General repression of enhanson activity by the adenovirus-2 E1A proteins

The Interplay between Adeno-Associated Virus and its Helper Viruses

The adeno-associated virus (AAV) is a small, nonpathogenic parvovirus, which depends on helper factors to replicate. Those helper factors can be provided by coinfecting helper viruses such as adenoviruses, herpesviruses, or papillomaviruses. We review the basic biology of AAV and its most-studied helper viruses, adenovirus type 5 (AdV5) and herpes simplex virus type 1 (HSV-1). We further outline the direct and indirect interactions of AAV with those and additional helper viruses.

Identification and analysis of the promoter region of the human DHCR24 gene: involvement of DNA methylation and histone acetylation

Mutations in the DHCR24 gene, which encodes the cholesterol biosynthesis enzyme 3ß-hydroxysterol-∆24 reductase, result in an autosomal recessive disease called desmosterolosis. Further, reduced expression of DHCR24 is found in the temporal cortex of Alzheimer's disease patients. This suggests that variability in the regulatory regions of DHCR24 may contribute to the development of this neurodegenerative disease. In this work, we functionally characterised the proximal fragment of the human DHCR24 gene, for the first time. We show that the transcription of DHCR24 is initiated from a single CpG-rich promoter that is regulated by DNA methylation in some cell types. An activator sequence was also uncovered in the -1203/-665 bp region by reporter gene assays. Furthermore, sodium butyrate (a well-known HDAC inhibitor) increased DHCR24 expression in SH-SY5Y cells by recruiting acetylated core histones H3 and H4 to the enhancer region, as demonstrated by transient transfection and chromatin immunoprecipitation assays. Understanding the regulation of the DHCR24 gene may lead to alternative therapeutic strategies in at least some Alzheimer's patients.

Structural basis for subversion of cellular control mechanisms by the adenoviral E1A oncoprotein

The adenovirus early region 1A (E1A) oncoprotein mediates cell transformation by deregulating host cellular processes and activating viral gene expression by recruitment of cellular proteins that include cyclic-AMP response element binding (CREB) binding protein (CBP)/p300 and the retinoblastoma protein (pRb). While E1A is capable of independent interaction with CBP/p300 or pRb, simultaneous binding of both proteins is required for maximal biological activity. To obtain insights into the mechanism by which E1A hijacks the cellular transcription machinery by competing with essential transcription factors for binding to CBP/p300, we have determined the structure of the complex between the transcriptional adaptor zinc finger-2 (TAZ2) domain of CBP and the conserved region-1 (CR1) domain of E1A. The E1A CR1 domain is unstructured in the free state and upon binding folds into a local helical structure mediated by an extensive network of intermolecular hydrophobic contacts. By NMR titrations, we show that E1A efficiently competes with the N-terminal transactivation domain of p53 for binding to TAZ2 and that pRb interacts with E1A at 2 independent sites located in CR1 and CR2. We show that pRb and the CBP TAZ2 domain can bind simultaneously to the CR1 site of E1A to form a ternary complex and propose a structural model for the pRb:E1A:CBP complex on the basis of published x-ray data for homologous binary complexes. These observations reveal the molecular basis by which E1A inhibits p53-mediated transcriptional activation and provide a rationale for the efficiency of cellular transformation by the adenoviral E1A oncoprotein.

Adenovirus small e1a alters global patterns of histone modification

Adenovirus small early region 1a (e1a) protein drives cells into S phase by binding RB family proteins and the closely related histone acetyl transferases p300 and CBP. The interaction with RB proteins displaces them from DNA-bound E2F transcription factors, reversing their repression of cell cycle genes. However, it has been unclear how the e1a interaction with p300 and CBP promotes passage through the cell cycle. We show that this interaction causes a threefold reduction in total cellular histone H3 lysine 18 acetylation (H3K18ac). CBP and p300 are required for acetylation at this site because their knockdown causes specific hypoacetylation at H3K18. SV40 T antigen also induces H3K18 hypoacetylation. Because global hypoacetylation at this site is observed in prostate carcinomas with poor prognosis, this suggests that processes resulting in global H3K18 hypoacetylation may be linked to oncogenic transformation.

The E1A proteins of all six human adenovirus subgroups target the p300/CBP acetyltransferases and the SAGA transcriptional regulatory complex

The N-terminal/conserved region 1 (CR1) portion of the human adenovirus (Ad) 5 E1A protein was previously shown to inhibit growth in the simple eukaryote Saccharomyces cerevisiae. We now demonstrate that the corresponding regions of the E1A proteins of Ad3,-4,-9,-12, and -40, which represent the remaining five Ad subgroups, also inhibit yeast growth. These results suggest that the E1A proteins of all six human Ad subgroups share a common cellular target(s) conserved in yeast. Growth inhibition induced by either full-length or the N-terminal/CR1 portion of Ad5 E1A was relieved by coexpression of the E1A binding portions of the mammalian p300, CBP, and pCAF acetyltransferases. Similarly, growth inhibition by the N-terminal/CR1 portions of the other Ad E1A proteins was suppressed by expression of the same regions of CBP or pCAF known to bind Ad5 E1A. The physical interaction of each of the different Ad E1A proteins with CBP, p300, and pCAF was confirmed in vitro. Furthermore, deletion of the gene encoding yGcn5, the yeast homolog of pCAF and a subunit of the SAGA transcriptional regulatory complex, restored growth in yeast expressing each of the different Ad E1A proteins. This indicates that the SAGA complex is a conserved target of all Ad E1A proteins. Our results demonstrate for the first time that the p300, CBP, and pCAF acetyltransferases are common targets for the E1A proteins of all six human Ad subgroups, highlighting the importance of these interactions for E1A function.

The multifunctional role of E1A in the transcriptional regulation of CREB/CBP-dependent target genes

Oncoproteins encoded by the early region 1A (E1A) of adenoviruses (Ads) have been shown to be powerful tools to study gene regulatory mechanisms. As E1A proteins lack a sequence-specific DNA-binding activity, they modulate viral and cellular gene expression by interacting directly with a diverse array of cellular factors, among them sequence-specific transcription factors, proteins of the general transcription machinery, co-activators and chromatin-modifying enzymes. By making use of these factors, E1A affects major cellular events such as cell cycle control, differentiation, apoptosis, and oncogenic transformation. In this review we will focus on the interaction of E1A with cellular components involved in the cAMP/PKA signal transduction pathway and we will discuss the consequences of these interactions in respect to the activation of CREB/CBP-dependent target genes.

Evidence for a function of CtBP in epithelial gene regulation and anoikis

Previously, we reported that adenovirus E1a protein behaves as a tumor suppressor in human cells. It apparently functions by transcriptionally inducing an array of epithelial cell adhesion genes, while repressing other cell-type specific genes, thus producing an epithelial phenotype. Concomitantly, the cells become sensitive to anoikis (apoptosis of epithelial cells detached from extracellular matrix), potentially causing tumor suppression. E1a protein interacts with the nuclear acetylases p300, CBP and P/CAF, and also with the co-repressor protein CtBP. In this study, we have determined the role of these interactions in E1a's phenotypic effects on human tumor cells. The results indicate that E1a's interaction with CtBP activates at least three epithelial cell adhesion gene promoters. The E-cadherin repressor appeared to be the CtBP-interacting protein delta EF1/ZEB, which bound the ras-repressible E-boxes of the E-cadherin promoter. The E1a-CtBP interaction also contributed to anoikis-sensitization. E1a's interactions with the nuclear acetylases conferred epithelial morphologies but did not activate epithelial genes. These latter interactions did not sensitize tumor cells to anoikis but nevertheless conferred tumor suppression. These results implicate CtBP as an antagonist of the epithelial phenotype and anoikis. They also indicate a new but undefined role for nuclear acetylases in maintaining the transformed phenotype.

Conserved cAMP responsive element and core promoter complex are critical for specificity of the distal T-cell receptor beta chain enhancer for its native promoter

The Vbeta 8.1 promoter is regulated by T-cell-specific and ubiquitous transcription factors, which bind immediately upstream of and inside the core promoter region. The various Vbeta promoters contain two conserved elements, a cAMP responsive element (CRE) located upstream of the core promoter and a basal initiator flanked by two regulatory motifs. Here we have studied the interplay between the distal enhancer and its native promoter. We show that the remote enhancer acts specifically through its native promoter. Specific enhancer-promoter interplay is mediated through the conserved regions of the Vbeta promoters. Importantly, the conserved CRE serves as a functional recognition element for the enhancer whereas it barely contributes to promoter activity. The other conserved regions surrounding the initiation site are critical for activators that bind at and function through the core promoter region and thereby regulate both promoter and enhancer activity. The enhancer is highly sensitive to E1A-12S, which represses both general and specific enhancer activities. Enhancer activity and promoter-enhancer specificity is, at least in part, mediated by the coactivators CBP/p300.

Cooperation between phosphorylation and acetylation processes in transcriptional control

We previously reported that the activation of the M promoter of the human choline acetyltransferase (ChAT) gene by butyrate and trapoxin in transfected CHP126 cells is blocked by PD98059, a specific mitogen-activated protein kinase kinase (MEK) inhibitor (E. Espinos and M. J. Weber, Mol. Brain Res. 56:118-124, 1998). We now report that the transcriptional effects of histone deacetylase inhibitors are mediated by an H7-sensitive serine/threonine protein kinase. Activation of the ChAT promoter by butyrate and trapoxin was blocked by 50 microM H7 in both transient- and stable-transfection assays. Overexpression of p300, a coactivator protein endowed with histone acetyltransferase activity, stimulated the ChAT promoter and had a synergistic effect on butyrate treatment. These effects were blocked by H7 and by overexpressed adenovirus E1A 12S protein. Moreover, both H7 and PD98059 suppressed the activation of the Rous sarcoma virus (RSV) and simian virus 40 promoters by butyrate in transfection experiments. Similarly, the induction of the cellular histone H1(0) gene by butyrate in CHP126 cells was blocked by H7 and by PD98059. Previous data (L. Cuisset, L. Tichonicky, P. Jaffray, and M. Delpech, J. Biol. Chem. 272:24148-24153, 1997) showed that the induction of the H1(0) gene by butyrate is blocked by okadaic acid, an inhibitor of protein phosphatases. We now show that the activation of the ChAT and RSV promoters by butyrate in transfected CHP126 cells is also blocked by 200 nM okadaic acid. Western blotting and in vivo metabolic labeling experiments showed that butyrate has a biphasic effect on histone H3 phosphorylation, i.e., depression for up to 16 h followed by stimulation. The data thus strongly suggest that the transcriptional effects of histone deacetylase inhibitors are mediated through the activation of MEK1 and of an H7-sensitive protein kinase in addition to protein phosphatases.

Differences in the interactions of oncogenic adenovirus 12 early region 1A and nononcogenic adenovirus 2 early region 1A with the cellular coactivators p300 and CBP

Association with the cellular coactivators p300 and CBP is required for the growth-regulatory function of adenoviral (Ad) early region 1A (E1A) proteins. E1A regions necessary for these interactions overlap with domains involved in the induction of tumours in immunocompetent rodents through highly oncogenic Ad12. Differences in the association of cellular factors with the respective E1A domains of Ad12 and nononcogenic Ad2 might therefore be involved in serotype-specific oncogenicity. We analyzed the interaction of the Ad12 E1A 235R protein with p300 and CBP. Here we demonstrate that in the case of Ad12, but not Ad2/5, amino acids (aa) 1-29 of E1A proteins are sufficient to bind the p300-C/H3 domain in vivo and wild-type p300 in vitro. The conserved arginine-2, which is essential for the interaction between Ad2 E1A and p300, was dispensable for the Ad12 E1A 235R-p300 interaction in vitro. In addition to the p300-C/H3 region, we identified a second domain within p300 (aa 1999-2200) binding to the 235R protein. Contrary to p300, the amino-terminus and CR1 are necessary to associate with CBP. The aa 1-29 of the 235R protein but not CR1 are essential for the repression of colTRE-driven gene expression. This repression function is strictly dependent on p300 but not on CBP.

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.

Viral transactivating proteins

Many viruses utilize the cellular transcription apparatus to express their genomes, and they encode transcriptional regulatory proteins that modulate the process. Here we review the current understanding of three viral regulatory proteins. The adenovirus E1A protein acts within the nucleus to regulate transcription through its ability to bind to other proteins. The herpes simplex type 1 virus VP16 protein acts within the nucleus to control transcription by binding to DNA in conjunction with cellular proteins. The human T-cell leukemia virus Tax protein influences transcription through interactions with cellular proteins in the nucleus as well as the cytoplasm.

Transcriptional coactivator cAMP response element binding protein mediates induction of the human proliferating cell nuclear antigen promoter by the adenovirus E1A oncoprotein

The proliferating cell nuclear antigen (PCNA), a crucial component of eukaryotic cell cycle and DNA replication complexes, is induced by the adenovirus E1A 243R oncoprotein through a cis-acting element termed the PERE (PCNA-E1A responsive element). The PERE contains a sequence homologous to an activating transcription factor (ATF) motif, and ATF-1 is a major component of PERE-protein complexes. We have identified a second PERE-binding protein, the cAMP response element binding protein (CREB) transcription factor, which forms heterodimers with ATF-1 at this site. CREB, but not ATF-1, is able to mediate transactivation of a minimal PCNA-chloramphenicol acetyltransferase reporter by E1A 243R. Further analysis revealed that the transcriptional coactivator, the CREB-binding protein (CBP), associates with PERE-related complexes, and that CBP is able to mediate a strong transactivation response to E1A 243R at the PCNA promoter. Experiments conducted with mutants in the E1A or CREB components support a model whereby E1A 243R transactivates the PCNA promoter via a CBP-CREB-PERE pathway. These findings delineate a paradigm by which E1A 243R can target and transactivate specific DNA promoter sequences.

Adenovirus E1a interferes with expression of vaccinia viral genes

The 12S and 13S cDNAs of the oncogene E1a encoded by the early region of adenovirus 12 (Ad12) were overexpressed using the T7/encephalomyocarditis (EMC)/vaccinia hybrid expression system. The E1a proteins were stable for at least 12 h in monkey epithelial BSC1 cells. The E1a proteins were recognized by a rabbit polyclonal antibody and displayed phosphorylation patterns similar to those displayed by the E1a proteins expressed in Ad12-transformed cells. Expression of E1a proteins by recombinant vaccinia virus led to inhibition of vaccinia viral protein synthesis which was observed as soon as 6 h after infection. This suppression was mediated by both the 12S and the 13S products of Ad12E1a and to a somewhat lesser extent by the 13S product of Ad2E1a. The inhibition of vaccinia virus gene expression resulted in enhanced survival of vaccinia virus-infected cells. These results suggest that the proteins encoded by the E1a sequester a viral or a cellular product(s) that is essential for the expression of vaccinia virus-encoded genes.

Dominant negative mutants of Myc inhibit cooperation of both Myc and adenovirus serotype-5 E1a with Ras

We have used dominant negative Myc mutants to analyze the Myc and E1a mechanisms of cooperation with Ras. We show that mutants of Myc with an altered basic region (BR; RR366, 367EE) or deletion of the leucine zipper (LZ; delta aa 414-439), changes which modify the DNA binding domain, or with deletions in the Myc amino terminal conserved regions box 1 (dlMB1; delta aa 46-55) and box 2 (dlMB2; delta aa 132-140) inhibit cooperation of wt Myc and activated Ras to transform rat embryo fibroblasts (REF). Expression of the amino terminal 104 aa had no effect whereas wt Myc stimulated focus formation. Mutant dlMB1 cooperated with Ras with one half wt efficiency while dlMB2 was inactive. No mutant tested was toxic during neomycin cotransformation of REF to G418 resistance. Interestingly, these Myc mutants exerted a parallel inhibition of E1a-Ras cooperation to transform REF. This suggests that the Myc-Ras and E1a-Ras cooperation pathways intersect and require common protein factors. A Myc box 2 deletion mutant which is a wt transactivator of the Myc responsive ornithine decarboxylase promoter, but unlike the wt does not repress the adenovirus-2 core promoter (Li et al., 1994, EMBO J., 13:4070-4079), inhibits Myc-Ras and E1a-Ras cooperation. This suggests that a box 2-dependent step, potentially gene repression, is required for both the E1a- and Myc-Ras cooperation mechanisms.

Reversal of malignancy by the adenovirus E1a gene

Tumor suppressor genes such as Rb and p53 usually kill tumor cells when overexpressed ectopically. This is a consequence of their normal cell cycle regulatory functions. By contrast, the E1a gene of adenovirus, a common cold virus, converts tumor cells into viable normal cells. This has advantages for investigation and control of cancer. In particular, E1a is a master programmer of the epithelial phenotype. This provides a new tool for understanding the molecular basis of the epithelial-mesenchymal transition, and how it goes awry in cancer cells. Furthermore, epithelial cells are sensitive to a form of apoptosis - 'anoikis' - that is induced by detachment from extracellular matrix. This property confers strict anchorage-dependence. Transcriptional programming, by E1a or the formation of cell-cell junctional complexes, programs epithelial cells to be sensitive to anoikis.

Transcriptional repression by human adenovirus E1A N terminus/conserved domain 1 polypeptides in vivo and in vitro in the absence of protein synthesis

The human adenovirus E1A 243R protein (243 residues) transcriptionally represses a set of cellular genes that regulate cellular growth and differentiation. We describe two lines of evidence that E1A repression does not require cellular protein synthesis but instead involves direct interaction with a cellular protein(s). First, E1A 243R protein represses an E1A-repressible promoter in the presence of inhibitors of protein synthesis, as shown by cell microinjection-in situ hybridization. Second, E1A 243R protein strongly represses transcription in vitro from promoters of the E1A-repressible genes, human collagenase, and rat insulin type II. Repression in vitro is promoter-specific, and an E1A polypeptide containing only the N-terminal 80 residues is sufficient for strong repression both in vivo and in vitro. By use of a series of E1A 1-80 deletion proteins, the E1A repression function was found to require two E1A sequence elements, one within the nonconserved E1A N terminus, and the second within a portion of conserved region 1 (40-80). These domains have been reported to possess binding sites for several cellular transcription regulators, including p300, Dr1, YY1, and the TBP subunit of TFIID. The in vitro transcription-repression system described here provides a powerful tool for the further analysis of molecular mechanism and the possible role of these cellular factors.

The transcriptional repressor even-skipped interacts directly with TATA-binding protein

The Drosophila homeodomain protein Even-skipped (Eve) has previously been shown to function as a sequence-specific transcriptional repressor, and in vitro and in vivo experiments have shown that the protein can actively block basal transcription. However, the mechanism of repression is not known. Here, we present evidence establishing a direct interaction between Eve and the TATA-binding protein (TBP). Using cotransfection assays with minimal basal promoters whose activity can be enhanced by coexpression of TBP, we found that Eve could efficiently block, or squelch, this enhancement. Squelching did not require Eve DNA-binding sites on the reporter plasmids but was dependent on the presence of the Eve repression domain. Further support for an in vivo interaction between the Eve repression domain and TBP was derived from a two-hybrid-type assay with transfected cells. Evidence that Eve and TBP interact directly was provided by in vitro binding assays, which revealed a specific protein-protein interaction that required an intact Eve repression domain and the conserved C terminus of TBP. The Eve homeodomain was also required for these associations, suggesting that it may function in protein-protein interactions. We also show that a previously characterized artificial repression region behaves in a manner similar to that of the Eve repression domain, including its ability to squelch TBP-enhanced expression in vivo and to bind TBP specifically in vitro. Our results suggest a model for transcriptional repression that involves an interaction between Eve and TBP.

Suppression of mutations in two Saccharomyces cerevisiae genes by the adenovirus E1A protein

The protein products of the adenoviral E1A gene are implicated in a variety of transcriptional and cell cycle events, involving interactions with several proteins present in human cells, including parts of the transcriptional machinery and negative regulators of cell division such as the Rb gene product and p107. To determine if there are functional homologs of E1A in Saccharomyces cerevisiae, we have developed a genetic screen for mutants that depend on E1A for growth. The screen is based on a colony color sectoring assay which allows the identification of mutants dependent on the maintenance and expression of an E1A-containing plasmid. Using this screen, we have isolated five mutants that depend on expression of the 12S or 13S cDNA of E1A for growth. A plasmid shuffle assay confirms that the plasmid-dependent phenotype is due to the presence of either the 12S or the 13S E1A cDNA and that both forms of E1A rescue growth of all mutants equally well. The five mutants fall into two classes that were named web1 and web2 (for "wants E1A badly"). Plasmid shuffle assays with mutant forms of E1A show that conserved region 1 (CR1) is required for rescue of the growth of the web1 and web2 E1A-dependent yeast mutants, while the N-terminal 22 amino acids are only partially required; conserved region 2 (CR2) and the C terminus are dispensable. The phenotypes of mutants in both the web1 and the web2 groups are due to a single gene defect, and the yeast genes that fully complement the mutant phenotypes of both groups were cloned. The WEB1 gene sequence encodes a 1,273-amino-acid protein that is identical to SEC31, a protein involved in the budding of transport vesicles from the endoplasmic reticulum. The WEB2 gene encodes a 1,522-amino-acid protein with homology to nucleic acid-dependent ATPases. Deletion of either WEB1 or WEB2 is lethal. Expression of E1A is not able to rescue the lethality of either the web1 or the web2 null allele, implying allele-specific mutations that lead to E1A dependence.

Repression of the c-Jun trans-activation function by the adenovirus type 12 E1A 52R protein correlates with the inhibition of phosphorylation of the c-Jun activation domain

The early region 1A 52R polypeptide, a protein expressed exclusively by the in vivo oncogenic adenovirus subtype 12, represses the trans-activating function of the cellular transcription factor complex AP-1 consisting of c-Jun-c-Jun homodimers. In this report we demonstrate that the repression in vivo correlates with a direct physical interaction of the adenovirus protein with c-Jun in vitro. Interestingly, the 52R protein binds to the bZIP domain of c-Jun essential for dimerization and DNA binding but not to the c-Jun activation domain. This interaction does not prevent the promoter binding of c-Jun/AP-1. Moreover, the physical association between c-Jun and the TATA box-binding protein TBP is not disturbed by the 52R polypeptide. In fact, we show evidence that down-regulation of c-Jun activity by the adenoviral protein is due to the inhibition of phosphorylation of the c-Jun trans-activation domain. In vivo phosphorylation of the c-Jun activation domain is necessary for the interaction of c-Jun with specific cofactors such as CBP and therefore a prerequisite for the activation of target genes. Due to these results we propose a model in which the 52R protein represses the trans-activating function of c-Jun by preventing its phosphorylation through a specific kinase necessary for the activation of the cellular transcription factor.

Repression in vitro, by human adenovirus E1A protein domains, of basal or Tat-activated transcription of the human immunodeficiency virus type 1 long terminal repeat

Human adenovirus E1A proteins can repress the expression of several viral and cellular genes. By using a cell-free transcription system, we demonstrated that the gene product of the E1A 12S mRNA, the 243-residue protein E1A243R, inhibits basal transcription from the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR). The HIV-1 transactivator protein Tat greatly stimulates transcription from the viral promoter in vitro. However, E1A243R can repress Tat-activated transcription in vitro. Strong repression of both basal and Tat-activated transcriptions requires only E1A N-terminal amino acid residues 1 to 80. Deletion analysis showed that E1A N-terminal amino acids 4 to 25 are essential for repression, whereas amino acid residues 30 to 49 and 70 to 80 are dispensable. Transcriptional repression by E1A in the cell-free transcription system is promoter specific, since under identical conditions, transcription of the adenovirus major late promoter and the Rous sarcoma virus LTR promoter was unaffected. The repression of transcription by small E1A peptides in vitro provides an assay for investigation of molecular mechanisms governing E1A-mediated repression of both basal and Tat-activated transcriptions of the HIV-1 LTR promoter.

E1a induces the expression of epithelial characteristics

Cells closely resembling epithelia constitute the first specific cell type in a mammalian embryo. Many other cell types emerge via epithelial-mesenchymal differentiation. The transcription factors and signal transduction pathways involved in this differentiation are being elucidated. I have previously reported (Frisch, 1991) that adenovirus E1a is a tumor suppressor gene in certain human cell lines. In the present report, I demonstrate that E1a expression caused diverse human tumor cells (rhabdomyosarcoma, fibrosarcoma, melanoma, osteosarcoma) and fibroblasts to assume at least two of the following epithelial characteristics: (a) epithelioid morphology; (b) epithelial-type intercellular adhesion proteins localized to newly formed junctional complexes; (c) keratin-containing intermediate filaments; and (d) down-regulation of non-epithelial genes. E1a thus appeared to partially convert diverse human tumor cells into an epithelial phenotype. This provides a new system for molecular analysis of epithelial-mesenchymal interconversions. This effect may also contribute to E1a's tumor suppression activity, possibly through sensitization to anoikis (Frisch, S.M., and H. Francis, 1994. J. Cell Biol. 124:619-626).

An adenovirus E1A transcriptional repressor domain functions as an activator when tethered to a promoter

The adenovirus E1A protein contains three well conserved regions, designated conserved region (CR) 1, 2 and 3, which are important for the multiple activities ascribed to E1A. The CR3 domain constitutes a prototypic transcription activator, consisting of a promoter targeting region and a transactivating region. Here we demonstrate the existence of a second transactivating region located within amino acids 28 to 90 (essentially the CR1 domain) of the E1A protein. A fusion protein, containing the Gal4 DNA binding domain linked to CR1, was as efficient as the classical CR3 transactivator in activating transcription from a reporter plasmid containing Gal4 binding sites. However, competition experiments suggest that Gal/CR1 and Gal/CR3 work through different cellular targets. The E1A-243R protein has previously been extensively characterized as a repressor of transcription. Here we show that a Gal4 fusion protein expressing the CR1 domain is indeed sufficient for repression of SV40 enhancer activity. Collectively, our results suggest that CR1 functions as an activator if tethered to a promoter and as a repressor in the absence of promoter association.

Human cancer cell lines express a negative transcriptional regulator of the interferon regulatory factor family of DNA binding proteins

Members of the interferon regulatory factor (IRF) family of DNA binding transcription factors have roles in growth regulation, antiviral responses, and transcriptional induction of interferon (IFN)-activated early response genes. The IRF family member ISGF3 gamma is the DNA binding component of IFN-stimulated gene factor 3 (ISGF3), a multicomponent complex responsible for the stimulation of IFN-alpha-responsive genes. IFN-alpha-stimulated formation of ISGF3 and subsequent gene expression can be inhibited by phorbol esters or expression of the adenovirus E1A protein. We have investigated IFN signaling in human malignant tumor cell lines of the lung, colon, ovary, cervix, and hematopoietic organs and found some of these cells to be defective for IFN-alpha-induced formation of ISGF3. In many cases, an inhibitory activity termed transcriptional knockout (TKO) correlated with nonresponsiveness. TKO purified from a human papillomavirus-negative cervical carcinoma cell line has a molecular size of 19 kDa. The purified protein interacted with the ISGF3 gamma component of ISGF3, preventing binding of ISGF3 to DNA. Purified TKO displaced ISGF3 from its DNA binding site in vitro and prevented ISGF3 gamma, IRF-1, and IRF-2 from interacting with the IFN-stimulated response element. Partially purified TKO can also directly interact with ISGF3 gamma in the absence of DNA. This protein may be involved with the development of malignancies and the inability of IFN to exert its antiproliferative and antiviral effects.

Human cancer cell lines express a negative transcriptional regulator of the interferon regulatory factor family of DNA binding proteins

Members of the interferon regulatory factor (IRF) family of DNA binding transcription factors have roles in growth regulation, antiviral responses, and transcriptional induction of interferon (IFN)-activated early response genes. The IRF family member ISGF3 gamma is the DNA binding component of IFN-stimulated gene factor 3 (ISGF3), a multicomponent complex responsible for the stimulation of IFN-alpha-responsive genes. IFN-alpha-stimulated formation of ISGF3 and subsequent gene expression can be inhibited by phorbol esters or expression of the adenovirus E1A protein. We have investigated IFN signaling in human malignant tumor cell lines of the lung, colon, ovary, cervix, and hematopoietic organs and found some of these cells to be defective for IFN-alpha-induced formation of ISGF3. In many cases, an inhibitory activity termed transcriptional knockout (TKO) correlated with nonresponsiveness. TKO purified from a human papillomavirus-negative cervical carcinoma cell line has a molecular size of 19 kDa. The purified protein interacted with the ISGF3 gamma component of ISGF3, preventing binding of ISGF3 to DNA. Purified TKO displaced ISGF3 from its DNA binding site in vitro and prevented ISGF3 gamma, IRF-1, and IRF-2 from interacting with the IFN-stimulated response element. Partially purified TKO can also directly interact with ISGF3 gamma in the absence of DNA. This protein may be involved with the development of malignancies and the inability of IFN to exert its antiproliferative and antiviral effects.

Modulation of transcriptional activation of the proliferating cell nuclear antigen promoter by the adenovirus E1A 243-residue oncoprotein depends on proximal activators

Previous analyses defined a proliferating cell nuclear antigen (PCNA) E1A-responsive element (PERE) in the PCNA promoter that is essential for transactivation by the 243-residue product of the adenovirus type 2 E1A 12S mRNA (E1A 243R). In this report, we show that the PERE activates a heterologous basal promoter and confers susceptibility to transactivation by E1A 243R, indicating that the PERE is both necessary and sufficient for the response of the PCNA promoter to this oncoprotein. Insertion of linker sequences between the PERE and the site of transcription initiation in the PCNA promoter severely impairs the promoter's response to E1A 243R transactivation. GAL4 sites can replace the function of the PERE in the E1A 243R response of the PCNA basal promoter if transcriptional activators of suitable strength are supplied as GAL4 fusion proteins. Weak transcriptional activators render the PCNA basal promoter subject to transactivation by E1A 243R but do not endow the adenovirus E1B basal promoter with a similar response. Strong transcriptional activators do not support transactivation by E1A 243R, however; instead, E1A reduces the ability of the strong activators to activate both the PCNA and E1B basal promoters. Although other mechanistic differences might determine the response, the data imply a relationship between the activation strength of promoter-proximal effectors and the response of the PCNA basal promoter to E1A 243R. These experiments indicate that the PERE can function autonomously in mediating transactivation by E1A 243R and that the PCNA basal promoter is configured in a manner that permits modulation by E1A 243R of transcriptional activation by promoter-proximal effectors.

Modulation of transcriptional activation of the proliferating cell nuclear antigen promoter by the adenovirus E1A 243-residue oncoprotein depends on proximal activators

Previous analyses defined a proliferating cell nuclear antigen (PCNA) E1A-responsive element (PERE) in the PCNA promoter that is essential for transactivation by the 243-residue product of the adenovirus type 2 E1A 12S mRNA (E1A 243R). In this report, we show that the PERE activates a heterologous basal promoter and confers susceptibility to transactivation by E1A 243R, indicating that the PERE is both necessary and sufficient for the response of the PCNA promoter to this oncoprotein. Insertion of linker sequences between the PERE and the site of transcription initiation in the PCNA promoter severely impairs the promoter's response to E1A 243R transactivation. GAL4 sites can replace the function of the PERE in the E1A 243R response of the PCNA basal promoter if transcriptional activators of suitable strength are supplied as GAL4 fusion proteins. Weak transcriptional activators render the PCNA basal promoter subject to transactivation by E1A 243R but do not endow the adenovirus E1B basal promoter with a similar response. Strong transcriptional activators do not support transactivation by E1A 243R, however; instead, E1A reduces the ability of the strong activators to activate both the PCNA and E1B basal promoters. Although other mechanistic differences might determine the response, the data imply a relationship between the activation strength of promoter-proximal effectors and the response of the PCNA basal promoter to E1A 243R. These experiments indicate that the PERE can function autonomously in mediating transactivation by E1A 243R and that the PCNA basal promoter is configured in a manner that permits modulation by E1A 243R of transcriptional activation by promoter-proximal effectors.

Comparison between E1A gene from oncogenic and non-oncogenic adenoviruses in cellular transformation (Ad E1A conserved region)

All adenoviruses transform primary BRK cells in vitro, but only cells transformed by oncogenic adenoviruses are tumorigenic for immunocompetent animals. The transforming E1 regions of human Ad 2 and Ad 12 also differ from each other in the frequency in which they can transform BRK cells. We have investigated these properties which can be assigned to the specific domain of the E1A region. For this purpose, chimeric E1A regions between Ad 2 and Ad 12 have been constructed. The efficiency of cell transformation appeared to be determined by the encoding region. The promoter sequences were not important for an efficient cellular transformation although the E1B region cis activated in E1A transcription in both cell transformation and transient expression. We show that sequences located in the E1B promoter were responsible for this effect. In the encoding region the CR 1 domain was essential for the cell transformation frequency.

Repression of RNA polymerase III transcription by adenovirus E1A

Adenovirus E1A encodes two major proteins of 289 and 243 amino acids (289R and 243R), which both have transcription regulatory properties. E1A-289R is a transactivator whereas E1A-243R primarily functions as a repressor of transcription. Here we show that E1A repression is not restricted to RNA polymerase II genes but also includes the adenovirus virus-associated (VA) RNA genes. These genes are transcribed by RNA polymerase III and have previously been suggested to be the target of an E1A-289R-mediated transactivation. Surprisingly, we found that during transient transfection both E1A proteins repressed VA RNA transcription. E1A repression of VA RNA transcription required both conserved regions 1 and 2 and therefore differed from the E1A-mediated inhibition of simian virus 40 enhancer activity which primarily required conserved region 1. The repression was counteracted by the E1B-19K protein, which also, in the absence of E1A, enhanced the accumulation of VA RNA. Importantly, we show that efficient VA RNA transcription requires expression of both E1A and the E1B-19K protein during virus infection.

Analysis of transcription factors binding to the duplicated PEA1 and PEA3 sites that are required for polyomavirus mutant expression in PCC4 embryonic carcinoma cells

Embryonic carcinoma (EC) cell lines, representative of early embryonic undifferentiated cells, are nonpermissive for polyomavirus (PyV) infection as a result of a blockade of viral DNA early transcription and replication. All enhancers of PyV mutants (Py EC-PCC4), selected for the ability to grow on PCC4 EC cells, display a duplication of PEA1 and PEA3 binding sites (sites 1 and 3). However, the Py EC-PCC4 rearrangement is complex and results in variable mutant enhancer activities. We demonstrate here that duplication of sites 1 and 3 is absolutely required for a cooperative cis activation of early Py EC-PCC4 mutant transcription in PCC4 EC cells. In addition, we detect in PCC4 EC cells significant amounts of site 1- and 3-binding proteins, which we characterize as related to the Fos/Jun and Ets protein families, respectively. Wild-type PyV restriction in PCC4 EC cells may be relieved by a cooperation between site 2- and 3-binding proteins that would thereby be activated. Since site 1- or 3-binding factors could be derepressed, we improved the analysis of UV cross-linked DNA-protein complexes and were able to detect a novel factor, called PEA1/2 (for PyV enhancer A site 1- and 2-binding factor). Its DNA binding sequence overlaps sites 1 and 2 (PEA2 binding site) and is not duplicated in the M1 mutant, which exhibits the highest Py EC-PCC4 enhancer activity. he suggest that PEA1/2 is also involved in the regulation of PyV enhancer activity by repressing the site 1-binding activity.

Identification of specific adenovirus E1A N-terminal residues critical to the binding of cellular proteins and to the control of cell growth

Adenovirus early region 1A (E1A) oncogene-encoded sequences essential for transformation- and cell growth-regulating activities are localized at the N terminus and in regions of highly conserved amino acid sequence designated conserved regions 1 and 2. These regions interact to form the binding sites for two classes of cellular proteins: those, such as the retinoblastoma gene product, whose association with the E1A products is specifically dependent on region 2, and another class which so far is known to include only a large cellular DNA-binding protein, p300, whose association with the E1A products is specifically dependent on the N-terminal region. Association between the E1A products and either class of cellular proteins can be disrupted by mutations in conserved region 1. While region 2 has been studied intensively, very little is known so far concerning the nature of the essential residues in the N-terminal region, or about the manner in which conserved region 1 participates in the binding of two distinct sets of cellular proteins. A combination of site-directed point mutagenesis and monoclonal antibody competition experiments reported here suggests that p300 binding is dependent on specific, conserved residues in the N terminus, including positively charged residues at positions 2 and 3 of the E1A proteins, and that p300 and pRB bind to distinct, nonoverlapping subregions within conserved region 1. The availability of precise point mutations disrupting p300 binding supports previous data linking p300 with cell cycle control and enhancer function.

Repression of liver-specific hepatitis B virus enhancer 2 activity by adenovirus E1A proteins

Two regions of the hepatitis B virus (HBV) genome have been shown to display properties of a transcriptional enhancer. Enhancer 1 is active in most hepatoma lines examined as well as in some non-hepatocyte-derived cell lines. In contrast, enhancer 2 activity is strictly liver specific. In this study, we show that adenovirus E1A expression in the highly differentiated human hepatoma line Huh6 strongly inhibits HBV enhancer 2-stimulated transcription while having no effect on HBV enhancer 1 activity. A sequence motif in HBV enhancer 2 which is essential for its enhancer function is the target for E1A-mediated repression. The repression of HBV enhancer 2 activity is mediated through the N-terminal region of the E1A proteins known to bind a 300-kDa cellular protein. Our results suggest that HBV enhancer function may be modulated by a cellular mechanism similar to E1A-mediated transcriptional repression.

A DNA element that regulates expression of an endogenous retrovirus during F9 cell differentiation is E1A dependent

The retinoic acid-induced differentiation of F9 cells into parietal endoderm-like cells activates transcription of the endogenous mouse retrovirus, the intracisternal A-particle (IAP). To investigate the elements that control IAP gene differentiation-specific expression, we used methylation interference, Southwestern (DNA-protein), and transient-transfection assays and identified the IAP-proximal enhancer (IPE) element that directs differentiation-specific expression. We find that the IPE is inactive in undifferentiated F9 cells and active in differentiated parietal endoderm-like PYS-2 cells. Three proteins of 40, 60, and 68 kDa bind to the sequence GAGTAGAC located between nucleotides -53 and -47 within the IPE. The 40- and 68-kDa proteins from both the undifferentiated and differentiated cells exhibit similar DNA-binding activities. However, the 60-kDa protein from differentiated cells has greater binding activity than that from undifferentiated cells, suggesting a role for this protein in F9 differentiation-specific expression of the IAP gene. The IAP gene is negatively regulated by the adenovirus E1A proteins, and the E1A sequence responsible for repression is located at the N terminus, between amino acids 2 and 67. The DNA sequence that is the target of E1A repression also maps to the IPE element. Colocalization of the differentiation-specific and E1A-sensitive elements to the same protein-binding site within the IPE suggests that the E1A-like activity functions in F9 cells to repress IAP gene expression. Activation of the IAP gene may result when the E1A-like activity is lost or inactivated during F9 cell differentiation, followed by binding of the 60-kDa positive regulatory protein to the enhancer element.

A DNA element that regulates expression of an endogenous retrovirus during F9 cell differentiation is E1A dependent

The retinoic acid-induced differentiation of F9 cells into parietal endoderm-like cells activates transcription of the endogenous mouse retrovirus, the intracisternal A-particle (IAP). To investigate the elements that control IAP gene differentiation-specific expression, we used methylation interference, Southwestern (DNA-protein), and transient-transfection assays and identified the IAP-proximal enhancer (IPE) element that directs differentiation-specific expression. We find that the IPE is inactive in undifferentiated F9 cells and active in differentiated parietal endoderm-like PYS-2 cells. Three proteins of 40, 60, and 68 kDa bind to the sequence GAGTAGAC located between nucleotides -53 and -47 within the IPE. The 40- and 68-kDa proteins from both the undifferentiated and differentiated cells exhibit similar DNA-binding activities. However, the 60-kDa protein from differentiated cells has greater binding activity than that from undifferentiated cells, suggesting a role for this protein in F9 differentiation-specific expression of the IAP gene. The IAP gene is negatively regulated by the adenovirus E1A proteins, and the E1A sequence responsible for repression is located at the N terminus, between amino acids 2 and 67. The DNA sequence that is the target of E1A repression also maps to the IPE element. Colocalization of the differentiation-specific and E1A-sensitive elements to the same protein-binding site within the IPE suggests that the E1A-like activity functions in F9 cells to repress IAP gene expression. Activation of the IAP gene may result when the E1A-like activity is lost or inactivated during F9 cell differentiation, followed by binding of the 60-kDa positive regulatory protein to the enhancer element.

Promoter-specific trans-activation by the adenovirus E1A12S product involves separate E1A domains

Recent studies have shown that the adenovirus E1A12S product can trans-activate transcription by activating the transcription factor E2F. However, E2F cannot be the only target for the E1A12S product, since several cellular promoters have been found to be activated by the E1A12S protein even though they lack E2F sites. Indeed, we now show that activation of the hsp70 promoter by the E1A12S product requires the TATAA sequence. Moreover, activation of the hsp70 promoter requires the N-terminal domain of the E1A protein and does not require the conserved region 2 sequences which are required for the E2F-dependent activation of transcription. We conclude that the targeting of distinct transcription factors, leading to trans-activation of transcription of multiple promoters, involves distinct domains of the E1A proteins that are also required for oncogenic activity.

Ability of adenovirus 5 E1A proteins to suppress differentiation of BC3H1 myoblasts correlates with their binding to a 300 kDa cellular protein

We have used deletion mutants to define the regions in Ad5 E1A proteins necessary to suppress differentiation of mouse BC3H1 myoblasts. We examined the differentiation of cells infected at a low multiplicity with viruses containing the E1A deletions and constructed so as to produce only the smaller of the two major E1A proteins. Only four of the mutant viruses containing deletions within the N-terminal 69 residues failed to suppress differentiation as judged by changes in morphology and in levels of muscle-specific alpha-actin mRNA and creatine kinase activity. The results were confirmed by analyses of lines of cells stably transfected with representative E1A mutants. The mouse cellular proteins to which mutant E1A proteins bound were identified by immunoprecipitating E1A proteins specifically from infected BC3H1 cells and by analyzing the precipitates on denaturing gels. Bands of proteins of 300, 130, 107, 105 (the retinoblastoma product), and 60 kDa (cyclin A) were distinguished. Failure to suppress differentiation correlated with loss of binding to the 300-kDa protein but not to any of the others. The regions of E1A defined in this way have been shown to be required for several other activities, including enhancer repression and transformation. One function of the 300-kDa protein appears to be to facilitate the action of transcriptional enhancers of differentiation-specific genes.

Promoter-specific trans-activation by the adenovirus E1A12S product involves separate E1A domains

Recent studies have shown that the adenovirus E1A12S product can trans-activate transcription by activating the transcription factor E2F. However, E2F cannot be the only target for the E1A12S product, since several cellular promoters have been found to be activated by the E1A12S protein even though they lack E2F sites. Indeed, we now show that activation of the hsp70 promoter by the E1A12S product requires the TATAA sequence. Moreover, activation of the hsp70 promoter requires the N-terminal domain of the E1A protein and does not require the conserved region 2 sequences which are required for the E2F-dependent activation of transcription. We conclude that the targeting of distinct transcription factors, leading to trans-activation of transcription of multiple promoters, involves distinct domains of the E1A proteins that are also required for oncogenic activity.

DNA-binding properties of the E1A-associated 300-kilodalton protein

One of the major E1A-associated cellular proteins is a 300-kDa product (p300) that binds to the N-terminal region of the E1A products. The p300 binding site is distinct from sequences involved in binding the retinoblastoma product and other E1A-associated cellular products such as p60-cyclin A and p107. p300 binding to E1A is linked genetically to the enhancer repression function of E1A and the other E1A-mediated gene-regulating functions as well as to the transforming functions of E1A. However, the biochemical properties of p300 have not yet been characterized. We report here that p300 has an intrinsic DNA-binding activity and shows a preferential affinity for specific DNA sequences. The sequences selectively bound by p300 are related to those of a series of enhancer elements that are recognized by NF-kappa B. The direct physical interaction of p300 with enhancer elements provides a biochemical basis for the genetic evidence linking the E1A-mediated enhancer repression function with the p300-binding activity of E1A.

DNA-binding properties of the E1A-associated 300-kilodalton protein

One of the major E1A-associated cellular proteins is a 300-kDa product (p300) that binds to the N-terminal region of the E1A products. The p300 binding site is distinct from sequences involved in binding the retinoblastoma product and other E1A-associated cellular products such as p60-cyclin A and p107. p300 binding to E1A is linked genetically to the enhancer repression function of E1A and the other E1A-mediated gene-regulating functions as well as to the transforming functions of E1A. However, the biochemical properties of p300 have not yet been characterized. We report here that p300 has an intrinsic DNA-binding activity and shows a preferential affinity for specific DNA sequences. The sequences selectively bound by p300 are related to those of a series of enhancer elements that are recognized by NF-kappa B. The direct physical interaction of p300 with enhancer elements provides a biochemical basis for the genetic evidence linking the E1A-mediated enhancer repression function with the p300-binding activity of E1A.

Functional diversity of E1A gene autoregulation among human adenoviruses

Autoregulation of the adenovirus E1A gene involves its constitutive expression and positively and negatively regulated transcription. Dissection of this process will identify basal-level cis elements and autoregulatory targets of the E1A promoter and functional domains within the trans-acting E1A gene products. In this report, the DNA sequence of the human subgroup B adenovirus type 3 (Ad3) E1A gene is presented and compared with that of the E1A genes of similar and distantly related human adenoviruses. The cDNA forms of the Ad3 E1A gene, corresponding to two major early mRNA species, are cloned, sequenced, and subcloned into plasmid expression vectors. Cotransfections of cell cultures are performed with Ad5 or Ad3 E1A gene expression plasmids and a reporter gene under control of the Ad5 or Ad3 E1A promoter. The Ad5 and Ad3 E1A promoters are similarly repressed by either serotype's 12S cDNA gene products. The Ad3 E1A promoter responds much more strongly than the Ad5 E1A promoter to transactivation by 13S cDNA gene products. In contrast, the 13S cDNA gene of Ad5 has greater transactivation activity than that of Ad3. Experiments with missense mutations of the Ad5 E1A gene indicate that transactivation of the Ad5 E1A promoter is weak, just reversing or balancing negative autorepression. Single amino acid substitutions in the conserved, repressive functional domain 2 of the E1A gene modulate transactivating activity that is usually associated with the separate and distal conserved functional domain 3. These results suggest a strong structure-function relationship influenced by the variable sequences separating these conserved domains.