|The prevalence of obesity is increasing in both developed and developing countries. Obesity is a disorder of energy homeostasis, associated with increased risk of diabetes mellitus, cardiovascular diseases and certain types of cancers. Hence, there is increasing interest in the molecular mechanisms of the pathogenesis of obesity. Although environmental factors play an important role in the pathogenesis of obesity, genetic factors confer an individual’s susceptibility to obesity. Mutations in several genes such as those encoding leptin, leptin receptor, proopiomelanocortin (POMC), and melanocortin-4 receptor (MC4R), have been identified to cause monogenic obesity.
Melanocortin-3 receptor (MC3R), another member of the melanocortin receptor family, and MC4R, are both expressed in central nervous system at sites potentially involved in regulating energy homeostasis. Rodent genetic studies demonstrate that MC3R is involved in regulating energy metabolism. It may serve as an auto-inhibitory receptor on POMC neurons to modulate food intake, potentially explaining the observation that Mc3r knock out mice do not exhibit hyperphagia. Mice lacking Mc3r gene have increased fat mass and decreased lean body mass, suggesting that clinical research addressing energy homeostasis and MC3R should focus on adiposity rather than body weight. Up to now, 24 mutations in MC3R gene have been identified. Detailed functional data were lacking for most of these mutations.
In the present study, detailed functional studies of 18 naturally occurring MC3R mutations were performed. Receptor expression, ligand binding and signaling properties were investigated. Ten mutants (S69C, A70T, F82S, I87T, M134I, L249V, A260V, M275T, T280S and L297V) had significantly decreased cell surface expression, and mutants A260V, M275T and L297V had decreased total expression. S69C, F82S, D158Y, R257S, T280S and L299V displayed significant defects in both ligand binding and signaling. M134I and M275T had decreased maximal binding, but exhibited similar signaling as wild-type (WT) MC3R, indicating the presence of spare receptors. L249F and L285V had normal maximal binding with decreased maximal signaling, suggesting that these mutants were defective in signaling. Data from multiple mutations at T280 demonstrated the necessity of Thr in different aspects of MC3R functions.
In conclusion, in the present study, detailed functional data of these naturally occurring mutations were obtained. D158Y, T280S and L299V might be the pathogenic causes for human obesity. S69C, F82S, and R257S, defective in function, were identified from lean subjects, suggesting that mutations in MC3R are not 100% penetrant in causing human obesity. F82, D158, L249, T280 and L299 played an important role in MC3R functions. These findings contribute to a better understanding of the structure-function relationship of MC3R. Whether and how other mutants that had normal signaling could affect energy homeostasis in vivo requires further investigation. More detailed studies, including in vivo studies and cosegregation analysis are needed.