Asparagine hydroxylation of the HIF transactivation domain a hypoxic switch

Up-down regulation of HIF-1α in cancer progression

Hypoxia induicible factor-1 alpha (HIF-1α) is a key transcription factor in cancer progression and target therapy in cancer. HIF-1α acts differently depending on presence or absence of Oxygen. In an oxygen-immersed environment, HIF-1α completely deactivated and destroyed by the ubiquitin proteasome pathway (UPP). In contrast, in the oxygen-free environment, it escapes destruction and enters to the nucleus of cells then upregulates many genes involved in cancer progression. Overexpressed HIF-1α and downstream genes support cancer progression through various mechanisms including angiogenesis, proliferation and survival of cells, metabolism reprogramming, invasion and metastasis, cancer stem cell maintenance, induction of genetic instability, and treatment resistance. HIF-1α can be provoked by signaling pathways unrelated to hypoxia during cancer progression. Therefore, cancer development and progression can be modulated by targeting HIF-1α and its downstream signaling molecules. In this regard, HIF-1α inhibitors which are categorized into the agents that regulate HIF-1α in gene, mRNA and protein levels used as an efficient way in cancer treatment. Also, HIF-1α expression can be negatively affected by the agents suppressing the activation of mTOR, PI3k/Akt and MAPK pathways.

LRP5 Regulates HIF-1α Stability via Interaction with PHD2 in Ischemic Myocardium

Low-density lipoprotein receptor-related protein 5 (LRP5) has been studied as a co-receptor for Wnt/β-catenin signaling. However, its role in the ischemic myocardium is largely unknown. Here, we show that LRP5 may act as a negative regulator of ischemic heart injury via its interaction with prolyl hydroxylase 2 (PHD2), resulting in hypoxia-inducible factor-1α (HIF-1α) degradation. Overexpression of LRP5 in cardiomyocytes promoted hypoxia-induced apoptotic cell death, whereas LRP5-silenced cardiomyocytes were protected from hypoxic insult. Gene expression analysis (mRNA-seq) demonstrated that overexpression of LRP5 limited the expression of HIF-1α target genes. LRP5 promoted HIF-1α degradation, as evidenced by the increased hydroxylation and shorter stability of HIF-1α under hypoxic conditions through the interaction between LRP5 and PHD2. Moreover, the specific phosphorylation of LRP5 at T1492 and S1503 is responsible for enhancing the hydroxylation activity of PHD2, resulting in HIF-1α degradation, which is independent of Wnt/β-catenin signaling. Importantly, direct myocardial delivery of adenoviral constructs, silencing LRP5 in vivo, significantly improved cardiac function in infarcted rat hearts, suggesting the potential value of LRP5 as a new target for ischemic injury treatment.

An Overview of the Recent Development of Anticancer Agents Targeting the HIF-1 Transcription Factor

Hypoxia, a characteristic feature of solid tumors, is associated with the malignant phenotype and therapy resistance of cancers. Hypoxia-inducible factor 1 (HIF-1), which is responsible for the metazoan adaptive response to hypoxia, has been recognized as a rational target for cancer therapy due to its critical functions in hypoxic regions. In order to efficiently inhibit its activity, extensive efforts have been made to elucidate the molecular mechanism underlying the activation of HIF-1. Here, we provide an overview of relevant research, particularly on a series of HIF-1 activators identified so far and the development of anticancer drugs targeting them.

Inhibition of the Oxygen-Sensing Asparaginyl Hydroxylase Factor Inhibiting Hypoxia-Inducible Factor: A Potential Hypoxia Response Modulating Strategy

Factor inhibiting hypoxia-inducible factor (FIH) is a JmjC domain 2-oxogluarate and Fe(II)-dependent oxygenase that catalyzes hydroxylation of specific asparagines in the C-terminal transcriptional activation domain of hypoxia-inducible factor alpha (HIF-α) isoforms. This modification suppresses the transcriptional activity of HIF by reducing its interaction with the transcriptional coactivators p300/CBP. By contrast with inhibition of the HIF prolyl hydroxylases (PHDs), inhibitors of FIH, which accepts multiple non-HIF substrates, are less studied; they are of interest due to their potential ability to alter metabolism (either in a HIF-dependent and/or -independent manner) and, provided HIF is upregulated, to modulate the course of the HIF-mediated hypoxic response. Here we review studies on the mechanism and inhibition of FIH. We discuss proposed biological roles of FIH including its regulation of HIF activity and potential roles of FIH-catalyzed oxidation of non-HIF substrates. We highlight potential therapeutic applications of FIH inhibitors.

eIF4E3 forms an active eIF4F complex during stresses (eIF4FS) targeting mTOR and re-programs the translatome

The eIF4E are a family of initiation factors that bind the mRNA 5' cap, regulating the proteome and the cellular phenotype. eIF4E1 mediates global translation and its activity is controlled via the PI3K/AKT/mTOR pathway. mTOR down-regulation results in eIF4E1 sequestration into an inactive complex with the 4E binding proteins (4EBPs). The second member, eIF4E2, regulates the translatome during hypoxia. However, the exact function of the third member, eIF4E3, has remained elusive. We have dissected its function using a range of techniques. Starting from the observation that it does not interact with 4EBP1, we demonstrate that eIF4E3 recruitment into an eIF4F complex occurs when Torin1 inhibits the mTOR pathway. Ribo-seq studies demonstrate that this complex (eIF4FS) is translationally active during stress and that it selects specific mRNA populations based on 5' TL (UTR) length. The interactome reveals that it associates with cellular proteins beyond the cognate initiation factors, suggesting that it may have 'moon-lighting' functions. Finally, we provide evidence that cellular metabolism is altered in an eIF4E3 KO background but only upon Torin1 treatment. We propose that eIF4E3 acts as a second branch of the integrated stress response, re-programming the translatome to promote 'stress resistance' and adaptation.

Hypoxia-dependent signaling in perioperative and critical care medicine

A critical goal of patient management for anesthesiologists and intensivists is to maintain oxygen homeostasis in patients admitted to operation theaters and intensive care units. For this purpose, it is imperative to understand the strategies of the body against oxygen imbalance—especially oxygen deficiency (hypoxia). Adaptation to hypoxia and maintenance of oxygen homeostasis involve a wide range of responses that occur at different organizational levels in the body. These responses are greatly influenced by perioperative patient management including factors such as perioperative drugs. Herein, the influence of perioperative patient management on the body's response to oxygen imbalance was reviewed with a special emphasis on hypoxia-inducible factors (HIFs), transcription factors whose activity are regulated by the perturbation of oxygen metabolism. The 2019 Nobel Prize in Physiology or Medicine was awarded to three researchers who made outstanding achievements in this field. While previous studies have reported the effect of perioperatively used drugs on hypoxia-induced gene expression mediated by HIFs, this review focused on effects of subacute or chronic hypoxia changes in gene expression that are mediated by the transcriptional regulator HIFs. The clinical implications and perspectives of these findings also will be discussed. Understanding the basic biology of the transcription factor HIF can be informative for us since anesthesiologists manage patients during the perioperative period facing the imbalances the oxygen metabolism in organ and tissue. The clinical implications of hypoxia-dependent signaling in critical illness, including Coronavirus disease (COVID-19), in which disturbances in oxygen metabolism play a major role in its pathogenesis will also be discussed.

Lessons from Comparison of Hypoxia Signaling in Plants and Mammals

Hypoxia is an important stress for organisms, including plants and mammals. In plants, hypoxia can be the consequence of flooding and causes important crop losses worldwide. In mammals, hypoxia stress may be the result of pathological conditions. Understanding the regulation of responses to hypoxia offers insights into novel approaches for crop improvement, particularly for the development of flooding-tolerant crops and for producing better therapeutics for hypoxia-related diseases such as inflammation and cancer. Despite their evolutionary distance, plants and mammals deploy strikingly similar mechanisms to sense and respond to the different aspects of hypoxia-related stress, including low oxygen levels and the resulting energy crisis, nutrient depletion, and oxidative stress. Over the last two decades, the ubiquitin/proteasome system and the ubiquitin-like protein SUMO have been identified as key regulators that act in concert to regulate core aspects of responses to hypoxia in plants and mammals. Here, we review ubiquitin and SUMO-dependent mechanisms underlying the regulation of hypoxia response in plants and mammals. By comparing and contrasting these mechanisms in plants and mammals, this review seeks to pinpoint conceptually similar mechanisms but also highlight future avenues of research at the junction between different fields of research.

HIF-α Prolyl Hydroxylase Inhibitors and Their Implications for Biomedicine: A Comprehensive Review

Oxygen is essential for the maintenance of the body. Living organisms have evolved systems to secure an oxygen environment to be proper. Hypoxia-inducible factor (HIF) plays an essential role in this process; it is a transcription factor that mediates erythropoietin (EPO) induction at the transcriptional level under hypoxic environment. After successful cDNA cloning in 1995, a line of studies were conducted for elucidating the molecular mechanism of HIF activation in response to hypoxia. In 2001, cDNA cloning of dioxygenases acting on prolines and asparagine residues, which play essential roles in this process, was reported. HIF-prolyl hydroxylases (PHs) are molecules that constitute the core molecular mechanism of detecting a decrease in the partial pressure of oxygen, or hypoxia, in the cells; they can be called oxygen sensors. In this review, I discuss the process of molecular cloning of HIF and HIF-PH, which explains hypoxia-induced EPO expression; the development of HIF-PH inhibitors that artificially or exogenously activate HIF by inhibiting HIF-PH; and the significance and implications of medical intervention using HIF-PH inhibitors.

Urinary l-erythro-β-hydroxyasparagine-a novel serine racemase inhibitor and substrate of the Zn2+-dependent d-serine dehydratase

In the present study, we identified l-erythro-β-hydroxyasparagine (l-β-EHAsn) found abundantly in human urine, as a novel substrate of Zn²⁺-dependent d-serine dehydratase (DSD). l-β-EHAsn is an atypical amino acid present in large amounts in urine but rarely detected in serum or most organs/tissues examined. Quantitative analyses of urinary l-β-EHAsn in young healthy volunteers revealed significant correlation between urinary l-β-EHAsn concentration and creatinine level. Further, for in-depth analyses of l-β-EHAsn, we developed a simple three-step synthetic method using trans-epoxysuccinic acid as the starting substance. In addition, our research revealed a strong inhibitory effect of l-β-EHAsn on mammalian serine racemase, responsible for producing d-serine, a co-agonist of the N-methyl-d-aspartate (NMDA) receptor involved in glutamatergic neurotransmission.

Vegfa promoter gene hypermethylation at HIF1α binding site is an early contributor to CKD progression after renal ischemia

Chronic hypoxia is a major contributor to Chronic Kidney Disease (CKD) after Acute Kidney Injury (AKI). However, the temporal relation between the acute insult and maladaptive renal response to hypoxia remains unclear. In this study, we analyzed the time-course of renal hemodynamics, oxidative stress, inflammation, and fibrosis, as well as epigenetic modifications, with focus on HIF1α/VEGF signaling, in the AKI to CKD transition. Sham-operated, right nephrectomy (UNx), and UNx plus renal ischemia (IR + UNx) groups of rats were included and studied at 1, 2, 3, or 4 months. The IR + UNx group developed CKD characterized by progressive proteinuria, renal dysfunction, tubular proliferation, and fibrosis. At first month post-ischemia, there was a twofold significant increase in oxidative stress and reduction in global DNA methylation that was maintained throughout the study. Hif1α and Vegfa expression were depressed in the first and second-months post-ischemia, and then Hif1α but not Vegfa expression was recovered. Interestingly, hypermethylation of the Vegfa promoter gene at the HIF1α binding site was found, since early stages of the CKD progression. Our findings suggest that renal hypoperfusion, inefficient hypoxic response, increased oxidative stress, DNA hypomethylation, and, Vegfa promoter gene hypermethylation at HIF1α binding site, are early determinants of AKI-to-CKD transition.

Anemia in Chronic Kidney Disease: From Pathophysiology and Current Treatments, to Future Agents

Anemia is a common complication in chronic kidney disease (CKD), and is associated with a reduced quality of life, and an increased morbidity and mortality. The mechanisms involved in anemia associated to CKD are diverse and complex. They include a decrease in endogenous erythropoietin (EPO) production, absolute and/or functional iron deficiency, and inflammation with increased hepcidin levels, among others. Patients are most commonly managed with oral or intravenous iron supplements and with erythropoiesis stimulating agents (ESA). However, these treatments have associated risks, and sometimes are insufficiently effective. Nonetheless, in the last years, there have been some remarkable advances in the treatment of CKD-related anemia, which have raised great expectations. On the one hand, a novel family of drugs has been developed: the hypoxia-inducible factor prolyl hydroxylase inhibitors (HIF-PHIs). These agents induce, among other effects, an increase in the production of endogenous EPO, improve iron availability and reduce hepcidin levels. Some of them have already received marketing authorization. On the other hand, recent clinical trials have elucidated important aspects of iron supplementation, which may change the treatment targets in the future. This article reviews the current knowledge of the pathophysiology CKD-related anemia, current and future therapies, the trends in patient management and the unmet goals.

Hypoxia, oxidative stress, and immune evasion: a trinity of the trichothecenes T-2 toxin and deoxynivalenol (DON)

T-2 toxin and deoxynivalenol (DON) are type A and B trichothecenes, respectively. They widely occur as pollutants in food and crops and cause a series of toxicities, including immunotoxicity, hepatotoxicity, and neurotoxicity. Oxidative stress is the primary mechanistic basis of these toxic effects. Increasing amounts of evidence have shown that mitochondria are significant targets of apoptosis caused by T-2 toxin- and DON-induced oxidative stress via regulation of Bax/B-cell lymphoma-2 and caspase-3/caspase-9 signaling. DNA methylation and autophagy are involved in oxidative stress related to apoptosis, and hypoxia and immune evasion are related to oxidative stress in this context. Hypoxia induces oxidative stress by stimulating mitochondrial reactive oxygen species production and regulates the expression of cytokines, such as interleukin-1β and tumor necrosis factor-α. Programmed cell death-ligand 1 is upregulated by these cytokines and by hypoxia-inducible factor-1, which allows it to bind to programmed cell death-1 to enable escape of immune cell surveillance and achievement of immune evasion. This review concentrates on novel findings regarding the oxidative stress mechanisms of the trichothecenes T-2 toxin and DON. Importantly, we discuss the new evidence regarding the connection of hypoxia and immune evasion with oxidative stress in this context. Finally, the trinity of hypoxia, oxidative stress and immune evasion is highlighted. This work will be conducive to an improved understanding of the oxidative stress caused by trichothecene mycotoxins.

Dynamic characterization of intestinal metaplasia in the gastric corpus mucosa of Atp4a-deficient mice

Parietal cells of the gastric mucosa contain a complex and extensive secretory membrane system that harbors gastric H⁺, K⁺-adenosine triphosphatase (ATPase), the enzyme primarily responsible for gastric lumen acidification. Here, we describe the characterization of mice deficient in the H⁺, K⁺-ATPase α subunit (Atp4a^−/−) to determine the role of this protein in the biosynthesis of this membrane system and the biology of the gastric mucosa. Atp4a^−/− mice were produced by gene targeting. Wild-type (WT) and Atp4a^−/− mice, paired for age, were examined at 10, 12, 14 and 16 weeks for histopathology, and the expression of mucin 2 (MUC2), α-methylacyl-CoA racemase (AMACR), Ki-67 and p53 proteins was analyzed by immunohistochemistry. For further information, phosphoinositide 3-kinase (PI3K), phosphorylated-protein kinase B (p-AKT), mechanistic target of rapamycin (mTOR), hypoxia-inducible factor 1α (HIF-1α), lactate dehydrogenase A (LDHA) and sirtuin 6 (SIRT6) were detected by Western blotting. Compared with the WT mice, hypochlorhydric Atp4a^−/− mice developed parietal cell atrophy and significant antral inflammation (lymphocyte infiltration) and intestinal metaplasia (IM) with elevated MUC2 expression. Areas of dysplasia in the Atp4a^−/− mouse stomach showed increased AMACR and Ki-67 expression. Consistent with elevated antral proliferation, tissue isolated from Atp4a^−/− mice showed elevated p53 expression. Next, we examined the mechanism by which the deficiency of the H⁺, K⁺-ATPase α subunit has an effect on the gastric mucosa. We found that the expression of phosphorylated-PI3K, p-AKT, phosphorylated-mTOR, HIF-1α, LDHA and SIRT6 was significantly higher in tissue from the Atp4a^−/− mice compared with the WT mice (P<0.05). The H⁺, K⁺-ATPase α subunit is required for acid-secretory activity of parietal cells in vivo, the normal development and cellular homeostasis of the gastric mucosa, and attainment of the normal structure of the secretory membranes. Chronic achlorhydria and hypergastrinemia in aged Atp4a^−/− mice produced progressive hyperplasia and mucolytic and IM, and activated the Warburg effect via PI3K/AKT/mTOR signaling.

Hypoxia/HIF Modulates Immune Responses

Oxygen availability varies throughout the human body in health and disease. Under physiological conditions, oxygen availability drops from the lungs over the blood stream towards the different tissues into the cells and the mitochondrial cavities leading to physiological low oxygen conditions or physiological hypoxia in all organs including primary lymphoid organs. Moreover, immune cells travel throughout the body searching for damaged cells and foreign antigens facing a variety of oxygen levels. Consequently, physiological hypoxia impacts immune cell function finally controlling innate and adaptive immune response mainly by transcriptional regulation via hypoxia-inducible factors (HIFs). Under pathophysiological conditions such as found in inflammation, injury, infection, ischemia and cancer, severe hypoxia can alter immune cells leading to dysfunctional immune response finally leading to tissue damage, cancer progression and autoimmunity. Here we summarize the effects of physiological and pathophysiological hypoxia on innate and adaptive immune activity, we provide an overview on the control of immune response by cellular hypoxia-induced pathways with focus on the role of HIFs and discuss the opportunity to target hypoxia-sensitive pathways for the treatment of cancer and autoimmunity.

Acute intermittent hypoxia enhances regeneration of surgically repaired peripheral nerves in a manner akin to electrical stimulation

The intrinsic repair response of injured peripheral neurons is enhanced by brief electrical stimulation (ES) at time of surgical repair, resulting in improved regeneration in rodents and humans. However, ES is invasive. Acute intermittent hypoxia (AIH) - breathing alternate cycles of regular air and air with ~50% normal oxygen levels (11% O₂), considered mild hypoxia, is an emerging, promising non-invasive therapy that promotes motor function in spinal cord injured rats and humans. AIH can increase neural activity and under moderately severe hypoxic conditions improves repair of peripherally crushed nerves in mice. Thus, we posited an AIH paradigm similar to that used clinically for spinal cord injury, will improve surgically repaired peripheral nerves akin to ES, including an impact on regeneration-associated gene (RAG) expression-a predictor of growth states. Alterations in early RAG expression were examined in adult male Lewis rats that underwent tibial nerve coaptation repair with either 2 days AIH or normoxia control treatment begun on day 2 post-repair, or 1 h ES treatment (20 Hz) at time of repair. Three days post-repair, AIH or ES treatments effected significant and parallel elevated RAG expression relative to normoxia control at the level of injured sensory and motor neuron cell bodies and proximal axon front. These parallel impacts on RAG expression were coupled with significant improvements in later indices of regeneration, namely enhanced myelination and increased numbers of newly myelinated fibers detected 20 mm distal to the tibial nerve repair site or sensory and motor neurons retrogradely labeled 28 mm distal to the repair site, both at 25 days post nerve repair; and improved return of toe spread function 5-10 weeks post-repair. Collectively, AIH mirrors many beneficial effects of ES on peripheral nerve repair outcomes. This highlights its potential for clinical translation as a non-invasive means to effect improved regeneration of injured peripheral nerves.

Hypoxia-inducible factors and diabetes

Hypoxia can be defined as a relative deficiency in the amount of oxygen reaching the tissues. Hypoxia-inducible factors (HIFs) are critical regulators of the mammalian response to hypoxia. In normal circumstances, HIF-1α protein turnover is rapid, and hyperglycemia further destabilizes the protein. In addition to their role in diabetes pathogenesis, HIFs are implicated in development of the microvascular and macrovascular complications of diabetes. Improving glucose control in people with diabetes increases HIF-1α protein and has wide-ranging benefits, some of which are at least partially mediated by HIF-1α. Nevertheless, most strategies to improve diabetes or its complications via regulation of HIF-1α have not currently proven to be clinically useful. The intersection of HIF biology with diabetes is a complex area in which many further questions remain, especially regarding the well-conducted studies clearly describing discrepant effects of different methods of increasing HIF-1α, even within the same tissues. This Review presents a brief overview of HIFs; discusses the range of evidence implicating HIFs in β cell dysfunction, diabetes pathogenesis, and diabetes complications; and examines the differing outcomes of HIF-targeting approaches in these conditions.

High-altitude deer mouse hypoxia-inducible factor-2α shows defective interaction with CREB-binding protein

Numerous mammalian species have adapted to the chronic hypoxia of high altitude. Recent genomic studies have identified evidence for natural selection of genes and associated genetic changes in these species. A major gap in our knowledge is an understanding of the functional significance, if any, of these changes. Deer mice (Peromyscus maniculatus) live at both low and high altitudes in North America, providing an opportunity to identify functionally important genetic changes. High-altitude deer mice show evidence of natural selection on the Epas1 gene, which encodes for hypoxia-inducible factor-2α (Hif-2α), a central transcription factor of the hypoxia-inducible factor pathway. An SNP encoding for a T755M change in the Hif-2α protein is highly enriched in high-altitude deer mice, but its functional significance is unknown. Here, using coimmunoprecipitation and transcriptional activity assays, we show that the T755M mutation produces a defect in the interaction of Hif-2α with the transcriptional coactivator CREB-binding protein. This results in a loss of function because of decreased transcriptional activity. Intriguingly, the effect of this mutation depends on the amino acid context. Interchanges between methionine and threonine at the corresponding position in house mouse (Mus musculus) Hif-2α are without effects on CREB-binding protein binding. Furthermore, transfer of a set of deer mouse–specific Hif-2α amino acids to house mouse Hif-2α is sufficient to confer sensitivity of house mouse Hif-2α to the T755M substitution. These findings provide insight into high-altitude adaptation in deer mice and evolution at the Epas1 locus.

The Oxidative Paradox in Low Oxygen Stress in Plants

Reactive oxygen species (ROS) are part of aerobic environments, and variations in the availability of oxygen (O2) in the environment can lead to altered ROS levels. In plants, the O2 sensing machinery guides the molecular response to low O2, regulating a subset of genes involved in metabolic adaptations to hypoxia, including proteins involved in ROS homeostasis and acclimation. In addition, nitric oxide (NO) participates in signaling events that modulate the low O2 stress response. In this review, we summarize recent findings that highlight the roles of ROS and NO under environmentally or developmentally defined low O2 conditions. We conclude that ROS and NO are emerging regulators during low O2 signalling and key molecules in plant adaptation to flooding conditions.

Insights into The Function and Regulation of Jumonji C Lysine Demethylases as Hypoxic Responsive Enzymes

Cellular responses to hypoxia (low oxygen) are governed by oxygen sensitive signaling pathways. Such pathways, in part, are controlled by enzymes with oxygen-dependent catalytic activity, of which the role of prolyl 4-hydroxylases has been widely reviewed. These enzymes inhibit hypoxic response by inducing the oxygen-dependent degradation of hypoxia-inducible factor 1α, the master regulator of the transcriptional hypoxic response. Jumonji C domain-containing lysine demethylases are similar enzymes which share the same oxygen-dependent catalytic mechanism as prolyl 4- hydroxylases. Traditionally, the role of lysine demethylases has been studied in relation to demethylation activity against histone substrates, however, within the past decade an increasing number of nonhistone protein targets have been revealed, some of which have a key role in survival in the hypoxic tumor microenvironment. Within this review, we highlight the involvement of methyllysine in the hypoxic response with a focus on the HIF signaling pathway, the regulation of demethylase activity by oxygen, and provide insights into notable areas of future hypoxic demethylase research.

The Effect of Hypoxia on the Expression of CXC Chemokines and CXC Chemokine Receptors-A Review of Literature

Hypoxia is an integral component of the tumor microenvironment. Either as chronic or cycling hypoxia, it exerts a similar effect on cancer processes by activating hypoxia-inducible factor-1 (HIF-1) and nuclear factor (NF-κB), with cycling hypoxia showing a stronger proinflammatory influence. One of the systems affected by hypoxia is the CXC chemokine system. This paper reviews all available information on hypoxia-induced changes in the expression of all CXC chemokines (CXCL1, CXCL2, CXCL3, CXCL4, CXCL5, CXCL6, CXCL7, CXCL8 (IL-8), CXCL9, CXCL10, CXCL11, CXCL12 (SDF-1), CXCL13, CXCL14, CXCL15, CXCL16, CXCL17) as well as CXC chemokine receptors—CXCR1, CXCR2, CXCR3, CXCR4, CXCR5, CXCR6, CXCR7 and CXCR8. First, we present basic information on the effect of these chemoattractant cytokines on cancer processes. We then discuss the effect of hypoxia-induced changes on CXC chemokine expression on the angiogenesis, lymphangiogenesis and recruitment of various cells to the tumor niche, including myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs), tumor-associated neutrophils (TANs), regulatory T cells (T_regs) and tumor-infiltrating lymphocytes (TILs). Finally, the review summarizes data on the use of drugs targeting the CXC chemokine system in cancer therapies.

Hypoxia-mediated drug resistance in breast cancers

Tissue hypoxia in solid tumors is caused by several pathological changes associated with tumor growth, including altered microvasculature structure, increased diffusional distances, and tumor-associated anemia. As the oxygen tension decreases, tumor cells adapt to the limited oxygen supply. Previous studies have shown that such adaptation leads to an aggressive phenotype that is resistant to many anti-cancer therapies. Induction of hypoxia inducible factors (HIFs) mediates many proteomic and genomic changes associated with tumor hypoxia. In breast cancers, HIFs not only predict poor prognosis, but also promote metastasis and drug resistance. Several studies have proposed HIF-1α as a druggable target in drug-resistant breast cancers, leading to the synthesis and development of small molecule inhibitors. Disappointingly, however, none of these small molecule inhibitors have progressed to clinical use. In this review, we briefly discuss the role of HIF-1α in breast cancer drug resistance and summarize the current and future approaches to targeting this transcription factor in breast cancer treatment.

Contribution of hypoxia inducible factor-1 during viral infections

Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that plays critical roles during the cellular response to hypoxia. Under normoxic conditions, its function is tightly regulated by the degradation of its alpha subunit (HIF-1α), which impairs the formation of an active heterodimer in the nucleus that otherwise regulates the expression of numerous genes. Importantly, HIF-1 participates in both cancer and infectious diseases unveiling new therapeutic targets for those ailments. Here, we discuss aspects related to the activation of HIF-1, the effects of this transcription factor over immune system components, as well as the involvement of HIF-1 activity in response to viral infections in humans. Although HIF-1 is currently being assessed in numerous clinical settings as a potential therapy for different diseases, up to date, there are no clinical studies evaluating the pharmacological modulation of this transcription factor as a possible new antiviral treatment. However, based on the available evidence, clinical trials targeting this molecule are likely to occur soon. In this review we discuss the role of HIF-1 in viral immunity, the modulation of HIF-1 by different types of viruses, as well as the effects of HIF-1 over their life cycle and the potential use of HIF-1 as a new target for the treatment of viral infections.

Hypoxic Regulation of Gene Transcription and Chromatin: Cause and Effect

Cellular responses to low oxygen (hypoxia) are fundamental to normal physiology and to the pathology of many common diseases. Hypoxia-inducible factor (HIF) is central to this by enhancing the transcriptional activity of many hundreds of genes. The cellular response to HIF is cell-type-specific and is largely governed by the pre-existing epigenetic landscape. Prior to activation, HIF-binding sites and the promoters of HIF-target genes are already accessible, in contact with each other through chromatin looping and display markers of activity. However, hypoxia also modulates the epigenetic environment, both in parallel to and as a consequence of HIF activation. This occurs through a combination of oxygen-sensitive changes in enzyme activity, transcriptional activation of epigenetic modifiers, and localized recruitment to chromatin by HIF and activated RNApol2. These hypoxic changes in the chromatin environment may both contribute to and occur as a consequence of transcriptional regulation. Nevertheless, they have the capacity to both modulate and extend the transcriptional response to hypoxia.

Molecular Crosstalk Between MYC and HIF in Cancer

The transcription factor c-MYC (MYC thereafter) is a global regulator of gene expression. It is overexpressed or deregulated in human cancers of diverse origins and plays a key role in the development of cancers. Hypoxia-inducible factors (HIFs), a central regulator for cells to adapt to low cellular oxygen levels, is also often overexpressed and activated in many human cancers. HIF mediates the primary transcriptional response of a wide range of genes in response to hypoxia. Earlier studies focused on the inhibition of MYC by HIF during hypoxia, when MYC is expressed at physiological level, to help cells survive under low oxygen conditions. Emerging evidence suggests that MYC and HIF also cooperate to promote cancer cell growth and progression. This review will summarize the current understanding of the complex molecular interplay between MYC and HIF.

Hypoxia and Oxygen-Sensing Signaling in Gene Regulation and Cancer Progression

Oxygen homeostasis regulation is the most fundamental cellular process for adjusting physiological oxygen variations, and its irregularity leads to various human diseases, including cancer. Hypoxia is closely associated with cancer development, and hypoxia/oxygen-sensing signaling plays critical roles in the modulation of cancer progression. The key molecules of the hypoxia/oxygen-sensing signaling include the transcriptional regulator hypoxia-inducible factor (HIF) which widely controls oxygen responsive genes, the central members of the 2-oxoglutarate (2-OG)-dependent dioxygenases, such as prolyl hydroxylase (PHD or EglN), and an E3 ubiquitin ligase component for HIF degeneration called von Hippel–Lindau (encoding protein pVHL). In this review, we summarize the current knowledge about the canonical hypoxia signaling, HIF transcription factors, and pVHL. In addition, the role of 2-OG-dependent enzymes, such as DNA/RNA-modifying enzymes, JmjC domain-containing enzymes, and prolyl hydroxylases, in gene regulation of cancer progression, is specifically reviewed. We also discuss the therapeutic advancement of targeting hypoxia and oxygen sensing pathways in cancer.

Association of an APBA3 Missense Variant with Risk of Premature Ovarian Failure in the Korean Female Population

Premature ovarian failure (POF) is a complex disease of which the etiology is influenced by numerous genetic variations. Several POF candidate genes have been reported. However, no causal genes with high odds ratio (OR) have yet been discovered. This study included 564 females of Korean ethnicity, comprising 60 patients with POF and 182 controls in the discovery set and 105 patients with POF and 217 controls in the replication set. We conducted genome-wide association analysis to search for novel candidate genes predicted to influence POF development using Axiom Precision Medicine Research Arrays and additive model logistic regression analysis. One statistically significant single nucleotide polymorphism (SNP), rs55941146, which encodes a missense alteration (Val > Gly) in the APBA3 gene, was identified with OR values for association with POF of 13.33 and 4.628 in the discovery and replication sets, respectively. No rs55941146 minor allele homozygotes were present in either cases or controls. The APBA3 protein binds FIH-1 that inhibits hypoxia inducible factor-1α (HIF-1α). HIF-1α contributes to granulosa cell proliferation, which is crucial for ovarian follicle growth, by regulating cell proliferation factors and follicle stimulating hormone-mediated autophagy. Our data demonstrate that APBA3 is a candidate novel causal gene for POF.

An allosteric peptide inhibitor of HIF-1α regulates hypoxia-induced retinal neovascularization

Retinal neovascularization (NV), a leading cause of vision loss, results from localized hypoxia that stabilizes the hypoxia-inducible transcription factors HIF-1α and HIF-2α, enabling the expression of angiogenic factors and genes required to maintain homeostasis under conditions of oxygen stress. HIF transcriptional activity depends on the interaction between its intrinsically disordered C-terminal domain and the transcriptional coactivators CBP/p300. Much effort is currently directed at disrupting protein-protein interactions between disease-associated transcription factors like HIF and their cellular partners. The intrinsically disordered protein CITED2, a direct product of HIF-mediated transcription, functions as a hypersensitive negative regulator that attenuates the hypoxic response by competing allosterically with HIF-1α for binding to CBP/p300. Here, we show that a peptide fragment of CITED2 is taken up by retinal cells and efficiently regulates pathological angiogenesis in murine models of ischemic retinopathy. Both vaso-obliteration (VO) and NV were significantly inhibited in an oxygen-induced retinopathy (OIR) model following intravitreal injection of the CITED2 peptide. The CITED2 peptide localized to retinal neurons and glia, resulting in decreased expression of HIF target genes. Aflibercept, a commonly used anti-VEGF therapy for retinal neovascular diseases, rescued NV but not VO in OIR. However, a combination of the CITED2 peptide and a reduced dose of aflibercept significantly decreased both NV and VO. In contrast to anti-VEGF agents, the CITED2 peptide can rescue hypoxia-induced retinal NV by modulating the hypoxic response through direct competition with HIF for CBP/p300, suggesting a dual targeting strategy for treatment of ischemic retinal diseases and other neovascular disorders.

The Role of Sumoylation in the Response to Hypoxia: An Overview

Sumoylation is the covalent attachment of the small ubiquitin-related modifier (SUMO) to a vast variety of proteins in order to modulate their function. Sumoylation has emerged as an important modification with a regulatory role in the cellular response to different types of stress including osmotic, hypoxic and oxidative stress. Hypoxia can occur under physiological or pathological conditions, such as ischemia and cancer, as a result of an oxygen imbalance caused by low supply and/or increased consumption. The hypoxia inducible factors (HIFs), and the proteins that regulate their fate, are critical molecular mediators of the response to hypoxia and modulate procedures such as glucose and lipid metabolism, angiogenesis, erythropoiesis and, in the case of cancer, tumor progression and metastasis. Here, we provide an overview of the sumoylation-dependent mechanisms that are activated under hypoxia and the way they influence key players of the hypoxic response pathway. As hypoxia is a hallmark of many diseases, understanding the interrelated connections between the SUMO and the hypoxic signaling pathways can open the way for future molecular therapeutic interventions.

Ascorbate and Tumor Cell Iron Metabolism: The Evolving Story and Its Link to Pathology

Significance: Vitamin C or ascorbate (Asc) is a water-soluble vitamin and an antioxidant that is involved in many crucial biological functions. Asc's ability to reduce metals makes it an essential enzyme cofactor. Recent Advances: The ability of Asc to act as a reductant also plays an important part in its overall role in iron metabolism, where Asc induces both nontransferrin-bound iron and transferrin-bound iron uptake at physiological concentrations (∼50 μM). Moreover, Asc has emerged to play an important role in multiple diseases and its effects at pharmacological doses could be important for their treatment. Critical Issues: Asc's role as a regulator of cellular iron metabolism, along with its cytotoxic effects and different roles at pharmacological concentrations, makes it a candidate as an anticancer agent. Ever since the controversy regarding the studies from the Mayo Clinic was finally explained, there has been a renewed interest in using Asc as a therapeutic approach toward cancer due to its minimal side effects. Numerous studies have been able to demonstrate the anticancer activity of Asc through selective oxidative stress toward cancer cells via H₂O₂ generation at pharmacological concentrations. Studies have demonstrated that Asc's cytotoxic mechanism at concentrations (>1 mM) has been associated with decreased cellular iron uptake. Future Directions: Recent studies have also suggested other mechanisms, such as Asc's effects on autophagy, polyamine metabolism, and the cell cycle. Clearly, more has yet to be discovered about Asc's mechanism of action to facilitate safe and effective treatment options for cancer and other diseases.

Hypoxia-inducible factors not only regulate but also are myeloid-cell treatment targets

Hypoxia describes limited oxygen availability at the cellular level. Myeloid cells are exposed to hypoxia at various bodily sites and even contribute to hypoxia by consuming large amounts of oxygen during respiratory burst. Hypoxia-inducible factors (HIFs) are ubiquitously expressed heterodimeric transcription factors, composed of an oxygen-dependent α and a constitutive β subunit. The stability of HIF-1α and HIF-2α is regulated by oxygen-sensing prolyl-hydroxylases (PHD). HIF-1α and HIF-2α modify the innate immune response and are context dependent. We provide a historic perspective of HIF discovery, discuss the molecular components of the HIF pathway, and how HIF-dependent mechanisms modify myeloid cell functions. HIFs enable myeloid-cell adaptation to hypoxia by up-regulating anaerobic glycolysis. In addition to effects on metabolism, HIFs control chemotaxis, phagocytosis, degranulation, oxidative burst, and apoptosis. HIF-1α enables efficient infection defense by myeloid cells. HIF-2α delays inflammation resolution and decreases antitumor effects by promoting tumor-associated myeloid-cell hibernation. PHDs not only control HIF degradation, but also regulate the crosstalk between innate and adaptive immune cells thereby suppressing autoimmunity. HIF-modifying pharmacologic compounds are entering clinical practice. Current indications include renal anemia and certain cancers. Beneficial and adverse effects on myeloid cells should be considered and could possibly lead to drug repurposing for inflammatory disorders.

Silibinin down-regulates PD-L1 expression in nasopharyngeal carcinoma by interfering with tumor cell glycolytic metabolism

The upregulation of checkpoint inhibitor PD-L1 expression has recently been associated with nasopharyngeal carcinoma (NPC) resistance to therapy. The mechanism of induction of PD-L1 has also been linked to enhanced aerobic glycolysis promoted by HIF1-α dysregulation and LDH-A activity in cancer. Here, we investigated the effect of the anti-tumoral compound Silibinin on HIF-1α/LDH-A mediated cancer cell metabolism and PD-L1 expression in NPC. Our results demonstrate that exposure to Silibinin potently inhibits tumor growth and promotes a shift from aerobic glycolysis toward oxidative phosphorylation. The EBV + NPC cell line C666-1 and glycolytic human tumor explants treated with Silibinin displayed a reduction in LDH-A activity which consistently associated with a reduction in lactate levels. This effect was accompanied by an increase in intracellular citrate levels in C666-1 cells. Accordingly, expression of HIF-1α, a critical regulator of glycolysis, was down-regulated after treatment. This event associated with a down-regulation in PD-L1. Altogether, our results provide evidence that silibinin can alter PD-L1 expression by interfering with HIF-1α/LDH-A mediated cell metabolism in NPC. These results provide a new perspective for Silibinin use to overcome PD-L1 mediated NPC resistance to therapy.

IDH1 Targeting as a New Potential Option for Intrahepatic Cholangiocarcinoma Treatment-Current State and Future Perspectives

Cholangiocarcinoma is a primary malignancy of the biliary tract characterized by late and unspecific symptoms, unfavorable prognosis, and few treatment options. The advent of next-generation sequencing has revealed potential targetable or actionable molecular alterations in biliary tumors. Among several identified genetic alterations, the IDH1 mutation is arousing interest due to its role in epigenetic and metabolic remodeling. Indeed, some IDH1 point mutations induce widespread epigenetic alterations by means of a gain-of-function of the enzyme, which becomes able to produce the oncometabolite 2-hydroxyglutarate, with inhibitory activity on α-ketoglutarate-dependent enzymes, such as DNA and histone demethylases. Thus, its accumulation produces changes in the expression of several key genes involved in cell differentiation and survival. At present, small-molecule inhibitors of IDH1 mutated enzyme are under investigation in preclinical and clinical phases as promising innovative treatments for IDH1-mutated intrahepatic cholangiocarcinomas. This review examines the molecular rationale and the results of preclinical and early-phase studies on novel pharmacological agents targeting mutant IDH1 in cholangiocarcinoma patients. Contextually, it will offer a starting point for discussion on combined therapies with metabolic and epigenetic drugs, to provide molecular support to target the interplay between metabolism and epigenetics, two hallmarks of cancer onset and progression.

Power to see-Drivers of aerobic glycolysis in the mammalian retina: A review

The mammalian retina converts most glucose to lactate rather than catabolizing it completely to carbon dioxide via oxidative phosphorylation, despite the availability of oxygen. This unusual metabolism is known as aerobic glycolysis or the Warburg effect. Molecules and pathways that drive aerobic glycolysis have been identified and thoroughly studied in the context of cancer but remain relatively poorly understood in the retina. Here, we review recent research on the molecular mechanisms that underly aerobic glycolysis in the retina, focusing on key glycolytic enzymes including hexokinase 2 (HK2), pyruvate kinase M2 (PKM2) and lactate dehydrogenase A (LDHA). We also discuss the potential involvement of cell signalling and transcriptional pathways including phosphoinositide 3-kinase (PI3K) signalling, fibroblast growth factor receptor (FGFR) signalling, and hypoxia-inducible factor 1 (HIF-1), which have been implicated in driving aerobic glycolysis in the context of cancer.

Hypoxia Alters the Expression of CC Chemokines and CC Chemokine Receptors in a Tumor-A Literature Review

Hypoxia, i.e., oxygen deficiency condition, is one of the most important factors promoting the growth of tumors. Since its effect on the chemokine system is crucial in understanding the changes in the recruitment of cells to a tumor niche, in this review we have gathered all the available data about the impact of hypoxia on β chemokines. In the introduction, we present the chronic (continuous, non-interrupted) and cycling (intermittent, transient) hypoxia together with the mechanisms of activation of hypoxia inducible factors (HIF-1 and HIF-2) and NF-κB. Then we describe the effect of hypoxia on the expression of chemokines with the CC motif: CCL1, CCL2, CCL3, CCL4, CCL5, CCL7, CCL8, CCL11, CCL13, CCL15, CCL16, CCL17, CCL18, CCL19, CCL20, CCL21, CCL22, CCL24, CCL25, CCL26, CCL27, CCL28 together with CC chemokine receptors: CCR1, CCR2, CCR3, CCR4, CCR5, CCR6, CCR7, CCR8, CCR9, and CCR10. To better understand the effect of hypoxia on neoplastic processes and changes in the expression of the described proteins, we summarize the available data in a table which shows the effect of individual chemokines on angiogenesis, lymphangiogenesis, and recruitment of eosinophils, myeloid-derived suppressor cells (MDSC), regulatory T cells (T_reg), and tumor-associated macrophages (TAM) to a tumor niche.

Hypoxia and HIF Signaling: One Axis with Divergent Effects

The correct concentration of oxygen in all tissues is a hallmark of cellular wellness, and the negative regulation of oxygen homeostasis is able to affect the cells and tissues of the whole organism. The cellular response to hypoxia is characterized by the activation of multiple genes involved in many biological processes. Among them, hypoxia-inducible factor (HIF) represents the master regulator of the hypoxia response. The active heterodimeric complex HIF α/β, binding to hypoxia-responsive elements (HREs), determines the induction of at least 100 target genes to restore tissue homeostasis. A growing body of evidence demonstrates that hypoxia signaling can act by generating contrasting responses in cells and tissues. Here, this dual and controversial role of hypoxia and the HIF signaling pathway is discussed, with particular reference to the effects induced on the complex activities of the immune system and on mechanisms determining cell and tissue responses after an injury in both acute and chronic human diseases related to the heart, lung, liver, and kidney.

Asparagine Hydroxylation is a Reversible Post-translational Modification

Amino acid hydroxylation is a common post-translational modification, which generally regulates protein interactions or adds a functional group that can be further modified. Such hydroxylation is currently considered irreversible, necessitating the degradation and re-synthesis of the entire protein to reset the modification. Here we present evidence that the cellular machinery can reverse FIH-mediated asparagine hydroxylation on intact proteins. These data suggest that asparagine hydroxylation is a flexible and dynamic post-translational modification akin to modifications involved in regulating signaling networks, such as phosphorylation, methylation and ubiquitylation.

Stapled Peptides as HIF-1α/p300 Inhibitors: Helicity Enhancement in the Bound State Increases Inhibitory Potency

Protein-protein interactions (PPIs) control virtually all cellular processes and have thus emerged as potential targets for development of molecular therapeutics. Peptide-based inhibitors of PPIs are attractive given that they offer recognition potency and selectivity features that are ideal for function, yet, they do not predominantly populate the bioactive conformation, frequently suffer from poor cellular uptake and are easily degraded, for example, by proteases. The constraint of peptides in a bioactive conformation has emerged as a promising strategy to mitigate against these liabilities. In this work, using peptides derived from hypoxia-inducible factor 1 (HIF-1α) together with dibromomaleimide stapling, we identify constrained peptide inhibitors of the HIF-1α/p300 interaction that are more potent than their unconstrained sequences. Contrary to expectation, the increased potency does not correlate with an increased population of an α-helical conformation in the unbound state as demonstrated by experimental circular dichroism analysis. Rather, the ability of the peptide to adopt a bioactive α-helical conformation in the p300 bound state is better supported in the constrained variant as demonstrated by molecular dynamics simulations and circular dichroism difference spectra.

Hypoxia-driven epigenetic regulation in cancer progression: A focus on histone methylation and its modifying enzymes

The mechanism underlying hypoxia-driven chromatin remodeling is a long-lasting question. For the last two decades, this question has been resolved in part. It is now widely agreed that hypoxia dynamically changes the methylation status of histones to control gene expression. Hypoxia-inducible factor (HIF) plays a central role in cellular responses to hypoxia through transcriptional activation of numerous genes. At least in part, the hypoxic regulation of histone methylation is attributed to the HIF-mediated expression of histone modifying enzymes. Protein hydroxylation and histone demethylation have emerged as the oxygen sensing processes because they are catalyzed by a family of 2-oxoglutarate (2OG)-dependent dioxygenases whose activities depend upon the ambient oxygen level. Recently, it has been extensively investigated that the 2OG dioxygenases oxygen-dependently regulate histone methylation. Nowadays, the hypoxic change in the histone methylation status is regarded as an important event to drive malignant behaviors of cancer cells. In this review, we introduced and summarized the cellular processes that govern hypoxia-driven regulation of histone methylation in the context of cancer biology. We also discussed the emerging roles of histone methyltransferases and demethylases in epigenetic response to hypoxia.

Quantitative MS-Based Proteomics: Comparing the MCF-7 Cellular Response to Hypoxia and a 2-Oxoglutarate Analogue

The hypoxia-inducible factors (HIFs) are key transcription factors in determining cellular responses involving alterations in protein levels in response to limited oxygen availability in animal cells. 2-Oxoglutarate-dependent oxygenases play key roles in regulating levels of HIF and its transcriptional activity. We describe MS-based proteomics studies in which we compared the results of subjecting human breast cancer MCF-7 cells to hypoxia or treating them with a cell-penetrating derivative (dimethyl N-oxalylglycine; DMOG) of the stable 2OG analogue N-oxalylglycine. The proteomic results are consistent with reported transcriptomic analyses and support the proposed key roles of 2OG-dependent HIF prolyl- and asparaginyl-hydroxylases in the hypoxic response. Differences between the data sets for hypoxia and DMOG might reflect context-dependent effects or HIF-independent effects of DMOG.

HIV-1 Tat-mediated astrocytic amyloidosis involves the HIF-1α/lncRNA BACE1-AS axis

Increased life expectancy of patients diagnosed with HIV in the current era of antiretroviral therapy is unfortunately accompanied with the prevalence of HIV-associated neurocognitive disorders (HANDs) and risk of comorbidities such as Alzheimer-like pathology. HIV-1 transactivator of transcription (Tat) protein has been shown to induce the production of toxic neuronal amyloid protein and also enhance neurotoxicity. The contribution of astrocytes in Tat-mediated amyloidosis remains an enigma. We report here, in simian immunodeficiency virus (SIV)+ rhesus macaques and patients diagnosed with HIV, brain region-specific up-regulation of amyloid precursor protein (APP) and Aβ (40 and 42) in astrocytes. In addition, we find increased expression of β-site cleaving enzyme (BACE1), APP, and Aβ in human primary astrocytes (HPAs) exposed to Tat. Mechanisms involved up-regulation of hypoxia-inducible factor (HIF-1α), its translocation and binding to the long noncoding RNA (lncRNA) BACE1-antisense transcript (BACE1-AS), resulting, in turn, in the formation of the BACE1-AS/BACE1 RNA complex, subsequently leading to increased BACE1 protein, and activity and generation of Aβ-42. Gene silencing approaches confirmed the regulatory role of HIF-1α in BACE1-AS/BACE1 in Tat-mediated amyloidosis. This is the first report implicating the role of the HIF-1α/lncRNABACE1-AS/BACE1 axis in Tat-mediated induction of astrocytic amyloidosis, which could be targeted as adjunctive therapies for HAND-associated Alzheimer-like comorbidity.

Hypoxia: Turning vessels into vassals of cancer immunotolerance

Hypoxia is a universal feature of solid cancers caused by a mismatch between cellular oxygen supply and consumption. To meet the increased demand for oxygen, hypoxic cancer cells (CCs) induce a multifaceted process known as angiogenesis, wherein new vessels are formed by the sprouting of pre-existing ones. In addition to providing oxygen for growth and an exit route for dissemination, angiogenic vessels and factors are co-opted by CCs to enable the generation of an immunotolerant, hypoxic tumor microenvironment, leading to therapeutic failure and mortality. In this review, we discuss how hypoxia-inducible factors (HIFs), the mechanistic target of rapamycin (mTOR), and the unfolded protein response (UPR) control angiogenic factors serving both vascular and immunomodulatory functions in the tumor microenvironment. Possible therapeutic strategies, wherein targeting oxygen sensing might enhance anti-angiogenic and immunologically-mediated anti-cancer responses, are suggested.

CITED2 and the modulation of the hypoxic response in cancer

CITED2 (CBP/p300-interacting transactivator with Glu/Asp-rich C-terminal domain, 2) is a ubiquitously expressed protein exhibiting a high affinity for the CH1 domain of the transcriptional co-activators CBP/p300, for which it competes with hypoxia-inducible factors (HIFs). CITED2 is particularly efficient in the inhibition of HIF-1α-dependent transcription in different contexts, ranging from organ development and metabolic homeostasis to tissue regeneration and immunity, being also potentially involved in various other physiological processes. In addition, CITED2 plays an important role in inhibiting HIF in some diseases, including kidney and heart diseases and type 2-diabetes. In the particular case of cancer, CITED2 either functions by promoting or suppressing cancer development depending on the context and type of tumors. For instance, CITED2 overexpression promotes breast and prostate cancers, as well as acute myeloid leukemia, while its expression is downregulated to sustain colorectal cancer and hepatocellular carcinoma. In addition, the role of CITED2 in the maintenance of cancer stem cells reveals its potential as a target in non-small cell lung carcinoma and acute myeloid leukemia, for example. But besides the wide body of evidence linking both CITED2 and HIF signaling to carcinogenesis, little data is available regarding CITED2 role as a negative regulator of HIF-1α specifically in cancer. Therefore, comprehensive studies exploring further the interactions of these two important mediators in cancer-specific models are sorely needed and this can potentially lead to the development of novel targeted therapies.

Activation of TWIST Transcription by Chromatin Remodeling Protein BRG1 Contributes to Liver Fibrosis in Mice

Liver fibrosis is a complex pathophysiological process to which many different cell types contribute. Endothelial cells play versatile roles in the regulation of liver fibrosis. The underlying epigenetic mechanism is not fully appreciated. In the present study, we investigated the role of BRG1, a chromatin remodeling protein, in the modulation of endothelial cells in response to pro-fibrogenic stimuli in vitro and liver fibrosis in mice. We report that depletion of BRG1 by siRNA abrogated TGF-β or hypoxia induced down-regulation of endothelial marker genes and up-regulation of mesenchymal marker genes in cultured endothelial cells. Importantly, endothelial-specific BRG1 deletion attenuated CCl₄ induced liver fibrosis in mice. BRG1 knockdown in vitro or BRG1 knockout in vivo was accompanied by the down-regulation of TWIST, a key regulator of endothelial phenotype. Mechanistically, BRG1 interacted with and was recruited to the TWIST promoter by HIF-1α to activate TWIST transcription. BRG1 silencing rendered a more repressive chromatin structure surrounding the TWIST promoter likely contributing to TWIST down-regulation. Inhibition of HIF-1α activity dampened liver fibrosis in mice. Similarly, pharmaceutical inhibition of TWIST alleviated liver fibrosis in mice. In conclusion, our data suggest that epigenetic activation of TWIST by BRG1 contributes to the modulation of endothelial phenotype and liver fibrosis. Therefore, targeting the HIF1α-BRG1-TWIST axis may yield novel therapeutic solutions to treat liver fibrosis.

A human protein hydroxylase that accepts D-residues

Factor inhibiting hypoxia-inducible factor (FIH) is a 2-oxoglutarate-dependent protein hydroxylase that catalyses C3 hydroxylations of protein residues. We report FIH can accept (D)- and (L)-residues for hydroxylation. The substrate selectivity of FIH differs for (D) and (L) epimers, e.g., (D)- but not (L)-allylglycine, and conversely (L)- but not (D)-aspartate, undergo monohydroxylation, in the tested sequence context. The (L)-Leu-containing substrate undergoes FIH-catalysed monohydroxylation, whereas (D)-Leu unexpectedly undergoes dihydroxylation. Crystallographic, mass spectrometric, and DFT studies provide insights into the selectivity of FIH towards (L)- and (D)-residues. The results of this work expand the potential range of known substrates hydroxylated by isolated FIH and imply that it will be possible to generate FIH variants with altered selectivities.

Cellular adaptation to hypoxia through hypoxia inducible factors and beyond

Molecular oxygen (O2) sustains intracellular bioenergetics and is consumed by numerous biochemical reactions, making it essential for most species on Earth. Accordingly, decreased oxygen concentration (hypoxia) is a major stressor that generally subverts life of aerobic species and is a prominent feature of pathological states encountered in bacterial infection, inflammation, wounds, cardiovascular defects and cancer. Therefore, key adaptive mechanisms to cope with hypoxia have evolved in mammals. Systemically, these adaptations include increased ventilation, cardiac output, blood vessel growth and circulating red blood cell numbers. On a cellular level, ATP-consuming reactions are suppressed, and metabolism is altered until oxygen homeostasis is restored. A critical question is how mammalian cells sense oxygen levels to coordinate diverse biological outputs during hypoxia. The best-studied mechanism of response to hypoxia involves hypoxia inducible factors (HIFs), which are stabilized by low oxygen availability and control the expression of a multitude of genes, including those involved in cell survival, angiogenesis, glycolysis and invasion/metastasis. Importantly, changes in oxygen can also be sensed via other stress pathways as well as changes in metabolite levels and the generation of reactive oxygen species by mitochondria. Collectively, this leads to cellular adaptations of protein synthesis, energy metabolism, mitochondrial respiration, lipid and carbon metabolism as well as nutrient acquisition. These mechanisms are integral inputs into fine-tuning the responses to hypoxic stress.

Hypoxia and Innate Immunity: Keeping Up with the HIFsters

Recent years have witnessed an emergence of interest in understanding metabolic changes associated with immune responses, termed immunometabolism. As oxygen is central to all aerobic metabolism, hypoxia is now recognized to contribute fundamentally to inflammatory and immune responses. Studies from a number of groups have implicated a prominent role for oxygen metabolism and hypoxia in innate immunity of healthy tissue (physiologic hypoxia) and during active inflammation (inflammatory hypoxia). This inflammatory hypoxia emanates from a combination of recruited inflammatory cells (e.g., neutrophils, eosinophils, and monocytes), high rates of oxidative metabolism, and the activation of multiple oxygen-consuming enzymes during inflammation. These localized shifts toward hypoxia have identified a prominent role for the transcription factor hypoxia-inducible factor (HIF) in the regulation of innate immunity. Such studies have provided new and enlightening insight into our basic understanding of immune mechanisms, and extensions of these findings have identified potential therapeutic targets. In this review, we summarize recent literature around the topic of innate immunity and mucosal hypoxia with a focus on transcriptional responses mediated by HIF.

The Genomics and Genetics of Oxygen Homeostasis

Human survival is dependent upon the continuous delivery of O₂ to each cell in the body in sufficient amounts to meet metabolic requirements, primarily for ATP generation by oxidative phosphorylation. Hypoxia-inducible factors (HIFs) regulate the transcription of thousands of genes to balance O₂ supply and demand. The HIFs are negatively regulated by O₂-dependent hydrox-ylation and ubiquitination by prolyl hydroxylase domain (PHD) proteins and the von Hippel-Lindau (VHL) protein. Germline mutations in the genes encoding VHL, HIF-2α, and PHD2 cause hereditary erythrocytosis, which is characterized by polycythemia and pulmonary hypertension and is caused by increased HIF activity. Evolutionary adaptation to life at high altitude is associated with unique genetic variants in the genes encoding HIF-2α and PHD2 that blunt the erythropoietic and pulmonary vascular responses to hypoxia.