Apoptosis signal-regulating kinase 1 is mediated in TNF-α-induced CCL2 expression in human synovial fibroblasts.

Tumor necrosis factor-α (TNF-α), a pro-inflammatory cytokine with a critical role in osteoarthritis (OA), was primarily produced by monocytes/macrophages and plays a crucial role in the inflammatory response. Here, we investigated the intracellular signaling pathways involved in TNF-α-induced monocyte chemoattractant protein 1 (MCP-1)/CCL2 expression in human synovial fibroblast cells. Stimulation of synovial fibroblasts (OASF) with TNF-α induced concentration- and time-dependent increases in CCL2 expression. TNF-α-mediated CCL2 production was attenuated by TNFR1 monoclonal antibody (Ab). Pretreatment with an apoptosis signal-regulating kinase 1 (ASK1) inhibitor (thioredoxin), JNK inhibitor (SP600125), p38 inhibitor (SB203580), or AP-1 inhibitor (curcumin or tanshinone IIA) also blocked the potentiating action of TNF-α. Stimulation of cells with TNF-α enhanced ASK1, JNK, and p38 activation. Treatment of OASF with TNF-α also increased the accumulation of phosphorylated c-Jun in the nucleus, AP-1-luciferase activity, and c-Jun binding to the AP-1 element on the CCL2 promoter. TNF-α-mediated AP-1-luciferase activity and c-Jun binding to the AP-1 element were inhibited by TNFR1 Ab, thioredoxin, SP600125, and SB203580. Our results suggest that the interaction between TNF-α and TNFR1 increases CCL2 expression in human synovial fibroblasts via the ASK1, JNK/p38, c-Jun, and AP-1 signaling pathway.