Melanocortin 3 receptors control crystal‐induced inflammation

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.

Targeting melanocortin receptors: an approach to treat weight disorders and sexual dysfunction

The melanocortin system has multifaceted roles in the control of body weight homeostasis, sexual behaviour and autonomic functions, and so targeting this pathway has immense promise for drug discovery across multiple therapeutic areas. In this Review, we first outline the physiological roles of the melanocortin system, then discuss the potential of targeting melanocortin receptors by using MC3 and MC4 agonists for treating weight disorders and sexual dysfunction, and MC4 antagonists to treat anorectic and cachectic conditions. Given the complexity of the melanocortin system, we also highlight the challenges and opportunities for future drug discovery in this area.

Mapping of a novel susceptibility locus suggests a role for MC3R and CTSZ in human tuberculosis

RATIONALE

Tuberculosis remains a major cause of morbidity and mortality in the developing world. A better understanding of the mechanisms of disease protection could allow novel strategies to disease management and control.

OBJECTIVES

To identify human genomic loci with evidence of linkage to tuberculosis susceptibility and, within these loci, to identify individual genes influencing tuberculosis susceptibility.

METHODS

Affected sibling pair analysis in South African and Malawian populations. Independent case-control study in West Africa.

MEASUREMENTS AND MAIN RESULTS

Two novel putative loci for tuberculosis susceptibility are identified: chromosome 6p21-q23 and chromosome 20q13.31-33--the latter with the strongest evidence for any locus reported to date in human tuberculosis (single point LOD score of 3.1, P = 10(-4), with a maximum likelihood score [MLS] of 2.8). An independent, multistage genetic association study in West African populations mapped this latter region in detail, finding evidence that variation in the melanocortin 3 receptor (MC3R) and cathepsin Z (CTSZ) genes play a role in the pathogenesis of tuberculosis.

CONCLUSIONS

These results demonstrate how a genomewide approach to the complex phenotype of human tuberculosis can identify novel targets for further research.

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.

Obesity-induced inflammation in white adipose tissue is attenuated by loss of melanocortin-3 receptor signaling

Metabolic syndrome, a complex of highly debilitating disorders that includes insulin resistance, hypertension, and dyslipidemia, is associated with the development of obesity in humans as well as rodent models. White adipose tissue (WAT) inflammation, caused in part by macrophage infiltration, and fat accumulation in the liver are both linked to development of the metabolic syndrome. Despite large increases in body fat, melanocortin 3-receptor (MC3-R)-deficient mice do not get fatty liver disease or severe insulin resistance. This is in contrast to obese melanocortin 4-receptor (MC4-R)-deficient mice and diet-induced obese (DIO) mice, which show increased adiposity, fatty liver disease, and insulin resistance. We hypothesized that defects in the inflammatory response to obesity may underlie the protection from metabolic syndrome seen in MC3-R null mice. MC4-R mice fed a chow diet show increased proinflammatory gene expression and macrophage infiltration in WAT, as do wild-type (WT) DIO mice. In contrast, MC3-R-deficient mice fed a normal chow diet show neither of these inflammatory changes, despite their elevated adiposity and a comparable degree of adipocyte hypertrophy to the MC4-R null and DIO mice. Furthermore, even when challenged with high-fat chow for 4 wk, a period of time shown to induce an inflammatory response in WAT of WT animals, MC3-R nulls showed an attenuated up-regulation in both monocyte chemoattractant protein-1 (MCP-1) and TNFalpha mRNA in WAT compared with WT high-fat-fed animals.