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

Partial rescue of defects in Cited2-deficient embryos by HIF-1alpha heterozygosity

Hypoxia inducible factor-1 (HIF-1) initiates key cellular and tissue responses to physiological and pathological hypoxia. Evidence from in vitro and structural analyses supports a critical role for Cited2 in down-regulating HIF-1-mediated transcription by competing for binding with oxygen-sensitive HIF-1alpha to transcriptional co-activators CBP/p300. We previously detected elevated expression of HIF-1 target genes in Cited2(-/-) embryonic hearts, indicating that Cited2 inhibits HIF-1 transactivation in vivo. In this study, we show for the first time that highly hypoxic cardiac regions in mouse embryos corresponded to the sites of defects in Cited2(-/-) embryos and that defects of the outflow tract, interventricular septum, cardiac vasculature, and hyposplenia were largely rescued by HIF-1alpha haploinsufficiency. The hypoxia of the outflow tract and interventricular septum peaked at E13.5 and dissipated by E15.5 in wild-type hearts, but persisted in E15.5 Cited2(-/-) hearts. The persistent hypoxia and abnormal vasculature in the myocardium of interventricular septum in E15.5 Cited2(-/-) hearts were rescued with decreased HIF-1alpha gene dosage. Accordingly, mRNA levels of HIF-1-responsive genes were reduced in Cited2(-/-) embryonic hearts by HIF-1alpha heterozygosity. These findings suggest that a precise level of HIF-1 transcriptional activity critical for normal development is triggered by differential hypoxia and regulated through feedback inhibition by Cited2.

The Transcriptional Activity of CITED1 Is Regulated by Phosphorylation in a Cell Cycle-dependent Manner

CITED1 is the founding member of the CITED family of cofactors that are involved in regulating a wide variety of CBP/p300-dependent transcriptional responses. In the present study, we show that the phosphorylation status of CITED1 changes during the cell cycle and affects its transcriptional cofactor activity. Tryptic mapping and mutagenesis studies identified five phosphorylated serine residues in CITED1. Phosphorylation of these residues did not affect CRM1-dependent nuclear export, but did decrease CITED1 binding to p300 and inhibited CITED1-dependent transactivation of Smad4 and p300. These results suggest that CITED1 functions as a cell cycle-dependent transcriptional cofactor whose activity is regulated by phosphorylation.

Role of ETS transcription factors in the hypoxia-inducible factor-2 target gene selection

Tumor hypoxia often directly correlates with aggressive phenotype, metastasis progression, and resistance to chemotherapy. Two transcription factors [hypoxia-inducible factor-1alpha (HIF-1alpha) and HIF-2alpha] are dramatically induced in hypoxic areas and regulate the expression of genes necessary for tumor adaptation to the conditions of low oxygen; however, the relative contribution of these factors is controversial. We used RNA interference-mediated inactivation of HIF-1alpha or HIF-2alpha followed by microarray analysis to identify genes specifically regulated by either HIF-1 or HIF-2 in hypoxia. We found that, in the MCF7 cell line, the vast majority of hypoxia-responsive genes (>80%) were dependent on the presence of HIF-1alpha. However, a small group of genes were preferentially regulated by HIF-2alpha. Promoter analysis for this group of genes revealed that all of them have putative binding sites for ETS family transcription factors, and 10 of 11 HIF-2alpha-dependent genes had at least one potential hypoxia-responsive element (HRE) in proximity to an ETS transcription factor binding site. Knockdown of ELK-1, the most often represented member of ETS family, significantly reduced hypoxic induction of the HIF-2alpha-dependent genes. Physical and functional interaction between ELK-1 and HIF-2alpha were supported by coimmunoprecipitation of these two proteins, luciferase reporter assay using CITED2 promoter, and binding of ELK-1 protein to the promoters of CITED2 and WISP2 genes in proximity to a HRE. These data suggest that the choice of the target genes by HIF-1 or HIF-2 depends on availability and cooperation of HIFs with other factors recognizing their cognate elements in the promoters.

Post-transcriptional Control of Cited2 by Transforming Growth Factor β

Cited2 is a transcription factor without typical DNA binding domains. Cited2 interacts with cAMP-responsive element-binding protein-binding protein (CBP)/p300, TFAP2, Lhx2, and nuclear receptors, such as peroxisome proliferator-activated receptor and estrogen receptor to function as a transcriptional modulator. Overexpression of Cited2 in Rat1 cells leads to tumor formation in nude mice, suggesting that Cited2 is a transforming gene. Through microarray analysis, Cited2 was found to be down-regulated by transforming growth factor beta1 (TGF-beta) in various cell lines. In this study, we confirmed that both mRNA and protein levels of Cited2 are down-regulated in MDA-MB-231 breast cancer cells. Overexpression of Smad7 or knockdown of Smad4 in MDA-MB-231 cells showed that the Smad pathway is involved in the down-regulation of Cited2. Based on nuclear run-on analysis and Cited2 promoter/reporter assay, Cited2 transcription was not affected by TGF-beta, supporting that down-regulation of Cited2 by TGF-beta is most likely through post-transcriptional regulation. By using transcriptional inhibitors, we demonstrated that the turnover of Cited2 transcripts appears to be accelerated during TGF-beta stimulation. Pharmacologic inhibition of translation with cycloheximide attenuated Cited2 down-regulation by TGF-beta. We examined the expression of recombinant Cited2 gene introduced into MDA-MB-231 cells by stable transfection, and we found that mRNA containing the Cited2 protein-coding region controlled by a heterologous promoter indeed responds to TGF-beta-mediated down-regulation. Study from Cited2 deletion mutants showed that the C-terminal conserved region of Cited2 coding sequence is essential for the down-regulation. This is the first demonstration that TGF-beta-mediated down-regulation of Cited2 is post-transcriptional, through the Smad pathway, and requires the presence of its coding sequence.

Differential regulation of the transcriptional activities of hypoxia-inducible factor 1 alpha (HIF-1alpha) and HIF-2alpha in stem cells

Transcriptional responses to hypoxia are primarily mediated by hypoxia-inducible factors (HIFs), HIF-1alpha and HIF-2alpha. 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 and require distinct transcriptional cofactors. Our previous results demonstrated that HIF-1alpha and HIF-2alpha regulate distinct target genes. Here, we report that HIF-2alpha is not transcriptionally active in embryonic stem (ES) cells, as well as possible inhibition by a HIF-2alpha-specific transcriptional repressor. Using DNA microarray analysis of hypoxia-inducible genes in wild-type (WT), Hif-1alpha(-)(/)(-), and Hif-2alpha(-)(/)(-) ES cells, we show that HIF-1alpha induces a large number of both confirmed and novel hypoxia-inducible genes, while HIF-2alpha does not activate any of its previously described targets. We further demonstrate that inhibition of HIF-2alpha function occurs at the level of transcription cofactor recruitment to endogenous target gene promoters. Overexpression of WT and, notably, a DNA-binding-defective HIF-2alpha mutant restores endogenous HIF-2alpha protein activity, suggesting that ES cells express a HIF-2alpha-specific corepressor that can be titrated by overexpressed HIF-2alpha protein. HIF-2alpha repression may explain why patients with mutations in the VHL tumor suppressor gene display cancerous lesions in specific tissue types.

Regulation of angiogenesis by hypoxia-inducible factor 1

Hypoxia is an imbalance between oxygen supply and demand that occurs in cancer and in ischemic cardiovascular disease. Hypoxia-inducible factor 1 (HIF-1) was originally identified as the transcription factor that mediates hypoxia-induced erythropoietin expression. More recently, the delineation of molecular mechanisms of angiogenesis has revealed a critical role for HIF-1 in the regulation of angiogenic growth factors. In this review, we discuss the role of HIF-1 in developmental, adaptive and pathological angiogenesis. In addition, potential therapeutic interventions involving modulation of HIF-1 activity in ischemic cardiovascular disease and cancer will be discussed.

Cited2 modulates TGF-beta-mediated upregulation of MMP9

Cited (CBP/p300-interacting transactivators with glutamic acid (E)/aspartic acid (D)-rich C-terminal domain) 2, which is a CBP/p300-binding transcription co-activator without typical DNA-binding domains, has been implicated in control of cell growth and malignant transformation in Rat1 cells. In this report, we provide evidence that Cited2 is an important regulator of transforming growth factor (TGF)-beta signaling. Overexpression of Cited2 enhanced TGF-beta-mediated transcription of a Smad-Binding Element-containing luciferase reporter construct, SBE4-Luc. This may occur through a direct physical association of Cited2 with Smads 2 and 3, as supported by co-immunoprecipitation, mammalian two-hybrid and glutathione S-transferase-pull down assays. The transcription factor p300, which binds to Smad3, was shown to further enhance the interaction between Cited2 and Smad3, and the transcriptional responses of Smad3 by Cited2 in reporter assays. Cited2 enhances TGF-beta-mediated upregulation of matrix metalloproteinase 9 (MMP9) in Cited2 inducible mouse embryo fibroblasts. Overexpression of Cited2 enhanced TGF-beta-mediated MMP9 promoter reporter activity. Moreover, knockdown of Cited2 in MDA-MB-231 cells attenuated TGF-beta-mediated upregulation of MMP9 and TGF-beta-mediated cell invasion. Chromatin immunoprecipitation showed that Cited2 and Smad3 were recruited to MMP9 promoter upon TGF-beta stimulation. This is the first demonstration that Cited2 functions as a Smad3/p300-interacting transcriptional co-activator in modulating the expression of MMP9, which could affect tumor cell invasion mediated by TGF-beta.

Loss of Cited2 affects trophoblast formation and vascularization of the mouse placenta

Cited2 is widely expressed in the developing embryo and in extraembryonic tissues including the placenta. Gene expression can be induced by a number of factors; most notably by the hypoxia inducible transcription factor, HIF1, under low oxygen conditions. Cited2 encodes for a transcriptional co-factor that in vitro can act as both a positive and negative regulator of transcription. This function is due to its interaction with CBP/p300 and appears to depend on whether Cited2 enables CBP/p300 to interact with the basic transcriptional machinery, or if its binding prevents such an interaction from occurring. Here, we report a novel function for Cited2 in placenta formation, following gene deletion in mouse. In the absence of Cited2 the placenta and embryo are significantly small from 12.5 and 14.5 dpc respectively, and death occurs in utero. Cited2 null placentas have fewer differentiated trophoblast cell types; specifically there is a reduction in trophoblast giant cells, spongiotrophoblasts and glycogen cells. In addition, the fetal vasculature of the placenta is disorganised and there are fewer anastomosing capillaries. Given that Cited2 is expressed in both trophoblasts and the fetal vasculature, the observed defects fit well with the sites of gene expression. We conclude that Cited2 is required for normal placental development and vascularisation, and hence for embryo viability.

Histone deacetylase inhibitors repress the transactivation potential of hypoxia-inducible factors independently of direct acetylation of HIF-alpha

Hypoxia-inducible factors (HIFs) are heterodimeric transcription factors regulating the oxygen supply, glucose metabolism, and angiogenesis. HIF function requires the recruitment of p300/CREB-binding protein, two coactivators with histone acetyltransferase activity, by the C-terminal transactivation domain of HIF-alpha (HIF-alphaCAD). Histone deacetylase inhibitors (HDAIs) induce differentiation or apoptosis and repress tumor growth and angiogenesis, hence being explored intensively as anti-cancer agents. Using combined pharmacological, biochemical, and genetic approaches, here we show that HDAIs repress the transactivation potential of HIF-alphaCAD. This repression is independent of the function of tumor suppressors von Hippel-Lindau or p53 or the degradation of HIF-alpha. We also demonstrate the sufficiency of low concentrations of HDAIs in repression of HIF target genes in tumor cells. We further show that HDAIs induce hyperacetylation of p300 and repress the HIF-1alpha.p300 complex in vivo. In vitro acetylation analysis reveals that the p300CH1 region, but not HIF-alphaCAD, is susceptible to acetylation. Taken together, our data demonstrate that a deacetylase activity is indispensable for the transactivation potential of HIF-alphaCAD and support a model that acetylation regulates HIF function by targeting HIF-alpha.p300 complex, not by direct acetylating HIF-alpha. The demonstration that HDAIs repress both HIF-1alpha and HIF-2alpha transactivation potential independently of von Hippel-Lindau tumor suppressor and p53 function indicates that HDAIs may have biological effects in a broad range of tissues in addition to tumors.

Linking tumor cell cytotoxicity to mechanism of drug action: an integrated analysis of gene expression, small-molecule screening and structural databases

An integrated, bioinformatic analysis of three databases comprising tumor-cell-based small molecule screening data, gene expression measurements, and PDB (Protein Data Bank) ligand-target structures has been developed for probing mechanism of drug action (MOA). Clustering analysis of GI50 profiles for the NCI's database of compounds screened across a panel of tumor cells (NCI60) was used to select a subset of unique cytotoxic responses for about 4000 small molecules. Drug-gene-PDB relationships for this test set were examined by correlative analysis of cytotoxic response and differential gene expression profiles within the NCI60 and structural comparisons with known ligand-target crystallographic complexes. A survey of molecular features within these compounds finds thirteen conserved Compound Classes, each class exhibiting chemical features important for interactions with a variety of biological targets. Protein targets for an additional twelve Compound Classes could be directly assigned using drug-protein interactions observed in the crystallographic database. Results from the analysis of constitutive gene expressions established a clear connection between chemo-resistance and overexpression of gene families associated with the extracellular matrix, cytoskeletal organization, and xenobiotic metabolism. Conversely, chemo-sensitivity implicated overexpression of gene families involved in homeostatic functions of nucleic acid repair, aryl hydrocarbon metabolism, heat shock response, proteasome degradation and apoptosis. Correlations between chemo-responsiveness and differential gene expressions identified chemotypes with nonselective (i.e., many) molecular targets from those likely to have selective (i.e., few) molecular targets. Applications of data mining strategies that jointly utilize tumor cell screening, genomic, and structural data are presented for hypotheses generation and identifying novel anticancer candidates.

Current understanding of HIF in renal disease

Hypoxia-inducible factors (HIF) are ubiquitous transcription factors regulated by oxygen-dependent proteolysis, and hence rapidly mount an adaptational response to hypoxia. The HIF system is apparently more complex than initially considered in the perspective of the increasing number of HIF target genes, and the inter-relationship with various additional regulatory pathways. Regional hypoxia is believed to play a major role in renal disease. Experimental data confirm a role for HIF in renal pathophysiology. The discovery of HIF prolyl-hydroxylases as key enzymes of oxygen sensing and HIF proteolysis offer new possibilities to therapeutically target HIF. Herein, we review basic concepts of HIF regulation, and existing data on HIF activation in renal disease.

Conditional knockout mice reveal distinct functions for the global transcriptional coactivators CBP and p300 in T-cell development

The global transcriptional coactivators CREB-binding protein (CBP) and the closely related p300 interact with over 312 proteins, making them among the most heavily connected hubs in the known mammalian protein-protein interactome. It is largely uncertain, however, if these interactions are important in specific cell lineages of adult animals, as homozygous null mutations in either CBP or p300 result in early embryonic lethality in mice. Here we describe a Cre/LoxP conditional p300 null allele (p300flox) that allows for the temporal and tissue-specific inactivation of p300. We used mice carrying p300flox and a CBP conditional knockout allele (CBPflox) in conjunction with an Lck-Cre transgene to delete CBP and p300 starting at the CD4- CD8- double-negative thymocyte stage of T-cell development. Loss of either p300 or CBP led to a decrease in CD4+ CD8+ double-positive thymocytes, but an increase in the percentage of CD8+ single-positive thymocytes seen in CBP mutant mice was not observed in p300 mutants. T cells completely lacking both CBP and p300 did not develop normally and were nonexistent or very rare in the periphery, however. T cells lacking CBP or p300 had reduced tumor necrosis factor alpha gene expression in response to phorbol ester and ionophore, while signal-responsive gene expression in CBP- or p300-deficient macrophages was largely intact. Thus, CBP and p300 each supply a surprising degree of redundant coactivation capacity in T cells and macrophages, although each gene has also unique properties in thymocyte development.

Angiogenesis in gliomas: biology and molecular pathophysiology

Glioblastoma multiforme (GBM) is characterized by exuberant angiogenesis, a key event in tumor growth and progression. The pathologic mechanisms driving this change and the biological behavior of gliomas remain unclear. One mechanism may involve cooption of native blood vessels by glioma cells inducing expression of angiopoietin-2 by endothelial cells. Subsequently, vascular apoptosis and involution leads to necrosis and hypoxia. This in turn induces angiogenesis that is associated with expression of hypoxia-inducible factor (HIF)-1alpha and vascular endothelial growth factor (VEGF) in perinecrotic pseudopalisading glioma cells. Here we review the molecular and cellular mechanisms implicated in HIF-1-dependent and HIF-1-independent glioma-associated angiogenesis. In GBMs, both tumor hypoxia and genetic alterations commonly occur and act together to induce the expression of HIF-1. The angiogenic response of the tumor to HIF-1 is mediated by HIF-1-regulated target genes leading to the upregulation of several proangiogenic factors such as VEGF and other adaptive response molecules. Understanding the roles of these regulatory processes in tumor neovascularization, tumor growth and progression, and resistance to therapy will ultimately lead to the development of improved antiangiogenic therapies for GBMs.

Induction of CITED2 expression in the rat hippocampus following transient global ischemia

CITED2 is implicated in the modulating the activity of HIF-1 which is a major transcription factor involved in ischemia-related gene expression. Following transient forebrain ischemia, we found that CITED2 was induced in a subset of brain regions including dentate gyrus of the hippocampal formation and piriform cortex. Because CITED2 was not induced in cultured neurons exposed to oxygen-glucose deprivation, we concluded that hypoxia is not sufficient to trigger its induction.

The hypoxic proteome is influenced by gene-specific changes in mRNA translation

BACKGROUND AND PURPOSE

Hypoxia causes a rapid reduction in mRNA translation efficiency. This inhibition does not affect all mRNA species to the same extent and can therefore contribute significantly to hypoxia-induced differential protein expression. Our aim in this study was to characterize changes in gene expression during acute hypoxia and evaluate the contribution of regulation via mRNA translation on these changes. For each gene, the contribution of changes in mRNA abundance versus mRNA translation was determined.

MATERIALS AND METHODS

DU145 prostate carcinoma cells were exposed to 4h of hypoxia (<0.02% O2). Efficiently translated mRNAs were isolated by sedimentation through a sucrose gradient. Affymetrix microarray technology was used to evaluate both the transcriptional and translational contribution to gene expression. Results were validated by quantitative PCR.

RESULTS

One hundred and twenty genes were more than 4-fold upregulated by hypoxia in the efficiently translated fraction of mRNA, in comparison to only 76 genes at the level of transcription. Of the 50 genes demonstrating the largest changes in translation, 11 were found to be more than 2-fold over represented in the translated fraction in comparison to their overall transcriptional level. The gene with the highest translational contribution to its induction was CITED-2, which is a negative regulator of HIF-1 transcriptional activity.

CONCLUSIONS

Gene-specific regulation of mRNA translation contributes significantly to differential gene expression during hypoxia.

The ribosomal protein rpL11 associates with and inhibits the transcriptional activity of peroxisome proliferator-activated receptor-alpha

Peroxisome proliferator-activated receptor alpha (PPARalpha) is a member of the nuclear receptor superfamily whose ligands, the peroxisome proliferators (PPs), are liver tumor promoters in rodents. Interaction cloning was performed using bacterially expressed PPARalpha to identify proteins involved in PP signaling. The ribosomal protein L11 (rpL11), a component of the large 60S subunit, was identified as a PPARalpha-associated protein. Since rpL11 is a regulator of p53 and the cell cycle, the association between this protein and PPARalpha was examined in detail. PPARalpha-rpL11 interaction was confirmed using yeast and mammalian two-hybrid systems as well as in vitro pull-down assays. The association with rpL11 occurs within the D-domain (hinge-region) of PPARalpha. Unlike PPARalpha, the two closely related isoforms PPARbeta and gamma do not interact with rpL11. Cotransfection of mammalian cells with rpL11 resulted in ligand-dependent inhibition of transcriptional activity of PPARalpha. Ribosomal protein L11-mediated inhibition of gene expression is associated with decreased binding to the PPAR-response element (PPRE) DNA sequence. Release of rpL11 from the ribosome by serum deprivation or low-dose actinomycin D did not dramatically affect PPRE-driven luciferase activity when PPARalpha was overexpressed by cotransfection. However, when endogenous levels of PPARalpha are examined and rpL11 concentration is manipulated by expression by small interference RNA, the ability of peroxisome proliferator to induce PPRE-driven reporter activity and target gene mRNA is affected. These studies show that rpL11 inhibits PPARalpha activity and adds further evidence that ribosomal proteins play roles in the control of transcriptional regulation.

Integration of oxygen signaling at the consensus HRE

The hypoxia-inducible factor 1 (HIF-1) was initially identified as a transcription factor that regulated erythropoietin gene expression in response to a decrease in oxygen availability in kidney tissue. Subsequently, a family of oxygen-dependent protein hydroxylases was found to regulate the abundance and activity of three oxygen-sensitive HIFalpha subunits, which, as part of the HIF heterodimer, regulated the transcription of at least 70 different effector genes. In addition to responding to a decrease in tissue oxygenation, HIF is proactively induced, even under normoxic conditions, in response to stimuli that lead to cell growth, ultimately leading to higher oxygen consumption. The growing cell thus profits from an anticipatory increase in HIF-dependent target gene expression. Growth stimuli-activated signaling pathways that influence the abundance and activity of HIFs include pathways in which kinases are activated and pathways in which reactive oxygen species are liberated. These pathways signal to the HIF protein hydroxylases, as well as to HIF itself, by means of covalent or redox modifications and protein-protein interactions. The final point of integration of all of these pathways is the hypoxia-response element (HRE) of effector genes. Here, we provide comprehensive compilations of the known growth stimuli that promote increases in HIF abundance, of protein-protein interactions involving HIF, and of the known HIF effector genes. The consensus HRE derived from a comparison of the HREs of these HIF effectors will be useful for identification of novel HIF target genes, design of oxygen-regulated gene therapy, and prediction of effects of future drugs targeting the HIF system.

Negative and positive regulation of HIF-1: a complex network

Hypoxia inducible factor-1 (HIF-1) is as a key transcriptional mediator of the hypoxic response in eukaryotic cells, regulating the expression of a myriad of genes involved in oxygen transport, glucose uptake and glycolysis and angiogenesis. Deregulation of HIF-1 activity occurs in many human cancers, usually at the level of the HIF-1alpha subunit. HIF-1 is regulated by a variety of mechanisms including transcription, translation post-translational modification, protein-protein interaction and degradation. Our understanding of the key signalling pathways that regulate HIF-1 has significantly progressed in recent years and has highlighted the potential for targeting the HIF-1 pathway as a basis for the development of new cancer therapies.

Suppression of hypoxia-inducible factor 1alpha (HIF-1alpha) transcriptional activity by the HIF prolyl hydroxylase EGLN1

The cellular response to hypoxia is, at least in part, mediated by the transcriptional regulation of hypoxia-responsive genes involved in balancing the intracellular ATP production and consumption. Recent evidence suggests that the transcription factor, HIF-1alpha, functions as a master regulator of oxygen homeostasis by controlling a broad range of cellular events in hypoxia. In normoxia, HIF-1alpha is targeted for destruction via prolyl hydroxylation, an oxygen-dependent modification that signals for recognition by the ubiquitin ligase complex containing the von Hippel-Lindau tumor suppressor. Three HIF prolyl hydroxylases (EGLN1, EGLN2, and EGLN3) have been identified in mammals, among which EGLN1 and EGLN3 are hypoxia-inducible at their mRNA levels in an HIF-1alpha-dependent manner. In this study, we demonstrated that apart from promoting HIF-1alpha proteolysis in normoxia, EGLN1 specifically represses HIF-1alpha transcriptional activity in hypoxia. Ectopic expression of EGLN1 inhibited HIF-1alpha transcriptional activity without altering its protein levels in a von Hippel-Lindau-deficient cell line, indicating a discrete activity of EGLN1 in transcriptional repression. Conversely, silencing of EGLN1 expression augmented HIF-1alpha transcriptional activity and its target gene expression in hypoxia. Thus, we proposed that the accumulated EGLN1 in hypoxia acts as a negative-feedback mechanism to modulate HIF-1alpha target gene expression. Our finding also provided new insight into the pharmacological manipulation of the HIF prolyl hydroxylase for ischemic diseases.

Hypoxia-dependent activation of HIF into a transcriptional regulator

Adaptation to conditions of limited oxygen availability (hypoxia) is a critical determinant of cell and tissue viability in several physiological and pathophysiological conditions. The hypoxia-inducible factor (HIF) is an oxygen-sensitive transcriptional activator that, under hypoxia, upregulates the expression of genes involved in the control of glucose metabolism, angiogenesis and cellular proliferation, among others. Activation of HIF to a fully competent transcriptional regulatory protein complex is a multi-step process that involves control of protein stability, subcellular localization, DNA-binding and interaction with transcriptional coregulators. The identity, regulation and hierarchy of interactions between several components of the HIF signal transduction pathway has been the object of intense study over the past decade and will be the subject of this review. Particular emphasis is given to the process of coordinated coactivator recruitment within the cell nucleus. The implications for future development of angiogenic/antiangiogenic therapeutic strategies of HIF activation/inactivation are discussed.

Hypoxia-activated metabolic pathway stimulates phosphorylation of p300 and CBP in oxygen-sensitive cells

Transcription co-activators and histone acetyltransferases, p300 and cyclic AMP responsive element-binding protein-binding protein (CBP), participate in hypoxic activation of hypoxia-inducible genes. Here, we show that exposure of PC12 and cells to 1-10% oxygen results in hyperphosphorylation of p300/CBP. This response is fast, long lasting and specific for hypoxia, but not for hypoxia-mimicking agents such as desferioxamine or Co2+ ions. It is also cell-type specific and occurs in pheochromocytoma PC12 cells and the carotid body of rats but not in hepatoblastoma cells. The p300 hyperphosphorylation specifically depends on the release of intracellular calcium from inositol 1,4,5-triphosphate (IP3)-sensitive stores. However, it is not inhibited by pharmacological inhibitors of any of the kinases traditionally known to be directly or indirectly calcium regulated. On the other hand, p300 hyperphosphorylation is inhibited by several different inhibitors of the glucose metabolic pathway from generation of NADH by glyceraldehyde 3-phosphate dehydrogenase, through the transfer of NADH through the glycerol phosphate shuttle to ubiquinone and complex III of the mitochondrial respiratory chain. Inhibition of IP3-sensitive calcium stores decreases generation of ATP, and this inhibition is significantly stronger in hypoxia than in normoxia. We propose that the NADH glycerol phosphate shuttle participates in generating a pool of ATP that serves either as a co-factor or a modulator of the kinases involved in the phosphorylation of p300/CBP during hypoxia.

Hypoxia and the hypoxia-inducible-factor pathway in glioma growth and angiogenesis

Glioblastomas, like other solid tumors, have extensive areas of hypoxia and necrosis. The importance of hypoxia in driving tumor growth is receiving increased attention. Hypoxia-inducible factor 1 (HIF-1) is one of the master regulators that orchestrate the cellular responses to hypoxia. It is a heterodimeric transcription factor composed of alpha and beta subunits. The alpha subunit is stable in hypoxic conditions but is rapidly degraded in normoxia. The function of HIF-1 is also modulated by several molecular mechanisms that regulate its synthesis, degradation, and transcriptional activity. Upon stabilization or activation, HIF-1 translocates to the nucleus and induces transcription of its downstream target genes. Most important to gliomagenesis, HIF-1 is a potent activator of angiogenesis and invasion through its upregulation of target genes critical for these functions. Activation of the HIF-1 pathway is a common feature of gliomas and may explain the intense vascular hyperplasia often seen in glioblastoma multiforme. Activation of HIF results in the activation of vascular endothelial growth factors, vascular endothelial growth factor receptors, matrix metalloproteinases, plasminogen activator inhibitor, transforming growth factors alpha and beta, angiopoietin and Tie receptors, endothelin-1, inducible nitric oxide synthase, adrenomedullin, and erythropoietin, which all affect glioma angiogenesis. In conclusion, HIF is a critical regulatory factor in the tumor microenvironment because of its central role in promoting proangiogenic and invasive properties. While HIF activation strongly promotes angiogenesis, the emerging vasculature is often abnormal, leading to a vicious cycle that causes further hypoxia and HIF upregulation.

Cited1 Is a Bifunctional Transcriptional Cofactor That Regulates Early Nephronic Patterning

In a screen to identify factors that regulate the conversion of mesenchyme to epithelium during the early stages of nephrogenesis, it was found that the Smad4-interacting transcriptional cofactor, Cited1, is expressed in the condensed cap mesenchyme surrounding the tip of the ureteric bud (UB), is downregulated after differentiation into epithelia, and has the capacity to block UB branching and epithelial morphogenesis in cultured metanephroi. Cited1 represses Wnt/beta-catenin but activates Smad4-dependent transcription involved in TGF-beta and Bmp signaling. By modifying these pathways, Cited1 may coordinate cellular differentiation and survival signals that regulate nephronic patterning in the metanephros.

Intrinsically unstructured proteins and their functions

Many gene sequences in eukaryotic genomes encode entire proteins or large segments of proteins that lack a well-structured three-dimensional fold. Disordered regions can be highly conserved between species in both composition and sequence and, contrary to the traditional view that protein function equates with a stable three-dimensional structure, disordered regions are often functional, in ways that we are only beginning to discover. Many disordered segments fold on binding to their biological targets (coupled folding and binding), whereas others constitute flexible linkers that have a role in the assembly of macromolecular arrays.

Cited2 is required both for heart morphogenesis and establishment of the left-right axis in mouse development

Establishment of the left-right axis is a fundamental process of vertebrate embryogenesis. Failure to develop left-right asymmetry leads to incorrect positioning and morphogenesis of numerous internal organs, and is proposed to underlie the etiology of several common cardiac malformations. The transcriptional modulator Cited2 is essential for embryonic development: Cited2-null embryos die during gestation with profound developmental abnormalities, including cardiac malformations, exencephaly and adrenal agenesis. Cited2 is also required for normal establishment of the left-right axis; we demonstrate that abnormal heart looping and right atrial and pulmonary isomerism are consistent features of the left-right-patterning defect. We show by gene expression analysis that Cited2 acts upstream of Nodal, Lefty2 and Pitx2 in the lateral mesoderm,and of Lefty1 in the presumptive floor plate.

Although abnormal left-right patterning has a major impact on the cardiac phenotype in Cited2-null embryos, laterality defects are only observed in a proportion of these embryos. We have therefore used a combination of high-resolution imaging and three-dimensional (3D) modeling to systematically document the full spectrum of Cited2-associated cardiac defects. Previous studies have focused on the role of Cited2in cardiac neural crest cell development, as Cited2 can bind the transcription factor Tfap2, and thus affect the expression of Erbb3 in neural crest cells. However, we have identified Cited2-associated cardiac defects that cannot be explained by laterality or neural crest abnormalities. In particular, muscular ventricular septal defects and reduced cell density in the atrioventricular (AV) endocardial cushions are evident in Cited2-null embryos. As we found that Cited2 expression tightly correlated with these sites, we believe that Cited2 plays a direct role in development of the AV canal and cardiac septa. We therefore propose that, in addition to the previously described reduction of cardiac neural crest cells, two other distinct mechanisms contribute to the spectrum of complex cardiac defects in Cited2-null mice; disruption of normal left-right patterning and direct loss of Cited2 expression in cardiac tissues.

The hypoxia-inducible factor and tumor progression along the angiogenic pathway

The hypoxia-inducible factor (HIF) is a transcription factor that plays a key role in the response of cells to oxygen levels. HIF is a heterodimer of alpha- and beta-subunits where the alpha-subunit is translated constitutively but has a very short half-life under normal oxygen concentrations. Negative regulation of the half-life and activity of the alpha-subunit is dependent on its posttranslational hydroxylation by hydroxylases that are dependent on oxygen for activity. Thus under low oxygen (hypoxic) conditions the hydroxylases are inactive and the alpha-subunit is stable and able to interact with the beta-subunit to bind and induce transcription of target genes. Hypoxic conditions are encountered in development and in disease states such as cancer. Tumors that have outstripped their blood supply become hypoxic and express high levels of HIF. HIF in turn targets genes that induce survival, glycolysis, and angiogenesis, a form of neovascularization, which ensures the tumor with a continued supply of oxygen and nutrients for further growth.

Identification and functional analysis ofCITED2 mutations in patients with congenital heart defects

Recent reports have demonstrated that mice lacking the transcription factor Cited2 die in utero showing various cardiac malformations. We present for the first time functionally relevant mutations of CITED2 in patients with congenital heart defects (CHDs). CITED2 encodes a CREBBP/EP300 interacting transcriptional modulator of HIF1A and TFAP2. To study the potential impact of sequence variations in CITED2 for CHDs in humans, we screened a cohort of 392 well-characterized patients and 192 control individuals using DHPLC, sequencing, and Amplifluor genotyping techniques. We identified 15 CITED2 nucleotide alterations. Seven of these alterations were found only in CHD patients and were not detected in controls, including three mutations leading to alterations of the amino acid sequence (p.Ser170_Gly178del, p.Gly178_Ser179ins9, and p.Ser198_Gly199del). All three of these amino acid changing mutations cluster in the serine-glycine-rich junction of the protein, to which no functionality had heretofore been assigned. Here we show that these mutations significantly reduce the capacity of CITED2 to transrepress HIF1A, and that the p.Ser170_Gly178del mutation significantly diminishes TFAP2C coactivation. This reveals a modifying role for the serine-glycine-rich region in CITED2 function. In summary, the observation of these mutations in patients with septal defects indicates that CITED2 has a causative impact in the development of CHD in humans.

The transcriptional response to hypoxic insult controlled by FRA-2

FRA-2 is involved in cellular differentiation and is also upregulated in response to ischemic injury to the brain. To shed light on the function of this transcription factor, a novel microarray analysis was utilized to identify FRA-2-dependent gene expression increased in the hypoxic response. Genes were identified that were upregulated by exposure of neuronally differentiated PC12 cells to hypoxia. Using a dominant negative construct to block FRA-2, a second subset of genes that were FRA-2 dependent was found. Cross comparison then allowed isolation of a list of genes that were induced in response to hypoxia in a FRA-2-dependent manner. These data suggest that FRA-2 is involved in the transcriptional control of neuroprotective genes and in the switch from aerobic to anaerobic metabolism.

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.

The CBP/p300 TAZ1 domain in its native state is not a binding partner of MDM2

The transcriptional co-activator CBP [CREB (cAMP-response-element-binding protein)-binding protein] and its paralogue p300 play a key role in the regulation of both activity and stability of the tumour suppressor p53. Degradation of p53 is mediated by the ubiquitin ligase MDM2 (mouse double minute protein) and is also reported to be regulated by CBP/p300. Direct protein-protein interaction between a central domain of MDM2 and the TAZ1 (transcriptional adaptor zinc-binding domain) [C/H1 (cysteine/histidine-rich region 1)] domain of p300 and subsequent formation of a ternary complex including p53 have been reported previously. We expressed and purified the proposed binding domains of HDM2 (human homologue of MDM2) and CBP, and examined their interactions using CD spectroscopy. The binding studies were extended by using natively purified GST (glutathione S-transferase)-p300 TAZ1 and GST-p53 fusion proteins, together with in vitro translated HDM2 fragments, under similar solution conditions to those in previous studies, but omitting added EDTA, which causes unfolding and aggregation of the zinc-binding TAZ1 domain. Comparing the binding properties of the known TAZ1 interaction partners HIF-1alpha (hypoxia-inducible factor 1), CITED2 (CBP/p300-interacting transactivator with glutamic- and aspartic-rich tail) and STAT2 (signal transducer and activator of transcription 2) with HDM2, our data suggest that TAZ1 in its native state does not serve as a specific recognition domain of HDM2. Rather, unfolded TAZ1 and HDM2 proteins have a high tendency to aggregate, and non-specific protein complexes are formed under certain conditions.

Cited2 controls left-right patterning and heart development through a Nodal-Pitx2c pathway

Malformations of the septum, outflow tract and aortic arch are the most common congenital cardiovascular defects and occur in mice lacking Cited2, a transcriptional coactivator of TFAP2. Here we show that Cited2(-/-) mice also develop laterality defects, including right isomerism, abnormal cardiac looping and hyposplenia, which are suppressed on a mixed genetic background. Cited2(-/-) mice lack expression of the Nodal target genes Pitx2c, Nodal and Ebaf in the left lateral plate mesoderm, where they are required for establishing laterality and cardiovascular development. CITED2 and TFAP2 were detected at the Pitx2c promoter in embryonic hearts, and they activate Pitx2c transcription in transient transfection assays. We propose that an abnormal Nodal-Pitx2c pathway represents a unifying mechanism for the cardiovascular malformations observed in Cited2(-/-) mice, and that such malformations may be the sole manifestation of a laterality defect.

HIF-1: master and commander of the hypoxic world. A pharmacological approach to its regulation by siRNAs

The hypoxia-inducible factor-1 (HIF-1) is primarily involved in the sensing and adapting of cells to changes in the O2 level, which is essential for their viability. It is important that this critical transcription factor be tightly regulated in order for cells to respond to a wide range of O2 concentrations. HIF-1 regulation by post-translational modification is the central theme of the scenario of O2 homeostasis. The alpha subunit of HIF-1 is the principal actor while the supporting actors (PHDs, FIH-1, ARD1, CITED2, p300...) all contribute to the complexity of the grand finale. It is well established that HIF-1 expression and activation correlates with tumor progression and resistance to cancer treatments. We will introduce the different actors involved in HIF-1 regulation, and their mechanisms of action via invalidation by siRNAs and discuss therapies targeting HIF-1, to selectively kill tumor cells that adapt to low O2 concentrations.

CITED4 inhibits hypoxia-activated transcription in cancer cells, and its cytoplasmic location in breast cancer is associated with elevated expression of tumor cell hypoxia-inducible factor 1alpha

The interaction of hypoxia-inducible factor 1alpha and the CH1 domain of the transcriptional coactivator p300/CBP is necessary for the expression of hypoxia responsive genes and tumor angiogenesis. The transcription factor CITED2 binds p300/CBP at the CH1 domain and functions as a negative regulator of hypoxia signaling by competing with hypoxia-inducible factor 1alpha. CITED4, a recently identified member of the CITED family, binds p300/CBP via the CH1 domain and functions as a coactivator for transcription factor AP-2. Here, we show that CITED4 blocks the binding of hypoxia-inducible factor 1alpha to p300 in vitro and inhibits hypoxia-inducible factor-1alpha transactivation and hypoxia-mediated reporter gene activation. These studies suggest that CITED4 might function as an inhibitor of hypoxia-inducible factor 1alpha. To explore the function of CITED4 in breast cancer, we determined its expression in normal, in situ and invasive breast cancers. We also correlated its expression in 286 invasive breast tumors with clinicopathological, hypoxia markers and survival. In contrast to the nuclear localization of CITED4 in normal breast tissue, breast tumors were characterized by cytoplasmic and nuclear localization. Nuclear CITED4 expression was significantly inversely associated with tumor hypoxia-inducible factor 1alpha (P < 0.05), tumor size (P = 0.03), tumor grade (P = 0.0001), and Chalkley vessel count (P = 0.04). CITED4 showed no significant correlation with patient age (P = 0.45), estrogen receptor (P = 0.11), or epidermal growth factor receptor (P = 0.48). These results show that breast cancer development is characterized by either nuclear loss or cytoplasmic translocation of CITED4, with consequent loss of hypoxia-inducible factor-1alpha transcriptional antagonist activity. This may be an important mechanism by which tumors enhance hypoxia-inducible factor expression and result in an aggressive phenotype.

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.

FOG-2 competes with GATA-4 for transcriptional coactivator p300 and represses hypertrophic responses in cardiac myocytes

A multizinc finger protein, FOG-2, associates with a cardiac transcription factor, GATA-4, and represses GATA-4-dependent transcription. GATA-4 is required not only for normal heart development but is also involved in hypertrophic responses in cardiac myocytes; however, the effects of FOG-2 on these responses are unknown. The interaction of GATA-4 with a transcriptional coactivator p300 is required for its full transcriptional activity and the activation of the embryonic program during myocardial cell hypertrophy. We show here that exogenous FOG-2 represses phenylephrine-induced hypertrophic responses such as myofibrillar organization, increases in cell size, and hypertrophy-associated gene transcription. Using immunoprecipitation Western blotting, we demonstrate that FOG-2 physically interacted with p300 and reduced the binding of GATA-4 to p300. In addition, in COS7 cells, in which the function of endogenous p300 is disrupted, FOG-2 is unable to repress the GATA-4-dependent transcriptional activities; however, FOG-2 markedly repressed the p300-mediated increase in the DNA-binding and transcriptional activities of GATA-4 in these cells. Similarly, FOG-2 inhibited a phenylephrine-induced increase in the p300/GATA-4 interaction, the GATA-4/DNA-binding, and transcriptional activities of GATA-4-dependent promoters in cardiac myocytes as well. These findings demonstrate that FOG-2 represses hypertrophic responses in cardiac myocytes and that p300 is involved in these repressive effects.

Ablation of the CLP-1 gene leads to down-regulation of the HAND1 gene and abnormality of the left ventricle of the heart and fetal death

We have recently reported that cardiac lineage protein-1 (CLP-1), a nuclear protein with an acidic region that constitutes a potential protein-protein interaction domain, regulates transcription of the cardiac myosin light chain-2v (MLC-2v) gene promoter in a manner consistent with its being a transcriptional co-activator or regulator. To test the postulate that CLP-1 is a regulator of cardiac genes we ablated the CLP-1 gene in mice. Past embryonic day (E)16.5, CLP-1 null alleles did not show Mendelian inheritance suggesting that absence of CLP-1 was lethal in late fetal stages. CLP-1 (-/-) fetal hearts exhibited a reduced left ventricular chamber with thickened myocardial walls, features suggestive of cardiac hypertrophy. Electron microscopic analysis of E16.5 CLP-1 (-/-) ventricular myocardium showed a marked decline in cell density and altered nuclear and myofibril morphologies similar to that seen in animal models of hypertrophic heart. Analysis of contractile and non-contractile protein genes known to be re-expressed during cardiac hypertrophy showed them to have higher expression levels in CLP-1 (-/-) hearts thereby confirming the hypertrophic phenotype at the molecular level. Analysis of cardiac development genes showed that expression of the HAND1 transcription factor, a gene involved in patterning of the heart tube and down-regulated in hypertrophic hearts, was also significantly reduced in CLP-1 (-/-) fetal hearts. CLP-1 and HAND1 have similar expression patterns in the developing heart ventricles. These data suggest that CLP-1 and the HAND transcription factors may be part of a genetic program critical to proper heart development, perturbation of which can lead to cardiomyopathy.

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.

Identification of the CREB-binding protein/p300-interacting protein CITED2 as a peroxisome proliferator-activated receptor alpha coregulator

Like other nuclear receptors, the peroxisome proliferator-activated receptors (PPARs) use a wide variety of protein-protein interactions to properly regulate transcription of target genes. In an attempt to identify novel PPAR-interacting proteins, a cDNA expression library was screened with bacterially expressed PPARalpha. One of the genes identified as a PPARalpha-associated protein by interaction cloning was the CREB-binding protein/p300-interacting transactivator with ED-rich tail 2 (CITED2, also called p35srj/mrg1/msg1). This coactivator interacted directly with PPARalpha in the presence or absence of ligand predominantly via the ligand binding domain of the nuclear receptor. In transient transfection reporter assays, CITED2 acted as a dose-dependent coactivator of PPARalpha-dependent transcriptional regulation in the presence of several exogenous ligands. CITED2 also increased PPARgamma-dependent regulation of reporter genes but had no effect on PPARbeta activity. To determine whether CITED2 affects endogenous gene expression, this protein was stably overexpressed (CITED2+) or repressed by small inhibitor RNA (CITED2-) in immortalized mouse hepatocytes. Relative to the control stably transfected or CITED2-cells, CITED2+ cells had an increased rate of cell proliferation. Microarray analysis and real time PCR showed that several genes are differentially affected by PPARalpha ligands in CITED2+ versus CITED2-cells. Genes that were affected by PPARalpha ligands in a CITED2-modulatory manner include angiopoietin-like protein 4, forkhead C2, hypoxia-inducible factor-1alpha, and MAPK phosphatase 1. Interestingly these genes share common functions in that they are known to promote vascularization and angiogenesis in response to hypoxia. The results described here suggest that CIT-ED2 is a coactivator of PPARalpha and that both proteins may participate in signaling cascades of hypoxic response and angiogenesis.

Cited1 is required in trophoblasts for placental development and for embryo growth and survival

Cited1 is a transcriptional cofactor that interacts with Smad4, estrogen receptors alpha and beta, TFAP2, and CBP/p300. It is expressed in a restricted manner in the embryo as well as in extraembryonic tissues during embryonic development. In this study we report the engineering of a loss-of-function Cited1 mutation in the mouse. Cited1 null mutants show growth restriction at 18.5 days postcoitum, and most of them die shortly after birth. Half the heterozygous females, i.e., those that carry a paternally inherited wild-type Cited1 allele, are similarly affected. Cited1 is normally expressed in trophectoderm-derived cells of the placenta; however, in these heterozygous females, Cited1 is not expressed in these cells. This occurs because Cited1 is located on the X chromosome, and thus the wild-type Cited1 allele is not expressed because the paternal X chromosome is preferentially inactivated. Loss of Cited1 resulted in abnormal placental development. In mutants, the spongiotrophoblast layer is irregular in shape and enlarged while the labyrinthine layer is reduced in size. In addition, the blood spaces within the labyrinthine layer are disrupted; the maternal sinusoids are considerably larger in mutants, leading to a reduction in the surface area available for nutrient exchange. We conclude that Cited1 is required in trophoblasts for normal placental development and subsequently for embryo viability.

Transcriptional coactivator Cited2 induces Bmi1 and Mel18 and controls fibroblast proliferation via Ink4a/ARF

Cited2 (CBP/p300 interacting transactivator with ED-rich tail 2) is required for embryonic development, coactivation of transcription factor AP-2, and inhibition of hypoxia-inducible factor 1 transactivation. Cited2 is induced by multiple growth factors and cytokines and oncogenically transforms cells. Here, we show that the proliferation of Cited2(-/-) mouse embryonic fibroblasts ceases prematurely. This is associated with a reduction in growth fraction, senescent cellular morphology, and increased expression of the cell proliferation inhibitors p16(INK4a), p19(ARF), and p15(INK4b). Deletion of INK4a/ARF (encoding p16(INK4a) and p19(ARF)) completely rescued the defective proliferation of Cited2(-/-) fibroblasts. However, the deletion of INK4a/ARF did not rescue the embryonic malformations observed in Cited2(-/-) mice, indicating that INK4a/ARF-independent pathways are likely to be involved here. We found that Cited2(-/-) fibroblasts had reduced expression of the polycomb-group genes Bmi1 and Mel18, which function as INK4a/ARF and Hox repressors. Complementation with CITED2-expressing retrovirus enhanced proliferation, induced Bmi1/Mel18 expression, and decreased INK4a/ARF expression. Bmi1- and Mel18-expressing retroviruses enhanced the proliferation of Cited2(-/-) fibroblasts, indicating that they function downstream of Cited2. Our results provide genetic evidence that Cited2 controls the expression of INK4a/ARF and fibroblast proliferation, at least in part via the polycomb-group genes Bmi1 and Mel18.

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.

Identification of genetic pathways activated by the androgen receptor during the induction of proliferation in the ventral prostate gland

The androgen receptor (AR), when complexed with 5alpha-dihydrotestosterone (DHT), supports the survival and proliferation of prostate cells, a process critical for normal development, benign prostatic hypertrophy, and tumorigenesis. However, the androgen-responsive genetic pathways that control prostate cell division and differentiation are largely unknown. To identify such pathways, we examined gene expression in the ventral prostate 6 and 24 h after DHT administration to androgen-depleted rats. 234 transcripts were expressed significantly differently from controls (p < 0.05) at both time points and were subjected to extensive data mining. Functional clustering of the data reveals that the majority of these genes can be classified as participating in induction of secretory activity, metabolic activation, and intracellular signaling/signal transduction, indicating that AR rapidly modulates the expression of genes involved in proliferation and differentiation in the prostate. Notably AR represses the expression of several key cell cycle inhibitors, while modulating members of the wnt and notch signaling pathways, multiple growth factors, and peptide hormone signaling systems, and genes involved in MAP kinase and calcium signaling. Analysis of these data also suggested that p53 activity is negatively regulated by AR activation even though p53 RNA was unchanged. Experiments in LNCaP prostate cancer cells reveal that AR inhibits p53 protein accumulation in the nucleus, providing a post-transcriptional mechanism by which androgens control prostate cell growth and survival. In summary these data provide a comprehensive view of the earliest events in AR-mediated prostate cell proliferation in vivo, and suggest that nuclear exclusion of p53 is a critical step in prostate growth.

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.

Investigating hypoxic tumor physiology through gene expression patterns

Clinical evidence shows that tumor hypoxia is an independent prognostic indicator of poor patient outcome. Hypoxic tumors have altered physiologic processes, including increased regions of angiogenesis, increased local invasion, increased distant metastasis and altered apoptotic programs. Since hypoxia is a potent controller of gene expression, identifying hypoxia-regulated genes is a means to investigate the molecular response to hypoxic stress. Traditional experimental approaches have identified physiologic changes in hypoxic cells. Recent studies have identified hypoxia-responsive genes that may define the mechanism(s) underlying these physiologic changes. For example, the regulation of glycolytic genes by hypoxia can explain some characteristics of the Warburg effect. The converse of this logic is also true. By identifying new classes of hypoxia-regulated gene(s), we can infer the physiologic pressures that require the induction of these genes and their protein products. Furthermore, these physiologically driven hypoxic gene expression changes give us insight as to the poor outcome of patients with hypoxic tumors. Approximately 1-1.5% of the genome is transcriptionally responsive to hypoxia. However, there is significant heterogeneity in the transcriptional response to hypoxia between different cell types. Moreover, the coordinated change in the expression of families of genes supports the model of physiologic pressure leading to expression changes. Understanding the evolutionary pressure to develop a 'hypoxic response' provides a framework to investigate the biology of the hypoxic tumor microenvironment.

Three conformational states of the p300 CH1 domain define its functional properties

Numerous transcription factors interact with the basal transcriptional machinery through the transcriptional co-activators p300 and CREB-binding protein (CBP). The Zn(2+)-binding cysteine/histidine-rich 1 (CH1) domain of p300/CBP binds many of these transcription factors, including hypoxia-inducible factor (HIF). We studied the structural and biophysical properties of the p300 CH1 domain alone and bound to the HIF-1 alpha C-terminal transactivation domain (TAD) to understand the diverse binding properties of CH1. The Zn(2+)-bound CH1 domain (CH1-Zn(2+)) and the HIF-1 alpha TAD-CH1 complex (CH1-Zn(2+)-HIF-1 alpha) are similarly helical, whereas metal-free CH1 is mostly random coil. CH1-Zn(2+) undergoes noncooperative thermal denaturation, does not have a near-UV elliptical signal, and binds the hydrophobic fluorophore ANS. In contrast, the CH1-Zn(2+)-HIF-1 alpha complex undergoes cooperative thermal denaturation, does produce a near-UV signal, and does not bind ANS. Addition of Zn(2+) ions to metal-free CH1 produced one conformational change, and subsequent addition of a HIF-1 alpha TAD peptide induced a second conformational change as detected by intrinsic tryptophan fluorescence spectroscopy. The NMR (1)H-(15)N HSQC spectrum of CH1-Zn(2+) exhibits few poorly dispersed peaks with broad line widths. Removal of metal ions produces more poorly dispersed peaks with sharper line widths. Addition of a HIF-1 alpha TAD peptide to CH1-Zn(2+) produces many well-dispersed peaks with sharp line widths. Taken together, these data support three conformational states for CH1, including an unstructured metal-free domain, a partially structured Zn(2+)-bound domain with molten globule characteristics, and a stable, well-ordered HIF-1 alpha TAD-CH1 complex.

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.

HIF prolyl and asparaginyl hydroxylases in the biological response to intracellular O(2) levels

Hypoxia-inducible factor (HIF) is a heterodimeric transcription factor that plays a crucial role in mediating cellular responses to oxygen. Oxygen availability influences multiple steps in HIF activation and recent studies have indicated that at least two steps in this process are governed by a novel mode of signal transduction involving enzymatic hydroxylation of specific amino acid residues in HIF-alpha subunits by a series of 2-oxoglutarate (2-OG)-dependent oxygenases. These enzymes are non-haem iron enzymes that use dioxygen in the hydroxylation reaction and therefore provide a direct link between the availability of molecular oxygen and regulation of HIF. Prolyl hydroxylation regulates proteolytic destruction of HIF-alpha by the von Hippel-Lindau ubiquitin ligase complex, whereas HIF-alpha asparaginyl hydroxylation regulates recruitment of transcriptional coactivators. The involvement of at least two distinct types of 2-OG-dependent oxygenase in oxygen-regulated transcription suggests that these enzymes may be well suited to a role in cellular oxygen sensing.