Osteocalcin Alleviates Lipopolysaccharide-Induced Acute Inflammation via Activation of GPR37 in Macrophages

The G protein-coupled receptor 37 (GPR37) has been reported to be expressed in macrophages and the activation of GPR37 by its ligand/agonist, and it can regulate macrophage-associated functions and inflammatory responses. Since our previous work identified that osteocalcin (OCN) acts as an endogenous ligand for GPR37 and can elicit various intracellular signals by interacting with GPR37, we thus hypothesized that OCN may also play a functional role in macrophage through the activation of GPR37. To verify the hypothesis, we conducted a series of in vivo and in vitro studies in lipopolysaccharide (LPS)-challenged mice and primary cultured macrophages. Our results reveal that the OCN gene deletion (OCN(−/−)) and wild type (WT) mice showed comparable death rates and inflammatory cytokines productions in response to a lethal dose of LPS exposure. However, the detrimental effects caused by LPS were significantly ameliorated by exogenous OCN treatments in both WT and OCN(−/−) mice. Notably, the protective effects of OCN were absent in GPR37(−/−) mice. In coordination with the in vivo results, our in vitro studies further illustrated that OCN triggered intracellular responses via GPR37 in peritoneal macrophages by regulating the release of inflammatory factors and macrophage phagocytic function. Finally, we exhibited that the adoptive transfer of OCN-treated macrophages from WT mice significantly inhibits the release of pro-inflammatory cytokines in GPR37(−/−) mice exposed to LPS. Taken together, these findings suggest a protective role of OCN against LPS-caused acute inflammation, by the activation of GPR37 in macrophages, and provide a potential application of the activation of the OCN/GPR37 regulatory axis as a therapeutic strategy for inflammatory diseases.