The hypoxia-responsive transcription factor EPAS1 is essential for catecholamine homeostasis and protection against heart failure during embryonic development

Hypoxia, pseudohypoxia and cellular differentiation

Tumor hypoxia correlates to aggressive disease, and while this is explained by a variety of factors, one clue to understand this phenomena was the finding that hypoxia induces a de-differentiated, stem cell-like phenotype in neuroblastoma and breast tumor cells. The hypoxia inducible transcription factors (HIFs) are regulated at the translational level by fluctuating oxygen concentrations, but emerging data reveal that both HIF-1α and HIF-2α expression can be induced by aberrantly activated growth factor signaling independently of oxygen levels. Furthermore, HIF-2α is regulated by hypoxia also at the transcriptional level in neuroblastoma and glioma cells. In cultured tumor cells, HIF-2α is stabilized at physiological oxygen concentrations followed by induced expression of classical hypoxia-driven genes, resulting in a pseudohypoxic phenotype. In addition, in neuroblastoma and glioma specimens, a small subset of HIF-2α positive, HIF-1α negative, tumor cells is found adjacent to blood vessels, i.e. in areas with presumably adequate oxygenation. These tumor niches are thus pseudohypoxic, and the HIF-2α expressing cells present immature features. We have postulated that this niche in neuroblastomas encompass the tumor stem cells. Oncogenes or tumor suppressor genes associated with pseudohypoxia are frequently mutated or deleted in the germline, implicating that the pseudohypoxic phenotype indeed is tumorigenic. In summary, the hypoxic and pseudohypoxic phenotypes of solid tumors are attractive therapeutic targets.

Pharmacological Regulation of In Situ Tissue Stem Cells Differentiation for Soft Tissue Calcification Treatment

Calcification of soft tissues, such as heart valves and tendons, is a common clinical problem with limited therapeutics. Tissue specific stem/progenitor cells proliferate to repopulate injured tissues. But some of them become divergent to the direction of ossification in the local pathological microenvironment, thereby representing a cellular target for pharmacological approach. We observed that HIF-2alpha (encoded by EPAS1 inclined form) signaling is markedly activated within stem/progenitor cells recruited at calcified sites of diseased human tendons and heart valves. Proinflammatory microenvironment, rather than hypoxia, is correlated with HIF-2alpha activation and promoted osteochondrogenic differentiation of tendon stem/progenitor cells (TSPCs). Abnormal upregulation of HIF-2alpha served as a key switch to direct TSPCs differentiation into osteochondral-lineage rather than teno-lineage. Notably, Scleraxis (Scx), an essential tendon specific transcription factor, was suppressed on constitutive activation of HIF-2alpha and mediated the effect of HIF-2alpha on TSPCs fate decision. Moreover, pharmacological inhibition of HIF-2alpha with digoxin, which is a widely utilized drug, can efficiently inhibit calcification and enhance tenogenesis in vitro and in the Achilles's tendinopathy model. Taken together, these findings reveal the significant role of the tissue stem/progenitor cells fate decision and suggest that pharmacological regulation of HIF-2alpha function is a promising approach for soft tissue calcification treatment.

The transcription factor EPAS1 links DOCK8 deficiency to atopic skin inflammation via IL-31 induction

Mutations of DOCK8 in humans cause a combined immunodeficiency characterized by atopic dermatitis with high serum IgE levels. However, the molecular link between DOCK8 deficiency and atopic skin inflammation is unknown. Here we show that CD4⁺ T cells from DOCK8-deficient mice produce large amounts of IL-31, a major pruritogen associated with atopic dermatitis. IL-31 induction critically depends on the transcription factor EPAS1, and its conditional deletion in CD4⁺ T cells abrogates skin disease development in DOCK8-deficient mice. Although EPAS1 is known to form a complex with aryl hydrocarbon receptor nuclear translocator (ARNT) and control hypoxic responses, EPAS1-mediated Il31 promoter activation is independent of ARNT, but in collaboration with SP1. On the other hand, we find that DOCK8 is an adaptor and negative regulator of nuclear translocation of EPAS1. Thus, EPAS1 links DOCK8 deficiency to atopic skin inflammation via IL-31 induction in CD4⁺ T cells.

Hypoxia promotes colon cancer dissemination through up-regulation of cell migration-inducing protein (CEMIP)

Hypoxic stress drives cancer progression by causing a transcriptional reprogramming. Recently, KIAA1199 was discovered to be a cell-migration inducing protein (renamed CEMIP) that is upregulated in human cancers. However, the mechanism of induction of CEMIP in cancer was hitherto unknown. Here we demonstrate that hypoxia induces CEMIP expression leading to enhanced cell migration. Immunohistochemistry of human colon cancer tissues revealed that CEMIP is upregulated in cancer cells located at the invasive front or in the submucosa. CEMIP localization inversely correlated with E-cadherin expression, which is characteristic of the epithelial-to-mesenchymal transition. Mechanistically, hypoxia-inducible-factor-2α (HIF-2α), but not HIF-1α binds directly to the hypoxia response element within the CEMIP promoter region resulting in increased CEMIP expression. Functional characterization reveals that CEMIP is a downstream effector of HIF-2α-mediated cell migration. Expression of CEMIP was demonstrated to negatively correlate with the expression of Jarid1A, a histone demethylase that removes methyl groups from H3K4me3 (an activation marker for transcription), resulting in altered gene repression. Low oxygen tension inhibits the function of Jarid1A, leading to increased presence of H3K4me3 within the CEMIP promoter. These results provide insight into the upregulation of CEMIP within cancer and can lead to novel treatment strategies targeting this cancer cell migration-promoting gene.

Enhanced BDNF signalling following chronic hypoxia potentiates catecholamine release from cultured rat adrenal chromaffin cells

KEY POINTS

We investigated the role of the neurotrophin BDNF signalling via the TrkB receptor in rat adrenomedullary chromaffin cells (AMCs) exposed to normoxia (Nox; 21% O2) and chronic hypoxia (CHox; 2% O2) in vitro for ∼ 48 h. TrkB receptor expression was upregulated in primary AMCs and in immortalized chromaffin (MAH) cells exposed to CHox; this effect was absent in MAH cells deficient in the transcription factor, hypoxia inducible factor (HIF)-2α. Relative to normoxic controls, activation of the TrkB receptor in chronically hypoxic AMCs led to a marked increase in membrane excitability, intracellular [Ca(2+)], and catecholamine secretion. The BDNF-induced rise of intracellular [Ca(2+)] in CHox cells was sensitive to the selective T-type Ca(2+) channel blocker TTA-P2 and tetrodotoxin (TTX), suggesting key roles of low threshold T-type Ca(2+) and voltage-gated Na(+) channels in the signalling pathway. Environmental stressors, including chronic hypoxia, enhance the ability of adrenomedullary chromaffin cells (AMCs) to secrete catecholamines; however, the underlying molecular mechanisms remain unclear. Here, we investigated the role of brain-derived neurotrophic factor (BDNF) signalling in rat AMCs exposed to chronic hypoxia. In rat adrenal glands, BDNF and its tropomyosin-related kinase B (TrkB) receptor are highly expressed in the cortex and medulla, respectively. Exposure of AMCs to chronic hypoxia (2% O2; 48 h) in vitro caused a significant increase to TrkB mRNA expression. A similar increase was observed in an immortalized chromaffin cell line (MAH cells); however, it was absent in MAH cells deficient in the transcription factor HIF-2α. A specific TrkB agonist, 7,8-dihydroxyflavone (7,8-DHF), stimulated quantal catecholamine secretion from chronically hypoxic (CHox; 2% O2) AMCs to a greater extent than normoxic (Nox; 21% O2) controls. Activation of TrkB by BDNF or 7,8-DHF increased intracellular Ca(2+) ([Ca(2+)]i), an effect that was significantly larger in CHox cells. The 7,8-DHF-induced [Ca(2+)]i rise was sensitive to the tyrosine kinase inhibitor K252a and nickel (2 mm), but not the Ca(2+) store-depleting agent cyclopiazonic acid. Blockade of T-type calcium channels with TTA-P2 (1 μm) or voltage-gated Na(+) channels with TTX inhibited BDNF-induced [Ca(2+)]i increases. BDNF also induced a dose-dependent enhancement of action potential firing in CHox cells. These data demonstrate that during chronic hypoxia, enhancement of BDNF-TrkB signalling increases voltage-dependent Ca(2+) influx and catecholamine secretion in chromaffin cells, and that T-type Ca(2+) channels play a key role in the signalling pathway.

The Endothelial Prolyl-4-Hydroxylase Domain 2/Hypoxia-Inducible Factor 2 Axis Regulates Pulmonary Artery Pressure in Mice

Hypoxia-inducible factors 1 and 2 (HIF-1 and -2) control oxygen supply to tissues by regulating erythropoiesis, angiogenesis and vascular homeostasis. HIFs are regulated in response to oxygen availability by prolyl-4-hydroxylase domain (PHD) proteins, with PHD2 being the main oxygen sensor that controls HIF activity under normoxia. In this study, we used a genetic approach to investigate the endothelial PHD2/HIF axis in the regulation of vascular function. We found that inactivation of Phd2 in endothelial cells specifically resulted in severe pulmonary hypertension (∼118% increase in right ventricular systolic pressure) but not polycythemia and was associated with abnormal muscularization of peripheral pulmonary arteries and right ventricular hypertrophy. Concurrent inactivation of either Hif1a or Hif2a in endothelial cell-specific Phd2 mutants demonstrated that the development of pulmonary hypertension was dependent on HIF-2α but not HIF-1α. Furthermore, endothelial HIF-2α was required for the development of increased pulmonary artery pressures in a model of pulmonary hypertension induced by chronic hypoxia. We propose that these HIF-2-dependent effects are partially due to increased expression of vasoconstrictor molecule endothelin 1 and a concomitant decrease in vasodilatory apelin receptor signaling. Taken together, our data identify endothelial HIF-2 as a key transcription factor in the pathogenesis of pulmonary hypertension.

Therapeutic targeting of hypoxia and hypoxia-inducible factors in cancer

Insufficient tissue oxygenation, or hypoxia, contributes to tumor aggressiveness and has a profound impact on clinical outcomes in cancer patients. At decreased oxygen tensions, hypoxia-inducible factors (HIFs) 1 and 2 are stabilized and mediate a hypoxic response, primarily by acting as transcription factors. HIFs exert differential effects on tumor growth and affect important cancer hallmarks including cell proliferation, apoptosis, differentiation, vascularization/angiogenesis, genetic instability, tumor metabolism, tumor immune responses, and invasion and metastasis. As a consequence, HIFs mediate resistance to chemo- and radiotherapy and are associated with poor prognosis in cancer patients. Intriguingly, perivascular tumor cells can also express HIF-2α, thereby forming a "pseudohypoxic" phenotype that further contributes to tumor aggressiveness. Therefore, therapeutic targeting of HIFs in cancer has the potential to improve treatment efficacy. Different strategies to target hypoxic cancer cells and/or HIFs include hypoxia-activated prodrugs and inhibition of HIF dimerization, mRNA or protein expression, DNA binding capacity, and transcriptional activity. Here we review the functions of HIFs in the progression and treatment of malignant solid tumors. We also highlight how HIFs may be targeted to improve the management of patients with therapy-resistant and metastatic cancer.

Single cell transcriptome analysis of mouse carotid body glomus cells

KEY POINTS

Carotid body (CB) glomus cells mediate acute oxygen sensing and the initiation of the hypoxic ventilatory response, yet the gene expression profile of these cells is not available. We demonstrate that the single cell RNA-Seq method is a powerful tool for identifying highly expressed genes in CB glomus cells. Our single cell RNA-Seq results characterized novel CB glomus cell genes, including members of the G protein-coupled receptor signalling pathway, ion channels and atypical mitochondrial electron transport chain subunits. A heterologous cell-based screening identified acetate (which is known to affect CB glomus cell activity) as an agonist for the most highly abundant G protein-coupled receptor (Olfr78) in CB glomus cells. These data established the first transcriptome profile of CB glomus cells, highlighting genes with potential implications in CB chemosensory function.

ABSTRACT

The carotid body (CB) is a major arterial chemoreceptor containing glomus cells whose activities are regulated by changes in arterial blood content, including oxygen. Despite significant advancements in the characterization of their physiological properties, our understanding of the underlying molecular machinery and signalling pathway in CB glomus cells is still limited. To overcome this, we employed the single cell RNA-Seq method by performing next-generation sequencing on single glomus cell-derived cDNAs to eliminate contamination of genes derived from other cell types present in the CB. Using this method, we identified a set of genes abundantly expressed in glomus cells, which contained novel glomus cell-specific genes. Transcriptome and subsequent in situ hybridization and immunohistochemistry analyses identified abundant G protein-coupled receptor signalling pathway components and various types of ion channels, as well as members of the hypoxia-inducible factors pathway. A short-chain fatty acid olfactory receptor Olfr78, recently implicated in CB function, was the most abundant G protein-coupled receptor. Two atypical mitochondrial electron transport chain subunits (Ndufa4l2 and Cox4i2) were among the most specifically expressed genes in CB glomus cells, highlighting their potential roles in mitochondria-mediated oxygen sensing. The wealth of information provided by the present study offers a valuable foundation for identifying molecules functioning in the CB.

Regulation of ventilatory sensitivity and carotid body proliferation in hypoxia by the PHD2/HIF-2 pathway

Ventilatory sensitivity to hypoxia increases in response to continued hypoxic exposure as part of acute acclimatisation. Although this process is incompletely understood, insights have been gained through studies of the hypoxia-inducible factor (HIF) hydroxylase system. Genetic studies implicate these pathways widely in the integrated physiology of hypoxia, through effects on developmental or adaptive processes. In keeping with this, mice that are heterozygous for the principal HIF prolyl hydroxylase, PHD2, show enhanced ventilatory sensitivity to hypoxia and carotid body hyperplasia. Here we have sought to understand this process better through comparative analysis of inducible and constitutive inactivation of PHD2 and its principal targets HIF-1α and HIF-2α. We demonstrate that general inducible inactivation of PHD2 in tamoxifen-treated Phd2(f/f);Rosa26(+/CreERT2) mice, like constitutive, heterozygous PHD2 deficiency, enhances hypoxic ventilatory responses (HVRs: 7.2 ± 0.6 vs. 4.4 ± 0.4 ml min(-1) g(-1) in controls, P < 0.01). The ventilatory phenotypes associated with both inducible and constitutive inactivation of PHD2 were strongly compensated for by concomitant inactivation of HIF-2α, but not HIF-1α. Furthermore, inducible inactivation of HIF-2α strikingly impaired ventilatory acclimatisation to chronic hypoxia (HVRs: 4.1 ± 0.5 vs. 8.6 ± 0.5 ml min(-1) g(-1) in controls, P < 0.0001), as well as carotid body cell proliferation (400 ± 81 vs. 2630 ± 390 bromodeoxyuridine-positive cells mm(-2) in controls, P < 0.0001). The findings demonstrate the importance of the PHD2/HIF-2α enzyme-substrate couple in modulating ventilatory sensitivity to hypoxia.

Oxygen sensing by the carotid body: mechanisms and role in adaptation to hypoxia

Oxygen (O2) is fundamental for cell and whole-body homeostasis. Our understanding of the adaptive processes that take place in response to a lack of O2(hypoxia) has progressed significantly in recent years. The carotid body (CB) is the main arterial chemoreceptor that mediates the acute cardiorespiratory reflexes (hyperventilation and sympathetic activation) triggered by hypoxia. The CB is composed of clusters of cells (glomeruli) in close contact with blood vessels and nerve fibers. Glomus cells, the O2-sensitive elements in the CB, are neuron-like cells that contain O2-sensitive K(+)channels, which are inhibited by hypoxia. This leads to cell depolarization, Ca(2+)entry, and the release of transmitters to activate sensory fibers terminating at the respiratory center. The mechanism whereby O2modulates K(+)channels has remained elusive, although several appealing hypotheses have been postulated. Recent data suggest that mitochondria complex I signaling to membrane K(+)channels plays a fundamental role in acute O2sensing. CB activation during exposure to low Po2is also necessary for acclimatization to chronic hypoxia. CB growth during sustained hypoxia depends on the activation of a resident population of stem cells, which are also activated by transmitters released from the O2-sensitive glomus cells. These advances should foster further studies on the role of CB dysfunction in the pathogenesis of highly prevalent human diseases.

Role of Intestinal HIF-2α in Health and Disease

The intestine is supported by a complex vascular system that undergoes dynamic and transient daily shifts in blood perfusion, depending on the metabolic state. Moreover, the intestinal villi have a steep oxygen gradient from the hypoxic epithelium adjacent to the anoxic lumen to the relative higher tissue oxygenation at the base of villi. Due to the daily changes in tissue oxygen levels in the intestine, the hypoxic transcription factors hypoxia-inducible factor (HIF)-1α and HIF-2α are essential in maintaining intestinal homeostasis. HIF-2α is essential in maintaining proper micronutrient balance, the inflammatory response, and the regenerative and proliferative capacity of the intestine following an acute injury. However, chronic activation of HIF-2α leads to enhanced proinflammatory response, intestinal injury, and colorectal cancer. In this review, we detail the major mechanisms by which HIF-2α contributes to health and disease of the intestine and the therapeutic implications of targeting HIF-2α in intestinal diseases.

Loss of Hif-2α Rescues the Hif-1α Deletion Phenotype of Neonatal Respiratory Distress In Mice

Hypoxia is a state of decreased oxygen reaching the tissues of the body. During prenatal development, the fetus experiences localized occurrences of hypoxia that are essential for proper organogenesis and survival. The response to decreased oxygen availability is primarily regulated by hypoxia-inducible factors (HIFs), a family of transcription factors that modulate the expression of key genes involved in glycolysis, angiogenesis, and erythropoiesis. HIF-1α and HIF-2α, two key isoforms, are important in embryonic development, and likely are involved in lung morphogenesis. We have recently shown that the inducible loss of Hif-1α in lung epithelium starting at E4.5 leads to death within an hour of parturition, with symptoms similar to neonatal respiratory distress syndrome (RDS). In addition to Hif-1α, Hif-2α is also expressed in the developing lung, although the overlapping roles of Hif-1α and Hif-2α in this context are not fully understood. To further investigate the independent role of Hif-2α in lung epithelium and its ability to alter Hif-1α-mediated lung maturation, we generated two additional lung-specific inducible Hif-α knockout models (Hif-2α and Hif-1α+Hif-2α). The intrauterine loss of Hif-2α in the lungs does not lead to decreased viability or observable phenotypic changes in the lung. More interestingly, survivability observed after the loss of both Hif-1α and Hif-2α suggests that the loss of Hif-2α is capable of rescuing the neonatal RDS phenotype seen in Hif-1α-deficient pups. Microarray analyses of lung tissue from these three genotypes identified several factors, such as Scd1, Retlnγ, and Il-1r2, which are differentially regulated by the two HIF-α isoforms. Moreover, network analysis suggests that modulation of hormone-mediated, NF-κB, C/EBPα, and c-MYC signaling are central to HIF-mediated changes in lung development.

Hypoxia Inducible Factor Pathway and Physiological Adaptation: A Cell Survival Pathway?

Oxygen homeostasis reflects the constant body requirement to generate energy. Hypoxia (0.1–1% O₂), physioxia or physoxia (∼1–13%), and normoxia (∼20%) are terms used to define oxygen concentration in the cellular environment. A decrease in oxygen (hypoxia) or excess oxygen (hyperoxia) could be deleterious for cellular adaptation and survival. Hypoxia can occur under both physiological (e.g., exercise, embryonic development, underwater diving, or high altitude) and pathological conditions (e.g., inflammation, solid tumor formation, lung disease, or myocardial infarction). Hypoxia plays a key role in the pathophysiology of heart disease, cancers, stroke, and other causes of mortality. Hypoxia inducible factor(s) (HIFs) are key oxygen sensors that mediate the ability of the cell to cope with decreased oxygen tension. These transcription factors regulate cellular adaptation to hypoxia and protect cells by responding acutely and inducing production of endogenous metabolites and proteins to promptly regulate metabolic pathways. Here, we review the role of the HIF pathway as a metabolic adaptation pathway and how this pathway plays a role in cell survival. We emphasize the roles of the HIF pathway in physiological adaptation, cell death, pH regulation, and adaptation during exercise.

HIF hydroxylase pathways in cardiovascular physiology and medicine

Hypoxia inducible factors (HIFs) are α/β heterodimeric transcription factors that direct multiple cellular and systemic responses in response to changes in oxygen availability. The oxygen sensitive signal is generated by a series of iron and 2-oxoglutarate-dependent dioxygenases that catalyze post-translational hydroxylation of specific prolyl and asparaginyl residues in HIFα subunits and thereby promote their destruction and inactivation in the presence of oxygen. In hypoxia, these processes are suppressed allowing HIF to activate a massive transcriptional cascade. Elucidation of these pathways has opened several new fields of cardiovascular research. Here, we review the role of HIF hydroxylase pathways in cardiac development and in cardiovascular control. We also consider the current status, opportunities, and challenges of therapeutic modulation of HIF hydroxylases in the therapy of cardiovascular disease.

Regulation of carotid body oxygen sensing by hypoxia-inducible factors

Oxygen (O2) sensing by the carotid body and its chemosensory reflex is critical for homeostatic regulation of breathing and blood pressure. Carotid body responses to hypoxia are not uniform but instead exhibit remarkable inter-individual variations. The molecular mechanisms underlying variations in carotid body O2 sensing are not known. Hypoxia-inducible factor-1 (HIF-1) and HIF-2 mediate transcriptional responses to hypoxia. This article reviews the emerging evidence that proper expression of the HIF-α isoforms is a key molecular determinant for carotid body O2 sensing. HIF-1α deficiency leads to a blunted carotid body hypoxic response, which is due to increased abundance of HIF-2α, elevated anti-oxidant enzyme activity, and a reduced intracellular redox state. Conversely, HIF-2α deficiency results in augmented carotid body sensitivity to hypoxia, which is due to increased abundance of HIF-1α, elevated pro-oxidant enzyme activity, and an oxidized intracellular redox state. Double heterozygous mice with equally reduced HIF-1α and HIF-2α showed no abnormality in redox state or carotid body O2 sensing. Thus, mutual antagonism between HIF-α isoforms determines the redox state and thereby establishes the set point for hypoxic sensing by the carotid body.

Regulation of lung development and regeneration by the vascular system

Blood vessels have been described a long time ago as passive circuits providing sufficient blood supply to ensure proper distribution of oxygen and nutrition. Blood vessels are mainly formed during embryonic development and in the early postnatal period. In the adult, blood vessels are quiescent, but can be activated and subsequently induced under pathophysiological conditions, such as ischemia and tumor growth. Surprisingly, recent data have suggested an active function for blood vessels, named angiocrine signaling, releasing trophogens which regulate organ development and organ regeneration including in the pancreas, lung, tumor cells, liver and bone. Lung development is driven by hypoxia as well as an intense endothelial-epithelial interaction, and important mechanisms contributing to these processes have recently been identified. This review aims to summarize recent developments and concepts about embryonic pulmonary vascular development and lung regeneration. We discuss hypoxia-inducible factor HIF-2α and vascular endothelial growth factor VEGF as important mediators in lung development and focus on endothelial-epithelial interactions and angiocrine signaling mechanisms.

Developmental Expression and Hypoxic Induction of Hypoxia Inducible Transcription Factors in the Zebrafish

The hypoxia inducible transcription factor (HIF) has been shown to coordinate the hypoxic response of vertebrates and is expressed in three different isoforms, HIF-1α, HIF-2α and HIF-3α. Knock down of either Hif-1α or Hif-2α in mice results in lethality in embryonic or perinatal stages, suggesting that this transcription factor is not only controlling the hypoxic response, but is also involved in developmental phenomena. In the translucent zebrafish embryo the performance of the cardiovascular system is not essential for early development, therefore this study was designed to analyze the expression of the three Hif-isoforms during zebrafish development and to test the hypoxic inducibility of these transcription factors. To complement the existing zfHif-1α antibody we expressed the whole zfHif-2α protein and used it for immunization and antibody generation. Similarly, fragments of the zfHif-3α protein were used for immunization and generation of a zfHif-3α specific antibody. To demonstrate presence of the Hif-isoforms during development [between 1 day post fertilization (1 dpf) and 9 dpf] affinity-purified antibodies were used. Hif-1α protein was present under normoxic conditions in all developmental stages, but no significant differences between the different developmental stages could be detected. Hif-2α was also present from 1 dpf onwards, but in post hatching stages (between 5 and 9 dpf) the expression level was significantly higher than prior to hatching. Similarly, Hif-3α was expressed from 1 dpf onwards, and the expression level significantly increased until 5 dpf, suggesting that Hif-2α and Hif-3α play a particular role in early development. Hypoxic exposure (oxygen partial pressure = 5 kPa) in turn caused a significant increase in the level of Hif-1α protein even at 1 dpf and in later stages, while neither Hif-2α nor Hif-3α protein level were affected. In these early developmental stages Hif-1α therefore appears to be more important for the coordination of hypoxic responsiveness.

Differential HIF-1α and HIF-2α Expression in Mammary Epithelial Cells during Fat Pad Invasion, Lactation, and Involution

The development and functional cycle of the mammary gland involves a number of processes that are caricatured by breast cancer cells during invasion and metastasis. Expression of the hypoxia-inducible transcription factors HIF-1 and HIF-2 has been associated with metastatic, poor prognosis, and high-grade breast cancers. Since hypoxia affects normal epithelial differentiation, we hypothesise that HIFs are important for normal breast epithelial development and regeneration as well as cancer initiation and progression. Here, we investigated the expression of the oxygen-sensitive HIF-alpha subunits during mouse mammary gland development, lactation, and involution. In breast epithelial cells, HIF-1α was expressed during early development, prior to cell polarisation. In contrast, expression of HIF-2α occurred later and was restricted to a subpopulation of luminal epithelial cells in the lactating gland. Mammary gland involution is a developmental stage that involves extensive tissue remodelling with cell death but survival of tissue stem/progenitor cells. At this stage, HIF-2α, but little HIF-1α, was expressed in CK14-positive epithelial cells. The temporal but differential expression of the HIF-alpha subunits during the mammary gland life cycle indicates that their expression is controlled by additional factors to hypoxia. Further functional studies of the roles of these proteins in the mammary gland and breast cancer are warranted.

Endothelial PAS domain-containing protein 1 confers TKI-resistance by mediating EGFR and MET pathways in non-small cell lung cancer cells

Mutations in epidermal growth factor receptor (EGFR) rendering it constitutively active is one of the major causes for metastatic non-small-cell lung cancer (NSCLC), and EGFR-targeted therapies utilizing tyrosine kinase inhibitors (TKIs) are often used clinically as the first-line treatment. But approximately half of NSCLC patients develop resistance to these therapies, where the MET proto-oncogene is amplified by EGFR through the hypoxia-inducible factor (HIF)-1α. Here we report that endothelial PAS domain-containing protein 1 (EPAS1), with 48% sequence identity to HIF-1α, specifically binds to TKI-resistant T790M EGFR, but not to wild-type EGFR, in NSCLC cell lines. Expression of EPAS1 enhances amplification of MET when simultaneously expressed with T790M EGFR but not with wild-type EGFR, and this enhancement is independent of ligand binding domain of EGFR. MET amplification requires EPAS1, since EPAS1 knock-down reduced MET levels. When NSCLC cells expressing T790M EGFR were treated with TKIs, reduced EPAS1 levels significantly enhanced the drug effect, whereas over-expression of EPAS1 increased the drug resistant effect. This EPAS1-dependent TKI-resistance was abolished by knocking-down MET, suggesting that EPAS1 does not cause TKI-resistance itself but functions to bridge EGFR and MET interactions. Our findings suggest that EPAS1 is a key factor in the EGFR-MET crosstalk in conferring TKI-resistance in NSCLC cases, and could be used as a potential therapeutic target in TKI-resistant NSCLC patients.

HIF1α is necessary for exercise-induced neuroprotection while HIF2α is needed for dopaminergic neuron survival in the substantia nigra pars compacta

Exercise reduces the risk of developing a number of neurological disorders and increases the efficiency of cellular energy production. However, overly strenuous exercise produces oxidative stress. Proper oxygenation is crucial for the health of all tissues, and tight regulation of cellular oxygen is critical to balance O2 levels and redox homeostasis in the brain. Hypoxia Inducible Factor (HIF)1α and HIF2α are transcription factors regulated by cellular oxygen concentration that initiate gene regulation of vascular development, redox homeostasis, and cell cycle control. HIF1α and HIF2α contribute to important adaptive mechanisms that occur when oxygen and ROS homeostasis become unbalanced. It has been shown that preconditioning by exposure to a stressor prior to a hypoxic event reduces damage that would otherwise occur. Previously we reported that 3 months of exercise protects SNpc dopaminergic (DA) neurons from toxicity caused by Complex I inhibition. Here, we identify the cells in the SNpc that express HIF1α and HIF2α and show that running exercise produces hypoxia in SNpc DA neurons, and alters the expression of HIF1α and HIF2α. In mice carrying a conditional knockout of Hif1α in postnatal neurons we observe that exercise alone produces SNpc TH+ DA neuron loss. Loss of HIF1α also abolishes exercise-induced neuroprotection. In mice lacking Hif2α in postnatal neurons, the number of TH+ DA neurons in the adult SNpc is diminished, but 3months of exercise rescues this loss. We conclude that HIF1α is necessary for exercise-induced neuroprotection and both HIF1α and HIF2α are necessary for the survival and function of adult SNpc DA neurons.

HIF signaling pathway in pheochromocytoma and other neuroendocrine tumors

Hypoxia-inducible factors (HIFs) are transcription factors controlling energy, iron metabolism, erythropoiesis, and development. Dysregulation of these proteins contributes to tumorigenesis and cancer progression. Recent findings revealed the important role of HIFs in the pathogenesis of neuroendocrine tumors, especially pheochromocytoma (PHEO) and paraganglioma (PGL). PHEOs and PGLs are catecholamine-producing tumors arising from sympathetic- or parasympathetic-derived chromaffin tissue. To date, eighteen PHEO/PGL susceptibility genes have been identified. Based on the main signaling pathways, PHEOs/PGLs have been divided into two clusters, pseudohypoxic cluster 1 and cluster 2, rich in kinase receptor signaling and protein translation pathways. Recent data suggest that both clusters are interconnected via the HIF signaling and its role in tumorigenesis is supported by newly described somatic and germline mutations in HIF2A gene in patients with PHEOs/PGLs associated with polycythemia, and in some of them also with somatostatinoma. Moreover, HIFalpha signaling has also been shown to be upregulated in neuroendocrine tumors other than PHEO/PGL. Some of these tumors are components of hereditary tumor syndromes which can be associated with PHEO/PGL, but also in ileal carcinoids or melanoma. HIF signaling appears to be one of the crucial players in tumorigenesis, which could suggest new therapeutic approaches for treatment of neuroendocrine tumors.

The role of hypoxia-inducible factor-2 in digestive system cancers

Hypoxia is an all but ubiquitous phenomenon in cancers. Two known hypoxia-inducible factors (HIFs), HIF-1α and HIF-2α, primarily mediate the transcriptional response to hypoxia. Despite the high homology between HIF-1α and HIF-2α, emerging evidence suggests differences between both molecules in terms of transcriptional targets as well as impact on multiple physiological pathways and tumorigenesis. To date, much progress has been made toward understanding the roles of HIF-2α in digestive system cancers. Indeed, HIF-2α has been shown to regulate multiple aspects of digestive system cancers, including cell proliferation, angiogenesis and apoptosis, metabolism, metastasis and resistance to chemotherapy. These findings make HIF-2α a critical regulator of this malignant phenotype. Here we summarize the function of HIF-2 during cancer development as well as its contribution to tumorigenesis in digestive system malignancies.

HIF2α signaling inhibits adherens junctional disruption in acute lung injury

Vascular endothelial barrier dysfunction underlies diseases such as acute respiratory distress syndrome (ARDS), characterized by edema and inflammatory cell infiltration. The transcription factor HIF2α is highly expressed in vascular endothelial cells (ECs) and may regulate endothelial barrier function. Here, we analyzed promoter sequences of genes encoding proteins that regulate adherens junction (AJ) integrity and determined that vascular endothelial protein tyrosine phosphatase (VE-PTP) is a HIF2α target. HIF2α-induced VE-PTP expression enhanced dephosphorylation of VE-cadherin, which reduced VE-cadherin endocytosis and thereby augmented AJ integrity and endothelial barrier function. Mice harboring an EC-specific deletion of Hif2a exhibited decreased VE-PTP expression and increased VE-cadherin phosphorylation, resulting in defective AJs. Mice lacking HIF2α in ECs had increased lung vascular permeability and water content, both of which were further exacerbated by endotoxin-mediated injury. Treatment of these mice with Fg4497, a prolyl hydroxylase domain 2 (PHD2) inhibitor, activated HIF2α-mediated transcription in a hypoxia-independent manner. HIF2α activation increased VE-PTP expression, decreased VE-cadherin phosphorylation, promoted AJ integrity, and prevented the loss of endothelial barrier function. These findings demonstrate that HIF2α enhances endothelial barrier integrity, in part through VE-PTP expression and the resultant VE-cadherin dephosphorylation-mediated assembly of AJs. Moreover, activation of HIF2α/VE-PTP signaling via PHD2 inhibition has the potential to prevent the formation of leaky vessels and edema in inflammatory diseases such as ARDS.

Blastocyst metabolism

The mammalian blastocyst exhibits an idiosyncratic metabolism, reflecting its unique physiology and its ability to undergo implantation. Glucose is the primary nutrient of the blastocyst, and is metabolised both oxidatively and through aerobic glycolysis. The production of significant quantities of lactate by the blastocyst reflects specific metabolic requirements and mitochondrial regulation; it is further proposed that lactate production serves to facilitate several key functions during implantation, including biosynthesis, endometrial tissue breakdown, the promotion of new blood vessel formation and induction of local immune-modulation of the uterine environment. Nutrient availability, oxygen concentration and the redox state of the blastocyst tightly regulate the relative activities of specific metabolic pathways. Notably, a loss of metabolic normality is associated with a reduction in implantation potential and subsequent fetal development. Even a transient metabolic stress at the blastocyst stage culminates in low fetal weights after transfer. Further, it is evident that there are differences between male and female embryos, with female embryos being characterised by higher glucose consumption and differences in their amino acid turnover, reflecting the presence of two active X-chromosomes before implantation, which results in differences in the proteomes between the sexes. In addition to the role of Hypoxia-Inducible Factors, the signalling pathways involved in regulating blastocyst metabolism are currently under intense analysis, with the roles of sirtuins, mTOR, AMP-activated protein kinase and specific amino acids being scrutinised. It is evident that blastocyst metabolism regulates more than the production of ATP; rather, it is apparent that metabolites and cofactors are important regulators of the epigenome, putting metabolism at centre stage when considering the interactions of the blastocyst with its environment.

Hypoxia-inducible factor as an angiogenic master switch

Hypoxia-inducible factors (HIFs) regulate the transcription of genes that mediate the response to hypoxia. HIFs are constantly expressed and degraded under normoxia, but stabilized under hypoxia. HIFs have been widely studied in physiological and pathological conditions and have been shown to contribute to the pathogenesis of various vascular diseases. In clinical settings, the HIF pathway has been studied for its role in inhibiting carcinogenesis. HIFs might also play a protective role in the pathology of ischemic diseases. Clinical trials of therapeutic angiogenesis after the administration of a single growth factor have yielded unsatisfactory or controversial results, possibly because the coordinated activity of different HIF-induced factors is necessary to induce mature vessel formation. Thus, manipulation of HIF activity to simultaneously induce a spectrum of angiogenic factors offers a superior strategy for therapeutic angiogenesis. Because HIF-2α plays an essential role in vascular remodeling, manipulation of HIF-2α is a promising approach to the treatment of ischemic diseases caused by arterial obstruction, where insufficient development of collateral vessels impedes effective therapy. Eukaryotic initiation factor 3 subunit e (eIF3e)/INT6 interacts specifically with HIF-2α and induces the proteasome inhibitor-sensitive degradation of HIF-2α, independent of hypoxia and von Hippel-Lindau protein. Treatment with eIF3e/INT6 siRNA stabilizes HIF-2α activity even under normoxic conditions and induces the expression of several angiogenic factors, at levels sufficient to produce functional arteries and veins in vivo. We have demonstrated that administration of eIF3e/INT6 siRNA to ischemic limbs or cold-injured brains reduces ischemic damage in animal models. This review summarizes the current understanding of the relationship between HIFs and vascular diseases. We also discuss novel oxygen-independent regulatory proteins that bind HIF-α and the implications of a new method for therapeutic angiogenesis using HIF stabilizers.

Two functional loci in the promoter of EPAS1 gene involved in high-altitude adaptation of Tibetans

EPAS1 involves in the hypoxic response and is suggested to be responsible for the genetic adaptation of high-altitude hypoxia in Tibetans. However, the detailed molecular mechanism remains unknown. In this study, a single nucleotide polymorphism rs56721780:G>C and an insertion/deletion (indel) polymorphism -742 indel in the promoter region showed divergence between Tibetans and non-Tibetan lowlanders. rs56721780:G>C regulated the transcription of EPAS1 by IKAROS family zinc finger 1 (IKZF1), which was identified as a new transcriptional repressor for EPAS1 gene. It demonstrated that the C allele of rs56721780:G>C decreased the binding of IKZF1, leading to the attenuated transcriptional repression of EPAS1 gene. The insertion at -742 indel provided a new binding site for Sp1 and was related to the activation of EPAS1 promoter. Further functional analysis revealed that lysyl oxidase (LOX) gene, which was reported to be responsible for extracellular matrix protein cross-linking of amnion previously, was a direct target of EPAS1. The CC genotype at rs56721780:G>C and the insertion genotype at -742 indel were found associated with higher EPAS1 and LOX expression levels in amnion, as well as higher birth weight of Tibetan newborns, suggesting that EPAS1 gene might play important roles in the development of amnion, fetus growth and high-altitude adaptation of Tibetans.

Signaling hypoxia by hypoxia-inducible factor protein hydroxylases: a historical overview and future perspectives

By the early 1900s, the close matching of oxygen supply with demand was recognized to be a fundamental requirement for physiological function, and multiple adaptive responses to environment hypoxia had been described. Nevertheless, the widespread operation of mechanisms that directly sense and respond to levels of oxygen in animal cells was not appreciated for most of the twentieth century with investigators generally stressing the regulatory importance of metabolic products. Work over the last 25 years has overturned that paradigm. It has revealed the existence of a set of “oxygen-sensing” 2-oxoglutarate dependent dioxygenases that catalyze the hydroxylation of specific amino acid residues and thereby control the stability and activity of hypoxia-inducible factor. The hypoxia-inducible factor hydroxylase pathway regulates a massive transcriptional cascade that is operative in essentially all animal cells. It transduces a wide range of responses to hypoxia, extending well beyond the classical boundaries of hypoxia physiology. Here we review the discovery and elucidation of these pathways, and consider the opportunities and challenges that have been brought into focus by the findings, including new implications for the integrated physiology of hypoxia and therapeutic approaches to ischemic/hypoxic disease.

Hypoxic signaling during tissue repair and regenerative medicine

In patients with chronic wounds, autologous tissue repair is often not sufficient to heal the wound. These patients might benefit from regenerative medicine or the implantation of a tissue-engineered scaffold. Both wound healing and tissue engineering is dependent on the formation of a microvascular network. This process is highly regulated by hypoxia and the transcription factors hypoxia-inducible factors-1α (HIF-1α) and -2α (HIF-2α). Even though much is known about the function of HIF-1α in wound healing, knowledge about the function of HIF-2α in wound healing is lacking. This review focuses on the function of HIF-1α and HIF-2α in microvascular network formation, wound healing, and therapy strategies.

Genetic modification of hypoxia signaling in animal models and its effect on cancer

Conditions that cause hypoxemia or generalized tissue hypoxia, which can last for days, months, or even years, are very common in the human population and are among the leading causes of morbidity, disability, and mortality. Therefore, the molecular pathophysiology of hypoxia and its potential deleterious effects on human health are important issues at the forefront of biomedical research. Generalized hypoxia is a consequence of highly prevalent medical disorders that can severely reduce the capacity for O2 exchange between the air and pulmonary capillaries. In recent years, some of the key O2-dependent signaling pathways have been characterized at the molecular level. In particular, the prolyl hydroxylase (PHD)-hypoxia-inducible factor (HIF) cascade has emerged as the master regulator of a general gene expression program involved in cell/tissue/organ adaptation to hypoxia. Hypoxia has emerged as a critical factor in cancer because it can promote tumor initiation, progression, and resistance to therapy. Beyond its role in neovascularization as a mechanism of tumor adaptation to nutrient and O2 deprivation, hypoxia has been linked to prolonged cellular lifespan and immortalization, the generation of "oncometabolites", deregulation of stem cell proliferation, and inflammation, among other tumor hallmarks. Hypoxia may contribute to cancer through several independent pathways, the inter-connections of which have yet to be elucidated. Furthermore, the relevance of chronic hypoxemia in the initiation and progression of cancer has not been studied in depth in the whole organism. Therefore, we explore here the contributions of hypoxia to the whole organism by reviewing studies on genetically modified mice with alterations in the key molecular factors regulating hypoxia.

HIF-1α and HIF-2α induce angiogenesis and improve muscle energy recovery

BACKGROUND

Cardiovascular patients suffer from reduced blood flow leading to ischaemia and impaired tissue metabolism. Unfortunately, an increasing group of elderly patients cannot be treated with current revascularization methods. Thus, new treatment strategies are urgently needed. Hypoxia-inducible factors (HIFs) upregulate the expression of angiogenic mediators together with genes involved in energy metabolism and recovery of ischaemic tissues. Especially, HIF-2α is a novel factor, and only limited information is available about its therapeutic potential.

METHODS

Gene transfers with adenoviral HIF-1α and HIF-2α were performed into the mouse heart and rabbit ischaemic hindlimbs. Angiogenesis was evaluated by histology. Left ventricle function was analysed with echocardiography. Perfusion in rabbit skeletal muscles and energy recovery after electrical stimulation-induced exercise were measured with ultrasound and (31)P-magnetic resonance spectroscopy ((31)P-MRS), respectively.

RESULTS

HIF-1α and HIF-2α gene transfers increased capillary size up to fivefold in myocardium and ischaemic skeletal muscles. Perfusion in skeletal muscles was increased by fourfold without oedema. Especially, AdHIF-1α enhanced the recovery of ischaemic muscles from electrical stimulation-induced energy depletion. Special characteristic of HIF-2α gene transfer was a strong capillary growth in muscle connective tissue and that HIF-2α gene transfer maintained left ventricle function.

CONCLUSIONS

We conclude that both AdHIF-1α and AdHIF-2α gene transfers induced beneficial angiogenesis in vivo. Transient moderate increases in angiogenesis improved energy recovery after exercise in ischaemic muscles. This study shows for the first time that a moderate increase in angiogenesis is enough to improve tissue energy metabolism, which is potentially a very useful feature for cardiovascular gene therapy.

Macrophage HIF-2α ameliorates adipose tissue inflammation and insulin resistance in obesity

In obesity, adipose tissue macrophages (ATMs) play a key role in mediating proinflammatory responses in the adipose tissue, which are associated with obesity-related metabolic complications. Recently, adipose tissue hypoxia has been implicated in the regulation of ATMs in obesity. However, the role of hypoxia-inducible factor (HIF)-2α, one of the major transcription factors induced by hypoxia, has not been fully elucidated in ATMs. In this study, we demonstrate that elevation of macrophage HIF-2α would attenuate adipose tissue inflammation and improve insulin resistance in obesity. In macrophages, overexpression of HIF-2α decreased nitric oxide production and suppressed expression of proinflammatory cytokines through induction of arginase 1. HIF-2α-overexpressing macrophages alleviated proinflammatory responses and improved insulin resistance in adipocytes. In contrast, knockdown of macrophage HIF-2α augmented palmitate-induced proinflammatory gene expression in adipocytes. Furthermore, compared with wild-type mice, Hif-2α heterozygous-null mice aggravated insulin resistance and adipose tissue inflammation with more M1-like ATMs upon high-fat diet (HFD). Moreover, glucose intolerance in HFD-fed Hif-2α heterozygous-null mice was relieved by macrophage depletion with clodronate treatment, implying that increase of proinflammatory ATMs is responsible for insulin resistance by haplodeficiency of Hif-2α upon HFD. Taken together, these data suggest that macrophage HIF-2α would counteract the proinflammatory responses to relieve obesity-induced insulin resistance in adipose tissue.

Hypoxia as a key player in the AKI-to-CKD transition

Recent clinical and animal studies have shown that acute kidney injury (AKI), even if followed by complete recovery of renal function, can eventually result in chronic kidney disease (CKD). Renal hypoxia is emerging as a key player in the pathophysiology of the AKI-to-CKD transition. Capillary rarefaction after AKI episodes induces renal hypoxia, which can in turn profoundly affect tubular epithelial cells, (myo)fibroblasts, and inflammatory cells, culminating in tubulointerstitial fibrosis, i.e., progression to CKD. Damaged tubular epithelial cells that fail to redifferentiate might supply a decreased amount of vascular endothelial growth factor and contribute to capillary rarefaction, thus aggravating hypoxia and forming a vicious cycle. Mounting evidence also shows that epigenetic changes are closely related to renal hypoxia in the pathophysiology of CKD progression. Animal experiments suggest that targeting hypoxia is a promising strategy to block the transition from AKI to CKD. However, the precise mechanisms by which hypoxia induces the AKI-to-CKD transition and by which hypoxia-inducible factor activation can exert a protective effect in this context should be clarified in further studies.

Hypoxia promotes uveal melanoma invasion through enhanced Notch and MAPK activation

The transcriptional response promoted by hypoxia-inducible factors has been associated with metastatic spread of uveal melanoma. We found expression of hypoxia-inducible factor 1α (HIF-1α) protein in well-vascularized tumor regions as well as in four cell lines grown in normoxia, thus this pathway may be important even in well-oxygenated uveal melanoma cells. HIF-1α protein accumulation in normoxia was inhibited by rapamycin. As expected, hypoxia (1% pO₂) further induced HIF-1α protein levels along with its target genes VEGF and LOX. Growth in hypoxia significantly increased cellular invasion of all 5 uveal melanoma lines tested, as did the introduction of an oxygen-insensitive HIF-1α mutant into Mel285 cells with low HIF-1α baseline levels. In contrast, HIF-1α knockdown using shRNA significantly decreased growth in hypoxia, and reduced by more than 50% tumor invasion in four lines with high HIF-1α baseline levels. Pharmacologic blockade of HIF-1α protein expression using digoxin dramatically suppressed cellular invasion both in normoxia and in hypoxia. We found that Notch pathway components, including Jag1-2 ligands, Hes1-Hey1 targets and the intracellular domain of Notch1, were increased in hypoxia, as well as the phosphorylation levels of Erk1-2 and Akt. Pharmacologic and genetic inhibition of Notch largely blocked the hypoxic induction of invasion as did the pharmacologic suppression of Erk1-2 activity. In addition, the increase in Erk1-2 and Akt phosphorylation by hypoxia was partially reduced by inhibiting Notch signaling. Our findings support the functional importance of HIF-1α signaling in promoting the invasive capacity of uveal melanoma cells in both hypoxia and normoxia, and suggest that pharmacologically targeting HIF-1α pathway directly or through blockade of Notch or Erk1-2 pathways can slow tumor spread.

The role of HIFs in ischemia-reperfusion injury

The reduction or cessation of the blood supply to an organ results in tissue ischemia. Ischemia can cause significant tissue damage, and is observed as a result of a thrombosis, as part of a disease process, and during surgery. However, the restoration of the blood supply often causes more damage to the tissue than the ischemic episode itself. Research is therefore focused on identifying the cellular pathways involved in the protection of organs from the damage incurred by this process of ischemia reperfusion (I/R). The hypoxia-inducible factors (HIFs) are a family of heterodimeric transcription factors that are stabilized during ischemia. The genes that are expressed downstream of HIF activity enhance oxygen-independent ATP generation, cell survival, and angiogenesis, amongst other phenotypes. They are, therefore, important factors in the protection of tissues from I/R injury. Interestingly, a number of the mechanisms already known to induce organ protection against I/R injury, including preconditioning, postconditioning, and activation of signaling pathways such as adenosine receptor signaling, converge on the HIF system. This review describes the evidence for HIFs playing a role in I/R protection mediated by these factors, highlights areas that require further study, and discuss whether HIFs themselves are good therapeutic targets for protecting tissues from I/R injury.

The role of hypoxia in stem cell potency and differentiation

Regenerative medicine relies on harnessing the capacity of stem cells to grow, divide and differentiate safely and predictably. This may be in the context of expanding stem cells in vitro or encouraging their expansion, mobilization and capacity to regenerate tissues either locally or remotely in vivo. In either case, understanding the stem cell niche is fundamental to recapitulating or manipulating conditions to enable therapy. It has become obvious that hypoxia plays a fundamental role in the maintenance of the stem cell niche. Low O2 benefits the self-renewal of human embryonic, hematopoietic, mesenchymal and neural stem cells, as well as improving the efficiency of genetic reprogramming to induced pluripotency. There is emerging evidence that harnessing or manipulating the hypoxic response can result in safer, more efficacious methodologies for regenerative medicine.

Hypoxia-inducible factor 2 alpha is essential for hepatic outgrowth and functions via the regulation of leg1 transcription in the zebrafish embryo

The liver plays a vital role in metabolism, detoxification, digestion, and the maintenance of homeostasis. During development, the vertebrate embryonic liver undergoes a series of morphogenic processes known as hepatogenesis. Hepatogenesis can be separated into three interrelated processes: endoderm specification, hepatoblast differentiation, and hepatic outgrowth. Throughout this process, signaling molecules and transcription factors initiate and regulate the coordination of cell proliferation, apoptosis, differentiation, intercellular adhesion, and cell migration. Hifs are already recognized to be essential in embryonic development, but their role in hepatogenesis remains unknown. Using the zebrafish embryo as a model organism, we report that the lack of Hif2-alpha but not Hif1-alpha blocks hepatic outgrowth. While Hif2-alpha is not involved in hepatoblast specification, this transcription factor regulates hepatocyte cell proliferation during hepatic outgrowth. Furthermore, we demonstrated that the lack of Hif2-alpha can reduce the expression of liver-enriched gene 1 (leg1), which encodes a secretory protein essential for hepatic outgrowth. Additionally, exogenous mRNA expression of leg1 can rescue the small liver phenotype of hif2-alpha morphants. We also showed that Hif2-alpha directly binds to the promoter region of leg1 to control leg1 expression. Interestingly, we discovered overrepresented, high-density Hif-binding sites in the potential upstream regulatory sequences of leg1 in teleosts but not in terrestrial mammals. We concluded that hif2-alpha is a key factor required for hepatic outgrowth and regulates leg1 expression in zebrafish embryos. We also proposed that the hif2-alpha-leg1 axis in liver development may have resulted from the adaptation of teleosts to their environment.

The role of HIF1α in renal cell carcinoma tumorigenesis

The transcription factor HIF1α is implicated in the development of clear cell renal cell carcinoma (ccRCC). Although HIF1α was initially believed to be essential for ccRCC development, recent studies hypothesize an oncogenic role for HIF2α in ccRCC, but a tumor suppressor role for HIF1α, leading to uncertainty as to the precise roles of the different HIF transcription factors in this disease. Using evidence available from studies with human ccRCC cell lines, mouse xenografts, murine models of ccRCC, and human ccRCC specimens, we evaluate the roles of HIF1α and HIF2α in the pathogenesis of ccRCC. We present a convergence of clinical and mechanistic data supporting an important role for HIF1α in promoting tumorigenesis in a clinically important and large subset of ccRCC. This indicates that current understanding of the exact roles of HIF1α and HIF2α is incomplete and that further research is required to determine the diverse roles of HIF1α and HIF2α in ccRCC.

KEY MESSAGES

The TRACK mouse ccRCC model with constitutively active HIF1α but not HIF2α expressed in proximal tubules develops RCC. HIF1α protein is expressed in the majority of human ccRCC specimens. Elevated HIF1α in ccRCC correlates with a worse prognosis. Many publications do not support a tumor suppressor role for HIF1α in ccRCC. HIF1α, but not HIF2α, is expressed in some types of cancer stem cells.

Hypoxia-inducible factor-2α is an essential catabolic regulator of inflammatory rheumatoid arthritis

Hypoxia-inducible factor-2α (HIF-2α) is sufficient to cause experimental rheumatoid arthritis and acts to regulate the functions of fibroblast-like cells from tissue surrounding joints, independent of HIF-1α.

Rheumatoid arthritis (RA) is a systemic autoimmune disorder that manifests as chronic inflammation and joint tissue destruction. However, the etiology and pathogenesis of RA have not been fully elucidated. Here, we explored the role of the hypoxia-inducible factors (HIFs), HIF-1α (encoded by HIF1A) and HIF-2α (encoded by EPAS1). HIF-2α was markedly up-regulated in the intimal lining of RA synovium, whereas HIF-1α was detected in a few cells in the sublining and deep layer of RA synovium. Overexpression of HIF-2α in joint tissues caused an RA-like phenotype, whereas HIF-1α did not affect joint architecture. Moreover, a HIF-2α deficiency in mice blunted the development of experimental RA. HIF-2α was expressed mainly in fibroblast-like synoviocytes (FLS) of RA synovium and regulated their proliferation, expression of RANKL (receptor activator of nuclear factor–κB ligand) and various catabolic factors, and osteoclastogenic potential. Moreover, HIF-2α–dependent up-regulation of interleukin (IL)-6 in FLS stimulated differentiation of T_H17 cells—crucial effectors of RA pathogenesis. Additionally, in the absence of IL-6 (Il6^−/− mice), overexpression of HIF-2α in joint tissues did not cause an RA phenotype. Thus, our results collectively suggest that HIF-2α plays a pivotal role in the pathogenesis of RA by regulating FLS functions, independent of HIF-1α.

Rheumatoid arthritis (RA) is a systemic autoimmune disorder characterized by chronic inflammation in joint tissues leading to destruction of cartilage and bone. Despite some therapeutic advances, the etiology of RA pathogenesis is not yet clear, and effective treatment of RA remains a significant, unmet medical need. Hypoxia is a prominent feature of inflamed tissue within RA-affected joints, and earlier work has implicated limited involvement of hypoxia-inducible factor (HIF)-1 α. We explored the role of a second HIF family member, HIF-2α, in RA pathogenesis. We showed that HIF-2α is markedly increased in the tissue lining the RA-affected joints. Notably and in contrast to HIF-1α, when overexpressed in normal mouse joint tissues, HIF-2α is sufficient to cause RA-like symptoms. Conversely, an HIF-2α deficiency blocks the development of experimental arthritis in mice. We discovered further that HIF-2α regulates RA pathogenesis by modulating various RA-associated functions of joint-specific fibroblast-like cells, including proliferation, expression of cytokines, chemokines, and matrix-degrading enzymes, and bone-remodeling potential. HIF-2α also increases the ability of these cells to promote interleukin-6–dependent differentiation of T_H17 cells, a known effector of RA pathogenesis. We thus show that HIF-1α and HIF-2α have distinct roles and act via different mechanisms in RA pathogenesis.

Current views on cell metabolism in SDHx-related pheochromocytoma and paraganglioma

Warburg's metabolic hypothesis is based on the assumption that a cancer cell's respiration must be under attack, leading to its damage, in order to obtain increased glycolysis. Although this may not apply to all cancers, there is some evidence proving that primarily abnormally functioning mitochondrial complexes are indeed related to cancer development. Thus, mutations in complex II (succinate dehydrogenase (SDH)) lead to the formation of pheochromocytoma (PHEO)/paraganglioma (PGL). Mutations in one of the SDH genes (SDHx mutations) lead to succinate accumulation associated with very low fumarate levels, increased glutaminolysis, the generation of reactive oxygen species, and pseudohypoxia. This results in significant changes in signaling pathways (many of them dependent on the stabilization of hypoxia-inducible factor), including oxidative phosphorylation, glycolysis, specific expression profiles, as well as genomic instability and increased mutability resulting in tumor development. Although there is currently no very effective therapy for SDHx-related metastatic PHEOs/PGLs, targeting their fundamental metabolic abnormalities may provide a unique opportunity for the development of novel and more effective forms of therapy for these tumors.

Overexpressed HIF-2α in Endothelial Cells Promotes Vascularization and Improves Random Pattern Skin Flap Survival

BACKGROUND

The local skin flap procedure is very useful for reconstruction. However, flap necrosis caused by circulatory failure can occur at its distal portion. Hypoxia-inducible factors (HIFs) in endothelial cells (ECs) help to maintain ECs and promote vascularization, and HIF-2α is abundantly expressed in ECs. However, the mechanisms of action of HIF-2α in ECs are not yet fully understood. The aim of this study was to evaluate the in vivo effects of overexpression of HIF-2α in ECs on skin flap survival.

METHODS

A random pattern skin flap (1.0 × 3.0 cm) was elevated on the dorsum of transgenic mice (Tg mice) with EC-specific HIF-2α conditional overexpression and wild-type littermate control mice (n = 6). Flap survival was evaluated on postoperative day 7. Tissue samples from the skin flaps were harvested and analyzed using Western blotting, quantitative reverse transcriptase-polymerase chain reaction, and immunohistochemistry.

RESULTS

The HIF-2α mRNA and protein levels were significantly increased in the Tg mice when compared with control mice. Tg mice had significantly increased skin flap survival areas (72.0% ± 2.7%) when compared with wild-type mice (45.7% ± 1.1%). Moreover, histological examination revealed an increase in the subcutaneous blood vessel counts in the Tg mice.

CONCLUSIONS

Specific overexpression of HIF-2α in ECs promoted vascularization and enhanced skin flap survival in vivo in a mouse model.

Kelch-like ECT2-interacting protein KLEIP regulates late-stage pulmonary maturation via Hif-2α in mice

Respiratory distress syndrome (RDS) caused by preterm delivery is a major clinical problem with limited mechanistic insight. Late-stage embryonic lung development is driven by hypoxia and the hypoxia-inducible transcription factors Hif-1α and Hif-2α, which act as important regulators for lung development. Expression of the BTB-and kelch-domain-containing (BTB-kelch) protein KLEIP (Kelch-like ECT2-interacting protein; also named Klhl20) is controlled by two hypoxia response elements, and KLEIP regulates stabilization and transcriptional activation of Hif-2α. Based on the available data, we hypothesized an essential role for KLEIP in murine lung development and function. Therefore, we have performed a functional, histological, mechanistic and interventional study in embryonic and neonatal KLEIP^−/− mice. Here, we show that about half of the KLEIP^−/− neonates die due to respiratory failure that is caused by insufficient aeration, reduced septal thinning, reduced glycogenolysis, type II pneumocyte immaturity and reduced surfactant production. Expression analyses in embryonic day (E) 18.5 lungs identified KLEIP in lung capillaries, and showed strongly reduced mRNA and protein levels for Hif-2α and VEGF; such reduced levels are associated with embryonic endothelial cell apoptosis and lung bleedings. Betamethasone injection in pregnant females prevented respiratory failure in KLEIP^−/− neonates, normalized lung maturation, vascularization, aeration and function, and increased neonatal Hif-2α expression. Thus, the experimental study shows that respiratory failure in KLEIP^−/− neonates is determined by insufficient angiocrine Hif-2α–VEGF signaling and that betamethasone activates this newly identified signaling cascade in late-stage embryonic lung development.

A new twist in neuroendocrine tumor research: Pacak-Zhuang syndrome, HIF-2α as the major player in its pathogenesis and future therapeutic options

Backround. There is increasing evidence of the role of hypoxia or pseudohypoxia in tumorigenesis, including pheochromocytoma (PHEO) and paraganglioma (PGL). (Pseudo)hypoxia leads to activation of hypoxia-inducible transcription factors (HIFs) and thus, promotes the transcription of hypoxia-responsive genes which are involved in tumorigenesis. Recently identified is a new syndrome consisting of multiple and recurrent PGLs or PHEOs, somatostatinoma, and congenital polycythemia, due to somatic hypoxia-inducible factor 2α gene (HIF2A) mutations.

METHODS AND RESULTS

PubMed and Web of Science online databases were used to search reviews and original articles on the HIF, PHEO/PGL, and Pacak-Zhuang syndrome.

CONCLUSIONS

The novel somatic and germline gain-of-function HIF2A mutations described latterly emphasize the role of the HIF-2α in the PHEO/PGL development and these findings designate HIF, especially HIF-2α, as a promising treatment target.

Opposing effects of HIF1α and HIF2α on chromaffin cell phenotypic features and tumor cell proliferation: Insights from MYC-associated factor X

Pheochromocytomas and paragangliomas (PPGLs) are catecholamine-producing chromaffin cell tumors with diverse phenotypic features reflecting mutations in numerous genes, including MYC-associated factor X (MAX). To explore whether phenotypic differences among PPGLs reflect a MAX-mediated mechanism and opposing influences of hypoxia-inducible factor (HIF)s HIF2α and HIF1α, we combined observational investigations in PPGLs and gene-manipulation studies in two pheochromocytoma cell lines. Among PPGLs from 140 patients, tumors due to MAX mutations were characterized by gene expression profiles and intermediate phenotypic features that distinguished these tumors from other PPGLs, all of which fell into two expression clusters: one cluster with low expression of HIF2α and mature phenotypic features and the other with high expression of HIF2α and immature phenotypic features due to mutations stabilizing HIFs. Max-mutated tumors distributed to a distinct subcluster of the former group. In cell lines lacking Max, re-expression of the gene resulted in maturation of phenotypic features and decreased cell cycle progression. In cell lines lacking Hif2α, overexpression of the gene led to immature phenotypic features, failure of dexamethasone to induce differentiation and increased proliferation. HIF1α had opposing actions to HIF2α in both cell lines, supporting evolving evidence of their differential actions on tumorigenic processes via a MYC/MAX-related pathway. Requirement of a fully functional MYC/MAX complex to facilitate differentiation explains the intermediate phenotypic features in tumors due to MAX mutations. Overexpression of HIF2α in chromaffin cell tumors due to mutations affecting HIF stabilization explains their proliferative features and why the tumors fail to differentiate even when exposed locally to adrenal steroids.

Hypoxia-inducible factors link iron homeostasis and erythropoiesis

Iron is required for efficient oxygen transport, and hypoxia signaling links erythropoiesis with iron homeostasis. Hypoxia induces a highly conserved signaling pathway in cells under conditions of low levels of O2. One component of this pathway, hypoxia-inducible factor (HIF), is a transcription factor that is highly active in hypoxic cells. The first HIF target gene characterized was EPO, which encodes erythropoietin-a glycoprotein hormone that controls erythropoiesis. In the past decade, there have been fundamental advances in our understanding of how hypoxia regulates iron levels to support erythropoiesis and maintain systemic iron homeostasis. We review the cell type-specific effects of hypoxia and HIFs in adaptive response to changes in oxygen and iron availability as well as potential uses of HIF modulators for patients with iron-related disorders.

Vascular endothelial growth factor signaling in hypoxia and inflammation

Infection, cancer and cardiovascular diseases are the major causes for morbidity and mortality in the United States according to the Center for Disease Control. The underlying etiology that contributes to the severity of these diseases is either hypoxia induced inflammation or inflammation resulting in hypoxia. Therefore, molecular mechanisms that regulate hypoxia-induced adaptive responses in cells are important areas of investigation. Oxygen availability is sensed by molecular switches which regulate synthesis and secretion of growth factors and inflammatory mediators. As a consequence, tissue microenvironment is altered by re-programming metabolic pathways, angiogenesis, vascular permeability, pH homeostasis to facilitate tissue remodeling. Hypoxia inducible factor (HIF) is the central mediator of hypoxic response. HIF regulates several hundred genes and vascular endothelial growth factor (VEGF) is one of the primary target genes. Understanding the regulation of HIF and its influence on inflammatory response offers unique opportunities for drug development to modulate inflammation and ischemia in pathological conditions.

Loss of Hand2 in a population of Periostin lineage cells results in pronounced bradycardia and neonatal death

The Periostin Cre (Postn-Cre) lineage includes endocardial and neural crest derived mesenchymal cells of the cardiac cushions, neural crest-derived components of the sympathetic and enteric nervous systems, and cardiac fibroblasts. In this study, we use the Postn-Cre transgenic allele to conditionally ablate Hand2 (H2CKO). We find that Postn-Cre H2CKOs die shortly after birth despite a lack of obvious cardiac structural defects. To ascertain the cause of death, we performed a detailed comparison of the Postn-Cre lineage and Hand2 expression at mid and late stages of embryonic development. Gene expression analyses demonstrate that Postn-Cre ablates Hand2 from the adrenal medulla as well as the sphenopalatine ganglia of the head. In both cases, Hand2 loss-of-function dramatically reduces expression of Dopamine Beta Hydroxylase (Dbh), a gene encoding a crucial catecholaminergic biosynthetic enzyme. Expression of the genes Tyrosine Hydroxylase (Th) and Phenylethanolamine N-methyltransferase (Pnmt), which also encode essential catecholaminergic enzymes, were severely reduced in postnatal adrenal glands. Electrocardiograms demonstrate that 3-day postnatal Postn-Cre H2CKO pups exhibit sinus bradycardia. In conjunction with the aforementioned gene expression analyses, these results strongly suggest that the observed postnatal lethality occurs due to a catecholamine deficiency and subsequent heart failure.