(1/132) Long term orexigenic effect of a novel melanocortin 4 receptor selective antagonist.
1. We designed and synthesized several novel cyclic MSH analogues and tested their affinities for cells expressing the MC1, MC3, MC4 and MC5 receptors. 2. One of the substances HS028 (cyclic [AcCys11, dichloro-D-phenylalanine14, Cys18, Asp-NH2(22)]-beta-MSH11-22) showed high affinity (Ki of 0.95nM) and high (80 fold) MC4 receptor selectivity over the MC3 receptor. HS028 thus shows both higher affinity and higher selectivity for the MC4 receptor compared to the earlier first described MC4 receptor selective substance HS014. 3. HS028 antagonised a alpha-MSH induced increase in cyclic AMP production in transfected cells expressing the MC3 and MC4 receptors, whereas it seemed to be a partial agonist for the MC1 and MC5 receptors. 4. Chronic intracerebroventricularly (i.c.v.) administration of HS028 by osmotic minipumps significantly increased both food intake and body weight in a dose dependent manner without tachyphylaxis for a period of 7 days. 5. This is the first report demonstrating that an MC4 receptor antagonist can increase food intake and body weight during chronic administration providing further evidence that the MC4 receptor is an important mediator of long term weight homeostasis. (+info)
(2/132) Altered energy balance causes selective changes in melanocortin-4(MC4-R), but not melanocortin-3 (MC3-R), receptors in specific hypothalamic regions: further evidence that activation of MC4-R is a physiological inhibitor of feeding.
We have examined the effects of underfeeding and obesity on the density of hypothalamic melanocortin MC3 and MC4 receptors (MC3-R and MC4-R, respectively), which may mediate the hypophagic effects of alpha-melanocyte-stimulating hormone (MSH) in the rat. MC3-R and MC4-R were measured by quantitative autoradiography in brain sections using 125I-labeled Nle4-D-Phe7-alpha-MSH (125I-NDP-MSH) and discriminated by masking MC3-R with excess unlabelled gamma2-MSH. High densities of MC4-R occurred in the ventromedial (VMH) and arcuate (ARC) nuclei, median eminence (ME), and medial habenular nucleus (MHb), with lower densities in the dorsomedial hypothalamus (DMH) and forebrain regions. MC3-R were confined to the VMH, ARC, and MHb. After 10-days of food restriction (14% weight loss), density of MC4-R was significantly increased by 20-65% in the VMH, ARC, ME, and DMH, with no changes elsewhere. Similarly, obese (fa/fa) Zucker rats showed 43-98% increases in MC4-R in the same regions. By contrast, rats with diet-induced obesity (18% heavier than controls) showed significantly decreased binding to MC4-R, especially in the VMH, ARC, and ME. MC3-R showed no significant alterations in any model. We suggest that increased density of MC4-R with food restriction and in obese Zucker rats reflects receptor upregulation secondary to decreased release of alpha-MSH, consistent with increased hunger in these models. Conversely, downregulation of MC4-R in diet-induced obesity may indicate increased alpha-MSH secretion in an attempt to limit overeating. This alpha-MSH/MC4-R system may be inhibited by leptin and/or insulin. MC3-R are not apparently involved in regulating feeding. (+info)
(3/132) Conformation of the core sequence in melanocortin peptides directs selectivity for the melanocortin MC3 and MC4 receptors.
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. (+info)
(4/132) POMC gene-derived peptides activate melanocortin type 3 receptor on murine macrophages, suppress cytokine release, and inhibit neutrophil migration in acute experimental inflammation.
To investigate the relevance of adrenocorticotrophic hormone (ACTH) therapy in human gouty arthritis, we have tested the effect of several ACTH-related peptides in a murine model of experimental gout. Systemic treatment of mice with ACTH4-10 (MEHFRWG) (10-200 microgram s. c.) inhibited neutrophil accumulation without altering peripheral blood cell counts or circulating corticosterone levels. A similar effect was seen with alpha- and beta-melanocyte stimulating hormones (1-30 microgram s.c.). In vivo release of the chemokine KC-(detected in the lavage fluids before maximal influx of neutrophils) was significantly reduced (-50 to -60%) by ACTH4-10. Macrophage activation in vitro, determined as phagocytosis and KC release, was inhibited by ACTH and ACTH4-10 with approximate IC50 values of 30 nM and 100 microM, respectively. The melanocortin receptor type 3/4 antagonist SHU9119 prevented the inhibitory actions of ACTH4-10 both in vitro and in vivo. However, melanocortin type 3, but not type 4, receptor mRNA was detected in mouse peritoneal macrophages by RT-PCR. Therefore, we propose that activation of this receptor type by ACTH4-10 and related amino acid sequences attenuates KC release (and possibly production of other cytokines) from macrophages with consequent inhibition of the host inflammatory response, thus providing a notional anti-inflammatory mechanism for ACTH that is unrelated to stimulation of glucocorticoid release. (+info)
(5/132) Anatomy of an endogenous antagonist: relationship between Agouti-related protein and proopiomelanocortin in brain.
Agouti-related protein (AGRP) is a recently discovered orexigenic neuropeptide that inhibits the binding and action of alpha-melanocyte-stimulating hormone derived from proopiomelanocortin (POMC) at the melanocortin 3 receptor (MC3R) and melanocortin 4 receptor (MC4R) and has been proposed to function primarily as an endogenous melanocortin antagonist. To better understand the interplay between the AGRP and melanocortin signaling systems, we compared their nerve fiber distributions with each other by immunohistochemistry and their perikarya distribution with MC3R and MC4R by double in situ hybridization. Although deriving from distinct cell groups, AGRP and melanocortin terminals project to identical brain areas. Both AGRP and melanocortin neurons selectively express the MC3R, which provides a neuroanatomical basis for a dual-input circuit with biological amplification and feedback inhibition. These studies highlight a broader complexity in POMC-mediated behavior in the brain. (+info)
(6/132) Type I beta-turn conformation is important for biological activity of the melanocyte-stimulating hormone analogues.
In order to define which structure of alpha-melanocyte-stimulating hormone (MSH) analogues plays a critical role for ligand-receptor interaction and selectivity, we analysed receptor-binding and cAMP-generating activity in Chinese hamster ovary cell lines stably transfected with rMC3R and hMC4R, as well as the NMR structures of chemically synthesized alpha-MSH analogues. Compared with [Ahx4]alpha-MSH, the linear MTII designated as alpha-MSH-ND revealed a preference for the MC4R, whereas its IC50 and EC50 values were comparable to those of MTII reported previously. Truncation of Ahx4 and Asp5 of alpha-MSH-ND remarkably decreased the receptor-binding and cAMP-generating activity. Meanwhile, maximum cAMP-generating activity was observed at a higher concentration (10(-5) M) of alpha-MSH-ND(6-10), and MC4R preference was changed into MC3R preference. In contrast, [Gln6]alpha-MSH-ND(6-10) lost its cAMP-generating activity almost completely, even though it bound to both receptors. Whereas the solution conformation of alpha-MSH-ND revealed a stable type I beta-turn structure, [Gln6]alpha-MSH-ND(6-10) revealed a tight gamma-turn composed of Gln6-D-Phe7-Arg8. Replacement of the His6 residue of alpha-MSH-ND by Gln, Asn, Arg or Lys decreased not only the receptor binding, but also the cAMP-generating activity in both the MC3R and the MC4R. The structure of [Gln6]alpha-MSH-ND exhibited a stable type I' beta-turn comprising Asp5, Gln6, D-Phe7 and Arg8. [Lys6]alpha-MSH-ND showed a greatly reduced binding affinity and cAMP-generating activity with the loss of MC4R selectivity. In NMR studies, [Lys6]alpha-MSH-ND also demonstrated a gamma-turn conformation around Lys6-DPhe7-Arg8. From the above results, we conclude that a type I beta-turn conformation comprising the residues Asp5-His6-(D-Phe7)-Arg8 was important for receptor binding and activation, as well as the selectivity of MSH analogues. (+info)
(7/132) Antagonism of the melanocortin system reduces cold and mechanical allodynia in mononeuropathic rats.
The presence of both pro-opiomelanocortin-derived peptides and melanocortin (MC) receptors in nociception-associated areas in the spinal cord suggests that, at the spinal level, the MC system might be involved in nociceptive transmission. In the present study, we demonstrate that a chronic constriction injury (CCI) to the rat sciatic nerve, a lesion that produces neuropathic pain, results in changes in the spinal cord MC system, as shown by an increased binding of (125)I-NDP-MSH to the dorsal horn. Furthermore, we investigated whether intrathecal administration (in the cisterna magna) of selective MC receptor ligands can affect the mechanical and cold allodynia associated with the CCI. Mechanical and cold allodynia were assessed by measuring withdrawal responses of the affected limb to von Frey filaments and withdrawal latencies upon immersion in a 4.5 degrees C water bath, respectively. We show that treatment with the MC receptor antagonist SHU9119 has a profound anti-allodynic effect, suggesting that the endogenous MC system has a tonic effect on nociception. In contrast, administration of the MC4 receptor agonists MTII and d-Tyr-MTII primarily increases the sensitivity to mechanical and cold stimulation. No antinociceptive action was observed after administration of the selective MC3 receptor agonist Nle-gamma-MSH. Together, our data suggest that the spinal cord MC system is involved in neuropathic pain and that the effects of MC receptor ligands on the responses to painful stimuli are exerted through the MC4 receptor. In conclusion, antagonism of the spinal melanocortin system might provide a new approach in the treatment of neuropathic pain. (+info)
(8/132) Differential regulation of cAMP-mediated gene transcription and ligand selectivity by MC3R and MC4R melanocortin receptors.
Melanocortins are known to be involved in the regulation of feeding behavior. These hormones mediate their effects through G-protein-coupled receptors by stimulating adenylate cyclase. In this study we describe the functional response of melanocortin 4 receptor (MC4R) and melanocortin 3 receptor (MC3R) in HEK 293T cells, by using a luciferase reporter gene under the transcriptional control of a cAMP-responsive element (CRE) as a monitor of intracellular cAMP levels and cAMP-regulated gene expression. We were able to show that MC4R and MC3R expressed in the human cell line HEK 293T stimulate transcription induced by stimulation with different analogs of alpha-melanocyte-stimulating hormone (alpha-MSH) at different levels. In our assay of CRE-mediated gene transcription activity, alpha-MSH-ND was the most efficient alpha-MSH analog for MC4R whereas NDP-MSH was the most efficient for MC3R. Changing the His6 residue of alpha-MSH-ND to Gln or Lys markedly decreased CRE-mediated luciferase activity for MC3R compared with MC4R. On analysis by modeling the receptor-ligand complex by NMR, [Gln6]alpha-MSH-ND and [Lys6]alpha-MSH-ND showed different conformational interactions between MC3R and MC4R. Furthermore, the maximum coupling efficiency of MC4R and MC3R to G proteins was different; MC4R showed only 30-50% of the maximum activity induced by MC3R. In total, our results suggest that a differential receptor-ligand interaction is involved and that the relative interactions of MC3R and MC4R with G protein are possibly quantitatively and qualitatively different. (+info)