[Show abstract][Hide abstract] ABSTRACT: The activity of melanocortin receptors (MCR) is regulated by melanocortin peptide agonists and by the endogenous antagonists,
Agouti protein and AgRP (Agouti-related protein). To understand how the selectivity for these structurally unrelated agonists
and antagonist is achieved, chimeric and mutants MC3R and MC4R were expressed in cell lines and pharmacologically analyzed.
A region containing the third extracellular loop, EC3, of MC4R was essential for selective Agouti protein antagonism. In addition,
this part of MC4R, when introduced in MC3R, conferred Agouti protein antagonism. Further mutational analysis of this region
of MC4R demonstrated that Tyr268 was required for the selective interaction with Agouti protein, because a profound loss of the ability of Agouti protein
to inhibit 125I-labeled [Nle4,d-Phe7]α-melanocyte-stimulating hormone (MSH) binding was observed by the single mutation of Tyr268 to Ile. This same residue conferred selectivity for the MC4R selective agonist, [d-Tyr4]MT-II, whereas it inhibited interaction with the MC3R-selective agonist, [Nle4]Lys-γ2-MSH. Conversely, mutation of Ile265 in MC3 (the corresponding residue of Tyr268) to Tyr displayed a gain of affinity for [d-Tyr4]MT-II, but not for Agouti protein, and a loss of affinity for [Nle4]Lys-γ2-MSH as compared with wild-type MC3R. This single amino acid mutation thus confers the selectivity of MC3R toward a pharmacological
profile like that observed for MC4R agonists but not for the antagonist, Agouti protein. Thus, selectivity for structurally
unrelated ligands with opposite activities is achieved in a similar manner for MC4R but not for MC3R.
Preview · Article · Feb 2001 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The central melanocortin (MC) system has been demonstrated to act downstream of leptin in the regulation of body weight. The system comprises alpha-MSH, which acts as agonist, and agouti-related protein (AgRP), which acts as antagonist at the MC3 and MC4 receptors (MC3R and MC4R). This property suggests that MCR activity is tightly regulated and that opposing signals are integrated at the receptor level. We here propose another level of regulation within the melanocortin system by showing that the human (h) MC4R displays constitutive activity in vitro as assayed by adenylyl cyclase (AC) activity. Furthermore, human AgRP(83-132) acts as an inverse agonist for the hMC4R since it was able to suppress constitutive activity of the hMC4R both in intact B16/G4F melanoma cells and membrane preparations. The effect of AgRP(83-132) on the hMC4R was blocked by the MC4R ligand SHU9119. Also the hMC3R and the mouse(m)MC5R were shown to be constitutively active. AgRP(83-132) acted as an inverse agonist on the hMC3R but not on the mMC5R. Thus, AgRP is able to regulate MCR activity independently of alpha-MSH. These findings form a basis to further investigate the relevance of constitutive activity of the MC4R and of inverse agonism of AgRP for the regulation of body weight.
Full-text · Article · Feb 2001 · Molecular Endocrinology
[Show abstract][Hide abstract] ABSTRACT: Since the melanocortin MC3 and melanocortin MC4 receptors are the main melanocortin receptor subtypes expressed in rat brain, we characterized the activity and affinity of nine melanocortin receptor ligands using these receptors in vitro, as well as their activity in a well-defined melanocortin-induced behavior in the rat: grooming behavior. We report here that [D-Tyr4]melanotan-II and RMI-2001 (Ac-cyclo-[Cys4, Gly5, D-Phe7, Cys10]alpha-MSH-NH2) have significantly higher affinity and potency on the rat melanocortin MC4 receptor as compared to the rat melanocortin MC3 receptor. Nle-gamma-MSH (melanocyte-stimulating hormone) was the only ligand with higher affinity and potency on the rat melanocortin MC3 receptor. The potency order of melanocortin MC4 receptor agonists, but not that of melanocortin MC3 receptor agonists, fitted with the potency of these ligands to stimulate grooming behavior, when administered intracerebroventricularly. SHU9119 (Ac-cyclo-[Nle4, Asp5, D-Nal(2)7, Lys10]alpha-MSH-(4-10)-NH2) and RMI-2005 (Ac-cyclo-[Cys4, Gly5, D-Na](2)7, Nal(2)9, Cys10]alpha-MSH-(4-10)-NH2) were able to inhibit alpha-MSH-induced melanocortin receptor activity in vitro, as well as alpha-MSH-induced grooming behavior. Melanotan-II, [Nle4-D-Phe7]alpha-MSH and RMI-2001 were also effective in inducing grooming behavior when administered intravenously. In the absence of purely selective melanocortin MC(3/4) receptor ligands, we demonstrated that careful comparison of ligand potencies in vitro with ligand potencies in vivo, could identify which melanocortin receptor subtype mediated alpha-MSH-induced grooming behavior. Furthermore, blockade of novelty-induced grooming behavior by SHU9119 demonstrated that this physiological stress response is mediated via activation of the melanocortin system.
No preview · Article · Sep 1999 · European Journal of Pharmacology
[Show abstract][Hide abstract] ABSTRACT: Melanocortin peptides regulate a variety of physiological processes. Five melanocortin receptors (MC-R) have been cloned and the MC3R and MC4R are the main brain MC receptors. The aim of this study was to identify structural requirements in both ligand and receptor that determine gamma-melanocyte-stimulating hormone (MSH) selectivity for the MC3R versus the MC4R. Substitution of Asp10 in [Nle4]Lys-gamma2-MSH for Gly10 from [Nle4]alpha-MSH, increased both activity and affinity for the MC4R while the MC3R remained unaffected. Analysis of chimeric MC3R/MC4Rs and mutant MC4Rs showed that Tyr268 of the MC4R mainly determined the low affinity for [Nle4]Lys-gamma2-MSH. The data demonstrate that Asp10 determines selectivity for the MC3R, however, not through direct side chain interactions, but probably by influencing how the melanocortin core sequence is presented to the receptor-binding pocket. This is supported by mutagenesis of Tyr268 to Ile in the MC4R which increased affinity and activity for [Nle4]Lys-gamma2-MSH, but decreased affinity for two peptides with constrained cyclic structure of the melanocortin core sequence, MT-II and [D-Tyr4]MT-II, that also displayed lower affinity for the MC3R. This study provides a general concept for peptide receptor selectivity, in which the major determinant for a selective receptor interaction is the conformational presentation of the core sequence in related peptides to the receptor-binding pocket.
[Show abstract][Hide abstract] ABSTRACT: The melanocortin MC3 and MC4 receptors are the main melanocortin receptors expressed in brain. Of the endogenous melanocortins, gamma2-melanocortin stimulating hormone (MSH) selectively activates the melanocortin MC3 receptor, whereas alpha- and beta-MSH activate all melanocortin receptors. The aim was to gain an insight into the contribution of amino acids in positions 5 and 10 of melanocortins to the selectivity of [Nle4]Lys-gamma2-MSH for the melanocortin MC3 receptor versus the melanocortin MC4 receptor. Introduction of Asp10 into [Nle4]alpha-MSH as in [Nle4,Gly5,Asp10]alpha-MSH selectively increased the EC50 value for the melanocortin MC4 receptor. Conversely, removal of Asp10, as in [Nle4,Gly10]Lys-gamma2-MSH, selectively decreased the EC50 value for the melanocortin MC4 receptor. Thus, Asp10 in Lys-gamma2-MSH determined selectivity for the melanocortin MC3 receptor versus the melanocortin MC4 receptor.
No preview · Article · Aug 1998 · European Journal of Pharmacology
[Show abstract][Hide abstract] ABSTRACT: Cyclic lactam analogs of alpha-melanocyte stimulating hormone (alpha-MSH) have been shown to be potent agonists in the frog skin bioassay [Al-Obeidi, F. et al., J. Med. Chem., 32 (1989) 2555], demonstrating melanocortin-1 (MC1) receptor activity. We synthesized cyclic alpha-MSH(1-13) and alpha-MSH(4-10) lactam analogs. The peptides were synthesized using Fmoc chemistry. We improved the cyclization procedure: side chains of Asp(5) and Lys(10) from the deprotected peptide were coupled in DMF to form a cyclic lactam, using an excess of PyBOP reagent and DIEA as a base. The cyclization reaction was completed within 1 h and was almost quantitative. We also synthesized an alpha-MSH analog cyclized via a disulphide bridge. The peptides were tested for their selectivity for the rat MC4 receptor. Cyclization and substitutions at position 7 dramatically influenced the selectivity for the rMC4 receptor.
No preview · Article · May 1998 · Letters in Peptide Science
[Show abstract][Hide abstract] ABSTRACT: Cyclic lactam analogs of α-melanocyte stimulating hormone (α-MSH) have been shown to be potent agonists in the frog skin bioassay
[Al-Obeidi, F. et al., J. Med. Chem., 32 (1989) 2555], demonstrating melanocortin-1 (MC1) receptor activity. We synthesized
cyclic α-MSH(1–13) and α-MSH(4–10) lactam analogs. The peptides were synthesized using Fmoc chemistry. We improved the cyclization
procedure: side chains of Asp5 and Lys10 from the deprotected peptide were coupled in DMF to form a cyclic lactam, using an excess of PyBOP reagent and DIEA as a
base. The cyclization reaction was completed within 1 h and was almost quantitative. We also synthesized an α-MSH analog cyclized
via a disulphide bridge. The peptides were tested for their selectivity for the rat MC4 receptor. Cyclization and substitutions
at position 7 dramatically influenced the selectivity for the rMC4 receptor.
No preview · Article · Apr 1998 · International Journal of Peptide Research and Therapeutics
[Show abstract][Hide abstract] ABSTRACT: The cloning of melanocortin receptors opened new avenues to identify selective ligands for this receptor family. gamma-MSH was characterized as a melanocortin-3 receptor selective agonist, [D-Arg8]ACTH-(4-10) and [Pro8,10, Gly9]ACTH-(4-10) were characterized as melanocortin-4 receptor antagonists. The application of these ligands in vivo revealed that melanocortin-4 receptors mediate melanocortin-induced grooming behaviour in the rat. Since we still lack potent and selective melanocortin receptor ligands, we performed homology modelling and site directed mutagenesis of the melanocortin-4 receptor, in order to understand how melanocortins bind melanocortin receptors. A histidine at position 260 in the melanocortin-4 receptor is important for normal receptor function. However this residue is not forming a salt bridge with a glutamate at position 92 to keep the receptor in an inactive conformation, nor with the glutamate in the melanocortin peptides as had been suggested before.
No preview · Article · Feb 1997 · Receptors and Channels
[Show abstract][Hide abstract] ABSTRACT: Antagonists for the melanocortin receptor family were identified by analysis of the effects of four melanocortin analogues on alpha-MSH(alpha-melanocyte-stimulating hormone)-induced cAMP accumulation in 293 human embryonal kidney (HEK) cells that expressed either the rat melanocortin MC3 receptor, the human melanocortin MC4 receptor or the ovine melanocortin MC5 receptor. Two peptides, [D-Arg8]ACTH(adrenocorticotrope hormone)-(4-10) and [Pro8,10,Gly9]ACTH-(4-10), antagonized the action of alpha-MSH on the melanocortin MC4 and MC5 receptors, but not the melanocortin MC3 receptor. [Ala6]ACTH-(4-10) inhibited the alpha-MSH activation of the melanocortin MC3 and MC5, but only weakly antagonized the activation of the melanocortin MC4 receptor. [Phe-I7]ACTH-(4-10) antagonized the melanocortin MC3, MC4 and MC5 receptors equally well. These antagonists were also tested to block a behavioral response induced by alpha-MSH. alpha-MSH-induced excessive grooming behavior in rats was inhibited by [Phe-I7]ACTH-(4-10), [D-Arg8]ACTH-(4-10) and [Pro8,10,Gly9]ACTH-(4-10), but not by [Ala6]ACTH-(4-10). This suggests that alpha-MSH-induced excessive grooming behavior is mediated by melanocortin MC4 receptors.
Full-text · Article · Dec 1994 · European Journal of Pharmacology