Reduced proliferative and differentiative activity of mouse pink-eyed dilution melanoblasts is related to apoptosis

The mouse pink-eyed dilution (p) locus is known to control the melanin content, melanosome morphology, and tyrosinase activity in melanocytes. However, it is not well known whether the p allele is involved in regulating melanocyte proliferation, differentiation, and death. The aim of this study is to investigate in detail the role of the p allele in melanocyte proliferation, differentiation, and death using a cell culture system. The epidermal cell suspensions of the neonatal dorsal skin derived from wild type mice at the p locus (black, C57BL/10JHir-P/P) and their congenic mutant (pink-eyed dilution, C57BL/10JHir-p/p) were cultured with serum-free melanoblast-proliferation medium (MDMDF) and melanocyte-proliferation medium (MDMD). The proliferation and differentiation of p/p melanoblasts in MDMDF or MDMD were greatly inhibited compared with those of P/P melanoblasts and melanocytes. It is possible that apoptosis is related to the reduced proliferative and differentiative activity of p/p melanoblasts/melanocytes. The addition of apoptosis-inhibitors, such as caspase-9 inhibitor (C9I) and Bax-inhibiting peptide (BIP) into MDMDF or MDMD stimulated the proliferation and differentiation of p/p melanoblasts. In contrast, in P/P melanoblasts and melanocytes, C9I and BIP failed to stimulate their proliferation and differentiation. The number of apoptotic keratinocytes and melanoblasts/melanocytes in p/p mice was greater than in P/P mice. Moreover, expression of C9 and Bax in keratinocytes and melanoblasts/melanocytes in p/p mice was greater than in P/P mice. These results suggest that the increased apoptosis in keratinocytes and melanoblasts/melanocytes is related to the reduced proliferative and differentiative activity of p/p melanoblasts.