Fatty acid sensing GPCR (GPR84) signaling safeguards cartilage homeostasis and protects against osteoarthritis

It is well known that free fatty acids (FFAs) have beneficial effects on the skeletal system, however, which fatty acid sensing GPCR(s) and how the GPCR(s) regulating cartilage development and osteoarthritis (OA) pathogenesis is largely unknown. In this study, we found Gpr84, a receptor for medium-chain FFAs (MCFA), was the only FFA-sensing GPCR in human and mouse chondrocytes that exhibited elevated expression when stimulated by interleukin (IL)-1β. Gpr84-deficiency upregulated cartilage catabolic regulator expression and downregulated anabolic factor expression in the IL-1β-induced cell model and the destabilization of the medial meniscus (DMM)-induced OA mouse model. Gpr84−/− mice exhibited an aggravated OA phenotype characterized by severe cartilage degradation, osteophyte formation and subchondral bone sclerosis. Moreover, activating Gpr84 directly enhanced cartilage extracellular matrix (ECM) generation while knockout of Gpr84 suppressed ECM-related gene expression. Especially, the agonists of GPR84 protected human OA cartilage explants against degeneration by inducing cartilage anabolic factor expression. At the molecular level, GPR84 activation inhibited IL-1β-induced NF-κB signaling pathway. Furthermore, deletion of Gpr84 had little effect on articular and spine cartilaginous tissues during skeletal growth. Together, all of our results demonstrated that fatty acid sensing GPCR (Gpr84) signaling played a critical role in OA pathogenesis, and activation of GPR84 or MCFA supplementation has potential in preventing the pathogenesis and progression of OA without severe cartilaginous side effect.