Expression of hypoxia-inducible factor 1: mechanisms and consequences

Tissue and plasma expression of the angiogenic peptide adrenomedullin in breast cancer

Adrenomedullin (ADM) is an angiogenic factor that has also been shown to be a mitogen and a hypoxia survival factor for tumour cells. These properties point to ADM as a potential promoter of human malignancies, but little data are available concerning the expression of ADM in human breast cancer. In the present work, we have examined ADM peptide expression in a series of malignant breast tumours by immunohistochemistry using a newly developed anti-ADM monoclonal antibody. In addition, ADM plasma concentrations in breast cancer patients and healthy controls were determined by radioimmunoassay. Of the examined breast cancer samples, 27/33 (82%) showed a moderate to strong staining intensity. ADM-peptide expression in breast tumours was significantly correlated with axillary lymph node metastasis (P=0.030). Analysis of ADM plasma concentrations showed no significant difference between the circulating ADM levels of breast cancer patients and healthy controls. However, a significant positive correlation was found between tumour size and plasma ADM levels (r=0.641, P=0.017). Moreover, ADM levels in breast cancer patients correlated with the presence of lymph node metastasis (P=0.002). In conclusion, we have shown for the first time that ADM peptide is widely expressed in breast cancer and that the degree of expression is associated with lymph node metastasis. ADM peptide in plasma of breast cancer patients reflects the size of the primary tumour, but is unlikely to be a useful tumour marker for the detection of breast cancer. Plasma ADM might represent an independent predictor of lymph node metastasis. The clinical implications of these findings remain to be evaluated.

Oxygen sensing: applications in humans

Our concepts of oxygen sensing have been transformed over the years. We now appreciate that oxygen sensing is not a unique property limited to “chemoreceptors” but is a common property of tissues and that responses to changes in oxygen levels are not static but can change over time. Respiratory responses initiated at the carotid body are modified by the excitatory and depressant effects of hypoxia at the brain and on the pathways connecting the carotid body to the brain. Equally important is that we are beginning to use our understanding of the cellular and molecular pathways triggered by hypoxia and hyperoxia to identify therapeutic targets to treat diseases such as cancer. We also have a better understanding of the complexities of the human respiratory responses to hypoxia; however, major deficiencies remain in our ability to alter or even measure human ventilatory responses to oxygen deficiency.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

2,3,7,8-tetrachlorodibenzo-p-dioxin reduces myocardial hypoxia and vascular endothelial growth factor expression during chick embryo development

BACKGROUND

Previous research has demonstrated that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces cardiomyocyte growth arrest, thinner ventricle walls, and reduced number and size of coronary arteries during chick embryogenesis. Coronary vascular development is believed to be mediated, in part, by myocardial oxygen gradients and a subsequent increase in hypoxia-inducible factor 1alpha (HIF-1alpha) and vascular endothelial growth factor-A (VEGF-A) expression. We investigated whether TCDD inhibition of coronary development was associated with altered myocardial oxygen status and reduced cardiac HIF-1alpha and VEGF-A.

METHODS

Chick embryos were exposed to 15% or 20% O2 for 24 hr from incubation days 9-10 or were injected with control (corn oil) or 0.24 pmol TCDD/gm egg on day 0. On day 9, embryos were injected with control (0.9% NaCl) or EF5, a tissue hypoxia marker, and cardiac binding of EF5 was determined by immunohistochemistry on day 10. In addition, embryo hearts were analyzed for VEGF-A mRNA by in situ hybridization and quantitative RT-PCR, and for HIF-1alpha mRNA by quantitative RT-PCR.

RESULTS

Cardiac binding of EF5 was significantly increased in embryos exposed to 15% O2, compared to embryos exposed to 20% O2. In contrast, TCDD-exposed embryos exhibited significantly reduced binding of EF5 in the heart, compared to controls. Similarly, cardiac expression of HIF-1alpha and VEGF-A were increased following hypoxia and tended to be decreased following TCDD exposure.

CONCLUSIONS

These results suggest that the myocardium may be a target of TCDD toxicity, resulting in reduced myocardial hypoxia, and HIF-1alpha and VEGF-A expression believed necessary for normal coronary development.

Redistribution of intracellular oxygen in hypoxia by nitric oxide: effect on HIF1alpha

Cells exposed to low oxygen concentrations respond by initiating defense mechanisms, including the stabilization of hypoxia-inducible factor (HIF) 1alpha, a transcription factor that upregulates genes such as those involved in glycolysis and angiogenesis. Nitric oxide and other inhibitors of mitochondrial respiration prevent the stabilization of HIF1alpha during hypoxia. In studies of cultured cells, we show that this effect is a result of an increase in prolyl hydroxylase-dependent degradation of HIF1alpha. With the use of Renilla luciferase to detect intracellular oxygen concentrations, we also demonstrate that, upon inhibition of mitochondrial respiration in hypoxia, oxygen is redistributed toward nonrespiratory oxygen-dependent targets such as prolyl hydroxylases so that they do not register hypoxia. Thus, the signaling consequences of hypoxia may be profoundly modified by nitric oxide.

GLUT1 and CAIX as intrinsic markers of hypoxia in bladder cancer: relationship with vascularity and proliferation as predictors of outcome of ARCON

Glucose transporter-1 protein (GLUT1) and carbonic anhydrase IX (CAIX) are regulated by hypoxia inducible factor-1 (HIF-1) and have been studied as putative intrinsic cellular markers for hypoxia. This study directly compares CAIX and GLUT1 with pimonidazole binding in a prospective series of bladder cancer patients and also studies the prognostic significance of the markers, in combination with vascularity and proliferation, in a retrospective series of bladder cancer patients treated in a phase II trial of radical radiotherapy with carbogen and nicotinamide (ARCON). A total of 21 patients with a diagnosis of transitional cell carcinoma of the bladder received 0.5 g m⁻² pimonidazole. Serial tumour sections were stained for pimonidazole, GLUT1 and CAIX and compared. Tissue sections obtained from a series of 64 patients previously treated for invasive bladder cancer using ARCON were stained for GLUT1 and CAIX together with Ki-67 and CD31/34. There was a good geographical colocalisation of both intrinsic markers with pimonidazole and a highly significant agreement in individual patients; correlation coefficients were 0.82 (P=0.0001) for GLUT1 and 0.74 (P<0.0001) for CAIX. In both series of patients, the intrinsic hypoxia markers were highly correlated with each other and a correlation with proliferation was also evident in the retrospective study. In univariate and multivariate analyses, GLUT1 and CAIX were independent predictors for overall and cause specific survival. The hypoxia markers did not predict for local control or metastases-free survival although higher Ki-67 indices showed a trend towards local failure. The data suggest that both hypoxia modification and accelerated treatment may be valid treatment options in bladder cancer.

Hypoxic induction of vascular endothelial growth factor is selectively impaired in mice carrying the mutant SOD1 gene

Localization and hypoxic induction of vascular endothelial growth factor (VEGF) was examined in the spinal cord of transgenic mice carrying a mutation in the superoxide dismutase 1 gene. Immunohistochemical and immunofluorescent study demonstrated that VEGF is mainly expressed in motor neurons before and after hypoxia. Baseline expression of VEGF was higher in transgenic (Tg) mice than in wild-type (Wt) littermates. However, VEGF was hardly induced after hypoxia in Tg mice, whereas Wt mice showed an approximate nine-fold increase. Impaired VEGF induction was evident in Tg mice at 12 weeks of age, when they were still presymptomatic. In contrast, baseline and hypoxic expression of brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor did not differ between Tg and Wt mice. Thus, the present study demonstrates that hypoxic induction of VEGF in Tg mice is selectively impaired from a very early stage, suggesting profound involvement in the pathogenesis of motor neuron degeneration in this animal model of amyotrophic lateral sclerosis.

Chemokine response of pulmonary artery endothelial cells to hypoxia and reoxygenation 1 1Presented at the annual meeting of the Association for Academic Surgery, Boston, MA, November 7–9, 2002

BACKGROUND

Chemokines are inflammatory mediators that activate and recruit specific leukocyte subpopulations. We have recently shown a role for certain chemokines in a warm in situ rat model of lung ischemia reperfusion injury. After hypoxic stress, rat pulmonary artery endothelial cells (RPAECS) potentiate and direct neutrophil sequestration, and, therefore, contribute to the development of tissue injury. The present studies were performed to determine whether RPAECS subjected to in vitro hypoxia and reoxygenation (H&R) secrete chemokines, and, if so, to define the regulatory mechanisms involved.

MATERIALS AND METHODS

RPAECS were isolated from 21-day-old Long-Evans rats and were rendered hypoxic (pO(2) 0.5%) for 2 hours and reoxygenated for up to 6 hours. Secreted chemokine content was quantified using sandwich enzyme-linked immunosorbent assay techniques. Mechanistic studies assessed chemokine messenger ribonucleic acid (mRNA) expression by Northern blot, as well as the nuclear translocation of proinflammatory transcription factors nuclear factor kappa beta (NFkappaB), early growth response (EGR), and activator protein-1 (AP-1) by electromobility shift assays. Supershift analysis for EGR-1 was also performed.

RESULTS

RPAECS showed a marked increase in the secretion of the chemokines cytokine induced neutrophil chemoattractant and monocyte chemoattractant protein-1 in response to H&R, which was dependent on de novo mRNA transcription and protein translation. Furthermore, in vitro H&R induced the nuclear translocation of the proinflammatory transcription factors NFkappaB and EGR-1 early during reoxygenation.

CONCLUSIONS

RPAECS secrete significant amounts of cytokine induced neutrophil chemoattractant and monocyte chemoattractant protein-1 in response to in vitro H&R. The secretion of both chemokines is dependant on de novo mRNA transcription and protein translation, and may be regulated by NFkappaB and EGR-1 activation.

Acid-base balance at exercise in normoxia and in chronic hypoxia. Revisiting the "lactate paradox"

Transitions between rest and work, in either direction, and heavy exercise loads are characterized by changes of muscle pH depending on the buffer power and capacity of the tissues and on the metabolic processes involved. Among the latter, in chronological sequence: (1). aerobic glycolysis generates sizeable amounts of lactate and H(+) by way of the recently described, extremely fast (20-100 ms) "glycogen shunt" and of the excess of glycolytic pyruvate supply; (2). hydrolysis of phosphocreatine, tightly coupled with that of ATP in the Lohmann reaction, is known to consume protons, a process undergoing reversal during recovery; (3). anaerobic glycolysis sustaining ATP production in supramaximal exercise as well as in conditions of hypoxia and ischemia, is responsible for the accumulation of large amounts of lactic acid (up to 1 mol for the whole body). The handling of metabolic acids, i.e., acid-base regulation, occurs both in blood and in tissues, mainly in muscles which are the main producers and consumers of lactic acid. The role of both blood and muscle bicarbonate and non-bicarbonate buffers as well as that of lactate/H(+) cotransport mechanisms is analyzed in relation to acid-base homeostasis in the course of exercise. A section of the review deals with the analysis of the acid-base state of humans exposed to chronic hypoxia. Particular emphasis is put on anaerobic glycolysis. In this context, the so-called lactate paradox is revisited and interpreted on the basis of the most recent findings on exercise at altitude.

Linking radiation oncology and imaging through molecular biology (or now that therapy and diagnosis have separated, it's time to get together again!)

Among the areas defined by the National Cancer Institute as "Extraordinary Opportunities for Research Investment" that are highly relevant to the technology-oriented disciplines within the broad field of radiology are cancer imaging, defining the signatures (ie, underlying molecular features) of cancer cells, and molecular targets of prevention and treatment. In molecular target credentialing, a specific molecular target is imaged, the molecular signature is defined, a treatment is given, and the effect of the intervention on the image findings and the signature is then evaluated. Such an approach is used to validate the proposed target as a legitimate one for cancer therapy or prevention and to provide the opportunity to ultimately individualize therapy on the basis of both the initial characteristics of the tumor and the tumor's response to an intervention. Therapeutic radiation is focused biology (ie, radiation produces molecular events in the irradiated tissue). Radiation can (a) kill cancer cells by itself, (b) be combined with cytotoxic or cytostatic drugs, and (c) serve to initiate radiation-inducible molecular targets that are amenable to treatment with drugs and/or biologic therapies. Focused biology can be anatomically confined with various types of external beams and with brachytherapy, and it can be used systemically with targeted radioisotopes. These new paradigms link diagnostic imaging, radiation therapy, and nuclear medicine in unique ways by way of basic biology. It is timely to develop new collaborative research, training, and education agendas by building on one another's expertise and adopting new fields of microtechnology, nanotechnology, and mathematical analysis and optimization.

Distinct role of fibroblast growth factor-2 and vascular endothelial growth factor on tumor growth and angiogenesis

Tumors express more than a single angiogenic growth factor. To investigate the relative impact of fibroblast growth factor-2 (FGF-2) and vascular endothelial growth factor (VEGF) on tumor growth and neovascularization, we generated tumor cell transfectants differing for VEGF and/or FGF-2 expression. Human endometrial adenocarcinoma HEC-1-B-derived Tet-FGF-2 cells that express FGF-2 under the control of the tetracycline-responsive promoter (Tet-off system) were further transfected with a VEGF(121) anti-sense (AS-VEGF) cDNA. Next, Tet-FGF-2 and AS-VEGF/Tet-FGF-2 cells were transplanted subcutaneously in nude mice that received tetracycline or not in the drinking water. Simultaneous expression of FGF-2 and VEGF in Tet-FGF-2 cells resulted in fast-growing lesions characterized by high blood vessel density, patency and permeability, and limited necrosis. Blood vessels were highly heterogeneous in size and frequently associated with pericytes. Inhibition of FGF-2 production by tetracycline caused a significant decrease in tumor burden paralleled by a decrease in blood vessel density and size. AS-VEGF expression resulted in a similar reduction in blood vessel density associated with a significant decrease in pericyte organization, vascular patency, and permeability. The consequent decrease in tumor burden was paralleled by increased tumor hypoxia and necrosis. A limited additional inhibitory effect was exerted by simultaneous down-regulation of FGF-2 and VEGF expression. These findings demonstrate that FGF-2 and VEGF stimulate vascularization synergistically but with distinctive effects on vessel functionality and tumor survival. Blockade of either one of the two growth factors results in a decrease in blood vessel density and, consequently, in tumor burden. However, inhibition of the expression of VEGF, but not of FGF-2, affects also vessel maturation and functionality, leading to tumor hypoxia and necrosis. Our experimental model represents an unique tool to investigate anti-neoplastic therapies in different angiogenic environments.

Decreased expression of the DNA mismatch repair gene Mlh1 under hypoxic stress in mammalian cells

The hypoxic tumor microenvironment has been shown to contribute to genetic instability. As one possible mechanism for this effect, we report that expression of the DNA mismatch repair (MMR) gene Mlh1 is specifically reduced in mammalian cells under hypoxia, whereas expression of other MMR genes, including Msh2, Msh6, and Pms2, is not altered at the mRNA level. However, levels of the PMS2 protein are reduced, consistent with destabilization of PMS2 in the absence of its heterodimer partner, MLH1. The hypoxia-induced reduction in Mlh1 mRNA was prevented by the histone deacetylase inhibitor trichostatin A, suggesting that hypoxia causes decreased Mlh1 transcription via histone deacetylation. In addition, treatment of cells with the iron chelator desferrioxamine also reduced MLH1 and PMS2 levels, in keeping with low oxygen tension being the stress signal that provokes the altered MMR gene expression. Functional MMR deficiency under hypoxia was detected as induced instability of a (CA)(29) dinucleotide repeat and by increased mutagenesis in a chromosomal reporter gene. These results identify a potential new pathway of genetic instability in cancer: hypoxia-induced reduction in the expression of key MMR proteins. In addition, this stress-induced genetic instability may represent a conceptual parallel to the pathway of stationary-phase mutagenesis seen in bacteria.

Dominant-negative hypoxia-inducible factor-1 alpha reduces tumorigenicity of pancreatic cancer cells through the suppression of glucose metabolism

In the tumor cells exposed to hypoxia, hypoxia-inducible factor-1 (HIF-1)-mediated adaptation responses such as angiogenesis and anaerobic metabolism are induced for their survival. We have recently reported that the constitutive expression of HIF-1 alpha renders pancreatic cancer cells resistant to apoptosis induced by hypoxia and glucose deprivation. We then established dominant-negative HIF-1 alpha (dnHIF-1 alpha) transfectants and examined their susceptibility to apoptosis and growth inhibition induced by hypoxia and glucose deprivation in vitro and their tumorigenicity in SCID mice. We further examined the expressions of aldolase A and Glut-1 in vitro and Glut-1 expression and glucose uptake in the tumor tissues and microvessel counts in the tumor tissues. As a result, dnHIF-1 alpha rendered the pancreatic cancer cells sensitive to apoptosis and growth inhibition induced by hypoxia and glucose deprivation. Also it abrogated the enhanced expression of Glut-1 and aldolase A mRNAs under hypoxia and reduced the expression of Glut-1 and the glucose uptake in the tumor tissues and consequently in vivo tumorigenicity. We found no significant difference in the microvessel counts among the tumor tissues. From these results, we suggest that the disruption of the HIF-1 pathway might be effective in the treatment of pancreatic cancers.

Hypoxia actively represses transcription by inducing negative cofactor 2 (Dr1/DrAP1) and blocking preinitiation complex assembly

Hypoxia is a growth inhibitory stress associated with multiple disease states. We find that hypoxic stress actively regulates transcription not only by activation of specific genes but also by selective repression. We reconstituted this bimodal response to hypoxia in vitro and determined a mechanism for hypoxia-mediated repression of transcription. Hypoxic cell extracts are competent for transcript elongation, but cannot assemble a functional preinitiation complex (PIC) at a subset of promoters. PIC assembly and RNA polymerase II C-terminal domain (CTD) phosphorylation were blocked by hypoxic induction and core promoter binding of negative cofactor 2 protein (NC2 alpha/beta, Dr1/DrAP1). Immunodepletion of NC2 beta/Dr1 protein complexes rescued hypoxic-repressed transcription without alteration of normoxic transcription. Physiological regulation of NC2 activity may represent an active means of conserving energy in response to hypoxic stress.

Animal response to drastic changes in oxygen availability and physiological oxidative stress

Oxygen is essential for most life forms, but it is also inherently toxic due to its biotransformation into reactive oxygen species (ROS). In fact, the development of many animal and plant pathological conditions, as well as natural aging, is associated with excessive ROS production and/or decreased antioxidant capacity. However, a number of animal species are able to tolerate, under natural conditions, situations posing a large potential for oxidative stress. Situations range from anoxia in fish, frogs and turtles, to severe hypoxia in organs of freeze-tolerant snakes, frogs and insect larvae, or diving seals and turtles, and mild hypoxia in organs of dehydrated frogs and toads or estivating snails. All situations are reminiscent of ischemia/reperfusion events that are highly damaging to most mammals and birds. This article reviews the responses of anoxia/hypoxia-tolerant animals when subjected to environmental and metabolic stresses leading to oxygen limitation. Abrupt changes in metabolic rate in ground squirrels arousing from hibernation, as well as snails arousing from estivation, may also set up a condition of increased ROS formation. Comparing the responses from these diverse animals, certain patterns emerge. The most commonly observed response is an enhancement of the antioxidant defense. The increase in the baseline activity of key antioxidant enzymes, as well as 'secondary' enzymatic defenses, and/or glutathione levels in preparation for a putative oxidative stressful situation arising from tissue reoxygenation seem to be the preferred evolutionary adaptation. Increasing the overall antioxidant capacity during anoxia/hypoxia is of relevance for species such as garter snakes (Thamnophis sirtalis parietalis) and wood fogs (Rana sylvatica), while diving freshwater turtles (Trachemys scripta elegans) appear to rely mainly upon high constitutive activities of antioxidant enzymes to deal with oxidative stress arising during tissue reoxygenation. The possibility that some animal species might control post-anoxic ROS generation cannot be excluded.

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.

Regulation of gene expression by oxygen: NF-kappaB and HIF-1, two extremes

Aerobic life is dependent on molecular oxygen for ATP regeneration, but only possible in a narrow range of oxygen concentrations. Increased oxygen tension is toxic through the generation of reactive oxygen species (ROS), while a decrease in oxygen concentration impairs energy availability and, hence, cell viability. Cells have developed strategies to respond to changes in oxygen tension: specific systems detect excessive ROS and hypoxia, leading to the activation of specific transcription factors and expression of appropriate target genes. The aim of this review is to describe how hypoxia-inducible factor-1 (HIF-1) and nuclear factor-kappaB (NF-kappaB) are regulated and what could be the sensors to the changes in oxygen levels. Some of the physiological responses initiated by these transcription factors are also mentioned.

Mitogen-inducible gene 6 (MIG-6), adipophilin and tuftelin are inducible by hypoxia

Adaptation to hypoxia is essential for tumor progression. Transcriptional activation of hypoxia-regulated genes is mediated by hypoxia-inducible factor 1 (HIF-1), a heterodimer of HIF-1alpha and ARNT (Ah receptor nuclear translocator; HIF-1beta). Using representational difference analysis, we identified three novel hypoxia-inducible genes: MIG-6 (gene 33), adipophilin and tuftelin. The mRNAs for these genes were inducible by 1% O(2) in the human HepG2 and MCF-7 cell lines. Hypoxic induction of the MIG-6 and tuftelin proteins was also observed. Induction was ARNT-dependent. Induction also occurred in livers of mice treated with CoCl(2), which mimics hypoxia. The potential roles of these genes in adaptation to hypoxia and in tumorigenesis will be of considerable interest.

Glycolysis as a metabolic marker in orthotopic breast cancer, monitored by in vivo (13)C MRS

Enhanced glycolysis represents a striking feature of cancers and can therefore serve to indicate a malignant transformation. We have developed a noninvasive, quantitative method to characterize tumor glycolysis by monitoring (13)C-labeled glucose and lactate with magnetic resonance spectroscopy. This method was applied in MCF7 human breast cancer implanted in the mammary gland of female CD1-NU mice and was further employed to assess tumor response to hormonal manipulation with the antiestrogen tamoxifen. Analysis of the kinetic data based on a unique physiological-metabolic model yielded the rate parameters of glycolysis, glucose perfusion, and lactate clearance in the tumor, as well as glucose pharmacokinetics in the plasma. Treatment with tamoxifen induced a twofold reduction in the rate of glycolysis and of lactate clearance but did not affect the other parameters. This metabolic monitoring can thus serve to evaluate the efficacy of new selective estrogen receptor modulators and may be further extended to improve diagnosis and prognosis of breast cancer.

Redox regulation of cytokine expression in Kupffer cells

Kupffer cells, resident macrophages in the liver, play a central role in the homeostatic response to liver injury. Ironically, this defensive mechanism, if dysregulated, also works against the liver in acute and chronic liver damage. Central to this response is activation of nuclear factor-kappaB (NF-kappaB), a redox-sensitive transcription factor that transactivates promoters of many inflammatory genes, including cytokines. Much research has been devoted to identification of upstream signaling for activation of NF-kappaB, but the precise mechanism by which oxidant stress participates in this signaling is yet to be determined. Clues to this key question may be attained through studies on the mechanisms of sustained and/or accentuated NF-kappaB activation in hepatic macrophages in chronic liver diseases. This article reviews the literature on redox regulation of cytokine gene expression by Kupffer cells.

Expression of hypoxia inducible factor-1 alpha and correlation with preoperative embolization of meningiomas

OBJECT

Vascular endothelial growth factor (VEGF) has been implicated in meningioma tumorigenesis and growth. The production of VEGF is regulated by hypoxia inducible factor-1alpha (HIF-1alpha), especially under conditions of hypoxia. In this study, the authors examine the expression of HIF-1alpha and VEGF in meningiomas, with a special emphasis on conditions of hypoxia, such as preoperative embolization, and on in vitro studies in cultured cells.

METHODS

Meningiomas obtained in 142 patients were studied using immunohistochemical methods to detect HIF-1alpha and the results were correlated with the extent or lack of preoperative embolization and expression of VEGF. Primary meningioma cell cultures were established and cell culture experiments were performed using a hypoxia chamber to stimulate HIF-1alpha and VEGF production. Expression of HIF-1alpha in primary meningioma cell cultures was measured using immunoblot assays. The VEGF secretion was measured using enzyme-linked immunosorbent assay. Half of the meningiomas studied were positive for HIF-1alpha, with a strong correlation between complete embolization and HIF-1alpha expression. Most of the meningiomas studied expressed VEGF protein, and VEGF expression did not correlate with the degree of embolization. A strong correlation was found between VEGF and HIF-1alpha expression in immunohistochemical studies. Secretion of VEGF is increased by hypoxia and growth factor stimulation. In meningiomas, growth factors stimulate HIF-1alpha expression. The role of hypoxia is less clear.

CONCLUSIONS

The expression of HIF-1alpha is increased by complete preoperative embolization of meningiomas. The expression of HIF-1alpha also correlates with VEGF secretion in meningiomas. Growth factor and hypoxic stimulation both contribute to VEGF control, but which is most important (or whether both are equally important) will require further studies.

The role of oxidative stress in the development of pulmonary arteriovenous malformations after cavopulmonary anastomosis

BACKGROUND

Cavopulmonary anastomosis is used for palliation of cyanotic heart disease. Clinically significant pulmonary arteriovenous malformations occur in up to 25% of patients after surgical intervention. Cavopulmonary anastomosis creates several modifications to pulmonary physiology that may contribute to the development of pulmonary arteriovenous malformations, including reduced pulmonary blood flow and the exclusion of inferior vena caval effluent.

OBJECTIVE

By comparing the expression of angiogenic and stress-related proteins after cavopulmonary anastomosis and pulmonary artery banding, we sought to determine which genes were upregulated independent of reduced pulmonary blood flow.

METHODS

Lambs aged 35 to 45 days were placed into 1 of 3 groups: cavopulmonary anastomosis (n = 6), pulmonary artery banding (n = 6), and sham control (n = 6) animals. In our model pulmonary arteriovenous malformations are detectable by means of bubble-contrast echocardiography 8 weeks after cavopulmonary anastomosis. Lung tissue was harvested for Western blotting at 2 and 5 weeks after surgery.

RESULTS

Cavopulmonary anastomosis and pulmonary artery banding both increased angiogenic gene expression, but only cavopulmonary anastomosis induced the expression of endothelial stress-related genes. Vascular endothelial growth factor was upregulated 2.5-fold after both cavopulmonary anastomosis (P =.002) and pulmonary artery banding (P =.007). Only cavopulmonary anastomosis upregulated 2 stress-related genes, HO1 and GLUT1, 2.7-fold (P =.002) and 3.8-fold (P =.03), respectively. Hypoxia-inducible factor was upregulated 4-fold (P =.003) after cavopulmonary anastomosis. Pulmonary artery banding failed to induce the increased expression of any of these proteins.

CONCLUSIONS

Reduced pulmonary blood flow induces a pulmonary angiogenic response but not an endothelial stress response. These results suggest that oxidative stress is more relevant to the formation of pulmonary arteriovenous malformations than angiogenic signaling alone because pulmonary artery banding does not result in pulmonary arteriovenous malformations. Oxidative stress of the pulmonary endothelium resulting from cavopulmonary anastomosis may predispose the affected vasculature to arteriovenous shunting.

O(2) sensing in hypoxic pulmonary vasoconstriction: the mitochondrial door re-opens

The identity of the O(2) sensor underlying the hypoxic pulmonary vasoconstriction (HPV) response has been sought for more than 50 years. Recently, the mitochondria have again come into sharp focus as the cellular organelle responsible for triggering the events that culminate in pulmonary artery constriction. Studies from different laboratories propose two disparate models to explain how mitochondria react to a decrease in P(O(2)). One model proposes that hypoxia slows or inhibits mitochondrial electron transport resulting in the accumulation of reducing equivalents and a decrease in the generation of reactive oxygen species (ROS). This is proposed to activate a redox-sensitive pathway leading to pulmonary vasoconstriction. A second and opposing model suggests that hypoxia triggers a paradoxical increase in mitochondrial ROS generation. This increase would then lead to the activation of an oxidant-sensitive signaling transduction pathway leading to HPV. This article summarizes the potential involvement of mitochondria in these two very different models.

Microvessel formation from mouse embryonic aortic explants is oxygen and VEGF dependent

To delineate the roles of O(2) and vascular endothelial growth factor (VEGF) in the process of angiogenesis from the embryonic aorta, we cultured mouse embryonic aorta explants (thoracic level to lateral vessels supplying the mesonephros and metanephros) in a three-dimensional type I collagen gel matrix. During 8 days of culture under 5% O(2), but not room air, the addition of VEGF to explants stimulated the formation of CD31-positive, Flk-1-positive, Gs-IB(4)-positive structures in a concentration-dependent manner. Electron microscopy showed the structures to be capillary-like. VEGF-induced capillary-like structure formation was inhibited by sequestration of VEGF via addition of soluble Flt-1 fusion protein or anti-VEGF antibodies. Expression of Flk-1, but not Flt-1, was increased in embryonic aorta cultured under 5% O(2) relative to room air. Our data suggest that low O(2) upregulates Flk-1 expression in embryonic aorta in vitro and renders it more responsive to VEGF.

Transferrin receptor gene expression and transferrin-bound iron uptake are increased during postischemic rat liver reperfusion

Iron-catalyzed production of reactive oxygen species is a cause of liver injury after ischemia/reperfusion (I/R). The aim of the present study was to address the regulation of transferrin receptor (TfR), which mediates cellular iron uptake, during I/R. The molecular mechanisms controlling TfR gene expression in vivo during I/R of rat liver were investigated by molecular biology procedures. We also analyzed transferrin-bound iron uptake into surviving liver slices. Increased amounts of TfR protein and messenger RNA (mRNA) were found 2 to 6 hours after reestablishment of blood supply. RNA bandshift analysis showed that iron regulatory protein (IRP) activity was decreased in the first hours of reperfusion, thus indicating that IRP-mediated mRNA stabilization was not involved in early TfR upregulation. On the contrary, increased transcription of the TfR gene in isolated nuclei was observed during reperfusion; during the ischemic phase this was preceded by enhanced binding of hypoxia inducible factor (HIF-1) to a DNA sequence derived from the TfR promoter. TfR2 mRNA levels were also enhanced after reperfusion. The increased expression of TfR at the cell surface resulted in increased uptake of transferrin-bound-iron into surviving liver slices; however, iron was not incorporated into ferritin. In conclusion, HIF-1 mediated activation of TfR gene transcription and IRP-mediated increase of TfR mRNA stability ensure a steady induction of TfR, and hence higher iron uptake in reperfused rat liver. TfR-mediated entry of the metal into liver cells may represent a source of catalytically active iron, which may play a role in reperfusion damage.

Role of nitric oxide in the regulation of HIF-1alpha expression during hypoxia

Hypoxia-inducible factor-1 (HIF-1), a heterodimeric transcription factor consisting of HIF-1alpha and HIF-1beta subunits, controls the expression of a large number of genes involved in the regulation of cellular responses to reduced oxygen availability. The oxygen-regulated subunit, HIF-1alpha, is stabilized in cells exposed to hypoxia. The regulation of hypoxic responses by nitric oxide (NO) is believed to have wide pathophysiological relevance, thus we investigated whether NO affects HIF-1 activation in hypoxic cells. Here we show that NO generated from NO donors prevented HIF-1alpha hypoxic accumulation in Hep 3B and PC-12 cells. Addition of a glutathione analog or peroxynitrite scavengers prevented the NO-induced inhibition of HIF-1alpha accumulation in both cell lines. Exposure to NO was associated with inhibition of mitochondrial electron transport and compensatory glycolysis, which maintained normal cellular ATP content. Succinate, a Krebs cycle intermediate and respiratory chain substrate, restored HIF-1alpha hypoxic induction in the cells, suggesting involvement of mitochondria in regulation of HIF-1alpha accumulation during hypoxia. Regulation of HIF-1alpha by NO is an additional important mechanism by which NO might modulate cellular responses to hypoxia in mammalian cells.

Hypoxia enhances the expression of autocrine motility factor and the motility of human pancreatic cancer cells

The incidence of distant metastases is higher in the tumours with low oxygen pressure than in those with high oxygen pressure. It is well known that hypoxia induces the transcription of various genes involved in angiogenesis and anaerobic metabolism necessary for the growth of tumour cells in vivo, suggesting that hypoxia may also induce the transcription of metastasis-associated genes. We sought to identify the metastasis-associated genes differentially expressed in tumour cells under hypoxic conditions with the use of a DNA microarray system. We found that hypoxia enhanced the expression of autocrine motility factor mRNA in various cancer cells and also enhanced the random motility of pancreatic cancer cells. Autocrine motility factor inhibitors abrogated the increase of motility under hypoxic conditions. In order to explore the roles of hypoxia-inducible factor-1alpha, we established hypoxia-inducible factor-1alpha-transfectants and dominant negative hypoxia-inducible factor-1alpha-transfectants. Transfection with hypoxia-inducible factor-1alpha and dominant-negative hypoxia-inducible factor-1alpha enhanced and suppressed the expression of autocrine motility factor/phosphohexase isomerase/neuroleukin mRNA and the random motility, respectively. These results suggest that hypoxia may promote the metastatic potential of cancer cells through the enhanced autocrine motility factor/phosphohexase isomerase/neuroleukin mRNA expression and that the disruption of the hypoxia-inducible factor-1 pathway may be an effective treatment for metastasis.

Genomic responses of the brain to ischemic stroke, intracerebral haemorrhage, kainate seizures, hypoglycemia, and hypoxia

RNA expression profiles in rat brain were examined 24 h after ischemic stroke, intracerebral haemorrhage, kainate-induced seizures, insulin-induced hypoglycemia, and hypoxia and compared to sham- or untouched controls. Rat oligonucleotide microarrays were used to compare expression of over 8000 transcripts from three subjects in each group (n = 27). Of the somewhat less than 4000 transcripts called 'present' in normal or treated cortex, 5-10% of these were up-regulated 24 h after ischemia (415), haemorrhage (205), kainate (187), and hypoglycemia (302) with relatively few genes induced by 6 h of moderate (8% oxygen) hypoxia (15). Of the genes induced 24 h after ischemia, haemorrhage, and hypoglycemia, approximately half were unique for each condition suggesting unique components of the responses to each of the injuries. A significant component of the responses involved immune-process related genes likely to represent responses to dying neurons, glia and vessels in ischemia; to blood elements in haemorrhage; and to the selectively vulnerable neurons that die after hypoglycemia. All of the genes induced by kainate were also induced either by ischemia, haemorrhage or hypoglycemia. This strongly supports the concept that excitotoxicity not only plays an important role in ischemia, but is an important mechanism of brain injury after intracerebral haemorrhage and hypoglycemia. In contrast, there was only a single gene that was down-regulated by all of the injury conditions suggesting there is not a common gene down-regulation response to injury.

Hypoxia-inducible factor-1alpha accumulation in the brain after experimental intracerebral hemorrhage

Hypoxia-inducible factor-1 (HIF-1), a transcription factor composed of HIF-1alpha and HIF-1beta protein subunits, has been implicated in cellular protection and cell death in cerebral ischemia. The extent to which HIF-1 plays a role in brain pathology during intracerebral hemorrhage (ICH) is unknown. This study determined whether HIF-1alpha is upregulated at different time points in a rat model of ICH and the role of thrombin and red blood cell lysis in upregulation. Recently, thrombin has been implicated as a nonhypoxic regulator of HIF-1alpha in cultured smooth-muscle cells. Male Sprague-Dawley rats received intracerebral infusions of saline, autologous whole blood, blood plus hirudin, thrombin, thrombin plus hirudin, or lysed erythrocytes. Rats were killed at different time points for Western blot analysis, immunohistochemistry, immunofluorescent double staining, and reverse transcription polymerase chain reaction measurements of HIF-1alpha. HIF-1alpha protein levels increased without changing HIF-1alpha messenger RNA levels after intracerebral infusions of blood, thrombin, and lysed erythrocytes. HIF-1alpha positive cells, which proved to be neurons, were found in the brain after ICH. Hirudin, a specific thrombin inhibitor, reduced HIF-1alpha upregulation in response to both thrombin and blood. This study demonstrates that perihematomal HIF-1alpha protein is upregulated after ICH. This phenomenon is an early response of brain parenchyma to the clot. Thrombin and erythrocyte lysate are involved in HIF-1alpha upregulation through reducing HIF-1alpha degradation.

Tumor hypoxia and the progression of prostate cancer

Tumor cell hypoxia is an innate environmental factor encountered during the development of many types of human tumors, including malignant prostate tumors. For prostate cancer, however, tumor cell hypoxia may be an even more critical element in tumor development and progression. Recent evidence suggests that androgenic steroids are important regulators of blood flow to prostate tumors and suppressors of tumor cell hypoxia. In addition, because prostate tumor cells are similar to other eukaryotic cells, they have the ability to respond to hypoxic conditions with drastic changes in gene expression mediated by the upregulation of a unique transcription factor, hypoxia-inducible factor-1. This response increases cancer cells' metabolic resistance to hypoxia, and also enhances the ability of prostate cancer cells to attract a more vigorous blood supply by upregulating the expression of pro-angiogenic factors. Because such changes would, in essence, increase the potential aggressiveness of affected prostate cancer cells, it is clear that tumor hypoxia has the potential for being a very important factor in prostate cancer cell biology. This review focuses on recent studies regarding the occurrence and potential role of hypoxia in prostate cancer, including hypoxia-inducible factor-1 and its related signaling pathways.

NO-mediated chemoresistance in C6 glioma cells

Expression of inducible nitric oxide synthase (iNOS) in malignant glioma and other tumors has been extensively documented. Massive production of NO by iNOS has been shown to exert tumoricidal effects. However, NO may enhance vasodilation and promote neovascularization, thereby facilitating tumor growth. Compared to the effects of NO on tumor cell death and survival, correlation between NO and cytotoxicity of chemotherapeutic reagents in glioma have been less well characterized. Another gene product often linked to tumor malignancy is hypoxia-inducible factor-1 (HIF-1). HIF-1 is a transcription factor that renders malignant tumors adaptive to hypoxic stress during massive vascularization and tumor invasion. Interestingly, HIF-1 also contributes to iNOS induction under hypoxia. We have characterized the interrelationship between iNOS, HIF-1 and chemoresistance. We note that increased NO synthesis by cytokine exposure or iNOS overexpression neutralized the cytotoxicity of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), but not cisplatin, in rat C6 glioma cells. Both BCNU and CCNU are chloroethylnitrosoureas that kill tumor cells via carbamoylating and alkylating actions. Further studies indicated that iNOS only neutralized carbamoylating action of chloroethylnitrosoureas. Expression of iNOS may inhibit HIF-1 activity under hypoxia in C6 glioma cells transfected with a VEGF promoter-driven luciferase gene. Pretreatment of C6 cells with N-acetyl-l-cysteine (NAC), an antioxidant, nullified the inhibitory effect of iNOS on HIF-1 binding. That NO generated by iNOS expression inhibits HIF-1 activity in hypoxic C6 cells reveals a negative feedback loop in the HIF-1 --> iNOS cascade. Together these results suggest a complicated role of NO in malignant tumor growth, survival and invasion.

Inhibitors of mitochondrial complex I attenuate the accumulation of hypoxia-inducible factor-1 during hypoxia in Hep3B cells

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription factor that regulates transcriptional activation of several genes that are responsive to oxygen lack, including erythropoietin, vascular endothelial growth factor, various glycolytic enzymes and the GLUT-1 glucose transporter. Because mitochondria have been postulated to be involved in the regulation of HIF-1, we tested the effects of mitochondrial electron transport chain complex I inhibitors, rotenone and 1-methyl-4-phenylpiridinium (MPP(+)), on hypoxic-induced accumulation of HIF-1 alpha, the regulated component of the dimer. We found, consistent with our previous observations in Cath.a and PC12 cells, that rotenone and MPP(+) attenuated the HIF-1 alpha hypoxic response. Thus, it can be concluded that an intact, functional mitochondrial respiratory chain is required for HIF-1 alpha accumulation.

Carboxyl-terminal transactivation activity of hypoxia-inducible factor 1 alpha is governed by a von Hippel-Lindau protein-independent, hydroxylation-regulated association with p300/CBP

Hypoxia-inducible factor 1 complex (HIF-1) plays a pivotal role in oxygen homeostasis and adaptation to hypoxia. Its function is controlled by both the protein stability and the transactivation activity of its alpha subunit, HIF-1 alpha. Hydroxylation of at least two prolyl residues in the oxygen-dependent degradation domain of HIF-1 alpha regulates its interaction with the von Hippel-Lindau protein (VHL) that targets HIF-1 alpha for ubiquitination and proteasomal degradation. Several prolyl hydroxylases have been found to specifically hydroxylate HIF-1 alpha. In this report, we investigated possible roles of VHL and hydroxylases in the regulation of the transactivation activity of the C-terminal activating domain (CAD) of HIF-1 alpha. We demonstrate that regulation of the transactivation activity of HIF-1 alpha CAD also involves hydroxylase activity but does not require functional VHL. In addition, stimulation of the CAD activity by a hydroxylase inhibitor, hypoxia, and desferrioxamine was severely blocked by the adenoviral oncoprotein E1A but not by an E1A mutant defective in targeting p300/CBP. We further demonstrate that a hydroxylase inhibitor, hypoxia, and desferrioxamine promote the functional and physical interaction between HIF-1 alpha CAD and p300/CBP in vivo. Taken together, our data provide evidence that hypoxia-regulated stabilization and transcriptional stimulation of HIF-1 alpha function are regulated through partially overlapping but distinguishable pathways.

Adriana and Luisa Castellucci Award lecture 2001. Hypoxia inducible factor-1: oxygen regulation of trophoblast differentiation in normal and pre-eclamptic pregnancies--a review

During early pregnancy, trophoblast differentiation occurs in an environment of relative low oxygen tension which is essential for normal embryonic and placental development. At around 10-12 weeks' gestation, when the intervillous space opens to maternal blood, there is an increase in Po(2). This increase correlates with the time of maximal trophoblast invasion into the maternal decidua, which allows extravillous trophoblast cells to access and remodel the maternal spiral arteries. Hypoxia Inducible Factor 1(HIF-1) is a transcription factor which activates gene transcription in response to varying oxygen concentration of cells. HIF-1 is a heterodimer composed of the inducible HIF-1alpha and the constitutively expressed HIF-1beta/ARNT subunits. Using villous explants, we have demonstrated that the oxygen-regulated events of early trophoblast differentiation are in part mediated by TGFbeta(3), an inhibitor of trophoblast differentiation, via HIF-1alpha. Pre-eclampsia is a disease of pregnancy that is characterized by shallow trophoblast invasion. Recently, we have reported that TGFbeta(3) is over-expressed in pre-eclamptic pregnancy and that its down-regulation restores invasive capability to trophoblast cells. Because TGFbeta(3) is downstream of HIF-1alpha, in the present study we investigated the expression of HIF-1alpha in pre-eclamptic placentae and age-matched controls using in situ hybridization and histochemical analyses. We found that HIF-1alpha mRNA and protein expression are abnormally elevated in pre-eclamptic placental tissue when compared to normal placental tissue. We conclude that pre-eclampsia may result from a developmental failure of oxygen to increase or of trophoblast cells to respond and/or sense an increase in oxygen. This will prevent the normal TGFbeta3 down-regulation and will lead to poor trophoblast invasion predisposing the pregnancy to pre-eclampsia.

Hypoxia in the thymus: role of oxygen tension in thymocyte survival

Our previous studies using oxygen microelectrodes showed that the thymus is grossly hypoxic under normal physiological conditions. We now have investigated how oxygen tension affects the thymus at the cellular and molecular level. Adducts of the hypoxia marker drug pimonidazole accumulated in foci within the cortex and medulla and at the corticomedullary junction, consistent with the presence of widespread cellular hypoxia in the normal thymus. Hypoxia-associated pimonidazole accumulation was decreased but not abrogated by oxygen administration. Genes previously reported to be induced by hypoxia were expressed at baseline levels in the normal thymus, indicating that physiological adaptation to hypoxia occurred. Despite changes in thymus size and cellularity, thymic PO(2) did not change with age. Combined assays for hypoxia and cell death showed that hypoxia achieved using either hypoxic gas mixtures or high-density culture in normoxia decreased spontaneous thymocyte apoptosis in vitro. Taken together, these data suggest that regulatory mechanisms exist to maintain thymic cellular hypoxia in vivo and that oxygen tension may regulate thymocyte survival both in vitro and in vivo.

Expression of angiogenic factors and hypoxia inducible factors HIF 1, HIF 2 and CA IX in non-Hodgkin's lymphoma

AIMS

Angiogenesis in solid tumour pathology is well established but less is known about its role in haematological malignancies. Our study investigated the immunohistochemical expression of a variety of angiogenic and hypoxic factors and microvessel densities on 110 cases of high- and low-grade non-Hodgkin's lymphomas and reactive lymphoid tissues. methods and results: Expression of vascular endothelial growth factor (VEGF) was present in 82 (96%) of the non-Hodgkin's cases and 35 (100%) of the reactive lymphoid tissue cases. Both hypoxia inducible factors 1 alpha and 2 alpha (HIF 1 alpha, 2 alpha) were weakly expressed in the majority of high- and low-grade lymphomas. Carbonic anhydrase IX (CA IX), a HIF-inducible membrane-bound enzyme, expression was not abundant with membranous staining being present in seven (8%) of the lymphoma cases and none of the reactive cases. Thymidine phosphorylase (TP) was distributed amongst macrophages and follicular dendritic cells but was not present in the neoplastic population. The vasculature was stained using CD34 which gave rise to a distinct vascular, predominantly paracortical network present in low-grade lymphomas and reactive lymphoid tissue but which was lost in high-grade lymphomas.

CONCLUSION

Our results suggest that non-Hodgkin's lymphomas may be less angiogenic and hypoxically driven than most solid tumours, which has implications for possible future therapies.

l-carnosine and verapamil inhibit hypoxia-induced expression of hypoxia inducible factor (HIF-1 alpha) in H9c2 cardiomyoblasts

Contractile failure of myocardial cells is a common cause of mortality in ischemic heart disease. In response to hypoxic conditions, cells upregulate the activity of hypoxia-inducible factor 1 (HIF-1) and express a number of genes encoding proteins that either enhance O (2)delivery or increase cellular ATP levels. HIF-1 is a heterodimer of bHLH-PAS proteins, HIF-1 alpha and HIF-1 beta. Both subunits are constitutively expressed under normoxic conditions, but HIF-1 alpha levels are kept low by proteolytic degradation, then stabilized under conditions of low O (2)by a mechanism that is poorly understood. Here we tested the hypothesis that expression of HIF-1 alpha in cardiac cells may be affected by two known cardioprotective agents. We tested l-carnosine, a naturally occurring dipeptide which has been shown to improve myocardial contractility during hypoxia, and verapamil, a calcium channel blocker frequently prescribed for the treatment of heart disease. The levels of HIF-1 alphamRNA remained relatively stable during time course hypoxia (1% O (2)) in H9c2 cardiomyoblasts, then increased slightly after 24 h. In cells pretreated with 1 microM carnosine, the levels of mRNA were transiently reduced, but then increased after 24 h similar to the controls. The levels of HIF-1 alpha protein increased rapidly in H9c2 cells within 30 min of hypoxia, but this induction was significantly reduced in cells treated with either carnosine or verapamil. In addition, treatment of cells with these agents further reduced the low levels of HIF-1 under normoxic conditions. These results suggest that l-carnosine and verapamil may affect the regulated proteolytic degradation of HIF-1 alpha in heart cells during hypoxia.

Hypoxia: the tumor's gateway to progression along the angiogenic pathway

Decreased aerobic (hypoxic) conditions in tumors induce the release of cytokines that promote vascularization and thereby enhance tumor growth and metastasis. Recent major advances have provided insight into the role hypoxia plays in cancer biology. The domain structure of the hypoxia-inducible factor 1alpha (HIF-1alpha) has been elucidated, as has the mechanism by which stabilization of HIF-1alpha leads to initiation of the transcription of target genes involved in growth of blood vessels.

Endotoxin-induced liver hypoxia: defective oxygen delivery versus oxygen consumption

In vivo EPR was used to investigate liver oxygenation in a hemodynamic model of septic shock in mice. Oxygen-sensitive material was introduced either (i) as a slurry of fine particles which localized at the liver sinusoids (pO2 = 44.39 +/- 5.13 mmHg) or (ii) as larger particles implanted directly into liver tissue to measure average pO2 across the lobule (pO2 = 4.56 +/- 1.28 mmHg). Endotoxin caused decreases in pO2 at both sites early (5-15 min) and at late time points (6 h after endotoxin; sinusoid = 11.22 +/- 2.48 mmHg; lobule = 1.16 +/- 0.42 mmHg). The overall pO2 changes observed were similar (74.56% versus 74.72%, respectively). Blood pressures decreased transiently between 5 and 15 min (12.88 +/- 8% decrease) and severely at 6 h (59 +/- 9% decrease) following endotoxin, despite volume replacement with saline. Liver and circulatory nitric oxide was elevated at these times. Liver oxygen extraction decreased from 44% in controls to only 15% following endotoxin, despite severe liver hypoxia. Arterial oxygen saturation, blood flow (hepatic artery), and cardiac output were unaffected. Pretreatment with l-NMMA failed to improve endotoxin-induced oxygen defects at either site, whereas interleukin-13 preserved oxygenation. These site-specific measurements of pO2 provide in vivo evidence that the principal cause of liver hypoxia during hypodynamic sepsis is reduced oxygen supply to the sinusoid and can be alleviated by maintaining sinusoidal perfusion.

Hypoxia-inducible factor-1alpha induces cell cycle arrest of endothelial cells

BACKGROUND

Hypoxia can induce tissue injury, including apoptosis of endothelial cells. However, little is known about the effects of hypoxia on endothelial cell function. We assessed the effects of hypoxia inducible factor (HIF)-1alpha on the functional characteristics of endothelial cells, particularly on cell cycle regulators, by cationic liposome-mediated transfection of HIF-1alpha-expression vector into the cells.

RESULTS

Transfection of the HIF-1alpha gene in endothelial cells resulted in (a) reduced proliferation and detachment of the cells; (b) up-regulation of intracellular p21waf1/cip1 and down-regulation of bcl-2; (c) reduced activities of cyclin-dependent kinase (CDK)-4 and CDK-6; (d) cell cycle arrest at G0/G1 phase; and (e) apoptosis of the cells.

CONCLUSIONS

HIF-1alpha can induce cell cycle arrest, resulting in the reduced proliferation and apoptosis of endothelial cells, and the hypoxia-induced cell death may be involved by suppression of anti-apoptotic molecule, bcl-2.

Endemic polycythemia in Russia: mutation in the VHL gene

Chuvash polycythemia (CP) is an autosomal recessive condition that is endemic in the Russian mid-Volga River region of Chuvashia. We previously found that CP patients may have increased serum erythropoietin (EPO) levels, ruled out linkage to both the EPO and EPO receptor (EPOR) gene loci, and hypothesized that the defect may lie in the oxygen homeostasis pathway. We now report a study of five multiplex Chuvash families which confirms that CP is associated with significant elevations of serum EPO levels and rules out a location for the CP gene on chromosome 11 as had been reported by other investigators or a mutation of the HIF-1 alpha gene. Using a genome-wide screen, we localized a region on chromosome 3 with a LOD score >2. After sequencing three candidate genes, we identified a C to T transition at nucleotide 598 (an R200W mutation) in the von Hippel-Lindau (VHL) gene. The VHL protein (pVHL) downregulates the alpha subunit of hypoxia-inducible factor 1 (HIF-1 alpha), the main regulator of hypoxia adaptation, by targeting the protein for degradation. In the simplest scenario, disruption of pVHL function causes a failure to degrade HIF-1 alpha resulting in accumulation of HIF-1 alpha, upregulation of downstream target genes such as EPO, and the clinical manifestation of polycythemia. These findings strongly suggest that CP is a congenital disorder of oxygen homeostasis.

Genotoxic and non-genotoxic pathways of p53 induction

Since the initial concept of p53 as a sensor of DNA-damage, the picture of the role of p53 has widened to include the sensing of much more diverse forms of stress, including hypoxia and constitutive activation of growth-promoting cascades. The pathways by which these processes regulate p53 are partially overlapping, but imply different patterns of post-translational modifications. In this review, we summarize current knowledge on post-translational modifications of p53, and we discuss how hypoxia and oncogene activation stresses may induce p53 independently of DNA damage.

Regulation of vascular endothelial growth factor expression by advanced glycation end products

Advanced glycation end products (AGEs) are generated during long term diabetes and are correlated with the development of diabetic complications, such as retinopathy. Diabetic retinopathy is characterized by an increased retinal neovascularization due to the action of the angiogenic factor, vascular endothelial growth factor (VEGF). In this report, we show that injection of insulin and glycated albumin (Alb-AGE) to mice increases VEGF mRNA expression in eyes. Insulin and Alb-AGE stimulate VEGF mRNA and protein expression in retinal epithelial cells (ARPE-19). Alb-AGE-induced VEGF expression is not modulated by the use of antioxidants, N-acetyl-l-cysteine or pyrrolidinedithiocarbamate, or by an inhibitor of phosphatidylinositol 3-kinase (PI3K), wortmannin. However, using an inhibitor of ERK activation, U0126, we show that Alb-AGE stimulates VEGF expression through an ERK-dependent pathway. Accordingly, we found that Alb-AGE activated mitogen-activate protein kinase, ERK1/2, JNK1/2, but not p38, and that Alb-AGE did not activate PI3K and PKB. Moreover, Alb-AGE activated the transcription factor, hypoxia inducible factor-1 (HIF-1) DNA binding activity. This activation is mediated by an increase in accumulation of the HIF-1alpha protein through an ERK-dependent pathway. Thus, stimulation of VEGF expression by Alb-AGE, through the activation of HIF-1, could play an important role in the development of diabetic retinopathy.

Hypoxia in cartilage: HIF-1alpha is essential for chondrocyte growth arrest and survival

Breakdown or absence of vascular oxygen delivery is a hallmark of many common human diseases, including cancer, myocardial infarction, and stroke. The chief mediator of hypoxic response in mammalian tissues is the transcription factor hypoxia-inducible factor 1 (HIF-1), and its oxygen-sensitive component HIF-1alpha. A key question surrounding HIF-1alpha and the hypoxic response is the role of this transcription factor in cells removed from a functional vascular bed; in this regard there is evidence indicating that it can act as either a survival factor or induce growth arrest and apoptosis. To study more closely how HIF-1alpha functions in hypoxia in vivo, we used tissue-specific targeting to delete HIF-1alpha in an avascular tissue: the cartilaginous growth plate of developing bone. We show here the first evidence that the developmental growth plate in mammals is hypoxic, and that this hypoxia occurs in its interior rather than at its periphery. As a result of this developmental hypoxia, cells that lack HIF-1alpha in the interior of the growth plate die. This is coupled to decreased expression of the CDK inhibitor p57, and increased levels of BrdU incorporation in HIF-1alpha null growth plates, indicating defects in HIF-1alpha-regulated growth arrest occurs in these animals. Furthermore, we find that VEGF expression in the growth plate is regulated through both HIF-1alpha-dependent and -independent mechanisms. In particular, we provide evidence that VEGF expression is up-regulated in a HIF-1alpha-independent manner in chondrocytes surrounding areas of cell death, and this in turn induces ectopic angiogenesis. Altogether, our findings have important implications for the role of hypoxic response and HIF-1alpha in development, and in cell survival in tissues challenged by interruption of vascular flow; they also illustrate the complexities of HIF-1alpha response in vivo, and they provide new insights into mechanisms of growth plate development.

Hypoxia: the tumor's gateway to progression along the angiogenic pathway

Decreased aerobic (hypoxic) conditions in tumors induce the release of cytokines that promote vascularization and thereby enhance tumor growth and metastasis. Recent major advances have provided insight into the role hypoxia plays in cancer biology. The domain structure of the hypoxia-inducible factor 1alpha (HIF-1alpha) has been elucidated, as has the mechanism by which stabilization of HIF-1alpha leads to initiation of the transcription of target genes involved in growth of blood vessels.

Chemoprotection from p53-dependent apoptosis: potential clinical applications of the p53 inhibitors

The p53 tumor suppressor pathway is a key mediator of stress response that protects the organism from accumulating genetically altered and potentially cancerous cells by inducing growth arrest or apoptosis in damaged cells. However, under certain stressful conditions, p53 activity can result in massive apoptosis in sensitive tissues, leading to severe pathological consequences for the organism. One such situation is anticancer therapy that is often associated with general genotoxic stress, leading to p53-dependent apoptosis in the epithelia of the digestive tract and in the hematopoietic system. A chemical inhibitor of p53, capable of suppressing p53-mediated apoptosis, was shown to protect mice from lethal doses of gamma-radiation, making pharmacological suppression of p53 a perspective therapeutic approach to reduce the side-effects of cancer treatment. There are other situations, besides anti-cancer therapy, when humans are exposed to stressful conditions known to involve p53 activation, which, in extreme cases, could result in the development of life-threatening diseases. Here we review the experimental evidence on the role of p53 in tissue injuries associated with hypoxia (heart and brain ischemias) and hyperthermia (fever and burns), comparing these pathologies with the consequences of genotoxic stress of cancer treatment. The accumulated information points to the involvement of p53 in the generation of the pathological outcome of the above stresses, making them potential targets for the therapeutic application of p53 inhibitors.

Nickel requires hypoxia-inducible factor-1 alpha, not redox signaling, to induce plasminogen activator inhibitor-1

Human epidemiological and animal studies have associated inhalation of nickel dusts with an increased incidence of pulmonary fibrosis. At the cellular level, particulate nickel subsulfide inhibits fibrinolysis by transcriptionally inducing expression of plasminogen activator inhibitor (PAI)-1, an inhibitor of the urokinase-type plasminogen activator. Because nickel is known to mimic hypoxia, the present study examined whether nickel transcriptionally activates PAI-1 through the hypoxia-inducible factor (HIF)-1 alpha signaling pathway. The involvement of the NADPH oxidase complex, reactive oxygen species, and kinases in mediating nickel-induced HIF-1 alpha signaling was also investigated. Addition of nickel to BEAS-2B human airway epithelial cells increased HIF-1 alpha protein levels and elevated PAI-1 mRNA levels. Pretreatment of cells with the extracellular signal-regulated kinase inhibitor U-0126 partially blocked HIF-1 alpha protein and PAI-1 mRNA levels induced by nickel, whereas antioxidants and NADPH oxidase inhibitors had no effect. Pretreating cells with antisense, but not sense, oligonucleotides to HIF-1 alpha mRNA abolished nickel-stimulated increases in PAI-1 mRNA. These data indicate that signaling through extracellular signal-regulated kinase and HIF-1 alpha is required for nickel-induced transcriptional activation of PAI-1.

Mitochondrial contributions to cancer cell physiology: potential for drug development

Mitochondria make an integral contribution to the regulation of several aspects of cell biology such as energy production, molecular metabolism, redox status, calcium signalling and programmed cell death. In accordance with an endosymbiotic origin, mitochondria rely upon the nucleus for synthesis and function. In addition, these organelles can respond to intra- and extracellular cues independently, and there exists a highly coordinated "cross talk" between mitochondrial and nuclear signals that can greatly influence cell behaviour. This review focuses upon the putative roles of altered mitochondrial physiology in the process of cellular transformation. Discussed are: mitochondria as targets of drug-induced cytotoxicity or cancer promotion, as regulators of apoptosis, as sources of cell signalling through reactive oxygen species, and mitochondrial control of specific nuclear responses.