HIF is a transcriptional complex that plays a central role in mammalian oxygen homeostasis, the posttranslational modification by prolyl hydroxylation as a key regulatory event that targets HIF-alpha subunits for proteasomal destruction via the von Hippel-Lindau ubiquitylation complex. The transcriptional complex is composed of an alpha-beta heterodimer; HIF-beta being a constitutive nuclear protein that dimerises with oxygen regulated HIF-alpha subunits. In normoxia, 4-hydroxylation of human HIF-alpha at Pro402 or Pro564 by a set of HIF prolyl hydroxylase isoenzymes (PHD 1-3) mediates HIF1-alpha recognition by von Hippel-Lindau ubiquitin ligase complex leading to its proteasomal destruction. In hypoxia (deprivation of oxygen), lack of hydroxylase activity enables HIF-alpha subunits to escape destruction and become transcriptionally active. Thus HIF hydroxylases provide a focus for understanding cellular responses to hypoxia and target for therapeutic manipulation. There are several HIF factors, which include HIF 1-alpha, HIF 1-beta, HIF 2-alpha. HIF 2-alpha/ EPAS 1, a 874aa each protein in mouse, rat and 870aa in human, The transcription factor involved in the induction of oxygen regulated genes. Binds to core DNA sequence 5'-[AG]CGTG-3' within the hypoxia response element (HRE) of target gene promoters. Regulates the vascular endothelial growth factor (VEGF) expression and seems to be implicated in the development of blood vessels and the tubular system of lung. May also play a role in the formation of the endothelium that gives rise to the blood brain barrier. Potent activator of the Tie-2 tyrosine kinase expression. Expressed in most tissues, with highest levels in lung, followed by heart, kidney, brain and liver.
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