HIFs, hypoxia, and vascular development

NHERF1/EBP50 in Breast Cancer: Clinical Perspectives

Na(+)/H(+) exchanger regulatory factor 1 (NHERF1) is a postsynaptic density 95/disc-large/zona occludens (PDZ) domain-containing protein that recruits membrane receptors/transporters and cytoplasmic signaling proteins into functional complexes. NHERF1 expression has been demonstrated to be altered in breast cancer, but its role in mammary cancerogenesis and progression remains still undefined. In this paper, we review what is known on the pathological role and the potential clinical application of NHERF1 protein in breast cancer. Recent evidence shows that an increased cytoplasmic expression of NHERF1 suggests a key role of its localization/compartmentalization in defining cancerogenesis, progression, and invasion. NHERF1 overexpression is associated with increasing tumor cytohistological grade, aggressive clinical behavior, unfavorable prognosis, and increased tumor hypoxia. Moreover, NHERF1 co-localizes with the oncogenic receptor HER2/neu in HER2/neu-overexpressing carcinoma and in distant metastases. These data make NHERF1 also a potential candidate of clinical relevance for anti-HER2/neu therapy.

Adaptation to oxygen deprivation in cultures of human pluripotent stem cells, endothelial progenitor cells, and umbilical vein endothelial cells

Hypoxia plays an important role in vascular development through hypoxia-inducible factor-1alpha (HIF-1alpha) accumulation and downstream pathway activation. We sought to explore the in vitro response of cultures of human embryonic stem cells (hESCs), induced pluripotent stem cells (iPSCs), human endothelial progenitor cells (hEPCs), and human umbilical cord vein endothelial cells (HUVECs) to normoxic and hypoxic oxygen tensions. We first measured dissolved oxygen (DO) in the media of adherent cultures in atmospheric (21% O(2)), physiological (5% O(2)), and hypoxic oxygen conditions (1% O(2)). In cultures of both hEPCs and HUVECs, lower oxygen consumption was observed when cultured in 1% O(2). At each oxygen tension, feeder-free cultured hESCs and iPSCs were found to consume comparable amounts of oxygen. Transport analysis revealed that the oxygen uptake rate (OUR) of hESCs and iPSCs decreased distinctly as DO availability decreased, whereas the OUR of all cell types was found to be low when cultured in 1% O(2), demonstrating cell adaptation to lower oxygen tensions by limiting oxygen consumption. Next, we examined HIF-1alpha accumulation and the expression of target genes, including VEGF and angiopoietins (ANGPT; angiogenic response), GLUT-1 (glucose transport), BNIP3, and BNIP3L (autophagy and apoptosis). Accumulations of HIF-1alpha were detected in all four cell lines cultured in 1% O(2). Corresponding upregulation of VEGF, ANGPT2, and GLUT-1 was observed in response to HIF-1alpha accumulation, whereas upregulation of ANGPT1 was detected only in hESCs and iPSCs. Upregulation of BNIP3 and BNIP3L was detected in all cells after 24-h culture in hypoxic conditions, whereas apoptosis was not detectable using flow cytometry analysis, suggesting that BNIP3 and BNIP3L can lead to cell autophagy rather than apoptosis. These results demonstrate adaptation of all cell types to hypoxia but different cellular responses, suggesting that continuous measurements and control over oxygen environments will enable us to guide cellular responses.

Neovascularization in diabetes

Diabetes and its complications are a major public health burden in the developed world. The major cause of diabetic complications is abnormal growth of new blood vessels. This dysfunctional neovascularization results in significant morbidity and mortality in patients with diabetes and, as such, is a major focus of basic and clinical investigation. It has become clear that hyperglycemia disrupts tissue-level signaling in response to hypoxia and ischemia, impairs the vasculogenic potential of circulating stem cells and fundamentally alters the structure and function of key neovascularization proteins, including hypoxia-inducible factor-1. These mechanistic and pathophysiologic studies have revealed new therapeutic targets to restore normal neovascularization and to ameliorate and prevent diabetic vascular complications.

Hypoxia-inducible factors as essential regulators of inflammation

Myeloid cells provide important functions in low oxygen (O(2)) 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 O(2). 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 O(2)-deprived conditions and mediates blood vessel extravasation by modulating β (2) 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.

Endothelial cell migration and vascular endothelial growth factor expression are the result of loss of breast tissue polarity

Recruiting a new blood supply is a rate-limiting step in tumor progression. In a three-dimensional model of breast carcinogenesis, disorganized, proliferative transformed breast epithelial cells express significantly higher expression of angiogenic genes compared with their polarized, growth-arrested nonmalignant counterparts. Elevated vascular endothelial growth factor (VEGF) secretion by malignant cells enhanced recruitment of endothelial cells (EC) in heterotypic cocultures. Significantly, phenotypic reversion of malignant cells via reexpression of HoxD10, which is lost in malignant progression, significantly attenuated VEGF expression in a hypoxia-inducible factor 1alpha-independent fashion and reduced EC migration. This was due primarily to restoring polarity: forced proliferation of polarized, nonmalignant cells did not induce VEGF expression and EC recruitment, whereas disrupting the architecture of growth-arrested, reverted cells did. These data show that disrupting cytostructure activates the angiogenic switch even in the absence of proliferation and/or hypoxia and restoring organization of malignant clusters reduces VEGF expression and EC activation to levels found in quiescent nonmalignant epithelium. These data confirm the importance of tissue architecture and polarity in malignant progression.

The molecular basis for impaired hypoxia-induced VEGF expression in diabetic tissues

Diabetes is associated with poor outcomes following acute vascular occlusive events. This results in part from a failure to form adequate compensatory microvasculature in response to ischemia. Since vascular endothelial growth factor (VEGF) is an essential mediator of neovascularization, we examined whether hypoxic up-regulation of VEGF was impaired in diabetes. Both fibroblasts isolated from type 2 diabetic patients, and normal fibroblasts exposed chronically to high glucose, were defective in their capacity to up-regulate VEGF in response to hypoxia. In vivo, diabetic animals demonstrated an impaired ability to increase VEGF production in response to soft tissue ischemia. This resulted from a high glucose-induced decrease in transactivation by the transcription factor hypoxia-inducible factor-1alpha (HIF-1alpha), which mediates hypoxia-stimulated VEGF expression. Decreased HIF-1alpha functional activity was specifically caused by impaired HIF-1alpha binding to the coactivator p300. We identify covalent modification of p300 by the dicarbonyl metabolite methylglyoxal as being responsible for this decreased association. Administration of deferoxamine abrogated methylglyoxal conjugation, normalizing both HIF-1alpha/p300 interaction and transactivation by HIF-1alpha. In diabetic mice, deferoxamine promoted neovascularization and enhanced wound healing. These findings define molecular defects that underlie impaired VEGF production in diabetic tissues and offer a promising direction for therapeutic intervention.

Shared circuitry: developmental signaling cascades regulate both embryonic and adult coronary vasculature

Ischemic heart disease is the most common cause of heart failure and is among the leading causes of mortality worldwide. Therapies used for the treatment of this disease aim to restore blood flow to severely narrowed or occluded coronary arteries by either catheter-based or surgical means. Although these strategies prove efficacious for many patients, a substantial number of individuals fail to improve following these procedures. Recently, a noninvasive strategy has been proposed, focusing on the use of endogenous growth factors that trigger the growth of new coronary arteries. Using the developing heart as a model, several groups have identified some of the key pathways that not only govern the development of the coronary vascular system but also promote the growth of the adult coronary vasculature. Here, we review the major morphological events and signaling cascades that mediate the formation of the coronary vasculature in the embryo. We further describe the mechanism by which many of these same pathways also regulate the adult coronary vasculature and their potential use in the treatment of ischemic heart disease.

The polyamines regulate endothelial cell survival during hypoxic stress through PI3K/AKT and MCL-1

Hypoxia-dependent angiogenesis is an inherent feature of solid tumors, and a better understanding of the molecular mechanisms of hypoxic cell-death should provide additional targets for cancer therapy. Here, we show a novel role of the polyamines in endothelial cell (EC) survival during hypoxia. Polyamine depletion by specific inhibition of ornithine decarboxylase was shown to protect ECs from hypoxia-induced apoptosis. Inhibition of the polyamines resulted in a significant induction of PI3K/AKT and its down-stream target MCL-1, i.e. an anti-apoptotic member of the BCL-2 family. Specific inhibitors of PI3K reversed the decrease of hypoxia-induced apoptosis as well as the induction of MCL-1 in polyamine-deprived cells. Moreover, siRNA-mediated down-regulation of MCL-1 was found to counter-act the protective effect of polyamine inhibition. We conclude that the polyamines regulate hypoxia-induced apoptosis in ECs through PI3K/AKT and MCL-1 dependent pathways. Our results may have important implications for the modulation of hypoxia-driven neovascularization.

Integrated transcriptomic response to cardiac chronic hypoxia: translation regulators and response to stress in cell survival

Complementary deoxyribonucleic acid microarray data from 36 mice subjected for 1, 2, or 4 weeks of their early life to normal atmospheric conditions (normoxia) or chronic intermittent (CIH) or constant (CCH) hypoxia were analyzed to extract organizational principles of the developing heart transcriptome and determine the integrated response to oxygen deprivation. Although both CCH and CIH regulated numerous genes involved in a wide diversity of processes, the changes in maturational profile, expression stability, and coordination were vastly different between the two treatments, indicating the activation of distinct regulatory mechanisms of gene transcription. The analysis focused on the main regulators of translation and response to stress because of their role in the cardiac hypertrophy and cell survival in hypoxia. On average, the expression of each heart gene was tied to the expression of about 20% of other genes in normoxia but to only 8% in CCH and 9% in CIH, indicating a strong decoupling effect of hypoxia. In contrast to the general tendency, the interlinkages among components of the translational machinery and response to stress increased significantly in CIH and much more in CCH, suggesting a coordinated response to the hypoxic stress. Moreover, the transcriptomic networks were profoundly and differently remodeled by CCH and CIH.

Biology of hypoxia-inducible factor-2alpha in development and disease

The transcriptional response to hypoxia is primarily mediated by two hypoxia-inducible factors--HIF-1alpha and HIF-2alpha. While these proteins are highly homologous, increasing evidence suggests they have unique transcriptional targets and differential impact on tumor growth. Furthermore, non-transcriptional effects of the HIF-alpha subunits, including effects on the Notch and c-Myc pathways, contribute to their distinct functions. HIF-2alpha transcriptional targets include genes involved in erythropoiesis, angiogenesis, metastasis, and proliferation. Therefore, HIF-2alpha contributes significantly to both normal physiology as well as tumorigenesis. Here, we summarize the function of HIF-2alpha during development as well as its contribution to pathologic conditions, such as tumors and vascular disease.

VHL inactivation in renal cell carcinoma: implications for diagnosis, prognosis and treatment

Clear cell renal cell carcinoma (ccRCC) provides a tumor paradigm for the integration of genetics, molecular biology, therapeutic target validation and the introduction of high-impact treatment strategies. Most cases of sporadic as well as familial ccRCC acquire somatic inactivating mutations of the von Hippel-Lindau tumor-suppressor gene, VHL. pVHL, VHL gene product and a protein member of the E3 ubiquitin ligase family, acts in normal cells to direct the degradation and clearance of the hypoxia inducible factor (HIF)alpha transcription factor family, such that in its absence, as in ccRCC, the HIF proteins stabilize, accumulate to supraphysiologic levels and activate the transcription of genes such as VEGF and PDGF, which contributes substantially to the physiology of the tumor, and has been assessed indirectly as a prognostic factor. Molecularly targeted therapy blocking components of this pathway has been successfully introduced to the clinic with a substantive impact on clinical parameters of RCC. This review will examine the regulation of these molecular pathways in RCC and discuss the impact on the clinical management of patients with RCC.

HIF-dependent antitumorigenic effect of antioxidants in vivo

The antitumorigenic activity of antioxidants has been presumed to arise from their ability to squelch DNA damage and genomic instability mediated by reactive oxygen species (ROS). Here, we report that antioxidants inhibited three tumorigenic models in vivo. Inhibition of a MYC-dependent human B lymphoma model was unassociated with genomic instability but was linked to diminished hypoxia-inducible factor (HIF)-1 levels in a prolyl hydroxylase 2 and von Hippel-Lindau protein-dependent manner. Ectopic expression of an oxygen-independent, stabilized HIF-1 mutant rescued lymphoma xenografts from inhibition by two antioxidants: N-acetylcysteine and vitamin C. These findings challenge the paradigm that antioxidants diminish tumorigenesis primarily through decreasing DNA damage and mutations and provide significant support for a key antitumorigenic effect of diminishing HIF levels.

A1 adenosine receptors play an essential role in protecting the embryo against hypoxia

Embryos can be exposed to environmental factors that induce hypoxia. Currently, our understanding of the effects of hypoxia on early mammalian development is modest. Potential mediators of hypoxia action include the nucleoside adenosine, which acts through A(1) adenosine receptors (A(1)ARs) and mediates adverse effects of hypoxia on the neonatal brain. We hypothesized that A(1)ARs may also play a role in mediating effects of hypoxia on the embryo. When pregnant dams were exposed to hypoxia (10% O(2)) beginning at embryonic day (E) 7.5 or 8.5 and continued for 24-96 h, A(1)AR+/+ embryos manifested growth inhibition and a disproportionate reduction in heart size, including thinner ventricular walls. Yet, when dams were exposed to hypoxia, embryos lacking A(1)ARs (A(1)AR-/-) had much more severe growth retardation than A(1)AR+/+ or +/- embryos. When levels of hypoxia-inducible factor 1alpha (HIF1alpha) were examined, A(1)AR-/- embryos had less stabilized HIF1alpha protein than A(1)AR+/- littermates. Normal patterns of cardiac gene expression were also disturbed in A(1)AR-/- embryos exposed to hypoxia. These results show that short periods of hypoxia during early embryogenesis can result in intrauterine growth retardation. We identify adenosine and A(1)ARs as playing an essential role in protecting the embryo from hypoxia.

The NHERF1 PDZ2 domain regulates PKA-RhoA-p38-mediated NHE1 activation and invasion in breast tumor cells

Understanding the signal transduction systems governing invasion is fundamental for the design of therapeutic strategies against metastasis. Na(+)/H(+) exchanger regulatory factor (NHERF1) is a postsynaptic density 95/disc-large/zona occludens (PDZ) domain-containing protein that recruits membrane receptors/transporters and cytoplasmic signaling proteins into functional complexes. NHERF1 expression is altered in breast cancer, but its effective role in mammary carcinogenesis remains undefined. We report here that NHERF1 overexpression in human breast tumor biopsies is associated with metastatic progression, poor prognosis, and hypoxia-inducible factor-1alpha expression. In cultured tumor cells, hypoxia and serum deprivation increase NHERF1 expression, promote the formation of leading-edge pseudopodia, and redistribute NHERF1 to these pseudopodia. This pseudopodial localization of NHERF1 was verified in breast biopsies and in three-dimensional Matrigel culture. Furthermore, serum deprivation and hypoxia stimulate the Na(+)/H(+) exchanger, invasion, and activate a protein kinase A (PKA)-gated RhoA/p38 invasion signal module. Significantly, NHERF1 overexpression was sufficient to induce these morphological and functional changes, and it potentiated their induction by serum deprivation. Functional experiments with truncated and binding groove-mutated PDZ domain constructs demonstrated that NHERF1 regulates these processes through its PDZ2 domain. We conclude that NHERF1 overexpression enhances the invasive phenotype in breast cancer cells, both alone and in synergy with exposure to the tumor microenvironment, via the coordination of PKA-gated RhoA/p38 signaling.

Cell-permeating alpha-ketoglutarate derivatives alleviate pseudohypoxia in succinate dehydrogenase-deficient cells

Succinate dehydrogenase (SDH) and fumarate hydratase (FH) are components of the tricarboxylic acid (TCA) cycle and tumor suppressors. Loss of SDH or FH induces pseudohypoxia, a major tumor-supporting event, which is the activation of hypoxia-inducible factor (HIF) under normoxia. In SDH- or FH-deficient cells, HIF activation is due to HIF1alpha stabilization by succinate or fumarate, respectively, either of which, when in excess, inhibits HIFalpha prolyl hydroxylase (PHD). To reactivate PHD, we focused on its substrate, alpha-ketoglutarate. We designed and synthesized cell-permeating alpha-ketoglutarate derivatives, which build up rapidly and preferentially in cells with a dysfunctional TCA cycle. This study shows that succinate- or fumarate-mediated inhibition of PHD is competitive and is reversed by pharmacologically elevating intracellular alpha-ketoglutarate. Introduction of alpha-ketoglutarate derivatives restores normal PHD activity and HIF1alpha levels to SDH-suppressed cells, indicating new therapy possibilities for the cancers associated with TCA cycle dysfunction.

A novel promoter element containing multiple overlapping xenobiotic and hypoxia response elements mediates induction of cytochrome P4502S1 by both dioxin and hypoxia

Cytochrome P4502S1 (CYP2S1) is expressed at high levels in epithelial tissues and is inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) via the aryl hydrocarbon receptor (AHR). Transcriptional initiation of mouse Cyp2s1 was found to occur at three regions, approximately 198, 102, and 22 nucleotides from the translational initiation codon. Approximately 400 nucleotides upstream of its translational initiation codon, mouse Cyp2s1 contains three overlapping xenobiotic-responsive element (XRE) sequences, which make a major contribution toward dioxin inducibility. Each XRE sequence in this trimeric XRE can bind the AHR/aryl hydrocarbon receptor nuclear translocator (ARNT) dimer in a dioxin-dependent fashion in vitro and can mediate dioxin-dependent transcription. Cyp2s1 is also markedly inducible by hypoxia. Induction is dependent on hypoxiainducible factor-1 (HIF-1) and is mediated in large part by three overlapping hypoxia response elements (HREs) embedded within the trimeric XRE segment. Although each HRE within this segment can bind HIF-1alpha/ARNT in vitro, the most 3' HRE contributes the most toward hypoxia inducibility. AHR/ARNT and HIF-1alpha/ARNT dimers bind to the region containing the trimeric XRE segment of the endogenous Cyp2s1 gene in vivo in a dioxin-dependent fashion and hypoxia-dependent fashion, respectively. These observations identify a novel regulatory cassette that mediates changes in Cyp2s1 expression.

Hypoxic regulation of Id-1 and activation of the unfolded protein response are aberrant in neuroblastoma

The Id proteins play an important role in proliferation, differentiation and tumorigenesis. Many tumors are hypoxic, but it is unknown if expression of Id proteins is regulated in hypoxic cells. Here we show that Id-1 is down-regulated in multiple primary, immortalized, and neoplastic hypoxic cell lines, and the transcriptional repressor ATF-3 is both necessary and sufficient for this hypoxia-induced repression of Id-1. Hypoxic up-regulation of ATF-3 is due in part to activation of the unfolded protein response, a cellular stress response. Remarkably, we observe that the unfolded protein response is de-regulated in all neuroblastoma cell lines tested. Indeed, in the absence of ATF-3 the hypoxia-induced transcription factor HIF-1 up-regulates Id-1 in hypoxic neuroblastoma cells. Hypoxic neuroblastoma cells diminish expression of some neuronal differentiation markers, and forced expression of ATF-3 in hypoxic neuroblastoma cells represses Id-1 and prevents the loss of these markers. The divergent regulation of Id proteins in distinct hypoxic cells may explain some of the varied effects hypoxia has on cellular differentiation and proliferation.

cDNA cloning, gene organization and variant specific expression of HIF-1 alpha in high altitude yak (Bos grunniens)

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic-helix-loop-helix-PER-ARNT-SIM (bHLH-PAS) transcription factor consisting of HIF-1alpha and HIF-1beta subunits. HIF-1alpha is the oxygen-regulated subunit of HIF-1, which regulates the transcription of genes involved in oxygen homeostasis in response to hypoxia. Yak (Bos grunniens), a mammal native to high altitude (HA) region ( approximately 3500-5500 m), has successfully adapted over many generations to the chronic hypoxia of HA. In the present work, cDNA encoding HIF-1alpha has been cloned from the blood of yak. Tissue specific expression of the mRNA was analyzed in blood, heart, lung, liver and kidney by RT-PCR with primers from three different regions of cDNA. The HIF-1alpha expression was liver and blood specific. The HIF-1alpha mRNA contains 823 bp long 3'UTR that is AU-rich and contains ten AUUUA pentamers and two overlapping copies of the nonamer UUAUUUAUUUAUU. Three potential microRNAs, hsa-miR-107/mmu-miR-107/rno-miR-107, hsa-miR-18b and hsa-miR-135a/mmu-miR-135a/rno-miR-135a, targeting 3'UTR of yak HIF-1alpha, were identified by using target prediction software. The CDS encodes for 823 residues of amino acids and showed 99%, 95%, 92%, 90% and 90% similarity to domestic cattle, human, plateau pika, mouse and rat HIF-1alpha, respectively. HIF-1alpha cDNA, cloned and sequenced in the present work has revealed the evolutionary conservation through multiple sequence alignment. Liver and blood specific stability of HIF-1alpha mRNA appears miR-107 regulated.

Histone deacetylase inhibitors induce VHL and ubiquitin-independent proteasomal degradation of hypoxia-inducible factor 1alpha

Adaptation to hypoxic microenvironment is critical for tumor survival and metastatic spread. Hypoxia-inducible factor 1alpha (HIF-1alpha) plays a key role in this adaptation by stimulating the production of proangiogenic factors and inducing enzymes necessary for anaerobic metabolism. Histone deacetylase inhibitors (HDACIs) produce a marked inhibition of HIF-1alpha expression and are currently in clinical trials partly based on their potent antiangiogenic effects. Although it has been postulated that HDACIs affect HIF-1alpha expression by enhancing its interactions with VHL (von Hippel Lindau), thus promoting its ubiquitination and degradation, the actual mechanisms by which HDACIs decrease HIF-1alpha levels are not clear. Here, we present data indicating that HDACIs induce the proteasomal degradation of HIF-1alpha by a mechanism that is independent of VHL and p53 and does not require the ubiquitin system. This degradation pathway involves the enhanced interaction of HIF-1alpha with HSP70 and is secondary to a disruption of the HSP70/HSP90 axis function that appears mediated by the activity of HDAC-6.

Oxygen-sensitive reset of hypoxia-inducible factor transactivation response: prolyl hydroxylases tune the biological normoxic set point

Cellular O(2) sensing enables physiological adjustments to variations in tissue pO(2). Under basal conditions, cells are adjusted to an O(2) environment biologically read as normoxia. Any sharp departure from that state of normoxia triggers O(2)-sensitive biological responses. The stabilization of hypoxia-inducible factor (HIF) signifies a robust biological readout of hypoxia. In the presence of sufficient O(2), HIF is hydroxylated and degraded. HIF prolyl hydroxylation is catalyzed by prolyl hydroxylase isoenzymes PHD1, 2, and 3. Using HT22 neurons stably transfected with a HIF reporter construct, we tested a novel hypothesis postulating that biological cells are capable of resetting their normoxic set point by O(2)-sensitive changes in PHD expression. Results of this study show that the pO(2) of the mouse brain cortex was 35 mm Hg or 5% O(2). Exposure of HT22, adjusted to growing in 20% O(2), to 5% O(2) resulted in HIF-driven transcription. However, cells adjusted to growing in 5% O(2) did not report hypoxia. Cells adjusted to growing in 30% O(2) reported hypoxia when acutely exposed to room air culture conditions. When grown under high O(2) conditions, cells reset their normoxic set point upward by down-regulating the expression of PHD1-3. When grown under low O(2) conditions, cells reset their normoxic set point downward by inducing the expression of PHD1-3. Exposure of mice in vivo to a hypoxic 10% O(2) environment lowered blood as well as brain pO(2). Such hypoxic exposure induced PHD1-3. Exposure of mice to a hyperoxic 50% O(2) ambience repressed the expression of PHD1-3, indicating that O(2)-sensitive regulation of PHD expression is effective in the brain in vivo. siRNA dependent knockdown of PHD expression revealed that O(2)-sensitive regulation of PHD may contribute to tuning the normoxic set point in biological cells.

Gene expression profiling reveals the profound upregulation of hypoxia-responsive genes in primary human astrocytes

Oxygen is vital for the development and survival of mammals. In response to hypoxia, the brain initiates numerous adaptive responses at the organ level as well as at the molecular and cellular levels, including the alteration of gene expression. Astrocytes play critical roles in the proper functioning of the brain; thus the manner in which astrocytes respond to hypoxia is likely important in determining the outcome of brain hypoxia. Here, we used microarray gene expression profiling and data-analysis algorithms to identify and analyze hypoxia-responsive genes in primary human astrocytes. We also compared gene expression patterns in astrocytes with those in human HeLa cells and pulmonary artery endothelial cells (ECs). Remarkably, in astrocytes, five times as many genes were induced as suppressed, whereas in HeLa and pulmonary ECs, as many as or more genes were suppressed than induced. More genes encoding hypoxia-inducible functions, such as glycolytic enzymes and angiogenic growth factors, were strongly induced in astrocytes compared with HeLa cells. Furthermore, gene ontology and computational algorithms revealed that many target genes of the EGF and insulin signaling pathways and the transcriptional regulators Myc, Jun, and p53 were selectively altered by hypoxia in astrocytes. Indeed, Western blot analysis confirmed that two major signal transducers mediating insulin and EGF action, Akt and MEK1/2, were activated by hypoxia in astrocytes. These results provide a global view of the signaling and regulatory network mediating oxygen regulation in human astrocytes.

The ABC transporter Abcg2/Bcrp: role in hypoxia mediated survival

ABC (ATP-binding cassette) transporters have diverse roles in many cellular processes. These diverse roles require the presence of conserved membrane spanning domains and nucleotide binding domains. Bcrp (Abcg2) is a member of the ATP binding cassette family of plasma membrane transporters that was originally discovered for its ability to confer drug resistance in tumor cells. Subsequent studies showed Bcrp expression in normal tissues and high expression in primitive stem cells. Bcrp expression is induced under low oxygen conditions consistent with its high expression in tissues exposed to low oxygen environments. Moreover, Bcrp interacts with heme and other porphyrins. This finding and its regulation by hypoxia suggests it may play a role in protecting cells/tissue from protoporphyrin accumulation under hypoxia. These observations are strengthened by the fact that porphyrins accumulate in tissues of the Bcrp knockout mouse. It is possible that humans with loss of function Bcrp alleles may be more susceptible to porphyrin-induced phototoxicity. We propose that Bcrp plays a role in porphyrin homoeostasis and regulates survival under low oxygen conditions.

Current status and perspective of antiangiogenic therapy for cancer: urinary cancer

Angiogenesis is considered a prerequisite for solid tumor growth. Antiangiogenic therapy reduces tumor size and extends host survival in a number of preclinical animal models. However, in humans antiangiogenic therapy is a poor promoter of tumor regression and has shown minimal effect on patient survival. In urinary cancers, such as renal cell cancer, prostate cancer, and bladder cancer, advanced refractory disease is a good candidate for antiangiogenic therapy because of its resistance to ordinary chemotherapy, radiotherapy, and hormonal therapy. Unique characteristics of molecular mechanisms underlie the induction of angiogenesis in urinary cancers. In this review, we summarize these unique mechanisms and review the results of clinical trials of antiangiogenic therapy for these cancers, discussing prospects and problems relating to antiangiogenic therapy.

Sensing and responding to hypoxia via HIF in model invertebrates

This past decade has brought considerable progress towards elucidating the molecular mechanisms of oxygen sensing pathways by which mammalian cells are able to detect and adjust, or succumb, to hypoxia. In contrast, far less is known about the protein and DNA constituents that endow many invertebrate species to withstand and recover from even more severe and prolonged O2 limitations. In spite of these differences in hypoxia tolerance, inadequacy in oxygen supply is, from mammals to insects to nematodes, signaled onto the DNA level predominantly by hypoxia-inducible factors (HIFs). Across the animal kingdom, HIF accumulates in hypoxic, but not normoxic, cells and functions in a remarkably conserved pathway. Using crustacean (Daphnia magna) and insect (Drosophila melanogaster) models, work by us and others has implicated HIF in restoring O2 delivery via stimulated hemoglobin synthesis (Daphnia) or tracheal remodeling (Drosophila). HIF is essential for these arthropods to adapt and survive during moderate O2 limitations. A similar life-preserving role for HIF-signaling in hypoxic, but not anoxic, environments had previously been established for another stress-tolerant invertebrate model, the nematode Caenorhabditis elegans. Exploring regulations of oxygen-dependent Daphnia and Drosophila genes in cell culture and in vivo have furthermore aided in uncovering novel HIF-targeting mechanisms that might operate to fine-tune the activity of this transcription factor under steadily hypoxic, rather than changing, oxygen tensions. We conclude our review with yet another addition to the growing list of HIF's many functions: the control of cellular growth during fly development.

A role for Hox A5 in regulating angiogenesis and vascular patterning

BACKGROUND

Homeobox (Hox) genes are transcriptional regulators which modulate embryonic morphogenesis and pathological tissue remodeling in adults via regulation of genes associated with cell-cell or cell extracellular matrix (ECM) interactions. We previously showed that while Hox 3 genes promote angiogenesis, Hox D10 inhibits this process.

METHODS AND RESULTS

Here we show that another Hox family gene, Hox A5, also blocks angiogenesis but accomplishes this by targeting different downstream genes than Hox D10. Sustained expression of Hox A5 leads to down regulation of many pro-angiogenic genes including VEGFR2, ephrin A1, Hif1alpha and COX-2. In addition, Hox A5 also upregulates expression of anti-angiogenic genes including Thrombospondin-2. Furthermore, we show that while Hox A5 mRNA is expressed in quiescent endothelial cells (EC), its expression is diminished or absent in active angiogenic EC found in association with breast tumors or in proliferating infantile hemangiomas.

CONCLUSIONS

Together our results suggest that restoring Hox A5 expression may provide a novel means to limit breast tumor growth or expansion of hemangiomas.

Hypoxic regulation of the fetal cerebral circulation

Fetal cerebrovascular responses to acute hypoxia are fundamentally different from those observed in the adult cerebral circulation. The magnitude of hypoxic vasodilatation in the fetal brain increases with postnatal age although fetal cerebrovascular responses to acute hypoxia can be complicated by age-dependent depressions of blood pressure and ventilation. Acute hypoxia promotes adenosine release, which depresses fetal cerebral oxygen consumption through action of adenosine on neuronal A1 receptors and vasodilatation through activation of A2 receptors on cerebral arteries. The vascular effect of adenosine can account for approximately half the vasodilatation observed in response to hypoxia. Hypoxia-induced release of nitric oxide and opioids can account for much of the adenosine-independent cerebral vasodilatation observed in response to hypoxia in the fetus. Direct effects of hypoxia on cerebral arteries account for the remaining fraction, although the vascular endothelium contributes relatively little to hypoxic vasodilatation in the immature cerebral circulation. In contrast to acute hypoxia, fetal cerebral blood flow tends to normalize during acclimatization to chronic hypoxia even though cardiac output is depressed. However, uncompensated chronic hypoxia in the fetus can produce significant changes in brain structure and function, alteration of respiratory drive and fluid balance, and increased incidence of intracranial hemorrhage and periventricular leukomalacia. At the level of the fetal cerebral arteries, chronic hypoxia increases protein content and depresses norepinephrine release, contractility, and receptor densities associated with contraction but also attenuates endothelial vasodilator capacity and decreases the ability of ATP-sensitive and calcium-sensitive potassium channels to promote vasorelaxation. Overall, fetal cerebrovascular adaptations to chronic hypoxia appear prioritized to conserve energy while preserving basic contractility. Many gaps remain in our understanding of how the effects of acute and chronic hypoxia are mediated in fetal cerebral arteries, but studies of adult cerebral arteries have produced many powerful pharmacological and molecular tools that are simply awaiting application in studies of fetal cerebral artery responses to hypoxia.

Essential role of B-Raf in ERK activation during extraembryonic development

The kinases of the Raf family have been intensively studied as activators of the mitogen-activated protein kinase kinase/extra-cellular signal-regulated kinase (ERK) module in regulated and deregulated proliferation. Genetic evidence that Raf is required for ERK activation in vivo has been obtained in lower organisms, which express only one Raf kinase, but was hitherto lacking in mammals, which express more than one Raf kinase. Ablation of the two best studied Raf kinases, B-Raf and Raf-1, is lethal at midgestation in mice, hampering the detailed study of the essential functions of these proteins. Here, we have combined conventional and conditional gene ablation to show that B-Raf is essential for ERK activation and for vascular development in the placenta. B-Raf-deficient placentae show complete absence of phosphorylated ERK and strongly reduced HIF-1alpha and VEGF levels, whereas all these parameters are normal in Raf-1-deficient placentae. In addition, neither ERK phosphorylation nor development are affected in B-raf-deficient embryos that are born alive obtained by epiblast-restricted gene inactivation. The data demonstrate that B-Raf plays a nonredundant role in ERK activation during extraembyronic mammalian development in vivo.

Cellular response to tissue hypoxia and its involvement in disease progression

Multicellular organisms show adaptive reactions for their survival when they are exposed to an atmosphere with reduced oxygen concentration. These reactions include increase in respiratory volume, switch from aerobic to anaerobic metabolism, erythropoiesis and angiogenesis. For these reactions, cells must change the expression of several hypoxia-responsive molecules such as erythropoietin and vascular endothelial growth factor. Hypoxia-responsible element (HRE) was delineated in the genes of hypoxia-responsive molecules as the sequence indispensable for their hypoxia-induced transcriptional activation, and hypoxia-inducible factor 1 (HIF-1) was identified as a transcriptional factor that binds to HRE and regulates the expression of various hypoxia-responsive molecules. Increasing evidence has revealed that HIF-1 is a key molecule regulating the cellular response to tissue hypoxia. HIF-1 is composed of two subunits, HIF-1alpha and HIF-1beta, and HIF-1 activity depends mainly on the intracellular level of HIF-1alpha protein, which is regulated to be in inverse relation to the oxygen concentration by an oxygen-dependent enzyme, prolyl hydroxylase 2 (PHD2). Thus, cells respond to tissue hypoxia by sensing the oxygen concentration as the enzyme activity of PHD2, regulating the HIF-1 activity and consequently changing the expression of various hypoxia-responsive molecules. Cellular response controlled by hypoxia-HIF-1 cascade is also involved in pathological situations such as solid tumor growth, diabetic retinopathy and rheumatoid arthritis. Under these pathological situations, the activation of hypoxia-HIF-1 cascade often leads to the acceleration of disease progression. Understanding an aspect of disease progression triggered by tissue hypoxia might provide a clue to new therapeutic strategies for intractable diseases.

Mitochondrial tumour suppressors: a genetic and biochemical update

Since the discovery 5 years ago that the D-subunit of succinate dehydrogenase (SDHD) can behave as a classic tumour suppressor, other nuclear-encoded mitochondrial proteins (SDHB, SDHC and fumarate hydratase) have been implicated in tumour susceptibility. Mutations in these proteins are principally involved in familial predisposition to benign tumours, but the spectrum of inherited lesions is increasingly recognized to include malignant tumours, such as malignant phaeochromocytomas and renal cell carcinomas. Here we review recent advances in the field of mitochondrial tumour suppressors, the biochemical pathway that links mitochondrial dysfunction with tumorigenesis, and potential therapeutic approaches to these malignancies.

Reversion of lethality and growth defects in Fatiga oxygen-sensor mutant flies by loss of hypoxia-inducible factor-alpha/Sima

Hypoxia-Inducible Factor (HIF) prolyl hydroxylase domains (PHDs) have been proposed to act as sensors that have an important role in oxygen homeostasis. In the presence of oxygen, they hydroxylate two specific prolyl residues in HIF-alpha polypeptides, thereby promoting their proteasomal degradation. So far, however, the developmental consequences of the inactivation of PHDs in higher metazoans have not been reported. Here, we describe novel loss-of-function mutants of fatiga, the gene encoding the Drosophila PHD oxygen sensor, which manifest growth defects and lethality. We also report a null mutation in dHIF-alpha/sima, which is unable to adapt to hypoxia but is fully viable in normoxic conditions. Strikingly, loss-of-function mutations of sima rescued the developmental defects observed in fatiga mutants and enabled survival to adulthood. These results indicate that the main functions of Fatiga in development, including control of cell size, involve the regulation of dHIF/Sima.

Hypoxia-dependent activation of HIF into a transcriptional regulator

Adaptation to conditions of limited oxygen availability (hypoxia) is a critical determinant of cell and tissue viability in several physiological and pathophysiological conditions. The hypoxia-inducible factor (HIF) is an oxygen-sensitive transcriptional activator that, under hypoxia, upregulates the expression of genes involved in the control of glucose metabolism, angiogenesis and cellular proliferation, among others. Activation of HIF to a fully competent transcriptional regulatory protein complex is a multi-step process that involves control of protein stability, subcellular localization, DNA-binding and interaction with transcriptional coregulators. The identity, regulation and hierarchy of interactions between several components of the HIF signal transduction pathway has been the object of intense study over the past decade and will be the subject of this review. Particular emphasis is given to the process of coordinated coactivator recruitment within the cell nucleus. The implications for future development of angiogenic/antiangiogenic therapeutic strategies of HIF activation/inactivation are discussed.

Role of HIF signaling on tumorigenesis in response to chronic low-dose arsenic administration

Trivalent inorganic arsenic (arsenite, arsenic trioxide, As(III)) is a primary contaminant of groundwater supplies worldwide. As(III), marketed as trisenox, is also an FDA-approved agent to treat cancer It has been previously shown by our laboratory that As(III) administered at doses lower than a therapeutic anticancer dose results in an increase in tumor formation and blood vessel density of tumors. In this work it was found that chronic administration of As(III) approaching the EPA action level of 10 ppb, given in the drinking water of mice 5 weeks prior to B16-F10 melanoma implantation, increased the growth rate of primary tumors and the number of metastases to the lung. Further, levels of arsenic in the tumor and lung were found to be much greater than those in the blood and similar to pro-angiogenic As(III) doses. Levels of hypoxia inducible factor-1alpha (HIF-1alpha) and vascular endothelial growth factor (VEGF) surrounding the blood vessels in the tumors of the As(III)-treated mice were also found to be increased. Exposure of isolated B16-F10 tumor cells to chronic (3 or 7 day) but not acute (4 h) low-dose As(III) was found to increase HIF-1alpha expression and secretion of VEGF. Finally, coadministration of an inhibitor of HIF (YC-1) or a VEGFR-2 kinase inhibitor (SU5416) was found to antagonize the pro-angiogenic effects of low-dose As(III). Together, these results suggest that chronic exposure to low-dose As(III) could stimulate growth of tumors through a HIF-dependent stimulation of angiogenesis.