R eview K E Y WORDS keratinization, psoriasis keratins, desmosome, cornified cell envelope, epidermal lipids, keratinocyte differentiation and activation Keratinization and psoriasis Keratinization arul psorias-is A. Kansky - --- --- --- --- - ------- --- -- S UM M ARY The modem concept of the epithelial keratinization includes at least five groups of biologic substrates and the respective molecular processes involved: 1. Keratin, 2. Oesmosomes, 3. Cornified cel/ envelope, 4. Lipids of the horny layer, 5. Signal reception transduction and transcription at the cellular level are also involved in the process of keratinization. Keratins are forming the cytoskeleton of the epidermal cells. Their structure has been extensively studied. A number of anomalies in the structure of keratins K1, K2e, K5, K6a, K9, K1 O and K14 have been recognized as causes ot various hereditary disorders ot keratinization. Mutations in keratin genes are responsible tor these events. In psoriatic lesions an increased expression of keratins K6, K16 and K17 was observed. Oesmosomes and its constituents are mainly responsible tor the intercellular adhesion specially in the basal and squamous layers of the epidermis. Deficient desmosomes and their constituents are responsible tor the pathology in Mb Darier, in tamilial benign pemphigus (Mb Hailey-Hailey) as well as in certain acquired bullous dermatoses. Cornified cel/ envelope (GEJ has been only lately recognized as an important structure enabling a normal barrier tunction. Hereditary deficiency ot the enzyme transglutaminase 1 linked to chromosome 14, has been made responsible tor about a half the cases ot lamellar ichthyosis. The Vohwinkel syndrome has been linked to a genetic anomaly of loricrin, a constituent of CE. Epidermal lipids play an important role in ensuring a normal barrier function ot str. corneum: inhibition ot penetration of foreign substances, transepidermal water loss. Ceramides and cholesterol are mainly responsible for the regeneration ot the barrier function tollowing exposition to solvents and detergents. Splitting of cholesterol esters is important for normal shedding of str. corneum. Activation of keratinocytes. The above mentioned biologic mechanisms are regulated by the very subtle processes at the molecular level. Two physiologic pathways are open to keratinocytes, differentiation and activation. Receptors on the cell membrane are accepting signals which are then transmitted through the cytoplasm to the nucleus and to the effectors. Transducing molecules and transcription factors are active in this process. acta dermatovenerologica A.PA. Vol 8, 99, No 3 ------- ------ ---- - ---- - --- --- 89 Keratinization and psoriasis Introduction In spite of the large amount of tirne and energy dedicated to the research of psoriasis it was stili not possible to solve the crucial problem concerning the etiology of this disorder. The aim of the present manuscript is to review the new data on the process of keratinization and to try singling out the items relevant for pathogenesis of psoriasis. The keratinization process in a broader sense does not involve strictly keratins , but includes also desmosomes, cornified celi envelopes, epidermal lipids as well as the signaling system concerning the epidermal cells (keratinocytes). In such a context genetics are also to be at least shortly mentioned. Keratinization The cytoskeleton of mammalian epidermal cells is composed of a three-filament system: microfilaments, micro tubules and intermediate fil aments (IF). IF represent the main component of the cytoskeleton, they are composed of keratins , which are polipeptide chains predominantly in a helical configuration (1,2). They are usually subdividecl into the smaller acidic type I keratins (KlO to K19) with a molecular weight of 40-56 kD (3) . Type II keratins (Kl to K9) are larger, neutral to basic w ith a molecular weight 52-67 kD. Within a keratin molecules a central rod domain of helical configuration w hich is pretty constant in all epithelia, and two lateral non -helical domains are to be distinguished . A schematic presentation of a keratin molecule is given in Figure l. Epitheli a l keratins are coexpressed in sp ecific pairings which consist of each one type I and type II molecules. In the basal cel! layer KS and K14 and in the squamo us layer Kl ancl KlO are coexpressecl, while in [0 . LI plantar skin K2 and K9 are also expressed. In psoriasis the expression of Kl ancl KlO is cle- creasecl in the spinous layer, while these keratins are replacecl by hyperproliferation associatecl keratins K6, Kl6 ancl Kl 7. Other less reproducible changes inclucle an increased expression of K7, Kl3 and Kl 7 (3) . Corn,jied cell envelope The cornified cel! envelope (CE) is present in keratinizing, terminally clifferentiatecl keratinocytes of the stratum corneum (SC) . It is a 7-15 mm thick structure on the inner sicle of the cel! membrane, composed of many cross-linkecl proteins: involucrin, cystatin, small protin rich proteins, loricrin, trichohyaline, filaggrin, keratin intermediatefilaments and elafin ( 4,5) as well as SI OOAl l and 1 O, annexin and plasminogen activa- tor inhibitor-2 (6). Together with the lipicls from the SC they are responsible for the barrier function (2). The problem has been lately discussecl in this journal (7) . Figure 2. In psoriasis the activity of the enzyme transgluta- minase 1 (TGM 1) as well as the synthesis of involucrin which rnay be normally localized to the granular layer (stratum granulosurn, SG) appear already in the lower- most portions of the spinous layer (8) . The TGM 1 activity is three to seven times that of normal skin (2) . It rnay be safely assurned that the rnentionecl shift in TGM 1 activity and involucrin synthesis rnay be responsible together with expression of the proliferating keratins (K6, Kl6) for the acceleratecl rate and incomplete keratinocyte rnaturation. It deserves to be rnentionecl that transcription of the TGM 1 gene can be activated in cultures of normal human keratinocytes by calcium ions, by protein kinase C and also by cholesterol sulfate (9). At the mo ment it is open to speculations if such rnechanisms coulcl be operative in psoriasis . N terminal lateral domain central rod domain 310-315 amina acids C tenninal lateral domain Figure 1. Simplified presentation of a keratine molecule Central rod domains:1 A, 1 B, 2A, 28: - main parts; L 1,L 1,2, L2: Lateral domains: E1 , V1, H1 - linkers R ev i ew 90 acta dermatovenerologica A.P.A. Vol 8, 99, No 3 R ev i ew Stratum corneum lipids The epidermal lipids are important for a proper functioning of the epidermis. In the basal and spinous layers the lipids are mainly components of the cel! membranes, in the upper SG they are contained in the lamellar bodies (LB, Oclland bodies), while in se they are present mainly as intercellular lip id bilayers. It was reportecl that in epidermis of atopic patients the content of phospholipicls was nearly twice as high as in healthy epidermis (10) . During the last few years a special attention has been paid to ceramides (1 1). Accorcling to Lavrij sen (1 2) the relative amou nt of ceramicle fra ctions was diffe re nt in patients w ith lamellar ichthyosis compared to normals; the relative amount of free fatty acicls was also lower. Steinert ancl Marekov stressecl the importance of the lipid envelope surroun- cling the e E of corneocytes (5) . It seems that it is com- posecl mainly of ceramides, a special role being attri- buted to the w-hydroxy-ceramides (13). eeramides are supp osecl also to be important in the keratinocyte signaling system. Barrier junction If the s e is exposecl to solvents or injurecl by tape stripping the epiclermal lipicls ancl the eE become Keratinization and psoriasis clamagecl, an increasecl permeation of substances from outside and an increased transepidermal water loss (TEWL) are observed at the injured skin site . Elias and his group have shown that a deficiency in essential, unsaturated fatty acids (linoleic and lanolenic acids) in food administered to nude mice caused symptoms simi- lar to ichthyosis and an increased TEWL. By including essential fa tty acids into the food, a goocl recovery was achieved (1 4). Ad clitio nally to acylglycerols and ceramicles cholesterol also seems to play an important role in the maintenance of the proper functioning of the barrier (9). In every-day life the skin of many people is exposecl to an abuse of detergents, solvents, abrasive substances, unfavorable weather conditions and further situations exhibiting detrimental effects in respect to the se barrier. Por this reason quite a few groups of investigators are searching for solutions how to improve barrier recove1y by applying adequate creams or ointments. Ghadially et al. have been able to show in aged mice after barrier disruption, that by application of an equimolar mixture of ceramides , cholesterol, linoleic acid and nonessential fatty acids, a goocl recovery of the barrier function was achieved after six hours (15) . In another article Elias and his grou p showed by using a three-lipid component system that either linoleic or palmitic acicls normalizecl the barrier function , thus proving that the structural requirement for free fatty acicls in the complete lipid mixture is not restrictecl to essential fatty acicls (16) . -~ ~-- EL ~ ""'~ CER IA-n-'r'trtt- PM '1"rt-'rr11- CE Figure 2. Simplified presentation of a human corneocyte structure EL: epidermal lipids (bilayers) CER: lipid envelope consisting mainly of ceramides, covalently bound to PM CE: cornified cell envelope PM: plasma membrane FG: filaggrin IF: intermediate filaments (keratins) acta dermatovenerologica A.P.A. Vo l 8, 99, No 3 91 Keratinization and psoriasis Activation oj keratinocytes Since the pioneer studies of the epidermal celi kinetics by Van Scott (17) it is known that the production of epidermal cells in psoriasis is increased and that the transit tirne from basal layer to SC is shortened about four times. Only recently it was realized however that keratinocytes not only react to, but also produce an abundance of cytokynes, chemoreactants and growth factors. Keratinocytes express receptors for many polypeptide factors and thus respond to various signals from the immune system as well as to autocrine stimu- lation. The signaling between keratinocytes and lymphocytes has been shown in psoriasis , delayed type hypersensitivity, atopic dermatitis and cutaneous T-cell lymphomas (18,19). Keratinocyte di/ferentiation is a complex process indicating the transformation of the basal celi into corneocyte. It includes various systems: different expression of keratins, transformation of keratohyline through profilaggrin to filaggrin as well as others. Keratinocyte activation designates primarily a response of keratinocytes to various stimuli e.g., extracellular signaling molecules, injuries, UV light, stress. The signals, which induce keratinocytes to start differen- tiating or become activated, are subject of intensive investigation. One well-known signal of activation is the release qfinterleukin 1 (IL-l)which is prestored in the cytoplasm of keratinocytes. Activated keratinocytes in the spinous layer can start to produce K6, K16 and Kl 7 instead of the normally present Kl and KlO (20). Signaling molecules bind to receptors on the celi membrane and activate various systems of transducing molecules (mainly kinases) in the cytoplasm which are then transmitting signals to the transcription factors in the nucleus. These in their turn activate segments of DNA and/or specific RNA molecules responsible for transcription of specific genes. At least three signaling pathways receiving signals from the extracellular envi- ronment are known to be important for keratinocyte activation: IFN-y, the EGF family and TNF-a/IL-1 (20). It was shown that IFN-y is a critical element in the induc- tion of keratinocyte proliferation on psoriasis (21). A significant proportion of lesional T cells in psoriatic epidermis and dermis , represent increased numbers of both IFN-y CD4+ and IFN-y CD8+ present in appropriate anatomk location to sustain the lesional pathology (22). From these and further data it is possible to deduce that the IFN-y signaling pathway is important for keratinocyte activation in psoriasis. One may hope that further studies of CD4+ and CD8+ cells , which are a major source of IFN-y in chronic psoriatic plaques, would provide clues for an efficient treatment. The role of genetics in the development of psoriasis remains to be additionally clarified, it seems that a number of genes are involved in the pathogenesis (23,24): class I and II MHC loci on chromosome 6p; keratin class II cluster of genes on chromosome 12q (K2,K6); keratin class I cluster of genes on chromosome 17p (Kl 6,Kl 7); epidermal differentiation complex on chromosome lq; lately the genetic defect was located to a locus on chromosome 17q. 1. Bowden PE. Keratins and other epidermal proteins. Molecular Aspects of Dermatology. Pristley GC ed, Wiley and Sons, 1993. 2. Smack DPH, Korge BP, James WD. Keratin and keratinization. J Am Acad Dermatol 1994;30:85-102, 3. Moll R, Franke WW, Schiller DL. _The catalogue of human cytokeratins: patterns and expression in normal epithelia, tumors and cultured cells. Cell 1982;31:11-24. 4. Sun TT, Green H. Differentiation of the epidermal keratinocyte in cell culture. Formati on of the cornified cell envelope. Cell 1976;19:511-21. 5. Steiner! PM, Marekov LN. The proteins Elafin, Filaggrin, Keratin Intermediate Filaments, Loricrin and Small Prolin-rich Proteins 1 and 2 are Isopeptide Cross-linked Components of the Human Epidermal Cornified Cell Envelope. J Biol Chem 1995;270: 17702-11. 6. Robinson NA, Lapic S, Welter JF et al. SlO0All, SlO0AlO, Annexin I, Desmosomal Proteins, Small prolin-rich proteins, Plasminogen Activator Inhibitor-Z and Involucrin are components of the Cornified Envelope of Cultured Human Epidermal Keratinocytes. J Biol Chem 1997;272:12035-46. 7. Kansky A, Kristl J. Importance of epidermal lipids for proper functioning of the stratum corneum barrier. Acta Dermatovenerol APA 1998:7:3-10. 8. Bernard BA, Reano A, Darmon YM et al. Precocious appearence of involucrin and epidermal TGM during differentiation of psoriatic skin. Br J Dermatol 1986;114:279-83. Review 92 acta dermatovenerologica A.P.A. Vol 8, 99, No 3 Review AUTHOR'S ADDRESS Keratinization and psoriasis 9. Kawabe Sh, Ikuta T, Ohba Metal. Cholesterol sulfate activates transcription of transglutaminase 1 gene in normal human keratinocytes. J Invest Dermatol 1998; 111: 1098-102. 1 O. Schaefer L, Kragballe K. Abnormalities in Epidermal Llpid Metabolism in Patients with Atopic Dermatitis. J InvestDermatol 1991;96:10-15. 11. Chujor SN, Seki T, Feingold et al. Glucosylceramide formation is critical for epidermal permeability barrier homeostasis. J Invest Dermatol 1998;11 l : IID Abstract 202. 12. Lavrijsen APM, Bouwstra JA, Gooris GS et al. Reduced Skin Barrier Function Parallels Abnormal Stratum Corneum Llpid Organization in Patientswith Lamellar Ichthyosis. J InvestDermatol 1995:105:619- 24. 13. Bene M, Ucluda Y, Seld T et al. Direct evidence that omega-hidroxy ceramides are important for epidermal barrier function. J Invest Dermatol 1998; 111: IID Abstract 203. 14. Elias PM, Cooper ER, Korc A, Brown BE. Percutaneous Transport in relation to Stratum Corneum Structure and Llpid Composition. J Invest Dermatol 1981;76:297-301 15. Ghadially R, Brown BE, Hanley K et al. Decreased Epidermal Lipid Synthesis Accounts far Altered Barrier Function in Aged Mice. J Invest Dermatol 1996; 106: 1064-9. 16. Mao-Qiang M, Feingold KR, Thornfeldt CR, Elias PM. Optimization on physiological lipid mixtures far barrier repait'. J Invest Dermatol 1996;106:1096-101. 17. Van Scott EJ, Eckel TM. Kinetics of Hyperplasia in Psoriasis. Arch Dermatol 1963;88:373-80. 18. NickoloffBJ, Griffiths CE, Bar ker JN. The Role of Adhesion Molecules chemotactic Factors and Cytokines in Inllan1matory and Neoplastic Skin Diseases - 1990 Update. J Invest Dermatol 1990;94: 1515-75. 19. Kamine M, Freedberg IM, Blumenberg M. The activated keratinocytes. Acta Dermatoven APA 1995;4: 169-73. 20. Blumenberg M. Keratinocyte differentiation and activation. Acta Dermatoven APA 1997;6: 127-35. 21. Bata-Csorgo Z, Hammerberg C, VoorheesJJ, Cooper KD. Kinetics and regulation ofhuman keratinocyte stem cel! growth in short-term primary ex vivo culture. Cooperative growth factors from psoriatic lesional T lymphocytes stimulate proliferation among psoriatic uninvolved, bul not normal keratinocytes. J Clin Invest 1995;95:317-27. 22. Szabo SS, Hanunerberg C, Yoshida Y et al. Identification and quantification of interferon-g producing T cells in psoriaticlesions: localization of both CD4• and CD8• subsets. J InvestDermatol 1998; 111: 1072- 8. 23. Henseler T. The genetics of psoriasis. J Am Acad Dermatol 1997;3 7:Suppl Sl-11. 24. Bhalerao J, BoucockAM, Menter MA et al. Genes in psoriasis.Roenigk HH, Maibach M. Psoriasis, 3rd ed, M Dekker, New York, 1998;177-85. Aleksej Kansky MD, PhD, projessor oj dermatology, Department oj Dermatology, University Medical Centre, Zaloška 2, 1525 Ljubljana, Slovenia acta dermatovenerologica A.P.A. Vol 8, 99, No 3 --- - - - -------- --- ---- - - - ------ 93