Defining MC1R Regulation in Human Melanocytes by Its Agonist ?-Melanocortin and Antagonists Agouti Signaling Protein and ?-Defensin 3
May 2012

Abstract
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 ?-defensin 3 (HBD3) acted as an antagonist for MC1R, inhibiting the ?-melanocortin (?-melanocyte-stimulating hormone (?-MSH or Melanotan 2))-induced increase in the activities of adenylate cyclase and tyrosinase, the rate-limiting enzyme for melanogenesis. ?-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 ?-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 ?-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 ?-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.