An essential role for p300/CBP in the cellular response to hypoxia

Oxidative stress is involved in inhibition of copper on histone acetylation in cells

Our previous study demonstrates that copper induces histone hypoacetylation by inhibiting histone acetyltransferase (HAT) activity. However, it lacks direct evidences whether copper-inhibited histone acetylation right contributes to the toxicity of copper. Exposure of human leukemia cells (HL-60) to Cu2+ resulted in cell proliferation arrest and a concentration- and time-dependent decrease of histone acetylation. At the same time, Cu2+-induced significant increase of H2O2 and O2.- generation via a concentration- and time-dependent manner too. The histone acetylation was efficiently suppressed by exogenous H2O2, and enhanced by superoxide dismutase (the scavenger of O2.-), catalase (the scavenger of H2O2) or the combination of both, indicating that Cu2+ at least partially inhibited histone acetylation through triggering oxidative stress. Further studies found that sodium butyrate, the inhibitor of histone deacetylase (HDAC), which had no obvious effect on oxidative stress but increased histone acetylation at the concentration of 50 microM, attenuated Cu2+-inhibited cell proliferation, indicating that histone acetylation inhibition is simultaneously involved in the cytotoxicity of Cu2+. Considering the important role of histone acetylation in gene transcription and regulation of cell fate, the present study may open a new door to further understand the mechanism of Cu2+-induced toxicity.

Histone hypoacetylation is involved in 1,10-phenanthroline?Cu2+-induced human hepatoma cell apoptosis

The 1,10-orthophenanthroline (OP)-Cu(2+) combination, one generally used reactive oxygen species (ROS) generation system, is known to induce cell apoptosis, but the mechanism of ROS generation in this process remains unclear. Here we found that in the presence of 5 microM Cu(2+), OP inhibited histone acetyltransferase (HAT) activity, resulting in decreased acetylation in both histone H3 and H4. This inhibition of histone acetylation and HAT activity was significantly attenuated by preventing or scavenging ROS generation with the Cu(2+) chelator of bathocuproine disulfonate, or the antioxidants of N-acetyl-cysteine and mannitol, respectively, indicating the involvement of ROS generation in OP-Cu(2+) -induced histone hypoacetylation. At the same time, this ROS generation is found to be involved in OP-Cu(2+) -induced apoptosis in human hepatoma Hep3B cells. The important role of histone hypoacetylation in the induction of apoptosis was also proven by the marked diminution of apoptosis by 100 nM trichostatin A, a specific inhibitor of histone deacetylase, or the overexpression of p300, an HAT protein. Collectively, these observations suggest that histone hypoacetylation represents one unrevealed mechanism involved in the in vivo function of OP-Cu(2+) -generated ROS, at least in their induction of cell apoptosis.

Activity of hypoxia-inducible factor 2alpha is regulated by association with the NF-kappaB essential modulator

The hypoxia-inducible factors 1alpha (HIF-1alpha) and 2alpha (HIF-2alpha) are key regulators of the transcriptional response to low oxygen and are closely related in domain architecture, DNA binding, and activation mechanisms. Despite these similarities, targeted disruption of the HIF-alpha genes in mice results in distinctly different phenotypes demonstrating nonredundancy of function, although the underlying mechanisms remain unclear. Here we report on the novel and specific interaction of HIF-2alpha, but not HIF-1alpha, with the NF-kappaB essential modulator (NEMO) using immunoprecipitation, mammalian two-hybrid, and in vitro protein interaction assays. Reporter gene assays demonstrate that this interaction specifically enhances normoxic HIF-2alpha transcriptional activity, independently of the HIF-2alpha transactivation domain, consistent with a model by which NEMO aids CBP/p300 recruitment to HIF-2alpha. In contrast, HIF-2alpha overexpression does not alter NF-kappaB signaling, suggesting that the functional consequence of the HIF-2alpha/NEMO interaction is limited to the HIF pathway. The specificity of NEMO for HIF-2alpha represents one of the few known differential protein-protein interactions between the HIF-alpha proteins, which has important implications for the activity of HIF-2alpha and is also the first postulated NF-kappaB-independent role for NEMO.

Retinoblastoma susceptibility gene product pRB activates hypoxia-inducible factor-1 (HIF-1)

Hypoxia-inducible factor-1 alpha (HIF-1alpha) constitutes a regulatory subunit of HIF-1, a major transcriptional activator of genes that coordinate physiological and pathological responses towards hypoxia. In order to identify novel interaction partners of HIF-1alpha we have applied T7 phage display system and identified a domain inherent in the retinoblastoma protein (pRB). The interaction between pRB and HIF-1alpha was confirmed by in vitro experiments and in transfected cells. Thereby, an HIF-1alpha domain spanning amino acids 530-694 was mapped to be required for pRB binding. Overexpression of pRB provoked transcriptional activation of HIF-1alpha under normoxia. Furthermore, the domain of pRB identified to bind HIF-1alpha in vitro is sufficient to cause HIF-1alpha transcriptional activation with the further notion that phosphorylation deficient pRB shows stronger HIF-1alpha transactivation. Using ChIP analysis, we show that HIF-1alpha responsive elements (HREs) are precipitated using alpha-pRB antibodies. Additionally, a functional interaction between pRB and HIF-1alpha is confirmed by showing that HIF-1alpha reverses the transcription repressor function of pRB.

Binding of Natively Unfolded HIF-1α ODD Domain to p53

Hypoxia-inducible factor-1 (HIF-1) is a heterodimeric transcription factor that plays a crucial role in mediating oxygen response in the cell. Using biophysical techniques, we characterized two fragments of the HIF-1alpha subunit, one the full-length ODD domain (residues 403-603) and the second comprising the N-TAD (N-transactivation domain) and inhibitory domain (residues 530-698). Both were unstructured in solution under physiological conditions and so belong to the family of natively unfolded proteins. The HIF-1alpha ODD domain binds weakly to the isolated p53 core domain but tightly to full-length p53 to give a complex of one HIF-1alpha ODD domain with a p53 dimer. By being unstructured, the HIF-1alpha ODD domain can thread both its binding sites through the p53 multimer and bind tightly by the "chelate effect." These results support the idea that hypoxic p53-mediated apoptosis does involve the direct binding of HIF-1alpha to p53.

HIF-1alpha: a valid therapeutic target for tumor therapy

Hypoxia plays a major role in the induction of angiogenesis during tumor development. One mechanism by which tumor cells respond to a reduced oxygen level is via the activation of hypoxia-inducible factor-1 (HIF-1). HIF-1 is an oxygen-dependent transcriptional activator that plays crucial roles in the angiogenesis of tumors and mammalian development. HIF-1 consists of a constitutively expressed HIF-1beta subunit and the highly regulated HIF-1alpha subunits. The stability and activity of HIF-1alpha are regulated by various post-translational modifications, hydroxylation, acetylation, phosphorylation and sumoyaltion. Therefore, HIF-1alpha interacts with several protein factors including PHD, pVHL, ARD-1, SUMO and p300/CBP. Under normoxia, the HIF-1alpha subunit is rapidly degraded via the von Hippel-Lindau tumor suppressor gene product (pVHL)-mediated ubiquitin/proteasome pathway. The association of pVHL and HIF-1alpha under normoxic conditions is triggered by the hydroxylation of prolines and the acetylation of lysine within a polypeptide segment known as the oxygen-dependent degradation (ODD) domain. On the contrary, under the hypoxia condition, the HIF-1alpha subunit becomes stable and interacts with coactivators such as p300/CBP to modulate its transcriptional activity. Under hypoxic conditions, HIF-1 eventually acts as a master regulator of numerous hypoxia-inducible genes. The target genes of HIF-1 are especially related to angiogenesis, cell proliferation and survival, and to glucose and iron metabolism. Moreover, it was reported that the activation of HIF-1alpha is closely associated with a variety of tumors and oncogenic pathways. Hence, the blocking of HIF-1alpha itself or the blocking of HIF-1alpha interacting proteins inhibits tumor growth. Based on these findings, HIF-1 can be a prime target for anticancer therapies. Therefore, this review summarizes the molecular mechanism of HIF-1alpha stability, the biological functions of HIF-1 and its potential applications for cancer therapies.

Role of CBP in regulating HIF-1-mediated activation of transcription

The hypoxia-inducible factor-1 (HIF-1) is a key regulator of oxygen homeostasis in the cell. We have previously shown that HIF-1alpha and the transcriptional coactivator CBP colocalize in accumulation foci within the nucleus of hypoxic cells. In our further exploration of the hypoxia-dependent regulation of HIF-1alpha function by transcriptional coactivators we observed that coexpression of SRC-1 (another important coactivator of the hypoxia response) and HIF-1alpha did not change the individual characteristic nuclear distribution patterns. Colocalization of both these proteins proved to be mediated by CBP. Biochemical assays showed that depletion of CBP from cell extracts abrogated interaction between SRC-1 and HIF-1alpha. Thus, in contrast to the current model for the assembly of complexes between nuclear hormone receptors and coactivators, the present data suggest that it is CBP that recruits SRC-1 to HIF-1alpha in hypoxic cells. We also observed that CBP, HIF-1alpha/Arnt and HIF-1alpha/CBP accumulation foci partially overlap with the hyperphosphorylated form of RNA polymerase II, and that CBP had a stabilizing effect on the formation of the complex between HIF-1alpha and its DNA-binding partner, Arnt. In conclusion, CBP plays an important role as a mediator of HIF-1alpha/Arnt/CBP/SRC-1 complex formation, coordinating the temporally and hierarchically regulated intranuclear traffic of HIF-1alpha and associated cofactors in signal transduction in hypoxic cells.

Bnip3L is induced by p53 under hypoxia, and its knockdown promotes tumor growth

p53-dependent apoptosis is a major determinant of its tumor suppressor activity and can be triggered by hypoxia. No p53 target is known to be induced by p53 or to mediate p53-dependent apoptosis during hypoxia. We report that p53 can directly upregulate expression of Bnip3L, a cell death inducer. During hypoxia, Bnip3L is highly induced in wild-type p53-expressing cells, in part due to increased recruitment of p53 and CBP to Bnip3L. Apoptosis is reduced in hypoxia-exposed cells with functional p53 following Bnip3L knockdown. In vivo, Bnip3L knockdown promotes tumorigenicity of wild-type versus mutant p53-expressing tumors. Thus, Bnip3L, capable of attenuating tumorigenicity, mediates p53-dependent apoptosis under hypoxia, which provides a novel understanding of p53 in tumor suppression.

Recruitment of Thyroid Hormone Receptor/Retinoblastoma-interacting Protein 230 by the Aryl Hydrocarbon Receptor Nuclear Translocator Is Required for the Transcriptional Response to Both Dioxin and Hypoxia

The aryl hydrocarbon receptor nuclear translocator/hypoxia-inducible factor (ARNT/HIF-1 beta) mediates an organism's response to various environmental cues, including those to chemical carcinogens, such as 2,3,7,8-tetrachlorodibenzo-rho-dioxin (TCDD or dioxin), via its formation of a functional transcription factor with the ligand activated aryl hydrocarbon receptor (AHR). Similarly, tissue responses to hypoxia are largely mediated through the HIF-1 heterodimeric transcription factor, comprising hypoxia-inducible factor-1 alpha (HIF-1 alpha) and ARNT. The latter response is essential for a metabolic switch from oxidative phosphorylation to glycolytic anaerobic metabolism as well as for angiogenesis and has been implicated as necessary for growth in many solid tumors. In this report, we demonstrate that the thyroid hormone receptor/retinoblastoma-interacting protein 230 (TRIP230) interacts directly with ARNT and is essential for both hypoxic and TCDD-mediated transcriptional responses. We initially identified TRIP230 as an ARNT-interacting protein in a yeast two-hybrid assay screen. This interaction was confirmed in mammalian cell systems using co-immunoprecipitation and in mammalian two-hybrid assays. Furthermore, TRIP230 could be recorded at sites of activated transcription of either TCDD- or hypoxia-inducible genes in a stimulus-dependent fashion by chromatin immunoprecipitation analysis. Finally, using single-cell microinjection and RNA interference assays, we demonstrate that TRIP230 is indispensable for TCDD- and hypoxia-dependent gene transcription.

p300 relieves p53-evoked transcriptional repression of hypoxia-inducible factor-1 (HIF-1)

HIF-1 (hypoxia-inducible factor-1), a heterodimeric transcription factor comprising HIF-1alpha and HIF-1beta subunits, serves as a key regulator of metabolic adaptation to hypoxia. HIF-1 activity largely increases during hypoxia by attenuating pVHL (von Hippel-Lindau protein)-dependent ubiquitination and subsequent 26 S-proteasomal degradation of HIF-1alpha. Besides HIF-1, the transcription factor and tumour suppressor p53 accumulates and is activated under conditions of prolonged/severe hypoxia. Recently, the interaction between p53 and HIF-1alpha was reported to evoke HIF-1alpha degradation. Destruction of HIF-1alpha by p53 was corroborated in the present study by using pVHL-deficient RCC4 (renal carcinoma) cells, supporting the notion of a pVHL-independent degradation process. In addition, low p53 expression repressed HIF-1 transactivation without affecting HIF-1alpha protein amount. Establishing that p53-evoked inhibition of HIF-1 reporter activity was relieved upon co-transfection of p300 suggested competition between p53 and HIF-1 for limiting amounts of the shared co-activator p300. This assumption was confirmed by showing competitive binding of in vitro transcription/translation-generated p53 and HIF-1alpha to the CH1 domain of p300 in vitro. We conclude that low p53 expression attenuates HIF-1 transactivation by competing for p300, whereas high p53 expression destroys the HIF-1alpha protein and thereby eliminates HIF-1 reporter activity. Thus once p53 becomes activated under conditions of severe hypoxia/anoxia, it contributes to terminating HIF-1 responses.

Regulation of vascular permeability factor/vascular endothelial growth factor (VPF/VEGF-A) expression in podocytes

BACKGROUND

Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF-A) is expressed constitutively in the adult glomerular podocytes at high levels; however, the regulation of its production is unclear. Recent data from podocyte-specific knockout mice suggest that VPF/VEGF-A is critical for the proper maintenance of glomerular filtration barrier and the glomerular endothelial fenestrae. We hypothesized that the glomerular basement membrane (GBM) matrix-podocyte interaction may play a role in the constitutive expression of VPF/VEGF-A in the adult glomerulus.

METHODS

VPF/VEGF-A mRNA levels in a human podocyte cell line grown in the presence of various extracellular matrices were quantitated by real-time polymerase chain reaction (PCR) experiments. VPF/VEGF-A protein levels in the culture supernatant from the same conditions were measured by enzyme-linked immunosorbent assay (ELISA). Promoter activity of VPF/VEGF-A gene in these cells was performed by transfecting full length (2.6 kb) VPF/VEGF-A promoter, which is fused with luciferase reporter gene. Immunoprecipitation and Western blot experiments were carried out in order to detect the association of hypoxia-inducible factor-alpha (HIF-alpha) and p300 in podocyte cells.

RESULTS

In this study, we provide preliminary evidence that signaling through the extracellular matrix proteins and, in particular, laminin and its receptor alpha(3)beta(1) integrin may regulate VPF/VEGF-A production in cultured podocytes in vitro. We also present data that increased activity of the transcription factor HIF-alphas in podocyte is not related to hypoxia and may lead to up-regulation of VPF/VEGF-A transcription. The classical type protein kinase C (PKC) may be a potential intermediate signaling molecule in this event.

CONCLUSION

These data suggest a novel nonhypoxic regulation of VPF/VEGF-A production in the glomerulus of the kidney during physiologic states. These observations may form the basis of more elaborate studies that will finally provide the detailed signaling pathway for VPF/VEGF-A synthesis in podocytes and will help our understanding of the pathogenesis of various VPF/VEGF-A-related diseases in the glomerulus of the kidney.

High altitude hypoxia: an intricate interplay of oxygen responsive macroevents and micromolecules

Physiological responses to high altitude hypoxia are complex and involve a range of mechanisms some of which occur within minutes of oxygen deprivation while others reset a cascade of biosynthetic and physiological programs within the cellular milieu. The O2 sensitive events occur at various organisational levels in the body: at the level of organism through an increase in alveolar ventilation involving interaction of chemoreceptors, the respiratory control centers in the medulla and the respiratory muscles and the lung/chest wall systems; at tissue level through the pulmonary vascular smooth muscle constriction and coronary and cerebral vessel vasodilation leading to optimized blood flow to tissues; at cellular level through release of neurotransmitters by the glomus cells of the carotid body, secretion of erythropoietin hormone by kidney and liver cells and release of vascular growth factors by parenchymal cells in many tissues; at molecular level there is expression/activation of an array of genes redirecting the metabolic and other cellular mechanisms to achieve enhanced cell survival under hypoxic environment. Transactivation of various oxygen responsive genes is regulated by the activation of various transcriptional factors which results in expression of genes in a highly coordinated manner. There is thus an intricate cascading interplay of biochemical pathways in response to hypoxia, which causes changes at the physiological and molecular levels. Added to this interplay is the possibility of genetic polymorphism and protein changes to adapt to environmental influences, which may allow a variability in the activity of the pathway. Our understanding of these interactions is growing and one may be close to the precise combination of genetic factors and protein factors that underlie the mechanism of what goes on under high altitude hypoxic stress and who will cope at high altitude.

17-beta estradiol attenuates hypoxic induction of HIF-1alpha and erythropoietin in Hep3B cells

Hypoxia inducible factor-1 (HIF-1) is a heterodimeric transcription factor that regulates expression of several hypoxia-inducible genes, including erythropoietin (EPO), by binding to hypoxia response elements (HREs) in their promoters/enhancers. Previously, we have shown that 17-beta estradiol (E2-beta) attenuates hypoxic induction of EPO in rats. We hypothesized that this response is mediated by E2-beta-induced attenuation of HIF-1alpha activity/expression. To test this hypothesis, we performed reporter gene assays in Hep3B cells to assess E2-beta effects on hypoxia-induced activity of a reporter gene driven by the HRE from a cloned EPO-enhancer element. Immunocytochemistry and Western blots were additionally used to determine effects of E2-beta on hypoxic increases in HIF-1alpha and EPO immunoreactivity. Finally, we examined potential influences of E2-beta on HIF-1alpha mRNA levels by real-time PCR. Consistent with our hypothesis, E2-beta (100 pM) inhibited hypoxic increases in HRE-mediated reporter gene activity. Furthermore, the estrogen-receptor antagonist ICI 182,780 (25 microM) eliminated these inhibitory effects of E2-beta. E2-beta similarly attenuated hypoxic induction of both EPO and HIF-1alpha protein in an estrogen-receptor dependent manner, but was without effect on HIF-1alpha mRNA expression. These findings suggest a role for E2-beta to attenuate EPO expression by interfering with hypoxic increases in HIF-1alpha protein through an estrogen receptor-dependent mechanism.

Roles of Brahma and Brahma/SWI2-related gene 1 in hypoxic induction of the erythropoietin gene

Upon hypoxia, the human erythropoietin (EPO) gene is transactivated by the heterodimeric hypoxia-inducible factor 1 (HIF-1). Mammalian SWI/SNF is a chromatin-remodeling complex involved in the modulation of gene expression. We demonstrate that Brahma (Brm) and Brahma/SWI2-related gene 1 (Brg-1), alternative ATPase subunits of SWI/SNF, potentiate reporter gene activation mediated by HIF-1 in an ATPase-dependent manner. Brm was more potent than Brg-1 in the reporter gene assays. Simultaneous depletion of both Brm and Brg-1 by small interfering RNAs significantly compromised the transcription of the endogenous EPO gene triggered by hypoxia. Whereas knocking down Brm alone resulted in a moderate reduction in transcription of the EPO gene, depletion of Brg-1 resulted in an augmentation of transcription of both the EPO gene and the Brm gene, indicating that Brm can compensate for loss of Brg-1. Chromatin immunoprecipitation (ChIP) and sequential ChIP (re-ChIP) analysis showed that both Brm and Brg-1 associate with the enhancer region of the EPO gene in vivo in a hypoxia-dependent fashion and that each is present in a complex with HIF-1. Brm and Brg-1 were also recruited to the promoter of the vascular endothelial growth factor (VEGF) gene in a hypoxia-dependent fashion, although hypoxic induction of VEGF transcription was not affected by depletions of either or both Brm and Brg-1. Together these studies reveal a novel role for SWI/SNF in the activation of transcription of the EPO gene, indicate an important communication and compensation between Brm and Brg-1, and suggest that the requirement for SWI/SNF during hypoxic induction is gene-specific.

Vascular endothelial growth factor and the nervous system

Vascular endothelial growth factor (VEGF) is an angiogenic factor essential for the formation of new blood vessels during embryogenesis and in many pathological conditions. A new role for VEGF as a neurotrophic factor has recently emerged. In the developing nervous system, VEGF plays a pivotal role not only in vascularization, but also in neuronal proliferation, and the growth of coordinated vascular and neuronal networks. After injury to the nervous system, activation of VEGF and its receptors may restore blood supply and promote neuronal survival and repair. There is a growing body of evidence that VEGF is essential for motor neurone survival, and that aberrant regulation of VEGF may play a role in the degeneration of neurones in diseases such as amyotrophic lateral sclerosis.

Strict regulation of CAIX(G250/MN) by HIF-1alpha in clear cell renal cell carcinoma

Renal cell carcinoma of the clear cell type (ccRCC) is associated with loss of functional von Hippel-Lindau (VHL) protein and high, homogeneous expression of the G250MN protein, an isoenzyme of the carbonic anhydrase family. High expression of G250MN is found in all ccRCCs, but not in most normal tissues, including normal human kidney. We specifically studied the mechanism of transcriptional regulation of the CAIXG250 gene in RCC. Previous studies identified Sp1 and hypoxia-inducible factor (HIF) as main regulatory transcription factors of G250MN in various non-RCC backgrounds. However, G250MN regulation in RCC has not been studied and may be differently regulated in view of the HIF accumulation under normoxic conditions due to VHL mutations. Transient transfection of different G250MN promoter constructs revealed strong promoter activity in G250MN -positive RCC cell lines, but no activity in G250MN -negative cell lines. DNase-I footprint and band-shift analysis demonstrated that Sp1 and HIF-1alpha proteins in nuclear extracts of RCC cells bind to the CAIX promoter and mutations in the most proximal Sp1 binding element or HIF binding element completely abolished CAIX promoter activity, indicating their critical importance for the activation of G250 expression in RCC. A close correlation between HIF-1alpha expression and G250MN expression was observed. In contrast, no relationship between HIF-2alpha expression and G250MN was seen. The participation of cofactor CBP/p300 in the regulation of G250 transcription was shown. In conclusion, HIF-1alpha and Sp1, in combination with CBP/p300, are crucial elements for G250MN expression in ccRCC, and CAIXG250 can be regarded as a unique HIF-1alpha target gene in ccRCC.

DNA damage is a prerequisite for p53-mediated proteasomal degradation of HIF-1alpha in hypoxic cells and downregulation of the hypoxia marker carbonic anhydrase IX

We investigated the relationship between the tumor suppressor p53 and the hypoxia-inducible factor-1 (HIF-1)-dependent expression of the hypoxia marker, carbonic anhydrase IX (CAIX). MCF-7 (wt p53) and Saos-2 (p53-null) cells displayed similar induction of CAIX expression and CA9 promoter activity under hypoxic conditions. Activation of p53 by the DNA damaging agent mitomycin C (MC) was accompanied by a potent repression of CAIX expression and the CA9 promoter in MCF-7 but not in Saos-2 cells. The activated p53 mediated increased proteasomal degradation of HIF-1alpha protein, resulting in considerably lower steady-state levels of HIF-1alpha protein in hypoxic MCF-7 cells but not in Saos-2 cells. Overexpression of HIF-1alpha relieved the MC-induced repression in MCF-7 cells, confirming regulation at the HIF-1alpha level. Similarly, CA9 promoter activity was downregulated by MC in HCT 116 p53(+/+) but not the isogenic p53(-/-) cells. Activated p53 decreased HIF-1alpha protein levels by accelerated proteasome-dependent degradation without affecting significantly HIF-1alpha transcription. In summary, our results demonstrate that the presence of wtp53 under hypoxic conditions has an insignificant effect on the stabilization of HIF-1alpha protein and HIF-1-dependent expression of CAIX. However, upon activation by DNA damage, wt p53 mediates an accelerated degradation of HIF-1alpha protein, resulting in reduced activation of CA9 transcription and, correspondingly, decreased levels of CAIX protein. A model outlining the quantitative relationship between p53, HIF-1alpha, and CAIX is presented.

Effect of neuronal nitric oxide synthase inhibition on CA2+/calmodulin kinase kinase and CA2+/calmodulin kinase IV activity during hypoxia in cortical nuclei of newborn piglets

The present study tests the hypothesis that cerebral tissue hypoxia results in increased Ca(2+)/calmodulin (CaM) kinase kinase activity and that the administration of nitric oxide synthase inhibitors (N-nitro-l-arginine [NNLA], or 7-nitroindazole sodium [7-NINA]) prior to the onset of hypoxia will prevent the hypoxia-induced increase in the enzyme activity. To test this hypothesis, CaM kinase kinase and CaM kinase IV activities were determined in normoxic, hypoxic, NNLA-treated hypoxic, and 7-NINA-treated hypoxic piglets. Hypoxia was induced (FiO(2)=0.05-0.08x1 h) and confirmed biochemically by tissue levels of ATP and phosphocreatine. CaM kinase kinase activity was determined in a medium containing protein kinase and phosphatase inhibitors, calmodulin, and a specifically designed CaM kinase kinase target peptide. CaM kinase IV activity was determined by (33)P-incorporation into syntide-2 in a buffer containing protein kinase and phosphatase inhibitors. Compared with normoxic animals, ATP and phosphocreatine levels were significantly lower in all hypoxic piglets whether or not pretreated with nitric oxide synthase inhibitors. There was a significant difference among CaM kinase kinase activity (pmol/mg protein/min) in normoxic (76.84+/-14.1), hypoxic (138.86+/-18.2, P<0.05 vs normoxia), NNLA-pretreated hypoxic (91.34+/-19.3; P=NS vs normoxia, P<0.05 vs hypoxia) and 7-NINA-pretreated hypoxic animals (100.12+/-23.3; P=NS vs normoxia, P<0.05 vs hypoxia). There was a significant difference among CaM kinase IV activity (pmol/mg protein/min) in normoxia (1270.80+/-126.1), hypoxia (2680.80+/-136.7; P<0.05 vs normoxia), NNLA-pretreated hypoxia (1666.00+/-154.8; P<0.05 vs normoxia, P<0.05 vs hypoxia), and 7-NINA-pretreated hypoxic (1712.9+/-231.5; P=NS vs normoxia, P<0.05 vs hypoxia). We conclude that the hypoxia-induced increase in CaM kinase kinase and CaM kinase IV activity is mediated by neuronal NOS-derived NO.

Perinatal asphyxia in the rat has lifelong effects on morphology, cognitive functions, and behavior

Perinatal asphyxia (PA) is a major determinant of neurological morbidity and mortality in the neonatal period. Many studies have been investigating neurological deficits following PA, including seizures, cerebral palsy, mental retardation, as well as psychiatric deficits. Most research performed so far has been focusing on acute or subacute sequelae and has uncovered a variety of morphological, neurochemical, behavioral, and cognitive changes following PA. However, information on long-term sequelae of animals that underwent a period of PA is scanty. Perinatally asphyxiated rats at the end of their life span present with immunohistochemical and synaptic changes as well as changes in brain protein expression. Furthermore, deficits in cognitive function tested in the Morris water maze and changes in social behavior were described. In this review, we are summarizing and discussing reported effects of global PA on morphology, cognitive functions, and behavior in rats at the end of their life span.

Involvement of histone hypoacetylation in Ni2+-induced bcl- 2 down-regulation and human hepatoma cell apoptosis

Although induction of cell apoptosis is known to be involved in the cytotoxicity of Ni(2+), little research has been aimed at the mechanism of Ni(2+)-induced apoptosis. Recent studies showed that Ni(2+) induces histone hypoacetylation in different cell lines. Since histone hypoacetylation plays important roles in the control of cell cycle progress and apoptosis, we hypothesized that histone hypoacetylation may be an unrevealed pathway in Ni(2+)-induced apoptosis. To address this, effects of Ni(2+) on cell apoptosis, bcl- 2 gene expression and histone acetylation were examined in human hepatoma Hep3B cells. We found that Ni(2+) treatment resulted in cell proliferation arrest, the appearance of detached cells, condensed chromatin, apoptotic bodies and specific DNA fragmentation, indicating the occurrence of cell apoptosis. At the same time, Ni(2+) induced a significant decrease in bcl- 2 expression and histone acetylation; the decrease of histone H4 acetylation in nucleosomes associated with the bcl- 2 promoter region was also proven by a chromatin immunoprecipitation assay, indicating the involvement of histone hypoacetylation in Ni(2+)-induced bcl- 2 down-regulation. Further studies showed that increasing histone acetylation by either 100 nM of trichostatin A or over-expressing histone acetyltranferase p300 in Hep3B cells obviously attenuated the bcl- 2 down-regulation and cell apoptosis caused by Ni(2+). Considering the importance of bcl- 2 in determining cell survival and apoptosis, the data presented here suggest that histone hypoacetylation may represent one unrevealed pathway in Ni(2+)-induced cell apoptosis, where bcl- 2 is one of its targets.

Small molecule blockade of transcriptional coactivation of the hypoxia-inducible factor pathway

Homeostasis under hypoxic conditions is maintained through a coordinated transcriptional response mediated by the hypoxia-inducible factor (HIF) pathway and requires coactivation by the CBP and p300 transcriptional coactivators. Through a target-based high-throughput screen, we identified chetomin as a disrupter of HIF binding to p300. At a molecular level, chetomin disrupts the structure of the CH1 domain of p300 and precludes its interaction with HIF, thereby attenuating hypoxia-inducible transcription. Systemic administration of chetomin inhibited hypoxia-inducible transcription within tumors and inhibited tumor growth. These results demonstrate a therapeutic window for pharmacological attenuation of HIF activity and further establish the feasibility of disrupting a signal transduction pathway by targeting the function of a transcriptional coactivator with a small molecule.

Copper induces histone hypoacetylation through directly inhibiting histone acetyltransferase activity

The abnormal accumulation of Cu2+ is closely correlated with the incidence of different diseases, such as Alzheimer's disease and Wilson disease. To study in vivo functions of Cu2+ will lead to a better understanding of the nature of these diseases. In the present study, effect of Cu2+ on histone acetylation was investigated in human hepatoma cells. Exposure of cells to Cu2+ resulted in a significant decrease of histone acetylation, as indicated by the decrease of the overall histone acetylation and the decrease of histone H3 and H4 acetylation. Since histone acetyltransferase (HAT) and histone deacetylase (HDAC) are the enzymes controlled the state of histone acetylation in vivo, we tested their contribution to the inhibition of Cu2+ on histone acetylation. One hundred nanomolar trichostatin A, the specific inhibitor of HDAC, did not attenuate the inhibitory effect of Cu2+ on histone acetylation. Combined with that Cu2+ showed no effect on the in vitro activity of HDAC, these results led to the conclusion that it is HAT, but not HDAC that is involved in Cu2+ -induced histone hypoacetylation. This conclusion was confirmed by the facts that (1) Cu2+ significantly inhibited the in vitro activity of HAT, (2) Cu2+ -treated cells possessed a lower HAT activity than control cells, and (3) 50 or 100 microM bathocuproine disulfonate, a chelator of Cu2+, significantly attenuated the inhibition of Cu2+ on HAT activity and histone acetylation in the similar pattern. Combined with that Cu2+ showed no or obvious cytotoxicity at 100 or 200 microM in human hepatoma cells, and the previous study that Cu2+ inhibits the histone H4 acetylation of yeast cells at nontoxic or toxic levels, the data presented here suggest that inhibiting histone acetylation is probably one general in vivo function of Cu2+, where HAT is its molecular target.

Oxygen-dependent and tissue-specific regulation of erythropoietin gene expression

Hypoxia-inducible expression of the gene encoding for the glycoprotein hormone erythropoietin (EPO) is the paradigm of oxygen-regulated gene expression. EPO is the main regulator of red blood cell production and more than 100 years of research on the regulation of EPO production have led to the identification of a widespread cellular oxygen sensing mechanism. Central to this signaling cascade is the transcription factor complex hypoxia-inducible factor-1 (HIF-1). Meanwhile, it is known that HIF-1 controls more than 50 oxygen-dependent genes and is now recognized as the main regulator of oxygen homoeostasis in the body. In addition to hypoxic induction, expression of the EPO gene is tightly regulated in a tissue-specific manner. During ontogeny, production of EPO required for erythropoiesis is switched from the fetal liver to the kidneys. Here EPO is mainly synthesized in adulthood. Production of EPO has also been found in organs where it has nonerythropoietic functions: EPO is important for development of the brain and is neuroprotective, whereas it stimulates angiogenesis in the reproductive tract and possibly in other organs. Understanding oxygen and tissue-specific regulation of EPO production is of high relevance for physiology. Moreover, this knowledge might be useful for new therapies to treat human diseases.

Growth factor-mediated induction of HDM2 positively regulates hypoxia-inducible factor 1alpha expression

The hypoxia-inducible factor 1 (HIF-1) transcriptional complex is regulated by cellular oxygen levels and growth factors. The phosphoinosotide 3-kinase (PI-3K)-Akt/protein kinase B (PKB) pathway has been shown to regulate HIF-1 activity in response to oncogenic signals and growth factors. We assessed whether the HDM2 oncoprotein, a direct target of Akt/PKB, could regulate HIF-1alpha expression and HIF-1 activity under normoxic conditions. We found that growth factor stimulation, overexpression of Akt/PKB, or loss of PTEN resulted in enhanced expression of both HIF-1alpha and HDM2. Growth factor-mediated induction of HIF-1alpha was ablated by transient expression of a dominant negative form of Akt/PKB or by treatment with LY294002. Transient expression of HDM2 led to increased expression of HIF-1alpha. Pulse-chase and cycloheximide experiments revealed that HDM2 did not significantly affect the half-life of HIF-1alpha. Growth factor-induced HIF-1alpha and HDM2 proteins were localized to the nucleus, and induction of both proteins was observed in both p53(+/+) and p53(-/-) HCT116 cells to comparable levels. Importantly, insulin-like growth factor 1-induced HIF-1alpha expression was observed in p53-null mouse embryo fibroblasts (MEFs) but was significantly impaired in p53 Mdm2 double-null MEFs, indicating a requirement for Mdm2 in this process. Finally, we showed that phosphorylation at Ser166 in HDM2 contributed in part to growth factor-mediated induction of HIF-1alpha. Our study has important implications for the role of the PI-3K-Akt/PKB-HDM2 pathway in tumor progression and angiogenesis.

HIFs and tumors--causes and consequences

For most organisms oxygen is essential fo life. When oxygen levels drop below those required to maintain the minimum physiological oxygen requirement of an organism or tissue it is termed hypoxia. To counter act possible deleterious effects of such a state, an immediate molecular response is initiated causing adaptation responses aimed at cell survival. This response is mediated by the hypoxia-inducible factor-1 (HIF-1), which is a heterodimer consisting of an alpha- and a beta-subunit. HIF-1 alpha protein is stabilized under hypoxic conditions and therefore confers selectivity to this response. Hypoxia is characteristic of tumors, mainly because of impaired blood supply resulting from abnormal growth. Over the past few years enormous progress has been made in the attempt to understand how the activation of the physiological response to hypoxia influences neoplastic growth. In this review some aspects of HIF-1 pathway activation in tumors and the consequences for pathophysiology and treatment of neoplasia are discussed.

Leu-574 of human HIF-1alpha is a molecular determinant of prolyl hydroxylation

Hypoxia-inducible factor (HIF)-1alpha, a master regulator of oxygen homeostasis, regulates genes crucial for cell growth and survival. In normoxia, HIF-1alpha is constantly degraded via the ubiquitin-proteasome pathway. The von Hippel-Lindau (VHL) E3 ubiquitin ligase binds HIF-1alpha through specific recognition of hydroxylated Pro-402 or Pro-564, both of which are modified by the oxygen-dependent HIF prolyl hydroxylases (PHDs/HPHs). Despite the identification of a conserved Leu-X-X-Leu-Ala-Pro motif, the molecular requirement of HIF-1alpha for PHDs/HPHs binding remains elusive. Recently, we demonstrated that Leu-574 of human HIF-1alpha--10 residues downstream of Pro-564--is essential for VHL recognition. We show here that the role of Leu-574 is to recruit PHD2/HPH2 for Pro-564 hydroxylation. An antibody specific for hydroxylated Pro-564 has been used to determine the hydroxylation status; mutation or deletion of Leu-574 results in a significant decrease in the ratio of the hydroxylated HIF-1alpha to the total amount. The nine-residue spacing between Pro-564 and Leu-574 is not obligatory for prolyl hydroxylation. Furthermore, mutation of Leu-574 disrupts the binding of PHD2/HPH2, a key prolyl hydroxylase for oxygen-dependent proteolysis of HIF-1alpha. Hence, our findings indicate that Leu-574 is essential for recruiting PHD2/HPH2, thereby providing a molecular basis for modulating HIF-1alpha activity.

Ref-1/Ape is critical for formation of the hypoxia-inducible transcriptional complex on the hypoxic response element of the rat pulmonary artery endothelial cell VEGF gene

The co-transcription factor and DNA repair enzyme, Redox effector factor-1/apurinic/apyrimidinic endonuclease (Ref-1/Ape), facilitates DNA binding and transcriptional activity of a number of transactivating factors, including those governing hypoxia-induced gene expression HIF-1. It is not known, however, whether Ref-1/Ape is a component of the hypoxic transcriptional complex. Electrophoretic mobility shift assays failed to detect direct DNA binding of Ref-1/Ape to either the HIF-1 or AP1 DNA recognition sequences present in the hypoxic response element of the VEGF gene. However, immunodepletion of Ref-1/Ape from nuclear extract prevented DNA binding of ATF/CREB and HIF-1 to the HIF-1 DNA recognition sequence. DNA affinity-precipitation analyses showed that Ref-1/Ape was part of the multiprotein transcriptional complex forming on a 64-mer sequence encompassing a minimal hypoxic response element. Immunodepletion of Ref-1/Ape prevented probe association with HIF-1, p300, ATF, and CREB. Co-immunoprecipitation experiments indicated that Ref-1/Ape present in nuclear extract interacted with HIF-1 and p300 but not ATF/CREB. However, when Ref-1/Ape was immunoprecipitated from the oligonucleotide probe, both HIF-1 and p300 remained probe-associated while ATF/CREB co-immunoprecipitated. These findings suggest that Ref-1/Ape is a critical component of the hypoxia-inducible transcriptional complex forming on the VEGF gene's hypoxic response element and that the presence of Ref-1/Ape in the complex is required for the apparent high affinity association between HIF-1 and its DNA recognition sequence.

Protein kinase C zeta transactivates hypoxia-inducible factor alpha by promoting its association with p300 in renal cancer

Hydroxylation at an asparagine residue at the COOH-terminal activation domain of hypoxia-inducible factor (HIF)-1/2 alphas is essential for its inactivation under normoxic condition. To date, the mechanism by which HIF-alpha avoids the inhibitory effect of asparagine hydroxylase in renal cell carcinoma (RCC) in normoxia is undefined. We have shown herein that protein kinase C (PKC) zeta has an important role in HIF-alpha activation in RCC. By using dominant negative mutant and small interference RNA approaches, we have demonstrated that the association between HIF-alpha and p300 is modulated by PKCzeta. Moreover, a novel signaling pathway involving phosphatidylinositol 3'-kinase and PKCzeta has been shown to be responsible for the activation of HIF-alpha by inhibiting the mRNA expression of FIH-1 (factor inhibiting HIF-1) in RCC and thereby promoting the transcription of hypoxia-inducible genes such as vascular permeability factor/vascular endothelial growth factor.

Hypoxia-induced Activation of the Retinoic Acid Receptor-related Orphan Receptor α4 Gene by an Interaction between Hypoxia-inducible Factor-1 and Sp1

Hypoxia plays a key role in the pathophysiology of many disease states, and expression of the retinoic acid receptor-related orphan receptor alpha (RORalpha) gene increases under hypoxia. We investigated the mechanism for this transient hypoxia-induced increase in RORalpha expression. Reverse transcription-coupled PCR analysis revealed that the steady-state level of mRNA for the RORalpha4 isoform, but not the RORalpha1 isoform, increased in HepG2 cells after 3 h of hypoxia. Transient transfection studies showed that the hypoxia-induced increase in RORalpha4 mRNA occurs at the transcriptional level and is dependent on a hypoxia-responsive element (HRE) located downstream of the promoter. A dominant-negative mutant of hypoxia-inducible factor-1alpha (HIF-1alpha) abrogates the transcription activated by hypoxia as well as the transcription activated by exogenously expressed HIF-1alpha, demonstrating the direct involvement of HIF-1alpha in the transcriptional activation. However, HIF-1 alone was not sufficient to activate transcription in hypoxic conditions but, rather, required Sp1/Sp3, which binds to a cluster of GC-rich sequences adjacent to the HRE. Deletion of one or more of these GC boxes reduced or eliminated the HIF-1-dependent transcription. Together, these results suggest that the hypoxia-responsive region of the RORalpha4 promoter is composed of the HRE and GC-rich sequences and that the transcriptional activation under hypoxia is conferred through the cooperation of HIF-1 with Sp1/Sp3.

Oncolytic virotherapy as a novel strategy for pancreatic cancer

We have developed a novel gene therapy that targets genetic alterations in pancreatic cancer using oncolytic replication-selective adenoviruses in tumor cells. E1B-55kDa-deleted adenovirus (AxE1AdB) can selectively replicate in TP53-deficient human cancer cells but not cells with functional TP53. Consecutive injection with AxE1AdB markedly inhibited the growth of human pancreatic tumors in severe combined immunodeficiency disease mice. Furthermore, AxE1AdB displayed the ability to enhance gene expression as a virus vector. It is reported that uracil phosphoribosyl transferase (UPRT) overcomes 5-FU resistance. The therapeutic advantage of a replication-selective adenovirus that expresses UPRT (AxE1AdB-UPRT) was thus evaluated in an intraperitoneum-disseminated tumor model. Combined treatment with 5-FU and AxE1AdB-UPRT dramatically reduced the disseminated tumor burden without causing toxicity in normal tissues. We also clarified the process of AxE1AdB-inhibited tumor angiogenesis through the preserved E1A region: an adenoviral E1A protein binds to pRB, forcing the quiescent cell into the S phase. We constructed a double-mutant, replication-selective adenovirus (AxdAdB-3) containing a mutation in the RB-binding motif of the E1A region and a deletion of large E1B-55kDa. AxdAdB-3 swiftly induced cancer cell death in vitro and showed a potent antitumor effect in vivo. These results strongly suggest that AxdAdB-3 possesses a wider therapeutic potential than previously believed, given that most pancreatic cancers have abnormalities in both the TP53 and RB pathways.

Hypoxia-Induced Activation of p38 Mitogen-Activated Protein Kinase and Phosphatidylinositol 3′-Kinase Signaling Pathways Contributes to Expression of Interleukin 8 in Human Ovarian Carcinoma Cells

PURPOSE

Overexpression of interleukin 8 (IL-8) is associated with disease progression in human ovarian cancer. Hypoxia, a common feature in solid tumors, induces IL-8 expression in human ovarian carcinoma cells through activation of nuclear factor-kappa B and activating protein-1. Here we show the upstream components of these signal transduction pathways that lead to IL-8 expression under hypoxia.

EXPERIMENTAL DESIGN

We incubated Hey-A8 human ovarian carcinoma cells under hypoxic condition (1% O(2)) and determined hypoxia regulation of phosphatidylinositol 3'-kinase (PI3K)/Akt pathway, mitogen-activated protein kinases (MAPKs), and effects of ras and vascular endothelial growth factor by Western and Northern blots, the use of specific inhibitors, in vitro kinase assays, luciferase reporter genes, and ELISA.

RESULTS

While investigating the upstream signaling pathways, we found that Akt kinase and p38 MAPK are activated by hypoxia. Both hypoxia-induced Akt and p38 MAPK functional activity, and IL-8 mRNA and protein expression were reduced with the inhibition of PI3K and p38 MAPK. Oncogenic ras overexpression resulted in an increase in the hypoxia-induced IL-8 expression, whereas the inhibition of ras by transfection of dominant-negative ras inhibited the hypoxia-induced IL-8 expression.

CONCLUSIONS

These results show that hypoxia activates ras, PI3K/Akt pathway, and p38 MAPK pathway to enhance IL-8 gene transcription under hypoxia, and suggest these signaling pathways as potential targets for controlling IL-8 expression and angiogenesis by human ovarian carcinoma cells.

Von Hippel-Lindau tumor suppressor protein and hypoxia-inducible factor in kidney cancer

The development of hereditary von Hippel-Lindau (VHL) disease and the majority of sporadic kidney cancers are due to the functional inactivation of the VHL gene. The product of the VHL gene, pVHL, in association with elongins B and C, cullin 2, and Rbx1 form an E3 ubiquitin-ligase complex VEC that targets the alpha subunits of hypoxia-inducible factor (HIF) for ubiquitination. Ubiquitin-tagged HIF-alpha proteins are subsequently degraded by the common 26S proteasome. pVHL functions as the substrate-docking interface that specifically recognizes prolyl-hydroxylated HIF-alpha. This hydroxylation occurs only in the presence of oxygen or normoxia. Thus, under hypoxia, HIF-alpha subunits are no longer subjected to degradation and are thereby able to dimerize with the common and constitutively stable beta subunits. The heterodimeric HIFs upregulate a myriad of hypoxia-inducible genes, triggering our physiologic response to hypoxia. Inappropriate accumulations of HIF-alpha in VHL disease are believed to contribute to the pathogenesis via the upregulation of several of these HIF target genes. Our current molecular understanding of the roles of HIF and pVHL in the development of VHL-associated clear-cell renal cell carcinoma (CC-RCC) is the focus of this review.

The Transcription Factors HIF-1 and HNF-4 and the Coactivator p300 Are Involved in Insulin-regulated Glucokinase Gene Expression via the Phosphatidylinositol 3-Kinase/Protein Kinase B Pathway

Glucokinase plays a key role in the regulation of glucose utilization in liver and its expression is strongly enhanced by insulin and modulated by venous pO(2). In primary rat hepatocytes, pO(2) modulated insulin-dependent glucokinase (GK) gene expression was abolished by wortmannin an inhibitor of phosphatidylinositol 3-kinase (PI3K). Transfection of vectors encoding the p110 catalytic subunit of PI3K or constitutively active protein kinase B (PKB) stimulated GK mRNA and protein expression. The transfection of GK promoter constructs together with expression vectors for p110 or constitutively active PKB revealed that the GK promoter region -87/-80 mediates the response to PI3K/PKB. Transfection experiments and gel shift assays show that this element is able to bind hypoxia-inducible factor-1 (HIF-1) in a hypoxia- and PKB-dependent manner. The ability of HIF-1alpha to activate the GK promoter was enhanced by hepatocyte nuclear factor-4alpha (HNF-4alpha), acting via the sequence -52/-39, and by the coactivator p300. Stimulation of the GK promoter by insulin was dependent on the intact -87/-80 region and maximal stimulation was achieved when HIF-1alpha, HNF-4, and p300 were cotransfected with the -1430 GK promoter Luc construct in primary hepatocytes. Maximal stimulation of GK promoter activity by insulin was inhibited when a p300 vector was used containing a mutation within a PKB phosphorylation site. Thus, a regulatory transcriptional complex consisting of HIF-1, HNF-4, and p300 appears to be involved in insulin-dependent GK gene activation.

Hypoxia and hypoxia inducible factors (HIF) as important regulators of tumor physiology

Regions of low oxygen tension are common findings in malignant tumors and are associated with increased frequency of tumor invasion and metastasis. Indeed, the ability to initiate homeostatic responses and adapt to hypoxia, e.g. by induction of angiogenesis, represents an important and crucial aspect in solid tumor growth. A significant advance in our understanding of the hypoxia response stems from the discovery of the hypoxia inducible factors (HIF) which act as key regulators of hypoxia-induced gene expression. Both, low levels of oxygen, apparently via reduced activity of a recently identified class of 2-oxoglutarate dependent oxygenases, and various tumor specific genetic alterations synergistically act to induce the HIF system. A widespread HIF activation can be observed in a variety of malignant tumors including brain tumors. The HIF system induces adaptive responses including angiogenesis, glycolysis, and pH regulation which confer increased resistance towards the hostile tumor microenvironment. Apart from protumorigenic the wide-ranging HIF pathway is known to harbor antitumorigenic components, which may, however, be disabled by tumor specific genetic alterations. Thus, mounting evidence has identified HIF as a crucial regulator of tumor growth and progression constituting an intriguing and novel target for therapeutic intervention.

HIFs, hypoxia, and vascular development

Cellular oxygen (O2) concentrations are tightly regulated to maintain ATP levels required for metabolic reactions in the human body. Responses to changes in O2 concentrations are primarily regulated by the transcription factor hypoxia inducible factor (HIF). HIF activates transcription of genes that increase systemic O2 delivery or provide cellular metabolic adaptation under conditions of hypoxia. HIF activity is essential for embryogenesis and various processes in postnatal life, and therefore, HIF levels need to be precisely controlled. Abnormal HIF expression is related to numerous diseases of the vascular system, including heart disease, cancer, and chronic obstructive pulmonary disease.

Anoxic fibroblasts activate a replication checkpoint that is bypassed by E1a

Little is known about cell cycle regulation in hypoxic cells, despite its significance. We utilized an experimentally tractable model to study the proliferative responses of rat fibroblasts when rendered hypoxic (0.5% oxygen) or anoxic (<0.01% oxygen). Hypoxic cells underwent G1 arrest, whereas anoxic cells also demonstrated S-phase arrest due to suppression of DNA initiation. Upon reoxygenation, only those cells arrested in G1 were able to resume proliferation. The oncoprotein E1a induced p53-independent apoptosis in anoxic cells, which when suppressed by Bcl-2 permitted proliferation despite anoxia. E1a expression led to marked increases in the transcription factor E2F, and overexpression of E2F-1 allowed proliferation in hypoxic cells, although it had minimal effect on the anoxic suppression of DNA initiation. We thus demonstrate two distinct cell cycle responses to low oxygen and suggest that alterations that lead to increased E2F can overcome hypoxic G1 arrest but that additional alterations, promoted by E1a expression, are necessary for neoplastic cells to proliferate despite anoxia.

The von Hippel Lindau/Hypoxia-inducible Factor (HIF) Pathway Regulates the Transcription of the HIF-Proline Hydroxylase Genes in Response to Low Oxygen

Most of the genes induced by hypoxia are regulated by a family of transcription factors termed hypoxia-inducible factors (HIF). Under normoxic conditions, HIFalpha proteins are very unstable due to hydroxylation by a recently described family of proline hydroxylases termed EGL-Nine homologs (EGLN). Upon hydroxylation, HIFalpha is recognized by the product of the tumor suppressor vhl and targeted for proteosomal degradation. Since EGLNs require oxygen to catalyze HIF hydroxylation, this reaction does not efficiently occur under low oxygen tension. Thus, under hypoxia, HIFalpha escapes from degradation and transcribes target genes. The mRNA levels of two of the three EGLNs described to date are induced by hypoxia, suggesting that they might be novel HIF target genes; however, no proof for this hypothesis has been reported. Here we show that the induction of EGLN1 and -3 by hypoxia is found in a wide range of cell types. The basal levels of EGLN3 are always well below those of EGLN1 and EGLN2, and its induction by hypoxia is larger than that found for EGLN1. The inhibitor of transcription, actinomycin D, prevents the increase of EGLN3 mRNA induced by hypoxia, indicating that it is due to enhanced gene expression. Interestingly, EGLN1 and EGLN3 mRNAs were also triggered by EGLN inhibitors, suggesting the involvement of HIFalpha in the control of its transcription. In agreement with this possibility, pVHL-deficient cell lines, which present high HIF activity under normoxia, also showed dramatically increased normoxic levels of EGLN3. Moreover, the overexpression of an oxygen-insensitive mutant form of HIFalpha resulted in increased normoxic levels of EGLN3 mRNA. Finally, hypoxic induction of EGLNs was not observed in cells lacking functional HIFalpha.

Differential roles of hypoxia-inducible factor 1alpha (HIF-1alpha) and HIF-2alpha in hypoxic gene regulation

Transcriptional responses to hypoxia are primarily mediated by hypoxia-inducible factor (HIF), a heterodimer of HIF-alpha and the aryl hydrocarbon receptor nuclear translocator subunits. The HIF-1alpha and HIF-2alpha subunits are structurally similar in their DNA binding and dimerization domains but differ in their transactivation domains, implying they may have unique target genes. Previous studies using Hif-1alpha(-/-) embryonic stem and mouse embryonic fibroblast cells show that loss of HIF-1alpha eliminates all oxygen-regulated transcriptional responses analyzed, suggesting that HIF-2alpha is dispensable for hypoxic gene regulation. In contrast, HIF-2alpha has been shown to regulate some hypoxia-inducible genes in transient transfection assays and during embryonic development in the lung and other tissues. To address this discrepancy, and to identify specific HIF-2alpha target genes, we used DNA microarray analysis to evaluate hypoxic gene induction in cells expressing HIF-2alpha but not HIF-1alpha. In addition, we engineered HEK293 cells to express stabilized forms of HIF-1alpha or HIF-2alpha via a tetracycline-regulated promoter. In this first comparative study of HIF-1alpha and HIF-2alpha target genes, we demonstrate that HIF-2alpha does regulate a variety of broadly expressed hypoxia-inducible genes, suggesting that its function is not restricted, as initially thought, to endothelial cell-specific gene expression. Importantly, HIF-1alpha (and not HIF-2alpha) stimulates glycolytic gene expression in both types of cells, clearly showing for the first time that HIF-1alpha and HIF-2alpha have unique targets.

Interaction of the TAZ1 domain of the CREB-binding protein with the activation domain of CITED2: regulation by competition between intrinsically unstructured ligands for non-identical binding sites

The TAZ1 domain of the homologous transcriptional coactivators CREB-binding protein (CBP) and p300 forms a complex with CITED2 (CBP/p300-interacting transactivator with ED-rich tail), inhibiting the activity of the hypoxia inducible factor (HIF-1alpha) and thereby attenuating the cellular response to low tissue oxygen concentration. We report the NMR structure of the CBP TAZ1 domain bound to the activation domain of CIT-ED2. The structure of TAZ1, consisting of four alpha-helices (alpha(1)-alpha(4)) stabilized by three zinc atoms, is very similar in the CITED2 and HIF-1alpha complexes. The activation domain of CITED2 is unstructured when free and folds upon binding, forming a helix (termed alpha(A)) and an extended structure that wraps around TAZ1. The CITED2 alpha(A) helix packs in the TAZ1 alpha(1)/alpha(4) interface, a site that forms weak interactions with the poorly defined aminoterminal alpha-helix of HIF-1alpha. CITED2 and HIF-1alpha both contain a four residue motif, LP(E/Q)L, which binds in the TAZ1 alpha(1)/alpha(2)/alpha(3) junction in each complex. The carboxyl-terminal region of CITED2 forms an extended structure with hydrophobic contacts in the TAZ1 alpha(1)/alpha(3) interface in the site occupied by the HIF-1alpha alpha(B) helix. CITED2 does not bind at all to the TAZ1 site occupied by the HIF-1alpha carboxyl-terminal helix. The HIF-1alpha and CITED2 domains utilize partly overlapping surfaces of TAZ1 to achieve high affinity binding and to compete effectively with each other for interaction with CBP/p300; CITED2 and HIF-1alpha use these binding sites differently to maintain similar binding affinities in order to displace each other in a feedback loop during the hypoxic response.

Molecular cloning and characterization of a hypoxia-responsive CITED3 cDNA from grass carp

We have isolated a 1586-bp full-length CITED3 cDNA from grass carp which specifies for a cAMP-responsive element-binding protein/p300-interacting transactivator with glutamic acid (E)/aspartic acid (D)-rich C-terminal domain protein. The cDNA, designated as gcCITED3, has an open reading frame of 762 bp and encodes a protein of 253 amino acids with a predicted molecular mass of 28.3 kDa and pI of 6.4. Pairwise comparison showed that gcCITED3 shares high sequence identity with the CITED3 of zebrafish (94%), chicken (72%) and Xenopus (59%). Northern blot analysis indicated that gcCITED3 is most highly expressed and responsive to hypoxia in the carp kidney. Hypoxic induction was also observed in heart, albeit at a lower level. This is the first report on the isolation of a hypoxia-responsive CITED3 gene from fish.

Overexpression of hypoxia-inducible-factor 1alpha(HIF-1alpha) in oesophageal squamous cell carcinoma correlates with lymph node metastasis and pathologic stage

The purpose of this study is to investigate the clinical and histopathologic significance of hypoxia-inducible-factor 1alpha (HIF-1alpha) expression in oesophageal squamous cell carcinoma. One hundred and thirty surgically resected specimens of OSCC were immunohistochemically assessed for HIF-1alpha expression with monoclonal antibody. High HIF-1alpha immunostaining was detected in 40 specimens. The percentage of high HIF-1alpha expression cases increased with tumour stage according to pTNM system. High HIF-1alpha expression correlated with pTNM stage, depth of tumour invasion, lymph node metastasis, distant metastasis, lymphatic invasion and positive surgical margin. The overall survival rate was worse in patients with high HIF-1alpha pattern than in patients with low-expression pattern. Univariate analyses identified high HIF-1alpha positivity, depth of tumour invasion, lymph node metastasis, distant metastasis, lymphatic invasion, and a positive surgical margin as risk factors. Multivariate analyses indicated that depth of tumour invasion, lymph node metastasis and positive surgical margin, but not HIF-1alpha, were independent prognostic factors. Survival in patients with a high HIF-1alpha expression was significantly worse than in those with low expression in patient treated with adjuvant therapy.

The forkhead transcription factor FOXO4 induces the down-regulation of hypoxia-inducible factor 1 alpha by a von Hippel-Lindau protein-independent mechanism

Tumors utilize hyperactivation of the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway to cope with deleterious environmental conditions. Activation of the PI3K/AKT pathway has been shown to increase protein expression of the alpha subunit of the hypoxia-inducible factor (HIF) 1, a key regulator of oxygen homeostasis. Elevated levels of HIF-1 alpha induce expression of genes with critical roles in angiogenesis, erythropoiesis, and glucose metabolism, processes that are essential for tumor expansion. Here we examine the involvement of FOXO4 (also known as AFX), a member of the forkhead transcription factor superfamily that is negatively regulated by the PI3K/AKT pathway, in the regulation of HIF-1 alpha protein expression. Nuclear expression of FOXO4 results in the suppression of various responses to hypoxia, including decreased vascular endothelial growth factor, glucose transporter 1, and erythropoietin expression. Interestingly, FOXO4 down-regulates the HIF-1 alpha protein levels, consistent with the lack of hypoxia responsiveness. Previous results have revealed a role for prolyl hydroxylation and resultant von Hippel-Lindau protein (pVHL) interactions in the ubiquitin-proteasome-mediated degradation of HIF-1 alpha. However, neither inhibition of prolyl hydroxylases nor mutation of HIF-1 alpha-hydroxylated prolines involved with pVHL-mediated binding inhibits the observed FOXO4-mediated down-regulation of HIF-1 alpha. These results suggest a novel alternate mechanism for hypoxic regulation that is dependent upon the level of activation of FOXO4-mediated transcription.

Nickel-induced histone hypoacetylation: the role of reactive oxygen species

The carcinogenicity of specific insoluble nickel compounds is mainly due to their intracellular generation of Ni2+ ion and its suppression on gene transcription, while the inhibition of Ni2+ on histone acetylation plays an important role in the suppression or silencing of genes. Recent studies on Ni2+ and histone H4 acetylation suggest that Ni2+ inhibits the acetylation of histone H4 through binding with its N-terminal histidine-18. It is well known that bound Ni2+ readily produces reactive oxygen species (ROS) in vivo, a critical factor inversely related with the occurrence of resistance of mammalian cells to Ni2+. Thus, we tried to find the possible role of ROS in the induction of Ni2+ on histone acetylation in the present study. We found that a high concentration of Ni2+ (no less than 600 microM) caused a significant decrease of histone acetylation in human hepatoma cells. This inhibition was shown to result mainly from the influence of Ni2+ on the overall histone acetyltransferase (HAT) activity indicated by the histone acetylation assay with the presence of a specific histone deacetylase (HDAC) inhibitor, trichostatin A (TSA). The in vitro HAT and HDAC assays further confirmed this result. At the same time, we found that the exposure of hepatoma cells to Ni2+ generated ROS. Coadministration of hydrogen peroxide with Ni2+ generated more ROS and more histone acetylation inhibition. Addition of the antioxidants 2-mercaptoethanol (2-ME) at 2 mM or N-acetyl-cysteine (NAC) at 1 mM, with Ni2+ together, completely suppressed ROS generation and significantly diminished the induced histone hypoacetylation. The data presented here prove that the ROS generation plays a role in the inhibition of histone acetylation, and, hence, the gene suppression and carcinogenesis caused by Ni2+ exposure, providing a new door for us to continuously understand the mechanism of ROS in the carcinogenicity of Ni2+ and the resistance of mammalian cells to Ni2+.

HIF-1 in cell cycle regulation, apoptosis, and tumor progression

Cells in a low oxygen, or hypoxic, microenvironment must have the ability to sense oxygen levels in the nucleus in order to maintain oxygen homeostasis by gene regulation. Hypoxia inducible factor-1 (HIF-1) serves as a molecular bridge between the sensation and utilization of oxygen, and thus functions as a key player in oxygen homeostasis. HIF-1 is a heterodimeric transcription factor and is composed of two subunits, the oxygen-sensitive HIF-1alpha and constitutively expressed HIF-1beta. HIF-1 regulates the expression of a broad range of genes that facilitate acclimation to low oxygen conditions by changes in protein levels in circulation, metabolism, and proliferation. Appropriate temporal and spatial activation of HIF-1 is crucial not only in developmental and physiological processes, characterized by programmed cellular proliferation, but also in pathophysiological conditions such as tumorigenesis, which exhibit unregulated cellular proliferation. However, many contradictory reports as to the role of HIF-1 in the regulation of cellular proliferation have been put forward in recent years. In this review, our first aim is to summarize the current knowledge of oxygen-dependent HIF-1 activation mechanisms based on its structure. Then we will describe the proposed mechanisms through which HIF-1 regulates cellular proliferation of different cell types, including tumor cells as well as non-transformed, nonimmortalized cells under normoxic and hypoxic conditions.