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

Abstract

The issue of which melanocortin receptor (MC-R) is responsible for the anti-inflammatory effects of melanocortin peptides is still a matter of debate. Here we have addressed this aspect using a dual pharmacological and genetic approach, taking advantage of the recent characterization of more selective agonists/antagonists at MC1 and MC3-R as well as of the existence of a naturally defective MC1-R mouse strain, the recessive yellow (e/e) mouse. RT-PCR and ultrastructural analyses showed the presence of MC3-R mRNA and protein in peritoneal macrophages (Mφ) collected from recessive yellow (e/e) mice and wild-type mice. This receptor was functional as Mφ incubation (30 min) with melanocortin peptides led to accumulation of cAMP, an effect abrogated by the MC3/4-R antagonist SHU9119, but not by the selective MC4-R antagonist HS024. In vitro Mφ activation, determined as release of the CXC chemokine KC and IL-1β, was inhibited by the more selective MC3-R agonist γ2-melanocyte stimulating hormone but not by the selective MC1-R agonist MS05. Systemic treatment of mice with a panel of melanocortin peptides inhibited IL-1β release and PMN accumulation elicited by urate crystals in the murine peritoneal cavity. MS05 failed to inhibit any of the inflammatory parameters either in wild-type or recessive yellow (e/e) mice. SHU9119 prevented the inhibitory actions of γ2-melanocyte stimulating hormone both in vitro and in vivo while HS024 was inactive in vivo. In conclusion, agonism at MC3-R expressed on peritoneal Mφ leads to inhibition of experimental nonimmune peritonitis in both wild-type and recessive yellow (e/e) mice.

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

“…These data and the fact that MC3‐R is expressed in skeletal muscle37 suggest that the inhibitory effect of αMSH on NF‐ k B activation is mediated through MC3‐R activation. Furthermore, it has been postulated that MC3‐R is the predominant anti‐inflammatory receptor for melanocortins 24, 25, 26, 38…”

A role for MC3R in modulating lung inflammation

“…We have done so by using a pharmacological, genetic and molecular approach utilising the selective MC3R ligand [D-TRP 8 ]-γ-MSH, [24,25], MC3R deficient mice [28,31] and the recessive yellow e/e (bearing a nonfunctional MC1R) mouse [14]. This study has focused on the MC1 and 3R given the wealth of data highlighting that both these receptors are involved in modulating the host inflammatory response [10] other MCR were not evaluated in this study due to the fact that they haven't been implicated in modulating peripheral inflammation.…”

“…To date five MCRs have been identified and they are all positively coupled to adenylate cyclase, signaling via protein kinase with agonism at these receptors leading to increases in intracellular cAMP [11]. Melanocortin peptides have long been reported to possess antiinflammatory effects in many experimental models of acute [12][13][14] and chronic inflammation, including inflammatory bowel disease [15], joint arthritis [16,17] and activation and consequent cytokine synthesis within the first few hours of treatment [20,21] and at later time points (>8h) induce the anti-inflammatory protein hemeoxygenase 1 [22] and IL-10 [24]. Only one study has so far addressed the role of α-MSH in modulating eosinophil accumulation levels in a murine model of allergic airway disease [19]: α-MSH treatment of allergic mice led to a significant reduction in inflammation and this protective effect was associated with augmented IL-10 production.…”

“…These were kindly provided by Dr Nancy Levin, Trega Biosciences, San Diego, USA) and have a C57 Bl.6 background [14]. Male C57 Bl.6 (wild-type) mice (20-22 g body weight) were obtained from a local supplier.…”

“…Recessive yellow e/e (mutant MC1R mice) which display a frameshift mutation in the MC1R gene at position 549; the outcome is a receptor with a premature termination in the fourth trans-membrane domain, thus unable to couple to adenylate cyclase and activate cAMP synthesis. and have a C57 Bl.6 background [14]. Male C57 Bl.6 (wild-type) mice (20-22 g body weight) were obtained from a local supplier.…”