Melanocortin receptors are novel effectors of macrophage efferocytosis and promote resolution of inflammation

T Montero-Melendez1, T EN Jonassen2, M Seed3, M Perretti1. 1William Harvey Research Institute, Barts and The London School of Medicine, Queen Mary University of London, Biochemical Pharmacology, EC1M 6BQ, UK, 2University of Copenhagen, Biochemical Sciences, Denmark, 3William Harvey Research Institute, Barts and The London School of Medicine, Queen Mary University of London, Experimental Medicine and Rheumatology, EC1M 6BQ, UK

Inflammation is a defensive response of the body against pathogens and injury in which immune cells and soluble factors take part to neutralize the injurious agent and initiate tissue repair. However, a loss of regulation of these mechanisms can prevent the final resolution of the inflammatory process leading to a chronic status. Our group has focused on melanocortin (MC) receptors (adenylate-cyclase associated GPCRs), which conveys the anti-inflammatory actions of endogenous and synthetic melanocortin agonists (1,2). We recently described the pan-MC receptor agonist AP214 - under development in Phase II clinical trials – promotes novel pro-resolving activities in macrophages and in experimental inflammation (3). Here we show that MCRs can also be targeted by positive allosteric modulators (PAM) to elicit potent biological actions burdened by less side effects.

We first characterized the MC system in the zymosan-induced peritonitis model in mice. Zymosan A (1mg/mouse) was administered intraperitoneally and mice were sacrificed at different time points. We found that αMSH was detected in the peritoneal fluid (~300pg/cavity) and was locally produced by macrophages and neutrophils. The MC receptors MC1, MC3 and MC5 were also expressed by immune cells, as assessed by PCR. Thus, we studied if AP1189, a PAM at MC receptors, was able to promote the resolution of inflammation by administering it at peak of disease (12h after zymosan). AP1189 (1 mg/kg i.p.) accelerated the resolution of inflammation, quantified using the resolution indices: T50 (time when peak neutrophil numbers are reduced to 50%): 21h and 38h; Ri (time from maximal response to T50): 9h and 26h, for AP1189- and PBS-treated mice, respectively. These effects of AP1189 were accompanied by significant reduction in inflammatory monocytes (-42% at 22h; 2.4x106 and 1.4x106 cells in vehicle and AP1189 treated mice respectively) and PGE2 levels (-80%; 413 and 88 pg/ml in vehicle and AP1189 treated mice respectively), together with enhanced extent of macrophages engulfed with apoptotic neutrophils (efferocytosis). Using biogel-elicited peritoneal macrophages, which also produce αMSH in vitro, we found that the pro-efferocytic effect of AP1189 (1-100 nM) was mainly driven by MC3, and to a less extent by MC1, as demonstrated with macrophages from MC1-/- and MC3-/- mice. In addition, this pro-resolving property of AP1189 was cAMP mediated, since the protein kinase A inhibitor PKI14-22 abrogated it.

We conclude MC receptor can be successfully harnessed with positive allosteric modulators to elicit pro-resolving actions with a potential beneficial application in chronic inflammatory diseases.

1. Montero-Melendez T et al. Curr Rheumatol Rep 2011, 13:138-145 (PMID: 21243457)

2. Patel HB et al. FASEB J 2010, 24:4835-4843 (PMID: 20702773)

3. Montero-Melendez T et al. Am J Pathol 2011, 179:259-269 (PMID: 21703408)

This project was funded by an Action Pharma/WHR Foundation collaboration