The melanocortin system

Discovery of (2S)-N-[(1R)-2-[4-cyclohexyl-4-[[(1,1-dimethylethyl)amino]carbonyl]-1-piperidinyl]-1-[(4-fluorophenyl)methyl]-2-oxoethyl]-4-methyl-2-piperazinecarboxamide (MB243), a potent and selective melanocortin subtype-4 receptor agonist

We report the discovery and optimization of substituted 2-piperazinecarboxamides as potent and selective agonists of the melanocortin subtype-4 receptor. Further in vivo development of lead agonist, MB243, is disclosed.

Cloning of two melanocortin (MC) receptors in spiny dogfish: MC3 receptor in cartilaginous fish shows high affinity to ACTH-derived peptides while it has lower preference to gamma-MSH

We report the cloning and characterization of two melanocortin receptors (MCRs) from the spiny dogfish (Squalus acanthias) (Sac). Phylogenetic analysis shows that these shark receptors are orthologues of the MC3R and MC5R subtypes, sharing 65% and 70% overall amino acid identity with the human counterparts, respectively. The SacMC3R was expressed and pharmacologically characterized in HEK293 cells. The radioligand binding results show that this receptor has high affinity for adrenocorticotropic hormone (ACTH)-derived peptides while it has comparable affinity for alpha- and beta-melanocyte stimulating hormone (MSH), and slightly lower affinity for gamma-MSH when compared with the human orthologue. ACTH(1-24) has high potency in a second-messenger cAMP assay while alpha- and gamma-MSH had slightly lower potency in cells expressing the SacMC3R. We used receptor-enhanced green fluorescence protein (EGFP) fusion to show the presence of SacMC3R in plasma membrane of Chinese hamster ovary and HEK293 cells but the SacMC5R was retained in intracellular compartments of these cells hindering pharmacological characterization. The anatomical distribution of the receptors were determined using reverse transcription PCR. The results showed that the SacMC3R is expressed in the hypothalamus, brain stem and telencephalon, optic tectum and olfactory bulbs, but not in the cerebellum of the spiny dogfish while the SacMC5R was found only in the same central regions. This report describes the first molecular characterization of a MC3R in fish. The study indicates that many of the important elements of the MC system existed before radiation of gnathostomes, early in vertebrate evolution, at least 450 million years ago.

Melanocortin 1 receptor variants, pigmentation, and skin cancer susceptibility

The melanocortin 1 receptor is a key regulator of variation in normal human pigmentation. Genetic variants of this receptor cause red hair and fair skin, and several case-control studies have demonstrated that these genetic variants increase the risk of skin cancer development in humans. The mechanism whereby the risks of skin cancer are increased is not entirely clear, and may be because of a combination of effects on pigmentation and non-pigmentary pathways.

Meal patterns and foraging in melanocortin receptor knockout mice

We report the meal patterns of mice with the deletion of either the melanocortin type 3 or 4 receptors (MC3RKO or MC4RKO) compared with that of the wild type (WT) under conditions of varying foraging costs. Mice lived in two-lever operant chambers; the completion of a designated number of responses (termed procurement fixed ratio or PFR) on the "foraging" lever activated the other lever. On this second lever, the completion of a designated number of responses (termed consumatory fixed ratio or CFR) caused the delivery of a 20-mg food pellet. Animals could complete as many CFRs as they wished to constitute a meal, but whenever 10 min elapsed without pressing on this second lever, the meal was terminated and pressing on the "foraging" lever was again required to initiate a new meal. At lower PFRs, mice of all three genotypes took 5-7 well-defined meals per day of approximately 35 pellets/meal. At the highest PFR, mice of all three groups took about half this number of meals, with some increase in meal size, and total intake was slightly reduced. MC4RKO mice were obese compared with WT or MC3RKO but failed to eat more food in the operant chambers and, as a consequence, lost weight, regardless of PFR. Thus, changes in meal-taking strategies as a function of imposed foraging cost are not critically dependent on either MC3 or MC4 receptors, but these conditions did not allow us to study meal patterns in MC4RKO mice that are hyperphagic.

The critical role of the melanocortin system in the control of energy balance

Animals have developed highly adaptive and redundant mechanisms to maintain energy balance by matching caloric intake to caloric expenditure. Recent evidence has pointed to a variety of peripheral signals that inform specific central nervous system (CNS) circuits about the status of peripheral energy stores as critical to the maintenance of energy balance. A critical component of these CNS circuits is the melanocortin system. Regulation of signaling by melanocortin 3 and melanocortin 4 receptors in the CNS is controlled via neuronal cell bodies in the arcuate nucleus of the hypothalamus that synthesize melanocortin receptor agonists such as alpha-melanocyte-stimulating hormone (alpha-MSH) or antagonists such as agouti-related protein (AgRP). The activity of these two populations of neurons is reciprocally regulated by a number of peripheral and central systems that influence energy balance. Further, increased melanocortin signaling via pharmacological or genetic means in the CNS causes potent reductions in food intake and weight loss. Decreased melanocortin signaling via pharmacological or genetic means results in increased food intake and weight gain. Reviewed here is the wide range of evidence that points to the melanocortin system as a critical node in the diverse neurocircuitry that regulates food intake and body weight.

MC-3 receptor and the inflammatory mechanisms activated in acute myocardial infarct

Investigation of the mechanisms activated by endogenous inhibitory pathways can lead to identification of novel targets for cardiovascular inflammatory pathologies. Here we exploited the potential protective role that melanocortin receptor type 3 (MC3-R) activation might play in a myocardial ischemia-reperfusion injury model. In resting conditions, mouse and rat heart extracts expressed MC3-R mRNA and protein, without changes following ischemia-reperfusion. At the cellular level heart macrophages, but not fibroblasts or cardiomyocytes, expressed this receptor, as demonstrated by immunogold labeling. In vivo, administration of the melanocortin agonist MTII (10 microg per mouse equivalent to 9.3 nmol) 30 min prior to ischemia (25 min) attenuated mouse heart 2 h reperfusion injury by approximately 40%, an effect prevented by the mixed MC3/4-R antagonist SHU9119 but not by the selective MC4-R antagonist HS204. Similar results were obtained when the compound was given at the beginning of the reperfusion period. Importantly, delayed myocardial damage as measured 24 h post-reperfusion was equally protected by administration of 10 microg MTII. The focus on MC3-R was also substantiated by analysis of the recessive yellow (e/e) mouse, bearing a mutated (inactive) MC1-R, in which MTII was fully protective. Myocardial protection was associated with reduced markers of systemic and local inflammation, including cytokine contents (interleukin-1 and KC) and myeloperoxidase activity. In conclusion, this study has highlighted a previously unrecognized protective role for MC3-R activation on acute and delayed heart reperfusion injury. These data may open new avenues for therapeutic intervention against heart and possibly other organ ischemia-reperfusion injury.

Design and syntheses of melanocortin subtype-4 receptor agonists: evolution of the pyridazinone archetype

The discovery and optimization of a new class of non-peptidyl, pyridazinone derived melanocortin subtype-4 receptor agonists is disclosed.

Functional characterization of melanocortin-3 receptor variants identify a loss-of-function mutation involving an amino acid critical for G protein-coupled receptor activation

Although melanocortin-4 receptor mutations are the cause of the most common monogenic form of obesity, the involvement of the melanocortin-3 receptor (MC3R) in the pathogenesis of obesity is unknown. Earlier studies failed to identify any mutations in obese patients except for the identification of two variants (K6T and I81V) that likely represent polymorphisms. However, a potential mutation (I183N) was recently reported from patients having high-fat contents. We report here the functional characterization of these variants. We show that K6T and I81V have ligand binding and signaling properties similar to wild-type (wt) MC3R, indicating that they are indeed polymorphisms. However, the other variant, I183N, completely lacks signaling in response to agonist stimulation, although it binds ligand with normal affinity and with only slightly decreased capacity. Coexpression of the wt and I183N MC3Rs showed that I183N does not exert dominant-negative activity on wt MC3R. These results provide supporting evidence for the hypothesis proposed in the original case report that MC3R mutation might be a genetic factor that confers susceptibility to obesity, likely due to haploinsufficiency. Further mutations at I183 revealed a discrete requirement for I183 in agonist-induced MC3R activation. The corresponding residue is also important for agonist-induced human melanocortin-4 receptor and lutropin receptor activation. In summary, we identify a residue that is critical for activation of G protein-coupled receptors.

Melanocortin 4 receptor-mediated hyperphagia and activation of neuropeptide Y expression in the dorsomedial hypothalamus during lactation

In several hyperphagic models, including lactation, in which hypothalamic melanocortin signaling is reduced, a novel expression of NPY mRNA in the dorsomedial hypothalamus (DMH) has been observed, suggesting that melanocortin signaling and the induced NPY in the DMH may constitute unique neurocircuitry in mediating energy balance. Using lactating rats as a model, the present study first showed that in the DMH abundant alpha-MSH and agouti-related protein fibers are in close apposition to NPY-positive cells. However, no NPY and MC4R (a melanocortin receptor) double-labeled neurons were observed. These data suggested that melanocortin input may synapse on presynaptic terminals that then synapse on DMH NPY cells. To study the function of DMH MC4Rs in energy balance, an MC3/4R-selective agonist, melanotan II (MTII), was injected bilaterally into the DMH. MTII injection significantly suppressed feeding induced by 24 hr fasting or suckling-induced hyperphagia. Furthermore, MTII treatment greatly attenuated suckling-induced NPY expression in the DMH. MTII treatment also stimulated uncoupling protein 1 activity in the brown adipose tissue of suckling female rats, indicative of increased sympathetic outflow. In summary, the present study demonstrated that the melanocortin system in the DMH not only plays an important role in inducing NPY expression in the DMH of lactating rats but also in regulating energy homeostasis, at least in part, by modulating appetite and energy expenditure.

Gamma-MSH, sodium metabolism, and salt-sensitive hypertension

Alpha-, beta-, and gamma-melanocyte stimulating hormones (MSHs) are melanotropin peptides that are derived from the ACTH/beta-endorphin prohormone proopiomelanocortin (POMC). They have been highly conserved through evolutionary development, although their functions in mammals have remained obscure. The identification in the last decade of a family of five membrane-spanning melanocortin receptors (MC-Rs), for which the melanotropins are the natural ligands, has permitted the characterization of a number of important actions of these peptides, although the physiological function(s) of gamma-MSH have remained elusive. Much evidence indicates that gamma-MSH stimulates sympathetic outflow and raises blood pressure through a central mechanism. However, this review focuses on newer cardiovascular and renal actions of the peptide, acting in most cases through the MC3-R. In rodents, a high-sodium diet (HSD) increases the pituitary abundance of POMC mRNA and of gamma-MSH content and results in a doubling of plasma gamma-MSH concentration. The peptide is natriuretic and acts through renal MC3-Rs, which are also upregulated by the HSD. Thus the system appears designed to participate in the integrated response to dietary sodium excess. Genetic or pharmacologic induction of gamma-MSH deficiency results in marked salt-sensitive hypertension that is corrected by the administration of the peptide, probably through a central site of action. Deletion of the MC3-R also produces salt-sensitive hypertension, which, however, is not corrected by infusion of the hormone. These observations in aggregate suggest the operation of a hormonal system important in blood pressure control and in the regulation of sodium excretion. The relationship of these two actions to each other and the significance of this system in humans are important questions for future research.

Targeting Melanocortin Receptors as a Novel Strategy to Control Inflammation

Adrenocorticotropic hormone and alpha-, beta-, and gamma-melanocyte-stimulating hormones, collectively called melanocortin peptides, exert multiple effects upon the host. These effects range from modulation of fever and inflammation to control of food intake, autonomic functions, and exocrine secretions. Recognition and cloning of five melanocortin receptors (MCRs) has greatly improved understanding of peptide-target cell interactions. Preclinical investigations indicate that activation of certain MCR subtypes, primarily MC1R and MC3R, could be a novel strategy to control inflammatory disorders. As a consequence of reduced translocation of the nuclear factor kappaB to the nucleus, MCR activation causes a collective reduction of the major molecules involved in the inflammatory process. Therefore, anti-inflammatory influences are broad and are not restricted to a specific mediator. Short half-life and lack of selectivity could be an obstacle to the use of the natural melanocortins. However, design and synthesis of new MCR ligands with selective chemical properties are already in progress. This review examines how marshaling MCR could control inflammation.

The melanocortin system in Fugu: determination of POMC/AGRP/MCR gene repertoire and synteny, as well as pharmacology and anatomical distribution of the MCRs

The G-protein-coupled melanocortin receptors (MCRs) play an important role in a variety of essential functions such as the regulation of pigmentation, energy homeostasis, and steroid production. We performed a comprehensive characterization of the MC system in Fugu (Takifugu rubripes). We show that Fugu has an AGRP gene with high degree of conservation in the C-terminal region in addition to a POMC gene lacking gamma-MSH. The Fugu genome contains single copies of four MCRs, whereas the MC3R is missing. The MC2R and MC5R are found in tandem and remarkably contain one and two introns, respectively. We suggest that these introns were inserted through a reverse splicing mechanism into the DRY motif that is widely conserved through GPCRs. We were able to assemble large blocks around the MCRs in Fugu, showing remarkable synteny with human chromosomes 16 and 18. Detailed pharmacological characterization showed that ACTH had surprisingly high affinity for the Fugu MC1R and MC4R, whereas alpha-MSH had lower affinity. We also showed that the MC2R gene in Fugu codes for an ACTH receptor, which did not respond to alpha-MSH. All the Fugu receptors were able to couple functionally to cAMP production in line with the mammalian orthologs. The anatomical characterization shows that the MC2R is expressed in the brain in addition to the head-kidney, whereas the MC4R and MC5R are found in both brain regions and peripheral tissues. This is the first comprehensive genomic and functional characterization of a GPCR family within the Fugu genome. The study shows that some parts of the MC system are highly conserved through vertebrate evolution, such as regions in POMC coding for ACTH, alpha-MSH, and beta-MSH, the C-terminal region of AGRP, key binding units within the MC1R, MC2R, MC4R, and MC5R, synteny blocks around the MCRs, pharmacological properties of the MC2R, whereas other parts in the system are either missing, such as the MC3R and gamma-MSH, or different as compared to mammals, such as the affinity of ACTH and MSH peptides to MC1R and MC4R and the anatomical expression pattern of the MCRs.