Hypoxia-inducible mammalian gene expression analyzed in vivo at a TATA-driven promoter and at an initiator-driven promoter

Role of glucose metabolism in amelogenesis

BACKGROUND

Cell energy metabolism plays a pivotal role in organ development and function by regulating cell behavior in pathophysiological conditions. Glucose metabolism is the central cascade for obtaining energy in mammalian cells, and cells alter the glucose metabolic pathway depending on intra- and extracellular environments. Therefore, glucose metabolism is closely associated with cell differentiation stages, and cell energy metabolism plays a vital role not only in energy production but also in cell fate regulation in organogenesis.

HIGHLIGHT

During enamel formation, the timing of the expression of passive and active glucose transporters, glycogen synthesis, and glycogen degradation is strictly regulated according to the energy demand of ameloblast-lineage cells. These glucose metabolic reactions are particularly active in the maturation stage of ameloblasts. Furthermore, autophagy, a key regulator of cellular energy homeostasis that modulates glucose metabolism, occurs during both the secretory and maturation stages of ameloblasts. Disruption of glucose metabolism cascade and autophagy induces enamel hypoplasia, as demonstrated in both in vitro and in vivo models.

CONCLUSION

Adequate energy supply via glucose metabolism is essential for enamel matrix secretion and maturation. A thorough understanding of the precise regulation of energy metabolism in amelogenesis facilitates comprehension of the normal enamel formation process and pathological conditions affecting it. This review summarizes glucose metabolic processes during amelogenesis, focusing on glucose uptake, glycogenesis, and glycogenolysis.

Regulation of phosphoglycerate kinase 1 and its critical role in cancer

Cells that undergo normal differentiation mainly rely on mitochondrial oxidative phosphorylation to provide energy, but most tumour cells rely on aerobic glycolysis. This phenomenon is called the "Warburg effect". Phosphoglycerate kinase 1 (PGK1) is a key enzyme in aerobic glycolysis. PGK1 is involved in glucose metabolism as well as a variety of biological activities, including angiogenesis, EMT, mediated autophagy initiation, mitochondrial metabolism, DNA replication and repair, and other processes related to tumorigenesis and development. Recently, an increasing number of studies have proven that PGK1 plays an important role in cancer. In this manuscript, we discussed the effects of the structure, function, molecular mechanisms underlying PGK1 regulation on the initiation and progression of cancer. Additionally, PGK1 is associated with chemotherapy resistance and prognosis in tumour patients. This review presents an overview of the different roles played by PGK1 during tumorigenesis, which will help in the design of experimental studies involving PGK1 and enhance the potential for the use of PGK1 as a therapeutic target in cancer. Video Abstract.

20-Hydroxyecdysone counteracts insulin to promote programmed cell death by modifying phosphoglycerate kinase 1

BACKGROUND

The regulation of glycolysis and autophagy during feeding and metamorphosis in holometabolous insects is a complex process that is not yet fully understood. Insulin regulates glycolysis during the larval feeding stage, allowing the insects to grow and live. However, during metamorphosis, 20-hydroxyecdysone (20E) takes over and regulates programmed cell death (PCD) in larval tissues, leading to degradation and ultimately enabling the insects to transform into adults. The precise mechanism through which these seemingly contradictory processes are coordinated remains unclear and requires further research. To understand the coordination of glycolysis and autophagy during development, we focused our investigation on the role of 20E and insulin in the regulation of phosphoglycerate kinase 1 (PGK1). We examined the glycolytic substrates and products, PGK1 glycolytic activity, and the posttranslational modification of PGK1 during the development of Helicoverpa armigera from feeding to metamorphosis.

RESULTS

Our findings suggest that the coordination of glycolysis and autophagy during holometabolous insect development is regulated by a balance between 20E and insulin signaling pathways. Glycolysis and PGK1 expression levels were decreased during metamorphosis under the regulation of 20E. Insulin promoted glycolysis and cell proliferation via PGK1 phosphorylation, while 20E dephosphorylated PGK1 via phosphatase and tensin homolog (PTEN) to repress glycolysis. The phosphorylation of PGK1 at Y194 by insulin and its subsequent promotion of glycolysis and cell proliferation were important for tissue growth and differentiation during the feeding stage. However, during metamorphosis, the acetylation of PGK1 by 20E was key in initiating PCD. Knockdown of phosphorylated PGK1 by RNA interference (RNAi) at the feeding stage led to glycolysis suppression and small pupae. Insulin via histone deacetylase 3 (HDAC3) deacetylated PGK1, whereas 20E via acetyltransferase arrest-defective protein 1 (ARD1) induced PGK1 acetylation at K386 to stimulate PCD. Knockdown of acetylated-PGK1 by RNAi at the metamorphic stages led to PCD repression and delayed pupation.

CONCLUSIONS

The posttranslational modification of PGK1 determines its functions in cell proliferation and PCD. Insulin and 20E counteractively regulate PGK1 phosphorylation and acetylation to give it dual functions in cell proliferation and PCD.

GLUT3/SLC2A3 Is an Endogenous Marker of Hypoxia in Prostate Cancer Cell Lines and Patient-Derived Xenograft Tumors

The microenvironment of solid tumors is dynamic and frequently contains pockets of low oxygen levels (hypoxia) surrounded by oxygenated tissue. Indeed, a compromised vasculature is a hallmark of the tumor microenvironment, creating both spatial gradients and temporal variability in oxygen availability. Notably, hypoxia associates with increased metastasis and poor survival in patients. Therefore, to aid therapeutic decisions and better understand hypoxia’s role in cancer progression, it is critical to identify endogenous biomarkers of hypoxia to spatially phenotype oncogenic lesions in human tissue, whether precancerous, benign, or malignant. Here, we characterize the glucose transporter GLUT3/SLC2A3 as a biomarker of hypoxic prostate epithelial cells and prostate tumors. Transcriptomic analyses of non-tumorigenic, immortalized prostate epithelial cells revealed a highly significant increase in GLUT3 expression under hypoxia. Additionally, GLUT3 protein increased 2.4-fold in cultured hypoxic prostate cell lines and was upregulated within hypoxic regions of xenograft tumors, including two patient-derived xenografts (PDX). Finally, GLUT3 out-performs other established hypoxia markers; GLUT3 staining in PDX specimens detects 2.6–8.3 times more tumor area compared to a mixture of GLUT1 and CA9 antibodies. Therefore, given the heterogeneous nature of tumors, we propose adding GLUT3 to immunostaining panels when trying to detect hypoxic regions in prostate samples.

Aryl hydrocarbon receptor nuclear translocator promotes the proliferation and invasion of clear cell renal cell carcinoma cells potentially by affecting the glycolytic pathway

Aryl hydrocarbon receptor nuclear translocator (ARNT) is a transcription factor that has been reported to play a vital role in regulating glycolysis, angiogenesis and apoptosis. Recently, ARNT has been reported to a play role in pancreatic-islet function in type 2 diabetes. However, the role of ARNT in kidney cancer has not yet been investigated. In the present study, ARNT expression was detected in tissues from patients with renal cell carcinoma (RCC) and in RCC cell lines. Oncomine, The Cancer Genome Atlas and cBioPortal were used to investigate the roles of ARNT in RCC. Cell migration and invasion assays were used to explore the molecular mechanisms involved. It was found that ARNT protein expression was elevated both in tissues from patients with clear cell RCC (ccRCC) and in different RCC cell lines. ARNT disruption using siRNA knockdown inhibited the migratory abilities and cell proliferation, potentially by altering the glycolysis pathway in vitro, as evidenced by decreased M2 type acetone kinase, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 and hexokinase 2 expression. Taken together, the findings in the present study revealed a novel function of ARNT in ccRCC and indicated that ARNT promotes the proliferation and invasion of ccRCC, possibly through changes to the glycolytic pathway.

Glut 1 in Cancer Cells and the Inhibitory Action of Resveratrol as A Potential Therapeutic Strategy

An important hallmark in cancer cells is the increase in glucose uptake. GLUT1 is an important target in cancer treatment because cancer cells upregulate GLUT1, a membrane protein that facilitates the basal uptake of glucose in most cell types, to ensure the flux of sugar into metabolic pathways. The dysregulation of GLUT1 is associated with numerous disorders, including cancer and metabolic diseases. There are natural products emerging as a source for inhibitors of glucose uptake, and resveratrol is a molecule of natural origin with many properties that acts as antioxidant and antiproliferative in malignant cells. In the present review, we discuss how GLUT1 is involved in the general scheme of cancer cell metabolism, the mechanism of glucose transport, and the importance of GLUT1 structure to understand the inhibition process. Then, we review the current state-of-the-art of resveratrol and other natural products as GLUT1 inhibitors, focusing on those directed at treating different types of cancer. Targeting GLUT1 activity is a promising strategy for the development of drugs aimed at treating neoplastic growth.

PGK1 Drives Hepatocellular Carcinoma Metastasis by Enhancing Metabolic Process

During the proliferation and metastasis, the tumor cells prefer glycolysis (Warburg effect), but its exact mechanism remains largely unknown. In this study, we demonstrated that phosphoglycerate kinase 1 (PGK1) is an important enzyme in the pathway of metabolic glycolysis. We observed a significant overexpression of PGK1 in hepatocellular carcinoma tissues, and a correlation between PGK1 expression and poor survival of hepatocellular carcinoma patients. Also, the depletion of PGK1 dramatically reduced cancer cell proliferation and metastasis, indicating an oncogenic role of PGK1 in liver cancer progression. Further experiments showed that PGK1 played an important role in MYC-induced metabolic reprogramming, which led to an enhanced Warburg effect. Our results revealed a new effect of PGK1, which can provide a new treatment strategy for hepatocellular carcinoma, as PGK1 is used to indicate the prognosis of hepatocellular carcinoma (HCC).

Insulin and mTOR Pathway Regulate HDAC3-Mediated Deacetylation and Activation of PGK1

Phosphoglycerate kinase 1 (PGK1) catalyzes the reversible transfer of a phosphoryl group from 1, 3-bisphosphoglycerate (1, 3-BPG) to ADP, producing 3-phosphoglycerate (3-PG) and ATP. PGK1 plays a key role in coordinating glycolytic energy production with one-carbon metabolism, serine biosynthesis, and cellular redox regulation. Here, we report that PGK1 is acetylated at lysine 220 (K220), which inhibits PGK1 activity by disrupting the binding with its substrate, ADP. We have identified KAT9 and HDAC3 as the potential acetyltransferase and deacetylase, respectively, for PGK1. Insulin promotes K220 deacetylation to stimulate PGK1 activity. We show that the PI3K/AKT/mTOR pathway regulates HDAC3 S424 phosphorylation, which promotes HDAC3-PGK1 interaction and PGK1 K220 deacetylation. Our study uncovers a previously unknown mechanism for the insulin and mTOR pathway in regulation of glycolytic ATP production and cellular redox potential via HDAC3-mediated PGK1 deacetylation.

Age-dependent metabolic dysregulation in cancer and Alzheimer's disease

Age is the main risk factor for cancer and neurodegeneration; two radically divergent diseases. Yet selective pressure to meet cellular metabolic needs may provide a common mechanism linking these two disorders. The exclusive use of glycolysis, despite the presence of oxygen, is commonly referred to as aerobic glycolysis and is the primary metabolic pathway of cancer cells. Recent evidence suggests that aerobic glycolysis is also a key regulator of synaptic plasticity in the brain that may positively influence cognition. Elevated aerobic glycolysis is a contributing factor to the development of cancer as increased glycolytic flux plays an important role in the biosynthesis of macromolecules and promotes proliferation. In contrast, decreased aerobic glycolysis in the brain occurs with age and could lead to a loss of cell survival mechanisms that counter pathogenic processes underlying neurodegeneration. In this review we discuss the recent findings from epidemiological studies demonstrating an inverse comorbidity of cancer and Alzheimer's disease. We summarize evidence linking the two diseases through changes in metabolism over the course of normal aging. We discuss the key steps and regulatory mechanisms of aerobic glycolysis and mitochondrial oxidative phosphorylation which could be exploited for the development of novel therapies. In addition, we outline the regulation of aerobic glycolysis at the transcriptional level by HIF-1α and Pin1 and their roles in cancer and neurodegeneration. Finally, we provide a possible explanation for metabolic dysregulation that occurs with age, and how it may be a contributing factor to age-related diseases. Determining how metabolism becomes dysregulated over time could lead to the development of effective interventions for ensuring metabolic homeostasis and healthy aging.

Dual‑sensitive HRE/Egr1 promoter regulates Smac overexpression and enhances radiation‑induced A549 human lung adenocarcinoma cell death under hypoxia

The aim of this study was to construct an expression vector carrying the hypoxia/radiation dual‑sensitive chimeric hypoxia response element (HRE)/early growth response 1 (Egr‑1) promoter in order to overexpress the therapeutic second mitochondria‑derived activator of caspases (Smac). Using this expression vector, the present study aimed to explore the molecular mechanism underlying radiotherapy‑induced A549 human lung adenocarcinoma cell death and apoptosis under hypoxia. The plasmids, pcDNA3.1‑Egr1‑Smac (pE‑Smac) and pcDNA3.1‑HRE/Egr-1‑Smac (pH/E‑Smac), were constructed and transfected into A549 human lung adenocarcinoma cells using the liposome method. CoCl2 was used to chemically simulate hypoxia, followed by the administration of 2 Gy X‑ray irradiation. An MTT assay was performed to detect cell proliferation and an Annexin V‑fluorescein isothiocyanate apoptosis detection kit was used to detect apoptosis. Quantitative polymerase chain reaction and western blot analyses were used for the detection of mRNA and protein expression, respectively. Infection with the pE‑Smac and pH/E‑Smac plasmids in combination with radiation and/or hypoxia was observed to enhance the expression of Smac. Furthermore, Smac overexpression was found to enhance the radiation‑induced inhibition of cell proliferation and promotion of cycle arrest and apoptosis. The cytochrome c/caspase‑9/caspase‑3 pathway was identified to be involved in this regulation of apoptosis. Plasmid infection in combination with X‑ray irradiation was found to markedly induce cell death under hypoxia. In conclusion, the hypoxia/radiation dual‑sensitive chimeric HRE/Egr‑1 promoter was observed to enhance the expression of the therapeutic Smac, as well as enhance the radiation‑induced inhibition of cell proliferation and promotion of cycle arrest and apoptosis under hypoxia. This apoptosis was found to involve the mitochondrial pathway.

Hypoxia- and radiation-induced overexpression of Smac by an adenoviral vector and its effects on cell cycle and apoptosis in MDA-MB-231 human breast cancer cells

A conditionally replicative adenoviral (CRAd) vector, designated as CRAd.pEgr-1-Smac, that promotes the overexpression of second mitochondria-derived activator of caspase (Smac) when stimulated by hypoxia and radiation was constructed. MDA-MB-231 cells were transfected with CRAd.pEgr-1-Smac and treated with 4-Gy X-rays. The hypoxic status in cancer cells was mimicked with the chemical reagent CoCl₂. Smac protein expression was measured by a western blotting assay and cell proliferation was detected with the MTT assay. The cell cycle progression and apoptotic percentage were measured by flow cytometry with PI and Annexin V-FITC staining kits, respectively, following the irradiation of the transfected cells with 4-Gy X-rays. The results showed that CRAd.pEgr-1-Smac was able to increase the Smac protein expression induced by hypoxia and radiation, inhibit cell proliferation and promote apoptosis. Therefore, this method of gene-radiotherapy is indicated to be an ideal strategy for the treatment of breast cancer.

Glucose uptake mediated by glucose transporter 1 is essential for early tooth morphogenesis and size determination of murine molars

Glucose is an essential source of energy for body metabolism and is transported into cells by glucose transporters (GLUTs). Well-characterized class I GLUT is subdivided into GLUTs1-4, which are selectively expressed depending on tissue glucose requirements. However, there is no available data on the role of GLUTs during tooth development. This study aims to clarify the functional significance of class I GLUT during murine tooth development using immunohistochemistry and an in vitro organ culture experiment with an inhibitor of GLUTs1/2, phloretin, and Glut1 and Glut2 short interfering RNA (siRNA). An intense GLUT1-immunoreaction was localized in the enamel organ of bud-stage molar tooth germs, where the active cell proliferation occurred. By the bell stage, the expression of GLUT1 in the dental epithelium was dramatically decreased in intensity, and subsequently began to appear in the stratum intermedium at the late bell stage. On the other hand, GLUT2-immunoreactivity was weakly observed in the whole tooth germs throughout all stages. The inhibition of GLUTs1/2 by phloretin in the bud-stage tooth germs induced the disturbance of primary enamel knot formation, resulting in the developmental arrest of the explants and the squamous metaplasia of dental epithelial cells. Furthermore, the inhibition of GLUTs1/2 in cap-to-bell-stage tooth germs reduced tooth size in a dose dependent manner. These findings suggest that the expression of GLUT1 and GLUT2 in the dental epithelial and mesenchymal cells seems to be precisely and spatiotemporally controlled, and the glucose uptake mediated by GLUT1 plays a crucial role in the early tooth morphogenesis and tooth size determination.

Choline phosphorylation and regulation of transcription of choline kinase α in hypoxia

Choline kinase catalyzes the phosphorylation of choline, the first step of phospholipid synthesis. Increased phosphorylation of choline is a hallmark characteristic of the malignant phenotype in a variety of neoplasms. However, in hypoxic cancer cells, choline phosphorylation is decreased. To understand the mechanism behind this altered metabolic state, we examined the expression and regulation of the major choline kinase isoform, choline kinase α (ChKα), in hypoxic PC-3 human prostate cancer cells. Hypoxia decreased choline phosphorylation, choline kinase activity, and ChKα mRNA and protein levels. Promoter analysis studies revealed a region upstream of the ChKα gene bearing a conserved DNA consensus binding motif, hypoxia response element-7 (HRE7), at position -222 relative to +1 translation start site, for binding the hypoxia dependent master regulator transcription factor, hypoxia-inducible factor 1α (HIF-1α). Electrophoretic mobility shift competition/supershift assay and chromatin immunoprecipitation assay confirmed binding of HIF-1α to HRE7. A putative promoter of ChKα was isolated from PC-3 genomic DNA and cloned into a luciferase-based reporter vector system. In PC-3 cells, hypoxia decreased the expression of luciferase under the control of the ChKα promoter. Mutation of HRE7 abrogated this hypoxia effect, further demonstrating the involvement of HRE7 in hypoxia-sensitive regulation of ChKα. The results strongly suggest that transcriptional control of choline phosphorylation is largely mediated via HIF-1α binding to the newly identified HRE7.

Hypoxia-induced oxidative DNA damage links with higher level biological effects including specific growth rate in common carp, Cyprinus carpio L

Both hypoxia and hyperoxia, albeit in different magnitude, are known stressors in the aquatic environment. Adopting an integrated approach, mirror carp (Cyprinus carpio L.), were exposed chronically (i.e. 30 days) to hypoxic (1.8 ± 1.1 mg O(2) l(-1)) and hyperoxic (12.3 ± 0.5 mg O(2) l(-1)) conditions and resultant biological responses or biomarkers were compared between these two treatments as well as with fish held under normoxic conditions (7.1 ± 1.04 mg O(2) l(-1)). The biomarkers determined included the activities of glutathione peroxidase (GPx), measurement of oxidative DNA damage (using modified Comet assay employing bacterial enzymes: Fpg and Endo-III), haematological parameters, histopathological and ultrastructural examination of liver and gills. Specific growth rate (SGR) of the fish, as an important ecotoxicological parameter was also determined over the exposure period. The study suggested that while the levels of hepatic GPx were unaffected, there was a significant difference in activity in the blood plasma under different exposure conditions; the hyperoxic group showed increased GPx activity by approximately 37% compared to normoxic group and the hypoxic group showed a decrease by approximately 38% than the normoxic group. Interestingly, oxidative DNA damage was significantly higher in both hypoxic and hyperoxic by approximately 25% compared to normoxic conditions, Fpg showing enhanced level of damage compared to the Endo-III treatment (P < 0.001). The haematological parameters showed enhanced values under hypoxic conditions. Transmission electron microscopic (TEM) studies revealed damage to liver and gill tissues for both the treatments. Interestingly, SGR of fish was significantly lowered in hypoxic by approx. 30% compared to normoxic condition and this was found to be correlated with DNA damage (R = -0.82; P = 0.02). Taken together, these results indicate that prolonged exposure to both hypoxic and hyperoxic conditions induce oxidative stress responses at both DNA and tissue levels, and hypoxia can result in compensatory changes in haematological and growth parameters which could influence Darwinian fitness of the biota with wider ecological implications.

The emerging role of hypoxia, HIF-1 and HIF-2 in multiple myeloma

Hypoxia is an imbalance between oxygen supply and demand, which deprives cells or tissues of sufficient oxygen. It is well-established that hypoxia triggers adaptive responses, which contribute to short- and long-term pathologies such as inflammation, cardiovascular disease and cancer. Induced by both microenvironmental hypoxia and genetic mutations, the elevated expression of the hypoxia-inducible transcription factor-1 (HIF-1) and HIF-2 is a key feature of many human cancers and has been shown to promote cellular processes, which facilitate tumor progression. In this review, we discuss the emerging role of hypoxia and the HIFs in the pathogenesis of multiple myeloma (MM), an incurable hematological malignancy of BM PCs, which reside within the hypoxic BM microenvironment. The need for current and future therapeutic interventions to target HIF-1 and HIF-2 in myeloma will also be discussed.

HIF-1α in epidermis: oxygen sensing, cutaneous angiogenesis, cancer, and non-cancer disorders

Besides lung, postnatal human epidermis is the only epithelium in direct contact with atmospheric oxygen. Skin epidermal oxygenation occurs mostly through atmospheric oxygen rather than tissue vasculature, resulting in a mildly hypoxic microenvironment that favors increased expression of hypoxia-inducible factor-1α (HIF-1α). Considering the wide spectrum of biological processes, such as angiogenesis, inflammation, bioenergetics, proliferation, motility, and apoptosis, that are regulated by this transcription factor, its high expression level in the epidermis might be important to HIF-1α in skin physiology and pathophysiology. Here, we review the role of HIF-1α in cutaneous angiogenesis, skin tumorigenesis, and several skin disorders.

Hypoxia-inducible factor-1alpha regulates the expression of nucleotide excision repair proteins in keratinocytes

The regulation of DNA repair enzymes is crucial for cancer prevention, initiation, and therapy. We have studied the effect of ultraviolet B (UVB) radiation on the expression of the two nucleotide excision repair factors (XPC and XPD) in human keratinocytes. We show that hypoxia-inducible factor-1α (HIF-1α) is involved in the regulation of XPC and XPD. Early UVB-induced downregulation of HIF-1α increased XPC mRNA expression due to competition between HIF-1α and Sp1 for their overlapping binding sites. Late UVB-induced enhanced phosphorylation of HIF-1α protein upregulated XPC mRNA expression by direct binding to a separate hypoxia response element (HRE) in the XPC promoter region. HIF-1α also regulated XPD expression by binding to a region of seven overlapping HREs in its promoter. Quantitative chromatin immunoprecipitation assays further revealed putative HREs in the genes encoding other DNA repair proteins (XPB, XPG, CSA and CSB), suggesting that HIF-1α is a key regulator of the DNA repair machinery. Analysis of the repair kinetics of 6-4 photoproducts and cyclobutane pyrimidine dimers also revealed that HIF-1α downregulation led to an increased rate of immediate removal of both photolesions but attenuated their late removal following UVB irradiation, indicating the functional effects of HIF-1α in the repair of UVB-induced DNA damage.

Phosphoglycerate kinase 1 expression responds to freezing, anoxia, and dehydration stresses in the freeze tolerant wood frog, Rana sylvatica

Natural freezing survival by wood frogs (Rana sylvatica) involves multiple organ-specific changes in gene expression. Screening of a cDNA library made from brain of frozen frogs revealed freeze-responsive up-regulation of the glycolytic enzyme, phosphoglycerate kinase 1 (PGK1). Northern blots showed an approximately two-fold increase in pgk1 transcripts in brain of frozen frogs whereas PGK1 protein levels rose by three- to five-fold within 4-8 hr of freezing. Freezing also elevated pgk1 transcripts in liver but not in skin. Both transcript and protein levels also rose in response to two of the component stresses of freezing (anoxia and dehydration) with a particularly pronounced (11-fold) increase in PGK1 protein in brain in response to anoxia. Amino acid sequence analysis showed 92.5% identity between wood frog and Xenopus laevis PGK1 and 86-88% identity with the zebrafish, chicken, and human protein. Four unique amino acid substitutions in the wood frog protein could be important in maintaining the functional conformation of the wood frog protein at low body temperatures. Elevated amounts of PGK1, one of the ATP-generating reactions of glycolysis, in wood frog brain during freezing would enhance the glycolytic capacity of the organ and support the maintenance of cellular energetics under the ischemic conditions of the frozen state.

An algorithm for identifying novel targets of transcription factor families: application to hypoxia-inducible factor 1 targets

Efficient and effective analysis of the growing genomic databases requires the development of adequate computational tools. We introduce a fast method based on the suffix tree data structure for predicting novel targets of hypoxia-inducible factor 1 (HIF-1) from huge genome databases. The suffix tree data structure has two powerful applications here: one is to extract unknown patterns from multiple strings/sequences in linear time; the other is to search multiple strings/sequences using multiple patterns in linear time. Using 15 known HIF-1 target gene sequences as a training set, we extracted 105 common patterns that all occur in the 15 training genes using suffix trees. Using these 105 common patterns along with known subsequences surrounding HIF-1 binding sites from the literature, the algorithm searches a genome database that contains 2,078,786 DNA sequences. It reported 258 potentially novel HIF-1 targets including 25 known HIF-1 targets. Based on microarray studies from the literature, 17 putative genes were confirmed to be upregulated by HIF-1 or hypoxia inside these 258 genes. We further studied one of the potential targets, COX-2, in the biological lab; and showed that it was a biologically relevant HIF-1 target. These results demonstrate that our methodology is an effective computational approach for identifying novel HIF-1 targets.

Hypoxia-inducible factor-1 target genes as indicators of tumor vessel response to vascular endothelial growth factor inhibition

Antiangiogenic therapy improves survival in patients with advanced stage cancers. Currently, there are no reliable predictors or markers for tumor vessel response to antiangiogenic therapy. To model effective antiangiogenic therapy, we disrupted the VEGF gene in three representative cancer cell lines. HCT116 xenografts had low proportions of endothelial tubes covered by pericytes that stained with alpha-smooth muscle actin (SMA) antibody. Upon disruption of VEGF, HCT116(VEGF-/-) xenografts had significantly decreased tumor microvessel perfusion compared with their parental counterparts. Furthermore, HCT116(VEGF-/-) xenografts mounted a tumor-reactive response to hypoxia, characterized by the induction of hypoxia-inducible factor-1 (HIF-1) target genes. One highly induced protein was DPP4, a measurable serum protein that has well-described roles in cancer progression. In contrast, LS174T and MKN45 tumor xenografts had high proportion of endothelial tubes that were covered by SMA+ pericytes. Upon disruption of VEGF, LS174T(VEGF-/-) and MKN45(VEGF-/-) xenografts maintained tumor microvessel perfusion. As such, there were no changes in intratumoral hypoxia or HIF-1 alpha induction. Together, these data show that the extent of tumor vessel response to angiogenic inhibition could be correlated with (a) the preexisting coverage of tumor endothelial tubes with SMA+ pericytes and (b) differential tumor induction of HIF-1 target genes. The data further show that DPP4 is a novel marker of HIF-1 induction. Altogether, these preclinical findings suggest novel clinical trials for predicting and monitoring tumor vessel responses to antiangiogenic therapy.

Differential gene expression of phosphoglyceric kinase (PGK) and hypoxic adaptation in chicken

Four single-nucleotide polymorphisms (SNP) of the Phosphoglyceric Kinase (PGK) gene were discovered based on comparison of the sequences from an altiplano chicken breed (Tibetan chicken) and two lowland breeds (White Leghorn and Shouguang chicken). Gel-shift results indicate that one of these SNPs, an A-->G mutation at position 59 in exon10, is able to bind hypoxia-induced factor-l (HIF-1), functioning as a hypoxia response element (HRE). The mutant gene results in M-->T mutation at position 379 amino acid. The combined activity of this HRE and HIF-1 could increase correspondingly under a hypoxic stimulus. Hypoxia leads to increased death rates of chicken embryos; while the M-->T mutation described herein is prevalent in healthy embryos grown under hypoxic conditions, thus it may represent an adaptation to hypoxia. Fluorescence quantitative reverse transcription PCR results revealed that HIF-1 upregulates the transcript level of the glycolytic enzyme PGK in the brain and skeletal muscle of animals subjected to hypoxia. Thus, a large amount of ATP is produced by increased glycolysis, allowing the organism to meet energy metabolism demands. As such, we believe this SNP to be an adaptation to the external anoxic environment.

The impact of hypoxic treatment on the expression of phosphoglycerate kinase and the cytotoxicity of troxacitabine and gemcitabine

beta-L-Dioxolane-cytidine (L-OddC, Troxacitabine, BCH-4556), a novel L-configuration deoxycytidine analog, is under clinical trials for treating cancer. The cytotoxicity of L-OddC is dependent on the amount of the triphosphate form (L-OddCTP) in nuclear DNA. Phosphoglycerate kinase (PGK), a downstream protein of hypoxia-inducible-factor-1alpha (HIF-1alpha), is responsible for the phosphorylation of the diphosphate to the triphosphate of L-OddC. In this study, we studied the impact of hypoxia on the metabolism and the cytotoxicity of L-OddC and beta-d-2',2'-difluorodeoxycytidine (dFdC) in several human tumor cell lines including HepG2, Hep3B, A673, Panc-1, and RKO. Hypoxic treatment induced the protein expression of PGK 3-fold but had no effect on the protein expression of APE-1, dCK, CMPK, and nM23 H1. Hypoxic treatment increased L-OddCTP formation and incorporation of L-OddC into DNA, but it decreased the uptake and incorporation of dFdC, which correlated with the reduction of hENT1, hENT2, and hCNT2 expression. Using a clonogenic assay, hypoxic treatment of cells made them 2- to 3-fold more susceptible to L-OddC but not to dFdC after exposure to drugs for one generation. Dimethyloxallyl glycine enhanced the cytotoxicity of L-OddC but not dFdC in Panc-1 cells under normoxic conditions. Overexpression or down-regulation of PGK using transient transfection of pcDNA5-PGK or inducible shRNA in RKO cells affected the cytotoxicity of L-OddC but not that of dFdC. The knockdown of HIF-1alpha in inducible shRNA in RKO cells reduced the cytotoxicity of L-OddC but not dFdC under hypoxic conditions. In conclusion, hypoxia is an important factor that may potentiate the activity of L-OddC.

Circulating Endothelial Progenitor Cells and Vascular Anomalies

Recent findings regarding pathways of stem/progenitor cell involvement in adult blood vessel growth (postnatal vasculogenesis) suggest new theories for the pathogenesis of vascular anomalies. The somatic growth of vascular malformations and the mysterious pattern of proliferation and involution in infantile hemangioma can no longer be purely understood through the paradigm of angiogenesis. Molecular signals for postnatal vasculogenesis are being discovered in numerous animal models of cancer and ischemia, yet little research has addressed the importance of vasculogenesis in the growth of vascular anomalies. In this review, we discuss early studies that have investigated stem/progenitor cell involvement in the pathophysiology of infantile hemangioma and other congenital vascular anomalies.

Integration of oxygen signaling at the consensus HRE

The hypoxia-inducible factor 1 (HIF-1) was initially identified as a transcription factor that regulated erythropoietin gene expression in response to a decrease in oxygen availability in kidney tissue. Subsequently, a family of oxygen-dependent protein hydroxylases was found to regulate the abundance and activity of three oxygen-sensitive HIFalpha subunits, which, as part of the HIF heterodimer, regulated the transcription of at least 70 different effector genes. In addition to responding to a decrease in tissue oxygenation, HIF is proactively induced, even under normoxic conditions, in response to stimuli that lead to cell growth, ultimately leading to higher oxygen consumption. The growing cell thus profits from an anticipatory increase in HIF-dependent target gene expression. Growth stimuli-activated signaling pathways that influence the abundance and activity of HIFs include pathways in which kinases are activated and pathways in which reactive oxygen species are liberated. These pathways signal to the HIF protein hydroxylases, as well as to HIF itself, by means of covalent or redox modifications and protein-protein interactions. The final point of integration of all of these pathways is the hypoxia-response element (HRE) of effector genes. Here, we provide comprehensive compilations of the known growth stimuli that promote increases in HIF abundance, of protein-protein interactions involving HIF, and of the known HIF effector genes. The consensus HRE derived from a comparison of the HREs of these HIF effectors will be useful for identification of novel HIF target genes, design of oxygen-regulated gene therapy, and prediction of effects of future drugs targeting the HIF system.

Loss of ARNT/HIF1beta mediates altered gene expression and pancreatic-islet dysfunction in human type 2 diabetes

beta cell dysfunction is a central component of the pathogenesis of type 2 diabetes. Using oligonucleotide microarrays and real-time PCR of pancreatic islets isolated from humans with type 2 diabetes versus normal glucose-tolerant controls, we identified multiple changes in expression of genes known to be important in beta cell function, including major decreases in expression of HNF4alpha, insulin receptor, IRS2, Akt2, and several glucose-metabolic-pathway genes. There was also a 90% decrease in expression of the transcription factor ARNT. Reducing ARNT levels in Min6 cells with small interfering RNA (siRNA) resulted in markedly impaired glucose-stimulated insulin release and changes in gene expression similar to those in human type 2 islets. Likewise, beta cell-specific ARNT knockout mice exhibited abnormal glucose tolerance, impaired insulin secretion, and changes in islet gene expression that mimicked those in human diabetic islets. Together, these data suggest an important role for decreased ARNT and altered gene expression in the impaired islet function of human type 2 diabetes.

Control of muscle bioenergetic gene expression: implications for allometric scaling relationships of glycolytic and oxidative enzymes

Muscle metabolic properties vary with body size, with larger animals relying relatively less on oxidative metabolism as a result of lower specific activities of mitochondrial enzymes and greater specific activities of glycolytic enzymes. While many have argued reasons why such relationships might be grounded in physical relationships, an explanation for the regulatory basis of the differences in enzyme levels remains unexplored. Focusing on skeletal muscle, we review potential cellular and genetic explanations for the relationship between bioenergetic enzymes and body mass. Differences in myonuclear domain (the ratio of fiber volume to nuclei number) in conjunction with constitutive expression may explain part of the variation in mitochondrial content among fiber types and species. Superimposed on such constitutive determinants are (1) extrinsic signalling pathways that control the muscle contractile and metabolic phenotype and (2) intrinsic signalling pathways that translate changes in cellular milieu (ions, metabolites, oxygen,redox) arising through the contractile phenotype into changes in enzyme synthesis. These signalling pathways work through transcriptional regulation,as well as post-transcriptional, translational and post-translational regulation, acting via synthesis and degradation.

The interaction between HIF-1 and AP-1 transcription factors in response to low oxygen

Hypoxia-inducible factor-1 (HIF-1) is a critical regulator of the transcriptional response to low oxygen conditions (hypoxia/anoxia) experienced by mammalian cells in both physiological and pathophysiological circumstances. As our understanding of the biology and biochemistry of HIF-1 has grown, it has become apparent that cells adapt to signals generated by low oxygen through a network of stress responsive transcription factors or complexes, which are influenced by HIF-1 activity. This review summarizes our current understanding of the interaction of HIF-1 with AP-1, a classic example of a family of pleiotropic transcription factors that impact on diverse cellular processes and phenotypes, including the adaptation to low oxygen stress. The review focuses on experimental studies involving cultured cells exposed to hypoxia/anoxia, and describes both established and possible interactions between HIF-1 and AP-1 at different levels of cellular organization.

Molecular and cellular regulation of glucose transporter (GLUT) proteins in cancer

Malignant cells are known to have accelerated metabolism, high glucose requirements, and increased glucose uptake. Transport of glucose across the plasma membrane of mammalian cells is the first rate-limiting step for glucose metabolism and is mediated by facilitative glucose transporter (GLUT) proteins. Increased glucose transport in malignant cells has been associated with increased and deregulated expression of glucose transporter proteins, with overexpression of GLUT1 and/or GLUT3 a characteristic feature. Oncogenic transformation of cultured mammalian cells causes a rapid increase of glucose transport and GLUT1 expression via interaction with GLUT1 promoter enhancer elements. In human studies, high levels of GLUT1 expression in tumors have been associated with poor survival. Studies indicate that glucose transport in breast cancer is not fully explained by GLUT1 or GLUT3 expression, suggesting involvement of another glucose transporter. Recently, a novel glucose transporter protein, GLUT12, has been found in breast and prostate cancers. In human breast and prostate tumors and cultured cells, GLUT12 is located intracellularly and at the cell surface. Trafficking of GLUT12 to the plasma membrane could therefore contribute to glucose uptake. Several factors have been implicated in the regulation of glucose transporter expression in breast cancer. Hypoxia can increase GLUT1 levels and glucose uptake. Estradiol and epidermal growth factor, both of which can play a role in breast cancer cell growth, increase glucose consumption. Estradiol and epidermal growth factor also increase GLUT12 protein levels in cultured breast cancer cells. Targeting GLUT12 could provide novel methods for detection and treatment of breast and prostate cancer.

Hypoxia-inducible factor regulates alphavbeta3 integrin cell surface expression

Hypoxia-inducible factor (HIF)-deficient placentas exhibit a number of defects, including changes in cell fate adoption, lack of fetal angiogenesis, hypocellularity, and poor invasion into maternal tissue. HIF is a heterodimeric transcription factor consisting of alpha and beta aryl hydrocarbon receptor nuclear translocator or ARNT) subunits. We used undifferentiated trophoblast stem (TS) cells to characterize HIF-dependent adhesion, migration, and invasion. Arnt(-/-) and Hifalpha(-/-) TS cells exhibit reduced adhesion and migration toward vitronectin compared with wild-type cells. Furthermore, this defect is associated with decreased cell surface expression of integrin alphavbeta3 and significantly decreased expression of this integrin in focal adhesions. Because of the importance of adhesion and migration in tumor progression (in addition to placental development), we examined the affect of culturing B16F0 melanoma cells in 1.5% oxygen (O(2)). Culturing B16F0 melanoma cells at 1.5% O(2) resulted in increased alphavbeta3 integrin surface expression and increased adhesion to and migration toward vitronectin. Together, these data suggest that HIF and O(2) tension influence placental invasion and tumor migration by increasing cell surface expression of alphavbeta3 integrin.

Inhibition of vascular endothelial growth factor with a sequence-specific hypoxia response element antagonist

Vascular endothelial growth factor (VEGF) and its receptors have been implicated as key factors in tumor angiogenesis that are up-regulated by hypoxia. We evaluated the effects of DNA-binding small molecules on hypoxia-inducible transcription of VEGF. A synthetic pyrrole-imidazole polyamide designed to bind the hypoxia response element (HRE) was found to disrupt hypoxia-inducible factor (HIF) binding to HRE. In cultured HeLa cells, this resulted in a reduction of VEGF mRNA and secreted protein levels. The observed effects were polyamide-specific and dose-dependent. Analysis of genome-wide effects of the HRE-specific polyamide revealed that a number of hypoxia-inducible genes were down-regulated. Pathway-based regulation of hypoxia-inducible gene expression with DNA-binding small molecules may represent a new approach for targeting angiogenesis.

HIFs, hypoxia, and vascular development

Cellular oxygen (O2) concentrations are tightly regulated to maintain ATP levels required for metabolic reactions in the human body. Responses to changes in O2 concentrations are primarily regulated by the transcription factor hypoxia inducible factor (HIF). HIF activates transcription of genes that increase systemic O2 delivery or provide cellular metabolic adaptation under conditions of hypoxia. HIF activity is essential for embryogenesis and various processes in postnatal life, and therefore, HIF levels need to be precisely controlled. Abnormal HIF expression is related to numerous diseases of the vascular system, including heart disease, cancer, and chronic obstructive pulmonary disease.

The presence of TATA-binding protein in the brainstem, correlated with sleep apnea in SIDS victims

BACKGROUND

Recent reports have indicated that the presence of transcription factors and RNA polymerase decreases in rat brains that suffer perinatal asphyxia from hypoxia. As hypoxia has been proposed as a causative factor in the Sudden Infant Death Syndrome (SIDS), the correlation between TATA-binding protein (TBP) in the brainstem of SIDS victims as a marker of transcription and the incidence of sleep apnea was investigated.

MATERIALS AND METHODS

A total of 38 infants, including 26 cases of SIDS, died under 6 months of age, in a cohort of 27,000 infants studied prospectively to characterize their sleep-wake behavior. The frequency and duration of sleep apnea was analyzed. Brainstem material was collected and immunohistochemistry of TBP was carried out. The density of TBP-positive neurons was measured quantitatively. Correlation analyses were carried out between the density of TBP-positive neurons and the data concerning sleep apnea.

RESULTS

One SIDS-specific positive correlation occurred between the density of TBP-positive neurons in the dorsal raphe nucleus of the midbrain and the duration of central apnea (p=0.049) and two SIDS-specific negative correlations between the density of TBP-positive neurons in the pars compacta and dissipata of the pedunculopontine tegmentum nucleus (PPTNc, PPTNd) in the midbrain and the duration of apnea (p=0.035).

CONCLUSIONS

The significant correlation between the findings of TBP-positive neurons in the midbrain arousal pathway and the characteristics of sleep apnea in SIDS victims is in agreement with the both association of apnea and arousal phenomenon in pathophysiology of SIDS.

Amifostine induces anaerobic metabolism and hypoxia-inducible factor 1α

PURPOSE

The cytoprotective mechanism of amifostine (WR-2721) implies free radical scavenging and DNA repair activities. We investigated additional cytoprotective pathways involving intracellular hypoxia and the activation of the hypoxia-inducible factor (HIF) pathway, a key transcription factor regulating glycolysis, angiogenesis and apoptosis, which is also linked with radioresistance.

MATERIALS AND METHODS

The glucose and oxygen levels in the peripheral blood of patients receiving 1000 mg amifostine were determined at various time-points in order to investigate the metabolic changes induced by amifostine. MDA468 breast tumor cell lines were incubated with a high amifostine concentration (10 m M) to overcome the natural resistance of cancer cells to influx of the non-hydrolyzed WR-2721, and the HIF1 alpha protein levels were determined by Western blot analysis. In vivo experiments with Wistar rats were performed in order to assess immunohistochemically changes in the intracellular accumulation of HIF1 alpha induced by amifostine (200 mg/kg).

RESULTS

By 30 min following amifostine administration, the hemoglobin oxygen saturation and pO(2) levels had increased in the peripheral blood while glucose levels had reduced, providing evidence that normal tissue metabolism switches to glycolytic pathways. Incubation of cell lines with amifostine resulted in HIF1 alpha induction. In Wistar rats administration of amifostine resulted in increased HIF1 alpha accumulation in normal tissues.

CONCLUSIONS

Since it is doubtful whether dephosphorylation of amifostine to the active metabolite WR-1065 occurs within tumoral tissues (an acidic environment that lacks vascular alkaline phosphatase activity), intracellular hypoxia and upregulation of HIF1 alpha represents an additional, normal tissue-specific, amifostine cytoprotective pathway.

Insulin and hypoxia share common target genes but not the hypoxia-inducible factor-1alpha

Both hypoxia and insulin induce common target genes, including vascular endothelial growth factors and several glycolytic enzymes. However, these two signals eventually trigger quite different metabolic pathways. Hypoxia induces glycolysis, resulting in anaerobic ATP production, while insulin increases glycolysis for energy storage. Hypoxia-induced gene expression is mediated by the hypoxia-inducible factor-1 (HIF-1) that consists of HIF-1alpha and the aromatic hydrocarbon nuclear translocator (Arnt). Hypoxia-induced gene expression is initiated by the stabilization of the HIF-1alpha subunit. Here we investigated whether insulin-induced gene expression also requires stabilization of HIF-1alpha. Our results indicate that hypoxia but not insulin stabilizes HIF-1alpha protein levels, whereas both insulin- and hypoxia-induced gene expression require the presence of the Arnt protein. Insulin treatment fails to inactivate proline hydroxylation of HIF-1alpha, which triggers recruitment of the von Hippel-Lindau protein and oxygen-dependent degradation of HIF-1alpha. Insulin-induced gene expression is inhibited by the presence of the phosphoinositide (PI) 3-kinase inhibitor LY294002 and the dominant negative mutant of the p85 subunit of PI 3-kinase, whereas hypoxia-induced gene expression is not. Pyrrolidine dithiocarbamate, a scavenger of H2O2, reduces insulin-induced gene expression but not hypoxia-induced gene expression. Although both hypoxia and insulin induce the expression of common target genes through a hypoxia-responsive element- and Arnt-dependent mechanism, insulin cannot stabilize the HIF-1alpha protein. We believe that insulin activates other putative partner proteins for Arnt in PI 3-kinase- and H2O2-dependent pathways.

Studies of renal injury IV: The GLUT1 gene protects renal cells from cyclosporine A toxicity

BACKGROUND

Renal cells activate the GLUT1 gene when exposed to stress. This response promotes glucose influx and glycolysis, which protects cells and preserves viability. We tested the hypothesis that cytotoxicity from cyclosporine A (CsA), a valuable but nephrotoxic immunosuppressor, also activated the GLUT1 gene. Methods and Results. GLUT1 nuclear transcription was increased in LLCPK1 cells injured with CsA, 10-5 mol/L or more for 24 hours, with increases of GLUT1 mRNA and protein levels, resulting in greater glucose consumption and glycolysis. The integrated stress response to CsA toxicity was cytoprotective, as blockade of glucose influx and glycolysis with 10-4 mol/L phloretin magnified CsA toxicity. Remarkably, whereas phloretin reduced GLUT1 transcription, it still increased GLUT1 protein and mRNA levels, and even amplified their responses to CsA. Interestingly, intracellular pH was preserved despite of greater lactic acid production in the face of Na+/H+ exchange inhibition from CsA toxicity. However, further inhibition of Na+/H+ exchange with amiloride greatly magnified CsA toxicity and GLUT1 gene transcription.

CONCLUSION

Activation of the GLUT1 gene during renal cell injury is mediated by at least two redundant systems. CsA stimulates GLUT1 gene transcription when membrane transport delivers glucose to the cell. However, when glucose delivery is compromised, GLUT1 gene expression is still supported by alternative mechanisms that remain operational even after cellular energy metabolism is compromised further by inhibition of glucose and glycolytic fluxes.

Hypoxia: from molecular responses to ecosystem responses

Hypoxia affects thousands of km2 of marine waters all over the world, and has caused mass mortality of marine animals, benthic defaunation and decline in fisheries production in many places. The severity, frequency occurrence and spatial scale of hypoxia have increased in the last few decades. Due to rapid human population growth and global warming, the problem of hypoxia is likely to become worse in the coming years. Molecular responses of marine animals to hypoxia are poorly known. In many animals, a haem protein probably serves as the cellular sensor for oxygen, and reactive oxygen species are generated as signaling molecules. In mammal and fish, a heterodimeric transcription factor, hypoxia-inducible factor 1 (HIF-1) has been identified. HIF-1 receives signals from the molecular oxygen senor through redox reactions and/or phosphorylation, and in turn, regulates the transcription of a number of hypoxia-inducible genes, including genes involved in erythropoiesis, angiogenesis and glycolysis. These molecular responses then cascade into a series of biochemical and physiological adjustments, enabling the animal to survive better under hypoxic conditions. Marine animals respond to hypoxia by first attempting to maintain oxygen delivery (e.g. increases in respiration rate, number of red blood cells, or oxygen binding capacity of hemoglobin), then by conserving energy (e.g. metabolic depression, down regulation of protein synthesis and down regulation/modification of certain regulatory enzymes). Upon exposure to prolonged hypoxia, animals must eventually resort to anaerobic respiration. Hypoxia reduces growth and feeding, which may eventually affect individual fitness. Effects of hypoxia on reproduction and development of marine animals, albeit important in affecting species survival, remain almost unknown. Many fish and marine organisms can detect, and actively avoid hypoxia. Some benthos may leave their burrows and move to sediment surface during hypoxia. These behaviorial changes may render the animals more vulnerable to predation. Hypoxia may eliminate sensitive species, thereby causing major changes in species composition of benthic, fish and phytoplankton communities. Decreases in species diversity and species richness are well documented, and changes in trophodynamics and functional groups have also been reported. Under hypoxic conditions, there is a general tendency for suspended feeders to be replaced by deposit feeders; demersal fish by pelagic fish; and macrobenthos by meiobenthos. Microflagellates and nanoplankton also tend to dominate in the phytoplankton community in hypoxic environments. Existing evidence suggest that recovery of benthic communities in temperate region take two to several years. Recovery however, appears to be much quicker in subtropical environments. In natural conditions, hypoxia is often associated with increases in ammonia, hydrogen sulphide and particulate organic materials. The inability to isolate effects of hypoxia from interactions of these compounding factors makes it difficult to attribute many of the observed ecological effects to hypoxia.

Mammalian oxygen sensing, signalling and gene regulation

Oxygen is essential to the life of all aerobic organisms. Virtually every cell type is able to sense a limited oxygen supply (hypoxia) and specifically to induce a set of oxygen-regulated genes. This review summarizes current concepts of mammalian oxygen-sensing and signal-transduction pathways. Since the discovery of the hypoxia-inducible factors (HIFs), a great deal of progress has been made in our comprehension of how hypoxia induces the expression of oxygen-regulated genes. The alpha subunit of the heterodimeric transcription factors HIF-1, 2 and 3 is unstable under normoxia but is rapidly stabilized upon exposure to hypoxic conditions. Following heterodimerization with the constitutively expressed beta subunit, HIFs activate the transcription of an increasing number of genes involved in maintaining oxygen homeostasis at the cellular, local and systemic levels.

Regulation of mammalian O2 homeostasis by hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic-helix-loop-helix-PAS transcription factor consisting of HIF-1 alpha and HIF-1 beta subunits. HIF-1 alpha expression and HIF-1 transcriptional activity increase exponentially as cellular O2 concentration is decreased. Several dozen target genes that are transactivated by HIF-1 have been identified, including those encoding erythropoietin, glucose transporters, glycolytic enzymes, and vascular endothelial growth factor. The products of these genes either increase O2 delivery or allow metabolic adaptation to reduced O2 availability. HIF-1 is required for cardiac and vascular development and embryonic survival. In fetal and postnatal life, HIF-1 is required for a variety of physiological responses to chronic hypoxia. HIF-1 expression is increased in tumor cells by multiple mechanisms and may mediate adaptation to hypoxia that is critical for tumor progression. HIF-1 thus appears to function as a master regulator of O2 homeostasis that plays essential roles in cellular and systemic physiology, development, and pathophysiology.