Molecular cloning, functional expression and pharmacological characterization of a mouse melanocortin receptor gene

We describe the cloning of the mouse HGMP01A gene that encodes a melanocortin receptor functionally distinct from the adrenal cortex corticotropin (adrenocorticotrophic hormone; ACTH) receptor and the melanocyte-stimulating hormone (MSH) receptor expressed in melanoma. The gene encodes a protein of 323 amino acids with a calculated molecular mass of 35,800 Da, displaying potential sites for N-linked glycosylation and phosphorylation by protein kinase C. An RNAase protection assay detected weak expression in the brain, but not in adrenal gland, skin, or any of the other tissues tested. Stable CHO cell lines expressing over 100,000 receptors per cell were generated. The recombinant receptor binds iodinated [Nle4,D-Phe7]alpha-MSH (NDP-MSH) with an apparent Kd of 700 pM. Displacement of the ligand by a variety of pro-opiomelanocortin-derived peptides revealed a pharmacological profile distinct from that of the classical ACTH and MSH receptors. NDP-MSH was the most powerful competitor (IC50 1.4 nM), followed by gamma-MSH (IC50 7 nM). alpha-MSH, beta-MSH and ACTH-(1-39) were significantly less potent, with IC50 values of 30, 19 and 21 nM respectively. ACTH-(4-10) was poorly active (IC50 2.4 microM), while corticotropin-like intermediate lobe peptide (CLIP) and beta-endorphin were totally ineffective. The recombinant receptor was found to stimulate adenylate cyclase. The potency order of the agonists in this assay was consistent with that of the binding displacement assays. This receptor represents the orthologue of the human melanocortin 3 receptor reported recently. The growing family of melanocortin receptors constitute the molecular basis for the variety of actions of melanocortins that have been described over the years. The availability of functionally expressed receptors from the melanocortin family will allow the development of a specific pharmacology, and a better understanding of the function of the pro-opiomelanocortin-derived peptides.