Structure-Activity Relationship Studies of Melanin-concentrating Hormone (MCH)-related Peptide Ligands at SLC-1, the Human MCH Receptor

Abstract Melanin-concentrating hormone(MCH) is a cyclic nonadecapeptide involved in the regulation of feeding behavior, which acts through a G protein-coupled receptor (SLC-1) inhibiting adenylcyclase activity. In this study, 57 analogues of MCH were investigated on the recently cloned human MCH receptor stably expressed in HEK293 cells, on both the inhibition of forskolin-stimulated cAMP production and guanosine-5′-O-(3-[35S]thiotriphosphate ([35S]- GTPγS) binding. The dodecapeptide MCH-(6–17) (MCH ring between Cys7 and Cys16, with a single extra amino acid at the N terminus (Arg6) and at the C terminus (Trp17)) was found to be the minimal sequence required for a full and potent agonistic response on cAMP formation and [35S]- GTPγS binding. We Ala-scanned this dodecapeptide and found that only 3 of 8 amino acids of the ring, namely Met8, Arg11, and Tyr13, were essential to elicit full and potent responses in both tests. Deletions inside the ring led either to inactivity or to poor antagonists with potencies in the micromolar range. Cys7 and Cys16 were substituted by Asp and Lys or one of their analogues, in an attempt to replace the disulfide bridge by an amide bond. However, those modifications were deleterious for agonistic activity. In [35S]- GTPγS binding, these compounds behaved as weak antagonists (K B 1–4 μm). Finally, substitution in MCH-(6–17) of 6 out of 12 amino acids by non-natural residues and concomitant replacement of the disulfide bond by an amide bond led to three compounds with potent antagonistic properties (K B = 0.1–0.2 μm). Exploitation of these structure-activity relationships should open the way to the design of short and stable MCH peptide antagonists.