The melanocortin-4 receptor (MC4R) is a G protein-coupled receptor (GPCR) expressed in the paraventricular nucleus of the hypothalamus where it exerts control on food intake. MC4R recycles constitutively and is endocytosed at the same rate in the presence or absence of the agonist, &agr;-MSH. Here, the effects of two distinct lipid stresses on MC4R are modeled. In immortalized Neuro2A cells stably expressing HA-MC4R-GFP, we model mild cholesterol depletion, similar to that observed in mouse models of insulin-deficient diabetes. We demonstrate that constitutive endocytosis of MC4R is clathrin-dependent and cholesterol-dependent and inhibition of receptor endocytosis decreases receptor responsiveness to &agr;-MSH. Therefore, mild cholesterol depletion induces a loss of MC4R function by inhibiting receptor endocytosis, leading to accumulation of non-functional receptor at the cell surface. Obese patients and high-fat diet-induced obese rodents exhibit elevated palmitate in the blood stream and hypothalamus. Here we have modeled exposure to high-fat diet by exposing immortalized hypothalamic neurons to elevated palmitate. We find that exposure to elevated palmitate impairs acute MC4R signaling and induces profound desensitization of the receptor. The loss of acute MC4R signaling occurs by reduction of MC4R protein abundance, while redistribution of the receptor depleted the endosomal pool of MC4R and increases receptor desensitization. In cells exposed to elevated palmitate, treatment with PBA restores MC4R protein abundance and distribution, thereby rescuing signaling of the receptor, suggesting a possible target for obesity therapy.
|Commitee:||Chambers, Timothy, Davies, David, Mock, Donald, Storrie, Brian|
|School:||University of Arkansas for Medical Sciences|
|Department:||Interdisciplinary Biomedical Sciences|
|School Location:||United States -- Arkansas|
|Source:||DAI-B 75/05(E), Dissertation Abstracts International|
|Subjects:||Molecular biology, Cellular biology|
|Keywords:||Diabetes, G protein-coupled receptors, Insulin, Lipid stress, MC4R recycles, Melanocortin-4 receptor|
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