In B16 melanoma cells, the inhibition of melanogenesis by TPA results from PKC activation and diminution of microphthalmia binding to the M-box of the tyrosinase promoter.

In B16 melanoma cells, cAMP-induced melanogenesis is inhibited by the tumor promoting phorbol ester, TPA. However, the role of PKC activation or depletion in the inhibition of melanogenesis by TPA remains controversial. In this report, using specific PKC inhibitors, we demonstrated that PKC inhibition does not impair cAMP-induced melanin synthesis and tyrosinase expression. Further, the inhibition of melanogenesis by TPA results from a decrease of the tyrosinase promoter transcriptional activity and this effect is mimicked by over-expression of a constitutively active form of PKC alpha. These findings clearly demonstrate that PKC activation accounts for the inhibition of melanin synthesis by TPA. Additional experiments were undertaken to elucidate the mechanism by which TPA inhibits the tyrosinase gene transcription. Deletions and mutation in the tyrosinase promoter showed that TPA acts on a M-box which is involved in tissue-specific expression and regulation by cAMP of the tyrosinase gene. We showed that TPA decreases the binding of microphthalmia, a basic helix-loop-helix transcription factor, to the M-box. Since microphthalmia, strongly stimulates the transcriptional activity of the promoter we propose that TPA, through PKC activation, decreases microphthalmia binding to the M-box of the tyrosinase promoter, thereby leading to a reduced tyrosinase expression and melanogenesis inhibition.