolding” ~ John O’Donahue.
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Table of Contents
Chapter 1 ......................................................................................................................................... 1
1 Introduction ................................................................................................................................. 2
1.1 Serine Proteases: Digesting the Basics ................................................................................ 2
1.1.1 General protease classification .................................................................................. 2
1.1.2 Serine proteases catalytic mechanism ........................................................................ 4
1.1.3 Human trypsin-like serine proteases of family S1 clan PA ....................................... 7
1.2 Kallikrein-related Peptidase-8 At a Glance .......................................................................... 7
1.2.1 Discovery of the Kallikrein-related peptidases .......................................................... 7
1.2.2 Genomic and proteomic structure of Kallikrein-related peptidases......................... 10
1.2.3 Molecular properties of Kallikrein-related peptidase-8 ........................................... 12
1.2.4 Kallikrein-related peptidase-8 knock out mouse ..................................................... 14
1.3 Kallikrein-related peptidases in Normal Human Epidermis .............................................. 15
1.3.1 Normal skin structure and function .......................................................................... 15
1.3.2 Kallikrein expression in the skin .............................................................................. 18
1.3.3 Dermatological roles of Kallikrein-related peptidases ............................................ 21
1.3.4 Regulation of epidermal Kallikrein-related peptidases ........................................... 23
1.4 Kallikrein-related peptidases in Skin Diseases .................................................................. 28
1.5 Psoriasis and Atopic Dermatitis ......................................................................................... 33
1.6 Kallikrein-related peptidase-8 in normal and inflamed skin 1.5 Psoriasis and Atopic
Dermatitis .......................................................................................................................... 36
1.7 Rationale, Hypotheses and Objectives ............................................................................... 37
1.7.1 Rationale .................................................................................................................. 37
1.7.2 Hypotheses ............................................................................................................... 38
1.7.3 Objectives ................................................................................................................ 38
Chapter 2 ....................................................................................................................................... 40
2 Kallikrein-related peptidase-8 (KLK8) is an active serine protease in human epidermis and
sweat and is involved in a skin barrier proteolytic cascade ..................................................... 41
2.1 Introduction ........................................................................................................................ 41
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2.2 Materials and Methods ....................................................................................................... 42
2.2.1 Cloning, expression, and purification of recombinant human KLK8 proteins ........ 43
2.2.2 Detection of recombinant mat-KLK8 and pro-KLK8 protein expression ............... 44
2.2.3 Gelatin-Zymography ............................................................................................... 45
2.2.4 AMC substrate profiling and kinetics constant determination ............................... 45
2.2.5 Cleavage of a positional-scanning library of FRET-quenched peptides ................ 46
2.2.6 pH profiling, divalent cation, and glycolsylation effect on KLK8 activity……... .. 46
2.2.7 Mat-KLK8 autodegradation assays .......................................................................... 47
2.2.8 Pro-KLK8 zymogen activation by KLK5, KLK1, and lysyl-endopeptidase .......... 47
2.2.9 Inhibition of KLK8 by epidermal inhibitors and general serpins ............................ 48
2.2.10 Activation of Pro-KLK1, pro-KLK11, and pro-KLK5 by KLK8 ......................... 48
2.2.11 Proteolytic processing of LL-37cathelicidin antimicrobial peptide ...................... 49
2.2.12 Calcium induction of keratinocyte differentiation and KLK8 expression ............ 50
2.2.13 Collection and preparation of sweat and stratum corneum (SC) extracts ............. 50
2.2.14 KLK8 expression in sweat, stratum corneum extracts, and skin cell cultures ...... 51
2.2.15 Immunocapture of KLK8 activity in sweat and SC epidermal extracts...… ......... 52
2.3 Results ................................................................................................................................ 52
2.3.1 Recombinant mat-KLK8 and pro-KLK8 protein characterization ......................... 52
2.3.2 Pro-KLK8 zymogen activation in an epidermal cascade ........................................ 57
2.3.3 Effect of cations on KLK8 activity 1.3.2 Kallikrein expression in the skin ............ 59
2.3.4 Differential inhibition by skin specific inhibitors and general serpins ................... 59
2.3.5 KLK8 AMC substrate profiling and steady-state kinetics… ................................... 61
2.3.6. Rapid endopeptidase library screening of KLK8 P2-P2’substrate specificity ....... 61
2.3.7 Activation of co-localized epidermal pro-KLKs by active KLK8 n ........................ 64
2.3.8 KLK8 processing of LL-37 antimicrobial peptide .................................................. 66
2.3.9 KLK8 is a keratinocyte-specific protease induced during terminal keratinocyte
differentiation… .................................................................................................... 68
2.3.10 KLK8 is expressed in a free form in human sweat and non-palmoplantar stratum
corneum ................................................................................................................ 68
2.3.11 KLK8 is catalytically active in normal human sweat and non-palmoplantar stratum
corneum ................................................................................................................ 70
2.4 Discussion .......................................................................................................................... 74
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Chapter 3 ....................................................................................................................................... 79
3 Kallikrein-related peptidase-8 is upregulated by TNFα and IL17A resulting in epidermal
hyperplasia and elevation of psoriasis-related innate immunity gene expression .................. 80
3.1 Introduction ........................................................................................................................ 80
3.2 Materials & Methods .......................................................................................................... 81
3.2.1 HaCat keratinocyte cell differentiation model and cytokine treatment ................... 82
3.2.2 Enzyme-linked immunosorbent assay (ELISA) and LDH assays ........................... 82
3.2.3 BrdU cell proliferation assay ................................................................................... 82
3.2.3 KLK8 treatment of full thickness 3D skin equivalents ............................................ 82
3.2.5 Immunocytochemistry and immuohistochemistry ................................................... 83
3.2.6 Reverse Transcription and quantitative PCR .......................................................... 84
3.2.7 Clinical samples from patients ................................................................................. 84
3.3 Results ................................................................................................................................ 85
3.3.1 Keratinocyte secretion of KLK8 is differentially regulated by Th1, Th17 and Th2
cytokines ............................................................................................................... 85
3.3.2 TNFα and IL17A-treated keratinocytes have an altered differentiation program and
mimic lesional psoriatic skin ................................................................................ 89
3.3.3 Overexpression of KLK8 alters keratinocyte differentiation program, induces
epidermal hyperplasia and up-regulates innate immunity gene expression .......... 91
3.3.4 KLK8 is significantly elevated in lesional psoriatic skin and reduced in lesional
acute atopic dermatitis skin………………………………………………………97
3.3.5 KLK8 elevation in psoriatic patients’ lesional skin and sera is significantly reduced
after effective treatment with the TNFα blockers................................................101
3.4 Discussion ........................................................................................................................ 105
Chapter 4 ..................................................................................................................................... 110
4 Serum kallikrein-related peptidase-8 levels correlate with skin activity, but not psoriatic
arthritis, in patients with psoriatic disease………………………………..… ....................... 111
4.1 Introduction ...................................................................................................................... 111
4.2 Materials & Methods ........................................................................................................ 112
4.2.1 Collection of synovial fluids (SF) from PsA and control patients ......................... 112
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4.2.2 Immunohistochemistry .......................................................................................... 112
4.2.3 Setting and participants .......................................................................................... 113
4.2.4 Enzyme-linked immunosorbent assays (ELISAs). ................................................ 114
4.2.5 Statistical Analysis. ................................................................................................ 114
4.3 Results .............................................................................................................................. 114
4.3.1 KLK6 and KLK8 are elevated in PsA synovial fluids and lesional psoriatic skin 114
4.3.2 PsA and PsC patients. ............................................................................................ 115
4.3.3 KLK8 is independently elevated in sera of patients with psoriatic disease… ....... 120
4.3.4 KLK8 serum levels in PsA correlate with the PASI score, but not inflamed joint
counts … ............................................................................................................. 122
4.4 Discussion ........................................................................................................................ 127
Chapter 5 ..................................................................................................................................... 130
5 Ongoing studies, general discussion and future directions…………………..… ................... 131
5.1 Introduction ...................................................................................................................... 131
5.2 Materials and methods ..................................................................................................... 131
5.2.1 KLK8-mediated PAR2 signaling by cell-based assays 131
5.2.2 Search for KLK8 inhibitors by high throughput screens of small molecule
compounds. ......................................................................................................... 132
5.3 Results .............................................................................................................................. 131
5.3.1 KLK8 displays differential PAR2 signaling compared to KLK14……………….135
5.3.2 Identification of KLK8-specific inhibitors by high throughput screens of small
molecule compounds. ......................................................................................... 139
5.4 General discussion ........................................................................................................... 143
5.5 Future directions ............................................................................................................... 148
References ................................................................................................................................... 150
Appendices .................................................................................................................................. 172
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List of Abbreviations
Acronym Actual word/phrase
AD Atopic dermatitis
AP-2 Activator protein-2
CDSN Corneodesmosin
DSC Desmocollin
DSG Desmoglein
ELISA Enzyme-linked immunosorbent assay
FDA Food and drug administration
FPLC Fast protein liquid chromatography
hBD4 Human beta-defensin-4
HPLC High pressure liquid chromatography
HTS High throughput screen
IL Interleukin
KLK Kallikrein/Kallikrein-related peptidase
LG Lamellar granules
LEKTI Lympho-epithelial Kazal-type inhibitor
NS Netherton syndrome
PAGE Polyacrylamide gel electrophoresis
PAR Proteinase-activated receptor
PASI Psoriasis and area and severity index
Ps Psoriasis
PsA Psoriatic Arthritis
RPC Reversed phase chromatography
RT Room temperature
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RT-PCR Reverse- transcription-polymerase chain reaction
S100A7 Psroiasin member of the S100 family
SBTI Soybean trypsin inhibitor
SC Stratum corneum
SCCE Stratum corneum chymotryptic enzyme
SCTE Stratum corneum tryptic enzyme
SG Stratum granulosum
SKALP Skin-derived antileukoproteinase
SLPI Secretory leukocyte protease inhibitor
SLS Sodium Lauryl Sulfate
SPI Serine protease inhibitor
SPINK Serine protease inhibitor Kazal-type
Th T Helper cells
TPA 12-O-tetradecanoylphorbol-13-acetate
List of Tables
Table Title Page
1.1
Relative trypsin-like KLK levels measured by enzyme-linked
immunosorbent assay (ELISA) of stratum corneum tissue extracts
and sweat of normal human epidermis
20
1.2 Summary of KLK involvement in skin disease pathologies 31
1.3 Differences between atopic dermatitis and psoriasis 35
2.1 Divalent ion effect on mat-KLK8 activity 60
3.1 KLK8 serum levels predict positive response to TNFα-blockers 104
4.1 Demographics and clinical characteristics of psoriatic disease patients 118
4.2 KLK levels in the serum of plaque-type cutaneous psoriasis (PsC) and
psoriasis arthritis (PsA) patients
121
4.3
Polychotomous logistic regression analysis to identify biomarkers
associated with patients having psoriasis alone and psoriatic arthritis
123
5.1 Libraries selected for KLK-inhibitor screening 134
5.2 Primary HTS assays identify potential KLK5, KLK8 and KLK14-specific
small molecule inhibitors
140
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5.3 KLK-specific inhibitors IC50’s 141
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List of Figures
Figure Title Page
1.1 Endoproteases and exproteases 3
1.2 The Schechter and Berger enzyme-substrate binding scheme 6
1.3 Schematic representation of serine protease catalytic mechanism 7
1.4 Kallikrein-related peptidase-8 in the protease family tree 9
1.5 Genomic and proteomic structure overview of Kallikrein-related
peptidases
11
1.6 Splice isoforms of the KLK8 gene 13
1.7 Stratified human epidermis 17
1.8 Kallikrein skin barrier proteolytic cascade 27
2.1 Activity and autodegradation of recombinant mat-KLK8 54
2.2 Pro-KLK8 activation by KLK5 54
2.3 KLK8 displays restricted substrate specificity based on cleavage of FRET
peptides
60
2.4 KLK8 activation of pro-KLK1 and pro-KLK11 62
2.5 Proteolytic processing of the LL-37 antimicrobial peptide by KLKs 64
2.6 KLK8 is a skin barrier protease 66
2.7 Immunocapture of KLK8 activity in normal human sweat and non-
palmoplantar stratum corneum ex vivo
71
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2.8 The majority of sweat and SC KLK8 is catalytically active 73
3.1
Differential Kallikrein-8 secretion by differentiating keratinocytes in
response to Th1 (TNFα and IFNγ), Th17 (IL17A, IL22) and Th2 (IL4,
IL13, IL25) cytokines
87
3.2 TNFα+IL-17A treatment induces changes in HaCat keratinocytes that
mimic psoriatic skin.
90
3.3 KLK8 treatment enhances differentiation and induces desquamation of
stratification domes in HaCat keratinocytes monolayers.
92
3.4 KLK8 treatment enhances differentiation of normal full thickness human
epidermis model and alters innate immunity gene expression.
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3.5 Alterations in proliferation and differentiation markers in KLK8-treated
full thickness epidermis model and psoriatic skin.
95
3.6
3.7
3.8
KLK8 overexpression induces drastic changes in full thickness epidermis
model
KLK8 is significantly overexpressed in lesional psoriatic skin washes
only, unlike other KLKs
KLK8 epidermal expression is elevated in lesional psoriasis and reduced
in lesional atopic dermatitis skin, compare to respective non-lesional
counterparts.
96
97
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3.9 KLK8 overexpression in lesional psoriatic skin, is not restricted to the
epidermis, but is also seen in dermis immune infiltrate near the epidermis,
unlike atopic dermatitis skin.
100
3.10 Expression of KLK8 and other innate immunity genes in lesional psoriatic
skin pre and post-treatment with the TNFα-blocker, etanercept
102
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4.1 Expression of KLK proteases in PsA inflamed joint synovial fluids and
control (osteoarthritis) synovial fluids.
116
4.2
4.3
Immunohistochemical expression of KLK6 and KLK8 in lesional
psoriatic skin.
KLK6 and KLK8 cannot function as screening biomarkers for arthritis in
psoriasis patients
117
125
4.4 KLK8 correlates positicely with PASI scores in psoriatic disease 126
5.1 KLK8 does not cause calcium signalling via either human PAR1 or
PAR2, but disarms thrombin-mediated human PAR1 signalling
136
5.2 KLK8 does not trigger human PAR2 and -arrestins interaction nor PAR2
internalization, unlike KLK14
137
5.3 Unlike KLK14, KLK8 does not activate P42/44 MAP kinase-signalling in
human PAR2-expressing cells
138
5.4
An example of KLK8-sepcific inhibitor identified from the high
throughput screen
136
5.5 KLK8 in normal and psoriatic skin 147
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List of Appendices
Label Title Page
2.1
Steady-state kinetic parameters for the hydrolysis of synthetic AMC
substrates by mat-KLK8 in optimal activity buffer
172
3.1 Demographics and disease characteristics of psoriasis patients pre and
post-treatment
173
3.2 KLK serum levels pre and post psoriasis treatment with TNFα-blockers 174
5.1 Differences in KLK5 and KLK8 active site pockets 175
1
Chapter 1
Introduction
Sections of this chapter were reproduced from the following published manuscripts:
Eissa A, and Diamandis E.P. Tissue Kallikrein-related peptidases as promiscuous modulators of
homeostatic barrier functions. Biol Chem. 2008; 286: 687-706
Eissa, A. and Diamandis, E. P. Kallikrein protease involvement in skin pathologies supports a
new view of the origin of inflamed itchy skin in Proteases and Their Receptors in Inflammation,
N. Vergnolle and M. Chignard Editors. Springer Basel. 2011
Eissa, A. and Diamandis, E. P. Kallikrein-related peptidase-8 (KLK8) in Handbook of
Proteolytic Enzymes. Third Edition. Neil Rawlings and Guy Salvesen Editors, Elsevier. 2013
2
1 Introduction
1.1 Proteases: Digesting the Basics
Various types of proteases are working diligently inside and outside our human cells and many
of them are incredibly fascinating. Proteases and peptidases (also known as proteolytic enzymes
or proteinases) are enzymes that breakdown peptide bonds linking amino acids together in
proteins or polypeptides, via a process known as proteolysis. Proteases are often depicted as
“mother nature’s swiss army knives” (Seife, 1997) due to their ability to cut proteins at specific
sites. Peptides can withstand hours of boiling heat in an acid, but they cannot endure more than a
few microseconds in the presence of a protease. Hence, protease activity must be tightly
regulated and blunted with endogenous inhibitors until physiologically required.
Proteases were primarily known for their roles as digestive enzymes, since early studies of their
roles date back to the 19th
century with the characterization of pepsin and trypsin in 1836 and
1856 (Drag and Salvesen, 2010 ). Our understanding of proteases since then has outgrown
digestion and degradation. Scientists are becoming increasingly more aware of protease roles as
important regulatory and signaling molecules with hormone-like and innate immune-like
properties in several tissues. Dysregulated protease activities impact various pathways and can
lead to devastating outcomes including cardiovascular diseases, neurodegeneration,
inflammation and cancer (Turk, 2006). Thus, proteases form up to 10% of currently approved
FDA-drugs, and many more are in development as potential disease drug targets including
proteases identified from the human genome project or from genomes of disease-causing
organisms (Bachovchin and Cravatt, 2012).
1.1.1 General protease classification
Based on the location of their cleavage site on a protein or a polypeptide, proteases/peptidases
are classified as endopeptidases or exopeptidases. Endopeptidases cleave their target protein or
polypeptide internally, while exopeptidases cut at the polypeptide terminals, as depicted in
Figure 1.1. Exopeptidases are further divided into aminopeptidases or carboxypeptidases
depending whether they cleave their target peptide bond, known as the scissile bond, near the N-
terminus or C-terminus, respectively, as shown in Figure 1.1.
3
C-terminal
Endopeptidase
Scissile peptide bond in the middle of polypeptide
N-terminal
Exopeptidase
Figure 1.1. Endopeptidases and exopeptidases. (A) Protease/peptidase classification based on
site of the scissile bond being cleaved by the protease (depicted as scissors in the Figure ). (B)
Classification of proteases based on scissile bond cleavage site.
4
Over 600 human proteases are identified to date, accounting for 2-4% of the human genome. The
availability of 3D structural information on these proteases facilitated their categorization into
five distinct classes based on their catalytic mechanism. Approximately 200 metalloproteinases,
178 serine proteases, 160 cysteine proteases, 30 threonine proteases and 25 aspartic acid
proteases have been identified, with the remaining proteases belonging to groups with an
unknown or unclassified catalytic mechanism (Drag and Salvesen, 2010; Turk, 2006). A sixth
class of proteases, known as glutamic proteases, exists but these proteases are not found in
mammals. Metalloproteases, aspartic acid and glutamic proteases utilize an activated water
molecule as a nucleophile to attack the peptide bond, whereas the nucleophile in serine, cysteine
and threonine proteases is a key amino acid residue (Ser, Cys or Thr, respectively) located in the
protease active site from which the class name is derived (Hartley, 1960; Turk, 2006).
Differences in the mechanisms are also based on the presence or absence of a covalent acyl-
enzyme intermediate in the reaction pathway. Serine and cysteine peptidases catalysis involves a
covalent intermediate (ester and thiolester, respectively), whereas aspartic and metallopeptidase
protease catalytic mechanisms do not. Proteases of the different catalytic types can be further
grouped into families and clans. Barrett and coworkers’ protease classification system of
protease families and clans forms the basis of the eminent peptidase database MEROPS
(Rawlings et al., 2008). In this classification, proteases are divided into ‘clans’ based on their 3D
structural homologies and into ‘families’ on the basis of common ancestry (Barrett and
Rawlings, 1995). This thesis focuses on Kallikrein-related peptidase-8, which is a serine
protease. Serine proteases are grouped into ~ 13 clans and 40 families, representing over one
third of all known proteolytic enzymes (Di Cera, 2009).
1.1.2 Serine proteases catalytic mechanism
Serine proteases bind their substrates in a groove or cleft where the peptide amide bond gets
hydrolyzed. According to the Schechter and Berger nomenclature, the substrate amino acid side
chains occupy enzyme sub-sites in the cleft, designated as S3, S2, S1, S1', S2' and S3', which
correspond with substrate residues P3, P2, P1, P1', P2' and P3' from the N-terminal to the C-
terminal, as shown in Figure 1.2. Cleavage occurs between P1 and P1’ positions of the substrate.
Thus, the S1 sub-site residue in the protease active-site determines the substrate specificity.
5
The mechanism of serine protease catalysis begins with the binding of the polypeptide or protein
substrate target to the surface of the serine protease. The scissile bond gets inserted into the
active site, with the carbonyl carbon of this bond positioned near the nucleophilic Ser 195. As
mentioned above, scissile bond refers to the covalent chemical bond that gets cleaved by a
protease. As shown in Figure 1.3, the mechanism involves aceylation and deacylation steps
through which several intermediates are formed. Stabilized by Asp102, the nitrogen of His57
accepts a proton from Ser195, allowing the nucleophilic oxygen atom of the hydroxyl group of
Ser195 to attack the carbonyl carbon of the scissile peptide bond, and a pair of electrons from the
double bond of the carbonyl oxygen moves to the oxygen. As a result, the scissile bond gets
broken, releasing the new amino-terminus and forming an ester bond between the enzyme and
the substrate called acyl enzyme tetrahedral intermediate. In the second deacylation step, a water
molecule hyrdolyzes the ester bond of the acyl-enzyme intermediate to liberate the protease and
a peptide with a free carboxyl group (Polgar, 2005).
Figure 1.2. The Schechter and Berger enzyme-substrate binding scheme. Cleavage site is
indicated with an arrow and substrate residues (P) binding the protease binding subsites (S) are
shown. Prime and non-prime designations indicate the C-side and the N-side of the cleavage site,
respectively.
6
Figure 1.3. Schematic representation of serine proteases catalytic mechanism. The
polypeptide R’-NH-CO-R scissile bond is cleaved by a serine protease in a sequential fashion involving
two major steps: acylation in which the oxygen of the hydroxyl group of serine acts as a neucleophile
resulting in formation of the acyl-enzyme intermediate and deacylation in which water acts as a
nucleophile to release the shorter broken peptide with a free carboxyl group.
Acyl-enzyme
intermediate
Amino-terminus Leaving group Substrate scissile bond
Water as a nucleophile Peptide with a free
carboxyl group
Acyl-enzyme
intermediate
Acylation
step
Deacylation
step
Enzyme
7
1.1.3 Serine proteases of family S1 clan PA(S)
Of the ~ 690 proteases described in man, 178 are serine proteases and 138 of them belong to the
S1 family (Di Cera, 2009). Over two thirds of the PA clan is comprised of the S1 family of
serine proteases, which bear the archetypal trypsin fold. These proteases have a two-domain
structure, with each domain containing a β barrel, and the active site cleft lying in-between.
Their catalytic triad is in the order Histidine, Aspartate, Serine, where the serine residue acts as a
nucleophile, the histidine as a proton donor, and the aspartate for proper orientation of the
imidazolium ring of the histidine, as shown in Figure 1.3. The trypsin-like peptidases of family
S1 and clan PA are the most abundant serine peptidases and among the best studied (Laskar et
al., 2012). Most clan PA(S) proteases are endopeptidases and have trypsin-like substrate
specificity and prefer cleaving at the carboxyl side of arginine (Arg) or lysine (Lys) side chains
at the P1 position. Some display chymotrypsin-like and elastase-like specificity. The majority of
these proteases are secreted in inactive latent forms and are activated via proteolytic activation
cascades to participate in important physiological processes, including digestion (trypsin,
chymotrypsin), immune responses (complement factors B, C, D), blood coagulation (factors
VIIa, IXa, Xa, XIIa), fibrinolysis (urokinase, tissue plasminogen activator, plasmin, kallikrein)
and fertilization allowing the sperm to penetrate the egg (acrosin) (Page and Di Cera, 2008).
Interestingly, the largest cluster of serine proteases is located on chromosome 19 and 16,
encoding Kallikrein-related peptidase family and the Tryptase family, respectively, which belong
1.2 Kallikrein-related peptidase-8 at a glance
1.2.1 Discovery of the Kallikrein-related peptidase family
Kallikrein-related peptidases (KLKs) or tissue Kallikrein-related peptidases are a family of 15
members belonging to the chymotrypsin-like serine endopeptidase family S1, clan PA
(Diamandis et al., 2000; Yousef and Diamandis, 2001). A list of the KLKs in the human protease
family tree is shown in Figure 1.4, along with their assigned OMIM reference numbers and
former alternate gene/protein names. The first member of this family, KLK1, was discovered in
the 1930s in the pancreas, known as ‘kallikreas’ in Greek. This protease is expressed in multiple
tissues and displays kinninogenase activity, whereby it cleaves kininogens to produce kinin
peptides, which bind to kinin receptors, triggering inflammation and several biological effects.
Another kinninogenase enzyme expressed solely in the liver and encoded by a single gene on
8
chromosome 4q35 was subsequently discovered and named a kallikrein as well, based on its
kinnogenase activity. However, these two kallikrein proteases share no genomic or proteomic
structural homologies. Hence, they were designated to two separate categories, whereby the
kallikrein encoded by chromosome 19q13.4 was dubbed human tissue kallikrein (KLK1) and the
one in chromosome 4q35 was dubbed plasma kallikrein (KLK1B) (Lundwall et al., 2006).
During the late 1980s, two additional tissue kallikrein genes (KLK) were discovered in the same
genomic vicinity as KLK1; the human glandular kallikrein (KLK2) and the prostate specific
antigen (PSA, KLK3) (Borgono and Diamandis, 2004). These two Kallikrein-related peptidases
exhibited very little to no kinninogenase activity despite sharing genomic and proteomic
homologies with KLK1. Accordingly, the traditional definition of a tissue kallikrein being a
kinninogenase acting on high molecular weight substrates to produce bioactive kinins was
modified. The term “tissue kallikrein” was then introduced to define the serine proteases encoded
by genes on chromosome 19q13.4 sharing extensive structural homologies to KLK1 at the DNA
and protein level, regardless of their enzymatic activities. KLK 1, 2, and 3 were referred to as
“classical tissue Kallikrein-related peptidases” as they share a loop region found in rodent
Kallikrein-related peptidases important for the enzyme’s substrate specificity (Borgono et al.,
2007a). The remaining eleven “non-classical” KLKs, including KLK8, do not have this loop as a
result of diverting further from rodent kallikrein genes during evolution. In addition to mouse
and rat, kallikrein gene families have been identified in the chimpanzee, dog, pig, and opossum
mammalian species (Elliott et al., 2006).
The last decade of the 20th
century culminated with the full characterization of the KLK locus
expanding the human tissue kallikrein family from three to fifteen genes and a pseudogene
(KLK1), tandemly mapped to a contiguous cluster of ~ 400 kbp on chromosome 19q13.4,
forming the largest protease gene cluster in the human genome (Clements et al., 2001;
Diamandis et al., 2000; Yousef and Diamandis, 2001). The most recent nomenclature of the
kallikrein family refers to KLK1 as “human tissue kallikrein”, while the remaining KLKs are
dubbed “kallikrein-related peptidases” (Lundwall et al., 2006).
9
Figure 1.4. Kallikrein-related peptidase-8 in the human protease family tree
10
1.2.2 Genomic and proteomic structure of Kallikrein-related peptidases
Kallikrein-related peptidases share a high degree of genomic and proteomic homology as
summarized in Figure 1.5. All KLK genes contain five coding exons of similar sizes and a
conserved intron-phase pattern of I-II-I-0 (Yousef and Diamandis, 2001). About 82 KLK mRNA
forms have been reported as each KLK gene has at least one alternative splice variant (Kurlender
et al., 2005). KLK genes contain both 5’ and 3’ untranslated regions (UTRs) of varying lengths,
except for the classical KLKs. Alternatively, kallikrein proteins (KLKs) have a characteristic
multidomain single chain structure consisting of an amino terminal pre-peptide, a pro-peptide
essential for maintaining the pro-KLK protein in a latent form, and a catalytic serine protease
domain containing a highly conserved triad of histidine (H), aspartic acid (D), and serine (S)
amino acids. All KLKs are secreted proteases, as shown in Figure 1.5. KLKs get secreted as pro-
KLK zymogens upon removal of their pre-peptide signal. Cleavage of the pro-peptide induces a
conformational change in the enzyme’s active site and substrate pocket, resulting in extracellular
activation of the mature enzyme (Borgono and Diamandis, 2004). Pro-KLK activation is a key
regulatory process postulated to occur via a proteolytic activation cascade similar to the
coagulation, fibrinolysis, and complement system activation cascades (Yoon et al., 2007). Once
active, Kallikreins employ a serine-directed nucleophilic attack mechanism to hydrolyze peptide
bonds of target substrates, resulting in substrate activation, inactivation, or degradation. The
majority of KLK proteins have acidic Asp residue at position 189, or Glu189 in the case of
KLK15, in their substrate binding pocket allowing them to interact with basic arginine or lysine
residues in their target substrates and rendering them to have trypsin-like substrate specificity.
On the other hand, KLKs 3, 7, and 9 function as chymotrypsin-like serine proteases as they
contain Ser189, Asn189, and Gly189 in their substrate binding pocket, respectively,
accommodating bulky non polar amino acids such as tyrosine or phenylalanine.
11
Figure 1.5. Genomic and proteomic structure overview of Kallikrein-related peptidases.
KLK genes are localized on chromosome 19q13.4 flanked by the testicular acid phosphatase
gene (ACPT) and the sialic acid–binding immunoglobulin-type lectin-type 9 (SIGLEC-9). Each
arrow represents a certain KLK gene with its direction of its transcription. The 5’ untranslated
region and 3’ untranslated region are shown in the primary mRNA transcript. H, D, and S
represent the catalytic histidine, aspartic acid, and serine triad residues. In the KLK mRNA
schematic, boxes indicate exons and lines indicate introns, whereby KLKs have 5 coding exons
and an intron phase pattern of I,II,I,0. KLK mRNAs are translated as inactive pre-proenzymes
which are directed to the endoplasmic reticulum for secretion via their prepeptide secretion
signal. Extracellular cleavage of the propeptide by a trypsin-like protease is required for enzyme
activation. Mat-KLK refers to the mature active form of the enzyme.
Zymogen
Secretion Pro-KLK
Mat-KLK
1 15 3 2 4 5 6 7 8 9 10 11 12 1 13 14
19q13.4
Chromosome 19
Extracellular
Intracellular
SIGLEC-9
ACPT
KLK mRNA
KLK protein
5’ 2 3 4 5 1
S D H I II I 0
5’UTR 3’UTR Coding exons
3’
C Pre Pro Serine protease domain
H57 D102 S195
N
C Pro Serine protease domain
H57 D102 S195
C Pre Pro Serine protease domain
H57 D102 S195
N N Enzyme
Activation
N
Centromere
12
1.2.3 Molecular properties of Kallikerin-related peptidase-8
Kalikrein-related peptidase-8 gene (KLK8) is located in the KLK locus between KLK7 and KLK9
on the long arm of chromosome 19q13.4 and is transcribed from telomere to centromere, as
shown in Figure 1.5. The KLK8 cDNA was initially cloned by Yoshida et.al (Yoshida et al.,
1998) from human skin keratinocytes as a homologue of a mouse brain protease called neuropsin
(Kishi et al., 1999). Its cDNA has a single open reading frame of 780 bp that encodes a 260
amino-acid protein. KLK8 gene consists of 6 exons and 5 introns, with the first exon being non-
coding. The first intron is interrupting the 5’-untranslated region while the remaining 4 interrupt
the coding sequence, as shown in Figure 1.6. Two repeat sequences are present in the promotor
region of KLK8, but neither one is a TATA or a known transcription-factor binding sequence
(Yoshida et al., 1998). Regulatory transcription factors of the KLK8 promoter are still unknown.
Yet, the transcription site of KLK8 gene seems tissue-specific (Lu et al., 2007).
At the RNA level, KLK8 has been shown to have at least 5 splice variants (Magklara et al., 2001)
forming KLK8 isoforms 1, 2 to 5. These isoforms have been detected in cancer tissues as
potential prognostic biomarkers (Planque et al., 2010). Of these isoforms, the regular isoform 1
and isoform 2 encode a functional protein. As shown in Figure 1.6, Isoform 1 and 2 transcripts
differ only in their exon 3 (or coding exon 2) sequences. KLK8 mRNA isoform 2 transcript has
extra 45 amino acids at the N-terminus of its coding exon 2 making this isoform encompass a
larger signal peptide, yet its pro activation sequence and full protease domain is the same as the
classical isoform (Lu et al., 2009). Although isoform 1 is homologous with the mouse neuropsin
mRNA, isoform 2 is absent in the mouse. Human KLK8 isoform 2 is believed to have risen later
in evolution. A human-specific T to A point mutation has led to this novel form in the human
brain, but not in the brains of chimpanzees or other species (Lu et al., 2007).
Human KLK8 and mouse neuropsin have 72% cDNA and amino acid similarity (Yoshida et al.,
1998). Work in this thesis focuses on canonical KLK8 protein encoded by isoform 1, which has a
secretion signal pre-peptide of 28 amino acids, followed by the pro-zymogen activation peptide
of 4 amino acids and the mature chain of 228 amino acids with 1 potential N-linked
glycosylation site and 12 Cys residues that participate in forming 6 disulfide bonds. Its catalytic
triad of His86, Asp120, Ser212 is conserved and is essential for proteolytic activity.
13
Figure 1.6. Splice isoforms of the KLK8 gene. The start site is indicated with * and the
termination codon with an arrow. The first exon is non-coding, and hence the only coding exons
are labeled 1 to 5 in the diagram. Coding exons 2, 3 and 5 are important for activity as they
encode H, D, and S, labeled to represent the positions of the catalytic triad. Only isoforms 1 and
2 produce functionally active proteins. The 28aa pre-signal , 4 pro-sequence and 228aa active
domain shown describes the 260 aa protein formed by isoform 1. KLK8 isoform 2 has a longer
pre-pro sequence but same 228 aa sequence of the active domain. Figure is modified from
(Kurlender et al., 2005).
BC040887
NM_007196
NM_144506
NM_144505
6 Exons
171 263 70 8 156 134 51
171 263
160
70 8 134 51
NM_144507
51 156 70 171
1377
1013
1148
456
590
32
119
305
260
260
bp AA
178
170
H S D
178 304 263 70 8 156 134 46
171
1
2
3
4
28 AA 4 AA 228 AA
295
2 3 4 5
8
1
70
156
14
1.2.4 Kallikerin-related peptidase-8 knock out mouse
Neuropsin is the mouse homologue of KLK8 which shares 72% similarity. The first studies of
neuropsin-deficient mice reported abnormalities in brain synapses and neurons (Chen et al.,
1995; Kitayoshi et al., 1999; Yoshida and Shiosaka, 1999). Currently, the KLK8/neuropsin
knock out mouse is the only available in vivo mouse model with targeted kallikrein-related
peptidase disruption. Abolishing the KLK8 gene is not embryonically lethal, as mice grow
without any major defects. The neuropsin-deficient mouse revealed that KLK8/neuropsin has
major roles in the brain and skin. Brain-related studies will be summarized below, while skin-
related findings will be discussed later in the skin section.
Neuropsin is expressed in limited regions in mouse brain including the hippocampus, lateral
nucleus of the amygdala and other areas involved in learning and memory (Hirata et al. 2001).
Neuropsin role in hippocampus plasticity is linked to kindling formation and long term
potentiation (LTP) (Kishi et al., 1999). Kindling is a model of epilepsy, whereby repeated
electric stimulations cause the brain to form synaptic structures and lowers its threshold, so that
weaker stimulations can cause convulsions. Neuropsin mRNA expression is increased in the
hippocampus after cumulative stimulations, and interference with neuropsin-specific antibody
delays kindling (Yoshida, 2010). Furthermore, neurpopsin was implicated in the LTP of the
synaptic plasticity process involved in learning and memory (Komai et al., 2000). Neuropsin
protease activity is induced by activation of the ion channel glutamate receptor, N-methyl-D-
aspartate (NMDA) and is blocked by NMDA receptor inhibitors (Matsumoto-Miyai et al., 2003).
Studies have indicated that the neuropsin KO mouse displays slower learning. It is assumed that
neuropsin remains inactive until stimulated by synaptic activation. An elegant recent study in
Nature showed that neuropsin is also critical for stress-related plasticity in the amygdala of mice,
as it regulates EphB2-NMDA-receptor interaction, Fkbp5 gene expression and anxiety-like
behavior (Attwood et al., 2011). In the central nervous system, neuropsin is not expressed in the
white matter or nerve fiber tract of healthy mice. Its expression is induced only after physical or
chemical injury to the spinal cord by oligodendrocytes near the neuronal lesion (Terayama et al.,
2007; Terayama et al., 2004). Neuropsin was also implicated in an experimental model of
multiple sclerosis, an autoimmune disease in which oligodendrocytes are dead and myelin is
degraded possibly through neuropsin actions (Terayama et al., 2005).
15
1.3 Kallikrein-related peptidases in normal human Epidermis
1.3.1 Normal skin structure and function
Human skin’s outer layer, the epidermis, is the body’s first line of defense against harsh
environment, water loss, chemical and physical damages, UV-radiation, allergen and pathogens
entry. It protects the body with its flexibility and toughness, all while acting as a sensory organ.
The skin is composed of 3 main layers: the epidermis, dermis and subcutaneous tissue. Mature
epidermis consists of 4 layers: the stratum basale (SB), stratum spinosum (SS), stratum
granulosum (SG) and stratum corneum (SC), in order of increased differentiation from the lowest
layer to the outer skin surface. The stratum basale displays characteristic small downward folds
into the dermis known as ‘rete ridges’. Keratinocytes form the overwhelming majority of cells in
the epidermis, with a few melanocytes and langerhan cells. Keratinocytes secrete over 30 types
of keratins during their differentiation into distinct epidermal layers or ‘strata’. The dermis layer
provides structural support, houses blood vessels and nerves, and is a base for skin appendages
such as hair follicles, sebaceous and sweat glands. The main cells in this layer are the fibroblasts
embedded in a matrix of collagen, glucosaminoglycans and glycoproteins which they produce.
Below the dermis lies a subcutaneous tissue containing fat cells, known as adipocytes, which
serves as a temperature insulator and a cushioning layer.
The majority of skin barrier functions are attributed to the uppermost epidermal layer, the
stratum corneum. The stratum corneum (SC) layer gets renewed every 2-4 weeks via an elegant
differentiation program of keratinocyte cells (Candi et al., 2005), which are the major cell
constituents of the epidermis (Simpson et al., 2011). The formation of the stratified epidermis
begins by the commitment of a single layer of multipotent ectodermal progenitor cells to a
keratinocyte cell fate in the lower stratum basale. Keratinocytes at the basal layer (SB) withdraw
from the cell cycle, detach from the basement membrane, and proliferate upwards to differentiate
into intermediate spinous (SS) and granular layers (SG), pushing already formed cells higher up.
At each stage the keratinocytes express a different set of proteins, and thus proliferating
keratinocytes and differentiating ones have a different set of markers. For instance the nuclear
Ki67 antigen is often used a marker of keratinocytes proliferation at the stratum basale, and
involucrin is a cytoplasmic marker of differentiation at the SG. As shown in Figure 1.7, basal
keratinocyte proliferation and spinous keratinocyte differentiation culminates with the processes
16
of terminal keratinocyte differentiation and cornification at the stratum granulosum/stratum
corneum (SG/SC) interface, where granular keratinocytes:
1. Transport and secrete cargos via their lamellar granules (LG) into the SG/SC extracellular
space. These cargos include structural proteins, adhesion proteins, lipids, lipid-processing
enzymes, antimicrobial peptides and a cocktail of proteases and protease inhibitors
2. Replace their plasma membrane with a tough insoluble protein and lipid envelope known
as the cornified envelope (CE)
3. Aggregate their keratin intermediate filaments via filaggrin (FLG), causing collapse of
their cytoskeleton into flattened squames
4. Lose their nuclei and sub-cellular organelles to get terminally-differentiated into non-
viable anucleated cells, known as ‘corneocytes’
The last step of skin barrier formation, shown in Figure 1.7, is known as ‘desquamation’. Skin
desquamation refers to corneocyte shedding off the skin surface as a result of regulated
degradation of adhesion proteins linking uppermost corneocytes, known as corneodesmosomes,
by endogenous proteases (Milstone, 2004). Inherent terminal keratinocyte differentiation and
corneocyte desquamation ensue in parallel to maintain the SC barrier thickness relatively
constant.
17
Figure 1.7. Stratified human epidermis. Human skin is composed of outer epidermis and inner
dermis. Normal human epidermis is a stratified epithelium of keratinocytes in the stratum basale
(SB), stratum spinosum (SS), stratum granulosum (SG), and corneocytes in the uppermost
stratum corneum (SC), in order of increasing differentiation. Common markers of keratinocyte
proliferation and differentiation are shown on the left of the H&E human skin image next to their
corresponding layer of expression. Ki67 is a marker of cell proliferation in the SB, keratin 10 or
K10 in an early differentiation marker in the SS, and involucrin is a late differentiation marker of
SG and SC layers. Epidermal programming is characterized by proliferation, differentiation,
cornification, and desquamation events.
Epidermis
Dermis
Ki67
K10
Inv
18
Up until the last two decades, the SC was viewed as a dead layer of non-viable corneocyte cells
embedded in a lamellar lipid sea, which was often represented by the skin barrier “brick and
mortar” model of vertically-stacked corneocyte ‘bricks’ held together by an extracellular lipid
‘mortar’ (Elias, 1983). With further studies, it became apparent that the SC layer is full of
exciting metabolic activity and is more dynamic than previously thought, as several terminal
keratinocyte differentiation products and metabolic processes take place in its extracellular
milieu to regulate barrier functions.
Although the co-localized SC barrier functions are highly inter-dependent, their molecular and
biochemical basis tend to differ. For example, in the SC, filaggrin serves as a template for the
assembly of corneocytes’ cornified envelope forming flat and tough corneocyte ‘bricks’ in the
outer barrier. Filaggrin ultimately dissociates to free amino acids that form the skin’s natural
moisturizing factor (NMF), which creates a hydrated and acidic skin surface (Hachem et al.,
2003; Rippke et al., 2004). Moreover, lipid precursor processing by β-glucocerebrosidase, acidic
sphingomyelinase and secretory phospholipase A2 enzymes into ceramides and free fatty acids
generates mature lipid lamellar-membranes that hamper transepidermal water loss (TEWL) and
form the SC permeability barrier (Hachem et al., 2010; Ohman and Vahlquist, 1994). In addition
to its hydrophobic content and acidic pH, the skin surface contains antimicrobial peptides (AP),
and certain keratinocyte, eccrine and sebaceous gland-derived proteases and protease inhibitors
which comprise its outermost antimicrobial shield (Braff et al., 2005a; Braff et al., 2005b; Lee et
al., 2008; Nizet et al., 2001). Additionally, specialized SC extracellular adhesion proteins, known
as corneodesmosomes (Candi et al., 2005; Simpson et al., 2011), are incorporated into the
corneocyte envelope to adhere corneocytes together and maintain the SC structural barrier. It is
important to note that KLK proteases co-localize with many of these barrier molecules in the
epidermis, as discussed below. Thus, epidermal KLK serine protease activity is implicated in
regulating many physiological features of a healthy skin barrier including lipid content, proper
antimicrobial shield formation and corneodesmosome degradation.
1.3.2 Kallikrein expression in the skin
Multiple Kallikrein-related peptidases are expressed in the skin epidermis and its associated
appendages. Of the 15 KLK-related peptidases present in the human body, eight KLKs co-
localize in human epidermis in addition to the parent tissue KLK1 (Komatsu et al., 2005b;
19
Komatsu et al., 2006b). These KLKs are expressed in the SC, upper SG, sebaceous glands,
eccrine sweat glands, hair follicles, and nerves. Kallikrein-related peptidases are detected by
immunohistochemistry in glandular epithelia secretions confirming their extracellular
localization (Komatsu et al., 2005b).
Kallikrein transcripts and/or proteins have been detected by RT-PCR and immunostained in the
SC, upper SG, sebaceous glands, eccrine sweat glands, hair follicles, and nerves (Komatsu et al.,
2005b; Komatsu et al., 2003). KLK mRNA expression in the inner and/or outer root sheath of
hair follicle epithelia (Komatsu et al., 2003), suggests KLK involvement in hair development.
KLK mRNAs and proteins are also intensely expressed in the basal layer of undifferentiated
sebocytes, indicating their potential participation in sebaceous gland differentitation and sebum
formation. Moreover, KLKs such as KLK6, 8, and 13 are found in the inner lumen of sweat
gland ducts and hence are expected to be secreted in sweat (Komatsu et al., 2005b). Indeed, the
expression of these three Kallikrein-related peptidases in human sweat and skin surface was
confirmed recently by ELISA quantification, verifying previous immunohistochemistry results.
KLK3 and 9 proteins have not been shown to be expressed in the epidermis (Komatsu et al.,
2005a), conferring the epidermal chymotrypsin-like serine protease activity to KLK7 solely. In
addition to the chymotrypsin-like KLK7, seven trypsin-like Kallikrein-related peptidases (KLKs
5, 6, 8, 10, 11, 13, and 14) have been detected in human SC and sweat from different body
regions (Komatsu et al., 2006b). Komatsu et al. detected a broad range of SC trypsin-like KLK
levels (KLK5, 6, 8, 10, 11, 13 and 14) where the most abundant KLKs (KLK8 and 11) are about
200-fold greater than the least abundant ones (KLK14 and 13). The levels of KLK8 and 11 are
similar in the SC, but KLK8 concentration is significantly higher in sweat, representing up to
60% of the total trypsin-like KLK levels, as shown in Table 1.1. In a different study, Komatsu
and colleagues showed that the total trypsin-like KLK concentration in the SC is approximately
double the total chymotrypsin-like concentration (Komatsu et al., 2005a); although this does not
necessarily signify a higher trypsin-like activity.
Studies have also reported visualizing KLKs, such as KLK5, KLK7 and KLK8, as they are
distinctly transported by lammelar granules (LG) of granular keratinocytes in the SG and
secreted to co-localize at SG and SC interstices (Ishida-Yamamoto et al., 2005; Ishida-
Yamamoto et al., 2004). To date, three KLKs, namely KLK5, 7, and 14 have been extracted in
active forms from SC tissues (Brattsand et al., 2005; Ekholm and Egelrud, 1999). Kallikrein-
20
related peptidases expression in human dermis had been less studied; nonetheless
immunohistochemical studies have indicated that KLKs can also be expressed by endothelial
cells in blood vessels. Our understanding of the expression and potential role of KLKs in dermal
endothelial and immune cells is rudimentary and needs to be studied.
Profiling of epidermal Kallikrein-related peptidases based on enzymatic activity instead of
protein levels is difficult, because activity results vary depending on the assay type used
(Brattsand et al., 2005). The majority of protease assays used to measure endogenous kallikrein
activity are not KLK-specific. Casein zymography analysis of SC tissue extracts indicates that
KLK5 and KLK7 are the major active KLKs in the SC, while chromogenic peptide substrate
studies attribute up to 50% of trypsin-like activity to KLK5, and the majority of the remaining
activity to KLK14 (Brattsand et al., 2005). A separate study indicated that KLK14 accounts for
50% of the overall SC trypsin-like KLK activity (Stefansson et al., 2006), although denoted a
minor SC trypsin-like kallikrein by Komatsu eta al. In general, quantification of epidermal KLKs
based on enzymatic activity is a difficult mission because KLK activity in the epidermis is
regulated by different endogenous and environmental factors.
Table 1.1 Relative trypsin-like KLK levels measured by enzyme-linked immunosorbent assay
(ELISA) of stratum corneum tissue extracts and sweat of normal human epidermis
21
1.3.3 Dermatological roles of epidermal Kallikrein-related peptidases
In general, KLKs have two major functions in human epidermis: regulation of normal skin
barrier thickness through differentiation and/or desquamation and modulation of innate and
adaptive immune responses. Their role in desquamation is the best studied thus far, however
mechanisms governing their roles in keratinocyte differentiation and innate/adaptive immunity
are emerging.
a) Skin Desquamation
Skin desquamation is a pH and calcium-dependent process, suggested to occur via initial
proteolysis of non-peripheral corneodesmosomes at the transition from inner stratum compactum
to outer stratum disjunctum of the stratum corneum layer, resulting in retention of
corneodesmosomes at the lateral edges of corneocytes. These corneodesmosomes are degraded
in the superficial SC leading to corneocyte shedding (Ishida-Yamamoto et al., 2005). KLK5 and
KLK7 degrade DSG1, DSG4, and DSC1 corneodesmosomal isoforms(Borgono et al., 2007b;
Descargues et al., 2006), as well as their CDSN glycoprotein(Simpson et al., 2011). KLK 6, 13,
and 14 are potential desquamatory enzymes since they digest DSG1 and/or are inhibited by the
epidermal KLK inhibitor, LEKTI (Borgono et al., 2007b).
b) Lipid Permeability Barrier
Skin barrier function depends on the formation of mature lamellar membranes in the SC
subsequent to proper extracellular lipid processing of lipid precursors secreted by LGs. Lipid
precursors such as glycosylceramides, sphingomyelin, and phospholipids are released into the
SG/SC interface to get processed by β-glucocerebrosidase, acidic sphingomyelinase, and
secretory phospholipase A2 into ceramides and free fatty acids. The resulting lipids, particularly
ceramides, are essential as they form extended hydrophobic lamellar sheets in SC extracellular
spaces limiting water and electrolyte loss and composing the skin’s lipid permeability barrier
(Hachem et al., 2006; Hachem et al., 2005).
Lamellar membrane organization is pH and calcium-dependent, as studies have shown that lipid
processing enzymes exhibit optimal activity at acidic pH (Hachem et al., 2003). Experimental
elevations of pH induce lipid processing defects visualized by the formation of immature
lamellar membranes in the SC. The pH-induced lipid barrier dysfunction is suggested to be
22
mediated by the actions of epidermal kallikreins, as KLKs may regulate lamellar membrane
formation via proteolytic degradation of SC lipid-processing enzymes (Hachem et al., 2003) or
downregulation of their secretion by LGs (Hachem et al., 2006). Earlier studies showed that
incubations of skin extracts with active recombinant KLK7 at an elevated pH of 7.6 results in
decreased immunoblotting of both β-glucocerebrosidase and acidic sphingomyelinase lipid
processing enzymes. However, later studies by the same group have revealed that inhibition of
serine protease activity leads to permeability barrier recovery by enhancing LG secretion of
lipids. Hence, the current premise suggested by Hachem et al. is that LG secretion, not lipid
processing, is down-regulated by pH-induced increases in kallikrein activity, leading to barrier
disruption. The deregulation of LG lipid secretion is suggested to occur via kallikrein-mediated
activation of proteinase activated receptors, PARs (Hachem et al., 2006).
c) Proteolytic Processing of Antimicrobial Peptides
Epidermal keratinocytes synthesize and secrete antimicrobial peptides that harbor the skin’s
innate immunity against bacterial, fungal, and viral infections. β-defensins and cathelicidins are
two major antimicrobial peptide families expressed by keratinocytes and neutrophils, where β-
defensins are constitutively expressed (Ong et al., 2002), and cathelicidins are induced and
deposited at inflammation sites upon infection (Braff et al., 2005b; Ong et al., 2002). Tight
control of cathelicidin peptides expression is required to ensure their activity when defense
against microbial invasion is required. Cathelicidins comprise a conserved N-terminal cathelin
pro-domain and a variable C-terminal antimicrobial domain of 30-40 amino acids that becomes
active after cleavage (Niyonsaba et al., 2010). KLK5 and KLK7 regulate cathelicidins’ pro-
inflammatory activity by processing either the nascent pro-cathelicidin (hCAP18) or the mature
peptide form (LL-37), serving as activators and inactivators. hCAP18 is biologically inactive
(Zaiou et al., 2003). KLK processing of hCAP18 to active antimicrobial peptide forms, such as
LL-37 stimulates host cell inflammatory reactions in response to infection as LL37 is known to
act as a chemoattractant of neutrophils, monocytes, mast and T-cells upon tissue insult
(Yamasaki et al., 2006). Subsequent to resolving the microbial challenge, KLK5 and KLK7
process LL-37 to peptide forms that lack pro-inflammatory activity, bringing the SC back to its
normal immuno-barrier setting. Furthermore, cathelicidin processing has been shown to be
altered in vivo in the absence of the epidermal serine protease inhibitor LEKTI (Yamasaki et al.,
23
2006), suggesting LEKTI involvement in antimicrobial peptide processing via its inhibitory
effect on kallikreins.
d) PAR-mediated effects
Proteinase-activated receptors (PARs 1-4) are members of the seven-transmembrane G-protein-
coupled receptor (GCPR) family, activated by proteases such as trypsin, mast cell tryptase,
cathepsin G, and thrombin. PARs are cell surface receptors expressed on keratinocytes (PAR2),
melanocytes (PAR1), fibroblasts (PAR2), neurons (PAR2), and dermal capillaries (PAR1)
(Rattenholl and Steinhoff, 2008). PAR activation occurs intramolecularly by irreversible
proteolytic cleavage of the extracellular N-terminal peptide exposing a tethered ligand that binds
the second extracellular loop of the receptor, initiating signaling. PARs mediate multiple
signaling pathways by coupling to G-proteins and stimulating a variety of downstream targets.
Growing evidence now attests to the role of kallikreins as modulators of PAR signaling. Recent
in vitro and in vivo work by Oikonomopoulou et al. has demonstrated that PAR activity may be
targeted by active KLK 5, 6, and 14. KLK5 and KLK6 were shown to activate PAR2, while
KLK14 was reported to inactivate PAR1 and activate PAR2 and PAR4 (Oikonomopoulou et al.,
2006). Among the four PARs, PAR2 is of prime interest as it is activated by trypsin cleavage and
co-localized with tissue kallikreins in the stratum granulosum and in keratinocytes of hair
follicles and sebaceous glands.
1.3.4 Regulation of epidermal Kallikrein-related peptidases
a) Proteolytic Activation Cascade
Pro-Kallikrein-related peptidases are believed to be activated in a step-wise manner forming an
activation cascade, where the active form of one kallikrein catalyzes the activation of the next
pro-KLK. The occurrence of such a cascade in the skin is supported by the co-expression of
multiple KLKs in upper epidermal layers and sweat at varying concentrations, and by the ability
of some of these KLKs to auto-activate and activate other pro-KLKs (Brattsand et al., 2005;
Yoon et al., 2007). A kallikrein may take on the role of the initiator, propagator, and/or executor
within the cascade, depending on its concentration, specificity, and activity level. However,
minute amounts of the initiator are sufficient to trigger the cascade, due to its catalytic nature. In
vitro kinetic studies demonstrated that pro-KLK5 is activated by KLK14 and KLK5 itself, and
24
active KLK5 activates pro-KLK7, which are the main players of the skin desquamation cascade,
as shown in Figure 1.8. Recently, Yoon and colleagues characterized the first extensive “KLK
activome” upon examining the hydrolysis of 15 pro-KLKs by mature recmobinant KLKs (Yoon
et al., 2007). Their novel findings allow expansion of the epidermal KLK activation cascade to
include additional KLK members and thrombostasis proteases (Yoon et al., 2009; Yoon et al.,
2008). The contribution of this in vito KLK activome to the skin barrier cascade remains to be
elucidated. Furthermore, the characterization of initiators, propagators, and executors in the KLK
proteolytic activation cascade poses a challenge that remains to be solved, although KLK5 is
believed to be the cascade initiator (Brattsand et al., 2005).
b) Lamellar Granule Trafficking
Lamellar granules (LGs), or lamellar bodies, are epidermal secretory granules that deliver cargos
synthesized in upper granulocytes to SC interstices, including kallikrein serine proteases. In
addition to epidermal transport and release of contents into intercellular spaces, LGs function as
cargo storage sites (Braff et al., 2005b). LGs fuse with the apical membranes of uppermost
granulocytes releasing their contents into extracellular spaces (Ishida-Yamamoto et al., 2005;
Ishida-Yamamoto et al., 2004). Secretion from the uppermost differentiated granular
keratinocytes allows KLKs to be delivered at close proximity to target substrates, such as
corneodesmosomes, regulating their degradation which results in skin desquamation. Cargos,
such as KLK5, 7, their epidermal inhibitor LEKTI, and target substrate corneodesmosin (CDSN),
are separately co-localized and transported in different LG vesicles before their release into the
SG/SC interface (Ishida-Yamamoto et al., 2005; Ishida-Yamamoto et al., 2004). LG separate
transport averts any possible premature enzymatic activties among cargos, such as the
proteolysis of CDSN by KLK5 or KLK7 which is known to occur at a pH of 5.5, similar to the
environmental pH of LGs (Ishida-Yamamoto et al., 2004). In addition to transport, LG’s regulate
the time and location of cargo secretion. For example, LEKTI inhibitor has been shown to be
secreted earlier than KLKs into the superficial SG layer, while KLK5 and 7 are secreted into SC
interstices. The exact mechanism for LG sorting, transport, and secretion of cargos is not fully
understood, but selective aggregation and condensation of cargos has been suggested (Ishida-
Yamamoto et al., 2005; Ishida-Yamamoto et al., 2004). Nonetheless, the temporal and spatial
transportation and secretion of KLKs by LGs are critical regulatory events controlling epidermal
kallikrein expression. Negative feedback loops regulating LG secretion of KLKs may also occur
25
in the epidermis as hyperactive Kallikrein-related peptidases have been reported to decrease LG
secretions (Hachem et al., 2006).
c) Colocalization with Epidermal Substrates and Inhibitors
Numerous skin-specific kallikrein substrates and inhibitors co-express with Kallikrein-related
peptidases in human SG and SC layers. Co-localization with myriad of substrates in the SC,
including: hCAP18 and LL37 cathelicidin antimicrobial peptides, and DSG1, DSG4, DSC1 and
CDSN corneodesmosomal cadherins, allows for their efficient proteolysis by Kallikrein-related
peptidases. On the other hand, co-localization with epidermal serine protease inhibitors, such as
lympho-epithelial Kazal type inhibitor (LEKTI), elafin, and secretory leukocyte protease
inhibitor (SLPI) results in regulation of KLK activity. Akin to other serine proteases, Kallikrein-
related peptidases can be inhibited or trapped by forming a stable covalent complex with
endogenous members of the superfamily of serine-protease inhibitors, serpins, such as α1-
antitrypsin, whereby the serpin acylates the protease active serine resulting in a conformational
change of the serpin’s reactive center and a destruction of the proteases’s active site. Elafin and
SLPI do not inhibit KLK5, 6, 13 and 14 (Borgono et al., 2007b), but these inhibitors result in
chymotrypsin-like KLK7 inhibition and cornocyte shedding in vitro (Franzke et al., 1996).
Alternatively, several KLKs including KLK5, 6, 7, 13, and 14 are inhibited by LEKTI inhibitory
domains (Borgono et al., 2007b; Deraison et al., 2007). Similar to KLKs, LEKTI is expressed in
normal stratum corneum, stratum granulosum and skin appendages (Bitoun et al., 2003;
Raghunath et al., 2004). LEKTI is a reversible inhibitor of 1064 amino acids encoded by
SPINK5 (serine protease inhibitor Kazal type 5) gene and organized into fifteen serine protease
inhibitory domains (D1-D15) (Bitoun et al., 2003). The full length protein is an inactive inhibitor
of KLKs. Its intracellular cleavage by furin generates single or multidomain inhibitory fragments
that get secreted by keratinocytes to inhibit KLKs (Deraison et al., 2007).
LEKTI domains display distinct inhibitory profiles as they are selective towards KLK (Egelrud
et al., 2005; Schechter et al., 2005). For instance, KLK5 inhibition can be achieved by all LEKTI
domain fragments, excluding D1. LEKTI domains D8-11 exhibited the strongest inhibition
towards trypsin-like KLK5 and 14 with low Ki values of 3.7 nM and 3.1 nM, respectively, and a
much lower inhibition of the chymotrypsin-like KLK7 with Ki of 34.8 nM (Deraison et al.,
2007). These results are consistent with Borgono et al. findings of D1-8 inhibition of multiple
26
KLKs, D12-15 selective inhibition of KLK5 only, and D9-12 highest inhibition specificity
towards KLK5 (Borgono et al., 2007b). Interestingly, neither KLK8 nor KLK1 were inhibited by
LEKTI. The strongest inhibitory capacity of multiple LEKTI fragments towards KLK5 supports
the paradigm of KLK5 being the initiator of the epidermal KLK activation cascade.
c) Epidermal pH and Calcium Gradients
It is important to note that SC integrity and the majority, if not all, of its barrier functions are
governed by its inherent calcium and pH gradients. Human SC extracellular calcium levels
increase and pH levels decrease from the lower SG/SC border to the uppermost skin surface. The
innate increase in epidermal calcium concentrations regulates terminal keratinocyte
differentiation, lamellar granule (LG) secretion and cornified envelope formation, while the
innate decrease in SC pH levels, from pH 7.0 to pH 5.0 at the skin surface, regulates
desquamation, lipid permeability and antimicrobial barrier integrity.
Epidermal pH regulation of Kallikrein-related peptidases is bidirectional, as it modulates KLK
inhibition, as well as activity. Experimental pH increase elevates KLK activity and results in
over-degradation of SC structural proteins, such as DSG1 corneodesmosome, and of SC lipid-
processing enzymes, such as β-glucocerebrosidase, leading to destruction of SC cohesion and
lipid barrier, respectively (Hachem et al., 2005). Epidermal pH also regulates the kinetics of the
interaction between KLKs and the serine protease inhibitor LEKTI. A recent in vitro study by
Deraison et al. has shown that LEKTI fragments D8-11 tightly bind KLK5 forming stable
complexes at pH 7.5, which mimics the pH of the SC/SG border. Increased dissociation of KLK5
from LEKTI fragments occurs upon decreasing pH from 7.5 to 4.5, akin to the SC pH gradient
(Deraison et al., 2007). Thus, the processes of KLK binding to LEKTI fragments in the deeper
SC and release of free KLKs in the superficial SC are intrinsically governed by the decreasing
pH gradient along the SC, as shown in Figure 1.8. The removal of KLK inhibition combined
with the retention of KLK proteolytic activity at acidic SC pH of 5.5 to 4.5 leads to regulated
corneodesmosomal degradation and proper skin desquamation from the superficial SC layer
(Caubet et al., 2004; Deraison et al., 2007). The pH-dependent regulation of KLK activity
implicates SC homeostatic barrier functions other than desquamation, such as antimicrobial
processing and lipid barrier formation.
27
Figure 1.8. Kallikrein skin barrier proteolytic activation cascade
Known activation interactions between the eight kallikreins expressed in the stratum corneum
(SC) forming a SC tissue-specific activation cascade are indicated with solid black arrows while
unknown ones are denoted with question marks at the top. Upon activation, KLKs (KLK5 and
KLK7) can target corneodesmosomes (DSG1, DSG4, DSC1, and CDSN) leading to skin
desquamation or (KLK5, 6, and 14) can target PAR2 activation in keratinocytes. Epidermal
serine protease inhibitors (LEKTI, elafin, and SLPI) regulate KLK activity in the epidermis.
pH ~ 5 pH ~ 7.5
High Ca2+ Low Ca2+
28
1.4 Kallikrein-related peptidases in inflammatory skin diseases
Aberrations in kallikrein levels and/or activity have been detected in inflammatory skin diseases
such as: Atopic Dermatisis (AD), Netherton syndrome (NS), Psoriasis, peeling skin syndrome
and Acne Rosacea. Accumulation of scales with increased numbers of persisting
corneodesmosomes in the upper SC has been detected in many xeroses and hyperkeratosis skin
states. Overexpression of trypsin- and chymotrypsin-like KLK levels was detected spanning the
SC, SG and the lower epidermis of AD skin lesions, with chymotrypsin-like elevations being
more prominent (Komatsu et al., 2007a). However, KLK overexpression in AD does not
translate into any significant increase in SC trypsin or chymotrypsin-like activities, presenting a
puzzling observation that remains to be illuminated (Komatsu et al., 2007a). Alternatively,
hyperactivity of KLK7 and elevated levels of many trypsin-like KLKs were detected in psoriasis.
Some trypsin-like KLKs (KLK6, 10, and 13) were also elevated in non-lesional SC of psoriatic
patients (Komatsu et al., 2007b). Simiarly, KLK overexpression was detected in the SC of
patients with peeling skin syndrome (Komatsu et al., 2006a). PAR2 receptors are overexpressed
in the epidermis of AD and NS skin lesions exhibiting similar co-localization as human tissue
kallikreins (Rattenholl and Steinhoff, 2008), which suggests KLK-PAR co-regulation and
involvement in the pathogenesis of inflammatory skin diseases. KLKs have been suggested to
induce inflammation in these skin disorders via PAR2 activation, in addition to inducing sweat-
mediated itch in AD (Stefansson et al., 2008; Steinhoff et al., 2003).
The paramount importance of maintaining a physiological regulatory balance between KLKs and
their epidermal inhibitors is demonstrated in the devastating rare skin disease Netherton
syndrome (NS). NS is characterized by severe barrier dysfunction, ‘bamboo hair’, and atopic
allergy-like symptoms resulting from SPINK5 gene mutations leading to loss or truncation of the
serine protease inhibitor LEKTI (Chavanas et al., 2000; Descargues et al., 2005). The skin
barrier dysfunction symptoms are mediated by KLK hyperactivity in the LEKTI-free NS
epidermis (Komatsu et al., 2002). By employing the Netherton Syndrome (NS) mouse model and
confirming results in human NS skin, Briot et.al demonstrated that KLK5 indeed induces
inflammation and atopic-like lesions in NS skin via a PAR2-NFκB mediated cytokine burst that
creates a pro-Th2 inflammatory microenvironment in the underlying dermis (Briot et al.,2010).
In LEKTI-deficient epidermis, hyperactive KLK5 activates PAR2 by proteolytic cleavage, and
29
induces NFκB-mediated ICAM, IL-8, TNF-α and TSLP cytokine overexpression.
Overexpression of the proallergic cytokine TSLP implicates KLK5 hyperactivity in an innate
allergy regulatory pathway, which may explain the susceptibility of the majority of NS patients
to develop AD.
KLKs are also known to mediate inflammation via their inherent ability to regulate skin
antimicrobial peptide processing, activity and function. Cathelicidins are important effectors of
the innate immune system, known for their role as ‘alarmins’ which protect the body from
bacterial and viral infections. Cathelicidin (hCAP18) is activated in human epidermis by KLK5
trypsin-like processing near the C-terminal to release a 37 amino acid long antimicrobial peptide,
named LL-37 (Yamasaki et al., 2006). LL-37 is a broad spectrum active antimicrobial peptide
against Escherichia coli, Staphylococcus aureus and group A Streptococcus and it has antiviral
activity. Trypsin-like KLK5 and chymotrypsin-like KLK7 target LL-37 antimicrobial peptide
processing in human epidermis to generate shorter antimicrobial peptides that are active against
Staphylococcus aureus. Individuals with the inflammatory skin disease ‘acne rosacea’ show
facial inflammation in response to stimuli and have an exacerbated response to irritants and
allergens as a result of barrier dysfunction. The facical skin of rosacea patients displays increased
serine protease activity, LL-37 antimicrobial peptide overexpression and increased inflammation,
compared to non-lesional areas. Furthermore, rosacea skin has unique cathelicidin peptides and
abundant KLK5 expression compared to normal skin, suggesting KLK5 activity in rosacea
pathogenesis. Subcutaneous injection of active KLK5 in mice, in amounts mimicking those
observed in rosacea, increases cathelicidin processing and induces leukocyte infiltration and
inflammation, confirming KLK5 pathogenic involvement in this disease (Yamasaki et al., 2007).
In addition to inhibition, KLK serine protease activity is also regulated by a pro-KLK zymogen
activation proteolytic cascade, as mentioned above. Recently, a seminal study highlighted a
novel role for a granular keratinocyte membrane-bound serine protease, known as matriptase, in
activating KLK5 and KLK7 in vitro and in vivo. The matriptase-KLK signaling pathway was
examined in LEKTI-deficient epidermis (Sales et al., 2010). Matriptase autoactivation is more
efficient than KLK5 autoactivation, which makes it a better activator of KLK5 and a more likely
‘initiator’ of KLK activation cascades in the lower epidermis of diseased skin. A summary of our
current knowledge of KLK roles in inflammatory skin diseases is provided in Table 1.2 and the
importance of KLK serine proteases/protease inhibitor balance in the skin is shown in figure 1.9.
30
Table 1.2. Summary of KLK involvement in inflammatory skin disease pathologies
Skin
Disease
Disease
Pathogenesis
KLK Levels
Possible pathological
pathways involving KLK
activity
Ref
Atopic
Dermatitis
(AD)
(OMIM
603165)
A common
chronic
inflammatory, dry,
itchy, allergic skin
disease involving
immune,
endocrine,
metabolic, and
infectious factors
Higher
trypsin and
chymotrypsin-like
KLK levels
No change in
total trypsin or
chymotrypsin like
activities
Higher SC pH (due to
epidermal FLG mutations
causing a decrease in SC
acidity or due to barrier
disruptions) leads to
increased kallikrein serine
protease activity??
KLK hyperactivity leads to
increased desquamation,
lipid permeability barrier
dysfunction, pain,
inflammation and allergy
(Briot et al.,
2009; Cork
et al., 2009;
Elias and
Schmuth,
2009;
Hansson et
al., 2002;
Komatsu et
al., 2007a)
Psoriasis
Vulgaris
(PV)
(OMIM
177900)
A common
chronic
inflammatory
dermatosis and
autoimmune skin
disease,
characterized by
erythematous
plaques and
keratinocyte
hyperproliferation
Higher
trypsin and
chymotrypsin-like
KLKs and
expanded lower
in the epidermis
Increased trypsin-
like activity in
lesional skin only
Higher SC pH leads to
increased kallikrein serine
protease activity
KLK7 hyperactivity leads to
increased desquamation, IL-
1β activation, inflammation
and itch
(Ekholm
and
Egelrud,
1999;
Komatsu et
al., 2007b;
Kuwae et
al., 2002)
31
Netherton
Syndrome
(NS)
(OMIM
256500)
A rare autosomal
recessive mutation
in SPINK5 gene
on chromosome
5q32 causing
truncation and/or
loss of the
epidermal serine
protease inhibitor
LEKTI
Higher
KLK expression,
expand lower in
the epidermis
Increased activity
of KLK5, KLK7
and KLK14, but
NOT KLK8
KLK hyperactivity (due to
SPINK5 mutations causing
LEKTI inhibitor
dysfunction) results in
overdesquamation,
inflammation and allergy
onset
Matriptase activation of pro-
KLK zymogens contributes
to the overwhelming KLK
hyperactivity in this disease
Aberrant cathelicidin
expression as a result of
KLK hyperactivity
(Briot et al.,
2009; Briot
et al., 2010;
Descargues
et al., 2005;
Sales et al.,
2010)
Rosacea
An inflammatory
skin disorder
characterized by
facial lesions with
erythema. The
etiology is
unknown, but
symptoms are
exacerbated by
factors that trigger
innate immune
responses
Higher
KLK expression,
where they
expand lower in
the epidermis, as
well as aberrant
expression of
cathelicidin
peptides,
compared to
normal skin
Abnormal processing of
hCAP18 and LL-37
cathelicidin peptides by
hyperactive KLKs, leading
to inflammation, barrier
dysfunction, and itching
(Yamasaki
et al., 2007;
Yamasaki
et al., 2006)
32
Activators Inhibitors
Figure 1.9. The innate balance of KLK activity is integral in maintaining a healthy skin
barrier. Future inflammatory skin disease therapy development may include targeting of KLK
activity in the epidermis. It is important to note than none of these inhibitors inhibit KLK8. Thus,
KLK8 hyperactivity may offset the serine protease/protease inhibitor balance in skin diseases.
33
1.5 Psoriasis and Atopic Dermatitis
Psoriasis and Atopic Dermatitis are two of the most common inflammatory skin diseases.
Psoriasis affects approximately 2% of the population, while Atopic Dermatitis affects about
10%. Both diseases involve complex genetics and environment interplay, along with clear
aberrations in the immune system and skin barrier. Similarities and contrasting features of these
two diseases in terms of clinical and pathological features (Guttman-Yassky et al., 2011a), and
dominating immune cell subsets (Guttman-Yassky et al., 2011b), have been extensively studied.
Most scientific research, including the work described here, refers to psoriasis vulgaris or
plaque-type psoriasis, which is the most common form of psoriasis affecting about 85% to 90%
of psoriatic patients (Nestle et al., 2009). About one-third of psoriasis vulgaris patients have
moderate-to-severe disease, clinically classified on the basis of the lesions’ surface area and
significant impact on patients quality of life, indicated with a Psoriasis Area and Severity Index
(PASI) score of greater than 12 (Feldman, 2004). Psoriasis is associated with a high degree of
morbidity. It mostly begins at a young age and is a lifelong condition. Patients feel depressed and
embarrassed by the appearance of their skin disease, have lower employment and income, and
often suffer from side effects and recurrence. The cost for long term treatment of this disease is
also a major economic burden. Among psoriasis patients, 30 to 40% have an inflammatory,
disabling joint arthritis known as psoriatic arthritis. Skin disease precedes joint disease by an
average of 10 years in 85% of patients with psoriatic arthritis. Thus, dermatologists have a key
role in the early detection and treatment of psoriatic arthritis (Gottlieb, 2005).
Psoriasis is characterized by red, raised scaly plaques that cover the body surface and immune
infiltrate into the dermis and epidermis. The scaly skin is a result of epidermal
hyperproliferation, premature differentiation of keratinocytes and incomplete cornification with
retention of nuclei in the stratum corneum (parakeratosis) (Nestle et al., 2009). Psoriasis is often
considered an autoimmune disease, yet the autoantigen remains unknown.
Two opposing paradigms have been proposed to explain concurrent barrier defects and
inflammatory symptoms in atopic dermatitis and psoriasis. The ‘inside-out’ theory postulates that
skin barrier breakdown is a secondary response to the inflammation process that occurs due to
34
activation of immune cells by autoantigens, allergen and/or irritants. On the other hand, the
‘outside-in’ theory postulates that skin barrier defects drive the inflammatory response.
Amounting evidence supports both theories for psoriasis and atopic dermatitis. The “inside-out”
theory of AD pathogenesis is supported by genetic defects leading to overproduction of T-helper
cells, Th2 cells, causing allergy via IgE overproduction, inflammation via cytokine release and
skin barrier defects via neutrophil proteases (Meyer-Hoffert et al., 2004). On the other hand,
environmental challenges (i.e. mechanical trauma, chemical detergents, pathogens, allergens like
Der P 1 protease produced by house dust mites, etc) and genetic abnormalities in skin barrier
proteins, such as filaggrin, affect one or more components of the stratum corneum barrier
causing its breakdown (Elias and Steinhoff, 2008). A defective barrier with abnormally SC
permits increased water loss and entry of pathogens and allergens (Ziegler and Artis, 2010). As a
result, stressed keratinocytes secrete cytokines that recruit leukocytes and activate inflammation
in response to barrier defects, hence the term ‘outside-in’. In psoriasis, the ‘outside in’ theory is
supported by the fact that skin wounding triggers formation of psoriatic plaques, known as the
Kobner phenomenon. On the other hand, aberrations in the immune system result in domination
of Th1 and Th17 cells in psoriatic epidermis, which induce dramatic changes on the skin
epidermal barrier. Effective psoriasis therapies are largely based on blocking T-cell derived
cytokines such as TNFα, which clear psoriatic plaques in support of the ‘inside oustide’
pathogenesis (Lowes et al., 2007).
These theories remain under intense debate. However, aberrant lymphocyte activation is still
viewed as the main root cause of psoriasis, as it remains considered an autoimmune disease in
line with the ‘inside-out’ theory. On the other hand, atopic dermatitis is viewed as an epidermal
disease where a defective thin skin barrier, results in hyperactivated immune system. Although
both psoriasis and AD have skin barrier and immune abnormalities, these two common diseases
are characterized by distinct and opposing expression of barrier proteins and innate/adaptive
immune players, as highlighted in Table 1.3 below. The mutual antagonism of T-helper (Th)
cells in psoriasis and atopic dermatitis pathogenesis is extensively studied in terms of
polarization of Th1 cells in psoriasis versus Th2 in Atopic Dermatitis (Eyerich et al., 2011). With
the discovery of Th17 cells in 2007, recent research is unraveling key roles of this new T-helper
cell subset in psoriasis (Martin et al., 2012).
35
Table 1.3. Differences between Atopic Dermatitis and Psoriasis
Atopic Dermatitis Psoriasis Vulgaris
Skin
Barrier
Characteristics
- Reduced keratinocyte
differentiation, cornification,
moisture and lipid content
- No scaling
- No parakeratosis
- Lipid depletion, with increased
differentiation and cornification
- Scaling and hyperplasia
- Parakeratosis
Neutrophils in
epidermis
- No - Accumulation of neutrophils
Eosinophils - Increased eosinophils and mast cells
in dermis
- No eosinophils, but mast cells
are present
T-helper cell
polarization
- Th2 cells secreting IL4 and IL13
- Th22 present
- Attenuated Th17 pathway
- Th1/Th17 polarization with
Th22 present, cytokines
implicated include TNFα, IFNγ,
IL17A and IL22
- Increased Th17 pathway
Host defense &
infection
frequency
- Reduced antimicrobial peptides
(AMP), leading to high frequency of
bacterial infections
- Increased antimicrobial peptides
(AMP), leading to lower
frequency of infections
Vasodilation and
Angiogenesis
- Evidnce of vasodilation but no
angiogenesis
- Few blood vessels
- Both vasodilation and
angiogenesis are well documented
- Dilated and tortuous blood
vessels near epidermis
36
1.6 Kallikrein-related peptidase-8 in normal and inflamed skin
KLK8/neurposin protein expression is mostly noted in mouse and human skin. Normal stratum
basale does not express KLK8/neuropsin mRNA, but it is expressed in the stratum spinosum and
granulosum of wild type mice. KLK8/neuropsin expression is highest in the skin during
development or under pathological conditions (Yoshida et al., 2010). Upon applying the phorbol
ester, TPA, to normal mouse skin, KLK8 protein expression was found to increase and to
correlate with increased thickness of mice epidermis (Kishibe et al., 2012). Phorbol esters induce
keratinocyte proliferation resulting in hyperkeratosis, and are often used to model psoriasis.
Administering the phorbol ester, TPA, to KLK8-KO mouse skin resulted in suppression of
KLK6, KLK7 and PAR2 expression indicating potential KLK8 participation in an epidermal
protease cascade upstream of KLKs and proteinase-activated receptors (PARs) (Kishibe et al.,
2012). The number of stratum corneum layers and proliferating cells was found to be higher in
KLK8-KO mouse compared to the wild type mice. Thus, KLK8/neuropsin is involved in
regulating normal keratinocyte proliferation, differentiation and desquamation. No degradation
of the corneodesmosome DSG1 or CDSN is apparent in the KLK8-KO mouse, endorsing its
involvement in desquamation (Kishibe et al., 2007). A recent study showed that KLK8 is also
involved in wound healing, as its expression is induced in regions near incisonal wounds and its
ability to heal the wound is associated upregulation of KLK6 and PAR2 (Kishibe et al., 2012).
With regards to understanding KLK8 involvement in inflamed skin, mouse studies seem to
suggest that it is involved in inducing hyperkeratosis in inflamed skin. SLS-induced
hyperkeratosis and acanthosis was largely inhibited in KLK8/neuropsin KO mouse (Shingaki et
al., 2010). SLS is an irritant used to induce skin inflammation. Two mechanisms were recently
identified to explain the KLK8-mediated hyperkeratosis: (1) via inhibition of the transcription
factor, activator protein-2α or AP-2α, resulting in induction of cell proliferation. Consistently,
studies have shown that AP-2α knockout mice have thick skin due to a hyperproliferative defect
(Wang et al., 2006). Alternatively, KLK8-mediated hyperkeratosis could be induced (2) via
stimulation by the nerve growth factor (NGF-p75) pathway which has also been shown to induce
hyperkeratosis in inflamed skin (Shingaki et al., 2012).
Thus, KLK8/neuropsin mouse studies have implicated KLK8 in the regulation of normal
epidermal stratification and induction of hyperkeratosis in inflamed skins. These findings place
37
KLK8 as a potential key player in normal skin barrier functions and inflammatory skin disease
pathologies such as psoriasis.
1.7 Rationale, hypotheses and objectives
1.7.1 Rationale
Kallikrein-related peptidase-8 is the most abundant KLK serine protease in human skin barrier,
yet it is one of the least studied epidermal KLKs. Studies have revealed significant roles for
KLK5, KLK7 and KLK14 in normal and diseased human skin, but our understanding of KLK8’s
role in normal and diseased skin is lagging behind. To date, KLK5, KLK7 and KLK14 are the
only KLKs isolated from human skin tissue extracts in active forms. The roles of these proteases
in the skin are well-studied because they are direct targets of the serine protease inhibitor LEKTI,
which is mutated in the rare skin disease Netherton Syndrome. Netherton Syndome is a great
model to study KLK5, KLK7 and KLK14 functions in the skin. However, KLK8 is not inhibited
by LEKTI or any of the currently known extracellular epidermal inhibitors, and thus its activity,
regulation and function in human epidermis is overlooked and understudied.
In recent years, studies in KLK8/neuropsin KO mice suggested KLK8 involvement in normal
epidermal proliferation, desquamation and wound healing. Despite being a barrier repair
protease, KLK8 could play a damaging role in inflamed skin, where it induces hyperkeratosis.
Hyperkeratosis and acanthosis in sodium lauryl sulphate (SLS)-stimulated skin is inhibited in
KLK8/neuropsin KO mice. These recent findings could have tremendous implications for
inflammatory skin diseases in humans, such as psoriasis. Consistently, the KLK8 gene was
recently listed as one of the 130 overexpressed core set of psoriasis disease-specific genes
(Ainali et al., 2012).
As outlined in Table 1.3, psoriasis and atopic dermatitis are characterized by opposing epidermal
and immune mechanisms, yet KLKs are reported to be generally overexpressed in both diseases.
Since KLK8 induces hyperkeratosis in inflamed mouse skin, then it should play a major role in
psoriasis, but not atopic dermatitis. The previously reported KLK overexpression in atopic
dermatitis (AD) lesions could simply be due to barrier disruption as a result of tape-stripping,
since no increase in total trypsin activity was noted in lesional AD skin compared to normal
(Komatsu et al., 2007a). In contrast, total trypsin-like activity was reported to be significantly
38
higher in psoriatic lesions compared to non-lesional and normal skin (Komatsu et al., 2007b).
Thus, it is very likely that KLK8 is an active protease in normal human skin, which is
overexpressed and hyperactive in lesional psoriatic skin, but not atopic dermatitis lesions.
In order to understand KLK8 role in normal skin and common inflammatory skin diseases such
as psoriasis and atopic dermatitis better, the following points need to be addressed:
(1) KLK8-specific activity in normal human skin surface, (2) KLK8 regulation by normal
epidermal factors, (3) KLK8 substrate specificity and epidermal targets, (4) KLK8 regulation by
immune cell subsets implicated in psoriasis and atopic dermatitis (5) the effect of KLK8
hyperactivity on normal skin and (6) KLK8 expression in psoriasis and atopic dermatitis skin
lesions. If KLK8 proves to be active in normal human skin surface and if its overexpression is
induced by immune factors governing psoriasis to play a pathogenic role in the disease, then it is
likely to be an attractive target for topical therapeutic development to hamper its skin surface
activity in psoriatic lesions.
1.7.2 Hypotheses
We hypothesized that Kallikrein-related peptidase-8 (KLK8) can be isolated from normal human
stratum corneum and sweat in its active form. Since psoriasis is characterized by altered
keratinocyte proliferation and differentiation, and by infiltration of T-helper Th1 and Th17 cells
into the epidermis, we hypothesized that KLK8 is overexpressed in psoriasis due to
keratinocytes’ cross talks with Th1 and Th17 immune cells, independent of barrier injury. KLK8
overexpression may induce epidermal hyperplasia and enhanced innate immune gene expression
in psoriatic lesions, giving further support to the ‘inside-outside’ dogma of psoriasis.
Consequently, KLK8 pathogenic role in psoriasis should be reduced by current systemic Th1 and
Th17 blocking treatments. Novel KLK8-specific small molecule inhibitors can be identified by
high throughput screening to be developed into topical psoriasis therapeutic agents.
1.7.3 Objectives
Objective 1: Characterization of previously unidentified KLK8 activity in normal skin
(Chapter 2)
a) To identify cellular sources of KLK8 protease in human epidermis by testing its expression by
epidermal keratinocytes, epidermal melanocytes and dermal fibroblasts
39
b) To produce recombinant human KLK8 proteases in both latent pro-KLK8 and active mature
KLK8 form as important reagents for in vitro assays
c) To examine regulation of KLK8 activity by normal epidermal pH and ion content, and identify
potential KLK8 activators and targets that augment its role in normal skin barrier function
d) To develop an immunocapture pull-down assay and elucidate KLK8-specific serine protease
activity in normal human epidermis and sweat ex vivo
Objective 2: Characterization of KLK8 in psoriasis (Chapter 3 and 4)
a) To profile KLK8 secretion by cultured epidermal keratinocytes in response to treatment with
T-helper cell-derived Th1, Th17 and Th2 cytokines, alone or in combination
b) To examine KLK8 effect on cultured epidermal keratinocytes and 3D full thickness human
skin model in terms of keratinocyte proliferation, differentiation and expression of innate defense
genes
c) To investigate in vivo KLK8 expression in psoriasis and atopic dermatitis patients and
correlate data with in vitro findings of KLK8 epidermal expression in response to different
cytokine subsets
d) To test KLK8 levels in skin and sera of psoriasis patients before and after psoriasis treatment
with common biologic TNFα and IL17A-blockers, and correlate levels with clinical measures of
skin improvement and psoriasis clearance
e) To examine the role of KLK8 as serum biomarker of psoriatic arthritis in psoriasis patients
Objective 3: Characterization of KLK8 signaling and inhibition patterns in comparison to
other trypsin-like epidermal KLKs (Chapter 5)
a) To demonstrate KLK8 differential signaling through proteinase-activated receptor-2 (PAR2)
b) To identify KLK8-specific small molecule inhibitors by high throughput screening
40
Chapter 2
Kallikrein-related peptidase-8 is an active serine protease in
human epidermis and sweat and is involved in a skin barrier
proteolytic cascade
Sections of this chapter were reproduced from the following published manuscripts:
Eissa, A., Amodeo, V., Smith, C.R., and Diamandis, E.P. Kallikrein-related Peptidase-8 (KLK8)
(2011). Is an Active Serine Protease in Human Epidermis and Sweat and Is Involved in a Skin
Barrier Proteolytic Cascade. The Journal of biological chemistry 286, 687-706.
41
2 Kallikrein-related peptidase-8 (KLK8) is an active serine
protease protease in human epidermis and sweat and is
involved in a skin barrier proteolytic cascade
2.1 Introduction
Multiple trypsin-like KLK peptidases are co-expressed in human epidermis and associated
appendages, such as hair follicles, sebaceous and sweat glands (Komatsu et al., 2005b; Komatsu
et al., 2006b). Interestingly, KLK8 was found to be among the most abundant trypsin-like KLKs
in normal human stratum corneum and sweat (Komatsu et al., 2006b).Work done in
Klk8/neuropsin-null mice suggested that Klk8/neuropsin plays an important role in neural
plasticity and skin barrier homeostasis. Healing of chemically-wounded or UV-irradiated mouse
skin is largely impaired in the absence of Klk8/neuropsin (Kirihara et al., 2003; Kitayoshi et al.,
1999). Additionally, the dramatic increase of KLK8 mRNA in hyperkeratotic skin of psoriasis
vulgaris, seborrheic keratosis, lichen planus, and squamous cell carcinoma patients, compared to
normal and basal cell carcinoma skin, suggested that human KLK8 is involved in keratinocyte
differentiation and skin barrier formation (Kuwae et al., 2002). KLK8 protein was also detected
expression in psoriasis, atopic dermatitis and peeling skin syndrome skin tissues (Komatsu et al.,
2007a; Komatsu et al., 2007b; Komatsu et al., 2006a). Although KLK8 involvement in normal
skin barrier formation and inflammatory skin disease pathology has recently become apparent,
our basic understanding of KLK8 enzymatic regulation and activity in normal skin remains
lacking. It is essential to probe KLK8 protease activity in normal human skin, in addition to
continuing the ongoing investigation of KLK8 function and regulation in Klk8/neuropsin knock
out (KO) mouse skin. This must be done while keeping in mind the anatomical and physiological
differences between mouse and human skin, as well as differences at the molecular level. Also,
the activation motif of human KLK8 (QEDK-VLGGH) differs from that of the mouse
Klk8/neuropsin (QGSK-ILEGR), suggesting that endogenous activators of KLK8 may differ
between mouse and human species, even though these proteases may play similar roles.
To investigate human KLK8 enzymatic properties and delineate its potential activators and
downstream targets in normal skin, we produced recombinant human KLK8 in its latent
zymogen (pro-KLK8) and active mature form (mat-KLK8) in yeast Pichia Pastoris for in vitro
42
activation and degradation assays. Recombinant KLK8 regulation by relevant epidermal pH and
cations, potential epidermal activators and inhibitors was investigated in a series of enzymatic
assays. We also examined recombinant mat-KLK8 substrate specificity via kinetic analysis of its
cleavage of a panel of fluorogenic AMC substrates and a small positional-scanning library of
internally-quenched FRET peptides. KLK8 ability to activate potential co-localized epidermal
pro-KLKs and LL-37 antimicrobial peptide was also examined. We performed these in vitro
biochemical characterization assays under the hypotheses that this protease is induced during
terminal keratinocyte differentiation and is activated in the SC extracellular space to participate
in barrier functions. Thus, we suspected that this serine protease is active in normal upper
epidermis and sweat. Herein, we investigated KLK8 expression during terminal keratinocyte
differentiation in culture and developed a sensitive and specific immunocapture assay to probe
its activity in human epidermal extracts and sweat ex vivo. Our findings shed light on the orphan
epidermal protease KLK8 and provide evidence that this KLK is indeed an active serine protease
in human stratum corneum and sweat and is an intriguing member of a proteolytic cascade
regulating skin barrier integrity.
2.2 Materials and Methods
Materials – The rapid endoprotease profiling library of fluorescence resonance energy transfer
(FRET) quenched peptides (PepSets™REPLi) was purchased from Mimotopes Pty Ltd
(Australia). The human antimicrobial LL-37 peptide (Leu-Leu-Gly-Asp-Phe-Phe-Arg-Lys-Ser-
Lys-Glu-Lys-Ile-Gly-Lys-Glu-Phe-Lys-Arg-Ile-Val-Gln-Arg-Ile-Lys-Asp-Phe-Leu-Arg-Asn-
Leu-Val-Pro-Arg-Thr-Glu-Ser) was purchased from Genemide Synthesis, Inc (San Antonio,
USA). The majority of synthetic fluorogenic AMC substrates were purchased from Bachem
Bioscience (King of Prussia, PA). AAPF-AMC and AAPV-AMC were obtained from
Calbiochem. All AMC substrates were diluted in DMSO at a final concentration of 80 mM and
stored at –20 °C. Recombinant KLK5 and KLK14 were produced in Pichia Pastoris as described
previously (Borgono et al., 2007c; Michael et al., 2005). Recombinant pro-KLK11 was produced
in Chinese Hamster Ovary cells and recombinant pro-KLK1 was produced in human embryonic
kidney cell line, HEK293, as described previously (Emami and Diamandis, 2008; Luo et al.,
2006).
43
2.2.1 Cloning, expression, and purification of recombinant human KLK8 proteins
Recombinant mature KLK8 protease was produced in the Original Pichia Pastoris expression
system (Invitrogen). Briefly, PCR-amplified DNA fragment encoding mature KLK8 isoform-1
(amino acids 33-260 of NCBI gene bank accession no. NP_009127) flanked by XhoI and EcoRI
restriction enzyme sites was cloned into pPIC9 expression vector, in-frame with its α-secretion
signal and the alcohol oxidase AOX1 gene. Purified mat-KLK8-pPIC9 DNA construct was
confirmed by sequencing using 5’-AOX1, 3’-AOX1, and α-secretion signal vector-specific
primers and NCBI BLAST Align program. The mat-KLK8-pPIC9 construct was linearized with
SacI and transformed into KM71 P.pastoris strain by electroporation. A stable KM71
transformant was grown in 1L BMGY media. After two days, yeast culture was centrifuged and
the cell pellet was suspended in 300mL BMMY media (OD600= 10). Recombinant KLK8
expression was induced with 1% methanol for 5 days at 30ºC in a shaking incubator (250 rpm).
Recombinant mat-KLK8 was purified from culture supernatant by ultra-concentration, serial
dialysis and centrifugation procedures, followed by cation-exchange chromatography. One liter
of culture containing secreted mat-KLK8 was centrifuged and supernatant was concentrated 10-
fold by positive pressure ultracentrifugation in an Amicon TM stirring chamber (Millipore
Corporation, Bedford, MA) with a 10 kDa cut-off regenerated cellulose membrane (Millipore).
A series of bench-top purification experiments using an aliquot of KLK8-containing supernatant
and SP Sepharose Fast Flow beads packed in Econo-Pac open column (Bio-Rad) were
performed. We determined the optimal binding buffer for cation exchange purification of mat-
KLK8 to be 0.01M acetic acid containing 50 mM NaCl (pH 4.76) and the optimal salt
concentration for elution to be 250 mM NaCl. These findings were translated into an automated
method where mat-KLK8 protein was purified using automated ÄKTA FPLC system on a pre-
equilibrated 5 ml cation exchange HiTrap high performance sepharose HP-SP column (GE
Healthcare), after serial dialysis (3X) against 0.01 M Acetic Acid, 50 mM NaCl (pH 4.76),
running buffer A. Mat-KLK8 was eluted in 4 ml fractions via a step-wise salt gradient using 1M
NaCl in 0.01M Acetic Acid (pH 4.76), Buffer B, at a flow rate of 1 ml/min, as follows: (a) 5% B
for 25 min, (b) 10% B for 25 mins, (c) 15% for 25 mins, (d) 25% for 15 mins, (d) followed by a
continuous gradient from 25%-100% B for 15 mins. Recombinant mat-KLK8 was further
purified using 10 ml cation exchange Source15S Tricorn™ Column (GE Healthcare), which
44
resulted in elution of a very pure protein in 3 fractions that were pooled, concentrated, and stored
at -80 ˚C.
Pro-KLK8 isoform 1 cDNA (amino acids 29 to 260) was cloned into pPIC9 Pichia Pastoris
yeast vector and transformed into a stable GS115 yeast strain as described above for mat-KLK8.
The recombinant colony was grown in 1L BMGY medium for 1 day and suspended in 2L
BMMY (OD600 =1.0). Pro-KLK8 expression of was induced with 1% methanol for 6 days at
30ºC in a shaking incubator (250 rpm). After concentrating the culture supernatant 20-fold,
recombinant pro-KLK8 was purified after serial dialysis by cation-exchange chromatography
using a HiTrap high performance sepharose HP-SP column connected to automated ÄKTA
FPLC system. Pro-KLK8 was eluted in 4 ml fractions using the same step-gradient described
above for mat-KLK8 purification. The protein eluted as a single peak and the protein-containing
fractions were pooled, concentrated, and stored at -80 ˚C in 0.01 M acetic acid containing 250
mM NaCl (pH 4.76).
2.2.2 Detection of active mat-KLK8 and latent pro-KLK8 recombinant protein expression
The purity of recombinant proteins produced was assessed on silver-stained SDS-PAGE.
Concentration was determined by the BCA method (Pierce), and protein identity was confirmed
by mass spectrometry and N-terminal sequencing. SDS-PAGE was performed using the
NuPAGE BisTris electroporesis system and precise 4-12% gradient polyacrylamide gels at 200
V for 45 min (Invitrogen). KLK8 Proteins were visualized with a Coomassie G-250 staining
solution, SimplyBlueTM
SafeStain (Invitrogen), and/or by silver staining with the Silver
XpressTM
Kit (Invitrogen), according to manufacturer’s instructions. For immunblotting of
KLK8, proteins resolved by SDS-PAGE were transferred onto a Hybond-C Extra nitrocellulose
membrane (GE Healthcare) at 30 V for 1 h. The membrane was blocked with Tris-buffered
saline/Tween (0.1 mol/liter Tris-HCl buffer (pH 7.5) containing 0.15 mol/liter NaCl and
0.1%
Tween 20) supplemented with 5% nonfat dry milk overnight at 4 °C and probed with a KLK8
polyclonal rabbit antibody (produced in-house; diluted 1:2000 in Tris-buffered saline/Tween)
for
1 h at room temperature. The membrane was washed three times for 15 min with Tris-buffered
saline/Tween and treated with alkaline phosphatase-conjugated goat anti-rabbit antibody (1:5,000
in Tris-buffered saline/Tween; Jackson ImmunoResearch) for 1 h at room temperature. Finally,
45
the membranes were washed again as above, and the signal was detected on x-ray film using
chemiluminescent substrate (Diagnostic Products Corp., Los Angeles).
Mass spectrometry analysis for positive identification of both recombinant pro- and mat-KLK8
proteins was performed. N-terminal sequencing was performed by the Edman degradation.
Briefly, proteins were transferred by electroblotting to polyvinylidene difluoride membrane and
visualized with Coomassie Blue Stain. The bands were excised and applied to the sequencer.
2.2.3 Gelatin Zymography
Mat-KLK8 proteolytic activity was visualized by gelatin zymography (Novex® 10% Zymogram,
Gelatin, Invitrogen), according to manufacturer’s instructions. Briefly, mat-KLK8 was diluted
1:1 in Tris-glycine SDS sample buffer and electrophoresed for 2 hrs at 125V at 4 ºC. After
electrophoresis, the gels were incubated in renaturing buffer for two- 30 min intervals at room
temperature, followed by incubation in developing buffer for 4 hrs at 37 ºC. Gels were stained
with SimplyBlueTM
SafeStain and destained until the white lytic bands corresponding to areas
of
protease activity were visible against a dark blue background.
2.2.4 AMC substrate profiling and kinetics constant determination
Mat-KLK8 hydrolysis of 16 fluorogenic AMC-peptides was investigated using the same KLK8
concentration (12nM) and increasing concentrations of AMC peptides (0.03, 0.06, 0.12, 0.25,
0.50, 0.75, 1.0 mM) in KLK8 activity buffer (100mM phosphate, 0.01% Tween 20, pH 8.5). The
trypsin-like substrate peptides tested were VPR-AMC, GGR-AMC, FSR-AMC, PFR-AMC,
LKR-AMC, LRR-AMC, QRR-AMC, QAR-AMC, QGR-AMC, GPR-AMC, GPK-AMC, EKK-
AMC, VLK-AMC. AAPF-AMC and LLVY-AMC were the chymotrypsin-like substrate peptides
tested. The known neutrophil elastase substrate AAPV-AMC was used as a negative control.
KLK8-free reactions, for each peptide concentration, were used as negative controls and
background counts were subtracted from each value. Free AMC fluorescence was measured on
the Wallac 1420 Victor2TM
fluorometer (PerkinElmer LifeSciences) with excitation and emission
filters set at 380 and 480 nm, respectively, at 1-min intervals for 20 min at 37 °C. A standard
curve was constructed using known concentrations of AMC in order to calculate the rate of free
AMC emission. The slope of the resultant AMC standard curve was 19.18 AMC fluorescence
counts/nM free AMC. The steady-state (Michaelis-Menten) kinetic constants (kcat/Km) were
46
then calculated by non-linear regression analysis using Enzyme Kinetics Module 1.1 (Sigma
Plot, SSPS, Chicago, IL). All experiments were performed in triplicate and repeated at least
twice.
2.2.5 Cleavage of a positional-scanning rapid endopeptidase library (RepLi) of FRET-quenched peptides
The RepLi library contains FRET-quenched peptide pools lyophilized into 512 wells in a total of
six 96-well plate format. Each well contains 8 peptides with the same amino acid combinations
in their variable tri-peptide core, which allows screening of 512 peptide pools with 3375
potential cleavage sites. The soluble peptide library pools (i.e. 512 wells) in six 96-well plates
were diluted with mat-KLK8 activity buffer, 100 mM sodium phosphate buffer without Tween-
20 (pH 8.50) to a final concentration of 50 µM. Tween-20 was not included because it is not
compatible with mass spectrometry analysis. After agitating the plate for 1 min, 20 µl aliquots of
each well were collected as background controls. Background readings were measured using
Envision 2103 Multilabel Reader (excitation λ=320 nm, emission λ=400 nm). After measuring
background readings, 10 µl of mat-KLK8 was added (10 nM final) to each well of the six 96-
well RepLi plates, prior to incubating plates at 37˚C for 1 hr. This library was incubated with
minimal amount of active enzyme (10 nM) for 1 hr to avoid selection of peptides containing non-
optimal cleavage sites. Fluorescence data were analyzed before and after protease addition.
Cleavage was determined by assigning “strong, moderate, weak, and no cleavage” identifiers to
wells generating a signal to background ratio (S:B) of “ ≥ 2, between 1.50-2.0, between 1.25-
1.50, and ≤1.25”, respectively. The cleavage sites of selected wells that showed the highest
fluorescence readings were determined by LC-MS analysis, comparing the sample before and
after mat-KLK8 addition.
2.2.6 pH, divalent cations, and glycolsylation effect on mat-KLK8 activity
Four buffer systems were assessed to determine the optimal pH for mat-KLK8 activity; 1 M
potassium phosphate buffer (pH 5.0-6.5), PBS ( pH 7.0-7.5), 50 mM Tris-HCl (pH 8.0-9.0), and
100 mM sodium phosphate (pH 7.0-9.0). Solutions prepared from salts of ZnCl2, MgCl2, CaCl2,
NaCl, and KCl were added to optimal activity buffer containing 0.25 mM VPR-AMC at a final
concentration of (0, 10
-2, 10
-3, 10
-4, 10
-5, 10
-6 and 10
-7 nM) in a final
volume of 100 µl. At this
point, KLK8 (12 nM) was applied to each reaction mixture, and the plate was agitated for 1 min.
47
Residual KLK8 activity against VPR-AMC after incubation in each buffer pH or with each
individual cation was calculated. Alternatively, mat-KLK8 was treated with PNGase F to remove
N-glycans without denaturation. The same amount of PNGase-treated mat-KLK8 and mock
treated mat-KLK8 (12nM) were added to activity buffer containing VPR-AMC (0.25 mM) in a
final volume of 100µl, and AMC fluorescence was measured to test the deglycosylation effect on
KLK8 activity.
2.2.7 Mat-KLK8 autodegradation
Aliquots of intact mat-KLK8 enzyme (100 ng) were incubated in KLK8 activity buffer at 4˚C,
25˚C, and 37 ˚C 0, 0.5, 1, 2, 4, 6, 12, 24 and 34 hrs. Autodegradation fragments were detected by
reduced silver-stained SDS-PAGE and their activity was tested against VPR-AMC substrate, as
described above.
2.2.8 Pro-KLK8 zymogen activation by KLK5, KLK1, and lysyl-endopeptidase
Activation studies of pro-KLK8 by potential recombinant activators were done in two
consecutive steps, an “activation step” followed by a “detection step”. In the activation step by
mat-KLK5, pro-KLK8 (200 nM) was added to 20 nM active KLK5 at increasing incubation
times at 37 °C (1, 3, 18, 24, 48 hr) and at 25 °C (day 1, 2 and 4) in KLK5-optimized activity
assay buffer (100 mM phosphate buffer, 0.01% Tween 20, pH 8.0) in a total volume of 35 µl. In
the detection step, pro-KLK8 activation was monitored as an increase in the fluorescence of
cleaved AMC, off VPR-AMC, after adding 10 µl of the activation mix to 90µl KLK8-optimized
assay buffer (100 mM phosphate buffer, 0.01% Tween 20, pH 8.5) containing 0.1 mM VPR-
AMC. To avoid confounding results due to KLK5 similar activity towards VPR-AMC, we
included a duplicate activation mix where α1-antitrypsin inhibitor (AT) is added to quench
KLK5 activity prior to detecting pro-KLK8 activation. α1-antitrypsin (AT) was added at a 5-fold
molar excess and incubated for an additional hour at 37 °C. Activity towards VPR-AMC was
measured for both reaction mixtures, with or without AT in triplicates. To test activation of pro-
KLK8 by KLK1, 200 nM pro-KLK8 was incubated with 20 nM active mat-KLK1 for 1 hr and 3
hr at 37 °C in 35 µl activity assay buffer (100 mM phosphate buffer, 0.01% Tween 20, pH 8.0).
Pro-KLK8 activation by lysyl endopeptidase was performed by incubating 200 nM pro-KLK8
with lysyl endopeptidase in its optimal activity buffer (50mM Tris, 10mM CaCl2,150 mM NaCl ,
48
pH 9.0) at an activator to pro-KLK8 molar ratio of 1:1000 for 1 hr at 37 oC in 35 µl total volume.
“Detection” of activation was done in triplicates where 10 µl of each activation mix was added to
90 µl KLK8-optimized assay buffer (100 mM phosphate buffer, 0.01% Tween 20, pH 8.5)
containing 0.1 mM VPR-AMC. The fluorescence values obtained for the activation mix, pro-
KLK8 alone and activator alone reaction controls were subtracted from raw values of the no
enzyme background control. The reaction rate was then calculated by measuring the slope in
FU/min and converting it to free AMC (nM)/min.
2.2.9 Inhibition of KLK8 by epidermal inhibitors and general serpins
Mat-KLK8 (12 nM and 3 nM) was incubated with 30 nM of each of the four LEKTI domains
(D1-6, D6-9, D9-12, D12-15) for 1 hr at 37˚C in optimal KLK8 activity buffer. To detect the
potential inhibitory effect of each LEKTI fragment, 10 µl of each mix was added to 90 µl KLK8-
optimized assay buffer containing 0.25 mM VPR-AMC. 12 nM of KLK5 was also incubated
with each inhibitory LEKTI fragment as a positive control. SLPI or elafin (60 nM and 600 nM)
were incubated separately with mat-KLK8 (6nM) in a final volume of 20 µl for 1hr at 37 °C.
Control reactions, i.e. elastase, elafin, and SLPI incubated alone, were also performed. 6 nM
neutrophil elastase (NE) was tested as a positive control for SLPI and elafin inhibition. KLK8
activity was also tested upon incubating with 0.1 mg/ml or 0.01 mg/ml soybean trypsin inhibitor
(STI) or aprotinin, 1 mM PMSF, 1 mg/mL α1-antitrypsin inhibitor and 1 mg/mL chymostatin for
1 hr at 37˚C in optimal KLK8 activity buffer. 10 µl aliquots of each inhibitor-treated and non-
treated reaction mix were added to 90 µl KLK8-optimized assay buffer containing 0.25 mM
VPR-AMC in triplicates in a 96-well plate, so that the final KLK8 concentration in each well is 6
nM. Enzyme free reactions were included to be used as background controls. KLK5 was also
treated with the same inhibitor concentration and incubation time as a control and for comparison
purposes.
2.2.10 Activation of Pro-KLK1, pro-KLK11, and pro-KLK5 by KLK8
Pro-KLK1 and active KLK8 were incubated at a 1:1 molar ratio in a total volume of 50 µl at 37
°C for 10 min and 30 min time points. Reactions were done in triplicates. Given the issues of
pro-KLK1 auto-activation and similar trypsin-like activity of mat-KLK8, KLK1-specific activity
was measured by detecting fluorescence release of pulled-down KLK1 before and after
incubation with mat-KLK8, as previously described (Emami and Diamandis, 2008).
49
Pro-KLK11 (1 µM) and active KLK8 (10 nM) were incubated at a 1:100 molar ratio in a total
volume of 50 µl at 37 °C for 0.5, 1.5 and 3 hr time points in optimal KLK8 activity buffer.
Detection of activation was done in triplicates where 10 µl of the activation mix was added to 90
µl KLK11-optimized assay buffer (50 mM Tris, 1.0 M NaCl, 0.01% Tween-20, pH 8.5)
containing 0.25 mM PFR-AMC.
KLK5 (20nM) and active KLK8 (20 nM) were incubated in a total volume of 50 µl at 37 °C for
1 hr and 3 hr time points in optimal KLK8 activity buffer. Detection of activation was tested in
triplicates where 10 µl of the activation mix was added to 90 µl KLK5-optimized assay buffer
(100 mM sodium phosphate, 0.01% Tween-20, pH 8.0) containing 0.1 mM FSR-AMC. The
increase in fluorescence signal was measured on a Wallac Victor fluorometer, as described
above.
2.2.11 Proteolytic processing of LL-37cathelicidin antimicrobial peptide
For analysis of LL-37 processing by mat-KLK8, l40 µM of LL-37 synthetic peptide was
incubated with 20 nM KLK8 for 0, 2, and 6 h at 37˚C in total volume of 400µl of 50mM Tris-
HCl buffer containing 2 M NaCl (pH 8.50). After incubation, peptides were separated by two
independent methods: 1) peptide separation by reverse-phase HPLC (Agilent Eclipse XDB-C18,
5µm) followed by peptide identification by LC/MS, and 2) direct peptide separation and
identification of the LL-37/KLK mixture by LC/MS/MS.
In the first method, the C18-column was equilibrated in 10% acetonitrile with 0.1%
trifluoroacetic acid at a flow rate of 0.8 ml/min for 10 min, and cleaved peptides were eluted
using a 30 min gradient of 10-100% acetonitrile. Column effluent was monitored at 214 nm and
280 nm, and one fraction was collected per minute. All collected 0.8 ml fractions were
lyophilized prior to pre-concentration using the OMIX C18MB (Varian) tips and eluted with 5
µL of buffer A (0.1% formic acid and 0.02% trifluoroacetic acid in 65% acetonitrile). In the
second method, the incubation mix was purified and pre-concentrated using the OMIX C18MB
(Varian) tips and immediately applied to a mass-spectrometer set-up with C18 trap column using
the EASY-nLC system (Proxeon Biosystems, Odense, Denmark). Recombinant KLK14 ability
to process LL-37 was tested given that this possibility was not previously investigated. KLK5
cleavage of LL-37 was included as a positive control.
50
2.2.12 Calcium induction of terminal keratinocyte differentiation and KLK8 secretion
HaCat cells were cultured in low-serum EpiLife medium (Invitrogen) containing 0.06 mM
calcium. One million HaCat cells were seeded in two T-175 flasks from the same passage
(passage 3). In the calcium-treated flasks, HaCat cells in low calcium (0.06 mM) basal condition
were switched to high calcium (2.0 mM) at 20% confluency. Calcium treated and non-treated
cells were allowed to reach 70% confluency prior to switching to Epilife serum-free medium
(SFM) containing no added supplements. One mL aliquots of the SFM were collected each day
for three days to measure total protein and KLK8 levels.
2.2.13 Collection and preparation of sweat and stratum corneum extracts
Sweat samples were collected from the face, arms, legs, stomach and abdomen of 7 healthy
donors during a dry sauna session. Volunteers showered the night before, and had not applied
any topical agents to their skin. Sweat samples were collected using 1mL pipettes into 15 mL
tubes and snap frozen on dry ice prior to storage at -20ºC. On the other hand, stratum corneum
flakes were collected from 8 volunteers using previously described ‘tape-stripping’ and
‘scraping’ methods (Bernard et al., 2003). The ‘scraping method’ generated a higher number of
total protein extracts during method optimization and hence this procedure was used for the
actual study. Briefly, scraping buffer (50mM sodium phosphate buffer (pH 7.2) containing 5mM
EDTA, 150mM NaCl, 0.1% Tween-20) was applied to each volunteer’s forearm and spread
evenly to moisten. A microscope slide was then used to scrape the skin surface until corneocyte
cells were visible on the slide. The corneocytes were washed off the slide into a 50mL tube with
10 ml buffer and soaked for 10 mins prior to storage at -20ºC. For processing, sweat and SC
samples were thawed, vortexed for 10 min, and centrifuged at 4000rpm, 4ºC for 15 min. Sweat
samples were pooled and dialyzed using a 3 kDa membrane to remove salts. Dialyzed pooled
sweat and pooled SC samples were next passed through a Millipore 0.22 µm filter (Nalgene
Syringe Filters, 0.2um; 25mm) and finally concentrated 20X using Amicon Ultra Centrifugal
Filters with a 3 kDa molecular mass cut-off. Concentrated and pooled sweat and SC tissue
extracts were stored at -20ºC until analysis. Sweat and SC solubilized total protein amounts were
quantified using the BCA assay (Pierce).
51
2.2.14 KLK8 expression in human sweat, stratum corneum extracts, and skin cell cultures
KLK8 concentration in sweat and SC tissue extracts, as well as in the culture media of HaCat
keratinocytes, primary human epidermal keratinocytes, epidermal melanocytes, and dermal
fibroblasts was determined by a monoclonal-monoclonal KLK8 sandwich-type ELISA. In the
case of culture media, primary epidermal keratinocytes (pHEK), primary epidermal melanocytes
(pHEM), and primary dermal fibroblasts (pHDF) from neonatal human foreskin were purchased
from Cascade Biologics (Invitrogen) and cultured according to the manufacturer’s instructions.
The HaCat keratinocyte cell line was grown in DMEM medium containing 10% FBS. HaCat
cells were plated at a seeding density of 3.0x104 cells/well while primary pHEK, pHEM and
pHDF cells were seeded at a density of 2.0x105 cells/well in 6-well plates. The medium was
changed after 48 hours, when cells were 60-80% confluent, to a fresh medium (5ml) and this day
was marked as day 0. Aliquots of the medium from each well of the 6-well plates were collected
daily for 12 days. Secretion of epidermal KLKs such as KLK5, 6, 7, 11, 13, and 14 was also
investigated by KLK-specific ELISAs. To control for the viability of cells, the levels of the
intracellular enzyme lactate dehydrogenase (LDH) were measured as an internal control in the
culture media to indicate cell membrane rupture and cell death over time.
2.2.15 Immunocapture of KLK8 activity in sweat and SC epidermal protein extracts
Physiologically-relevant KLK8-specific activity was measured by detecting AMC fluorescence
emission increase after adding VPR-AMC substrate to wells containing immunocaptured or
pulled-down KLK8 compared to background. Briefly, 500 ng of KLK8-specific monoclonal
antibody (Mono Ab 19-10) were immobilized overnight on a 96-well plate in coating buffer (50
mmol/liter Tris, 0.05% Tween 20, pH 7.8). The plate was washed three times with washing
buffer (50 mmol/liter Tris, 150 mmol/liter NaCl, 0.05% Tween 20, pH 7.8). About 30 µL of
sweat (~40 ng KLK8), 40 µL of SC extracts (~4 ng KLK8), and 30 µL of recombinant mat-
KLK8 (~250 ng), as well as 10X diluted samples in a total volume of 100 µL 100mM sodium
phosphate buffer (pH 7.5) were loaded per KLK8-antibody-coated well in triplicates. The plate
was incubated at room temperature with gentle shaking for 3 hrs, and then washed six times with
the washing buffer, above, to remove contaminants as well as non-bound KLK8. Subsequently,
200 µL of 0.50 mM VPR-AMC substrate in KLK8-activity buffer at optimal pH 8.5 or 5.0 was
52
added to each well. The substrate was incubated with the immunocaptured KLK8 for a total of
24 hrs at 37 ºC. The increase in fluorescence release was measured in real time at 20 minute-
intervals on a Wallac Victor fluorometer, set at 355 nm for excitation and 460 nm for emission.
For controls, recombinant active mat-KLK8, mat-KLK5, and lysyl endopeptidase were loaded
into wells coated with KLK8-antibody in the same plate. To test if the sweat and SC contain
latent pro-KLK8 that can be activated by potential activators, sweat and SC samples were spiked
with active KLK5 or lysyl endopeptidase at 1:100 and 1:1000 molar ratios, respectively,
overnight at 37ºC prior to loading into wells coated with KLK8-antibody.
2.3 Results
2.3.1 Recombinant mat-KLK8 and pro-KLK8 enzyme production and characterization
Mature KLK8 and pro-KLK8 proteins were produced in their native forms, without any fusion
tags, in yeast (P. pastoris). Both proteins were secreted in the yeast culture supernatant after 1
day of 1% methanol induction, with the highest levels produced on day 6. Recombinant proteins
were obtained with > 95% purity, as verified by silver-stained reduced SDS-PAGE and
confirmed by mass-spectrometry. The yield of purified mat-KLK8 and pro-KLK8 from 1L
culture supernatants was in the range of 0.8-1.5 mg, as determined by ELISA and BCA total
protein assays. The apparent mass of both purified pro-KLK8 and mat-KLK8 recombinant
proteins on a reduced SDS-PAGE was higher (~31 kDa) than their predicted molecular mass
(~28 kDa). KLK8 has one predicted glycosylation site at N100
SS. We detected recombinant
KLK8 glycosylation upon treating both mat-KLK8 and pro-KLK8 enzymes with PNGase F. The
non-glycosylated reduced form of KLK8 shifted lower from an apparent molecular mass of 31
kDa to 28 kDa.
The purified pro-KLK8 was visualized as a single glycosylated band of 31 kDa, with an N-
terminal sequence of QEDKV as expected. Although purified mat-KLK8 appeared as an intact
31 kDa band at the time of purification, we detected 3 lower bands at 21, 11, and 8 kDa after
keeping the enzyme at 4˚C for a week in PBS buffer (pH of 7.4) (Figure 2.1A lanes 1-3).
Western blotting and N-terminal sequence analysis identified the low molecular mass (<28 kDa)
bands as internal fragments of mat-KLK8, likely arising from auto-proteolytic cleavage, (Figure
2.1B and C). The N-terminal sequence of the top two bands, band I and band II, was determined
53
to be VLGGHE by Edman degradation, which corresponds to the N-terminal sequence of active
mature KLK8. The lower two bands were identified as autodegradation products of mat-KLK8
having an internal N-terminal sequence of ENFPDT, indicating auto-cleavage after Arg164
(Figure 2.1C). By homology modeling using the pymol software, we found that Arg164
resides in
an exposed, solvent-accessible surface loop, which is consistent with being susceptible to
autolysis, Figure 2.1D. It is highly unlikely that the N-glycan attached to N100
of KLK8
participates in its substrate binding as it is directed away from the catalytic triad and substrate
binding pocket (Figure 2.1D). Our results confirmed that deglycosylation had no effect on mat-
KLK8 ability to cleave VPR-AMC.
54
A) B)
C)
D)
1 MGRPRPRAAK TWMFLLLLGG AWAGHSRA28
QE DK32
VLGGHECQ PHSQPWQAAL FQGQQLLCG
61 VLVGGNWVLT AAHCKKPKYT VRLGD(H86
)*
SLQN KDGPEQEIPV VQSIPHPCY N110
SSDVEDHNH (D120
)*
121 LMLLQLRDQA SLGSKVKPIS LADHCTQPGQ KCTVSGWGTV TSPR164
ENFPDTLNCAEVKIFP
181 QKKCEDAYPG QITDGMVCAG SSKGA D206
TCQG D (S212
)*
GGPLVCDG ALQGITSWGS DPCGRSDKPG
241 VYTNICRYLD WIKKIIGSK
1 2 3 4
38
28
17
14
6
kDa
31
21
Silver WB Gelatin
Zymogram
kDa
I
II
III
IV
55
E) F)
G)
Figure 2.1. Activity and autodegradation of recombinant mat-KLK8. A. Silver-stained
reduced SDS-PAGE of purified mat-KLK8 (lanes 1-4). Lane 4 represents intact purified
recombinant mat-KLK8 protein at 31 kDa. Lanes 1-3 represent the purified mat-KLK8 protease
after storage in PBS buffer (pH 7.4) at 4˚C for 7 days. B. Detection of active mat-KLK8 and
degraded fragments in replicate silver-stained SDS-PAGE, western Blotting, and gelatin-
Mat
-KLK
8 *
Mat
-KLK
8
Cock
tail
Inhib
itor
1 m
M P
MSF
1 m
M E
DTA
1 m
g/m
L A
T
1 m
g/m
L c
hym
ost
atin
0
200
400
600
800
1000
1200Yeast mat-KLK8
Ra
te o
f V
PR
-AM
C h
yd
roly
sis
(nM
AM
C/m
in)
56
zymography. Intact KLK8 corresponded to the 31 kDa band. Lower molecular weight
autodegradation fragments of KLK8 are labeled II-IV having an apparent molecular mass of 21,
11, and 8 kDa. Only bands I and II were active as revealed by the two white bands in the gelatin
zymogram. C. Location of the N-terminal sequences obtained by Edman degradation of each
KLK8 autodegradation fragment within the primary KLK8 protein sequence. Pre-pro-KLK8 is
formed of a signal peptide, followed by a short 4 amino acid pro-peptide (bolded and underlined)
and the mature KLK8 N-terminal sequence (bolded and boxed). The N-terminal sequence of
KLK8 degradation fragments II, III, IV is boxed, with the corresponding label above. The R164
amino acid where autolytic cleavage occurs is bolded. The catalytic triad (H86, D120, S212) is
indicated with an asterisk (*). The site of putative processing by a signal peptidase is C terminal
to A28, K32 marks the activation site of the pro-KLK8 protease, N110 glycosylation site, and
D206, which confers trypsin-like specificity of the mat-KLK8 protease are bolded and labelled
with their position in the KLK8 primary sequence. KLK8 sequence is numbered from the N-
terminus of pre-pro-KLK8 based on NCBI gene bank accession no. NP_009127. D. Location of
key residues and the autolytic cleavage site within the theoretical tertiary structure of mature
KLK8, as predicted by Pymol homology modeling. The ribbon plot of mature KLK8 is shown in
the traditional serine protease standard orientation (i.e. looking into the active site cleft).
Secondary structure elements are displayed as arrows (β-strands) and ribbons (α-helices). N- and
C-terminal residues are shown in black. The side chains of the catalytic triad residues are shown
in green. D206 is shown in red at the base of the active-site pocket. The glycosylation site is
coloured orange. KLK8 auto-cleavage site after R164 at P1 is shown in magenta. E. Silver-
stained SDS-PAGE displaying mat-KLK8 autodegradation in a time-course study. Cleaved
fragments represented the majority of KLK8 detected after 12 hr incubations at 37˚C. F. Mat-
KLK8 autolysis resulted in enzymatic inactivation detected by the drastic decrease in residual
KLK8 activity towards VPR-AMC at time points 12, 24, and 34 hrs, corresponding to residual
activity of 78%, 18% and 9%, respectively. G. Mat-KLK8 activity in the presence of inhibitors
of different protease classes. The mat-KLK8 labeled with an asterisk was not incubated at 37˚C,
while the remaining samples were incubated for 12 hr at 37˚C.
57
2.3.2 Pro-KLK8 zymogen activation in an epidermal cascade
To avoid confounding activity results due to overlapping KLK5 and KLK8 trypsin-like activity,
α1-antitrypsin (AT) inhibitor was used in this study to quench KLK5 activity prior to detecting
pro-KLK8 activation. As a control, we showed that AT decreased mat-KLK5 activity to less that
3% of its original activity, and had no inhibitory effect on mat-KLK8 activity at all (Figure 2.2).
The significant increase in activity in the pro-KLK8/mat-KLK5 mix detected after 18 hrs at
37˚C, post quenching KLK5 activity with AT, indicated activation of pro-KLK8 (Figure 2.2).
Although KLK5 is indeed an in vitro activator of pro-KLK8, this activation process was slow as
it occurred after 18 hr incubation at 37˚C (Figure 2.2) or 1 day at 25˚C. Nonetheless, KLK5
activation of pro-KLK8 may be important physiologically as normal human epidermal cell
turnover occurs in the span of 2-4 weeks. Furthermore, expression data suggest that
immunoreactive KLK8 concentration in normal human SC tissue extracts is about 4-fold higher
than KLK5 (Komatsu et al., 2005a). Hence, this activation process may occur quicker had we
used a pro-KLK8: KLK5 molar ratio of 1:4 or 1:1 instead of 1:10.
We also tested if KLK8 is activated by KLK1 given that KLK1 is active in normal human sweat
(Hibino et al., 1994) and co-localizes with pro-KLK8 in the SC. We used a pro-KLK1
preparation known to autoactivate and found that the pro-KLK8 was not activated by KLK1.
Autoactivation of pro-KLK8 was not reported previously, however, we detected a time and
concentration dependent increase in AMC fluorescence emission after incubating pro-KLK8 for
48 hr at 37˚C. This very slow autoactivation could be facilitated by host proteases. We thus
carried out a stability time-course study using two recombinant pro-KLK8 enzymes produced in
baculovirus and yeast expression systems. Pro-KLK8 proteases were incubated in the presence
of inhibitors of different protease classes, similar to the experiment done above to investigate
mat-KLK8 autodegradation. We detected increase in activity upon incubating both proteases
alone at 37˚C for 48 hours. But unlike mat-KLK8 autodegradation, pro-KLK8 activation was
facilitated by host serine proteases that were inhibited by AT treatment. For in vitro activation of
pro-KLK8, lysyl endopeptidase was the best pro-KLK8 activator, due to its specific cleavage
after lysine residues, where it resulted in rapid activation of pro-KLK8 (16.5-fold increase)
within 1 hr incubation at 37˚C with 1:1000 (activator:pro-KLK8) ratio (data not shown). Thus,
our data show that pro-KK8 does not autoactivate or get activated by KLK1, but it is activated by
KLK5 and lysyl-endopeptidase in vitro.
58
37 C
KLK
5 + p
ro-K
LK
8 (1
hr)
KLK
5 + p
ro-K
LK
8 (3
hr)
KLK
5 + p
ro-K
LK
8 (1
8hr)
KLK
5 (1h
r)K
LK
5 (3h
r)K
LK
5 (18
hr)
pro
-KLK
8 (1
hr)
pro
-KLK
8 (3
hr)
pro
-KLK
8 (1
8 h)
0
100
200
300
400
500 - AT
+ ATR
ate
of
VP
R-A
MC
hy
dro
lys
is
(nM
AM
C/m
in)
Figure 2.2. Pro-KLK8 activation by active KLK5. Activation or pro-KLK8 by mat-KLK5 was
carried at a molar ratio of (10:1), where a duplicate mix was included with α1-antitrypsin
inhibitor (AT) as a KLK5 activity quencher. Each activation mix was incubated for varying time
points (1, 3, 18, at 37 °C) with or without an additional 1 hr incubation with AT. Activity
towards VPR-AMC substrate was measured for the same reaction mixures with or without AT in
triplicates.
59
2.3.3 Effect of cations on KLK8 activity
KLK8 activity ability to hydrolyze VPR-AMC in the presence of the relevant epidermal cations
Zn2+
, Ca2+
, Mg2+
, Na+, and K
+ was examined, as shown in Table 2.1. KLK8 was activated to a
significant extent by Ca2+
ions at all concentrations examined. Mg2+
ions activated KLK8 but to
a lower extent compared to Ca2+
and at higher concentrations. On the other hand, Zn2+
attenuated
mat-KLK8 activity as expected given its inhibitory effect on numerous metal binding enzymes
including metalloproteases and other KLKs. Zn2+
inhibition of 10 nM mat-KLK8 was
pronounced in the µM and mM range but not in the nM range. Na+
and K+ cations had no
significant effect on KLK8 activity at the concentrations tested. These results support the
involvement of metal ions, particularly Ca2+
and Zn2+
, at the active site of mat-KLK8. To date,
the crystal structure of human KLK8 remains to be resolved, thus the exact mechanisms by
which zinc and calcium bind human KLK8 active site remain to be elucidated.
2.3.4 Differential inhibition by skin specific inhibitors and general serpins
We investigated the inhibitory effects of three serine protease inhibitors known to be present in
human SC: the lympho-epithelial kazal type inhibitor (LEKTI), SLPI, and elafin. SLPI and elafin
did not inhibit mat-KLK8 activity. These two inhibitors inhibit chymotrypsin-like KLK7
activity, but do not exert any inhibitory effect on epidermal trypsin-like KLKs (Borgono et al.,
2007b). Alternatively, inhibitory LEKTI domains (D1-15) inhibit distinct trypsin-like KLKs and
chymotrypsin-like KLK7 with different potencies (Deraison et al., 2007; Descargues et al.,
2005). Unlike other epidermal KLKs such as KLK5, KLK6, KLK7, KLK13, and KLK14, and
similar only to KLK1 (Borgono et al., 2007b), all LEKTI domains had no inhibitory effect on
mat-KLK8 activity, even at 10-fold higher molarity. All LEKTI domains tested inhibited KLK5
as a positive control.
We further tested mat-KLK8 inhibition by general serine protease inhibitors (serpins). We
confirmed KLK8 inhibition by α2-antiplasmin, protein C inhibitor, and aprotinin. Unlike KLK5
and KLK14, mat-KLK8 was inhibited by the chymotrypsin-like inhibitor chymostatin, but not
inhibited by the trypsin-like inhibitor α1-antitrypsin. Taken together, our results show that KLK8
regulation by endogenous inhibitors and general serpins is different from the two other epidermal
trypsin-like KLK5 and KLK14.
60
Table 2.1. Divalent Ion effect on mat-KLK8 activity
Ion 1
Concentration 2
(mM)
Molar ratio
(KLK8:cation)
Residual Activity
(100%)
Ca 2+
0.0001 (1:10) 114.9
0.001 (1:100) 116.5
0.10 (1:10,000) 121.0
1.00 (1:100,000) 145.0
10.0 (1:1000,000) 150.2
Mg 2+
0.10
1.00
(1:10,000)
(1:100,000)
106.1
112.8
10.0 (1:1000,000) 142.0
Zn 2+
Na +
K +
0.0001
0.001
0.01
0.10
1.00
10.0
10.0
10.0
(1:10)
(1:1,00)
(1:1,000)
(1:10,000)
(1:100,000)
(1:1000,000)
(1:1000,000)
(1:1000,000)
105.4
101.2
107.3
76.6
39.6
28.6
100
100
1. Ca2+
and Na+
data corresponds to incubation of KLK8 with CaCl2
or NaCl, respectively.
2. KLK8 final concentration was 0.00001 mM (or 10 nM) in all experiments
61
2.3.5 KLK8 AMC substrate profiling and steady-state kinetic constants
The substrate specificity of KLK8 was assessed by profiling its kinetic parameters (i.e. Km and
kcat) against a panel of 16 tripeptide synthetic substrates containing an AMC fluorogenic leaving
group. Among these AMC-peptides, thirteen were candidate trypsin-like enzyme substrates (10
with Arg and 3 with Lys basic residues at P1 position according to the Schechter and Berger
notation), and two for chymotrypsin-like enzymes (with bulky, hydrophobic amino acids Tyr and
Phe), and one substrate for neutrophil elastase (with small aliphatic Val at P1 position). As
predicted by the presence of Asp206
, close to Ser212
of the catalytic triad, KLK8 was confirmed to
have trypsin-like, but not chymotrypsin-like activity, since no reaction was observed for the two
chymotrypsin-like enzyme substrates (AAPF-AMC, LLVY-AMC). Mat-KLK8 displayed
trypsin-like specificity with a greater catalytic efficiency for Arg vs. Lys at the P1 position, as
substrates with the highest kcat/Km values contained P1-Arg. Although KLK8 did not cleave
GPK, it cleaved VLK, indicating that is it capable of cleaving after lysine, depending on adjacent
residues. The P2 specificity of KLK8 was examined by comparing the kcat/Km values among
substrates with invariable P1 and P3 residues as follows: 1) QAR-AMC, QGR-AMC, and QRR-
AMC; 2) LKR-AMC and LRR-AMC; and 3) GPR-AMC and GRR-AMC. KLK8 preferred
A>R>G, R>K, G>P at P2. This suggested that P2 position may not influence mat-KLK8
specificity significantly, but this observation was based on comparing a small number of
fluorogenic tripeptides. The P3 preference of KLK8 was assessed by examining kcat/Km values
in substrates bearing the same P1 and P2 amino acids as follows: 1) VPR-AMC and GPR-AMC;
2) QGR-AMC, and GGR-AMC; and 3) QRR-AMC and LRR-AMC. Glycine was a highly
unfavored residue at this position. The best substrates for mat-KLK8 were VPR-AMC, also a
substrate for α-thrombin, and QAR-AMC, also a substrate for trypsin.
2.3.6 Rapid endopeptidase library screening of mat-KLK8 P2-P2’ substrate specificity
A rapid endopeptidase profiling library of quenched FRET peptides was utilized to reveal
information about non-prime and prime side substrate specificity of mat-KLK8. The principle of
this assay and how it differed from non-prime AMC substrate profiling is illustrated in Figure 4.
Our data suggest that mat-KLK8 is a specific trypsin-like enzyme, where all top 29 peptide pools
displaying strong and moderate cleavage contained R/K. Of the 57 peptide pools cleaved, 47
contained R/K in at least one variable core region (Figure 2.3A), consistent with the trypsin-like
62
specificity of KLK8 on the C-terminal side of basic residues. Interestingly, 10 of the peptide
pools that showed weak cleavage (S:B = 1.25-1.50) contained no R/K, where 9 pools contained
F/Y and one peptide pool contained I/L, suggesting restricted weak chymotrypsin-like activity of
mat-KLK8. Mat-KLK8 enzyme is likely to function as a regulatory trypsin-like protease in upper
epidermis rather than a degrading enzyme with broad specificity, as 122 out of the total 169
peptide pools containing at least one R/K (~ 72% of trypsin-like peptides) remained uncleaved
after 1 hr incubation (S:B< 1.25). KLK8 selectivity can be seen by its in vitro ability to activate
the pro-form of merpin-β, but not merpin-α, even though activation of both epidermal
metalloproteases requires cleavage after arginine (Ohler et al., 2010). The top peptide hit for
KLK8 with highest fluorescence increase contained the core motif (RK)-(S/T)-(A/V) and 6 out
of the top 14 peptide hits contained (R/K)-(S/T) bond, indicating a strong preference for Ser/Thr
at P1'. Using this peptide as a reference for positional-scanning profiling of mat-KLK8
specificity we found that mat-KLK8 displayed strong preference for hydrophobic I/L and F/Y
amino acids at P2, specific preference for R/K at P1, restricted preference for S/T and F/Y at P1',
and A/V, R/K, and F/Y at P2', (Figure 2.3B). All peptides containing negatively charged amino
acids (D or E) at P'1 or P'2 in this library were not hydrolyzed by mat-KLK8. Furthermore, we
detected potential subsite cooperativity in the active site pocket, where the presence of two (R/K)
residues close to each other enhanced cleavage. These results were consistent with M. Debela,
E.L. Schinder, C.S. Craik unpublished findings regarding mat-KLK8 non-prime substrate
specificity where preference for R over K at P1, hydrophobic residues at P2, and R/K residues at
P3 was observed (Ohler et al., 2010). Herein, we also show that the prime side of the scissile
bond is important, where the highest fluorescence signal was obtained for the (R/K)-(S/T)-(A/V)
FRET-quenched peptides. We confirmed KLK8-cleavage of this eight peptide pool after arginine
by mass-spectrometry MS1 and identified the fluorescent tag and the cleaved N-terminal side by
MS2 analysis.
63
A)
B)
Figure 2.3. Kallikrein-related peptidase-8 displays restricted substrate specificity based on
cleavage of FRET-tripeptides. (A) Pie chart of mat-KLK8 cleavage of peptides after 1 hr incubations
with 10 nM mat-KLK8 at 37˚C. (B) Mat-KLK8 specificity based on positional scanning of the top
peptide hit result from the Rapid Endoprotease Library (RepLi) screenin
64
2.3.7 KLK8 activation of co-localized epidermal pro-KLKs
Pro-KLK5, pro-KLK11, and pro-KLK1 are potential downstream targets of mat-KLK8 given
their co-localization with KLK8 in normal human sweat and SC and the requirement for
cleavage after Arg for their activation. Pro-KLK10 is an unlikely substrate of mat-KLK8 because
its activation sequence NDTRLDP contains two acidic D residues, P3-D and P2’-D, which are
highly unfavored residue for mat-KLK8 based on our RepLi screening. Unfortunately, we were
unable to draw any conclusions regarding KLK5 activation by KLK8, due to the confounding
effect of KLK5 rapid autoactivation. In the case of pro-KLK1, which autoactivates, we utilized a
sandwitch-type pull-down assay to capture KLK1 and completely eliminate mat-KLK8 activity.
Although KLK8 exhibits activity towards PFR-AMC, no enzymatic activity was observed for the
pulled-down KLK8 on KLK1 antibody-coated wells (Figure 2.4A). KLK8 ability to activate
KLK1 was detected in a time-dependent manner, Figure 2.4A. In the case of pro-KLK11, time-
dependent pro-KLK8 activation was seen within 30 min incubation period at 37˚C at 1:100
(KLK8: pro-KLK11) molar ratio Figure 2.4B. Hence, we demonstrated KLK8 ability to target in
vitro activation of two potential substrates, pro-KLK11 and pro-KLK1, in human SC and sweat.
65
Figure 2.4. KLK8 activation of pro-KLK1 and pro-KLK11. (A) Pro-KLK1 activation by
mat-KLK8 was performed using a sandwitch-type pull-down activity assay of pro-KLK1.
Enzymatic activity of pulled down KLK1 was measured, using 0.25 mM PFR-AMC. (B) Pro-
KLK11 activation by mat-KLK8. Time-dependent activation of pro-KLK11 was detected at a
1:100 molar ratio at 37 °C in optimal KLK8 activity buffer. Pro-KLK11 activation is measured
by detecting fluorescence release off 0.25 mM PFR-AMC over time for the pro-KLK11 alone,
mat-KLK8 alone, and the pro-KLK11/mat-KLK8 activation mixture as shown above.
66
2.3.8 KLK8 processing of LL-37 antimicrobial peptide
Cathelicidin antimicrobial peptides (APs) are effector molecules of the innate immune system
detected in human SC, sweat, and wound secretions with microbicidal and proinflammatory
activities (43 Gallo -45 Murakami). The 37-amino acid-long C-terminus of cathelicidin is
referred to as LL-37 and represents the active peptide which displays direct antimicrobial activity
by interaction with the cell membranes of microorganisms. We investigated KLK8 ability to
process LL-37 synthetic peptide into shorter active antimicrobial peptides. We detected a single
peak by RP-HPLC on a C18 column eluting at 55% B, Acetonitrile, corresponding to the LL-37
peptide alone which remained present after 2 and 6 hrs incubations at 37˚C. Processing of the
LL37 peptide was observed upon incubation with 20 nM mat-KLK8, where the LL-37 peak seen
at 25 minute decreased and broadened, and two earlier sharp peaks were detected indicating
cleavage of LL-37. The two peaks corresponded to small peptides (charge state z = 2) with m/z
ratio of 434.2390 and 458.2481, which were identified by LC/MS to represent the LL-7 and NL-
8 peptides, respectively. However, the abundance of the remaining cleavage products in the
lyophilized RP-HPLC fractions was insufficient to be detected by LC/MS. Thus, we used an
alternative method where the LL-37 control peptide and LL-37/KLK incubation mixtures were
purified by C18 OMIX tips prior to separating on nano-LC C18 column attached directly to
LTQ/Orbitrap mass-spectrometer. This sensitive method allowed us to identify all peptide
fragments present in each sample. We identified the LL-7 and NL-8 fragments as well as IK14,
LL-23, and LL-37 in the KLK8-treated sample. LL-37 peptide was treated with active KLK5 as
a positive control and LL-7, NL-8, and IK-6 were identified as cleavage products, in addition to
their other halves, KS-30*, LL-29*, KS-22*, which represents the active antimicrobial peptides
marked with asterisks, respectively (Lundstrom et al., 1994). Our results show that active KLK8
and KLK14 process LL-37 AP after Arg residues generating KS-30*/LL-7, LL-29*/NL-8, and
IK-14/LL-23* peptides. Figure 6 displays identified fragments in each LL-37/KLK incubation
mix versus LL-37 peptide incubated alone for 6 hours. IK-14/LL-23* was not identified in the
KLK5-treated sample because the IK-14 peptide was likely further proteolysed to IK-6 and NL-
8.
67
NO Enzyme
KLK5
c:\chris\...\ll-37-klk5-6hrdigest_01 11/17/2009 3:06:37 PM
RT: 0.00 - 23.03 SM : 7B
0 5 10 15 20
Time (min)
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50
100
0
50
100
0
50
100
Rela
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50
100
RT: 15.16
AA: 150613447
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RT: 15.64
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m/ z= 899.5137-899.5317 MS Genesis ll-37-cont rol-6hdigat 37c_01
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NL: 3.41E6
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NL: 4.04E6
m/ z= 899.5137-899.5317 MS Genesis ll-37-klk14-6hrdigest _01
RT: 0.00 - 23.03 SM : 7B
0 5 10 15 20
Time (min)
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50
100
0
50
100
0
50
100
Rela
tive A
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0
50
100
RT: 17.55
AA: 292366
RT: 9.92
AA: 602461
RT: 14.11
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RT: 9.86
AA: 177596707
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NL: 1.98E7
m/ z= 434.2352-434.2438 MS Genesis ll-37-klk5-6hrdigest _01
NL: 7.30E5
m/ z= 434.2352-434.2438 MS Genesis LL-37-KLK8-6hrdigest _01
NL: 6.40E7
m/ z= 434.2352-434.2438 MS Genesis ll-37-klk14-6hrdigest _01
RT: 0.00 - 23.03 SM : 7B
0 5 10 15 20
Time (min)
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50
100
0
50
100
0
50
100
Rela
tive A
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50
100
RT: 12.85
AA: 474056
RT: 4.89
AA: 85652
RT: 12.51
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RT: 4.80
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m/ z= 458.2430-458.2522 MS Genesis ll-37-cont rol-6hdigat 37c_01
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m/ z= 458.2430-458.2522 MS Genesis ll-37-klk5-6hrdigest _01
NL: 3.83E5
m/ z= 458.2430-458.2522 MS Genesis LL-37-KLK8-6hrdigest _01
NL: 6.56E6
m/ z= 458.2430-458.2522 MS Genesis ll-37-klk14-6hrdigest _01
RT: 0.00 - 23.03 SM : 7B
0 5 10 15 20
Time (min)
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50
100
0
50
100
0
50
100
Rela
tive A
bundance
0
50
100
RT: 8.67
AA: 390314
RT: 8.76
AA: 146250RT: 12.62
AA: 40697RT: 7.66
AA: 63449 RT: 15.28
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AA: 3311
RT: 8.44
AA: 1387272
RT: 7.93
AA: 17746784
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m/ z= 457.6086-457.6178 MS Genesis ll-37-cont rol-6hdigat 37c_01
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m/ z= 457.6086-457.6178 MS Genesis ll-37-klk5-6hrdigest _01
NL: 8.66E4
m/ z= 457.6086-457.6178 MS Genesis LL-37-KLK8-6hrdigest _01
NL: 2.03E6
m/ z= 457.6086-457.6178 MS Genesis ll-37-klk14-6hrdigest _01
ll-37-klk5-6hrdigest_01 # 1801-2421 RT: 12.39-15.63 AV: 90
T: FTM S + p NSI Full ms [350.00-1455.22]
899 900 901
m/z
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Rela
tive A
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899.5231
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899.7229
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899.3228
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899.9231
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900.1234
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900.5287
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900.7253
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ll-37-klk14-6hrdigest_01 # 1212-1890 RT: 8.15-11.43 AV: 98 NL:
T: FTM S + p NSI Full ms [350.00-1455.22]
433.5 434.0 434.5
m/z
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Rela
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434.2396
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434.0819
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434.3971
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433.7677
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434.6701
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433.5831
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ll-37-klk14-6hrdigest_01 # 237-1068 RT: 2.53-7.51 AV: 124 NL:
T: FTM S + p NSI Full ms [350.00-1455.22]
458.0 458.2 458.4
m/z
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40
45
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65
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Rela
tive A
bundance
458.2480
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457.9447
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458.2790
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458.4187
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458.2162
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ll-37-klk14-6hrdigest_01 # 806-1752 RT: 6.29-10.80 AV: 136
T: FTM S + p NSI Full ms [350.00-1455.22]
563.0 563.2 563.4 563.6 563.8
m/z
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5
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55
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Rela
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bundance
563.6496
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563.3203
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563.8162
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c:\chris\...\ll-37-klk5-6hrdigest_01 11/17/2009 3:06:37 PM
RT: 0.00 - 23.03 SM : 7B
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Time (min)
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50
100
Rela
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m/ z= 899.5137-899.5317 MS Genesis ll-37-cont rol-6hdigat 37c_01
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NL: 3.41E6
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m/ z= 899.5137-899.5317 MS Genesis ll-37-klk14-6hrdigest _01
RT: 0.00 - 23.03 SM : 7B
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Time (min)
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50
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Rela
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AA: 292366
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m/ z= 434.2352-434.2438 MS Genesis ll-37-klk5-6hrdigest _01
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NL: 6.40E7
m/ z= 434.2352-434.2438 MS Genesis ll-37-klk14-6hrdigest _01
RT: 0.00 - 23.03 SM : 7B
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Time (min)
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m/ z= 458.2430-458.2522 MS Genesis ll-37-klk5-6hrdigest _01
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m/ z= 458.2430-458.2522 MS Genesis LL-37-KLK8-6hrdigest _01
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Time (min)
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50
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50
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Rela
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100
RT: 8.67
AA: 390314
RT: 8.76
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AA: 63449 RT: 15.28
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m/ z= 457.6086-457.6178 MS Genesis ll-37-klk14-6hrdigest _01
ll-37-klk5-6hrdigest_01 # 1801-2421 RT: 12.39-15.63 AV: 90
T: FTM S + p NSI Full ms [350.00-1455.22]
899 900 901
m/z
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Rela
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900.1234
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900.3238
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900.5287
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900.7253
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ll-37-klk14-6hrdigest_01 # 1212-1890 RT: 8.15-11.43 AV: 98 NL:
T: FTM S + p NSI Full ms [350.00-1455.22]
433.5 434.0 434.5
m/z
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Rela
tive A
bundance
434.2396
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434.0819
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434.3971
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433.7677
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434.6701
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433.5831
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ll-37-klk14-6hrdigest_01 # 237-1068 RT: 2.53-7.51 AV: 124 NL:
T: FTM S + p NSI Full ms [350.00-1455.22]
458.0 458.2 458.4
m/z
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Rela
tive A
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458.2480
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457.9447
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458.2790
z=3458.0771
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458.4187
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458.2162
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ll-37-klk14-6hrdigest_01 # 806-1752 RT: 6.29-10.80 AV: 136
T: FTM S + p NSI Full ms [350.00-1455.22]
563.0 563.2 563.4 563.6 563.8
m/z
0
5
10
15
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25
30
35
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45
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55
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Rela
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563.6496
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563.8162
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c:\chris\...\ll-37-klk5-6hrdigest_01 11/17/2009 3:06:37 PM
RT: 0.00 - 23.03 SM : 7B
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Time (min)
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50
100
0
50
100
0
50
100
Rela
tive A
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100
RT: 15.16
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RT: 0.00 - 23.03 SM : 7B
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Time (min)
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50
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50
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Rela
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100
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AA: 292366
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NL: 6.40E7
m/ z= 434.2352-434.2438 MS Genesis ll-37-klk14-6hrdigest _01
RT: 0.00 - 23.03 SM : 7B
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Time (min)
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100
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50
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50
100
Rela
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100
RT: 12.85
AA: 474056
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m/ z= 458.2430-458.2522 MS Genesis ll-37-klk14-6hrdigest _01
RT: 0.00 - 23.03 SM : 7B
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Time (min)
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100
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50
100
0
50
100
Rela
tive A
bundance
0
50
100
RT: 8.67
AA: 390314
RT: 8.76
AA: 146250RT: 12.62
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AA: 63449 RT: 15.28
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RT: 8.44
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RT: 7.93
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NL: 3.21E4
m/ z= 457.6086-457.6178 MS Genesis ll-37-cont rol-6hdigat 37c_01
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m/ z= 457.6086-457.6178 MS Genesis ll-37-klk5-6hrdigest _01
NL: 8.66E4
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m/ z= 457.6086-457.6178 MS Genesis ll-37-klk14-6hrdigest _01
ll-37-klk5-6hrdigest_01 # 1801-2421 RT: 12.39-15.63 AV: 90
T: FTM S + p NSI Full ms [350.00-1455.22]
899 900 901
m/z
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Rela
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900.1234
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900.3238
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900.5287
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900.7253
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ll-37-klk14-6hrdigest_01 # 1212-1890 RT: 8.15-11.43 AV: 98 NL:
T: FTM S + p NSI Full ms [350.00-1455.22]
433.5 434.0 434.5
m/z
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25
30
35
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Rela
tive A
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434.2396
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434.0819
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434.3971
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433.7677
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434.6701
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433.5831
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ll-37-klk14-6hrdigest_01 # 237-1068 RT: 2.53-7.51 AV: 124 NL:
T: FTM S + p NSI Full ms [350.00-1455.22]
458.0 458.2 458.4
m/z
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Rela
tive A
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458.2480
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457.9447
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458.2790
z=3458.0771
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458.4187
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458.2162
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ll-37-klk14-6hrdigest_01 # 806-1752 RT: 6.29-10.80 AV: 136
T: FTM S + p NSI Full ms [350.00-1455.22]
563.0 563.2 563.4 563.6 563.8
m/z
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5
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Rela
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c:\chris\...\ll-37-klk5-6hrdigest_01 11/17/2009 3:06:37 PM
RT: 0.00 - 23.03 SM : 7B
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Time (min)
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50
100
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50
100
0
50
100
Rela
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50
100
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Time (min)
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Rela
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100
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50
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Rela
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100
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m/ z= 457.6086-457.6178 MS Genesis ll-37-klk14-6hrdigest _01
ll-37-klk5-6hrdigest_01 # 1801-2421 RT: 12.39-15.63 AV: 90
T: FTM S + p NSI Full ms [350.00-1455.22]
899 900 901
m/z
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Rela
tive A
bundance
899.5231
z=5
899.7229
z=5
899.3228
z=5
899.9231
z=5
900.1234
z=5
899.1226
z=5
900.3238
z=5
900.5287
z=5
900.7253
z=5898.5205
z=?
ll-37-klk14-6hrdigest_01 # 1212-1890 RT: 8.15-11.43 AV: 98 NL:
T: FTM S + p NSI Full ms [350.00-1455.22]
433.5 434.0 434.5
m/z
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Rela
tive A
bundance
434.2396
z=2
434.0819
z=51
434.3971
z=6
433.7677
z=?
434.6701
z=?
433.5831
z=2
ll-37-klk14-6hrdigest_01 # 237-1068 RT: 2.53-7.51 AV: 124 NL:
T: FTM S + p NSI Full ms [350.00-1455.22]
458.0 458.2 458.4
m/z
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Rela
tive A
bundance
458.2480
z=2
457.9447
z=3
458.2790
z=3458.0771
z=?
458.4187
z=?
458.2162
z=?
ll-37-klk14-6hrdigest_01 # 806-1752 RT: 6.29-10.80 AV: 136
T: FTM S + p NSI Full ms [350.00-1455.22]
563.0 563.2 563.4 563.6 563.8
m/z
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Rela
tive A
bundance
563.6496
z=3
563.3203
z=3
563.8162
z=?563.5550
z=?563.0768
z=?
563.2133
z=?
LL-37 LL-7 NL-8 IK-14m/z = 899.1226
z = 5
m/z = 432.2390
z = 2
m/z =458.2481
z = 2
m/z = 563.3206
z = 3
KLK8
KLK14
Figure 2.5. Proteolytic processing of the LL-37 antimicrobial peptide by kallikreins.
Identification of cleaved fragments off LL-37 peptide after 6 hr incubation at 37°C alone or with
active KLK5, KLK8, or KLK14. The MS1 elution profile with integrated peaks is shown for the
parent LL-37 peptide, and the LL-7, NL-8, and IK-14 cleaved fragments (labelled at the top of
the Figure with their corresponding m/z ratio and z charge state) for each of the LL-37 peptide
incubated with no enzyme, KLK5, KLK8, and KLK14 as indicated on the left side of the Figure .
The retention times (RT) and integrated area under curve (AA) are displayed as well. Each of
these peptides was confirmed by MS2 in the enzyme-treated samples (data not shown).
68
2.3.9 Human KLK8 is a keratinocyte-specific protease induced during terminal keratinocyte differentiation
In order to support KLK8 involvement in normal skin barrier functions, we investigated KLK8
secretion by skin cells in culture. Given that KLKs are constitutively expressed in the granular
layer of epidermis, we cultured HaCat keratinocytes in low versus high calcium medium, a
manipulation thought to mimic the calcium gradient of upper epidermis (Hennings and
Holbrook, 1983). We found that increasing exogenous Ca2+
in the medium changed keratinocyte
cell morphology, induced corneocyte formation as expected, Figure 2.6B (Menon and Elias,
1991; Menon et al., 1994) and significantly increased KLK8 protein secretion into the medium in
a time-dependent manner. We detected over 14-fold increase in KLK8 levels in high versus low
calcium serum-free medium in day 3, (Figure 2.6A). We also investigated the ability of primary
epidermal keratinocytes to co-secrete KLK proteins, including KLK8, as well as primary
melanocytes and fibroblasts. We found that KLK8 was exclusively secreted by keratinocytes,
whereby primary epidermal melanocytes (pHEM) and dermal fibroblasts (pHDF) secreted no
detectable KLK8 even after day 12 in culture. KLK8 levels in the HaCat immortalized
keratinocyte and primary neonatal keratinocyte culture media continued to increase with
increased keratinocyte proliferation, while LDH levels remained constant. This confirmed that
the elevation in KLK8 levels was due to increased secretion rather that cell rupture or death over
time. Hence, our data suggest that KLK8 is a keratinocyte-specific protease secreted during
epidermal keratinocyte proliferation and abundantly secreted upon their terminal differentiation
in culture, Figure 2.6A. However, not all epidermal KLKs evaluated were keratinocyte-specific.
KLK7 was secreted by all epidermal cells investigated, while KLK5 and KLK8 were by far the
most abundant secreted keratinocyte-specific KLKs.
2.3.10 KLK8 is expressed in a free form in human sweat and non-palmoplantar stratum corneum
We pooled processed normal human sweat and non-palmo-plantar stratum corneum (SC)
extracts to study KLK8 expression and activity ex vivo. The sweat and SC samples contained 8.0
mg and 3.5 mg total protein, respectively. KLK8 was detected in both biological samples at a
molecular mass similar to recombinant mat-KLK8 (~31 kDa) in a reduced SDS-PAGE, and
processed froms of KLK8 in the sweat (~31 and 21 kDa) were also detected, (Figure 2.6C).
69
A) B) C)
Figure 2.6. KLK8 is a skin barrier protease. (A) KLK8 secretion is induced by calcium-
mediated stimulation of terminal keratinocyte differentiation and cornification in culture. The
black bars represent undifferentiated keratinocytes and the grey bars represent differentiated
keratinocytes grown in high calcium medium over 3 days period. (B) Terminal keratinocyte
differentiation and cornification in culture (high calcium), compared proliferating keratinocytes
(low calcium) in day 3. (C) KLK8 is expressed in a free non-complexed form in normal human
sweat and non-palmoplantar stratum corneum skin extracts.
70
2.3.11 KLK8 is catalytically active in human sweat and non-palmoplantar stratum corneum
We sought to investigate KLK8-specific activity using an ex vivo skin model and recombinant
mat-KLK8 as a positive control. We first confirmed the co-expression of multiple KLKs in sweat
and SC epidermal extracts. According to our KLK-specific ELISA results, the pooled sweat
sample contained 172 ng KLK8/mg total protein, 35 ng KLK11/mg total protein and 2.2 ng
KLK5/mg total protein. On the other hand, the pooled SC epidermal extract contained 28 ng
KLK8/mg total protein, 63 ng KLK11/mg total protein, and 210 ng KLK5/mg total protein.
Hence, we detected 78-fold higher concentration of KLK8 compared to KLK5 in sweat, and 7.5-
fold lower concentration of KLK8 compared to KLK5 in the stratum corneum extracts. KLK11
was in the middle range between KLK5 and KLK8, as indicated above.
To detect KLK8 activity specifically, and no other KLK or serine protease activity, we
immunocaptured KLK8 and tested its ability to cleave fluorogenic VPR-AMC substrate at
different pH levels. The principle of the KLK8 immuocapture-activity assay we developed for
this purpose is shown in Figure 2.7A. We validated first that the monoclonal KLK8 antibody we
used to coat the plate was a non-neutralizing antibody. Then, we performed a series of
optimization steps using active recombinant mat-KLK8 protein. The sensitivity of the KLK8
immunocapture assay was optimized to detect a concentration-dependent increase in activity
when increased amount of active recombinant mat-KLK8 was loaded into KLK8-Ab coated
wells. Assay specificity was also confirmed upon detecting no activity as a result of loading
active KLK5 into wells coated with KLK8 antibody.
Approximately 40 ng of sweat KLK8, 4 ng of SC KLK8, and 250 ng of mat-KLK8 as well as
10X diluted samples of each were loaded in triplicate wells in the same 96-well plate coated with
KLK8-antibody. We demonstrated a concentration and time dependent increase in AMC-
fluorescence emission in the non-diluted (1X) and (10X) diluted sweat and SC samples,
confirming KLK8 activity in normal human skin surface (Figure 2.7B and 2.7C). The
immunocaptured recombinant mat-KLK8 positive control reached saturation quicker than the
immunocaptured sweat and SC KLK8, as expected, due to loading a significantly higher amount
into the wells (5-10 fold higher than sweat-KLK8 and SC-KLK8). Immunocaptured SC and
sweat KLK8 displayed optimal activity at pH 8.5, Figure 2.7B and 2.7C, and retained lower, yet
71
significant, activity at pH 5.0. Hence, we elucidated KLK8 activity in upper skin surface within
the normal physiological pH gradient of human stratum corneum.
We also investigated if sweat and SC KLK8 contains some inactive pro-KLK8 proportion that
can be activated by exogenous KLK5 or lysyl-endopeptidase. Our results indicate that the
majority of sweat and SC KLK8 was catalytically active (Figure 2.8A and 2.8B). These results,
combined with our immunodetection of KLK5 in the same sweat and SC samples, suggest that
KLK5 may have already activated a large proportion of sweat and SC KLK8. When recombinant
pro-KLK8 was incubated with active KLK5 (1:100 molar ratio) or lysyl endopeptidase (1:1000
molar ratio), and immunocaptured as a positive control in the same KLK8 antibody-coated plate,
we detected pro-KLK8 activation by both proteases (Figure 2.8C).
72
A)
B) C)
Figure 2.7. Immunocapture of KLK8 activity in normal human sweat and non-
palmoplantar stratum corneum ex vivo. (A) A schematic of the principle of the assay. KLK8
is immunocaptured using a specific non-neutralizing monoclonal antibody. Potential non-specific
binding of other protease contaminants is eliminated through a series of stringent washes. The
activity of the immobilized sweat or SC KLK8 is measured by monitoring fluorescence emission
of VPR-AMC substrate compared to no-enzyme background control. (B) Time and
concentration-dependent activity of immunocaptured sweat and SC KLK8 monitored in real-time
for a total of 24 hours at 37ºC and pH 8.5. (C) Time and concentration-dependent activity of
immunocaptured recombinant mat-KLK8 activity as a positive control monitored in the same
plate in real-time for a total of 24 hours at 37ºC and pH 8.5.
73
Figure 2.8. The majority of sweat and SC KLK8 is catalytically active. No significant
increase in sweat and SC KLK8 activity was detected upon adding active KLK5 (A) or active
lysyl-endopeptidase (B) to the samples prior to the KLK8 immunocapture-activity assays,
compared to the immunocaptured recombinant pro-KLK8 positive control (C). KLK5 and lysyl
endopeptidase alone reaction rates were zero ensuring assay specificity
74
2.4 Discussion
We sought in this study to investigate KLK8 activity, regulation, and downstream targets in
order to support its potential functional involvement in a proteolytic activation cascade
regulating skin desquamation and antimicrobial defence. We previously described that multiple
KLKs are co-localized in human epidermis and sweat and noted particularly an abundant
expression of KLK8 (Komatsu et al., 2006a). KLK8 is transported and exocytosed by lamellar
granules into the stratum granulosum/stratum corneum (SG/SC) interface (Ishida-Yamamoto et
al., 2004), and is thus likely activated in the SC extracellular space to play a role in SC barrier
functions. Once activated, KLK8 activity is possibly regulated by many of the factors that
control SC barrier integrity, such as epidermal pH and calcium ion gradients as well as
endogenous serine protease/serine protease inhibitors. Herein, we produced recombinant KLK8
in its precursor (pro-KL8) and active (mat-KLK8) forms and investigated a potential SC
cascade-mediated role of KLK8 by examining in vitro its potential to: 1) be regulated by pH and
ions, 2) be activated by co-localized serine proteases, 3) be inhibited by co-localized serine
protease inhibitors, 3) activate co-localized pro-KLKs, and to 4) target co-localized LL-37
antimicrobial peptide activation.
Our results showed that recombinant KLK8 activity is pH-dependent, since it displayed maximal
activity at alkali pH of 8.5 and retained lower activity at pH 5. This suggests a role for KLK8 in
human SC barrier where the pH decreases from 7.5 to 5 at the uppermost surface. Although the
alkaline pH (where KLK8 activity is optimal) occurs in lower SC, it is possible that less active
enzyme is present there where the majority is in a latent pro-KLK8 form or is bound to
endogenous inhibitors. More active enzyme may be present in the upper SC as a result of pro-
KLK8 activation, being freed of a potential inhibitor, and/or the calcium ion gradient shift. Our
studies indicate that KLK8 activity is enhanced by calcium and to a lesser degree by magnesium,
two ions known to have higher concentrations in uppermost SC compared to lower layers.
Interestingly, topical application of 10 mM MgCl2 and CaCl2 was shown to accelerate mouse
skin barrier recovery after acute chemical disruptions (Denda et al., 1999). It is possible that
magnesium and calcium ions influence barrier recovery via their activation of serine proteases
such as KLK8. This possibility requires further investigation.
75
Since Zn2+
levels in human sweat (Crew et al., 2008) and in the skin of mammals may reach the
millimolar range (Nitzan et al., 2004), this ion could be an important regulator of KLK8 activity
in the skin. We showed that zinc ions, within physiological range, attenuate KLK8 activity. Zinc
had a significantly lower inhibitory effect on KLK8 activity compared to other active epidermal
KLKs, as 1:10 molar ratio of KLK to zinc ions resulted in 97.5% and 95% inhibition of KLK5
and KLK14 activity (Michael et al., 2005; Borgono et al., 2007c), respectively, compared to 0%
inhibition of KLK8. This suggests a possibly different binding mode of zinc to KLK8 structure.
However, the crystal structure of human KLK8 has not been resolved to date and the
mechanisms of zinc or calcium binding remain unknown.
Another important mode of regulating a protease irreversible cleavage of substrates is its pro-
zymogen activation to generate the mature active form. We examined the ability of recombinant
pro-KLK8 to be activated by epidermal tryspin-like serine proteases. Akin to pro-KLK8, the pro-
enzymes of KLK7 and 14 require cleavage after lysine for activation. Pro-KLK7 and pro-KLK14
are activated by KLK5 in human epidermis (Brattsand et al., 2005). Given that KLK5 is active in
human SC as a result of autoactivation, we suspected that it activates pro-KLK8 in the deepest
layers of the SC at the close-to-neutral pH. Our results show slow activation of pro-KLK8 by
KLK5 within a day at room temperature at 10:1 molar ratio. This slow activation is similar to
previous reports of KLK5 activation of pro-KLK7 (Brattsand et al., 2005), which may be
important in normal skin physiology where SC layers get renewed every 2-4 weeks (Milstone,
2004). Our in vitro results suggest that pro-KLK8 does not undergo autoactivation or activation
by KLK1, but that it is activated slowly by KLK5 and rapidly by lysl-endopeptidase. Lysyl-
endopeptidase was a better activator of pro-KLK8 due to it specific cleavage after lysine
residues. Hence, we put forward the lysine-specific enteropeptidase, discovered recently in
human skin granular layer (Nakanishi et al., 2010), as a potent potential endogenous activator of
pro-KLK8.
KLK8 activity was not inhibited by any of the currently known endogenous epidermal inhibitors
(LEKTI domains, SLPI, or elafin) in our study. Mat-KLK8 in vitro activity is inhibited by
general serine protease inhibitors such as α2-antiplasmin, aprotinin, protein C inhibitor,
chymostatin and to a low extent by soybean trypsin inhibitor. Unlike KLK5 and KLK14, α1-
antitrypsin does not inhibit mat-KLK8 activity at all (Luo and Jiang, 2006). The P1-Arg
preference of KLK8 and the presence of P1-Arg in α2-antiplasmin and P1-Met in α1-antitrypsin
76
in these serpins may explain their different inhibitory potencies towards KLK8. Perhaps more
interesting from a dermatological perspective, is the finding that unlike epidermal KLK5, 6, 7,
13, and 14, and similar only to KLK1, KLK8 was not inhibited by any of the LEKTI domains
implicated in the devastating skin disease, Netherton Syndrome (Deraison et al., 2007),
(Descargues et al., 2006). KLK8 activity was also recently shown to be completely unaffected by
the new epidermal LEKTI inhibitors encoded by SPINK6 and SPINK9, even though they inhibit
KLK5, 7, and 14 (Brattsand et al., 2009; Meyer-Hoffert et al. 2010). We demonstrated here that
KLK8 activity could be inhibited by auto-cleavage after a solvent-exposed Arg164
in its
computed tertiary structure, which was not previously reported. It is plausible that active co-
localized trypsin-like KLKs, such as KLK14, may degrade KLK8 after Arg164
and reduce its
activity. However, this possibility requires further assessment.
A recent study described KLK8 as a desquamatory enzyme which regulates corneodesmosome’s
degradation, through the actions of other kallikreins, in normal and barrier-disrupted mouse
epidermis (Kishibe et al., 2007). KLK5 has not been detected in mouse epidermis to date,
although a major active enzyme in human SC and a potential activator of KLK8 as we showed
here. We thus investigated the possibility that active KLK8 may target activation of pro-KLK11,
which is also not yet detected in mouse epidermis, but has been detected in human epidermis and
sweat with unknown activators and undemonstrated activity/functionality to date. We elucidated
KLK8 ability to activate pro-KLK11 in vitro using full recombinant KLK proteins which retain
their native tertiary structure. KLK8 was also able to activate pro-KLK1 in vitro. Tissue
kallikrein (KLK1) is active in human sweat (Hibino et al., 1994) and our study is the first to
identify KLK8 as a possible endogenous protease activator of KLK1 in sweat. Our activation
results are in accord with Yoon et al findings with pro-KLK peptide-fusion proteins (Yoon et al.,
2007), except for our finding that KLK1 does not activate pro-KLK8. Overall, our data implicate
KLK5 as an activation initiator and KLK8 as an upstream activator of KLK1 and KLK11 in a SC
and sweat activation cascade.
SC serine proteases can also target LL-37 antimicrobial peptide processing in human skin to
generate shorter antimicrobial peptides that are active against Staphylococcus aureus (Yamasaki
et al., 2006). We demonstrated KLK8 ability to process LL-37 synthetic peptide in vitro, leading
to the formation of active KS-30, LL-29, and LL-23 antimicrobial peptides by trypsin-like
cleavage. Yamasaki et al. previously showed that trypsin-like KLK5 and chymotrypsin-like
77
KLK7 are responsible for LL-37 processing in human skin surface (Yamasaki et al., 2006),
although other yet unknown serine proteases may be equally important in this process. Our in
vitro data suggest KLK8 as a new potential regulator of LL-37 antimicrobial activity in human
skin and sweat.
We showed here that chymostatin inhibits KLK8 activity and previously demonstrated that this
chymotrypsin-like inhibitor is a better inhibitor of KLK8 than the trypsin-like inhibitor,
leupeptin, with IC50 of 8 µM compared to 66 µM, respectively (Kishi et al., 2006). Epidermal
KLK5 and KLK14 trypsin-like activity is efficiently inhibited by leupeptin, but not by
chymostatin (Brattsand et al., 2005). Interestingly, Yamasaki et al detected 80% reduction in
skin surface serine protease activity by chymostatin and a minor reduction by leupeptin, and
suggested accordingly that the chymotrypsin-like KLK7 is a more potent enzyme than trypsin-
like KLKs in human skin surface. Our data lead us to infer that trypsin-like KLK8 was present
in the skin samples tested by Yamasaki et al., contributing to the major serine protease activity
inhibited by chymostatin and leading to LL-37 antimbicrobial peptide trypsin-like processing.
Furthermore, our detection of enhanced KLK8 in vitro expression and activity as a result of
calcium induction of terminal keratinocyte differentiation, and of pro-KLK8 activation by KLK5
and lysyl-endopeptidase, also suggested that KLK8 is active in uppermost epidermis.
We sought after elucidating KLK8 activity in non-palmoplantar stratum corneum and sweat ex
vivo by a designing a KLK8-specific immunocapture-activity assay using fluoreogenic VPR-
AMC substrate. This method allowed us to pull down KLK8 and identify its serine protease
activity in human epidermis and sweat for the first time, supporting our in vitro findings. The
majority of SC and sweat KLK8 was present in a catalytically-active form, displaying optimal
activity at pH 8.5 and retaining activity at pH 5, similar to our recombinant human KLK8. Given
that soap-treated, inflamed, atopic-dermatitis and scaly psoriatic skins show alkali to neutral pH
near KLK8 activity optimum (Hachem et al., 2010), KLK8 activity may contribute to their over-
desquamation and inflammation symptoms, which remains to be studied.
To date, few active serine proteases were demonstrated in human sweat including tissue
kallikrein (KLK1) and kininase II (Hibino et al., 1994). In the stratum corneum (SC), kallikrein-
related peptidase 5, 7, and 14 are the only described active serine proteases (Brattsand et al.,
2005). Herein, we elucidated the presence of KLK8 as an active serine protease in human sweat
78
and non-palmoplantar stratum corneum, raising its potential functional involvement in skin
desquamation and antimicrobial proteolytic cascades. We identified the substrate-specificity as
well as potential endogenous activators and targets of this new active epidermal protease. As
mentioned above, none of the currently known endogenous serine protease inhibitors exert an
effect on KLK8 activity. Thus, KLK8 activity may affect the serine protease/serine protease
inhibitor balance in normal human epidermis and skin diseases. Further understanding of
KLK8’s ‘unique’ inhibition mechanism is needed for the development KLK8 activity-based
probing tools to study its function in vivo and for the development of KLK8-specific inhibitors as
potential skin care and disease drug agents.
79
Chapter 3
Kallikrein-related peptidase-8 is induced by TNFα and IL17A
resulting in epidermal hyperplasia and elevation of psoriasis-
related innate immunity gene expression
This chapter contains original unpublished data for a first author manuscript in preparation.
80
3 Kallikrein-related peptidase-8 (KLK8) is induced by TNFα
and IL17A resulting in epidermal hyperplasia and elevation
of psoriasis-related innate immunity gene expression
3.1 Introduction
Psoriasis is a complex multifactorial autoimmune skin disease with a devastating negative impact
on patients’ quality of life. This chronic disease primarily affects keratinocytes in the epidermis.
Psoriatic keratinocytes proliferate about ten times higher than normal skin keratinocytes (Crow,
2012) and fail to undergo proper differentiation and cornification. Recent genomic studies
associated psoriasis pathogenesis to epidermal innate immunity and listed the Kallikrein-related
peptidase-8 (KLK8) gene as one of the core set of 130 overexpressed psoriasis disease-specific
genes (Ainali et al., 2012). Further, KLK8 mRNA and protein overexpression was reported in
multiple skin diseases, including psoriasis and atopic dermatitis (Bowcock et al., 2001; Komatsu
et al., 2007b; Komatsu et al., 2005b). However, the molecular mechanisms behind KLK8
dysregulation in psoriasis and other skin diseases remain unknown.
We investigated herein KLK8 expression in psoriasis and atopic dermatitis skin under the
hypothesis that this protease is differentially regulated, by the distinct T-helper (Th) cell immune
milieu in these common inflammatory skin diseases, independent of barrier injury or infections.
Unlike AD lesions, psoriatic lesions are characterized by absence of a functional stratum
granulosum (SG), hyperplasia and retention of nuclei in the stratum corneum (parakeratosis)
(Guttman-Yassky et al., 2011a). Since granular keratinocytes of the SG are the major producers
of KLK8 in normal human skin barrier, we asked the question: how is KLK8 overexpressed in
psoriatic lesions then? Is it possible that KLK8 overexpression in psoriasis is an epidermal
response to ‘inside’ T-cell mediated-mechanisms rather than ‘outside’ epidermal mechanisms
such as skin injury? The expression of KLK8 is controlled by epidermal variables such as
differentiation, calcium, vitamin D, retinoic acids, and wounds, all of which are known to
influence human skin innate immunity (Morizane et al., 2010). Yet, KLK8 epidermal regulation
by major cytokine subsets implicated in psoriasis and atopic dermatitis was never examined and
the role of KLK8 protease activity in modulating psoriatic lesions architecture and innate
immunity remains unclear.
81
Since psoriasis is characterized by altered keratinocyte proliferation and differentiation, and by
infiltration of T-helper Th1 and Th17 cells into the epidermis (Lowes et al., 2007), we postulated
that KLK8 is overexpressed in psoriasis due to keratinocytes’ cross talks with Th1 and Th17
immune cells, independent of barrier injury. In turn, KLK8 overexpression may induce
epidermal hyperplasia and enhanced innate immune gene expression in psoriatic lesions, giving
further support to the ‘inside-outside’ dogma of psoriasis (Elias et al., 2008). On the other hand,
prototypic atopic dermatitis-associated Th2 cytokines likely reduce KLK8 expression by
keratinocytes, given that AD is characterized by a reduction in barrier function and terminal
keratinocyte differentiation.
Using HaCat keratinocytes as a differentiation model, we profiled keratinocyte secretion of
KLK8 post-treatment with Th1, Th17 and Th2 cytokines alone or in combination, and
investigated the effect of KLK8 overexpression on terminal keratinocyte differentiation and
innate immunity gene expression in cultured keratinocytes and 3D full-thickness human skin
equivalents. KLK8 expression was also examined in lesional and non-lesional skin of psoriasis
and atopic dermatitis patients. Moreover, psoriasis treatment with common biologic drugs that
target specific aberrant immune pathways in psoriasis should decrease KLK8 levels in the skin
and serum of psoriatic patients. This study addresses the missing gaps in our understanding of
KLK8 regulation and role in diseased human skin by focusing on keratinocytes’ cross-talks with
immune cells. We provide new insights into the regulation and function of KLK8 in normal and
psoriatic skin, which expand beyond KLK8 role in barrier repair responses, to support its
pathogenic involvement in the ‘inside-outside’ dogma of psoriasis pathogenesis.
3.2 Materials and Methods
3.2.1 HaCat keratinocyte cell differentiation model and cytokine stimulations
Immortalized HaCat keratinocyte cells were grown in 6-well plates in DMEM media with
10%FBS, until they reach full confluence. Cells were then washed with PBS three times before
adding serum-free chemically-defined CD-CHO (Life Technologies, Cat# 10743-029) medium.
Calcium levels and cell confluence were constant to allow cells to undergo adhesion-mediated
82
differentiation post-confluency, as previously described (Alameda et al., 2011). All cytokines
were purchased from R&D and stored in aliquots at -20ºC according to manufactures
instructions. The only variable was the cytokine(s) included in the serum-free CD-CHO medium.
After 3X washes with PBS, confluent cells were treated with 10ng/mL of a panel of cytokines,
alone or in combination with other cytokines, in 4mL CD-CHO medium for 6 days. 500 µL of
medium were collected on day 3 and day 6 for ELISA measurements and cells were collected 24
and 48 hrs later and frozen at – 80 ºC for mRNA analysis. To confirm that the effects of Th1,
Th17 or Th2 cytokines occur in proliferating keratinocytes, instead of differentiating ones, we
performed similar cytokine stimulation experiments in serum-containing media, with dose-
response cytokine stimulations. All experiments were done in triplicates and repeated at least
twice.
3.2.2 Enzyme-linked immunosorbent assays (ELISAs) and LDH assays
KLKs were measured via ELISAs, as described previously (Komatsu et al., 2005a), in skin
washes, serum samples and media of cytokine-treated keratinocytes. To control for the viability
of cultured keratinocytes during cytokine stimulation experiments, the levels of the intercellular
enzyme lactate dehydrogenase (LDH) were measured in the culture media as an internal control
to indicate cell membrane rupture and cell death.
3.2.3 BrdU cell proliferation assay
BrdU Cell Proliferation Assay Kit (Cat #6813) was purchased from Cell Signaling Technologies
and used in quadruplicate wells per condition, according to manufacturer’s instructions.
3.2.4 KLK8 treatment of human full thickness 3D epidermis equivalents
Active recombinant human KLK8 protease was produced and purified as previously done (Eissa
et al., 2011). EpiDerm-FT™ full-thickness human skin equivalents (EFT-400) and serum-free
EFT-400-MM medium were purchased from MatTek Corporation (Ashland, MA). Mattek
Epiderm full thickness model is composed of neonatal human-derived dermal fibroblasts and
epidermal keratinocytes co-cultured in a collagen matrix to form a multi-layered, highly
differentiated model of the human dermis and epidermis at the air-liquid interface, with the
epidermis differentiating up and the dermis remaining below at the liquid/medium level,
separated by a functional skin basement membrane (Kubilus et al., 2004). The skin equivalents
83
were placed in 2 ml of EFT-400-MM in 6-well culture plates for overnight equilibration at 37° C
in a humidified incubator. The next morning, medium was aspirated and the skin equivalents
were removed from the plates and immediately placed in the same 6-well plates in fresh EFT-
400-MM. After 24 hr incubation, cells were washed in PBS and 5 ml of fresh medium or KLK8-
containing medium (200 ng/mL or 2000 ng/mL KLK8) was added to wells containing tissues.
The cultures were then incubated at 37°C for 5 days for IHC analysis or for 2 days for PCR
analysis. Analysis of the effects of KLK8 on Epiderm-FT™ was performed in triplicates. For
histological analysis, tissues were removed from the supports and placed in 3%
paraformaldehyde in PBS and paraffin embedded. Tissue sections (4 μm) were stained with
hematoxylin and eosin (H&E). In the 2 day-treatment experiments, the epidermis was placed in
RNA later for mRNA analysis.
3.2.5 Immunohistochemistry and immunocytochemistry
4m formalin-fixed paraffin-embedded sections were dewaxed in 5 changes of xylene and
brought down to water through graded alcohols. Endogenous peroxidase and biotin activities
were blocked respectively using 3% hydrogen peroxide and avidin/ biotin blocking kit (Vector
Labs Cat#SP2001) and then followed 10% normal serum (from the species where the secondary
antibody is obtained) blocking for 10 min. Sections were incubated accordingly at room
temperature with the appropriate primary antibodies using conditions previously optimized. This
was followed with a biotinylated secondary (Vector labs) for 30 min and alkaline phosphatase
streptavidin labeling reagent (Vector labs Cat#SA5100.) for 30 min. After washing well in TBS,
color development was done with freshly prepared Vector Red solution (Vector labs Cat#
SK5100) for involucrin or KLK8 while DAB (Vector labs Cat# SK4100) was used for Ki67.
Finally, sections were counterstained lightly with Mayer’s Hematoxylin, dehydrated in alcohols,
cleared in xylene and mounted in Permount (Fisher, cat# SP15-500). For the single and double
immunocytochemistry staining of stimulated HaCat cells, we used the ImmPress universal
antibody (anti-mouse Ig/anti-rabbit Ig, peroxidase) polymer detection kit (Vector Laboratories,
Cat # MP-7500), following manfacturers’ instructions. Primary antibodies for Ki67 and
involucrin (Abcam) and KLK8 (R&D) were applied at ratios of 1:100; 1:1380; 1:200,
respectively. In double immunocytochemistry experiment, VIP stain (dark red) was used for
cytoplasmic Involucrin and DAB for nuclear Ki67 (brown).
84
3.2.6 Reverse-transcription and quantitative PCR
Briefly, FT epiderm tissues were homogenized (Polytron PT3100, Capitol Scientific, USA) and
RNA was isolated using QIAGEN RNeasy purification kit following the manufacturer's
protocol. To obtain high quality samples, genomic DNA contaminants were removed by DNase I
treatment followed by the RNA Cleanup kit. Total RNA was isolated from control and KLK8-
treated Epiderm tissues or cells using an RNeasy Kit (Qiagen Hilden, Germany). cDNA was
generated from 1 μg of total RNA using the Superscript II cDNA synthesis kit (Invitrogen).
Quantitative PCR was conducted using 1X SYBR reagent (Applied Biosystems, Foster City,
CA), and transcript levels of S100A7, hBD4, IL6, IL17, TSLP, FLG were measured on a 7500
ABI system. All quantitative PCR data were normalized to TATA-binding protein expression
and analyzed via delta delta Ct method.
3.2.7 Clinical samples from patients
Skin washes from lesional and non-lesional skin of patients with psoriasis (n=12) and atopic
dermatitis (n=4) were collected as previously described (Harder et al., 2010). Lesional and non-
lsional skin tissues from age and localization-matched psoriatic (n=4) and atopic dermatitis
patients (n=4) were provided by Dr. Ulf Meyer-Hoffert for immunohistochemical analysis. Our
study also included frozen lesional skin tissues from psoriatic patients before and after treatment
with etanercept (n=3) provided by Dr. Martin Steinhoff. Serum samples from a large cohort
(n=60) of sex and age-matched psoriatic patients before and after psoriasis treatment with TNFα-
blockers were collected from the Toronto Western Hospital with research ethics board approval.
The demographics of the patients are listed in Appendix Table 3.1.
3.2.8 Statistical analysis
Statistical analysis was done using GraphPad Prism software version 4.03 (GraphPad Software
Inc., La Jolla, CA). One-way ANOVA was used for comparison of different treatment regimens.
If two groups were compared, Student's t test was applied. p values <0.05 were considered
significant (*). In figures with bar graphs or tables, data are expressed as means ± S.E. of at least
three independent experiments unless stated otherwise.
85
3.3 Results
3.3.1 Keratinocyte secretion of KLK8 is differentially-regulated by Th1, Th17 and Th2 cytokines
We previously showed that proliferating HaCat and primary keratinocyte cells secrete KLK8,
with calcium induction of terminal keratinocyte differentiation causing a dramatic increase in
KLK8 secretion. In this study, we profiled the secretion of KLK8 into the media of confluent
HaCat cells treated with various cytokines in day 3 and day 6 post-treatment. All treatments
were performed in serum-free media to confluent cells unless otherwise indicated. As shown in
Figure 3.1A, individual treatments with Th1 and Th17/22 cytokines: TNF-α, IL-22 and IL17A
cytokines enhanced KLK8 secretion in day 3, while IFNγ and the Th2 cytokines: IL4, IL13 and
IL25 caused no significant changes in day 3. Combined treatments of TNFα+IL22 doubled
KLK8 levels the media, while TNFα+IL17A treatments and TNFα +IL17A+IL22 treatments
induced more than 3-fold increase in KLK8 secretion. Interestingly, more dramatic changes were
observed for the combined TNFα and IL17A treatment by day 6. TNFα+IL17A induced
synergistic and potent secretion of KLK8 compared to un-treated control cells and those treated
with TNFα only and IL17A only. This synergistic effect was detected at two different doses of
TNFα+IL17A, 10 ng/mL of TNFα with 10ng/mL IL17A and also when 100 ng/mL IL17A was
combined with 10ng/mL TNFα. Combining TNFα and IL-22 increased KLK8 secretion
significantly compared to control cells, but it did not induce KLK8 secretion synergistically
when compared with TNFα and IL22 individual treatments (Figure 3.1A). TNFα+IFNγ treatment
resulted in keratinocyte apoptosis characterized by clear morphological features, such as
membrane blebbing, nuclear fragmentation and a dramatic elevation of LDH levels in the media
compared to control cells (data not shown). On the other hand, the Th2 cytokines, IL4, IL13 and
IL25 reduced KLK8 secretion significantly by day 6 compared to the untreated cells, Figure
3.1B. Thus, KLK8 secretion by keratinocytes is differentially regulated by Th1, Th17 and Th2
cytokines, with TNFα+IL17A inducing a significant overexpression of this epidermal protease.
It is important to note that KLK8 hypersecretion by TNFα+IL17A-stimulated keratinocytes was
accompanied with unique and dramatic morphological changes, Figure 3.1B, C and D. We
observed accumulation of cells in circular web-like structures of enucleated cells conntected to
nodes that resembled the ‘stratification domes’ previously reported to be induced by adhesion-
mediated terminal differentiation of HaCat keratincoytes in serum-free medium and cyclinD
86
overexpression in HaCat cells. Cell remodelling and formation of the web-like structures was
visible in the TNFα+IL17A-treated cells in day 3 post-treatment and these structures became
very pronounced with enhanced differentiation in culture (day 6 versus day 3). Unlike the
TNFα+IFNγ-treated cells, the TNFα+IL17A-treated cells had lower levels of LDH levels
compared to their respective control cells in day 3 and day 6, indicating that KLK8 hyper-
secretion by these cells was due to enhanced keratinocyte proliferation or differentiation, but not
cell rupture and death.
Furthermore, we confirmed that this combined cytokine effect on KLK8 hyper-secretion was
stratification and serum independent. We observed similar results of TNFα+IL17A stimulatory
effect on KLK8 secretion by proliferating keratinocytes in serum-containing media (data not
shown), although no effect on cell polarity or stratification domes formation was observed when
TNFα+IL17A cytokines were added in serum-containing media. Significant KLK8 reduction in
media of proliferating cells in serum-containing media, treated with combined IL4/IL13/IL25
was also detected (data not shown) confirming that keratinocytes respond to these cytokines
regardless of growth factor signalling. To our knowledge, the synergistic effect of TNFα and
IL17A cytokines on HaCat keratinocyte differentiation in serum-free medium, cell remodelling
into web-like structures, stratification dome formation (Figure 3.1E and 3.1F), and KLK8
secretion (Figure 3.1A) were never reported before. Based on these in vitro results, we suspected
that TNFα and IL17A-treated keratinocytes likely recapitulate the psoriatic phenotype of altered
keratinocyte differentiation and that the web-like structures may resemble psoriatic rete ridges
and epidermal scaling.
87
A)
B)
Figure 3.1. Differential Kallikrein-8 secretion by differentiating keratinocytes in response
to Th1 (TNFα and IFNγ), Th17 (IL17A, IL22) and Th2 (IL4, IL13, IL25) cytokines. (A)
KLK8 secretion in HaCat conditioned medium on day 3 and day 6 after individual and cocktail
II
IV V
*** ***
*** ***
** ** **
***
**
***
* * * * * * *
* *
**
88
cytokine stimulation. All cytokines were added at 10ng/mL in SFM, except in the case of
IL17A* where the asterisk indicates higher concentration of 100ng/mL (***p<0.001, **p<0.01,
***p<0.05). (B) Microscopic images (4X) of cells: I) HaCat control, day 6. II) 10 ng/mL TNFα-
treated, day 6. III) 10 ng/mL IL17A-treated, day 6. IV) 10 ng/mL TNFα + 10 ng/mL IL17A, day
6. V) 10 ng/mL TNFα + 100 ng/mL IL17A*, day 6. Note the web-like structures and
stratification domes indicated with arrows in TNFα + IL17 and TNFα + IL17A*-treated cells.
89
3.3.2 TNFα and IL17A-treated keratinocytes have an altered differentiation program and mimic lesional psoriatic skin
To confirm that the web-like structure seen after TNFα+IL17A treatment represent epidermal
scales, we performed BrDU cell proliferation assays and double-immunocytochemistry with
Ki67 proliferation and involucrin terminal differentiation marker. All treatments, except for
TNFα which is a known anti-proliferative, increased BrdU incorporation compared to untreated
control cells, Figure 3.2A. However, unlike the consistently increased proliferation induced by
IL17 alone, the TNFα+IL17A treatment reduced BrDU incorporation over time, resulting in a
negative slope in Figure 3.2A, indicating reduced proliferation and perhaps an accelerated
differentiation program. Consistently, TNFα+IL17A–treated cells displayed intense staining for
the terminal differentiated marker, involucrin, in the web-like structures and stratification domes
(Figure 3.2B). Over 70% of the TNFα+IL17A-keratinocytes were differentiated by day 6 as
indicated by involucrin staining in the enucleated stratification domes where it displays the
highest intensity. Hence, our data suggests that KLK8 overexpression by TNFα+IL17A-treated
keratinocytes is a likely outcome of altered keratinocyte differentiation rather than
hyperproliferation. We immunolocalized KLK8 and LL-37 proteins to the epidermal scales of
TNFα+IL17A-treated HaCat cells in Figure 3.2C. Finally, to prove that TNFα+IL17A-treated
HaCat keratinocytes indeed mimics psoriatic skin, we measured gene expression of psoriasin
(S100A7) and human β-defensin-4 (hBD4), IL6 and filaggrin. Synergistic upregulation of the
psoriasis-related genes S100A7 and hBD4 to 114-fold and 218-fold, respectively, was detected
in addition to elevation of IL6 by 6-fold and reduction of filaggrin to 0.25-fold compared to
untreated controls, Figure 3.2D. The S100A7 and hBD4 gene expression was significantly
reduced in IL4-treated keratinocytes to 0.01-fold and 0.05-fold, (date not shown) confirming
their differential response to Th1/Th17 versus Th2 cytokine milieu and their psoriasis specificity.
90
A) B)
C) D)
Figure 3.2. TNFα+IL-17A treatment induces changes in HaCat keratinocytes that mimic
psoriatic skin. (A) TNFα+IL-17A stimulation enhances keratinocyte proliferation and
accelerates differentiation program over time. (B) Web-like structures and stratification domes
display intense positive staining for involucrin as well as (C) KLK8 and LL-37 vs. NT controls.
(D) TNFα+IL-17 treated keratinocyte-induced gene expression of psoriasis-related genes. (***
p<0.001)
NT
TNFα+IL17A
Anti-
KLK8
Anti-
LL37
4X
4X 4X
NT TNFα+IL17A
Anti-
INV
91
3.3.3 Overexpression of KLK8 alters keratinocyte differentiation program, induces epidermal hyperplasia and up-regulates innate immunity gene expression
Upon examining the kinetics of KLK8 treatment on HaCat keratinocyte proliferation in culture,
we observed that KLK8 induces keratinocyte proliferation compared to non-treated controls on
day 3 and reduces proliferation significantly by day 6, similar to the kinetics of TNFα and IL17A
effect on keratinocyte proliferation shown in Figure 3.2A. As shown in Figure 3.3, the KLK8-
mediated reduction of BrdU incorporation and cell proliferation was concentration dependent
and it was paralleled with enhanced involucrin staining on day 6, consistent with having a role in
inducing keratinocyte differentiation. Thus our data show that KLK8 effect on keratinocyte
proliferation and differentiation is dose-dependent, where higher KLK8 concentrations reduce
proliferation and enhance differentiation.
92
Figure 3.3. KLK8 treatment reduces cell proliferation and enhances differentiation of
HaCat keratinocytes in a concentration-dependent manner.
93
Given that HaCat keratinocytes are an immortalized cell line, we also studied the role of KLK8
in epidermal differentiation and remodelling in a 3D epidermis model, where full-thickness
human epidermis equivalents of normal primary epidermal keratinocytes and fibroblasts from a
single donor were grown in collagen matrix and treated with two doses of active KLK8 protease
(200 ng/mL and 2000 ng/mL). The lower dose of KLK8 increased SC and overall epidermis
thickness, as shown in Figure 3.4. KLK8 treatment enhanced gene expression of S100A7 by 3-
fold, hBD4 by 1.8-fold and IL6 by 1.5-fold in HaCat keratinocytes within 24 hrs (data not
shown), and a similar trend was noted in the human FT-epiderm model, where KLK8 treatment
resulted in 3.2-fold, 7.8-fold and 2-fold increase in S100A7, hBD4 and IL6 genes after 48 hrs,
respectively. Interestingly, we also detected a 2.5-fold increase in TSLP gene.
Further, the keratinocytes of KLK8-treated epidermis displayed increased and more intense
staining of involucrin in multiple layers and reduced Ki67 staining in the stratum basale on day
5, whereby basal keratinocytes appeared more differentiated and columnar in shape compared to
control (Figure 3.6B versus Figure 3.6A). Interestingly, unlike the non-treated epidermis, we
observed co-localization of Ki67 and involucrin in cytoplasmic regions of upper layers of the
KLK8-treated epidermis, suggesting nuclear rupture and accelerated differentiation program.
These results were consistent with our BrdU cell proliferation assay results of KLK8-treated
keratinocytes (Figure 3.3A). Hence, the KLK8-treated epidermis exhibited altered differentiation
and expression of innate immunity genes.
Treating the epidermis model with a higher KLK8 dose, on the other hand, resulted in a dramatic
destruction of the barrier visualized by absence of proper differentiation of the basal layer, SC
detachment, epidermal thickening and retention of nuclei in upper layers (Figure 3.5B and 3.5C
compared to 3.5A). Interestingly, we also detected changes in dermal fibroblasts in epidermis
treated with the higher KLK8-dose. Although the dermal fibroblasts were similar in numbers
between KLK8-treated and non-treated epidermis, they were elongated in shape and their focal
length was significantly higher than normal human epidermis fibroblasts as shown in the H&E
stained epidermis in Figure 3.6.
94
Stratum corn
eum
Epidermis
0
50
100
150
200
250Control
KLK8-treated
p<0.01
p<0.01
Mean
th
ickn
ess (
m)
IL6
hBD4
TSLP
S10
0A7
0
2
4
6
8
10
*
**
** **
No
rm
alized
gen
e f
old
ch
an
ge
Figure 3.4. KLK8 treatment enhances differentiation of normal full thickness human
epidermis model and alters innate immunity gene expression. (A) Normal human full-
thickness epidermis (FT-Epiderm) equivalent after 5 days of culture at the air-liquid interface.
Alterations in basal keratinocytes following KLK8 treatment are indicated with an arrow. (B)
KLK8-treated epidermis equivalent after 5 days of culture at the air-liquid interface. (C) Mean
stratum corneum and epidermis thickness of control and KLK8-treated epidermis equivalents.
(D) Changes in innate immune gene expression 48 hrs post-KLK8-treatment of FT-Epiderm
normalized to GAPDH and non-treated controls via the ∆∆Ct method.
A) B)
C) D)
SC
SG
SS
SB
95
Figure 3.5. Alterations in proliferation and differentiation markers in KLK8-treated full
thickness epidermis model and psoriatic skin. Double immunohitochemical staining of
cytoplasmic involucrin (pink red) differentiation marker and nuclear Ki67 (brown) proliferation
markers in: (A) Normal human Epidermis equivalent after 5 days of culture at the air-liquid
interface. (B) KLK8-treated epidermis equivalent after 5 days of culture at the air-liquid
interface. Intense involucrin staining in all layers and arrow indicates brown staining in
cytoplasmic regions in the KLK8-treated epidermis equivalent, further suggesting enhanced
differentiation and nuclear rupture to form a thicker stratum corneum compared to untreated
control where Ki67 is restricted to the SB. (C) Non-lesional psoriatic skin and (D) matched
lesional psoriatic skin from the same psoriatic patient display enhanced involucrin and Ki67
staining.
C)
A) B)
C) D)
96
Figure 3.6. KLK8 overexpression induces drastic changes in full thickness epidermis
model. KLK8 induces abnormal differentiation, SC detachment, retention of nuclei in upper
layers (parakeratosis), and elongation of fibroblasts in normal full-thickness epidermis
equivalents. H&E stained 20X images of (A) Normal epidermis. (B and C) Epidermis tissues
treated with high KLK8 dose. (D) Mean fibroblast count and focal length as a measure of
morphological change upon KLK8-treatment.
Fibro
blasts Count
Foca
l interfac
e length
0
50
100
200
300
400
500Control
KLK8
p>0.05
p<0.01
A)
B)
C) D)
97
3.3.4 KLK8 is significantly elevated in lesional psoriatic skin and reduced in lesional acute atopic dermatitis skin
Our observed TNFα and IL17A synergistic effects on keratinocytes may explain KLK8
overexpression in psoriatic lesions. Further, the reduction of KLK8 secretion by Th2 cytokines
(IL4, IL13 and combined IL4/IL13/IL25 treatments) suggests that KLK8 is reduced in atopic
dermatitis lesions. To validate that our above in vitro organotypic tissue culture findings are
relevant in vivo, we analyzed KLK8 expression in skin washes and tissue biopsies from non-
lesional and lesional skins of psoriatic and atopic dermatitis patients. Our results show that
KLK8 is significantly elevated in skin washes of lesional psoriatic compared to non-lesional skin
of the same patient. Conversely, KLK8 levels seemed to be reduced or comparable in lesional
versus non-lesional skins of atopic dermatitis patients Figure 3.7. Since we had a smaller number
of AD (n=4) compared to psoriatic patient volunteers (n=12) for skin wash experiments, we also
investigated the expression pattern of KLK8 in lesional and non-lesional skins of psoriasis and
AD skins by immunohistochemistry. KLK8 overexpression in lesional psoriatic skin and
reduction in lesional AD skin was also observed compared to non-lesional skin from the same
patients, as shown in Figure 3.8. Intriguingly, unlike acute AD lesions, the dermis of psoriatic
lesions displayed intense staining of KLK8, in the immune cell infiltrate near the epidermis as
shown in Figure 3.9.
98
Figure 3.7. KLK8 is significantly overexpressed in lesional psoriatic skin washes only,
unlike other KLKs. ELISA levels of (A) KLK5, (B) KLK7 and (C) KLK8 in lesional (L) and
non lesional (NL) skin washes from psoriasis (n=12) and atopic dermatitis patients (n=4). In
Figure s, Ps refers to psoriasis and AD to atopic dermatitis.
P<0.05
P<0.05
P<0.05
B)
C)
99
Figure 3.8. KLK8 epidermal expression is elevated in lesional psoriasis and reduced in
lesional atopic dermatitis skin, compare to respective non-lesional counterparts.
Microscopic IHC images (20X) of KLK8 staining in skin biopsies from (A) a psoriatic patient
and (B) an atopic dermatitis patient indicated with arrows.
ATOPIC
DERMATITIS
B)
A)
NON-LESIONAL
PSORIASIS
LESIONAL
100
B)
Figure 3.9. KLK8 overexpression in lesional psoriatic skin, is not restricted to the
epidermis, but is also seen in dermis immune infiltrate near the epidermis, unlike atopic
dermatitis skin. Representative microscopic images (20X) of KLK8 IHC staining in lesional
(A) psoriasis and (B) atopic dermatitis skin.
PSORIASIS
ATOPIC
DERMATITIS
Epidermis
Dermis
A)
101
3.3.5 KLK8 elevation in psoriatic patients’ lesional skin and sera is significantly reduced after effective treatment with the TNFα –blockers
Since our data strongly suggest that KLK8 is linked to major immune players in psoriasis, TNFα
and IL17A, we hypothesized that common biologic psoriasis therapy, such as etanercept, will
reduce KLK8 expression. Since KLK8 overexpression is restricted to lesional psoriatic lesions
only, its reduction by psoriatic therapy targeting TNFα and IL17A should correlate positively
with psoriasis clearance. Indeed, we found that KLK8 gene expression was reduced in the skins
of psoriatic post-etanercept treatment (p<0.05), and confirmed dramatic reduction of psoriasis-
related and KLK8-induced genes such as hBD4 (p<0.001) post-treatment. SA1007 was also
reduced post-treatment, but with p=0.07, as shown in Figure 3.10.
102
Figure 3.10. Expression of KLK8 and other innate immunity genes in lesional psoriatic skin
pre and post-treatment with the TNFα-blocker, etanercept. KLK8 and other innate immunity
gene (such as S100A7 and hBD4) expression are reduced in pooled psoriatic patients (n=3) skin
lesions post-treatment with the TNF-blocker etanercept. Changes in gene expression were
normalized to GAPDH and normal skin control via the ∆∆Ct method.
KLK
8
S10
0A7
hBD4
IL17
FLG
050
100150200
10002000300040005000
5000
10000
15000
20000
Pre-treatment
Post-treatment
p<0.05
p<0.001
Gen
e f
old
no
rmalized
to
no
rmal skin
103
Furthermore, consistent with our clinical findings (discussed in Chapter 4 of this thesis) of
significant KLK8 elevation in the sera of psoriatic patients and its positive correlation with
clinical variables of psoriasis severity measured by the Psoriasis Area and Severity Index (PASI)
score, our data here show that indeed KLK8 levels in the sera of 60 psoriatic patients are
significantly reduced after treatment with TNFα-blockers (p=0.006), unlike other KLKs, as
shown in Appendix Table 3.2. High sensitivity C-reactive protein (hs-CRP) was also reduced in
serum samples post-treatment and was included in the analysis as an internal control of treatment
effect and correlation with PASI. KLK8 reduction in the sera of patients correlated positively
with psoriasis clearance and PASI reduction with a correlation coefficient of 0.544 and a p-
value<0.0001 (data not shown). Our results show that KLK8 decreases with treatment
(p=0.0001), and is a better indicator than hsCRP for patients’ response to treatment as shown in
Table 3.1.
104
Table 3.1. KLK8 serum levels predict positive response to TNFα-blockers
POPULATION: patients who take anti-TNF and have pre and post-treatment medication
sample values (n=60); 55 patients had pre and post treatment information on PASI scores
RESPONSE: PASI score
COVARIATES: hsCRP and KLK8 reduction adjusting for age, sex and duration of Ps;
MODEL: Multiple Linear regression models with reduced model obtained by stepwise
elimination and the R-squared method.
Univaraite Multivariate
Covariate Estimate SE P-value Estimate SE P-value
Age (1 year
increase)
0.088 0.100 0.383 0.029 0.099 0.769
Sex (Males vs.
Females)
-3.500 2.499 0.167 0.077 2.349 0.674
Psoriasis
Duration
(1 year increase)
-0.036 0.094 0.701 -0.034 0.090 0.704
KLK8 reduction
(1 unit increase)
-1.004 0.213 <0.000
1
-0.931 0.220 0.0001
hsCRP reduction
(1 unit increase)
-0.179 0.077 0.024 -0.140 0.072 0.057
105
3.4 Discussion
Psoriatic epidermal response shares many similar features with epidermal regeneration during
wound healing. Not only do keratinocytes get activated, but also angiogenesis and inflammatory
responses are stimulated and antimicrobial peptides are induced to prevent infections around the
wound (Nickoloff et al., 2006). Psoriasis is often triggered with skin injury that develops later
into plaques, known as the Kobner phenomenon. Thus, wound repair pathways are implicated in
psoriasis pathogenesis.
The role of KLK8 in epidermal regeneration and wound healing was recently reported (Kishibe
et al., 2012). The mouse homologue of KLK8 is able to induce hyperkeratosis in SLS-induced
mouse skin inflammation (Shingaki et al., 2012). Here, we hypothesized that human KLK8 is
overexpressed in psoriasis to induce hyperkeratosis (or hyperplasia) and host defense gene
expression, which are characteristics of psoriatic lesions. We postulated that KLK8
overexpression in psoriatic lesions is an ‘outside’ exacerbated epidermal response to ‘inside’
immune aberrations known to dominate this devastating autoimmune disease. The T helper 17
cells (Th17) are active new players in psoriasis. Th17 are developmentally distinct from Th1 and
Th2 cells, which have been shown to drive psoriasis and atopic dermatitis pathogenesis,
respectively (Guttman-Yassky et al., 2011). Our results demonstrated that KLK8 epidermal
expression in psoriasis and atopic dermatitis is a differential response to Th1/Th17 and Th2
cytokines, independent of barrier injury, and identified ways in which KLK8 may contribute to
psoriasis skin architecture and innate immunity.
HaCat keratinocytes were used as a differentiation model as they recapitulate the process of
terminal keratinocyte differentiation in culture through the formation of visible stratification
domes of enucleated corneocytes. Overexpressing the cell cycle regulator, cyclin D, implicated
in psoriasis hyperproliferation (Belso et al., 2008), in HaCat cells induced visible stratification
domes, underscoring the role of cyclin D as an epidermal differentiation regulator in a recent
study (Alameda et al., 2011). Here, we show original data demonstrating that combined TNFα
and IL17A treatment induces dramatic changes in keratinocytes cell polarity and differentiation
106
through formation of unique ‘web-like structures’ and stratification domes. To our knowledge,
this observation has never been shown. Yet, it provides strong evidence for the immune bases of
psoriasis. Our cell proliferation assays and immunohistochemical expression of the terminal
differentiation marker involucrin indicate a key role for TNFα and IL17A cytokines in
accelerating keratinocyte differentiation and inducing scaling of cultured keratinocytes. Th1 and
Th17 immune cells, which secrete TNFα and IL17A, respectively, are usually absent in normal
human epidermis, but they are abundant in psoriatic epidermis. Certainly, our TNFα+IL17A-
treated keratinocytes mimicked psoriatic kerationcytes as we detected synergistic elevation of the
psoriasis-related host defense genes: psoriasin (S100A7) and human beta-defensin-4 (hBD4) and
upregulation of IL6, but not filaggrin, consistent with previous findings (Chiricozzi et al., 2011).
TNFα and IL17A acted synergistically to induce the most potent upregulation of KLK8 protein
by keratinocytes, compared to non-treated cells and all other cytokine treatments, including
TNFα+IL22. Combining TNFα+IL17A or TNFα+IL17A+IL22 resulted in comparable levels of
KLK8 secretion suggesting a key role for TNFα+IL17A in modulating KLK8 expression.
Overexpression of KLK8, LL37 also immunolocalized to the web-like structures and
stratification domes of TNFα and IL17A-stimulated cells. Interestingly, KLK8 hypersecretion
by keratinocytes post TNFα and IL17A treatment was independent of keratinocytes’ state of
differentiation. Thus, our data suggest that TNFα+IL17A induce proliferating and differentiating
keratinocytes to secrete KLK8 and provide a conceivable explanation for KLK8 overexpression
in lesional psoriatic skin characterized by absence of a functional stratum granulosum. Our
analysis of patients’ skin biopsies and skin surface washes confirmed that our in vitro findings
are relevant in vivo, as we detected significant KLK8 overexpression in lesional psoriatic skin
only, compared to matched non-lesional control and lesional and non-lesional atopic dermatitis
(AD) skin. We noted significant reduction in KLK8 secretion by keratinocytes post-treatment
with Th2 cytokines (IL4, IL13 and IL25) implicated in AD, which was mirrored by reduced
KLK8 immunohistochemical expression in lesional AD skin compared to its matched control.
Our data is consistent with a previous study indicating enhanced trypsin-like activity in psoriatic
lesions, but not AD lesions. Although KLK8 was reported to be elevated in AD stratum
corneum, the study was based on analysis of tape-stripped skin, which could have activated the
wound healing response. Our current study investigated KLK8 expression in cultured
keratinocytes and skin biopsies independent of barrier disruption.
107
We hypothesized that KLK8 overexpression in lesional psoriatic skin contributes to the
development and/or maintenance of psoriatic lesions. We confirmed this notion upon examining
the effect of KLK8 on the differentiation, proliferation and innate immune gene expression in
cultured normal keratinocytes and full-thickness 3D skin equivalents of epidermal keratinocytes
and dermal fibroblasts. Keratinocytes within these skin equivalents differentiate, forming distinct
basal, spinous and granular layers, as well as a stratum corneum, when placed at an air-liquid
interface. Our data show that KLK8 activity induces keratinocyte differentiation leading to
increased thickness of the stratum corneum and full epidermis, and enhanced involucrin terminal
differentiation marker expression in all differentiating layers. The Ki67-positive proliferating
cells, which were at the basal layer moved to suprabasal layers and their nuclei ruptured to
release their contents, confirming enhanced differentiation. However, adding ten-fold higher
concentration of KLK8 to the skin model induced dramatic changes that mimicked psoriatic skin,
such as stratum corneum detachment, epidermal thickening and retention of keratinocytes’ nuclei
in the stratum corneum (parakeratosis).
KLK8 expression must be relevant to host defense as it enhances differentiation and thickening
of the stratum corneum, thereby providing increased protection from injury or infection. We
show that KLK8 induces expression of the psoriasis-related antimicrobial S100A7 and hBD4
genes in cultured keratinocytes and 3D full thickness equivalents, confirming its role in
regulating cutaneous innate immunity. Thus, given KLK8 ability to induce a psoriatic phenotype
in vitro, it is very likely to induce a similar effect in response to TNFα and IL17A in vivo.
Our findings support the recently elucidated role of KLK8 in inducing hyperkeratosis in
inflamed mouse skin. We provide a missing regulatory link and a possible upstream mechanism
for KLK8 overexpression and induction of hyperkeratosis in psoriatic lesions. Our work
demonstrates that KLK8 is a key epidermal protease regulated by immune and epidermal barrier
crosstalks in psoriatic lesions, to induce epidermal hyperplasia and enhanced host defense gene
expression. The mechanisms by which KLK8 can cause a thickened epidermis may involve
inhibition of AP-2α expression leading to hyperproliferation (Shingaki et al., 2010).
Consistently, AP-2α knockout mice have thick skin due to a hyperproliferative defect (Wang et
al., 2006). Another possible mechanism may include IL6, which is known to act as an autocrine
regulator of keratinocytes differentiation and mediator of hyperplasia in active psoriatic lesions
(Gottlieb, 1990; Lindroos et al., 2011). IL6 is overexpressed in psoriatic lesions compared to
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non-lesional and normal skin as well as in the sera of psoriatic patients (Lo et al., 2010; Suttle et
al., 2012). Almost all classical psoriatic therapies normalize IL6 expression in psoriasis. Further,
IL6 activity and its receptor are involved in the Koebner phenomenon and wound healing
response (Suttle et al., 2012). TNFα and IL1 activate epidermal keratinocytes to produce IL6
(Fujisawa et al., 1997) and our data here suggest KLK8 as a new inducer of IL6 expression by
keratinocytes. A third possible mechanism is that the altered skin epidermis phenotype following
KLK8 treatment reflects increased proteolytic degradation of structures responsible for basal cell
cohesion in the basement membrane such as collagen IV, which is expressed in full thickness
epidermis and is a known substrate for KLK8. Keratinocyte differentiation and proliferation
were recently shown to be regulated by adhesion to the 3D meshwork of type IV collagen in
reconstructed skin equivalents (Fujisaki et al., 2008) and KLK8 proteolytic activity may affect
this regulation. Further studies are required to confirm this possibility.
We also detected elevation of the Thymic Stromal Lymphopoietin (TSLP) gene, which serves as
a link between innate and adaptive immunity. TSLP elevation has been reported in lesional
psoriatic skin although not associated with inducing allergy in psoriatic patients. Furthermore,
KLK8 overexpression induced elongation of dermal fibroblasts in full thickness epidermis
equivalents. We previously showed that KLK8 is not secreted by primary dermal fibroblasts
(Eissa et al., 2011), but it is possible that KLK8 is involved in paracrine signalling in the dermis
where it remodels dermal fibroblasts and/or endothelial cells (which are not present in the full-
thickness epidermis model) but are dilated in psoriatic lesions. Further studies are required to
investigate KLK8 role in dermal psoriatic lesions and its effect on dermal fibroblasts and
endothelial cells, as well as its expression by these cells post TNFα+IL17A stimulation. Our data
pointed to a dramatic KLK8 overexpression by dermal cells in psoriatic lesions compared to
matched non-lesional skin. Potential candidates may include dendritic cells, endothelial cells,
mast cells or neutrophils known to secrete serine proteases. KLK8 was previously localized in
mast cells of mice (Wong et al., 2003). IL17A-positive mast cells and neutrophils are found in
high levels at sites of skin and joint disease in humans (Kirkham et al., 2013). Thus, the
possibility of KLK8 overexpression by any of these cells in psoriatic lesions is not farfetched.
Our data show that effective biologic psoriasis treatment diminished KLK8 gene expression in
lesional psoriatic skin post-treatment, along with the hBD4 gene. Unlike other KLKs, KLK8
protein levels in the serum were also significantly reduced in psoriatic patients post treatment
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with common TNFα-blockers in the clinic, such as etanercept. The reduction in KLK8 correlated
significantly with psoriasis clearance. Etanercept is an immunoglobulin fusion protein that
blocks tumor necrosis factorα (TNFα) receptor. Blockade of TNFα is considered to be its
primary action, but recent clinical trials showed that effective treatment of psoriasis with
etanercept is a result of its early inhibitory effects on the newly discovered Th17 cells (Zaba et
al., 2007). Newly developed IL17A antagonists in clinical trials will also likely reduce KLK8
epidermal and serum levels in psoriasis, given that it is one of the synergistically-induced
epidermal proteins by these two cytokines in lesional psoriatic skin. Together, our data provide
new insights into KLK8 distinct regulation and pathogenic involvement in psoriatic skin.
Inhibiting KLK8-specific activity in lesional psoriatic skin will reduce psoriatic plaques, and
opens a new future avenue for topical psoriasis drug development.
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Chapter 4
Serum Kallikrein-related peptidase-8 levels correlate with skin
activity, but not psoriatic arthritis, in patients with psoriatic disease
Sections of this chapter were reproduced from the following published manuscripts:
Eissa, A. Cretu, D., Soosaipillai, A., Thavaneswaran, A., Pellett, F., Diamandis, E.P., Cevikbas,
F., Steinhoff, M., Gladman, D., Chandran, Vinod. Serum kallikrein-8 correlates with skin
activity, but not psoriatic arthritis, in patients with psoriatic disease. 2012. Clinical Chemistry
Laboratory Medicine: 1434-6621
111
4 Serum Kallikrein-related peptidase-8 levels correlate with
skin activity, but not psoriatic arthritis, in patients with
psoriatic disease
4.1 Introduction
As mentioned in previous chapters, psoriasis is a very common inflammatory skin disease
affecting about 2% of the North American population (Kurd and Gelfand, 2009). This
multifactorial chronic disease is characterized by epidermal hyper-proliferation and dermal
inflammation that vary in severity from minor, localized patches to involvement of the entire
skin surface (Lowes et al., 2007). About one-third of patients with psoriasis suffer from
moderate-to-severe disease and report that the disease has a substantial negative impact on their
quality of life. The concept of ‘psoriatic disease’ encompasses additional manifestations often
associated with the occurrence of psoriatic skin lesions, including musculoskeletal and
cardiovascular systems (Nograles et al., 2009; Scarpa et al., 2010). Approximately 30% of
psoriasis patients develop arthritis which contributes additional morbidity to psoriasis patients
(Langley et al., 2005). Psoriatic arthritis (PsA) is an inflammatory joint disease associated with
cutaneous psoriasis and seronegative for rheumatoid factor. There is a high prevalence of
undiagnosed PsA among psoriasis patients seen in dermatology clinics (Reich et al., 2009). The
diagnosis of PsA is usually made by a rheumatologist after a clinical evaluation; no diagnostic
test is available. Soluble biomarkers of PsA are of particular interest to dermatologists and
rheumatologists, as they may aid in screening, early detection and treatment, leading to
amelioration of progressive joint damage and disability, and improvement in the quality of life
and function.
Given that the majority of PsA patients initially present with cutaneous psoriasis, we
hypothesized that epidermal proteins implicated in skin barrier function and innate immunity,
such as KLKs, may act as serum biomarkers of psoriasis severity and may aid in screening and
early detection of PsA.
Currently, there is no validated single or panel of blood/serum biomarkers for PsA in the clinic.
The diagnosis of PsA is considered when inflammatory musculoskeletal disease is recognised in
the presence of psoriasis. PsA is classified using the CASPAR (ClASsification criteria for
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Psoriatic ARthritis) criteria (Taylor et al., 2006). PsA disease activity is primarily assessed by
counting the number of tender or swollen joints. Since early diagnosis of PsA is associated with
less joint damage progression (Gladman et al., 2011), it is imperative that psoriasis patients are
screened regularly for the presence of PsA. Screening tools for PsA including questionnaires,
imaging, genetic and cellular biomarkers are being investigated (Chandran and Gladman, 2012).
Preliminary studies have suggested soluble PsA markers, including acute phase reactants (such
as hsCRP), markers of extracellular matrix-destruction (MMPs) and cytokines such as tumour
necrosis factor (TNF) may discriminate patients with PsA from those with psoriasis alone
(Chandran and Gladman, 2012). These serum biomarkers tend to be overexpressed in the
lesional psoriatic skin and/or the inflamed synovial fluid of psoriasis and PsA patients,
respectively (Gladman, 2009; Myers et al., 2006). Herein, we examined KLK expression in
inflamed PsA synovial fluids and psoriatic skin under the hypothesis that KLKs may mediate
both skin and joint inflammation in PsA and hence may be useful as screening biomarkers of
PsA in psoriasis patients. We then measured the levels of a panel of epidermal and synovial
fluid-expressed KLKs in 152 serum samples of well-phenotyped psoriasis patients, with or
without PsA, to determine the utility of KLKs as soluble biomarkers for screening PsA.
4.2 Materials and Methods
4.2.1 Collection of synovial fluids (SF) from PsA and control patients
Synovial fluids were aspirated from inflamed knee joints of three PsA patients and three non-
inflammatory (early osteoarthritis) controls. Each sample was subjected to BCA total protein
assay prior to loading 800 µg protein into antibody-coated plates to measure KLK levels by
ELISAs.
4.2.2 Immunohistochemistry
Rabbit anti-human Kallikrein 6 and 8 antibodies were purchased from R&D Systems. Formalin-
fixed paraffin sections were deparaffinized with 2x xylene, and serials dilutions of ethanol
(100%, 95%, 80%, 50%) and water, 5 minutes per step. Slides were heated at 80°C for 20
minutes in autoclave, and cooled on ice for 20 minutes. Endogenous peroxidase activity was
quenched with Dako peroxidase quenching buffer for 20 minutes at room temperature. After
PBS washing, sections were blocked with 2% BSA in PBS for 1 hour and incubated with
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antibodies diluted in 2% BSA (1:250, 1:400, 1:500, 1:1000 dilutions) overnight at 4°C in a
humid chamber. After rinsing with PBS, slides were incubated for 1 hour at RT in a humid
chamber with rabbit horseradish peroxidase-conjugated secondary antibodies diluted 1:400 in
blocking buffer. Nuclei were counterstained with Hematoxylin QS (Vector Laboratories,
Burlingame, CA) and mounted with Aquamount (BDH, Poole, UK). The immunoreactivity was
detected with the Liquid DAB+ Substrate Chromogen System (Dako Cytomation).
4.2.3 Setting and participants
We recruited 152 age and sex-matched patients with active psoriasis, with or without psoriatic
arthritis (PsA), from the University of Toronto PsA and psoriasis clinics. The study protocol and
informed consent forms were approved by the University Health Network research ethics board
and all patients signed a written consent form. Consenting patients were recruited into on
observational cohort and evaluated according to a standard protocol every 6-12 months.
At each visit, symptoms, physical examination (including complete musculoskeletal examination
and assessment of psoriasis severity), current use of medications and laboratory findings were
recorded. The data were entered and stored in a computerized database. In phase I, 52 psoriasis
patients and 26 healthy controls were recruited. Of the 52 psoriasis patients, 26 were diagnosed
with PsA by a rheumatologist and satisfied the ClASsification of Psoriatic ARthritis (CASPAR)
criteria. PsA was excluded by a rheumatologist in the remaining 26 psoriasis patients. Psoriasis
severity was evaluated using the psoriasis area and severity index (PASI) score. Phase I patients
had mild-to-moderate psoriasis as indicated by PASI < 8. Controls were recruited from healthy
volunteers who did not have psoriasis or inflammatory arthritis. Patients with psoriasis and PsA
were group-matched for age, sex and psoriasis duration, while controls were matched for age and
sex. In phase II of the study, KLKs showing promise, were further investigated in a second
independent cohort of 100 patients with moderate-to-severe psoriasis (PASI scores >8), 50 of
whom had PsA. None of the 152 patients were treated with TNF inhibitors at the time of study
participation. All recruited patients reported European ethnicity. Blood samples were drawn at
the time of clinical assessment, processed immediately, and serum aliquots stored at -80°C until
laboratory analysis.
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4.2.4 Enzyme linked immunosorbent assays (ELISAs)
Serum levels of KLK5, KLK6, KLK7, KLK8, KLK10, KLK11 and KLK13 were determined
using KLK-specific and sensitive ELISAs developed in-house. The assays were performed on
stored serum samples linked to phenotypic information collected prospectively. The reader of the
laboratory tests were blinded to the diagnosis and clinical information.
4.2.5 Statistical Analysis
In Phase I, logistic regression models were fit with disease classification as the outcome, using
KLKs as explanatory variables while controlling for age and sex. Univariate, full multivariate
and reduced multivariate logistic regression models were used to identify KLKs that are
independently associated with disease class. Patients with PsC and PsA were grouped into one
group to identify biomarkers for ‘psoriatic disease’. Subsequently, patients with PsC, PsA, and
controls were compared using polychotomous logistic regression. Finally, biomarkers that
differentiate PsA from psoriasis were investigated by comparing patients with PsA to those with
psoriasis alone. Since phase I of the study was exploratory, results were considered to be
statistically significant at p<0.05 and correction for multiple testing was not done. In phase II
logistic regression models were fit to examine the relationship between serum KLK6 and 8 levels
and disease class. In order to determine the association between disease activity and KLK6 and 8
levels, correlation analyses and linear regression analyses were done with PASI score and joint
counts as outcome and KLK6 and 8 levels, age, sex and disease duration as explanatory
variables.
4.3 Results
4.3.1 KLK6 and KLK8 are elevated in PsA synovial fluids and lesional psoriatic skin
Synovial fluid samples were aspirated from the knee joints of three PsA patients and three
patients with early osteoarthritis (OA). OA samples are commonly used as controls in
rheumatology studies, since synovial fluid is not usually available from healthy individuals. We
explored the presence of KLK proteases in the inflammatory synovial fluid from PsA patients,
compared to non-inflammatory fluid from patients with OA. As shown in Figure 4.1, many of
the epidermal KLKs (such as KLK5, 6, 7, 8, 13 and 14) were detected in the synovial fluids of
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all three PsA patients, with significant elevation of KLK6 (p=0.039) and KLK8 (p=0.013) in PsA
compared to OA. To our knowledge, this is the first report of KLK serine protease expression in
human synovial fluids, although metalloproteases (MMPs) were previously detected in PsA
synovial fluids and have been investigated as serum PsA biomarkers (Chandran et al., 2010).
Since KLK6 and KLK8 were significantly elevated in the synovial fluids of PsA patients among
the KLKs tested, we next investigated their expression in psoriatic skin and confirmed the
overexpression of these two KLKs in lesional psoriatic skin tissues by immunohistochemistry.
Compared to normal skin, where KLKs are normally expressed in the uppermost stratum
corneum (SC) layer, KLK6 and KLK8 expression expanded below the SC into the spinous layer
of the elongated rete ridges of psoriatic skin, Figure 4.2.
4.3.2 PsA and PsC Patients
PsC and PsA patients had well established disease and were matched for age, sex and psoriasis
duration. The patients’ demographics and clinical characteristics are summarized in Table 4.1.
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Figure 4.1. Expression of KLK proteases in PsA inflamed joint synovial fluids and control
(osteoarthritis) synovial fluids. Asterisk denotes statistically significant differences between
groups (p<0.05)
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Figure 4.2. Immunohistochemical expression of KLK6 and KLK8 in lesional psoriatic skin.
(a) Normal skin. (b) Psoriatic skin. SC: stratum corneum, RG: rete ridges. Note expression of
both KLK6 and KLK8 below the SC into the rete ridges (RG) of psoriatic skin indicated with an
arrow.
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Table 4.1. Demographics and clinical characteristics of psoriatic disease patients
Phase I
Characteristic PsA1 (N=26)
PsC
2 (N=26) Controls (N=26)
Females/males 14/12 14/12 14/12
Age (years) 46.9 ± 10.4 3 45.0 ± 12.1 42.5 ± 14.1
Duration of psoriasis (years) 16.9 ± 13.8 16.7 ± 13.8 -
Duration of PsA (years) 13.4 ± 10.7 - -
Psoriasis Area and Severity Index
score (PASI)
3.70 ± 3.50 4.90 ± 5.20 -
Number of tender or swollen joints 15.7 ± 12.3 - -
Number of Swollen joints 4.90 ± 4.20 - -
Phase II
Characteristic PsA (N=50) PsC (N=50) Controls (N=26) 4
Females/males 18/32 17/33 14/12
Age (years) 51.8 ± 12.2 45.9 ± 13.0 42.5 ± 14.1
Duration of psoriasis (years) 23.0 ± 11.7 17.4 ± 12.4 -
Duration of PsA (years) 16.9 ± 11.7 - -
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1. PsA: psoriasis arthritis.
2. PsC: cutaneous psoriasis without arthritis
3. For all continuous variables mean ± standard deviation is reported.
4. Controls are the same for Phase I (mild to moderate psoriasis) and PhaseII (Moderate to severe
psoriasis) patients.
Psoriasis Area and Severity Index
score (PASI)
18.1 ± 10.0 16.5 ± 9.40 -
Number of tender and/or swollen
joints
11.4 ± 12.0 - -
Number of Swollen joints 2.20 ± 3.0 - -
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4.3.3 KLK8 is independently elevated in sera of patients with psoriatic disease
We investigated KLKs as soluble biomarkers of psoriatic disease by measuring their
concentration in the serum of psoriasis patients (PsC and PsA) and healthy controls in two
subsequent phases (Phase I and Phase II). In phase I pilot study, epidermal and synovial fluid-
expressed KLK5, 6, 7, 8, 11, 13 were measured in 52 psoriatic disease serum samples compared
to 26 healthy controls, as shown in Table 4.2. Among all the KLKs tested, only KLK8 levels
were significantly elevated in psoriatic disease patients compared to controls in univariate
logistic regression analyses adjusted for age and sex. Increased serum levels of KLK8 associated
with psoriatic disease in a multivariate reduced model adjusted for age and sex [Odds ratio per
unit increase (OR) 2.56, 95% CI (1.08, 6.12), p = 0.03]. Polychotomous logistic regression
analysis showed that only KLK8 had significantly different effects when modelling PsC and PsA
separately, controlling for age, sex and the other KLKs, as shown in Table 4.3. Binomial logistic
regression analyses did not demonstrate a difference in serum KLK levels between PsC and PsA.
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Table 4.2. KLK levels in the serum of plaque-type cutaneous psoriasis (PsC) and psoriasis
arthritis (PsA) patients
1. Mild to moderate psoriasis with mean PASI scores 3.7-4.9.
2. Severe psoriasis with mean PASI scores 16.5 – 18.1
3. For all KLK concentrations measured in µg/L, the mean ± standard deviation is reported
Phase I samples 1
(N=52)
PsA
(N=26)
PsC
(N=26)
Controls
(N=26)
KLK5 0.26 ± 0.09 3
0.30 ± 0.17 0.29 ± 0.16
KLK6 1.85 ± 0.55 1.95 ± 0.49 1.78 ± 0.45
KLK7 8.00 ± 22.6 7.43 ± 7.93 4.37 ± 5.68
KLK8 2.03 ± 0.95 2.20 ± 0.93 1.67 ± 0.50
KLK10 1.08 ± 0.57 1.03 ± 0.47 1.12 ± 0.43
KLK11 0.64 ± 0.45 0.53 ± 0.12 0.53 ± 0.15
KLK13 0.20 ± 0.0 0.21 ± 0.07 0.20 ± 0.02
Phase II samples 2
(N=100)
PsA
(N=50)
PsC
(N=50)
Controls
(N=26)
KLK6 5.48 ± 2.64 4.68 ± 1.78 1.78 ± 0.45
KLK8 4.74 ± 4.19 3.89 ± 2.15 1.67 ± 0.50
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4.3.4 KLK8 serum levels in PsA correlate with PASI score but not inflamed joint counts
As mentioned above, we detected significant elevation of KLK8 in the sera of a small set of PsC
and PsA patients with mild-to-moderate plaque-type psoriasis. Furthermore, KLK6 and KLK8
were significantly elevated in the synovial fluids of PsA patients and lesional psoriatic skin.
Thus, we subsequently aimed to re-examine and validate KLK6 and KLK8 as PsA screening
biomarkers in a larger patient cohort consisting of 100 patients with moderate-to-severe psoriasis
(PASI >8), with or without PsA (Phase II). Both KLK6 and KLK8 were elevated in the sera of
these patients (Figure 3a). As listed in Table 4.2, mean KLK6 level in patients with mild-to-
moderate psoriasis (1.95 ± 0.49 µg/L) was similar to age and sex-matched healthy controls (1.78
± 0.45 µg/L), but was significantly higher in the sera of moderate-to-severe psoriasis patients
(4.68 ± 1.78 µg/L). Alternatively, mean KLK8 level was increased in both PsA patients with
mild-to-moderate psoriasis (2.20 ± 0.93 µg/L) and patients with moderate-to-severe psoriasis
(3.89 ± 2.15 µg/L), compared to healthy controls (1.67±0.5 µg/L). A similar trend was observed
for mean KLK6 and KLK8 serum levels in PsA patients. Logistic regression analysis showed
that neither KLK8 nor KLK6 levels distinguished PsA from PsC, as indicted in Table 4.3 and
Figure 4.3.
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Table 4.3. Polychotomous logistic regression analysis to identify biomarkers associated with
patients having psoriasis alone and psoriatic arthritis
Homogeneity Psoriatic Arthritis (PsA) Cutaneous Psoriasis (PsC)
Covariate P value 1
OR 2
95% CI 3
P-value OR 95% CI P-value
KLK5 0.36 0.01 (<0.001, 5.3) 0.16 0.071 (<0.001, 16.0) 0.34
KLK6 0.47 1.36 (0.321, 5.7) 0.67 2.490 (0.566, 10.9) 0.23
KLK7 0.70 1.06 (0.928, 1.2) 0.40 1.054 (0.926, 1.2) 0.43
KLK8 0.03 4.27 (1.024, 17.8) 0.05 6.623 (1.62, 26.9) 0.01
KLK10 0.10 0.46 (0.099, 2.1) 0.33 0.143 (0.024. 0.85) 0.03
KLK11 0.49 1.88 (0.102, 34.6) 0.67 0.284 (0.006, 12.5) 0.51
1. The homogeneity p values indicate whether the markers have significantly different
effects when modelling psoriasis and PsA separately, controlling for age, sex and the
other KLKs listed. The p values associated with PsA and psoriasis indicates the
significance of difference between either patient group compared to controls. KLK13
levels were un-measurable and are not reported. Only KLK8 was statistically significant,
for discussion see text.
2. OR: odds ratio
3. CI: confidence interval
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Furthermore, we investigated the association between KLK6 and KLK8 serum levels with
clinical parameters of PsA skin and joint activity, such as PASI scores and actively inflamed
(tender and/or swollen) and swollen joint counts. As shown in Figure 4.4, both KLK6 (r = 0.52)
and KLK8 (r = 0.42) serum levels correlated with the PASI scores of all patients (p <0.0001).
KLK8 correlated positively with PASI scores when patients with PsA (r=0.60, p <0.0001) and
PsC (r= 0.43, p=0.001) were considered separately. KLK6 correlated with PASI scores in PsA
patients (r=0.63, p <0.0001) but not in PsC patients (r=0.036, p=0.8). Only KLK8 association
with the PASI score was significant (β=1.153, p=0.0003) after adjusting for age, sex, psoriasis
duration and disease group (PsA and PsC) in a linear regression model with PASI score as the
outcome. However, there was no correlation between KLK8 and actively inflamed joint count
(p=0.35) or swollen joint count (p=0.12) in PsA patients. Thus, KLK8 serum level in PsA
correlates with skin, but not arthritis, activity.
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Figure 4.3. KLK6 and KLK8 cannot function as screening biomarkers for arthritis in
psoriasis patients. (A) KLK6 and KLK8 levels are elevated in moderate-to-severe psoriatic
disease patients (combined PsC and PsA) compared to healthy controls. (B) KLK6 and KLK8
serum levels do not distinguish PsA from PsC patients.
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Figure 4.4. KLK8 correlates positively with the PASI scores in psoriatic disease patients
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4.4 Discussion
Psoriasis (PsC) and psoriatic arthritis (PsA) are relatively common inflammatory diseases of the
skin and joints, respectively. Psoriatic arthritis (PsA) is an inflammatory arthritis that develops in
up to one-third of patients with psoriasis (Langley et al., 2005). The varied manifestations of PsA
make it sometimes difficult to recognize. Dermatologists managing a patient’s psoriasis often do
not inquire about symptoms of arthritis. Thus, the presence of PsA in psoriasis patients is often
overlooked in dermatology clinics. Early diagnosis of PsA is essential to prevent joint damage
progression and disability. The key to early diagnosis of PsA is better recognition of the presence
of PsA in patients with psoriasis (Chandran et al., 2010), but the diagnosis of PsA is difficult due
to the lack of specific diagnostic tests. Soluble biomarkers have the potential to provide means
for screening PsA in psoriasis patients so that appropriate referral to a rheumatologist for early
diagnosis is made. Candidate serum PsA biomarkers are likely to originate from target tissues
such as inflamed skin and joints. To date, few potential PsA screening biomarkers have been
identified, none validated and additional biomarkers await discovery. The main aim of this study
was to evaluate various kallikreins (KLKs) as potential PsA serum biomarkers in a cohort of
psoriasis patients with or without PsA, with special emphasis on the relationships between KLKs
and clinical parameters of PsA skin and joint activity.
Given that the majority of PsA patients initially present with cutaneous psoriasis, we
hypothesized that kallikrein proteases may be overexpressed in the inflamed skin, joints and sera
of PsA patients and may thus function as serum markers of PsA. Kallikreins are secreted serine
proteases that are expressed in a wide range of tissues, including skin epidermis. They regulate
skin barrier integrity via their ability to degrade adhesion molecules and activate antimicrobial
peptides and the immune system (Borgono et al., 2007b; Briot et al., 2009; Eissa and Diamandis,
2008; Yamasaki et al., 2006). KLK upregulation has been implicated in a number of
inflammatory skin diseases including psoriasis (Komatsu et al., 2007b; Kuwae et al., 2002), but
their involvement in inflammatory joint arthritis, including PsA, has not been studied. The
potential role of serum Kallikreins as soluble biomarkers of psoriasis severity and PsA has not
been examined.
We detected multiple KLKs in synovial fluids, with increased expression of KLK6 and KLK8 in
PsA compared to non-inflammatory fluid (osteoarthritis). The expression of multiple KLKs in
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PsA synovial fluids has not been previously reported. In PsA, the synovial membrane becomes
inflamed in response to proinflammatory cytokines, such as TNF-α, leading to the secretion of
cartilage-digesting enzymes, such as matrix metalloproteinases (MMPs) by synovial fibroblasts
(Chandran and Gladman, 2012 ; Ribbens et al., 2002). Similarly, KLK6 and KLK8 may be
induced by cytokines to degrade collagenous and non-collagenous structural molecules in the
joint.This hypothesis requires further scrutiny. These two KLKs were also elevated in lesional
psoriatic skin in which they expanded lower into the spinous layer of the elongated rete ridges.
Our immunohistochemistry results are consistent with previous reports of elevation of KLK6 and
KLK8 transcripts within two separate gene clusters in a large-scale psoriasis gene expression
analysis (Bowcock et al., 2001).
After examining KLK expression in the synovial fluids and lesional psoriatic skin, we measured
the concentrations of KLK5, 6, 7, 8, 10, 11 and 13 in the sera of a cohort of PsA and PsC patients
(N=52) with mild-to-moderate psoriasis (PASI <8). Among all the KLKs tested, patients with
PsC (N=26) and PsA (N=26) displayed significantly higher KLK8 serum levels compared to
healthy controls (N=26). The remaining tested KLKs did not significantly vary in these patients.
Thus, our initial exploratory analysis of KLK expression in the synovial fluids, lesional skin and
serum of patients with psoriatic disease suggested KLK6 and KLK8 as candidate psoriatic
disease biomarkers. Therefore, we next measured the levels of these two KLKs in a larger
independent cohort of serum samples of moderate-to-severe psoriasis patients, with or without
arthritis (N=100, PASI >8). Although both KLK6 and KLK8 were elevated in the sera of these
patients compared to healthy controls, the increase in KLK6 serum levels was not significant
when we controlled for sex, age and disease duration as well as KLK8.
Given that KLK8 was independently and significantly elevated in the sera of PsC and PsA
patients, we next examined its correlation with clinical parameters of skin and joint activity.
KLK8 correlated positively with the PASI score, but there was no significant correlation between
KLK8 levels in the serum and actively inflamed joint or swollen joint counts, indicating that
KLK8 is a soluble marker of skin, but not arthritis, activity in PsA patients. We recently showed
that KLK8 is a physiologically-active trypsin-like serine protease in normal skin epidermis.
KLK8 protein levels are elevated in skin extracts from lesional psoriatic skin compared to non-
lesional and normal skin, whereby the lesional skin exhibits significantly higher trypsin-like
activity (Komatsu et al., 2007b). Hence, the significant correlation of KLK8 serum levels with
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both PsC and PsA patients’ PASI scores reported herein indicates that KLK8 trypsin-like activity
is related to the progressive skin barrier dysfunction in both PsC and PsA.
To summarize, our study illuminates the importance of KLK8 as a marker of cutaneous psoriasis
severity. We demonstrate that although some KLKs are elevated in PsA synovial fluid, lesional
psoriatic skin and serum, none of these KLKs, including KLK8, can distinguish PsA from PsC
patients. Identifying PsA-specific soluble biomarkers remains a challenge to date. For instance,
biomarkers from genetic and genomic studies such as HLA-Cw06, IL12B and IL23R are
associated with PsA, but their primary association is with psoriasis susceptibility (Chandran and
Gladman, 2012; Gladman and Farewell, 2003; Veale et al., 2005). TNF-α is present in the sera
and skin of psoriasis patients and in the synovial fluids of PsA patients (Slobodin et al., 2009;
Veale et al., 2005). However, TNF-α serum levels are not useful for identifying PsA in psoriasis
patients. Nonetheless, anti-TNF agents are effective in controlling skin and joint manifestations
in PsA patients and preventing progression of joint damage (Weger, 2010). Since we
demonstrate that KLK8 can act as a surrogate serum biomarker of PASI, KLK8 may hold
promise as a biomarker for monitoring response to psoriasis therapies and relapse. Furthermore,
the detection of higher levels of KLK8 in the sera of both PsC and PsA patients, as well as in the
skin and synovial fluid of PsA patients, suggests that blockade of this protease may be beneficial
in both skin and musculoskeletal manifestations of psoriasis.
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Chapter 5
Ongoing Studies, General Discussion and Future Directions
Sections of this chapter were reproduced from the following manuscript with copy right
permission:
Ramachandran R., Eissa, A. et al. (2012). Proteinase-activated receptors (PARs): differential
signalling by kallikrein-related peptidases KLK8 and KLK14. Biological Chemistry. 393(5):421
Eissa, A., ……, Diamanids E.P. High through put screening of large small molecule compound
libraries reveals novel KLK8-specific inhibitors which exhibit potential as topical drug targets in
psoriasis. (Unpublished data, ongoing studies, manuscript in preparation)
131
5 Ongoing Studies, General Discussion and Future Directions
5.1 Introduction
We and others have recently begun to challenge the prevailing dogma that trypsin-like KLKs
play similar roles in the skin. The lack of KLK8 inhibition by all three physiological serine
protease inhibitors from the LEKTI family (LEKTI-1, LEKTI-2, LEKTI-3) attests to the fact that
trypsin-like KLKs may be regulated differently to play overlapping and/or distinct
dermatological functions (Brattsand et al, 2009). One of the important functional pathways
KLKs target in the skin is the activation of the keratinocyte cell surface-expressed receptors
known as the proteinase-activated receptors or PARs. KLK5, KLK6 and KLK14 are able to
cleave specific recognition sites in the extracellular amino termini of PARs to reveal a motif
known as the tethered ligand (TL). The exposed tethered ligand then interacts with the
extracellular loops of the receptor to initiate the recruitment of G-proteins and other signaling
molecules to the intracellular domains of the receptors (Oikonomopoulou et al., 2006). Both
PAR1, PAR2 are expressed in epidermal keratinocytes (Santulli et al., 1995), and PAR2 has been
heavily studied in the context of keratinocyte proliferation and differentiation, skin hydration and
inflammation (Steinhoff et al., 2005; Rattenholl et al., 2008). Recent seminal studies enhanced
our understanding of KLK-PAR2 signaling pathways in inflamed skin of Netherton syndrome
human patients and mouse models, but the main focus of these studies was KLK5 (Briot et al.,
2009). A study by Stefansson et al, indicated that KLK5 and KLK14 can induce a calcium signal
through PAR2, while KLK7 and KLK8 can not (Stefansson et al., 2008). In terms of signaling,
PAR2 is able to activate multiple downstream pathways including MAPKinase, as well as
stimulating elevations of intracellular calcium. It was recently shown that PAR2 signaling can
occur by a G-protein-independent mechanism to activate MAPKinase by interacting with -
arrestins (Ramachandran et al., 2011). Whether KLK8 is able to signal through PAR2 via
calcium/G-protein independent mechanisms remains unknown.
The limitations in our current understanding of KLK8 signaling mechanisms in the skin is partly
due to the lack of KLK-specific inhibitors or activity-based probes to distinguish its proteolytic
activity from other epidermal trypsin-like KLKs in vivo. Identification of KLK8-specific
inhibitors is of major interest as they may serve as important biochemical tools in functional
assays and may have a future therapeutic potential in reducing psoriasis flares.
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Hence, the ongoing studies described below address the missing gaps in KLK8-skin research by
(1) investigating KLK8-mediated proteinase-activated receptors (PARs) signaling and (2)
searching for KLK8 inhibitors by high throughput screening of small molecule compound
libraries.
5.2 Materials and Methods
5.2.1 KLK8-mediated PAR2 signaling in cell-based assays
Active recombinant KLK8 and KLK14 were prepared as previously documented (Eissa et al,
2011; Oikonomopoulou et al., 2006) and the specific activities in standard samples of each
enzyme U/ml (micromoles substrate cleaved/min/ml) were determined using the synthetic
substrate: VPR-aminomethylcoumarin. Both KLK8 and KLK14 were used at comparable levels
of enzyme catalytic activity. Suspensions of human embryonic kidney (HEK) cells were
harvested from cultured monolayers and calcium signalling was measured as documented
previously for studies of KLK14 (Oikonomopoulou et al., 2006). Calcium assays were
performed as previously described (Ramachandran et al, 2009).
To investgate if KLK8 can induce PAR2 and β-arrestin interactions, Bioluminescence Resonance
Energy Transfer (BRET) between YFP-tagged human PAR2 and renilla luciferase (rLuc)-tagged
-arrestin1 and -arrestin2 was measured as described in detail previously (Ramachandran et al.,
2009; Ramachandran et al., 2011) following a 20 minute incubation with the indicated agonists.
Data show the mean ± s.e.m. (bars) for triplicate measurements in two different cultures of cells
transfected with PAR2 and arrestins. To visualize receptor internalization, YFP-tagged human
PAR2 (C-terminal YFP) was transfected in HEK cells in glass bottom petridishes. 48 hrs after
transfection cells were treated with proteases for 20 minutes at 37 C. Cells were fixed with 10%
formalin in phosphate-buffered isotonic saline, pH7.4 and the localization of YFP-tagged
receptor was determined by confocal microscopy. Receptor activation is indicated by the
formation of punctate YFP fluorescent speckles both internalized and adjacent to the plasma
membrane. Activation/internalization was quantified by morphometric analysis of speckle
formation (speckles/cell). For MAPK activation studies, YFP-tagged human PAR2 (C-terminal
YFP) was transfected in KRNK cells in six well plates. Fourty-eight hours after transfection,
cells were quiesced in serum-free media for 2 h and either treated or exposed to trypsin (10nM),
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2f-LIGRLO-NH2 (10 mM), KLK14 (10 U/ml) or KLK8 (10 U/ml). MAPK activation was
monitored as described previously (Ramachandran et al, 2009).
5.2.2 Search for KLK8 inhibitors by high throughput screening of small molecule compound libraries
A library of 13,569 small molecule compounds were screened against KLK5, KLK8 and
KLK14, including pharmacologically-active, FDA-approved, natural and synthetic small
molecules from 5 different libraries as indicated in Table 5.1, below. Our previously reported
protease assays (100 μl) (Eissa et al, 2011) were adapted for a 384-well plate format (50 μl). The
primary screens for each enzyme were performed in 384-well plates (catalog no. 3370; Corning,
NY) in duplicates using a fully automated Beckman/Coulter Sagian core system in high
throughput screening facility. Microtiter plates were loaded sequentially with small molecule
compounds first (10µM final concentration in 0.2% DMSO), followed by the reaction buffer 100
mM Sodium Phosphate +0.1% Tween (pH 8.5) containing 0.2 mM of the fluorogneic substrate
VPR-AMC, and finally by adding the KLK protease of interest (20 nM KLK5, 10 nM KLK8 or
10 nM KLK14 in 3 parallel screens). Fluorescence was measured immediately at excitation and
emission wavelengths of 385 nm and 465 nm, respectively, every 30 seconds for the first 7
minutes in an Envision plate reader. Assay mixtures containing DMSO only were used as
controls. The average of fluorescence values in duplicate wells for a given compound was used
to determine the residual activity by taking the values obtained with DMSO controls as 100%.
Compounds that reduced the protease activity by ≥50% were selected for further analysis.
After applying filtering criteria such as excluding compounds with reactive groups, and
including compounds that inhibited only one KLK but not the other two, a total of 59 putative
KLK-specific inhibitors with RA <20% as well as 13 general inhibitory compounds were
subjected to a secondary dose-dependent validation screen to confirm findings and determine the
compounds’ IC50. Known general serine protease inhibitors acted as pan-KLK inhibitors
validating the sensitivity of our HTS-assays. Compound-specific IC50 values were determined
by nonlinear regression to the following four-parameter equation, as previously described.
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According to the multi-parametric equation (1), dose-response relationships were classified into
one of 4 categories: no-inhibition (NI: little to no decrease in activity), good dose-response
inhibition (GI: Y-range close to 100, slope factor close to 1, background close to zero), moderate
dose-response inhibition (MI: A good sigmoidal curve seen, but either with a Y-range
significantly less than 100, or with a slope factor significantly higher or lower than 1) and poor
dose-response inhibition (PI: low fit curve parameters, inhibition seen only in the highest doses).
Table 5.1. Libraries selected for KLK-inhibitor screening
Compound
Library
Distributor Description
DIVERSet Chembridge Corp., San Diego,USA 9,989 synthetic small molecules
PRESTWICK Prestwick Chemical, Illkirch,France 1,214 off-patent small molecules
SPECTRUM Microsource Discovery System,
Gaylordsville, USA
1,120 natural products and
bioactives
LOPAC Sigma-Aldrich, Canada Ltd., Oakville,
Canada
885 pharmacologically active
Natural
Products
Library
Biomol International L.P., Plymouth
Meeting, USA
361 natural products
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5.3 Results
5.3.1 KLK8 displays differential PAR2 signaling compared to KLK14
Recent studies indicated that PAR2 signaling can occur without calcium induction. We
hypothesized here that KLK8 may exhibit differential PAR signaling compared to KLK14,
giving support to our hypothesis that trypsin-like epidermal KLKs are not redundant. Potential
KLK8-mediated and KLK14-mediated PAR2 signaling was examined in collaboration with the
Dr. Hollenberg lab, in terms of calcium release in human embryonic kidney cells (HEK) cells,
PAR2 and -arrestins interaction and receptor internalization, and MAPK activation.
As shown in Figure 5.1, we confirmed that KLK8 does not induce calcium release through
PAR2, while KLK14 induces calcium release via PAR2, in a different cell background from the
one used by Stefansson et al (Stefansson et al., 2008).
Since PAR2 signaling can occur by a G-protein-independent mechanism to activate MAPKinase
by interacting with -arrestins, we next aimed to determine if PAR2 activation by KLKs 14 and
8 could trigger interactions of PAR2 with -arrestins, as does trypsin. Our results showed that
KLK14 is able to promote the interaction of the receptor with both -arrestin1 and 2. However,
KLK8 was unable to promote interaction of PAR2 with neither -arrestin1 or 2, even at high
enzyme concentrations, Figure 5.2. In terms of PAR2 internalization, KLK14, like trypsin and
the PAR-activating peptide, 2-furoyl-LIGRLO-NH2, was able to stimulate PAR2 clustering and
internalization, as illustrated by the formation of visible membrane-associated and internalized
fluorescent speckles, but KLK8 was unable to do so, as shown in Figure 5.2. Thus, our results
confirm that KLK8 is a unique trypsin-like epidermal KLK that cannot signal through PAR2 via
calcium dependent or independent mechanisms. This was further confirmed as we observed no
downstram MAPK activation upon treating cells with KLK8, Figure 5.3.
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Figure 5.1: KLK8 does not cause calcium signalling via either human PAR1 or PAR2, but
disarms thrombin-mediated human PAR1 signalling. The effects of thrombin and KLK8 on
calcium signalling in human HEK cell suspensions (E530 fluorescence emission) was measured as
outlined in the materials and methods. KLK8 at 4 U/ml did not cause a calcium signal, as would
be expected for the activation of either PAR1 or PAR2 in the HEK cells. The signal generated by
1 U/ml thrombin either before (right-hand-tracing) or after exposure of cells to 4 U/ml KLK8
(left-hand tracing) shows a reduction in fluorescence signal due to receptor dis-arming (double
arrow pointing to the thrombin-generated signal prior to (right) and after (left) KLK8 treatment).
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Figure 5.2. KLK8 does not trigger human PAR2 and -arrestins interaction, or PAR2
internalization, unlike KLK14. YFP-tagged human PAR2 (C-terminal YFP) was transfected in HEK
cells in glass bottom petridishes. 48 hrs after transfection cells were either (A) untreated (No Treatment,
NT) or exposed to (B) 2f-LIGRLO-NH2 (50M), (C) Trypsin (20nM), (D) KLK14 (20U/ml) or (E) KLK8
(20U/ml) for 20 minutes at 37 C. Receptor interaction is indicated by the BRET ratio and receptior
internatlization is indicated by the formation of punctate YFP fluorescent speckles both internalized and
adjacent to the plasma membrane (A-E). Internaliztion was quantified by morphometric analysis of
speckle formation (speckles/cell) (F). * indicates a significant increase in the number of internalized
receptors (speckles/cell) over the untreated (NT) cells.
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Figure 5.3. Unlike KLK14, KLK8 does not activate P42/44 MAP kinase-signalling in
human PAR2-expressing cells. The densitometry intensities representing
ctivated/phosphorylated MAPK (p-P42/44) were normalized to the intensity of total MAPK for
each sample. Histograms represent the averages +/- SEM for data obtained from two separately
transfected and stimulated cells.
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5.3.2 Identification of KLK8-specific inhibitors by high throughput screens (HTS) of small molecule compounds
Our primary screens of ~13,000 compound libraries resulted in 181, 226 and 196 small
molecules that reduced residual activity (RA %) of KLKs 5, 8 and 14 to less than 50%,
respectively, as shown in Table 5.2. After applying filtering criteria such as excluding
compounds with reactive groups, and including compounds that inhibited only one KLK but not
the other two, a total of 59 putative KLK-specific inhibitors with RA <20% as well as 13 general
inhibitory compounds were subjected to a dose-dependent validation screen to confirm findings
and determine the compounds’ IC50. Known general serine protease inhibitors acted as pan-
KLK inhibitors validating the sensitivity of our HTS-assays. Our secondary screen results
indicate that the three epidermal trypsin-like KLKs exhibited differential inhibition profiles
despite their significant sequence homology. Our dose-dependent secondary screens identified 1
KLK5-specific inhibitor (IC50: 17.298 µM), 9 KLK8-specific inhibitors (IC50s: 0.08-1.6 µM)
and 7 KLK14-specific inhibitors (IC50s: 0.08-1.7µM), as listed in Table 5.3. An example of a
good KLK8-specific inhibitor is shown in Figure 5.4.
Moreover, there was a clear functional classification among the inhibitors specific for each
enzyme. For instance, we discovered that several XXX natural derivatives as KLK14-specific,
while they exhibited no inhibition against the two other KLKs. Furthermore, there was a high
structural homology among potent KLK8-specific inhibitors. Differences in the 3D structures of
these enzymes might account for the formation of distinct active site cleavages with unique steric
properties. Further structure-activity relationship (SAR) studies are required to understand the
structure-based inhibition mechanism of these compounds. Fortunately, the crystal structure of
KLK8 was resolved very recently, although not published yet. We obtained the crystal structure
by collaboration with Dr. Peter Goettig and docked our top 9 compound hits into it. Interestingly,
the active site pockets of KLK8 versus KLK5 exhibited different electrostatic potentials, which
may explain their different inhibition profiles towards some small molecules. The majority of the
novel KLK8-specific small molecule inhibitors we identified docked into KLK8’s active site
pocket, as expected. These compounds represent the starting point for further biochemical kinetic
characterization assays and development into potential therapeutic targets.
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Table 5.2. Primary HTS assays identify potential KLK5, KLK8 and KLK14-specific small
molecule inhibitors
Total Statistical
actives
Unique
actives
Active against 1
other KLK
Active against
2 other KLK's
KLK5 181 (1.4%) 113 38 30
KLK8 226 (1.7%) 115 81 30
KLK14 196 (1.5%) 99 67 30
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A. KLK5-specific
Compound Name KLK5 Inhibition (IC50) KLK8 Inhibition (IC50) KLK14 Inhibition (IC50)
CHEMBRIDGE (17.29 uM) NI NI
B. KLK8-specific
Compound Name KLK5 Inhibition (IC50) KLK8 Inhibition (IC50) KLK14 Inhibition (IC50)
CHEMBRIDGE 6627234 W (0.08 uM) NI
CHEMBRIDGE 6625809 W (0.08 uM) NI
CHEMBRIDGE 5757756 W (0.09 uM) NI
CHEMBRIDGE 6623448 W (0.49 uM) W
CHEMBRIDGE 6623548 NI (0.65 uM) NI
CHEMBRIDGE 6631370 NI (0.66 uM) NI
CHEMBRIDGE 5653470 NI (1.01 uM) NI
CHEMBRIDGE 6625888 NI (1.09 uM) NI
CHEMBRIDGE 6625410 NI (1.68 uM) NI
C. KLK14-specific
Compound Name KLK5 Inhibition (IC50) KLK8 Inhibition (IC50) KLK14 Inhibition (IC50)
Thiaflavin Digallate NI NI (0.08 uM)
CHEMBRIDGE 5965041 W W (0.08 uM)
Hematein NI NI (0.09 uM)
Epigallocatechin NI NI (0.66 uM)
CHEMBRIDGE 5318527 NI NI (1.01 uM)
Gossypol NI NI (1.68 uM)
Teaflavin Monogallate NI NI (1.97 uM)
D. general-KLK
Compound Name KLK5 Inhibition (IC50) KLK8 Inhibition (IC50) KLK14 Inhibition (IC50)
Compound 40 (5.53 uM) (2.28 uM) (6.76 uM)
CHEMBRIDGE 5578375 (8.89 uM) (9.59 uM) (4.52 uM)
Gabexate Mesylate (16.08 uM) (0.57 uM) (4.12 uM)
Table 5.3. KLK-specific inhibitors IC50’s
(A) KLK5-specific, (B) KLK8-specific, (C) KLK14-specific, (D) pan-inhibitors. NI: No
inhibition. W:Weak inhibition.
A)
B)
C)
D)
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[cmd] (uM)
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Parameter Value Std. Error
Y Range 96.6313 4.7118
IC 50 0.0812 0.0104
Slope factor 1.5064 0.2505
Background -8.7613 2.0557
Parameter Value Std. Error
Y Range 98.5004 4.9437
IC 50 0.0887 0.0110
Slope factor 1.8011 0.3549
Background -6.3812 2.3187
KLK8
KLK5 KLK14
Figure 5.3. An example of a KLK8-specific inhibitor identified from the high throughput
screen. Small molecule compound displays no significant inhibitory effect on KLK5 and KLK14
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5.4 General Discussion
Our data shed light, at the molecular level, on the activity, regulation and putative role of
Kallikrein-related peptidase-8 (KLK8) in normal and psoriatic skin. In Chapter 2, recombinant
human KLK8 proteases were produced in active and latent zymogen forms to perform in vitro
biochemical kinetic assays to: (1) understand pro-KLK8 activation by other epidermal KLKs
known to be present in active forms in human stratum corneum, (2) understand regulation by
important epidermal ions such as calcium and by the epidermal pH gradient, which regulate
keratinocyte differentiation and healthy skin barrier function, and to (3) identify KLK8
enzymatic substrate specificity and potential substrate targets in the skin surface. In order to
confirm the physiological relevance of this protease in human skin surface, we designed a
sandwich-type KLK8-specific immunocapture-activity assay that elucidated the presence of
KLK8 as a physiologically active trypsin-like serine protease in normal human epidermal
stratum corneum extracts and sweat ex vivo. Our analysis of the immunocaptured skin and sweat
KLK8 demonstrated that it exhibits optimal activity at pH 8.5 and retains activity at pH 5, within
the physiological pH gradient of upper human epidermis. Our data showed that KLK8 in vitro
expression and activity is enhanced by calcium-induced terminal keratinocyte differentiation and
cornification in culture. Hence, KLK8 can be considered a marker of terminal keratinocyte
differentiation with an active innate immune role in normal human skin surface and sweat. Our
in vitro biochemical assays placed KLK5 as an activator of pro-KLK8 and pro-KLK1, pro-
KLK11 and LL-37 antimicrobial peptide as downstream targets of active KLK8, augmenting its
functional involvement in a proteolytic cascade regulating skin desquamation and antimicrobial
peptide activation. Interestingly, a recent study showed that the C-terminal peptide of KLK8 has
an antimicrobial and antifungal function against P. aeruginosa, S.aureus and C. albicans and a
positive effect on an LPS-treated mouse model (Kasetty et al., 2011). Thus, in addition to
regulating terminal keratinocyte differentiation, KLK8 most likely plays a role in modulating the
skin’s antimicrobial function through LL37 cathelicidins and/or human beta-defensins.
Identifying KLK8-specific activity in healthy skin surface and sweat indicated potential
important implications for inflammatory skin diseases, such as psoriasis. For one, unlike healthy
skin surface which has a slightly acidic pH of ~5, psoriatic patients’ skin surface has an elevated
pH of ~8 near the pH optimum of KLK serine proteases, suggesting enhanced activity of
physiologically-active KLK5, KLK7, KLK8 or KLK14 in normal skin surface. Secondly, KLK8
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mRNA expression levels are dramatically elevated in hyperkeratotic skin of psoriasis vulgaris,
followed by seborrheic keratosis, lichen planus, and squamous cell carcinoma patients, compared
to normal and basal cell carcinoma skin suggesting KLK8 involvement in abnormal and
excessive keratinocyte differentiation (Kuwae et al., 2002). Since KLK8 is not inhibited by
LEKTI or any other known epidermal serine protease inhibitors, unlike other trypsin-like KLKs,
then this barrier repair protease is more likely to play a distinct epidermal role compared to other
KLKs, which is yet to be specified. We demonstrated here that KLK8 cannot signal through
PAR2 via calcium dependent or independent means, yet it can process LL-37 antimicrobial
peptide. KLK8 inhibition profile is consistent with the results obtained by Yamasaki et al when
they first identified KLK5 and KLK7 as regulators of cathelicidin LL37 processing (Yamasaki et
al, 2006), suggesting that its endogenous activity is linked in skin innate immunity. In this thesis,
we identified specific elevation of KLK8 in lesional skin and serum of psoriasis patients, further
suggesting an innate immune role for this protease.
KLK8 overexpression and hyperactivity in normal skin can be a trauma-induced epidermal stress
signal that aims to restore the barrier by inducing terminal keratinocyte differentiation and
desquamation to enhance cell turnover and barrier recovery. Consistently, KLK8 knock-out mice
skin suffers from delayed recovery after chemical, physical, and UV-induced barrier impairment
(Kirihara et al., 2003; Kitayoshi et al., 1999). Studies have shown that serine protease activity
increases in the uppermost stratum corneum following barrier disruption, consistent with KLK8
role in wound healing (Hachem et al., 2005; Kishibi et al., 2012). Paradoxically, inhibition of
serine proteases, but no other protease types, was also shown to accelerate barrier recovery
(Hachem et al., 2006). This improvement in barrier function can be due to inhibition of KLK8-
mediated accelerated differentiation program. Hence, KLK8 seems to play opposing protective
and damaging roles during barrier breaches. It is likely that initial increases in KLK8 activity
upon acute barrier breaches are beneficial as a stress response or repair mechanism. However,
sustained KLK hyperactivity is likely to cause the skin to enter a pathological state as alluded to
by our investigation of KLK8 regulation and role in psoriasis.
In Chapter 3 of this thesis, we hypothesized that KLK8 overexpression and hyperactivity is
induced and sustained by Th1 and/or Th17 immune cell-mediated effects on epidermal
keratinocytes. Since psoriasis and atopic dermatitis have opposing polarization of T-cell
subpopulations, we hypothesized that KLK8 is distinctly regulated by the immune subsets
145
governing these two common skin diseases. In particular, psoriasis was an interesting model to
study KLK8 function in the skin, given the scaly lesions appearance and their intensified innate
immune nature. Psoriatic lesions are characterized by absence of a functional granular layer
which normally secretes KLK8 and absence of cornification-related proteins (such as caspase-14
and loricin). Yet, KLK8 is abundant in psoriatic lesions indicating its secretion by non-granular
keratinocytes. Limited attention has been given to investigating the mechanisms leading to
KLK8 upregulation in psoriatic lesions. Thus, we tested the hypothesis that secreted immune-
related factors in psoriatic skin induce KLK8 overexpression by keratinocytes in lower stratum
spinosum. Chapter 3 results supported the inside-outside theory of psoriasis pathogenesis, where
‘inside’ T-cell immune aberrations result in ‘outside’ epidermal barrier dysfunction. We show
that synergy of TNFα and IL17A cytokines induces dramatic morphological changes in
epidermal polarity and keratinocytes cell differentiation, indicated by the TNFα+IL17A-induced
‘web-like’ structures and ‘stratification domes’. TNFα and IL17A treatment of cultured
keratinocytes mimicked psoriatic keratincoytes, as it induced synergistic upregulation of the
psoriasis-related innate immune psoriasin (S100A7) and human beta-defensin-4 (hBD4) gene
expression, as well as IL6. Our results support a recent study reporting S100A7 and hBD4 as
synergistic IL-17A and TNF-α response genes in human keratinocytes (Chiricozzi et al., 2011).
Although this study demonstrated a list of key synergistic/additive TNFα and IL17A response
epidermal genes, it did not investigate IL-17A and TNF-α effect on keratinocyte differentiation
in culture and did not point KLKs as potential key pathogenic players in psoriasis circuits.
Our data show that TNFα and IL17A induce significant KLK8 hypersecretion by cultured
keratinocytes, suggesting that KLK8 overexpression in lesional psoriatic skin is an immune-
related response that contributes to the development and/or maintenance of psoriatic lesions. We
confirmed this notion upon examining the effect of KLK8 treatment on full-thickness 3D
epidermis tissue equivalents’ expression of keratinocyte differentiation and proliferation markers
and innate host defense genes. KLK8 induced keratinocyte differentiation leading to increased
stratum corneum and full epidermis thickness, as well as enhanced involucrin terminal
differentiation marker expression in all layers. However, adding a 10-fold higher KLK8 dose
resulted in dramatic changes that mimic lesional psoriatic skin, such as stratum corneum
detachment, epidermal thickening, retention of keratinocytes’ nuclei in the stratum corneum
(parakeratosis) and elongation of dermal fiboblasts. KLK8 also induced significant upregulation
146
of the psoriasis-related genes S100A7, hBD4 and IL6. Our study extends data from recent
genomic findings of KLK8 gene specific involvement in psoriasis (Ainali et al., 2012) to KLK8
protein levels and activity. We link KLK8-mediated induction of hyperkeratosis in inflamed
mouse skin to psoriasis skin disease in humans, and provide a possible explanation for KLK8
epidermal overexpression and induction of hyperkeratosis in psoriatic lesions.
The mechanisms by which KLK8 can cause a thickened epidermis may involve inhibition of AP-
2α protein expression resulting in keratinocyte hyperproliferation, induction of IL6-mediated
keratinoytes growth and differentiation, or increased proteolytic degradation of structures
responsible for basal cell cohesion in the basement membrane such as collagen IV, as discussed
in chapter 3. Further studies are required to dissect KLK8 signaling mechanisms in psoriasis. For
instance, our data pointed a dramatic KLK8 overexpression by dermal cells in psoriatic, but not
atopic dermatitis, lesions compared to matched non-lesional psoriatic and normal skin. However,
the identity of these cells remains unclear. Potential candidates include T17-cells and/or
neutrophils known to secrete serine proteases. KLK8 was previously localized in mast cells of
mouse skin (Wong et al., 2003) but was not studied in any other immune cell types. Virtually all
mast cells in the skin are known to secrete tryptase and IL17A-positive mast cells and
neutrophils are found in high levels at sites of skin and joint disease in humans (Kirkham et al.,
2013). Thus, it is possible that immune cells express KLK8 in psoriatic lesions. Interestingly, we
noted KLK8 overexpression in synovial fluids of psoriatic arthritis patients, suggesting a
possibly similar TNFα and IL17A regulation of KLK8 expression in the skin and joint of
psoriatic patients.
This thesis also highlights KLK8 potential as a therapeutic target for psoriasis topical treatment.
We show that current TNFα and IL17A blocking biologic therapies used to treat psoriasis in the
clinic reduce KLK8 expression in the skin and serum significantly. However, immune-based
therapies are often used intermittently due to their tendency to cause immunosuppression in
psoriatic patients. Thus, hampering KLK8-specific activity in the skin surface of psoriatic lesions
may be a beneficial alternative for psoriasis patients to use as a topical agent during the
intermittent halt periods of TNFα and IL17A immune-blocking therapeutic use. The KLK8-
specific small inhibitors we identified in Chapter 5 represent the starting points for future studies
aiming at developing and fine-tuning KLK8-inhibitors as topical psoriasis therapeutic targets.
147
Figure 5.5. KLK8 in normal and psoriatic skin. KLK8 is secreted by granular keratinocytes in
normal upper skin epidermis as indicated by the packman. (1) KLK8 is an active trypsin-like
serine protease in stratum corneum extracellular environment. (2-3) In psoriatic skin, the
abundance of immune cells secreting TNFα and IL17A induces KLK8 hypersecretion (and
increased trypsin-like activity) by keratinocytes in lower layers. KLK8 protein overexpression
correlates with the accelerated keratinocyte differentiation program, epidermal scaling, area and
severity of skin lesions in psoriasis patients. Inhibition of TNFα and IL17A (by biologic drugs,
such as etanercept which inhibits TNFα-binding to its receptor), reduces psoriasis scaling and
KLK8 expression. (4) Future studies should investigate the effect of topical application of
KLK8-specific inhibitors to lesional psoriatic skin as an adjuvant and/or an alternative to the use
of TNFα and IL17A-blockers. Figure is adapted by permission from Macmillan Publishers Ltd:
Nature (Crow, 2012), copyright (2012).
148
5.5 Future Directions
Our knowledge of KLK8 activity and functionality in normal human skin and psoriasis is
emerging. This work has contributed to a significant improvement in the understanding of KLK8
characteristics and functions in normal and psoriatic skin, and has identified KLK8 as an
attractive therapeutic target in psoriasis. Despite considerable progress, many important
questions remain elusive. For instance, what are the physiological substrate targets of KLK8 in
normal and psoriatic skin?? It is clear that this protease is unique compared to other KLKs in
terms of its PAR-signaling and inhibition properties. Thus, future directions should apply
unbiased advanced quantitative mass-spectrometry-based proteomic methods, in the field of
‘degradomics’, to identify natural KLK8-specific skin substrates and pathways, in comparison to
KLK5, KLK7 and KLK14. A recent approach known as terminal amine isotopic labeling of
substrates (TAILS) was developed to identify natural substrates of orphan proteases in tissues
and conditioned cell culture media. The recombinant human KLK proteases produced in this
thesis are available reagents that can be used to treat cultured keratinocytes and 3D skin
equivalents, prior to applying the TAILS protocol to identify KLK substrates by differential
display, as described previously (Kleifeld et al, 2010).
Seminal research in the KLK world focused on their roles as epidermal proteases, but limited
consideration was given to KLKs as immune proteases. Future research should characterize KLK
protein expression by immune cells in skin diseases and investigate their in vivo roles as innate
immune proteases. In the case of KLK8, immunofluorescence and flow cytometry studies
investigating KLK8 co-localization with immune cell markers in psoriatic lesions and sera are
warranted.
Finally, in the drug development arena, more research is needed in terms of fine-tunning the
small molecule compounds we identified here as KLK8 inhibitors. When the peptide inhibitor
leupeptin is docked into KLK5 and KLK8 active site pockets, these two trypsin-like proteases
exhibited different electorstatic potentials in their active site pockets, as shown in Appendix
Figure 5.1. Thus, molecular docking studies may reveal subtle differences between trypsin-like
KLK5 and KLK8. Furthermore, biochemical kinetic analyses of the inhibition mechanisms of the
top-identified KLK8 inhibitors are underway. Inhibitors’ specificity and selectivity towards other
epidermal proteases will also be investigated, prior to characterizing the inhibitor’s biological
149
effect in normal and psoriatic 3D skin epidermis models. The KLK8 recombinant proteases and
KLK8-immunocapture activity assays designed in this thesis are valuable biochemical tools to
test inhibitors’ potency in inhibiting physiological KLK8 activity in SC tissue extracts and sweat.
In conclusion, this work raises several important questions, such as what is the natural inhibitor
of KLK8 activity in the skin? What are the downstream targets of KLK8 and which in vivo
proteolytic cascades is it involved in? What are KLK8 epidermal and immune functions? How
will the structural information brought by the recent crystallisation of the protein help in
understanding its function and in fine-tunning inhibitors? Inhibitors of active skin-surface KLKs
are attractive targets for development of cosmetic agents that improve normal barrier function
and targeted topical therapies of chronic skin diseases that evade systemic immunosuppression.
The future of KLK8 dermatological research is promising with specific and intriguing basic
questions to explore and clinical applications to unravel.
150
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Appendices
Appendix Table 2.1. Steady-state kinetic parameters for the hydrolysis of synthetic AMC
substrates by mat-KLK8 in optimal activity buffer (100 mM Na2HPO4 pH 8.5)
a Specific activity, Vmax (µmol/min/mg) = [Adjusted Vmax (FU/min) x Calibration factor (µmol
AMC/FU)] ÷ Amount of enzyme added (mg)
b NR indicates no reaction
c Normalized activity = Kcat/Km (substrate) ÷ Kcat/Km (Boc-VPR-AMC) × 100
Substrate Vmax a
(µmol/min/
mg)
Km
(mM)
kcat
(min-1
)
kcat/Km
( mM-1
min-1
)
Normalized
Activity c
(%)
Trypsin-like
Boc-VPR-AMC 13.08 0.0238 ±
0.025
405.38 17004.31 100
Boc-QAR-AMC 11.14 0.1844 ±
0.009
345.24 1872.253 11
H-PFR-AMC 8.610 0.1533 ±
0.017
266.89 1740.994 10
Boc-QRR-AMC 6.566 0.2247 ±
0.017
203.53 905.7995 5.0
Boc-LRR-AMC 3.136 0.1165 ±
0.004
97.216 834.4745 4.9
Boc-FSR-AMC 6.903 0.2983 ±
0.071
213.98 717.3365 4.2
Boc-LKR-AMC 3.028 0.1407 ±
0.017
93.881 667.2431 3.9
Boc-VLK-AMC 7.039 0.3326 ±
0.023
218.21 656.0810 3.8
Boc-QGR-AMC 2.766 0.3699 ±
0.018
85.751 231.8222 1.4
Benzlyoxy-GGR-AMC 2.881 0.4312 ±
0.060
89.307 207.1133 1.2
Tos-GPR-AMC 4.076 0.6897 ±
0.125
126.36 183.2210 1.0
Tos-GPK-AMC NR b
NR b
Boc-EKK-AMC NR b NR
b
Chymotrypsin-like
AAPF-AMC NR b NR
b
LLVY-AMC NR b NR
b
Negative Control
AAPV-AMC NR b NR
b
173
Appendix Table 3.1. Demographics and disease characteristics of psoriasis patients pre and
post-treatment
Characteristics of patients at study entry
(Pre-treatment sample 1)
Total number of patients
(n = 60)
Males/ Females 42 (70.0%)/18 (30.0%)
Age (years) 47.2 (11.7)
Age at diagnosis of Psoriasis (years) 27.5 (12.4)
Age at diagnosis of PsA (years) 33.3 (11.2)
Psoriasis duration (years) 20.4 (12.4)
PsA duration (years) 14.6 (9.7)
Actively inflamed joint count 10.5 (9.9)
Swollen joint count 3.7 (4.6)
PASI score 6.5 (9.4)
Mean duration from Sample1 to Sample2 1.49 (0.76)
174
Appendix Table 3.2. KLK serum levels pre and post psoriasis treatment with TNFα-
blockers
Variable
Mean
Reduction
Sd Reduction
P-value
KLK5 Reduction -0.075 0.336 0.1672
KLK6 Reduction 0.1228 0.896 0.3916
KLK7 Reduction 0.2312 3.197 0.6500
KLK8 Reduction 1.629 4.484 0.0066
KLK10 Reduction 0.0471 0.7011 0.6736
KLK11 Reduction 0.0345 0.1479 0.1483
KLK13 Reduction -0.0455 0.291 0.3289
hsCRP Reduction 9.543 16.717 <0.0001
175
KLK5 KLK8
Appendix Figure 5.1. Differences in KLK5 and KLK8 active site pockets. Docking of the
general serine protease inhibitor leupeptin into active site pockets is shown indicating orientation
differences. Electrostatic potential (ESP) calculated on the surface of the catalytic site of trypsin-
like KLK5 and KLK8 clearly displays differences in the active site pocket.
