Hypoxia-reoxygenation induces a multitude of cascades of cellular and molecular events which may threaten endothelial survival. Hypoxia-inducible factor (Hif-1a) is stabilized under hypoxic conditions and is involved in transcriptional regulation of adaptation to hypoxia. With the onset of reoxygenation Hif-1a is rapidly degraded and its effect on hypoxic adaptation is ebbing. Hif-1a stability is controlled by prolyl 4-hydroxylase 2 (PHD2), targeting Hif-1a for degradation in presence of oxygen. Here, we hypothesized that preservation of Hif-1a beyond the period of hypoxia can attenuate endothelial hypoxia-reperfusion-induced injury. To analyse the protective effect of Hif-1a endothelial cells from human umbilical veins (HUVEC) were exposed to 1 h of hypoxia (Po₂ < 5 mm Hg) followed by 24 hrs of reperfusion (Po₂ = 140 mm Hg). Exposure of HUVEC to hypoxia causes a 3-fold increase in Hif-1a compared to normoxic control (Western blot; P<0.05, n = 5, for all further parameters). During reoxygenation Hif-1a rapidly declined towards basal level. This maneuver induced apoptosis in 47% (annexin V staining) of HUVEC after 24 hrs of reoxygenation. Silencing of PHD2 by siRNA reduced PHD2 content to 53%. Under this condition Hif-1a content was increased during hypoxia and maintained its level during ongoing reoxygenation. Furthermore, apoptosis was reduced by 20%. Downregulation of Hif-1a by siRNA (64% reduction of Hif-1a content) increased apoptosis to 66%. Similar effects on Hif-1a content and reduction of apoptosis were achieved by addition of DMOG (0.5 mM dimethyloxalylglycine), a PHD inhibitor, at the onset of reoxygenation.

Conclusions: 

Pharmacological inhibition as well as siRNA silencing of PDH2 protects endothelial cells against hypoxia-reperfusion induced apoptosis via Hif-1a stabilisation. The data suggest that strategies targeting PHD activity may provide a new therapeutic option to prevent endothelial injury.