Expression of cannabinoid receptor 2 and its inhibitory effects on synovial fibroblasts in rheumatoid arthritis

Objective. Recent studies have suggested immunomodulatory and anti-inflammatory effects of cannabinoid receptor 2 (CB2R) activation, which shows no psychoactivity. However, it is still unclear whether CB2R is expressed in fibroblast-like synoviocytes (FLS) of RA. In this study we investigated whether CB2R is expressed in FLS of RA, and whether CB2R activation modulates the function of RA-FLS. Methods. Expression of CB2R in synovial tissue and FLS was studied by immunohistochemistry, western blotting and RT-PCR. mRNA expression levels of CB2R, IL-6 and MMPs were analysed by quantitative real-time RT-PCR. The protein concentrations of IL-6 and MMPs in culture supernatants were determined by ELISA. The protein levels of signal transducing molecules were assayed by western blotting. Results. Both mRNA and protein expression of CB2R were found in synovial tissue and cultured FLS with slightly higher levels in RA patients than in OA patients. In cultured RA-FLS, the expression level of CB2R was up-regulated by stimulation with IL-1 beta, TNF-alpha or lipopolysaccharide. In vitro, HU-308, a selective CB2R agonist, inhibited IL-1 beta-induced proliferation of RA-FLS as well as IL-1 beta-induced production of MMP-3, MMP-13 and IL-6 in RA-FLS in a dose-dependent manner. HU-308 also suppressed IL-1 beta-induced activation of extracellular signal-regulated kinase 1/2 and p38 mitogen-activated protein kinase in FLS. Conclusion. In RA-FLS, proinflammatory mediators up-regulate the expression of CB2R, which negatively regulates the production of proinflammatory cytokines and MMPs. These data suggest that CB2R may be a potential therapeutic target of RA.