Changes in the Proliferation and Differentiation of Neonatal Mouse Pink-Eyed Dilution Melanocytes in the Presence of Excess Tyrosine

Changes in the proliferation and differentiation of epidermal melanocytes derived from newborn mice wild-type at the pink-eyed dilution (p) locus (P/P) and from congenic mice mutant at that locus (p/p) were investigated in serum-free primary culture, with or without the addition of L-Tyr. Incubation with added L-Tyr inhibited the proliferation of P/P melanocytes in a concentration-dependent manner and that inhibition was gradually augmented as the donor mice aged. In contrast, L-Tyr stimulated the proliferation of p/p melanoblasts/melanocytes derived from 0.5 -day-old mice, but inhibited their proliferation when derived from 3.5- or from 7.5-day-old mice. L-Tyr stimulated the differentiation of P/P melanocytes. However, almost all cells were undifferentiated melanoblasts in control cultures derived from 0.5-, 3.5- and 7.5-day-old p/p mice, but L-Tyr induced their differentiation as the age of the donor mice advanced. The content of the eumelanin marker, pyrrole-2,3,5-tricarboxylic acid (PTCA) as well as the pheomelanin marker, 4-amino-3-hydroxyphenylalanine (4-AHP) in p/p melanocytes was greatly reduced compared with P/P melanocytes. However, the content of eumelanin and its precursor, 5,6-dihydroxyindole-2-carboxylic acid (DHICA), as well as the content of pheomelanin and its precursor, 5-S-cysteinyldopa (5-S-CD) in culture media from p/p melanocytes were similar to those of P/P melanocytes at all ages tested. L-Tyr increased the content of eumelanin and pheomelanin 2- to 3-fold in cultured cells and in media derived from 0.5-, 3.5- and 7.5-day-old mice. These results suggest that the proliferation of p/p melanoblasts/melanocytes is stimulated by L-Tyr, and that the differentiation of melanocytes is induced by L-Tyr as the age of the donor mice advanced, even though eumelanin and pheomelanin fail to accumulate in p/p melanocytes and are released from them at all ages of skin development