α-Melanocyte stimulating hormone and its analogue Nle4DPhe7α-MSH affect morphology, tyrosinase activity and melanogenesis in cultured human melanocytes

Although melanocyte stimulating hormone (MSH) peptides are known to stimulate pigmentation in man, previous reports suggest that human melanocytes are relatively unresponsive to these peptides in vitro. This may be related to the conditions under which the melanocytes were cultured. Thus, we have re-investigated the in vitro effects of MSH peptides using human melanocytes cultured in the absence of artificial mitogens. Human melanocytes were incubated with α-MSH or its potent analogue Nle4DPhe7αMSH for 3 days. After 18 hours, melanocyte morphology had evolved from mainly bipolar to dendritic in approximately 66% of cultures. Nle4DPhe7α-MSH produced doserelated increases in both tyrosinase activity and melanin content although the degree of response was variable and tyrosinase activity was the relatively more responsive to the peptide. Similar results were obtained with α-MSH, but, although the effect on melanin content was similar to that of Nle4DPhe7α-MSH, the effect on tyrosinase activity was less marked. The preliminary EC50 values for the actions of the MSH peptides suggest that they may be equipotent in their actions on human melanocytes. In addition, we have demonstrated that the common melanocyte mitogens 12-O-tetradecanoyl phorbol-13-acetate (TPA) and cholera toxin affect basal melanogenesis and modulate the effects of the MSH peptides. However, not all melanocyte cultures showed melanogenic responses to the MSH peptides. Ability to respond was unrelated to basal levels of tyrosinase activity or melanin content. In at least some cultures, morphological and melanogenic responses appear to be independent of one another. For reasons that are unknown, a small number of human melanocyte cultures show neither morphological nor melanogenic responses to the MSH peptides. Neither peptide had any effect on cell numbers, regardless of whether the cultures were able to respond morphologically or melanogenically. The present findings are particularly interesting in light of recent developments in the molecular biology of the MSH receptor.