INTERACTIONS OF ALPHA-MELANOTROPIN AND AGOUTI ON B16 MELANOMA-CELLS -EVIDENCE FOR INVERSE AGONISM OF AGOUTI

alpha-Melanocyte-stimulating hormone (alpha-MSH, alpha-melanotropin) a nd agouti control the switch between eumelanin and pheomelanin synthes is in mammalian melanocytes. Here we investigated interactions between alpha-MSH, agouti protein, cAMP elevating agents and phorbol ester on mouse B16 melanoma cells. Agouti (K-d 3.7 nmol/l) and alpha-MSH (K-d 2.3 nmol/l) had similar affinities to the MCl melanocortin receptor. B oth alpha-MSH and agouti induced MCl receptor down-regulation. Agouti antagonized melanogenesis induced by alpha-MSH, forskolin, cholera tox in (CT), and pertussis toxin (PT). It also reduced the constitutive me lanin formation of long-term cultures. Cell proliferation was inhibite d by agouti (43% at 100 nM). This effect was reversed by alpha-MSH, fo rskolin, or CT. B16-G4F. cells, a cell variant that lacks the MCl rece ptor, did not respond to agouti. From these results we conclude that a gouti shows the characteristics of an inverse agonist acting through t he MCl receptor.