Molecular determinants of human melanocortin-4 receptor responsible for antagonist SHU9119 selective activity.

The hypothalamic melanocortin-4 receptor (MC4R), a seven transmembrane G-protein-coupled receptor, plays an important role in the regulation of body weight. The synthetic melanocortin analog SHU9119 has been widely used to characterize the physiological role of MC4R in feeding behavior and energy homeostasis. Previous studies indicated that SHU9119 is an agonist at the melanocortin-1 receptor (MC1R) but an antagonist at the MC4R. However, the molecular basis of the interaction between hMC4R and SHU9119 has not been clearly defined. To gain insight into the molecular determinants of hMC4R in the selectivity of SHU9119 chimeras and mutants hMC1R and hMC4R were expressed in cell lines and pharmacologically analyzed. A region of receptor containing the third transmembrane of hMC4R was found to be required for selective SHU9119 antagonism. Further mutagenesis studies of this region of hMC4R demonstrated that the amino acid residue leucine 133 in the third transmembrane was critical for the selective antagonist activity of SHU9119. The single substitution of leucine 133 to methionine did not affect SHU9119 binding to hMC4R. However, this substitution did convert SHU9119 from an antagonist to an agonist. Conversely, exchange of Met(128) in hMC1R to Leu, the homologous residue 133 of hMC4R, displayed a reduction in SHU9119 binding affinity and potency. This report provides the details of the molecular recognition of SHU9119 antagonism at hMC4R and shows that amino acid Leu(133) of hMC4R plays a key role in melanocortin receptor subtype specificity.