Preface Hypoxia inducible factors (HIFs) are broadly expressed in human cancers, and HIF1α and HIF2α were previously suspected of promoting tumor progression through largely overlapping functions. However, this relatively simple model has now been challenged in light of recent data from genome-wide analyses of human tumors, genetically engineered mouse models of cancer, and systems biology approaches that reveal unique and sometimes opposing HIFa activities in both normal physiology and disease. These effects are mediated in part through regulation of unique target genes, as well as direct and indirect interactions with important oncoproteins and tumor suppressors, including MYC and p53. As HIF inhibitors are currently under clinical evaluation as cancer therapeutics, a more thorough understanding of unique roles performed by HIF1α and HIF2α in human neoplasia is warranted. This Review summarizes our rapidly changing understanding of shared and independent HIF1α and HIF2α activities in tumor growth and progression, and the implications for using selective HIF inhibitors as cancer therapeutics.

Transcriptional responses to hypoxia are primarily mediated by hypoxia-inducible factor (HIF), a heterodimer of HIF-␣ and the aryl hydrocarbon receptor nuclear translocator subunits. The HIF-1␣ and HIF-2␣ 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-1␣ ؊/؊ embryonic stem and mouse embryonic fibroblast cells show that loss of HIF-1␣ eliminates all oxygen-regulated transcriptional responses analyzed, suggesting that HIF-2␣ is dispensable for hypoxic gene regulation. In contrast, HIF-2␣ 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-2␣ target genes, we used DNA microarray analysis to evaluate hypoxic gene induction in cells expressing HIF-2␣ but not HIF-1␣. In addition, we engineered HEK293 cells to express stabilized forms of HIF-1␣ or HIF-2␣ via a tetracycline-regulated promoter. In this first comparative study of HIF-1␣ and HIF-2␣ target genes, we demonstrate that HIF-2␣ 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-1␣ (and not HIF-2␣) stimulates glycolytic gene expression in both types of cells, clearly showing for the first time that HIF-1␣ and HIF-2␣ have unique targets.Oxygen (O 2 ), the final electron acceptor during oxidative phosphorylation, is absolutely required for invertebrate and vertebrate life. The immediate response to O 2 deprivation (hypoxia) is a defense phase, which suppresses ATP consumption by arresting protein translation and ion channel activity, two major ATP sinks during normoxia. During a rescue phase, in spite of a general reduction in RNA synthesis, transcription of some genes increases dramatically under low O 2 (21, 34). These hypoxia-responsive genes are involved in glucose transport, glycolysis, erythropoiesis, angiogenesis, vasodilation, and respiratory rate, and together they function to minimize the effects caused by low O 2 at cellular, tissue and systemic levels (93, 106).The activation of many O 2 -regulated genes is mediated by hypoxia-inducible factor (HIF), a heterodimer consisting of HIF-1␣ and HIF-1␤ (also called the aryl hydrocarbon receptor nuclear translocator [ARNT]) in most cells (52,104,105). Both HIF-1␣ and ARNT belong to the basic helix-loop-helix (bHLH)-Per-Arnt-Sim (PAS) family of transcription factors, which share several conserved structural domains, including a bHLH region for DNA binding and two PAS domains for target gene specificity and dimerization (102). Although ARNT is absolutely required for HIF activity (63, 110), HIF function is primarily regulated by HIF-1␣ protein stability (37,46,84). Under normoxia, HIF-1␣ is ubiquitinated through interaction with the von Hippel-Lindau tumor suppr...

Regions of severe oxygen deprivation (hypoxia) arise in tumors due to rapid cell division and aberrant blood vessel formation. The hypoxia-inducible factors (HIFs) mediate transcriptional responses to localized hypoxia in normal tissues and in cancers and can promote tumor progression by altering cellular metabolism and stimulating angiogenesis. Recently, HIFs have been shown to activate specific signaling pathways such as Notch and the expression of transcription factors such as Oct4 that control stem cell self renewal and multipotency. As many cancers are thought to develop from a small number of transformed, self-renewing, and multipotent "cancer stem cells," these results suggest new roles for HIFs in tumor progression.

Low levels of oxygen (O 2 ) occur naturally in developing embryos. Cells respond to their hypoxic microenvironment by stimulating several hypoxia-inducible factors (and other molecules that mediate O 2 homeostasis), which then coordinate the development of the blood, vasculature, placenta, nervous system, and other organs. Furthermore, embryonic stem and progenitor cells frequently occupy hypoxic 'niches' and low O 2 regulates their differentiation. Recent work has revealed an important link between factors involved in regulating stem/progenitor cell behaviour and hypoxiainducible factors, which provides a molecular framework for hypoxic control of differentiation and cell fate. These findings have important implications for the development of therapies for tissue regeneration and disease.