Uncontrolled proliferation of epidermal cells is the most prominent characteristic of psoriasis. This widespread skin disease can be effectively treated with the microbial substance FK506, which acts by modulating gene expression. We, therefore, asked if the drug changes the expression of genes involved in growth regulation (the mitogenic cytokine interleukin-8 (IL-8) and p53, a negative cell cycle regulator) and signal transduction (protooncogenes c-ras, c-raf, and HER-2). Gene expression was monitored by semiquantitative mRNA-PCR and for p53 by immunocytochemistry in cultured primary keratinocytes (KC). In addition, p53 expression was analysed in skin biopsies of psoriatic patients. After 1-3 hr, IL-8 mRNA levels were dose-dependently decreased in tacrolimus (FK506)-treated cells. Protooncogene expression was not significantly altered. Interestingly, p53 transcription was clearly induced by FK506 treatment. This tendency could be verified on the protein level by immunocytochemistry. In contrast, p53 expression was decreased in lesional psoriatic as compared to normal skin, providing evidence that not only posttranslational modification of the p53 protein, but also transcriptional modulation of the p53 gene, are involved in pathological processes and pharmacological drug action in skin. Together with earlier results showing downmodulation for IL-8 receptor type A expression in cultured KC treated with FK506, these results suggest that both the mitogenic IL-8/IL-8R system and the cell cycle inhibitor p53 represent potential targets for the antipsoriatic action of the drug, whereas protooncogenes acting downstream in mitogenic signal transduction cascades are unaffected. The differential modulation of an entire set of genes provides evidence for the specificity of the drug effects and rules out nonspecific toxic effects on KC.