• angiogenesis;
  • HIF prolyl-hydroxylases;
  • hypoxia signalling;
  • oxygen sensor;
  • small interfering RNA

Hypoxia-inducible factor (HIF), a transcriptional complex conserved from Caenorhabditis elegans to vertebrates, plays a pivotal role in cellular adaptation to low oxygen availability. In normoxia, the HIF-α subunits are targeted for destruction by prolyl hydroxylation, a specific modification that provides recognition for the E3 ubiquitin ligase complex containing the von Hippel–Lindau tumour suppressor protein (pVHL). Three HIF prolyl-hydroxylases (PHD1, 2 and 3) were identified recently in mammals and shown to hydroxylate HIF-α subunits. Here we show that specific ‘silencing’ of PHD2 with short interfering RNAs is sufficient to stabilize and activate HIF-1α in normoxia in all the human cells investigated. ‘Silencing’ of PHD1 and PHD3 has no effect on the stability of HIF-1α either in normoxia or upon re-oxygenation of cells briefly exposed to hypoxia. We therefore conclude that, in vivo, PHDs have distinct assigned functions, PHD2 being the critical oxygen sensor setting the low steady-state levels of HIF-1α in normoxia. Interestingly, PHD2 is upregulated by hypoxia, providing an HIF-1-dependent auto-regulatory mechanism driven by the oxygen tension.