CYCLOSPORINE-A, FH506 AND DITHRANOL ALTER TYROSINE-SPECIFIC PROTEIN-PHOSPHORYLATION IN HACAT KERATINOCYTES

Protein tyrosine kinases (PTKs) are closely related to cell growth, pr oliferation and differentiation. In keratinocytes, various growth fact or receptors and cytosolic proteins, including the EGF and IGF recepto rs, the proteins of the src family and others, exhibit PTK activity. I n psoriatic epidermis an increased level of EGF receptors and their li gand TOP-alpha has been found, and this is thought to be one reason fo r the pathological hyperproliferation of keratinocytes in this disease . Oral treatment with cyclosporin A (CsA) and FK506 or topical treatme nt with dithranol lead to an improvement in psoriasis. In the present study we examined the effect of these three drugs on the cellular cont ent of phosphorylated tyrosines in highly proliferative HaCaT keratino cytes. HaCaT keratinocytes can be used as a model for highly prolifera tive epidermis, e.g. psoriatic epidermis. CsA had no effect whereas FK 506 and dithranol reduced the phosphorylation of tyrosine residues in HaCaT keratinocytes. The activation of serine/threonine protein kinase C (PKC) is known to downregulate PTKs. Therefore we incubated keratin ocytes with the selective PKC inhibitor Po 31-8220 in addition to the other drugs. Only after the addition of Po 31-8220 to FK506-treated ke ratinocytes was the phosphotyrosine (p-tyr) level elevated, but this w as only one-third of the increase measured without additional therapeu tic drugs. We assume that an induction of PKC alone is not responsible for the reduced p-tyr level after treatment with dithranol and FK506. FK506 and CsA affect the p-tyr signal transduction pathways of HaCaT keratinocytes in different ways, indicating distinct mechanisms of act ion of these drugs on keratinocytes.