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

Multiple domains in the 50 kDa form of E4F1 regulate promoter-specific repression and E1A trans-activation

The 50 kDa N-terminal product of the cellular transcription factor E4F1 (p50E4F1) mediates E1A289R trans-activation of the adenovirus E4 gene, and suppresses E1A-mediated transformation by sensitizing cells to cell death. This report shows that while both E1A289R and E1A243R stimulate p50E4F1 DNA binding activity, E1A289R trans-activation, as measured using GAL-p50E4F1 fusion proteins, involves a p50E4F1 transcription regulatory (TR) region that must be promoter-bound and is dependent upon E1A CR3, CR1 and N-terminal domains. Trans-activation is promoter-specific, as GAL-p50E4F1 did not stimulate commonly used artificial promoters and was strongly repressive when competing against GAL-VP16. p50E4F1 and E1A289R stably associate in vivo using the p50E4F1 TR region and E1A CR3, although their association in vitro is indirect and paradoxically disrupted by MAP kinase phosphorylation of E1A289R, which stimulates E4 trans-activation in vivo. Multiple cellular proteins, including TBP, bind the p50E4F1 TR region in vitro. The mechanistic implications for p50E4F1 function are discussed.

Cloning and expression analysis of heat shock cognate 70 gene promoter in tiger shrimp (Penaeus monodon)

Heat shock cognate 70 (HSC70) functions as a molecular chaperon and plays an important role in protein folding. HSC70 cDNA of tiger shrimp (Penaeus monodon) was cloned and characterized in our previous study. After shrimps were treated with the 1-hr heat shock, the HSC70 mRNA level in hemocytes increased (approximately 8 fold) using real-time quantitative PCR. An hsc70 clone was obtained from genomic library screening. The gene contains 2 exons separated by a 1557-bp intron. The 5'-flanking region sequence (approximately 1 kb) ahead of the hsc70 gene contains a putative core promoter region and transcription elements including perfect heat shock element (HSE), imperfect HSE, CAAT elements, SP1, NF-kappaB and GC box. In insect Sf21 cells, the region could drive expression of the enhanced green fluorescent protein (EGFP) and luciferase gene to verify its promoter function. In the luciferase assay system, the effects of serial deletions on the hsc70 promoter were elucidated. Autographa californica multiple nuclear polyhedrosis virus infection (MOI=0.1) on Sf21 cells significantly increased the hsc70 promoter activity. In addition, the effects of amino acid analogs and arsenic acid incubation with the cells on the hsc70 promoter activity were examined.

The steroid receptor co-activator-1 (SRC-1) potentiates TGF-beta/Smad signaling: role of p300/CBP

The three related 160-kDa proteins, SRC-1, TIF-2 and RAC-3, were initially identified as factors interacting with nuclear receptors. They have also been reported to potentiate the activity of other transcription factors such as AP-1 or NF-kappaB. The aim of this work was to identify whether SRC-1 interferes with the TGF-beta/Smad signaling pathway, and if so, to identify its underlying mechanisms of action. Using transient cell transfection experiments performed in human dermal fibroblasts with the Smad3/4-specific (SBE)4-lux reporter construct, as well as the human PAI-1 promoter, we determined that SRC-1 enhances TGF-beta-induced, Smad-mediated, transcription. Likewise, SRC-1 overexpression potentiated TGF-beta-induced upregulation of PAI-1 steady-state mRNA levels. Using a mammalian two-hybrid system, we demonstrated that SRC-1 interacts with the transcriptional co-activators p300/CBP, but not with Smad3. Overexpression of the adenovirus E1A oncoprotein, an inhibitor of CBP/p300 activity, prevented the enhancing effect of SRC-1 on Smad3/4-mediated transcription, indicating that p300/CBP may be required for SRC-1 effect. Such hypothesis was validated, as expression of a mutant form of SRC-1 lacking the CBP/p300-binding site failed to upregulate Smad3/4-dependent transcription, while full-length SRC-1 potentiated p300.Smad3 interactions. These results identify SRC-1 as a novel Smad3/4 transcriptional partner, facilitating the functional link between Smad3 and p300/CBP.

Tumor induction by an endogenous K-ras oncogene is highly dependent on cellular context

We have targeted a K-ras allele in mouse embryonic stem (ES) cells to express a K-Ras(V12) oncoprotein along with a marker protein (beta-geo) from a single bicistronic transcript. Expression of this oncogenic allele requires removal of a knocked in STOP transcriptional cassette by Cre recombinase. Primary mouse embryonic fibroblasts expressing this K-ras(V12) allele do not undergo proliferative senescence and proliferate as immortal cells. In mice, expression of K-ras(V12) throughout the body fails to induce unscheduled proliferation or other growth abnormalities for up to eight months. Only a percentage of K-ras(V12)-expressing lung bronchiolo-alveolar cells undergo malignant transformation leading to the formation of multiple adenomas and adenocarcinomas. These results indicate that neoplastic growth induced by an endogenous K-ras oncogene depends upon cellular context.

The co-activator p300 associates physically with and can mediate the action of the distal enhancer of the FGF-4 gene

Distal enhancers commonly regulate gene expression. However, the mechanisms of transcriptional mediation by distal enhancers remain largely unknown. To better understand distal enhancer-mediated transcription, we examined the regulation of the FGF-4 gene. The FGF-4 gene is regulated during early development by a powerful distal enhancer located downstream of the promoter in exon 3. Sox-2 and Oct-3 bind to the enhancer and are required for the activation of the FGF-4 gene. Previously, we implicated the co-activator p300 as a mediator of Sox-2/Oct-3 synergistic activation of a heterologous promoter, suggesting that p300 may play a role in mediating enhancer activation of the FGF-4 gene. In this study, we provide both functional and physical evidence that p300 plays an important role in the action of the FGF-4 enhancer. Specifically, we show that E1a, but not a mutant form of E1a that is unable to bind p300, inhibits enhancer activation of the FGF-4 promoter. We also demonstrate that Gal4/p300 fusion proteins can stimulate the FGF-4 promoter when bound to the FGF-4 enhancer. Additionally, we present evidence that p300 mediation of the FGF-4 enhancer requires acetyltransferase activity. Importantly, we also show that Sox-2 and p300 are physically associated with the endogenous FGF-4 enhancer but weakly associated with the endogenous FGF-4 promoter. These results are consistent with a model of transitory interaction between the distal enhancer and the FGF-4 promoter. Our results also suggest that intragenic distal enhancers may use mechanisms that differ from extragenic distal enhancers.

Effects of B-Myb on gene transcription: phosphorylation-dependent activity ans acetylation by p300

The transcription factor B-Myb is a cell-cycle regulated phosphoprotein involved in cell cycle progression through the transcriptional regulation of many genes. In this study, we show that the promoter of the fibroblast growth factor-4 (FGF-4) gene is strongly activated by B-Myb in HeLa cells and it can serve as a novel diagnostic tool for assessing B-Myb activity. Specifically, B-Myb deletion mutants were examined and domains of B-Myb required for activation of the FGF-4 promoter were identified. Using phosphorylation-deficient mutant forms of B-Myb, we also show that phosphorylation is essential for B-Myb activity. Moreover, a mutant form of B-Myb, which lacks all identified phosphorylation sites and which has little activity, can function as a dominant-negative and suppress wild-type B-Myb activity. Acetylation is another post-translational modification known to affect the activity of other Myb family members. We show that B-Myb is acetylated by the co-activator p300. We also show that the bromo and histone acetyltransferase domains of p300 are sufficient to interact with and acetylate B-Myb. These data indicate that phosphorylation of B-Myb is an essential modification for activity and that acetylation of B-Myb may play a role in B-Myb activity.

Core promoter involvement in the induction of rat ornithine decarboxylase by phorbol esters

Overexpression of ornithine decarboxylase (ODC) is an important oncogenic event in tumorigenesis. Although ODC was one of the first genes described whose product is inducible by 12-O-tetradecanoylphorbol-13-acetate (TPA), the mechanisms of ODC transcriptional regulation have remained elusive. In this study, we systematically analyzed the rat ODC core promoter region for novel TPA response elements. Analysis of linker scanning mutants of the ODC promoter from the TATA box to the transcription start site demonstrated that mutation of the TATA box reduced the TPA induction ratio by 40%, while the basal ODC promoter activity was not significantly changed. A novel region between nt - 20 to - 10 was shown to be critical for both basal promoter activity and induction by TPA. Random mutagenesis of this region showed that conversion of the GC-rich wild-type sequence into a T-rich sequence could either substantially increase the basal promoter activity and decrease the TPA induction ratio or dramatically reduce the basal promoter activity, depending on the T content. Mutant R5, containing an ATTT sequence at nt - 15 to - 12, caused a more than twofold increase of basal promoter activity and 80% reduction of TPA induction ratio. We suggest that this region interacts with components of the general transcription machinery and that the strength of this interaction is mediated by the T-content in this region.

Acetylation control of the retinoblastoma tumour-suppressor protein

The retinoblastoma tumour-suppressor protein (pRb) and p300/CBP co-activator proteins are important for control of proliferation and in tumour cells these are sequestered by viral oncoproteins such as E1A. pRb is involved in negatively regulating growth, and p300/CBP proteins have histone acetyltransferase (HAT) activity, which influences gene expression. Although it is known that phosphorylation by G1 cyclin-dependent kinases (CDKs) regulates pRb activity, the nature and role of other post-translational modifications is not understood. Here we identify acetylation as a new type of modification and level of control in pRb function. Adenovirus E1A, which binds p300/CBP through an amino-terminal transformation-sensitive domain, stimulates the acetylation of pRb by recruiting p300 and pRb into a multimeric-protein complex. Furthermore, pRb acetylation is under cell-cycle control, and acetylation hinders the phosphorylation of pRb by cyclin-dependent kinases. pRb binds more strongly when acetylated to the MDM2 oncoprotein, which indicates that acetylation may regulate protein-protein interactions in the pRb pathway. The acetylation of pRb defines a new level of cell-cycle control mediated by HAT. Furthermore, our results establish a relationship between p300, pRb and acetylation in which E1A acts to recruit and target a cellular HAT activity to pRb.

p300/CBP proteins: HATs for transcriptional bridges and scaffolds

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

RACK1 interacts with E1A and rescues E1A-induced yeast growth inhibition and mammalian cell apoptosis

The adenoviral E1A proteins are able to promote proliferation and transformation, inhibit differentiation, induce apoptosis, and suppress tumor growth. The extreme N terminus and conserved region one of E1A, which are indispensable for transcriptional regulation and for binding to p300/CBP, TBP, and pCAF, play essential roles in these abilities. These observations strongly suggest an intrinsic link between E1A-mediated transcriptional regulation and other effects. In this report, we show that E1A inhibits the normal growth of Saccharomyces cerevisiae HF7c, and this inhibition also depends on the domains required for transcriptional regulation. We demonstrate that E1A associates with histone acetyltransferase activity and represses the transactivation activity of transcription factor in S. cerevisiae, suggesting that E1A may suppress the expression of genes required for normal growth. Based on yeast growth rescue, we present a genetic screening strategy that identified RACK1 as an E1A antagonizing factor. Expression of human RACK1 efficiently relieves E1A-mediated growth inhibition in HF7c and protects human tumor cells from E1A-induced apoptosis. Finally, we show that RACK1 decreases E1A-associated histone acetyltransferase activity in yeast and mammalian cells, and physically interacts with E1A. Our data demonstrate that RACK1 is a repressor of E1A, possibly by antagonizing the effects of E1A on host gene transcription.

Zinc finger and carboxyl regions of adenovirus E1A 13S CR3 are important for transactivation of the cytomegalovirus major immediate early promoter by adenovirus

Reactivation of latent cytomegalovirus (CMV) is an important cause of disease in susceptible patients. We previously demonstrated that an adenovirus early gene product can transactivate the CMV major immediate early (IE) promoter in inflammatory cells. This effect was due to the conserved region 3 (CR3) of the adenovirus E1A 13S gene product. There are two domains in the CR3 region, a zinc finger (aa 147-177) and a carboxyl (aa 180-188) domain. Both are crucial for transactivation of downstream promoter elements of adenovirus in E1A 13S. We sought to determine if either or both of these specific domains is also necessary for transactivation of the CMV IE promoter by the adenovirus E1A 13S gene product. We cotransfected T-lymphocyte Jurkat cells and monocyte/macrophage-like THP-1 cells with plasmids expressing wild-type (WT) or CR3 mutant E1A 13S and a CMV IE chloramphenicol acetyltransferase (CAT) reporter construct. With extracts of cells coinfected with E1A WT set to 100%, mutation in the zinc finger domain, the carboxyl domain, or both domains decreased CMV IE CAT activity by >/= 96%. In contrast, a mutation in the region between the zinc finger and carboxyl domains reduced CMV IE CAT activity by only 24 to 26%. Mixing studies in Jurkat cells confirmed the importance of these domains. We also evaluated the active site of the CMV IE promoter involved in transactivation in THP-1 cells using CMV IE promoter deletions and single promoter element constructs. These studies showed that progressive deletion of the 19-bp CMV IE repeats containing cyclic AMP response element binding protein/activating transcription factor (CREB/ATF) sites resulted in progressive loss of activity. The importance of this element was confirmed using single promoter elements containing CMV IE 16-, 18-, 19-, and 21-bp repeats. Finally, using a 19-bp single promoter element construct and the CR3 mutants we demonstrated that mutations in the zinc finger (C171S) carboxyl region (S185N) or both regions (C171S/ S185N) resulted in significant (83, 94, and 85%) loss of activity. We conclude that the zinc finger and carboxyl domains of the CR3 region of E1A 13S are necessary for transactivation of the CMV promoter and that this occurs mainly through activation of the 19-bp CREB/ATF site of the promoter.

Smad-dependent stimulation of type I collagen gene expression in human skin fibroblasts by TGF-beta involves functional cooperation with p300/CBP transcriptional coactivators

Transforming growth factor-beta (TGF-beta) stimulation of Type I collagen gene (COL1A2) transcription involves the Smad signal transduction pathway, but the mechanisms of Smad-mediated transcriptional activation are not fully understood. We now demonstrate that the ubiquitous transcriptional coactivators p300 and CREB-binding protein (CBP) enhanced basal as well as TGF-beta- or Smad3-induced COL1A2 promoter activity, and stimulated the expression of endogenous Type I collagen. The adenoviral E1A oncoprotein abrogated stimulation of COL1A2 activity in transfected fibroblasts, and reduced the basal level of collagen gene expression. This effect was due to specific interaction of E1A with cellular p300/CBP because (a) a mutant form of E1A defective in p300 binding failed to abrogate stimulation, and (b) forced expression of p300/CBP restored the ability of TGF-beta to stimulate COL1A2 promoter activity in the presence of E1A. The effect of p300 on COL1A2 transcription appeared to be due, in part, to its intrinsic acetyltransferase activity, as stimulation induced by a histone acetyltransferase-deficient mutant p300 was substantially reduced. Transactivation of COL1A2 by p300 involved the Smad signaling pathway, as Smad4-deficient cells failed to respond to p300, and stimulation was rescued by overexpression of Smad4. Furthermore, minimal constructs containing only the Smad-binding CAGACA element of COL1A2 were transactivated by p300 in the presence of TGF-beta. These results indicate, for the first time, that the multifunctional p300/CBP coactivators play a major role in Smad-dependent TGF-beta stimulation of collagen gene expression in fibroblasts. Oncogene (2000) 19, 3546 - 3555

Identification of the transactivation domain of the transcription factor Sox-2 and an associated co-activator

The importance of interactions between Sox and POU transcription factors in the regulation of gene expression is becoming increasingly apparent. Recently, many examples of the involvement of Sox-POU partnerships in transcription have been discovered, including a partnership between Sox-2 and Oct-3. Little is known about the mechanisms by which these factors modulate transcription. To better understand the molecular interactions involved, we mapped the location of the transactivation domain of Sox-2. This was done in the context of its interaction with Oct-3, as well as its ability to transactivate as a fusion protein linked to the DNA-binding domain of Gal4. Both approaches demonstrated that Sox-2 contains a transactivation domain in its C-terminal half, containing a serine-rich region and the C terminus. We also determined that the viral oncoprotein E1a inhibits the ability of the Gal4/Sox-2 fusion protein to transactivate, as well as the transcriptional activation mediated by the combined action of Sox-2 and Oct-3. In contrast, a mutant form of E1a, unable to bind p300, lacks both of these effects. Importantly, we determined that p300 overcomes the inhibitory effects of E1a in both assays. Together, these findings suggest that Sox-2 mediates its effects, at least in part, through the co-activator p300.

The Smad4 activation domain (SAD) is a proline-rich, p300-dependent transcriptional activation domain

Transforming growth factor-beta (TGF-beta) family members signal through a unique set of intracellular proteins called Smads. Smad4, previously identified as the tumor suppressor DPC4, is functionally distinct among the Smad family, and is required for the assembly and transcriptional activation of diverse, Smad-DNA complexes. We previously identified a 48-amino acid proline-rich regulatory element within the middle linker domain of this molecule, the Smad4 activation domain (SAD), which is essential for mediating these signaling activities. We now characterize the functional activity of the SAD. Mutants lacking the SAD are still able to form complexes with other Smad family members and associated transcription factors, but cannot activate transcription in these complexes. Furthermore, the SAD itself is able to activate transcription in heterologous reporter assays, identifying it as a proline-rich transcriptional activation domain, and indicating that the SAD is both necessary and sufficient to activate Smad-dependent transcriptional responses. We show that transcriptional activation by the SAD is p300-dependent, and demonstrate that this activity is associated with a physical interaction of the SAD with the amino terminus of p300. These data identify a novel function of the middle linker region of Smad4, and define the role of the SAD as an important locus determining the transcriptional activation of the Smad complex.

Activation of Xenopus genes required for lateral inhibition and neuronal differentiation during primary neurogenesis

XNGN-1, a member of the neurogenin family of basic helix-loop-helix proteins, plays a critical role in promoting neuronal differentiation in Xenopus embryos. When ectopically expressed, XNGN-1 induces the expression of a set of genes required for neuronal differentiation such as XMyT1 and NeuroD. At the same time, however, XNGN-1 induces the expression of genes that antagonize neuronal differentiation by a process called lateral inhibition. Here, we present evidence that XNGN-1 activates the expression of genes required for differentiation and lateral inhibition by recruiting transcriptional coactivators p300/CBP (CREB-binding protein) or PCAF (p3OO/CBP-associated protein), both of which contain histone acetyltransferase (HAT) activity. Significantly, transcriptional activation of the genes in the lateral inhibitory pathway is less dependent on the HAT activity than is the activation of the genes that mediate differentiation. We propose that this difference enables the genes in the lateral inhibition pathway to be induced prior to the genes that promote differentiation, thus enabling lateral inhibition to establish a negative feedback loop and restrict the number of cells undergoing neuronal differentiation.

Promoter specificity and stability control of the p53-related protein p73

The p53 family of proteins play instrumental roles in mediating the cellular response to stress. The p53-related gene product, p73, occurs as two distinct protein isoforms, referred to as alpha and beta, which differ in the length of the C-terminal region and arise through alternative splicing of the p73 RNA. Here, we describe an analysis of the transcription properties of p73 and show that although there are certain similarities between transcriptional activation mediated by p73 and p53, such as in their sensitivity to adenovirus E1A and the requirement for p300/CBP co-activator proteins, significant differences are apparent in the response mechanisms. Thus, we find that p73 shows a degree of specificity for the promoters of target genes that is quantitatively distinct from the response mediated by p53. For example, p73 activates the GADD45 gene more efficiently than p53, whereas the reverse situation was apparent for the p21 gene. These effects are, in part, due to the influence of a regulatory domain present in the extended C-terminal of the alpha isoform. Moreover, we provide evidence that this domain regulates protein abundance by influencing the proteasome-dependent degradation of p73. These data define a novel level of isoform-specific control in regulating p73 activity, and thereby highlight a significant difference between the mechanisms that govern the transcriptional activity of p53 and p73.

TGF-beta-induced phosphorylation of Smad3 regulates its interaction with coactivator p300/CREB-binding protein

Smads are intermediate effector proteins that transduce the TGF-beta signal from the plasma membrane to the nucleus, where they participate in transactivation of downstream target genes. We have shown previously that coactivators p300/CREB-binding protein are involved in TGF-beta-mediated transactivation of two Cdk inhibitor genes, p21 and p15. Here we examined the possibility that Smads function to regulate transcription by directly interacting with p300/CREB-binding protein. We show that Smad3 can interact with a C-terminal fragment of p300 in a temporal and phosphorylation-dependent manner. TGF-beta-mediated phosphorylation of Smad3 potentiates the association between Smad3 and p300, likely because of an induced conformational change that removes the autoinhibitory interaction between the N- and C-terminal domains of Smad3. Consistent with a role for p300 in the transcription regulation of multiple genes, overexpression of a Smad3 C-terminal fragment causes a general squelching effect on multiple TGF-beta-responsive reporter constructs. The adenoviral oncoprotein E1A can partially block Smad-dependent transcriptional activation by directly competing for binding to p300. Taken together, these findings define a new role for phosphorylation of Smad3: in addition to facilitating complex formation with Smad4 and promoting nuclear translocation, the phosphorylation-induced conformational change of Smad3 modulates its interaction with coactivators, leading to transcriptional regulation.

Role of p300, a transcriptional coactivator, in signalling of TGF-beta

BACKGROUND

Smad proteins are novel transcriptional regulators mediating the signalling of the transforming growth factor-beta (TGF-beta) superfamily. Coactivators such as p300/CBP promote transactivation by various transcription factors through a direct interaction with them. Adenoviral oncoprotein E1A, which binds p300, was shown to inhibit the signalling of TGF-beta. These findings raise the possibility that p300 may be involved in TGF-beta signalling.

RESULTS

We investigated whether p300 is involved in transactivation by Smads. p300 enhanced the Smad-induced transactivation of p3TP-Lux, a TGF-beta responsive reporter. E1A inhibited this enhancement, and the inhibition required its ability to bind p300/CBP. p300 and Smad3, as well as Smad2, interacted in vivo in a ligand-dependent manner. The binding region in Smad3 was its C-terminal half that was previously shown to possess an intrinsic transactivation activity. The binding region in p300 was mapped to its C-terminal 678 amino acids. The minimal Smad2/3-interacting region, as well as the rest of the p300, inhibited the transactivation of p3TP-Lux in a dominant-negative fashion.

CONCLUSION

p300 interacted with Smad2 and Smad3 in a ligand-dependent manner, and enhanced the transactivation by Smads. Our results present the molecular basis of the transactivation by Smad proteins.

The Pumilio RNA-binding domain is also a translational regulator

Posterior patterning in the Drosophila embryo requires the action of Nanos (Nos) and Pumilio (Pum), which collaborate to regulate the translation of maternal hunchback (hb) mRNA. Previous work demonstrated that Pum recognizes sites in the 3' UTR of hb mRNA. In this report, we first define the RNA-binding domain of Pum and then show that residues essential for translational repression are embedded within this domain. We also show that Nos and Pum can repress cap-independent translation from an internal ribosome entry site (IRES) in vivo, suggesting that they act downstream of the initial steps of normal, cap-dependent translation.

E1A represses apolipoprotein AI enhancer activity in liver cells through a pRb- and CBP-independent pathway

The apolipoprotein AI (apoAI) promoter/enhancer contains multiple cis -acting elements on which a variety of hepatocyte-enriched and ubiquitous transcription factors function synergistically to regulate liver-specific transcription. Adenovirus E1A proteins repress tissue-specific gene expression and disrupt the differentiated state in a variety of cell types. In this study expression of E1A 12Sor 13S in hepatoblastoma HepG2 cells repressed apoAI enhancer activity 8-fold. Deletion mapping analysis showed that inhibition by E1A was mediated by the apoAI promoter site B. E1A selectively inhibited the ability of HNF3beta and HNF3alpha to transactivate reporter genes controlled by the apoAI site B and the HNF3 binding site from the transthyretin promoter. The E1A-mediated repression of HNF3 activity was not reversed by overexpression of HNF3beta nor did E1A alter nuclear HNF3beta protein levels or inhibit HNF3 binding to DNA in mobility shift assays. Overexpression of two cofactors known to interact with E1A, pRb and CBP failed to overcome inhibition of HNF3 activity. Similarly, mutations in E1A that disrupt its interaction with pRb or CBP did not compromise its ability to repress HNF3beta transcriptional activity. These data suggest that E1A inhibits HNF3 activity by inactivating a limiting cofactor(s) distinct from pRb or CBP.

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.

Transforming region of 243R E1A contains two overlapping but distinct transactivation domains

Conserved regions 1 and 2 as well as the amino terminus of E1A are required for the transforming activity of the E1A oncoprotein. We show here that the amino terminus of 243R E1A has transactivation activity when brought to a promoter in yeast. Recruitment to a specific promoter is essential. Mutagenesis studies correlated the transactivation function with the extreme amino terminus and the conserved region 1 of E1A. Cotransfection assays in rodent cells confirmed that two overlapping but distinguishable domains, amino acids 1-65 and 37-80, can transactivate independently when targeted to a promoter. We also observed that when recruited to the proliferating cell nuclear antigen (PCNA) promoter, the amino-terminal region was sufficient to transactivate the PCNA promoter. On the other hand, deletion of the amino terminus of E1A resulted in failure to induce PCNA expression. Fusion of VP16 with the amino-terminal-deleted E1A mutant was able to restore the ability to induce the PCNA promoter. We further show that the amino-terminal region also is required for 243R E1A to repress the transactivation mediated by a universal transactivator DBD.VP16 and DBD.E1A. This repression could be specifically relieved by overexpression of TBP but not TFIIB. In addition, we show that the amino terminus of E1A is involved in in vitro interaction with the TATA binding protein (TBP). Thus the amino-terminal transforming region of E1A may regulate cellular gene expression in species that are distant in evolution via a common mechanism, functionally targeting TBP.

Adenovirus mediated-gene transfer into cardiomyocytes

To circumvent limitations imposed by conventional gene transfer techniques into cardiac muscle cells, we studied whether replication defective adenovirus would obviate this limitation to basic studies of signal transduction and transcriptional control processes in the heart. We demonstrate here the utility of adenovirus mediated gene transfer to introduce foreign DNA into post-mitotic terminally differentiated ventricular myocytes with uniformity and high efficiency. We also provide evidence for the genetic modification of neonatal ventricular myocytes by adenovirus early region 1 (E1) proteins and their impact on cardiac gene transcription and DNA synthesis respectively. Thus, for studies of transcriptional control processes in the heart, which until now have been restricted to neonatal ventricular myocytes; adenovirus mediated gene transfer provides a means to genetically manipulate adult cardiac muscle cells. The advent of adenovirus gene transfer will extend our understanding of the molecular mechanisms that mediate basic cardiac disease and may ultimately provide a means to therapeutically mitigate the disease process.

Extreme N terminus of E1A oncoprotein specifically associates with a new set of cellular proteins

By interacting with key regulatory proteins such as the pRb family, cyclins, cyclin-dependent kinases and p300/CBP of host cells, adenoviral E1A interferes with various cellular processes to provide a suitable environment for the replication of viruses. E1A may promote DNA synthesis and cell cycle progression, immortalize rodent cells in culture and transform cultured cells in cooperation with E1B, Ras, or other oncoproteins. Both extreme N terminus and conserved region 1 of E1A are required for the immortalization and the transformation of rodent cells, transcriptional repression and specific induction of the expression of cellular genes such as the proliferating cell nuclear antigen (PCNA) and heat shock protein 70 (HSP70). Although the molecular mechanisms of these functions of E1A are not fully understood, it is believed that protein-protein interactions may play essential roles. In this communication, we report that a new set of cellular proteins with apparent molecular weight of 200, 90, 45, 30, and 28 specifically associate with the extreme N terminus of E1A. Further analysis demonstrate that these associations do not depend on E1A's association with p300 or pRB. Neither the 30 kDa nor the 28 kDa polypeptide is identical to Cdc2 or Cdk2. The region of E1A required for the protein interaction is also required for the recently identified N-terminal transactivation activity of E1A. Our observations suggest that in addition to p300/CBP, the new set of cellular proteins may be involved in the functional complexity of the N terminus of E1A, thus predicting a p300/CBP independent pathway.

Cooperation of Stat2 and p300/CBP in signalling induced by interferon-alpha

The transcription factor ISGF3 transduces interferon (IFN)-alpha signals and activates the transcription of cellular antiviral defence genes. Adenovirus E1A blocks the IFN-alpha response, allowing unhindered viral replication. ISGF3 consists of Stat1, Stat2 and p48. Here we show that p300 and/or CBP (CREB-binding protein), which are transcription adaptors targeted by E1A, interact specifically with Stat2. Binding occurs between the first cysteine-histidine-rich region of p300/CBP and the carboxy-terminal segment of Stat2, a domain essential for ISGF3 function. We find that this domain of Stat2 has transactivation potential, which correlates with its binding to p300/CBP. Moreover, E1A represses Stat2 transactivation and IFN-alpha-activated transcription by inhibiting p300/CBP function. This provides a new mechanism for inhibition of the IFN-alpha-activated antiviral response by E1A, and supports the view that E1A binding to p300/CBP has functional significance for adenovirus replication in its natural host.

The human papillomavirus type 16 E7 protein complements adenovirus type 5 E1A amino-terminus-dependent transactivation of adenovirus type 5 early genes and increases ATF and Oct-1 DNA binding activity

We have previously shown that conserved region 1 (CR1) of the adenovirus type 5 (Ad5) E1A protein synergizes with CR3 in the transactivation of Ad5 early genes (H.K. Wong and E. B. Ziff, J. Virol. 68:4910-4920, 1994). CR1 lies within the E1A amino terminus and binds host regulatory proteins such as the RB protein, p107, p130, and p300. Since simian virus 40 (SV40) large T antigen and human papillomavirus type 16 (HPV16) E7 protein also bind host regulatory factors, we investigated whether these viral proteins can complement E1A mutants which are defective in early gene activation. We show that the HPV16 E7 protein but not SV40 T antigen can complement mutations in the Ad5 E1A CR1 in the transactivation of viral early promoters. The inability of SV40 T antigen to complement suggests that RB binding on its own is not sufficient for early promoter transactivation by the E1A amino terminus. Nuclear runoff assays show that complementation by HPV16 E7 restores the ability of the E1A mutants to stimulate early gene expression at the level of transcription. Furthermore, nuclear extracts from the E7-transformed cells show increased binding activity of ATF and Oct-1, factors that can recognize the elements of Ad5 early genes, consistent with gene activation by E1A and E7 at the transcriptional level.

The adenoviral transcription factor, E1A 13S, trans-activates the human tumor necrosis factor-alpha promoter

The 1311 bp TNF-alpha promoter region fused to a luciferase reporter vector was used in a transient transfection system to study the regulation of TNF-alpha promoter activity by E1A 13S in the U937 macrophage cell line and the MLA 144 T cell line. Co-transfections of the TNF-alpha promoter with an E1A expression vector resulted in a strong trans-activation of the promoter in both cell lines. Sequential truncation of the promoter mapped the E1A responsive region to sequences contained between -120 bp and the transcription start site. Truncation to -95 bp caused a dramatic 87% reduction of E1A activation in MLA 144 cells and further truncation to -36 bp caused a complete loss of E1A activation. In U937 cells, each truncation lowered E1A responsiveness but activity was never completely abolished. Site-directed mutagenesis of putative cis-acting sequences in the TNF-alpha promoter identified the AP-1 site as important for E1A trans-activation in the U937 cell line; the AP-2 and CRE sites also appeared to contribute to a lesser degree. In contrast, only the CRE mutation caused a reduction in E1A induced activity in the MLA 144 cell line. Co-transfection of the E1A expression vector with expression vectors for the cellular transcription factors AP-1, AP-2 and CREB indicated that none of these transcription factors showed any co-operativity with E1A. Thus, cis-acting sequences which contribute to E1A trans-activation of the TNF-alpha promoter have been delineated.

Induction of susceptibility to tumor necrosis factor by E1A is dependent on binding to either p300 or p105-Rb and induction of DNA synthesis

The introduction of the adenovirus early region 1A (E1A) gene products into normal cells sensitizes these cells to the cytotoxic effects of tumor necrosis factor (TNF). Previous studies have shown that the region of E1A responsible for susceptibility is CR1, a conserved region within E1A which binds the cellular proteins p300 and p105-Rb at nonoverlapping sites. Binding of these and other cellular proteins by E1A results in the induction of E1A-associated activities such as transformation, immortalization, DNA synthesis, and apoptosis. To investigate the mechanism by which E1A induces susceptibility to TNF, the NIH 3T3 mouse fibroblast cell line was infected with viruses containing mutations within E1A which abrogate binding of some or all of the cellular proteins to E1A. The results show that TNF susceptibility is induced by E1A binding to either p300 or p105-Rb. E1A mutants that bind neither p300 nor p105-Rb do not induce susceptibility to TNF. Experiments with stable cell lines created by transfection with either wild-type or mutant E1A lead to these same conclusions. In addition, a correlation between induction of DNA synthesis and induction of TNF sensitivity is seen. Only viruses which induce DNA synthesis can induce TNF sensitivity. Those viruses which do not induce DNA synthesis also do not induce TNF sensitivity. These data suggest that the mechanisms underlying induction of susceptibility to TNF by E1A are intimately connected to E1A's capacity to override cell cycle controls.

Adenovirus E1A243 disrupts the ATF/CREB-YY1 complex at the mouse c-fos promoter

The adenovirus E1A243 protein can activate transcription of the mouse c-fos gene in a manner that depends on treatment of cells with inducers or analogs of cyclic AMP (cAMP). Activation requires conserved region 1 and the N-terminal domain of E1A243 and is mediated by a 22-bp E1A response element containing a cAMP response element (CRE) at -67 and a binding site for transcription factor YY1 at -54. In the absence of E1A243, YY1 represses CRE-dependent transcription of c-fos by physically interacting with ATF/CREB proteins bound to the -67 CRE. Here we present evidence that expression of E1A243 leads to relief of YY1-mediated repression by a disruption of the ATF/CREB-YY1 complex. Addition of E1A243 to in vitro binding assays prevented binding of ATF-2 to glutathione S-transferase-YY1. Similarly, expression of E1A243 in HeLa cells prevented the association of a YY1-VP16 fusion protein with endogenous ATF/CREB proteins bound to the -67 CRE of a transfected c-fosCAT reporter plasmid. In each case, the N-terminal domain of E1A243, which mediates a direct interaction with YY1, was responsible for disruption of the ATF/CREB-YY1 complex. On the basis of these and previously published results, we present a model for the synergistic transcriptional activation of the c-fos gene by E1A243 and cAMP.

Induction by adenovirus-5 E1A of the differentiation phenotype of F9 teratocarcinoma cells involves a conserved region (CR1) of E1A

The effects of the E1A protein of adenovirus-5 on the differentiation program of F9 teratocarcinoma cells were examined by the stable introduction of plasmids that expressed wild-type or mutated forms of E1A. Constitutive expression of plasmids for most of the mutant E1As induced loss of expression of the cell-surface antigen SSEA-1 and the enhanced expression of genes specific for the differentiated phenotype of F9 cells, such as genes for laminin B1, tissue-type plasminogen activator (tPA) and type IV collagen, as well as the altered cell morphology that is associated with the differentiated state. However, such changes were not observed in the case of genes for mutant proteins from which a conserved region (CR1) of E1A had been deleted. Furthermore, no significant induction of expression of the c-jun gene or transactivation of the c-jun-CAT reporter gene were observed when the sequence that encodes CR1 of E1A had been deleted. A palindromic sequence element (DRE) of the c-jun promoter was essential for the E1A-mediated up-regulation of the c-jun gene. These results imply that CR1 is required for activation of the c-jun gene and that it is implicated in the growth arrest, expression of parietal endoderm-specific functions and the orderly differentiation of F9 cells.

Identification of a novel E1A response element in the mouse c-fos promoter

Transcriptional activation of the c-fos gene in mouse S49 cells by the adenovirus 243-amino-acid E1A protein depends on domains of E1A that are also required for transformation and that bind the cellular protein p300. Activation additionally depends on stimulation of endogenous cyclic AMP (cAMP)-dependent protein kinase by analogs or inducers of cAMP. Transient transfection assays were used to analyze the c-fos promoter for sequences that confer responsiveness to E1A. Linker substitution and point mutants revealed that transcriptional activation by E1A depended on a cAMP response element (CRE) located at -67 relative to the start site of transcription and a neighboring binding site for transcription factor YY1 located at -54. A 22-bp sequence containing the -67 CRE and the -54 YY1 site was sufficient to confer responsiveness to a minimal E1B promoter and was termed the c-fos E1A response element (ERE). Function of the c-fos ERE depended on both the CRE and the YY1 site, since mutation of either site resulted in a loss of responsiveness to E1A. These results imply a specific functional interaction between CRE-binding proteins, transcription factor YY1, and E1A in the regulation of the c-fos gene.

The effect of E1A transfection on MMP-9 expression and metastatic potential

The expression of MMP-9 in rat embryo fibroblasts (REF) transformed with Ha-ras or with Ha-ras and v-myc is associated with metastatic behavior. In contrast, REF transformed with Ha-ras and the adenovirus E1A genes (E1A) are tumorigenic, do not release MMP-9 and are rarely metastatic. In this report, we establish that E1A expression results in decreased levels of MMP-9 mRNA in an Ha-ras and v-myc transformed cell line and examine which of the functional domains of E1A participate in the inhibition of MMP-9 expression and which contribute to the suppression of metastasis. The metastatic 2.10 REF line, derived by co-transfection with v-myc and Ha-ras, constitutively expresses high levels of MMP-9 (92 kDa gelatinase). Transfection of E1A wild-type plasmids into this cell line eliminates detectable MMP-9 mRNA expression and greatly reduces MMP-9 activity. Transfection of 2.10 with E1A plasmids encoding mutations in conserved region 2 (CR2) retained inhibition of MMP-9 similar to the inhibition seen with wild-type E1A. Transfection with E1A containing mutations in CRI or the amino terminal region diminished, but did not fully inhibit, MMP-9 expression. In contrast, inhibition of MMP-9 was lost in with E1A mutations in CR3. Cells transfected with E1A mutants in CR1, the amino terminal region or CR3 retained metastatic behavior. Our findings delineate the regions of E1A responsible for MMP-9 inhibition and further define the domains of E1A responsible for inhibition of metastasis.

Adenoviral E1A-associated protein p300 as a functional homologue of the transcriptional co-activator CBP

The 265K nuclear protein CBP was initially identified as a co-activator for the protein kinase A (PKA)-phosphorylated form of the transcription factor CREB. The domains in CBP that are involved in CREB binding and transcriptional activation are highly related to the adenoviral E1A-associated cellular protein p300 (refs 2, 3), and to two hypothetical proteins from Caenorhabditis elegans, R10E11.1 and K03H1.10 (refs 4 and 5, respectively), whose functions are unknown. Here, we show that CBP and p300 have similar binding affinity for the PKA-phosphorylated form of CREB, and that p300 can substitute for CBP in potentiating CREB-activated gene expression. We find that E1A binds to CBP through a domain conserved with p300 and represses the CREB-dependent co-activator functions of both CBP and p300. Our results indicate that the gene repression and cell immortalization functions associated with E1A involve the inactivation of a family of related proteins that normally participate in second-messenger-regulated gene expression.

Functional importance of complex formation between the retinoblastoma tumor suppressor family and adenovirus E1A proteins as determined by mutational analysis of E1A conserved region 2

Adenovirus early region 1A (E1A) products induce DNA synthesis, transform primary rodent cells, and activate transcription factor E2F through complex formation with an array of cellular proteins via the E1A amino terminus and conserved regions 1 and 2 (CR1 and CR2). Interactions with the retinoblastoma tumor suppressor, pRb, and related proteins p107 and p130 rely somewhat on CR1 but largely on CR2, which contains a core binding sequence Leu-122-X-Cys-X-Glu. We introduced point mutations in CR2 to define such interactions more precisely. In human cells, alteration of any of the conserved residues within the binding core eliminated complex formation with pRb. Conversion of nonconserved Thr-123 to Pro (but not to either Ala or Ser) disrupted binding of pRb, presumably because of conformational changes in the binding core. No single E1A point mutant was completely defective in binding p107, suggesting that molecular interactions between E1A proteins and p107 clearly differ from those with pRb and p130. In general, the patterns of complex formation by E1A mutants in rat, monkey, and human cells were quite similar. All mutants which failed to bind significant amounts of pRb also failed to transform primary rat cells. Several mutants demonstrated selective binding to pRb, p107, and p130, but transforming activity corresponded largely with complex formation with pRb, regardless of the levels of interactions with p107 and p130. Mutants defective for binding of both pRb and p107 failed to induce the activity of transcription factor E2F; however, quite high levels were activated by E1A mutants that interacted with p107 alone. These results suggested that both pRb and p107 are important regulators of E2F activity but that complex formation with and activation of E2F by p107 are insufficient for cell transformation.

The E1A products of oncogenic adenovirus serotype 12 include amino-terminally modified forms able to bind the retinoblastoma protein but not p300

The cell growth-regulating properties of the adenovirus type 5 (Ad5) E1A oncogene correlate closely with the binding of the E1A products to specific cellular proteins. These proteins include the products of the retinoblastoma tumor susceptibility gene and a 300-kDa product, p300. pRB binds to E1A sequences that are highly conserved among the E1A products of various serotypes, while p300 binding requires sequences in the E1A amino terminus, a region that is not highly conserved. To help evaluate the roles of the E1A-associated proteins in cell growth control, we have compared the p300-binding abilities of the E1A products of Ad5 and of the more oncogenic Ad12 serotype. We show here that despite encoding a sequence that varies somewhat from the p300-binding sequences of Ad5 E1A, the Ad12 E1A products associate with p300 with an affinity similar to that of the Ad5 E1A products. Both the 12S and 13S splice products of Ad12 E1A, like those of Ad5 E1A, encode proteins able to associate with p300. Interestingly, though, both also give rise to prominent forms that are amino terminally modified and unable to associate with p300. This modification, at least in the 13S product, does not appear to diminish the affinity of this product for the retinoblastoma protein.

Proteins with transcription regulatory properties encoded by human adenoviruses

Of the more than 30 genes encoded by the adenovirus genome, no less than six have been shown to encode proteins that have transcription regulatory properties. None of them is a sequence-specific DNA-binding protein. They act to modulate the activity of cellular transcription factors by causing their phosphorylation or dephosphorylation, by physically interacting with them, or by dissociating transcription factor inhibitory protein complexes.

Adenovirus-E1A proteins transform cells by sequestering regulatory proteins

Cell transformation by adenovirus-E1A proteins is mediated by binding to cellular proteins whose functions are thereby inactivated or altered. The various properties of the E1A proteins are reviewed in relation to their binding to cellular proteins. A number of the cellular proteins which associate to E1A have been identified: the retinoblastoma-susceptibility protein (Rb), the p107 protein, cyclin A and the p33cdk2 kinase. Recent data have shown that those proteins are also able to bind to transcription factor E2F. Binding of Rb to E2F represses the transcription-activating potential of E2F. E1A can sequester the regulatory proteins, like Rb, and thereby release free, active E2F. The domains in E1A that are essential for this transcriptional regulation are also required for the transforming properties of E1A.