Interaction of melanin-concentrating hormone (MCH), neuropeptide E-I (NEI), 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 hormone (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-Rpc) and the different melanocortin (MC) receptors. Radioreceptor assays using [125I]MCH, [125l]alpha-MSH and [125I]NEI as radioligands and bioassays were performed with MCI-R-positive and MC1-R-negative mouse B16 melanoma cells and with COS cells expressing the different MC receptors. The IC50s of alpha-MSH and NEI or NGE for [125I]MCH displacement from mouse MCH-Rpc were 80-fold and, respectively, >300-fold higher than that of MCH, and the IC50s for MCH and NEI or NGE for [125I]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 B16 melanoma cells and there was no significant displacement of [1251]alpha-MSH by MCH, NEI or NGE with MC3-R, MC4-R and MC5-R expressed in COS cells. At concentrations of 100 nM to 10 microM, however, MCH, NEI and NGE induced cAMP formation and melanin synthesis which could be blocked by agouti protein or inhibitors 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.

Expression of the melanin-concentrating hormone receptor in porcine and human ciliary epithelial cells

PURPOSE

To evaluate whether the receptors for melanin-concentrating hormone (MCH) and its functional antagonist alpha-melanocyte-stimulating hormone (alpha-MSH) are expressed in the ciliary epithelium. Furthermore, to examine whether MCH, a neuropeptide involved in fluid and electrolyte homeostasis, may influence ion flux mediated by Na,K (adenosine triphosphatase)-ATPase in a ciliary epithelial cell line.

METHODS

Expression of MCH receptors (MCH-R) and alpha-MSH receptors (MSH-R) on primary porcine ciliary pigmented epithelial (PE) cells and on a human nonpigmented ciliary epithelial (NPE) cell line, ODM-2 was investigated by radioligand binding studies and reverse transcription-polymerase chain reaction (RT-PCR). The MCH-R was further characterized by photocrosslinking. Influence of MCH on Na, K-ATPase activity was evaluated by an Rb(+) transport assay.

RESULTS

MCH-R expression was observed at both the mRNA and protein levels in PE and NPE cells. In contrast, MSH-Rs were not detectable. At the mRNA level, expression of slc-1 was shown and with crosslinking, a 44-kDa protein was labeled. MCH showed no effect on Na,K-ATPase activity of NPE cells.

CONCLUSIONS

The presence of MCH-R in ciliary epithelial cells of both human and porcine origin but the absence of MSH-Rs indicates that in these cells, MCH and alpha-MSH do not form a functionally antagonistic hormonal pair as they do in several other systems. Although effects of MCH on intestinal water and ion transport have been documented, a direct control of Na,K-ATPase activity was not detected in human NPE cells in vitro.