Differential Roles of Hypoxia-Inducible Factor 1α (HIF-1α) and HIF-2α in Hypoxic Gene Regulation

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.