Redundancy of a functional melanocortin 1 receptor in the anti-inflammatory actions of melanocortin peptides: studies in the recessive yellow (e/e) mouse suggest an important role for melanocortin 3 receptor

The multifunctional human ocular melanocortin system

Immune privilege in the eye involves physical barriers, immune regulation and secreted proteins that together limit the damaging effects of intraocular immune responses and inflammation. The neuropeptide alpha-melanocyte stimulating hormone (α-MSH) normally circulates in the aqueous humour of the anterior chamber and the vitreous fluid, secreted by iris and ciliary epithelium, and retinal pigment epithelium (RPE). α-MSH plays an important role in maintaining ocular immune privilege by helping the development of suppressor immune cells and by activating regulatory T-cells. α-MSH functions by binding to and activating melanocortin receptors (MC1R to MC5R) and receptor accessory proteins (MRAPs) that work in concert with antagonists, otherwise known as the melanocortin system. As well as controlling immune responses and inflammation, a broad range of biological functions is increasingly recognised to be orchestrated by the melanocortin system within ocular tissues. This includes maintaining corneal transparency and immune privilege by limiting corneal (lymph)angiogenesis, sustaining corneal epithelial integrity, protecting corneal endothelium and potentially enhancing corneal graft survival, regulating aqueous tear secretion with implications for dry eye disease, facilitating retinal homeostasis via maintaining blood-retinal barriers, providing neuroprotection in the retina, and controlling abnormal new vessel growth in the choroid and retina. The role of melanocortin signalling in uveal melanocyte melanogenesis however remains unclear compared to its established role in skin melanogenesis. The early application of a melanocortin agonist to downregulate systemic inflammation used adrenocorticotropic hormone (ACTH)-based repository cortisone injection (RCI), but adverse side effects including hypertension, edema, and weight gain, related to increased adrenal gland corticosteroid production, impacted clinical uptake. Compared to ACTH, melanocortin peptides that target MC1R, MC3R, MC4R and/or MC5R, but not adrenal gland MC2R, induce minimal corticosteroid production with fewer adverse systemic effects. Pharmacological advances in synthesising MCR-specific targeted peptides provide further opportunities for treating ocular (and systemic) inflammatory diseases. Following from these observations and a renewed clinical and pharmacological interest in the diverse biological roles of the melanocortin system, this review highlights the physiological and disease-related involvement of this system within human eye tissues. We also review the emerging benefits and versatility of melanocortin receptor targeted peptides as non-steroidal alternatives for inflammatory eye diseases such as non-infectious uveitis and dry eye disease, and translational applications in promoting ocular homeostasis, for example, in corneal transplantation and diabetic retinopathy.

Molecular cloning, tissue distribution, and pharmacologic function of melanocortin-3 receptor in common carp (Cyprinus carpio)

Melanocortin-3 receptor (MC3R) not only regulates energy homeostasis in animals, but also is an important regulator of inflammation. As one of the most widely farmed freshwater fish, common carp has attracted great interest for its feeding and inflammation regulation. In this study, we cloned the coding sequence (CDS) of common carp Mc3r (ccMc3r), examined its tissue expression profile, and investigated the function of this receptor in mediating downstream signaling pathways. The results showed that the CDS of ccMc3r was 975 bp, encoding a putative protein of 324 amino acids. Homology, phylogeny, and chromosomal synteny analyses revealed that ccMc3r is evolutionarily close to the orthologs of cyprinids. Quantitative real-time PCR (qPCR) indicated that ccMc3r was highly expressed in the brain and intestine. The luciferase reporter systems showed that four ligands, ACTH (1-24), α-MSH, β-MSH, and NDP-MSH, were able to activate the cAMP and MAPK/ERK signaling pathways downstream of ccMc3r with different potencies. For the cAMP signaling pathway, ACTH (1-24) had the highest activation potency; while for the MAPK/ERK signaling pathway, β-MSH had the greatest activation effect. In addition, we found that the four agonists were able to inhibit TNF-α-induced NF-κB signaling in approximately the same order of potency as cAMP signaling activation. This study may facilitate future studies on the role of Mc3r in common carp feed efficiency and immune regulation.

Modulation of Canine Melanocortin-3 and -4 Receptors by Melanocortin-2 Receptor Accessory Protein 1 and 2

The neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), have crucial roles in regulating energy homeostasis. The melanocortin-2 receptor accessory proteins (MRAPs, MRAP1 and MRAP2) have been shown to regulate neural MCRs in a species-specific manner. The potential effects of MRAP1 and MRAP2 on canine neural MCRs have not been investigated before. Herein, we cloned canine (c) MC3R and identified one canine MRAP2 splice variant, MRAP2b, with N-terminal extension of cMRAP2a. Canine MC3R showed higher maximal responses to five agonists than those of human MC3R. We further investigated the modulation of cMRAP1, cMRAP2a, and cMRAP2b, on cMC3R and cMC4R pharmacology. For the cMC3R, all MRAPs had no effect on trafficking; cMRAP1 significantly decreased B_max whereas cMRAP2a and cMRAP2b significantly increased B_max. Both MRAP1 and MRAP2a decreased R_maxs in response to α-MSH and ACTH; MRAP2b only decreased α-MSH-stimulated cAMP generation. For the MC4R, MRAP1 and MRAP2a increased cell surface expression, and MRAP1 and MRAP2a increased B_maxs. All MRAPs had increased affinities to α-MSH and ACTH. MRAP2a increased ACTH-induced cAMP levels, whereas MRAP2b decreased α-MSH- and ACTH-stimulated cAMP production. These findings may lead to a better understanding of the regulation of neural MCRs by MRAP1 and MRAP2s.

Regulation of Melanocortin-3 and -4 Receptors by Isoforms of Melanocortin-2 Receptor Accessory Protein 1 and 2

The neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), play essential non-redundant roles in the regulation of energy homeostasis. Interaction of neural MCRs and melanocortin-2 receptor accessory proteins (MRAPs, MRAP1 and MRAP2) is suggested to play pivotal roles in MC3R and MC4R signaling. In the present study, we identified two new human (h) MRAP2 splice variants, MRAP2b (465 bp open reading frame) and MRAP2c (381 bp open reading frame). Human MRAP2s are different in C-termini. We investigated the effects of five isoforms of MRAPs, hMRAP1a, hMRAP1b, hMRAP2a, hMRAP2b, and hMRAP2c, on MC3R and MC4R pharmacology. At the hMC3R, hMRAP1a and hMRAP2c increased and hMRAP1b decreased the cell surface expression. hMRAP1a increased affinity to ACTH. Four MRAPs (hMRAP1a, hMRAP1b, hMRAP2a, and hMRAP2c) decreased the maximal responses in response to α-MSH and ACTH. For hMC4R, hMRAP1a, hMRAP2a, and hMRAP2c increased the cell surface expression of hMC4R. Human MRAP1b significantly increased affinity to ACTH while MRAP2a decreased affinity to ACTH. Human MRAP1a increased ACTH potency. MRAPs also affected hMC4R basal activities, with hMRAP1s increasing and hMRAP2s decreasing the basal activities. In summary, the newly identified splicing variants, hMRAP2b and hMRAP2c, could regulate MC3R and MC4R pharmacology. The two MRAP1s and three MRAP2s had differential effects on MC3R and MC4R trafficking, binding, and signaling. These findings led to a better understanding of the regulation of neural MCRs by MRAP1s and MRAP2s.

Naturally occurring mutations in G protein-coupled receptors associated with obesity and type 2 diabetes mellitus

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors involved in the regulation of almost all known physiological processes. Dysfunctions of GPCR-mediated signaling have been shown to cause various diseases. The prevalence of obesity and type 2 diabetes mellitus (T2DM), two strongly associated disorders, is increasing worldwide, with tremendous economical and health burden. New safer and more efficacious drugs are required for successful weight reduction and T2DM treatment. Multiple GPCRs are involved in the regulation of energy and glucose homeostasis. Mutations in these GPCRs contribute to the development and progression of obesity and T2DM. Therefore, these receptors can be therapeutic targets for obesity and T2DM. Indeed some of these receptors, such as melanocortin-4 receptor and glucagon-like peptide 1 receptor, have provided important new drugs for treating obesity and T2DM. This review will focus on the naturally occurring mutations of several GPCRs associated with obesity and T2DM, especially incorporating recent large genomic data and insights from structure-function studies, providing leads for future investigations.

Topmouth culter melanocortin-3 receptor: regulation by two isoforms of melanocortin-2 receptor accessory protein 2

Melanocortin-3 receptor (MC3R) is a regulator of energy homeostasis, and interaction of MC3R and melanocortin-2 receptor accessory protein 2 (MRAP2) plays a critical role in MC3R signaling of mammals. However, the physiological roles of MC3R in teleosts are not well understood. In this study, qRT-PCR was used to measure gene expression. Radioligand binding assay was used to study the binding properties of topmouth culter MC3R (caMC3R). Intracellular cAMP generation was determined by RIA, and caMC3R expression was quantified with flow cytometry. We showed that culter mc3r had higher expression in the CNS. All agonists could bind and stimulate caMC3R to increase dose dependently intracellular cAMP accumulation. Compared to human MC3R, culter MC3R showed higher constitutive activity, higher efficacies, and Rmax to alpha-melanocyte-stimulating hormone (α-MSH), des-α-MSH, and adrenocorticotrophic hormone. Both caMRAP2a and caMRAP2b markedly decreased caMC3R basal cAMP production. However, only caMRAP2a significantly decreased cell surface expression, Bmax, and Rmax of caMC3R. Expression analysis suggested that MRAP2a and MRAP2b might be more important in regulating MC3R/MC4R signaling during larval period, and reduced mc3r, mc4r, and pomc expression might be primarily involved in modulation of MC3R/MC4R in adults. These data indicated that the cloned caMC3R was a functional receptor. MRAP2a and MRAP2b had different effects on expression and signaling of caMC3R. In addition, expression analysis suggested that MRAP2s, receptors, and hormones might play different roles in regulating culter development and growth.

Neuropeptides and neurohormones in immune, inflammatory and cellular responses to ultraviolet radiation

Humans are exposed to varying amounts of ultraviolet radiation (UVR) through sunlight. UVR penetrates into human skin leading to release of neuropeptides, neurotransmitters and neuroendocrine hormones. These messengers released from local sensory nerves, keratinocytes, Langerhans cells (LCs), mast cells, melanocytes and endothelial cells (ECs) modulate local and systemic immune responses, mediate inflammation and promote differing cell biologic effects. In this review, we will focus on both animal and human studies that elucidate the roles of calcitonin gene-related peptide (CGRP), substance P (SP), nerve growth factor (NGF), nitric oxide and proopiomelanocortin (POMC) derivatives in mediating immune and inflammatory effects of exposure to UVR as well as other cell biologic effects of UVR exposure.

Activation of Melanocortin Receptors as a Potential Strategy to Reduce Local and Systemic Reactions Induced by Respiratory Viruses

The clinical hallmarks of infections caused by critical respiratory viruses consist of pneumonia, which can progress to acute lung injury (ALI), and systemic manifestations including hypercoagulopathy, vascular dysfunction, and endotheliitis. The disease outcome largely depends on the immune response produced by the host. The bio-molecular mechanisms underlying certain dire consequences of the infection partly arise from an aberrant production of inflammatory molecules, an event denoted as "cytokine storm". Therefore, in addition to antiviral therapies, molecules able to prevent the injury caused by cytokine excess are under investigation. In this perspective, taking advantage of melanocortin peptides and their receptors, components of an endogenous modulatory system that exerts marked anti-inflammatory and immunomodulatory influences, could be an effective therapeutic strategy to control disease evolution. Exploiting the melanocortin system using natural or synthetic ligands can form a realistic basis to counteract certain deleterious effects of respiratory virus infections. The central and peripheral protective actions exerted following melanocortin receptor activation could allow dampening the harmful events that trigger the cytokine storm and endothelial dysfunction while sustaining the beneficial signals required to elicit repair mechanisms. The long standing evidence for melanocortin safety encourages this approach.

Emerging roles of melanocortin receptor accessory proteins (MRAP and MRAP2) in physiology and pathophysiology

Melanocortin-2 receptor accessory protein (MRAP) has an unusual dual topology and influences the expression, localisation, signalling and internalisation of the melanocortin receptor 2 (MC₂); the adrenocorticotropic hormone (ACTH) receptor. Mutations in MRAP are associated with familial glucocorticoid deficiency type-2 and evidence is emerging of the importance of MRAP in adrenal development and ACTH signalling. Human MRAP has two functional splice variants: MRAP-α and MRAP-β, unlike MRAP-β, MRAP-α has little expression in brain but is highly expressed in ovary. MRAP2, identified through whole human genome sequence analysis, has approximately 40% sequence homology to MRAP. MRAP2 facilitates MC2 localisation to the cell surface but not ACTH signalling. MRAP and MRAP2 have been found to regulate the surface expression and signalling of all melanocortin receptors (MC_1-5). Additionally, MRAP2 moderates the signalling of the G-protein coupled receptors (GCPRs): orexin, prokineticin and GHSR1a; the ghrelin receptor. Whilst MRAP appears to be mainly involved in glucocorticoid synthesis, an important role is emerging for MRAP2 in regulating appetite and energy homeostasis. Transgenic models indicate the importance of MRAP in adrenal gland formation. Like MC3R and MC4R knockout mice, MRAP2 knockout mice have an obese phenotype. In vitro studies indicate that MRAP2 enhances the MC3 and MC4 response to the agonist αMSH, which, like ACTH, is produced through precursor polypeptide proopiomelanocortin (POMC) cleavage. Analysis of cohorts of individuals with obesity have revealed several MRAP2 genetic variants with loss of function mutations which are causative of monogenic hyperphagic obesity with hyperglycaemia and hypertension. MRAP2 may also be associated with female infertility. This review summarises current knowledge of MRAP and MRAP2, their influence on GPCR signalling, and focusses on pathophysiology, particularly familial glucocorticoid deficiency type-2 and obesity.

Melanocortin 3 receptor activation with [D-Trp8]-γ-MSH suppresses inflammation in apolipoprotein E deficient mice

The melanocortin MC₁ and MC₃ receptors elicit anti-inflammatory actions in leukocytes and activation of these receptors has been shown to alleviate arterial inflammation in experimental atherosclerosis. Thus, we aimed to investigate whether selective targeting of melanocortin MC₃ receptor protects against atherosclerosis. Apolipoprotein E deficient (ApoE^-/-) mice were fed high-fat diet for 12 weeks and randomly assigned to receive either vehicle (n = 11) or the selective melanocortin MC₃ receptor agonist [D-Trp(8)]-gamma-melanocyte-stimulating hormone ([D-Trp8]-γ-MSH; 15 μg/day, n = 10) for the last 4 weeks. Lesion size as well as macrophage and collagen content in the aortic root plaques were determined. Furthermore, leukocyte counts in the blood and aorta and cytokine mRNA expression levels in the spleen, liver and aorta were quantified. No effect was observed in the body weight development or plasma cholesterol level between the two treatment groups. However, [D-Trp8]-γ-MSH treatment significantly reduced plasma levels of chemokine (C-C motif) ligands 2, 4 and 5. Likewise, cytokine and adhesion molecule expression levels were reduced in the spleen and liver of γ-MSH-treated mice, but not substantially in the aorta. In line with these findings, [D-Trp8]-γ-MSH treatment reduced leukocyte counts in the blood and aorta. Despite reduced inflammation, [D-Trp8]-γ-MSH did not change lesion size, macrophage content or collagen deposition of aortic root plaques. In conclusion, the findings indicate that selective activation of melanocortin MC₃ receptor by [D-Trp8]-γ-MSH suppresses systemic and local inflammation and thereby also limits leukocyte accumulation in the aorta. However, the treatment was ineffective in reducing atherosclerotic plaque size.

Pharmacology of the giant panda (Ailuropoda melanoleuca) melanocortin-3 receptor

The melanocortin-3 receptor (MC3R) is a member of the G protein-coupled receptor superfamily that plays a critical role in controlling energy balance and metabolism. Although pharmacological characterization of MC3R has been reported previously in several other species, there is no report on the MC3R from giant panda (Ailuropoda melanoleuca). This ancient species is known as a 'living fossil' and is among the most endangered animals in the world. Giant panda survive on a specialized diet of bamboo despite possessing a typical carnivorous digestive system. We report herein the molecular cloning and pharmacological characterization of amMC3R. Homology and phylogenetic analysis showed that amMC3R was highly homologous (>85%) to several other mammalian MC3Rs. Using human MC3R (hMC3R) as a control, the binding of five agonists, [Nle⁴, D-Phe⁷]-α-melanocyte stimulating hormone (NDP-MSH), α-, β-, γ-, and D-Trp⁸-γ-MSH, was investigated, as well as Gs-cAMP and pERK1/2 signaling. The results showed that amMC3R bound NDP- and D-Trp⁸-γ-MSH with the highest affinity, followed by α-, β-, and γ-MSH, with the same rank order as hMC3R. When stimulated with agonists, amMC3R displayed increased intracellular cAMP and activation of pERK1/2. These data suggest that the cloned amMC3R was a functional receptor. The availability of amMC3R and knowledge of its pharmacological functions will assist further investigation of its role in controlling energy balance and metabolism.

Generation of an MC3R knock-out pig by CRSPR/Cas9 combined with somatic cell nuclear transfer (SCNT) technology

BACKGROUND

Melanocortin 3 receptor (MC3R), a rhodopsin-like G protein-coupled receptor, is an important regulator of metabolism. Although MC3R knock-out (KO) mice and rats were generated in earlier studies, the function of MC3R remains elusive. Since pig models have many advantages over rodents in metabolism research, we generated an MC3R-KO pig using a CRSPR/Cas9-based system combined with somatic cell nuclear transfer (SCNT) technology.

METHOD

Four CRSPR/Cas9 target vectors were constructed and then their cleavage efficiency was tested in porcine fetal fibroblasts (PFFs). The pX330-sgRNA1 and pX330-sgRNA4 vectors were used to co-transfect PFFs to obtain positive colonies. PCR screening and sequencing were conducted to identify the genotype of the colonies. The biallelically modified colonies and wild-type control colonies were used simultaneously as donor cells for SCNT. A total of 1203 reconstructed embryos were transferred into 6 surrogates, of which one became pregnant. The genotypes of the resulting piglets were determined by PCR and sequencing, and off-target effects in the MC3R KO piglets were detected by sequencing. Then, offspring were obtained through breeding and six male KO pigs were used for the growth performance analysis.

RESULTS

Four vectors were constructed successfully, and their cleavage efficiencies were 27.96, 44.89, 32.72 and 38.86%, respectively. A total of 21 mutant colonies, including 11 MC3R-/- and 10 MC3R+/- clones, were obtained, corresponding to a gene targeting efficiency of 29.17%, with 15.28% biallelic mutations. A total of 6 piglets were born, and only two MC3R KO piglets were generated, one with malformations and a healthy one. No off-target effects were detected by sequencing in the healthy mutant. Six male MC3R KO pigs were obtained in the F2 generation and their body weight and body fat were both increased compared to wild-type full siblings.

CONCLUSION

A MC3R KO pig strain was generated using the CRSIPR/Cas9-based system, which makes it possible to study the biological function of MC3R in a non-rodent model.

Melanocortin Regulation of Inflammation

Adrenocorticotropic hormone (ACTH), and α-, β-, and γ-melanocyte-stimulating hormones (α-, β-, γ-MSH), collectively known as melanocortins, together with their receptors (melanocortin receptors), are components of an ancient modulatory system. The clinical use of ACTH in the treatment of rheumatoid arthritis started in 1949, originally thought that the anti-inflammatory action was through hypothalamus-pituitary-adrenal axis and glucocorticoid-dependent. Subsequent decades have witnessed extensive attempts in unraveling the physiology and pharmacology of the melanocortin system. It is now known that ACTH, together with α-, β-, and γ-MSHs, also possess glucocorticoid-independent anti-inflammatory and immunomodulatory effects by activating the melanocortin receptors expressed in the brain or peripheral immune cells. This review will briefly introduce the melanocortin system and highlight the action of melanocortins in the regulation of immune functions from in vitro, in vivo, preclinical, and clinical studies. The potential therapeutic use of melanocortins are also summarized.

Negative regulators that mediate ocular immune privilege

The ocular microenvironment has adapted several negative regulators of inflammation to maintain immune privilege and health of the visual axis. Several constitutively produced negative regulators within the eye TGF-β2, α-melanocyte stimulating hormone (α-MSH), Fas ligand (FasL), and PD-L1 standout because of their capacity to influence multiple pathways of inflammation, and that they are part of promoting immune tolerance. These regulators demonstrate the capacity of immune privilege to prevent the activation of inflammation, and to suppress activation of effector immune cells even under conditions of ocular inflammation induced by endotoxin and autoimmune disease. In addition, these negative regulators promote and expand immune cells that mediate regulatory and tolerogenic immunity. This in turn makes the immune cells themselves negative regulators of inflammation. This provides for a greater understanding of immune privilege in that it includes both molecular and cellular negative regulators of inflammation. This would mean that potentially new approaches to the treatment of autoimmune disease can be developed through the use of molecules and cells as negative regulators of inflammation.

Bioelectric regulation of innate immune system function in regenerating and intact Xenopus laevis

Two key inputs that regulate regeneration are the function of the immune system, and spatial gradients of transmembrane potential (Vmem). Endogenous bioelectric signaling in somatic tissues during regenerative patterning is beginning to be understood, but its role in the context of immune response has never been investigated. Here, we show that Vmem levels modulate innate immunity activity in Xenopus laevis embryos. We developed an assay in which X. laevis embryos are infected with a uropathogenic microorganism, in the presence or absence of reagents that modify Vmem, prior to the ontogenesis of the adaptive immune system. General depolarization of the organism's Vmem by pharmacological or molecular genetic (ion channel misexpression) methods increased resistance to infection, while hyperpolarization made the embryos more susceptible to death by infection. Hyperpolarized specimens harbored a higher load of infectious microorganisms when compared to controls. We identified two mechanisms by which Vmem mediates immune function: serotonergic signaling involving melanocytes and an increase in the number of primitive myeloid cells. Bioinformatics analysis of genes whose transcription is altered by depolarization revealed a number of immune system targets consistent with mammalian data. Remarkably, amputation of the tail bud potentiates systemic resistance to infection by increasing the number of peripheral myeloid cells, revealing an interplay of regenerative response, innate immunity, and bioelectric regulation. Our study identifies bioelectricity as a new mechanism by which innate immune response can be regulated in the context of infection or regeneration. Vmem modulation using drugs already approved for human use could be exploited to improve resistance to infections in clinical settings.

Melanocortin 1 Receptor Signaling Regulates Cholesterol Transport in Macrophages

BACKGROUND

The melanocortin 1 receptor (MC1-R) is expressed by monocytes and macrophages, where it exerts anti-inflammatory actions on stimulation with its natural ligand α-melanocyte-stimulating hormone. The present study was designed to investigate the specific role of MC1-R in the context of atherosclerosis and possible regulatory pathways of MC1-R beyond anti-inflammation.

METHODS

Human and mouse atherosclerotic samples and primary mouse macrophages were used to study the regulatory functions of MC1-R. The impact of pharmacological MC1-R activation on atherosclerosis was assessed in apolipoprotein E-deficient mice.

RESULTS

Characterization of human and mouse atherosclerotic plaques revealed that MC1-R expression localizes in lesional macrophages and is significantly associated with the ATP-binding cassette transporters ABCA1 and ABCG1, which are responsible for initiating reverse cholesterol transport. Using bone marrow-derived macrophages, we observed that α-melanocyte-stimulating hormone and selective MC1-R agonists similarly promoted cholesterol efflux, which is a counterregulatory mechanism against foam cell formation. Mechanistically, MC1-R activation upregulated the levels of ABCA1 and ABCG1. These effects were accompanied by a reduction in cell surface CD36 expression and in cholesterol uptake, further protecting macrophages from excessive lipid accumulation. Conversely, macrophages deficient in functional MC1-R displayed a phenotype with impaired efflux and enhanced uptake of cholesterol. Pharmacological targeting of MC1-R in atherosclerotic apolipoprotein E-deficient mice reduced plasma cholesterol levels and aortic CD36 expression and increased plaque ABCG1 expression and signs of plaque stability.

CONCLUSIONS

Our findings identify a novel role for MC1-R in macrophage cholesterol transport. Activation of MC1-R confers protection against macrophage foam cell formation through a dual mechanism: It prevents cholesterol uptake while concomitantly promoting ABCA1- and ABCG1-mediated reverse cholesterol transport.

Polymorphisms and mutations in the melanocortin-3 receptor and their relation to human obesity

Inactivating mutations in the melanocortin 3 receptor (Mc3r) have been described as causing obesity in mice, but the physiologic effects of MC3R mutations in humans have been less clear. Here we review the MC3R polymorphisms and mutations identified in humans, and the in vitro, murine, and human cohort studies examining their putative effects. Some, but not all, studies suggest that the common human MC3R variant T6K+V81I, as well as several other rare, function-altering mutations, are associated with greater adiposity and hyperleptinemia with altered energy partitioning. In vitro, the T6K+V81I variant appears to decrease MC3R expression and therefore cAMP generation in response to ligand binding. Knockin mouse studies confirm that the T6K+V81I variant increases feeding efficiency and the avidity with which adipocytes derived from bone or adipose tissue stem cells store triglycerides. Other MC3R mutations occur too infrequently in the human population to make definitive conclusions regarding their clinical effects. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Effects of Melanocortin 3 and 4 Receptor Deficiency on Energy Homeostasis in Rats

Melanocortin-3 and 4 receptors (MC3R and MC4R) can regulate energy homeostasis, but their respective roles especially the functions of MC3R need more exploration. Here Mc3r and Mc4r single and double knockout (DKO) rats were generated using CRISPR-Cas9 system. Metabolic phenotypes were examined and data were compared systematically. Mc3r KO rats displayed hypophagia and decreased body weight, while Mc4r KO and DKO exhibited hyperphagia and increased body weight. All three mutants showed increased white adipose tissue mass and adipocyte size. Interestingly, although Mc3r KO did not show a significant elevation in lipids as seen in Mc4r KO, DKO displayed even higher lipid levels than Mc4r KO. DKO also showed more severe glucose intolerance and hyperglycaemia than Mc4r KO. These data demonstrated MC3R deficiency caused a reduction of food intake and body weight, whereas at the same time exhibited additive effects on top of MC4R deficiency on lipid and glucose metabolism. This is the first phenotypic analysis and systematic comparison of Mc3r KO, Mc4r KO and DKO rats on a homogenous genetic background. These mutant rats will be important in defining the complicated signalling pathways of MC3R and MC4R. Both Mc4r KO and DKO are good models for obesity and diabetes research.

D-TRP(8)-γMSH Prevents the Effects of Endotoxin in Rat Skeletal Muscle Cells through TNFα/NF-KB Signalling Pathway

Sepsis induces anorexia and muscle wasting secondary to an increase in muscle proteolysis. Melanocyte stimulating hormones (MSH) is a family of peptides that have potent anti-inflammatory effects. Melanocortin receptor-3 (MC3-R) has been reported as the predominant anti-inflammatory receptor for melanocortins. The aim of this work was to analyse whether activation of MC3-R, by administration of its agonist D-Trp(8)-γMSH, is able to modify the response of skeletal muscle to inflammation induced by lipopolysaccharide endotoxin (LPS) or TNFα. Adult male rats were injected with 250 μg/kg LPS and/or 500 μg/kg D-Trp(8)-γMSH 17:00 h and at 8:00 h the following day, and euthanized 4 hours afterwards. D-Trp(8)-γMSH decreased LPS-induced anorexia and prevented the stimulatory effect of LPS on hypothalamic IL-1β, COX-2 and CRH as well as on serum ACTH and corticosterone. Serum IGF-I and its expression in liver and gastrocnemius were decreased in rats injected with LPS, but not in those that also received D-Trp(8)-γMSH. However, D-Trp(8)-γMSH was unable to modify the effect of LPS on IGFBP-3. In the gastrocnemius D-Trp(8)-γMSH blocked LPS-induced decrease in pAkt, pmTOR, MHC I and MCH II, as well as the increase in pNF-κB(p65), FoxO1, FoxO3, LC3b, Bnip-3, Gabarap1, atrogin-1, MuRF1 and in LC3a/b lipidation. In L6 myotube cultures, D-Trp(8)-γMSH was able to prevent TNFα-induced increase of NF-κB(p65) phosphorylation and decrease of Akt phosphorylation as well as of IGF-I and MHC I expression. These data suggest that MC3-R activation prevents the effect of endotoxin on skeletal wasting by modifying inflammation, corticosterone and IGF-I responses and also by directly acting on muscle cells through the TNFα/NF-κB(p65) pathway.

The melanocortin receptor type 3 agonist d-Trp(8)-γMSH decreases inflammation and muscle wasting in arthritic rats

BACKGROUND

Chronic inflammatory diseases induce cachexia that increases mortality and morbidity of the illness. Adjuvant-induced arthritis is an experimental model of rheumatoid arthritis that is associated with body weight loss and muscle wasting. Alpha-melanocyte stimulating hormone has an anti-inflammatory effect in arthritic rats and decreases muscle wasting. The aim of this work was to elucidate whether the anti-cachectic action of alpha-melanocyte stimulating hormone is mediated by the melanocortin receptor type 3 pathway.

METHODS

Arthritis was induced in male Wistar rats by intradermal injection of Freund's adjuvant, and 6 days afterwards, arthritic rats were injected with the selective melanocortin receptor type 3 agonist d-Trp(8)-gammaMSH ( d-Trp(8)-γMSH) 500 µg/kg subcutaneously. or saline twice a day, for 10 days.

RESULTS

d-Trp(8)-γMSH decreased the external signs of inflammation and body weight loss, but it was not able to modify the anorexigenic effect of arthritis or the increase in hypothalamic cyclooxygenase-2 (COX-2) expression. In contrast, d-Trp(8)-γMSH prevented arthritis-induced increase in hypothalamic IL-1β and serum corticosterone levels and the decrease in serum IGF-I levels. d-Trp(8)-γMSH treatment also prevented arthritis-induced NF-kB(p65) phosphorylation and tumour necrosis factor-α mRNA increase in the gastrocnemius. d-Trp(8)-γMSH administration to arthritic rats increased gastrocnemius mass, its cross-sectional area, and mean fast fibre area. Those effects of d-Trp(8)-γMSH were associated with a decreased expression of atrogin-1 and muscle ring-finger protein-1 in the gastrocnemius. In rats treated with saline, arthritis increased the expression of autophagy marker genes LC3b, Bnip-3, and Gabarap1 as well as the conversion of LC3b I to LC3b II by lipidation in the gastrocnemius. d-Trp(8)-γMSH decreased gastrocnemius LC3b, Bnip-3, and Gabarap1 mRNA expression and prevented the increase in LC3b II in arthritic rats.

CONCLUSION

These data suggest that d-Trp(8)-γMSH administration prevents the effect of arthritis on corticosterone and insulin-like growth factor-I serum levels and decreases muscle wasting, by down-regulating atrogenes and autophagy through modifying the NF-kB(p65)/tumour necrosis factor-α signalling transduction pathway.

The Role of Alpha-MSH as a Modulator of Ocular Immunobiology Exemplifies Mechanistic Differences between Melanocortins and Steroids

Melanocortins are a highly conserved family of peptides and receptors that includes multiple proopiomelanocortin-derived peptides and five defined melanocortin receptors. The melanocortins have an important role in maintaining immune homeostasis and in suppressing inflammation. Within the healthy eye, the melanocortins have a central role in preventing inflammation and maintaining immune privilege. A central mediator of the anti-inflammatory activity is the non-steroidogenic melanocortin peptide alpha-melanocyte stimulating hormone. In this review we summarize the major findings of melanocortin regulation of ocular immunobiology with particular interest in the ability of melanocortin to induce immune tolerance and cytoprotection. The melanocortins have therapeutic potential because their mechanisms of action in regulating immunity are distinctly different from the actions of steroids.

Functions of DPLIY motif and helix 8 of human melanocortin-3 receptor

The melanocortin-3 receptor (MC3R) is a member of the family A G protein-coupled receptors (GPCRs). The MC3R remains the most enigmatic of the melanocortin receptors with regard to its physiological functions, especially its role in energy homeostasis. The N/DPxxY motif and the eighth helix (helix 8) in the carboxyl terminus of GPCRs have been identified to be important for receptor expression, ligand binding, signal transduction and internalization. To gain a better understanding of the structure-function relationship of MC3R, we performed a systematic study of all 20 residues in this domain using alanine-scanning mutagenesis. We showed that although all mutants were expressed normally on the cell surface, eleven residues were important for ligand binding and one was indispensable for downstream cAMP generation. F347A showed constitutive activity in cAMP signaling while all the other mutants had normal basal activities. We studied the signaling capacity of nine mutants in the ERK1/2 signaling pathway. All of these mutants showed normal basal ERK1/2 phosphorylation levels. The pERK1/2 levels of six binding- or signaling-defective mutants were enhanced upon agonist stimulation. The unbalanced cAMP and pERK1/2 signaling pathways suggested the existence of biased signaling in MC3R mutants. In summary, we showed that the DPLIY motif and helix 8 was important for MC3R activation and signal transduction. Our data led to a better understanding of the structure-function relationship of MC3R.

Improved cutaneous wound healing after intraperitoneal injection of alpha-melanocyte-stimulating hormone

Skin wound healing is a complex process involving many types of cells and molecules and often results in scar tissue formation in adult mammals. However, scarless healing occurs in foetal skin and minimal scars may occur after cutaneous healing in the adult with reduced inflammation. Alpha-melanocyte-stimulating hormone (α-MSH) is widely distributed within the central nervous system and in other body regions, such as the skin, and has strong anti-inflammatory activity. The aim in the present experiments was to learn whether intraperitoneal (i.p) injection of α-MSH just before skin wounds antagonize inflammation and improves skin wound healing in adult mice. C57BL/6 young adult mice received an i.p. injection of 1 mg/kg of α-MSH and, 30 min later, two circular through-and-through holes (6.5 mm diameter) were made in their dorsal skin under anaesthesia. Control mice were wounded after vehicle injection. The wound healing process was analysed macroscopically and microscopically at 3, 7, 40 and 60 days. Skin samples were fixed in formalin, embedded in paraffin, sectioned at 5 μm, stained with H&E or toluidine blue for cell analysis or Gomori's trichrome for extracellular matrix (ECM) analysis. Other samples were fixed in DMSO+methanol, embedded in paraplast and incubated with anti-CD45, antismooth muscle actin, anticollagen-I and anticollagen-III for immunofluorescence analysis. Alpha-MSH significantly reduced the number of leucocytes, mast cells and fibroblasts at 3 and 7 days after injury. On days 40 and 60, α-MSH reduced scar area and improved the organization of the collagen fibres indicating that it may direct the healing into a more-regenerative/less-scarring pathway.

Is Mc1r an important regulator of non-pigmentary responses to UV radiation?

MC1R is recognized for its role in the regulation of melanin pigmentation. In addition, many investigators believe that it also plays a crucial role in immunomodulation (immunosuppression) and in melanogenesis-independent protective responses against ultraviolet radiation (UVR). Surprisingly, Wolnicka-Glubisz et al. have shown that loss of function in the MC1R has no effect on inflammatory responses and immunosuppression induced by UVR in C57BL/6 mice as well as on the degree of UVA-induced DNA damage in the epidermis and dermis. These findings, by challenging the existing dogmas on the precise role of MC1R in non-pigmentary responses to the UVR, mandate further research to either validate the presented data or to define to which degree these phenomena are restricted to the C57BL/6 mouse model or are applicable to other species including humans. The alternative target for immunomodulation is represented by MC3R. However, cutaneous expression of MC3R remains to be demonstrated.

Is MC1 dispensable for regulation of cutaneous inflammatory and immune responses?

The melanocortin-1 receptor (MC1 ) - being most abundantly expressed in the skin by melanocytes - has a physiological role for melanin pigmentation in many vertebrate species. MC1 has also been implicated in regulation of skin inflammation as this receptor is detectable in the majority of non-melanocytic cell types and its ligand α-melanocyte-stimulating hormone (α-MSH) exerts immunoregulatory and anti-inflammatory effects. However, in vivo studies on mice with targeted disruption of MC1 have been missing in the context of skin inflammation until recently. Wolnicka-Glubisz et al. now reported that the course of ultraviolet (UV)-induced inflammation, contact hypersensitivity, neonatal immune tolerance and UV-induced immunosuppression is similar in MC1 signal-deficient (C57BL/6-Mc1r(e/e)) and wild-type mice. These unexpected findings are supported by own observations in experimentally induced immune-complex-mediated vasculitis: Mc1r(e/e) mice exhibited a similar extent of the reverse passive cutaneous Arthus reaction compared with wild-type animals. Future studies are thus needed to clarify whether these findings are due to limitations in the chosen mouse model and/or point to additional MC subtypes that may regulate inflammatory and immune responses in the skin.

Functional melanocortin 1 receptor Mc1r is not necessary for an inflammatory response to UV radiation in adult mouse skin

The G-protein-coupled receptor, Mc1r, plays a major role in pigment production and has been reported to be important in the inflammatory response. We have investigated the effect of deficiency in Mc1r on UV-induced inflammation. Mice on the same genetic background were used - C57BL/6-c (albino), C57BL/6 (black), C57BL/6-Mc1r(e/e) deficient (yellow). FACS analysis of disaggregated skin showed a similar dose-dependent increase in Ly6G(+) and CD11b(+) cells in response to UV radiation in all groups. No differences in UV-induced edema or in DNA damage were detected between groups. The contact hypersensitivity response, neonatal immune tolerance and UV immunosuppression were all similar in C57BL/6 and C57BL/6-Mc1r(e/e) mice. We conclude that the absence of Mc1r does not impair the inflammatory response to UV radiation or the generation of immunosuppression.

Curbing Inflammation through Endogenous Pathways: Focus on Melanocortin Peptides

The resolution of inflammation is now known to be an active process, armed with a multitude of mediators both lipid and protein in nature. Melanocortins are peptides endowed with considerable promise with their proresolution and anti-inflammatory effects in preclinical models of inflammatory disease, with tissue protective effects. These peptides and their targets are appealing because they can be seen as a natural way of inducing these effects as they harness endogenous pathways of control. Whereas most of the information generated about these mediators derives from several acute models of inflammation (such as zymosan induced peritonitis), there is some indication that these mediators may inhibit chronic inflammation by modulating cytokines, chemokines, and leukocyte apoptosis. In addition, proresolving mediators and their mimics have often been tested alongside therapeutic protocols, hence have been tested in settings more relevant to real life clinical scenarios. We provide here an overview on some of these mediators with a focus on melanocortin peptides and receptors, proposing that they may unveil new opportunities for innovative treatments of inflammatory arthritis.

Melanocortin receptors as novel effectors of macrophage responses in inflammation

Macrophages have crucial functions in initiating the inflammatory reaction in a strict temporal and spatial manner to provide a “clear-up” response required for resolution. Hormonal peptides such as melanocortins modulate macrophage reactivity and attenuate inflammation ranging from skin inflammation to joint disease and reperfusion injury. The melanocortins (e.g., adrenocorticotrophin, ACTH and αMSH) elicit regulatory properties through activation of a family of GPCRs, the melanocortin (MC) receptors; MC₁–MC₅. Several studies have focused on MC₁ and MC₃ as anti-inflammatory receptors expressed on cells of the macrophage lineage. We review here elements of the melanocortin pathway with particular attention to macrophage function in anti-inflammatory and pro-resolving inflammatory settings. Evidence shows that ACTH, αMSH, and other MC agonists can activate MC₁ and MC₃ on macrophage through cAMP and/or NFκB-dependent mechanisms to abrogate pro-inflammatory cytokines, chemokines, and NO and enhance anti-inflammatory mediators such as IL-10 and HO-1. Melanocortins and their receptors regulate inflammation by inhibiting leukocyte recruitment to and interaction with inflamed tissue. An intensely exciting addition to this field of research has been the ability of an αMSH analog; AP214 to activate MC₃ expressed on macrophage to enhance their clearance of both zymosan particles and apoptotic neutrophils thus putting melanocortins in line with other pro-resolving mediators. The use of mouse colonies mutated or nullified for MC₁ or MC₃, respectively as well as availability of selective MC receptor agonist/antagonists have been key to deciphering mechanisms by which elements of the melanocortin system play a role in these phenomena. We review here melanocortin pathway components with attention to the macrophage, reiterating receptor targets required for pro-resolving properties. The overall outcome will be identification of selective MC agonists as a strategy for innovative anti-inflammatory therapeutics.

Physiological roles of the melanocortin MC₃ receptor

The melanocortin MC(3) receptor remains the most enigmatic of the melanocortin receptors with regard to its physiological functions. The receptor is expressed both in the CNS and in multiple tissues in the periphery. It appears to be an inhibitory autoreceptor on proopiomelanocortin neurons, yet global deletion of the receptor causes an obesity syndrome. Knockout of the receptor increases adipose mass without a readily measurable increase in food intake or decrease in energy expenditure. And finally, no melanocortin MC(3) receptor null humans have been identified and associations between variant alleles of the melanocortin MC(3) receptor and diseases remain controversial, so the physiological role of the receptor in humans remains to be determined.

Melanocortin 1 receptor agonists reduce proteinuria

Membranous nephropathy is one of the most common causes of nephrotic syndrome in adults. Recent reports suggest that treatment with adrenocorticotropic hormone (ACTH) reduces proteinuria, but the mechanism of action is unknown. Here, we identified gene expression of the melanocortin receptor MC1R in podocytes, glomerular endothelial cells, mesangial cells, and tubular epithelial cells. Podocytes expressed most MC1R protein, which colocalized with synaptopodin but not with an endothelial-specific lectin. We treated rats with passive Heymann nephritis (PHN) with MS05, a specific MC1R agonist, which significantly reduced proteinuria compared with untreated PHN rats (P < 0.01). Furthermore, treatment with MC1R agonists improved podocyte morphology and reduced oxidative stress. In summary, podocytes express MC1R, and MC1R agonism reduces proteinuria, improves glomerular morphology, and reduces oxidative stress in nephrotic rats with PHN. These data may explain the proteinuria-reducing effects of ACTH observed in patients with membranous nephropathy, and MC1R agonists may provide a new therapeutic option for these patients.

The melanocortin MC(1) receptor agonist BMS-470539 inhibits leucocyte trafficking in the inflamed vasculature

BACKGROUND AND PURPOSE

Over three decades of research evaluating the biology of melanocortin (MC) hormones and synthetic peptides, activation of the MC type 1 (MC(1)) receptor has been identified as a viable target for the development of novel anti-inflammatory therapeutic agents. Here, we have tested a recently described selective agonist of MC(1) receptors, BMS-470539, on leucocyte/post-capillary venule interactions in murine microvascular beds.

EXPERIMENTAL APPROACH

Intravital microscopy of two murine microcirculations were utilized, applying two distinct modes of promoting inflammation. The specificity of the effects of BMS-470539 was assessed using mice bearing mutant inactive MC(1) receptors (the recessive yellow e/e colony).

KEY RESULTS

BMS-470539, given before an ischaemia-reperfusion protocol, inhibited cell adhesion and emigration with no effect on cell rolling, as assessed 90 min into the reperfusion phase. These properties were paralleled by inhibition of tissue expression of both CXCL1 and CCL2. Confocal investigations of inflamed post-capillary venules revealed immunostaining for MC(1) receptors on adherent and emigrated leucocytes. Congruently, the anti-inflammatory properties of BMS-470539 were lost in mesenteries of mice bearing the inactive mutant MC(1) receptors. Therapeutic administration of BMS-470539 stopped cell emigration, but did not affect cell adhesion in the cremasteric microcirculation inflamed by superfusion with platelet-activating factor.

CONCLUSIONS AND IMPLICATIONS

Activation of MC(1) receptors inhibited leucocyte adhesion and emigration. Development of new chemical entities directed at MC(1) receptors could be a viable approach in the development of novel anti-inflammatory therapeutic agents with potential application to post-ischaemic conditions.

The melanocortin receptor agonist NDP-MSH impairs the allostimulatory function of dendritic cells

As alpha-melanocyte-stimulating hormone (alpha-MSH) is released by immunocompetent cells and has potent immunosuppressive properties, it was determined whether human dendritic cells (DCs) express the receptor for this hormone. Reverse transcription-polymerase chain reaction detected messenger RNA specific for all of the known melanocortin receptors in DCs. Mixed lymphocyte reactions also revealed that treatment with [Nle(4), DPhe(7)]-alpha-MSH (NDP-MSH), a potent alpha-MSH analogue, significantly reduced the ability of DCs to stimulate allogeneic T cells. The expression of various cell surface adhesion, maturation and costimulatory molecules on DCs was also investigated. Although treatment with NDP-MSH did not alter the expression of CD83 and major histocompatibility complex class I and II, the surface expression of CD86 (B7.2), intercellular adhesion molecule (ICAM-1/CD54) and CD1a was reduced. In summary, our data indicate that NDP-MSH inhibits the functional activity of DCs, possibly by down-regulating antigen-presenting and adhesion molecules and that these events may be mediated via the extracellular signal-regulated kinase 1 and 2 pathway.

Substitution of arginine with proline and proline derivatives in melanocyte-stimulating hormones leads to selectivity for human melanocortin 4 receptor

A new series of melanotropin analogues with His or Arg residues in the core pharmacophores of MTII, SHU9119, and Ac-NDP-gamma-MSH-NH(2) replaced by Pro or trans-/cis-4-guanidinyl-Pro derivatives were designed and synthesized to introduce selectivity toward the human melanocortin 4 receptor (hMC4R). Analogues 1, 2, 3, 6, 7, 8 were found to be hMC4R selective. Second messenger studies have demonstrated that analogues 1 and 2 are insurmountable inhibitors of MTII agonist activity at the hMC4R. Molecular modeling studies suggest that the hMC4R selectivity is due to a beta-turn shift induced by the Pro ring that makes the global minimum structures of these analogues resemble the NMR solution structure of the hASIP melanocortin receptor binding motif. Substitution of His in MTII also provided functional selectivity for the hMC3R or the hMC4R. These findings are important for a better understanding of the selectivity mechanism at the hMC3R/hMC4R and the development of therapeutic ligands selectively targeting the hMC4R.

A role for MC3R in modulating lung inflammation

In this study we set out to ascertain whether melanocortin peptides could be potential therapeutic agents in allergic and non-allergic models of lung inflammation by identifying the receptor(s) involved using a molecular, genetic and pharmacological approach. Western blot analyses revealed expression of the melanocortin receptor (MCR) type 1 and 3 on alveolar macrophages from wild-type mice. Alveolar macrophage incubation, with the selective MC3R agonist [D-TRP(8)]-gamma-MSH and pan-agonist alpha-MSH but not the selective MC1R agonist MS05, led to an increase in cAMP in wild-type macrophages. This increase occurred also in macrophages taken from recessive yellow (e/e; bearing a mutant and inactive MC1R) mice but not from MC3R-null mice. In an allergic model of inflammation, the pan-agonist alpha-MSH and selective MC3R agonist [D-TRP(8)]-gamma-MSH displayed significant attenuation of both eosinophil and lymphocyte accumulation but not IL-5 levels in wild-type and recessive yellow e/e mice. However in MC3R-null mice, alpha-MSH failed to cause a significant inhibition in these parameters, highlighting a preferential role for MC3R in mediating the anti-inflammatory effects of melanocortins in this model. Utilising a non-allergic model of LPS-induced lung neutrophilia, the pan-agonist alpha-MSH and selective MC3R agonist [D-TRP(8)]-gamma-MSH displayed significant attenuation of neutrophil accumulation and inhibition of TNF-alpha release. Thus, this study highlights that melanocortin peptides inhibit leukocyte accumulation in a model of allergic and non-allergic inflammation and this protective effect is associated with activation of the MC3R. The inhibition of leukocyte accumulation is via inhibition of TNF-alpha in the non-allergic model of inflammation but not IL-5 in the allergic model. These data have highlighted the potential for selective MC3R agonists as novel anti-inflammatory therapeutics in lung inflammation.

Inflamed phenotype of the mesenteric microcirculation of melanocortin type 3 receptor-null mice after ischemia-reperfusion

The existence of anti-inflammatory circuits centered on melanocortin receptors (MCRs) has been supported by the inhibitory properties displayed by melanocortin peptides in models of inflammation and tissue injury. Here we addressed the pathophysiological effect that one MCR, MCR type 3 (MC3R), might have on vascular inflammation. After occlusion (35 min) and reopening of the superior mesenteric artery, MC3R-null mice displayed a higher degree of plasma extravasation (45 min postreperfusion) and cell adhesion and emigration (90 min postreperfusion). These cellular alterations were complemented by higher expression of mesenteric tissue CCL2 and CXCL1 (mRNA and protein) and myeloperoxydase, as compared with wild-type animals. MC1R and MC3R mRNA and protein were both expressed in the inflamed mesenteric tissue; however, no changes in vascular responses were observed in a mouse colony bearing an inactive MC1R. Pharmacological treatment of animals with a selective MC3R agonist ([d-Trp⁸]-γ-melanocyte-stimulating hormone; 10 μg i.v.) produced marked attenuation of cell adhesion, emigration, and chemokine generation; such effects were absent in MC3R-null mice. These new data reveal the existence of a tonic inhibitory signal provided by MC3R in the mesenteric microcirculation of the mouse, acting to down-regulate cell trafficking and local mediator generation.—Leoni, G., Patel, H. B., Sampaio, A. L. F., Gavins, F. N. E., Murray, J. F., Grieco, P., Getting, S. J., Perretti, M. Inflamed phenotype of the mesenteric microcirculation of melanocortin type 3 receptor-null mice after ischemia-reperfusion.

Functions of acidic transmembrane residues in human melanocortin-3 receptor binding and activation

The melanocortin-3 receptor (MC3R) is an important regulator of energy homeostasis, inflammation, and cardiovascular function. Inactivating mutations in MC3R gene are associated with childhood obesity. How MC3R binds to its ligands has rarely been studied. In the present study, we systematically mutated all ten acidic residues in transmembrane (TM) domains and measured the cell surface expression levels as well as ligand binding and signaling properties of these mutants. Our results showed that of the 19 mutants stably expressed in HEK293 cells, all were expressed on the cell surface, although some mutants had decreased levels of cell surface expression. We showed that with the superpotent analog [Nle(4), D-Phe(7)]-alpha-melanocyte stimulating hormone (MSH), E92, E131, D154, D158, D178, and D332 are important for ligand binding. D121 and D332 are important for binding and signaling. Further experiments using other ligands such as D-Trp(8)-gamma-MSH, alpha-MSH and gamma-MSH showed that different ligands induce or select different conformations. In summary, we showed that acidic residues in TMs 1 and 3 are important for ligand binding whereas the acidic residues in TMs 2 and 7 are important for both ligand binding and signaling.

Diminishment of alpha-MSH anti-inflammatory activity in MC1r siRNA-transfected RAW264.7 macrophages

The neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) is a powerful suppressor of inflammation mediated by macrophages, which express at least two receptors, melanocortin 1 and 3 receptors (MC1r and MC3r) that bind alpha-MSH. Albeit, the anti-inflammatory activity of alpha-MSH has been well documented in macrophages, the mechanisms of alpha-MSH activity in macrophages are not clearly understood. This study is to investigate which of the MCr expressed on macrophages is associated with the immunosuppressive activities of alpha-MSH on LPS-stimulated macrophages. To address this question, we transfected RAW264.7 macrophage cells with MC1r small interfering (si)RNA, which specifically targets mouse MC1r mRNA. The diminution of MC1r mRNA expression was 82% at 24 h and 67% at 48 h after transfection. There was a significant loss in alpha-MSH suppression of NO generation and TNF-alpha production by MC1r siRNA-transfected macrophages stimulated with LPS. There was an equally diminished alpha-MSH suppression of LPS-stimulated intracellular activation of NF-kappaB and p38 phosphorylation. In addition, the diminishment of MC1r expression by siRNA transfection had no influence on MC3r expression and function in the macrophages. These findings demonstrate that alpha-MSH suppression of LPS-induced inflammatory activity in macrophages requires expression of MC1r. The results imply that although all of the MCr are G-coupled proteins, they may not necessarily function through the same intracellular pathways in macrophages.

Adrenocorticotrope hormone fragment (4-10) attenuates the ischemia/reperfusion-induced cardiac injury in isolated rat hearts

The aim of our study was to investigate the contribution of the adrenocorticotropic hormone fragment, ACTH (4-10), on the recovery of postischemic cardiac function. Effects of ACTH (4-10) on caspase-3 activity, cardiomyocyte and endothelial apoptosis, and HO-1 protein expression were studied. Rats were treated with various doses of ACTH (4-10), and then 12 h later, anesthetized, hearts were isolated, perfused, and subjected to 30-min ischemia followed by 120-min reperfusion. Cardiac function including heart rate, coronary flow, aortic flow, and left ventricular developed pressure were recorded. After 120-min reperfusion, 200 mug/kg of ACTH (4-10) significantly improved the recovery of aortic flow, coronary flow, and left ventricular developed pressure from their untreated control values of 15.3 +/- 0.9 ml/min, 6.5 +/- 0.9 ml/min, and 10 +/- 0.6 kPa to 20.7 +/- 1.3 ml/min, 24.8 +/- 1.8 ml/min and 13.7 +/- 0.7 kPa, respectively. Heart rate did not show significant changes during reperfusion. ACTH (4-10) treatment resulted in a reduction in infarct size, caspase 3 activity, apoptosis, and an increase in HO-1 expression. When ACTH (4-10) was given at the moment of reperfusion, the drug failed to improve the postischemic recovery of the myocardium. Thus, ACTH (4-10) can be a useful tool for the prevention of the development of ischemia/reperfusion-induced injury.

Activation and endocytic internalization of melanocortin 3 receptor in neuronal cells

Melanocortins play a central role in autonomic modulation of metabolism by acting through a family of highly homologous G protein-coupled receptors. Studies with gene knockout mice have implicated neural melanocortin receptors, MC3R and MC4R, in the etiology of obesity, insulin resistance, and salt-sensitive hypertension. In an attempt to better understand the mechanisms of function of these receptors, we expressed MC3R and MC4R in neuronal cells and demonstrated their co-localization to several membrane regions. We now show that in cultured neuronal cells, MC3R localizes to lipid rafts and undergoes endocytic internalization upon activation by gamma-MSH through a protein kinase-sensitive pathway. The appearance of the internalized receptor in lysosomes suggests that it is subsequently degraded. The expression of protein kinase A regulatory subunits and of c-Jun and c-Fos was analyzed by either immunoblotting or real-time PCR. No discernable changes were observed in the expression levels of these protein kinase A and protein kinase C responsive genes. Immunohistochemical studies showed a robust expression of MC3R protein in brain nuclei with relevance to cardiovascular function and fluid homeostasis further supporting the notion that the physiological effects of melanocortins on the cardiovascular system arise from effects on the central nervous system.

Selective melanocortin MC4 receptor agonists reverse haemorrhagic shock and prevent multiple organ damage

BACKGROUND AND PURPOSE

In circulatory shock, melanocortins have life-saving effects likely to be mediated by MC4 receptors. To gain direct insight into the role of melanocortin MC4 receptors in haemorrhagic shock, we investigated the effects of two novel selective MC4 receptor agonists.

EXPERIMENTAL APPROACH

Severe haemorrhagic shock was produced in rats under general anaesthesia. Rats were then treated with either the non-selective agonist [Nle4, D-Phe7]-melanocyte-stimulating hormone (NDP--MSH) or with the selective MC4 agonists RO27-3225 and PG-931. Cardiovascular and respiratory functions were continuously monitored for 2 h; survival rate was recorded up to 24 h. Free radicals in blood were measured using electron spin resonance spectrometry; tissue damage was evaluated histologically 25 min or 24 h after treatment.

KEY RESULTS

All shocked rats treated with saline died within 30-35 min. Treatment with NDP--MSH, RO27-3225 and PG-931 produced a dose-dependent (13-108 nmol kg-1 i.v.) restoration of cardiovascular and respiratory functions, and improved survival. The three melanocortin agonists also markedly reduced circulating free radicals relative to saline-treated shocked rats. All these effects were prevented by i.p. pretreatment with the selective MC4 receptor antagonist HS024. Moreover, treatment with RO27-3225 prevented morphological and immunocytochemical changes in heart, lung, liver, and kidney, at both early (25 min) and late (24 h) intervals.

CONCLUSIONS AND IMPLICATIONS

Stimulation of MC4 receptors reversed haemorrhagic shock, reduced multiple organ damage and improved survival. Our findings suggest that selective MC4 receptor agonists could have a protective role against multiple organ failure following circulatory shock.

Design, synthesis and biological evaluation of ligands selective for the melanocortin-3 receptor

The processed products of the proopiomelanocortin gene (ACTH, alpha-MSH, beta-MSH, gamma-MSH, etc.) interact with five melanocortin receptors, the MC1R, MC2R, MC3R, MC4R, and MC5R to modulate and control many important biological functions crucial for good health both peripherally (as hormones) and centrally (as neurotransmitters). Pivotal biological functions include pigmentation, adrenal function, response to stress, fear/flight, energy homeostasis, feeding behavior, sexual function and motivation, pain, immune response, and many others, and are believed to be involved in many disease states including pigmentary disorders, adrenal disorders, obesity, anorexia, prolonged and neuropathic pain, inflammatory response, etc. The melanocortin-3 receptor (MC3R) is found primarily in the brain and spinal cord and also in the periphery, and its biological functions are still not well understood. Here we review some of the biological functions attributed to the MC3R, and then examine in more detail efforts to design and synthesize ligands that are potent and selective for the MC3R, which might help resolve the many questions still remaining about its function. Though some progress has been made, there is still much to be done in this critical area.

Melanocortin-3 receptor activates MAP kinase via PI3 kinase

HEK 293 cells stably expressing human melanocortin-3 receptor (MC3R) were exposed to melanocortin receptor agonist, NDP-MSH (10(-)(10)-10(-)(6) M). ERK1/2 was phosphorylated in a dose-dependent manner with an EC(50) of 3.3+/-1.5 x 10(-)(9) M, similar to the IC(50) of NDP-MSH binding to the MC3R. ERK1/2 phosphorylation was blocked by the melanocortin receptor antagonists SHU9119. NDP-MSH-induced ERK1/2 phosphorylation was sensitive to pertussis toxin and the PI3K inhibitor, wortmannin. Rp-cAMPS, BAPTA-AM and Myr-PKC did not inhibit the NDP-MSH-induced ERK1/2 phosphorylation. NDP-MSH stimulated cellular proliferation in a dose-dependent manner with a similar EC(50) to ERK1/2 phosphorylation, 2.1+/-0.6 x 10(-)(9) M. Cellular proliferation was blocked by AGRP (86-132) and by the MEK inhibitor, PD98059. The NDP-MSH did not inhibit serum deprivation-induced apoptosis. MC3R activation induces ERK1/2 phosphorylation via PI3K and this pathway is involved in cellular proliferation in HEK cells expressing MC3R.

[D-Trp8]-gamma-melanocyte-stimulating hormone exhibits anti-inflammatory efficacy in mice bearing a nonfunctional MC1R (recessive yellow e/e mouse)

Two melanocortin receptors (MC1 and MC3R) have been identified as main transducers of the anti-inflammatory effects of natural and synthetic melanocortins. In this study, we have taken advantage of the recent description of the selective MC3R agonist [d-Trp(8)]-gamma-melanocyte-stimulating hormone (MSH) and of the recessive yellow (e/e) mouse, bearing a nonfunctional MC1R, thereby incrementing our knowledge on this topic. Culturing peritoneal macrophages of recessive yellow (e/e) mice with [d-Trp(8)]-gamma-MSH led to accumulation of cAMP, indicating MC3R receptor functionality: this effect was blocked by a neutralizing antibody against MC3R. Likewise, release of the chemokine KC by urate crystals was attenuated by [d-Trp(8)]-gamma-MSH, and this effect was prevented by synthetic [Ac-Nle(4)-c[Asp(5)-2'-Nal(7),Lys(10)]alpha-MSH(4-10)-NH(2) (SHU9119)] and natural [agouti-related protein (AGRP)] MC3R antagonists but not by the MC4R antagonist Ac-Cys-Nle-Arg-His-d-2-Nal-Arg-Trp-Cys-NH(2) (HS024). Systemic treatment of mice with [d-Trp(8)]-gamma-MSH inhibited KC release and polymorphonuclear cell accumulation elicited by urate crystals in the murine peritoneal cavity. SHU9119 and AGRP prevented the inhibitory actions of [d-Trp(8)]-gamma-MSH, whereas HS024 was inactive. We also demonstrate here that [d-Trp(8)]-gamma-MSH displays a dual mechanism of action by inducing the anti-inflammatory protein heme-oxygenase 1 (HO-1). Treatment with the HO-1 inhibitor zinc protoporphyrin IX exacerbated the inflammatory response elicited by urate crystals and abrogated the anti-inflammatory effects of [d-Trp(8)]-gamma-MSH. In conclusion, these data support the development of the selective MC3R agonist [d-Trp(8)]-gamma-MSH for the treatment of inflammatory pathologies, based on a dual mechanism of cytokine/chemokine inhibition and induction of the anti-inflammatory protein HO-1.

Role of the melanocortin system in inflammation

In recent years, scientific interest in melanocortins (MC) has progressively increased due to their wide range of effects and expression on various tissues. Primarily discovered as mediators of skin pigmentation, recent research has shown their important roles in various body functions, such as energy homeostasis, sexual function, and inflammation. The anti-inflammatory and immunomodulatory properties discovered so far have led to the hypothesis that alpha-melanocyte-stimulating hormone (MSH) and its cognate receptors might present potential anti-inflammatory treatment options.

Melanocortin 3 receptors control crystal‐induced inflammation

In this study we have characterized the anti-inflammatory profile of a selective melanocortin type 3 receptor (MC3-R) ligand [D-Trp8]-gamma-MSH, validating in vitro results with analyses in mice deficient for this receptor subtype. In wild-type (WT) macrophages, [D-Trp8]-gamma-MSH activated MC3-R (as tested by accumulation of cyclic AMP) and inhibited (approximately 50%) the release of interleukin (IL)-1 and the chemokine KC (CXCL1), but was ineffective in cells taken from MC3-R null mice. In vivo, administration of 3-30 microg [D-Trp8]-gamma-MSH significantly inhibited leukocyte influx and cytokine production in a model of crystal-induced peritonitis, and these effects were absent in MC3-R null mice or blocked by coadministration of an MC3-R antagonist. Finally, in a model of gouty arthritis, direct injection of urate crystals into the rat joint provoked a marked inflammatory reaction that was significantly inhibited (approximately 70%) by systemic or local administration of [D-Trp8]-gamma-MSH. In conclusion, using an integrated transgenic and pharmacological approach, we provide strong proof of concept for the development of selective MC3-R agonists as novel anti-inflammatory therapeutics.