Defining MC1R Regulation in Human Melanocytes by Its Agonist α-Melanocortin and Antagonists Agouti Signaling Protein and β-Defensin 3

The melanocortin 1 receptor (MC1R), a G(s) protein-coupled receptor, has an important role in human pigmentation. We investigated the regulation of expression and activity of the MC1R in primary human melanocyte cultures. Human beta-defensin 3 (HBD3) acted as an antagonist for MC1R, inhibiting the alpha-melanocortin (alpha-melanocyte-stimulating hormone (alpha-MSH))-induced increase in the activities of adenylate cyclase and tyrosinase, the rate-limiting enzyme for melanogenesis. alpha-Melanocortin and forskolin, which activate adenylate cyclase, and 12-O-tetradecanoylphorbol-13-acetate, which activates protein kinase C, increased, whereas exposure to UV radiation reduced, MC1R gene and membrane protein expression. Brief treatment with alpha-MSH resulted in MC1R desensitization, whereas continuous treatment up to 3 hours caused a steady rise in cAMP, suggesting receptor recycling. Pretreatment with agouti signaling protein or HBD3 prohibited responsiveness to a-MSH, but not forskolin, suggesting receptor desensitization by these antagonists. Melanocytes from different donors expressed different levels of the G protein-coupled receptor kinases (GRKs) 2, 3, 5, and 6, as well as beta-arrestin 1. Therefore, in addition to the MC1R genotype, regulation of MC1R expression and activity is expected to affect human pigmentation and the responses to UV.