Melanocortin peptides: signal transduction pathways

Abstract

The melanocortin peptides, α-melanocyte-stimulating-hormone (α-MSH) and desacetyl-α-MSH are two endogenous hormones arising through post-translational processing from the precursor protein, proopiomelanocortin (POMC). These peptides signal through centrally and peripherally expressed melanocortin receptors (MCRs) to affect physiological processes including food intake and body weight control. While α-MSH and desacetyl-α-MSH can couple four MCR subtypes similarly in vitro, the functional significance of N-terminal acetylation of desacetyl-α-MSH in vivo remains unknown. This project aimed to better understand signalling transduction pathways downstream of α-MSH and desacetyl-α-MSH. First, hypothalamic proteins that were changed by central and peripheral administration of the peptides were identified using two-dimensional gel electrophoresis-based proteomic approach. Second, the ability of the peptides to induce internalisation of the human MC4R was compared using a cellular model in which human MC4R, tagged with Green Fluorescent Protein (hMC4R-GFP) to visualise the protein, was overexpressed in HEK293 cells. α-MSH was more potent than desacetyl-α-MSH at decreasing food intake 3 hours after intracerebroventricular injections into adult rat brain, while desacetyl-α-MSH was more potent than α-MSH at decreasing body weight change in neonatal rats when the peptides were injected subcutaneously for 14 days. Proteomic analysis of hypothalamic tissue from both animal models showed that α-MSH and desacetyl- α-MSH induced different patterns of protein changes. Proteins that were significantly changed in expression levels have functional roles associated with modulation of the cytoskeleton, stress response, and metabolism, all of which may underlie the neurotrophic, metabolic and other central behavioural effects of MSH. α-MSH was more potent than desacetyl- α-MSH at inducing hMC4R internalisation. AGRP, the endogenous antagonist at MC3R and MC4R, was equally potent at antagonising α-MSH and desacetyl-α-MSH induced adenylyl cyclase activity, but it was more effective at antagonising hMC4R internalisation induced by α-MSH than that induced by desacetyl-α-MSH. AGRP alone had no effect on receptor internalisation. Genetic evidence showed that mutations in the hMC4R that reduce hMC4R cell-surface expression lead to an obese phenotype. Thus, the expression level of MC4R in the cell-surface membrane is important for MC4R signaling and the different regulation of hMC4R signaling by α-MSH and desacetyl-αMSH may ultimately influence energy homeostasis.