The melanocortin 1 receptor is the principal mediator of the effects of agouti signaling protein on mammalian melanocytes

The agouti gene codes for agouti signaling protein (ASP), which is temporal ly expressed in wild-type mouse follicular melanocytes where it induces phe omelanin synthesis. Studies using purified full-length agouti signaling pro tein has shown that it competes with alpha -melanocyte stimulating hormone for binding to the melanocortin 1 receptor. We have investigated whether AS P binds exclusively to the melanocortin 1 receptor expressed on mouse melan ocytes in primary culture, or additionally activates a receptor that has no t been identified yet. We have compared the responses of congenic mouse mel anocytes derived from C57 BL/6J-E+/E+, e/e, or E-so/E-so mice to alpha -MSH and/or ASP. E+/E+ melanocytes express the wild-type melanocortin 1 recepto r, e/e melanocytes express a loss-of-function mutation in the melanocortin 1 receptor that results in a yellow coat color, and E-so/E-so is a mutation that causes constitutive activation of the melanocortin 1 receptor and ren ders melanocytes unresponsive to a-melanocyte stimulating hormone. Mouse E/E+ melanocytes, but not e/e or E-so/E-so melanocytes, respond to agouti si gnaling protein with decreased basal tyrosinase activity, and reduction in levels of tyrosinase and tyrosinase-related proteins 1 and 2, Only in E+/E melanocytes does agouti signaling protein abrogate the stimulatory effects of alpha -melanocyte stimulating hormone on cAMP formation and tyrosinase activity. These results indicate that a functional melanocortin 1 receptor is obligatory for the response of mammalian melanocytes to agouti signaling protein.