Adaptive transcriptional responses to oxygen deprivation (hypoxia) are mediated by the hypoxia-inducible factors (HIFs), heterodimeric transcription factors composed of two basic helix-loop-helix-PAS family proteins. The transcriptional activity of HIF is determined by the hypoxic stabilization of the HIF-␣ proteins. HIF-1␣ and HIF-2␣ exhibit high sequence homology but have different mRNA expression patterns; HIF-1␣ is expressed ubiquitously whereas HIF-2␣ expression is more restricted to certain tissues, e.g., the endothelium, lung, brain, and neural crest derivatives. Germ-line deletion of either HIF subunit is embryonic lethal with unique features suggesting important roles for both HIF-␣ isoforms. Global deletion of Hif-2␣ results in distinct phenotypes depending on the mouse strain used for the mutation, clearly demonstrating an important role for HIF-2␣ in mouse development. The function of HIF-2␣ in adult life, however, remains incompletely understood. In this study, we describe the generation of a conditional murine Hif-2␣ allele and the effect of its acute postnatal ablation. Under very stringent conditions, we ablate Hif-2␣ after birth and compare the effect of acute global deletion of Hif-2␣ and Hif-1␣. Our results demonstrate that HIF-2␣ plays a critical role in adult erythropoiesis, with acute deletion leading to anemia. Furthermore, although HIF-1␣ was first purified and cloned based on its affinity for the human erythropoietin (EPO) 3 enhancer hypoxia response element (HRE) and regulates Epo expression during mouse embryogenesis, HIF-2␣ is the critical ␣ isoform regulating Epo under physiologic and stress conditions in adults.Epo ͉ hypoxia-inducible factor ͉ hypoxia ͉ red blood cell H ypoxia-inducible factors (HIFs), members of the basic helix-loop-helix (bHLH)-PAS family of transcription factors, are master regulators of oxygen (O 2 ) homeostasis and stimulate genes important for angiogenesis, erythropoiesis and glucose metabolism (1-3). HIFs are heterodimeric factors consisting of ␣ (HIF-1␣, -2␣, and -3␣) and ␤ subunits [HIF-1␤ or ARNT (arylhydrocarbon-receptor nuclear translocator)].

Mammalian cells are believed to have a cell-intrinsic ability to increase glucose metabolism in response to hypoxia. Here we show that the ability of hematopoietic cells to up-regulate anaerobic glycolysis in response to hypoxia is dependent on receptor-mediated signal transduction. In the absence of growth factor signaling, hematopoietic cells fail to express hypoxia-inducible transcription factor (Hif-1␣) mRNA. Growth factor-deprived hematopoietic cells do not engage in glucose-dependent anabolic synthesis and neither express Hif-1␣ mRNA nor require HIF-1␣ protein to regulate cell survival in response to hypoxia. However, HIF-1␣ is adaptive for the survival of growth factor-stimulated cells, as suppression of HIF-1␣ results in death when growing cells are exposed to hypoxia. Growth factor-dependent HIF-1␣ expression reprograms the intracellular fate of glucose, resulting in decreased glucose-dependent anabolic synthesis and increased lactate production, an effect that is enhanced when HIF-1␣ protein is stabilized by hypoxia. Together, these data suggest that HIF-1␣ contributes to the regulation of growth factor-stimulated glucose metabolism even in the absence of hypoxia.[Keywords: HIF-1␣; hypoxia; growth factor signaling; glucose metabolism; cell survival] Supplemental material is available at http://www.genesdev.org.

Peri-operative SARS-CoV-2 infection increases postoperative mortality. The aim of this study was to determine the optimal duration of planned delay before surgery in patients who have had SARS-CoV-2 infection. This international, multicentre, prospective cohort study included patients undergoing elective or emergency surgery during October 2020. Surgical patients with pre-operative SARS-CoV-2 infection were compared with those without previous SARS-CoV-2 infection. The primary outcome measure was 30-day postoperative mortality. Logistic regression models were used to calculate adjusted 30-day mortality rates stratified by time from diagnosis of SARS-CoV-2 infection to surgery. Among 140,231 patients (116 countries), 3127 patients (2.2%) had a pre-operative SARS-CoV-2 diagnosis. Adjusted 30-day mortality in patients without SARS-CoV-2 infection was 1.5% (95%CI 1.4-1.5). In patients with a pre-operative SARS-CoV-2 diagnosis, mortality was increased in patients having surgery within 0-2 weeks, 3-4 weeks and 5-6 weeks of the diagnosis (odds ratio (95%CI) 4.1 (3.3-4.8), 3.9 (2.6-5.1) and 3.6 (2.0-5.2), respectively). Surgery performed ≥ 7 weeks after SARS-CoV-2 diagnosis was associated with a similar mortality risk to baseline (odds ratio (95%CI) 1.5 (0.9-2.1)). After a ≥ 7 week delay in undertaking surgery following SARS-CoV-2 infection, patients with ongoing symptoms had a higher mortality than patients whose symptoms had resolved or who had been asymptomatic (6.0% (95%CI 3.2-8.7) vs. 2.4% (95%CI 1.4-3.4) vs. 1.3% (95%CI 0.6-2.0), respectively). Where possible, surgery should be delayed for at least 7 weeks following SARS-CoV-2 infection. Patients with ongoing symptoms ≥ 7 weeks from diagnosis may benefit from further delay.

Solid tumors exhibit heterogeneous microenvironments, often characterized by limiting concentrations of oxygen (O2), glucose, and other nutrients. How oncogenic mutations alter stress response pathways, metabolism, and cell survival in the face of these challenges is incompletely understood. Here we report that constitutive mammalian target of rapamycin complex 1 (mTORC1) activity renders hypoxic cells dependent on exogenous desaturated lipids, as levels of de novo synthesized unsaturated fatty acids are reduced under low O2. Specifically, we demonstrate that hypoxic Tsc2−/− (tuberous sclerosis complex 2−/−) cells deprived of serum lipids exhibit a magnified unfolded protein response (UPR) but fail to appropriately expand their endoplasmic reticulum (ER), leading to inositol-requiring protein-1 (IRE1)-dependent cell death that can be reversed by the addition of unsaturated lipids. UPR activation and apoptosis were also detected in Tsc2-deficient kidney tumors. Importantly, we observed this phenotype in multiple human cancer cell lines and suggest that cells committed to unregulated growth within ischemic tumor microenvironments are unable to balance lipid and protein synthesis due to a critical limitation in desaturated lipids.