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.
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.
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.
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.