Tumor necrosis factor receptor-associated factor (TRAF) 6 inhibition mitigates the pro-inflammatory roles and proliferation of rheumatoid arthritis fibroblast-like synoviocytes

Background: Rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLSs) play a crucial role in RA through producing inflammatory cytokines and proteases which could cause cartilage destruction. We showed previously that elevated expression of tumor necrosis factor receptor-associated factor 6 (TRAF6) in RA synovium correlated significantly with the severity of synovitis and the number of infiltrated inflammatory cells. The aims of this study are to detect the roles of TRAF6 in RA-FLSs. Methods: Synovium were collected by closed needle biopsy from inflamed knees of active RA patients, and FLSs were isolated by modified tissue culture method. Expression of TRAF6 and CD55 in RA synivium was tested by double immunofluorescence (IF) staining. TRAF6 in RA-FLSs was inhibited using Lentiviral-TRAF6-shRNA transfection. Real-time PCR and ELISA were used to detect the mRNA expression and secretion of matrix metalloproteinase (MMP) and pro-inflammatory cytokines. Cell Counting Kit-8 was used to detect cell proliferation, flow cytometry was used to detect cell cycle, and Annexin V assay was used to detect cell apoptosis. Results: We showed that in the intimal and subintimal area of RA synovium, TRAF6 was expressed obviously not only in CD55+ cells, but also in some other CD55- cells. TRAF6 expression in RA-FLSs was suppressed effectively by Lentiviral-TRAF6-shRNA transfection. Inhibition of TRAF6 in RA-FLSs mitigated the mRNA levels and secretion of pro-inflammatory cytokines and MMPs, such as IL-1 beta, IL-8, IL-6, TNF-alpha, MMP-13, and MMP-3. In addition, it decreased the proliferation of RA-FLSs, blocked RA-FLSs in G0/G1-phase, and inhibited the cells to go into S-phase and G2/M-phase, but not facilitated apoptosis of RA-FLSs. Conclusion: TRAF6 plays direct roles in the pro-inflammatory effects and proliferation of RA-FLSs. TRAF6 may serve as a potential treatment target in RA.