Treatment of murine skin and cultures of murine epidermal or hepatoma cells with 12-0-tretradecanoylphorbol-13-acetate (TPA) reduces basal cytochrome P450-dependent monooxygenase activities, and suppresses the rise in the monooxygenase 7-ethoxyresorufin O-deethylase (7-ERD) that normally occurs after treatment with P450 inducers. TPA is a potent activator of protein kinase C (PKC). The overall hypothesis of the proposal is that PKC regulates the expression/activities of some P-450 species. This hypothesis will be examined by analyzing the effects of PKC inhibitors and PKC down regulation on the TPA-dependent suppression of 7-ERD induction in the aforementioned systems. PKC activators structurally unrelated to TPA, and analogs of TPA incapable of activating PKC will also be surveyed for their abilities to modulate 7-ERD (cytochrome CYPIA1) expression. Analyses of 7-ERD induction in cultured epidermal and hepatoma cells will also be made following reduction or elevation of cellular PKC activities by transfection of PKC antisense oligodeoxynucleotides or PKC cDNA expression vectors, respectively. Antibodies and cDNA probes to P-450IAl will be used to determine whether the TPA-mediated effects on 7-ERD activity reflect transcriptional or post-transcriptional regulation. If steady state RNA levels are affected by TPA, rates of IA1 transcription and IA1 mRNA half lifes will be determined. Relevance of PKC mediated suppression of IA1 induction to the processes of in vivo xenobiotic metabolism and chemical carcinogenesis will be assessed by analyses of epidermal DNA-adducts and the numbers of skin tumors that develop in an initiation-promotion protocol employing benzo[a]pyrene as the initiator. Lastly, the effects of TPA and other PKC activators on the inductions of P-450 cytochromes IA1, IA2, IIB1, IIB2, and IIE1 in rat liver will be determined to assess the generality of PKC as a regulator of P450 expression. These studies will characterize a new and novel mechanism for the regulation of the P-450 system, and may proyide a general explanation for the suppressive effects of a variety ot agents on the P-450 system.