Hypoxia-inducible factor-1alpha (HIF-1alpha) is a master regulator in the cellular response to hypoxic conditions, and rheumatoid synovial tissue is known to exist under hypoxic conditions. Therefore, this study was conducted to determine the contribution of HIF-1alpha to hypoxia-induced MMP and cytokine production in fibroblast-like synoviocytes (FLS).
RA FLS were transfected with either a plasmid that expresses HIF-1alpha or an empty vector as a control, and then cultured under normoxia (21% O(2)). Also, FLS were transfected with either HIF-1alpha small interfering RNA (siRNA) or control siRNA, and cultured under hypoxic conditions (1% O(2)). Following transfection, the amounts of MMP and cytokine mRNAs and HIF-1alpha protein were examined using real-time RT-PCR and western blotting, respectively.
The expression of HIF-1alpha, MMP-1, MMP-3, IL-6 and IL-8 was markedly enhanced in FLS that were cultured under hypoxia. We confirmed that transient transfection of HIF-1alpha overexpressing vector or siRNA had occurred using western blotting, and in vitro studies conducted using FLS transfected with HIF-1alpha overexpression vector showed that they had significantly increased MMP-1, MMP-3 and IL-8 expression levels. Further, hypoxia-induced MMP-3 expression was significantly attenuated by knock-down of HIF-1alpha, whereas hypoxia-induced IL-8 or MMP-1 expression was not significantly repressed by HIF-1alpha siRNA.
Hypoxia-induced MMP-3 expression is exclusively regulated by HIF-1alpha, and hypoxia-induced MMP-1 or IL-8 expression appears to have salvage pathways other than the HIF-1alpha pathway. Together, these data provide new insight regarding the mechanism by which hypoxia participates in joint inflammation and destruction in RA.