Humans, like other complex aerobic organisms, possess highly evolved systems for the delivery of dioxygen to all the cells of the body. These systems are regulated since excessive levels of dioxygen are toxic. In animals hypoxia causes an increase in the transcription levels of specific genes, including those encoding for vascular endothelial growth factor and erythropoietin. At the transcriptional level, the hypoxic response is mediated by hypoxia-inducible factor (HIF), an α,β-heterodimeric protein. HIF-β is constitutively present, but HIF-α levels are regulated by dioxygen. Under hypoxic conditions, levels of HIF-a rise, allowing its dimerization with HIF-b and enabling transcriptional activation. Under normoxic conditions both the level of HIF-α and its ability to enable transcription are directly controlled by its post-translational oxidation by oxygenases. Hydroxylation of HIF-α at either of two conserved prolyl residues enables its recognition by the von Hippel-Lindau tumour suppressor protein which targets it for proteasomal degradation. Hydroxylation of an asparaginyl residue in the C-terminal transactivation domain of HIF-a directly prevents its interaction with the coactivator p300 from the transcription complex. Hydroxylation of HIF-α is catalysed by members of the iron (II) and 2-oxoglutarate dependent oxygenase family. In humans, three prolyl-hydroxylase isozymes (PHD1-3, for prolyl hydroxylase domain enzymes) and an asparaginyl hydroxylase (FIH, for factor inhibiting HIF) have been identified. Recent studies have identified additional post-translational modifications of HIF-α including acetylation and phosphorylation. Modulation of the HIF mediated hypoxic response is of potential use in a wide range of disease states including cardiovascular disease and cancer. Here we review current knowledge of the HIF pathway focusing on its regulation by dioxygen and discussion of potential targets and challenges in attempts to modulate the pathway for medicinal application.
Keywords: angiogenesis, erythropoietin, hydroxylase, hypoxia, hypoxia-inducible factor (hif), oxygenase, oxygen sensing, transcription
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