Increased prolyl 4-hydroxylase domain proteins compensate for decreased oxygen levels. Evidence for an autoregulatory oxygen-sensing system

Selective Hypoxia-Sensitive Oxomer Formation by FIH Prevents Binding of the NF-κB Inhibitor IκBβ to NF-κB Subunits

Pharmacologic inhibitors of cellular hydroxylase oxygen sensors are protective in multiple preclinical in vivo models of inflammation. However, the molecular mechanisms underlying this regulation are only partly understood, preventing clinical translation. We previously proposed a new mechanism for cellular oxygen sensing: oxygen-dependent, (likely) covalent protein oligomer (oxomer) formation. Here, we report that the oxygen sensor factor inhibiting HIF (FIH) forms an oxomer with the NF-κB inhibitor β (IκBβ). The formation of this protein complex required FIH enzymatic activity and was prevented by pharmacologic inhibitors. Oxomer formation was highly hypoxia-sensitive and very stable. No other member of the IκB protein family formed an oxomer with FIH, demonstrating that FIH-IκBβ oxomer formation was highly selective. In contrast to the known FIH-dependent oxomer formation with the deubiquitinase OTUB1, FIH-IκBβ oxomer formation did not occur via an IκBβ asparagine residue, but depended on the amino acid sequence VAERR contained within a loop between IκBβ ankyrin repeat domains 2 and 3. Oxomer formation prevented IκBβ from binding to its primary interaction partners p65 and c-Rel, subunits of NF-κB, the master regulator of the cellular transcriptional response to pro-inflammatory stimuli. We therefore propose that FIH-mediated oxomer formation with IκBβ contributes to the hypoxia-dependent regulation of inflammation.

PHD1-3 oxygen sensors in vivo-lessons learned from gene deletions

Oxygen sensors enable cells to adapt to limited oxygen availability (hypoxia), affecting various cellular and tissue responses. Prolyl-4-hydroxylase domain 1-3 (PHD1-3; also called Egln1-3, HIF-P4H 1-3, HIF-PH 1-3) proteins belong to the Fe2+- and 2-oxoglutarate-dependent dioxygenase superfamily and utilise molecular oxygen (O2) alongside 2-oxoglutarate as co-substrate to hydroxylate two proline residues of α subunits of the dimeric hypoxia inducible factor (HIF) transcription factor. PHD1-3-mediated hydroxylation of HIF-α leads to its degradation and inactivation. Recently, various PHD inhibitors (PHI) have entered the clinics for treatment of renal anaemia. Pre-clinical analyses indicate that PHI treatment may also be beneficial in numerous other hypoxia-associated diseases. Nonetheless, the underlying molecular mechanisms of the observed protective effects of PHIs are only partly understood, currently hindering their translation into the clinics. Moreover, the PHI-mediated increase of Epo levels is not beneficial in all hypoxia-associated diseases and PHD-selective inhibition may be advantageous. Here, we summarise the current knowledge about the relevance and function of each of the three PHD isoforms in vivo, based on the deletion or RNA interference-mediated knockdown of each single corresponding gene in rodents. This information is crucial for our understanding of the physiological relevance and function of the PHDs as well as for elucidating their individual impact on hypoxia-associated diseases. Furthermore, this knowledge highlights which diseases may best be targeted by PHD isoform-selective inhibitors in case such pharmacologic substances become available.

Recent Advances and Therapeutic Implications of 2-Oxoglutarate-Dependent Dioxygenases in Ischemic Stroke

Ischemic stroke is a common disease with a high disability rate and mortality, which brings heavy pressure on families and medical insurance. Nowadays, the golden treatments for ischemic stroke in the acute phase mainly include endovascular therapy and intravenous thrombolysis. Some drugs are used to alleviate brain injury in patients with ischemic stroke, such as edaravone and 3-n-butylphthalide. However, no effective neuroprotective drug for ischemic stroke has been acknowledged. 2-Oxoglutarate-dependent dioxygenases (2OGDDs) are conserved and common dioxygenases whose activities depend on O2, Fe2+, and 2OG. Most 2OGDDs are expressed in the brain and are essential for the development and functions of the brain. Therefore, 2OGDDs likely play essential roles in ischemic brain injury. In this review, we briefly elucidate the functions of most 2OGDDs, particularly the effects of regulations of 2OGDDs on various cells in different phases after ischemic stroke. It would also provide promising potential therapeutic targets and directions of drug development for protecting the brain against ischemic injury and improving outcomes of ischemic stroke.

Hypoxia and renal fibrosis

Renal fibrosis is the final stage of most progressive kidney diseases. Chronic kidney disease (CKD) is associated with high comorbidity and mortality. Thus, preventing fibrosis and thereby preserving kidney function increases the quality of life and prolongs the survival of patients with CKD. Many processes such as inflammation or metabolic stress modulate the progression of kidney fibrosis. Hypoxia has also been implicated in the pathogenesis of renal fibrosis, and oxygen sensing in the kidney is of outstanding importance for the body. The dysregulation of oxygen sensing in the diseased kidney is best exemplified by the loss of stimulation of erythropoietin production from interstitial cells in the fibrotic kidney despite anemia. Furthermore, hypoxia is present in acute or chronic kidney diseases and may affect all cell types present in the kidney including tubular and glomerular cells as well as resident immune cells. Pro- and antifibrotic effects of the transcription factors hypoxia-inducible factors 1 and 2 have been described in a plethora of animal models of acute and chronic kidney diseases, but recent advances in sequencing technologies now allow for novel and deeper insights into the role of hypoxia and its cell type-specific effects on the progression of renal fibrosis, especially in humans. Here, we review existing literature on how hypoxia impacts the development and progression of renal fibrosis.

L3MBTL3 is induced by HIF-1α and fine tunes the HIF-1α degradation under hypoxia in vitro

HIF-1α plays a crucial part in hypoxia response by transcriptionally upregulating genes to adapt the hypoxic condition. HIF-1α is under severe cellular control as its exceptional activation is always associated with tumorigenesis and tumor progression. Here, we report L3MBTL3 serves as a novel negative regulator of HIF-1α. It is upregulated during hypoxia and acts as a transcriptional target of HIF-1α. In the nuclei, L3MBTL3 makes an interaction with HIF-1α and promotes its ubiquitination and degradation. These findings indicate L3MBTL3 forms a negative feedback loop with HIF-1α in vitro to dampen the hypoxic response.

Hypoxia-mediated activation of hypoxia-inducible factor-1α in head and neck squamous cell carcinoma: A review

Since the 1950s, hypoxia has been recognized as a crucial characteristic of cancer cells and their microenvironment. Indeed, hypoxia promotes the growth, survival, and metastasis of cancer cells. In the early 1990s, we found that as many phenomena in hypoxia can occur through hypoxia-inducible factor-1α (HIF1α). HIF1α is known as an angiogenesis converter in hypoxia, which promotes tumorigenesis, development, immune escape, recurrence, etc; This page goes into great detail on how HIF1α is activated during hypoxia and how the 2 signaling channels interact. It specifically emphasizes the significance of reactive oxygen species, the function of the PI3K/the serine/threonine kinase Akt/mammalian target of rapamycin cascade, and outlines the similarities between the 2 important factors (reactive oxygen species and PI3K/the serine/threonine kinase Akt/mammalian target of rapamycin cascade), nuclear factor κB, for HIF1α Important implications, in an effort to offer fresh views for the treatment of head and neck squamous cell carcinoma and HIF1α research.

Modeling hypoxia-related inflammation scenarios

Cells respond to hypoxia via the activation of three isoforms of Hypoxia Inducible Factors (HIFs), that are characterized by different activation times. HIF overexpression has many effects on cell behavior, such as change in metabolism, promotion of angiogenic processes and elicitation of a pro-inflammatory response. These effects are driving forces of malignant progression in cancer cells. In this work we study in detail hypoxia-induced dynamics of HIF1α and HIF2α, which are the most studied isoforms, comparing available experimental data on their evolution in tumor cells with the results obtained integrating the deduced mathematical model. Then, we examine the possible scenarios that characterize the link between hypoxia and inflammation via the activation of NFkB (Nuclear Factor k-light-chain-enhancer of activated B cells) when the dimensionless groups of parameters of the mathematical model change. In this way we are able to discuss why and when hypoxic conditions lead to acute or chronic inflammatory states.

Hypoxia inducible factor prolyl hydroxylases in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic disease that is characterized by intestinal inflammation. Epithelial damage and loss of intestinal barrier function are believed to be the hallmark pathologies of the disease. In IBD, the resident and infiltrating immune cells consume much oxygen, rendering the inflamed intestinal mucosa hypoxic. In hypoxia, the hypoxia-inducible factor (HIF) is induced to cope with the lack of oxygen and protect intestinal barrier. Protein stability of HIF is tightly controlled by prolyl hydroxylases (PHDs). Stabilization of HIF through inhibition of PHDs is appearing as a new strategy of IBD treatment. Studies have shown that PHD-targeting is beneficial to the treatment of IBD. In this Review, we summarize the current understanding of the role of HIF and PHDs in IBD and discuss the therapeutic potential of targeting PHD-HIF pathway for IBD treatment.

The transition from HIF-1 to HIF-2 during prolonged hypoxia results from reactivation of PHDs and HIF1A mRNA instability

The hypoxia-inducible factors (HIF) are transcription factors that activate the adaptive hypoxic response when oxygen levels are low. The HIF transcriptional program increases oxygen delivery by inducing angiogenesis and by promoting metabolic reprograming that favors glycolysis. The two major HIFs, HIF-1 and HIF-2, mediate this response during prolonged hypoxia in an overlapping and sequential fashion that is referred to as the HIF switch. Both HIF proteins consist of an unstable alpha chain and a stable beta chain. The instability of the alpha chains is mediated by prolyl hydroxylase (PHD) activity during normoxic conditions, which leads to ubiquitination and proteasomal degradation of the alpha chains. During normoxic conditions, very little HIF-1 or HIF-2 alpha-beta dimers are present because of PHD activity. During hypoxia, however, PHD activity is suppressed, and HIF dimers are stable. Here we demonstrate that HIF-1 expression is maximal after 4 h of hypoxia in primary endothelial cells and then is dramatically reduced by 8 h. In contrast, HIF-2 is maximal at 8 h and remains elevated up to 24 h. There are differences in the HIF-1 and HIF-2 transcriptional profiles, and therefore understanding how the transition between them occurs is important and not clearly understood. Here we demonstrate that the HIF-1 to HIF-2 transition during prolonged hypoxia is mediated by two mechanisms: (1) the HIF-1 driven increase in the glycolytic pathways that reactivates PHD activity and (2) the much less stable mRNA levels of HIF-1α (HIF1A) compared to HIF-2α (EPAS1) mRNA. We also demonstrate that the alpha mRNA levels directly correlate to the relative alpha protein levels, and therefore to the more stable HIF-2 expression during prolonged hypoxia.

The Asparagine Hydroxylase FIH: A Unique Oxygen Sensor

Significance: Limited oxygen availability (hypoxia) commonly occurs in a range of physiological and pathophysiological conditions, including embryonic development, physical exercise, inflammation, and ischemia. It is thus vital for cells and tissues to monitor their local oxygen availability to be able to adjust in case the oxygen supply is decreased. The cellular oxygen sensor factor inhibiting hypoxia-inducible factor (FIH) is the only known asparagine hydroxylase with hypoxia sensitivity. FIH uniquely combines oxygen and peroxide sensitivity, serving as an oxygen and oxidant sensor. Recent Advances: FIH was first discovered in the hypoxia-inducible factor (HIF) pathway as a modulator of HIF transactivation activity. Several other FIH substrates have now been identified outside the HIF pathway. Moreover, FIH enzymatic activity is highly promiscuous and not limited to asparagine hydroxylation. This includes the FIH-mediated catalysis of an oxygen-dependent stable (likely covalent) bond formation between FIH and selected substrate proteins (called oxomers [oxygen-dependent stable protein oligomers]). Critical Issues: The (patho-)physiological function of FIH is only beginning to be understood and appears to be complex. Selective pharmacologic inhibition of FIH over other oxygen sensors is possible, opening new avenues for therapeutic targeting of hypoxia-associated diseases, increasing the interest in its (patho-)physiological relevance. Future Directions: The contribution of FIH enzymatic activity to disease development and progression should be analyzed in more detail, including the assessment of underlying molecular mechanisms and relevant FIH substrate proteins. Also, the molecular mechanism(s) involved in the physiological functions of FIH remain(s) to be determined. Furthermore, the therapeutic potential of recently developed FIH-selective pharmacologic inhibitors will need detailed assessment. Antioxid. Redox Signal. 37, 913-935.

IOX1 Fails to Reduce α-Globin and Mediates γ-Globin Silencing in Adult β0-Thalassemia/Hemoglobin E Erythroid Progenitor Cells

The accumulation of unbound α-globin chains in red blood cells is a crucial pathophysiology of β-thalassemia. IOX1 (5-carboxy-8-hydroxyquinoline) is a broad-spectrum 2-oxoglutarate (2OG)-dependent oxygenase inhibitor that can reduce α-globin mRNA expression in human cord blood erythroid progenitor cells. Therefore, IOX1 has been proposed as a potential compound for β-thalassemia treatment through the decrease inα-globin chain synthesis. However, there is no empirical evidence regarding the consequences of IOX1 in β-thalassemia. In this study, the therapeutic effects of IOX1 were investigated in β⁰-thalassemia/hemoglobin E (HbE) erythroid progenitor cells during in vitro erythropoiesis. The results indicated that IOX1 had no impact on α-globin gene expression, but it led instead to significant decreases in γ-globin and fetal hemoglobin (HbF, α₂γ₂) production without affecting well-known globin regulators: KLF1, BCL11A, LRF, and GATA1. In addition, differential mRNA expression of several genes in the hypoxia response pathway revealed the induction of EGLN1, the PHD2-encoding gene, as a result of IOX1 treatment. These findings suggested that IOX1 fails to lower α-globin gene expression; on the contrary, it mediates γ-globin and HbF silencing in β⁰-thalassemia/HbE erythroid progenitor cells. Because of the negative correlation of EGLN1 and γ-globin gene expression after IOX1 treatment, repurposing IOX1 to study the hypoxia response pathway and γ-globin regulation may provide beneficial information for β-thalassemia.

Fount, fate, features, and function of renal erythropoietin-producing cells

Renal erythropoietin (Epo)-producing (REP) cells represent a rare and incompletely understood cell type. REP cells are fibroblast-like cells located in close proximity to blood vessels and tubules of the corticomedullary border region. Epo mRNA in REP cells is produced in a pronounced “on–off” mode, showing transient transcriptional bursts upon exposure to hypoxia. In contrast to “ordinary” fibroblasts, REP cells do not proliferate ex vivo, cease to produce Epo, and lose their identity following immortalization and prolonged in vitro culture, consistent with the loss of Epo production following REP cell proliferation during tissue remodelling in chronic kidney disease. Because Epo protein is usually not detectable in kidney tissue, and Epo mRNA is only transiently induced under hypoxic conditions, transgenic mouse models have been developed to permanently label REP cell precursors, active Epo producers, and inactive descendants. Future single-cell analyses of the renal stromal compartment will identify novel characteristic markers of tagged REP cells, which will provide novel insights into the regulation of Epo expression in this unique cell type.

Hypoxia-inducible factor (HIF)-3a2 serves as an endothelial cell fate executor during chronic hypoxia

The adaptive response to hypoxia involves the transcriptional induction of three transcription factors called hypoxia inducible factor alpha 1, 2 and 3 (HIF-1α, HIF-2α, and HIF-3α) which dimerize with constitutively expressed beta chains that together form the HIF-1, -2 and -3 transcription factors. During normoxic conditions, the alpha chain is expressed at low levels since its stability is regulated by prolyl-hydroxylation that promotes subsequent ubiquitination and degradation. During hypoxic conditions, however, the prolyl hydroxylases are less active, and the alpha chain accumulates through elevated protein stability and the elevated induction of expression. Two of the three HIFs isoforms present in mammals, HIF-1 and HIF-2, are well characterized and have overlapping functions that promote cell survival, whereas HIF-3's role remains less clear. The HIF-3 response is complicated because the HIF3A gene can utilize different promotors and alternate splicing sites that result in a number of different HIF-3α isoforms. Here, using human umbilical vein endothelial cells (HUVECs), we demonstrate that one of the isoforms of HIF-3α, isoform 2 (HIF-3α2) accumulates at a late stage of hypoxia and induces the expression of DNA damage inducible transcript 3 (DDIT4), a gene known to promote apoptosis. We also demonstrate that caspase 3/7 activity is elevated, supporting that the role of the HIF-3α2 isoform is to promote apoptosis. Furthermore, we provide evidence that HIF-3α2 is also expressed in seven other primary endothelial cell types, suggesting that this may be a common feature of HIF-3α2 in endothelial cells.

Elucidating the role of hypoxia-inducible factor in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic multifactorial disease, provocative, and degenerative autoimmune condition that impacts millions of individuals around the globe. As a result of this understanding, anti-inflammatory drugs have been created, perhaps widely effective (like steroids) and highly specialized methods (including anti-TNF antibody) using biological therapies (including TNF inhibitors). Despite this, the connections between inflammatory response, articular development, and intracellular responsiveness to changes in oxygen concentration are undervalued in rheumatoid arthritis. Hypoxia, or a lack of oxygen, is thought to cause enhanced synovial angiogenesis in RA, which is mediated by some of the hypoxia-inducible factors like vascular endothelial growth factor (VEGF). Substantial genetic alterations occur when the HIF regulatory factors signaling cycle is activated, allowing organelles, tissues, and species to acclimatize to decreasing oxygen saturation. The most well-characterized hypoxia-responsive transcripts are the angiogenic stimulant VEGF, whose production is greatly elevated by hypoxia in several types of cells, especially RA synovium fibroblasts. Blocking vascular endothelial growth factors has been demonstrated to be helpful in murine models of rheumatism, indicating how hypoxia could trigger the angiogenesis process, resulting in the progression of RA. These mechanisms highlight the intimate affiliation amongst hypoxia, angiogenesis, and inflammation in rheumatoid arthritis. This review will look at how hypoxia activates molecular pathways and how other pathways involving inflammatory signals develop and sustain synovitis in rheumatoid arthritis.

Adaptation of the Oxygen Sensing System during Lung Development

During gestation, the most drastic change in oxygen supply occurs with the onset of ventilation after birth. As the too early exposure of premature infants to high arterial oxygen pressure leads to characteristic diseases, we studied the adaptation of the oxygen sensing system and its targets, the hypoxia-inducible factor- (HIF-) regulated genes (HRGs) in the developing lung. We draw a detailed picture of the oxygen sensing system by integrating information from qPCR, immunoblotting, in situ hybridization, and single-cell RNA sequencing data in ex vivo and in vivo models. HIF1α protein was completely destabilized with the onset of pulmonary ventilation, but did not coincide with expression changes in bona fide HRGs. We observed a modified composition of the HIF-PHD system from intrauterine to neonatal phases: Phd3 was significantly decreased, while Hif2a showed a strong increase and the Hif3a isoform Ipas exclusively peaked at P0. Colocalization studies point to the Hif1a-Phd1 axis as the main regulator of the HIF-PHD system in mouse lung development, complemented by the Hif3a-Phd3 axis during gestation. Hif3a isoform expression showed a stepwise adaptation during the periods of saccular and alveolar differentiation. With a strong hypoxic stimulus, lung ex vivo organ cultures displayed a functioning HIF system at every developmental stage. Approaches with systemic hypoxia or roxadustat treatment revealed only a limited in vivo response of HRGs. Understanding the interplay of the oxygen sensing system components during the transition from saccular to alveolar phases of lung development might help to counteract prematurity-associated diseases like bronchopulmonary dysplasia.

Neurogenic and pericytic plasticity of conditionally immortalized cells derived from renal erythropoietin-producing cells

In adult mammals, the kidney is the main source of circulating erythropoietin (Epo), the master regulator of erythropoiesis. In vivo data in mice demonstrated multiple subtypes of interstitial renal Epo‐producing (REP) cells. To analyze the differentiation plasticity of fibroblastoid REP cells, we used a transgenic REP cell reporter mouse model to generate conditionally immortalized REP‐derived (REPD) cell lines. Under nonpermissive conditions, REPD cells ceased from proliferation and acquired a stem cell‐like state, with strongly enhanced hypoxia‐inducible factor 2 (HIF‐2α), stem cell antigen 1 (SCA‐1), and CD133 expression, but also enhanced alpha‐smooth muscle actin (αSMA) expression, indicating myofibroblastic signaling. These cells maintained the “on‐off” nature of Epo expression observed in REP cells in vivo, whereas other HIF target genes showed a more permanent regulation. Like REP cells in vivo, REPD cells cultured in vitro generated long tunneling nanotubes (TNTs) that aligned with endothelial vascular structures, were densely packed with mitochondria and became more numerous under hypoxic conditions. Although inhibition of mitochondrial oxygen consumption blunted HIF signaling, removal of the TNTs did not affect or even enhance the expression of HIF target genes. Apart from pericytes, REPD cells readily differentiated into neuroglia but not adipogenic, chondrogenic, or osteogenic lineages, consistent with a neuronal origin of at least a subpopulation of REP cells. In summary, these results suggest an unprecedented combination of differentiation features of this unique cell type.

Enhanced glucose metabolism through activation of HIF-1α covers the energy demand in a rat embryonic heart primordium after heartbeat initiation

The initiation of heartbeat is an essential step in cardiogenesis in the heart primordium, but it remains unclear how intracellular metabolism responds to increased energy demands after heartbeat initiation. In this study, embryos in Wistar rats at embryonic day 10, at which heartbeat begins in rats, were divided into two groups by the heart primordium before and after heartbeat initiation and their metabolic characteristics were assessed. Metabolome analysis revealed that increased levels of ATP, a main product of glucose catabolism, and reduced glutathione, a by-product of the pentose phosphate pathway, were the major determinants in the heart primordium after heartbeat initiation. Glycolytic capacity and ATP synthesis-linked mitochondrial respiration were significantly increased, but subunits in complexes of mitochondrial oxidative phosphorylation were not upregulated in the heart primordium after heartbeat initiation. Hypoxia-inducible factor (HIF)-1α was activated and a glucose transporter and rate-limiting enzymes of the glycolytic and pentose phosphate pathways, which are HIF-1α-downstream targets, were upregulated in the heart primordium after heartbeat initiation. These results suggest that the HIF-1α-mediated enhancement of glycolysis with activation of the pentose phosphate pathway, potentially leading to antioxidant defense and nucleotide biosynthesis, covers the increased energy demand in the beating and developing heart primordium.

Prolyl-4-hydroxylases 2 and 3 control erythropoietin production in renin-expressing cells of mouse kidneys

Activation of the hypoxia-signalling pathway induced by deletion of the ubiquitin-ligase von Hippel-Lindau protein causes an endocrine shift of renin-producing cells to erythropoietin (EPO)-expressing cells. However, the underlying mechanisms have not yet been investigated. Since oxygen-regulated stability of hypoxia-inducible transcription factors relevant for EPO expression is dependent on the activity of prolyl-4-hydroxylases (PHD) 2 and 3, this study aimed to determine the relevance of different PHD isoforms for the EPO expression in renin-producing cells in vivo. For this purpose, mice with inducible renin cell-specific deletions of different PHD isoforms were analysed. Our study shows that there are two subgroups of renal renin-expressing cells, juxtaglomerular renin⁺ cells and platelet-derived growth factor receptor-β⁺ interstitial renin⁺ cells. These interstitial renin⁺ cells belong to the cell pool of native EPO-producing cells and are able to express EPO and renin in parallel. In contrast, co-deletion of PHD2 and PHD3, but not PHD2 deletion alone, induces EPO expression in juxtaglomerular and hyperplastic renin⁺ cells and downregulates renin expression. A strong basal PHD3 expression in juxtaglomerular renin⁺ cells seems to prevent the hypoxia-inducible transcription factor-2-dependent phenotype shift into EPO cells. In summary, PHDs seem important for the stabilization of the juxtaglomerular renin cell phenotype. Moreover, these findings reveal tubulointerstitial cells as a novel site of renal renin expression and suggest a high endocrine plasticity of these cells. Our data concerning the distinct expression patterns and functions of PHD2 and PHD3 provide new insights into the regulation of renin-producing cells and highlight the need for selective PHD inhibitors. KEY POINTS: Renal renin-expressing cells can be clearly distinguished into two subgroups, the typical juxtaglomerular renin-producing cells and interstitial renin⁺ cells. Interstitial renin⁺ cells belong to the cell pool of native erythropoietin (EPO)-producing cells, show a fast EPO response to acute hypoxia-inducible factor-2 (HIF-2) stabilization and are able to express EPO and renin in parallel. Only co-deletion of the prolyl-4-hydroxylases (PHD) 2 and 3, but not PHD2 deletion alone, induces EPO expression in juxtaglomerular renin⁺ cells. Chronic HIF-2 stabilization in juxtaglomerular renin-expressing cells leads to their phenotypic shift into EPO-producing cells. A strong basal PHD3 expression in juxtaglomerular renin⁺ cells seems to prevent a HIF-2-dependent phenotype shift into EPO cells suggesting PHD3 fulfils a stabilizer function for the juxtaglomerular renin cell phenotype.

Influence of hypoxia on biochemical aspects and on expression of genes related to oxygen-homeostasis of the Amazonian cichlid Astronotus ocellatus (Agassiz, 1831)

Variations in dissolved oxygen levels are common in the Amazonian aquatic environments and the aquatic organisms that inhabit these environments developed a variety of adaptive responses to deal with such conditions. Some Amazonian fish species are tolerant to low oxygen levels and the cichlid Astronotus ocellatus is one of the most hypoxia-tolerant species. Herein, we aimed to unveil the biochemical and molecular responses that A. ocellatus presents when submitted to hypoxia. Hypoxia indicators were measured, such as plasma glucose, plasma lactate, hepatic glycogen and relative transcript levels of prolyl hydroxylase 2 (phd2) and hypoxia-inducible factor-1α (hif-1α) in juveniles of approximately 50 g exposed to 1, 3, and 5 hours of hypoxia (0.7 mg O₂.L^-1), followed by 3 hours of recovery in normoxia (6 mg O₂.L^-1). Fish exposed to hypoxia reduced liver glycogen levels within 3 hours of hypoxia, when comparing with 1 hour, and increased plasma glucose and lactate. Under the same condition, phd2 transcripts levels increased in gills, but decreased in liver. In contrast, hypoxia did not affect relative gene expression of hif-1α in both tissues. Based on the transcription pattern of phd2, these results showed that liver and gills of A. ocellatus have different molecular strategies to cope with environmental hypoxia.

Chronic and Cycling Hypoxia: Drivers of Cancer Chronic Inflammation through HIF-1 and NF-κB Activation: A Review of the Molecular Mechanisms

Chronic (continuous, non-interrupted) hypoxia and cycling (intermittent, transient) hypoxia are two types of hypoxia occurring in malignant tumors. They are both associated with the activation of hypoxia-inducible factor-1 (HIF-1) and nuclear factor κB (NF-κB), which induce changes in gene expression. This paper discusses in detail the mechanisms of activation of these two transcription factors in chronic and cycling hypoxia and the crosstalk between both signaling pathways. In particular, it focuses on the importance of reactive oxygen species (ROS), reactive nitrogen species (RNS) together with nitric oxide synthase, acetylation of HIF-1, and the action of MAPK cascades. The paper also discusses the importance of hypoxia in the formation of chronic low-grade inflammation in cancerous tumors. Finally, we discuss the effects of cycling hypoxia on the tumor microenvironment, in particular on the expression of VEGF-A, CCL2/MCP-1, CXCL1/GRO-α, CXCL8/IL-8, and COX-2 together with PGE₂. These factors induce angiogenesis and recruit various cells into the tumor niche, including neutrophils and monocytes which, in the tumor, are transformed into tumor-associated neutrophils (TAN) and tumor-associated macrophages (TAM) that participate in tumorigenesis.

Protective effects of carbonic anhydrase inhibition in brain ischaemia in vitro and in vivo models

Ischaemic stroke is a leading cause of death and disability. One of the major pathogenic mechanisms after ischaemia includes the switch to the glycolytic pathway, leading to tissue acidification. Carbonic anhydrase (CA) contributes to pH regulation. A new generation of CA inhibitors, AN11-740 and AN6-277 and the reference compound acetazolamide (ACTZ) were investigated in two models of brain ischaemia: in rat hippocampal acute slices exposed to severe oxygen, glucose deprivation (OGD) and in an in vivo model of focal cerebral ischaemia induced by permanent occlusion of the middle cerebral artery (pMCAo) in the rat. In vitro, the application of selective CAIs significantly delayed the appearance of anoxic depolarisation induced by OGD. In vivo, sub-chronic systemic treatment with AN11-740 and ACTZ significantly reduced the neurological deficit and decreased the infarct volume after pMCAo. CAIs counteracted neuronal loss, reduced microglia activation and partially counteracted astrocytes degeneration inducing protection from functional and tissue damage.

Role of Carbonic Anhydrase in Cerebral Ischemia and Carbonic Anhydrase Inhibitors as Putative Protective Agents

Ischemic stroke is a leading cause of death and disability worldwide. The only pharmacological treatment available to date for cerebral ischemia is tissue plasminogen activator (t-PA) and the search for successful therapeutic strategies still remains a major challenge. The loss of cerebral blood flow leads to reduced oxygen and glucose supply and a subsequent switch to the glycolytic pathway, which leads to tissue acidification. Carbonic anhydrase (CA, EC 4.2.1.1) is the enzyme responsible for converting carbon dioxide into a protons and bicarbonate, thus contributing to pH regulation and metabolism, with many CA isoforms present in the brain. Recently, numerous studies have shed light on several classes of carbonic anhydrase inhibitor (CAI) as possible new pharmacological agents for the management of brain ischemia. In the present review we summarized pharmacological, preclinical and clinical findings regarding the role of CAIs in strokes and we discuss their potential protective mechanisms.

Downregulation of both mismatch repair and non-homologous end-joining pathways in hypoxic brain tumour cell lines

Glioblastoma, a grade IV astrocytoma, has a poor survival rate in part due to ineffective treatment options available. These tumours are heterogeneous with areas of low oxygen levels, termed hypoxic regions. Many intra-cellular signalling pathways, including DNA repair, can be altered by hypoxia. Since DNA damage induction and subsequent activation of DNA repair mechanisms is the cornerstone of glioblastoma treatment, alterations to DNA repair mechanisms could have a direct influence on treatment success. Our aim was to elucidate the impact of chronic hypoxia on DNA repair gene expression in a range of glioblastoma cell lines. We adopted a NanoString transcriptomic approach to examine the expression of 180 DNA repair-related genes in four classical glioblastoma cell lines (U87-MG, U251-MG, D566-MG, T98G) exposed to 5 days of normoxia (21% O₂), moderate (1% O₂) or severe (0.1% O₂) hypoxia. We observed altered gene expression in several DNA repair pathways including homologous recombination repair, non-homologous end-joining and mismatch repair, with hypoxia primarily resulting in downregulation of gene expression. The extent of gene expression changes was dependent on hypoxic severity. Some, but not all, of these downregulations were directly under the control of HIF activity. For example, the downregulation of LIG4, a key component of non-homologous end-joining, was reversed upon inhibition of the hypoxia-inducible factor (HIF). In contrast, the downregulation of the mismatch repair gene, PMS2, was not affected by HIF inhibition. This suggests that numerous molecular mechanisms lead to hypoxia-induced reprogramming of the transcriptional landscape of DNA repair. Whilst the global impact of hypoxia on DNA repair gene expression is likely to lead to genomic instability, tumorigenesis and reduced sensitivity to anti-cancer treatment, treatment re-sensitising might require additional approaches to a simple HIF inhibition.

HIF-1α Enhances Vascular Endothelial Cell Permeability Through Degradation and Translocation of Vascular Endothelial Cadherin and Claudin-5 in Rats With Burn Injury

The mechanism underlying burn injury-induced enhanced vascular endothelial permeability and consequent body fluid extravasation is unclear. Here, the rat aortic endothelial cells (RAECs) were treated with the serum derived from rats with burn injury to elucidate the mechanism. Sprague-Dawley (SD) rats were grouped as follows (10 rats/group): control, 2, 4, 8, 12, and 24 hours postburn groups. The heart, liver, kidney, lung, jejunum, and ileum of rats injected with 2% Evans blue (EB) through the tail vein were excised to detect the EB level in each organ. The serum levels of hypoxia-inducible factor-1α (HIF-1α) and endothelin-1 (ET-1) were examined using enzyme-linked immunosorbent assay (ELISA). The effect of serum from 12-hour postburn group on the membrane permeability of RAEC monolayer, as well as on the mRNA and protein levels of ET-1, endothelin receptor A (ETA), ETB, and zonula occludens (ZO-1), was analyzed using quantitative real-time polymerase chain reaction (qRT-PCR) and western blotting. The membrane permeability of GV230/HIF-1α-transfected or shRNA-HIF-1α-transfected RAECs, as well as the expression levels of HIF-1α, ET-1, ETA, ETB, vascular endothelial (VE)-cadherin, and claudin-5, was analyzed using qRT-PCR and western blotting, whereas the localization of VE-cadherin and claudin-5 was examined using immunofluorescence. The serum HIF-1α and ET-1 levels in the burn groups, which peaked at 12 hours postburn, were significantly upregulated (P < .01) when compared with those in the control group. Additionally, the serum HIF-1α levels were positively correlated with vascular permeability. Compared with the shRNA-negative control-transfected RAECs, the shRNA-II/HIF-1α-transfected RAECs exhibited downregulated expression of HIF-1α, ET-1, ETA, and ETB (P < .01), and upregulated expression of ZO-1, claudin-5, and VE-cadherin (P < .05). Compared with the GV230-transfected RAECs, the GV230/HIF-1α-transfected RAECs exhibited upregulated expression of HIF-1α, ET-1, ETA, and ETB (P < .01), and downregulated expression of ZO-1, claudin-5, and VE-cadherin (P < .05). The GV230/HIF-1α-transfected RAECs exhibited degradation and translocation of VE-cadherin and claudin-5. In addition to degradation of VE-cadherin and claudin-5, HIF-1α mediated enhanced endothelial cell permeability through upregulation of ET-1, ETA, and ETB, and downregulation of ZO-1 and VE-cadherin in rats with burn injury.

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.

Inactivation of HIF-prolyl 4-hydroxylases 1, 2 and 3 in NG2-expressing cells induces HIF2-mediated neurovascular expansion independent of erythropoietin

AIM

NG2 cells in the brain are comprised of pericytes and NG2 glia and play an important role in the execution of cerebral hypoxia responses, including the induction of erythropoietin (EPO) in pericytes. Oxygen-dependent angiogenic responses are regulated by hypoxia-inducible factor (HIF), the activity of which is controlled by prolyl 4-hydroxylase domain (PHD) dioxygenases and the von Hippel-Lindau (VHL) tumour suppressor. However, the role of NG2 cells in HIF-regulated cerebral vascular homeostasis is incompletely understood.

METHODS

To examine the HIF/PHD/VHL axis in neurovascular homeostasis, we used a Cre-loxP-based genetic approach in mice and targeted Vhl, Epo, Phd1, Phd2, Phd3 and Hif2a in NG2 cells. Cerebral vasculature was assessed by immunofluorescence, RNA in situ hybridization, gene and protein expression analysis, gel zymography and in situ zymography.

RESULTS

Vhl inactivation led to a significant increase in angiogenic gene and Epo expression. This was associated with EPO-independent expansion of capillary networks in cortex, striatum and hypothalamus, as well as pericyte proliferation. A comparable phenotype resulted from the combined inactivation of Phd2 and Phd3, but not from Phd2 inactivation alone. Concomitant PHD1 function loss led to further expansion of the neurovasculature. Genetic inactivation of Hif2a in Phd1/Phd2/Phd3 triple mutant mice resulted in normal cerebral vasculature.

CONCLUSION

Our studies establish (a) that HIF2 activation in NG2 cells promotes neurovascular expansion and remodelling independently of EPO, (b) that HIF2 activity in NG2 cells is co-controlled by PHD2 and PHD3 and (c) that PHD1 modulates HIF2 transcriptional responses when PHD2 and PHD3 are inactive.

The Role of Hypoxia-Inducible Factor Post-Translational Modifications in Regulating Its Localisation, Stability, and Activity

The hypoxia signalling pathway enables adaptation of cells to decreased oxygen availability. When oxygen becomes limiting, the central transcription factors of the pathway, hypoxia-inducible factors (HIFs), are stabilised and activated to induce the expression of hypoxia-regulated genes, thereby maintaining cellular homeostasis. Whilst hydroxylation has been thoroughly described as the major and canonical modification of the HIF-α subunits, regulating both HIF stability and activity, a range of other post-translational modifications decorating the entire protein play also a crucial role in altering HIF localisation, stability, and activity. These modifications, their conservation throughout evolution, and their effects on HIF-dependent signalling are discussed in this review.

In vivo longitudinal and multimodal imaging of hypoxia-inducible factor 1α and angiogenesis in breast cancer

Background

Angiogenesis and hypoxia-inducible factor 1α (HIF-1α) play major roles in solid tumors. This study aimed to establish a longitudinal and multimodal imaging model for in vivo evaluation of HIF1α and angiogenesis in breast cancer.

Methods

By transfection of a 5 hypoxia-responsive element (HRE)/green fluorescent protein (GFP) plasmid, the cell line Ca761-hre-gfp was established, which emitted green fluorescence triggered by HIF-1α under hypoxia. The cells were subjected to CoCl₂-simulated hypoxia to confirm the imaging strategy. We grew Ca761-hre-gfp cells in the left rear flanks of twelve 615 mice. Experiments were conducted on days 4, 9, 15, and 19. For in vivo analysis, Ca761-hre-gfp subcutaneous allografted tumors were imaged in vivo using contrast-enhanced ultrasound (CEUS) and fluorescence imaging (FLI) during tumor development. The tumor size, CEUS peak intensity, and FLI photons were measured to evaluate tumor growth, angiogenesis, and HIF-1α activity, respectively. After each experiment, three mice were randomly sacrificed and tumor specimens were collected to examine HIF-1α activity and the microvessel density (MVD).

Results

In vitro, both green fluorescence and HIF-1α expression were detected in Ca761-hre-gfp cells treated with CoCl₂, indicating the suitability of the cells to detect HIF-1α activity. In vivo, HIF-1α activity first increased and then decreased, which was significantly correlated with angiogenic changes (r = 0.803, P = 0.005). These changes were confirmed by immunohistochemical staining of HIF-1α and MVD.

Conclusions

The findings validated the Ca761-hre-gfp murine allograft model for reliable evaluation of HIF-1α activity and angiogenesis longitudinally using both molecular and pre-clinical non-invasive imaging modalities. The cell line may be useful for studies of anti-HIF pathway therapies.

Orchestration of lincRNA-p21 and miR-155 in Modulating the Adaptive Dynamics of HIF-1α

Hypoxia-inducible factor-1 (HIF-1) is the key regulator of cellular adaptive response to hypoxia. Accumulating evidence shows that HIF-1 induces some non-coding RNAs (ncRNAs) including lncRNAs and miRNAs to modulate its own activity, enclosing several feedback loops. How the two classes of ncRNAs are orchestrated in the HIF-1-dependent adaptive response to hypoxia is poorly understood. By selecting lincRNA-p21 and miR-155 as the representatives, we develop an integrated model of the HIF-1 network comprising interlinked positive and negative feedback loops to clarify the interplay between the two ncRNAs in the hypoxic response. By numerical simulations, we find that coordination of lincRNA-p21 and miR-155 shapes the adaptive dynamics of HIF-1α: lincRNA-p21 induction in the early phase stimulates the upregulation of HIF-1α via stabilizing it, while miR-155 induction in the late phase promotes the recovery of HIF-1α via enhancing the degradation of its mRNA. Moreover, HIF-1α-induced PHD2 plays an auxiliary role in the decline of HIF-1α. In addition, lincRNA-p21 and miR-155 modulate each other via regulating HIF-1α activity. Together, lincRNA-p21 and miR-155 coordinate in modulating HIF-1α dynamics, and our work may shed light on the role for ncRNAs in the cellular adaptation to hypoxia.

The role of α-ketoglutarate and the hypoxia sensing pathway in the regulation of pancreatic β-cell function

Anaplerosis and the associated mitochondrial metabolite transporters generate unique cytosolic metabolic signaling molecules that can regulate insulin release from pancreatic β-cells. It has been shown that mitochondrial metabolites, transported by the citrate carrier (CIC), dicarboxylate carrier (DIC), oxoglutarate carrier (OGC), and mitochondrial pyruvate carrier (MPC) play a vital role in the regulation of glucose-stimulated insulin secretion (GSIS). Metabolomic studies on static and biphasic insulin secretion, suggests that several anaplerotic derived metabolites, including α-ketoglutarate (αKG), are strongly associated with nutrient regulated insulin secretion. Support for a role of αKG in the regulation of insulin secretion comes from studies looking at αKG dependent enzymes, including hypoxia-inducible factor-prolyl hydroxylases (PHDs) in clonal β-cells, and rodent and human islets. This review will focus on the possible link between defective anaplerotic-derived αKG, PHDs, and the development of type 2 diabetes (T2D).

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.

Inhibition of firefly luciferase activity by a HIF prolyl hydroxylase inhibitor

The three hypoxia-inducible factor (HIF) prolyl-4-hydroxylase domain (PHD) 1-3 enzymes confer oxygen sensitivity to the HIF pathway and are novel therapeutic targets for treatment of renal anemia. Inhibition of the PHDs may further be beneficial in other hypoxia-associated diseases, including ischemia and chronic inflammation. Several pharmacologic PHD inhibitors (PHIs) are available, but our understanding of their selectivity and its chemical basis is limited. We here report that the PHI JNJ-42041935 (JNJ-1935) is structurally similar to the firefly luciferase substrate D-luciferin. Our results demonstrate that JNJ-1935 is a novel inhibitor of firefly luciferase enzymatic activity. In contrast, the PHIs FG-4592 (roxadustat) and FG-2216 (ICA, BIQ, IOX3, YM 311) did not affect firefly luciferase. The JNJ-1935 mode of inhibition is competitive with a K_i of 1.36 μM. D-luciferin did not inhibit the PHDs, despite its structural similarity to JNJ-1935. This study provides insights into a previously unknown JNJ-1935 off-target effect as well as into the chemical requirements for firefly luciferase and PHD inhibitors and may inform the development of novel compounds targeting these enzymes.

Genetically Encoded Tools for Research of Cell Signaling and Metabolism under Brain Hypoxia

Hypoxia is characterized by low oxygen content in the tissues. The central nervous system (CNS) is highly vulnerable to a lack of oxygen. Prolonged hypoxia leads to the death of brain cells, which underlies the development of many pathological conditions. Despite the relevance of the topic, different approaches used to study the molecular mechanisms of hypoxia have many limitations. One promising lead is the use of various genetically encoded tools that allow for the observation of intracellular parameters in living systems. In the first part of this review, we provide the classification of oxygen/hypoxia reporters as well as describe other genetically encoded reporters for various metabolic and redox parameters that could be implemented in hypoxia studies. In the second part, we discuss the advantages and disadvantages of the primary hypoxia model systems and highlight inspiring examples of research in which these experimental settings were combined with genetically encoded reporters.

Adapt and conquer: Metabolic flexibility in cancer growth, invasion and evasion

BACKGROUND

It has been known for close to a century that, on average, tumors have a metabolism that is different from those found in healthy tissues. Typically, tumors show a biosynthetic metabolism that distinguishes itself by engaging in large scale aerobic glycolysis, heightened flux through the pentose phosphate pathway, and increased glutaminolysis among other means. However, it is becoming equally clear that non tumorous tissues at times can engage in similar metabolism, while tumors show a high degree of metabolic flexibility reacting to cues, and stresses in their local environment.

SCOPE OF THE REVIEW

In this review, we want to scrutinize historic and recent research on metabolism, comparing and contrasting oncogenic and physiological metabolic states. This will allow us to better define states of bona fide tumor metabolism. We will further contextualize the stress response and the metabolic evolutionary trajectory seen in tumors, and how these contribute to tumor progression. Lastly, we will analyze the implications of these characteristics with respect to therapy response.

MAJOR CONCLUSIONS

In our review, we argue that there is not one single oncogenic state, but rather a diverse set of oncogenic states. These are grounded on a physiological proliferative/wound healing program but distinguish themselves due to their large scale of proliferation, mutations, and transcriptional changes in key metabolic pathways, and the adaptations to widespread stress signals within tumors. We find evidence for the necessity of metabolic flexibility and stress responses in tumor progression and how these responses in turn shape oncogenic progression. Lastly, we find evidence for the notion that the metabolic adaptability of tumors frequently frustrates therapeutic interventions.

Bclaf1 is a direct target of HIF-1 and critically regulates the stability of HIF-1α under hypoxia

Hypoxic stress is intimately connected with tumor progression, with hypoxia-inducible factor-1α (HIF-1α) being a critical regulator in this process. HIF-1α is stabilized in response to hypoxia, which is required for the induction of gene transcriptions important for hypoxic adaptation. Bclaf1 is a multifunctional protein involved in tumorigenesis, however, its role in this process is not well characterized. Here we report Bclaf1 is a direct transcriptional target of HIF-1 and upregulated in multiple cell lines during hypoxia. Importantly, we found Bclaf1 is involved in the stabilization of HIF-1α during long-term hypoxic treatments. Compared with the control cells, the protein level and stability of HIF-1α in Bclaf1 knockdown or knockout cells is greatly compromised after long-term hypoxic treatments, concomitant with the impaired inductions of HIF-1 target gene transcription. Bclaf1 knockout HeLa cells exhibit a reduced tumor growth in mice xenografts, in which the expressions of HIF-1α and its target genes are also decreased. Bclaf1 binds to HIF-1α in the nucleus, and this interaction is required for Bclaf1 to stabilize HIF-1α in hypoxic condition. These results uncover a positive feedback loop, HIF-1-Bclaf1, that sustains HIF-1 activity during long-term hypoxic conditions by binding to and protecting HIF-1α from degradation, and suggest that Bclaf1 may promote tumor progression by enhancing HIF-1α stability.

HIF hydroxylase inhibitors decrease cellular oxygen consumption depending on their selectivity

Pharmacologic HIF hydroxylase inhibitors (HIs) are effective for the treatment of anemia in chronic kidney disease patients and may also be beneficial for the treatment of diseases such as chronic inflammation and ischemia-reperfusion injury. The selectivities of many HIs for HIF hydroxylases and possible off-target effects in cellulo are unclear, delaying the translation from preclinical studies to clinical trials. We developed a novel assay that discriminates between the inhibition of HIF-α prolyl-4-hydroxylase domain (PHD) enzymes and HIF-α asparagine hydroxylase factor inhibiting HIF (FIH). We characterized 15 clinical and preclinical HIs, categorizing them into pan-HIF-α hydroxylase (broad spectrum), PHD-selective, and FIH-selective inhibitors, and investigated their effects on HIF-dependent transcriptional regulation, erythropoietin production, and cellular energy metabolism. While energy homeostasis was generally maintained following HI treatment, the pan-HIs led to a stronger increase in pericellular pO₂ than the PHD/FIH-selective HIs. Combined knockdown of FIH and PHD-selective inhibition did not further increase pericellular pO₂ . Hence, the additional increase in pericellular pO₂ by pan- over PHD-selective HIs likely reflects HIF hydroxylase independent off-target effects. Overall, these analyses demonstrate that HIs can lead to oxygen redistribution within the cellular microenvironment, which should be considered as a possible contributor to HI effects in the treatment of hypoxia-associated diseases.

Mechanisms of hypoxia signalling: new implications for nephrology

Studies of the regulation of erythropoietin (EPO) production by the liver and kidneys, one of the classical physiological responses to hypoxia, led to the discovery of human oxygen-sensing mechanisms, which are now being targeted therapeutically. The oxygen-sensitive signal is generated by 2-oxoglutarate-dependent dioxygenases that deploy molecular oxygen as a co-substrate to catalyse the post-translational hydroxylation of specific prolyl and asparaginyl residues in hypoxia-inducible factor (HIF), a key transcription factor that regulates transcriptional responses to hypoxia. Hydroxylation of HIF at different sites promotes both its degradation and inactivation. Under hypoxic conditions, these processes are suppressed, enabling HIF to escape destruction and form active transcriptional complexes at thousands of loci across the human genome. Accordingly, HIF prolyl hydroxylase inhibitors stabilize HIF and stimulate expression of HIF target genes, including the EPO gene. These molecules activate endogenous EPO gene expression in diseased kidneys and are being developed, or are already in clinical use, for the treatment of renal anaemia. In this Review, we summarize information on the molecular circuitry of hypoxia signalling pathways underlying these new treatments and highlight some of the outstanding questions relevant to their clinical use.

In chronic hypoxia, glucose availability and hypoxic severity dictate the balance between HIF-1 and HIF-2 in astrocytes

Astrocyte function is an important contributor to cellular viability during brain hypoxia and ischemia. Levels of the hypoxia-inducible transcription factors (HIFs) HIF-1 and HIF-2 are increased in hypoxic conditions and impact the neuroprotective properties of astrocytes. For example, HIF-2 induces levels of erythropoietin (EPO), a neuroprotectant, by astrocytes. In contrast, HIF-1 activity in astrocytes diminishes the viability of neurons in cocultures during hypoxia. Thus, HIF-1 and HIF-2 may have opposing effects on astrocytes. In this study, we explore the balance of HIF-1 and HIF-2 signaling in astrocytes during chronic (1-7 d) hypoxia while altering the degree of hypoxia and glucose availability. In addition, we investigate the effects of these conditions on neuron apoptosis. During exposure to chronic moderate hypoxia (2% O₂) and plentiful glucose (10 mM), HIF-2 and EPO abundance increases from d 1 to 7. Similarly, pretreatment with moderate hypoxia markedly increases the abundance of HIF-2 and EPO when astrocytes are subsequently exposed to severe hypoxia (0.5% O₂; 24 h) in 10 mM glucose, which inhibits neuron apoptosis in coculture. Although HIF-1 targets the expression increase during the 7 d in chronic moderate hypoxia (2% O₂) and limited glucose (2 mM), further exposure to severe hypoxia (0.5% O₂; 24 h) induces a decrease of most HIF-1 targets in astrocytes. Notably, in astrocyte exposure to 2% O₂ prior to 0.5% O₂, the expression of iNOS, an HIF-1-regulated protein, keeps increasing when glucose is limited, whereas EPO and VEGF abundance is suppressed, inducing increased apoptosis of neurons in coculture under limited glucose (2 mM). Thus, both hypoxic severity and glucose abundance regulate the balance of HIF-1 and HIF-2 activity in astrocytes, leading to diverse effects on neurons. These results could have important implications on the adaptive or pathologic role of astrocytes during chronic hypoxia and ischemia.-Guo, M., Ma, X., Feng, Y., Han, S., Dong, Q., Cui, M., Zhao, Y. In chronic hypoxia, glucose availability and hypoxic severity dictate the balance between HIF-1 and HIF-2 in astrocytes.

Metabolic control of gene transcription in non-alcoholic fatty liver disease: the role of the epigenome

Non-alcoholic fatty liver disease (NAFLD) is estimated to affect 24% of the global adult population. NAFLD is a major risk factor for the development of cirrhosis and hepatocellular carcinoma, as well as being strongly associated with type 2 diabetes and cardiovascular disease. It has been proposed that up to 88% of obese adults have NAFLD, and with global obesity rates increasing, this disease is set to become even more prevalent. Despite intense research in this field, the molecular processes underlying the pathology of NAFLD remain poorly understood. Hepatic intracellular lipid accumulation may lead to dysregulated tricarboxylic acid (TCA) cycle activity and associated alterations in metabolite levels. The TCA cycle metabolites alpha-ketoglutarate, succinate and fumarate are allosteric regulators of the alpha-ketoglutarate-dependent dioxygenase family of enzymes. The enzymes within this family have multiple targets, including DNA and chromatin, and thus may be capable of modulating gene transcription in response to intracellular lipid accumulation through alteration of the epigenome. In this review, we discuss what is currently understood in the field and suggest areas for future research which may lead to the development of novel preventative or therapeutic interventions for NAFLD.

Modeling the regulation of p53 activation by HIF-1 upon hypoxia

As a famous tumor suppressor, p53 is also activated under hypoxic conditions. Hypoxia-inducuble factor 1, HIF-1, is involved in the activation of p53 upon hypoxia. However, how p53 is modulated by the HIF-1 pathway to decide cell fate is less understood. In this work, we developed a network model including p53 and HIF-1 pathways to clarify the mechanism of cell fate decision in response to hypoxia. We found that HIF-1α and p53 are activated under different conditions. Under moderate hypoxia, HIF-1α is activated to induce glycolysis or angiogenesis, and promotes partial accumulation of p53 by inducing PNUTS. Under severe hypoxia, p53 rises to high levels due to ATR-dependent stabilization and promotes Mdm2-dependent HIF-1α degradation. As a result, fully activated p53 triggers apoptosis. Of note, competition for p300 between HIF-1α and p53 plays a key role in regulating their transcriptional activities. This work may advance the understanding of the mechanism for p53 regulation by HIF-1 in the hypoxic response.

Oxygen-dependent bond formation with FIH regulates the activity of the client protein OTUB1

Protein:protein interactions are the basis of molecular communication and are usually of transient non-covalent nature, while covalent interactions other than ubiquitination are rare. For cellular adaptations, the cellular oxygen and peroxide sensor factor inhibiting HIF (FIH) confers oxygen and oxidant stress sensitivity to the hypoxia inducible factor (HIF) by asparagine hydroxylation. We investigated whether FIH contributes to hypoxia adaptation also through other mechanisms and identified a hypoxia sensitive, likely covalent, bond formation by FIH with several client proteins, including the deubiquitinase ovarian tumor domain containing ubiquitin aldehyde binding protein 1 (OTUB1). Biochemical analyses were consistent with a co-translational amide bond formation between FIH and OTUB1, occurring within mammalian and bacterial cells but not between separately purified proteins. Bond formation is catalysed by FIH and highly dependent on oxygen availability in the cellular microenvironment. Within cells, a heterotrimeric complex is formed, consisting of two FIH and one covalently linked OTUB1. Complexation of OTUB1 by FIH regulates OTUB1 deubiquitinase activity. Our findings reveal an alternative mechanism for hypoxia adaptation with remarkably high oxygen sensitivity, mediated through covalent protein-protein interactions catalysed by an asparagine modifying dioxygenase.

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FIH forms a (likely amide) bond with client proteins.

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Bond formation is highly hypoxia sensitive and occurs co-translationally.

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FIH forms a heterotrimer with the client protein OTUB1 (FIH₂OTUB1₁).

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Complex formation between OTUB1 and FIH regulates OTUB1 deubiquitinase activity.

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Bond formation by hydroxylases is an alternative mechanism for hypoxia adaptation.

Aspergillus fumigatus Challenged by Human Dendritic Cells: Metabolic and Regulatory Pathway Responses Testify a Tight Battle

Dendritic cells (DCs) are antigen presenting cells which serve as a passage between the innate and the acquired immunity. Aspergillosis is a major lethal condition in immunocompromised patients caused by the adaptable saprophytic fungus Aspergillus fumigatus. The healthy human immune system is capable to ward off A. fumigatus infections however immune-deficient patients are highly vulnerable to invasive aspergillosis. A. fumigatus can persist during infection due to its ability to survive the immune response of human DCs. Therefore, the study of the metabolism specific to the context of infection may allow us to gain insight into the adaptation strategies of both the pathogen and the immune cells. We established a metabolic model of A. fumigatus central metabolism during infection of DCs and calculated the metabolic pathway (elementary modes; EMs). Transcriptome data were used to identify pathways activated when A. fumigatus is challenged with DCs. In particular, amino acid metabolic pathways, alternative carbon metabolic pathways and stress regulating enzymes were found to be active. Metabolic flux modeling identified further active enzymes such as alcohol dehydrogenase, inositol oxygenase and GTP cyclohydrolase participating in different stress responses in A. fumigatus. These were further validated by qRT-PCR from RNA extracted under these different conditions. For DCs, we outlined the activation of metabolic pathways in response to the confrontation with A. fumigatus. We found the fatty acid metabolism plays a crucial role, along with other metabolic changes. The gene expression data and their analysis illuminate additional regulatory pathways activated in the DCs apart from interleukin regulation. In particular, Toll-like receptor signaling, NOD-like receptor signaling and RIG-I-like receptor signaling were active pathways. Moreover, we identified subnetworks and several novel key regulators such as UBC, EGFR, and CUL3 of DCs to be activated in response to A. fumigatus. In conclusion, we analyze the metabolic and regulatory responses of A. fumigatus and DCs when confronted with each other.

Decrease of Nibrin expression in chronic hypoxia is associated with hypoxia-induced chemoresistance in some brain tumour cells

Background

Solid tumours are less oxygenated than normal tissues. This is called tumour hypoxia and leads to resistance to radiotherapy and chemotherapy. The molecular mechanisms underlying such resistance have been investigated in a range of tumour types, including the adult brain tumours glioblastoma, yet little is known for paediatric brain tumours. Medulloblastoma (MB) is the most common malignant brain tumour in children. We aimed to elucidate the impact of hypoxia on the sensitivity of MB cells to chemo- and radiotherapy.

Methods

We used two MB cell line (D283-MED and MEB-Med8A) and a widely used glioblastoma cell line (U87MG) for comparison. We applied a range of molecular and cellular techniques to measure cell survival, cell cycle progression, protein expression and DNA damage combined with a transcriptomic micro-array approach in D283-MED cells, for global gene expression analysis in acute and chronic hypoxic conditions.

Results

In D283-MED and U87MG, chronic hypoxia (5 days), but not acute hypoxia (24 h) induced resistance to chemotherapy and X-ray irradiation. This acquired resistance upon chronic hypoxia was present but less pronounced in MEB-Med8A cells. Using transcriptomic analysis in D283-MED cells, we found a large transcriptional remodelling upon long term hypoxia, in particular the expression of a number of genes involved in detection and repair of double strand breaks (DSB) was altered. The levels of Nibrin (NBN) and MRE11, members of the MRN complex (MRE11/Rad50/NBN) responsible for DSB recognition, were significantly down-regulated. This was associated with a reduction of Ataxia Telangiectasia Mutated (ATM) activation by etoposide, indicating a profound dampening of the DNA damage signalling in hypoxic conditions. As a consequence, p53 activation by etoposide was reduced, and cell survival enhanced. Whilst U87MG shared the same dampened p53 activity, upon chemotherapeutic drug treatment in chronic hypoxic conditions, these cells used a different mechanism, independent of the DNA damage pathway.

Conclusion

Together our results demonstrate a new mechanism explaining hypoxia-induced resistance involving the alteration of the response to DSB in D283-MED cells, but also highlight the cell type to cell type diversity and the necessity to take into account the differing tumour genetic make-up when considering re-sensitisation therapeutic protocols.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5476-9) contains supplementary material, which is available to authorized users.

Source and microenvironmental regulation of erythropoietin in the kidney

PURPOSE OF REVIEW

Historically, the identity of O2-sensing renal erythropoietin (Epo)-producing (REP) cells was a matter of debate. This review summarizes how recent breakthroughs in transgenic mouse and in-situ hybridization techniques have facilitated sensitive and specific detection of REP cells and accelerated advancements in the understanding of the regulation of renal Epo production in health and disease.

RECENT FINDINGS

REP cells are a dynamically regulated unique subpopulation of tubulointerstitial cells with features of fibroblasts, pericytes and neurons. Under normal conditions, REP cells are located in the corticomedullary border region within a steep decrement in O2 availability. During the progression of chronic kidney disease (CKD), REP cells cease Epo production, dedifferentiate and contribute to the progression of renal fibrosis. However, CKD patients with renal anaemia still respond with elevated Epo production following treatment with hypoxia-mimicking agents.

SUMMARY

We hypothesize that REP cells are neuron-like setpoint providers and controllers, which integrate information about blood O2 concentration and local O2 consumption via tissue pO2, and combine these inputs with intrinsic negative feedback loops and perhaps tubular cross-talk, converging in Epo regulation.

Generation of renal Epo-producing cell lines by conditional gene tagging reveals rapid HIF-2 driven Epo kinetics, cell autonomous feedback regulation, and a telocyte phenotype

Erythropoietin (Epo) is essential for erythropoiesis and is mainly produced by the fetal liver and the adult kidney following hypoxic stimulation. Epo regulation is commonly studied in hepatoma cell lines, but differences in Epo regulation between kidney and liver limit the understanding of Epo dysregulation in polycythaemia and anaemia. To overcome this limitation, we have generated a novel transgenic mouse model expressing Cre recombinase specifically in the active fraction of renal Epo-producing (REP) cells. Crossing with reporter mice confirmed the inducible and highly specific tagging of REP cells, located in the corticomedullary border region where there is a steep drop in oxygen bioavailability. A novel method was developed to selectively grow primary REP cells in culture and to generate immortalized clonal cell lines, called fibroblastoid atypical interstitial kidney (FAIK) cells. FAIK cells show very early hypoxia-inducible factor (HIF)-2α induction, which precedes Epo transcription. Epo induction in FAIK cells reverses rapidly despite ongoing hypoxia, suggesting a cell autonomous feedback mechanism. In contrast, HIF stabilizing drugs resulted in chronic Epo induction in FAIK cells. RNA sequencing of three FAIK cell lines derived from independent kidneys revealed a high degree of overlap and suggests that REP cells represent a unique cell type with properties of pericytes, fibroblasts, and neurons, known as telocytes. These novel cell lines may be helpful to investigate myofibroblast differentiation in chronic kidney disease and to elucidate the molecular mechanisms of HIF stabilizing drugs currently in phase III studies to treat anemia in end-stage kidney disease.

Glycolytic Response to Inflammation Over Time: Role of Myeloid HIF-1alpha

The in vivo response to lipopolysaccharide (LPS) occurs rapidly and has profound physiological and metabolic effects. The hypoxia inducible (HIF) transcription factor is an intrinsic and essential part of inflammation, and is induced by LPS. To determine the importance of the HIF response in regulating metabolism following an LPS response, glucose uptake was quantified in a time dependent manner in mice lacking HIF-1α in myeloid cells. We found that deletion of HIF-1α has an acute protective effect on LPS-induced hypoglycemia. Furthermore, reduced glucose uptake was observed in the heart and brown fat, in a time dependent manner, following loss of HIF-1α. To determine the physiological significance of these findings, cardiovascular, body temperature, and blood pressure changes were subsequently quantified in real time using radiotelemetry measurements. These studies reveal the temporal aspects of HIF-1α as a regulator of the metabolic response to acute LPS-induced inflammation.

A HIF-LIMD1 negative feedback mechanism mitigates the pro-tumorigenic effects of hypoxia

The adaptive cellular response to low oxygen tensions is mediated by the hypoxia-inducible factors (HIFs), a family of heterodimeric transcription factors composed of HIF-α and HIF-β subunits. Prolonged HIF expression is a key contributor to cellular transformation, tumorigenesis and metastasis. As such, HIF degradation under hypoxic conditions is an essential homeostatic and tumour-suppressive mechanism. LIMD1 complexes with PHD2 and VHL in physiological oxygen levels (normoxia) to facilitate proteasomal degradation of the HIF-α subunit. Here, we identify LIMD1 as a HIF-1 target gene, which mediates a previously uncharacterised, negative regulatory feedback mechanism for hypoxic HIF-α degradation by modulating PHD2-LIMD1-VHL complex formation. Hypoxic induction of LIMD1 expression results in increased HIF-α protein degradation, inhibiting HIF-1 target gene expression, tumour growth and vascularisation. Furthermore, we report that copy number variation at the LIMD1 locus occurs in 47.1% of lung adenocarcinoma patients, correlates with enhanced expression of a HIF target gene signature and is a negative prognostic indicator. Taken together, our data open a new field of research into the aetiology, diagnosis and prognosis of LIMD1-negative lung cancers.

HIF1 mediates a switch in pyruvate kinase isoforms after myocardial infarction

Alternative splicing of RNA is an underexplored area of transcriptional response. We expect that early changes in alternatively spliced genes may be important for responses to cardiac injury. Hypoxia inducible factor 1 (HIF1) is a key transcription factor that rapidly responds to loss of oxygen through alteration of metabolism and angiogenesis. The goal of this study was to investigate the transcriptional response after myocardial infarction (MI) and to identify novel, hypoxia-driven changes, including alternative splicing. After ligation of the left anterior descending artery in mice, we observed an abrupt loss of cardiac contractility and upregulation of hypoxic signaling. We then performed RNA sequencing on ischemic heart tissue 1 and 3 days after infarct to assess early transcriptional changes and identified 89 transcripts with altered splicing. Of particular interest was the switch in Pkm isoform expression (pyruvate kinase, muscle). The usually predominant Pkm1 isoform was less abundant in ischemic hearts, while Pkm2 and associated splicing factors (hnRNPA1, hnRNPA2B1, Ptbp1) rapidly increased. Despite increased Pkm2 expression, total pyruvate kinase activity remained reduced in ischemic myocardial tissue. We also demonstrated HIF1 binding to PKM by chromatin immunoprecipitation, indicating a direct role for HIF1 in mediating this isoform switch. Our study provides a new, detailed characterization of the early transcriptome after MI. From this analysis, we identified an HIF1-mediated alternative splicing event in the PKM gene. Pkm1 and Pkm2 play distinct roles in glycolytic metabolism and the upregulation of Pkm2 is likely to have important consequences for ATP synthesis in infarcted cardiac muscle.