MC3-R as a novel target for antiinflammatory therapy.

To date five melanocortin receptors (MC-R) have been cloned, identified and shown to have a wide distribution throughout the body and likely many diverse functions. MC1-R, found on melanocytes, is involved in pigmentation, while MC2-R is the classic adrenocorticotropic (ACTH) receptor found on the adrenal cortex and adipocytes. MC3-R, MC4-R and MC5-R are in their infancy with regard to their characterization. MC4-R has generated wide interest for its involvement in obesity, whereas our own studies have indicated a role for MC3-R in experimental inflammation. An ACTH fragment unable to alter circulating corticosterone, ACTH-4-10, acts at murine MC3-R present on peritoneal macrophage to inhibit cytokine formation and subsequent neutrophil extravasation. These findings were confirmed using agonists with a higher degree of selectivity toward MC3-R, such as gamma-2-MSH and the synthetic mixed MC3/4-R agonist MTII. In vitro, all these agents were able to affect macrophage functions, including phagocytosis and production of the CXC chemokine KC. Besides using RT-PCR and cAMP formation assays, the involvement of MC3-R in the antiinflammatory actions of these melanocortins was validated with the antagonist SHU-9119. Together these experimental data support the notion that agonism at MC3-R can be used for the design of novel therapeutics for inflammatory conditions.