Under conditions of limited O₂ supply oxygen homeostasis requires hypoxia-dependent gene expression regulated by the transcription factor hypoxia-inducible factor (HIF-1). Under normoxic conditions oxygen-dependent hydroxylation of HIF-1 alpha subunits (HIF-1a) by a family of prolyl hydroxylases (PHD 1–3) leads to their polyubiquitination and proteosomal degradation. Thus PHDs play a crucial role as oxygen-sensors in regulation of HIF-1a protein stability. The different isoforms, PHD1–3, may differently contribute to specific hypoxia-inducible processes, including angiogenesis, erythropoiesis, tumorigenesis and cell growth, differentiation and survival. To explore the role of the PHD1–3 as oxygen sensors in several biological processes, we studied different aspects of PHD regulation: The role of PHD1–3 isoenzymes, their different induction in response to reduced oxygen or nitric oxide and their distributions in cell compartments.

Our data suggest that PHD activity of cell extracts obtained from cells incubated under hypoxia and nitric oxide was induced. Induction of PHD activity correlated with upregulation of PHD2 and PHD3 levels in both cytoplasmic and cell nuclei extracts. Fractions of cell nuclei exhibit higher PHD activity than the respective cytoplasmic fractions. The different subcellular distribution of PHD activity was essentially due to PHD2. PHD2 activity was strictly O₂-dependent below oxygen concentration of 100 mM implicating PHD2 as an oxygen sensor for the HIF-pathway in cytoplasm and cell nuclei. It remains subject of further investigation whether cytoplasmic and nuclear PHD2 activity differently contribute to specific hypoxia-inducible processes like tumorigenesis and cell growth.