Molecular mechanism of the constitutive activation of the L250Q human melanocortin-4 receptor polymorphism.

Department of Medicinal Chemistry, University of Florida, PO Box 100485, Gainesville, FL 32610-0485, USA.
Chemical Biology &amp Drug Design (Impact Factor: 2.51). 04/2006; 67(3):215-29. DOI: 10.1111/j.1747-0285.2006.00362.x
Source: PubMed

ABSTRACT The Melanocortin-4 Receptor is a G-protein coupled receptor that has been physiologically linked to participate in the regulation of energy homeostasis. The Melanocortin-4 Receptor is stimulated by endogenous melanocortin agonists derived from the pro-opiomelanocortin gene transcript and antagonized by the endogenous antagonist agouti-related protein. Central administration of melanocortin agonists has been demonstrated to decrease food intake and conversely, treatment with antagonists resulted in increased food intake. Deletion of the Melanocortin-4 Receptor gene from the mouse genome results in an obese and hyperphagic phenotype. Polymorphisms of the human Melanocortin-4-Receptor have been found in severely obese individuals, suggesting that Melanocortin-4 Receptor malfunction might be involved in human obesity and obesity-associated diabetes. Herein, we have performed experiments to understand the molecular mechanisms associated with the L250Q human Melanocortin-4-Receptor polymorphism discovered in an extremely obese woman. This L250Q human Melanocortin-4-Receptor has been pharmacologically characterized to result in a constitutively active receptor. The fact that a constitutively active human Melanocortin-4-Receptor mutation was found in an obese person is a physiologic contradiction, as chronic activation of the human Melanocortin-4-Receptor and subsequently high cyclic adenosine monophosphate levels should theoretically result in a normal or lean phenotype. In this study, we demonstrated that agouti-related protein acts as an inverse agonist at this constitutively active receptor, and we propose a mechanism by which agouti-related protein might contribute to the obese phenotype in the L250Q patient. In addition, using receptor mutagenesis, pharmacology, and computer modeling approaches, we investigated the molecular mechanism by which modification of the L250 residue results in constitutive activation of the human Melanocortin-4-Receptor.

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    ABSTRACT: The melanocortin 4 receptor (MC4R) critically regulates feeding and satiety. Rare variants in MC4R are predominantly found in obese individuals. Though some rare variants in MC4R discovered in patients have defects in localization, ligand binding and signaling to cAMP, many have no recognized defects. In our cohort of 1433 obese subjects that underwent Roux-en-Y Gastric Bypass (RYGB) surgery, we found fifteen variants of MC4R. We matched rare variant carriers to patients with the MC4R reference alleles for gender, age, starting BMI and T2D to determine the variant effect on weight-loss post-RYGB. In vitro, we determined expression of mutant receptors by ELISA and western blot, and cAMP production by microscopy. While carrying a rare MC4R allele is associated with obesity, carriers of rare variants exhibited comparable weight-loss after RYGB to non-carriers. However, subjects carrying three of these variants, V95I, I137T or L250Q, lost less weight after surgery. In vitro, the R305Q mutation caused a defect in cell surface expression while only the I137T and C326R mutations showed impaired cAMP signaling. Despite these apparent differences, there was no correlation between in vitro signaling and pre- or post-surgery clinical phenotype. These data suggest that subtle differences in receptor signaling conferred by rare MC4R variants combined with additional factors predispose carriers to obesity. In the absence of complete MC4R deficiency, these differences can be overcome by the powerful weight-reducing effects of bariatric surgery. In a complex disorder such as obesity, genetic variants that cause subtle defects that have cumulative effects can be overcome after appropriate clinical intervention.
    PLoS ONE 04/2014; 9(4):e93629. DOI:10.1371/journal.pone.0093629 · 3.53 Impact Factor
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    ABSTRACT: The melanocortin-4 receptor gene (MC4R) has intensively been analyzed in molecular genetic obesity research. Decreased MC4R activity leads to obesity. Currently 166 nonsynonymous, nonsense, deletion, and frameshift MC4R mutations have been described. Vast numbers of these mutations were identified in (extremely) obese individuals. Total or partial loss of MC4R function results from most detected mutations, as shown by in vitro analyses. The heterozygote frequency for these mutations in (extremely) obese individuals cumulates to approximately 2-5%. Surprisingly, two polymorphisms in the MC4R are associated with a slight protection from obesity. Large study groups were screened to be able to pinpoint these rather small effects on body weight. Recently, the advent of genome-wide association studies led to the discovery of association of polymorphisms in the 3' region of the MC4R with obesity. The functional implication of the finding is still unresolved; an effect on gene expression is the most likely mechanism. Synthetic association could not be detected for the MC4R region.
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