Stromal cell-derived factor-1/CXCR4 promotes IL-6 production in human synovial fibroblasts

The production of chemokine stromal cell-derived factor (SDF)-1 is significantly higher in synovial fluid of patients with osteoarthritis (OA). IL-6 is a multifunctional cytokine that plays a central role in both OA and rheumatoid arthritis. However, the effects of SDF-1α on human synovial fibroblasts are largely unknown. In this study, we investigated the intracellular signaling pathway involved in SDF-1α-induced IL-6 production in human synovial fibroblast cells. SDF-1α caused concentration- and time-dependent increases in IL-6 production. SDF-1α also increased the mRNA and surface expression of CXCR4 receptor in human synovial fibroblasts. CXCR4-neutralizing antibody, CXCR4-specific inhibitor (AMD3100), or small interfering RNA against CXCR4 inhibited the SDF-1α-induced increase of IL-6 expression. The transcriptional regulation of IL-6 by SDF-1α was mediated by phosphorylation of phosphatidylinositol 3-kinase (PI3K)/Akt and activation of the activator protein (AP)-1 component of c-Jun. The binding of c-Jun to the AP-1 element on the IL-6 promoter and the increase in AP-1 luciferase activity was enhanced by SDF-1α. Co-transfection with CXCR4, PI3K, Akt, and c-Jun mutants or siRNA inhibited the potentiating action of SDF-1α on AP-1 promoter activity. Taken together, our results suggest that SDF-1α-increased IL-6 production in human synovial fibroblasts via the CXCR4 receptor, PI3K, Akt, c-Jun, and AP-1 signaling pathways.