Protein Hydroxylation by Hypoxia-Inducible Factor (HIF) Hydroxylases: Unique or Ubiquitous?

Hypoxia-Inducible Factor (HIF) in Ischemic Stroke and Neurodegenerative Disease

Hypoxia is one of the most common pathological conditions, which can be induced by multiple events, including ischemic injury, trauma, inflammation, tumors, etc. The body's adaptation to hypoxia is a highly important phenomenon in both health and disease. Most cellular responses to hypoxia are associated with a family of transcription factors called hypoxia-inducible factors (HIFs), which induce the expression of a wide range of genes that help cells adapt to a hypoxic environment. Basic mechanisms of adaptation to hypoxia, and particularly HIF functions, have being extensively studied over recent decades, leading to the 2019 Nobel Prize in Physiology or Medicine. Based on their pivotal physiological importance, HIFs are attracting increasing attention as a new potential target for treating a large number of hypoxia-associated diseases. Most of the experimental work related to HIFs has focused on roles in the liver and kidney. However, increasing evidence clearly demonstrates that HIF-based responses represent an universal adaptation mechanism in all tissue types, including the central nervous system (CNS). In the CNS, HIFs are critically involved in the regulation of neurogenesis, nerve cell differentiation, and neuronal apoptosis. In this mini-review, we provide an overview of the complex role of HIF-1 in the adaptation of neurons and glia cells to hypoxia, with a focus on its potential involvement into various neuronal pathologies and on its possible role as a novel therapeutic target.

The iron chelator, PBT434, modulates transcellular iron trafficking in brain microvascular endothelial cells

Iron and other transition metals, such as copper and manganese, are essential for supporting brain function, yet over-accumulation is cytotoxic. This over-accumulation of metals, particularly iron, is common to several neurological disorders; these include Alzheimer’s disease, Parkinson’s disease, Friedrich’s ataxia and other disorders presenting with neurodegeneration and associated brain iron accumulation. The management of iron flux by the blood-brain barrier provides the first line of defense against the over-accumulation of iron in normal physiology and in these pathological conditions. In this study, we determined that the iron chelator PBT434, which is currently being developed for treatment of Parkinson’s disease and multiple system atrophy, modulates the uptake of iron by human brain microvascular endothelial cells (hBMVEC) by chelation of extracellular Fe²⁺. Treatment of hBMVEC with PBT434 results in an increase in the abundance of the transcripts for transferrin receptor (TfR) and ceruloplasmin (Cp). Western blot and ELISA analyses reveal a corresponding increase in the proteins as well. Within the cell, PBT434 increases the detectable level of chelatable, labile Fe²⁺; data indicate that this Fe²⁺ is released from ferritin. In addition, PBT434 potentiates iron efflux likely due to the increase in cytosolic ferrous iron, the substrate for the iron exporter, ferroportin. PBT434 equilibrates rapidly and bi-directionally across an hBMVEC blood-brain barrier. These results indicate that the PBT434-iron complex is not substrate for hBMVEC uptake and thus support a model in which PBT434 would chelate interstitial iron and inhibit re-uptake of iron by endothelial cells of the blood-brain barrier, as well as inhibit its uptake by the other cells of the neurovascular unit. Overall, this presents a novel and promising mechanism for therapeutic iron chelation.

Erythropoietin and derivatives: Potential beneficial effects on the brain

Erythropoietin (Epo), the main erythropoiesis-stimulating factor widely prescribed to overcome anemia, is also known nowadays for its cytoprotective action on non-hematopoietic tissues. In this context, Epo showed not only its ability to cross the blood-brain barrier, but also its expression in the brain of mammals. In clinical trials, recombinant Epo treatment has been shown to stimulate neurogenesis; improve cognition; and activate antiapoptotic, antioxidant, and anti-inflammatory signaling pathways. These mechanisms, proposed to characterize a neuroprotective property, opened new perspectives on the Epo pharmacological potencies. However, many questions arise about a possible physiological role of Epo in the central nervous system (CNS) and the factors or environmental conditions that induce its expression. Although Epo may be considered a strong candidate to be used against neuronal damage, long-term treatments, particularly when high Epo doses are needed, may induce thromboembolic complications associated with increases in hematocrit and blood viscosity. To avoid these adverse effects, different Epo analogs without erythropoietic activity but maintaining neuroprotection ability are currently being investigated. Carbamylated erythropoietin, as well as alternative molecules like Epo fusion proteins and partial peptides of Epo, seems to match this profile. This review will focus on the discussion of experimental evidence reported in recent years linking erythropoietin and CNS function through investigations aimed at finding benefits in the treatment of neurodegenerative diseases. In addition, it will review the proposed mechanisms for novel derivatives which may clarify and, eventually, improve the neuroprotective action of Epo.

Comparative Biology of Oxygen Sensing in Plants and Animals

Aerobic respiration is essential to almost all eukaryotes and sensing oxygen is a key determinant of survival. Analogous but mechanistically different oxygen-sensing pathways were adopted in plants and metazoan animals, and include ubiquitin-mediated degradation of transcription factors and direct sensing via non-heme iron(Fe²⁺)-dependent-dioxygenases. Key roles for oxygen sensing have been identified in both groups, with downstream signalling focussed on regulating gene transcription and chromatin modification to control development and stress responses. Components of sensing systems are promising targets for human therapeutic intervention and developing stress-resilient crops. Here, we review current knowledge about the origins, commonalities and differences between oxygen sensing in plants and animals.

Regulatory Mechanisms of LncRNAs in Cancer Glycolysis: Facts and Perspectives

Cancer cells exhibit distinct metabolic characteristics that employ glycolysis to provide energy and intermediary metabolites. This aberrant metabolic phenotype favors cancer progression. LncRNAs are transcripts longer than 200 nucleotides that do not encode proteins. LncRNAs contribute to cancer progression and therapeutic resistance and affect aerobic glycolysis via multiple mechanisms, including modulating glycolytic transporters and enzymes. Further, dysregulated signaling pathways are vital for glycolysis. In this review, we highlight regulatory mechanisms for lncRNAs in aerobic glycolysis that provide novel insights into cancer development. Moreover, a comprehensive understanding of the regulatory mechanisms of lncRNAs in aerobic glycolysis can provide new strategies for clinical cancer management.

Stresses in the metastatic cascade: molecular mechanisms and therapeutic opportunities

In this review, Shen and Kang provide an overview of the tumor-intrinsic and microenvironment- and treatment-induced stresses that tumor cells encounter in the metastatic cascade and the molecular pathways they develop to relieve these stresses.

Metastasis is the ultimate “survival of the fittest” test for cancer cells, as only a small fraction of disseminated tumor cells can overcome the numerous hurdles they encounter during the transition from the site of origin to a distinctly different distant organ in the face of immune and therapeutic attacks and various other stresses. During cancer progression, tumor cells develop a variety of mechanisms to cope with the stresses they encounter, and acquire the ability to form metastases. Restraining these stress-releasing pathways could serve as potentially effective strategies to prevent or reduce metastasis and improve the survival of cancer patients. Here, we provide an overview of the tumor-intrinsic, microenvironment- and treatment-induced stresses that tumor cells encounter in the metastatic cascade and the molecular pathways they develop to relieve these stresses. We also summarize the preclinical and clinical studies that evaluate the potential therapeutic benefit of targeting these stress-relieving pathways.

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.

Inhibition of HIF-prolyl hydroxylases improves healing of intestinal anastomoses

Anastomotic leakage (AL) accounts for a major part of in-house mortality in patients undergoing colorectal surgery. Local ischemia and abdominal sepsis are common risk factors contributing to AL and are characterized by upregulation of the hypoxia-inducible factor (HIF) pathway. The HIF pathway is critically regulated by HIF-prolyl hydroxylases (PHDs). Here, we investigated the significance of PHDs and the effects of pharmacologic PHD inhibition (PHI) during anastomotic healing. Ischemic or septic colonic anastomoses were created in mice by ligation of mesenteric vessels or lipopolysaccharide-induced abdominal sepsis, respectively. Genetic PHD deficiency (Phd1-/-, Phd2+/-, and Phd3-/-) or PHI were applied to manipulate PHD activity. Pharmacologic PHI and genetic PHD2 haplodeficiency (Phd2+/-) significantly improved healing of ischemic or septic colonic anastomoses, as indicated by increased bursting pressure and reduced AL rates. Only Phd2+/- (but not PHI or Phd1-/-) protected from sepsis-related mortality. Mechanistically, PHI and Phd2+/- induced immunomodulatory (M2) polarization of macrophages, resulting in increased collagen content and attenuated inflammation-driven immune cell recruitment. We conclude that PHI improves healing of colonic anastomoses in ischemic or septic conditions by Phd2+/--mediated M2 polarization of macrophages, conferring a favorable microenvironment for anastomotic healing. Patients with critically perfused colorectal anastomosis or abdominal sepsis could benefit from pharmacologic PHI.

Remodeling of Cancer-Specific Metabolism under Hypoxia with Lactate Calcium Salt in Human Colorectal Cancer Cells

Hypoxic cancer cells meet their growing energy requirements by upregulating glycolysis, resulting in increased glucose consumption and lactate production. Herein, we used a unique approach to change in anaerobic glycolysis of cancer cells by lactate calcium salt (CaLac). Human colorectal cancer (CRC) cells were used for the study. Intracellular calcium and lactate influx was confirmed following 2.5 mM CaLac treatment. The enzymatic activation of lactate dehydrogenase B (LDHB) and pyruvate dehydrogenase (PDH) through substrate reaction of CaLac was investigated. Changes in the intermediates of the tricarboxylic acid (TCA) cycle were confirmed. The cell viability assay, tube formation, and wound-healing assay were performed as well as the confirmation of the expression of hypoxia-inducible factor (HIF)-1α and vascular endothelial growth factor (VEGF). In vivo antitumor effects were evaluated using heterotopic and metastatic xenograft animal models with 20 mg/kg CaLac administration. Intracellular calcium and lactate levels were increased following CaLac treatment in CRC cells under hypoxia. Then, enzymatic activation of LDHB and PDH were increased. Upon PDH knockdown, α-ketoglutarate levels were similar between CaLac-treated and untreated cells, indicating that TCA cycle restoration was dependent on CaLac-mediated LDHB and PDH reactivation. CaLac-mediated remodeling of cancer-specific anaerobic glycolysis induced destabilization of HIF-1α and a decrease in VEGF expression, leading to the inhibition of the migration of CRC cells. The significant inhibition of CRC growth and liver metastasis by CaLac administration was confirmed. Our study highlights the potential utility of CaLac supplementation in CRC patients who display reduced therapeutic responses to conventional modes owing to the hypoxic tumor microenvironment.

Remodeling of Cancer-Specific Metabolism under Hypoxia with Lactate Calcium Salt in Human Colorectal Cancer Cells

Simple Summary

This study was to prove the changes in cancer-specific metabolism caused by the introduction of lactate calcium salt into human colorectal cancer cells from the viewpoint of remodeling in anaerobic glycolysis and the tricarboxylic acid cycle under hypoxia. An influx of lactate calcium salt-induced enzymatic activation of lactate dehydrogenase B reacting to lactate followed by the decrease in the transcriptional activation of hypoxia-inducible factor-1α to suppress the expression of the oncogenes. Thereby, it was possible to induce anti-cancer effects on the colorectal cancer xenograft animal model.

Abstract

Hypoxic cancer cells meet their growing energy requirements by upregulating glycolysis, resulting in increased glucose consumption and lactate production. Herein, we used a unique approach to change in anaerobic glycolysis of cancer cells by lactate calcium salt (CaLac). Human colorectal cancer (CRC) cells were used for the study. Intracellular calcium and lactate influx was confirmed following 2.5 mM CaLac treatment. The enzymatic activation of lactate dehydrogenase B (LDHB) and pyruvate dehydrogenase (PDH) through substrate reaction of CaLac was investigated. Changes in the intermediates of the tricarboxylic acid (TCA) cycle were confirmed. The cell viability assay, tube formation, and wound-healing assay were performed as well as the confirmation of the expression of hypoxia-inducible factor (HIF)-1α and vascular endothelial growth factor (VEGF). In vivo antitumor effects were evaluated using heterotopic and metastatic xenograft animal models with 20 mg/kg CaLac administration. Intracellular calcium and lactate levels were increased following CaLac treatment in CRC cells under hypoxia. Then, enzymatic activation of LDHB and PDH were increased. Upon PDH knockdown, α-ketoglutarate levels were similar between CaLac-treated and untreated cells, indicating that TCA cycle restoration was dependent on CaLac-mediated LDHB and PDH reactivation. CaLac-mediated remodeling of cancer-specific anaerobic glycolysis induced destabilization of HIF-1α and a decrease in VEGF expression, leading to the inhibition of the migration of CRC cells. The significant inhibition of CRC growth and liver metastasis by CaLac administration was confirmed. Our study highlights the potential utility of CaLac supplementation in CRC patients who display reduced therapeutic responses to conventional modes owing to the hypoxic tumor microenvironment.

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.

Gene transcription and chromatin regulation in hypoxia

Oxygen sensing is an essential feature of metazoan biology and reductions in oxygen availability (hypoxia) have both physiological and pathophysiological implications. Co-ordinated mechanisms have evolved for sensing and responding to hypoxia, which involve diverse biological outputs, with the main aim of restoring oxygen homeostasis. This includes a dynamic gene transcriptional response, the central drivers of which are the hypoxia-inducible factor (HIF) family of transcription factors. HIFs are regulated in an oxygen-dependent manner and while their role in hypoxia is well established, it is apparent that other key players are required for gene expression control in hypoxia. In this review, we highlight the current understanding of the known and potential molecular mechanisms underpinning gene transcriptional responses to hypoxia in mammals, with a focus on oxygen-dependent effects on chromatin structure.

Vitamin C-Sources, Physiological Role, Kinetics, Deficiency, Use, Toxicity, and Determination

Vitamin C (L-ascorbic acid) has been known as an antioxidant for most people. However, its physiological role is much larger and encompasses very different processes ranging from facilitation of iron absorption through involvement in hormones and carnitine synthesis for important roles in epigenetic processes. Contrarily, high doses act as a pro-oxidant than an anti-oxidant. This may also be the reason why plasma levels are meticulously regulated on the level of absorption and excretion in the kidney. Interestingly, most cells contain vitamin C in millimolar concentrations, which is much higher than its plasma concentrations, and compared to other vitamins. The role of vitamin C is well demonstrated by miscellaneous symptoms of its absence-scurvy. The only clinically well-documented indication for vitamin C is scurvy. The effects of vitamin C administration on cancer, cardiovascular diseases, and infections are rather minor or even debatable in the general population. Vitamin C is relatively safe, but caution should be given to the administration of high doses, which can cause overt side effects in some susceptible patients (e.g., oxalate renal stones). Lastly, analytical methods for its determination with advantages and pitfalls are also discussed in this review.

Development of a colorimetric α-ketoglutarate detection assay for prolyl hydroxylase domain (PHD) proteins

Since the discovery of the prolyl hydroxylases domain (PHD) proteins and their canonical hypoxia-inducible factor (HIF) substrate two decades ago, a number of in vitro hydroxylation (IVH) assays for PHD activity have been developed to measure the PHD-HIF interaction. However, most of these assays either require complex proteomics mass spectrometry methods that rely on the specific PHD-HIF interaction or require the handling of radioactive material, as seen in the most commonly used assay measuring [¹⁴C]O₂ release from labeled [¹⁴C]α-ketoglutarate. Here, we report an alternative rapid, cost-effective assay in which the consumption of α-ketoglutarate is monitored by its derivatization with 2,4-dinitrophenylhydrazine (2,4-DNPH) followed by treatment with concentrated base. We extensively optimized this 2,4-DNPH α-ketoglutarate assay to maximize the signal-to-noise ratio and demonstrated that it is robust enough to obtain kinetic parameters of the well-characterized PHD2 isoform comparable with those in published literature. We further showed that it is also sensitive enough to detect and measure the IC₅₀ values of pan-PHD inhibitors and several PHD2 inhibitors in clinical trials for chronic kidney disease (CKD)-induced anemia. Given the efficiency of this assay coupled with its multiwell format, the 2,4-DNPH α-KG assay may be adaptable to explore non-HIF substrates of PHDs and potentially to high-throughput assays.

Hypoxia-inducible factor (HIF) inhibitors: a patent survey (2016-2020)

Introduction: Hypoxia-inducible factor (HIF) is a master regulator of oxygen homeostasis. The increased expression of genes targeted by HIF is associated with many human diseases, including ischemic cardiovascular disease, stroke, chronic lung disease, and cancer.Areas covered: This patent survey summarizes the information about patented HIF inhibitors over the last 5 years.Expert opinion: HIF inhibitors have shown promise for the treatment of hypoxic pulmonary hypertension, a circadian rhythm disorder, calcific aortic valve disease, cerebrovascular accident, and heterotopic ossification. In addition, HIF-2α inhibitors can be used for the treatment or prevention of iron overload disorders, Crohn's disease, ulcerative colitis, and thyroid eye disease, or to improve muscle generation and repair. PT2385 completed phase I clinical trials for the treatment of clear cell renal cell carcinoma. It exerted a higher synergistic inhibitory effect on tumor growth in combination with anti-PD-1 antibody, in comparison with each treatment alone, indicating that effective immunotherapy for solid tumors counteracts of the immunosuppression induced by hypoxia. Therefore, considering the effects of hypoxia on cancer cells, stromal cells, and effector immune cells, it is important to develop inhibitors of molecular pathways activated by hypoxia for successful treatments.

The Landscape of Interactions between Hypoxia-Inducible Factors and Reactive Oxygen Species in the Gastrointestinal Tract

The gastrointestinal tract (GT) is the major organ involved in digestion, absorption, and immunity, which is prone to oxidative destruction by high levels of reactive oxygen species (ROS) from luminal oxidants, such as food, drugs, and pathogens. Excessive ROS will lead to oxidative stresses and disrupt essential biomolecules, which also act as cellular signaling molecules in response to growth factors, hormones, and oxygen tension changes. Hypoxia-inducible factors (HIFs) are critical regulators mediating responses to cellular oxygen tension changes, which are also involved in energy metabolism, immunity, renewal, and microbial homeostasis in the GT. This review discusses interactions between HIF (mainly HIF-1α) and ROS and relevant diseases in the GT combined with our lab's work. It might help to develop new therapies for gastrointestinal diseases associated with ROS and HIF-1α.

Evolution and Adaptation of Legionella pneumophila to Manipulate the Ubiquitination Machinery of Its Amoebae and Mammalian Hosts

The ubiquitin pathway is highly conserved across the eukaryotic domain of life and plays an essential role in a plethora of cellular processes. It is not surprising that many intracellular bacterial pathogens often target the essential host ubiquitin pathway. The intracellular bacterial pathogen Legionella pneumophila injects into the host cell cytosol multiple classes of classical and novel ubiquitin-modifying enzymes that modulate diverse ubiquitin-related processes in the host cell. Most of these pathogen-injected proteins, designated as effectors, mimic known E3-ubiquitin ligases through harboring F-box or U-box domains. The classical F-box effector, AnkB targets host proteins for K⁴⁸-linked polyubiquitination, which leads to excessive proteasomal degradation that is required to generate adequate supplies of amino acids for metabolism of the pathogen. In contrast, the SidC and SdcA effectors share no structural similarity to known eukaryotic ligases despite having E3-ubiquitin ligase activity, suggesting that the number of E3-ligases in eukaryotes is under-represented. L. pneumophila also injects into the host many novel ubiquitin-modifying enzymes, which are the SidE family of effectors that catalyze phosphoribosyl-ubiquitination of serine residue of target proteins, independently of the canonical E1-2-3 enzymatic cascade. Interestingly, the environmental bacterium, L. pneumophila, has evolved within a diverse range of amoebal species, which serve as the natural hosts, while accidental transmission through contaminated aerosols can cause pneumonia in humans. Therefore, it is likely that the novel ubiquitin-modifying enzymes of L. pneumophila were acquired by the pathogen through interkingdom gene transfer from the diverse natural amoebal hosts. Furthermore, conservation of the ubiquitin pathway across eukaryotes has enabled these novel ubiquitin-modifying enzymes to function similarly in mammalian cells. Studies on the biological functions of these effectors are likely to reveal further novel ubiquitin biology and shed further lights on the evolution of ubiquitin.

The protein tyrosine phosphatase PTP-PEST mediates hypoxia-induced endothelial autophagy and angiogenesis via AMPK activation

Global and endothelial loss of PTP-PEST (also known as PTPN12) is associated with impaired cardiovascular development and embryonic lethality. Although hypoxia is implicated in vascular remodelling and angiogenesis, its effect on PTP-PEST remains unexplored. Here we report that hypoxia (1% oxygen) increases protein levels and catalytic activity of PTP-PEST in primary endothelial cells. Immunoprecipitation followed by mass spectrometry revealed that α subunits of AMPK (α1 and α2, encoded by PRKAA1 and PRKAA2, respectively) interact with PTP-PEST under normoxia but not in hypoxia. Co-immunoprecipitation experiments confirmed this observation and determined that AMPK α subunits interact with the catalytic domain of PTP-PEST. Knockdown of PTP-PEST abrogated hypoxia-mediated tyrosine dephosphorylation and activation of AMPK (Thr172 phosphorylation). Absence of PTP-PEST also blocked hypoxia-induced autophagy (LC3 degradation and puncta formation), which was rescued by the AMPK activator metformin (500 µM). Because endothelial autophagy is a prerequisite for angiogenesis, knockdown of PTP-PEST also attenuated endothelial cell migration and capillary tube formation, with autophagy inducer rapamycin (200 nM) rescuing angiogenesis. In conclusion, this work identifies for the first time that PTP-PEST is a regulator of hypoxia-induced AMPK activation and endothelial autophagy to promote angiogenesis.

Abscisic Acid as an Emerging Modulator of the Responses of Plants to Low Oxygen Conditions

Different environmental and developmental cues involve low oxygen conditions, particularly those associated to abiotic stress conditions. It is widely accepted that plant responses to low oxygen conditions are mainly regulated by ethylene (ET). However, interaction with other hormonal signaling pathways as gibberellins (GAs), auxin (IAA), or nitric oxide (NO) has been well-documented. In this network of interactions, abscisic acid (ABA) has always been present and regarded to as a negative regulator of the development of morphological adaptations to soil flooding: hyponastic growth, adventitious root emergence, or formation of secondary aerenchyma in different plant species. However, recent evidence points toward a positive role of this plant hormone on the modulation of plant responses to hypoxia and, more importantly, on the ability to recover during the post-hypoxic period. In this work, the involvement of ABA as an emerging regulator of plant responses to low oxygen conditions alone or in interaction with other hormones is reviewed and discussed.

A distinctive distribution of hypoxia-inducible factor-1α in cultured renal tubular cells with hypoperfusion simulated by coverslip placement

Chronic hypoxia in the renal tubulointerstitium plays a key role in the progression of chronic kidney disease (CKD). It is therefore important to investigate tubular hypoxia and the activity of hypoxia-inducible factor (HIF)-1α in response to hypoxia. Rarefaction of the peritubular capillary causes hypoperfusion in CKD; however, the effect of hypoperfusion on HIFs has rarely been investigated. We induced hypoperfusion caused by coverslip placement in human kidney-2 cells, and observed an oxygen gradient under the coverslip. Immunocytochemistry of HIF-1α showed a doughnut-shaped formation on the edge of a pimonidazole-positive area, which we named the "HIF-ring". The oxygen tension of the HIF-ring was estimated to be between approximately 4 mmHg and 20 mmHg. This result was not compatible with those of past research showing HIF-1α accumulation in the anoxic range with homogeneous oxygen tension. We further observed the presence of a pH gradient under a coverslip, as well as a shift of the HIF ring due to changes in the pH of the culture medium, suggesting that the HIF ring was formed by suppression of HIF-1α related to low pH. This research demonstrated that HIF-1α activation mimics the physiological state in cultured cells with hypoperfusion.

Acriflavine, a Potent Inhibitor of HIF-1α, Disturbs Glucose Metabolism and Suppresses ATF4-Protective Pathways in Melanoma under Non-Hypoxic Conditions

Simple Summary

Hypoxia is a common feature in solid tumors such as melanoma, contributing locally and systemically to tumor progression. Although the hypoxia response in tumor cells is well understood, the role of constitutively activated hypoxia-inducible factor (HIF)-1α in normoxic conditions is less known. Here, we used acriflavine, a chemical inhibitor of HIF-1α, to investigate the role of this transcription factor on the progression of melanoma under normoxic conditions. The data indicated that acriflavine disturbs glucose metabolism and induces melanoma cell death under normoxia. As a result, we describe a possible clinical option that may target melanoma cells irrespective of the hypoxic microenvironment of the tumors. However, the translational importance of these findings should be confirmed in pre-clinical models.

Abstract

Hypoxia-inducible factor (HIF)-1α is constitutively expressed in melanoma cells under normoxic conditions and its elevated expression correlates with the aggressiveness of melanoma tumors. Here, we used acriflavine, a potent inhibitor of HIF-1α dimerization, as a tool to investigate whether HIF-1α-regulated pathways contribute to the growth of melanoma cells under normoxia. We observed that acriflavine differentially modulated HIF-1α-regulated targets in melanoma under normoxic conditions, although acriflavine treatment resulted in over-expression of vascular endothelial growth factor (VEGF), its action clearly downregulated the expression of pyruvate dehydrogenase kinase 1 (PDK1), a well-known target of HIF-1α. Consequently, downregulation of PDK1 by acrifavine resulted in reduced glucose availability and suppression of the Warburg effect in melanoma cells. In addition, by inhibiting the AKT and RSK2 phosphorylation, acriflavine also avoided protective pathways necessary for survival under conditions of oxidative stress. Interestingly, we show that acriflavine targets activating transcription factor 4 (ATF4) for proteasomal degradation while suppressing the expression of microphthalmia-associated transcription factor (MITF), a master regulator of melanocyte development and a melanoma oncogene. Since acriflavine treatment results in the consistent death of melanoma cells, our results suggest that inhibition of HIF-1α function in melanoma could open new avenues for the treatment of this deadly disease regardless of the hypoxic condition of the tumor.

Targeting H3K9 methyltransferase G9a and its related molecule GLP as a potential therapeutic strategy for cancer

H3K9 methyltransferase (G9a) and its relevant molecule GLP are the SET domain proteins that specifically add mono, di and trimethyl groups on to the histone H3K9, which lead to the transcriptional inactivation of chromatin and reduce the expression of cancer suppressor genes, which trigger growth and progress of several cancer types. Various studies have demonstrated that overexpression of H3K9 methyltransferase G9a and GLP in different kinds of tumors, like lung, breast, bladder, colon, cervical, gastric, skin cancers, hepatocellular carcinoma and hematological malignancies. Several G9a and GLP inhibitors such as BIX-01294, UNC0642, A-366 and DCG066 were developed to combat various cancers; however, there is a need for more effective and less toxic compounds. The current molecular docking study suggested that the selected new compounds such as ninhydrin, naphthoquinone, cysteamine and disulfide cysteamine could be suitable molecules as a G9a and GLP inhibitors. Furthermore, detailed cell based and preclinical animal studies are required to confirm their properties. In the current review, we discussed the role of G9a and GLP mediated epigenetic regulation in the cancers. A thorough literature review was done related to G9a and GLP. The databases used extensively for retrieval of information were PubMed, Medline, Scopus and Science-direct. Further, molecular docking was performed using Maestro Schrodinger version 9.2 software to investigate the binding profile of compounds with Human G9a HMT (PDB ID: 3FPD, 3RJW) and Human GLP MT (PDB ID: 6MBO, 6MBP).

The Relationship Between Reactive Oxygen Species and Endothelial Cell Metabolism

Cardiovascular disease (CVD) has been the leading cause of death for many decades, highlighting the importance of new research and treatments in the field. The role of hypoxia and subsequent free radical production [reactive oxygen species (ROS)] have become an area of particular interest in CVD. Interestingly, our laboratory and other laboratories have recently reported positive roles of subcellular ROS in modulating endothelial cell (EC) metabolism, proliferation, and angiogenesis. This bidirectional relationship between ROS and EC metabolism, as well as functional changes, continues to be an area of active research. Interestingly, ECs have been shown to rely on anaerobic processes for ATP generation, despite their direct access to oxygen. This paradox has proven to be beneficial as the major reliance on glycolysis produces ATP faster, preserves oxygen, and results in reduced ROS levels in contrast to oxidative phosphorylation. This review will address the relationship between ROS and carbohydrate, lipid, and nitrogen metabolism in ECs, and their effects on EC phenotype such as sprouting angiogenesis.

Characterizing Ischaemic Tolerance in Rat Pheochromocytoma (PC12) Cells and Primary Rat Neurons

Preconditioning tissue with sublethal ischaemia or hypoxia can confer tolerance (protection) against subsequent ischaemic challenge. In vitro ischaemic preconditioning (IPC) is typically achieved through oxygen glucose deprivation (OGD), whereas hypoxic preconditioning (HPC) involves oxygen deprivation (OD) alone. Here, we report the effects of preconditioning of OGD, OD or glucose deprivation (GD) in ischaemic tolerance models with PC12 cells and primary rat neurons. PC12 cells preconditioned (4 h) with GD or OGD, but not OD, prior to reperfusion (24 h) then ischaemic challenge (OGD 6 h), showed greater mitochondrial activity, reduced cytotoxicity and decreased apoptosis, compared to sham preconditioned PC12 cells. Furthermore, 4 h preconditioning with reduced glucose (0.565 g/L, reduced from 4.5 g/L) conferred protective effects, but not for higher concentrations (1.125 or 2.25 g/L). Preconditioning (4 h) with OGD, but not OD or GD, induced stabilization of hypoxia inducible factor 1α (HIF1α) and upregulation of HIF1 downstream genes (Vegf, Glut1, Pfkfb3 and Ldha). In primary rat neurons, only OGD preconditioning (4 h) conferred neuroprotection. OGD preconditioning (4 h) induced stabilization of HIF1α and upregulation of HIF1 downstream genes (Vegf, Phd2 and Bnip3). In conclusion, OGD preconditioning (4 h) followed by 24 h reperfusion induced ischaemic tolerance (against OGD, 6 h) in both PC12 cells and primary rat neurons. The OGD preconditioning protection is associated with HIF1α stabilization and upregulation of HIF1 downstream gene expression. GD preconditioning (4 h) leads to protection in PC12 cells, but not in neurons. This GD preconditioning-induced protection was not associated with HIF1α stabilization.

Impact of inflammation and immunotherapy in renal cell carcinoma

Substantial research attention has been directed at exploring the mechanisms and treatment of renal cell carcinoma (RCC). Indeed, the association between inflammation and tumor phenotypes has been at the center of cancer research. Concomitant with research on the inflammation response and inflammatory molecules involved in RCC, new breakthroughs have emerged. A large body of knowledge now shows that treatments targeting inflammation and immunity in RCC provide substantial clinical benefits. Adequate analysis and a better understanding of the mechanisms of inflammatory factors in the occurrence and progression of RCC are highly desirable. Currently, numerous RCC treatments targeted at inflammation and immunotherapy are available. The current review describes in detail the link between inflammation and RCC.

Transcription factor-mediated signaling pathways' contribution to the pathology of acute lung injury and acute respiratory distress syndrome

The 2019 novel coronavirus (2019-nCoV) is still spreading rapidly around the world, and one cause of lethality for patients infected with 2019-nCoV is acute respiratory distress syndrome (ARDS). ARDS is a severe syndrome of acute lung injury (ALI) that is predominantly triggered by inflammation and results in a sudden loss of, or damage to, kidney function. Emerging studies reveal that multiple transcription factor-associated signaling pathways are activated in the pathology of ALI/ARDS. Of these pathways, the activation of NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells), AP-1 (activator protein 1), IRFs (interferon regulatory factors), STATs (signal transducer and activator of transcription), Wnt/β-catenin-TCF/LEF (T-cell factor/lymphoid enhancer-binding factor), and CtBP2 (C-Terminal binding protein 2)-associated transcriptional complex contributes to ALI/ARDS pathology through diverse mechanisms, such as inducing proinflammatory cytokine levels and mediating macrophage polarization. In this review, we present an updated summary of the mechanisms underlying these signaling activations and regulations, as well as their contribution to the pathogenesis of ALI/ARDS. We aim to develop a better understanding of how ALI/ARDS occurs and improve ALI/ARDS therapy.

Novel Insights into Beta 2 Adrenergic Receptor Function in the rd10 Model of Retinitis Pigmentosa

Background: In retinitis pigmentosa (RP), inherited rod death is followed by cone loss and blindness. Why cones die is still a matter of consideration. Here, we investigate the pathogenic role of the sympathetic transmission in the rd10 mouse model of RP. Methods: Retinal levels of beta adrenergic receptor (BAR) 2 and norepinephrine (NE) were measured. After administration of the BAR1/2 blocker propranolol or the hypoxia-inducible factor (HIF)-1 activator dimethyloxalylglycine (DMOG), retinal levels of HIF-1α, BAR2 or proteins involved in BAR2 desensitization were also measured. In DMOG treated mice, expression and localization of BAR2, inflammatory markers and cone arrestin were determined. Finally, rd10 mice were subjected to electroretinogram (ERG) analysis to assess rod and cone function. Results: In the rd10 retina, BAR2 overexpression and NE accumulation were found, with BAR2 immunoreactivity localized to Müller cells. BAR2 overexpression was likely due to desensitization defects. Upregulated levels of BAR2 were drastically reduced by propranolol that also restored desensitization defects. Due to the low level of HIF-1 consequent to the hyperoxic environment in the rd10 retina, we hypothesized a link between HIF-1 and BAR2. HIF-1α stabilization with DMOG resulted in i. increased HIF-1α accumulation, ii. decreased BAR2 levels, iii. restored desensitization processes, iv. reduced expression of inflammatory markers and v. increased cone survival without improved retinal function. Conclusions: Our results support a pathogenic role of the sympathetic system in RP that might help to understand why rd10 mice show a positive response to BAR blockers.

Hypoxic gastric cancer-derived exosomes promote progression and metastasis via MiR-301a-3p/PHD3/HIF-1α positive feedback loop

Hypoxic tumor microenvironment(TME) is a universal feature in solid carcinoma and is associated with unfavorable prognosis. Tumor-derived exosomes are now significantly implicating in mediating cellular communication and interactions in TME. The aim of this study was to identify exosomal miR-301a-3p involved in gastric cancer(GC) progression and metastasis. Here, we found hypoxia promote GC exosomes release and miR-301a-3p expression in an HIF-1α-dependent manner. In hypoxic TME, enriched miR-301a-3p could be transmitted between GC cells via exosomes and then contributed to inhibit HIF-1α degradation through targeting PHD3, that were capable to hydroxylate HIF-1α subunits to ubiquitinate degradation. This synergistical positive feedback loop between HIF-1α and miR-301a-3p facilitated GC proliferation, invasion, migration, and epithelial-mesenchymal transition. In clinical samples, we further discovered circulating exosomal miR-301a-3p in serum was positively related with peritoneal metastasis. Collectively, these data indicate that GC cells could generate miR-301a-3p-rich exosomes in the hypoxic TME, which then help to HIF-1α accumulation and promote GC malignant behaviors and metastasis. Exosomal miR-301a-3p/HIF-1α signaling axis may serve as a promising predictor and potential therapeutic target of GC with metastasis.

Loss of miR-145-5p Causes Ceruloplasmin Interference with PHD-Iron Axis and HIF-2α Stabilization in Lung Adenocarcinoma-Mediated Angiogenesis

For decades, lung cancer has been the leading cause of cancer-related death worldwide. Hypoxia-inducible factors (HIFs) play critical roles in mediating lung cancer development and metastasis. The present study aims to clarify how HIF’s over-activation affects lung cancer angiogenesis not only in a normoxic condition, but also a hypoxic niche. Our study shows that human lung cancer exhibits elevated levels of ceruloplasmin (CP), which has a negative impact on the prognosis of patients. CP affects the cellular Fe²⁺ level, which inactivates prolyl hydroxylase (PHD) 1 and 2, resulting in HIF-2α enhancement. Increased HIF-2α leads to vascular endothelial growth factor-A (VEGF-A) secretion and angiogenesis. The expression of CP is under the epigenetic control of miR-145-5p. Restoration of miR-145-5p by miRNA mimics transfection decreases CP expression, increases Fe²⁺ and PHD1/2 levels and HIF hydroxylation while reduced HIF-2α levels resulting in the inhibition of tumor angiogenesis. In contrast, inhibition of miR-145-5p by miRNA inhibitors increases the expression of CP and VEGF-A in lung cancer cells. Significantly, miR-145-5p expression is lost in the tumor samples of lung cancer patients, and low miR-145-5p expression is strongly correlated with a shorter overall survival time. In conclusion, the current study reveals the clinical importance and prognostic value of miR-145-5p and CP. It identifies a unique mechanism of HIF-2α over-activation, which is mediated by iron imbalance of the iron-PHD coupling that modulates tumor angiogenesis.

Molecular regulation and function of FoxO3 in chronic kidney disease

PURPOSE OF REVIEW

FOXOs are transcription factors that regulate downstream target genes to counteract to cell stress. Here we review the function and regulation of FOXO transcription factors, the mechanism of FOXO3 activation in the kidney, and the role of FOXO3 in delaying the development of chronic kidney disease (CKD).

RECENT FINDINGS

Progressive renal hypoxia from vascular dropout and metabolic perturbation is a pathogenic factor for the initiation and development of CKD. Hypoxia and low levels of α-ketoglutarate generated from the TCA cycle inhibit prolyl hydroxylase domain (PHD)-mediated prolyl hydroxylation of FoxO3, thus reducing FoxO3 protein degradation via the ubiquitin proteasomal pathway, similar to HIF stabilization under hypoxic conditions. FoxO3 accumulation and nuclear translocation activate two key cellular defense mechanisms, autophagy and antioxidative response in renal tubular cells, to reduce cell injury and promote cell survival. FoxO3 directly activates the expression of Atg proteins, which replenishes core components of the autophagic machinery to allow sustained autophagy in the chronically hypoxic kidney. FoxO3 protects mitochondria by stimulating the expression of superoxide dismutase 2 (SOD2), as tubular deletion of FoxO3 in mice results in reduced SOD2 levels and profound mitochondrial damage.

SUMMARY

Knowledge gained from animal studies may help understand the function of stress responsive transcription factors that could be targeted to prevent or treat CKD.

Inhibition of prolyl hydroxylases increases hepatic insulin and decreases glucagon sensitivity by an HIF-2α-dependent mechanism

Objective

Recent evidence indicates that inhibition of prolyl hydroxylase domain (PHD) proteins can exert beneficial effects to improve metabolic abnormalities in mice and humans. However, the underlying mechanisms are not clearly understood. This study was designed to address this question.

Methods

A pan-PHD inhibitor compound was injected into WT and liver-specific hypoxia-inducible factor (HIF)-2α KO mice, after onset of obesity and glucose intolerance, and changes in glucose and glucagon tolerance were measured. Tissue-specific changes in basal glucose flux and insulin sensitivity were also measured by hyperinsulinemic euglycemic clamp studies. Molecular and cellular mechanisms were assessed in normal and type 2 diabetic human hepatocytes, as well as in mouse hepatocytes.

Results

Administration of a PHD inhibitor compound (PHDi) after the onset of obesity and insulin resistance improved glycemic control by increasing insulin and decreasing glucagon sensitivity in mice, independent of body weight change. Hyperinsulinemic euglycemic clamp studies revealed that these effects of PHDi treatment were mainly due to decreased basal hepatic glucose output and increased liver insulin sensitivity. Hepatocyte-specific deletion of HIF-2α markedly attenuated these effects of PHDi treatment, showing PHDi effects are HIF-2α dependent. At the molecular level, HIF-2α induced increased Irs2 and cyclic AMP-specific phosphodiesterase gene expression, leading to increased and decreased insulin and glucagon signaling, respectively. These effects of PHDi treatment were conserved in human and mouse hepatocytes.

Conclusions

Our results elucidate unknown mechanisms for how PHD inhibition improves glycemic control through HIF-2α-dependent regulation of hepatic insulin and glucagon sensitivity.

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PHD inhibitor treatment improves glycemic control in obese glucose-intolerant mice.

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PHD inhibitor treatment decreases liver glucagon sensitivity in obese mice.

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The effects of PHD inhibition on glycemic control is hepatocyte HIF-2α-dependent.

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PHD inhibitor treatment stimulates HIF-2α-dependent cAMP-specific PDE expression.

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In human and mouse hepatocytes, PHD inhibitor treatment suppresses glucagon signaling.

Oxygen-sensing mechanisms in cells

The importance of oxygen for the survival of multicellular and aerobic organisms is well established and documented. Over the years, increased knowledge of its use for bioenergetics has placed oxygen at the centre of research on mitochondria and ATP-generating processes. Understanding the molecular mechanisms governing cellular oxygen sensing and response has allowed for the discovery of novel pathways oxygen is involved in, culminating with the award of the Nobel Prize for Medicine and Physiology in 2019 to the pioneers of this field, Greg Semenza, Peter Ratcliffe and William Kaelin. However, it is now beginning to be appreciated that oxygen can be a signalling molecule involved in a vast array of molecular processes, most of which impinge on gene expression control. This review will focus on the knowns and unknowns of oxygen as a signalling molecule, highlighting the role of 2-oxoglutarate-dependent dioxygenases as central players in the cellular response to deviations in oxygen tension.

Prolyl hydroxylase domain 2 reduction enhances skeletal muscle tissue regeneration after soft tissue trauma in mice

The transcription factor Hypoxia-inducible factor 1 (HIF-1) plays a pivotal role in tissue regeneration. HIF-1 is negatively controlled by O₂-dependent prolyl hydroxylases with a predominant role of prolyl hydroxylase 2 isoform (Phd2). Transgenic mice, hypomorphic for this isoform, accumulate more HIF-1 under normoxic conditions. Using these mice, we investigated the influence of Phd2 and HIF-1 on the regenerative capability of skeletal muscle tissue after myotrauma. Phd2-hypomorphic and wild type mice (on C57Bl/6 background) were grouped with regeneration times from 6 to 168 hours after closed mechanic muscle trauma to the hind limb. Tissue samples were analysed by immuno-staining and real-time PCR. Bone marrow derived macrophages of wild type and Phd2-hypomorphic mice were isolated and analysed via flow cytometry and quantitative real-time PCR. Phd2 reduction led to a higher regenerative capability due to enhanced activation of myogenic factors accompanied by induction of genes responsible for glucose and lactate metabolism in Phd2-hypomorphic mice. Macrophage infiltration into the trauma areas in hypomorphic mice started earlier and was more pronounced compared to wild type mice. Phd2-hypomorphic mice also showed higher numbers of macrophages in areas with sustained trauma 72 hours after myotrauma application. In conclusion, we postulate that the HIF-1 pathway is activated secondary to a Phd2 reduction which may lead to i) higher activation of myogenic factors, ii) increased number of positive stem cell proliferation markers, and iii) accelerated macrophage recruitment to areas of trauma, resulting in faster muscle tissue regeneration after myotrauma. With the current development of prolyl hydroxylase domain inhibitors, our findings point towards a potential clinical benefit after myotrauma.

A guiding torch at the poles: the multiple roles of spindle microtubule-organizing centers during cell division

The spindle constitutes the cellular machinery that enables the segregation of the chromosomes during eukaryotic cell division. The microtubules that form this fascinating and complex genome distribution system emanate from specialized structures located at both its poles and known as microtubule-organizing centers (MTOCs). Beyond their structural function, the spindle MTOCs play fundamental roles in cell cycle control, the activation and functionality of the mitotic checkpoints and during cellular aging. This review highlights the pivotal importance of spindle-associated MTOCs in multiple cellular processes and their central role as key regulatory hubs where diverse intracellular signals are integrated and coordinated to ensure the successful completion of cell division and the maintenance of the replicative lifespan.

Recent advances in understanding the role of hypoxia-inducible factor 1α in renal fibrosis

Renal fibrosis is the most common pathological manifestation of chronic kidney disease (CKD), and with numerous influencing factors, its pathogenesis is complex. Epithelial-mesenchymal transition (EMT) is known to promote the progression of renal fibrosis via alterations in the secreted proteome. Moreover, blocking or even reversing EMT can effectively reduce the degree of fibrosis. As such, targeting the key molecules responsible for promoting EMT may be an effective strategy for inhibiting renal fibrosis. Research in recent years has demonstrated that hypoxia-inducible factor 1α (HIF-1α) acts to promote renal fibrosis through regulation of EMT. However, the relationship between HIF-1α and EMT remains incompletely understood. In the present review, the underlying mechanism of the interaction between HIF-1α and EMT is explored to provide novel insight into the pathogenesis of renal fibrosis and new ideas for early targeted intervention.

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.

HIF-1β Positively Regulates NF-κB Activity via Direct Control of TRAF6

NF-κB signalling is crucial for cellular responses to inflammation but is also associated with the hypoxia response. NF-κB and hypoxia inducible factor (HIF) transcription factors possess an intense molecular crosstalk. Although it is known that HIF-1α modulates NF-κB transcriptional response, very little is understood regarding how HIF-1β contributes to NF-κB signalling. Here, we demonstrate that HIF-1β is required for full NF-κB activation in cells following canonical and non-canonical stimuli. We found that HIF-1β specifically controls TRAF6 expression in human cells but also in Drosophila melanogaster. HIF-1β binds to the TRAF6 gene and controls its expression independently of HIF-1α. Furthermore, exogenous TRAF6 expression is able to rescue all of the cellular phenotypes observed in the absence of HIF-1β. These results indicate that HIF-1β is an important regulator of NF-κB with consequences for homeostasis and human disease.

Lysophosphatidic Acid and Hematopoiesis: From Microenvironmental Effects to Intracellular Signaling

Vertebrate hematopoiesis is a complex physiological process that is tightly regulated by intracellular signaling and extracellular microenvironment. In recent decades, breakthroughs in lineage-tracing technologies and lipidomics have revealed the existence of numerous lipid molecules in hematopoietic microenvironment. Lysophosphatidic acid (LPA), a bioactive phospholipid molecule, is one of the identified lipids that participates in hematopoiesis. LPA exhibits various physiological functions through activation of G-protein-coupled receptors. The functions of these LPARs have been widely studied in stem cells, while the roles of LPARs in hematopoietic stem cells have rarely been examined. Nonetheless, mounting evidence supports the importance of the LPA-LPAR axis in hematopoiesis. In this article, we have reviewed regulation of hematopoiesis in general and focused on the microenvironmental and intracellular effects of the LPA in hematopoiesis. Discoveries in these areas may be beneficial to our understanding of blood-related disorders, especially in the context of prevention and therapy for anemia.

Now a Nobel gas: oxygen

The recent bestowal of the Nobel Prize 2019 in Physiology or Medicine to Gregg L. Semenza, Sir Peter J. Ratcliffe, and William G. Kaelin Jr. celebrates a series of remarkable discoveries that span from the physiological research question on how oxygen deficiency (hypoxia) induces the red blood cell forming hormone erythropoietin (Epo) to the first clinical application of a novel family of Epo-inducing drugs to treat patients suffering from renal anemia. This review looks back at the most important findings made by the three Nobel laureates, highlights current research trends, and sheds an eye on future perspectives of hypoxia research, including emerging and potential clinical applications.

Mechanisms and Consequences of Oxygen and Carbon Dioxide Sensing in Mammals

Molecular oxygen (O₂) and carbon dioxide (CO₂) are the primary gaseous substrate and product of oxidative phosphorylation in respiring organisms, respectively. Variance in the levels of either of these gasses outside of the physiological range presents a serious threat to cell, tissue, and organism survival. Therefore, it is essential that endogenous levels are monitored and kept at appropriate concentrations to maintain a state of homeostasis. Higher organisms such as mammals have evolved mechanisms to sense O₂ and CO₂ both in the circulation and in individual cells and elicit appropriate corrective responses to promote adaptation to commonly encountered conditions such as hypoxia and hypercapnia. These can be acute and transient nontranscriptional responses, which typically occur at the level of whole animal physiology or more sustained transcriptional responses, which promote chronic adaptation. In this review, we discuss the mechanisms by which mammals sense changes in O₂ and CO₂ and elicit adaptive responses to maintain homeostasis. We also discuss crosstalk between these pathways and how they may represent targets for therapeutic intervention in a range of pathological states.

When is a target not a target?

Cells rely on prolyl hydroxylase enzymes to sense low levels of oxygen, but they might act on fewer targets than previously thought.

Hypoxia-adaptive pathways: A pharmacological target in fibrotic disease?

Wound healing responses are physiological reactions to injuries and share common characteristics and phases independently of the injured organ or tissue. A major hallmark of wound healing responses is the formation of extra-cellular matrix (ECM), mainly consisting of collagen fibers, to restore the initial organ architecture and function. Overshooting wound healing responses result in unphysiological accumulation of ECM and collagen deposition, a process called fibrosis. Importantly, hypoxia (oxygen demand exceeds supply) plays a significant role during wound healing responses and fibrotic diseases. Under hypoxic conditions, cells activate a gene program, including the stabilization of hypoxia-inducible factors (HIFs), which induces the expression of HIF target genes counteracting hypoxia. In contrast, in normoxia, so-called HIF-prolyl hydroxylases (PHDs) oxygen-dependently hydroxylate HIF-α, which marks it for proteasomal degradation. Importantly, PHDs can be pharmacologically inhibited (PHI) by so-called PHD inhibitors. There is mounting evidence that the HIF-pathway is continuously up-regulated during the development of tissue fibrosis, and that pharmacological (HIFI) or genetic inhibition of HIF can prevent organ fibrosis. By contrast, initial (short-term) activation of the HIF pathway via PHI during wound healing seems to be beneficial in several models of inflammation or acute organ injury. Thus, timing and duration of PHI and HIFI treatment seem to be crucial. In this review, we will highlight the role of hypoxia-adaptive pathways during wound healing responses and development of fibrotic disease. Moreover, we will discuss whether PHI and HIFI might be a promising treatment option in fibrotic disease, and consider putative pitfalls that might result from this approach.