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.

The division, differentiation, and function of stem cells and multipotent progenitors are influenced by complex signals in the microenvironment, including oxygen availability. Using a genetic "knock-in" strategy, we demonstrate that targeted replacement of the oxygen-regulated transcription factor HIF-1␣ with HIF-2␣ results in expanded expression of HIF-2␣-specific target genes including Oct-4, a transcription factor essential for maintaining stem cell pluripotency. We show that HIF-2␣, but not HIF-1␣, binds to the Oct-4 promoter and induces Oct-4 expression and transcriptional activity, thereby contributing to impaired development in homozygous Hif-2␣ KI/KI embryos, defective hematopoietic stem cell differentiation in embryoid bodies, and large embryonic stem cell (ES)-derived tumors characterized by altered cellular differentiation. Furthermore, loss of HIF-2␣ severely reduces the number of embryonic primordial germ cells, which require Oct-4 expression for survival and/or maintenance. These results identify Oct-4 as a HIF-2␣-specific target gene and indicate that HIF-2␣ can regulate stem cell function and/or differentiation through activation of Oct-4, which in turn contributes to HIF-2␣'s tumor promoting activity.[Keywords: HIF; hypoxia; HIF-2␣; Oct-4; VEGF; TGF-␣; stem cells; cancer] Supplemental material is available at http://www.genesdev.org.

Chimeric antigen receptors (CARs) redirect T cell cytotoxicity against cancer cells, providing a promising approach to cancer immunotherapy. Despite extensive clinical use, the attributes of CAR co-stimulatory domains that impact persistence and resistance to exhaustion of CAR-T cells remain largely undefined. Here, we report the influence of signaling domains of coreceptors CD28 and 4-1BB on the metabolic characteristics of human CAR T cells. Inclusion of 4-1BB in the CAR architecture promoted the outgrowth of CD8(+) central memory T cells that had significantly enhanced respiratory capacity, increased fatty acid oxidation and enhanced mitochondrial biogenesis. In contrast, CAR T cells with CD28 domains yielded effector memory cells with a genetic signature consistent with enhanced glycolysis. These results provide, at least in part, a mechanistic insight into the differential persistence of CAR-T cells expressing 4-1BB or CD28 signaling domains in clinical trials and inform the design of future CAR T cell therapies.

Clear cell renal cell carcinoma (ccRCC), the most frequent form of kidney cancer1, is characterized by elevated glycogen and fat deposition2. These consistent metabolic alterations are associated with normoxic stabilization of hypoxia inducible factors (HIFs)3, secondary to von hippel-lindau (VHL) mutations that occur in over 90% of ccRCC tumours4. However, kidney-specific VHL deletion in mice fails to elicit ccRCC-specific metabolic phenotypes and tumour formation5, suggesting that additional mechanisms are essential. Recent large-scale sequencing analyses revealed loss of several chromatin remodelling enzymes in a subset of ccRCC (polybromo 1 [PBRM1] ~40%, SET domain containing 2 [SETD2] ~15%, BRCA1 associated protein-1 [BAP1] ~15%, etc.)6–9, indicating that epigenetic perturbations are likely important contributors to the natural history of this disease. Here we utilized an integrative approach comprising pan-metabolomic profiling and metabolic gene set analysis, and determined that the gluconeogenic enzyme fructose-1, 6-bisphosphatase 1 (FBP1)10 is uniformly depleted in over six hundred ccRCC tumours examined. Importantly, the human FBP1 locus resides on chromosome 9q22, whose loss is associated with poor prognosis for ccRCC patients11. Our data further indicate that FBP1 inhibits ccRCC progression through two distinct mechanisms: 1) FBP1 antagonizes glycolytic flux in renal tubular epithelial cells, the presumptive ccRCC cell of origin12, thereby inhibiting a potential “Warburg effect”13,14, and 2) in pVHL-deficient ccRCC cells, FBP1 restrains cell proliferation, glycolysis, and the pentose phosphate pathway in a catalytic activity-independent manner, by inhibiting nuclear HIF function via direct interaction with the HIF “inhibitory domain”. This unique dual function of the FBP1 protein explains its ubiquitous loss in ccRCC, distinguishing FBP1 from previously-identified tumour suppressors (PBRM1, SETD2, BAP1, etc.) which are not consistently mutated in all tumours6,7,15.

Oxygen (O2) deprivation, or hypoxia, has profound effects on cell metabolism and growth. Cells can adapt to low O2 in part through activation of hypoxia-inducible factor (HIF). We report here that hypoxia inhibits mRNA translation by suppressing multiple key regulators, including eIF2alpha, eEF2, and the mammalian target of rapamycin (mTOR) effectors 4EBP1, p70S6K, and rpS6, independent of HIF. Hypoxia results in energy starvation and activation of the AMPK/TSC2/Rheb/mTOR pathway. Hypoxic AMP-activated protein kinase (AMPK) activation also leads to eEF2 inhibition. Moreover, hypoxic effects on cellular bioenergetics and mTOR inhibition increase over time. Mutation of the TSC2 tumor suppressor gene confers a growth advantage to cells by repressing hypoxic mTOR inhibition and hypoxia-induced G1 arrest. Together, eIF2alpha, eEF2, and mTOR inhibition represent important HIF-independent mechanisms of energy conservation that promote survival under low O2 conditions.