HIF hydroxylation and cellular oxygen sensing

Hypoxiainducible factor (HIF) is a transcriptional complex that mediates a broad range of cellular and systemic responses to hypoxia. Analysis of HIF-alpha subunits has demonstrated that its activity is regulated by a series of oxygendependent enzymatic hydroxylations at specific prolyl and asparaginyl residues. Combined structural/genetic approaches have identified the relevant enzymes as members of the 2-oxoglutaratedependent dioxygenase superfamily, possessing a beta-barrel jellyroll conformation that aligns a 2-histidine/1-carboxylate iron coordination motif at the catalytic centre. HIF prolyl hydroxylation is performed by a closely related set of isoenzymes (PHD1-3) that differ in abundance and subcellular localisation. Hydroxylation of either human HIF-1alpha Pro402 or Pro564 promotes interaction with the von HippelLindau tumour suppressor protein (pVHL). In oxygenated cells this process targets HIF-alpha for rapid proteasomal destruction. HIF asparaginyl hydroxylation is performed by a protein termed factor inhibiting HIF (FIH). In oxygenated cells hydroxylation of human HIF-1alpha Asn803 prevents interaction with the p300 transcriptional coactivator, providing a second mechanism by which HIFmediated transcription is inactivated. Genetic studies demonstrate a critical function for both types of enzyme in regulating the HIF transcriptional cascade. Limitation of activity in hypoxia supports a central role of these hydroxylases in cellular oxygen sensing. Regulation of the amount of hydroxylase protein, and the supply of other cosubstrates and cofactors, particularly the cellular availability of iron, also contribute to tuning the physiological response to hypoxia.