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

A node between proliferation, apoptosis, and growth arrest

Paradoxically, oncogenes and growth factors can induce proliferation and promote cellular survival but can also cause apoptosis and growth arrest. What determines whether a cell decides to proliferate, arrest growth, or die? Mitogens and activators of mitogen-activated pathways initiate the simultaneous production of proliferative (cyclins) and anti-proliferative (CDK inhibitors such as p21WAF1/CIP1) signals. Quiescent cells may respond to these signals by proliferation whereas proliferating cells may respond by growth arrest. Although pro-apoptotic oncoproteins, which constitute the downstream pathway (cyclin D, E2F, c-myc) directly induce proliferation, the activation of the upstream steps (growth factor receptors, Ras, cytoplasmic kinases) is required to prevent apoptosis.

p300/cAMP-responsive element-binding protein interactions with ets-1 and ets-2 in the transcriptional activation of the human stromelysin promoter

In this paper we show that transcription factors Ets-1 and Ets-2 recruit transcription adapter proteins p300 and CBP (cAMP-responsive element-binding protein) during the transcriptional activation of the human stromelysin promoter, which contains palindromic Ets-binding sites. Ets-2 and p300/CBP exist as a complex in vivo. Two regions of p300/CBP between amino acids (a.a.) 328 and 596 and a. a. 1678 and 2370 independently can interact with Ets-1 and Ets-2 in vitro and in vivo. Both these regions of p300/CBP bind to the transactivation domain of Ets-2, whereas the C-terminal region binds only to the DNA binding domain of Ets-2. The N- and the C-terminal regions of CBP (a.a. 1-1097 and 1678-2442, respectively) which lack histone acetylation activity independently are capable of coactivating Ets-2. Other Ets family transcription factors failed to cooperate with p300/CBP in stimulating the stromelysin promoter. The LXXLL sequence, reported to be important in receptor-coactivator interactions, does not appear to play a role in the interaction of Ets-2 with p300/CBP. Previous studies have shown that the stimulation of transcriptional activation activity of Ets-2 requires phosphorylation of threonine 72 by the Ras/mitogen-activated protein kinase signaling pathway. We show that mutation of this site does not affect its capacity to bind to and to cooperate with p300/CBP.

Models for oxygen sensing in yeast: implications for oxygen-regulated gene expression in higher eucaryotes

Adaptation to changes in oxygen tension in cells, tissues, and organisms depends on changes in the level of expression of a large and diverse set of proteins. It is likely that most cells and tissues possess an oxygen sensing apparatus and signal transduction pathways for regulating expression of oxygen-responsive genes. Although progress has been made in understanding the transcriptional machinery involved in oxygen-regulated gene expression of eucaryotic genes the underlying mechanism(s) of oxygen sensing and the signaling pathways that connect oxygen sensor(s) to the transcription machinery of eucaryotes are still poorly understood. The yeast Saccharomyces cerevisiae is ideal for addressing these problems. Indeed, it is well-suited for broadly based studies on oxygen sensing at the cellular level because it lends itself well to genetic and biochemical studies and because its genome has been completely sequenced. This review focuses on oxygen-regulated gene expression and current models for oxygen sensing in this yeast and then considers their applicability for understanding oxygen sensing in mammals.

Molecular mechanisms of transcription activation by HLF and HIF1alpha in response to hypoxia: their stabilization and redox signal-induced interaction with CBP/p300

Hypoxia-inducible factor 1 alpha (HIF1alpha) and its related factor, HLF, activate expression of a group of genes such as erythropoietin in response to low oxygen. Transfection analysis using fusion genes of GAL4DBD with various fragments of the two factors delineated two transcription activation domains which are inducible in response to hypoxia and are localized in the C-terminal half. Their sequences are conserved between HLF and HIF1alpha. One is designated NAD (N-terminal activation domain), while the other is CAD (C-terminal activation domain). Immunoblot analysis revealed that NADs, which were rarely detectable at normoxia, became stabilized and accumulated at hypoxia, whereas CADs were constitutively expressed. In the mammalian two-hybrid system, CAD and NAD baits enhanced the luciferase expression from a reporter gene by co-transfection with CREB-binding protein (CBP) prey, whereas CAD, but not NAD, enhanced beta-galactosidase expression in yeast by CBP co-expression, suggesting that NAD and CAD interact with CBP/p300 by a different mechanism. Co-transfection experiments revealed that expression of Ref-1 and thioredoxin further enhanced the luciferase activity expressed by CAD, but not by NAD. Amino acid replacement in the sequences of CADs revealed a specific cysteine to be essential for their hypoxia-inducible interaction with CBP. Nuclear translocation of thioredoxin from cytoplasm was observed upon reducing O2 concentrations.

Inhibition of hypoxia-inducible factor 1 activation by carbon monoxide and nitric oxide. Implications for oxygen sensing and signaling

It has been proposed that cells sense hypoxia by a heme protein, which transmits a signal that activates the heterodimeric transcription factor hypoxia-inducible factor 1 (HIF-1), thereby inducing a number of physiologically relevant genes such as erythropoietin (Epo). We have investigated the mechanism by which two heme-binding ligands, carbon monoxide and nitric oxide, affect oxygen sensing and signaling. Two concentrations of CO (10 and 80%) suppressed the activation of HIF-1 and induction of Epo mRNA by hypoxia in a dose-dependent manner. In contrast, CO had no effect on the induction of HIF-1 activity and Epo expression by either cobalt chloride or the iron chelator desferrioxamine. The affinity of CO for the putative sensor was much lower than that of oxygen (Haldane coefficient, approximately 0.5). Parallel experiments were done with 100 microM sodium nitroprusside, a nitric oxide donor. Both NO and CO inhibited HIF-1 DNA binding by abrogating hypoxia-induced accumulation of HIF-1alpha protein. Moreover, both NO and CO specifically targeted the internal oxygen-dependent degradation domain of HIF-1alpha, and also repressed the C-terminal transactivation domain of HIF-1alpha. Thus, NO and CO act proximally, presumably as heme ligands binding to the oxygen sensor, whereas desferrioxamine and perhaps cobalt appear to act at a site downstream.

Functional role of p35srj, a novel p300/CBP binding protein, during transactivation by HIF-1

Recruitment of p300/CBP by the hypoxia-inducible factor, HIF-1, is essential for the transcriptional response to hypoxia and requires an interaction between the p300/CBP CH1 region and HIF-1alpha. A new p300-CH1 interacting protein, p35srj, has been identified and cloned. p35srj is an alternatively spliced isoform of MRG1, a human protein of unknown function. Virtually all endogenous p35srj is bound to p300/CBP in vivo, and it inhibits HIF-1 transactivation by blocking the HIF-1alpha/p300 CH1 interaction. p35srj did not affect transactivation by transcription factors that bind p300/CBP outside the CH1 region. Endogenous p35srj is up-regulated markedly by the HIF-1 activators hypoxia or deferoxamine, suggesting that it could operate in a negative-feedback loop. In keeping with this notion, a p300 CH1 mutant domain, defective in HIF-1 but not p35srj binding, enhanced endogenous HIF-1 function. In hypoxic cells, p35srj may regulate HIF-1 transactivation by controlling access of HIF-1alpha to p300/CBP, and may keep a significant portion of p300/CBP available for interaction with other transcription factors by partially sequestering and functionally compartmentalizing cellular p300/CBP.

Signal transduction in hypoxic cells: inducible nuclear translocation and recruitment of the CBP/p300 coactivator by the hypoxia-inducible factor-1alpha

In response to decreased cellular oxygen concentrations the basic helix-loop-helix (bHLH)/PAS (Per, Arnt, Sim) hypoxia-inducible transcription factor, HIF-1alpha, mediates activation of networks of target genes involved in angiogenesis, erythropoiesis and glycolysis. Here we demonstrate that the mechanism of activation of HIF-1alpha is a multi-step process which includes hypoxia-dependent nuclear import and activation (derepression) of the transactivation domain, resulting in recruitment of the CREB-binding protein (CBP)/p300 coactivator. Inducible nuclear accumulation was shown to be dependent on a nuclear localization signal (NLS) within the C-terminal end of HIF-1alpha which also harbors the hypoxia-inducible transactivation domain. Nuclear import of HIF-1alpha was inhibited by either deletion or a single amino acid substitution within the NLS sequence motif and, within the context of the full-length protein, these mutations also resulted in inhibition of the transactivation activity of HIF-1alpha and recruitment of CBP. However, nuclear localization per se was not sufficient for transcriptional activation, since fusion of HIF-1alpha to the heterologous GAL4 DNA-binding domain generated a protein which showed constitutive nuclear localization but required hypoxic stimuli for function as a CBP-dependent transcription factor. Thus, hypoxia-inducible nuclear import and transactivation by recruitment of CBP can be functionally separated from one another and play critical roles in signal transduction by HIF-1alpha.

cAMP-response-element-binding-protein-binding protein (CBP) and p300 are transcriptional co-activators of early growth response factor-1 (Egr-1)

Egr-1 (early-growth response factor-1) is a sequence-specific transcription factor that plays a regulatory role in the expression of many genes important for cell growth, development and the pathogenesis of disease. The transcriptional co-activators CBP (cAMP-response-element-binding-protein-binding protein) and p300 interact with sequence-specific transcription factors as well as components of the basal transcription machinery to facilitate RNA polymerase II recruitment and transcriptional initiation. Here we demonstrate a unique way in which Egr-1 physically and functionally interacts with CBP/p300 to modulate gene transcription. CBP/p300 potentiated Egr-1 mediated expression of 5-lipoxygenase (5-LO) promoter-reporter constructs, and the degree of trans-activation was proportional to the number of Egr-1 consensus binding sites present in wild-type and naturally occurring mutants of the 5-LO promoter. The N- and C-terminal domains of CBP interact with the transcriptional activation domain of Egr-1, as demonstrated by a mammalian two-hybrid assay. Direct protein-protein interactions between CBP/p300 and Egr-1 were demonstrated by glutathione S-transferase fusion-protein binding and co-immunoprecipitation/Western-blot studies. These data suggest that CBP and p300 act as transcriptional co-activators for Egr-1-mediated gene expression and that variations between individuals in such co-activation could serve as a genetic basis for variability in gene expression.

p300/MDM2 complexes participate in MDM2-mediated p53 degradation

Control of p53 turnover is critical to p53 function. E1A binding to p300/CBP translates into enhanced p53 stability, implying that these coactivator proteins normally operate in p53 turnover control. In this regard, the p300 C/H1 region serves as a specific in vivo binding site for both p53 and MDM2, a naturally occurring p53 destabilizer. Moreover, most of the endogenous MDM2 is bound to p300, and genetic analysis implies that specific interactions of p53 and MDM2 with p300 C/H1 are important steps in the MDM2-directed turnover of p53. A specific role for p300 in endogenous p53 degradation is underscored by the p53-stabilizing effect of overproducing the p300 C/H1 domain. Taken together, the data indicate that specific interactions between p300/CBP C/H1, p53, and MDM2 are intimately involved in the MDM2-mediated control of p53 abundance.

Hypoxia-inducible mammalian gene expression analyzed in vivo at a TATA-driven promoter and at an initiator-driven promoter

We have analyzed protein-DNA interactions in vivo at transcriptional control elements for two hypoxia-inducible genes in mouse hepatoma cells. The promoter for the phosphoglycerate kinase 1 (PGK1) gene contains an initiator element, but no TATA sequence, whereas the promoter for the glucose transporter 1 (Glut1) gene contains a TATA element but no initiator sequence. Our findings reveal hypoxia-inducible, Arnt-dependent occupancy of DNA recognition sites for hypoxia-inducible factor 1 (HIF-1) upstream of both target genes. The conserved recognition motif among the five recognition sites is 5'-CGTG-3'. The PGK1 promoter exhibits constitutive occupancy of a binding site for an unknown protein(s); however, we detect no protein-DNA interaction at the initiator element, in either uninduced or induced cells. The Glut1 promoter also exhibits constitutive protein binding; in addition, the TATA element exhibits partial occupancy in uninduced cells and increased occupancy under hypoxic conditions. We find no evidence for hypoxia-induced changes in chromatin structure of either gene. Time-course analyses of the Glut1 gene reveal a temporal relationship between occupancy of HIF-1 sites and TATA element occupancy. Our findings suggest that the promoters for both hypoxia-responsive genes constitutively maintain an accessible chromatin configuration and that HIF-1 facilitates transcription by recruiting and/or stabilizing a transcription factor(s), such as TFIID, at both promoters.

Mechanisms of ligand-induced aryl hydrocarbon receptor-mediated biochemical and toxic responses

The ubiquitous environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) is a member of a broad group of halogenated aromatic hydrocarbons (HAHs) that is known to induce a wide range of toxic and biochemical responses in laboratory animals and humans. The effects of HAH exposure are mediated by binding to the cytosolic aryl hydrocarbon receptor (AhR), which is expressed in a tissue- and cell type-specific manner. The AhR is a ligand-activated transcription factor belonging to the basic helix-loop-helix/Per-AhR-Arnt-Sim (bHLH/PAS) superfamily of proteins. The mechanism of induction of gene transcription by TCDD involves ligand recognition and binding by the AhR, nuclear translocation, and dimerization with the AhR cofactor, AhR nuclear translocator (Arnt). The nuclear heterodimer interacts with cognate xenobiotic responsive elements (XREs) in promoter/enhancer regions of multiple Ah-responsive genes. Subsequent changes in chromatin structure and/or interaction of the AhR complex with the basal transcriptional machinery play a significant role in AhR-mediated gene expression. Although Arnt is a necessary component of a functional nuclear AhR complex, this protein also forms transcriptionally active heterodimers with other bHLH/PAS factors, including those involved in the transcriptional response to hypoxia. Arnt is ubiquitously expressed in mammalian systems, and results from transgenic mouse studies suggest that this protein plays a vital role in early mammalian embryonic development. Similar experiments suggest that the AhR may be involved in development of various organ systems. Thus, molecular mechanistic studies of TCDD action have contributed significantly to an improved understanding of the role of at least 2 bHLH/PAS proteins, as well as organ- and tissue-specific biochemical and toxic responses to this class of environmental toxins.

CREB binding protein is a required coactivator for Smad-dependent, transforming growth factor beta transcriptional responses in endothelial cells

The transforming growth factor-beta (TGF-beta) superfamily of growth factors and cytokines has been implicated in a variety of physiological and developmental processes within the cardiovascular system. Smad proteins are a recently described family of intracellular signaling proteins that transduce signals in response to TGF-beta superfamily ligands. We demonstrate by both a mammalian two-hybrid and a biochemical approach that human Smad2 and Smad4, two essential Smad proteins involved in mediating TGF-beta transcriptional responses in endothelial and other cell types, can functionally interact with the transcriptional coactivator CREB binding protein (CBP). This interaction is specific in that it requires ligand (TGF-beta) activation and is mediated by the transcriptional activation domains of the Smad proteins. A closely related, but distinct endothelial-expressed Smad protein, Smad7, which does not activate transcription in endothelial cells, does not interact with CBP. Furthermore, Smad2,4-CBP interactions involve the COOH terminus of CBP, a region that interacts with other regulated transcription factors such as certain signal transduction and transcription proteins and nuclear receptors. Smad-CBP interactions are required for Smad-dependent TGF-beta-induced transcriptional responses in endothelial cells, as evidenced by inhibition with overexpressed 12S E1A protein and reversal of this inhibition with exogenous CBP. This report demonstrates a functional interaction between Smad proteins and an essential component of the mammalian transcriptional apparatus (CBP) and extends our insight into how Smad proteins may regulate transcriptional responses in many cell types. Thus, functional Smad-coactivator interactions may be an important locus of signal integration in endothelial cells.

Hypoxia induces phosphorylation of the cyclic AMP response element-binding protein by a novel signaling mechanism

To investigate signaling mechanisms by which hypoxia regulates gene expression, we examined the effect of hypoxia on the cyclic AMP response element-binding protein (CREB) in PC12 cells. Exposure to physiological levels of hypoxia (5% O2, approximately 50 mm Hg) rapidly induced a persistent phosphorylation of CREB on Ser133, an event that is required for CREB-mediated transcriptional activation. Hypoxia-induced phosphorylation of CREB was more robust than that induced by any other stimulus tested, including forskolin, depolarization, and osmotic stress. Furthermore, this effect was not mediated by any of the previously known signaling pathways that lead to phosphorylation of CREB, including protein kinase A, calcium/calmodulin-dependent protein kinase, protein kinase C, ribosomal S6 kinase-2, and mitogen-activated protein kinase-activated protein kinase-2. Hypoxic activation of a CRE-containing reporter (derived from the 5'-flanking region of the tyrosine hydroxylase gene) was attenuated markedly by mutation of the CRE. Thus, a physiological reduction in O2 levels induces a functional phosphorylation of CREB at Ser133 via a novel signaling pathway.

Microenvironmental control of gene expression: implications for tumor angiogenesis, progression, and metastasis

Low oxygen tension (hypoxia) is an important prognostic factor in cancer treatment because it affects tumor formation and malignant progression. Many genes governing these complex processes have been found to be oxygen regulated. This article reviews the present knowledge of hypoxia-inducible gene expression and how this affects angiogenesis, progression, and metastasis. Of particular importance are hypoxia-regulated transcription factors because they can modulate expression of countless different genes. Additional genes analyzed in some detail include those encoding angiogenic growth factors, factors controlling blood flow, and those involved in metastasis. Although hypoxia is generally perceived as a hindrance to cancer therapy, it is possibly exploitable because severe oxygen deficiency is tumor specific. Strategies aimed at using the presence of hypoxia in solid tumors include oxygen sensitive chemotherapy and gene therapy.

Regulation of transcription by hypoxia requires a multiprotein complex that includes hypoxia-inducible factor 1, an adjacent transcription factor, and p300/CREB binding protein

Molecular adaptation to hypoxia depends on the binding of hypoxia-inducible factor 1 (HIF-1) to cognate response elements in oxygen-regulated genes. In addition, adjacent sequences are required for hypoxia-inducible transcription. To investigate the mechanism of interaction between these cis-acting sequences, the multiprotein complex binding to the lactate dehydrogenase A (LDH-A) promoter was characterized. The involvement of HIF-1, CREB-1/ATF-1, and p300/CREB binding protein (CBP) was demonstrated by techniques documenting in vitro binding, in combination with transient transfections that test the in vivo functional importance of each protein. In both the LDH-A promoter and the erythropoietin 3' enhancer, formation of multiprotein complexes was analyzed by using biotinylated probes encompassing functionally critical cis-acting sequences. Strong binding of p300/CBP required interactions with multiple DNA binding proteins. Thus, the necessity of transcription factor binding sites adjacent to a HIF-1 site for hypoxically inducible transcription may be due to the requirement of p300 to interact with multiple transcription factors for high-affinity binding and activation of transcription. Since it has been found to interact with a wide range of transcription factors, p300 is likely to play a similar role in other genes, mediating interactions between DNA binding proteins, thereby activating stimulus-specific and tissue-specific gene transcription.

p53 inhibits hypoxia-inducible factor-stimulated transcription

p53 is required for hypoxia-induced apoptosis in vivo, although the mechanism by which this occurs is not known. Conversely, induction of the hypoxia-inducible factor-1 (HIF-1) transactivator stimulates transcription of a number of genes crucial to survival of the hypoxic state. Here we demonstrate that p53 represses HIF-1-stimulated transcription. Although higher levels of p53 are required to inhibit HIF than are necessary to transcriptionally activate p53 target genes, these levels of p53 are similar to those that stimulate cleavage of poly(ADP-ribose) polymerase, an early event in apoptosis. Transfection of full-length p300 stimulates both p53-dependent and HIF-dependent transcription but does not relieve p53-mediated inhibition of HIF. In contrast, a p300 fragment, which binds to p53 but not to HIF-1, prevents p53-dependent repression of HIF activity. Transcriptionally inactive p53, mutated in its DNA binding domain, retains the ability to block HIF transactivating activity, whereas a transcriptionally inactive double point mutant defective for p300 binding does not inhibit HIF. Finally, depletion of doxorubicin-induced endogenous p53 by E6 protein attenuates doxorubicin-stimulated inhibition of HIF, suggesting that a p53 level sufficient for HIF inhibition can be achieved in vivo. These data support a model in which stoichiometric binding of p53 to a HIF/p300 transcriptional complex mediates inhibition of HIF activity.

Hypoxia regulates expression of the endothelin-1 gene through a proximal hypoxia-inducible factor-1 binding site on the antisense strand

Endothelin-1 (ET-1) is a peptide hormone with potent vasoconstrictor properties that is synthesized and secreted predominantly by vascular endothelial cells. Its production is regulated by numerous stimuli including ischemia and hypoxia, and the enhanced levels that occur during myocardial ischemia may contribute to the progression of heart failure. We previously reported that ET-1 expression was induced by both hypoxia and transition metals in endothelial cells (ECs). Here we define an element in the proximal promoter of the ET-1 gene that is responsible for this induction. By using deletions and site directed mutagenesis of the human ET-1 promoter, in combination with electrophoretic gel mobility shifts and transient expression assays in human ECs, we identified an active hypoxia-inducible factor 1 (HIF-1) binding site starting at position -118 upstream of the transcription start site on the non-coding DNA strand. Mutation of this site eliminated induction by hypoxia without affecting basal (aerobic) expression, and the mutated sequence did not display hypoxia-specific binding of HIF-1.

Stabilization of wild-type p53 by hypoxia-inducible factor 1alpha

Although hypoxia (lack of oxygen in body tissues) is perhaps the most physiological inducer of the wild-type p53 gene, the mechanism of this induction is unknown. Cells may detect low oxygen levels through a haem-containing sensor protein. The hypoxic state can be mimicked by using cobalt chloride and the iron chelator desferrioxamine: like hypoxia, cobalt chloride and desferrioxamine activate hypoxia-inducible factor 1alpha (HIF-1alpha), which stimulates the transcription of several genes that are associated with hypoxia. Here we show that these treatments induce accumulation of wild-type p53 through HIF-1alpha-dependent stabilization of p53 protein. Induction of p53 does not occur in either a mutant hepatoma cell line that is unable to induce HIF-1alpha or embryonic stem cells derived from mice lacking HIF-1beta. HIF-1alpha is found in p53 immunoprecipitates from MCF7 cells that express wild-type p53 and are either hypoxic or have been exposed to desferrioxamine. Similarly, anti-haemagglutinin immunoprecipitates from lysates of normoxic PC3M cells that had been co-transfected with haemagglutinin-tagged HIF-1alpha and wild-type p53 also contain p53. Transfection of normoxic MCF7 cells with HIF-1alpha stimulates a co-transfected p53-dependent reporter plasmid and increases the amount of endogenous p53. Our results suggest that hypoxic induction of transcriptionally active wild-type p53 is achieved as a result of the stabilization of p53 by its association with HIF-1alpha.

The basic helix-loop-helix protein BETA2 interacts with p300 to coordinate differentiation of secretin-expressing enteroendocrine cells

The major epithelial cell types lining the intestine comprise a perpetually self-renewing population of cells that differentiate continuously from a stem cell in the intestinal crypts. Secretin-producing enteroendocrine cells represent a nondividing subpopulation of intestinal epithelial cells, suggesting that expression of the hormone is coordinated with cell cycle arrest during the differentiation of this cell lineage. Here we report that the basic helix-loop-helix protein BETA2 associates functionally with the coactivator, p300 to activate transcription of the secretin gene as well as the gene encoding the cyclin-dependent kinase inhibitor p21. Overexpression of BETA2 in cell lines induces both cell cycle arrest and apoptosis suggesting that BETA2 may regulate proliferation of secretin cells. Consistent with this role, we observed both reentry of normally quiescent cells into the cell cycle and disrupted cell number regulation in the small intestine of BETA2 null mice. Thus, BETA2 may function to coordinate transcriptional activation of the secretin gene, cell cycle arrest, and cell number regulation, providing one of the first examples of a transcription factor that controls terminal differentiation of cells in the intestinal epithelium.

Hypoxic stabilization of vascular endothelial growth factor mRNA by the RNA-binding protein HuR

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor whose expression is dramatically induced by hypoxia due in large part to an increase in the stability of its mRNA. Here we show that HuR binds with high affinity and specificity to the element that regulates VEGF mRNA stability by hypoxia. Inhibition of HuR expression abrogates the hypoxia-mediated increase in VEGF mRNA stability. Overexpression of HuR increases the stability of VEGF mRNA. However, this only occurs efficiently in hypoxic cells. We further show that the stabilization of VEGF mRNA can be recapitulated in vitro. Using an S-100 extract, we show that the addition of recombinant HuR stabilizes VEGF mRNA markedly. These data support the critical role of HuR in mediating the hypoxic stabilization of VEGF mRNA by hypoxia.

Hypoxia and the regulation of gene expression

The optimal delivery of oxygen to tissues is essential both to ensure adequate energy provision and to avoid the toxic effects of higher oxygen concentrations. For this to occur, organisms must be able to sense oxygen and respond to changes in oxygen tension by altering gene expression. The analysis of the regulation of erythropoiesis has provided important insights into the mechanisms of oxygen-regulated gene expression. These mechanisms have a role in the regulation of many genes, in many cell types and appear to be of relevance to many common pathologies in which disturbances of oxygen supply are central.

Identification of N-terminal minimal transactivation domain of CBP, p300 and caenorhabditis elegans homologues

CBP/p300 is a multidomain transcriptional cofactor that acts in junction with other factors to regulate transcription. To elucidate the domain function of CBP, we fused its dissected fragments to Ga14 DNA-binding domain and transfected the deletion mutants into several cell lines. First, we found that the minimal transactivation domain (MTD) at the N-terminal portion maps to between 344 and 451 aa, and shows activity in a cell-type dependent manner. Second, we cloned C. elegans homologues corresponding to the MTD by RT-PCR and identified the three related products, two of which exhibited weak transcriptional activity. Finally, by means of the yeast two hybrid screening using MTD as a bait, we cloned hypoxia-inducible factor (HIF) 1 alpha and Stat2 cDNAs. These results suggested a functional role of MTD located at the N-terminal region of CBP/p300 in connecting to transcriptional factors.

p300 functions as a coactivator for the peroxisome proliferator-activated receptor alpha

The integrator protein, p300, was demonstrated to interact with mouse peroxisome proliferator-activated receptor alpha in a ligand-enhanced manner. The PPARalpha-interacting domain of p300 was mapped to amino acids 39-117 which interacted strongly with PPARalpha but did not interact with retinoic acid receptor-gamma or retinoid X receptor-alpha. Amino acids within the carboxyl terminus of PPARalpha as well as residues within the hinge region were required for ligand-dependent interaction with p300. p300 enhanced the transcriptional activation properties of PPARalpha and, therefore, can be considered a bona fide coactivator for this nuclear receptor. These observations extend the group of p300-interacting proteins to include mPPARalpha and further characterize the molecular mechanisms of PPARalpha-mediated transcriptional regulation.

Molecular aspects of oxygen sensing in physiological adaptation to hypoxia

Oxygen is an essential substrate in aerobic metabolism for most eukaryotic organisms. Thus organisms and cells have developed numerous immediate and long-term compensatory mechanisms for dealing with oxygen deprivation. Adaptation to hypoxia at the organismal level includes reflex hyperventilation, polycythemia and angiogenesis, which lead to increased O2 delivery to the tissues. Adaptation at the cellular level involves a shift from oxidative phosphorylation to anaerobic glycolysis, increased glucose metabolism, and expression of hypoxic stress-related proteins. Regulation of many proteins participating in adaptation to hypoxia occurs at the level of gene expression. The most widespread molecular mechanism of hypoxia-dependent regulation is transcriptional induction via the binding of a transcription factor, hypoxia-inducible factor-1 (Hif-1), to the specific sequences on the regulated genes. Long-term induction of many proteins also requires an increase in mRNA stability, which is mediated by the binding of regulatory proteins to specific sequences within the mRNAs. The current theories of coupling between the O2 sensor and mechanisms controlling gene expression are discussed.

Activator-protein-1 binding potentiates the hypoxia-induciblefactor-1-mediated hypoxia-induced transcriptional activation of vascular-endothelial growth factor expression in C6 glioma cells

The endothelial cell-specific mitogen vascular-endothelial growth factor (VEGF) plays a key role in both physiological and pathological angiogenesis. The up-regulation of VEGF expression in response to reduced oxygen tension occurs through transcriptional and post-transcriptional mechanisms. To investigate the molecular mechanisms of transcriptional activation by hypoxia (1% oxygen), fine mapping of a hypoxia-responsive region of the human VEGF promoter was carried out using luciferase reporter-gene constructs in C6 glioma cells. Here, we report that the binding site of hypoxia-inducible factor 1 (HIF1) is crucial for the hypoxic induction of VEGF gene expression. However, an enhancer subfragment containing the HIF1 binding site was not sufficient to confer full hypoxia responsiveness. Addition of upstream sequences restored the full sensitivity to hypoxia induction. This potentiating effect is due to activator protein 1 binding. The 'potentiating' sequences are unable to confer hypoxia responsiveness on their own. Our results strongly suggest that in C6 glioma cells a complex array of trans-acting factors facilitates full transcriptional induction of VEGF gene expression by hypoxia.

Isolation and characterization of PBP, a protein that interacts with peroxisome proliferator-activated receptor

In an attempt to identify cofactors that could possibly influence the transcriptional activity of peroxisome proliferator-activated receptors (PPARs), we used a yeast two-hybrid system with Gal4-PPARgamma as bait to screen a mouse liver cDNA library and have identified steroid receptor coactivator-1 (SRC-1) as a PPAR transcriptional coactivator. We now report the isolation of a cDNA encoding a 165-kDa PPARgamma-binding protein, designated PBP which also serves as a coactivator. PBP also binds to PPARalpha, RARalpha, RXR, and TRbeta1, and this binding is increased in the presence of specific ligands. Deletion of the last 12 amino acids from the carboxyl terminus of PPARgamma results in the abolition of interaction between PBP and PPARgamma. PBP modestly increased the transcriptional activity of PPARgamma, and a truncated form of PBP (amino acids 487-735) acted as a dominant-negative repressor, suggesting that PBP is a genuine coactivator for PPAR. In addition, PBP contains two LXXLL signature motifs considered necessary and sufficient for the binding of several coactivators to nuclear receptors. In situ hybridization and Northern analysis showed that PBP is expressed in many tissues of adult mice, including the germinal epithelium of testis, where it appeared most abundant, and during ontogeny, suggesting a possible role for this cofactor in cellular proliferation and differentiation.

Adenovirus E1A inhibits cardiac myocyte-specific gene expression through its amino terminus

Adenovirus E1A oncoproteins inhibit muscle-specific gene expression and myogenic differentiation by suppressing the transcriptional activating functions of basic helix-loop-helix proteins. As one approach to identifying cardiac-specific gene regulatory proteins, we analyzed the functional regions of E1A proteins that are required for muscle gene repression in cardiac cells. Myocyte-specific promoters, including the alpha-actins and alpha-myosin heavy chain, were selectively and potently inhibited (>90%) by E1A, while the ubiquitously expressed beta-actin promoter was only partially ( approximately 30%) repressed; endogenous gene expression was also affected. Distinct E1A protein binding sites mediated repression of muscle-specific and ubiquitous actin promoters. E1A-mediated inhibition of beta-actin required both an intact binding site for the tumor repressor proteins pRb and p107 and a second E1A domain (residues 15-35). In contrast, cardiac-specific promoter repression required the E1A amino-terminal residues 2-36. The proximal skeletal actin promoter (3' to base pair -153) was a target for repression by E1A. Although E1A binding to p300 was not required for inhibition of either promoter, co-expression of p300 partially reversed E1A-mediated transcriptional repression. We conclude that cardiac-specific and general promoter inhibition by E1A occurs by distinct mechanisms and that cardiac-specific gene expression is modulated by cellular factors interacting with the E1A p300/CBP-binding domain.

Ca2+-dependent regulation in neuronal gene expression

Ca2+ is an important signal-transduction molecule that plays a role in many intracellular signaling pathways. Recent advances have indicated that in neurons, Ca2+-controlled signaling mechanisms cooperate in order to discriminate amongst incoming cellular inputs. Ca2+-dependent transcriptional events can thereby be made selectively responsive to bursts of synaptic activity of specific intensity or duration.

Sequencing of the human vascular endothelial growth factor (VEGF) 3' untranslated region (UTR): conservation of five hypoxia-inducible RNA-protein binding sites

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor whose mRNA expression is induced by hypoxia. This induction is due in large part to an increase in the stability of its mRNA. The RNA sequences and cognate proteins responsible for this increased stability with hypoxia are not well understood. In order to identify regions of functional importance in the 3'UTR of VEGF mRNA, we have sequenced the human VEGF 3'UTR and compared it to the rat sequence. Overall sequence homology was 82% with complete conservation of all four potential polyadenylation signals and both nonameric instability elements. Five hypoxia-inducible RNA protein-binding (HI-RPB) sites were identified by RNA electromobility shift assay (EMSA) in the human and rat genes. EMSA and competition studies suggest that these sites bind a similar or related protein complex. On average, the five sites were 95% conserved at the nucleotide level between the rat and corresponding human sequence. This conservation taken together with several previously described, independent correlations between the presence of these RNA-protein complexes and an increase in VEGF mRNA stability suggest an important functional role for these sites in mediating hypoxia-inducible VEGF mRNA stability.

Activation of hypoxia-inducible factor 1alpha: posttranscriptional regulation and conformational change by recruitment of the Arnt transcription factor

In response to hypoxia the hypoxia-inducible factor-1 (HIF-1) mediates transcriptional activation of a network of genes encoding erythropoietin, vascular endothelial growth factor, and several glycolytic enzymes. HIF-1 consists of a heterodimer of two basic helix-loop-helix PAS (Per/Arnt/Sim) proteins, HIF-1alpha and Arnt. HIF-1alpha and Arnt mRNAs are constitutively expressed and were not altered upon exposure of HeLa or HepG2 cells to hypoxia, suggesting that the activity of the HIF-1alpha-Arnt complex may be regulated by some as yet unknown posttranscriptional mechanism. In support of this model, we demonstrate here that Arnt protein levels were not increased under conditions that induce an hypoxic response in HeLa and HepG2 cells. However, under identical conditions, HIF-1alpha protein levels were rapidly and dramatically up-regulated, as assessed by immunoblot analysis. In addition, HIF-1alpha acquired a new conformational state upon dimerization with Arnt, rendering HIF-1alpha more resistant to proteolytic digestion in vitro. Dimerization as such was not sufficient to elicit the conformational change in HIF-1alpha, since truncated forms of Arnt that are capable of dimerizing with HIF-1alpha did not induce this effect. Moreover, the high affinity DNA binding form of the HIF-1alpha-Arnt complex was only generated by forms of Arnt capable of eliciting the allosteric change in conformation. In conclusion, the combination of enhanced protein levels and allosteric change by dimerization defines a novel mechanism for modulation of transcription factor activity.

Construction of a 1.2-Mb contig surrounding, and molecular analysis of, the human CREB-binding protein (CBP/CREBBP) gene on chromosome 16p13.3

In the interest of cloning and analyzing the genes responsible for two very different diseases, the Rubinstein-Taybi syndrome (RTS) and acute myeloid leukemia (AML) associated with the somatic translocation t(8;16)(p11;p13.3), we constructed a high-resolution restriction map of contiguous cosmids (contig) covering 1.2 Mb of chromosome 16p13.3. By fluorescence in situ hybridization and Southern blot analysis, we assigned all tested RTS and t(8;16) translocation breakpoints to a 100-kb region. We have previously reported exact physical locations of these 16p breakpoints, which all disrupt one gene we mapped to this interval: the CREB-binding protein (CBP or CREBBP) gene. Intriguingly, mutations in the CBP gene are responsible for RTS as well as the t(8;16)-associated AML. CBP functions as an integrator in the assembly of various multiprotein regulatory complexes and is thus necessary for transcription in a broad range of transduction pathways. We report here the cloning, physical mapping, characterization, and full cDNA nucleotide sequence of the human CBP gene.

Erythropoietin gene regulation depends on heme-dependent oxygen sensing and assembly of interacting transcription factors

Studies on erythropoietin (Epo) gene expression have been useful in investigating the mechanism by which cells and tissues sense hypoxia. Both in vivo and in Hep3B cells. Epo production is induced not only by hypoxia but also by certain transition metal (cobalt and nickel) and by iron chelation. When Hep3B cells were incubated in an iron deficient medium, Epo mRNA expression was enhanced fourfold compared to Hep3B cells in iron enriched medium. Epo induction by cobalt was inversely related to iron concentration in the medium, indicating competition between the two metals. Under hyperbaric oxygen, cobalt induction of erythropoietin mRNA was modestly suppressed while nickel induction was markedly enhanced. These recent observations support the proposal that the oxygen sensor is a heme protein in which cobalt and nickel can substitute for iron in the porphyrin ring. The up-regulation of Epo gene transcription by hypoxia depends on at least two known DNA binding transcription factors, HIF-1 and HNF-4, which bind to cognate response elements in a critical approximately 50 bp 3' enhancer. Hypoxia induces HIF-1 binding. HNF-4, an orphan nuclear receptor constitutively expressed in kidney and liver, binds downstream of HIF-1 and cooperates with HIF-1, contributing importantly to high level and perhaps tissue specific expression. The C-terminal activation domain of HNF-4 binds to the beta subunit of HIF-1. The C-terminal portion of the alpha subunit of HIF-1 binds specifically to p300, a general transcriptional activator. Hypoxic induction of the endogenous Epo gene in Hep3B cells as well as an Epo-reporter gene was fully inhibited by E1A, an adenovirus protein that binds to and inactivates p300, but only slightly by a mutant E1A that fails to bind to p300. Moreover, overexpression of p300 enhanced hypoxic induction. Thus, it is likely that in hypoxic cells, p300 or a related family member plays a critical role in forming a macromolecular assembly with HIF-1 and HNF-4, enabling transduction from the Epo 3' enhancer to the apparatus on the promoter responsible for the initiation of transcription.