Evidence suggests that the activation of the transcription factor hypoxia-inducible factor 1α (HIF-1α) may promote cell survival in hypoxic or ischemic brain. To help understand the role of HIF-1α in neonatal hypoxic-ischemic brain injury, mice with conditional neuron-specific inactivation of HIF-1α underwent hypoxia-ischemia (HI). Mice heterozygous for Cre recombinase under the control of the calcium/calmodulin-dependent kinase II promoter were bred with homozygous ‘floxed’ HIF-1α transgenic mice. The resulting litters produced mice with a forebrain predominant neuronal deletion of HIF-1α (HIF-1αΔ/Δ), as well as littermates without the deletion. In order to verify reduction of HIF-1α at postnatal day 7, HIF-1αΔ/Δ and wild-type mice were exposed to a hypoxic stimulus (8% oxygen) or room air for 1 h, followed by immediate collection of brain cortices for determination of HIF-1α expression. Results of Western blotting of mouse cortices exposed to hypoxia stimulus or room air confirmed that HIF-1αΔ/Δ cortex expressed a minimal amount of HIF-1α protein compared to wild-type cortex with the same hypoxic stimulus. Subsequently, pups underwent the Vannucci procedure of HI at postnatal day 7: unilateral ligation of the right common carotid artery followed by 30 min of hypoxia (8% oxygen). Immunofluorescent staining of brains 24 h after HI confirmed a relative lack of HIF-1α in the HIF-1αΔ/Δ cortex compared to the wild type, and that HIF-1α in the wild type is located in neurons. HIF-1α expression was determined in mouse cortex 24 h after HI. Histological analysis for the degree of injury was performed 5 days after HI. HIF-1α protein expression 24 h after HI showed a large increase of HIF-1α in the hypoxic-ischemic cortex of the wild-type compared to the hypoxic only cortex. Histological analysis revealed that HI injury was increased in the neuronally deficient HIF-1αΔ/Δ mouse brain (p < 0.05) and was more severe in the cortex. Genetic reduction of neuronal HIF-1α results in a worsening of injury after neonatal HI, with a region-specific role for HIF-1α in the setting of neonatal brain injury.
Brain injury; Transcription factor; Hypoxia-inducible factor 1α; Neonatal hypoxia-ischemia
Hypoxia-Inducible Factor (HIF)-1 is a dimeric protein complex that plays an integral role in the body's response to low oxygen concentrations, or hypoxia. HIF-1 is among the primary genes involved in the homeostatic process, which can increase vascularization in hypoxic areas such as localized ischemia and tumors. It is a transcription factor for dozens of target genes; HIF-1 is also essential for immunological responses and is a crucial physiological regulator of homeostasis, vascularization, and anaerobic metabolism. Furthermore, HIF-1 is increasingly studied because of its perceived therapeutic potential. As it causes angiogenesis, enhancement of this gene within ischemic patients could promote the vessel proliferation needed for oxygenation. In contrast, as HIF-1 allows for survival and proliferation of cancerous cells due to its angiogenic properties, inhibition potentially could prevent the spread of cancer. With a growing understanding of the HIF-1 pathway, the inhibition and stimulation of its transcriptional activity via small molecules is now an attractive goal. Gene therapy to achieve both vessel proliferation and tumor regression has been demonstrated in animal studies but requires significant improvement and modification before becoming commercially available. This review focuses on the potential of the HIF-1 pathway in therapeutic intervention for the treatment of diseases such as cancer and ischemia.
Hypoxia-inducible factor-1 (HIF-1) is the major hypoxia-regulated transcription factor that regulates cellular responses to low oxygen environments. HIF-1 is composed of two subunits: hypoxia-inducible HIF-1α and constitutively-expressed HIF-1β. During hypoxic conditions, HIF-1α heterodimerizes with HIF-1β and translocates to the nucleus where the HIF-1 complex binds to the hypoxia-response element (HRE) and activates expression of target genes implicated in cell growth and survival. HIF-1α protein expression is elevated in many solid tumors, including those of the cervix and brain, where cells that are the greatest distance from blood vessels, and therefore the most hypoxic, express the highest levels of HIF-1α. Therapeutic blockade of the HIF-1 signaling pathway in cancer cells therefore provides an attractive strategy for development of anticancer drugs. To identify small molecule inhibitors of the HIF-1 pathway, we have developed a cell-based reporter gene assay and screened a large compound library by using a quantitative high-throughput screening (qHTS) approach.
The assay is based upon a β-lactamase reporter under the control of a HRE. We have screened approximate 73,000 compounds by qHTS, with each compound tested over a range of seven to fifteen concentrations. After qHTS we have rapidly identified three novel structural series of HIF-1 pathway Inhibitors. Selected compounds in these series were also confirmed as inhibitors in a HRE β-lactamase reporter gene assay induced by low oxygen and in a VEGF secretion assay. Three of the four selected compounds tested showed significant inhibition of hypoxia-induced HIF-1α accumulation by western blot analysis.
The use of β-lactamase reporter gene assays, in combination with qHTS, enabled the rapid identification and prioritization of inhibitors specific to the hypoxia induced signaling pathway.
Glioblastomas are lethal cancers characterized by florid angiogenesis promoted in part by glioma stem cells (GSCs). As hypoxia regulates angiogenesis, we examined hypoxic responses in GSCs. We now demonstrate that hypoxia-inducible factor HIF2α and multiple HIF-regulated genes are preferentially expressed in GSCs in comparison to nonstem tumor cells and normal neural progenitors. In tumor specimens, HIF2α co-localizes with cancer stem cell markers. Targeting HIFs in GSCs inhibits self-renewal, proliferation and survival in vitro, and attenuates tumor initiation potential of GSCs in vivo. Analysis of a molecular database reveals that HIF2A expression correlates with poor glioma patient survival. Our results demonstrate that GSCs differentially respond to hypoxia with distinct HIF induction patterns and HIF2α may represent a promising target for anti-glioblastoma therapies.
Recent evidence supports the presence of cancer stem cell populations that contribute to tumor progression through preferential resistance to radiation and chemotherapy, and promotion of tumor angiogenesis, invasion, and metastasis. Therefore, the elucidation of molecular regulators of cancer stem cells may translate into improved anti-neoplastic therapies. Our work demonstrates that cancer stem cells derived from glioblastomas differentially respond to hypoxia with a distinct induction of HIF2α. We find that HIFs are critical to cancer stem cell maintenance and angiogenic drive, and that expression of HIF2α is significantly associated with poor glioma patient survival. These data further suggest that anti-angiogenic therapies can be designed to target cancer stem cell specific molecules involved in neoangiogenesis, including HIF2α and its regulated factors.
Hypoxia-inducible factor (HIF) is a nuclear transcription factor that responds to environmental and pathological hypoxia to induce metabolic adaptation, vascular growth, and cell survival. Here we found that HIF subunits and HIF2α in particular were normally expressed in the mediobasal hypothalamus of mice. Hypothalamic HIF was up-regulated by glucose to mediate the feeding control of hypothalamic glucose sensing. Two underlying molecular pathways were identified, including suppression of PHDs by glucose metabolites to prevent HIF2α degradation and the recruitment of AMPK and mTOR/S6K to regulate HIF2α protein synthesis. HIF activation was found to directly control the transcription of POMC gene. Genetic approach was then employed to develop conditional knockout mice with HIF inhibition in POMC neurons, revealing that HIF loss-of-function in POMC neurons impaired hypothalamic glucose sensing and caused energy imbalance to promote obesity development. The metabolic effects of HIF in hypothalamic POMC neurons were independent of leptin signaling or pituitary ACTH pathway. Hypothalamic gene delivery of HIF counteracted overeating and obesity under conditions of nutritional excess. In conclusion, HIF controls hypothalamic POMC gene to direct the central nutrient sensing in regulation of energy and body weight balance.
The hypothalamus in the brain is a master regulator of feeding and body weight. The regulation of it is mediated by the ability of the hypothalamus to sense nutrients (most importantly glucose) and hormones (such as insulin and leptin). While hormone has been extensively studied, we know less about how the hypothalamus can sense nutrients. It is also unclear whether changes in hypothalamic nutrient sensing can influence the development of obesity and related disease, and could therefore be targeted for disease intervention. In this study, we show that a protein termed hypoxia-inducible factor (HIF) is normally present in the hypothalamus and able to respond to glucose. This glucose response leads to the up-regulation of a hypothalamic neuropeptide, POMC, a pivotal molecule that controls feeding and body weight balance. We then developed a mouse model in which HIF is disrupted in hypothalamic cells that express POMC. These mice displayed reduced hypothalamic sensitivity to glucose, resulting in overeating and susceptibility to obesity. Furthermore, we found that delivery of the HIF gene into the hypothalamus has strong anti-obesity effects in mice. We conclude that HIF is a molecular mediator of hypothalamic glucose sensing and can be potentially targeted for obesity therapeutics.
Hypoxia is a characteristic feature of the tissue microenvironment during bacterial infection. Here we report on our use of conditional gene targeting to examine the contribution of hypoxia-inducible factor 1, α subunit (HIF-1α) to myeloid cell innate immune function. HIF-1α was induced by bacterial infection, even under normoxia, and regulated the production of key immune effector molecules, including granule proteases, antimicrobial peptides, nitric oxide, and TNF-α. Mice lacking HIF-1α in their myeloid cell lineage showed decreased bactericidal activity and failed to restrict systemic spread of infection from an initial tissue focus. Conversely, activation of the HIF-1α pathway through deletion of von Hippel–Lindau tumor-suppressor protein or pharmacologic inducers supported myeloid cell production of defense factors and improved bactericidal capacity. HIF-1α control of myeloid cell activity in infected tissues could represent a novel therapeutic target for enhancing host defense.
Hypoxia has been proposed as an important microenvironmental factor in the development of tissue fibrosis; however, the underlying mechanisms are not well defined. To examine the role of hypoxia-inducible factor–1 (HIF-1), a key mediator of cellular adaptation to hypoxia, in the development of fibrosis in mice, we inactivated Hif-1α in primary renal epithelial cells and in proximal tubules of kidneys subjected to unilateral ureteral obstruction (UUO) using Cre-loxP–mediated gene targeting. We found that Hif-1α enhanced epithelial-to-mesenchymal transition (EMT) in vitro and induced epithelial cell migration through upregulation of lysyl oxidase genes. Genetic ablation of epithelial Hif-1α inhibited the development of tubulointerstitial fibrosis in UUO kidneys, which was associated with decreased interstitial collagen deposition, decreased inflammatory cell infiltration, and a reduction in the number of fibroblast-specific protein–1–expressing (FSP-1–expressing) interstitial cells. Furthermore, we demonstrate that increased renal HIF-1α expression is associated with tubulointerstitial injury in patients with chronic kidney disease. Thus, we provide clinical and genetic evidence that activation of HIF-1 signaling in renal epithelial cells is associated with the development of chronic renal disease and may promote fibrogenesis by increasing expression of extracellular matrix–modifying factors and lysyl oxidase genes and by facilitating EMT.
Sphingosine-1-phosphate (S1P) is a bioactive lipid, which regulates several cancer-related processes including migration and angiogenesis. We have previously shown S1P to induce migration of follicular ML-1 thyroid cancer cells. Hypoxia-induced factor-1 (HIF-1) is an oxygen-sensitive transcription factor, which adapts cells to hypoxic conditions through increased survival, motility and angiogenesis. Due to these properties and its increased expression in response to intratumoral hypoxia, HIF-1 is considered a significant regulator of tumor biology. We found S1P to increase expression of the regulatory HIF-1α subunit in normoxic ML-1 cells. S1P also increased HIF-1 activity and expression of HIF-1 target genes. Importantly, inhibition or knockdown of HIF-1α attenuated the S1P-induced migration of ML-1 cells. S1P-induced HIF-1α expression was mediated by S1P receptor 3 (S1P3), Gi proteins and their downstream effectors MEK, PI3K, mTOR and PKCβI. Half-life measurements with cycloheximide indicated that S1P treatment stabilized the HIF-1α protein. On the other hand, S1P activated translational regulators eIF-4E and p70S6K, which are known to control HIF-1α synthesis. In conclusion, we have identified S1P as a non-hypoxic regulator of HIF-1 activity in thyroid cancer cells, studied the signaling involved in S1P-induced HIF-1α expression and shown S1P-induced migration to be mediated by HIF-1.
Hypoxia-inducible factor-1 (HIF-1) has been reported to promote tumour radioresistance; therefore, it is recognised as an excellent target during radiation therapy. However, the inhibition of HIF-1 in unsuitable timing can suppress rather than enhance the effect of radiation therapy because its anti-angiogenic effect increases the radioresistant hypoxic fraction. In this study, we imaged changes of HIF-1 activity after treatment with radiation and/or an HIF-1 inhibitor, YC-1, and optimised their combination. Hypoxic tumour cells were reoxygenated 6 h postirradiation, leading to von Hippel-Lindau (VHL)-dependent proteolysis of HIF-1α and a resultant decrease in HIF-1 activity. The activity then increased as HIF-1α accumulated in the reoxygenated regions 24 h postirradiation. Meanwhile, YC-1 temporarily but significantly suppressed HIF-1 activity, leading to a decrease in microvessel density and an increase in tumour hypoxia. On treatment with YC-1 and then radiation, the YC-1-mediated increase in tumour hypoxia suppressed the effect of radiation therapy, whereas on treatment in the reverse order, YC-1 suppressed the postirradiation upregulation of HIF-1 activity and consequently delayed tumour growth. These results indicate that treatment regimen determines whether an HIF-1 inhibitor enhances or inhibits the therapeutic effect of radiation, and the suppression of the postirradiation upregulation of HIF-1 activity is important for the best therapeutic benefit.
radiation therapy; tumour hypoxia; hypoxia-inducible factor-1; molecular imaging
Cellular metabolism depends on the availability of oxygen and the major regulator of oxygen homeostasis is hypoxia-inducible factor 1 (HIF-1), a highly conserved transcription factor that plays an essential role in cellular and systemic homeostatic responses to hypoxia. HIF-1 is a heterodimeric transcription factor composed of hypoxia-inducible HIF-1α and constitutively expressed HIF-1β. Under hypoxic conditions, the two subunits dimerize, allowing translocation of the HIF-1 complex to the nucleus where it binds to hypoxia-response elements (HREs) and activates expression of target genes implicated in angiogenesis, cell growth, and survival. The HIF-1 pathway is essential to normal growth and development, and is involved in the pathophysiology of cancer, inflammation, and ischemia. Thus, there is considerable interest in identifying compounds that modulate the HIF-1 signaling pathway. To assess the ability of environmental chemicals to stimulate the HIF-1 signaling pathway, we screened a National Toxicology Program collection of 1408 compounds using a cell-based β-lactamase HRE reporter gene assay in a quantitative high-throughput screening (qHTS) format. Twelve active compounds were identified. These compounds were tested in a confirmatory assay for induction of vascular endothelial growth factor, a known hypoxia target gene, and confirmed compounds were further tested for their ability to mimic the effect of a reduced-oxygen environment on hypoxia-regulated promoter activity. Based on this testing strategy, three compounds (o-phenanthroline, iodochlorohydroxyquinoline, cobalt sulfate heptahydrate) were confirmed as hypoxia mimetics, whereas two compounds (7-diethylamino-4-methylcoumarin and 7,12-dimethylbenz(a)anthracence) were found to interact with HIF-1 in a manner different from hypoxia. These results demonstrate the effectiveness of qHTS in combination with secondary assays for identification of HIF-1α inducers and for distinguishing among inducers based on their pattern of activated hypoxic target genes. Identification of environmental compounds having HIF-1α activation activity in cell-based assays may be useful for prioritizing chemicals for further testing as hypoxia-response inducers in vivo.
cobalt sulfate heptahydrate; 7-diethylamino-4-methylcoumarin; 7,12-dimethylbenz(a)anthracence; HIF-1α; inducers; iodochlorohydroxyquinoline; NTP 1408 compound library; o-phenanthroline; qHTS
HIF-1α activates genes under hypoxia and was hypothesized to regulate bone regeneration. Surprisingly, HIF-1α+/− fracture calluses are larger, stronger, and stiffer than HIF-1α+/+ calluses because of decreased apoptosis. These data identify apoptosis inhibition as a means to enhance bone regeneration.
Bone regeneration subsequent to fracture involves the synergistic activation of multiple signaling pathways. Localized hypoxia after fracture activates hypoxia-inducible factor 1α (HIF-1α), leading to increased expression of HIF-1 target genes. We therefore hypothesized that HIF-1α is a key regulator of bone regeneration.
Materials and Methods
Fixed femoral fractures were generated in mice with partial HIF-1α deficiency (HIF-1α+/−) and wildtype littermates (HIF-1α+/+). Fracture calluses and intact contralateral femurs from postfracture days (PFDs) 21 and 28 (N = 5–10) were subjected to μCT evaluation and four-point bending to assess morphometric and mechanical properties. Molecular analyses were carried out on PFD 7, 10, and 14 samples (N = 3) to determine differential gene expression at both mRNA and protein levels. Finally, TUNEL staining was performed on PFD 14 samples (N = 2) to elucidate differential apoptosis.
Surprisingly, fracture calluses from HIF-1α+/− mice exhibited greater mineralization and were larger, stronger, and stiffer. Microarray analyses focused on hypoxia-induced genes revealed differential expression (between genotypes) of several genes associated with the apoptotic pathway. Real-time PCR confirmed these results, showing higher expression of proapoptotic protein phosphatase 2a (PP2A) and lower expression of anti-apoptotic B-cell leukemia/lymphoma 2 (BCL2) in HIF-1α+/+ calluses. Subsequent TUNEL staining showed that HIF-1α+/+ calluses contained larger numbers of TUNEL+ chondrocytes and osteoblasts than HIF-1α+/− calluses.
We conclude that partial HIF-1α deficiency results in decreased chondrocytic and osteoblastic apoptosis, thereby allowing the development of larger, stiffer calluses and enhancing bone regeneration. Furthermore, apoptosis inhibition may be a promising target for developing new treatments to accelerate bone regeneration.
bone; fracture; regeneration; hypoxia; inducible factor 1α; apoptosis
Vascular endothelial cells (ECs) play a critical role in angiogenesis and organogenesis, especially in embryonic liver development. Hypoxia inducible transcription factors (Hif) are a key trigger for hypoxic signals, a primary stimulus for angiogenesis. Aryl hydrocarbon receptor nuclear translocator (Arnt), also called Hif-1β, serves as an obligate heterodimerization partner for Hif-1α and Hif-2α. Using Cre-Lox technology the mouse Arnt gene, specifically disrupted in endothelial cells. The resulting mice, designated ArntΔEC, developed impaired hepatic vasculature, liver necrosis and cardiac myocyte degenerative lesions at the late embryonic stage (E16.5–18.5) leading to approximately 90% neonatal lethality. Low serum glucose, down-regulation of glucose transporter-1 and glucose 6- phosphatase mRNAs, and hepatic proliferation were observed in ArntΔEC embryos. Magnetic resonance imaging on E16.5 embryonic livers revealed that ArntΔEC mice had significant volume of avascular region. ArntΔEC mice that survived to the adult stage were fertile, showed normal behavioral activity, but had smaller livers with mild portal fibrosis, dilated blood vessels, extra abnormal collagen accumulation and remarkable iron deposition. ArntΔEC mice had reduced adiposity, impaired serum lipid homeostasis, and a higher respiratory exchange ratio, indicating that they utilize relatively more carbohydrates than their ArntF/F counterparts. In conclusion, the endothelial Arnt plays a pivotal role in embryonic liver development. Adult ArntΔEC mice carrying embryonic hepatic defects, developed a possible early stage of cirrhosis with consequences of limited oxygen availability and altered lipid metabolism.
aryl hydrocarbon receptor; hypoxia inducible factor; endothelial cell; magnetic resonance imaging
While the functions of hypoxia-inducible factor 1α (HIF1α)/aryl hydrocarbon receptor nuclear translocator (ARNT) and HIF2α/ARNT (HIF2) proteins in activating hypoxia-inducible genes are well established, the role of other transcription factors in the hypoxic transcriptional response is less clear. We report here for the first time that the basic helix-loop-helix-leucine-zip transcription factor upstream stimulatory factor 2 (USF2) is required for the hypoxic transcriptional response, specifically, for hypoxic activation of HIF2 target genes. We show that inhibiting USF2 activity greatly reduces hypoxic induction of HIF2 target genes in cell lines that have USF2 activity, while inducing USF2 activity in cells lacking USF2 activity restores hypoxic induction of HIF2 target genes. Mechanistically, USF2 activates HIF2 target genes by binding to HIF2 target gene promoters, interacting with HIF2α protein, and recruiting coactivators CBP and p300 to form enhanceosome complexes that contain HIF2α, USF2, CBP, p300, and RNA polymerase II on HIF2 target gene promoters. Functionally, the effect of USF2 knockdown on proliferation, motility, and clonogenic survival of HIF2-dependent tumor cells in vitro is phenocopied by HIF2α knockdown, indicating that USF2 works with HIF2 to activate HIF2 target genes and to drive HIF2-depedent tumorigenesis.
Hypoxia Inducible Factor (HIF) is a master heterodimeric transcriptional regulator of oxygen (O2) homeostasis critical to proper angiogenic responses. Due to the distinctive coexpression of HIF-1α and HIF-2α subunits in endothelial cells, our goal was to examine the genetic elimination of HIF transcriptional activity in response to physiological hypoxic conditions by using a genetic model in which the required HIF-β subunit (ARNT, Aryl hydrocarbon Receptor Nuclear Translocator) to HIF transcriptional responses was depleted. Endothelial cells (ECs) and aortic explants were isolated from ArntloxP/loxP mice and infected with Adenovirus -Cre/GFP or control -GFP. We observed that moderate levels of 2.5% O2 promoted vessel sprouting, growth, and branching in control aortic ring assays while growth from Adenovirus -Cre infected explants was compromised. Primary Adenovirus -Cre infected EC cultures featured adverse migration and tube formation phenotypes. Primary pulmonary or cardiac ARNT-deleted ECs also failed to proliferate and survive in response to 8 or 2.5% O2 and hydrogen peroxide treatment. Our data demonstrates that ARNT promotes EC migration and vessel outgrowth and indispensible for the proliferation and preservation of ECs in response to the physiological environmental cue of hypoxia. Thus, these results demonstrate that ARNT plays a critical intrinsic role in ECs and support a critical role for the collaboration of HIF-1 and HIF-2 transcriptional activity in these cells.
Angiogenesis; ARNT; HIF; physiological hypoxia; endothelium
Tumor hypoxia induces the up-regulation of Hif-1alpha which in turn induces the expression of genes including VEGF to recruit new blood vessel outgrowth, enabling tumor growth and metastasis. Interference with the Hif-1 pathway and neoangiogenesis is an attractive anti-tumor target. The hydroxylation of Hif-1alpha by PHD proteins during normoxia serves as a recognition motif for its proteasomal degradation. However, under hypoxic conditions, hydroxylation is inhibited and furthermore, PHD proteins are themselves poly-ubiquitylated and degraded by Siah ubiqiuitin ligases. Our data demonstrate for the first time that inhibition of the interaction between Siah and PHD proteins using a peptide derived from a Drosophila protein interferes with the PHD degradation. Furthermore, cells stably expressing the inhibitor display reduced up-regulation of Hif-1alpha protein levels and Hif-1 mediated gene expression under hypoxia. In a syngeneic mouse model of breast cancer, the inhibitor reduced tumor growth and neoangiogenesis and prolonged survival of the mice. In addition, levels of Hif-1alpha and its target Glut-1 are reduced in the inhibitor expressing tumors. These data demonstrate, in a proof-of-principle study, that Siah protein, the most upstream component of the hypoxia pathway yet identified, is a viable drug target for anti-tumor therapies.
Hypoxia-inducible factor (HIF)-1α, is a transcription factor that controls energy metabolism and angiogenesis under hypoxic conditions, and a potent regulator of innate immunity. The studies described herein examined the role of HIF-1α in disease resolution in BALB/c (resistant, cornea heals) mice after ocular infection with Pseudomonas (P.) aeruginosa. Furthermore, the current studies focused on the neutrophil (PMN), the predominant cell infiltrate in keratitis. Using both siRNA and an antagonist (17-DMAG), the role of HIF-1α was assessed in P. aeruginosa-infected BALB/c mice. Clinical score and slit lamp photography indicated HIF-1α inhibition exacerbated disease and corneal destruction. Real time RT-PCR, immunohistochemistry, ELISA, Greiss and MPO assays, bacterial load, intracellular killing, phagocytosis and apoptosis assays further tested the regulatory role of HIF-1α. Despite increased pro-inflammatory cytokine expression and increased MPO levels after knocking down HIF-1α expression, in vivo studies revealed a decrease in NO production and higher bacterial load. In vitro studies using PMN provided evidence that although inhibition of HIF-1α did not affect phagocytosis, both bacterial killing and apoptosis were significantly affected, as was production of antimicrobial peptides. Overall, data provide evidence that inhibition of HIF-1α converts a normally resistant disease response to susceptible (corneal thinning and perforation) after induction of bacterial keratitis. Although this inhibition does not appear to affect PMN transmigration or phagocytosis, both in vivo and in vitro approaches indicate that the transcriptional factor is essential for effective bacterial killing, apoptosis and antimicrobial peptide production.
Infections of the eye, especially the cornea, often result in vision loss and may require corneal transplantation. Pseudomonas aeruginosa is a common, opportunistic bacterium that can infect the cornea, especially in extended wear contact lens users. There are more than 30 million such individuals in the US alone and they account for over 40% of corneal bacterial infection (keratitis) cases. The cornea itself is a specialized tissue that must effectively combat infections, while preserving intact visual acuity. Using various techniques to mimic in an animal model, what can occur in the human eye, we test how cells and molecules of the immune response react to the bacteria, leading to either its eradication or blindness. By better understanding the functions of the immune system during infection, rational design of more effective treatments for this disease, which preserve a patient's eyesight, are feasible. In addition, what we have learned about the different cells and molecules of the immune response in the eye may be applicable to other infectious diseases, as well.
A key adaptation to environmental hypoxia is an increase in erythropoiesis, driven by the hormone erythropoietin (EPO) through what is traditionally thought to be primarily a renal response. However, both neurons and astrocytes (the largest subpopulation of glial cells in the CNS) also express EPO following ischemic injury, and this response is known to ameliorate damage to the brain. To investigate the role of glial cells as a component of the systemic response to hypoxia, we created astrocyte-specific deletions of the murine genes encoding the hypoxia-inducible transcription factors HIF-1α and HIF-2α and their negative regulator von Hippel–Lindau (VHL) as well as astrocyte-specific deletion of the HIF target gene Vegf. We found that loss of the hypoxic response in astrocytes does not cause anemia in mice but is necessary for approximately 50% of the acute erythropoietic response to hypoxic stress. In accord with this, erythroid progenitor cells and reticulocytes were substantially reduced in number in mice lacking HIF function in astrocytes following hypoxic stress. Thus, we have demonstrated that the glial component of the CNS is an essential component of hypoxia-induced erythropoiesis.
Members of the hypoxia-inducible factor (HIF) family of transcription factors regulate the cellular response to hypoxia. In non–small cell lung cancer (NSCLC), high HIF2α levels correlate with decreased overall survival, and inhibition of either the protein encoded by the canonical HIF target gene VEGF or VEGFR2 improves clinical outcomes. However, whether HIF2α is causal in imparting this poor prognosis is unknown. Here, we generated mice that conditionally express both a nondegradable variant of HIF2α and a mutant form of Kras (KrasG12D) that induces lung tumors. Mice expressing both Hif2a and KrasG12D in the lungs developed larger tumors and had an increased tumor burden and decreased survival compared with mice expressing only KrasG12D. Additionally, tumors expressing both KrasG12D and Hif2a were more invasive, demonstrated features of epithelial-mesenchymal transition (EMT), and exhibited increased angiogenesis associated with mobilization of circulating endothelial progenitor cells. These results implicate HIF2α causally in the pathogenesis of lung cancer in mice, demonstrate in vivo that HIF2α can promote expression of markers of EMT, and define HIF2α as a promoter of tumor growth and progression in a solid tumor other than renal cell carcinoma. They further suggest a possible causal relationship between HIF2α and prognosis in patients with NSCLC.
Hypoxia inducible factor-1 (HIF-1) monitors the cellular response to the oxygen levels in solid tumors. Under hypoxia conditions, HIF-1α protein is stabilized and forms a heterodimer with the HIF-1β subunit. The HIF-1 complex activates the transcription of numerous target genes in order to adapt the hypoxic environment in human cancer cells. In gastric cancer patients, HIF-1α activation following extended hypoxia strongly correlates with an aggressive tumor phenotype and a poor prognosis. HIF-1α activation has been also reported to occur via hypoxia-independent mechanisms such as PI3K/AKT/mTOR signaling and ROS production. This article argues for the critical roles of HIF-1α in glucose metabolism, carcinogenesis, angiogenesis, invasion, metastasis, cell survival and chemoresistance, focusing on gastric cancer.
HIF-1α; hypoxia; gastric cancer
Myeloid cells provide important functions in low oxygen (O2) environments created by pathophysiological conditions, including sites of infection, inflammation, tissue injury and solid tumors. Hypoxia-inducible factors (HIFs) are principle regulators of hypoxic adaptation, regulating gene expression involved in glycolysis, erythropoiesis, angiogenesis, proliferation and stem cell function under low O2. Interestingly, increasing evidence accumulated over recent years suggests an additional important regulatory role for HIFs in inflammation. In macrophages, HIFs not only regulate glycolytic energy generation, but also optimize innate immunity, control pro-inflammatory gene expression, mediate bacterial killing and influence cell migration. In neutrophils, HIF-1α promotes survival under O2-deprived conditions and mediates blood vessel extravasation by modulating β2 integrin expression. Additionally, HIFs contribute to inflammatory functions in various other components of innate immunity, such as dendritic cells, mast cells and epithelial cells. This review will dissect the role of each HIF isoform in myeloid cell function and discuss their impact on acute and chronic inflammatory disorders. Currently, intensive studies are being conducted to illustrate the connection between inflammation and tumorigenesis. Detailed investigation revealing interaction between microenvironmental factors such as hypoxia and immune cells is needed. We will also discuss how hypoxia and HIFs control properties of tumor-associated macrophages and their relationship to tumor formation and progression.
Obesity, insulin resistance, and type 2 diabetes form a tightly correlated cluster of metabolic disorders in which adipose is one of the first affected tissues. The role of hypoxia and hypoxia-inducible factor 1 (HIF1) in the development of high-fat diet (HFD)–induced obesity and insulin resistance was investigated using animal models.
RESEARCH DESIGN AND METHODS
Mice with adipocyte-specific targeted disruption of the genes encoding the HIF1 obligatory subunits Hif1α or Arnt (Hif1β) were generated using an aP2-Cre transgene with the Cre/LoxP system. The mice were fed an HFD for 12 weeks and their metabolic phenotypes were determined. Gene expression patterns in adipose tissues were also determined by microarray and quantitative PCR.
On an HFD, adipocyte-specific ARNT knockout mice and adipocyte-specific HIF1α knockout mice exhibit similar metabolic phenotypes, including reduced fat formation, protection from HFD-induced obesity, and insulin resistance compared with similarly fed wild-type controls. The cumulative food intake remained similar; however, the metabolic efficiency was lower in adipocyte-specific HIF1α knockout mice. Moreover, indirect calorimetry revealed respiratory exchange ratios were reduced in adipocyte-specific HIF1α knockout mice. Hyperinsulinemic-euglycemic clamp studies demonstrated that targeted disruption of HIF1α in adipocytes enhanced whole-body insulin sensitivity. The improvement of insulin resistance is associated with decreased expression of Socs3 and induction of adiponectin.
Inhibition of HIF1 in adipose tissue ameliorates obesity and insulin resistance. This study reveals that HIF1 could provide a novel potential therapeutic target for obesity and type 2 diabetes.
Intratumoral hypoxia is a poor prognostic factor associated with reduced disease-free survival in many cancer types, including breast cancer. Hypoxia encourages tumor cell proliferation, stimulates angiogenesis and lymphangiogenesis, and promotes epithelial-mesenchymal transition and metastasis. Tumor cells respond to a hypoxic state by stabilizing the Hif-1α subunit of the Hypoxia-Inducible Factor (HIF) transcription factor to promote expression of various tumor- and metastasis-promoting hypoxic response genes. The antioxidant N-acetylcysteine (NAC) was recently shown to prevent Hif-1α stabilization under hypoxia, and has been identified as a potential alternative method to target the hypoxic response in tumors. We utilized three orthotopic syngeneic murine models of breast cancer, the PyMT, EO771 and 4T1.2 models, to investigate the ability of NAC to modulate the hypoxic response in vitro and in vivo. While NAC prevented Hif-1α stabilization under hypoxia in vitro and increased levels of glutathione in the blood of mice in vivo, this did not translate to a difference in tumor growth or the hypoxic state of the tumor compared to untreated control mice. In addition, NAC treatment actually increased metastatic burden in an experimental metastasis model. This work raises questions regarding the validity of NAC as an anti-tumorigenic agent in breast cancer, and highlights the need to further investigate its properties in vivo in different cancer models.
Macrophage secretion of VEGF in response to hypoxia contributes to tumor growth and angiogenesis. In addition to VEGF, hypoxic macrophages stimulated with GM-CSF secrete high levels of a soluble form of the VEGF receptor (sVEGFR-1), which neutralizes VEGF and inhibits its biological activity. Using mice with a monocyte/macrophage-selective deletion of HIF-1α or HIF-2α, we recently demonstrated that the anti-tumor response to GM-CSF was dependent on HIF-2α-driven sVEGFR-1 production by tumor-associated macrophages, while HIF-1α specifically regulated VEGF production. We therefore hypothesized that chemical stabilization of HIF-2α using an inhibitor of prolyl hydroxylase 3 (PHD3; an upstream inhibitor of HIF-2α activation) would increase sVEGFR-1 production from GM-CSF-stimulated macrophages. Treatment of macrophages with the PHD3 inhibitor AKB-6899 stabilized HIF-2α and increased sVEGFR-1 production from GM-CSF-treated macrophages, with no effect on HIF-1α accumulation or VEGF production. Treatment of B16F10 melanoma-bearing mice with GM-CSF and AKB-6899 significantly reduced tumor growth compared to either drug alone. Increased levels of sVEGFR-1 mRNA, but not VEGF mRNA, were detected within the tumors of GM-CSF- and AKB-6899-treated mice, correlating with decreased tumor vascularity. Finally, the anti-tumor and anti-angiogenic effects of AKB-6899 were abrogated when mice were simultaneously treated with a sVEGFR-1 neutralizing antibody. These results demonstrate that AKB-6899 decreases tumor growth and angiogenesis in response to GM-CSF by increasing sVEGFR-1 production from tumor-associated macrophages. Specific activation of HIF-2α can therefore decrease tumor growth and angiogenesis.
Hypoxia-inducible factor 1α (HIF1α) is an important cellular survival protein under hypoxic conditions, regulating the cellular response to low oxygen tension via recruitment of a transcriptional co-activator, p300/CBP. p300/CBP induces expression of multiple genes involved in cell survival, proliferation, angiogenesis, and tumor development. Thus, a strategy to inhibit hypoxic responses in tumors may be to target the protein-protein interaction between HIF1α and p300/CBP. Here, we document, for the first time, that the aminocoumarin antibiotic, novobiocin, directly blocks the protein-protein interaction between the HIF1α C-terminal activation domain (CTAD) and the cysteine-histidine rich (CH1) region of p300/CBP. Also, novobiocin down-regulated HIF1α-controlled gene expression, specifically CA9, which is related to tumorigenesis. In a monolayer cell culture, novobiocin inhibited cell proliferation and colony formation in the MCF-7 human breast adenocarcinoma cell line and the A549 human lung cancer cell line. Rescue experiments revealed that the recombinant CTAD fragment of HIF1α partially reversed novobiocin’s inhibitory effects on cell proliferation and colony formation in MCF-7 cells. These findings suggest a novel mechanism of action for novobiocin which has the potential for innovative therapeutic use in tumor treatment.
Cited2 (CBP/p300-interacting transactivators with glutamic acid (E) and aspartic acid (D)-rich tail 2) is a member of a new family of transcriptional modulators. Cited2 null embryos exhibit hyaloid hypercellularity consisting of aberrant vasculature in the eye. The purpose of the study is to address whether abnormal lenticular development is a primary defect of Cited2 deletion and whether deletion of hypoxia inducible factor (HIF)-1α or an HIF-1α target gene, vascular endothelial growth factor (VEGF), could rescue abnormal hyaloid vascular system (HVS) in Cited2 deficient adult eyes.
Le-Cre specific Cited2 knockout (Cited2CKO) mice with or without deletion of HIF-1α or VEGF were generated by standard Cre-Lox methods. Eyes collected from six-eight weeks old mice were characterized by Real Time PCR and immunohistological staining.
Cited2CKO mice had smaller lenses, abnormal lens stalk formation, and failed regression of the HVS in the adult eye. The eye phenotype had features similar to persistent hyperplastic primary vitreous (PHPV), a human congenital eye disorder leading to abnormal lenticular development. Deletion of HIF-1α or VEGF in Cited2 knockout eyes partially rescued the abnormal HVS but had no effect on the smaller lens and abnormal lens stalk differentiation. Intravitreal injection of Topotecan (TPT), a compound that inhibits HIF-1α expression, partially eliminated HVS defects in Cited2CKO lenses.
Abnormal HVS is a primary defect in Cited2 knockout mice, resulting in part from dysregulated functions of HIF-1 and VEGF. The Cited2CKO mouse line could be used as a novel disease model for PHPV and as an in vivo model for testing potential HIF-1 inhibitors.