DIFFERENT CIS-ACTING ELEMENTS ARE INVOLVED IN THE REGULATION OF TRP1 AND TRP2 PROMOTER ACTIVITIES BY CYCLIC-AMP - PIVOTAL ROLE OF M-BOXES (GTCATGTGCT) AND OF MICROPHTHALMIA

In melanocytes and in melanoma cells, cyclic AMP (cAMP)-elevating agen ts stimulate melanogenesis and increase the transcription of tyrosinas e, the rate-limiting enzyme in melanin synthesis. However, two other e nzymes, tyrosinase-related protein I (TRP1) and TRP2, are required for a normal melanization process leading to eumelanin synthesis. In B16 melanoma cells, we demonstrated that stimulation of melanogenesis by c AMP-elevating agents results in an increase in tyrosinase, TRP1, and T RP2 expression, cAMP, through a cAMP-dependent protein kinase pathway, stimulates TRP1 and TRP2 promoter activities in both B16 mouse melano ma cells and normal human melanocytes. Regulation of the TRP1 and TRP2 promoters by cAMP involves a hi box and an E box. Further, a classica l cAMP response element-like motif participates in the cAMP responsive ness of the TRP2 promoter, demonstrating that the TRP2 gene is subject ed to different regulatory processes, which could account for its diff erent expression patterns during: embryonic development or under speci fic physiological and pathological conditions. We also found that micr ophthalmia, a basic helix-loop-helix transcription factor, strongly st imulates the transcriptional activities of the TRP1 and TRP2 promoters , mainly through binding to the M boxes. Additionally, we demonstrated that cAMP increases microphthalmia expression and thereby its binding to TRP1 and TRP2 M boxes. These convergent and compelling results dis close at least a part of the molecular mechanism involved in the regul ation of melanogenic gene expression by cAMP and emphasize the pivotal role of microphthalmia in this process.