Specificity of cyclin E-Cdk2, TFIIB, and E1A interactions with a common domain of the p300 coactivator

Proteomic Landscape of Tissue-Specific Cyclin E Functions in Vivo

E-type cyclins (cyclins E1 and E2) are components of the cell cycle machinery that has been conserved from yeast to humans. The major function of E-type cyclins is to drive cell division. It is unknown whether in addition to their 'core' cell cycle functions, E-type cyclins also perform unique tissue-specific roles. Here, we applied high-throughput mass spectrometric analyses of mouse organs to define the repertoire of cyclin E protein partners in vivo. We found that cyclin E interacts with distinct sets of proteins in different compartments. These cyclin E interactors are highly enriched for phosphorylation targets of cyclin E and its catalytic partner, the cyclin-dependent kinase 2 (Cdk2). Among cyclin E interactors we identified several novel tissue-specific substrates of cyclin E-Cdk2 kinase. In proliferating compartments, cyclin E-Cdk2 phosphorylates Lin proteins within the DREAM complex. In the testes, cyclin E-Cdk2 phosphorylates Mybl1 and Dmrtc2, two meiotic transcription factors that represent key regulators of spermatogenesis. In embryonic and adult brains cyclin E interacts with proteins involved in neurogenesis, while in adult brains also with proteins regulating microtubule-based processes and microtubule cytoskeleton. We also used quantitative proteomics to demonstrate re-wiring of the cyclin E interactome upon ablation of Cdk2. This approach can be used to study how protein interactome changes during development or in any pathological state such as aging or cancer.

Transcriptional/epigenetic regulator CBP/p300 in tumorigenesis: structural and functional versatility in target recognition

In eukaryotic cells, gene transcription is regulated by sequence-specific DNA-binding transcription factors that recognize promoter and enhancer elements near the transcriptional start site. Some coactivators promote transcription by connecting transcription factors to the basal transcriptional machinery. The highly conserved coactivators CREB-binding protein (CBP) and its paralog, E1A-binding protein (p300), each have four separate transactivation domains (TADs) that interact with the TADs of a number of DNA-binding transcription activators as well as general transcription factors (GTFs), thus mediating recruitment of basal transcription machinery to the promoter. Most promoters comprise multiple activator-binding sites, and many activators contain tandem TADs, thus multivalent interactions may stabilize CBP/p300 at the promoter, and intrinsically disordered regions in CBP/p300 and many activators may confer adaptability to these multivalent complexes. CBP/p300 contains a catalytic histone acetyltransferase (HAT) domain, which remodels chromatin to 'relax' its superstructure and enables transcription of proximal genes. The HAT activity of CBP/p300 also acetylates some transcription factors (e.g., p53), hence modulating the function of key transcriptional regulators. Through these numerous interactions, CBP/p300 has been implicated in complex physiological and pathological processes, and, in response to different signals, can drive cells towards proliferation or apoptosis. Dysregulation of the transcriptional and epigenetic functions of CBP/p300 is associated with leukemia and other types of cancer, thus it has been recognized as a potential anti-cancer drug target. In this review, we focus on recent exciting findings in the structural mechanisms of CBP/p300 involving multivalent and dynamic interactions with binding partners, which may pave new avenues for anti-cancer drug development.

African swine fever virus controls the host transcription and cellular machinery of protein synthesis

Throughout a viral infection, the infected cell reprograms the gene expression pattern in order to establish a satisfactory antiviral response. African swine fever virus (ASFV), like other complex DNA viruses, sets up a number of strategies to evade the host's defense systems, such as apoptosis, inflammation and immune responses. The capability of the virus to persist in its natural hosts and in domestic pigs, which recover from infection with less virulent isolates, suggests that the virus displays effective mechanisms to escape host defense systems. ASFV has been described to regulate the activation of several transcription factors, thus regulating the activation of specific target genes during ASFV infection. Whereas some reports have concerned about anti-apoptotic ASFV genes and the molecular mechanisms by which ASFV interferes with inducible gene transcription and immune evasion, less is yet known regarding how ASFV regulates the translational machinery in infected cells, although a recent report has shown a mechanism for favored expression of viral genes based on compartmentalization of viral mRNA and ribosomes with cellular translation factors within the virus factory. The viral mechanisms involved both in the regulation of host genes transcription and in the control of cellular protein synthesis are summarized in this review.

Increased expression of transcription initiation factor IIB after rat traumatic brain injury

The protein TFIIB is a general transcription initiation factor that plays a pivotal role in the preinitiation complex (PIC) and selects the transcription initiation site. However, its distribution and function in the central nervous system (CNS) remains unclear. In the present study, we mainly investigated the expression and cellular localization of TFIIB during traumatic brain injury (TBI). Western blot analysis revealed that TFIIB was present in normal rat brain cortex. It gradually increased, reached a peak at the 5th day after TBI, and then decreased. Importantly, more TFIIB was colocalized with astrocytes and microglia, which are largely proliferated. In addition, Western blot detection showed that the 5th day post injury was also the proliferation peak indicated by the elevated expression of PCNA. Importantly, injury-induced expression of TFIIB was colabelled by proliferating cell nuclear antigen (proliferating cells marker). These data suggested that TFIIB may be implicated in the proliferation of astrocytes and microglia and the recovery of neurological outcomes. But the inherent mechanisms remained unknown. Further studies are needed to confirm the exact role of TFIIB after brain injury.

The interferon antagonist ML protein of thogoto virus targets general transcription factor IIB

The ML protein of Thogoto virus, a tick-transmitted orthomyxovirus, is a splice variant of the viral matrix protein and antagonizes the induction of antiviral type I interferon (IFN). Here we identified the general RNA polymerase II transcription factor IIB (TFIIB) as an ML-interacting protein. Overexpression of TFIIB neutralized the inhibitory effect of ML on IRF3-mediated promoter activation. Moreover, a recombinant virus expressing a mutant ML protein unable to bind TFIIB was severely impaired in its ability to suppress IFN induction. We concluded that TFIIB binding is required for the IFN antagonist effect exerted by ML. We further demonstrate that the ML-TFIIB interaction has surprisingly little impact on gene expression in general, while a strong negative effect is observed for IRF3- and NF-kappaB-regulated promoters.

Dysregulation of CREB binding protein triggers thrombin-induced proliferation of vascular smooth muscle cells

Thrombin is a potent mitogen for vascular smooth muscle cells (VSMCs). CBP has been regarded as a potential therapeutic target on the basis of its ability to affect cell growth. Therefore we hypothesized that CBP mediates thrombin-induced proliferation of VSMCs. We constructed recombinant adenoviral vector that expresses four short hairpin RNA (shRNA) targeting rat CBP mRNA (CBP-shRNA/Ad). VSMCs were infected with CBP-shRNA/Ad and treated with thrombin. CBP level were analyzed by quantitative real-time PCR and Western blot. To evaluate VSMC proliferation, the cell cycle and DNA synthesis were analyzed by flow cytometry and (3)H-thymidine incorporation, respectively. CBP-shRNA/Ad infection inhibited thrombin-induced CBP expression in a dose-dependent manner concomitant with a decrease in the percentage of cells in the S phase and in DNA synthesis. These findings suggest that CBP plays a pivotal role in the S phase progression of VSMCs.

Screening for novel human genes associated with CRE pathway activation with cell microarray

In this study, cell microarray technology is used to identify novel human genes associated with CRE pathway activation. By reverse transfection, expression plasmids containing full-length cDNAs were cotransfected with the reporter plasmid pCRE-d2EGFP to monitor the activation of the CRE pathway via enhanced green fluorescence protein (EGFP) expression. Of the 575 predominantly novel genes screened, 22 exhibited relatively higher EGFP fluorescence compared with a negative control. After a functional validation with a dual luciferase reporter system that included both cis- and trans-luciferase assays, 4 of the 22 genes (RNF41, C8orf32, C6orf208, and MEIS3P1) were confirmed as CRE-pathway activators. Western blot analysis revealed that RNF41 can promote CREB phosphorylation. These results demonstrate the successful combination of cell microarray technology with this reporting system and the potential of this tool to characterize functions of novel genes in a highly parallel format.

A novel role of VIP in colonic motility function: induction of excitation-transcription coupling in smooth muscle cells

BACKGROUND & AIMS

Vasoactive intestinal polypeptide (VIP) relaxes smooth muscle by generation of cAMP and activation of protein kinase A (PKA). However, PKA activation also phosphorylates the transcription factor CREB. The aim of this study was to investigate whether the phosphorylation of CREB induces gene expression of the pore-forming alpha(1C) subunit of Ca(v)1.2 channels (L-type calcium channels), whose promoter has 2 binding sites for CREB.

METHODS

The experiments were performed on primary cultures of human colonic circular smooth muscle cells and freshly obtained human and rat colonic circular muscle strips.

RESULTS

The incubation of human colonic circular smooth muscle cells or muscle strips with VIP for 24 hours enhanced the expression of alpha(1C) protein and mRNA as well as the contractile response to acetylcholine and KCl. On the contrary, incubation of the muscle strips with VIP antagonist for 24 hours suppressed cell contractility. The incubation of the cells with VIP caused sustained generation of cAMP for 24 hours, but PKA activation and CREB phosphorylation were transient. The inhibition of PKA by H-89 or silencing of CREB gene with targeted RNAi blocked the transcription of alpha(1C). Progressive 5' deletions of halpha(1C)1b promoter and site-directed mutations of the 2 CREB binding cis-elements indicated that most of alpha(1C) transcription was mediated by the 5' cAMP response element.

CONCLUSIONS

The excitation-transcription coupling stimulated by VIP induces expression of the Ca(v)1.2 channels. The influx of calcium through these channels is a critical step in excitation-contraction coupling in smooth muscle cells.

Role of the N-terminal activation domain of the coiled-coil coactivator in mediating transcriptional activation by beta-catenin

The coiled-coil coactivator (CoCoA) is involved in transcriptional activation of target genes by nuclear receptors and the xenobiotic aryl hydrocarbon receptor, as well as target genes of the Wnt signaling pathway, which is mediated by the lymphocyte enhancer factor (LEF)/T cell factor transcription factors and the coactivator beta-catenin. The recruitment of CoCoA by nuclear receptors is accomplished by the interaction of the central coiled-coiled domain of CoCoA with p160 coactivators; the C-terminal activation domain (AD) of CoCoA is used for downstream signaling, whereas the function of the N-terminal region is undefined. Here we report that the N terminus of CoCoA contains another AD, which is necessary and sufficient for synergistic activation of LEF1-mediated transcription by CoCoA and beta-catenin. The N-terminal AD contains a p300 binding motif, which is important for synergistic cooperation of CoCoA and p300 as coactivators for LEF1 and beta-catenin. p300 contributes to the function of the CoCoA N-terminal AD primarily through its histone acetyltransferase activity. Moreover, in cultured cells, endogenous p300 is recruited to the promoter of an integrated reporter gene by the N terminus of CoCoA. Thus, the coactivator function of CoCoA for nuclear receptors and LEF1/beta-catenin involves differential utilization of two different CoCoA ADs.

Targeting of p300/CREB binding protein coactivators by simian virus 40 is mediated through p53

The primary transforming functions of simian virus 40 large T antigen (SV40 LT) are conferred primarily through the binding and inactivation of p53 and the retinoblastoma family members. Normal p53 function requires an association with the CREB binding protein (CBP)/p300 coactivators, and a ternary complex containing SV40 LT, p53, and CBP/p300 has been identified previously. In this report, we have evaluated a secondary function of p53 bound to the SV40 LT complex in mediating the binding of human CBP/p300. We demonstrate that p53 associated with SV40 LT was posttranslationally modified in a manner consistent with the binding of CBP/p300. Furthermore, expression of SV40 LT induced the proportion of p53 phosphorylated on S15. An essential function for p53 in bridging the interaction between SV40 LT and CBP/p300 was identified through the reconstitution of the SV40 LT-CBP/p300 complex upon p53 reexpression in p53-null cells. In addition, the SV40 LT-CBP/p300 complex was disrupted through RNA interference-mediated depletion of endogenous p53. We also demonstrate that SV40 LT was acetylated in a p300- and p53-dependent manner, at least in part through the CH3 domain of p300. Therefore, the binding of p53 serves to modify SV40 LT by targeting CBP and p300 binding to direct the acetylation of SV40 LT.

Mouse models of cell cycle regulators: new paradigms

In yeast, a single cyclin-dependent kinase (Cdk) is able to regulate diverse cell cycle transitions (S and M phases) by associating with multiple stage-specific cyclins. The evolution of multicellular organisms brought additional layers of cell cycle regulation in the form of numerous Cdks, cyclins and Cdk inhibitors to reflect the higher levels of organismal complexity. Our current knowledge about the mammalian cell cycle emerged from early experiments using human and rodent cell lines, from which we built the current textbook model of cell cycle regulation. In this model, the functions of different cyclin/Cdk complexes were thought to be specific for each cell cycle phase. In the last decade, studies using genetically engineered mice in which cell cycle regulators were targeted revealed many surprises. We discovered the in vivo functions of cell cycle proteins within the context of a living animal and whether they are essential for animal development. In this review, we discuss first the textbook model of cell cycle regulation, followed by a global overview of data obtained from different mouse models. We describe the similarities and differences between the phenotypes of different mouse models including embryonic lethality, sterility, hematopoietic, pancreatic, and placental defects. We also describe the role of key cell cycle regulators in the development of tumors in mice, and the implications of these data for human cancer. Furthermore, animal models in which two or more genes are ablated revealed which cell cycle regulators interact genetically and functionally complement each other. We discuss for example the interaction of cyclin D1 and p27 and the compensation of Cdk2 by Cdc2. We also focus on new functions discovered for certain cell cycle regulators such as the regulation of S phase by Cdc2 and the role of p27 in regulating cell migration. Finally, we conclude the chapter by discussing the limitations of animal models and to what extent can the recent findings be reconciled with the past work to come up with a new model for cell cycle regulation with high levels of redundancy among the molecular players.

A novel estradiol/estrogen receptor alpha-dependent transcriptional mechanism controls expression of the human prolactin receptor

Prolactin exerts diverse functions in target tissues through its membrane receptors, and is a potent mitogen in normal and neoplastic breast cells. Estradiol (E(2)) induces human prolactin receptor (hPRLR) gene expression through stimulation of its generic promoter (PIII). This study identifies a novel E(2)-regulated non-estrogen responsive element-dependent transcriptional mechanism that mediates E(2)-induced hPRLR expression. E(2) stimulated transcriptional activity in MCF7A(2) cells transfected with PIII lacking an estrogen responsive element, and increased hPRLR mRNA and protein. The abolition of the E(2) effect by mutation of Sp1 or C/EBP elements that bind Sp1/Sp3 and C/EBPbeta within PIII indicated the cooperation of these transfactors in E(2)-induced transcription of the hPRLR. DNA affinity protein assay showed that E(2) induced estrogen receptor alpha (ERalpha) binding to Sp1/Sp3 and C/EBPbeta DNA-protein complexes. The ligand-binding domain of ERalpha was essential for its physical interaction with C/EBPbeta, and E(2) promoted this association, and its DNA binding domain was required for transactivation of PIII. Co-immunoprecipitation studies revealed tethering of C/EBPbeta to Sp1 by E(2)-activated ERalpha. Chromatin immunoprecipitation analysis showed that E(2) induced recruitment of C/EBPbeta, ERalpha, SRC1, p300, pCAF, TFIIB, and Pol II, with no change in Sp1/Sp3. E(2) also induced promoter-associated acetylation of H3 and H4. These findings demonstrate that an E(2)/ERalpha, Sp1, and C/EBPbeta complex with recruitment of coactivators and TFIIB and Pol II are required for E(2)-activated transcriptional expression of the hPRLR through PIII. Estradiol produced in breast stroma and adipose tissue, which are major sources of estrogen in post-menopausal women, could up-regulate hPRLR gene expression and stimulate breast tumor growth.

Transcriptional analysis of avian embryonic tissues following infection with avian infectious bronchitis virus

Avian infectious bronchitis virus (IBV) infection is one of the major viral respiratory diseases of chickens. Better understanding of the molecular basis of viral pathogenesis should contribute significantly towards the development of improved prophylactic, therapeutic and diagnostic reagents to control infections. In the present investigation, transcriptional profiles were analyzed by using RNA recovered from the lung tissue of IBV infected 18-day-old chicken embryos at 6, 24, 48 and 72 h post IBV infection. This microarray analysis was completed using avian cDNA arrays comprised of fragments of 1191 unique chicken and turkey gene transcripts. These arrays were generated from normalized cDNA subtraction libraries that were derived from avian pneumovirus (APV) infected chicken embryo fibroblast (CEF) cultures and tissues obtained from APV infected turkeys subtracted with their respective uninfected cultures and tissues. Of the 1191 unique genes represented on the array, the expression of a total of 327 genes (27% of total) were altered by two-fold or more from 6 through 72 h post-infection. A comparative analysis of IBV regulated genes with genes previously reported to change in expression following infection with other avian respiratory viruses revealed both conserved and unique changes. Real-time qRT-PCR was used to confirm the regulated expression of genes related to several functional classes including kinases, interferon induced genes, chemokines and adhesion molecules, vesicular trafficking and fusion protein genes, extracellular matrix protein genes, cell cycle, metabolism, cell physiology and development, translation, RNA binding, lysosomal, protein degradation and ubiquitination related genes. Microarray analysis served as an efficient tool in facilitating a comparative analysis of avian respiratory viral infections and provided insight into host transcriptional changes that were conserved as well as those which were unique to individual pathogens.

Model of the TBP–TFIIB Complex from Plasmodium falciparum: Interface Analysis and Perspectives as a New Target for Antimalarial Design

BACKGROUND

Malaria affects 200-300 million individuals per year worldwide. Plasmodium falciparum is the causative agent of the most severe and mortal type of malaria. The need for new antimalarials comes from the widespread resistance to those in current use. New antimalarial targets are required to increase chemical diversity and effectiveness of the drugs. The research for such new targets and drug chemotypes is aided by structure-based drug design. We present a model of the TBP-TFIIB complex from P. falciparum (pfTBP-pfTFIIB) and a detailed study of the interactions at the TBP-TFIIB interface.

METHODS

The model was built using standard methodology, optimized energetically and evaluated structurally. We carried out an analysis of the interface considering its evolution, available experimental data on TBP and TFIIB mutants, and the main conserved and non-conserved interactions. To support the perspective of using this complex as a new target for rational antimalarial design, we present the comparison of the pfTBP-pfTFIIB interface with its human homolog.

RESULTS

Despite the high residue conservation at the interface, we identified a potential region, composed of species-specific residues that can be used for rational antimalarial design.

CONCLUSIONS

Currently there are no antimalarial drugs targeted to stop the nuclear transcription process, a vital event for all replication stages of P. falciparum. Due to its absolute requirement in transcription initiation, we consider the pfTBP-pfTFIIB interface as a new potential target for novel antimalarial chemotypes.

Cell cycle in mouse development

Mice likely represent the most-studied mammalian organism, except for humans. Genetic engineering in embryonic stem cells has allowed derivation of mouse strains lacking particular cell cycle proteins. Analyses of these mutant mice, and cells derived from them, facilitated the studies of the functions of cell cycle apparatus at the organismal and cellular levels. In this review, we give some background about the cell cycle progression during mouse development. We next discuss some insights about in vivo functions of the cell cycle proteins, gleaned from mouse knockout experiments. Our text is meant to provide examples of the recent experiments, rather than to supply an extensive and complete list.

Dynamic interaction of p220(NPAT) and CBP/p300 promotes S-phase entry

Cajal bodies contain cyclin E/cdk2 and the substrate p220(NPAT) to regulate the transcription of histones, which is essential for cell proliferation, however, recent mouse knockout studies indicate that cyclin E and cdk2 are dispensable for these events. Because the CBP/p300 histone acetyltransferase are also known to be involved in cell proliferation, we examined the molecular and functional interactions of p220(NPAT) with the CBP/p300 at the G1/S boundary as cell cycle regulators. The subnuclear localization of p220(NPAT) and CBP/p300 proteins showed that their foci partially overlapped in a cell cycle dependent manner. Overexpression of p220(NPAT) and CBP/p300 cooperatively enhanced G1/S transition and DNA synthesis even without cdk2 phosphorylation site. Finally, molecular alignment analysis indicated that p220(NPAT) contains several potential substrate sites for CBP/p300. Overall, our findings demonstrate that p220(NPAT) and CBP/p300 form a transient complex at the G1/S boundary to play cooperative roles to promote the S-phase entry.

Phosphorylation of progesterone receptor serine 400 mediates ligand-independent transcriptional activity in response to activation of cyclin-dependent protein kinase 2

Human progesterone receptors (PR) are phosphorylated by cyclin-dependent protein kinase 2 (CDK2) at multiple sites, including Ser400. Herein, we have addressed the significance of phosphorylation of this residue. PR phospho-Ser400-specific antibodies revealed regulated phosphorylation of Ser400 in response to progestins and mitogens, and this correlated with increased CDK2 levels and activity. Expression of cyclin E elevated CDK2 activity and downregulated PR independently of ligand. Similarly, overexpression of activated mutant CDK2 increased PR transcriptional activity in the absence and presence of progestin. Mutation of PR Ser400 to alanine (S400A) blocked CDK2-induced PR activity in the absence, but not in the presence, of progestin. PR was unresponsive to activated CDK2 in breast cancer cells with elevated p27, and RNA interference knock-down of p27 partially restored CDK2-induced ligand-independent PR activation. Similarly, in p27(-/-) mouse embryonic fibroblasts, elevated CDK2 activity increased wild-type (wt) but not S400A PR transcriptional activity in the absence of progestin. CDK2 induced nuclear localization of unliganded wt but not S400A PR; liganded S400A PR exhibited delayed nuclear accumulation. These studies demonstrate that CDK2 regulates PR in the absence of progestins via phosphorylation of Ser400, thus revealing a novel mechanism for upregulated PR transcriptional activity in human breast cancer cells expressing altered cell cycle regulatory molecules.

HIV-1 Tat interactions with p300 and PCAF transcriptional coactivators inhibit histone acetylation and neurotrophin signaling through CREB

The human immunodeficiency virus type-1 (HIV-1) infects microglia, macrophages, and astrocytes in the central nervous system (CNS) and may cause severe neurological diseases, such as AIDS-related dementias or progressive encephalopathies, as a result of CNS inflammation and neurotrophin signaling defects associated with expression of viral antigens and HIV-1 replication in the brain. The HIV Tat protein can be endocytosed by surrounding uninfected cells; interacts with transcriptional coactivators/acetyltransferases, p300/CREB-binding protein, and p300/CREB-binding protein-associated factor (PCAF); and induces neuronal apoptosis. Since nerve growth factor (NGF) receptor and brain-derived neurotrophic factor receptor signaling through CREB requires p300 and PCAF histone acetyltransferases, we sought to determine whether HIV-1 Tat coactivator interactions interfere with neurotrophin receptor signaling in neuronal cells. Here, we demonstrate that Tat-coactivator interactions inhibit NGF- and brain-derived neurotrophic factor-responsive CRE trans-activation and neurotrophin protection against apoptosis in PC12 and IMR-32 neuroblastoma cells. Purified recombinant Tat or Tat-derived synthetic peptides, spanning p300- and PCAF-binding sequences, inhibit histone H3/H4 acetylation in vitro. A Tat mutant, TatK28A/K50A, defective for binding p300 and PCAF, neither repressed NGF-responsive CRE transactivation nor inhibited histone acetylation. HIV-1 Tat interacts in PCAF complexes in post-mortem CNS tissues from donor neuro-AIDS patients, as determined by fluorescence resonance energy transfer immunoconfocal microscopy. Importantly, these findings suggest that HIV-1 Tat-coactivator interactions may contribute to neurotrophin signaling impairments and neuronal apoptosis associated with HIV-1 infections of the CNS.

Stability of the Hepatocyte Nuclear Factor 6 Transcription Factor Requires Acetylation by the CREB-binding Protein Coactivator

We previously demonstrated that the formation of complexes between the DNA binding domains of the hepatocyte nuclear factor 6 (HNF6) and Forkhead Box a2 (Foxa2) transcription factors resulted in synergistic transcriptional activation of a Foxa2 target promoter. This Foxa2.HNF6 transcriptional synergy was mediated by the recruitment of CREB-binding protein (CBP) coactivator through the HNF6 Cut-Homeodomain sequences. Although the HNF6 DNA binding domain sequences are sufficient to recruit CBP coactivator for HNF6.Foxa2 transcriptional synergy, paradoxically these HNF6 Cut-Homeodomain sequences were unable to stimulate the transcription of an HNF6-dependent reporter gene. Here, we investigated whether the CBP coactivator protein played a different role in regulating HNF6 transcriptional activity. We showed that acetylation of the HNF6 protein by CBP increased both HNF6 protein stability and its ability to stimulate transcription of the glucose transporter 2 promoter. Mutation of the HNF6 Cut domain lysine 339 residue to an arginine residue abrogated CBP acetylation, which is required for HNF6 protein stability. Furthermore, the HNF6 K339R mutant protein, which failed to accumulate detected protein levels, was transcriptionally inactive and could not be stabilized by inhibiting the ubiquitin proteasome pathway. Finally, increased HNF6 protein levels stabilized the Foxa2 protein, presumably through the formation of the Foxa2.HNF6 complex. These studies show for the first time that HNF6 protein stability is controlled by CBP acetylation and provides a novel mechanism by which the activity of the CBP coactivator may regulate steady levels of two distinct liver-enriched transcription factors.

Cell-type-specific binding of the transcription factor CREB to the cAMP-response element

The cAMP-response element-binding protein (CREB) transcription factor was initially identified as a mediator of cAMP-induced gene expression. CREB binds to a target sequence termed the cAMP-response element (CRE) found in many cellular and viral gene promoters. One of the best-characterized CREs resides in the promoter of the gene encoding the neuropeptide somatostatin, and this element has served as a model for studies of CREB function. Phosphorylation of CREB by protein kinase A allows recruitment of the coactivator CREB-binding protein (CBP). A central tenet of the CREB-CBP model is that CREB binds constitutively to the CRE and that regulation occurs through the phosphorylation-dependent recruitment of CBP. In this report, we use chromatin immunoprecipitation assays to show that CREB does not interact in vivo with the somatostatin CRE, or similar elements in several other genes, in PC12 cells, a standard model for studies of CREB function. Rather, CREB binding in vivo is regulated in a cell-specific manner, a finding that was confirmed by using in vivo genomic footprinting assays. The CREs in other genes were also found to interact differentially with CREB in PC12 cells, hepatoma cells, and cortical neurons. We conclude that the family of CREB target genes differs from one cell type to another and that the ability of CREB to bind to a particular CRE represents an important component of gene regulation.

Effect of HIV-1 Vpr on cell cycle regulators

Cell cycle is one of the most complex processes in the life of a dividing cell. It involves numerous regulatory proteins, which direct the cell through a specific sequence of events for the production of two daughter cells. Cyclin-dependent kinases (cdks), which complex with the cyclin proteins, are the main players in the cell cycle. They can regulate the progression of the cells through different stages regulated by several proteins including p53, p21(WAF1), p19, p16, and cdc25. Downstream targets of cyclin-cdk complexes include pRB and E2F. A cell cycle can be altered to the advantage of many viral agents, most notably polyomaviruses, papillomaviruses, adenoviruses, and retroviruses. In addition, viral protein R (Vpr) is a protein encoded by the human immunodeficiency virus type 1 (HIV-1). HIV-1, the causative agent of acquired immunodeficiency syndrome (AIDS), is a member of the lentivirus class of retroviruses. This accessory protein plays an important role in the regulation of the cell cycle by causing G(2) arrest and affecting cell cycle regulators. Vpr prevents infected cells from proliferating, and collaborates with the matrix protein (MA) to enable HIV-1 to enter the nucleus of nondividing cells. Studies from different labs including ours showed that Vpr affects the functions of cell cycle proteins, including p53 and p21(WAF1). Thus, the replication of HIV-1, and ultimately its pathogenesis, are intrinsically tied to cell-cycle control.

Deacetylase recruitment by the C/H3 domain of the acetyltransferase p300

The balance between acetylation and deacetylation of histone and nonhistone proteins controls gene expression in a variety of cellular processes, with transcription being activated by acetyltransferases and silenced by deacetylases. We report here the formation and enzymatic characterization of a complex between the acetyltransferase p300 and histone deacetylases. The C/H3 region of p300 was found to co-purify deacetylase activity from nuclear cell extracts. A prototype of class I histone deacetylases, HDAC1, interacts with p300 C/H3 domain in vitro and in vivo. The p300-binding protein E1A competes with HDAC1 for C/H3 binding; and, like E1A, HDAC1 overexpression interferes with either activation of Gal4p300 fusion protein or p300-dependent co-activation of two C/H3-binding proteins, MyoD and p53. The exposure to deacetylase inhibitors could reverse the dominant-negative effect of a C/H3 fragment insulated from the rest of the molecule, on MyoD- and p53-dependent transcription, whereas inhibition by E1A was resistant to trichostatin A. These data support the hypothesis that association between acetyltransferases and deacetylases can control the expression of genes implicated in cellular growth and differentiation, and suggest that the dominant-negative effect of the p300 C/H3 fragment relies on deacetylase recruitment.

Role of p160 coactivator complex in the activation of liver X receptor

OBJECTIVE

Liver X receptor (LXR) is a member of a nuclear receptor family regulating the expression of several key proteins involved in lipid metabolism and inflammation. In contrast to several other nuclear receptors, very little is known about the coactivators needed for the agonist-mediated transactivation by LXR. In this study, we have investigated the role of p160 coactivator complex in the regulation of ATP-binding transporter A1 (ABCA1), a clinically important gene transcriptionally upregulated by LXR/RXR (retinoid X receptor) heterodimer.

METHODS AND RESULTS

Overexpression of LXRalpha, SRC-1, and p300, either alone or in combination, increased the luciferase activity driven by the wild-type ABCA1 promoter. The same coactivators bound to the ABCA1 promoter on oxysterol induction in chromatin immunoprecipitation assays. To the contrary, CARM-1 and P/CAF had no effect on ABCA1 transactivation, nor do they bind the promoter. When the DR-4 element was mutated from the ABCA1 promoter, only p300 was able to activate ABCA1 transcription in a ligand-independent manner.

CONCLUSIONS

The p160 coactivator complex members SRC-1 and p300, but not CARM-1 and P/CAF, coactivate LXR-mediated transcription of ABCA1 gene. In addition, p300 activates ABCA1 transcription independently of DR-4 element and LXR/RXR.

Transcriptional coactivation of nuclear factor-kappaB-dependent gene expression by p300 is regulated by poly(ADP)-ribose polymerase-1

Nuclear factor kappaB (NF-kappaB) plays an important role in the transcriptional regulation of genes involved in inflammation and cell survival. In this study, we demonstrated that NF-kappaB-dependent gene expression was inhibited by E1A in poly(ADP)-ribose polymerase-1 knock out (PARP-1 (-/-)) cells complemented with wild type PARP-1 after tumor necrosis factor alpha (TNFalpha) or lipopolysaccharide (LPS) treatment. PARP-1 and p300 synergistically coactivated NF-kappaB-dependent gene expression in response to TNFalpha and LPS. Furthermore, PARP-1 interacted directly with p300 and enhanced the interaction of NF-kappaB1/p50 to p300. The C terminus, harboring the catalytic domain of PARP-1 but not its enzymatic activity, was required for complete transcriptional coactivation of NF-kappaB by p300 in response to TNFalpha and LPS. Together, these results indicate that PARP-1 acts synergistically with p300 and plays an essential regulatory role in NF-kappaB-dependent gene expression.

p300 is involved in formation of the TBP-TFIIA-containing basal transcription complex, TAC

We have recently identified a novel basal transcription complex, TAC, that is present and active in embryonal carcinoma (EC) cells but not in other adult cells such as COS7. In the search for factors involved in TAC formation, we found that expression of the adenoviral 12S E1A oncoprotein abolishes TAC formation in EC cells. This effect of E1A depends on its N-terminal domain that is essential for cell differentiation and that targets the transcriptional coactivators p300 and PCAF. Expression of p300 lacking its major E1A interaction domain, CH3, restores TAC formation in the presence of E1A, in a bromodomain- and HAT domain-dependent manner. Consistently, the unprocessed TFIIAalphabeta precursor that is selectively assembled into TAC is acetylated preferentially compared with the processed subunits present in 'free' TFIIA. Intriguingly, expression of p300 in COS7 cells that do not contain detectable levels of TAC instigates formation of TAC from endogenous components. Our data suggest that p300 plays a role in formation of the TBP-TFIIA-containing basal transcription complex, TAC.

Fine tuning the transcriptional regulation of the CXCL1 chemokine

Constitutive activation of the transcription factor nuclear factor-κB (NF-κB) plays a major role in inflammatory diseases as well as cancer by inducing the endogenous expression of many proinflammatory proteins such as chemokines, and facilitating escape from apoptosis. The constitutive expression of chemokines such as CXCL1 has been correlated with growth, angiogenesis, and metastasis of cancers such as melanoma. The transcription of CXCL1 is regulated through interactions of NF-κB with other transcriptional regulatory molecules such as poly(ADP-ribose) polymerase-1 (PARP-1) and cAMP response element binding protein (CREB)-binding protein (CBP). It has been proposed that these two proteins interact with NF-κB and other enhancers to form an enhanceosome at the promoter region of CXCL1 and modulate CXCL1 transcription. In addition to these positive cofactors, a negative regulator, CAAT displacement protein (CDP), may also be involved in the transcriptional regulation of CXCL1. It has been postulated that the elevated expression of CXCL1 in melanomas is due to altered interaction between these molecules. CDP interaction with the promoter down-regulates transcription, whereas PARP and/or CBP interactions enhance transcription. Thus, elucidation of the interplay between components of the enhanceosome of this gene is important in finding more efficient and new therapies for conditions such as cancer as well as acute and chronic inflammatory diseases.

Linking cyclins to transcriptional control

Cell cycle activation is coordinated by D-type cyclins which are rate limiting and essential for the progression through the G1 phase of the cell cycle. D-type cyclins bind to and activate the cyclin-dependent kinases Cdk4 and Cdk6, which in turn phosphorylate their downstream target, the retinoblastoma protein Rb. Upon Rb phosphorylation, the E2F transcription factors activate the expression of S-phase genes and thereby induce cell cycle progression. The raise of cyclin D levels in early G1 also serves to titrate Kip/Cip proteins away from cyclinE/Cdk2 complexes, further accelerating cell cycle progression. Therefore, cyclin D plays essential roles in the response to mitogens, transmitting their signal to the Rb/E2F pathway. Surprisingly, cyclin D1-deficient animals are viable and have developmental abnormalities limited to restricted tissues, such as retina, the nervous system and breast epithelium. This observation, combined with several other studies, have raised the possibility that cyclin D1 may have new activities that are unrelated to its function as a cdk regulatory subunit and as regulator of Rb. Effectively, cyclin D has been reported to have transcriptional functions since it interacts with several transcription factors to regulate their activity. Most often, this effect does not rely on the kinase function of Cdk4, indicating that this function is probably independent of cell cycle progression. Further extending its role in gene regulation, cyclin D interacts with histone acetylases such as P/CAF or NcoA/SRC1a but also with components of the transcriptional machinery such as TAF(II)250. Therefore, these studies suggest that the functions of cyclin D might need to be reevaluated. They have established a new cdk-independent role of cyclin D1 as a transcriptional regulator, indicating that cyclin D1 can act via two different mechanisms, as a cdk activator it regulates cell cycle progression and as a transcriptional regulator, it modulates the activity of transcription factors.

Acetylation of the adenovirus-transforming protein E1A determines nuclear localization by disrupting association with importin-alpha

Posttranslational modifications may alter the biochemical functions of a protein by modifying associations with other macromolecules, allosterically altering intrinsic catalytic activities, or determining subcellular localization. The adenovirus-transforming protein E1A is acetylated by its cellular targets, the co-activators CREB-binding protein, p300, and p300/CREB-binding protein-associated factor in vitro and also in vivo at a single lysine residue (Lys(239)) within a multifunctional carboxyl-terminal domain necessary for both nuclear localization and interaction with the transcriptional co-repressor carboxyl-terminal binding protein (CtBP). In contrast to a previous report, we demonstrate that acetylation of Lys(239) does not disrupt CtBP binding and that 12 S E1A-mediated repression of CREB-binding protein-dependent transcription does not require recruitment of CtBP. Instead we find that the cytoplasmic fraction of E1-transformed 293 cells is enriched for acetylated E1A with relative exclusion from the nuclear compartment. Whereas wild type 12 S E1A binds importin-alpha 3, binding affinity was markedly reduced both by single amino acid substitution mutations and acetylation at Lys(239). This is the first demonstration that acetylation may alter nuclear partitioning by direct interference with nuclear import receptor recognition. The finding that the cytoplasmic fraction of E1A is acetylated indicates that E1A may exert its pleiotropic effects on cellular transformation in part by affecting cytoplasmic processes.

Transcriptional activation by the oestrogen receptor alpha is modulated through inhibition of cyclin-dependent kinases

We have investigated the interaction between the expression of p21(WAF1/CIP1/SDI1), a stoichiometric inhibitor of Cdk, and the transcriptional activity of the oestrogen receptor alpha (ER(alpha). Transient transfection experiments demonstrated that the expression of p21(WAF1/CIP1/SDI1) amplified the transcriptional activation by ER(alpha). A dominant negative mutant of Cdk2 also enhanced the ER(alpha) transcriptional activity, indicating that the underlying mechanism relies on the inhibition of Cdk2 activity and cell cycle arrest. In agreement with this conclusion, experiments with p21(WAF1/CIP1/SDI1) mutants demonstrated that the domain involved in the binding of p21(WAF1/CIP1/SDI1) to Cdks was indispensable for the modulation of ER(alpha) activity. In addition, we show that expression of p21(WAF1/CIP1/SDI1) alleviates the block on CBP function mediated by Cdk2 and in turn stimulates transcriptional activation by ER(alpha) in a CBP-histone acetyltransferase (HAT)-dependent manner. These results suggest a novel mechanism by which p21(WAF1/CIP1/SDI1) functions as an enhancer of ER(alpha) activity through the modulation of CBP function.

Inhibition of HTLV-1 transcription by cyclin dependent kinase inhibitors

HTLV-1 is the etiologic agent for adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), where viral replication and transformation are largely dependent upon modification of regulatory and host cell cycle proteins. The mechanism of HTLV-1 transformation appears to be distinct from that of many known chronic or acute leukemia viruses and is related to the viral activator Tax. Here we show that cyclin E, can associate tightly with the coactivator p300 and Pol II complex in HTLV-1 infected cells. The cyclin E associated complex is kinase active and phosphorylates the carboxy terminal domain of RNA Pol II. More importantly, p21/Waf1, a well-known cdk inhibitor at the G1/S border, inhibits transcription of HTLV-1 in both transfections and in in vitro transcription assays. Finally, specific cdk chemical inhibitors, functionally similar to cellular cdkIs, such as p21/Waf1 which inhibits cyclin E/cdk2 activity, also inhibit transcription of the HTLV-1 promoter. In particular, Purvalanol A, with an IC50 of 0.035 microm inhibits activated, but not basal transcription, as well as HTLV-1 infected cells. Collectively, the role of cyclin E/cdk2 in HTLV-1 infected cells and its involvement in RNA Pol II phosphorylation is discussed.

Dynamics of reporter gene stimulation by HMG box proteins

Overcoming local DNA rigidity is required to perform three-dimensional DNA-protein configuration at promoter regions. The abundant architectural nonhistone chromosomal HMG box proteins are nonsequence-specific; however, they have been established to specifically recognize distorted DNA. Using transient transfection to overexpress two different members of the HMGB-1/2 family of DNA architectural factors, we demonstrate that these proteins provide a general enhancement in reporter gene expression irrespective of the promoter being considered. Evidences are also provided indicating that stimulation may not be achieved by recruitment of the proteins by regulatory factors or as a consequence of major chromatin unfolding as previously suggested. Interestingly, the influence of the HMG box proteins under study was overridden when the promoters were either induced or stimulated by Trichostatin A (TSA) but recovered upon extended induction period. These results also support the concept that the architectural role of these proteins can contribute to the preinitiation complex assembly required for basal transcription, but to a much lesser extent to the poised promoter scaffolding characteristic of activated transcription.

Histone acetylation by p300 is involved in CREB-mediated transcription on chromatin

Signal transduction through cAMP to activate gene expression via the cAMP-responsive element (CRE) is one of the most intensively studied transcription pathways. In this pathway, transcription factor CRE-binding protein (CREB) recognizes the CRE enhancer on DNA. The CREB protein is activated via phosphorylation at serine 133 by protein kinase A and then is able to recruit coactivator CREB-binding protein (CBP) and its homologue p300. This recruitment of CBP/p300 is required for transcription activation. The mechanism for CBP/p300 to participate in this transcription process is still unclear. CBP and p300 are histone acetyltransferases (HAT) and able to associate with other HAT proteins. It has been reported that the regulation of nuclear receptor-mediated transcription initiation by p300 requires chromatin and its HAT function. The data shown here indicate that the requirements for chromatin and p300 HAT activity also apply to the activation of CREB-mediated transcription. Serine 133-phosphorylated CREB recruits p300 onto chromatin for efficient acetylation of nucleosomes. This targeted acetylation by p300 is essential to CREB-dependent transcription pathway.

A role for coactivators and histone acetylation in estrogen receptor alpha-mediated transcription initiation

Transcriptional regulation by estrogen receptor alpha (ERalpha) involves protein-protein interactions among the receptor, its associated coactivators and the RNA polymerase II transcriptional machinery. We have used an in vitro chromatin assembly and transcription system to examine the biochemistry of interactions among ERalpha, the SRC proteins and p300/CBP. Using polypeptides designed to block specific receptor- cofactor or cofactor-cofactor interactions, we show that interactions among ERalpha, its coactivators and the RNA pol II machinery are all required for ERalpha- mediated transcription. Furthermore, we show that ERalpha-SRC-p300/CBP interactions are necessary and sufficient for the targeted acetylation of nucleosomal histones on estrogen-responsive promoters in the absence of transcription. The protein-protein interactions required for histone acetylation constitute a subset of the interactions required for transcriptional activation. Finally, we show that the major role of SRC-p300/CBP interactions is to enhance ERalpha- mediated transcription initiation, and they have little or no role in stimulating subsequent rounds of transcription. Together, our results indicate a specific role for the SRC and p300/CBP coactivators, as well as targeted histone acetylation, in ERalpha-mediated transcription.

The POU domain factor Skin-1a represses the keratin 14 promoter independent of DNA binding. A possible role for interactions between Skn-1a and CREB-binding protein/p300

The genes encoding keratin 5 and 14 are highly expressed in the basal cell layer keratinocytes of the epidermis, but both genes are silenced when keratinocytes move into the suprabasal compartment. The POU homeodomain factors Skn-1a and Tst-1, which are expressed in epidermis, may play a role in the suprabasal repression of the keratin 5 and 14 genes because keratin 14 mRNA expression persists in suprabasal cells in Skn-1/Tst-1 double knockout mice. In transfection experiments, both Skn-1a and Tst-1 repress the keratin 14 promoter, with the POU domain being sufficient for repression. The region of the keratin 14 gene sufficient and required for repression by Skn-1a is a 100-base pair sequence lacking POU-binding sites adjacent to the transcription start site. DNA-binding defective mutants of Skn-1a and Tst-1 are as effective at mediating repression as the wild type proteins, suggesting that protein-protein interactions rather than direct DNA binding are important for repression. We also show that CREB-binding protein (CBP)/p300 co-activators are strong activators of keratin 14 gene expression, acting through sequences close to the keratin 14 promoter. Further, CBP interacts directly with the POU domain of Skn-1a, and increasing concentrations of CBP can overcome Skn-1a-mediated repression, suggesting that POU domain factors may repress keratin 14 gene expression by interfering with the activity of co-activators such as CBP/p300.

p300/CBP proteins: HATs for transcriptional bridges and scaffolds

p300/CBP transcriptional co-activator proteins play a central role in co-ordinating and integrating multiple signal-dependent events with the transcription apparatus, allowing the appropriate level of gene activity to occur in response to diverse physiological cues that influence, for example, proliferation, differentiation and apoptosis. p300/CBP activity can be under aberrant control in human disease, particularly in cancer, which may inactivate a p300/CBP tumour-suppressor-like activity. The transcription regulating-properties of p300 and CBP appear to be exerted through multiple mechanisms. They act as protein bridges, thereby connecting different sequence-specific transcription factors to the transcription apparatus. Providing a protein scaffold upon which to build a multicomponent transcriptional regulatory complex is likely to be an important feature of p300/CBP control. Another key property is the presence of histone acetyltransferase (HAT) activity, which endows p300/CBP with the capacity to influence chromatin activity by modulating nucleosomal histones. Other proteins, including the p53 tumour suppressor, are targets for acetylation by p300/CBP. With the current intense level of research activity, p300/CBP will continue to be in the limelight and, we can be confident, yield new and important information on fundamental processes involved in transcriptional control.

Differential control of transcription by DNA-bound cyclins

Different cyclins mediate different cell-cycle transitions. Some cyclins, such as cyclin A and cyclin E, form stable complexes with proteins that bind directly or indirectly to DNA and thus might be recruited to certain regions of the genome at specific times in the cell cycle. Furthermore, cyclins contain structural motifs that are also present in known transcriptional modulators. We found that cyclin A is a potent transcriptional repressor and cyclin E is a potent transcriptional activator when bound to DNA via a heterologous DNA binding domain. The former activity was linked to the integrity of the cyclin A cyclin fold, whereas the latter activity related to the ability of cyclin E to activate cdk2 and recognize substrates. Furthermore, we found that cyclin E, but not cyclin A, activated transcription in a cell-cycle-dependent manner when present in physiological concentrations as an unfused protein. These results suggest that cyclin A and cyclin E intrinsically differ with respect to their ability to modulate transcription when tethered to DNA.

Epstein-Barr virus immediate-early protein BRLF1 interacts with CBP, promoting enhanced BRLF1 transactivation

The Epstein-Barr virus (EBV) immediate-early protein BRLF1 is a transcriptional activator that mediates the switch from latent to lytic viral replication. Many transcriptional activators function, in part, due to an interaction with histone acetylases, such as CREB-binding protein (CBP). Here we demonstrate that BRLF1 interacts with the amino and carboxy termini of CBP and that multiple domains of the BRLF1 protein are necessary for this interaction. Furthermore, we show that the interaction between BRLF1 and CBP is important for BRLF1-induced activation of the early lytic EBV gene SM in Raji cells.

Insulin suppresses transactivation by CAAT/enhancer-binding proteins beta (C/EBPbeta). Signaling to p300/CREB-binding protein by protein kinase B disrupts interaction with the major activation domain of C/EBPbeta

CAAT/enhancer-binding proteins (C/EBPs) play an important role in the regulation of gene expression in insulin-responsive tissues. We have found that a complex containing C/EBPbeta interacts with an insulin response sequence in the insulin-like growth factor-binding protein-1 (IGFBP-1) gene and that a C/EBP-binding site can mediate effects of insulin on promoter activity. Here, we examined mechanisms mediating this effect of insulin. The ability of insulin to suppress promoter activity via a C/EBP-binding site is blocked by LY294002, a phosphatidylinositol 3-kinase inhibitor, but not by rapamycin, which blocks activation of p70(S6 kinase). Dominant negative phosphatidylinositol 3-kinase and protein kinase B (PKB) block the effect of insulin, while activated PKB suppresses promoter function via a C/EBP-binding site, mimicking the effect of insulin. Coexpression studies indicate that insulin and PKB suppress transactivation by C/EBPbeta, but not C/EBPalpha, and that N-terminal transactivation domains in C/EBPbeta are required. Studies with Gal4 fusion proteins reveal that insulin and PKB suppress transactivation by the major activation domain in C/EBPbeta (AD II), located between amino acids 31 and 83. Studies with E1A protein indicate that interaction with p300/CBP is required for transactivation by AD II and the effect of insulin and PKB. Based on a consensus sequence, we identified a PKB phosphorylation site (Ser(1834)) within the region of p300/CBP known to bind C/EBPbeta. Mammalian two-hybrid studies indicate that insulin and PKB disrupt interactions between this region of p300 and AD II and that Ser(1834) is critical for this effect. Signaling by PKB and phosphorylation of Ser(1834) may play an important role in modulating interactions between p300/CBP and transcription factors and mediate effects of insulin and related growth factors on gene expression.

Acetylation of adenovirus E1A regulates binding of the transcriptional corepressor CtBP

Adenovirus E1A mediates its effects on cellular transformation and transcription by interacting with critical cellular proteins involved in cell growth and differentiation. The amino terminus of E1A binds to CBP/p300 and associated histone acetyltransferases such as P/CAF. The carboxyl terminus binds to the carboxyl-terminal binding protein (CtBP), which associates with histone deacetylases. We show that 12S E1A can be acetylated by p300 and P/CAF and map one of the acetylation sites to Lys-239. This Lys residue is adjacent to the consensus CtBP binding motif, PXDLS. Mutation of Lys-239 to Gln or Ala blocks CtBP binding in vitro and disrupts the E1A-CtBP interaction in vivo. Peptide competition assays demonstrated that the interaction of E1A with CtBP is also blocked by Lys-239 acetylation. Supporting a functional role for Lys-239 in CtBP binding, mutation of this residue to Ala decreases the ability of E1A to block cAMP-regulated enhancer (CRE)-binding protein (CREB)-stimulated gene expression. Finally, we demonstrate that Lys-239 is acetylated in cells by using an antibody directed against an acetyl-Lys-239 E1A peptide. CtBP interacts with a wide variety of other transcriptional repressors through the PXDLS motif, and, in many instances, this motif is followed by a Lys residue. We suggest that acetylation of this residue by histone acetyltransferases, and the consequent disruption of repressor complexes, might be a general mechanism for gene activation.

Acetylation of HIV-1 Tat by CBP/P300 increases transcription of integrated HIV-1 genome and enhances binding to core histones

The HIV-1 Tat protein is required for viral replication and is a potent stimulator of viral transcription. Although Tat has been extensively studied in various reductive paradigms, to date there is little information as to how this activator mediates transcription from natural nucleosomally packaged long terminal repeats. Here we show that CREB-binding protein (CBP)/p300 interacts with the HIV-1 Tat protein and serves as a coactivator of Tat-dependent HIV-1 gene expression on an integrated HIV-1 provirus. The site of acetylation of Tat was mapped to the double-lysine motif in a highly conserved region, (49)RKKRRQ(54), of the basic RNA-binding motif of Tat. Using HLM1 cells (HIV-1(+)/Tat(-)), which contain a single copy of full-length HIV-1 provirus with a triple termination codon at the first AUG of the Tat gene, we find that only wild type, and not K50A, K51A, or K50A/K51A alone or in combination of ectopic CBP/p300, is able to produce full-length infectious virions, as measured by p24 gag ELISAs. Tat binds CBP/p300 in the minimal histone acetyltransferase domain (1253-1710) and the binding is stable up to 0.85 M salt wash conditions. Interestingly, wild-type peptide 41-54, and not other Tat peptides, changes the conformation of the CBP/p300 such that it can acquire and bind better to basal factors such as TBP and TFIIB, indicating that Tat may influence the transcription machinery by helping CBP/p300 to recruit new partners into the transcription machinery. Finally, using biotinylated wild-type or acetylated peptides, we find that acetylation decreases Tat's ability to bind the TAR RNA element, as well as to bind basal factors such as TBP, CBP, Core-Pol II, or cyclin T. However, the acetylated Tat peptide is able to bind to core histones on a nucleosome assembled HIV-1 proviral DNA.

Regulation of beta -catenin transformation by the p300 transcriptional coactivator

The beta-catenin protein plays a critical role in embryonic development and mature tissue homeostasis through its effects on E-cadherin-mediated cell adhesion and Wnt-dependent signal transduction. In colon and other cancers, mutations of beta-catenin or the adenomatous polyposis coli (APC) tumor suppressor appear to stabilize beta-catenin and enhance its interaction with T cell factor (TCF) or lymphoid enhancer factor (Lef) transcription factors. At present, a complete picture of the means by which beta-catenin's interactions with TCF/Lef proteins contribute to neoplastic transformation is lacking. We report that the transcriptional coactivator p300 interacts with beta-catenin in vitro and in vivo and is critical for beta-catenin-mediated neoplastic transformation. p300 synergistically activates beta-catenin/TCF transcription, and their biochemical association requires the CH1 domain of p300 and a region of beta-catenin that includes its NH(2)-terminal transactivation domain and the first two armadillo repeats. Lowering of cellular p300 levels by using a ribozyme directed against p300 reduced TCF transcriptional activity and inhibited the neoplastic growth properties of a beta-catenin-transformed rat epithelial cell line and a human colon carcinoma line with a beta-catenin mutation. These findings demonstrate a critical role for p300 in beta-catenin/TCF transcription and in cancers arising from defects in beta-catenin regulation.

Modulation of histone acetyltransferase activity through interaction of epstein-barr nuclear antigen 3C with prothymosin alpha

The Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is essential for EBV-dependent immortalization of human primary B lymphocytes. Genetic analysis indicated that amino acids 365 to 992 are important for EBV-mediated immortalization of B lymphocytes. We demonstrate that this region of EBNA3C critical for immortalization interacts with prothymosin alpha (ProTalpha), a cellular protein previously identified to be important for cell division and proliferation. This interaction maps to a region downstream of amino acid 365 known to be involved in transcription regulation and critical for EBV-mediated transformation of primary B lymphocytes. Additionally, we show that EBNA3C also interacts with p300, a cellular acetyltransferase. This interaction suggests a possible role in regulation of histone acetylation and chromatin remodeling. An increase in histone acetylation was observed in EBV-transformed lymphoblastoid cell lines, which is consistent with increased cellular gene expression. These cells express the entire repertoire of latent nuclear antigens, including EBNA3C. Expression of EBNA3C in cells with increased acetyltransferase activity mediated by the EBV transactivator EBNA2 results in down-modulation of this activity in a dose-responsive manner. The interactions of EBNA3C with ProTalpha and p300 provide new evidence implicating this essential EBV protein EBNA3C in modulating the acetylation of cellular factors, including histones. Hence, EBNA3C plays a critical role in balancing cellular transcriptional events by linking the biological property of mediating inhibition of EBNA2 transcription activation and the observed histone acetyltransferase activity, thereby orchestrating immortalization of EBV-infected cells.

Histone binding protein RbAp48 interacts with a complex of CREB binding protein and phosphorylated CREB

A CREB-CREB binding protein (CBP) complex was used as bait to screen a mouse embryo cDNA library in yeast. One of the strongest interactions identified the histone binding protein RbAp48. RbAp48 also interacted weakly with CBP alone but did not interact with phosphorylated or nonphosphorylated CREB. CBP (or its homologue p300) from HeLa cell nuclear extracts coimmunoprecipitated with RbAp48 and its homologue RbAp46 and bound to a glutathione S-transferase-RbAp48 fusion protein. This interaction was stimulated by the addition of phosphorylated CREB and allowed the association of core histones and mononucleosomes in an acetylation-dependent manner. RbAp48 lowered the K(m) of CBP histone acetylase activity and facilitated p300-mediated in vitro transcription of a chromatinized template in the presence of acetylcoenzyme A. These data indicate that the association of phosphorylated CREB with CBP promotes the binding of RbAp48 and its homologue RbAp46, allowing the formation of a complex that facilitates histone acetylation during transcriptional activation.

The p300/CBP acetyltransferases function as transcriptional coactivators of beta-catenin in vertebrates

Wnt growth factors regulate a variety of developmental processes by altering specific gene expression patterns. In vertebrates beta-catenin acts as transcriptional activator, which is needed to overcome target gene repression by Groucho/TLE proteins, and to permit promoter activation as the final consequence of Wnt signaling. However, the molecular mechanisms of transcriptional activation by beta-catenin are only poorly understood. Here we demonstrate that the closely related acetyltransferases p300 and CBP potentiate beta-catenin-mediated activation of the siamois promoter, a known Wnt target. beta-catenin and p300 also synergize to stimulate a synthetic reporter gene construct, whereas activation of the cyclin D1 promoter by beta-catenin is refractory to p300 stimulation. Axis formation and activation of the beta-catenin target genes siamois and Xnr-3 in Xenopus embryos are sensitive to the E1A oncoprotein, a known inhibitor of p300/CBP. The C-terminus of beta-catenin interacts directly with a region overlapping the CH-3 domain of p300. p300 could participate in alleviating promoter repression imposed by chromatin structure and in recruiting the basal transcription machinery to promoters of particular Wnt target genes.

A novel transcriptional repression domain mediates p21(WAF1/CIP1) induction of p300 transactivation

The transcriptional coactivators p300 and CREB binding protein (CBP) are important regulators of the cell cycle, differentiation, and tumorigenesis. Both p300 and CBP are targeted by viral oncoproteins, are mutated in certain forms of cancer, are phosphorylated in a cell cycle-dependent manner, interact with transcription factors such as p53 and E2F, and can be found complexed with cyclinE-Cdk2 in vivo. Moreover, p300-deficient cells show defects in proliferation. Here we demonstrate that transcriptional activation by both p300 and CBP is stimulated by coexpression of the cyclin-dependent kinase inhibitor p21(WAF/CIP1). Significantly this stimulation is independent of both the inherent histone acetyltransferase (HAT) activity of p300 and CBP and of the previously reported carboxyl-terminal binding site for cyclinE-Cdk2. Rather, we describe a previously uncharacterized transcriptional repression domain (CRD1) within p300. p300 transactivation is stimulated through derepression of CRD1 by p21. Significantly p21 regulation of CRD1 is dependent on the nature of the core promoter. We suggest that CRD1 provides a novel mechanism through which p300 and CBP can switch activities between the promoters of genes that stimulate growth and those that enhance cell cycle arrest.

Recruitment of CREB binding protein is sufficient for CREB-mediated gene activation

Phosphorylation of the transcription factor CREB leads to the recruitment of the coactivator, CREB binding protein (CBP). Recent studies have suggested that CBP recruitment is not sufficient for CREB function, however. We have identified a conserved protein-protein interaction motif within the CBP-binding domains of CREB and another transcription factor, SREBP (sterol-responsive element binding protein). In contrast to CREB, SREBP interacts with CBP in the absence of phosphorylation. We have exploited the conservation of this interaction motif to test whether CBP recruitment to CREB is sufficient for transcriptional activation. Substitution of six nonconserved amino acids from SREBP into the activation domain of CREB confers high-affinity, phosphorylation-independent CBP binding. The mutated CREB molecule, CREB(DIEDML), activates transcription in F9 teratocarcinoma and PC12 cells even in the absence of protein kinase A (PKA). Addition of exogenous CBP augments the level of transcription mediated by CREB(DIEDML), and adenovirus 12S E1A blocks transcription, implicating CBP in the activation process. Thus, recruitment of CBP to CREB is sufficient for transcriptional activation. Addition of PKA stimulates transcription induced by CREB(DIEDML) further, suggesting that a phosphorylation event downstream from CBP recruitment augments CREB signaling.