Sinomenine increases adenosine A2A receptor and inhibits NF-κB to inhibit arthritis in adjuvant-induced-arthritis rats and fibroblast-like synoviocytes through α7nAChR

Sinomenine (SIN) is a clinical drug for treating rheumatoid arthritis (RA) in China. Our previous study found SIN inhibited inflammation via alpha7 nicotinic acetylcholine receptor (α7nAChR) in macrophages in vitro. Adenosine receptor A2A has anti-inflammatory and immunosuppressive function. However, the mechanisms of SIN acting on α7nAChR and the effect on adenosine A2A receptor (A2AR) in RA are not clear. In the present study, the effects of SIN on adjuvant-induced-arthritis (AIA) rats in vivo and on fibroblast-like synoviocytes (FLSs) in vitro were investigated. Indomethacin (Indo) and methotrexate (MTX), the clinical anti-arthritis drugs, were used as controls. Nicotine (Nic), a specific agonist of α7nAChR, was used as a control for targeting α7nAChR. Alpha-bungarotoxin (α-BTX), the antagonist of α7nAChR or small interference RNA (siRNA) was used to block or knock down α7nAChR. Results showed that SIN decreased arthritis index, hind paw volume, erythrocyte sedimentation (ESR) and serum TNF-α in AIA rats, and α-BTX attenuated the earlier-mentioned effects of SIN and Nic, but not Indo and MTX. The expressions of A2AR in synovium declined in AIA rats, but remarkably increased after the intervention of SIN. The expression of A2AR decreased by LPS or TNF-α, but increased by SIN; cAMP also increased by SIN in FLSs in vitro. SIN inhibited the expression of MCP-1, IL-6, and vascular endothelial growth factor in LPS-induced FLSs. SIN inhibited the activation of NF-κB. Meanwhile, α-BTX or α7nAChR siRNA blocked the earlier-mentioned effects of SIN in FLSs. Results suggested the expressions of A2AR in synovium and FLSs are negatively correlated with the arthritis progression of AIA rats and the activation of FLSs. SIN increases A2AR and inhibits the activation of NF-κB pathway via α7nAChR in AIA rats and FLSs.