Interactions of alpha-melanotropin and agouti on B16 melanoma cells: Evidence for inverse agonism of agouti

alpha-Melanocyte-stimulating hormone (alpha-MSH, alpha-melanotropin) and agouti control the switch between eumelanin and pheomelanin synthesis 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. Both alpha-MSH and agouti induced MCl receptor down-regulation. Agouti antagonized melanogenesis induced by alpha-MSH, forskolin, cholera toxin (CT), and pertussis toxin (PT). It also reduced the constitutive melanin formation of long-term cultures. Cell proliferation was inhibited by agouti (43% at 100 nM). This effect was reversed by alpha-MSH, forskolin, or CT. B16-G4F. cells, a cell variant that lacks the MCl receptor, did not respond to agouti. From these results we conclude that agouti shows the characteristics of an inverse agonist acting through the MCl receptor.