Serine proteinases activate the G protein-coupled receptor, proteinase-activated receptor 2 (PAR-2), via cleavage and exposure of a tethered ligand. PAR-2 is known to exert proinflammatory actions in a murine model of arthritis, since PAR-2-deficient mice exhibit strikingly reduced articular inflammation. This study was undertaken to examine synovial PAR-2 expression and to determine the effect of a novel PAR-2 antagonist on synovial cytokine production, in order to investigate the hypothesis that PAR-2 plays a critical role in the pathogenesis of rheumatoid arthritis (RA).
Using a monoclonal antibody to human PAR-2, expression in RA synovium and cultured synovial fibroblasts was characterized. The novel PAR-2 antagonist, ENMD-1068, was added to primary cultures of RA synovial tissue, from which spontaneous cytokine release was measured.
PAR-2 was substantially up-regulated in RA synovium compared with control synovial tissue from patients with osteoarthritis or seronegative inflammatory arthritis, neither of which exhibited significant PAR-2 expression. Importantly, spontaneous release of tumor necrosis factor alpha and interleukin-1beta from RA synovium was substantially inhibited by ENMD-1068, in a dose-dependent manner.
These findings identify PAR-2 as a novel upstream regulator of proinflammatory cytokine production in RA and indicate its potential as a novel therapeutic target in inflammatory arthritis.