The heterodimeric transcription factor Hypoxia-inducible factor-1 (HIF-1) plays a major role in cellular adaptation to low oxygen levels by inducing expression of various genes involved in cellular response to hypoxia such as alteration of glucose metabolism or angiogenesis. HIF-1 consists of the two subunits, the constitutively expressed HIF-1ß and the oxygen labile HIF-1a. Under normoxia the a-subunit is targeted for proteasomal degradation by prolyl-hydroxylase 2 (PHD2)-mediated hydroxylation and binding to the von Hippel-Lindau protein associated ubiquitin ligase. Under hypoxia, however, PHD2 activity is blocked and HIF-1a accumulates, dimerizes with the ß-subunit to form the active HIF-1.

HIF-1-mediated oxygen sensing of the cell is known to be influenced by low amounts of reactive oxygen species (ROS). So far contrary results have been found, indicating reduction as well as induction of HIF-1a by ROS. Therefore we studied the effects of H₂O₂ and its derived reactive oxygen species (ROS) on the HIF-1a and its oxygen sensor PHD2 in an osteosarcoma cell line (U2OS). We found that under normoxia and mild hypoxia H₂O₂ initially stabilized but later reduced HIF-1a protein in U2OS cells, while PHD2 was constantly diminished. Luciferase reporter gene assays revealed that reduction of HIF-1a by H₂O₂ was accompanied by a decrease in HIF-1 transcriptional activity. Whether reduction of the HIF-1 target gene PHD2 by H₂O₂ is due to the lowered HIF-1 activity remains subject of further investigation.