Cyclosporine A (CsA) is efficacious in the treatment of psoriasis. Although CsA is known to inhibit T-lymphocyte proliferation in vitro, whether this is its mode of action in psoriasis is uncertain. 12-0-tetradecanoylphorbol-13-acetate (TPA) induces an inflammatory, hyperplastic response in mouse skin, with many of the biochemical and histologic aberrations that occur in psoriatic epidermis. Protein kinase C is the major cellular phorbol ester receptor, and most responses of cells to TPA are mediated by PK-C, which is directly activated by TPA. We therefore have investigated the effects of CsA on pleiotypic responses induced by TPA and whether CsA acts in vivo as a direct inhibitor of PK-C. Simultaneous application of CsA (1.7 mumol) and TPA (10 nmol) to mouse skin significantly reduced inflammatory cell infiltration and increased epidermal thickness induced by TPA treatment alone. CsA had to be applied within 30 min of TPA application in order to have a significant inhibitory effect. Optimal doses of CsA inhibited TPA-induced ODC activity, TGase activity, arachidonic acid release, and interleukin-1 beta (IL-1 beta) mRNA to the same degree (approximately 80%), despite measurement at widely different times (30 min-12 h) required to obtain maximal induction by TPA. CsA did not, however, directly inhibit activation of PK-C by TPA. These data demonstrate that CsA blocks the pleiotypic responses of mouse skin to TPA treatment involving biochemical events, inflammatory cell infiltration, and epidermal hyperplasia. The molecular site(s) of action of CsA appears to be distal to the initial activation of PK-C by TPA and clearly inhibits PK-C inducible events. Furthermore, the above data suggest that CsA may directly affect keratinocytes in vivo.