Differential gene up-regulation by hypoxia-inducible factor-1alpha and hypoxia-inducible factor-2alpha in HEK293T cells

Hypoxia-associated markers in gastric carcinogenesis and HIF-2alpha in gastric and gastro-oesophageal cancer prognosis

The study investigated hypoxia-associated markers (HIF-2α, Epo, Epo-R, Glut-1 and VEGF) along with Ki-67 in a gastric carcinogenesis model, and the prognostic significance of hypoxia-inducible factor (HIF)-2α in surgically treated gastro-oesophageal cancer. Protein expression was examined using immunohistochemistry on formalin-fixed, paraffin-embedded biopsies of normal mucosa (n=20), Helicobacter pylori-associated gastritis (n=24), intestinal metaplasia (n=24), dysplasia (n=12) and intestinal (n=19) and diffuse (n=21) adenocarcinoma. Relationships between HIF-2α expression and prognosis were assessed in resection specimens from 177 patients with gastric and gastro-oesophageal junction adenocarcinoma. Expression of all markers increased with progression along the gastric carcinogenesis sequence (P=0.0001). Hypoxia-inducible factor-2α was expressed in 63% of 177 resection specimens and at a high level in 44%. The median overall survival in patients with HIF-2α-expressing tumours was 22 (95% CI 18−26) months, whereas those with HIF-2α-negative tumours had a median survival of 37 (95% CI 29−44) months (P=0.015). Hypoxia-inducible factor-2α had no independent prognostic significance in multivariate analysis. In view of the lack of independent prognostic significance, HIF-2α has no role as a routine prognostic indicator. However, the high expression of HIF-2α suggests that it may be of value as a potential therapeutic target.

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.

Hypoxia, via stabilization of the hypoxia-inducible factor HIF-1alpha, is a direct and sufficient stimulus for brain-type natriuretic peptide induction

BNP (brain-type natriuretic peptide) is a cardiac hormone with systemic haemodynamic effects as well as local cytoprotective and antiproliferative properties. It is induced under a variety of pathophysiological conditions, including decompensated heart failure and myocardial infarction. Since regional hypoxia is a potential common denominator of increased wall stretch and myocardial hypoperfusion, we investigated the direct effects of hypoxia on BNP expression, and the role of the HIF (hypoxia-inducible transcription factor) in BNP regulation. Using an RNase protection assay we found a strong hypoxic induction of BNP mRNA expression in different cell lines and in cultured adult rat cardiomyocytes. Systemic hypoxia and exposure to 0.1% CO induced BNP expression in the rodent myocardium in vivo, although this was at a lower amplitude. BNP promoter-driven luciferase expression increased 10-fold after hypoxic stimulation in transient transfections. Inactivation of four putative HREs (hypoxia-response elements) in the promoter by site-directed mutagenesis revealed that the HRE at -466 nt was responsible for hypoxic promoter activation. A functional CACAG motif was identified upstream of this HRE. The HIF-1 complex bound specifically and inducibly only to the HRE at -466 nt, as shown by EMSA (electrophoretic mobility-shift assay) and ChIP (chromatin immunoprecipitation). siRNA (small interfering RNA)-mediated knockdown of HIF-1alpha, but not HIF-2alpha, interfered with hypoxic BNP mRNA induction and BNP promoter activation, confirming that BNP is a specific HIF-1alpha target gene. In conclusion, BNP appears to be part of the protective program steered by HIF-1 in response to oxygen deprivation. Induction of BNP may therefore contribute to the potential benefits of pharmacological HIF inducers in the treatment of ischaemic heart disease and heart failure.

Prolyl 4-hydroxylases, key enzymes in the synthesis of collagens and regulation of the response to hypoxia, and their roles as treatment targets

Prolyl 4-hydroxylases (P4Hs) have central roles in the synthesis of collagens and the regulation of oxygen homeostasis. The 4-hydroxyproline residues generated by the endoplasmic reticulum (ER) luminal collagen P4Hs (C-P4Hs) are essential for the stability of the collagen triple helix. Vertebrate C-P4Hs are alpha2beta2 tetramers with three isoenzymes differing in their catalytic alpha subunits. Another P4H family, the HIF-P4Hs, hydroxylates specific prolines in the alpha subunit of the hypoxia-inducible transcription factor (HIF), a master regulator of hypoxia-inducible genes, and controls its stability in an oxygen-dependent manner. The HIF-P4Hs are cytoplasmic and nuclear enzymes, likewise with three isoenzymes in vertebrates. A third vertebrate P4H type is an ER transmembrane protein that can act on HIF-alpha but not on collagens. All P4Hs require Fe2+, 2-oxoglutarate, O2, and ascorbate. C-P4Hs are regarded as attractive targets for pharmacological inhibition to control excessive collagen accumulation in fibrotic diseases and severe scarring, while HIF-P4H inhibitors are believed to have beneficial effects in the treatment of diseases such as myocardial infarction, stroke, peripheral vascular disease, diabetes, and severe anemias. Studies with P4H inhibitors in various animal models of fibrosis, anemia, and ischemia and ongoing clinical trials with HIF-P4H inhibitors support this hypothesis by demonstrating efficacy in many applications.

Transcriptional adaptation of neuroblastoma cells to hypoxia

Low oxygen pressure (hypoxia) is a physiological condition that has been linked to tumor progression and increased malignancy in several cancer forms. Cells of the childhood neoplasm neuroblastoma respond to hypoxia by attaining a lower grade of differentiation, which clinically is associated with poor prognosis. Furthermore, expression of the hypoxia inducible factor-2alpha correlates to poor outcome in neuroblastoma patients. In this report we have by microarray analysis studied transcriptional changes in seven neuroblastoma cell lines subjected to long term hypoxia. We find the gene regulatory response to be highly dependent on cell line background, however, a set of genes was coherently regulated by hypoxia and these genes are correlated to known hypoxia-induced transcriptional profiles.

Hypoxia-inducible factor-1alpha and hypoxia-inducible factor-2alpha are expressed in kaposi sarcoma and modulated by insulin-like growth factor-I

PURPOSE

Neoangiogenesis is essential for tumor development. Hypoxia-inducible factor (HIF), a transcriptional factor composed of two subunits (alpha and beta), plays a key role in this process, activating proangiogenic factors such as vascular endothelial growth factor (VEGF). The HIF alpha subunits are critically regulated by oxygen and are also modulated by growth factors. Kaposi sarcoma (KS) is a highly vascular tumor that releases large amounts of VEGF and for which we have recently described an essential role for the insulin-like growth factor (IGF) system. We therefore investigated the expression of HIF alpha subunits in biopsies from KS tumors and their modulation by IGF-I in KSIMM, a KS cell line.

RESULTS

Both HIF-1alpha and HIF-2alpha were expressed in KS biopsies in all tumoral stages. HIF-1alpha immunopositivity increased through the tumor development with highest expression in the late nodular stages. In KSIMM cells, IGF-I induced accumulation of both HIF alpha subunits. The induction suggests a translation mechanism as documented by cycloheximide chase experiment coupled with constant RNA levels as evaluated by quantitative real-time PCR. IGF-I-induced HIF alpha accumulation was followed by an increase in HIF function as assessed both by reporter gene assay and by induction of endogenous target gene expression (VEGF-A). Specific blockade of IGF-I receptor with alphaIR3 antibody or with picropodophyllin, a specific IGF-IR tyrosine kinase inhibitor, diminishes the basal and IGF-I-dependent induction of both HIF alpha congeners.

CONCLUSION

These novel findings show the coupling between the IGF and HIF signaling in KS and suggest a coordinated contribution by these pathways to the characteristic vascular phenotype of this tumor.

Sphingosine kinase 1 is up-regulated during hypoxia in U87MG glioma cells. Role of hypoxia-inducible factors 1 and 2

Sphingosine 1-phosphate (S1P), a sphingolipid metabolite that plays an important role in the regulation of cell survival, growth, migration, and angiogenesis, acts both inside the cells and as an extracellular mediator through binding to five G protein-coupled receptors (S1P(1-5)). Sphingosine kinase 1 (SK1), the enzyme responsible for S1P production, is overexpressed in many solid tumors, including gliomas. One common feature of these tumors is the presence of "hypoxic regions," characterized by cells expressing high levels of hypoxia-inducible factors HIF-1alpha and HIF-2alpha, two transcription regulators that modulate the levels of proteins with crucial roles in tumor progression. So far, nothing is known about the role and the regulation of SK1 during tumor-induced hypoxia or about SK1 regulation and HIFs. Here we investigated the role of HIF-1alpha and HIF-2alpha in the regulation of SK1 during hypoxic stress in glioma-derived U87MG cells. We report that hypoxia increases SK1 mRNA levels, protein expression, and enzyme activity, followed by intracellular S1P production and S1P release. Interestingly, knockdown of HIF-2alpha by small interfering RNA abolished the induction of SK1 and the production of extracellular S1P after CoCl(2) treatment, whereas HIF-1alpha small interfering RNA resulted in an increase of HIF-2alpha and of SK1 protein levels. Moreover, using chromatin immunoprecipitation analysis, we demonstrate that HIF-2alpha binds the SK1 promoter. Functionally, we demonstrate that conditioned medium from hypoxia-treated tumor cells results in neoangiogenesis in human umbilical vein endothelial cells in a S1P receptor-dependent manner. These studies provide evidence of a link between S1P production as a potent angiogenic agent and the hypoxic phenotype observed in many tumors.

Chronic oxidative stress causes amplification and overexpression of ptprz1 protein tyrosine phosphatase to activate beta-catenin pathway

Ferric nitrilotriacetate induces oxidative renal tubular damage via Fenton-reaction, which subsequently leads to renal cell carcinoma (RCC) in rodents. Here, we used gene expression microarray and array-based comparative genomic hybridization analyses to find target oncogenes in this model. At the common chromosomal region of amplification (4q22) in rat RCCs, we found ptprz1, a tyrosine phosphatase (also known as protein tyrosine phosphatase zeta or receptor tyrosine phosphatase beta) highly expressed in the RCCs. Analyses revealed genomic amplification up to eightfold. Despite scarcity in the control kidney, the amounts of PTPRZ1 were increased in the kidney after 3 weeks of oxidative stress, and mRNA levels were increased 16 approximately 552-fold in the RCCs. Network analysis of the expression revealed the involvement of the beta-catenin pathway in the RCCs. In the RCCs, dephosphorylated beta-catenin was translocated to nuclei, resulting in the expression of its target genes cyclin D1, c-myc, c-jun, fra-1, and CD44. Furthermore, knockdown of ptprz1 with small interfering RNA (siRNA), in FRCC-001 and FRCC-562 cell lines established from the induced RCCs, decreased the amounts of nuclear beta-catenin and suppressed cellular proliferation concomitant with a decrease in the expression of target genes. These results demonstrate that chronic oxidative stress can induce genomic amplification of ptprz1, activating beta-catenin pathways without the involvement of Wnt signaling for carcinogenesis. Thus, iron-mediated persistent oxidative stress confers an environment for gene amplification.

Hypoxia and osteoarthritis: how chondrocytes survive hypoxic environments

PURPOSE OF REVIEW

This review summarizes the current knowledge about hypoxia and hypoxia-inducible factor-1 (HIF-1) for chondrocyte survival, energy generation and matrix synthesis of articular chondrocytes during cartilage homeostasis and disease.

RECENT FINDINGS

In recent years increasing evidence of a pivotal role of hypoxia and the transcription factor HIF-1alpha in cartilaginous tissues has been published. Growth plates with functionally inactivated hypoxia-inducible factor-1alpha display great defects in their central areas caused by massive cell death. This very important observation indicates that hypoxia-inducible factor-1alpha is absolutely necessary for chondrocytes to survive extremely low oxygen tensions. Furthermore, hypoxia-inducible factor-1alpha has been shown to have very important functions for the regulation of glucose transport, anaerobic energy generation and matrix synthesis by articular chondrocytes. Besides hypoxia, other factors such as proinflammatory mediators and mechanical load have been shown to increase hypoxia-inducible factor-1alpha activity in articular chondrocytes. All these factors are known to be involved in the pathogenesis of osteoarthritis. Thus, a dependence of osteoarthritis chondrocytes on hypoxia-inducible factor-1alpha to survive and function properly is a reasonable assumption.

SUMMARY

Low oxygen tensions and hypoxia-inducible factor-1alpha are important factors in articular chondrocyte behaviour during cartilage homeostasis and osteoarthritis. Hypoxia-inducible factor-1alpha is a highly conserved transcription factor that has key functions in controlling energy generation, cell survival and matrix synthesis by articular and growth-plate chondrocytes.

Hypoxia-inducible factors and cancer

Decreased oxygen availability is a common feature during embryonic development as well of malignant tumours. Hypoxia regulates many transcription factors, and one of the most studied is the hypoxia-inducible factor (HIF). As a consequence of HIF stabilisation, the cell constitutively upregulates the hypoxic programme resulting in the expression of genes responsible for global changes in cell proliferation, angiogenesis, metastasis, invasion, de-differentiation and energy metabolism. Of the three known alpha subunits of HIF transcription factors, HIF-1alpha and HIF-2alpha have been the most studied. Their differential expression and function have been widely discussed, however no clear picture has been drawn on how these two transcription factors differently regulate common and unique target genes. Their role as oncogenes has also been suggested in several studies. In this review we provide an overview of the current knowledge on some of the most important aspects of HIFalpha regulation, its role in tumour angiogenesis and energetic metabolism. We also give an overview of how the modulation of HIF regulating pathways is a potential therapeutic target that may have benefits in the treatment of cancer.

The TWIST1 oncogene is a direct target of hypoxia-inducible factor-2alpha

Hypoxia-inducible factors (HIFs) are highly conserved transcription factors that play a crucial role in oxygen homeostasis. Intratumoral hypoxia and genetic alterations lead to HIF activity, which is a hallmark of solid cancer and is associated with poor clinical outcome. HIF activity is regulated by an evolutionary conserved mechanism involving oxygen-dependent HIFalpha protein degradation. To identify novel components of the HIF pathway, we performed a genome-wide RNA interference screen in Caenorhabditis elegans, to suppress HIF-dependent phenotypes, like egg-laying defects and hypoxia survival. In addition to hif-1 (HIFalpha) and aha-1 (HIFbeta), we identified hlh-8, gska-3 and spe-8. The hlh-8 gene is homologous to the human oncogene TWIST1. We show that TWIST1 expression in human cancer cells is enhanced by hypoxia in a HIF-2alpha-dependent manner. Furthermore, intronic hypoxia response elements of TWIST1 are regulated by HIF-2alpha, but not HIF-1alpha. These results identify TWIST1 as a direct target gene of HIF-2alpha, which may provide insight into the acquired metastatic capacity of hypoxic tumors.

Metastasis and stem cell pathways

Recent studies have described a small population of self-renewing and multipotent cells within tumors termed "cancer stem cells." These cells share many traits with somatic and embryonic stem cells and are thought to be responsible for driving tumor progression in a growing list of neoplastic diseases. Cells within solid tumors encounter hypoxia due to poor vascular function. Both long-standing and emerging data describe hypoxic effects on somatic and embryonic stem cells, and it is likely that hypoxia also has profound effects on cancer stem cells. These effects include the activation of pathways that induce the dedifferentiation of cancer cells, the maintenance of stem cell identity, and increased metastatic potential. Hypoxia may contribute to tumor progression by specifically impacting these pathways in cancer stem cells.

The N-terminal transactivation domain confers target gene specificity of hypoxia-inducible factors HIF-1alpha and HIF-2alpha

The basic helix-loop-helix-Per-ARNT-Sim-proteins hypoxia-inducible factor (HIF)-1alpha and HIF-2alpha are the principal regulators of the hypoxic transcriptional response. Although highly related, they can activate distinct target genes. In this study, the protein domain and molecular mechanism important for HIF target gene specificity are determined. We demonstrate that although HIF-2alpha is unable to activate multiple endogenous HIF-1alpha-specific target genes (e.g., glycolytic enzymes), HIF-2alpha still binds to their promoters in vivo and activates reporter genes derived from such targets. In addition, comparative analysis of the N-terminal DNA binding and dimerization domains of HIF-1alpha and HIF-2alpha does not reveal any significant differences between the two proteins. Importantly, replacement of the N-terminal transactivation domain (N-TAD) (but not the DNA binding domain, dimerization domain, or C-terminal transactivation domain [C-TAD]) of HIF-2alpha with the analogous region of HIF-1alpha is sufficient to convert HIF-2alpha into a protein with HIF-1alpha functional specificity. Nevertheless, both the N-TAD and C-TAD are important for optimal HIF transcriptional activity. Additional experiments indicate that the ETS transcription factor ELK is required for HIF-2alpha to activate specific target genes such as Cited-2, EPO, and PAI-1. These results demonstrate that the HIF-alpha TADs, particularly the N-TADs, confer HIF target gene specificity, by interacting with additional transcriptional cofactors.

Asparaginyl Hydroxylation of the Notch Ankyrin Repeat Domain by Factor Inhibiting Hypoxia-inducible Factor

The stability and activity of hypoxia-inducible factor (HIF) are regulated by the post-translational hydroxylation of specific prolyl and asparaginyl residues. We show that the HIF asparaginyl hydroxylase, factor inhibiting HIF (FIH), also catalyzes hydroxylation of highly conserved asparaginyl residues within ankyrin repeat (AR) domains (ARDs) of endogenous Notch receptors. AR hydroxylation decreases the extent of ARD binding to FIH while not affecting signaling through the canonical Notch pathway. ARD proteins were found to efficiently compete with HIF for FIH-dependent hydroxylation. Crystallographic analyses of the hydroxylated Notch ARD (2.35A) and of Notch peptides bound to FIH (2.4-2.6A) reveal the stereochemistry of hydroxylation on the AR and imply that significant conformational changes are required in the ARD fold in order to enable hydroxylation at the FIH active site. We propose that ARD proteins function as natural inhibitors of FIH and that the hydroxylation status of these proteins provides another oxygen-dependent interface that modulates HIF signaling.

HIF-1alpha and HIF-2alpha play a central role in stretch-induced but not shear-stress-induced angiogenesis in rat skeletal muscle

Angiogenesis, which is essential for the physiological adaptation of skeletal muscle to exercise, occurs in response to the mechanical forces of elevated capillary shear stress and cell stretch. Increased production of VEGF is a characteristic of endothelial cells undergoing either stretch- or shear-stress-induced angiogenesis. Because VEGF production is regulated by hypoxia inducible factors (HIFs), we examined whether HIFs play a significant role in the angiogenic process initiated by these mechanical forces. Rat extensor digitorum longus (EDL) muscles were overloaded to induce stretch, or exposed to the dilator prazosin to elevate capillary shear stress, and capillaries from these muscles were isolated by laser capture microdissection for RNA analysis. HIF-1alpha and HIF-2alpha transcript levels increased after 4 and 7 days of stretch, whereas a transient early induction of HIF-1alpha and HIF-2alpha transcripts was detected in capillaries from prazosin-treated muscles. Skeletal muscle microvascular endothelial cells exposed to 10% stretch in vitro showed an elevation in HIF-1alpha and HIF-2alpha mRNA, which was preceded by increases in HIF-binding activity. Conversely, HIF-1alpha and HIF-2alpha mRNA were reduced significantly, and HIF-alpha proteins were undetectable, after 24 h exposure to elevated shear stress (16 dyn cm(-2) (16 x10(-5) N cm(-2)). Given the disparate regulation of HIFs in response to these mechanical stimuli, we tested the requirement of HIF-alpha proteins in stretch- and shear-stress-induced angiogenesis by impeding HIF accumulation through use of the geldanamycin derivative 17-DMAG. Treatment with 17-DMAG significantly impaired stretch-induced, but not shear-stress-induced, angiogenesis. Together, these results illustrate that activation of HIF-1alpha and HIF-2alpha contributes significantly to stretch- but not to shear-stress-induced capillary growth.

The hypoxia-inducible factor alpha pathway couples angiogenesis to osteogenesis during skeletal development

Skeletal development and turnover occur in close spatial and temporal association with angiogenesis. Osteoblasts are ideally situated in bone to sense oxygen tension and respond to hypoxia by activating the hypoxia-inducible factor alpha (HIF alpha) pathway. Here we provide evidence that HIF alpha promotes angiogenesis and osteogenesis by elevating VEGF levels in osteoblasts. Mice overexpressing HIF alpha in osteoblasts through selective deletion of the von Hippel-Lindau gene (Vhl) expressed high levels of Vegf and developed extremely dense, heavily vascularized long bones. By contrast, mice lacking Hif1a in osteoblasts had the reverse skeletal phenotype of that of the Vhl mutants: long bones were significantly thinner and less vascularized than those of controls. Loss of Vhl in osteoblasts increased endothelial sprouting from the embryonic metatarsals in vitro but had little effect on osteoblast function in the absence of blood vessels. Mice lacking both Vhl and Hif1a had a bone phenotype intermediate between those of the single mutants, suggesting overlapping functions of HIFs in bone. These studies suggest that activation of the HIF alpha pathway in developing bone increases bone modeling events through cell-nonautonomous mechanisms to coordinate the timing, direction, and degree of new blood vessel formation in bone.

Oxygen, a source of life and stress

Oxygen is an essential element in the survival of complex organisms, however the level of oxygen, low or high, can be a source of stress depending on the biological context. Low levels of oxygen in tissues (hypoxia) can be the consequence of a number of pathophysiological conditions including ischemic disorders and cancer while relative, higher levels (hyperoxia) can lead to retinopathy of prematurity. The local oxygen environment and oxygen consumption dictate vascular homeostasis, vaso-proliferation and vaso-cessation, which is deregulated in these diseases through oxygen-dependent growth factors. In this review, we will introduce aspects of the physiology and biology of oxygen partial pressure and the molecular mechanisms implicated in oxygen sensing. We will outline the regulation and function of the key operator in cellular signalling of hypoxia, the transcription factor, hypoxia-inducible factor. In addition, we will focus on cancer cell hypoxia and on its role in driving cell metabolism, pH regulation and survival.

Role of hypoxia-inducible transcription factors 1alpha and 2alpha in the regulation of plasminogen activator inhibitor-1 expression in a human trophoblast cell line

The plasminogen activator inhibitors (PAIs) play critical roles in regulating hemostatic and invasive functions of trophoblasts through suppression of plasmin-dependent fibrinolysis and extracellular matrix degradation. The expression of PAI-1 is increased under hypoxic conditions, although the mechanism remains incompletely understood. In the current study we used HTR-8/SVneo cells, a first trimester extravillous trophoblast cell line, and siRNA technology to examine the role of hypoxia-inducible transcription factors (HIFs)-1alpha and -2alpha in the regulation of PAI-1 expression. Using serum-containing and serum-free media culture media it was initially noted that levels of PAI-1, but not PAI-2 protein, were markedly induced by hypoxic (2-3% oxygen) treatment. Under hypoxic conditions, Western blotting revealed that the presence of siRNAs to HIF-1alpha and HIF-2alpha suppressed expression of their respective proteins, whereas treatment with non-targeting and cyclophilin B siRNAs did not. Importantly, incubation with siRNA to HIF-1alpha or HIF-2alpha alone reduced PAI-1 protein levels to a similar extent, with the combined treatment inducing a more profound effect. The presence of HIF siRNAs reduced levels of PAI-1 mRNA as measured by quantitative real-time PCR, indicating that HIF-1alpha and HIF-2 alpha regulate PAI-1 expression at a transcriptional level. These results indicate that both HIF-1alpha and HIF-2alpha play important and similar roles in hypoxia-mediated stimulation of PAI-1 expression in HTR-8/SVneo cells. Our findings provide insight into the physiological regulation of trophoblast PAI-1 expression in early pregnancy when placental oxygen levels are low, as well as a mechanism for over-expression of placental PAI-1 noted in pregnancies with preeclampsia.

HIF-2alpha specifically activates the VE-cadherin promoter independently of hypoxia and in synergy with Ets-1 through two essential ETS-binding sites

The mechanisms that are responsible for the restricted pattern of expression of the VE-cadherin gene in endothelial cells are not clearly understood. Regulation of expression is under the control of an approximately 140 bp proximal promoter that provides basal, non-endothelial specific expression. A larger region contained within the 2.5 kb genomic DNA sequence located ahead of the transcription start is involved in the specific expression of the gene in endothelial cells. We show here that the VE-cadherin promoter contains several putative hypoxia response elements (HRE) which are able to bind endothelial nuclear factors under normoxia. The VE-cadherin gene is not responsive to hypoxia but hypoxia-inducible factor (HIF)-2alpha specifically activates the promoter while HIF-1alpha does not. The HRE, that are involved in this activity have been identified. Further, we show that HIF-2alpha cooperates with the Ets-1 transcription factor for activation of the VE-cadherin promoter and that this synergy is dependent on the binding of Ets-1 to DNA. This cooperative action of HIF-2alpha with Ets-1 most probably participates to the transcriptional regulation of expression of the gene in endothelial cells. This mechanism may also be involved in the expression of the VE-cadherin gene by tumor cells in the process of vascular mimicry.

The transcription factor HIF-1alpha plays a critical role in the growth factor-dependent regulation of both aerobic and anaerobic glycolysis

Mammalian cells are believed to have a cell-intrinsic ability to increase glucose metabolism in response to hypoxia. Here we show that the ability of hematopoietic cells to up-regulate anaerobic glycolysis in response to hypoxia is dependent on receptor-mediated signal transduction. In the absence of growth factor signaling, hematopoietic cells fail to express hypoxia-inducible transcription factor (Hif-1alpha) mRNA. Growth factor-deprived hematopoietic cells do not engage in glucose-dependent anabolic synthesis and neither express Hif-1alpha mRNA nor require HIF-1alpha protein to regulate cell survival in response to hypoxia. However, HIF-1alpha is adaptive for the survival of growth factor-stimulated cells, as suppression of HIF-1alpha results in death when growing cells are exposed to hypoxia. Growth factor-dependent HIF-1alpha expression reprograms the intracellular fate of glucose, resulting in decreased glucose-dependent anabolic synthesis and increased lactate production, an effect that is enhanced when HIF-1alpha protein is stabilized by hypoxia. Together, these data suggest that HIF-1alpha contributes to the regulation of growth factor-stimulated glucose metabolism even in the absence of hypoxia.

In cultured astrocytes, p53 and MDM2 do not alter hypoxia-inducible factor-1alpha function regardless of the presence of DNA damage

A principal molecular mechanism by which cells respond to hypoxia is by activation of the transcription factor hypoxia-inducible factor 1alpha (HIF-1alpha). Several studies describe a binding of p53 to HIF-1alpha in a protein complex, leading to attenuated function, half-life, and abundance of HIF-1alpha. However, these reports almost exclusively utilized transformed cell lines, and many employed transfection of p53 or HIF-1alpha plasmid constructs and/or p53 and HIF-1alpha reporter constructs as surrogates for endogenous protein activity and target expression, respectively. Thus, it remains an open and important question as to whether p53 inhibits HIF-1alpha-mediated transactivation of endogenous HIF-1alpha targets in nontransformed cells. After determining in primary astrocyte cultures the HIF-1alpha targets that were most dependent on HIF-1alpha function, we examined the effect of the loss of p53 function either alone or in combination with MDM2 on expression of these targets. Although p53 null astrocyte cultures resulted in markedly increased HIF-1alpha-dependent target expression compared with controls, this altered expression was determined to be the result of increased cell density of p53 null cultures and the accompanying acidosis, not loss of p53 protein. Although activation of p53 by DNA damage induced p53 target expression in astrocytes, it did not alter hypoxia-induced HIF-1alpha target expression. Finally, a combined loss of MDM2 and p53 did not alter HIF-1alpha target expression compared with loss of p53 alone. These data strongly suggest that p53 and MDM2 do not influence the hypoxia-induced transactivation of HIF-1alpha targets, regardless of p53 activation, in primary astrocytes.

HIF-2alpha promotes hypoxic cell proliferation by enhancing c-myc transcriptional activity

HIF-2alpha promotes von Hippel-Lindau (VHL)-deficient renal clear cell carcinoma (RCC) tumorigenesis, while HIF-1alpha inhibits RCC growth. As HIF-1alpha antagonizes c-Myc function, we hypothesized that HIF-2alpha might enhance c-Myc activity. We demonstrate here that HIF-2alpha promotes cell-cycle progression in hypoxic RCCs and multiple other cell lines. This correlates with enhanced c-Myc promoter binding, transcriptional effects on both activated and repressed target genes, and interactions with Sp1, Miz1, and Max. Finally, HIF-2alpha augments c-Myc transformation of primary mouse embryo fibroblasts (MEFs). Enhanced c-Myc activity likely contributes to HIF-2alpha-mediated neoplastic progression following loss of the VHL tumor suppressor and influences the behavior of hypoxic tumor cells.

Target gene selectivity of hypoxia-inducible factor-alpha in renal cancer cells is conveyed by post-DNA-binding mechanisms

Inactivation of the von Hippel–Lindau tumour suppressor in renal cell carcinoma (RCC) leads to failure of proteolytic regulation of the α subunits of hypoxia-inducible factor (HIF), constitutive upregulation of the HIF complex, and overexpression of HIF target genes. However, recent studies have indicated that in this setting, upregulation of the closely related HIF-α isoforms, HIF-1α and HIF-2α, have contrasting effects on tumour growth, and activate distinct sets of target genes. To pursue these findings, we sought to elucidate the mechanisms underlying target gene selectivity for HIF-1α and HIF-2α. Using chromatin immunoprecipitation to probe binding to hypoxia response elements in vivo, and expression of chimaeric molecules bearing reciprocal domain exchanges between HIF-1α and HIF-2α molecules, we show that selective activation of HIF-α target gene expression is not dependent on selective DNA-binding at the target locus, but depends on non-equivalent C-terminal portions of these molecules. Our data indicate that post-DNA binding mechanisms that are dissimilar for HIF-1α and HIF-2α determine target gene selectivity in RCC cells.

Hypoxia-inducible factor-1 (HIF-1) is a transcriptional activator of the TrkB neurotrophin receptor gene

Neurotrophins and their cognate receptors play a pivotal role in the development and function of the nervous system. High expression levels of the neurotrophin receptor TrkB and its ligands in neuroblastomas are associated with an unfavorable outcome. We report here that NTRK2, which encodes the TrkB receptor tyrosine kinase, is an oxygen-regulated gene, whose expression is stimulated by the hypoxia-inducible factor-1 (HIF-1). TrkB mRNA and protein levels were elevated nearly 30-fold in neuroblastoma-derived Kelly cells in hypoxia (1% O(2)) versus normoxia (21% O(2)). A luciferase reporter construct containing approximately 2.1 kilobases of the human TrkB promoter was activated about 6-fold both in hypoxia and after stimulation with the hypoxia mimetic 2,2'-dipyridyl (100 microm) at 21% O(2). Luciferase activity in the presence of 2,2'-dipyridyl was reduced significantly upon small interfering RNA knockdown of HIF-1alpha but not of HIF-2alpha. Accordingly, hypoxia failed to stimulate the TrkB promoter in mouse embryonic fibroblasts that lacked HIF-1alpha. The hypoxia-responsive promoter region could be mapped to three HIF-1 binding elements that were located between -923 and -879 bp relative to the transcription start site. The migration of cultured neuroblastoma cells was increased approximately 2-fold upon incubation at 1 versus 21% O(2). This effect of hypoxia was abrogated with the tyrosine kinase inhibitor K252a (200 nm). Our findings indicate that transcription of the NTRK2 gene is stimulated at low oxygen tension through a HIF-1-dependent mechanism. In conclusion, enhanced expression of TrkB could represent a critical switch for the previously reported dedifferentiation of neuroblastoma cells under hypoxic conditions.

A long-term "memory" of HIF induction in response to chronic mild decreased oxygen after oxygen normalization

Background

Endothelial dysfunction (ED) is functionally characterized by decreased vasorelaxation, increased thrombosis, increased inflammation, and altered angiogenic potential, has been intimately associated with the progression and severity of cardiovascular disease. Patients with compromised cardiac function oftentimes have a state of chronic mild decreased oxygen at the level of the vasculature and organs, which has been shown to exacerbate ED. Hypoxia inducible factor (HIF) is a transcription factor complex shown to be the master regulator of the cellular response to decreased oxygen levels and many HIF target genes have been shown to be associated with ED.

Methods

Human endothelial and aortic smooth muscle cells were exposed either to A) normoxia (21% O₂) for three weeks, or to B) mild decreased oxygen (15% O₂) for three weeks to mimic blood oxygen levels in patients with heart failure, or to C) mild decreased oxygen for two weeks followed by one week of normoxia ("memory" treatment). Levels of HIF signaling genes (HIF-1α, HIF-2α, VEGF, BNIP3, GLUT-1, PAI-1 and iNOS) were measured both at the protein and mRNA levels.

Results

It was found that chronic exposure to mild decreased oxygen resulted in significantly increased HIF signaling. There was also a "memory" of HIF-1α and HIF target gene induction when oxygen levels were normalized for one week, and this "memory" could be interrupted by adding a small molecule HIF inhibitor to the last week of normalized oxygen. Finally, levels of ubiquitylated HIF-1α were reduced in response to chronic mild decreased oxygen and were not full restored after oxygen normalization.

Conclusion

These data suggest that HIF signaling may be contributing to the pathogenesis of endothelial dysfunction and that normalization of oxygen levels may not be enough to reduce vascular stress.

Hypoxia-inducible factors: central regulators of the tumor phenotype

Low oxygen levels are a defining characteristic of solid tumors, and responses to hypoxia contribute substantially to the malignant phenotype. Hypoxia-induced gene transcription promotes characteristic tumor behaviors, including angiogenesis, invasion, metastasis, de-differentiation and enhanced glycolytic metabolism. These effects are mediated, at least in part, by targets of the hypoxia-inducible factors (HIFs). The HIFs function as heterodimers comprising an oxygen-labile alpha-subunit and a stable beta-subunit also referred to as ARNT. HIF-1alpha and HIF-2alpha stimulate the expression of overlapping as well as unique transcriptional targets, and their induction can have distinct biological effects. New targets and novel mechanisms of dysregulation place the HIFs in an ever more central role in tumor biology and have led to development of pharmacological inhibitors of their activity.

HIF gene expression in cancer therapy

Tumor hypoxia is a feature common to almost all solid tumors due to malformed vasculature and inadequate perfusion. Tumor cells have evolved mechanisms that allow them to respond and adapt to a hypoxic microenvironment. The hypoxia-inducible transcription factor (HIF) family is comprised of oxygen-sensitive alpha (alpha) subunits that respond rapidly to decreased oxygen levels and oxygen-insensitive beta (beta) subunits. HIF binds to specific recognition sequences in the genome and increases the transcription of genes involved in a variety of metabolic and enzymatic pathways that are necessary for cells to respond to an oxygen-poor environment. The critical role of this family of transcriptional regulators in maintaining oxygen homeostasis is supported by multiple regulatory mechanisms that allow the cell to control the levels of HIF as well as its transcriptional activity. This review will focus on how the transcriptional activity of HIF is studied and how it can be exploited for cancer therapy.

Hypoxia-inducible factor-1alpha expression in the gastric carcinogenesis sequence and its prognostic role in gastric and gastro-oesophageal adenocarcinomas

Hypoxia-inducible factor-1 (HIF-1)α expression was studied in the gastric carcinogenesis sequence and as a prognostic factor in surgically resected gastric and gastro-oesophageal junction tumours. Protein expression was examined using immunohistochemistry on formalin-fixed biopsies of normal mucosa (n=20), Helicobacter pylori associated gastritis (n=24), intestinal metaplasia (n=24), dysplasia (n=12) and intestinal (n=19) and diffuse (n=21) adenocarcinoma. The relationship between HIF-1α expression and prognosis was assessed in resection specimens from 177 patients with gastric and gastro-oesophageal junction adenocarcinoma. Hypoxia-inducible factor-1α expression was not observed in normal gastric mucosa but increased in density (P<0.01) and intensity (P<0.01) with progression from H. pylori-associated gastritis, intestinal metaplasia, dysplasia to adenocarcinoma. The pattern of staining in the resection specimens was focally positive in 49 (28%) and at the invasive tumour edge in 41 (23%). Invasive edge expression was associated with lymph node metastases (P=0.034), advanced TNM stage (P=0.001) and was an adverse prognostic factor for cancer-specific survival (P=0.019). In univariate analysis and in comparison with tumours not expressing HIF-1α, invasive edge staining was associated with a hazard ratio of 1.6 (95% CI 1.0−2.5) and focally positive staining a hazard ratio of 0.7 (95% CI 0.5−1.2). Hypoxia-inducible factor-1α lost prognostic significance in multivariate analysis. The results suggest HIF-1α is involved in gastric carcinogenesis and disease progression, but is only a weak prognostic factor for survival.

EBV latent membrane protein 1 up-regulates hypoxia-inducible factor 1alpha through Siah1-mediated down-regulation of prolyl hydroxylases 1 and 3 in nasopharyngeal epithelial cells

Hypoxia-inducible factor 1 (HIF1) is up-regulated in most malignant tumors usually via interruption of ubiquitination and proteasomal degradation of its subunit alpha. Recently, we have shown that the principal EBV oncoprotein, latent membrane protein 1 (LMP1), activates HIF1alpha and subsequently expression of HIF1-responsive genes in epithelial cells. Here, we explore the mechanism for HIF1alpha activation by LMP1 in nasopharyngeal epithelial cells: LMP1 up-regulates the level of Siah1 E3 ubiquitin ligase by enhancing its stability, which subsequently induces proteasomal degradation of prolyl HIF-hydroxylases 1 and 3 that normally mark HIF1alpha for degradation. As a result, LMP1 prevents formation of von Hippel-Lindau/HIF1alpha complex, as shown by coimmunoprecipitation analyses. Thus, Siah1 is implicated in the regulation of HIF1alpha and is involved in a recently appreciated aspect of EBV-mediated tumorigenesis, namely, the angiogenesis process triggered by LMP1.

Recruitment of HIF-1alpha and HIF-2alpha to common target genes is differentially regulated in neuroblastoma: HIF-2alpha promotes an aggressive phenotype

In neuroblastoma specimens, HIF-2alpha but not HIF-1alpha is strongly expressed in well-vascularized areas. In vitro, HIF-2alpha protein was stabilized at 5% O2 (resembling end capillary oxygen conditions) and, in contrast to the low HIF-1alpha activity at this oxygen level, actively transcribed genes like VEGF. Under hypoxia (1% O2), HIF-1alpha was transiently stabilized and primarily mediated acute responses, whereas HIF-2alpha protein gradually accumulated and governed prolonged hypoxic gene activation. Knockdown of HIF-2alpha reduced growth of neuroblastoma tumors in athymic mice. Furthermore, high HIF-2alpha protein levels were correlated with advanced clinical stage and high VEGF expression and predicted poor prognosis in a clinical neuroblastoma material. Our results demonstrate the relevance of HIF-2alpha in neuroblastoma progression and have general tumor biological implications.

Identification of novel VHL targets that are associated with the development of renal cell carcinoma

von Hippel-Lindau (VHL) disease is a dominantly inherited family cancer syndrome characterized by the development of retinal and central nervous system haemangioblastomas, renal cell carcinoma (RCC) and phaeochromocytoma. Specific germline VHL mutations may predispose to haemangioblastomas, RCC and phaeochromocytoma to a varying extent. Although dysregulation of the hypoxia-inducible transcription factor-2 and JunB have been linked to the development of RCC and phaeochromocytoma, respectively, the precise basis for genotype-phenotype correlations in VHL disease have not been defined. To gain insights into the pathogenesis of RCC in VHL disease we compared gene expression microarray profiles in a RCC cell line expressing a Type 1 or Type 2B mutant pVHL (RCC-associated) to those of a Type 2A or 2C mutant (not associated with RCC). We identified 19 differentially expressed novel VHL target genes linked to RCC development. Eight targets were studied in detail by quantitative real-time polymerase chain reaction (three downregulated and five upregulated by wild-type VHL) and for six genes the effect of VHL inactivation was mimicked by hypoxia (but hypoxic-induction of smooth muscle alpha-actin 2 was specific for a RCC cell line). The potential role of four RCC-associated VHL target genes was assessed in vitro. NB thymosin beta (TMSNB) and proteinase-activated receptor 2 (PAR2) (both downregulated by wt pVHL) increased cell growth and motility in a RCC cell line, but aldehyde dehydrogenase (ALDH)1 and ALDH7 had no effect. These findings implicate TMSNB and PAR2 candidate oncogenes in the pathogenesis of VHL-associated RCC.

Failure to prolyl hydroxylate hypoxia-inducible factor alpha phenocopies VHL inactivation in vivo

Many functions have been assigned to the von Hippel-Lindau tumor suppressor gene product (pVHL), including targeting the alpha subunits of the heterodimeric transcription factor HIF (hypoxia-inducible factor) for destruction. The binding of pVHL to HIFalpha requires that HIFalpha be hydroxylated on one of two prolyl residues. We introduced HIF1alpha and HIF2alpha variants that cannot be hydroxylated on these sites into the ubiquitously expressed ROSA26 locus along with a Lox-stop-Lox cassette that renders their expression Cre-dependent. Expression of the HIF2alpha variant in the skin and liver induced changes that were highly similar to those seen when pVHL is lost in these organs. Dual expression of the HIF1alpha and HIF2alpha variants in liver, however, more closely phenocopied the changes seen after pVHL inactivation than did the HIF2alpha variant alone. Moreover, gene expression profiling confirmed that the genes regulated by HIF1alpha and HIF2alpha in the liver are overlapping but non-identical. Therefore, the pathological changes caused by pVHL inactivation in skin and liver are due largely to dysregulation of HIF target genes.

Genetic organization and hypoxic activation of the Kaposi's sarcoma-associated herpesvirus ORF34-37 gene cluster

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent for Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL). We previously reported that hypoxia activates KSHV lytic replication and that the promoter for open reading frame 34 (ORF34) contains a functional hypoxia-responsive element (HRE). ORF34 is part of a cluster of lytic genes (ORF34-37) that includes ORF36, a phosphotransferase, and ORF37, a shutoff exonuclease. Rapid amplification of cDNA ends analysis revealed that they share a common polyadenylation signal but have two start sites. Two transcripts were identified, one 3.4 kb encoding ORF35-37, and the other 4.2 kb encoding ORF34 and also having coding potential for ORF35-37. Exposure of PEL cell lines to hypoxia induced messages of lengths consistent with those of these transcripts. Reporter assays with Hep3B cells showed activation of both transcripts by hypoxia. The ORF34-37 promoter region has six consensus HREs. Sequential deletion, site-directed mutagenesis experiments, and Northern blot analysis of RNA produced by constructs indicated that the second HRE (HRE-2) plays a critical role in the hypoxic activation of both RNA transcripts. The ORF35-37 transcript was upregulated by cotransfected hypoxia-inducible factor (HIF). Electrophoretic mobility shift assays demonstrated that HRE-2 and ancillary sequences bind and compete for HIF with hypoxic Hep3B nuclear extract. The activation of this gene cluster by hypoxia may have implications for the pathogenesis of PEL and KS. Moreover, the activation of ORF36 by hypoxia might be exploited to develop targeted therapy for PEL, which arises in a hypoxic environment (pleural effusions).

Oxygen sensing by mitochondria at complex III: the paradox of increased reactive oxygen species during hypoxia

All eukaryotic cells utilize oxidative phosphorylation to maintain their high-energy phosphate stores. Mitochondrial oxygen consumption is required for ATP generation, and cell survival is threatened when cells are deprived of O(2). Consequently, all cells have the ability to sense O(2), and to activate adaptive processes that will enhance the likelihood of survival in anticipation that oxygen availability might become limiting. Mitochondria have long been considered a likely site of oxygen sensing, and we propose that the electron transport chain acts as an O(2) sensor by releasing reactive oxygen species (ROS) in response to hypoxia. The ROS released during hypoxia act as signalling agents that trigger diverse functional responses, including activation of gene expression through the stabilization of the transcription factor hypoxia-inducible factor (HIF)-alpha. The primary site of ROS production during hypoxia appears to be complex III. The paradoxical increase in ROS production during hypoxia may be explained by an effect of O(2) within the mitochondrial inner membrane on: (a) the lifetime of the ubisemiquinone radical in complex III; (b) the relative release of mitochondrial ROS towards the matrix compartment versus the intermembrane space; or (c) the ability of O(2) to access the ubisemiquinone radical in complex III. In summary, the process of oxygen sensing is of fundamental importance in biology. An ability to control the oxygen sensing mechanism in cells, potentially using small molecules that do not disrupt oxygen consumption, would open valuable therapeutic avenues that could have a profound impact on a diverse range of diseases.

HIF-dependent hematopoietic factors regulate the development of the embryonic vasculature

Hypoxia inducible factors (HIFs) regulate adaptive responses to changes in oxygen (O(2)) tension during embryogenesis, tissue ischemia, and tumorigenesis. Because HIF-deficient embryos exhibit a number of developmental defects, the precise role of HIF in early vascular morphogenesis has been uncertain. Using para-aortic splanchnopleural (P-Sp) explant cultures, we show that deletion of the HIF-beta subunit (ARNT) results in defective hematopoiesis and the inhibition of both vasculogenesis and angiogenesis. These defects are rescued upon the addition of wild-type Sca-1(+) hematopoietic cells or recombinant VEGF. Arnt(-/-) embryos exhibit reduced levels of VEGF protein and increased numbers of apoptotic hematopoietic cells. These results suggest that HIF coordinates early endothelial cell emergence and vessel development by promoting hematopoietic cell survival and paracrine growth factor production.

Role of ETS transcription factors in the hypoxia-inducible factor-2 target gene selection

Tumor hypoxia often directly correlates with aggressive phenotype, metastasis progression, and resistance to chemotherapy. Two transcription factors [hypoxia-inducible factor-1alpha (HIF-1alpha) and HIF-2alpha] are dramatically induced in hypoxic areas and regulate the expression of genes necessary for tumor adaptation to the conditions of low oxygen; however, the relative contribution of these factors is controversial. We used RNA interference-mediated inactivation of HIF-1alpha or HIF-2alpha followed by microarray analysis to identify genes specifically regulated by either HIF-1 or HIF-2 in hypoxia. We found that, in the MCF7 cell line, the vast majority of hypoxia-responsive genes (>80%) were dependent on the presence of HIF-1alpha. However, a small group of genes were preferentially regulated by HIF-2alpha. Promoter analysis for this group of genes revealed that all of them have putative binding sites for ETS family transcription factors, and 10 of 11 HIF-2alpha-dependent genes had at least one potential hypoxia-responsive element (HRE) in proximity to an ETS transcription factor binding site. Knockdown of ELK-1, the most often represented member of ETS family, significantly reduced hypoxic induction of the HIF-2alpha-dependent genes. Physical and functional interaction between ELK-1 and HIF-2alpha were supported by coimmunoprecipitation of these two proteins, luciferase reporter assay using CITED2 promoter, and binding of ELK-1 protein to the promoters of CITED2 and WISP2 genes in proximity to a HRE. These data suggest that the choice of the target genes by HIF-1 or HIF-2 depends on availability and cooperation of HIFs with other factors recognizing their cognate elements in the promoters.

Endogenous markers of hypoxia/anaerobic metabolism and anemia in primary colorectal cancer

Anemia has been implicated in the decreased oxygen tension noted within the tumor environment. In a series of 79 colorectal adenocarcinomas we investigated the role of anemia in activating molecular pathways regulated by hypoxia. Preoperative Hb levels were correlated with the immunohistochemical expression of HIF1alpha and HIF2alpha, LDH5, GLUT1, VEGF, DEC1 and BNIP3, and with angiogenesis and the cancer cell proliferation index. Upregulation of HIF1alpha and HIF2alpha proteins, found in 43% and 44.3% of cases, respectively, was not related to anemia (Hb < 10 g%). This is in agreement with other studies suggesting that HIF activation occurs for various reasons, such as poor or irregular vascularity, or oncogene activation. Nevertheless, low Hb levels (<10 g%) were linked to activated anaerobic metabolism (LDH5 overexpression) in a subset of tumors not expressing HIF1alpha (P < 0.01). Overexpression of HIFs, whether linked to anemia or not, was associated with a number of factors related to tumor aggressiveness (assessed as local invasion and nodal metastasis), anaerobic metabolism and intratumoral acidosis (LDH5, GLUT1; increased glucose metabolism to lactate), activation of genes related to necrosis (BNIP3) and angiogenesis (VEGF). Expression of BNIP3 emerged as the strongest independent factor related to transmural invasion and metastasis to lymph nodes. Identification of specific patterns of the hypoxia molecular cascade activated in cancer cells might help in developing specific therapeutic policies.

Mitochondrial disorders in renal tumors

As early as 1930, Warburg discovered that metabolic alterations were associated with carcinogenesis and that cancer cells fermented even in the presence of oxygen using glycolysis to fulfill their energy needs, though less efficiently than with respiration. The kidney requiring a very active energy production for its pumping functions has a high mitochondrial activity. Kidney tumors can exist either in relatively benign forms, as for example, in oncocytomas that are crowded with mitochondria or in very aggressive forms such as in clear cell renal carcinomas that exhibit strongly down-regulated mitochondrial activities. These carcinomas can produce metastases that are resistant to anti-mitotic drugs and current treatments only delay the fatal issue. In this review, the mitochondrial alterations observed in various forms of renal tumors will be discussed with the aim of understanding how the knowledge of mitochondrial impairment mechanisms could be helpful to develop new anti-cancer strategies.

Expression of Vascular Endothelial Growth Factor Receptor 1 in Bone Marrow-derived Mesenchymal Cells Is Dependent on Hypoxia-inducible Factor 1

Bone marrow-derived cells are recruited to sites of ischemia, where they promote tissue vascularization. This response is dependent upon the expression of vascular endothelial growth factor (VEGF) receptor 1 (VEGFR1), which mediates cell migration in response to VEGF or placental growth factor (PLGF). In this study, we found that exposure of cultured mouse bone marrow-derived mesenchymal stromal cells (MSC) to hypoxia or an adenovirus encoding a constitutively active form of hypoxia-inducible factor 1 (HIF-1) induced VEGFR1 mRNA and protein expression and promoted ex vivo migration in response to VEGF or PLGF. MSC in which HIF-1 activity was inhibited by a dominant negative or RNA interference approach expressed markedly reduced levels of VEGFR1 and failed to migrate or activate AKT in response to VEGF or PLGF. Thus, loss-of-function and gain-of-function approaches demonstrated that HIF-1 activity is necessary and sufficient for basal and hypoxia-induced VEGFR1 expression in bone marrow-derived MSC.

Global gene expression analysis of ERK5 and ERK1/2 signaling reveals a role for HIF-1 in ERK5-mediated responses

ERK5 is a recently characterized MAPK, which is most similar to the well studied ERK1/2 subfamily but uses distinct mechanisms to elicit responses. To understand the specificity of signaling through ERK5 versus ERK1/2, we examined global gene expression changes in response to each pathway. Microarray measurements in retinal pigment epithelial cells revealed 36 genes regulated by ERK5, all which were novel targets for this pathway. 39 genes were regulated by ERK1/2, which included 11 known genes. Of these genes, 19 were regulated by both pathways. Inspection of the 17 genes uniquely regulated by ERK5 revealed that 14 genes (82%) were previously associated with hypoxia via regulation by HIF-1. In contrast, 16 genes (84%) regulated by either ERK5 or ERK1/2 were implicated in hypoxia, most through mechanisms independent of HIF-1. Of the 20 genes regulated by ERK1/2, only 9 were implicated in hypoxia and were not well characterized hypoxia targets. Thus, unlike ERK5, a mechanistic link between ERK1/2 and HIF-1/HRE could not be established on the basis of gene regulation. Activation of both pathways enhanced transcription from a hypoxia-response element and increased HIF-1alpha protein expression. In contrast, ERK5 but not ERK1/2 elevated transcription through GAL4-HIF-1. Most interestingly, ERK5 is not significantly activated by hypoxia in retinal pigment epithelial cells, indicating that ERK5 regulation of these genes is relevant in normoxia rather than hypoxia. Thus, ERK5 and ERK1/2 differ in their mechanisms of gene regulation, and indicate that ERK5 may control hypoxia-responsive genes by a mechanism independent of HIF-1alpha expression control.

Differential regulation of the transcriptional activities of hypoxia-inducible factor 1 alpha (HIF-1alpha) and HIF-2alpha in stem cells

Transcriptional responses to hypoxia are primarily mediated by hypoxia-inducible factors (HIFs), HIF-1alpha and HIF-2alpha. The HIF-1alpha and HIF-2alpha subunits are structurally similar in their DNA binding and dimerization domains but differ in their transactivation domains, implying they may have unique target genes and require distinct transcriptional cofactors. Our previous results demonstrated that HIF-1alpha and HIF-2alpha regulate distinct target genes. Here, we report that HIF-2alpha is not transcriptionally active in embryonic stem (ES) cells, as well as possible inhibition by a HIF-2alpha-specific transcriptional repressor. Using DNA microarray analysis of hypoxia-inducible genes in wild-type (WT), Hif-1alpha(-)(/)(-), and Hif-2alpha(-)(/)(-) ES cells, we show that HIF-1alpha induces a large number of both confirmed and novel hypoxia-inducible genes, while HIF-2alpha does not activate any of its previously described targets. We further demonstrate that inhibition of HIF-2alpha function occurs at the level of transcription cofactor recruitment to endogenous target gene promoters. Overexpression of WT and, notably, a DNA-binding-defective HIF-2alpha mutant restores endogenous HIF-2alpha protein activity, suggesting that ES cells express a HIF-2alpha-specific corepressor that can be titrated by overexpressed HIF-2alpha protein. HIF-2alpha repression may explain why patients with mutations in the VHL tumor suppressor gene display cancerous lesions in specific tissue types.

The oxygen sensor factor-inhibiting hypoxia-inducible factor-1 controls expression of distinct genes through the bifunctional transcriptional character of hypoxia-inducible factor-1alpha

The function of the hypoxia-inducible factor-1 (HIF-1), the key transcription factor involved in cellular adaptation to hypoxia, is restricted to low oxygen tension (pO(2)). As such, this transcription factor is central in modulating the tumor microenvironment, sensing nutrient availability, and controlling anaerobic glycolysis, intracellular pH, and cell survival. Degradation and inhibition of the limiting HIF-1alpha subunit are intimately connected in normoxia. Hydroxylation of two proline residues by prolyl hydroxylase domain (PHD) 2 protein earmarks the protein for degradation, whereas hydroxylation of an asparagine residue by factor-inhibiting HIF-1 (FIH-1 or FIH) reduces its transcriptional activity. Indeed, silencing of either PHD2 or FIH in normoxia partially induced hypoxic genes, whereas combined PHD2/FIH silencing generated a full hypoxic gene response. Given the fact that HIF-1alpha possesses two transcriptional activation domains [TAD; NH(2)-terminal (N-TAD) and COOH-terminal (C-TAD)], we hypothesized on a possible bifunctional activity of HIF-1alpha that could be discriminated by FIH, an inhibitor of the C-TAD. In human cell lines engineered to overexpress or silence FIH in response to tetracycline, we show by quantitative reverse transcription-PCR that a set of hypoxic genes (ca9, phd3, pgk1, and bnip3) respond differently toward FIH expression. This finding, extended to 26 hypoxia-induced genes, indicates differential gene expression by the N-TAD and C-TAD in response to the hypoxic gradient. We propose that the oxygen-sensitive attenuator FIH, together with two distinct TADs, is central in setting the gene expression repertoire dictated by the cell pO(2).

HIF-2alpha regulates Oct-4: effects of hypoxia on stem cell function, embryonic development, and tumor growth

The division, differentiation, and function of stem cells and multipotent progenitors are influenced by complex signals in the microenvironment, including oxygen availability. Using a genetic "knock-in" strategy, we demonstrate that targeted replacement of the oxygen-regulated transcription factor HIF-1alpha with HIF-2alpha results in expanded expression of HIF-2alpha-specific target genes including Oct-4, a transcription factor essential for maintaining stem cell pluripotency. We show that HIF-2alpha, but not HIF-1alpha, binds to the Oct-4 promoter and induces Oct-4 expression and transcriptional activity, thereby contributing to impaired development in homozygous Hif-2alpha KI/KI embryos, defective hematopoietic stem cell differentiation in embryoid bodies, and large embryonic stem cell (ES)-derived tumors characterized by altered cellular differentiation. Furthermore, loss of HIF-2alpha severely reduces the number of embryonic primordial germ cells, which require Oct-4 expression for survival and/or maintenance. These results identify Oct-4 as a HIF-2alpha-specific target gene and indicate that HIF-2alpha can regulate stem cell function and/or differentiation through activation of Oct-4, which in turn contributes to HIF-2alpha's tumor promoting activity.

Concordant regulation of gene expression by hypoxia and 2-oxoglutarate-dependent dioxygenase inhibition: the role of HIF-1alpha, HIF-2alpha, and other pathways

Studies of gene regulation by oxygen have revealed novel signal pathways that regulate the hypoxia-inducible factor (HIF) transcriptional system through post-translational hydroxylation of specific prolyl and asparaginyl residues in HIF-alpha subunits. These oxygen-sensitive modifications are catalyzed by members of the 2-oxoglutarate (2-OG) dioxygenase family (PHD1, PHD2, PHD3, and FIH-1), raising an important question regarding the extent of involvement of these and other enzymes of the same family in directing the global changes in gene expression that are induced by hypoxia. To address this, we compared patterns of gene expression induced by hypoxia and by a nonspecific 2-OG-dependent dioxygenase inhibitor, dimethyloxalylglycine (DMOG), among a set of 22,000 transcripts, by microarray analysis of MCF7 cells. By using short interfering RNA-based suppression of HIF-alpha subunits, we also compared responses that were dependent on, or independent of, the HIF system. Results revealed striking concordance between patterns of gene expression induced by hypoxia and by DMOG, indicating the central involvement of 2-OG-dependent dioxygenases in oxygen-regulated gene expression. Many of these responses were suppressed by short interfering RNAs directed against HIF-1alpha and HIF-2alpha, with HIF-1alpha suppression manifesting substantially greater effects than HIF-2alpha suppression, supporting the importance of HIF pathways. Nevertheless, the definition of genes regulated by both hypoxia and DMOG, but not HIF, distinguished other pathways most likely involving the action of 2-OG-dependent dioxygenases on non-HIF substrates.

No relationship between thymidine phosphorylase (TP, PD-ECGF) expression and hypoxia in carcinoma of the cervix

The expression of hypoxia-regulated genes promotes an aggressive tumour phenotype and is associated with an adverse cancer treatment outcome. Thymidine phosphorylase (TP) levels increase under hypoxia, but the protein has not been studied in association with hypoxia in human tumours. An investigation was made, therefore, of the relationship of tumour TP with hypoxia, the expression of other hypoxia-associated markers and clinical outcome. This retrospective study was carried out in patients with locally advanced cervical carcinoma who underwent radiotherapy. Protein expression was evaluated with immunohistochemistry. Hypoxia was measured using microelectrodes and the level of pimonidazole binding. There was no relationship of TP expression with tumour pO₂ (r=−0.091, P=0.59, n=87) or pimonidazole binding (r=0.13, P=0.45, n=38). There was no relationship between TP and HIF-1α, but there was a weak borderline significant relationship with HIF-2α expression. There were weak but significant correlations of TP with the expression of VEGF, CA IX and Glut-1. In 119 patients, the presence of TP expression predicted for disease-specific (P=0.032) and metastasis-free (P=0.050) survival. The results suggest that TP is not a surrogate marker of hypoxia, but is linked to the expression of hypoxia-associated genes and has weak prognostic power.

Effects of geldanamycin on HIF-1α mediated angiogenesis and invasion in prostate cancer cells

Geldanamycin (GA), a benzoquinone ansamycin, is a naturally occurring inhibitor of heat shock protein (Hsp90), which regulates the transcription activity of hypoxia-inducible factor 1 (HIF-1alpha). Under hypoxia, HIF-1alpha is activated in tumor cells, and induces the transcription of vascular endothelial growth factor (VEGF), which is the prime regulator for angiogenesis. VEGF promotes the formation of new blood vessels by stimulating endothelial cell division and migration. This eventually forms a vascular network that allows for tumor growth and metastasis. In this study, we used GA to inhibit HIF-1alpha transcription function. Human prostate cancer DU-145 cells were incubated in a hypoxic chamber at 1% O(2) and 37 degrees C for different durations. Both mRNA and protein levels of HIF-1alpha and VEGF were upregulated under hypoxic conditions. We demonstrated that GA treatment of hypoxic DU-145 cells abolished the induction of HIF-1alpha protein in a time-dependent manner and decreased VEGF mRNA and its protein levels. The transient transfection of DU-145 cells with luciferase reporter gene construct (5HRE/hCMVmp-luc) showed that the transcriptional activity of HIF-1alpha was significantly induced in response to hypoxia, but inhibited by GA. In addition, using conditioned medium from GA-treated hypoxic cells led to a significant decrease in cell invasion in comparison with using conditioned medium from nontreated hypoxic cells. These data provide evidence for the important role of GA in inhibition of angiogenesis and also invasion mediated by HIF-1alpha in prostate cancer cells.

The hypoxic proteome is influenced by gene-specific changes in mRNA translation

BACKGROUND AND PURPOSE

Hypoxia causes a rapid reduction in mRNA translation efficiency. This inhibition does not affect all mRNA species to the same extent and can therefore contribute significantly to hypoxia-induced differential protein expression. Our aim in this study was to characterize changes in gene expression during acute hypoxia and evaluate the contribution of regulation via mRNA translation on these changes. For each gene, the contribution of changes in mRNA abundance versus mRNA translation was determined.

MATERIALS AND METHODS

DU145 prostate carcinoma cells were exposed to 4h of hypoxia (<0.02% O2). Efficiently translated mRNAs were isolated by sedimentation through a sucrose gradient. Affymetrix microarray technology was used to evaluate both the transcriptional and translational contribution to gene expression. Results were validated by quantitative PCR.

RESULTS

One hundred and twenty genes were more than 4-fold upregulated by hypoxia in the efficiently translated fraction of mRNA, in comparison to only 76 genes at the level of transcription. Of the 50 genes demonstrating the largest changes in translation, 11 were found to be more than 2-fold over represented in the translated fraction in comparison to their overall transcriptional level. The gene with the highest translational contribution to its induction was CITED-2, which is a negative regulator of HIF-1 transcriptional activity.

CONCLUSIONS

Gene-specific regulation of mRNA translation contributes significantly to differential gene expression during hypoxia.

Gene expression profile of B 16(F10) murine melanoma cells exposed to hypoxic conditions in vitro

Hypoxia is an important feature of tumor microenvironment, exerting far-reaching effects on cells and contributing to cancer progression. Previous studies have established substantial differences in hypoxia response between various cell lines. Investigating this phenomenon in melanoma cells contributes to a better understanding of cell lineage-specific hypoxia response and could point out novel hypoxia-regulated genes. We investigated transcriptional activity of B 16(F10) murine melanoma cells cultured for 24 h under hypoxic (nominal 1% O2, 15 samples including controls) and hypoxia-mimicking conditions (cobalt chloride, 100 or 200 microM, 6 samples including controls). Gene expression profiles were analyzed using MG-U74Av2 oligonucleotide microarrays. Data analysis revealed 2541 probesets (FDR <5%) for 1% oxygen experiment and 364 probesets (FDR <5%) for cobalt chloride, which showed differences in expression levels. Analysis of hypoxia-regulated genes (true hypoxia, 1% O2) by stringent Family-Wise Error Rate estimation indicated 454 significantly changed transcripts (p < 0.05). The most upregulated genes were Lgals3, Selenbpl, Nppb (more than ten-fold increase). We observed significant differences in expression levels of genes regulating glycolysis (Pfkp, Hk2, Aldo3, Eno2), apoptosis (Bnip3, Bnip31, Cdknla), transcription (Bhlhb2, Sap30, Atf3, Mxil), angiogenesis (Vegfa, Adm, Anxa2, Ctgf), adhesion (Pkp2, Itga4, Mcam), migration (Cnn2, Tmsb4x), and other processes. Both true hypoxia and hypoxia mimicry induced HIF-1-regulated genes. However, unsupervised analysis (Singular Value Decomposition) revealed distinct differences in gene expression between these two experimental conditions. Contrary to hypoxia, cobalt chloride caused suppression of gene expression rather than stimulation, especially concerning transcripts related to proliferation, immune response, DNA repair, and melanin biosynthesis.

Gold(III) compound is a novel chemocytotoxic agent for hepatocellular carcinoma

Recently, a series of gold(III) meso-tetraarylporphyrins that are stable against demetallation in physiological conditions have been synthesized. In the present study, the antitumor effects of one of these compounds, gold(III) meso-tetraarylporphyrin 1a (gold-1a) was investigated in an orthotopic rat hepatocellular carcinoma (HCC) model as well as using a HCC cell line. The rat HCC model was induced by injection of rat hepatoma cells, McA-RH7777, into the left lobe of the liver. Seven days after tumor cell inoculation, gold-1a was injected directly into the tumor nodule at different doses, followed by the same doses via intraperitoneal injection twice a week. Gold-1a administration significantly prolonged the survival of HCC-bearing rats. Importantly, gold-1a induced necrosis as well as apoptosis in the tumor tissues, but not in the normal liver tissues. Furthermore, gold-1a treatment neither caused significant drop in body weight of the rats nor affected plasma aspartate aminotransferase level. In the in vitro studies, we observed that gold-1a treatment inhibited the proliferation of McA-RH7777 cells. Gold-1a upregulated genes that increase apoptosis, stabilize p53, decrease proliferation and downregulated genes playing roles in angiogenesis, invasion, and metabolism, as demonstrated by microarray. In particular, the compound upregulated 2 members of the growth arrest and DNA damage (Gadd) inducible gene family, Gadd34 and Gadd153. Suppression of Gadd34 and Gadd153 in McA-RH7777 cells by small hairpin RNA reduced the gold-1a-induced apoptosis and growth inhibition, indicating that gold-1a mediated its effects via upregulation of Gadd34 and Gadd153. Results from our study demonstrated that gold-1a might be a novel promising chemocytotoxic agent for treating HCC.

The von Hippel-Lindau protein, HIF hydroxylation, and oxygen sensing

The heterodimeric transcription factor HIF (hypoxia-inducible factor), consisting of a labile alpha-subunit and a stable beta-subunit, is a master regulator of genes involved in acute or chronic adaptation to low oxygen. Studies performed over the past 5 years revealed that HIFalpha-subunits are enzymatically hydroxylated in an oxygen-dependent manner. Hydroxylation of either of two conserved prolyl residues targets HIFalpha for destruction by a ubiquitin ligase containing the von Hippel-Lindau tumor suppressor protein whereas hydroxylation on a C-terminal asparagine affects HIF transactivation function. Pharmacological manipulation of HIF activity might be beneficial in diseases characterized by abnormal tissue oxygenation including myocardial infarction, cerebrovascular disease, and cancer.