Objective: Stromal cell-derived factor 1 (SDF-1; or, CXCL12) is a potent chemotactic and angiogenic factor that has been proposed to play a role in the recruitment of lymphocytes into rheumatoid arthritis (RA) synovium. We tested the hypothesis that synovial SDF-1 expression is regulated by cytokine and hypoxic stimulation, the latter being mediated by hypoxia-inducible factor 1alpha (HIF-1alpha). These factors regulate the expression of vascular endothelial growth factor (VEGF), itself an important angiogenic mediator.
Methods: RA and osteoarthritic synovial fibroblasts and whole tissue explants were cultured under normoxic or hypoxic (1% O(2)) conditions for up to 72 hours in the presence or absence of interleukin-1beta (IL-1beta), tumor necrosis factor (TNF), or transforming growth factor beta (TGFbeta). Expression of HIF-1alpha, VEGF, and SDF-1 was detected in synovial tissue and cells by immunohistochemistry and Western blotting. VEGF and SDF-1 expression by cultured synovial fibroblasts was evaluated by reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay.
Results: Immunohistochemistry revealed the presence of HIF-1alpha, VEGF, and SDF-1 in RA synovium. Patchy expression of HIF-1alpha was detected primarily in the synovial lining and sublining areas; expression in synovial fibroblasts and in the lining cells of whole synovial tissue explants was markedly augmented by hypoxic culture conditions. Hypoxia enhanced the expression of VEGF and SDF-1 messenger RNA in synovial fibroblasts. The production of VEGF and SDF-1 protein by synovial fibroblasts was augmented by 50% and 132%, respectively, after 24 hours of hypoxia. VEGF production was potently induced by TGFbeta, and to a lesser extent by IL-1beta and TNF, and was further augmented by hypoxia. In contrast, none of the tested cytokines induced SDF-1 production.
Conclusion: As with VEGF, SDF-1 expression is induced by hypoxia; however, cytokines induce VEGF but not SDF-1. Hypoxic conditions in RA synovium, which are likely to be transient and episodic, may contribute to the persistence of synovitis by inducing VEGF and SDF-1.