HER2 (neu) signaling increases the rate of hypoxia-inducible factor 1alpha (HIF-1alpha) synthesis: novel mechanism for HIF-1-mediated vascular endothelial growth factor expression

NO and TNF-alpha released from activated macrophages stabilize HIF-1alpha in resting tubular LLC-PK1 cells

Hypoxic/ischemic conditions provoke activation of the transcription factor hypoxia-inducible factor-1 (HIF-1). HIF-1 is composed of HIF-1alpha (subjected to protein stability regulation) and constitutively expressed HIF-1beta. Besides hypoxia, diverse agonists are identified that stabilize HIF-1alpha during normoxia. Here we used a coculture system of RAW 264.7 macrophage cells and tubular LLC-PK(1) cells to establish that lipopolysaccharide- and interferon-gamma-stimulated but not resting macrophages elicited HIF-1alpha accumulation in LLC-PK(1) cells. Via pharmacological interventions such as blockade of nitric oxide (NO) production in macrophages, scavenging of NO with the use of 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, or application of tumor necrosis factor-alpha (TNF-alpha)-neutralizing antibodies, we identified NO and TNF-alpha as signaling molecules. Working in concert, NO and TNF-alpha have a stronger response when allowed direct cell-to-cell contact instead of contact with only the cell supernatant of activated macrophages. We show that signal transmission by NO with TNF-alpha in LLC-PK(1) cells is mediated via the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway, because it is blocked by wortmannin or dominant-negative forms of PI3-K as well as protein kinase B. We conclude that NO and TNF-alpha, derived from activated macrophages, provoke HIF-1alpha stabilization in LLC-PK(1) cells under normoxic conditions, which underscores HIF-1alpha stabilization due to intercellular regulation.

Hypoxia-inducible factor-1 and hypoxia response elements mediate the induction of plasminogen activator inhibitor-1 gene expression by insulin in primary rat hepatocytes

The expression of the plasminogen activator inhibitor-1 (PAI-1) gene is enhanced by insulin both in vivo and in various cell types. Because insulin exerts a number of its biologic activities via the phosphatidylinositol 3-kinase and protein kinase B (PI3K/PKB) signaling pathway, it was the aim of the present study to investigate the role of the PI3K/PKB pathway in the expression of the PAI-1 gene and to identify the insulin responsive promoter sequences. It was shown that the induction of PAI-1 mRNA and protein expression by insulin and mild hypoxia could be repressed by the PI3K inhibitor wortmannin. Overexpression of a constitutively active PKB led to induction of PAI-1 mRNA expression and of luciferase (Luc) activity from a gene construct containing 766 bp of the rat PAI-1 promoter. Mutation of the hypoxia response elements (HRE-1 and HRE-2) in rat PAI-1 promoter, which could bind hypoxia inducible factor-1 (HIF-1), abolished the induction of PAI-1 by insulin and PKB. Insulin and the constitutive active PKB also induced Luc expression in cells transfected with the pGl3EPO-HRE Luc construct, containing 3 copies of the HRE from the erythropoietin gene in front of the SV40 promoter. Furthermore, insulin and the active PKB enhanced all 3 HIF alpha-subunit protein levels and HIF-1 DNA-binding activity, as shown by electrophoretic mobility shift assays (EMSAs). Thus, the insulin-dependent activation of the PAI-1 gene expression can be mediated via the PI3K/PKB pathway and the transcription factor HIF-1 binding to the HREs in the PAI-1 gene promoter.

Impaired oxygen-dependent reduction of HIF-1alpha and -2alpha proteins in pre-eclamptic placentae

Pre-eclamptic (PE) placentae overexpress hypoxia inducible transcription factors-1alpha and -2alpha proteins (Biol. Repro. 64: 499-506, 2001; Ibid 1019-1020). Possible explanations include (a) impaired oxygen-dependent reduction, and/or (b) enhanced sensitivity to reduced oxygen. After 18 h equilibration under 21 per cent O(2) atmosphere, we subjected villous explants prepared from placentae of normal pregnant (NP) and pre-eclamptic (PE) women (n=8 each) to 4h of hypoxia (2 per cent oxygen), and then studied the disappearance of HIF-1alpha and -2alpha proteins during subsequent oxygenation over 90 min (21 per cent oxygen). The disappearance of these HIF proteins as assessed by Western analysis was significantly impaired in the pre-eclamptic tissues. Even after 18h equilibration under a 21 per cent O(2) atmosphere, and then a further 4h at 21 per cent O(2), HIF-1alpha and -2alpha protein expression remained increased in villous explants from PE women (both P< 0.04 vs NP). To address whether chronic hypoxia per se (which is believed to exist in the pre-eclamptic placenta) might contribute to these findings, we subjected villous explants from normal placentae (n=6) to 18 h preincubation under 2 per cent or 21 per cent oxygen prior to subsequent incubation for 4h at 2 per cent oxygen and then 90 min at 21 per cent oxygen. The time course of disappearance of HIF proteins during oxygenation was similar irrespective of the 2 per cent or 21 per cent preconditioning. To evaluate oxygen sensitivity, we exposed villous explants from NP and PE women (n=6 each) to different oxygen atmospheres for 4h and measured HIF protein induction. Although the data showed a significant inverse relationship between HIF expression and oxygen concentration, there was no significant difference between the slopes of this relationship for the two groups of women. We conclude that villous explants from PE placentae fail to adequately downregulate HIF protein expression upon oxygenation. This abnormality may contribute to their overexpression in vivo.

The von Hippel-Lindau tumor suppressor, hypoxia-inducible factor-1 (HIF-1) degradation, and cancer pathogenesis

Recently, work on the mechanism of action of the von Hippel-Lindau tumour suppressor protein (pVHL) and studies on hypoxic gene regulation have converged, providing insights into both cellular oxygen sensing and cancer pathogenesis. pVHL is the recognition component of the E3-ubiquitin ligase complex involved in the degradation of hypoxia-inducible factor-1 (HIF) alpha-subunits, a process regulated by oxygen availability and blocked by disease causing pVHL mutations. In normoxic cells, pVHL targeting of HIF-alpha subunits follows hydroxylation of critical HIF prolyl residues by a group of oxygen, 2-oxoglutarate- and iron-dependent enzymes. In this review, we outline current understanding of HIF/pVHL/prolyl hydroxylase pathway and consider the implications for VHL-associated cancer.

Imidazole ring-opened DNA purines and their biological significance

Fragmentation of purine imidazole ring and production of formamidopyrimidines in deoxynucleosides (Fapy lesions) occurs upon DNA oxidation as well as upon spontaneous or alkali-triggered rearrangement of certain alkylated bases. Many chemotherapeutic agents such as cyclophosphamide or thiotepa produce such lesions in DNA. Unsubstituted FapyA and FapyG, formed upon DNA oxidation cause moderate inhibition of DNA synthesis, which is DNA polymerase and sequence dependent. Fapy-7MeG, a methylated counterpart of FapyG-, a efficiently inhibits DNA replication in vitro and in E.coli, however its mutagenic potency is low. This is probably due to preferential incorporation of cytosine opposite Fapy-7MeG and preferential extension of Fapy-7MeG:C pair. In contrast, FapyA and Fapy-7MeA possess miscoding potential. Both lesions in SOS induced E.coli preferentially mispair with cytosine giving rise to A-->G transitions. Fapy lesions substituted with longer chain alkyl groups also show simult aneous lethal and mutagenic properties. Fapy lesions are actively eliminated from DNA by repair glycosylases specific for oxidized purines and pyrimidines both in bacteria and eukaryotic cells. Bacterial enzymes include E.coli formamidopyrimidine-DNA-glycosylase (Fpg protein), endonuclease III (Nth protein) and endonuclease VIII (Nei protein).

A critical evaluation of DNA adducts as biological markers for human exposure to polycyclic aromatic compounds

The causative role of polycyclic aromatic hydrocarbons (PAH) in human carcinogenesis is undisputed. Measurements of PAH-DNA adduct levels in easily accessible white blood cells therefore represent useful early endpoints in exposure intervention or chemoprevention studies. The successful applicability of DNA adducts as early endpoints depends on several criteria: i. adduct levels in easily accessible surrogate tissues should reflect adduct levels in target-tissues, ii. toxicokinetics and the temporal relevance should be properly defined. iii. sources of interand intra-individual variability must be known and controllable, and finally iv. adduct analyses must have advantages as compared to other markers of PAHexposure. In general, higher DNA adduct levels or a higher proportion of subjects with detectable DNA adduct levels were found in exposed individuals as compared with nonexposed subjects, but saturation may occur at high exposures. Furthermore, DNA adduct levels varied according to changes in exposure, for example smoking cessation resulted in lower DNA adduct levels and adduct levels paralleled seasonal variations of air-pollution. Intraindividual variation during continuous exposure was low over a short period of time (weeks), but varied significantly when longer time periods (months) were investigated. Inter-individual variation is currently only partly explained by genetic polymorphisms in genes involved in PAH-metabolism and deserves further investigation. DNA adduct measurements may have three advantages over traditional exposure assessment: i. they can smooth the extreme variability in exposure which is typical for environmental toxicants and may integrate exposure over a longer period of time. Therefore, DNA adduct assessment may reduce the monitoring effort. ii. biological monitoring of DNA adducts accounts for all exposure routes. iii. DNA adducts may account for inter-individual differences in uptake, elimination, distribution, metabolism and repair amongst exposed individuals. In conclusion, there is now a sufficiently large scientific basis to justify the application of DNA adduct measurements as biomarkers in exposure assessment and intervention studies. Their use in risk-assessment, however, requires further investigation.

Hypoxia-induced angiogenesis during carcinogenesis

The formation of new blood vessels, angiogenesis, is an essential process during development and disease. Angiogenesis is well known as a crucial step in tumor growth and progression. Angiogenesis is induced by hypoxic conditions and regulated by the hypoxia-inducible factor 1 (HIF-1). The expression of HIF-1 correlates with hypoxia-induced angiogenesis as a result of the induction of the major HIF-1 target gene, vascular endothelial cell growth factor (VEGF). In this review, a brief overview of the mechanism of angiogenesis is discussed, focusing on the regulatory processes of the HIF-1 transcription factor. HIF-1 consists of a constitutively expressed HIF-1 beta (HIF-1beta) subunit and an oxygen-regulated HIF-1 alpha (HIF-1a) subunit. The stability and activity of HIF-1alpha are regulated by the interaction with various proteins, such as pVHL, p53, and p300/CBP as well as by post-translational modifications, hydroxylation, acetylation, and phosphorylation. It was recently reported that HIF-1alpha binds a co-activator of the AP-1 transcription factor, Jab-1, which inhibits the p53-dependent degradation of HIF-1 and enhances the transcriptional activity of HIF-1 and the subsequent VEGF expression under hypoxic conditions. ARD1 acetylates HIF-1alpha and stimulates pVHL-mediated ubiquitination of HIF-1alpha. With a growing knowledge of the molecular mechanisms in this field, novel strategies to prevent tumor angiogenesis can be developed, and from these, new anticancer therapies may arise.

PTEN encoding product: A marker for tumorigenesis and progression of gastric carcinoma

AIM: To detect the expression of PTEN encoding product in normal mucosa, intestinal metaplasia (IM), dysplasia and carcinoma of the stomach, and to investigate its clinical implication in tumorigenesis and progression of gastric carcinoma.

METHODS: Formalin-fixed paraffin embedded specimens from 184 cases of gastric carcinoma, their adjacent normal mucosa, IM and dysplasia were evaluated for PTEN protein expression by SABC immunohistochemistry. PTEN expression was compared with tumor stage, lymph node metastasis, Lauren’s and WHO’s histological classification of gastric carcinoma. Expression of VEGF was also detected in 60 cases of gastric carcinoma and its correlation with PTEN was concerned.

RESULTS: The positive rates of PTEN protein were 100% (102/102), 98.5% (65/66), 66.7% (4/6) and 47.8% (88/184) in normal mucosa, IM, dysplasia and carcinoma of the stomach, respectively. The positive rates in dysplasia and carcinoma were lower than in normal mucosa and IM (P < 0.01). Advanced gastric cancers expressed less frequent PTEN than early gastric cancer (42.9% vs 67.6%, P < 0.01). The positive rate of PTEN protein was lower in gastric cancer with than without lymph node metastasis (40.3% vs 63.3%, P < 0.01). PTEN was less expressed in diffuse-type than in intestinal-type gastric cancer (41.5% vs 57.8%, P < 0.05). Signet ring cell carcinoma showed the expression of PTEN at the lowest level (25.0%, 7/28); less than well and moderately differentiated ones (P < 0.01). Expression of PTEN was not correlated with expression of VEGF (P > 0.05).

CONCLUSION: Loss or reduced expression of PTEN protein occures commonly in tumorigenesis and progression of gastric carcinoma. It is suggested that PTEN can be an objective marker for pathologically biological behaviors of gastric carcinoma.

Breast cancer-induced angiogenesis: multiple mechanisms and the role of the microenvironment

Growth and progression of breast cancers are accompanied by increased neovascularization (angiogenesis). A variety of factors, including hypoxia and genetic changes in the tumor cells, contribute to increased production of angiogenic factors. Furthermore, cells within the activated tumor stroma also contribute to the increase in production of vascular endothelial growth factor and other angiogenic factors, including basic fibroblast growth factor and platelet-derived growth factor. The contribution of the microenvironment to tumor-induced angiogenesis is underscored by findings that breast tumors implanted into different tissue sites show marked differences in the extent and nature of the angiogenic response. These findings have important implications for designing anti-angiogenic therapies.

Angiogenesis relates to estrogen receptor negativity, c-erbB-2 overexpression and early relapse in node-negative ductal carcinoma of the breast

Tumor angiogenic activity is an important process linked to tumor growth, metastasis, and invasion. In the present study we investigated whether intratumoral microvessel density (MVD), as assessed with immunohistochemistry, is of prognostic relevance in a series of 77 breast cancer patients with node-negative disease. The mean MVD in the hot spots ranged from 9 to 106 (median 31) vessels per x200 optical field. Patients were grouped into 3 categories of low (27 pts), medium (26 pts), and high (24 pts) MVD. Angiogenesis was not related to the primary tumor dimensions (T-stage) or the histology differentiation. An inverse association of MVD with estrogen receptor (ER) expression was noted (p=0.0007), while high MVD was directly related to c-erbB-2 overexpression (p=0.04) and high MIB1 proliferation index (p=0.02). In univariate and multivariate analysis of relapse-free survival, MVD was the only variable significantly and independently linked to relapse. It is concluded that high intratumoral angiogenic activity is linked with early relapse in node-negative breast cancer.

Hypoxia and High Altitude

Increased erythropoietin plasma levels and the consequent augmented production of red blood cells is the best known systemic adaptation to reduced oxygen partial pressure (pO2). Intensive research during the last years revealed that the molecular mechanism behind the regulation of erythropoietin is ubiquitous and has far more implications than first thought. Erythropoietin regulation results from the activation of the hypoxia-inducible factor-1 (HIF-1) pathway under hypoxic conditions. HIF-1 is a heterodimer consisting of an oxygen sensitive--HIF-1--and an oxygen-independent subunit--HIF-1beta (also known as the aryl hydrocarbon receptor nuclear translocator--ARNT). In addition to erythropoietin, more than 30 genes are now known to be up-regulated by HIF-1. Recently, the critical involvement of HIF-1alpha post-translational modifications in the cellular oxygen sensing mechanism was discovered. In this review we will focus on the regulation of the HIF-1 pathway and the cellular oxygen sensor and discuss their implications in high altitude hypoxia.

Induction of hypoxia-inducible factor-1alpha by transcriptional and translational mechanisms

Hypoxia-inducible factor-1 (HIF-1) regulates the transcription of many genes induced by low oxygen conditions. Recent studies have demonstrated that non-hypoxic stimuli can also activate HIF-1 in a cell-specific manner. Here, we define two key mechanisms that are implicated in increasing the active subunit of the HIF-1 complex, HIF-1alpha, following the stimulation of vascular smooth muscle cells (VSMC) with angiotensin II (Ang II). We show that, in contrast to hypoxia, the induction of HIF-1alpha by Ang II in VSMC is dependent on active transcription and ongoing translation. We demonstrate that stimulation of VSMC by Ang II strongly increases HIF-1alpha gene expression. The activation of diacylglycerol-sensitive protein kinase C (PKC) plays a major role in the increase of HIF-1alpha gene transcription. We also demonstrate that Ang II relies on ongoing translation to maintain elevated HIF-1alpha protein levels. Ang II increases HIF-1alpha translation by a reactive oxygen species (ROS)-dependent activation of the phosphatidylinositol 3-kinase pathway, which acts on the 5'-untranslated region of HIF-1alpha mRNA. These results establish that the non-hypoxic induction of the HIF-1 transcription factor via vasoactive hormones (Ang II and thrombin) is triggered by a dual mechanism, i.e. a PKC-mediated transcriptional action and a ROS-dependent increase in HIF-1alpha protein expression. Elucidation of these signaling pathways that up-regulate the vascular endothelial growth factor (VEGF) could have a strong impact on different aspects of vascular biology.

NF-kappaB plays a key role in hypoxia-inducible factor-1-regulated erythropoietin gene expression

OBJECTIVE

The aim of this study was to further define the signal transduction pathways leading to hypoxia-inducible factor-1 (HIF-1) erythropoietin (EPO) gene expression.

MATERIALS AND METHODS

Human hepatocellular carcinoma cells (Hep3B) were exposed to hypoxia (1% oxygen) and examined for mRNA expression, as well as gene transactivation with RT-PCR and luciferase reporter gene assays, respectively.

RESULTS

Treatment with LY294002 (a selective pharmacological inhibitor of phosphatidylinositol 3-kinase) significantly inhibited EPO protein and mRNA expression in Hep3B cells exposed to hypoxia for 24 hours, while treatment with PD098059 or SB203580 (selective pharmacological inhibitors of the MEK and p38 mitogen-activated protein kinase pathways, respectively) had no significant effects. The activity of AKT, a downstream target of PI3K, was increased by hypoxia and was also inhibited by LY294002. Genetic inhibition of AKT resulted in significant inhibition of NF-kappaB and HIF-1-mediated transactivation, as well as EPO gene expression, in response to hypoxia. Overexpression of constitutively active AKT resulted in increased NF-kappaB and HIF-1 transactivation. The selective inhibitor of NF-kappaB, pyrrolidine dithiocarbamate (PDTC), significantly blocked HIF-1 protein expression. Inhibition of NF-kappaB with a superrepressor dominant negative IkappaBalpha genetic construct also significantly blocked NF-kappaB and HIF-1 transactivation, as well as EPO gene expression.

CONCLUSION

We propose a key role for NF-kappaB in EPO gene regulation in response to hypoxia.

A novel mechanism of repression of the vascular endothelial growth factor promoter, by single strand DNA binding cold shock domain (Y-box) proteins in normoxic fibroblasts

Overexpression of vascular endothelial growth factor (VEGF) is implicated in a number of diseases. It is therefore critical that mechanisms exist to strictly regulate VEGF expression. A hypoxia-responsive (HR) region of the VEGF promoter which binds the HIF-1 transcription factor is a target for many signals that up-regulate VEGF transcription. Repressors targeting the HIF-1 transcription factor have been identified but no repressors directly binding the HR promoter region had been reported. We now report a novel mechanism of repression of the VEGF HR region involving DNA binding. We find that single strand DNA-specific cold shock domain (CSD or Y-box) proteins repress the HR region via a binding site downstream of the HIF-1 site. The repressor site is functional in unstimulated, normoxic fibroblasts and represents a novel means to prevent expression of VEGF in the absence of appropriate stimuli. We characterized complexes forming on the VEGF repressor site and identified a previously unreported nuclear CSD protein complex containing dbpA. Nuclear dbpA appears to bind as a dimer and we determined a means by which nuclear CSD proteins may enter double strand DNA to bind to their single strand sites to bring about repression of the VEGF HR region.

BCR/ABL induces expression of vascular endothelial growth factor and its transcriptional activator, hypoxia inducible factor-1alpha, through a pathway involving phosphoinositide 3-kinase and the mammalian target of rapamycin

Recent data suggest that vascular endothelial growth factor (VEGF), a cytokine involved in autocrine growth of tumor cells and tumor angiogenesis, is up-regulated and plays a potential role in myelogenous leukemias. In chronic myelogenous leukemia (CML), VEGF is expressed at high levels in the bone marrow and peripheral blood. We show here that the CML-associated oncogene BCR/ABL induces VEGF gene expression in growth factor-dependent Ba/F3 cells. Whereas starved cells were found to contain only baseline levels of VEGF mRNA, Ba/F3 cells induced to express BCR/ABL exhibited substantial amounts of VEGF mRNA. BCR/ABL also induced VEGF promoter activity and increased VEGF protein levels in Ba/F3 cells. Moreover, BCR/ABL was found to promote the expression of functionally active hypoxia-inducible factor-1 (HIF-1), a major transcriptional regulator of VEGF gene expression. BCR/ABL-induced VEGF gene expression was counteracted by the phosphoinositide 3-kinase (PI3-kinase) inhibitor LY294002 and rapamycin, an antagonist of mammalian target of rapamycin (mTOR), but not by inhibition of the mitogen-activated protein kinase pathway. Similarly, BCR/ABL-dependent HIF-1alpha expression was inhibited by the addition of LY294002 and rapamycin. Together, our data show that BCR/ABL induces VEGF- and HIF-1alpha gene expression through a pathway involving PI3-kinase and mTOR. BCR/ABL-induced VEGF expression may contribute to the pathogenesis and increased angiogenesis in CML.

Activated pp60c-Src leads to elevated hypoxia-inducible factor (HIF)-1alpha expression under normoxia

Hypoxia-inducible factor (HIF)-1 is a master transcription factor, which up-regulates glycolysis, erythropoiesis, and angiogenesis under hypoxia. HIF-1alpha accumulates in normoxic tumor cells, leading to glycolysis under aerobic conditions. This phenomenon, known as the "Warburg effect," is caused by a yet unknown mechanism. Here we show that transformed cells that express constitutively active pp60(c-Src) (Src) express HIF-1alpha protein under normoxia, which results in the expression of multiple HIF-1alpha target genes. We show that this occurrence is due to an enhanced rate of HIF-1alpha protein synthesis and not due to reduced HIF-1alpha degradation. Furthermore, we show that the Src-induced increase in protein synthesis is due to the global increase in the rate of cap-dependent translation and does not involve inhibition of HIF-1alpha degradation.

Epigallocatechin-3-gallate decreases VEGF production in head and neck and breast carcinoma cells by inhibiting EGFR-related pathways of signal transduction

In a recent study on head and neck squamous cell carcinoma (HNSCC) cells we found that epigallocatechin-3-gallate (EGCG), a major biologically active component of green tea, inhibited activation of the epidermal growth factor receptor (EGFR) and related signaling pathways. Since activation of EGFR signaling pathways is associated with angiogenesis, we examined the effects of EGCG on vascular endothelial growth factor (VEGF) production by YCU-H891 HNSCC and MDA-MB-231 breast carcinoma cell lines, because we found that both of these cell lines display autocrine activation of transforming growth factor-alpha (TGF-alpha)/EGFR signaling and produce high levels of VEGF. Treatment with EGCG inhibited the constitutive activation of the EGFR, Stat3, and Akt in both cell lines. These changes were associated with inhibition of VEGF promoter activity and cellular production of VEGF. Mechanistic studies indicated that inhibition of Stat3, but not mitogen-activated protein kinase kinase (MEK)1 or phosphatidylinositol 3'-kinase (PI3K), significantly decreased VEGF promoter activity. However, the inhibitory effects of a dominant negative Stat3 on VEGF expression was not as strong as that produced by EGCG. An analysis of alternative pathways indicated that EGCG strongly inhibited the constitutive activation of NF-kappa B in both cell lines, and an NF-kappa B inhibitor strongly inhibited VEGF production. These results suggest that EGCG inhibits VEGF production by inhibiting both the constitutive activation of Stat3 and NF-kappa B, but not extracellular-signal-regulated kinase (ERK) or Akt, in these cells. Therefore, EGCG may be useful in treating HNSCC and breast carcinoma because it can exert both antiproliferative and antiangiogenic activities.

Differential regulation of tumor angiogenesis by distinct ErbB homo- and heterodimers

Interactions between cancer cells and their microenvironment are critical for the development and progression of solid tumors. This study is the first to examine the role of all members of the ErbB tyrosine kinase receptors (epidermal growth factor receptor [EGFR], ErbB-2, ErbB-3, or ErbB-4), expressed singly or as paired receptor combinations, in the regulation of angiogenesis both in vitro and in vivo. Comparison of all receptor combinations reveals that EGFR/ErbB-2 and ErbB-2/ErbB-3 heterodimers are the most potent inducers of vascular endothelial growth factor (VEGF) mRNA expression compared with EGFR/ErbB-3, EGFR/ErbB-4, ErbB-2/ErbB-4, and ErbB-3/ErbB-4. Immunohistochemistry of tumor xenografts overexpressing these heterodimers shows increased VEGF expression and remarkably enhanced vascularity. Enhanced VEGF expression is associated with increased VEGF transcription. Deletional analysis reveals that ErbB-mediated transcriptional up-regulation of VEGF involves a hypoxia-inducible factor 1-independent responsive region located between nucleotides -88 to -66 of the VEGF promoter. Mutational analysis reveals that the Sp-1 and AP-2 transcription factor binding elements within this region are required for up-regulation of VEGF by heregulin beta1 and that this up-regulation is dependent on the activity of extracellular signal-related protein kinases. These results emphasize the biological implications of cell signaling diversity among members of the ErbB receptor family in regulation of the tumor microenvironment.

Regulation of hypoxia-inducible factor 1alpha expression and function by the mammalian target of rapamycin

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription factor containing an inducibly expressed HIF-1alpha subunit and a constititutively expressed HIF-1beta subunit. Under hypoxic conditions, the HIF-1alpha subunit accumulates due to a decrease in the rate of proteolytic degradation, and the resulting HIF-1alpha-HIF-1beta heterodimers undergo posttranslational modifications that promote transactivation. Recent studies suggest that amplified signaling through phosphoinositide 3-kinase, and its downstream target, mTOR, enhances HIF-1-dependent gene expression in certain cell types. In the present study, we have explored further the linkage between mTOR and HIF-1 in PC-3 prostate cancer cells treated with hypoxia or the hypoxia mimetic agent, CoCl(2). Pretreatment of PC-3 cells with the mTOR inhibitor, rapamycin, inhibited both the accumulation of HIF-1alpha and HIF-1-dependent transcription induced by hypoxia or CoCl(2). Transfection of these cells with wild-type mTOR enhanced HIF-1 activation by hypoxia or CoCl(2), while expression of a rapamycin-resistant mTOR mutant rendered both HIF-1alpha stabilization and HIF-1 transactivating function refractory to inhibition by rapamycin. Studies with GAL4-HIF-1alpha fusion proteins pinpointed the oxygen-dependent degradation domain as a critical target for the rapamycin-sensitive, mTOR-dependent signaling pathway leading to HIF-1alpha stabilization by CoCl(2). These studies position mTOR as an upstream activator of HIF-1 function in cancer cells and suggest that the antitumor activity of rapamycin is mediated, in part, through the inhibition of cellular responses to hypoxic stress.

Insulin-like growth factor 1 induces hypoxia-inducible factor 1-mediated vascular endothelial growth factor expression, which is dependent on MAP kinase and phosphatidylinositol 3-kinase signaling in colon cancer cells

Stimulation of human colon cancer cells with insulin-like growth factor 1 (IGF-1) induces expression of the VEGF gene, encoding vascular endothelial growth factor. In this article we demonstrate that exposure of HCT116 human colon carcinoma cells to IGF-1 induces the expression of HIF-1 alpha, the regulated subunit of hypoxia-inducible factor 1, a known transactivator of the VEGF gene. In contrast to hypoxia, which induces HIF-1 alpha expression by inhibiting its ubiquitination and degradation, IGF-1 did not inhibit these processes, indicating an effect on HIF-1 alpha protein synthesis. IGF-1 stimulation of HIF-1 alpha protein and VEGF mRNA expression was inhibited by treating cells with inhibitors of phosphatidylinositol 3-kinase and MAP kinase signaling pathways. These inhibitors also blocked the IGF-1-induced phosphorylation of the translational regulatory proteins 4E-BP1, p70 S6 kinase, and eIF-4E, thus providing a mechanism for the modulation of HIF-1 alpha protein synthesis. Forced expression of a constitutively active form of the MAP kinase kinase, MEK2, was sufficient to induce HIF-1 alpha protein and VEGF mRNA expression. Involvement of the MAP kinase pathway represents a novel mechanism for the induction of HIF-1 alpha protein expression in human cancer cells.

Molecular targets as therapeutic strategies in the management of breast cancer

Therapy directed against specific biologic targets has long been used in the treatment of breast cancer; the estrogen receptor is a validated prognostic and therapeutic target, and antiestrogen therapy has been used effectively for decades. Recently, scientific progress and increased comprehension of mechanisms of breast cancer pathogenesis have led to the proliferation of both potential molecular targets and new therapeutic agents. The success of traztuzumab (Herceptin, Genentech, South San Francisco, CA), an anti-HER2 antibody, has spurred the development of other biologically directed therapeutics. In this overview, I discuss three targets relevant to breast cancer (the epidermal growth factor receptor family, angiogenesis, and NF-kappa B), and therapeutic approaches directed against these targets are discussed.

New oncolytic adenoviruses with hypoxia- and estrogen receptor-regulated replication

Oncolytic adenoviruses with restricted replication can be produced if the expression of crucial transcription units of the virus is controlled by tissue- or tumor-specific promoters. Here we describe a method for the rapid incorporation of exogenous promoters into the E1A and E4 regions of the human adenovirus type 5 genome. Using this system, we have generated AdEHT2 and AdEHE2F, two conditionally replicative adenoviruses for the treatment of breast cancer. The expression of the E1A gene in both viruses is controlled by a minimal dual-specificity promoter that responds to estrogens and hypoxia. The tight regulation of E1A expression correlated with the ability of these viruses to replicate and kill human cancer cells that express estrogen receptors, or are maintained under hypoxic conditions. The telomerase reverse transcriptase (TERT) promoter and the E2F-1 promoter are preferentially activated in cancer cells. They were introduced into the E4 region of AdEHT2 and AdEHE2F, respectively. The telomerase core promoter failed to block the replication of the virus in telomerase-negative cells. In contrast, AdEHE2F was attenuated in nontransformed quiescent cells growing under normoxic conditions, suggesting that an intact pRB pathway with low levels of E2F transcription factors acts as a negative modulator for the virus. These data indicate that the simultaneous regulation of E1A and E4 viral transcription units by the appropriate combination of promoters can increase the tumor selectivity of oncolytic adenoviruses.

Hypoxia-induced pathways in breast cancer

Hypoxia, a common consequence of solid tumor growth in breast cancer and other cancers, serves to propagate a cascade of molecular pathways which include angiogenesis, glycolysis, and alterations in microenvironmental pH. Hypoxia-inducible factors, heterodimeric DNA binding complexes composed of two subunits, provide critical regulation of this response. This review presents a synopsis of the genes induced by hypoxia in the context of breast cancer and discusses how upregulation of HIF-1 activity, and the homologous factor HIF-2, are not only fundamental for the adaptation to hypoxia but also may be critical for tumor progression.

Signal transduction to hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF-1) is a transcriptional activator that functions as a master regulator of O2 homeostasis. HIF-1 target genes encode proteins that increase O2 delivery and mediate adaptive responses to O2 deprivation. HIF-1 activity is regulated by the cellular O2 concentration and by the major growth factor-stimulated signal transduction pathways. In human cancer cells, both intratumoral hypoxia and genetic alterations affecting signal transduction pathways lead to increased HIF-1 activity, which promotes angiogenesis, metabolic adaptation, and other critical aspects of tumor progression.

HIF-1 and tumor progression: pathophysiology and therapeutics

Hypoxia-inducible factor 1 (HIF-1) controls oxygen delivery (via angiogenesis) and metabolic adaptation to hypoxia (via glycolysis). HIF-1 consists of a constitutively expressed HIF-1 beta subunit and an oxygen- and growth-factor-regulated HIF-1 alpha subunit. In xenografts, tumor growth and angiogenesis are correlated with HIF-1 expression. In human cancers, HIF-1 alpha is overexpressed as a result of intratumoral hypoxia and genetic alterations affecting key oncogenes and tumor suppressor genes. HIF-1 alpha overexpression in biopsies of brain, breast, cervical, esophageal, oropharyngeal and ovarian cancers is correlated with treatment failure and mortality. Increased HIF-1 activity promotes tumor progression, and inhibition of HIF-1 could represent a novel approach to cancer therapy.

Endothelin-1 induces vascular endothelial growth factor by increasing hypoxia-inducible factor-1alpha in ovarian carcinoma cells

Angiogenesis is an essential prerequisite for tumor growth, invasion, and metastasis. In ovarian carcinoma cells, endothelin-1 (ET-1) stimulates the secretion of vascular endothelial growth factor (VEGF), a major mediator of tumor angiogenesis. In OVCA 433 and HEY ovarian carcinoma cell lines, ET-1 treatment increases VEGF mRNA expression and induces VEGF protein levels in a time- and dose-dependent fashion, and do so to a greater extent under hypoxic conditions. ET-1 also increases hypoxia-inducible factor-1alpha (HIF-1alpha) accumulation and activates the HIF-1 transcription complex under both normoxic and hypoxic conditions, suggesting a role for HIF-1 in the induction of VEGF expression. These effects are inhibited by the selective ET(A) receptor (ET(A)R) antagonist, BQ123. The ET-1-induced increase in HIF-1alpha protein levels is due to the enhanced HIF-1alpha stabilization. These results implicate HIF-1alpha in the induction of VEGF expression in ET-1-stimulated ovarian carcinoma cells, and provide a mechanism whereby ET-1 acting selectively through ET(A)R can interact with the HIF-1alpha-dependent machinery of angiogenesis. Our results suggest that new therapeutic strategies using specific ET(A)R antagonists could provide an additional approach to the treatment of ovarian carcinoma by inhibiting neovascularization as well as tumor cell growth.

Insulin stimulates hypoxia-inducible factor 1 through a phosphatidylinositol 3-kinase/target of rapamycin-dependent signaling pathway

Hypoxia-inducible factor 1 (HIF-1) is a transcription factor involved in normal mammalian development and in the pathogenesis of several disease states. It consists of two subunits, HIF-1alpha, which is degraded during normoxia, and HIF-1beta, which is constitutively expressed. Activated HIF-1 induces the expression of genes involved in angiogenesis, erythropoiesis, and glucose metabolism. We have previously reported that insulin stimulates vascular endothelial growth factor (VEGF) expression (). In this study, we show that insulin activates HIF-1, leading to VEGF expression in retinal epithelial cells. Insulin activates HIF-1alpha protein expression in a dose-dependent manner with a maximum reached within 6 h. The expression of HIF-1alpha is correlated with the activation of HIF-1 DNA binding activity and the transactivation of a HIF-1-dependent reporter gene. Insulin does not appear to affect HIF-1alpha mRNA transcription but regulates HIF-1alpha protein expression through a translation-dependent pathway. The expression of an active form of protein kinase B and treatment of cells with specific inhibitors of phosphatidylinositol 3-kinase (PI3K), MAPK, and target of rapamycin (TOR) show that mainly PI3K and to a lesser extent TOR are required for insulin-induced HIF-1alpha expression. HIF-1 activity and VEGF expression are also dependent on PI3K- and TOR-dependent signaling. In conclusion, we show here that insulin regulates HIF-1 action through a PI3K/TOR-dependent pathway, resulting in increased VEGF expression.

Cellular adaptation to hypoxia: O2-sensing protein hydroxylases, hypoxia-inducible transcription factors, and O2-regulated gene expression

Although it was known for a long time that oxygen deprivation leads to the transcriptional induction of the gene encoding erythropoietin, the molecular mechanisms behind this process remained enigmatic. The cloning of the hypoxia-inducible factors (HIFs), the finding that HIF-1 regulates the expression of many more genes apart from erythropoietin, and the elucidation of the oxygen-dependent mechanisms degrading the HIF alpha subunits recently led to the spectacular discovery of the molecular principles of oxygen sensing. This review aims to summarize our current knowledge of oxygen-regulated gene expression..

Nuclear expression of hypoxia-inducible factor 1alpha protein is heterogeneous in human malignant cells under normoxic conditions

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription factor composed of alpha and beta subunits, which plays an essential role in cancer cell hypoxia adaptation, glycolysis, and angiogenesis. Normally, HIF-1alpha protein, the dominant subunit of HIF-1, is accumulated in nuclei when cells are exposed to hypoxia (1% O2) and rapidly degraded when cells are re-oxygenated. Here, we found that constitutive nuclear expression of HIF-1alpha protein was a general phenomenon in vitro under normoxic conditions in human malignant cells including those derived from the hematopoietic system, such as lymphoma and leukemia cells. In addition, the constitutive expression and induction of HIF-1alpha protein were more heterogeneous and dynamic compared with other transcription factors tested. HIF-2alpha and HIF-1beta proteins showed a limited range of varieties among different cell lines and different extracellular stimuli. Mechanisms involved in sustaining constitutive expression of HIF-1alpha protein in malignant cells at normal oxygen tension warrant further investigation.

Hypoxic induction of the hypoxia-inducible factor is mediated via the adaptor protein Shc in endothelial cells

Tyrosine kinase cascades may play a role in the hypoxic regulation of hypoxia-inducible factor (HIF)-1. We investigated the role of tyrosine kinase phosphorylation and of the Shc/Ras cascade on hypoxic HIF-1 stabilization. Exposure of human umbilical vein endothelial cells to hypoxia results in HIF protein stabilization as early as 10 minutes, with a maximum at 3 hours, and also in Shc tyrosine phosphorylation, with a maximum at 10 minutes. To test whether Shc directly mediates hypoxia-induced HIF stabilization, human umbilical vein endothelial cells were transfected with a dominant-negative Shc mutant (dnShc), resulting in significantly reduced HIF protein levels compared with control. Similar results were obtained with cells transfected with dominant-negative Ras, a known downstream effector of Shc. Hypoxia-induced Ras activity was significantly reduced in cells transfected with dnShc compared with control levels, indicating that Ras indeed acts downstream from Shc. Moreover, cells pretreated with a specific Raf-1 kinase inhibitor, a known downstream effector of Ras, exhibited reduced HIF protein levels. To examine the functional consequences of Shc in hypoxic signaling, HIF-1 ubiquitination, protein stabilization, and endothelial cell migration were assessed. Overexpression of dnShc increased ubiquitination of HIF-1 and reduced the half-life of the protein. Moreover, dnShc, dominant-negative Ras, or the Raf-1 kinase inhibitor significantly inhibited migration under hypoxia. Thus, Shc in concert with Ras and Raf-1 contributes to hypoxia-induced HIF-1alpha protein stabilization and endothelial cell migration.

Role of X-linked inhibitor of apoptosis protein in chemoresistance in ovarian cancer: possible involvement of the phosphoinositide-3 kinase/Akt pathway

Although cisplatin derivatives are first-line chemotherapeutic agents for the treatment of epithelial ovarian cancer, chemoresistance remains a major hurdle to successful therapy and the molecular mechanisms involved are poorly understood. Apoptosis is the cellular underpinning of cisplatin-induced cell death, which is associated with expression of specific "death" genes and down-regulation of "survival" counterparts. The X-linked inhibitor of apoptosis proteins (Xiap), an intracellular anti-apoptotic protein, plays a key role in cell survival by modulating death signaling pathways and is a determinant of cisplatin resistance in ovarian cancer cells in vitro. This review focuses on the role of Xiap and its interactions with the phosphoinositide-3 kinase (PI3K)/Akt cell survival pathway in conferring resistance of ovarian cancer cells to chemotherapeutic agents and discusses potential therapeutic strategies in overcoming chemoresistant ovarian cancer.

ErbB2 overexpression correlates with increased expression of vascular endothelial growth factors A, C, and D in human breast carcinoma

BACKGROUND

The angiogenic factor vascular endothelial growth factor (VEGF)-A plays an important role in breast cancer progression. However, the involvement of VEGF-C and VEGF-D, two newer members of the VEGF family, in breast carcinoma and their relationship with clinicopathologic parameters have not been clearly demonstrated.

METHODS

In this study, the expression levels of VEGF-A, VEGF-C, and VEGF-D protein in 107 breast carcinoma cases and 22 nonmalignant breast tissue samples were examined by immunohistochemistry and quantitated by image analysis.

RESULTS

Higher expression of VEGF-C and VEGF-D was found in breast carcinomas than in nonmalignant breast tissue samples. Moreover, expression of VEGF-A, VEGF-C, and VEGF-D was significantly and positively correlated with ErbB2 expression. High levels of VEGF-A expression were associated with shorter disease-free survival (DFS). Patients with tumors expressing high levels of VEGF-C or VEGF-D showed a notable trend for worse DFS, however, it was not statistically significant. The combination of VEGF-A and VEGF-C status predicted survival better than either marker alone.

CONCLUSIONS

Our study suggests that expression of the angiogenic and lymphangiogenic factors (i.e., VEGFs) might be regulated at least in part by ErbB2. In addition, the combination of VEGF-A and VEGF-C status may better predict prognosis of patients with breast carcinoma than VEGF-A alone.

AKT2 is frequently upregulated in HER-2/neu-positive breast cancers and may contribute to tumor aggressiveness by enhancing cell survival

Amplification or overexpression of the HER-2/neu gene in breast cancers is associated with aggressive behavior and resistance to therapeutic regimens. The molecular mechanisms that contribute to therapeutic resistance/survival of HER-2/neu-overexpressing tumor cells have not been well defined. To determine if phosphatidylinositol 3-kinase/AKT signaling contributes to cell survival in HER-2/neu-positive breast cancers, we performed immunohistochemical analyses to evaluate expression of HER-2/neu and AKT in a series of 52 breast carcinomas. Elevated expression of HER-2/neu was found to correlate with overexpression of AKT2 protein and activation of AKT kinase. HER-2/neu-overexpressing breast cancer cell lines were resistant to apoptosis induced by UV treatment and hypoxia, which was suppressed in the presence of the phosphatidylinositol 3-kinase inhibitors LY294002 and wortmannin, indicating a link between AKT activation and stress resistance in HER-2/neu-overexpressing cells. These observations suggest that AKT signaling augments resistance to stress-induced apoptosis in breast cancer cells overexpressing HER-2/neu.

The protein kinase B/Akt signalling pathway in human malignancy

Protein kinase B or Akt (PKB/Akt) is a serine/threonine kinase, which in mammals comprises three highly homologous members known as PKBalpha (Akt1), PKBbeta (Akt2), and PKBgamma (Akt3). PKB/Akt is activated in cells exposed to diverse stimuli such as hormones, growth factors, and extracellular matrix components. The activation mechanism remains to be fully characterised but occurs downstream of phosphoinositide 3-kinase (PI-3K). PI-3K generates phosphatidylinositol-3,4,5-trisphosphate (PIP(3)), a lipid second messenger essential for the translocation of PKB/Akt to the plasma membrane where it is phosphorylated and activated by phosphoinositide-dependent kinase-1 (PDK-1) and possibly other kinases. PKB/Akt phosphorylates and regulates the function of many cellular proteins involved in processes that include metabolism, apoptosis, and proliferation. Recent evidence indicates that PKB/Akt is frequently constitutively active in many types of human cancer. Constitutive PKB/Akt activation can occur due to amplification of PKB/Akt genes or as a result of mutations in components of the signalling pathway that activates PKB/Akt. Although the mechanisms have not yet been fully characterised, constitutive PKB/Akt signalling is believed to promote proliferation and increased cell survival and thereby contributing to cancer progression. This review surveys recent developments in understanding the mechanisms and consequences of PKB/Akt activation in human malignancy.

Phosphatidylinositol 3-kinase/Akt signaling is neither required for hypoxic stabilization of HIF-1 alpha nor sufficient for HIF-1-dependent target gene transcription

The serine/threonine kinase Akt/PKB and the oxygen-responsive transcription factor HIF-1 share the ability to induce such processes as angiogenesis, glucose uptake, and glycolysis. Akt activity and HIF-1 are both essential for development and implicated in tumor growth. Upon activation by products of phosphatidylinositol 3-kinase (PI3K), Akt phosphorylates downstream targets that stimulate growth and inhibit apoptosis. Previous reports suggest that Akt may achieve its effects on angiogenesis and glucose metabolism by stimulating HIF-1 activity. We report here that, whereas serum stimulation can induce a slight accumulation of HIF-1 alpha protein in a PI3K/Akt pathway-dependent fashion, hypoxia induces much higher levels of HIF-1 alpha protein and HIF-1 DNA binding activity independently of PI3K and mTOR activity. In addition, we find the effects of constitutively active Akt on HIF-1 activity are cell-type specific. High levels of Akt signaling can modestly increase HIF-1 alpha protein, but this increase does not affect HIF-1 target gene expression. Therefore, the PI3K/Akt pathway is not necessary for hypoxic induction of HIF-1 subunits or activity, and constitutively active Akt is not itself sufficient to induce HIF-1 activity.

Lack of evidence for the involvement of the phosphoinositide 3-kinase/Akt pathway in the activation of hypoxia-inducible factors by low oxygen tension

Hypoxia-inducible factors (HIF) belong to an evolutionary conserved family of transcription factors, the activity of which is tightly regulated by oxygen levels. We have recently demonstrated that hypoxia activates the phosphoinositide 3-kinase (PI3K)/Akt pathway in some cell types, and other works have suggested that this pathway is involved in the activation of HIF. In the present work we studied the role of this pathway in the induction of HIF by hypoxia. Under hypoxic conditions the PI3K/Akt pathway was activated in some (PC12 and HeLa) but not all cell types (HepG2) tested, whereas the HIF protein was induced by hypoxia in all cases. Kinetics analysis showed that, when observed, the activation of PI3K/Akt occurred after HIF induction. In addition, the chemical inhibition of PI3K had no significant effect on the induction of the HIF protein or its transcriptional activity but prevented Akt activation. Accordingly, transient overexpression of a dominant negative form of the regulatory subunit of PI3K in HEK293T cells did not interfere with the induction of the HIF-alpha protein by hypoxia or affect HIF-mediated transcription in any of the cell types tested. Moreover, forced activation of the PI3K/Akt pathway did not affect the transcriptional activity of HIF under normoxic or hypoxic conditions. Thus, our data suggest that the activation of PI3K/Akt by hypoxia is cell type-specific and, when observed, lies downstream of HIF activation or in a parallel pathway. Furthermore, the activity of the PI3K/Akt is not sufficient for the activation of HIF nor is it essential for its induction by hypoxia.

VEGF165 antisense RNA suppresses oncogenic properties of human esophageal squamous cell carcinoma

AIM: To investigate the effect of antisense RNA to vascular endothelial growth factor165 (VEGF₁₆₅) on human esophageal squamous cell carcinoma cell line EC109 and the feasibility of gene therapy for esophageal carcinoma.

METHODS: By using subclone technique, the full length of VEGF₁₆₅ amino acid cDNA, which was cut from pGEM-3Zf(+), was cloned inversely into the eukaryotic expression vector pCEP4.The recombinant plasmid pCEP-AVEGF₁₆₅ was transfected into EC109 cell with lipofectamine. After a stable transfection, dot blot, enzyme-linked immunosorbent assay (ELISA), laser confocal imaging system analysis, transmission electron microscopy and flow cytometry were performed to determine the biological characteristics of EC109 cell line before and after transfection in vitro and whether there was a reversion in the tumorigenic properties of the EC109 cell in vivo.

RESULTS: The eukaryotic expression vector pCEP-AVEGF₁₆₅ was successfully constructed and transfected into EC109 cells. The expression of VEGF₁₆₅ was significantly decreased in the transfected cells while the biological characteristics of the cells were not influenced by the expression of antisense gene. The tumorigenic and angiogenic capabilities were greatly reduced in nude mice, as demonstrated by reduced tumor end volume (820 ± 112.5) mm³vs (7930 ± 1035) mm³ and (7850 ± 950) mm³,P£¼0.01£½ and microvessel density(8.5 ± 1.2) mm^-2vs (44.3 ± 9.4) mm^-2 and (46.4 ± 12.6) mm^-2,P < 0.01) in comparison between experimental groups empty vector transfected group and control group.

CONCLUSION: The angiogenesis and tumorigenicity of human esophageal squamous cell carcinoma were effectively inhibited by VEGF₁₆₅ antisense RNA. Antisense RNA to VEGF₁₆₅ can potentially be used as an adjuvant therapy for solid tumors.

Early expression of myocardial HIF-1alpha in response to mechanical stresses: regulation by stretch-activated channels and the phosphatidylinositol 3-kinase signaling pathway

Vascular endothelial growth factor (VEGF) expression is upregulated by hypoxia-inducible factor-1 (HIF-1) in ischemic tissues and growing tumors. Normally, HIF-1 activity depends on the amount of HIF-1alpha subunit, which is tightly regulated by the oxygen tension. In the myocardium, VEGF expression has been shown to be induced under nonhypoxic conditions by mechanical stresses. However, the cellular mechanism of stress-mediated VEGF induction remains unclear. Therefore, we examined the possible involvement of HIF-1 in stress-mediated VEGF induction in rat hearts. In this study, we increased the left ventricular wall tension using 3 different methods, namely by inducing regional ischemia, by expanding an intraventricular balloon, and by producing hemodynamic overload using an aortocaval shunt. In all cases, HIF-1alpha accumulated in the nuclei of cardiac myocytes in the early phase, and this was followed by VEGF induction. Phosphatidylinositol 3-kinase (PI3K)-dependent Akt phosphorylation was found to be activated by mechanical stress and completely blocked by wortmannin (a PI3K inhibitor). Moreover, the stress-mediated induction of HIF-1alpha and VEGF was suppressed by gadolinium (a stretch-activated channel inhibitor), wortmannin, and rapamycin (a FRAP inhibitor). Our results suggest that HIF-1alpha plays an important role in the induction of VEGF in nonischemic and mechanically stressed myocardium, and that this is regulated by stretch-activated channels and the PI3K/Akt/FRAP pathway. Moreover, this signaling pathway, which induces HIF-1alpha, seems to play an important role in the adaptation of the myocardium to stresses. The full text of this article is available at http://www.circresaha.org.

Genetic analysis of Pten and Ink4a/Arf interactions in the suppression of tumorigenesis in mice

Dual inactivation of PTEN and INK4a/ARF tumor suppressor genes is a common feature observed in a broad spectrum of human cancer types. To validate functional collaboration between these genes in tumor suppression, we examined the biological consequences of Pten and/or Ink4a/Arf deficiency in cells and mice. Relative to single mutant controls, Ink4a/Arf-/-Pten+/- mouse embryonic fibroblast cultures exhibited faster rates of growth in reduced serum, grew to higher saturation densities, produced more colonies upon low density seeding, and showed increased susceptibility to transformation by oncogenic H-Ras. Ink4a/Arf deficiency reduced tumor-free survival and shortened the latency of neoplasias associated with Pten heterozygosity, specifically pheochromocytoma, prostatic intraepithelial neoplasia, and endometrial hyperplasia. Compound mutant mice also exhibited an expanded spectrum of tumor types including melanoma and squamous cell carcinoma. Functional synergy between Ink4a/Arf and Pten manifested most prominently in the development of pheochromocytoma, prompting an analysis of genes and loci implicated in this rare human neoplasm. The classical pheochromocytoma genes Ret, Vhl, and Nf-1 remained intact, a finding consistent with the intersection of these genes with pathways engaged by Pten and Ink4a/Arf. Notably, conventional and array-comparative genomic hybridization revealed frequent loss of distal mouse chromosome 4 in a region syntenic to human chromosome 1p that is implicated in human pheochromocytoma. This study provides genetic evidence of collaboration between Pten and Ink4a/Arf in constraining the growth and oncogenic transformation of cultured cells and in suppressing a wide spectrum of tumors in vivo.

The bHLH/PAS factor MOP3 does not participate in hypoxia responses

The basic helix-loop-helix/PAS (bHLH/PAS) family of proteins regulates transcriptional responses during development and in response to environmental stimuli. bHLH/PAS factors act as heterodimers, and genetic and biochemical data indicate that multiple heterodimeric combinations are found in vivo to regulate hypoxic gene expression. For example, HIF1alpha heterodimerizes with the highly related proteins ARNT or ARNT2 in neurons. In vivo, MOP3 interacts with CLOCK to regulate circadian rhythms; however, its role in hypoxia responses is unclear. We show here that unlike ARNT and ARNT2, MOP3 does not effectively form HIF-1 complexes or restore HIF-1 target gene expression in response to low oxygen when expressed in Arnt(-/-) ES cells. Furthermore, Mop3(-/-) day 9.5 embryos exhibit no angiogenic defects as shown for Arnt(-/-), Hif1alpha(-/-), and Hif2alpha(-/-) embryos. Therefore, by a variety of criteria, we show that MOP3 has little if any role in the regulation of hypoxia responses in vivo.

Coexistence of hypercholesterolemia and hypertension impairs adventitial vascularization

We have shown that adventitial vasa vasorum (AVV) formation is enhanced in hypertensive rat aorta to compensate hypoxia in the thickened media and that hypercholesterolemia impairs angiogenesis in rat ischemic hindlimb. Thus, we examined the effects of coexistence of hypercholesterolemia and hypertension on AVV formation. In Wistar rats, hypercholesterolemia was established by high-cholesterol diet from Day -14 (HC rats), and hypertension was induced by a suprarenal aortic constriction at Day 0 (HT rats). At Day 28, we studied AVV density, adventitial area, and medial thickness in the ascending aorta of control (standard diet+sham operation), HC, HT, and HC+HT rats (n=5/group). In HC rats, although the adventitial area was modestly increased, the AVV density and medial thickness were unchanged versus controls. In addition to medial thickening, marked enlargement of the adventitial area accompanied by increased AVV density was observed in HT rats, compared with controls. HC+HT rats showed lower AVV density, despite larger adventitial area, than HT rats, whereas the medial thickness was similar in HT and HC+HT rats. Immunohistostaining revealed hypoxia-inducible factor-1alpha expression in the media only in HC+HT rats but not in the other 3 groups, suggesting persistent medial hypoxia in HC+HT rats. In conclusion, it is suggested that coexistence of hypercholesterolemia and hypertension impairs AVV formation, resulting in insufficient compensation for hypoxia in the thickened media. Our findings provide an insight into the mechanism of the aggravation of arteriosclerosis when both hypercholesterolemia and hypertension are present.

Stretch-induced retinal vascular endothelial growth factor expression is mediated by phosphatidylinositol 3-kinase and protein kinase C (PKC)-zeta but not by stretch-induced ERK1/2, Akt, Ras, or classical/novel PKC pathways

Stretch-induced expression of vascular endothelial growth factor (VEGF) is thought to be important in mediating the exacerbation of diabetic retinopathy by systemic hypertension. However, the mechanisms underlying stretch-induced VEGF expression are not fully understood. We present novel findings demonstrating that stretch-induced VEGF expression in retinal capillary pericytes is mediated by phosphatidylinositol (PI) 3-kinase and protein kinase C (PKC)-zeta but is not mediated by ERK1/2, classical/novel isoforms of PKC, Akt, or Ras despite their activation by stretch. Cardiac profile cyclic stretch at 60 cpm increased VEGF mRNA expression in a time- and magnitude-dependent manner without altering mRNA stability. Stretch increased ERK1/2 phosphorylation, PI 3-kinase activity, Akt phosphorylation, and PKC-zeta activity. Signaling pathways were explored using inhibitors of PKC, MEK1/2, and PI 3-kinase; adenovirus-mediated overexpression of ERK, PKC-alpha, PKC-delta, PKC-zeta, and Akt; and dominant negative (DN) mutants of ERK, PKC-zeta, Ras, PI 3-kinase and Akt. Although stretch activated ERK1/2 through a Ras- and PKC classical/novel isoform-dependent pathway, these pathways were not responsible for stretch-induced VEGF expression. Overexpression of DN ERK and Ras had no effect on VEGF expression in these cells. In contrast, DN PI 3-kinase as well as pharmacologic inhibitors of PI 3-kinase blocked stretch-induced VEGF expression. Although stretch-induced PI 3-kinase activation increased both Akt phosphorylation and activity of PKC-zeta, VEGF expression was dependent on PKC-zeta but not Akt. In addition, PKC-zeta did not mediate stretch-induced ERK1/2 activation. These results suggest that stretch-induced expression of VEGF involves a novel mechanism dependent upon PI 3-kinase-mediated activation of PKC-zeta that is independent of stretch-induced activation of ERK1/2, classical/novel PKC isoforms, Ras, or Akt. This mechanism may play a role in the well documented association of concomitant hypertension with clinical exacerbation of neovascularization and vascular permeability.

Hypoxia--a key regulatory factor in tumour growth

Cells undergo a variety of biological responses when placed in hypoxic conditions, including activation of signalling pathways that regulate proliferation, angiogenesis and death. Cancer cells have adapted these pathways, allowing tumours to survive and even grow under hypoxic conditions, and tumour hypoxia is associated with poor prognosis and resistance to radiation therapy. Many elements of the hypoxia-response pathway are therefore good candidates for therapeutic targeting.

Hypoxia-inducible factor-1alpha/vascular endothelial growth factor pathway for adventitial vasa vasorum formation in hypertensive rat aorta

The roles of adventitial vasa vasorum have been highlighted in vascular wall homeostasis. Vascular endothelial growth factor (VEGF) is a potent angiogenic factor in physiological and pathophysiological conditions. However, little is known regarding the changes in adventitial vasa vasorum and the mechanism of the formation in hypertensive arteries. Accordingly, endothelial cell proliferation, adventitial vasa vasorum count, and expression of VEGF signaling axis proteins were examined in the ascending aorta of hypertensive Wistar rats that underwent suprarenal aortic constriction. Hypertension not only induced medial and adventitial thickening but also significantly increased adventitial vasa vasorum count by day 28. Preceding the medial thickening, BrdU(+)-proliferative endothelial cells were observed in the adventitia but not in the media and intima after day 3; they peaked at day 7 and remained modestly increased at day 28. The BrdU(+) endothelial cells showed induction of Ets-1, a transcription factor mediating angiogenic response of VEGF. Furthermore, concomitant expression of VEGF and a hypoxia-inducible transcription factor (HIF-1alpha) was observed in the outer layers of medial smooth muscle cells at day 3 and extended to the middle layers of medial smooth muscle cells at day 7, returning to lower levels by day 28. In conclusion, adventitial vasa vasorum formation was induced by hypertension through the HIF-1alpha/VEGF/Ets-1 pathway during hypertensive remodeling.

Induction of vascular endothelial growth factor expression and hypoxia-inducible factor 1alpha protein by the oxidative stressor arsenite

Recent evidence suggests that vascular endothelial growth factor (VEGF) expression is up-regulated by oxidative stressors through activation of hypoxia-inducible Factor 1 (HIF-1). To investigate whether this is a general phenomenon, we studied the effects of the sulfhydryl reagent arsenite on VEGF expression in human ovarian cancer cells. Arsenite potently induces the production of reactive oxygen species (ROS) in several cell systems and directly interacts with sulfhydryl groups of cellular thiols. We report that arsenite induces VEGF mRNA and protein levels in normoxic H134 and OVCAR-3 cells. Arsenite also increases HIF-1alpha protein levels, suggesting a role for HIF-1 in the induction of VEGF expression. Pretreatment with the ROS inhibitors catalase and mannitol attenuated arsenite-induced ROS production, but did not affect induction of VEGF mRNA and HIF-1alpha protein. In contrast, pretreatment with the thiol antioxidants glutathione or N-acetylcysteine completely abrogated both effects, whereas a potentiation was observed by depletion of intracellular glutathione. These results demonstrate that arsenite-induced VEGF mRNA and HIF-1alpha protein expression is independent of increased ROS production but critically regulated by the cellular reduced glutathione content. In addition, these data suggest the involvement of a thiol-sensitive mechanism in the regulation of VEGF mRNA expression and HIF-1alpha protein in human ovarian cancer cells.

FIH-1: a novel protein that interacts with HIF-1alpha and VHL to mediate repression of HIF-1 transcriptional activity

Hypoxia-inducible factor 1 (HIF-1) is a master regulator of oxygen homeostasis that controls angiogenesis, erythropoiesis, and glycolysis via transcriptional activation of target genes under hypoxic conditions. O(2)-dependent binding of the von Hippel-Lindau (VHL) tumor suppressor protein targets the HIF-1alpha subunit for ubiquitination and proteasomal degradation. The activity of the HIF-1alpha transactivation domains is also O(2) regulated by a previously undefined mechanism. Here, we report the identification of factor inhibiting HIF-1 (FIH-1), a protein that binds to HIF-1alpha and inhibits its transactivation function. In addition, we demonstrate that FIH-1 binds to VHL and that VHL also functions as a transcriptional corepressor that inhibits HIF-1alpha transactivation function by recruiting histone deacetylases. Involvement of VHL in association with FIH-1 provides a unifying mechanism for the modulation of HIF-1alpha protein stabilization and transcriptional activation in response to changes in cellular O(2) concentration.

FIH-1: a novel protein that interacts with HIF-1alpha and VHL to mediate repression of HIF-1 transcriptional activity

Hypoxia-inducible factor 1 (HIF-1) is a master regulator of oxygen homeostasis that controls angiogenesis, erythropoiesis, and glycolysis via transcriptional activation of target genes under hypoxic conditions. O(2)-dependent binding of the von Hippel-Lindau (VHL) tumor suppressor protein targets the HIF-1alpha subunit for ubiquitination and proteasomal degradation. The activity of the HIF-1alpha transactivation domains is also O(2) regulated by a previously undefined mechanism. Here, we report the identification of factor inhibiting HIF-1 (FIH-1), a protein that binds to HIF-1alpha and inhibits its transactivation function. In addition, we demonstrate that FIH-1 binds to VHL and that VHL also functions as a transcriptional corepressor that inhibits HIF-1alpha transactivation function by recruiting histone deacetylases. Involvement of VHL in association with FIH-1 provides a unifying mechanism for the modulation of HIF-1alpha protein stabilization and transcriptional activation in response to changes in cellular O(2) concentration.