The adenovirus E1A repression domain disrupts the interaction between the TATA binding protein and the TATA box in a manner reversible by TFIIB

Enhanced MYC association with the NuA4 histone acetyltransferase complex mediated by the adenovirus E1A N-terminal domain activates a subset of MYC target genes highly expressed in cancer cells

The proto-oncogene MYC is a transcription factor over-expressed in many cancers and required for cell survival. Its function is regulated by histone acetyltransferase (HAT) complexes, such as the GCN5 complex and the NuA4/Tip60 complex. However, the roles of the HAT complexes during MYC function in cancer have not been well characterized. We recently showed that adenovirus E1A and its N-terminal 80 aa region, E1A 1-80, interact with the NuA4 complex, through the E1A TRRAP-targeting (ET) domain, and enhance MYC association with the NuA4 complex. We show here that the ET domain mainly targets the MYC-NuA4 complex. By global gene expression analysis using E1A 1-80 and deletion mutants, we have identified a panel of genes activated by targeting the MYC-NuA4 complex and notably enriched for genes involved in ribosome biogenesis and gene expression. A second panel of genes is activated by E1A 1-80 targeting of both the MYC-NuA4 complex and p300, and is enriched for genes involved in DNA replication and cell cycle processes. Both panels of genes are highly expressed in cancer cells. Since the ET domain is essential for E1A-mediated cellular transformation, our results suggest that MYC and the NuA4 complex function cooperatively in cell transformation and cancer.

An integrated ontology resource to explore and study host-virus relationships

Our growing knowledge of viruses reveals how these pathogens manage to evade innate host defenses. A global scheme emerges in which many viruses usurp key cellular defense mechanisms and often inhibit the same components of antiviral signaling. To accurately describe these processes, we have generated a comprehensive dictionary for eukaryotic host-virus interactions. This controlled vocabulary has been detailed in 57 ViralZone resource web pages which contain a global description of all molecular processes. In order to annotate viral gene products with this vocabulary, an ontology has been built in a hierarchy of UniProt Knowledgebase (UniProtKB) keyword terms and corresponding Gene Ontology (GO) terms have been developed in parallel. The results are 65 UniProtKB keywords related to 57 GO terms, which have been used in 14,390 manual annotations; 908,723 automatic annotations and propagated to an estimation of 922,941 GO annotations. ViralZone pages, UniProtKB keywords and GO terms provide complementary tools to users, and the three resources have been linked to each other through host-virus vocabulary.

Expression of the Adenovirus Early Gene 1A Transcription-Repression Domain Alone Downregulates HER2 and Results in the Death of Human Breast Cancer Cells Upregulated for the HER2 Proto-Oncogene

Adenovirus (Ad) early gene 1A 243 residue protein (E1A 243R) possesses a potent transcription-repression function within the N-terminal 80 amino acids (E1A 1-80). We examined the ability of E1A 243R and E1A 1-80 to repress transcription of both an exogenous and the endogenous HER2 promoter in a human breast cancer cell line upregulated for the HER2 proto-oncogene (SK-BR-3). Both moieties repressed HER2 expression by over 90%. When E1A 1-80 was expressed from a nonreplicative Ad vector, levels of expression were lower than anticipated. Addition of nonspecific sequences to the E1A 1-80 C-terminus (E1A 1-80 C+) enhanced its expression 10- to 20-fold. Because "oncogene addiction" suggests that repression of HER2 could kill HER2 upregulated cells, we examined the ability of full-length E1A 243R and E1A 1-80 C+ delivered by an Ad vector to kill HER2 upregulated SK-BR-3 cells. Expression of both E1A 243R and E1A 1-80 C+ killed SK-BR-3 cells but not normal breast cells. E1A 1-80 C+ is a particularly effective killer of SK-BR-3 cells. At 144 h post infection, over 85% of SK-BR-3 cells were killed by a 100 moi of the Ad vector expressing E1A 1-80 C+. As controls, Ad vectors expressing E1A 243R with deletion of all known functional domains or expressing unrelated β-galactosidase had no effect. Three additional human breast cancer cells lines reported to be upregulated for HER2 or another EGF family member (EGFR) were found to be efficiently killed by expression of E1A 1-80 C+, whereas three additional "normal" cell lines (two derived from breast and one from foreskin) were not. The ability of the E1A transcription-repression domain alone to kill HER2 upregulated breast cancer cells has potential for development of therapies for treatment of aggressive human breast cancers and potentially other human cancers that overexpress HER2.

The adenovirus E1A N-terminal repression domain represses transcription from a chromatin template in vitro

The adenovirus repression domain of E1A 243R at the E1A N-terminus (E1A 1-80) transcriptionally represses genes involved in differentiation and cell cycle progression. E1A 1-80 represses transcription in vitro from naked DNA templates through its interaction with p300 and TFIID. E1A 1-80 can also interact with several chromatin remodeling factors and associates with chromatin in vivo. We show here that E1A 243R and E1A 1-80 can repress transcription from a reconstituted chromatin template in vitro. Temporal analysis reveals strong repression by E1A 1-80 when added at pre-activation, activation and early transcription stages. Interestingly, E1A 1-80 can greatly enhance transcription from chromatin templates, but not from naked DNA, when added at pre-initiation complex (PIC) formation and transcription-initiation stages. These data reveal a new dimension for E1A 1-80's interface with chromatin and may reflect its interaction with key players in PIC formation, p300 and TFIID, and/or possibly a role in chromatin remodeling.

Transcriptional control by adenovirus E1A conserved region 3 via p300/CBP

The human adenovirus type 5 (HAdV-5) E1A 13S oncoprotein is a potent regulator of gene expression and is used extensively as a model for transcriptional activation. It possesses two independent transcriptional activation domains located in the N-terminus/conserved region (CR) 1 and CR3. The protein acetyltransferase p300 was previously identified by its association with the N-terminus/CR1 portion of E1A and this association is required for oncogenic transformation by E1A. We report here that transcriptional activation by 13S E1A is inhibited by co-expression of sub-stoichiometric amounts of the smaller 12S E1A isoform, which lacks CR3. Transcriptional inhibition by E1A 12S maps to the N-terminus and correlates with the ability to bind p300/CBP, suggesting that E1A 12S is sequestering this limiting factor from 13S E1A. This is supported by the observation that the repressive effect of E1A 12S is reversed by expression of exogenous p300 or CBP, but not by a CBP mutant lacking actyltransferase activity. Furthermore, we show that transcriptional activation by 13S E1A is greatly reduced by siRNA knockdown of p300 and that CR3 binds p300 independently of the well-characterized N-terminal/CR1-binding site. Importantly, CR3 is also required to recruit p300 to the adenovirus E4 promoter during infection. These results identify a new functionally significant interaction between E1A CR3 and the p300/CBP acetyltransferases, expanding our understanding of the mechanism by which this potent transcriptional activator functions.

The transcription-repression domain of the adenovirus E1A oncoprotein targets p300 at the promoter

Extensive mutational/functional analysis of the transcription-repression domain encoded in the N-terminal 80 amino acids of the adenovirus E1A 243R oncoprotein suggests a model for the molecular mechanism of E1A repression: E1A accesses transcriptional co-activators such as p300 on specific promoters and then interacts with TBP to disrupt the TBP-TATA complex. In support of this model, as reported here, a basal core promoter activated by tethering p300 is repressible by E1A at the promoter level as shown by chromatin immunoprecipitation (ChIP) analysis. Sequestration of p300 by E1A does not play a significant role, as indicated by dose-response measurements. Furthermore, when the core promoter is transcriptionally activated by tethering activation domains of several transcription factors that can recruit p300 (p65, MyoD, cMyb and TFE3), transcription is repressible by E1A. However, when the core promoter is activated by factors not known to recruit p300 (USF1 and USF2), transcription is resistant to E1A repression. Finally, tethering p300 to the non-repressible adenovirus major late promoter (MLP) renders it repressible by E1A. ChIP analysis shows that E1A occupies the repressed MLP. These findings provide support for the hypothesis that p300 can serve as a scaffold for the E1A repression domain to access specific cellular gene promoters involved in growth regulation.

Adenovirus E1A proteins are closely associated with chromatin in productively infected and transformed cells

The adenovirus E1A 243R oncoprotein encodes a potent transcription-repression function within the N-terminal 80 amino acids. Our proposed model of E1A repression predicts that E1A interacts with important cellular proteins on chromatin. Consistent with this idea, we report here that E1A proteins from in vivo formaldehyde cross-linked 293 cells are closely associated with chromatin even after several stringent purification steps including double isopycnic CsCl density gradient centrifugation and size exclusion chromatography. Likewise, E1A proteins expressed from virus during productive infection of HeLa cells are closely associated with chromatin starting at early times after infection. No other adenoviral proteins are necessary for E1A 243R protein to associate with chromatin. Analyses of chromatin from HeLa cells infected with adenovirus vectors expressing E1A 243R protein with deletions in different E1A functional domains indicate that sequences within the E1A N-terminal repression domain are needed for the majority of E1A's interactions with chromatin.

Vaccinia virus intermediate and late promoter elements are targeted by the TATA-binding protein

Vaccinia virus replicates in the cytoplasm of the host cell and encodes its own RNA polymerase and transcription factors. The proteins that target the poxvirus RNA polymerase to intermediate- and late-class promoters have not been identified. In this study, representatives of the intermediate and late promoters were characterized at the nucleotide level to identify essential motifs. Both intermediate and late viral promoters are shown to have an essential element suggestive of TATA boxes, which are potential targets for the TATA-binding protein (TBP). Several approaches were used to test for TBP requirement in vaccinia virus transcription, including overexpression of TBP, expression of a dominant negative mutant of TBP, RNA interference, and expression of adenovirus E1A protein, which inactivates TBP. In each case, the results support an essential role for TBP in vaccinia virus intermediate- and late-gene transcription. These findings indicate that poxviruses have integrated TBP as a central feature into an otherwise heterologous transcription system. A model for transcriptional switching, in which both intermediate and late promoter elements are targeted by TBP that recruits viral transcription factors to assemble a functional complex on their cognate promoters and a dysfunctional, repressed complex on the other class, is proposed.

Mutational and functional analysis of an essential subdomain of the adenovirus E1A N-terminal transcription repression domain

Adenovirus early gene 1A (E1A) possesses a potent transcriptional repression function within the first 80 amino acids (E1A 1-80). Our previous analysis of subdomain 1 (residues 1 to 30) revealed strong correlations between residues required for repression and for disruption of TBP-TATA complexes. Here, we report a functional analysis of subdomain 2 (48 to 60) by alanine-scanning mutagenesis. 53Ala, 54Pro, 55Glu, and 56Asp are required for repression in vitro and in vivo and for efficient interaction with p300 but not for disruption of TBP-TATA. These combined results suggest a model for E1A transcription repression. E1A through subdomains 1 and 2 uses coactivators like p300 as scaffolds to access E1A repressible promoters. At the promoter, subdomain 1 interacts with TBP to disrupt TBP-TATA and abort transcription initiation. In further support of this model, we show that E1A 1-80 bound to the p300-binding site retains the ability to interact with TBP.

Varicella-zoster virus IE63 protein represses the basal transcription machinery by disorganizing the pre-initiation complex

Using transient transfection assays, regulation properties of varicella-zoster virus (VZV)-encoded IE63 protein were analyzed on several VZV immediate early (ORF4), early (ORF28) and late (ORF67) promoters. IE63 was shown to repress the basal activity of most of the promoters tested in epithelial (Vero) and neuronal (ND7) cells to various extents. Trans-repressing activities were also observed on heterologous viral and cellular promoters. Since a construct carrying only a TATA box sequence and a series of wild-type or mutated interleukin (IL)-8 promoters was also repressed by IE63, the role of upstream regulatory elements was ruled out. Importantly, the basal activity of a TATA-less promoter was not affected by IE63. Using a series of IE63 deletion constructs, amino acids 151-213 were shown to be essential to the trans-repressing activity in Vero cells, while in ND7 cells the essential region extended to a much larger carboxy-terminal part of the protein. We also demonstrate that IE63 is capable of disrupting the transcriptional pre-initiation complex and of interacting with several general transcription factors. The central and carboxy-terminal domains of IE63 are important for these effects. Altogether, these results demonstrate that IE63 protein is a transcriptional repressor whose activity is directed towards general transcription factors.

Accumulation of p53 in response to adenovirus early region 1A sensitizes human cells to tumor necrosis factor alpha-induced apoptosis

Many tumor cells are resistant to tumor necrosis factor alpha (TNFalpha)-induced apoptosis. Adenovirus early region 1A (AdE1A) sensitizes the otherwise resistant cells to TNFalpha. AdE1A also stabilizes the p53 protein. The present study demonstrates a correlation between AdE1A-induced sensitization and stabilization of p53 in TNFalpha-induced apoptosis since the N-terminal and CR2 regions, the binding sites for CBP/p300, Rb and 26S proteasome regulatory components, are required for both these actions of AdE1A. TNFalpha does not induce apoptosis and AdE1A fails to sensitize TNFalpha cytotoxicity in p53-negative cells. However, introduction of exogenous p53 overcomes the cellular resistance to TNFalpha toxicity and enhances AdE1A sensitization, demonstrating that AdE1A sensitizes TNFalpha-induced apoptosis by its stabilization of p53. A proteasome inhibitor, lactacystin, enhances TNFalpha cytotoxicity in p53-positive and -negative cells, suggesting that accumulation of cellular proteins other than p53 might also regulate the cellular response to TNFalpha signaling.

Recruitment of CBP/p300, TATA-binding protein, and S8 to distinct regions at the N terminus of adenovirus E1A

The N-terminal region of the adenovirus (Ad) 12S E1A gene product targets several cellular proteins that are essential for the induction of S phase, cellular immortalization, cellular transformation, transcriptional repression, and transcriptional activation. The precise binding sites for these proteins, however, remain to be resolved. We therefore undertook an extensive site-directed mutagenesis approach to generate specific point mutants and to precisely map the binding sites for CBP, p300, TATA-binding protein (TBP), S4, S8, hGcn5, P/CAF, and Ran within the first 30 amino acids of the Ad5 12S E1A protein. We determined that although common residues within the N-terminal region can form partial binding sites for these proteins, point mutants were also generated that could discriminate between binding sites. These data indicate that AdE1A can target each of these proteins individually through distinct binding sites. It was evident, however, that the mutation of specific hydrophobic residues typically had the greatest effect upon AdE1A's ability to bind individual partners. Indeed, the mutation of L at positions 19 and 20 eliminated the ability of AdE1A to interact with any of the N-terminal binding proteins studied here. Interestingly, although TBP and S8 or CBP/p300 can exist as functional complexes, RNA interference revealed that the recruitment of either TBP, S8, or CBP/p300 to AdE1A was not dependent upon the expression of the other proteins. These data further indicate that AdE1A can target individual partner proteins in vivo and that it does not necessarily recruit these proteins indirectly as components of larger macromolecular complexes. Finally, we took advantage of the fine-mapping data to ascertain which proteins were targeted during the transformation process. Consistent with previous studies, CBP/p300 was found to be targeted by AdE1A during this process, although our data suggest that binding to other N-terminal proteins is also important for transformation.

Cell Cycle-mediated Regulation of Smooth Muscle α-Actin Gene Transcription in Fibroblasts and Vascular Smooth Muscle Cells Involves Multiple Adenovirus E1A-interacting Cofactors

Expression of smooth muscle alpha-actin in growth factor-induced myofibroblasts and in differentiated vascular smooth muscle cells is transcriptionally controlled by multiple positive or negative trans-acting factors interacting with distinct cis-elements in the 5'-flanking region of the gene. Because none of the transcriptional regulators reported to date is smooth muscle cell- or myofibroblast-specific per se, the dynamic interplay among many factors interacting at specific sites along the promoter appears to be a signature feature of smooth muscle alpha-actin gene regulation in these cell types. Herein, the ability of the adenovirus E1A 12 S protein to bind and functionally inactivate specific cell regulatory factors has been exploited to identify several previously unknown coactivators of the mouse smooth muscle alpha-actin promoter in rodent fibroblasts and vascular smooth muscle cells. In transient cotransfection assays, ectopic expression of wild type E1A suppressed promoter activity in a dose- and cis-element-dependent manner. In asynchronous cells, N-terminal E1A mutants defective in CREB-binding protein (CBP) and p300 binding capacity exhibited markedly reduced inhibitory activity toward a smooth muscle alpha-actin promoter driven by a composite TEF-1-, SRF-, and Sp1/3-regulated enhancer. In synchronized cells, however, a more complex mutant E1A inhibitory pattern indicated that collaboration between CBP/p300 and the retinoblastoma family of pocket proteins was required to produce a fully functional enhancer. Cotransfection experiments conducted with Rb(-/-) fibroblasts demonstrated the necessity of pRB in augmenting smooth muscle alpha-actin enhancer/promoter activity. Physical interaction studies with the use of purified wild type and mutant E1A proteins confirmed that CBP, p300, and pRB were targets of E1A binding in nuclear extracts of vascular smooth muscle cells and/or fibroblasts. Collectively, these results suggest that a repertoire of E1A-interacting proteins, namely CBP/p300 and pRB, serve to integrate the activities of multiple trans-acting factors to control smooth muscle alpha-actin gene transcription in a cell type- and cell cycle-dependent manner.

Comprehensive sequence analysis of the E1A proteins of human and simian adenoviruses

Despite extensive study of human adenovirus type 5 E1A, surprisingly little is known about the E1A proteins of other adenoviruses. We report here a comprehensive analysis of the sequences of 34 E1A proteins. These represent all six primate adenovirus subgroups and include all human representatives of subgroups A, C, E, and F, eight from subgroup B, nine from subgroup D, and seven simian adenovirus E1A sequences. We observed that many, but not all, functional domains identified in human adenovirus type 5 E1A are recognizably present in the other E1A proteins. Importantly, we identified highly conserved sequences without known activities or binding partners, suggesting that previously unrecognized determinants of E1A function remain to be uncovered. Overall, our analysis forms a solid foundation for future study of the activities and features of the E1A proteins of different serotypes and identifies new avenues for investigating E1A function.

ZNF76, a Novel Transcriptional Repressor Targeting TATA-binding Protein, Is Modulated by Sumoylation

Direct interaction of positive and negative regulators with the general transcription machinery modulates transcription. The TATA-binding protein (TBP) is one target for transcriptional regulators. In this study, we identified ZNF76 as a novel transcriptional repressor that targets TBP. ZNF76 interacts with TBP through both its N and C termini, and both regions are required for ZNF76 to exert its inhibitory function on p53-mediated transactivation. The inhibitory effect of ZNF76 on p53 activity was demonstrated by reporter assays and endogenous target gene expression. We mapped the TBP-interacting region in the C terminus of ZNF76 to a glutamic acid-rich domain, which acts in a dominant negative manner to enhance p53-mediated transactivation in reporter assays. Mutagenesis study for ZNF76 suggests a correlation between interaction with TBP and effect on p53-mediated transactivation, supporting the conclusion that ZNF76 targets TBP for transcriptional repression. Chromatin immunoprecipitation experiments suggest that ZNF76 prevents TBP from occupying the endogenous p21 promoter. ZNF76 is sumoylated by PIAS1 at lysine 411, which is in the minimal TBP-interacting region. Overexpression of PIAS1 and SUMO-1 abolishes the interaction between ZNF76 and TBP and partially relieves the repressive effect of ZNF76. These results suggest that ZNF76 functions as a transcriptional repressor through its interaction with TBP and that sumoylation modulates its transcriptional repression activity.

The targeting of the proteasomal regulatory subunit S2 by adenovirus E1A causes inhibition of proteasomal activity and increased p53 expression

Although adenovirus early region 1A (AdE1A) can modulate protein expression through its interaction with transcriptional regulators it can also influence the ability of the cell to degrade proteins by binding to components of the 26 S proteasome. We demonstrate here that AdE1A interacts with the S2 subunit of the 19 S regulatory complex in addition to the ATPase subunits S4 and S8 previously identified. S2 forms complexes with both the 13 and 12 S AdE1A proteins both in vivo and in vitro. Mutational analysis has shown direct binding through a short sequence toward the N terminus of conserved region 2 of AdE1A, which encompasses the LXCXE motif, involved in interaction with the pRb family of proteins. In vivo, additional contacts are made between AdE1A and proteasomal components, as well as within the proteasome, such that deletion of the N-terminal region of E1A as well as part of conserved region 2 is required to completely disrupt S2 binding. Mutation of AdE1A, which disrupts complex formation with S2, results in the loss of its ability to stabilize the p53 protein. Similarly down-regulation of S2 expression using small interfering RNAs leads to the inhibition of p53 degradation. These effects were observed in normally growing cells and those subjected to UV irradiation. Furthermore, AdE1A had no effect on the Mdm2-mediated ubiquitination of p53. We suggest therefore that interaction of AdE1A with S2, as well as with the ATPases S4 and S8, directly causes inhibition of proteasomal activity and consequent increase in the protein levels of p53.

TFIIA abrogates the effects of inhibition by HMGB1 but not E1A during the early stages of assembly of the transcriptional preinitiation complex

Successful assembly of the transcriptional preinitiation complex (PIC) is prerequisite to transcriptional initiation. At each stage of PIC assembly, regulation may occur as repressors and activators compete with and influence the incorporation of general transcription factors (GTFs). Both TFIIA and HMGB1 bind individually to the TATA-binding protein (TBP) to increase the rate of binding and to stabilize TBP binding to the TATA element. The competitive binding between these two cofactors for TBP/TATA was examined to show that TFIIA binds preferentially to TBP and inhibits HMGB1 binding. TFIIA can also readily dissociate HMGB1 from the preestablished HMGB1/TBP/TATA complex. This suggests that TFIIA and HMGB1 may bind to the same or overlapping sites on TBP and/or compete for similar DNA sites that are 5' to the TATA element. In addition, EMSA studies show that adenovirus E1A(13S) oncoprotein is unable to disrupt either the preestablished TFIIA/TBP/TATA or TFIIA/TFIIB/TBP/TATA complexes, but does inhibit complex formation when all transcription factors were simultaneously added. The inhibitory effect of E1A(13S) on the assembly of the PIC is overcome when excess TBP is added back in the reaction, while addition of either excess TFIIA or TFIIB were ineffective. This shows that the main target for E1A(13S) is free TBP and emphasizes the primary competition between E1A and the TATA-element for unbound TBP. This may be the principal point, if not the only point, at which E1A can target TBP to exert its inhibitory effect. This work, coupled with previous findings in our laboratory, indicates that TFIIA is much more effective than TFIIB in reversing the inhibitory effect of HMGB1 binding in the early stages of PIC assembly, which is consistent with the in vitro transcription results.

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.

Transgenic expression in mouse lung reveals distinct biological roles for the adenovirus type 5 E1A 243- and 289-amino-acid proteins

Little is known about the biological significance of human adenovirus type 5 (Ad5) E1A in vivo. However, Ad5 E1A is well defined in vitro and can be detected frequently in the lungs of patients with pulmonary disease. Transgenic expression of the Ad5 E1A gene targeted to the mouse lung reveals distinct biological effects caused by two Ad5 E1A products. Either of two Ad5 E1A proteins was preferentially expressed in vivo in the transgenic lungs. The preferential expression of the Ad5 E1A 243-amino-acid (aa) protein at a moderate level was associated with cellular hyperplasia, nodular lesions of proliferating lymphocyte-like cells, and a low level of p53-dependent apoptosis in the lungs of transgenic mice. In contrast, the preferential expression of the Ad5 E1A 289-aa protein at a moderate level resulted in a proapoptotic injury and an acute pulmonary proinflammation in the lungs of transgenic mice, mediated by multiple apoptotic pathways, as well as an enhancement of the host immune cell response. Expression of the Ad5 E1A 243-aa protein resulted in proliferation-stimulated p53 upregulation, while expression of the Ad5 E1A 289-aa protein led to DNA damage-induced p53 activation. These data suggest that the Ad5 E1A 243- and 289-aa proteins lead to distinct biological roles in vivo.

Suppression of tissue factor expression, cofactor activity, and metastatic potential of murine melanoma cells by the N-terminal domain of adenovirus E1A 12S protein

Tissue factor, the cellular initiator of blood coagulation, has been implicated as a determinant of metastatic potential in human melanoma cells. Here, we report that differential expression of tissue factor in murine melanoma cell lines of known metastatic behavior is mediated by AP-1-dependent and 12S E1A oncoprotein-repressible gene transcription. When compared to weakly metastatic C10 cells, highly metastatic M4 cells possessed elevated levels of tissue factor cofactor activity, transfected promoter activity, and heterodimeric AP-1 DNA-binding complexes containing Fra-1. Transient co-expression of the adenovirus E1A 12S oncoprotein strongly repressed transcription of an AP-1-driven tissue factor reporter gene indicating the additional requirement of N-terminal E1A-interacting coactivators. Stable expression of E1A mutants defective in CBP/p300-binding failed to suppress tissue factor expression and experimental metastasis by M4 cells while clones expressing wild type E1A exhibited greatly reduced tissue factor cofactor activity and metastatic potential in vivo. Overexpression of functional tissue factor in cells containing wild type E1A failed to restore the highly metastatic M4 phenotype suggesting that additional E1A-responsive and CBP/p300-dependent genes are required to facilitate metastasis of murine melanoma cells demonstrating high tissue factor expression and cofactor activity.

Functional interaction between coactivators CBP/p300, PCAF, and transcription factor FKLF2

The Sp1/KLF family of factors regulates diverse cellular processes, including growth and development. Fetal Krüppel-like factor (FKLF2) is a new member of this family. In this study, we characterized the coactivators involved in FKLF2 transcriptional activation. Our results show that both CBP/p300 and p300/CBP-associated factor (PCAF) enhance FKLF2 transcriptional activity. We demonstrate that the acetyltransferase activity of PCAF but not that of CBP/p300 is required for stimulating FKLF2 transcription activity. We further show that p300 and PCAF act cooperatively in stimulating FKLF2 transcriptional activation. FKLF2 interacts with both CBP and PCAF through specific domains, and CBP and PCAF acetylate FKLF2. Both CBP/p300 and PCAF stimulate FKLF2 DNA binding activity. The integrity of the acetyltransferase domain of PCAF but not that of CBP/p300 is required for stimulating FKLF2 DNA binding activity. These results demonstrate that CBP/p300 and PCAF stimulate FKLF2 transcriptional activity at least by enhancing its DNA binding. The acetyltransferase activities of CBP/p300 and PCAF play a distinct role in stimulating FKLF2 transcription and DNA binding.

Adenovirus E1A N-terminal amino acid sequence requirements for repression of transcription in vitro and in vivo correlate with those required for E1A interference with TBP-TATA complex formation

The adenovirus (Ad) E1A 243R oncoprotein encodes an N-terminal transcription repression domain that is essential for early viral functions, cell immortalization, and cell transformation. The transcription repression function requires sequences within amino acids 1 to 30 and 48 to 60. To elucidate the roles of the TATA-binding protein (TBP), p300, and the CREB-binding protein (CBP) in the mechanism(s) of E1A repression, we have constructed 29 amino acid substitution mutants and 5 deletion mutants spanning the first 30 amino acids within the E1A 1-80 polypeptide backbone. These mutant E1A polypeptides were characterized with regard to six parameters: the ability to repress transcription in vitro and in vivo, to disrupt TBP-TATA box interaction, and to bind TBP, p300, and CBP. Two regions within E1A residues 1 to 30, amino acids 2 to 6 and amino acid 20, are critical for E1A transcription repression in vitro and in vivo and for the ability to interfere with TBP-TATA interaction. Replacement of 6Cys with Ala in the first region yields the most defective mutant. Replacement of 20Leu with Ala, but not substitutions in flanking residues, yields a substantially defective phenotype. Protein binding assays demonstrate that replacement of 6Cys with Ala yields a mutant completely defective in interaction with TBP, p300, and CBP. Our findings are consistent with a model in which the E1A repression function involves interaction of E1A with p300/CBP and interference with the formation of a TBP-TATA box complex.

Binding of PKA-RIIalpha to the Adenovirus E1A12S oncoprotein correlates with its nuclear translocation and an increase in PKA-dependent promoter activity

The adenovirus type 12 (Ad12) E1A12S oncoprotein utilizes the cAMP/protein kinase A (PKA) signal transduction pathway to activate expression of the viral E2 gene, the products of which are essential for viral replication. A central unsolved question is, however, whether E1A12S interacts directly with PKA in the process of promoter activation. We show here that E1A12S binds to the regulatory subunits (R) of PKA in vitro and in vivo. Interaction depends on the N-terminus and the conserved region 1 (CR1) of E1A12S. Both domains are also essential for the activation of viral E2 gene expression. Infection of cells with Ad12 leads to the cellular redistribution of RIIalpha from the cytoplasm into the nucleus. Furthermore, RIIalpha is also located in the nucleus of cells transformed by E1 of Ad12 and transient expression of E1A12S leads to the redistribution of RIIalpha into the nucleus in a N-terminus- and CR1-dependent manner. Cotransfection of E1A12S with RIIalpha results in strong activation of the E2 promoter. Based on these results we conclude that E1A12S functions as a viral A-kinase anchoring protein redistributing RIIalpha from the cytoplasm into the nucleus where it is involved in E1A12S-mediated activation of the E2 promoter.

Regulation of parathyroid hormone-related protein gene expression in murine keratinocytes by E1A isoforms: a role for basal promoter and Ets-1 site

PTHrP gene expression was evaluated in a murine keratinocyte line, Pam 212K, transformed with E1A and ras. We found that the 12S-E1A oncogene, with or without ras transformation, markedly reduced PTHrP mRNA expression. Using transient transfection assays, we found that the 12S isoform repressed activity from a 5'PTHrP-driven reporter gene. E1A-induced repression of PTHrP reporter constructs appears to be mediated by sequences within minimal promoter region. The 13S-E1A isoform did not repress PTHrP reporter gene activity, and a 13S-deletion mutant that lacked the repressor domains activated a subset of reporter constructs. Mutation of an Ets-1 binding site upstream of the basal promoter substantially decreased activation of reporter constructs by this 13S-deletion mutant. These findings suggest that the E1A oncoprotein may serve as a model for both activation and repression of PTHrP gene expression.

Even-skipped represses transcription by binding TATA binding protein and blocking the TFIID-TATA box interaction

The Drosophila homeodomain protein Even-skipped (Eve) is a transcriptional repressor, and previous studies have suggested that it functions by interfering with the basal transcription machinery. Here we describe experiments indicating that the mechanism of Eve repression involves a direct interaction with the TATA binding protein (TBP) that blocks binding of TBP-TFIID to the promoter. We first compared Eve activities in in vitro transcription systems reconstituted with either all the general transcription factors or only TBP, TFIIB, TFIIF30, and RNA polymerase II. In each case, equivalent and very efficient levels of repression were observed, indicating that no factors other than those in the minimal system are required for repression. We then show that Eve can function efficiently when its recognition sites are far from the promoter and that the same regions of Eve required for repression in vivo are necessary and sufficient for in vitro repression. This includes, in addition to an Ala-Pro-rich region, residues within the homeodomain. Using GAL4-Eve fusion proteins, we demonstrate that the homeodomain plays a role in repression in addition to DNA binding, which is to facilitate interaction with TBP. Single-round transcription experiments indicate that Eve must function prior to TBP binding to the promoter, suggesting a mechanism whereby Eve represses by competing with the TATA box for TBP binding. Consistent with this, excess TATA box-containing oligonucleotide is shown to specifically and efficiently disrupt the TBP-Eve interaction. Importantly, we show that Eve binds directly to TFIID and that this interaction can also be disrupted by the TATA oligonucleotide. We conclude that Eve represses transcription via a direct interaction with TBP that blocks TFIID binding to the promoter.

p300 mediates transcriptional stimulation by the basic helix-loop-helix activators of the insulin gene

Pancreatic beta-cell-type-specific and glucose-inducible transcription of the insulin gene is mediated by the basic helix-loop-helix factors that bind to and activate expression from an E-box element within its enhancer. The E-box activator is a heteromeric complex composed of a beta-cell-enriched factor, BETA2/NeuroD, and ubiquitously distributed proteins encoded by the E2A and HEB genes. Previously, we demonstrated that the adenovirus type 5 E1A proteins repressed stimulation by the E-box activator in beta cells. In this study, our objective was to determine how E1A repressed activator function. The results indicate that E1A reduces activation by binding to and sequestering the p300 cellular coactivator protein. Thus, we show that expression of p300 in beta cells can relieve inhibition by E1A, as well as potentiate activation by the endogenous insulin E-box transcription factors. p300 stimulated activation from GAL4 (amino acids 1 to 147) fusion constructs of either BETA2/NeuroD or the E2A-encoded E47 protein. The sequences spanning the activation domains of BETA2/NeuroD (amino acids 156 to 355) and E47 (amino acids 1 to 99 and 325 to 432) were required for this response. The same region of BETA2/NeuroD was shown to be important for binding to p300 in vitro. The sequences of p300 involved in E47 and BETA2/NeuroD association resided between amino acids 1 and 1257 and 1945 and 2377, respectively. A mutation in p300 that abolished binding to BETA2/NeuroD also destroyed the ability of p300 to activate insulin E-box-directed transcription in beta cells. Our results indicate that physical and functional interactions between p300 and the E-box activator factors play an important role in insulin gene transcription.

Amino acids 1-29 of the adenovirus serotypes 12 and 2 E1A proteins interact with rap30 (TF(II)F) and TBP in vitro

Early region 1A (E1A) gene products of adenoviruses (Ad) play an essential role in both productive infection and cellular transformation. Besides their function to induce the expression of all other adenoviral genes they modulate the expression of specific cellular genes to ensure an efficient viral reproduction. Gene regulatory functions of E1A proteins are mainly located in the conserved regions 1-3 (CRs) and in the non-conserved amino terminal end and are mediated via protein/protein interactions with cellular factors. We could show recently, that the E1A N-terminus (amino acids [aa] 1-29) of oncogenic Ad12 contains a unique 'trans'-activation domain. Here we demonstrate that this region binds to rap30/TF(II)F and to the TATA-box binding protein TBP in vitro. Mutation analyses suggest that binding to rap30 and 'trans'-activation are two independent functions as a mutant which failed to interact with rap30 was still able to induce gene expression with wildtype efficiency. Moreover loss of transcriptional activity does not correlate with a loss of TBP binding suggesting that this association is not necessary for the N-terminal 'trans'-activating activity. Interestingly, aa 1-29 of Ad2 E1A binds also to rap30 indicating that this interaction might be a common feature of E1A proteins from different serotypes.

Involvement of negative cofactor NC2 in active repression by zinc finger-homeodomain transcription factor AREB6

The transcription factor AREB6 contains a homeodomain flanked by two clusters of Krüppel type C2H2 zinc fingers. AREB6 binds to the E-box consensus sequence, CACCTGT, through either the N- or the C-terminal zinc finger cluster. To gain insights into the molecular mechanism by which AREB6 activates and represses gene expression, we analyzed the domain structure of AREB6 in the context of a heterologous DNA-binding domain by transient-transfection assays. The C-terminal region spanning amino acids 1011 to 1124 was identified as a conventional acidic activation domain. The region containing amino acids 754 to 901, which was identified as a repression domain, consists of 40% hydrophobic amino acids displaying no sequence similarities to other known repression domains. This region repressed transcription in vitro in a HeLa nuclear extract but not in reconstituted transcription systems consisting of transcription factor IID (TFIID), TFIIB, TFIIE, TFIIH/F, and RNA polymerase II. The addition of recombinant negative cofactor NC2 (NC2alpha/DRAP1 and NC2beta/Dr1) to the reconstituted transcription system restored the activity of the AREB6 repression domain. We further demonstrated interactions between the AREB6 repression domain and NC2alpha in yeast two-hybrid assay. Our findings suggest a mechanism of transcriptional repression that is mediated by the general cofactor NC2.