Co-transactivation of the 3' erythropoietin hypoxia inducible enhancer by the HIF-1 protein

Long-term HIF-1α stabilization reduces respiration, promotes mitophagy, and results in retinal cell death

Ocular hypertension during glaucoma can lead to hypoxia, activation of the HIF transcription factors, and a metabolic shift toward glycolysis. This study aims to test whether chronic HIF activation and the attendant metabolic reprogramming can initiate glaucoma-associated pathology independently of ocular hypertension. HIF-1α stabilization was induced in mice for 2 and 4 weeks by inhibiting prolyl hydroxylases using the small molecule Roxadustat. HIF-1α stabilization and the expression of its downstream bioenergetic targets were investigated in the retina by immunofluorescence, capillary electrophoresis, and biochemical enzyme activity assays. Roxadustat dosing resulted in significant stabilization of HIF-1α in the retina by 4 weeks, and upregulation in glycolysis-associated proteins (GLUT3, PDK-1) and enzyme activity in both neurons and glia. Accordingly, succinate dehydrogenase, mitochondrial marker MTCO1, and citrate synthase activity were significantly decreased at 4 weeks, while mitophagy was significantly increased. TUNEL assay showed significant apoptosis of cells in the retina, and PERG amplitude was significantly decreased with 4 weeks of HIF-1α stabilization. A significant increase in AMPK activation and glial hypertrophy, concomitant with decreases in retinal ganglion cell function and inner retina cell death suggests that chronic HIF-1α stabilization alone is detrimental to retina metabolic homeostasis and cellular survival.

Fetal Oxygenation from the 23rd to the 36th Week of Gestation Evaluated through the Umbilical Cord Blood Gas Analysis

The embryo and fetus grow in a hypoxic environment. Intrauterine oxygen levels fluctuate throughout the pregnancy, allowing the oxygen to modulate apparently contradictory functions, such as the expansion of stemness but also differentiation. We have recently demonstrated that in the last weeks of pregnancy, oxygenation progressively increases, but the trend of oxygen levels during the previous weeks remains to be clarified. In the present retrospective study, umbilical venous and arterial oxygen levels, fetal oxygen extraction, oxygen content, CO2, and lactate were evaluated in a cohort of healthy newborns with gestational age < 37 weeks. A progressive decrease in pO2 levels associated with a concomitant increase in pCO2 and reduction in pH has been observed starting from the 23rd week until approximately the 33-34th week of gestation. Over this period, despite the increased hypoxemia, oxygen content remains stable thanks to increasing hemoglobin concentration, which allows the fetus to become more hypoxemic but not more hypoxic. Starting from the 33-34th week, fetal oxygenation increases and ideally continues following the trend recently described in term fetuses. The present study confirms that oxygenation during intrauterine life continues to vary even after placenta development, showing a clear biphasic trend. Fetuses, in fact, from mid-gestation to near-term, become progressively more hypoxemic. However, starting from the 33-34th week, oxygenation progressively increases until birth. In this regard, our data suggest that the placenta is the hub that ensures this variable oxygen availability to the fetus, and we speculate that this biphasic trend is functional for the promotion, in specific tissues and at specific times, of stemness and intrauterine differentiation.

Mechanisms governing target search and binding dynamics of hypoxia-inducible factors

Transcription factors (TFs) are classically attributed a modular construction, containing well-structured sequence-specific DNA-binding domains (DBDs) paired with disordered activation domains (ADs) responsible for protein-protein interactions targeting co-factors or the core transcription initiation machinery. However, this simple division of labor model struggles to explain why TFs with identical DNA-binding sequence specificity determined in vitro exhibit distinct binding profiles in vivo. The family of hypoxia-inducible factors (HIFs) offer a stark example: aberrantly expressed in several cancer types, HIF-1α and HIF-2α subunit isoforms recognize the same DNA motif in vitro - the hypoxia response element (HRE) - but only share a subset of their target genes in vivo, while eliciting contrasting effects on cancer development and progression under certain circumstances. To probe the mechanisms mediating isoform-specific gene regulation, we used live-cell single particle tracking (SPT) to investigate HIF nuclear dynamics and how they change upon genetic perturbation or drug treatment. We found that HIF-α subunits and their dimerization partner HIF-1β exhibit distinct diffusion and binding characteristics that are exquisitely sensitive to concentration and subunit stoichiometry. Using domain-swap variants, mutations, and a HIF-2α specific inhibitor, we found that although the DBD and dimerization domains are important, another main determinant of chromatin binding and diffusion behavior is the AD-containing intrinsically disordered region (IDR). Using Cut&Run and RNA-seq as orthogonal genomic approaches, we also confirmed IDR-dependent binding and activation of a specific subset of HIF target genes. These findings reveal a previously unappreciated role of IDRs in regulating the TF search and binding process that contribute to functional target site selectivity on chromatin.

Differential placental DNA methylation of VEGFA and LEP in small-for-gestational age fetuses with an abnormal cerebroplacental ratio

Background

In Fetal Growth Restriction ‘fetal programming’ may take place via DNA methylation, which has implications for short-term and long-term health outcomes. Small-for-gestational age fetuses are considered fetal growth restricted, characterized by brain-sparing when fetal Doppler hemodynamics are abnormal, expressed as a cerebroplacental ratio (CPR) <1. We aimed to determine whether brain-sparing is associated with altered DNA methylation of selected genes.

Methods

We compared DNA methylation of six genes in 41 small-for-gestational age placentas with a normal or abnormal CPR. We selected EPO, HIF1A, VEGFA, LEP, PHLDA2, and DHCR24 for their role in angiogenesis, immunomodulation, and placental and fetal growth. DNA methylation was analyzed by pyrosequencing.

Results

Growth restricted fetuses with an abnormal CPR showed hypermethylation of the VEGFA gene at one CpG (VEGFA-309, p = .001) and an overall hypomethylation of the LEP gene, being significant at two CpGs (LEP-123, p = .049; LEP-51, p = .020). No differences in methylation were observed for the other genes.

Conclusions

VEGFA and LEP genes are differentially methylated in placentas of small-for-gestational age fetuses with brain-sparing. Hypermethylation of VEGFA-309 in abnormal CPR-placentas could indicate successful compensatory mechanisms. Methylation of LEP-51 is known to suppress LEP expression. Hypomethylation in small-for-gestational age placentas with abnormal CPR may result in hyperleptinemia and predispose to leptin-resistance later in life.

The hypoxia inducible factor/erythropoietin (EPO)/EPO receptor pathway is disturbed in a rat model of chronic kidney disease related anemia

Objectives

Anemia is a known driver for hypoxia inducible factor (HIF) which leads to increased renal erythropoietin (EPO) synthesis. Bone marrow (BM) EPO receptor (EPOR) signals are transduced through a JAK2-STAT5 pathway. The origins of anemia of chronic kidney disease (CKD) are multifactorial, including impairment of both renal EPO synthesis as well as intestinal iron absorption. We investigated the HIF- EPO- EPOR axis in kidney, BM and proximal tibia in anemic juvenile CKD rats.

Methods

CKD was induced by 5/6 nephrectomy in young (20 days old) male Sprague-Dawley rats while C group was sham operated. Rats were sacrificed 4 weeks after CKD induction and 5 minutes after a single bolus of IV recombinant human EPO. An additional control anemic (C-A) group was daily bled for 7 days.

Results

Hemoglobin levels were similarly reduced in CKD and C-A (11.4 ± 0.3 and 10.8±0.2 Vs 13.5±0.3 g/dL in C, p<0.0001). Liver hepcidin mRNA was decreased in CA but increased in CKD. Serum iron was unchanged while transferrin levels were mildly decreased in CKD. Kidney HIF2α protein was elevated in C-A but unchanged in CKD. Kidney EPO protein and mRNA levels were unchanged between groups. However, BM EPO protein (which reflects circulating EPO) was increased in C-A but remained unchanged in CKD. BM and proximal tibia EPOR were unchanged in C-A but decreased in CKD. Proximal tibial phospho-STAT5 increased after the EPO bolus in C but not in CKD.

Conclusions

Compared to blood loss, anemia in young CKD rats is associated with inappropriate responses in the HIF-EPO-EPO-R axis: kidney HIF2α and renal EPO are not increased, BM and bone EPOR levels, as well as bone pSTAT5 response to EPO are reduced. Thus, anemia of CKD may be treated with additional therapeutic avenues beyond iron and EPO supplementation.

Hypoxia inducible factor-1 alpha and prolinhydroxlase 2 polymorphisms in patients with severe sepsis: a prospective observational trial

BACKGROUND

Hypoxia-inducible-factor-1α (HIF-1α) and HIF-1 degrading prolyl-hydroxylases (PHD) are key regulators of the hypoxic-inflammatory response. Functionally active genetic variants in the HIF-1α (C/T; Single Nucleotide Polymorphism (SNP) rs11549465) and the PHD2 gene (EGLN1; C/T; SNP rs516651 and T/C; SNP rs480902) are associated with altered HIF-1α mRNA nuclear translocation and an altered adaptation to hypoxia. Furthermore, the HIF system is important in surviving inflammatory disorders and sepsis. Thus, we tested the hypotheses, that SNPs in the HIF-1α or PHD2 genes are (1) common in Caucasians, with 2) the HIF-1α genetic variant being associated with an altered HIF-1α mRNA expression; and 3) independent risk factors for 30-day mortality in severe sepsis.

METHODS

After ethics approval, 128 septic patients (Caucasian descent) were included prospectively within 24 h after first diagnosing sepsis. Patients characteristics and severity of illness (simplified acute physiology score II), genotypes (Taqman assay), and their influence on leukocyte HIF-1α-mRNA-expression (Real-Time PCR) and 30-day mortality were determined.

RESULTS

Frequencies were 0.8 % for homozygous HIF-1α TT-carriers (CT 17.6 %; CC 81.6 %), 2.5 % for homozygous PHD2 SNP rs516651 TT-allele carriers (CT 17.5 % and CC 80 %), and 9.4 % for homozygous PHD2 SNP rs480902 TT-allele carriers (CT 34.4 % and CC 56.3 %). While HIF-1α T-allele carriers had a borderline decrease in HIF-1α-mRNA-expression (p = 0.06) neither HIF-1α nor PHD2 SNPs were (independent) risk factors for 30-day mortality.

CONCLUSIONS

Genetic variants in HIF-1α and PHD2 genes exist in Caucasians but do not appear to alter 30-day mortality in sepsis.

Genome-wide mapping of Hif-1α binding sites in zebrafish

Background

Hypoxia Inducible Factor (HIF) regulates a cascade of transcriptional events in response to decreased oxygenation, acting from the cellular to the physiological level. This response is evolutionarily conserved, allowing the use of zebrafish (Danio rerio) as a model for studying the hypoxic response. Activation of the hypoxic response can be achieved in zebrafish by homozygous null mutation of the von Hippel-Lindau (vhl) tumour suppressor gene. Previous work from our lab has focused on the phenotypic characterisation of this mutant, establishing the links between vhl mutation, the hypoxic response and cancer. To further develop fish as a model for studying hypoxic signalling, we examine the transcriptional profile of the vhl mutant with respect to Hif-1α. As our approach uses embryos consisting of many cell types, it has the potential to uncover additional HIF regulated genes that have escaped detection in analogous mammalian cell culture studies.

Results

We performed high-density oligonucleotide microarray analysis of the gene expression changes in von Hippel-Lindau mutant zebrafish, which identified up-regulation of well-known hypoxia response genes and down-regulation of genes primarily involved in lipid processing. To identify the dependency of these transcriptional changes on HIF, we undertook Chromatin Immunoprecipitation linked next generation sequencing (ChIP-seq) for the transcription factor Hypoxia Inducible Factor 1α (HIF-1α). We identified HIF-1α binding sites across the genome, with binding sites showing enrichment for an RCGTG motif, showing conservation with the mammalian hypoxia response element.

Conclusions

Transcriptome analysis of vhl mutant embryos detected activation of key hypoxia response genes seen in human cell models of hypoxia, but also suppression of many genes primarily involved in lipid processing. ChIP-seq analysis of Hif-1α binding sites unveiled an unprecedented number of loci, with a high proportion containing a canonical hypoxia response element. Whether these sites are functional remains unknown, nevertheless their frequent location near transcriptional start sites suggests functionality, and will allow for investigation into the potential hypoxic regulation of genes in their vicinity. We expect that our data will be an excellent starting point for analysis of both fish and mammalian gene regulation by HIF.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2169-x) contains supplementary material, which is available to authorized users.

Erythropoietin improves motor and cognitive deficit, axonal pathology, and neuroinflammation in a combined model of diffuse traumatic brain injury and hypoxia, in association with upregulation of the erythropoietin receptor

Background

Diffuse axonal injury is a common consequence of traumatic brain injury (TBI) and often co-occurs with hypoxia, resulting in poor neurological outcome for which there is no current therapy. Here, we investigate the ability of the multifunctional compound erythropoietin (EPO) to provide neuroprotection when administered to rats after diffuse TBI alone or with post-traumatic hypoxia.

Methods

Sprague–Dawley rats were subjected to diffuse traumatic axonal injury (TAI) followed by 30 minutes of hypoxic (Hx, 12% O₂) or normoxic ventilation, and were administered recombinant human EPO-α (5000 IU/kg) or saline at 1 and 24 hours post-injury. The parameters examined included: 1) behavioural and cognitive deficit using the Rotarod, open field and novel object recognition tests; 2) axonal pathology (NF-200); 3) callosal degradation (hematoxylin and eosin stain); 3) dendritic loss (MAP2); 4) expression and localisation of the EPO receptor (EpoR); 5) activation/infiltration of microglia/macrophages (CD68) and production of IL-1β.

Results

EPO significantly improved sensorimotor and cognitive recovery when administered to TAI rats with hypoxia (TAI + Hx). A single dose of EPO at 1 hour reduced axonal damage in the white matter of TAI + Hx rats at 1 day by 60% compared to vehicle. MAP2 was decreased in the lateral septal nucleus of TAI + Hx rats; however, EPO prevented this loss, and maintained MAP2 density over time. EPO administration elicited an early enhanced expression of EpoR 1 day after TAI + Hx compared with a 7-day peak in vehicle controls. Furthermore, EPO reduced IL-1β to sham levels 2 hours after TAI + Hx, concomitant to a decrease in CD68 positive cells at 7 and 14 days.

Conclusions

When administered EPO, TAI + Hx rats had improved behavioural and cognitive performance, attenuated white matter damage, resolution of neuronal damage spanning from the axon to the dendrite, and suppressed neuroinflammation, alongside enhanced expression of EpoR. These data provide compelling evidence of EPO’s neuroprotective capability. Few benefits were observed when EPO was administered to TAI rats without hypoxia, indicating that EPO’s neuroprotective capacity is bolstered under hypoxic conditions, which may be an important consideration when EPO is employed for neuroprotection in the clinic.

Suppression of the dual-specificity phosphatase MKP-1 enhances HIF-1 trans-activation and increases expression of EPO

Hypoxia-inducible factor 1 (HIF-1) is a phosphorylated protein and its phosphorylation is involved in HIF-1alpha subunit stabilization as well as in the regulation of HIF-1 transcriptional activity. In a variety of cell lines, the phosphorylation of HIF-1alpha is dependent on ERK or p38, two members of the mitogen-activated protein kinase (MAPK) superfamily. In addition, active MAPK could be inactivated through dephosphorylation by mitogen-activated protein kinase phosphatase-1 (MKP-1). MKP-1 has been identified as a hypoxia responsive gene, but its role in the response of cells to hypoxia is poorly understood. Here we found that hypoxia induces MKP-1 expression in human hepatoma cells HepG2 in a time-dependent manner. Inhibition of MKP-1 expression using siRNA technique could enhance HIF-1alpha phosphorylation, accompanied by an increase in transcriptionally active HIF-1 as well as a rise in the levels of HIF-1-induced erythropoietin expression.

Interleukin-1β and Tumor Necrosis Factor- Stimulate DNA Binding of Hypoxia-Inducible Factor-1

The rate of transcription of several genes encoding proteins involved in O2 and energy homeostasis is controlled by hypoxia-inducible factor-1 (HIF-1), a heterodimeric DNA binding complex composed of  and β subunits. HIF-1 is considered the primarytrans-acting factor for the erythropoietin (EPO) and vascular endothelial growth factor (VEGF) genes. Since EPO gene expression is inhibited by the proinflammatory cytokines interleukin-1β (IL-1β) and tumor necrosis factor- (TNF-), while no such effect has been reported with respect to the VEGF gene, we investigated the effects of IL-1β and TNF- on the activation of the HIF-1 DNA-binding complex and the amount of HIF-1 protein in human hepatoma cells in culture. Under normoxic conditions, both cytokines caused a moderate activation of HIF-1 DNA binding. In hypoxia, cytokines strongly increased HIF-1 activity compared with the effect of hypoxia alone. Only IL-1β increased HIF-1 protein levels. In transient transfection experiments, HIF-1–driven reporter gene expression was augmented by cytokines only under hypoxic conditions. In contrast to their effect on EPO synthesis, neither IL-1β nor TNF- decreased VEGF production. The mRNA levels of HIF-1 and VEGF were unaffected. Thus, cytokine-induced inhibition of EPO production is not mediated by impairment of HIF-1 function. We propose that HIF-1 may be involved in modulating gene expression during inflammation.

Interleukin-1β and Tumor Necrosis Factor- Stimulate DNA Binding of Hypoxia-Inducible Factor-1

The rate of transcription of several genes encoding proteins involved in O2 and energy homeostasis is controlled by hypoxia-inducible factor-1 (HIF-1), a heterodimeric DNA binding complex composed of  and β subunits. HIF-1 is considered the primarytrans-acting factor for the erythropoietin (EPO) and vascular endothelial growth factor (VEGF) genes. Since EPO gene expression is inhibited by the proinflammatory cytokines interleukin-1β (IL-1β) and tumor necrosis factor- (TNF-), while no such effect has been reported with respect to the VEGF gene, we investigated the effects of IL-1β and TNF- on the activation of the HIF-1 DNA-binding complex and the amount of HIF-1 protein in human hepatoma cells in culture. Under normoxic conditions, both cytokines caused a moderate activation of HIF-1 DNA binding. In hypoxia, cytokines strongly increased HIF-1 activity compared with the effect of hypoxia alone. Only IL-1β increased HIF-1 protein levels. In transient transfection experiments, HIF-1–driven reporter gene expression was augmented by cytokines only under hypoxic conditions. In contrast to their effect on EPO synthesis, neither IL-1β nor TNF- decreased VEGF production. The mRNA levels of HIF-1 and VEGF were unaffected. Thus, cytokine-induced inhibition of EPO production is not mediated by impairment of HIF-1 function. We propose that HIF-1 may be involved in modulating gene expression during inflammation.

Modulation of erythropoiesis in rat bone marrow erythroblastic islands by cyclooxygenase inhibition

We designed our study to explore how the inhibition of prostaglandins (PGs) could affect erythropoiesis in bone marrow erythroblastic islands (EIs). To this end, we used hypoxic-stimulated rats-hypobaric hypoxia (42.55 kPa/6 h)-pretreated or not with indomethacin (4 mg/kg/3 days). Blood sampling was done at 0 h, 24 h, and 72 h after hypoxia. The study included estimations of the plasma erythropoietin (EPO) level (by radioimmunoassay), peripheral blood, number of EI from classes I to V per femur, rate of immature cell's differentiation into erythroblasts, and rate of repeated participation of macrophages in new EI reconstruction. Plasma EPO rose significantly (p < 0.01) in all hypoxic rats: 40.5+/-10.15 mU/ml and 46.75+/-16.28 mU/ml and at 0 h versus 13.83+/-6.82 mU/ml in controls. An increased rate of cell differentiation into erythroblasts in EIs (p < 0.01), an enhanced reconstruction in involuted EIs, and a reduced number of maturing EIs (p < 0.01) were observed in all hypoxic animals. However, in indomethacin-pretreated rats, the stimulation of bone marrow erythropoiesis was better expressed. Our results favor the concept that PG inhibition does not attenuate the erythropoietic response to hypoxia and support the hypothesis about the important role of EI macrophages as a local regulator of bone marrow erythropoiesis.

Microenvironmental control of gene expression: implications for tumor angiogenesis, progression, and metastasis

Low oxygen tension (hypoxia) is an important prognostic factor in cancer treatment because it affects tumor formation and malignant progression. Many genes governing these complex processes have been found to be oxygen regulated. This article reviews the present knowledge of hypoxia-inducible gene expression and how this affects angiogenesis, progression, and metastasis. Of particular importance are hypoxia-regulated transcription factors because they can modulate expression of countless different genes. Additional genes analyzed in some detail include those encoding angiogenic growth factors, factors controlling blood flow, and those involved in metastasis. Although hypoxia is generally perceived as a hindrance to cancer therapy, it is possibly exploitable because severe oxygen deficiency is tumor specific. Strategies aimed at using the presence of hypoxia in solid tumors include oxygen sensitive chemotherapy and gene therapy.