EPAS1

Abstract: Here we describe the cloning and characterization of a PAS domain transcription factor termed endothelial PAS-1 (EPAS1). This protein shares 48% sequence identity with hypoxia inducible factor (HIF-1alpha) and lesser similarity with other members of the basic helix-loop-helix/PAS domain family of transcription factors. Like HIF-1alpha, EPAS1 binds to and activates transcription from a DNA element originally isolated from the erythropoietin gene and containing the sequence 5'-GCCCTACGTGCTGTCTCA-3'. Activation by both HIF-1alpha and EPAS1 is stimulated by hypoxic conditions. EPAS1 forms a heterodimeric complex with the aryl hydrocarbon nuclear transporter prior to transcriptional activation of target genes. EPAS1 expression is limited to the endothelium of mouse embryos and, in agreement with its cell type-specific expression pattern, is capable of specifically activating the transcription of the endothelial tyrosine kinase gene Tie-2. These observations raise the possibility that EPAS1 may represent an important regulator of vascularization, perhaps involving the regulation of endothelial cell gene expression in response to hypoxia.

Abstract: Transcriptional responses to hypoxia are primarily mediated by hypoxia-inducible factor (HIF), a heterodimer of HIF-α and the aryl hydrocarbon receptor nuclear translocator subunits. The HIF-1α and HIF-2α subunits are structurally similar in their DNA binding and dimerization domains but differ in their transactivation domains, implying they may have unique target genes. Previous studies using Hif-1α−/− embryonic stem and mouse embryonic fibroblast cells show that loss of HIF-1α eliminates all oxygen-regulated transcriptional responses analyzed, suggesting that HIF-2α is dispensable for hypoxic gene regulation. In contrast, HIF-2α has been shown to regulate some hypoxia-inducible genes in transient transfection assays and during embryonic development in the lung and other tissues. To address this discrepancy, and to identify specific HIF-2α target genes, we used DNA microarray analysis to evaluate hypoxic gene induction in cells expressing HIF-2α but not HIF-1α. In addition, we engineered HEK293 cells to express stabilized forms of HIF-1α or HIF-2α via a tetracycline-regulated promoter. In this first comparative study of HIF-1α and HIF-2α target genes, we demonstrate that HIF-2α does regulate a variety of broadly expressed hypoxia-inducible genes, suggesting that its function is not restricted, as initially thought, to endothelial cell-specific gene expression. Importantly, HIF-1α (and not HIF-2α) stimulates glycolytic gene expression in both types of cells, clearly showing for the first time that HIF-1α and HIF-2α have unique targets.

Abstract: Although it was known for a long time that oxygen deprivation leads to the transcriptional induction of the gene encoding erythropoietin, the molecular mechanisms behind this process remained enigmatic. The cloning of the hypoxia-inducible factors (HIFs), the finding that HIF-1 regulates the expression of many more genes apart from erythropoietin, and the elucidation of the oxygen-dependent mechanisms degrading the HIF alpha subunits recently led to the spectacular discovery of the molecular principles of oxygen sensing. This review aims to summarize our current knowledge of oxygen-regulated gene expression..