Tumor necrosis factor-α promotes the expression of osteoprotegerin in rheumatoid synovial fibroblasts

Objective. To clarify the regulation of osteoprotegerin (OPG) expression in rheumatoid synovial fibroblasts by investigating the effect of tumor necrosis factor-alpha (TNF-alpha) and the mechanism of TNF-alpha-induced OPG expression. Methods. OPG expression was examined by Northern blot hybridization and reverse transcriptase-polymerase chain reaction in synovial fibroblasts from patients with rheumatoid arthritis (RA) and osteoarthritis (OA) and subjects with no inflammatory condition. Amounts of OPG in conditioned medium were determined by ELISA. The effect of OPG on TNF-alpha-induced osteoclastogenesis was investigated in primary cultures of RA synovial cells. Results. OPG was highly expressed in RA synovial fibroblasts compared to OA and noninflammatory synovial fibroblasts. Different levels of OPG expression were found among patients with RA. TNF-alpha induced OPG expression in all synovial fibroblasts, even OA and noninflammatory fibroblasts, and expression occurred to a remarkable degree in RA fibroblasts. The OPG expression was upregulated by TNF-alpha in a time- and dose-dependent manner. TNF-alpha-induced OPG expression was inhibited by hymenialdisine, a nuclear factor-kappaB inhibitor, in a dose-dependent manner, and expression was inhibited by soluble TNF receptor/Fc fusion protein I (TNFs-RI/Fc), not by TNFs-RII/Fc. In contrast, TNF-alpha-induced osteoclastogenesis in primary cultures of RA synovial cells was inhibited by the addition of OPG. Conclusion. These results suggest that OPG is highly expressed and is upregulated by TNF-alpha in rheumatoid synovial fibroblasts. TNF-alpha-induced OPG expression is mediated predominately through TNF-RI. Although TNF-alpha is known to stimulate bone destruction, TNF-alpha-induced upregulation of OPG may contribute to self-protection from the bone destruction in RA.