Significance of the melanocortin 1 receptor in regulating human melanocyte pigmentation, proliferation, and survival

The characterization of the melanocortin 1 receptor (MC1R) expressed on human melanocytes and the findings that certain mutations in the POMC gene or the MC1R gene result in red hair phenotype underscore the significance of melanocortins and MC1R in regulating human pigmentation. We demonstrated that human melanocytes respond to alpha-melanocortin (alpha-MSH) or ACTH with increased proliferation and melanogenesis, and to agouti signaling protein by abrogation of these effects. alpha-MSH and ACTH were equipotent and more potent than beta-MSH, and gamma-MSH was the least potent in activating the MC1R and stimulating melanogenesis and proliferation of human melanocytes. We characterized the MC1R genotype in a panel of human melanocyte cultures and identified three cultures that were homozygous for Arg160Trp, heterozygous for Arg151Cys and Asp294His, and heterozygous for Arg160Trp and Asp294His substitutions, respectively. Those cultures failed to respond to alpha-MSH with increase in cAMP levels, tyrosinase activity, or proliferation and had an exaggerated response to the cytotoxic effect of ultraviolet (UV) radiation. These loss-of-function mutations have been associated with red hair phenotype and increased risk for skin cancer. Melanocytes homozygous for Val29Met substitution in MC1R responded normally to alpha-MSH and UVB, suggesting that this variant is a polymorphism. We observed that alpha-MSH promotes human melanocyte survival by inhibiting the UV-induced apoptosis independently of melanin synthesis. This effect was absent in human melanocytes with loss of function MC1R mutations. We predict that the survival effect of alpha-MSH is caused by reduction of UV-induced DNA damage and contributes to the prevention of melanoma.