Tyrosine levels regulate the melanogenic response to alpha-melanocyte-stimulating hormone in human melanocytes: Implications for pigmentation and proliferation

Melanocyte-stimulating hormone (alpha -MSH) increases cytosolic levels of c AMP as well as tyrosinase activity in murine melanocytes. These activities depend upon the presence of melanin precursors and may differ in human mela nocytes. In this study, me demonstrate that high levels of tyrosine (3.7 mM ), the chief melanin precursor, reduced the proliferative effect of alpha - MSH and altered human melanocyte morphology as compared to treatment with l ow (25-30 muM, half-physiological) levels of tyrosine. The anti-proliferati ve effect of high levels of tyrosine was not restricted to alpha -MSH; tyro sine also reduced proliferation induced by forskolin, a direct activator of the cAMP pathway. Exposure to low tyrosine levels and alpha -MSH induced a dendritic morphology; in the presence of high tyrosine and alpha -MSH, mel anocytes displayed large, pigmented cell bodies and less dendricity. Exposu re to alpha -MSH in the presence of low tyrosine for up to 9 days did not a ppreciably increase melanin levels, but culturing the human melanocytes in high levels of tyrosine with alpha -MSH increased melanin levels 10-50-fold , depending on the pigmentation background of the donor. A greater inductio n of melanin accumulation was observed in melanocytes derived from light-sk inned donors than was observed in cells obtained from dark-skinned donors. The poor ability of alpha -MSH to stimulate melanin synthesis was not cause d by a lack of induction of melanogenic proteins, as alpha -MSH increased t he expression of microphthalmia (MITF), tyrosinase, dopachrome tautomerase (DCT), and Pmel-17, compared to untreated cells or cells stimulated by phor bol ester alone, regardless of tyrosine levels. DCT levels were greatly ind uced by low tyrosine with alpha -MSH, but were dramatically decreased by hi gh tyrosine with alpha -MSH. Interestingly, in this same medium thigh tyros ine), MITF levels also decreased after 2 weeks and were barely detectable b y the third meek. Despite the absence of MITF at 3 weeks of treatment in hi gh tyrosine medium, tyrosinase levels remained high, thereby suggesting tha t additional factors must be responsible for tyrosinase transcription in hu man melanocytes. Our results indicate that tyrosine levels can regulate the proliferative activity induced by alpha -MSH, as well as the extent of mel anogenesis in normal human melanocytes. The significance of this work is th at tyrosine levels may be part of the mechanism that switches melanocytes o ut of a proliferative status and into a melanin-synthesizing, terminally di fferentiated phenotype.