The role of iron and 2-oxoglutarate oxygenases in signalling

Sensing of ambient dioxygen levels and appropriate feedback mechanisms are essential processes for all multicellular organisms. In animals, moderate hypoxia causes an increase in the transcription levels of specific genes, including those encoding vascular endothelial growth factor and erythropoietin. The hypoxic response is mediated by hypoxia-inducible factor (HIF), an alphabeta heterodimeric transcription factor in which both the HIF subunits are members of the basic helix-loop-helix PAS (PER-ARNT-SIM) domain family. Under hypoxic conditions, levels of HIFalpha rise, allowing dimerization with HIFbeta and initiating transcriptional activation. Two types of dioxygen-dependent modification to HIFalpha have been identified, both of which inhibit the transcriptional response. Firstly, HIFalpha undergoes trans -4-hydroxylation at two conserved proline residues that enable its recognition by the von Hippel-Lindau tumour-suppressor protein. Subsequent ubiquitinylation, mediated by an ubiquitin ligase complex, targets HIFalpha for degradation. Secondly, hydroxylation of an asparagine residue in the C-terminal transactivation domain of HIFalpha directly prevents its interaction with the co-activator p300. Hydroxylation of HIFalpha is catalysed by enzymes of the iron(II)- and 2-oxoglutarate-dependent dioxygenase family. In humans, three prolyl hydroxylase isoenzymes (PHD1-3) and an asparagine hydroxylase [factor inhibiting HIF (FIH)] have been identified. The role of 2-oxoglutarate oxygenases in the hypoxic and other signalling pathways is discussed.