INVOLVEMENT OF MICROPHTHALMIA IN THE INHIBITION OF MELANOCYTE LINEAGEDIFFERENTIATION AND OF MELANOGENESIS BY AGOUTI SIGNAL PROTEIN

In mouse follicular melanocytes, production of eumelanins (brown-black pigments) and pheomelanins (yellow-brownish pigments) is under the co ntrol of two intercellular signaling molecules that exert opposite act ions, alpha-melanocyte-stimulating hormone (alpha MSH) which preferent ially increases the synthesis of eumelanins, and agouti signal protein (ASP) whose expression favors the production of hair containing pheom elanins. In this study, we report that ASP does not only affect mature melanocytes but can also inhibit the differentiation of melanoblasts. We show that both alpha MSH and forskolin promote the differentiation of murine melanoblasts into mature melanocytes and that ASP inhibits this process. We present evidence that the expression of a specific me lanogenic transcription factor, microphthalmia, and its binding to an M box regulatory element, is inhibited by ASP. We also show that, in B 16 murine melanoma cells, ASP inhibits alpha MSH-stimulated expression of tyrosinase, tyrosine-related proteins 1 and 2 through an inhibitio n of the transcription activity of their respective promoters. Further , ASP inhibits alpha MSH-induced expression of the microphthalmia gene and reduces the level of microphthalmia in the cells. Our data demons trate that ASP can regulate both melanoblast differentiation and melan ogenesis, pointing out the key role of microphthalmia in the control o f these processes.