Hypoxia-inducible factor 1: oxygen homeostasis and disease pathophysiology

Expression of proinflammatory cytokines via HIF-1alpha and NF-kappaB activation on desferrioxamine-stimulated HMC-1 cells

We investigated the expression and the role of hypoxia-inducible factor 1alpha (HIF-1alpha) on the desferrioxamine (DFX)-induced cytokine production in human mast cells, HMC-1 cells. HIF-1alpha mRNA was constitutively expressed in mast cell lines including the P815, RBL-2H3, and HMC-1. DFX (100 microM) resulted in a great increase in protein levels of HIF-1alpha in HMC-1 cells, but it did not affect HIF-1alpha mRNA expression. Iron (HIF-1 inhibitor) inhibited increase of HIF-1alpha and NF-kappaB protein levels. Pyrriolidine-dithiocarbamate (PDTC, NF-kappaB inhibitor) inhibited increase of NF-kappaB protein levels, but it slightly increased HIF-1alpha protein levels. In addition, DFX significantly increased the production of IL-6, IL-8, and TNF-alpha in HMC-1 (P<0.05). These increased cytokine levels were significantly inhibited by treatment of iron or PDTC in a dose-dependent manner (P<0.05). We demonstrated the regulatory effects of HIF-1alpha on the DFX-induced proinflammatory cytokine production in human mast cells for the first time. These data indicate that inflammatory cytokines seem to be under HIF-1alpha or NF-kappaB transcriptional regulation in the hypoxic conditions on mast cells.

pVHL-independent ubiquitination of HIF1alpha and its stabilization by cobalt ion

Recent studies have shown that hypoxia-inducible factor1alpha (HIF1alpha) is ubiquitinated by an E3-ligase complex containing von Hippel-Lindau gene product (pVHL) after which it is targeted for proteasomal degradation. In this study, we showed that HIF1alpha was stabilized in the pVHL-deficient cell line 786-0 treated with a proteasome inhibitor or Co(2+). This suggests that HIF1alpha is also ubiquitinated by a pVHL-independent pathway and that its stability is regulated by Co(2+). Indeed, using the COS cell expression system, we confirmed that HIF1alpha is ubiquitinated at the N-terminal region by a pVHL-independent pathway and that its degradation is inhibited by Co(2+). We also demonstrated that Co(2+) binds to both PAS domains in the N-terminal region of HIF1alpha. These observations imply that the stability of HIF1alpha is regulated by an additional pathway through the cobalt binding of PAS domains.

Structural basis for negative regulation of hypoxia-inducible factor-1alpha by CITED2

Expression of hypoxia-responsive genes is mediated by the heterodimeric transcription factor hypoxia-inducible factor-1 (HIF-1) in complex with the p300/CREB-binding protein (p300/CBP) transcriptional coactivator. The protein CITED2, which binds p300/CBP, is thought to be a negative regulator of HIF-1 transactivation. We show that the CITED2 transactivation domain (TAD) disrupts a complex of the HIF-1alpha C-terminal TAD (C-TAD) and the cysteine-histidine-rich 1 (CH1) domain of p300/CBP by binding CH1 with high affinity. The high-resolution solution structure of the CITED2 TAD-p300 CH1 complex shows that the CITED2 TAD, like the HIF-1alpha C-TAD, folds on a helical, Zn2+-containing CH1 scaffold. The CITED2 TAD binds a different, more extensive surface of CH1 than does the HIF-1alpha C-TAD. However, a conserved 'LPXL' sequence motif in CITED2 and HIF-1alpha interacts with an overlapping binding site on CH1. Mutation of the LPEL sequence in full-length CITED2 abolishes p300 binding in vivo. These findings reveal that CITED2 regulates HIF-1 by competing for a hot spot on the p300 CH1 domain.

Molecular mechanisms of cerebral ischemia-induced neuronal death

The mode of neuronal death caused by cerebral ischemia and reperfusion appears on the continuum between the poles of catastrophic necrosis and apoptosis: ischemic neurons exhibit many biochemical hallmarks of apoptosis but remain cytologically necrotic. The position on this continuum may be modulated by the severity of the ischemic insult. The ischemia-induced neuronal death is an active process (energy dependent) and is the result of activation of cascades of detrimental biochemical events that include perturbion of calcium homeostasis leading to increased excitotoxicity, malfunction of endoplasmic reticulum and mitochondria, elevation of oxidative stress causing DNA damage, alteration in proapoptotic gene expression, and activation of the effector cysteine proteases (caspases) and endonucleases leading to the final degradation of the genome. In spite of strong evidence showing that brain infarction can be reduced by inhibiting any one of the above biochemical events, such as targeting excitotoxicity, up-regulation of an antiapoptotic gene, or inhibition of a down-stream effector caspase, it is becoming clear that targeting a single gene or factor is not sufficient for stroke therapeutics. An effective neuroprotective therapy is likely to be a cocktail aimed at all of the above detrimental events evoked by cerebral ischemia and the success of such therapeutic intervention relies upon the complete elucidation of pathways and mechanisms of the cerebral ischemia-induced active neuronal death.

Inhibitory effect of high molecular weight water-soluble chitosan on hypoxia-induced inflammatory cytokine production

Chitosan is widely used to treat patients with hypoxia-induced diseases such as ischemia, neuronal death, cerebral stroke, and cerebral infarction. Using the ELISA method, we examined the effect of high molecular weight water-soluble chitosan (WSC) on inflammatory cytokine production in the desferrioxamine (DFX, known to mimic hypoxia)-stimulated human mast cell line HMC-1. DFX significantly increased interleukin (IL)-6, IL-8, and tumor necrosis factor (TNF)-alpha production compared with the control in a time-dependent manner (p<0.05), but did not affect IL-1alpha production and mRNA expression. The increase in IL-6, IL-8, and TNF-alpha levels was significantly inhibited by WSC in a dose-dependent manner with IC(50) values of 0.77, 0.88, and 2.5 microg/ml, respectively. The maximal inhibition rate of IL-6, IL-8, and TNF-alpha production by WSC was 64+/-9.7%, 80+/-9.4% and 54+/-4.5%, respectively. In addition, WSC inhibited DFX-induced activation of nuclear factor (NF)-kappaB. In conclusion, these results suggest that WSC is an inhibitor of NF-kappaB under hypoxic conditions, which might explain its beneficial effect in the treatment of hypoxia-induced inflammatory diseases.

Redox side reactions of haemoglobin and cell signalling mechanisms

Cell-free chemically modified or recombinant haemoglobins developed as oxygen therapeutics are designed to correct oxygen deficit caused by ischaemia in a variety of clinical settings. Oxidative processes, which are in some cases enhanced when modifications are introduced that lower oxygen affinity, can limit the safety of these proteins. Direct cytotoxic effects associated with haemoglobins have been ascribed to the redox reactions between haemoglobin and biological peroxides [i.e. hydrogen peroxide (H2O2), lipid peroxides (LOOH) and peroxynitrite (ONOO-)]. Biochemical changes at the cellular, tissue and organ levels have been documented to occur in response to haemoglobin oxidative reactions. These peroxides have been implicated as regulators of redox sensitive cell signalling pathways. The effects of reactions between haemoglobin and biologically relevant peroxides may be more subtle than oxidative damage and may thus involve perturbation of redox sensitive signalling pathways. In this review, a brief outline of the role of cell-free haemoglobin in oxidative and cell-signalling pathways and the implications of these reactions on the safety and efficacy evaluation of haemoglobin-based oxygen carries are presented.

Inhibition of prolyl-4-hydroxylase ameliorates chronic rejection of mouse kidney allografts

Interstitial fibrosis, glomerulosclerosis and arteriosclerosis are the major components of chronic allograft nephropathy (CAN), the leading cause of late graft failure after transplantation. To investigate the mechanism of collagen deposition in CAN, we studied the effects of prolyl-hydroxylase inhibitor (PHI), an enzyme essential for collagen formation, using a mouse model of kidney transplantation. Kidneys from H-2b mice were transplanted into MHC-incompatible H-2d recipients (allografts) and at 3 weeks post-transplant, received either PHI or vehicle treatment daily for 3 weeks. At 6 weeks post-transplant, GFR was significantly improved in the allografts receiving PHI (3.3 +/- 0.5 mL/min/kg) compared with those receiving vehicle (1.8 +/- 0.5 mL/min/kg, p < 0.05), while renal function was relatively unimpaired in the nonrejecting isografts (6.45 +/- 0.53 mL/min/kg). Allografts had histologic changes of CAN but the severity was significantly reduced with PHI treatment compared with vehicle, with reductions in interstitial inflammation and fibrosis. Furthermore, TGFâ and connective tissue growth factor mRNA expression was enhanced in both allograft groups compared with the isografts. In conclusion, PHI-treated allografts had improved renal function and reduced the severity of renal injury as a result of CAN. Inhibition of matrix synthesis may be a useful adjunct in ameliorating the development of CAN in humans.

HIF-1alpha is essential for myeloid cell-mediated inflammation

Granulocytes and monocytes/macrophages of the myeloid lineage are the chief cellular agents of innate immunity. Here, we have examined the inflammatory response in mice with conditional knockouts of the hypoxia responsive transcription factor HIF-1alpha, its negative regulator VHL, and a known downstream target, VEGF. We find that activation of HIF-1alpha is essential for myeloid cell infiltration and activation in vivo through a mechanism independent of VEGF. Loss of VHL leads to a large increase in acute inflammatory responses. Our results show that HIF-1alpha is essential for the regulation of glycolytic capacity in myeloid cells: when HIF-1alpha is absent, the cellular ATP pool is drastically reduced. The metabolic defect results in profound impairment of myeloid cell aggregation, motility, invasiveness, and bacterial killing. This role for HIF-1alpha demonstrates its direct regulation of survival and function in the inflammatory microenvironment.

The subtle side to hypoxia inducible factor (HIFalpha) regulation

The transcription factor hypoxia inducible factor alpha-subunit (HIFalpha) is pivotal in the cellular response to the stress of hypoxia. Post-translational modification of HIFalpha by hydroxylase enzymes has recently been identified as a key "oxygen sensing" mechanism within the cell. The absence of the substrate oxygen prevents the hydroxylases from modifying HIFalpha during hypoxia and allows dramatic up-regulation of both HIFalpha protein stability and transcriptional activation capability. In addition to this oxygen-dependent response, increased HIFalpha protein levels and/or enhanced transcriptional activity during normoxic conditions can be stimulated by various receptor-mediated factors such as growth-factors and cytokines (insulin, insulin-like growth factor 1 or 2, endothelial growth factor, tumour necrosis factor alpha, angiotensin-2). Oncogenes are also capable of HIFalpha activation. This induction is generally less intense than that stimulated by hypoxia and although not fully elucidated, appears to occur via hypoxia-independent, receptor-mediated signal pathways involving either phosphatidyl-inositol-3-kinase/Akt or mitogen activated protein kinase (MAPK) pathways, depending on the cell-type. Activation of Akt increases HIFalpha protein synthesis in the cell and results in increased HIFalpha protein and transcriptional activity. MAPK also activates HIFalpha protein synthesis and additionally may potentiate HIF1alpha transcriptional activity via a separate mechanism that does not necessarily require protein stabilization. Here we review the mechanisms and function of receptor-mediated signals in the multifaceted regulation of HIFalpha.

Identification of residues critical for regulation of protein stability and the transactivation function of the hypoxia-inducible factor-1alpha by the von Hippel-Lindau tumor suppressor gene product

Under normoxic conditions the hypoxia-inducible factor-1alpha (HIF-1alpha) protein is targeted for degradation by the von Hippel-Lindau (pVHL) tumor suppressor protein acting as an E3 ubiquitin ligase. Binding of pVHL to HIF-1alpha is dependent on hydroxylation of specific proline residues by O(2)-dependent prolyl 4-hydroxylases. Upon exposure to hypoxia the hydroxylase activity is inhibited, resulting in stabilization of HIF-1alpha protein levels and activation of transcription of target genes. One of the two critical proline residues, Pro(563) in mouse HIF-1alpha, is located within a bifunctional domain, the N-terminal transactivation domain (N-TAD), which mediates both pVHL-dependent degradation at normoxia and transcriptional activation at hypoxia. Here we have identified two N-TAD residues, Tyr(564) and Ile(565), which, in addition to Pro(563), were critical for pVHL-mediated degradation at normoxia. We have also identified D568A/D569A/D570A, F571A, and L573A as mutations of the N-TAD that abrogated binding to pVHL both in vitro and in vivo, and constitutively stabilized N-TAD against degradation. Moreover, the mutations Y564G, L556A/L558A, and F571A/L573A drastically reduced the transactivation function of either the isolated N-TAD or full-length HIF-1alpha in hypoxic cells. Interestingly, the P563A mutant exhibited a constitutively active and potent transactivation function that was enhanced by functional interaction with the transcriptional coactivator protein CREB-binding protein. In conclusion, we have identified by mutation analysis several residues that are critical for either one or both of the interdigitated and conditionally regulated degradation and transactivation functions of the N-TAD of HIF-1alpha.

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.

Regulation of vascular endothelial growth factor synthesis by nitric oxide: facts and controversies

Vascular endothelial growth factor (VEGF) is the major molecule governing angiogenesis, defined as the growth of blood vessels from vascular structure. There is abundant evidence that nitric oxide (NO) is an effector molecule mediating the activity of VEGF. By binding to its receptors, VEGF initiates the signaling cascades leading to NO production and angiogenic activation of endothelial cells. Recent data show that NO induces VEGF synthesis in numerous cell types, including vascular smooth muscle cells, macrophages, keratinocytes, and tumor cells. NO enhances VEGF production by augmenting its expression through activation of Akt kinase, followed by induction of several transcription factors, of which stabilization of hypoxia-inducible factor (HIF-1) is the critical step. With respect to its effect on VEGF expression, NO mimics hypoxia, the classical activator of HIF-1 and VEGF synthesis. The effect of NO on VEGF production is also mediated by heme oxygenase, an enzyme generating carbon monoxide, which appears to stimulate VEGF release. In this review, we attempt to elucidate the molecular mechanisms underlying the effects of NO on VEGF synthesis. We also discuss some discrepant data and suggest explanations for various aspects of the NO-VEGF relationship.

Downregulation of hnRNP A2/B1 expression in tumor cells under prolonged hypoxia

Heterogeneous nuclear ribonucleoprotein (hnRNP) A2/B1 has been previously shown to be overexpressed in breast and lung tumors. Because hypoxia is a feature inherent in solid tumors, the regulation of hnRNP A2/B1 expression and subcellular localization under hypoxic conditions was studied on human lung and breast carcinoma cell lines. We found that sustained hypoxic treatment downregulated hnRNP A2/B1 expression in MCF7 and H157 cell lines. Northern blot analysis showed that this decay: (i) was observed as a marked diminution of transcript levels after 24-48 h of exposure to low oxygen tension; (ii) is not mediated by the transcription factor, hypoxia inducible factor-1; and (iii) is partially dependent on a higher hnRNP A2/B1 messenger RNA turnover under hypoxic than normoxic conditions. Immunocytochemical staining also showed a significant diminution of hnRNP A2/B1 staining in these cell lines after 24-48 h of hypoxia, together with a predominant loss of cytoplasmic staining. Further investigations are warranted to evaluate the relevance of modulation of hnRNP A2/B1 in hypoxic environments relative to its previously reported utility as a marker of early lung carcinogenesis.

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.

Identification of functional hypoxia response elements in the promoter region of the DEC1 and DEC2 genes

Adaptation to hypoxia is a crucial process both physiologically (i.e. in chondrocytes) and pathologically (i.e. in tumor cells). Previous studies have shown that DEC1, a basic helix-loop-helix transcription factor, is induced by hypoxia in glioma cells (Ivanova, A. V., Ivanov, S. V., Danilkovitch-Miagkova, A., and Lerman, M. I. (2001) J. Biol. Chem. 276, 15306-15315). In the present study, we found that hypoxia or CoCl(2) enhanced the mRNA expression of DEC2, as well as DEC1, within 24 h in chondrogenic ATDC5, 293T, and HeLa cells. In luciferase assays, the regions between -524 and -401 in the DEC1 promoter, and between -863 and -258 in the DEC2 promoter, were responsible for the hypoxia- or hypoxia-inducible factor-1alpha (HIF-1alpha)-induced transcription. In these regions, we identified functional hypoxia response elements (HREs) that bound to HIF-1alpha and HIF-1beta. In addition to an HIF-1 binding site consensus sequence, the DEC1 HRE had cAMP response element-like and CACAG sequences, which were also involved in the transcription activation in response to HIF-1alpha. Although the DEC2 HRE did not have a cAMP response element-like or CACAG sequence, it showed a higher affinity for HIF-1 than did the DEC1 HRE. Because DEC1 and DEC2 are directly inducible by HIF-1, these transcription factors may be crucial for the adaptation to hypoxia.

The aryl hydrocarbon receptor nuclear transporter is modulated by the SUMO-1 conjugation system

The aryl hydrocarbon receptor nuclear transporter (ARNT) is a member of the basic helix-loop-helix/PAS (Per-ARNT-Sim) family of transcription factors, which are important for cell regulation in response to environmental conditions. ARNT is an indispensable partner of the aryl hydrocarbon receptor (AHR) or hypoxia-inducible factor-1alpha. This protein is also able to form homodimers such as ARNT/ARNT. However, the molecular mechanism that regulates the transcriptional activity of ARNT remains to be elucidated. Here, we report that ARNT is modified by SUMO-1 chiefly at Lys(245) within the PAS domain of this protein, both in vivo and in vitro. Substitution of the target lysine with alanine enhanced the transcriptional potential of ARNT per se. Furthermore, green fluorescent protein-fused ARNT tended to form nuclear foci in approximately 20% of the transfected cells, and the foci partly colocalized with PML nuclear bodies. PML, one of the well known substrates for sumoylation, was found to augment the transcriptional activities of ARNT. ARNT bound AHR or PML, whereas the sumoylated form of ARNT associated with AHR, but not with PML, resulting in a reduced effect of PML on transactivation by ARNT. Our data suggest that the sumoylation of ARNT modulates its transcriptional role through affecting the ability of ARNT to interact with cooperative molecules such as PML. This exemplifies a crucial role of protein sumoylation in modulating protein-protein interactions.

Glucocorticoids inhibit vascular endothelial growth factor expression in growth plate chondrocytes

Vascular endothelial growth factor (VEGF) plays an essential role in angiogenesis in the growth plate and ultimately in regulating endochondral ossification. Since longitudinal bone growth is often disturbed in children who are treated with glucocorticoids, we investigated the effects of dexamethasone on VEGF expression by epiphyseal chondrocytes. Cells were cultured from tibial growth plates of neonatal piglets. Using Northern blotting and RT-PCR techniques, the chondrocyte-specific markers aggrecan, collagen II and CD-RAP were detected. Also the glucocorticoid receptor (GR) was expressed. VEGF protein secreted from these cells was examined by ELISA and Western immunoblotting. The VEGF(121) and VEGF(165) isoforms were detected in the supernatant. As determined by RT-PCR, all three major mRNA splice variants were produced, including the species encoding VEGF(189). Dexamethasone (100 nM) inhibited both protein and mRNA expression by approximately 45%. Hydrocortisone (cortisol) and prednisolone also inhibited VEGF secretion, but they were less active than dexamethasone. The inhibitory actions of dexamethasone were almost completely blocked by the GR antagonist Org34116, indicating that the GR mediates these actions. Degradation of the VEGF mRNA was not accelerated by dexamethasone. Therefore, a transcriptional mechanism seems likely. Downregulation of this important growth factor could lead to disruption of the normal invasion of blood vessels in the growth plate, which could contribute to disturbed endochondral ossification and growth.

Reduction of hypoxia-induced transcription through the repression of hypoxia-inducible factor-1alpha/aryl hydrocarbon receptor nuclear translocator DNA binding by the 90-kDa heat-shock protein inhibitor radicicol

Under low oxygen tension, cells increase the transcription of specific genes involved in angiogenesis, erythropoiesis, and glycolysis. Hypoxia-induced gene expression depends primarily on stabilization of the alpha subunit of hypoxia-inducible factor-1 (HIF-1alpha), which acts as a heterodimeric trans-activator with the nuclear protein known as the aryl hydrocarbon receptor nuclear translocator (Arnt). The resulting heterodimer (HIF-1alpha/Arnt) interacts specifically with the hypoxia-responsive element (HRE), thereby increasing transcription of the genes under HRE control. Our results indicate that the 90-kDa heat-shock protein (Hsp90) inhibitor radicicol reduces the hypoxia-induced expression of both endogenous vascular endothelial growth factor (VEGF) and HRE-driven reporter plasmids. Radicicol treatment (0.5 microg/ml) does not significantly change the stability of the HIF-1alpha protein and does not inhibit the nuclear localization of HIF-1alpha. However, this dose of radicicol significantly reduces HRE binding by the HIF-1alpha/Arnt heterodimer. Our results, the first to show that radicicol specifically inhibits the interaction between the HIF-1alpha/Arnt heterodimer and HRE, suggest that Hsp90 modulates the conformation of the HIF-1alpha/Arnt heterodimer, making it suitable for interaction with HRE. Furthermore, we demonstrate that radicicol reduces hypoxia-induced VEGF expression to decrease hypoxia-induced angiogenesis.

Functional analysis of hypoxia-inducible factor-1 alpha-mediated transactivation. Identification of amino acid residues critical for transcriptional activation and/or interaction with CREB-binding protein

The hypoxia-inducible factor-1 alpha (HIF-1 alpha) is a key regulator of adaptive responses to hypoxia. HIF-1 alpha has two independent transactivation domains (TADs). Whereas the N-terminal TAD (N-TAD) also constitutes a degradation box, the C-terminal TAD (C-TAD) functions in a strictly hypoxia-inducible fashion. Oxygen-dependent hydroxylation of an asparagine residue has recently been reported to regulate C-TAD function by disrupting the interaction with the CH1 domain of the p300/CBP coactivator at normoxia. Here we have performed alanine-scanning mutagenesis of a predicted alpha-helix within the C-TAD of mouse HIF-1 alpha to identify residues important for transactivation and interaction of the C-TAD with transcriptional coactivators. We observed that several hydrophobic residues, Ile(802), Leu(808), Leu(814), Leu(815), and Leu(818), were critical for transactivation and binding to the CH1 domain of CBP in hypoxic cells. Moreover, E812A/E813A and D819A mutations impaired hypoxia-dependent transactivation without disrupting binding to CH1. In the context of full-length HIF-1 alpha, mutation of the leucine residues conferred conformational changes to the protein and significantly reduced the transactivation function as well as functional interaction with the transcriptional coactivators CBP and SRC-1. These mutations also affected intranuclear redistribution of HIF-1 alpha in the presence of CBP, indicating that the integrity of the C-TAD is critical for intracellular localization of mouse HIF-1 alpha.

MRI of the tumor microenvironment

The microenvironment within tumors is significantly different from that in normal tissues. A major difference is seen in the chaotic vasculature of tumors, which results in unbalanced blood supply and significant perfusion heterogeneities. As a consequence, many regions within tumors are transiently or chronically hypoxic. This exacerbates tumor cells' natural tendency to overproduce acids, resulting in very acidic pH values. The hypoxia and acidity of tumors have important consequences for antitumor therapy and can contribute to the progression of tumors to a more aggressive metastatic phenotype. Over the past decade, techniques have emerged that allow the interrogation of the tumor microenvironment with high resolution and molecularly specific probes. Techniques are available to interrogate perfusion, vascular distribution, pH, and pO(2) nondestructively in living tissues with relatively high precision. Studies employing these methods have provided new insights into the causes and consequences of the hostile tumor microenvironment. Furthermore, it is quite exciting that there are emerging techniques that generate tumor image contrast via ill-defined mechanisms. Elucidation of these mechanisms will yield further insights into the tumor microenvironment. This review attempts to identify techniques and their application to tumor biology, with an emphasis on nuclear magnetic resonance (NMR) approaches. Examples are also discussed using electron MR, optical, and radionuclear imaging techniques.

Hypoxia signaling to genes: significance in Alzheimer's disease

Aberrations in neural signaling, converging to and diverging from oxidative metabolism and blood supply, contribute to the initiation and maintenance of inflammatory responses, neuronal degeneration, and age-related cognitive decline in Alzheimer's disease (AD). Hypoxia/ischemia triggers phospholipase A2, leading to the accumulation of free arachidonic and docosahexaenoic acids (AA, DHA), as well as that of lysophospholipids. Some of these bioactive lipid messengers in turn give rise to several downstream lipid messengers, such as platelet-activating factor (PAF) and ecosanoids (prostaglandins and leukotrienes). Eicosanoid synthesis is highly regulated in hypoxia and in reperfusion subsequent to ischemia. As one of the consequences, mitochondrial function is disrupted and reactive oxygen species (ROS) both contribute to the expansion of cellular inflammatory responses and reduce the expression of genes required to maintain synaptic structure and function. On the other hand, pro-inflammatory genes are up-regulated. One of these, the inducible cyclooxygenase-2 (COX-2), along with oxygen-starved mitochondria, comprise the major sources of ROS in the brain during hypoxia, ischemia, and reperfusion. One outcome is a sustained metabolic stress that drives progressive dysfunction, apoptosis and/or necrosis, and brain cell death. How hypoxia modulates oxygen-sensitive gene expression is not well understood. Pro-inflammatory gene families that contribute to neurodegeneration are transiently activated in part by the heterodimeric oxygen-sensitive DNA-binding proteins nuclear factor for kappa B (NF-kappaB) and hypoxia-inducible factor-alpha (HIF-1alpha). Here the authors summarize current studies supporting the hypothesis that synaptically-derived lipid messengers play significant roles in ischemic stroke and that hypoxia is an important contributor to the onset and progression of AD neurodegeneration.

Oxygen homeostasis, thiol equilibrium and redox regulation of signalling transcription factors in the alveolar epithelium

There is growing evidence linking the pathophysiology of lung disease to an imbalance state of reduction-oxidation (redox) equilibrium. The therapeutic potential of glutathione, an ubiquitous sulfhydryl thiol, and its immunopharmacological properties in the airway epithelium bears clinical consequences for the paediatric treatment of respiratory distress (RD). Dynamic variation in alveolar pO(2) and its effect on redox state may impose a direct role in modulating the pattern of gene expression in lung tissues and, accordingly, could be pivotal in determining cellular fate under these conditions. Hypoxia-inducible factor-1alpha (HIF-1alpha) and nuclear factor-kappaB (NF-kappaB) are redox-sensitive transcription factors of particular importance because their differential activation by reducing and oxidizing signals, respectively, regulate the expression/suppression of O(2)-responsive genes. The regulation of these transcription factors, therefore, which is redox sensitive, is consistent with their roles in coordinating adaptive homeostatic responses to oxidative stress. Functionally, the relationship between O(2), glutathione biosynthesis and transcription factor activity bears typical implications for the pattern of cellular survivorship and alveolarization on exposure to O(2)-linked stresses. In this review, I discuss (1) the HIF-1alpha/NF-kappaB responsiveness to dynamic changes in pO(2) characteristic of the transition period from placental to pulmonary-based respiration, (2) the capacity of the alveolar epithelium to engage in glutathione biosynthesis and redox shuttling, effectively forming a feedback mechanism governing gene expression, (3) the restitution of antioxidant/prooxidant equilibrium following oxidative challenge and its dependency on the adaptive coordination of responses between redox-associated signalling pathways controlling apoptosis and genetic regulatory factors and (4) a likely association between oxidative stress and the evolution of an inflammatory signal through the pleiotropic O(2)-sensitive cytokines.

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.

Cyclooxygenase-2 and presenilin-1 gene expression induced by interleukin-1beta and amyloid beta 42 peptide is potentiated by hypoxia in primary human neural cells

Lipid messengers and amyloid beta (Abeta) peptides generated by cyclooxygenase-2 (COX-2) and presenilin-1 (PS1) mediate pro-inflammatory signaling and neural degeneration in Alzheimer's disease (AD) brain. This study provides data showing that the COX-2 and PS1 genes each transcribe rare, highly labile RNA species that display early response gene behavior in human neural (HN) cells in primary culture, down-regulation during human neural development, and up-regulation in AD neocortex and hippocampal CA1. Together, interleukin-1beta and amyloid beta42 peptide [IL-1beta+Abeta42] synergistically activated COX-2 and PS1 gene expression preceded by increases in AP1-, STAT1alpha-, and in particular NF-kappaBp50/p65- and HIF-1alpha-DNA binding. These events were markedly potentiated by hypoxia and blocked by the antioxidant alpha-phenyl-N-tert-butyl nitrone. Broad transcription profiling further indicated that hypoxia-induced, [IL-1beta+Abeta42]-treated HN cells display robust induction of COX-2 and PS1 as well as a pro-inflammatory gene family that includes NF-kappaBp50/p105, IL-1beta precursor, and cytosolic phospholipase A2 genes. These findings indicate a novel [IL-1beta+Abeta42]-mediated, hypoxia-enhanced, free radical-triggered gene program that drives inflammatory gene signaling and suggest a mechanism by which hypoxia during aging contributes episodically to amyloidogenesis, inflammation, and AD pathophysiology.

Oxygen-sensitive enzyme-prodrug gene therapy for the eradication of radiation-resistant solid tumours

Overwhelming clinical and experimental data demonstrate that tumour hypoxia is associated with aggressive disease and poor treatment outcome as hypoxic cells are refractive to radiotherapy and some forms of chemotherapy. However, hypoxia is rare in physiologically normal tissues representing a tumour-specific condition. To selectively target this therapeutically refractive cell population, we have combined bioreductive chemotherapy with hypoxia-directed gene therapy. We have transfected the human fibrosarcoma cell line, HT1080, with a hypoxia-regulated expression vector encoding the human flavoprotein cytochrome c P450 reductase (HRE-P450R). This conferred hypoxia-dependent sensitivity to the alkylating nitroimidazole prodrug RSU1069 in vitro, with a greater than 30-fold increase in oxic/hypoxic cytotoxicity ratio compared with controls. Xenografts of both the HRE-P450R and empty vector transfectants had comparable hypoxic fractions and were refractive to single dose radiotherapy of up to 15 Gy. However, combining a prodrug of RSU1069 with a reduced radiotherapy dose of 10 Gy represents a curative regimen (50% tumour-free survival; day 100) in the HRE-P450R xenografts. In complete contrast, 100% mortality was apparent by day 44 in the empty vector control xenografts treated in the same way. Thus, an oxygen-sensitive gene-directed enzyme prodrug therapy approach may have utility when incorporated into conventional radiotherapy and/or chemotherapy protocols for loco-regional disease in any tissue where hypoxia is a contra-indication to treatment success. doi:10.1038/sj.gt.3301702

Hypoxia-inducible factor 1 activation by aerobic glycolysis implicates the Warburg effect in carcinogenesis

Cancer cells display high rates of aerobic glycolysis, a phenomenon known historically as the Warburg effect. Lactate and pyruvate, the end products of glycolysis, are highly produced by cancer cells even in the presence of oxygen. Hypoxia-induced gene expression in cancer cells has been linked to malignant transformation. Here we provide evidence that lactate and pyruvate regulate hypoxia-inducible gene expression independently of hypoxia by stimulating the accumulation of hypoxia-inducible Factor 1alpha (HIF-1alpha). In human gliomas and other cancer cell lines, the accumulation of HIF-1alpha protein under aerobic conditions requires the metabolism of glucose to pyruvate that prevents the aerobic degradation of HIF-1alpha protein, activates HIF-1 DNA binding activity, and enhances the expression of several HIF-1-activated genes including erythropoietin, vascular endothelial growth factor, glucose transporter 3, and aldolase A. Our findings support a novel role for pyruvate in metabolic signaling and suggest a mechanism by which high rates of aerobic glycolysis can promote the malignant transformation and survival of cancer cells.

The use of dioxygen by HIF prolyl hydroxylase (PHD1)

The hypoxic response in animals is mediated by hydroxylation of proline residues in the alpha-subunit of hypoxia inducible factor (HIF). Hydroxylation is catalysed by prolyl-4-hydroxylases (PHD isozymes in humans) which are iron(II) and 2-oxoglutarate dependent oxygenases. Mutation of the arginine proposed to bind 2-oxoglutarate and of the 2His-1-carboxylate iron(II) binding motif in PHD1 dramatically reduces its activity. The source of the oxygen of the product alcohol is (>95%) dioxygen.

Structural basis for recruitment of CBP/p300 by hypoxia-inducible factor-1 alpha

Adaptation to hypoxia is mediated by transactivation of hypoxia-responsive genes by hypoxia-inducible factor-1 (HIF-1) in complex with the CBP and p300 transcriptional coactivators. We report the solution structure of the cysteine/histidine-rich 1 (CH1) domain of p300 bound to the C-terminal transactivation domain of HIF-1 alpha. CH1 has a triangular geometry composed of four alpha-helices with three intervening Zn(2+)-coordinating centers. CH1 serves as a scaffold for folding of the HIF-1 alpha C-terminal transactivation domain, which forms a vise-like clamp on the CH1 domain that is stabilized by extensive hydrophobic and polar interactions. The structure reveals the mechanism of specific recognition of p300 by HIF-1 alpha, and shows how HIF-1 alpha transactivation is regulated by asparagine hydroxylation.

Structural basis for Hif-1 alpha /CBP recognition in the cellular hypoxic response

The cellular response to low tissue oxygen concentrations is mediated by the hypoxia-inducible transcription factor HIF-1. Under hypoxic conditions, HIF-1 activates transcription of critical adaptive genes by recruitment of the general coactivators CBP/p300 through interactions with its alpha-subunit (Hif-1 alpha). Disruption of the Hif-1 alpha/p300 interaction has been linked to attenuation of tumor growth. To delineate the structural basis for this interaction, we have determined the solution structure of the complex between the carboxy-terminal activation domain (CAD) of Hif-1 alpha and the zinc-binding TAZ1 (CH1) motif of cyclic-AMP response element binding protein (CREB) binding protein (CBP). Despite the overall similarity of the TAZ1 structure to that of the TAZ2 (part of the CH3) domain of CBP, differences occur in the packing of helices that can account for differences in specificity. The unbound CAD is intrinsically disordered and remains relatively extended upon binding, wrapping almost entirely around the TAZ1 domain in a groove through much of its surface. Three short helices are formed upon binding, stabilized by intermolecular interactions. The Asn-803 side chain, which functions as a hypoxic switch, is located on the second of these helices and is buried in the molecular interface. The third helix of the Hif-1 alpha CAD docks in a deep hydrophobic groove in TAZ1, providing extensive intermolecular hydrophobic interactions that contribute to the stability of the complex. The structure of this complex provides new insights into the mechanism through which Hif-1 alpha recruits CBP/p300 in response to hypoxia.

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.

Oxygen sensors and angiogenesis

Local oxygen tension has a profound effect on the vasculature, which compensates vascular insufficiency through the induction of angiogenesis. An important mediator in this process is the hypoxia-inducible factor (HIF) complex, which is activated in hypoxic cells and increases transcription of a broad range of genes including angiogenic growth factors such as VEGF. HIF is primarily regulated through oxygen-dependent proteasomal destruction of the regulatory subunit, HIF-1 alpha or HIF-2 alpha. Regulation is through the modification of specific prolines in HIF- alpha chains which are hydroxylated by a recently identified family of enzymes which require molecular oxygen and 2-oxoglutarate as cosubstrates, and iron as a cofactor. Following modification HIF- alpha chains are captured by a ubiquitin ligase E3 complex containing the von Hippel-Lindau (VHL) tumour suppressor protein. The HIF prolyl hydroxylases (PHD enzymes) act as oxygen sensors regulating HIF, and hence angiogenesis. The PHD-HIF-VHL system provides a range of opportunities for therapeutic manipulation.

Defective carotid body function and impaired ventilatory responses to chronic hypoxia in mice partially deficient for hypoxia-inducible factor 1 alpha

To investigate whether the transcriptional activator hypoxia-inducible factor 1 (HIF-1) is required for ventilatory responses to hypoxia, we analyzed mice that were either wild type or heterozygous for a loss-of-function (knockout) allele at the Hif1a locus, which encodes the O(2)-regulated HIF-1 alpha subunit. Although the ventilatory response to acute hypoxia was not impaired in Hif1a(+/-) mice, the response was primarily mediated via vagal afferents, whereas in wild-type mice, carotid body chemoreceptors played a predominant role. When carotid bodies isolated from wild-type mice were exposed to either cyanide or hypoxia, a marked increase in sinus nerve activity was recorded, whereas carotid bodies from Hif1a(+/-) mice responded to cyanide but not to hypoxia. Histologic analysis revealed no abnormalities of carotid body morphology in Hif1a(+/-) mice. Wild-type mice exposed to hypoxia for 3 days manifested an augmented ventilatory response to a subsequent acute hypoxic challenge. In contrast, prior chronic hypoxia resulted in a diminished ventilatory response to acute hypoxia in Hif1a(+/-) mice. Thus partial HIF-1 alpha deficiency has a dramatic effect on carotid body neural activity and ventilatory adaptation to chronic hypoxia.