Interaction of melanin-concentrating hormone (MCH), neuropeptide E-I (NEI)9 neuropeptide G-E (NGE), and alpha-msh with melanocortin and MCH receptors on mouse B16 melanoma cells

Melanin-concentrating hormone (MCH) and alpha -melanocyte-stimulating hormo ne (alpha -MSH) are known to exhibit mostly functionally antagonistic, but in some cases agonistic activities, e.g., in pigment cells and in the brain . Neuropeptide E-I (NEI) displays functional MCH-antagonist and MSH-agonist activity in different behavioral paradigms; the role of neuropeptide G-E ( NGE) is not known. This study addressed the question of possible molecular interactions between alpha -MSH, MCH and the MCH-precursor-derived peptides NEI and NGE at the level of the pigment cell MCH receptor subtype (MCH-R-p c) and the different melanocortin (MC) receptors. Radioreceptor assays usin g [I-125]MCH, [I-125]alpha -MSH and [I-125]NEI as radioligands and bioassay s were performed with MC1-R-positive and MC1-R-negative mouse B16 melanoma cells and with COS cells expressing the different MC receptors. The IC(50)s of alpha -MSH and NEI or NGE for [I-125]MCH displacement from mouse MCH-R- pc were 80-fold and, respectively, > 300-fold higher than that of MCH, and the IC(50)s for MCH and NEI or NGE for [I-125]alpha -MSH displacement from mouse MC1-R were 50,000-fold and > 200,000-fold higher than that of alpha - MSH. No high-affinity binding sites for NEI were detected on 16 melanoma ce lls and there was no significant displacement of [I-125]alpha -MSH by MCH, NEI or NGE with MC3-R. MC4-R and MC5-R expressed in COS cells. At concentra tions of 100 nM to 10 muM, however, MCH, NEI and NGE induced cAMP formation and melanin synthesis which could be blocked by agouti protein or inhibito rs of adenylate cyclase or protein kinase A. This shows that mammalian MCH- precursor-derived peptides may mimic MSH signalling via MC1-R activation at relatively high, but physiologically still relevant concentrations, as e.g . found in autocrine/paracrine signalling mechanisms.