Rheumatoid arthritis (RA) is the most common type of autoimmune arthritis. TET2 and HIF-1a take part in various of biological processes. The role of TET2-HIF-1α interactions in RA inflammation remains unclear.

We analyzed the expression of HIF-1α in synovial tissue from RA patients and in fibroblast-like synoviocytes (FLSs) under hypoxic and inflammatory conditions using immunofluorescence staining. Immunohistochemical staining was performed to analyze the expression of TET2 in synovial tissue from RA patients. Bisulfite sequencing (BSP-seq) was used to determine the DNA methylation in HIF-1α promoter (-1000bp to +2000bp) region in peripheral blood mononuclear cells (PBMCs). Following the treatment of FLSs with TET2-specific siRNA, quantitative real-time polymerase chain reaction (qRT-PCR) was performed to analyze TET2 and HIF-1α expression. Enzyme-linked immunosorbent assays (ELISA) were conducted to measure concentrations of inflammatory cytokines including interleukin (IL)-6, IL-17, IL-10, and tumor necrosis factor (TNF)-α in cell culture supernatants. We evaluated whether siRNA silencing of TET2 can alleviate the clinical manifestations of RA and suppress inflammatory expression in a collagen-induced arthritis (CIA) rat model.

Compared with that in healthy individuals, HIF-1α and TET2 expressions were significantly upregulated in the synovial tissue of RA patients. In additionally, HIF-1α was markedly elevated in FLSs cultured under hypoxic and inflammatory conditions in vitro. BSP-seq detected that DNA demethylation in the HIF-1α promoter region of PBMCs cells obtained from RA patients. The concentrations of inflammatory cytokines, such as IL-6, IL-17, and TNF-α in the peripheral blood serum of RA patients were significantly elevated compared with those in healthy individuals. However, after silencing TET2 in vitro in FLSs, these inflammatory cytokines showed a marked decrease compared with the control group. Silencing TET2 in the synovial cavity of collagen-induced arthritis rats significantly alleviated swelling and joint destruction and suppressed inflammatory expression.

We demonstrated that TET2 mediates low methylation levels in the HIF-1α promoter region, and silencing TET2 suppresses HIF-1α expression and inhibits the production of proinflammatory. These findings suggest that the TET2/HIF-1α pathway may serve as a therapeutic target for controlling inflammation in RA.