Poor Cell Surface Expression of Human Melanocortin-4 Receptor Mutations Associated with Obesity

The melanocortin-4 receptor (MC4R) plays an important role in the regulation of body weight in rodents. Mutations in the coding region of the MC4R are found more frequently in obese individuals, supporting the hypothesis that also in humans deficient melanocortin signaling may lead to obesity. Family studies that were carried out to demonstrate the relevance of single mutations for obesity were mostly inconclusive, most likely due to small sample size and complexity of the trait. In addition, the existing pharmacological data of the mutant receptors are limited in that for most mutations the effect on receptor expression level and Agouti-related protein (AgRP) pharmacology have not been studied. The aim of the present study was to gain further insight into the impact of the MC4R mutations on receptor function. Eleven missense mutations were tested for cell surface expression, affinity for α-melanocyte-stimulating hormone (α-MSH) and AgRP-(83–132), and the biological response to α-MSH. All mutants were poorly expressed at the cell surface, as measured by 125 I-[Nle 4 - D -Phe 7 ]α-MSH binding, and only a few mutants showed altered pharmacology for α-MSH and AgRP. Hemagglutinin-tagged mutant receptors were retained in the intracellular environment. These pharmacological data provide a basis to estimate the quantitative effect of MC4R mutations for the development of obesity.