Mechanism of regulation of melanin synthesis by coat color genes and ultraviolet radiation in mice and human

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In black melanocytes (Dct^+), almost all melanosomes were elliptical stage IV melanosomes. However, in slaty (Dctslt) melanocytes, numerous spherical stage III melanosomes with globular depositions of pigment in addition to elliptical stage III melanosomes with intralumenal fibrils were observed. In slaty melanocytes, spherical melanosomes were gradually decreased after birth, whereas elliptical melanosomes were gradually increased. Stage IV melanosomes were very few in slaty melanocytes, and were not increased after birth. L-tyrosine increased the elliptical stage IV melanosomes, and eumelanin and pheomelanin synthesis. These results suggest that the slaty mutation blocks the melanosome maturation at stage III and affects the melanosome morphology and L-tyrosine restores these abnormalities.The recessive yellow (Mclre) melanocytes produced mainly eumelanin in our culture system. The proliferation of recessive yellow melanocytes was decreased compared with that of wild-type at the e-locus, black melanocytes. The differentiation of melanocytes was also delayed and inhibited in recessive yellow mice. Tyrosinase (TYR) activity and TYR-related protein 1 (TRP1) and TRP2 (dopachrome tautomerase, DCT) expressions were decreased and, in addition, the maturation of stage IV melanosomes was inhibited. Excess L-tyrosine (Tyr) added to the culture media rescued the reduced activity of proliferation of melanocytes. L-Tyr also stimulated TYR activity and TRP1 and TRP2 expressions as well as the maturation of stage IV melanosomes and pigmentation. These results suggest that the Mclre mutation affects the proliferation and differentiation of melanocytes and L-Tyr rescues the reduced proliferative and differentiative activities by stimulating TYR activity and TRP1 and TRP2 expressions as well as melanosome maturation.