Hypoxia and TNF-α Synergistically Induce Expression of IL-6 and IL-8 in Human Fibroblast-like Synoviocytes via Enhancing TAK1/NF-κB/HIF-1α Signaling

Hypoxia and increased levels of inflammatory cytokines in the joints are characteristics of rheumatoid arthritis (RA). However, the effects of hypoxia and tumor necrosis factor-alpha (TNF-alpha) on interleukin (IL)-6 and IL-8 production on fibroblast-like synoviocytes (FLSs) remain to be clarified. This study aimed to explore how hypoxia and TNF-alpha affect the expression of IL-6 and IL-8 in human FLSs isolated from RA patients. Hypoxia or TNF-alpha treatment alone significantly increased the expression and promoter activity of IL-6, IL-8, and hypoxia-inducible factor-1 alpha (HIF-1 alpha). Treatment of hypoxic FLSs with TNF-alpha further significantly elevated the expression of these cytokines and enhanced promoter activity of HIF-1 alpha, which was abrogated by treatment with the HIF-1 alpha inhibitor YC-1. Similarly, TNF-alpha alone elevated the phosphorylation and promoter activity of nuclear factor-kappa Bp65 (NF-kappa Bp65) in the FLSs. These effects were further enhanced by the combined treatment of hypoxia and TNF alpha but were attenuated by the NF-kappa B inhibitor BAY11-7082. NF-kappa B-p65 inhibition decreased the effect of TNF-alpha on HIF-1 alpha upregulation in the FLSs in response to hypoxia. The combination of hypoxia and TNF-alpha also significantly upregulated transforming growth factor-beta-activated kinase 1 (TAK1) expression, and silencing TAK1 dramatically decreased NF-kappa B-p65, HIF-1 alpha, IL-6, and IL-8 expression under the same conditions. Our results indicate that hypoxia and TNF-alpha synergistically increase IL-6 and IL-8 expression in human FLSs via enhancing TAK1/NF-kappa B/HIF-1 alpha signaling.