Interleukin-13 reduces bone erosion in rheumatoid arthritis by up-regulating osteoprotegerin expression in fibroblast-like synoviocytes : an in vitro and in vivo study.

OBJECTIVES: Bone erosion in rheumatoid arthritis (RA) is partly caused by excessive activation of osteoclasts. Osteoclasts can be derived from RA synovium and their differentiation can be inhibited by osteoprotegerin (OPG), a decoy receptor of the osteoclastogenesis-promoting cytokine receptor activator of nuclear factor κB ligand (RANKL). Fibroblast-like synoviocytes (FLSs) are the main stromal cells in the synovium that can secret OPG. The OPG secretion of FLSs can be modulated by various cytokines. Interleukin (IL)-13 can alleviate bone erosion in RA mouse models, but the mechanisms remain unclear. Therefore, we aimed to investigate whether IL-13 can induce OPG secretion by RA-FLSs, thus ameliorating bone destruction in RA by inhibiting osteoclast differentiation.

METHODS: OPG, RANKL, and IL-13 receptors expression by RA-FLSs were evaluated by RT-qPCR. OPG secretion was determined by ELISA. Western blot was performed to analyse OPG expression and the activation of the STAT6 pathway. IL-13 and (or) OPG siRNA pre-treated RA-FLSs conditioned medium were used in osteoclast induction to test if IL-13 can inhibit osteoclastogenesis by up-regulating OPG in RA-FLSs. Micro-CT and immunofluorescence were performed to determine if IL-13 can induce OPG expression and alleviate bone erosion in vivo.

RESULTS: IL-13 can promote OPG expression of RA-FLSs, and the promotion can be overcome by IL-13Rα1 or IL-13Rα2 siRNA transfection, or STAT6 inhibitor. Osteoclast differentiation can be inhibited by IL-13 pre-treated RA-FLSs conditioned medium. The inhibition can be reversed by OPG siRNA transfection. IL-13 injection can increase OPG expression in the joints while reducing bone destruction in collagen-induced arthritis mice.

CONCLUSIONS: IL-13 can inhibit osteoclastogenesis by up-regulating OPG in RA-FLSs through IL-13 receptors via the STAT6 pathway, thus may ameliorate bone erosion in RA.