Oxygen sensing by the prolyl-4-hydroxylase PHD2 within the nuclear compartment and the influence of compartmentalisation on HIF-1 signalling.

Hypoxia-inducible factors (HIFs) regulate more than 200 genes involved in cellular adaptation to reduced oxygen availability. HIFs are heterodimeric transcription factors that consist of one of three HIF-α subunits and a HIF-ß subunit. Under normoxic conditions the HIF-α subunit is hydroxylated by members of a family of prolyl-4-hydroxylase domain (PHD) proteins, PHD1, PHD2 and PHD3, resulting in recognition by von-Hippel-Lindau protein, ubiquitylation and proteasomal degradation. It has been suggested that PHD2 is the key regulator of HIF-1α stability in vivo. Previous studies on the intracellular distribution of PHD2 have provided evidence for a predominant cytoplasmic localisation but also nuclear activity of PHD2. Here, we investigated functional nuclear transport signals in PHD2 and identified amino acids 196-205 as having a crucial role in nuclear import, whereas amino acids 6-20 are important for nuclear export. Fluorescence resonance energy transfer (FRET) showed that an interaction between PHD2 and HIF-1α occurs in both the nuclear and cytoplasmic compartments. However, a PHD2 mutant that is restricted to the cytoplasm does not interact with HIF-1α and shows less prolyl hydroxylase activity for its target HIF-1α than wild-type PHD2 located in the nucleus. Here, we present a new model by which PHD2-mediated hydroxylation of HIF-1α predominantly occurs in the cell nucleus and is dependent on very dynamic subcellular trafficking of PHD2.

Cellular oxygen sensing: Importins and exportins are mediators of intracellular localisation of prolyl-4-hydroxylases PHD1 and PHD2.

Hypoxia-inducible factors are crucial in the regulatory process of oxygen homeostasis of vertebrate cells. Inhibition of prolyl hydroxylation of HIF-alpha subunits by prolyl-hydroxylases (PHD1, PHD2 and PHD3) leads to transcription of a greater number of hypoxia responsive genes. We have investigated the subcellular distribution and the molecular mechanisms regulating the intracellular allocation of PHD1 and PHD2. As reported earlier we find PHD1 located exclusively in the nucleus. We demonstrate that nuclear import of PHD1 occurs importin alpha/beta dependently and relies on a nuclear localisation signal (NLS). By contrast PHD2 is cycling between nucleus and cytoplasm, and nuclear import seems to be independent of "classical" importin alpha/beta receptors. Furthermore, we reveal that the exit of PHD2 from the nucleus requires CRM1 and the N-terminal 100 amino acids of the protein. Our findings provide new insights into the mechanisms of the regulation of the oxygen sensor cascade of PHDs in different cellular compartments.