Connie W. Lam

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 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.

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]‐γ‐MSH, validating in vitro results with analyses in mice deficient for this receptor subtype. In wild‐type (WT) macrophages, [D‐Trp8]‐γ‐MSH activated MC3‐R (as tested by accumulation of cyclic AMP) and inhibited (~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 μg [D‐Trp8]‐γMSH 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 (~70%) by systemic or local administration of [D‐Trp8]‐γ‐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.—Getting, S. J., Lam, C. W., Chen, A. S., Grieco, P., Perretti, M. Melanocortin 3 receptors control crystal‐induced inflammation. FASEB J. 20, 2234–2241 (2006)

In vitro and in vivo induction of heme oxygenase 1 in mouse macrophages following melanocortin receptor activation

RAW264.7 cell incubation with adrenocorticotrophin (ACTH) led to a time-dependent (4–24 h) and concentration-related (1–100 ng/ml) induction of heme oxygenase (HO)-1, and this was a specific effect, because the pattern of expression of other cellular proteins (HO-2, heat shock proteins 70 and 90) was not modified by ACTH. Combined RT-PCR and Western blot analyses revealed expression of the melanocortin receptor (MC-R) types 1 and 3, but not 4, in these cells. However, use of more selective agonists (including melanotan (MTII)) indicated a predominant role for MC3-R in the induction of HO-1 expression and activity. Relevantly, ACTH and MTII incubation with primary peritoneal macrophages (Mφ) also induced HO-1 expression. The potential link between MC3-R dependent cAMP formation and HO-1 induction was ascertained by the following: 1) ACTH and MTII produced a concentration-dependent accumulation of cAMP in RAW264.7 cells, and 2) whereas a selective inhibitor of cAMP-dependent protein kinase A abrogated ACTH- and MTII-induced HO-1 expression, a soluble cAMP derivative promoted HO-1 induction both in RAW264.7 cells and primary Mφ. HO-1 induction in peritoneal Mφ was also detected following in vivo administration of MTII, and appeared to be functionally related to the antimigratory effect of this melanocortin, as determined with a specific inhibitor (zinc protoporphyrin IX). In conclusion, this study highlights a biochemical link between MC-R activation and HO-1 induction in the Mφ, and proposes that this may be of functional relevance in determining MC-R-dependent control of the host inflammatory response.

Melanocortin receptor signaling in RAW264.7 macrophage cell line

Melanocortin peptides modulate cytokine release and adhesion molecule expression. Here we have investigated the early cell-signaling pathway responsible for the induction of interleukin-10 (IL-10) in RAW264.7 cells. Cell incubation with ACTH(1-39) or MTII (melanotan II) did not alter ERK1/2 and JNK phosphorylation, while p38 phosphorylation and intracellular cAMP accumulation occurred within minutes. ACTH(1-39) and MTII provoked a time-dependent accumulation of IL-10 that was abrogated by the PKA inhibitor H-89 and only partially blocked by the p38 MAPK inhibitor SB203580. Thus, in RAW264.7 cells, IL-10 induction by the melanocortins is via the PKA pathway, and this mechanism could contribute to their anti-inflammatory profile.

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 μg per mouse equivalent to 9.3 nmol) 30 min prior to ischemia (25 min) attenuated mouse heart 2 h reperfusion injury by ∼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 μg 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.

[D -Trp8]-γ -melanocyte -stimulating hormone exhibits anti-inflammatory efficacy in mice bearing a nonfunctional MC1 R (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-Trp8]-γ-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-Trp8]-γ-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-Trp8]-γ-MSH, and this effect was prevented by synthetic [Ac-Nle4-c[Asp5-2′-Nal7,Lys10]α-MSH(4-10)-NH2 (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-NH2 (HS024). Systemic treatment of mice with [d-Trp8]-γ-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-Trp8]-γ-MSH, whereas HS024 was inactive. We also demonstrate here that [d-Trp8]-γ-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-Trp8]-γ-MSH. In conclusion, these data support the development of the selective MC3R agonist [d-Trp8]-γ-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.

Melanocortin Receptor Type 3 as a Potential Target for Anti-Inflammatory Therapy

Alpha-melanocyte stimulating hormone (alpha-MSH) and other melanocortin peptides are potent anti-inflammatory agents exhibiting efficacy in many animal models of acute and chronic inflammation. They are derived from a larger precursor molecule known as the POMC prohormone and are produced both centrally and peripherally. They exert their effect by binding to melanocortin receptors, of which five have been cloned and partially characterised. Agonism at these receptors leads to adenylate cyclase activation and subsequent increases in cAMP formation. Two receptors to date have been proposed to mediate the actions of the melanocortin peptides in an inflammatory scenario, the MC1 and 3-R, and here we discuss our findings proposing the MC3-R as a novel therapeutic target. The potential anti-inflammatory role for MC3-R is in its infancy, however, recent studies have shown that melanocortin peptides are effective in mice bearing a non-functional MC1-R (recessive yellow e/e mice). This ability to inhibit cell migration appears to be via inhibition in cytokine and adhesion molecule expression and is due to their abilities to interfere with cell signalling pathways. Identification of endogenous mediators of anti-inflammation, their receptors and the pathologies they are effective in, is of benefit to the medical community, and will hopefully have reduced side effects. We believe that specific small molecule agonists directed at MC3-R could be potential novel therapeutics for inflammatory conditions.