REGULATION OF MELANOGENESIS IN B16 MOUSE MELANOMA-CELLS BY PROTEIN-KINASE-C

Melanogenesis is regulated by a variety of environmental and hormonal factors. In this study, we showed that protein kinase C (PKC) plays a major role in regulating melanogenesis in B16 mouse melanoma cells. Ch ronic treatment of B16 cells with phorbol dibutyrate resulted in a con centration-dependent loss of density-dependent induction of tyrosinase activity, which correlated positively with a concentration-dependent loss of PKC enzyme activity. In contrast, B16 clones overexpressing PK C alpha had increased tyrosinase activity. Different phorbol derivativ es inhibited tyrosinase activity and depleted cellular PKC alpha in a manner that reflected their reported tumor-promoting activity. Western blotting analysis showed that phorbol dibutyrate decreased the amount of the brown locus gene product (TRP-1) by 50% and lowered the amount of the albino locus gene product (tyrosinase) to undetectable levels. None of the phorbol derivatives affected the level of the slaty locus protein (TRP-2). The decrease in tyrosinase and TRP-1 protein levels was found to be due to a decrease in the mRNA encoded by these genes. In addition to inhibiting the density-dependent increase in tyrosinase activity, phorbol dibutyrate inhibited some, but not all, of the 8-br omocyclic AMP-induced increase in tyrosinase activity. This was accomp anied by a decrease in the amount of tyrosinase protein induced by 8-b romocyclic AMP. Although 8-bromocyclic AMP did not change the level of TRP-1, it did reverse the decrease in the amount of this protein indu ced by phorbol dibutyrate. The amount of TRP-2 was not altered by any of these agents. These data suggest that PKC regulates melanogenesis p rimarily by controlling the constitutive expression of tyrosinase and, to a lesser extent, TRP-1. (C) 1996 Wiley-Liss, Inc.