Novel Agouti-Related-Protein-Based Melanocortin-1 Receptor Antagonist

Abstract

The melanocortin receptors are G-protein coupled receptors (GPCRs) that activate the cAMP signal transduction pathway and are stimulated by the melanocortin agonist alpha-melanocyte stimulating hormone (alpha-MSH). Members of these melanocortin receptors are antagonized by agouti (ASP) and agouti-related protein (AGRP), which are the only known endogenous antagonists of GPCRs identified to date. Structure-function studies of the hAGRP(109-118) decapeptide, Tyr-c[Cys-Arg-Phe-Phe-Asn-Ala-Phe-Cys]-Tyr-NH(2), by replacing the 26-membered disulfide Cys(2)-Cys(9) ring with lactam bridges resulted in the identification of a novel peripheral skin melanocortin-1 receptor (MC1R) antagonist. This antagonist, Tyr-c[Glu-Arg-Phe-Phe-Asn-Ala-Phe-Dpr]-Tyr-NH(2), possesses a 27-membered ring with the lactam bridge being formed from the Calpha-carboxyl moiety of Glu (instead of the typical side chain carboxyl moiety) with the amine of the diaminopropionic acid (Dpr) residue. This mouse MC1 receptor antagonist (pA(2) = 5.9) is also an antagonist at the brain melanocortin-4 receptor (pA(2) = 6.9), with no observable pharmacology at the melanocortin-3 or -5 receptors. This MC1R hAGRP(109-118) based decapeptide is novel in that AGRP(83-132) itself does not bind to, agonize, or antagonize the skin MC1R. Structural analysis has been performed using two-dimensional (1)H NMR and computer-assisted molecular modeling (CAMM) techniques in attempts to identify structural features of this Tyr-c[Glu-Arg-Phe-Phe-Asn-Ala-Phe-Dpr]-Tyr-NH(2) (cyclo Glu alphaCOOH-Dpr betaNH) peptide that can differentially result in antagonist versus agonist properties at the mMC1R.

Structure−Activity Relationships of the Unique and Potent Agouti-Related Protein (AGRP)−Melanocortin Chimeric Tyr-c[β-Asp-His-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH2 Peptide Template

“…31 Precedent regarding the use of stereochemical conversion of L-Phe to D-Phe to stabilize a putative reverse turn in the melanocortin peptides has been previously documented with great success. [21][22][23] Herein using this peptide template and DPhe at the 9 position, full nM melanocortin receptor agonists resulted that possessed 5-to 14-fold decreased potency, as compared to 1.…”

Bone morphogenetic protein (BMP) signaling controls hair pigmentation by means of cross-talk with the melanocortin receptor-1 pathway

“…Synthetic MC-1R antagonist (4 mg͞kg; ref. 28) or PBS (as vehicle control) were administered to TG mice (n ϭ 8) intracutaneously twice a day on days 3-7 of the depilation-induced hair cycle. Hairs were plucked from the injection sites on day 21 after depilation, and melanin content was determined as described above.…”

The Melanocortin System and its Role in Obesity and Cachexia

“…When the nature of the cyclic linking group was changed from Cys-Cys to a 27-membered lactam formed from the α-carboxylic acid of a Glu residue and diaminopropionic acid with the γ -carboxylic acid of Glu also participating in the ring (10, n=2, Fig. 8) [28], an antagonist for mMC4-R with K i 100 nM resulted which was similar in potency to the Cys-Cys decapeptide. Interestingly, a 26-ring analog using the β-carboxylic acid group of Asp (11, n=1) was six-fold less active at the MC4-R but behaved as an agonist (EC 50 6 µM) at MC1-R and, in this respect again behaved like the Cys-Cys decapeptide [29].…”