Sphingosine 1‐phosphate (S1P) is involved in various pathologic conditions and has been implicated as an important mediator of angiogenesis, inflammation, cancer, and autoimmunity. This study was undertaken to examine the role of S1P/S1P₁ signaling in the pathogenesis of rheumatoid arthritis (RA).

We examined S1P₁ messenger RNA (mRNA) and protein levels in RA synoviocytes and MH7A cells by reverse transcriptase–polymerase chain reaction and Western blotting. We also performed S1P₁ immunohistochemistry analysis in synovial tissue from 28 RA patients and 18 osteoarthritis (OA) patients. We investigated the effects of S1P on proliferation by WST‐1 assay, and its effects on tumor necrosis factor α (TNFα)– or interleukin‐1β (IL‐1β)–induced cyclooxygenase 2 (COX‐2) expression and prostaglandin E₂ (PGE₂) production in RA synoviocytes and MH7A cells by Western blotting and enzyme‐linked immunosorbent assay, respectively. Finally, we examined whether these effects of S1P were sensitive to pertussis toxin (PTX), an inhibitor of the Gi/Go proteins.

S1P₁ mRNA and protein were detected in RA synoviocytes and MH7A cells. S1P₁ was more strongly expressed in synovial lining cells, vascular endothelial cells, and inflammatory mononuclear cells of RA synovium compared with OA synovium. S1P increased the proliferation of RA synoviocytes and MH7A cells. S1P alone significantly enhanced COX‐2 expression and PGE₂ production. Moreover, S1P enhanced expression of COX‐2 and production of PGE₂ induced by stimulation with TNFα or IL‐1β in RA synoviocytes and MH7A cells. These effects of S1P were inhibited by pretreatment with PTX.

These findings suggest that S1P signaling via S1P receptors plays an important role in cell proliferation and inflammatory cytokine–induced COX‐2 expression and PGE₂ production by RA synoviocytes. Thus, regulation of S1P/S1P₁ signaling may represent a novel therapeutic target in RA.