Expression, regulation and signaling of the pattern-recognition receptor NOD2 in rheumatoid arthritis synovial fibroblasts

Objective: Since pattern-recognition receptors (PRRs) in particular Toll-like receptors were found to be over-expressed in the synovium of rheumatoid arthritis (RA) patients and to play a role in the production of disease relevant molecules, we analyzed the expression, regulation and function of the PRR NOD2 in RA. Methods: Expression of NOD2 in synovial tissues was analyzed by immunohistochemistry. Expression and induction of NOD2 in RASFs was measured by conventional and Real-time PCR. Levels of IL-6 and IL-8 were measured by ELISA, expression of MMPs by ELISA and/or Real-time PCR. The expression of NOD2 was silenced with siRNA. Western blots with antibodies against phosphorylated and total p38, JNK and ERK, and inhibitors of p38, JNK and ERK were applied. Activation of NF-κB was measured by EMSA. Results: NOD2 was expressed by fibroblasts and macrophages in the synovium of RA patients, predominantly at sites of invasion into articular cartilage. In cultured RASFs no basal expression of mRNA for NOD2 was detectable, but was induced by polyI-C, LPS and TNF. After upregulation of NOD2 by TLR ligands, its ligand MDP increased the expression of IL-6 and IL-8 via p38 and NF-κB. Stimulation with MDP further induced the expression of MMP1, MMP3 and MMP13. Conclusion: Not only TLRs but also the PRR NOD2 is expressed in the synovium of RA patients and activation of NOD2 synergistically acts with TLRs in the production of proinflammatory and destructive mediators. Therefore NOD2 might contribute to the initiation and perpetuation of chronic, destructive inflammation in RA.