In melanocytes and in melanoma cells, upregulation of melanogenesis, by cAMP elevating agents, results from a stimulation of tyrosinase activity that has been ascribed to an increase in tyrosinase protein and messenger amount. However, the mechanism by which cAMP elevating agents increase tyrosinase mRNA remains to be elucidated. In this study, using a luciferase reporter plasmid containing the 2.2-kb fragment 5' of the transcriptional start site of the mouse tyrosinase gene, we showed that cAMP elevating agents lead to a strong stimulation (20-fold) of transcriptional activity of the tyrosinase promoter. Deletions and mutations in the mouse tyrosinase promoter showed that the M-box 70-bp upstream from the TATA-box and the E-box located downstream the TATA-box, near to the initiator site, are involved in the regulation of the tyrosinase promoter activity by cAMP. Additionally, we showed that microphthalmia, a b-HLH transcription factor associated with pigmentation disorders in mouse, binds to these regulatory elements and modulates the transcriptional activity of the tyrosinase promoter. Since cAMP stimulates the binding of microphthalmia to the M-box and to the E-box; it is tempting to propose that microphthalmia, through its interaction with cis-acting elements surrounding the TATA-box, plays a key role in the regulation of the mouse tyrosinase gene expression by cAMP.

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