TNF-α increases inflammatory factor expression in synovial fibroblasts through the toll-like receptor-3-mediated ERK/AKT signaling pathway in a mouse model of rheumatoid arthritis

Abstract

Osteoarthritis is a type of joint disease that may lead to other joint diseases. Previous research has demonstrated that tumor necrosis factor (TNF)-α is associated with osteoarthritis activity and pathology. The possible mechanisms of the TNF-α-mediated signaling pathway have not been clearly elaborated in synovial fibroblasts. The present study aimed to investigate the potential mechanisms of TNF-α in a mouse model of iodoacetate-induced osteoarthritis. Reverse transcription-quantitative polymerase chain reaction, ELISA, western blotting and immunohistochemistry were performed to evaluate the role of TNF-α in the progression of osteoarthritis. The results revealed that the serum levels of TNF-α, interleukin (IL)-1β, IL-4 and IL-6 were significantly upregulated in a mouse model of iodoacetate-induced osteoarthritis compared with healthy mice (P<0.01). TNF-α, IL-1β, IL-4 and IL-6 mRNA and protein levels were also significantly upregulated in synovial fibroblasts in the experimental mice (P<0.01). It was demonstrated that TNF-α increased pro-inflammation factors matrix metalloproteinase (MMP)-3, MMP-9, nuclear factor (NF)-kB and receptor activator of NF-kB ligand (RANKL) in synovial fibroblasts. It was also observed that the toll-like receptor (TLR)-3 was significantly upregulated and extracellular signal-regulated kinase (ERK) and protein kinase B (AKT) were significantly downregulated in synovial fibroblasts in osteoarthritis mice (P<0.01). An in vitro assay demonstrated that TNF-α inhibitor decreased mRNA and protein levels of IL-1β, IL-4 and IL-6 in synovial fibroblasts. The knockdown of TLR-3 abolished the TNF-α upregulated mRNA and protein levels of IL-1β, IL-4 and IL-6 in synovial fibroblasts. In addition, the knockdown of TLR-3 also reversed TNF-α-upregulated ERK and AKT expression in synovial fibroblasts. In vivo assays demonstrated that TNF-α inhibitor significantly decreased the deposition of IL-1β, IL-4 and IL-6 as well as bone destruction and significantly increased the body weight and osteoarthritis score for osteoarthritic mice (P<0.01). TNF-α inhibitor decreased TLR-3 and significantly increased the expression and phosphorylation of ERK and AKT in articular cartilage (P<0.01). In conclusion the results of the present study indicate that TNF-α serves an essential role in synovial fibroblasts in osteoarthritis, suggesting that inhibition of TNF-α may decrease inflammation via the TLR-3-mediated ERK/AKT signaling pathway in a mouse model of monosodium iodoacetate-induced osteoarthritis.