Hypoxia-inducible factor-1 as regulator of angiogenesis in rheumatoid arthritis - therapeutic implications

Angiogenesis plays an important role in the pathogenesis of inflammatory diseases, including rheumatoid arthritis (RA). The site and extent of inflammation and subsequent joint destruction in the rheumatoid synovium is dependent on the development of new vasculature. Inhibition of angiogenesis, extensively studied in cancer, might therefore be of interest as treatment option for RA. Hypoxia-inducible factor-1 (HIF-1) has been reported to play a critical role in the regulation of hypoxia driven angiogenesis. HIF-1 is a transcription factor that is constitutively expressed in many cells. It gains transcriptional activity in hypoxic cells leading to the expression of genes involved in angiogenesis. The synovium is hypoxic, but also in an inflammatory environment such as seen in RA, inflammatory cytokines may be important inducers of HIF-1 expression and/or activation. Many drugs currently used in the treatment of RA have anti-angiogenic effects, which are exerted at different levels. Blocking of TNF-alpha, for instance, reduces TNF-alpha induced VEGF production. Studies aiming at direct inhibition of pro-angiogenic factors, such as inhibiting VEGF- or FGF-receptor signalling or blocking VEGF by monoclonal anti-VEGF antibody therapeutics, are in progress. Inhibition of HIF-1 expression or activation, either by blocking signal transduction pathways leading to HIF-1 induction or by inhibiting accumulation of HIF -1 protein, represents a new strategy, which is of interest for the treatment of RA. This review will concisely summarize the general knowledge on the molecular control of gene expression by HIF-1, its involvement in RA, and potential for therapeutic intervention at the level of HIF-1 activity.