The effects of co-treatment of C3H10T1/2 (10T1/2) cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 12-O-tetradecanoylphorbol-13-acetate (TPA) on the expression of the novel cytochrome P4501B1 (CYP1B1) were investigated. As monitored by CYP1B1-catalyzed 7,12-dimethylbenzanthracene (DMBA) metabolism, TPA suppressed basal and TCDD-induced DMBA metabolism in a concentration-dependent manner, with a maximum inhibitory concentration of 100 nM. The suppression of CYP1B1 catalytic activity occurred at two time points during which protein kinase C (PKC) was activated and down-regulated in these cells as judged by analyses of cellular PKC content and PKC-inhibitor (chelerythrine chloride)-influenced suppression of CYP1B1 catalytic activity. Experiments in which TCDD and benzanthracene (BA)-induced DMBA metabolism were monitored in PKCbeta1-overexpressing 10T1/2 cells revealed that the suppression of CYP1B1 activity is a consequence of cellular PKC elevation. This suppression phenomenon could be accounted for by PKC-mediated suppression of TCDD-induced CYP1B1 mRNA and apoprotein and of nuclear translocation of the Ah-receptor. In contrast, the mitogen-activated protein kinase (MAPK) proteins ERKs 1 and 2 were stimulated by TCDD under conditions in which PKC was activated. Collectively, our results suggest that PKC participates in the regulation of CYP1B1 in 10T1/2 cells, positively by directly suppressing the Ah-receptor signaling pathway, followed by an indirect or negative activation of the MAPK signaling pathway.