GLUCOCORTICOIDS Glucocorticoids: mechanisms of action...

Glucocorticoids: mechanisms ofaction and anti-inflammatorypotential in asthma

V. H. J. van der Velden

Department of Immunology, Erasmus University,PO Box 1738, 3000 DR Rotterdam, The Netherlands

Te l: (+31) 10 408 7192Fax : (+31) 10 436 7601Email: [email protected]

GLUCOCORTICOIDS are potent inh ibitors of in flam m atoryprocesse s and are w ide ly used in th e treatm ent ofas thm a. Th e an ti-inflam m atory effects are m ediatedeither by direct binding of the glucocorticoid/gluco-corticoid receptor com plex to glucocorticoid respon-sive e lem ents in th e prom oter region of genes , or byan in te raction of th is com plex w ith other tran scrip-tion factors , in particular activating prote in-1 ornuclear factor -k B. Glucocorticoids in hibit m anyin flam m ation-associa ted m olecules such as cyto-kines , chem okines , arachidon ic acid m etabolites , andadhesion m olecules . In contras t, an ti-inflam m atorym ediators often are up-regulated by glucocorticoids .In vivo studies have shown that tr eatm ent of asth -m atic patients w ith in haled glucocorticoids in h ibitsthe bronchial in flam m ation and s im ultaneouslyim proves th eir lung function . In th is r evie w , ourcurren t kn owledge of th e m echan ism of action ofglucocor ticoids and their anti-in flam m atory poten tialin as thm a is described. Since bronch ial epithe lialcells m ay be im portan t targe ts for glucocorticoidtherapy in asth m a, the effects of glucocor ticoids onepithe lial ex pre ssed inflam m atory genes w ill beem phasized.

Key words : Glucocorticoids, Asthma, Bronchial epithe lialce lls, Inflammation

Introduction

Glucocorticoids are hormones synthe sized in theadrenal cortex and sec rete d into the blood, w here theleve ls of glucocorticoids fluctuate in a circadianmode . In humans, the naturally occurring gluco-corticoid is hydrocortisone (cortisol), w hich is syn-thesized from its precursor cortisone .

The be neficial e ffec ts of glucocorticoids in as th-matic patients w ere first de scribed in 1950.1 Sincethen on, many studie s have focused on the ther-apeutic potential of glucocorticoids. Several syntheticglucocorticoids, much more potent than cortisol andw ithout the unw anted mine ralocorticoid side e ffec ts,have been deve loped. Now adays, glucocorticoids arepow erful agents in the tre atment of inflammatorydiseases and are by far the most effective anti-inflammatory drugs used in the treatment ofasthma.

Mechanism of ActionAlthough glucocorticoids have been know n for a longperiod of time , the ir prec ise mechanism of ac tion isstill not complete ly unde rstood. However, recentstudies have inc reased our understanding of theircomplex mechanisms of action.

Glucocorticoid receptor

To ex ert the ir effe cts, glucocorticoids ne ed to bind toa spec ific cytoplasmic glucocorticoid receptor (GR).Almost all ce lls of the body ex pre ss the GR, but thenumber of re ceptors may vary be tw een differe nt ce lltype s.2 Cloning of the GR has re vealed that the GRcons is ts of approx imately 800 amino ac id residues,and that ce rtain areas of the molecule show homol-ogy w ith othe r steroid re ceptors, receptors forthyroid hormones, and receptors for re tinoic ac id.3 –7

All members of the nuclear hormone receptor familyshare a charac teristic three-domain structure , firstdesc ribed for the human GR. The C-terminal domainis equal in size in all nuclear receptors studied (about250 amino ac ids) and its main function is to bind thesteroid.8 It also contains the binding site s for the heatshock prote ins (hsp) 90.9,1 0 Removal of the steroid-binding domain results in a constitutively ac tive GRmolecule, indicating that this part of the moleculeacts as a repressor of the transcription-activationfunction. The most conserved central domain isinvolved in direct binding of the re ceptor to DNA. Itcontains tw o distinct loops of prote in, each bound attheir base via four cyste ine re sidue s to a single zincion, the so-called zinc fingers.1 1 These zinc cluste rsare involved in binding of the GR to the major grooveof the DNA double he lix and play a role in dimeriza-

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Review

Mediators of Inflammation, 7, 229–237 (1998)

tion of tw o GR molecule s.1 2,1 3 In addition, the centralDNA-binding domain has a transcription-activationfunction.4 ,14 The ste roid-binding and DNA-bindingdomains are separated by the ‘hinge-region’, w hichcontains sequence s that are important for nucleartranslocation and dimerization.9,10 The N-te rminaldomain is ex treme ly variable in size (24–600 aminoacids). Its pre cise role is still uncertain, but it isrequired in transc riptional activation.15

Tw o different forms of the human GR have be endesc ribe d.3 ,1 6 These tw o highly homologous isoforms,te rmed GRa and GRb , are gene rated by alternativesplic ing of the human GR pre -mRNA. The GRbisoform diffe rs from the GRa isoform only in itsC-terminal domain, in w hich the last 50 amino ac idsof the latte r are replaced by a unique 15 amino ac idsequence . How ever, this re placement has dramaticfunctional consequence s, since the GRb isoform isunable to bind glucocorticoids and to transduceligand-dependent transac tivation. How eve r, the phys-iological significance of the GRb isoform remainsquestionable, s ince some recent studies indicate thatthis form is not conse rved among spec ie s and nodominant negative inhibition of GRa activity could befound.17 ,1 8 Nevertheless , abundant ex pre ssion of GRbprote in can be found in the epithe lial cells lining thete rminal bronchioli of the lung.19

The ex press ion of the GR may be re gulated bynumerous fac tors e ither at the transc riptional, transla-tional or post-translational leve l.2 0,21 Glucocorticoidshave be en show n to dow n-regulate the ex pression ofthe GR, both in v itro and in vivo .22 ,23 In contrast,inflammatory mediators like interleukin (IL)-1 b , IL-4,tumour ne crosis fac tor (TNF)-a , lipopolysac charide(LPS) and interferon (IFN)-g have been show n toinc re ase glucocorticoid binding in vitro .24 – 28 How -eve r, the inc rease in GR numbers may be ac com-panied by a reduced affinity for glucocorticoids.24 ,2 8

Analysis of GR localization in normal and asthmaticlung has not revealed differenc es in the leve l or sitesof GR ex pre ssion.2 9

Regulation of gene transcription

In the absence of glucocorticoids, the GR is presentin the cytoplasm of the cell as a he te ro-oligomercons is ting of the GR itse lf, tw o molecule s of hsp90, one molecule hsp 70, and one molecule of hsp56 (w hich probably does not interact w ith the GRitself, but interacts w ith hsp 90).30 – 3 4 Glucocor-ticoids enter the cytoplasm of the ce ll by passivediffusion through the cell membrane. In the cyto-plasm they bind to the GR complex , w hich subse-quently undergoes conformational change s, re sult-ing in the dissoc iation of the hsp 90 and hsp 56molecules . Upon this activation, the glucocorticoid-GR complex passe s the nuc lear membrane, entersthe nuc leus, and the hsp 70 molecule is dissoc iated.

Furthe rmore , in the nucleus liganded GR form hom-odimers (Fig. 1).

Within the nucleus, the GR homodimers mayregulate gene transc ription in seve ral ways: (1) viabinding of the glucocorticoid-GR complex to spec ificDNA sequences, the reby directly ac tivating or re pre s-sing gene s; (2) via inte raction w ith other transcriptionfactors; and (3) via modulating the stability of specificmRNA molecule s.3 5 – 3 9

Bin ding to DNA s eque nce sSeve ral steroid-re sponsive genes contain glucocor-ticoid re sponsive e lements (GRE) in their promoterregion.3 5,4 0 Binding of GR homodimers to GRE mayeither re sult in transcriptional activation of the gene(via a po s itive GRE) or repression of the gene (via anega tive GRE) (Fig. 1). The consensus sequence for(positive) GRE is the palindromic 15-base-pairsequence GGTACAnnnTGTTCT, w hereas the negativeGRE has a more variable sequence.36 The rate oftransc riptional regulation of steroid-re sponsive genesis dependent on both the numbers of GRE, the affinityof the glucocorticoid-GR complex to the GRE, and theposition of the GRE re lative to the transcriptional startsite . Binding of the complex to GRE may re sult inconformationa l change s in the DNA and ex posure ofpreviously masked areas, re sulting in increased bind-ing of othe r transcription factors.4 1– 44

In te ra c tio n w ith o th e r tra n s criptio n fa c to r sMany ste roid-responsive gene s do not have GRE intheir promoter region. How ever, binding site s forothe r transcription factors, including nuclear factor(NF)-k B, ac tivating prote in (AP)-1, and cAMP-respon-sive e lement binding protein (CREB), often can befound.45

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230 Mediators of Inflammation · Vol 7 · 1998

FIG. 1. Schematic representation of the cellular events afteradministration of glucocorticoids (adapted from Ref. 39).

AP-1, w hich is a dimer of tw o proto-oncogene s(members of the c-jun and c-fos family),46 ,47 isinvolved in the regulation of several genes, includingadhesion molecules and cytokines (review ed in Ref.47). Direct protein–prote in inte raction betw een AP-1and the glucocorticoid-GR complex results in rec ipro-cal repre ssion of one another’s transc riptional activa-tion by pre venting binding of the AP-1 and gluco-corticoid-GR complex to AP-1 sites and GRE,respective ly (Fig. 1).3 7,4 8,4 9

Comparable to AP-1, NF-k B (a hete rodimer of p50and p65 subunits5 0,51 ) re gulate s the transcription ofseveral gene s involved in inflammatory reac-tions.5 0,52,5 3 In unstimulated ce lls , NF-k B is re tained inthe cytoplasm of the cells through the interaction w iththe inhibitor s Ik Ba and Ik Bb .5 4 – 56 Upon cell stimula-tion, for ex ample by IL-1 b or TNF-a , I k B are rapidlyphosphorylated, ubiquitinated, and consequently pro-te olysed.53 ,57 The liberated NF-k B dimers translocateto the nucleus w here they can ac tivate target genes.Glucocorticoids may inhibit NF-k B-stimulated gene sby a direct interaction betw een the glucocorticoid-GRcomplex and the p65 subunit of NF-k B, re sulting intransrepre ssion (Fig. 1).5 1,5 5,5 8,59 Furthe rmore , gluco-corticoids may indirec tly antagonize NF-k B mediate dtransc ription by up-regulating the synthesis of theinhibitory protein Ik Ba , w hich traps NF-k B in inac tivecytoplasmic complex e s.39 ,54 ,5 5 A large number ofimmunoregulatory genes, w hose ex pre ssion isinduced by a variety of pro-inflammatory mediators,contain NF-k B sites in their promote rs/re gulatoryregions. Therefore , it is no w onder that glucocorticoidshave been found to prevent the ex pre ssion of the segenes, including those coding for IL-1 b , IL-2, IL-6, IL-8,monocyte chemoattrac tant prote in (MCP)-1, RANTES(Regulated upon Activation, Normal T ce ll Ex pre ssed,and presumably Sec rete d), granulocyte macrophagecolony-stimulating factor (GM-CSF), the IL-2 receptor,inte rcellular adhe sion molecule (ICAM)-1, andE-se lectin (review ed in Ref. 45). Probably, interactionsbetw een glucocorticoids and NF-k B or AP-1 w illex plain most of the anti-inflammatory and immuno-suppressive activities of glucocorticoids.

An inte rac tion be tw een CREB and the gluco-corticoid-GR complex has also been suggeste d.6 0,6 1

b -agonists, w hich are used as bronchodilators in thetre atment of asthma, inc rease cAMP formation andsubsequently ac tivate CREB. Therefore , s imultaneoustre atment of as thmatic patients w ith glucocorticoidsand b -agonists may result in reduced re sponsivene ssof the airw ays for steroids.6 1– 63

Mo dula tio n o f m RNA s ta bility.A third mechanism by w hich glucocorticoids mayregulate the synthesis of prote ins is via enhancedtransc ription of spec ific ribonuclease s w hich are ableto degrade mRNA containing constitutive AU-richsequence s in the untranslated 3 9 -re gion.64 Such gluco-

corticoid-mediated modulation of post-translationalevents (resulting in decreased mRNA stability andreduced half-life time) has be en observed for IL-1 b , IL-6 and GM-CSF.6 5,6 6

Glucocorticoid Regulated GenesGlucocorticoids are able to modulate the transcrip-tion of a varie ty of gene s, including cytokines andchemokines, re ceptors, enzymes, adhesion mole-cules , and inhibitory prote ins (Table 1). Since epithe-lial c ells may be one of the most important targets forglucocorticoid the rapy in asthma, the effe cts ofglucocorticoids on epithe lial ex pressed inflammatorygenes w ill be emphas ized in this re view.

Cytokines and chemokines

Glucocorticoids inhibit the transcription of mostcytokines and chemokines that are re levant in asthma,including IL-1 b , TNF-a , GM-CSF, IL-3, IL-4, IL-5, IL-6, IL-8, IL-11, IL-12, IL-13, RANTES, eotax in, and mac ro-phage inhibitory prote in (MIP)-1 a .45 ,66 In general,reduced synthe sis of these mediators may re sult in adecre ased re cruitment and ac tivation of leukocytes,also indire ctly due to e ffec ts on adhe sion molecule sand cell survival. Since many cytokine gene pro-mote rs do not contain a ne gative GRE, the effects ofglucocorticoids on cytokine and chemokine produc-tion are probably mediated via an e ffec t on a critic altransc ription factor (e spec ially NF-k B and AP-1).6 7

Cross-signalling betw een NF-k B and AP-1 w ith gluco-corticoid/GR complex have indeed be en demon-strated in bronchial ep ithe lial cells.6 7

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Mediators of Inflammation · Vol 7 · 1998 231

Table 1. Influence of glucocorticoids on the synthesis ofproteins with inflammatory effects by bronchial epithelialcells

Protein Glucocorticoideffect

CytokinesIL-1b , IL-6, IL-11, TNF- a , GM-CSF ¯IL-10, LIF ?G-CSF =

ChemokinesMCP-1, eotaxin, IL-8, RANTES, MIP-1a ¯

ReceptorsNK, GR ¯IL-1R II, IL-6R, b 2-adrenergic receptor

EnzymesiNOS, COX-2, cPLA2 ¯NEP

Adhesion moleculesICAM-1 ¯

Inhibitory proteinsLc-1 =/IL-1RA type I, SPLI

Bronchial epithelial cells are capable of producing avarie ty of cytokine s and chemokine s that may contrib-ute to the initiation and pe rpetuation of airw ayinflammation. Several studies have show n that cyto-kine-induced ex pression of eotax in, IL-6, IL-8, GM-CSF, and RANTES can be diminished by gluco-corticoids in v itro .68 –76 In contrast, glucocorticoidsdid not modulate the sec re tion of G-CSF by humanbronchial epithelial cells.7 6 In v ivo studie s haveshow n that treatment w ith inhaled steroids decre ase sboth the ex press ion of GM-CSF,77 IL-1 b ,78 IL-8,79 andRANTES80 by the bronchial ep ithe lium, toge the r w iththe number of ac tivated eosinophils in theepithe lium.

Receptors

Glucocorticoids may modulate the ex press ion ofseveral re ceptors. The ex pre ssion of the neurokinin(NK)1 re ceptor, w hich mediate s many effects ofsubstance P (SP) in the airw ays and is be lieved to beup-regulated in asthma,8 1 is dow n-re gulated by gluco-corticoids.8 2 Since the NK1 re ceptor gene promoterregion has no GRE but has an AP-1 re sponse e lement,this e ffec t probably w ill be mediated via an inte r-action of the glucocorticoid-GR complex w ith AP-1.

In contrast to NK1 re ceptors, ex press ion of theb 2-adrene rgic receptor is increased by glucocor-tic oids.8 3 Since the human b 2-adrene rgic receptorgene contains three potential GRE, this effect ofglucocorticoids probably is a dire ct one .83 Up-regula-tion of b 2-adrene rgic re ceptors by glucocorticoidsmay be re levant in as thma as it may prevent dow n-regulation in re sponse to prolonged tre atment w ithb 2-agonists .84

The IL-1 receptor type II, w hich func tions as adecoy re ceptor,8 5 may also be up-regulated by gluco-corticoids, there by re ducing the functional ac tivity ofIL-1 agonists.8 6,8 7 Soluble TNF-re ceptor type I (p55)re lease by human bronchial epithelial c ells, bothconstitutive as w ell as IL-1 b -induced, has been show nto be re duced by glucocorticoids.8 8 In contrast,glucocorticoids up-re gulate the ex pre ssion of IL-6receptors in rat hepatoma and human epithelialce lls .89 ,90 Thus far little is know n about this proce ss inhuman bronchial epithe lial ce lls, w hich constitutivelyex pre ss these re ceptors.91

Glucocorticoids also modulate the ex pre ssion oftheir ow n receptor. In a re cent study it w as show nthat ex press ion of the a -form (but not the b -form) ofthe GR in human bronchial ep ithe lial ce lls w as dow n-regulated in healthy subjects after 4 weeks of budeso-nide inhalation.2 3

Enzymes

Glucocorticoids inhibit the synthe sis of severalinflammatory mediators implicated in the pathogene-

sis of asthma through an inhibitory effect on enzymeinduction. The synthe sis of inducible nitric ox idesynthase (iNOS) by human airw ay epithelial cells isinhibited by glucocorticoids, both in vitro and inv ivo .92 – 9 4 This effe ct seems to be mediated viainactivation of NF-k B.9 5,9 6 Since nitric ox ide (NO)may contribute to skew ing of Th lymphocytestow ards a Th2 phenotype , there by promoting IgEproduction and eosinophil rec ruitment, inhibition ofiNOS may be of importance in anti-inflammatorytherapy in as thma.9 7

Glucocorticoids also inhibit the gene transcriptionof a cytosolic form of phospholipase A2 induced bycytokine s98 and inhibit the gene ex pre ssion ofcycloox ygenase-2, re sulting in reduced formation ofprostaglandins and thrombox anes.99

In contrast to the enzymes mentioned above,glucocorticoids have be en show n to increase theex pre ssion of neutral endopeptidase (NEP),10 0 –10 2

thereby pote ntially limiting neurogenic inflammatoryresponses.1 03 In ac cordance w ith the se results, it w asfound that the ex press ion of NEP on bronchialepithe lial c ells w as higher in as thmatic s treatedw ith steroids compared w ith nonste roid-treatedasthmatics.1 04

Endothelins

Endothelins are a family of highly homologous21-amino ac id peptide s, charac terized by tw o intra-chain disulphide chains, a hairpin loop cons isting ofpolar amino ac ids, and a hydrophobic C-te rminalchain.10 5 Human bronchial epithelial c ells have be enshow n to produce ET-1,1 06 –1 08 w hich promotes theprolife ration of smooth muscle ce lls , is a pote ntconstrictor of both vascular and non-vascular smoothmuscle ce lls , inc reases the se cretion of mucus, andmay ac tivate inflammatory ce lls.1 05 ,1 07 ,10 9 ET-1 alsostimulates collagen gene ex pression and through itsinhibitory ac tions on collagenase w ill promote airw ayw all collagen deposition, there by contributing toairw ay w all thickening w hich underlies bronchialhyperresponsiveness.11 0 –11 2 Inc re ased leve ls of ET-1-immunoreac tivity we re detected in airw ay epithe-lium and vascular endothelium of bronchial biopsyspecimens from nonste roid-tre ated asthmatic s com-pared w ith healthy subjects .10 6,1 13 ,11 4 In contrast, noincre ased ET-1 ex pression w as found in the bronchialepithe lium of asthmatic patients treated w ithglucocorticoids.11 5

Adhesion molecules

Adhesion molecules play an important role in therec ruitment of inflammatory ce lls to the inflamma-tory locus. Ex pression of adhesion molecule s onendothelial, epithelial or inflammatory ce lls is often



induced by cytokine s, w hereas glucocorticoidsreduce surface ex pression of adhe sion molecules.This effect may be due either to inhibition of cytokinesynthesis or to a direc t effect of glucocorticoids onadhesion molecule gene transcription. It has be enshow n that the ex pression of ICAM-1, endothelialleukocyte adhesion molecule (ELAM)-1, and E-selec tinis dow n-re gulated by ste roids.11 6 Basal and cytokine-stimulated ICAM-1 ex pre ssion on human bronchialepithe lial c ell lines is inhibited by glucocor-tic oids.1 17 ,11 8 Howeve r, inhaled glucocorticoids didnot modulate ICAM-1 ex pre ssion by bronchial epithe-lial ce lls from asthmatic s in vivo .11 9

Eosinophil adhe sion to cytokine-stimulated bron-chial ep ithe lial c ells w as show n to be inhibited by thesynthetic glucocorticoid dex amethasone .120 Althoughcytokine-activated epithelial c ells show ed increasedex pre ssion of ICAM-1, this molecule did not seem tobe involved in the dec reased adhe sion of eosinophilsobserved in the presenc e of dex amethasone .12 0

Inhibitory proteins

The anti-inflammatory e ffec ts of glucocorticoids maybe mediated by increasing the production of inhibi-tory prote ins, such as lipocortins. Lipocortins aremembers of a supe rfamily of proteins characterizedby the ir ability to bind calcium and anionic phospho-lipids, now know n as the ‘annex ins’.12 1,12 2 In severalce ll type s, but not all, glucocorticoids are inducers oflipocortins, w hich have an inhibitory effec t on theactivity of phospholipase A2 .123 ,1 24 As a re sult, thesynthesis of lipid mediators, including prostaglandinsand e icosanoids, w ill be re duced. How ever, in humanbronchial epithelial c ells glucocorticoids do not se emto up-regulate the ex pre ssion of lipocortins.12 5 Fur-thermore , no significant difference w as foundbetw een lipocortin-1 concentration in BAL fluid fromasthmatic patients re ce iving inhaled glucocorticoidtherapy and those w ho were not treated w ithglucocorticoids.12 6

Recently, glucocorticoids have also been show n toinc re ase the ex pre ssion of intracellular IL-1 receptorantagonist type I by human bronchial epithelial c ellsin vitro .1 27 Increased production of this mediatormay inhibit the effe cts of IL-1 agonists , the rebyreducing inflammation. Howeve r, glucocorticoidtreatment of asthmatic patients did not affe ct theex pre ssion of IL-1 receptor antagonist by the bron-chial epithe lium.78

To provide prote ction against potentially injuriousagents, airw ay epithe lial cells se cre te a number ofmediators, including antiprote ase s. Secretory leuko-cyte protease inhibitor (SLPI) is the pre dominantantiprotease in the airw ays. Its ex pre ssion has be enshow n to be inc reased in airw ay epithelial ce lls afterstimulation w ith glucocorticoids.12 8

Cellular and Clinical Effects ofGlucocorticoids in AsthmaSeve ral s tudie s have determined the effe cts of inhaledglucocorticoids on bronchial inflammation, e ithe r bymeasurements in BAL fluid, sputum, or ex haled air, orby performing bronchial biopsie s. Although differ-ences can be observed betw een differe nt trials , the sestudies have confirmed that glucocorticoid treatmentof asthmatic patients reduces the number and activa-tion of inflammatory ce lls in the airw ays, togetherw ith an improvement of lung func tion. Now adays,the potent anti-inflammatory ac tions of glucocor-ticoids are thought to underlie the clinical e ffic acy oforal glucocorticoids.1 29

Effects of glucocorticoids on immunopathology

Inhaled glucocorticoids decre ase the number andactivation status of most inflammatory cells in thebronchus, including mast cells, dendritic ce lls , eosino-phils , and T lymphocytes . Changes in cellular infiltra-tion are ac companied by modulated ex press ion ofseveral cytokines. Inhaled glucocorticoids have be enshow n to decrease mRNA ex pre ssion of GM-CSF, IL-13, IL-4, and IL-5, w hereas mRNA leve ls of IL-12 andIFN-g inc reased, sugge sting a shift from a Th2-tow ards a more Th1-like environment.77 ,13 0,1 31

Glucocorticoid tre atment is as sociated w ith areduction in m a st c e ll numbers in the bron-chus7 9,1 29 ,1 32 –13 5 and w ith reduced mast ce ll asso-ciated mediators in BAL fluid.13 5,1 36 This may be dueto a re duc tion in IL-3 and ste m ce ll factor production,w hich are necessary for the survival of mast c ells intissue. The (IgE-dependent) re lease of mediators frommast cells does not se em to be affec ted by gluco-corticoid treatment.1 37,13 8

Dendritic cells play an important role in presentingantigens to (naive) T ce lls .139 ,1 40 Inhaled gluco-corticoids have been show n to re duce the numberof dendritic c ells in the human bronchialepithe lium.1 41

Increased numbers of eos inophils are a prominentfeature of asthmatic airw ays.14 2 –148 In v itro studie shave show n that many eosinophil functions, includ-ing adhe renc e and chemotax is, are diminished follow -ing glucocorticoid treatment.1 38 How ever, most datasuggest that eos inophil re sponses to steroids are likelyto be indire ct, s ince eosinophil func tion is markedlyaffec ted by cytokine s e laborated from T lymphocytes(IL-3, IL-4, IL-5, GM-CSF), endothelial c ells (GM-CSF)and epithelial c ells (GM-CSF).1 49 –1 53 In vivo studie sindicate that tre atment w ith inhaled steroids re ducesthe number of eosinophils and eosinophil-re latedmediators in BAL fluid79 ,1 48,15 4 and the numberof (activated) eosinophils in bronchial biop-sie s.79 ,1 29 ,13 2,1 33 ,1 55 Recently, induced sputum hasbeen suggeste d as a useful tool for evaluating the



effects of the rapy on airw ay mucosal inflammation.Thus far, most studie s have focused on the pre senc eof eosinophils and eosinophil-re lated mediators . Inaccordance w ith the findings in BAL fluid andbronchial biopsie s, glucocorticoid treatment w asassociated w ith a re duction in sputum eosinophilnumbers , eosinophil cationic protein (ECP), andeosinophil pe rox idase (EPO).156

Glucocorticoids also re duce the number of acti-vated T lymphocyte s in bronchial biopsie s and BALfluid.1 29 ,13 3,1 34 ,1 55,1 5 7 In addition, inhaled corticoste-roids reduced the number of ce lls ex pressing mRNAfor IL-4 or IL-5, and increased the number of cellsex pre ssing mRNA for IFN-g ,1 31 ,13 2 the reby favouringthe deve lopment of Th1 cells.15 8

In addition to the e ffec ts of glucocorticoids onepithe lial ce lls described above, inhaled glucocor-ticoid therapy has be en show n to reduce the shed-ding of epithe lial cells.1 55 ,15 9,16 0 No consistent e ffec tof corticosteroids on the thickness of the basementmembrane has been obse rved.79 ,16 0,161

Besides the suppre ssive e ffec ts on inflammatoryce lls , inhaled glucocorticoids have also show n toinhibit mucus secre tion and mic rovascular leakage (asdete rmined by the dow n-re gulation of plasma pro-te ins in BAL fluid).1 60 ,16 2 –16 6 At present it is not clearw hether this is mediated via a dire ct effe ct ofglucocorticoids on endothe lial or mucous cells, or viaa reduction of inflammatory mediators that inc re asemucus se cretion and vascular leakage.

Effects of glucocorticoids on lung function

Treatment w ith glucocorticoids has be en consiste ntlyshow n not only to re duce the symptoms of as thma,but also bronchial hyperresponsiveness.13 4,16 7–16 9 Incontrast to the rapid inhibitory effe cts of b 2-agonis ts,glucocorticoids given in a s ingle dose are not effe ctivein pre venting early alle rgen-invoked bronchoconstric-tion, but inhibition of the late re sponse has be enclearly demonstrated.1 70 ,17 1 In contrast, chronic treat-ment w ith either oral or inhaled steroids attenuate seven the early bronchoconstric tion to alle rgen,17 1–17 3

an effe ct that probably is mediated via the anti-inflammatory actions of glucocorticoids alre adydesc ribed. Although inhaled glucocorticoids con-sis tently re duce airw ay hyperreac tivity in asthmat-ics ,16 9 even after several months of treatment re spon-siveness fails to re turn to the normal range. This mayreflec t persis tence of structural changes that cannotbe re versed by steroids (such as the thickening of thebasement membrane ), despite of suppress ion of theinflammatory and immunologic al processes.

Concluding RemarksGlucocorticoids are w idely used in the tre atment ofasthma and have anti-inflammatory e ffec ts. The se

effects are mediated either by direc t binding of theglucocorticoid/GR complex to GRE in the promoterregion of respons ive gene s, or by an inte raction of thiscomplex w ith transcription fac tors such as AP-1 andNF-k B. Glucocorticoids inhibit the ex pression of alarge number of inflammation-assoc iated molecules,including cytokines, chemokines, arachidonic ac idmetabolite s, and adhesion molecules. These e ffec tspredominantly are mediated via inhibition of NF-k Bactivity. In contrast, anti-inflammatory mediators,such as NEP and IL-1 receptor antagonist, often areup-regulated by glucocorticoids. The bene fic ial e ffec tsof glucocorticoid the rapy in asthma is demonstratedby in vivo studie s show ing that treatment of as th-matic patients w ith inhaled glucocorticoids inhibitsthe inflammation of the airw ays and s imultaneouslyimprove s the ir lung func tion. The se effe cts may bemediated in part by modulation of epithelial ce llfunctions, since many studies, both in v itro and inv ivo , have show n that glucocorticoids are able tomodulate the inflammatory func tions of bronchialepithe lial ce lls. Further studies on the mechanism ofaction of glucocorticoids w ill e ventually le ad to thedeve lopment of drugs w hich spec ifically inhibit thetransc ription of inflammatory genes w ithout havingne gative side e ffec ts, and w ill contribute to a moreeffic ient tre atment of asthmatic patients .

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ACKNOWLEDGEMENTS. I acknow le dge Mr T. M. van Os for preparing thefigure .

Received 28 April 1998;accepted 7 May 1998



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GLUCOCORTICOIDS Glucocorticoids: mechanisms of action...

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