Protease-activated receptor 2, rather than protease-activated receptor 1, contributes to the aggressive properties of synovial fibroblasts in rheumatoid arthritis.

OBJECTIVE: To investigate whether protease-activated receptor 1 (PAR-1) and/or PAR-2 promotes the invasiveness/proliferation of synovial fibroblasts (SFs) and to determine the signaling mechanisms of these pathways.

METHODS: SFs were isolated from the synovial tissue of patients with rheumatoid arthritis (RA), patients with osteoarthritis (OA), and PAR-1- or PAR-2-knockout (KO) mice. Expression of PAR-1 and PAR-2 was detected by immunofluorescence and Western blotting. The invasion and proliferation of SFs were measured by invasion assay and MTT assay, respectively. Matrix metalloproteinase 2 (MMP-2) and MMP-9 were detected by zymography, and cytokines were measured by enzyme-linked immunosorbent assay.

RESULTS: PAR-1 and PAR-2 were colocalized with SFs in RA and OA synovium and, to a considerably lesser extent, in normal synovium. Inhibition of PAR-2 by small interfering RNA (siRNA) inhibited RASF invasion and proliferation, whereas blocking of PAR-1 by siRNA had the reverse effects. SFs from PAR-2-KO mice exhibited slower rates of proliferation and invasion. SFs from PAR-1-KO mice produced less MMP-2 and, in response to tumor necrosis factor α (TNFα) stimulation, had increased MMP-9 secretion when compared to SFs from wild-type and PAR-2-KO mice. Inhibition of PAR-1, but not PAR-2, stimulated the secretion of interleukin-17 (IL-17) and TNFα by RASFs. Furthermore, PAR-1 and PAR-2 had opposing effects on the activation of ERK, p38, and NF-κB.

CONCLUSION: Activation of PAR-1 stimulates MMP-2 secretion, inhibits RASF growth and invasion, and decreases production of IL-17 and TNFα by RASFs, whereas activation of PAR-2 stimulates RASF growth and invasion and increases production of TNFα. Thus, although PAR-1 and PAR-2 are coexpressed by RASFs, PAR-2 alone appears to be responsible for the aggressive properties of RASFs and is likely to contribute to the pathologic progression of RA.