The pattern‐recognition receptor nucleotide‐binding oligomerization domain–containing protein 1 promotes production of inflammatory mediators in rheumatoid arthritis synovial fibroblasts

Objective Pattern‐recognition receptors (PRRs), such as Toll‐like receptors (TLRs) and nucleotide‐binding oligomerization domain–containing protein 2 (NOD‐2), have been shown to contribute to the pathogenesis of rheumatoid arthritis (RA). The aim of this study was to analyze the expression, regulation, and function of the PRR NOD‐1 in RA synovial fibroblasts (RASFs), and to examine its interaction with other PRRs. Methods Expression of NOD‐1 was analyzed by immunohistochemistry in synovial tissue from RA patients, psoriatic arthritis patients, gout patients, and osteoarthritis (OA) patients. RASFs and human monocyte‐derived macrophages (HMDMs) were stimulated with L ‐alanyl‐γ‐ D ‐glutamyl‐meso‐diaminopimelic acid, palmitoyl‐3‐cysteine‐serine‐lysine‐4, poly(I‐C), lipopolysaccharide, heat‐inactivated bacteria, tumor necrosis factor α (TNFα), or interleukin‐1β (IL‐1β). Expression levels of IL‐6, CCL5, matrix metalloproteinases (MMPs), NODs, and TLRs were measured by real‐time reverse transcription–polymerase chain reaction and/or enzyme‐linked immunosorbent assay. NOD‐1 and NOD‐2 were silenced with target‐specific small interfering RNA. Phosphorylation of IL‐1 receptor–associated kinase 1 (IRAK‐1) was measured by Western blotting. Results Expression of NOD‐1 protein was significantly increased in RA synovium compared to OA synovium. The basal expression of NOD‐1 was similar in RASFs, OASFs, healthy control peripheral blood mononuclear cells, and healthy control HMDMs. Stimulation of RASFs with TLR‐3 up‐regulated the expression of NOD‐1. Expression of IL‐6, CCL5, MMPs, TLR‐2, and NOD‐2 was significantly up‐regulated in RASFs by stimulation with the NOD‐1 ligand. A synergistic effect on IL‐6 production was observed in cells stimulated with NOD‐1 and TLR‐2 ligands or NOD‐1 and TLR‐4 ligands. Silencing of NOD‐1, but not NOD‐2, decreased the levels of IL‐6 in RASFs after stimulation with TLR‐2 and IL‐1β, and blocked the phosphorylation of IRAK‐1. Conclusion NOD‐1 is strongly expressed in different cell types in the synovial tissue of patients with RA. These results indicate that NOD‐1, either alone or interacting with other inflammatory mediators, can play an important role in the chronic and destructive inflammation of the joints in RA.