Metal Ion-mediated Agonism and Agonist Enhancement in Melanocortin MC1 and MC4 Receptors

• Published in: The Journal of biological chemistry Vol. 277; no. 49; pp. 47662 - 47670
• Main Authors: Holst, Birgitte, Elling, Christian E., Schwartz, Thue W.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 06.12.2002; American Society for Biochemistry and Molecular Biology
• Subjects: Amino Acid Sequence; Animals; Binding, Competitive; Brain - metabolism; COS Cells; Cyclic AMP - metabolism; Cysteine - chemistry; DNA, Complementary - metabolism; Dose-Response Relationship, Drug; Humans; Inhibitory Concentration 50; Ions; Kinetics; Ligands; Metals - chemistry; Mice; Models, Chemical; Molecular Sequence Data; Mutation; Peptides; Protein Binding; Protein Structure, Secondary; Receptor, Melanocortin, Type 4; Receptors, Corticotropin - agonists; Receptors, Corticotropin - metabolism; Receptors, Melanocortin; Transfection; Tumor Cells, Cultured; Zinc - chemistry; Zinc - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M202103200

An endogenous metal-ion site in the melanocortin MC1 and MC4 receptors was characterized mainly in transiently transfected COS-7 cells. ZnCl2 alone stimulated signaling through the Gs pathway with a potency of 11 and 13 μm and an efficacy of 50 and 20% of that of α-melanocortin stimulating hormone (α-MSH) in the MC1 and MC4 receptors, respectively. In the presence of peptide agonist, Zn(II) acted as an enhancer on both receptors, because it shifted the dose-response curves to the left: most pronounced was a 6-fold increase in α-MSH potency on the MC1 receptor. The effect of the metal ion appeared to be additive, because the maximal cAMP response for α-MSH in the presence of Zn(II) was 60% above the maximal response for the peptide alone. The affinity of Zn(II) could be increased through binding of the metal ion in complex with small hydrophobic chelators. The binding affinities and profiles were similar for a number of the 2,2′-bipyridine and 1,10-phenanthroline analogs in complex with Zn(II) in the MC1 and MC4 receptors. However, the potencies and efficacies of the metal-ion complexes were very different in the two receptors, and close to full agonism was obtained in the MC1 receptor. Metal ion-chelator complexes having antagonistic properties were also found. An initial attempt to map the metal-ion binding site in the MC1 receptor indicated that Cys271 in extracellular loop 3 and possibly Asp119 at the extracellular end of TM-III, which are both conserved among all MC receptors, are parts of the site. It is concluded that the function of the MC1 and MC4 receptors can be positively modulated by metal ions acting both as partial agonists and as potentiators for other agonists, including the endogenous peptide ligand α-MSH at Zn(II) concentrations that could be physiological. Furthermore, the metal ion-chelator complexes may serve as leads in the development of novel melanocortin receptor modulators.