title: Interactions of alpha-melanotropin and agouti on B16 melanoma cells: evidence for inverse agonism of agouti.
abstract: 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 (Kd 3.7 nmol/l) and alpha-MSH (Kd 2.3 nmol/l) had similar affinities to the MC1 melanocortin receptor. Both alpha-MSH and agouti induced MC1 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 MC1 receptor, did not respond to agouti. From these results we conclude that agouti shows the characteristics of an inverse agonist acting through the MC1 receptor.
authors.affiliation: Department of Research, University Hospital, Basel, Switzerland.
authors.forename: W
authors.initials: W
authors.lastname: Siegrist
authors.forename: R
authors.initials: R
authors.lastname: Drozdz
authors.forename: R
authors.initials: R
authors.lastname: Cotti
authors.forename: D H
authors.initials: DH
authors.lastname: Willard
authors.forename: W O
authors.initials: WO
authors.lastname: Wilkison
authors.forename: A N
authors.initials: AN
authors.lastname: Eberle
mesh_terms: D054366:Agouti Signaling Protein; D000818:Animals; D000970:Antineoplastic Agents / Q000378:metabolism*; D002455:Cell Division; D015536:Down-Regulation; D036341:Intercellular Signaling Peptides and Proteins; D008543:Melanins / Q000096:biosynthesis; D009074:Melanocyte-Stimulating Hormones / Q000037:antagonists & inhibitors; D008546:Melanoma, Experimental / Q000378:metabolism*; D051379:Mice; D011506:Proteins / Q000378:metabolism*; D011979:Receptors, Pituitary Hormone / Q000378:metabolism; D015398:Signal Transduction; D014407:Tumor Cells, Cultured; D000521:alpha-MSH / Q000378:metabolism*
chemical_list: D054366:Agouti Signaling Protein; D000970:Antineoplastic Agents; D036341:Intercellular Signaling Peptides and Proteins; D008543:Melanins; D011506:Proteins; D011979:Receptors, Pituitary Hormone; C515183:a protein, mouse; D000521:alpha-MSH; C029446:MSH receptor; D009074:Melanocyte-Stimulating Hormones