Cytochrome P450, CYP26AI, is expressed at low levels in human epidermal keratinocytes and is not...

Cytochrome P450, CYP26AI, is expressed at low levels in humanepidermal keratinocytes and is not retinoic acid-inducible

C.POPA, A.J.DICKER, A.L.DAHLER AND N.A.SAUNDERS

Epithelial Pathobiology Group, Centre for Immunology and Cancer Research, University of Queensland Department of Medicine,

Princess Alexandra Hospital, Brisbane, Queensland 4102, Australia

Accepted for publication 29 April 1999

Summary Retinoids, and their synthetic analogues, are well-established regulators of the squamous differ-

entiation programme both in vivo and in vitro. Despite this, very few studies have focused on themechanism by which retinoid action is terminated, e.g. metabolism. Recently, a new cytochrome

P450 family member (CYP26AI) was cloned. CYP26AI was reported to have substrate speci®city for

retinoids and to be retinoid-inducible. In this study, we have examined the expression and retinoicacid (RA) inducibility of CYP26AI in human epidermis and cultured keratinocytes. We found very

low levels of CYP26AI mRNA expression in both epidermis and keratinocytes. Furthermore, we

found no evidence for RA inducibility of CYP26 mRNA expression. This lack of RA inducibility wasnot due to inactivity of the retinoids, as we show that transglutaminase was still repressed by RA in

the same cultures. Despite the low levels of CYP26AI expression in the keratinocytes, the

keratinocytes were still capable of signi®cant RA metabolism. In conclusion, our study reports, forthe ®rst time, that CYP26AI is unlikely to contribute to RA metabolism in keratinocytes. These

studies also indicate that as yet unknown isoforms of cytochrome P450 may be involved in RA

metabolism in keratinocytes.

Key words: cytochrome P450, CYP26AI, human keratinocytes, retinoic acid

Squamous differentiation of keratinocytes is a tightlyregulated multistep process. Both in vivo and in vitro,

this process is initiated by the irreversible growth arrest

of proliferating cells and the subsequent induction ofsquamous differentiation-speci®c activities. The initia-

tion of growth arrest is characterized by inhibition of

expression of proliferation-associated genes such asE2F1 or cdk1.1±3 The induction of differentiation is

characterized by the expression of various squamous-speci®c genes such as transglutaminase type 1 (TG 1) or

corni®n.3±6 To a large extent, the tightly co-ordinated

changes in gene expression that accompany differentia-tion are transcriptional in mechanism.4,7 Disruption to

the control of this differentiation programme is

frequently associated with the development of variouspathologies.

It is well established that natural and synthetic

retinoids are potent regulators of the squamous differ-entiation programme.8 The actions of these retinoids

are mediated by a family of ligand-dependent nuclear

retinoic acid (RA) receptors (RARa, RARb and RARg)and retinoid X receptors (RXRa, RXRb and RXRg). Of

relevance to epidermal physiology are the reports that

keratinocytes selectively express RARg (90%), RARa

(10%) and RXRa.9,10 As retinoids are such potent

regulators of the differentiation programme, they have

extensive use in the treatment of various derma-topathologies such as neoplasms, psoriasis and acne.11

Although the action of retinoids is well understood,the mechanism by which their actions can be termin-

ated in the skin remain unclear. In this regard, the

recent cloning of a novel member of the cytochromeP450 family, CYP26AI, is of considerable interest.

CYP26AI was originally cloned by differential display

of RNA from RA-treated zebra®sh ®ns.12 This wasfollowed closely by the cloning of the mouse and

human homologues.13,14 CYP26AI was found to be

able to metabolize RA when transfected into Cos-1cells.15 Furthermore, the metabolism of RA could be

induced by pretreatment of F9 cells with RA.15 Meta-

bolite analysis indicated that the RA was degraded byoxidative metabolism consistent with that of the

cytochrome P450 family.12±14,16 Furthermore, these

British Journal of Dermatology 1999; 141: 460±468.

460 q 1999 British Association of Dermatologists

Correspondence: Nicholas Saunders. E-mail:NSaunders@medicine. pa.uq.edu.au

metabolites were consistent with RA metabolitesobserved in vivo.

Several observations suggest that CYP26AI may have

a part in RA metabolism in skin. First, it has beenreported that CYP26AI is expressed in a keratinocyte-

derived cell line.15,16 Secondly, it has been shown that

CYP26AI is selectively induced in the wounded epi-thelium of RA-treated zebra®sh ®ns.12 Thirdly, studies

of the RA inducibility of CYP26AI showed it to be

mediated by isoforms of receptors that are the majorforms also found in skin, namely RARg and RXRa.9,10

These ®ndings, coupled with the observation that the

epidermis is a major target of retinoid action, promptedus to examine the expression of CYP26AI in human

epidermal keratinocytes.

Materials and methods

Cell culture and treatment

Human epidermal keratinocytes (HEKs) and human

dermal ®broblasts were isolated from neonatal foreskinsand cultured as described.1,2 The culture of the kerati-

nocyte-derived squamous carcinoma cell line SCC25,

and the spontaneously immortalized keratinocyte cellline HaCaT, have also been described.17,18

The effects of the retinoid all-trans-RA, or its isomers9-cis-RA and 13-cis-RA, on CYP26AI and TG 1 mRNA

expression were investigated by incubating proliferative

and con¯uent keratinocytes with 1 mmol/L retinoid(Sigma, Sydney, Australia) in the presence or absence of

the generic P450 inhibitor, ketoconazole19 (10 mmol/L;

ICN Biomedicals, Melbourne, Australia) for 48 h. Insome instances, cells were pretreated with the generic

P450 inhibitor proadifen20 (Sigma) for 12 h prior to

incubation with [11,12±3H(N)-]RA ([3H]RA). All treat-ments involving RA were performed in reduced light

conditions to minimize isomerization to the 9-cis-RA or

13-cis-RA forms.

RNA isolation and reverse transcription±polymerase chain

reaction analysis

Total RNA was extracted from cells using Trizol reagent

(Life Technologies, Sydney, Australia), as described.1

Single-stranded cDNA was then synthesized from 2 mg

total RNA using SuperScript II RNaseHÿ reverse tran-

scriptase, as described.1 The expression of CYP26AI,actin and TG 1 mRNA were determined by polymerase

chain reaction (PCR) ampli®cation of 1 mL of the reverse

transcription (RT) reaction. All PCR ampli®cations were

performed under conditions of linearity with respect tocycle number.1 Actin primer sequences were: forward

primer, 50 GAA ATC GTG CGT GAC ATT AAG 30 and

reverse primer, 50 CTA GAA GCA TTT GCG GTG GACGAT GGA GGG GCC 30. These primers spanned exons 3±

5 and generated an mRNA-speci®c PCR product of

507 bp.21 TG 1 primer sequences were: forwardprimer, 50 TGG AGG CAC AGG ACG CGG TGA 30 and

reverse primer, 50 GAG AGC TGT GGG CTG TCC AAG 30,

which generated a PCR product of 580 bp. TheCYP26AI primer sequences were: forward primer, 50

AGG CAC TAA AGC AAT CTT CAA 30 and reverse

primer, 50 CAT GGA AAT GGG TGA ATC TTG 30,which generated a PCR product of 621 bp. PCR ampli-

®cations were carried out at a denaturation tempera-

ture of 94 8C for 40 s, an annealing temperature of 55 8C(actin and CYP26AI) or 65 8C (TG 1) for 40 s and an

extension temperature of 72 8C for 45 s. The PCR pro-

ducts were fractionated by agarose gel electrophoresisand then blotted on to a nylon membrane (Schleicher

and Schuell Inc., Keene, NH, U.S.A.), as previously

described.22 The DNA was ultraviolet cross-linked onto the membrane and probed for 24 h with the respec-

tive [g-32P]dCTP (Bresatec, Adelaide, Australia) cDNA

probes, and washed at high stringency, as described.23

The image was then analysed on an SF phosphorimager

(Molecular Dynamics, Melbourne, Australia) andinequalities in RT and PCR ampli®cation were normal-

ized to the actin expression of the same sample. The

range of linearity for determining mRNA levels for eachgene product was then determined by performing tem-

plate titration analysis.

Sequencing

To con®rm that the CYP26AI PCR product was genuineCYP26AI, the PCR product was puri®ed and subjected

to automated sequencing on an ABI automated sequen-

cer (Dr Brenda Cheung, Grif®th University, Australia).

Metabolism studies

The ability of proliferative keratinocytes to metabolize

high RA concentrations (1 mmol/L RA spiked with

33´33 nmol/L [3H]RA; New England Nuclear, Boston,MA, U.S.A.) and low RA concentrations (33´33 nmol/L

[3H]RA) over a 6-h period was examined. In other

experiments, cells were studied following either no RApretreatment, pretreatment with 1 mmol/L RA for 48 h

or pretreatment with 1 mmol/L RA plus 10 mmol/L

ketoconazole for 48 h. As a control in these experiments,

CYP26AI IN HUMAN EPIDERMAL KERATINOCYTES 461

q 1999 British Association of Dermatologists, British Journal of Dermatology, 141, 460±468

media were spiked with 33´33 nmol/L [3H]RA, added tocells and removed immediately.

Following the incubations, media and keratinocytes

were harvested separately. The media were collectedand the cells trypsinized and resuspended in 500 mL

phosphate-buffered saline. Lipid-soluble retinoids were

separated from water-soluble retinoids by a modi®edBligh and Dyer method of total lipid extraction.24 All

organic solvents for this extraction contained the anti-

oxidant butylated hydroxytoluene (0´05%; Bioscienti®cPty Ltd, Brisbane, Australia). All tubes and tips were

coated with SurfaSil siliconizing ¯uid (Pierce, U.S.A.) to

prevent binding of lipids to surfaces; the extractionef®ciency by this method was 87 6 2% and 99 6 2%

when high and low RA concentrations were used,

respectively. Transformation of [3H]RA to water-solubleproducts was measured by liquid scintillation spectrom-

etry of aliquots of the aqueous phase of both media and

cell extracts. In some experiments, lipid-soluble extractswere evaporated to dryness and resuspended in mobile

phase (80 : 20 methanol/H2O v/v with 10 mmol/L

ammonium acetate) for high-performance liquidchromatography (HPLC) analysis.

Organic phase-soluble retinoids were separated by

HPLC on a Waters 3´9 ´ 150 mm C18 NovaPak analy-tical column using an isocratic mobile phase consisting

of 80 : 20 methanol/H2O with 10 mmol/L ammoniumacetate at a ¯ow rate of 0´5 mL/min. The HPLC system

consisted of a Waters Model M 45 solvent delivery

system, Waters Model 710B Wisp, Waters 490Programmable Multiwavelength detector set at

350 nm and Waters System Interface Module. The

computer package used for processing of acquireddata was the MilleniumTM 2010 Chromatography

Manager. All HPLC components were from Millipore

Corporation Waters Association (Milford, MA, U.S.A.).Column ef¯uent was monitored for 3H by scintillation

counting of collected fractions.

Results

CYP26AI expression in human epidermal keratinocytesand other tissues and cell lines

Several human cell types and tissues were screened forthe expression of CYP26AI mRNA by RT±PCR. HaCaT

cells and human liver were shown to express high levels

of CYP26AI mRNA, whereas the epidermis, SCC25cells, proliferating HEKs, differentiated HEKs or HEKs

treated for 48 h with RA showed very little CYP26AI

mRNA expression (approximately 50-fold less than the

liver; Fig. 1). The levels of CYP26AI expression observed

in the epidermis and HEKs could only be detected byprobing blots of PCR products (i.e. no PCR product

visible on an ethidium bromide-stained gel after 40

ampli®cation cycles). The low level of CYP26AI mRNAexpression was not due to contaminating DNA, as the

product was present in DNase I-treated RNA samples.

These data suggested that it is unlikely that the con-stitutive CYP26AI expression in the epidermis or cul-

tured keratinocytes is catalytically active. These data

also indicated that CYP26AI mRNA expression was notdependent upon the differentiation status of the HEKs,

as CYP26AI mRNA expression in proliferative HEKs was

not signi®cantly different from that observed in differ-entiated HEKs. Con®rmation of the differentiation status

of the HEKs was shown by a fourfold induction in TG 1

mRNA expression in these cultures (not shown).Furthermore, treatment of either proliferative or con-

¯uent HEKs with RA did not signi®cantly alter the

CYP26AI mRNA expression, even after a 5±8 day treat-ment (Fig. 2A,B). The bioactivity of the added RA was

con®rmed by its ability to downregulate the expressionof TG 1 mRNA in the same cultures of differentiated

HEKs (Fig. 2B). This demonstrated that the inability of

RA to induce CYP26AI mRNA was not due to theinactivity of the RA used. Combined, these data suggest

that the expression of CYP26AI mRNA in cultured

HEKs and in the epidermis is much lower than that

462 C.POPA et al.


Figure 1. Expression of CYP26AI in human tissue and cells. Total

RNA was extracted from cells or tissue and 2 mg used in reverse

transcription±polymerase chain reaction analysis. CYP26AI mRNA

levels were estimated by phosphorimage analysis as a percentage ofliver expression. Data expressed as mean 6 SEM of no less than three

experiments. All samples normalized to actin expression. Control, no

DNA; HaCaT, spontaneously derived keratinocyte cell line; SCC25,keratinocyte-derived squamous cell carcinoma cell line; Epid, epidermis;

Prol, proliferating keratinocytes; Diff, differentiated keratinocytes, RA,

proliferating keratinocytes treated with 1 mmol/L retinoic acid for 48 h.

observed in the liver and that CYP26AI mRNA is not RA

responsive in cultured HEKs.

Retinoic acid metabolism

Relatively low levels of CYP26AI mRNA were detected

in cultured HEKs by RT-PCR. Paradoxically, cytochrome

P450-dependent RA metabolism has previously beenreported in both cultured keratinocytes and skin.19,25,26

These previous studies demonstrated that the uptake of

RA, its metabolism and the release of RA metabolitesback into the media, can occur within 6 h. To examine

whether the low level of CYP26AI mRNA expression

observed in HEKs was associated with an inability tometabolize RA, proliferative keratinocytes were incu-

bated with either high (1 mmol/L) or low (33´33 nmol/L)

RA concentrations for 6 h.Treatment of HEKs with [3H]RA for 6 h resulted in a

large increase in the water-soluble radioactivity

recovered in the media (mean 6 SEM 28´8 6 0´86%)compared with the control cells (5´7 6 0´39%; Fig. 3A).

This was re¯ected in the loss of radioactivity recovered

in the lipid phase (97´2 6 0´18% to 62´3 6 3´08%;Fig. 3B). HEKs that had been pretreated with 1 mmol/L

cold RA for 48 h prior to being exposed to [3H]RA for 6 h

also showed an increase (16´7 6 0´67%) in the produc-tion of water-soluble radioactivity recovered in the

media relative to control cells (5´7 6 0´39%), but this

increase was lower than that observed in cells whichhad not been pretreated (Fig. 3A,B). Again, the increase

in water-soluble radioactivity correlated well with the

loss of radioactivity in the lipid phase (Fig. 3A,B).Furthermore, the production of water-soluble retinoid

derivatives was signi®cantly reduced in HEKs treated

with ketoconazole irrespective of whether the cells hadbeen pretreated with RA or not (Fig. 3A,B). Ketoconazole

has previously been shown to inhibit cytochrome P450-

dependent metabolism.26±28 These data indicated thatthe conversion of RA to water-soluble derivatives in

HEKs was inhibited by ketoconazole and, as such, wasconsistent with a cytochrome P450-dependent event.

However, other ketoconazole-sensitive pathways could

not be excluded. To address this issue, we preincubatedproliferating HEKs with the generic P450 inhibitor

proadifen (5 mmol/L) for 12 h, followed by incubation

with 33´33 nmol/L [3H]RA for 6 h. This resulted in areduction of water-soluble RA from 34´6 6 1´7% in the

untreated cells to 21´7 6 0´5% in the proadifen-treated

cells. The water-soluble RA in the spiked controls was10´4 6 0´5%. These data strongly suggest that P450-

dependent catalysis is involved in RA metabolism in

human keratinocytes.It is possible that RA-inducible RA conversion was

not detected in HEKs because the high concentration of

cold RA (1 mmol/L) used in the pretreatment (that mayhave remained after extensive washing) may have

saturated the converting enzymes or reduced the spe-

ci®c activity of the added [3H]RA. To address thisquestion, the previous experiments were repeated

using a high (1 mmol/L) RA concentration. Similar to

the studies conducted using low RA concentrations, theHEKs converted RA to water-soluble derivatives (Fig. 3C).

This conversion was inhibited by ketoconazole, and

there was no evidence for RA-induced RA conversion



Figure 2. Time-dependent alterations in CYP26AI or transglutamin-

ase type 1 (TG 1) mRNA expression following retinoic acid (RA)treatment. Proliferative (A) or differentiated (B) human epidermal

keratinocytes (HEKs) were treated with 1 mmol/L RA for varying

lengths of time and then assayed for CYP26AI or TG 1 mRNA

expression. Total RNA was extracted and 2 mg used in the reversetranscription±polymerase chain reaction. mRNA levels were then

quanti®ed, following blotting and probing, by phosphorimage analy-

sis. Data presented as a percentage of untreated HEKs (day 0) and as

mean 6 SEM from three experiments. All samples normalized to actinexpression. Large SEMs were associated with estimating CYP26AI

mRNA levels, possibly due to its low expression in HEKs and the

sensitivity limits of the assay.

(Fig. 3C,D). This suggests that RA metabolism in culturedHEKs is cytochrome P450-dependent but not RA indu-

cible. Similar results were obtained using cell extracts ofthese same cultures (Fig. 4A±D). Ninety per cent of the

recoverable 3H retinoids were associated with the media.

The previous experiments established that culturedHEKs could metabolize RA despite the low expression of

the RA-speci®c cytochrome P450, CYP26AI. We next

examined the lipid extracts from HEKs reported inFigures 3 and 4 by HPLC analysis. Under the conditions

used in this laboratory, the retention time of RA was

approximately 10´2 min (corresponding to peak 4 inFig. 5) and for the isomers 9-cis-RA (peak 3, Fig. 5) and

13-cis-RA (peak 2, Fig. 5) were 9´02 min and 8´2 min,

respectively. Despite carrying out all experiments involving

RA in reduced light conditions, RA isomerization wasdif®cult to prevent (Fig. 5B). These data indicate that

isomerization of RA occurs readily in the cultures ofHEKs or during the extraction. This is consistent with

reports of an isomerase being present in epidermis.29

Despite the presence of the 9-cis and 13-cis isomers ofRA, we could detect no metabolites of RA by HPLC. The

lack of RA metabolites may be due either to their

transient nature or due to `masking' under peak 1which was predominantly non-enzymatic in origin.

This applied to samples from all the conditions shown

in Figures 3 and 4. The peak shown in Figure 5 (peak 1)was present in all samples, including the control, and

was not reduced following ketaconazole treatment (not

shown). This suggested that peak 1 was the product of

464 C.POPA et al.


Figure 3. Aqueous and lipid-soluble

derivatives of [3H]retinoic acid (RA) in

media following a 6-h incubation. Human

epidermal keratinocytes (HEKs) weretreated with either 33´33 nmol/L RA (A,B)

or 1 mmol/L RA (C,D) for 6 h. Media were

then extracted and lipid (B,D) or aqueous(A,C) soluble RA derivatives counted by

liquid scintillation spectrometry. Control,

media spiked with either a low

(33´33 nmol/L [3H]RA) or high (1 mmol/Lcold RA�33´33 nmol/L [3H]RA) RA

concentration; 6 h, HEKs treated with RA

for 6 h; keto, HEKs treated with RA plus

10 mmol/L ketoconazole for 6 h; pretreated,HEKs pretreated with 1 mmol/L RA for 48 h

then either a low or high RA concentration

([3H]) for 6 h, pre� keto, HEKs pretreated

with 1 mmol/L RA and 10 mmol/Lketoconazole for 48 h followed by a 6-h

treatment with either a low or high [3H]RA

concentration� ketoconazole. Datapresented as mean 6 SEM of triplicate

determinations from two experiments. All

data presented as a percentage of total

recoverable radioactivity.

non-enzymatic breakdown. These data indicated that:(i) RA is readily isomerized; (ii) there is a possibility that

the suppression of TG 1 mRNA by RA in HEKs is mediatedby 9-cis-RA or 13-cis-RA; (iii) qualitatively, RA intermedi-

ates could not be de®nitively identi®ed, as differences in

peaks between treatment groups and control cells couldnot be shown; (iv) ketoconazole did not alter the HPLC

pro®le signi®cantly; and (v) the RA derivative in peak 1

may be a non-catalytic conversion product of RA, as itwas present in the control sample as well.

Biological activity of retinoic acid derivatives

The previous ®ndings raised the possibility that the

retinoid effects observed in this study are mediated by

the parent compound, all-trans-RA, or by RA isomers.To address this question, the ability of the 9-cis-RA and

13-cis-RA isomers to suppress TG 1 mRNA expressionin con¯uent cultures of HEKs was tested. Figure 6 shows

that the mRNA expression of TG 1 markedly decreased

following a 48-h exposure to 1 mmol/L 9-cis-RA, 13-cis-RA or all-trans-RA. These data indicate that all-trans-

RA, 9-cis-RA or 13-cis-RA have similar abilities to

suppress TG 1 mRNA expression in differentiated HEKs.

Discussion

The present study shows, for the ®rst time, that the

newly cloned CYP26AI enzyme, thought to contribute

to RA metabolism, is poorly expressed and not inducible



Figure 4. Aqueous and lipid-solublederivatives of [3H]retinoic acid (RA) in

human epidermal keratinocyte (HEK)

extracts following a 6-h incubation. HEKs

were treated with either 33´33 nmol/L RA(A,B) or 1 mmol/L RA (C,D) for 6 h. HEKs

were harvested and then extracted. Lipid

(B,D) or aqueous (A,C) soluble RAderivatives were then counted by liquid

scintillation spectrometry. Control, media

spiked with either a low (33´33 nmol/L

[3H]RA) or high (1 mmol/L coldRA�33´33 nmol/L [3H]RA) RA

concentration; 6 h, HEKs treated with RA

for 6 h; keto, HEKs treated with RA plus

10 mmol/L ketoconazole for 6 h; pretreated,HEKs pretreated with 1 mmol/L RA for 48 h

then either a low or high RA concentration

([3H]) for 6 h; pre� keto, HEKs pretreated

with 1 mmol/L RA and 10 mmol/Lketoconazole for 48 h followed by a 6-h

treatment with either a low or high [3H]RA

concentration� ketoconazole. Datapresented as mean 6 SEM of triplicate

determinations from two experiments. All

data presented as a percentage of total

recoverable radioactivity.

in HEKs. This is despite the observation that keratino-cytes possess signi®cant RA-metabolizing activity. This

®nding has signi®cant implications for our understand-

ing of normal keratinocyte biology as well as retinoid

therapeutics. For instance, it is established that kera-tinocytes are a major target for retinoids. However, our

study shows that the termination of retinoid action in

the keratinocyte may be mediated by isoform(s) ofcytochrome P450 other than CYP26AI. Our studies

also show that if modulation of retinoid levels is to be

achieved by modulating RA-catabolizing enzymes, thenenzymes other than CYP26AI should be considered.

In this study, we demonstrated that CYP26AI is

unlikely to be a major contributor to endogenousmetabolism of retinoids in the skin. In the ®rst instance,

we found that CYP26AI mRNA levels in both epidermis

and cultured keratinocytes were approximately 50-foldless than those in the liver. In contrast, the sponta-

neously derived immortal keratinocyte cell line, HaCaT,

had similar levels of expression to that of the liver. Thiswould explain the earlier ®nding that a keratinocyte cell

line could express CYP26AI.16 Furthermore, the expres-

sion of CYP26AI is not a characteristic of all establishedkeratinocyte cell lines, as the SCC25 cells reported in

this study showed little CYP26AI expression. The levels

of CYP26AI mRNA expression in keratinocytes werenot visible on an ethidium bromide-stained gel and

would appear to be expressed at a lower level than

`rare' mRNA species (e.g. E2F1) that we have previously

466 C.POPA et al.


Figure 5. High-performance liquid chromatography (HPLC) pro®les ofmedia derived from [3H]retinoic acid (RA)-treated human epidermal

keratinocytes (HEKs). (A) An HPLC pro®le of [3H]RA. Proliferating

HEKs were treated with [3H]RA for 6 h in the absence (B) or presence

(C) of 10 mmol/L ketoconazole. Following incubation, lipid extractswere taken and the sample injected on an HPLC. Eluted fractions were

collected and subjected to liquid scintillation spectrometry. Data

presented as disintegrations per minute eluted vs. the time of thefraction. The ®gures are representative of at least two independent

experiments. (1) Putative non-enzymatic degradation product; (2)

peak comigrating with authentic 13-cis RA; (3) peak comigrating

with authentic 9-cis RA; (4) peak comigrating with authentic RA.

Figure 6. Activity of retinoids in suppressing transglutaminase type 1

(TGase1) mRNA expression. Con¯uent (CONFL) human epidermal

keratinocytes (HEKs) were treated with 1 mmol/L all-trans, 9-cis or 13-

cis retinoic acid (RA) for 48 h. Total RNA was extracted and 2 mg usedin reverse transcription±polymerase chain reaction (PCR) analy-sis.

The mRNA levels for TGase1 were estimated by phosphorimage

analysis of Southern blots from the TGase1 PCR product. Datapresented as a percentage of the con¯uent HEKs and represent mean -

6 SEM of four experiments. Proliferating HEKs (PROL) are provided to

con®rm the differentiated status of the con¯uent cells. All samples

normalized to actin expression.

reported.1 These data suggest that the low levels ofCYP26AI mRNA expression reported in this study are

unlikely to be associated with catalytically active

CYP26AI. Further support for this comes from theobservation that the CYP26AI mRNA expression in

HEKs was not modulated by pretreatment with RA.

Previous studies have established that cells in whichCYP26AI appears to be catalytically active are also

subject to RA induction.16,30 The HEKs used in this

study were responsive to RA, as characterized by thesuppression of TG 1 mRNA and the induction of CRAB-

PII mRNA (not shown), but not with respect to

CYP26AI. Combined, these data strongly suggest thatCYP26AI is likely to be catalytically inactive in HEKs.

However, it may be possible that functional levels of the

CYP26AI protein are present despite little mRNAexpression.

Despite the low levels of CYP26AI mRNA expression,

HEKs maintained the capacity to metabolize RA. It hasbeen previously reported that microsomes isolated from

both skin and cultured keratinocytes are able to meta-

bolize RA extensively.25,29,31,32 The major metabolitesreported in these studies were 4-OH and 4-oxo RA.25

There have also been reports of 5,6-epoxy RA,25 but its

formation remains contentious. Formation of these meta-bolites appeared to be cytochrome P450-dependent, as

their production was inhibited by both ketoconazoleand liarozole.25,31 Although metabolites of RA have

been reported, there still is no clear evidence as to

what isoform of cytochrome P450 may be responsible.In fact, there is more evidence excluding isoforms than

there is evidence identifying isoforms. For instance,

there is now evidence to exclude the following isoforms:CYP26AI, CYP1AI, CYP3f and CYP1A2´32 These data

indicate the need for an exhaustive examination of skin-

associated cytochrome P450s. Only by this means willwe be able to identify potential cytochrome P450 iso-

forms involved in RA metabolism. This is an important

question to address given the extensive use of therapeu-tic retinoids, the importance of retinoids in normal skin

homeostasis and the continuing evidence implicating

retinoid metabolism in the development of retinoidinsensitivity in various tumours.

Acknowledgments

The authors extend their gratitude to Paul Taylor, Paul

Salm and Drs Paul Masci and Mark Bowles from theDepartment of Clinical Pharmacology for their expert

assistance with the HPLC. The authors also thank

Dr Julie Joyner from the Department of Surgery for the

generous gift of human liver RNA. This work wassupported by a grant from the Princess Alexandra

Hospital Foundation. Dr Dicker is supported by an

Australian NH & MRC Predoctoral Fellowship andMs C.Popa was supported by a CICR Honours Research

Scholarship.

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Cytochrome P450, CYP26AI, is expressed at low levels in human epidermal keratinocytes and is not...

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