Antagonist and agonist activities of the mouse agouti protein fragment (91-131) at the melanocortin-1 receptor.
Antagonist and agonist activities of chemically synthetized mouse agouti protein fragment (91-131) (AP91-131) at the melanocortin type-1 receptor (MC1-R) were assessed using B 16-F1 mouse melanoma cells in vitro and the following assay systems: (i) receptor binding, (ii) adenylate cyclase, (iii) tyrosinase, (iv) melanin production, and (v) cell proliferation. In competition binding studies AP91-131 was about 3-fold less potent than the natural agonist alpha-melanocyte-stimulating hormone (alpha-MSH) in displacing the radioligand [125I]-[Nle4, D-Phe7]-alpha-MSH (Ki 6.5 +/- 0.8 nmol/l). Alpha-MSH-induced tyrosinase activation and melanin production were completely inhibited by a 100-fold higher concentration of AP9 l -131; the IC50 values for AP91-131 in thetwo assay systems were 91 +/- 22 nM and 95 +/- 15 nM respectively. Basal melanin production and adenylate cyclase activity in the absence of agonist were decreased by AP91-131 with IC50 values of 9.6+/-1.8 nM and 5.0+/-2.4 nM, respectively. This indicates inverse agonist activity of AP91-131 similar to that of native AP. The presence of 10 nM melanin-concentrating hormone (MCH) slightly potentiated the inhibitory activity of AP91-131 in the adenylate cyclase and melanin assays. On the other hand, AP91-131 inhibited cell growth similar to alpha-MSH (IC50 11.0 +/- 2.1 nM; maximal inhibition 1.8-fold higher than that of alpha-MSH). Furthermore, MC1-R was down-regulated by AP91-131 with about the same potency and time-course as with alpha-MSH. These results demonstrate that AP91-131 displays both agonist and antagonist activities at the MC1-R and hence that it is the cysteine-rich region of agouti protein which inhibits and mimics the different alpha-MSH functions, most likely by simultaneous modulation of different intracellular signalling pathways.
Eberle
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AN
Laboratory of Endocrinology, Department of Research, University Hospital and University Children's Hospital, CH-4031 Basel, Switzerland.
Bódi
J
J
Orosz
G
G
Süli-Vargha
H
H
Jäggin
V
V
Zumsteg
U
U
D000262:Adenylyl Cyclases / Q000378:metabolism; D054366:Agouti Signaling Protein; D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D002455:Cell Division / Q000187:drug effects; D036341:Intercellular Signaling Peptides and Proteins; D007700:Kinetics; D008024:Ligands; D008543:Melanins / Q000096:biosynthesis; D008546:Melanoma, Experimental / Q000378:metabolism / Q000473:pathology; D051379:Mice; D008969:Molecular Sequence Data; D014442:Monophenol Monooxygenase / Q000378:metabolism; D010446:Peptide Fragments / Q000138:chemical synthesis / Q000378:metabolism / Q000494:pharmacology*; D011506:Proteins / Q000138:chemical synthesis / Q000378:metabolism / Q000494:pharmacology*; D018043:Receptors, Corticotropin / Q000819:agonists* / Q000037:antagonists & inhibitors* / Q000378:metabolism; D044101:Receptors, Melanocortin; D014407:Tumor Cells, Cultured; D000521:alpha-MSH / Q000819:agonists / Q000037:antagonists & inhibitors / Q000378:metabolism
D054366:Agouti Signaling Protein; D036341:Intercellular Signaling Peptides and Proteins; D008024:Ligands; D008543:Melanins; D010446:Peptide Fragments; D011506:Proteins; D018043:Receptors, Corticotropin; D044101:Receptors, Melanocortin; C515183:a protein, mouse; D000521:alpha-MSH; D014442:Monophenol Monooxygenase; D000262:Adenylyl Cyclases