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Melanocortin Receptor

Melanocortins, primarily consisting of a-melanocyte stimulating hormone (MSH), bMSH, g-MSH, and adrenocorticotropin (ACTH), are small peptide hormones derived from tissue-specific post-translational processing of a pre-prohormone, proopiomelanocortin(POMC).  Functions of melanocortins on pigmentation and adrenal steroidogenesis are mediated by cell surface receptors that are coupled to the stimulatory heterotrimeric G protein, Gs. With the cloning of the classical MSH receptor and ACTH receptor, it was soon realized that three additional G protein-coupled receptors (GPCRs) with significant homology to the MSH receptor and ACTH receptor exist in mammalian genomes. The five cell-surface GPCRs that mediate melanocortin action are named MC1R (MSH receptor), MC2R (ACTH receptor), MC3R to MC5R, based on the sequence of their cloning. All five MCRs are coupled to the stimulatory G protein, increasing intracellular cAMP levels when the receptors are activated. MCRs also activate other signaling pathways, especially MAPKs, as shown extensively in other GPCRs. When the signaling in the canonical Gs-cAMP and non-canonical pathways are compared, differential response is observed in some instances. For example, the endogenous antagonist AgRP, while decreasing cAMP generation, stimulates ERK1/2 phosphorylation, therefore serving as agonist in the ERK1/2 pathway in both MC3R and MC4R. 

References

1.Tao YX. Biochim Biophys Acta Mol Basis Dis. 2017;1863(10 Pt A):2411–2413.