Ligand-Dependent Activation of the Melanocortin 5 Receptor: cAMP Production and Ryanodine Receptor-Dependent Elevations of [Ca2+]i (original) (raw)
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New aspects on the melanocortins and their receptors
Pharmacological Research, 2000
Knowledge of melanocortins and their receptors has increased tremendously over the last few years. The cloning of five melanocortin receptors, and the discovery of two endogenous antagonists for these receptors, agouti and agouti-related peptide, have sparked intense interest in the field. Here we give a comprehensive review of the pharmacology, physiology and molecular biology of the melanocortins and their receptors. In particular, we review the roles of the melanocortins in the immune system, behaviour, feeding, the cardiovascular system and melanoma. Moreover, evidence is discussed suggesting that while many of the actions of the melanocortins are mediated via melanocortin receptors, some appear to be mediated via mechanisms distinct from melanocortin receptors.
The Melanocortin 5 Receptor is Expressed in Human Sebaceous Glands and Rat Preputial Cells
Journal of Investigative Dermatology, 2000
Melanocortins regulate pigmentation, adrenal hormone secretion, immune functions, lipid metabolism, and feeding behaviors in rodents. These peptides include adrenocorticotrophic hormone, melanocyte stimulating hormone, b-lipotrophin, and the endorphins. Lipid metabolism in sebaceous glands and preputial glands of rodents is regulated by a-melanocyte stimulating hormone, the major agonist for melanocortin receptors. Five melanocortin receptor subtypes have been identi®ed that differ in their tissue localization and af®nities for melanocortin ligands. Targeted disruption of the melanocortin 5 receptor in transgenic mice results in widespread dysfunction of exocrine glands, including a marked decrease in sebum production. A role for melanocortins in the modulation of human sebum production has not been established. The goal of this study is to determine which melanocortin receptors are expressed in human sebaceous glands. Messenger RNA was isolated from human sebaceous glands and the reverse transcriptase polymerase chain reaction was performed using primers speci®c for each of the melanocortin receptor subtypes. Transcripts were detected for the melanocortin 5 receptor. A polyclonal chicken antihuman antibody to the melanocortin 5 receptor localized to sebaceous glands, eccrine glands, hair follicles, and epidermis in human skin, rat skin, cultured human sebocytes, and rat preputial cells. Presence of the melanocortin 5 receptor protein in human sebaceous glands and rat preputial glands was further veri®ed by Western blotting. These data support further investigation of the role of melanocortins in the regulation of human sebum production and support the use of the rat preputial system as an experimental model in sebaceous gland physiology.
Biochemical and Biophysical Research Communications, 1996
The TM4, EL2 and TM5 show low amino acid homology within the MC receptor family. Three mutants of the human MC3 receptor were created in order to investigate the participation of these regions in ligand binding. The TM4, EL2 and TM5 were separately changed by multiple mutagenesis so that their amino acid sequences became identical with the human MC1 receptor. The mutants were expressed in COS cells and they bound peptide ligands in the same fashion as the wild type MC3 receptor clone. Our results indicate that the amino acids that were mutated in the MC3 receptor do not affect the binding of MSH peptides. The data provide further evidence, that the mutated regions may not participate at all in ligand binding, as indicated by modelling experiments and homology comparison. ᭧ 1996 Academic Press
Journal of Medicinal Chemistry, 2007
The physiological role of melanocortin receptor 5 (MC5R) in humans is not clear despite its broad presence in various peripheral sites and in the brain, cortex, and cerebellum. To differentiate between functions of this receptor and those of the other melanocortin receptors (hMC1,3,4R), peptides with improved receptor subtype selectivity are needed. The endogenous ligands, melanocortins, and their various synthetic analogues are not particularly selective for hMC5R. In this study, cyclic peptides derived from MTII, Ac-Nle-cyclo-(Asp-His 6-D-Phe 7-Arg 8-Trp-Lys)-NH 2 (a pan-agonist at the melanocortin receptors) were prepared and tested in binding and functional assays on CHO cells expressing hMC1b,3-5R. The analogues included in their structures sterically constrained hydrophobic amino acids in positions 6 (His) and 8 (Arg), and the D-4,4′biphenyl residue in position 7 (D-Phe). Several of the new compounds were selective potent agonists at hMC5R. They are exemplified by peptide 29, Ac-Nle-cyclo(Asp-Oic 6-D-4,4′-Bip 7-Pip 8-Trp-Lys)-NH 2 (Oic) octahydroindole-2-COOH; 4,4′-Bip) 4,4′-biphenylalanine; Pip) pipecolic acid) of IC 50) 0.95 nM and EC 50) 0.99 nM at hMC5R and selectivity for this receptor with respect to the other melanocortin receptors greater than 5000-fold.
Melanocyte Function and Its Control by Melanocortin Peptides
Journal of Histochemistry & Cytochemistry, 2002
S U M M A R Y Melanocytes are cells of neural crest origin. In the human epidermis, they form a close association with keratinocytes via their dendrites. Melanocytes are well known for their role in skin pigmentation, and their ability to produce and distribute melanin has been studied extensively. One of the factors that regulates melanocytes and skin pigmentation is the locally produced melanocortin peptide ␣ -MSH. The effects of ␣ -MSH on melanogenesis are mediated via the MC-1R and tyrosinase, the rate-limiting enzyme in the melanogenesis pathway. Binding of ␣ -MSH to its receptor increases tyrosinase activity and eumelanin production, which accounts for the skin-darkening effect of ␣ -MSH. Other ␣ -MSH-related melanocortin peptides, such as ACTH1-17 and desacetylated ␣ -MSH, are also agonists at the MC-1R and could regulate melanocyte function. Recent evidence shows that melanocytes have other functions in the skin in addition to their ability to produce melanin. They are able to secrete a wide range of signal molecules, including cytokines, POMC peptides, catecholamines, and NO in response to UV irradiation and other stimuli. Potential targets of these secretory products are keratinocytes, lymphocytes, fibroblasts, mast cells, and endothelial cells, all of which express receptors for these signal molecules. Melanocytes may therefore act as important local regulators of a range of skin cells. It has been shown that ␣ -MSH regulates NO production from melanocytes, and it is possible that the melanocortins regulate the release of other signalling molecules from melanocytes. Therefore, the melanocortin signaling system is one of the important regulators of skin homeostasis.
Signaling Mechanisms Controlled by Melanocortins in Melanoma, Lacrimal, and Brain Astroglial Cells
Annals of the New York Academy of Sciences, 1993
SALOMON et af.: SIGNALING MECHANISMS 365 12-16 amino acids, respectively, and arise in various mammalian tissues along with other bioactive peptides by posttranscriptional processing from the 3 1-36-kDa POMC precursor. The melanocortin peptides contain a consensus 7 amino acid sequence flanked by peptide-specific sequences of considerable homology on both the Nand C-terminal ends.'O Other common features of these peptides relate to (1) their unique dependence on extraceflular calcium required for interaction with their respective receptor molecules2' and (2) their ability to regulate target cell function via G-p r~t e i n s~~*~~ and adenylate cyclase (AC),20724 as well as other signaling pathways as further discussed below. A requirement of extracellular Ca2+ ions for the activity of ACTH was first noted in the early 50s by Birmingham and associates,2s studying ACTH-induced steroidogenesis in rat adrenal cortical tissue. ACTH stimulation of lipolysis was also shown to be Ca2+ dependent.26 Understanding of this phenomenon increased when Birnbaumer and Rodbell demonstrated that AC in fat cells serves as the common effector enzyme for several lipolytic hormones (epinephrine, glucagon, ACTH).*' Among these hormones, only ACTH appeared to require Ca2+ ions and its stimulatory effect was reduced to basal levels by EGTA. The difficulty of obtaining biologically active radiolabeled ACTH deferred further progress in the analysis of this phenomenon until, in 1985, Cheitlin and his associates first synthesized [1251][Tyr23 Phe2 Nle41ACTH-(1-38)-a derivative that maintained full biological activity.28 Binding of this peptide to adrenal glomerulosa cells required physiological calcium concentration, Furthermore, transfer of the cells to Ca2+-free medium induced the release of the receptor-bound hormone. Studies performed in lower vertebrates on the stimulation of skin melanophores, in which MSH induces melanosome dispersion, also revealed that the response to this hormone required extracellular calcium.20~29*30 Pigment dispersion could, however, be induced in the absence of calcium by exogenous cyclic AMP (CAMP), dibutyryl cAMP or other agents, such as theophylline, that nonspecifically raise intracellular CAMP."-^^ These observations, therefore, suggested that the elevation of cAMP and, hence, the cellular response depend on extracellular calcium only when stimulated by MSH. In experiments performed with photoreactive a-MSH, De Graan et al. noted that covalent cross-linking of MSH to the MSH receptor (MSH-R), in the presence but not in the absence of calcium, resulted in persistent activation of amphibian m e l a n o p h~r e s .~~-~~ However, removal of calcium reversed the stimulation, presumably while maintaining the active covalently cross-linked receptor-MSH complex. This was inferred since replenishment of calcium in MSH-free medium restored the cellular response. These authors, therefore, suggested a dual role for Ca2+ ions in the amphibian melanocyte, at the receptor level and at sites in the MSH-responsive pathways that are distal to the MSH-R itself. In this short review, we will discuss our recent work on MSH receptors in melanoma, lacrimal, and rat brain astrocytes, tissues in which MSH or ACTH appears to control rather different cellular responses. Furthermore, the various cell-specific responses seem to be mediated by different signaling mechanisms. M2R MOUSE MELANOMA The Calcium Dependence of MSH Receptor Function in Mouse Melanoma To further understand the role of Ca2+ in MSH-R function, particularly in mammalian systems, the mouse melanoma M2R cell line was chosen as a model.
Melanocortins: Anti-Inflammatory and Neuroprotective Peptides
Neurodegeneration, 2012
The melanocortin system includes the melanocortins, their receptors and two endogenous antagonists. This system is involved in several physiological processes in the brain. Melanocortins have potent anti-inflammatory and neuroprotective effects in the central nervous system (CNS). Therefore, they are suitable candidates for the treatment of inflammatory and neurodegenerative disorders within the brain. 2. Melanocortins Melanocortins include , , and -melanocyte stimulating homones (MSH), and adrenocorticotropin (ACTH). These neuropeptides derive from pro-opiomelanocortin (POMC) proteolytic cleavage (Fig. 1). POMC is a 31 KDa pro-hormone that is processed by pro-hormone convertases (PCs) in secretory vesicles of the cell. PCs belong to the family of serine proteases and recognize pairs of basic amino acid residues. PC expression is tissue specific and their presence induces secretion of different products generated from POMC, thereby determining melanocortins' selective expression (Bicknell, 2008). Intact POMC was also shown to be released into the circulation (Gibson et al., 1994). -MSH is produced in the presence of PC1 and PC2 (Benjannet et al., 1991) and additional modifications such as glycosylation, phosphorylation, amidation, and acetylation may occur. For example, acetylation gives -MSH increased resistance to degradation, this modification resulting in increased biological activity (Wilkinson, 2006). Although -MSH is synthesized in several tissues such as skin, placenta, testis, ovary, kidney, and adrenal gland, the main source of -MSH is the pars intermedia of the pituitary gland (Usategui et al., 1976). In the CNS, -MSH is produced in the arcuate nucleus of the hypothalamus (O´Donohue & Dorsa, 1982) and in the nucleus of the solitary tract in the brain stem (Bronstein et al., 1992). Melanocortin fibers project from these sites to the paraventricular nucleus, the lateral hypothalamus, and throughout the brain, e.g., amygdala, hippocampus, nucleus accumbes, and spinal cord (Bagnol et al., 1999). ACTH is produced mainly in the anterior pituitary gland and released into circulation, although it is www.intechopen.com Neurodegeneration 94 also expressed in the skin (Wakamatsu et al., 1997). -MSH was detected in adrenal medulla (Bjartell et al., 1987), intestine neurons (Wolter, 1985), and the brain (Kawai et al., 1984) whereas -MSH was found in human hypothalamus (Bertagna et al., 1986), but not in rodent brain. All melanocortins share a conserved sequence of aminoacids: Met-Glu(Gly)-His-Phe-Arg-Trp necessary for their biological activity. Rare mutations in POMC gene have been found in humans, and are associated with severe early-onset obesity, adrenal insufficiency and red hair pigmentation (Krude et al., 1998).