E1A-mediated inhibition of myogenesis correlates with a direct physical interaction of E1A12S and basic helix-loop-helix proteins

Hacking the Cell: Network Intrusion and Exploitation by Adenovirus E1A

As obligate intracellular parasites, viruses are dependent on their infected hosts for survival. Consequently, viruses are under enormous selective pressure to utilize available cellular components and processes to their own advantage. As most, if not all, cellular activities are regulated at some level via protein interactions, host protein interaction networks are particularly vulnerable to viral exploitation. Indeed, viral proteins frequently target highly connected “hub” proteins to “hack” the cellular network, defining the molecular basis for viral control over the host. This widespread and successful strategy of network intrusion and exploitation has evolved convergently among numerous genetically distinct viruses as a result of the endless evolutionary arms race between pathogens and hosts. Here we examine the means by which a particularly well-connected viral hub protein, human adenovirus E1A, compromises and exploits the vulnerabilities of eukaryotic protein interaction networks. Importantly, these interactions identify critical regulatory hubs in the human proteome and help define the molecular basis of their function.

The critical protein interactions and structures that elicit growth deregulation in cancer and viral replication

One of the greatest challenges in biomedicine is to define the critical targets and network interactions that are subverted to elicit growth deregulation in human cells. Understanding and developing rational treatments for cancer requires a definition of the key molecular targets and how they interact to elicit the complex growth deregulation phenotype. Viral proteins provide discerning and powerful probes to understand both how cells work and how they can be manipulated using a minimal number of components. The small DNA viruses have evolved to target inherent weaknesses in cellular protein interaction networks to hijack the cellular DNA and protein replication machinery. In the battle to escape the inevitability of senescence and programmed cell death, cancers have converged on similar mechanisms, through the acquisition and selection of somatic mutations that drive unchecked cellular replication in tumors. Understanding the dynamic mechanisms through which a minimal number of viral proteins promote host cells to undergo unscheduled and pathological replication is a powerful strategy to identify critical targets that are also disrupted in cancer. Viruses can therefore be used as tools to probe the system-wide protein-protein interactions and structures that drive growth deregulation in human cells. Ultimately this can provide a path for developing system context-dependent therapeutics. This review will describe ongoing experimental approaches using viruses to study pathways deregulated in cancer, with a particular focus on viral cellular protein-protein interactions and structures.

The N terminus of adenovirus type 12 E1A inhibits major histocompatibility complex class I expression by preventing phosphorylation of NF-kappaB p65 Ser276 through direct binding

The immune-escape strategy employed by human oncogenic adenovirus type 12 (Ad12) involves downregulation of major histocompatibility complex class I (MHC-I) transcription by disabling the transactivator NF-kappaB (p50/p65). This is accomplished by the Ad12 E1A protein (E1A-12), which prevents NF-kappaB from becoming phosphorylated by the protein kinase A catalytic subunit (PKAc). In this study, we examined the interactions between E1A-12 and NF-kappaB. Our data show that an E1A-12 mutant retaining the N-terminal 66 amino acids was as effective as the wild-type E1A-12 protein (266 amino acids) in binding p65, preventing phosphorylation of p65-Ser(276), and inhibiting transactivation. In contrast, the nontumorigenic adenovirus type 5 E1A protein (E1A-5) and other E1A-12 mutants lacking the N-terminal regions were severely defective in these activities. Further studies revealed that an N-terminal peptide consisting of residues 1 to 40 of E1A-12 was able to associate directly with p65 in vitro and prevent PKAc from phosphorylating p65-Ser(276). In the absence of the N terminus, there is an almost complete loss of E1A-12 binding to p65. These findings provide solid evidence for the role of the E1A-12 N terminus as an NF-kappaB binding domain. Significantly, this study indicates that the E1A-12 N terminus prevents PKAc from gaining access to p65 to account for Ser(276) hypophosphorylation. The E1A-12 N terminus interaction with p65 serves as a key explanation of how Ad12 downregulates MHC-I transcription and contributes to oncogenesis by escaping cytotoxic T lymphocytes.

Muscle-regulated expression and determinants for neuromuscular junctional localization of the mouse RIalpha regulatory subunit of cAMP-dependent protein kinase

In skeletal muscle, transcription of the gene encoding the mouse type Ialpha (RIalpha) subunit of the cAMP-dependent protein kinase is initiated from the alternative noncoding first exons 1a and 1b. Here, we report that activity of the promoter upstream of exon 1a (Pa) depends on two adjacent E boxes (E1 and E2) in NIH 3T3-transfected fibroblasts as well as in intact muscle. Both basal activity and MyoD transactivation of the Pa promoter require binding of the upstream stimulating factors (USF) to E1. E2 binds either an unknown protein in a USF/E1 complex-dependent manner or MyoD. Both E2-bound proteins seem to function as repressors, but with different strengths, of the USF transactivation potential. Previous work has shown localization of the RIalpha protein at the neuromuscular junction. Using DNA injection into muscle of plasmids encoding segments of RIalpha or RIIalpha fused to green fluorescent protein, we demonstrate that anchoring at the neuromuscular junction is specific to RIalpha subunits and requires the amino-terminal residues 1-81. Mutagenesis of Phe-54 to Ala in the full-length RIalpha-green fluorescent protein template abolishes localization, indicating that dimerization of RIalpha is essential for anchoring. Moreover, two other hydrophobic residues, Val-22 and Ile-27, are crucial for localization of RIalpha at the neuromuscular junction. These amino acids are involved in the interaction of the Caenorhabditis elegans type Ialpha homologue R(CE) with AKAP(CE) and for in vitro binding of RIalpha to dual A-kinase anchoring protein 1. We also show enrichment of dual A-kinase anchoring protein 1 at the neuromuscular junction, suggesting that it could be responsible for RIalpha tethering at this site.

The retinoblastoma gene family members pRB and p107 coactivate the AP-1-dependent mouse tissue factor promoter in fibroblasts

Serum-stimulation of quiescent mouse fibroblasts results in transcriptional activation of tissue factor (TF), the cellular initiator of blood coagulation. This requires the rapid entry of c-Fos into specific AP-1 DNA-binding complexes and can be strongly inhibited by the adenovirus EIA 12S gene product. In this study, we utilized a panel of E1A mutants deficient in cellular protein binding to analyse the molecular basis for EIA inhibition of a minimal, c-Fos-dependent TF promoter/ reporter construct in mouse AKR-2B fibroblasts. Mutations which impaired binding of the retinoblastoma tumor suppressor protein family members pRB, p107, and p130 relieved E1A-mediated inhibition of transcription in response to serum-stimulation or c-Fos overexpression. Inhibition was restricted to the G0 to G1 transition, consistent with the specificity of E1A for hypophosphorylated forms of RB proteins. Although E1A mutants deficient in CBP/p300 binding retained the ability to inhibit TF transcription, deletion of the amino-terminal portion of the CBP/p300 interaction domain was required to permit rescue of TF promoter activity by coexpression of pRB. Moreover, ectopic p107 could effectively substitute for pRB in relieving E1A-mediated repression. In primary mouse embryo fibroblasts, activity of the minimal AP-1-dependent TF promoter was suppressed in Rb(-/-) cells compared to parallel Rb(+/-) and Rb(+/+) transfectants. Ectopic expression of either pRB or p107 markedly enhanced TF promoter activity in Rb(-/-) fibroblasts. Collectively, these data imply that pRB and p107 can cooperate with c-Fos to activate TF gene transcription in fibroblasts and suggest a requirement for another, as yet unidentified, E1A-binding protein.

Suppression of E1A-mediated transformation by the p50E4F transcription factor

The adenovirus E1A gene can act as an oncogene or a tumor suppressor, with the latter effect generally arising from the induction of apoptosis or the repression of genes that provide oncogenic growth stimuli (e.g., HER-2/c-erbB2/neu) or increased metastatic invasiveness (e.g., metalloproteases). In this study, coexpression of E1A and p50E4F, a cellular transcription factor whose DNA binding activity is stimulated by E1A, suppressed colony formation by NIH 3T3 cells and transformation of primary rat embryo fibroblasts but had no observed effect in the absence of E1A. Domains in p50E4F required for stimulation of the adenovirus E4 promoter were required for the suppressive effect, indicating a transcriptional mechanism. In serum-containing media, retroviral expression of p50E4F in E1A13S/ras-transformed NIH 3T3 fibroblasts had little effect on subconfluent cultures but accelerated a decline in viability after the cultures reached confluence. Cell death occurred by both apoptosis and necrosis, with the predominance of each process determined by culture conditions. In serum-free media, p50E4F accelerated E1A-induced apoptosis. The results suggest that p50E4F sensitizes cells to signals or conditions that cause cell death.

p21(CIP1) and p57(KIP2) control muscle differentiation at the myogenin step

Cell-cycle arrest is thought to be required for differentiation of muscle cells. However, the molecules controlling cell-cycle exit and the differentiation step(s) dependent on cell-cycle arrest are poorly understood. Here we show that two Cdk inhibitors, p21(CIP1) and p57(KIP2), redundantly control differentiation of skeletal muscle and alveoli in the lungs. Mice lacking both p21 and p57 fail to form myotubes, display increased proliferation and apoptotic rates of myoblasts, and display endoreplication in residual myotubes. This point of arrest during muscle development is identical to that of mice lacking the myogenic transcription factor myogenin, indicating a role for cell-cycle exit in myogenin function. Expression of myogenin, p21, and p57 is parallel but independent, and in response to differentiation signals, these proteins are coordinately regulated to trigger both cell-cycle exit and a dependent muscle-specific program of gene expression to initiate myoblast terminal differentiation and muscle formation.

Suppression of adenovirus E1A-induced apoptosis by mutated p53 is overcome by coexpression with Id proteins

The rat 3Y1 derivative cell lines, EId10 and EId23, established by introducing the adenovirus E1A12S, Id-1H, and Id-2H cDNAs linked to the hormone-inducible promoter, express these proteins upon treatment with dexamethasone and elicit apoptosis, although these cell lines express mutated p53. The E1A mutants containing a deletion in either the N terminus or the conserved region 1 were unable to induce apoptosis in cooperation with Ids. Western blot analysis of the immunoprecipitates prepared from the dexamethasone-treated EId10 cell extract showed that Id-2H preferentially binds to E1A and E2A (E12/E47) helix-loop-helix transcription factors in vivo, but scarcely to the retinoblastoma protein. After induction of E1A and Ids, EId10 and EId23 cells began to accumulate in S phase and undergo apoptosis before entering G2 phase, suggesting that abnormal synthesis of DNA induced by coexpression of E1A, Id-1H, and Id-2H results in the induction of apoptosis. Apoptosis also is induced in mouse A40 (p53-/-) cells by E1A alone or E1A plus Ids after transient transfection of the expression vectors. The induction of apoptosis is stimulated by coexpression with wild-type p53; however, apoptosis induced by E1A alone was suppressed completely by coexpression with mutated p53, whereas apoptosis induced by E1A plus Ids was stimulated by the mutated p53 as done by wild-type p53. These results suggest that the suppressive function of mutated p53 is overcome by Ids.

A cellular repressor of E1A-stimulated genes that inhibits activation by E2F

The adenovirus E1A protein both activates and represses gene expression to promote cellular proliferation and inhibit differentiation. Here we report the identification and characterization of a cellular protein that antagonizes transcriptional activation and cellular transformation by E1A. This protein, termed CREG for cellular repressor of E1A-stimulated genes, shares limited sequence similarity with E1A and binds both the general transcription factor TBP and the tumor suppressor pRb in vitro. In transfection assays, CREG represses transcription and antagonizes 12SE1A-mediated activation of both the adenovirus E2 and cellular hsp70 promoters. CREG also antagonizes E1A-mediated transformation, as expression of CREG reduces the efficiency with which E1A and the oncogene ras cooperate to transform primary cells. Binding sites for E2F, a key transcriptional regulator of cell cycle progression, were found to be required for repression of the adenovirus E2 promoter by CREG, and CREG was shown to inhibit activation by E2F. Since both the adenovirus E1A protein and transcriptional activation by E2F function to promote cellular proliferation, the results presented here suggest that CREG activity may contribute to the transcriptional control of cell growth and differentiation.

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.

Database of mutations within the adenovirus 5 E1A oncogene

The Ad5 E1A database is a listing of mutations affecting the early region 1A (E1A) proteins of human adenovirus type 5. The database contains the name of the mutation, the nucleic acid sequence changes, the resulting alterations in amino acid sequence and reference. Additional notes and references are provided on the effect of each mutation on E1A function. The database is contained within the Adenovirus 5 E1A page on the World Wide Web at: http://www.geocities.com/CapeCanaveral/Hangar /2541/

The basic domain of myogenic basic helix-loop-helix (bHLH) proteins is the novel target for direct inhibition by another bHLH protein, Twist

In vertebrates, the basic helix-loop-helix (bHLH) protein Twist may be involved in the negative regulation of cellular determination and in the differentiation of several lineages, including myogenesis, osteogenesis, and neurogenesis. Although it has been shown that mouse twist (M-Twist) (i) sequesters E proteins, thus preventing formation of myogenic E protein-MyoD complexes and (ii) inhibits the MEF2 transcription factor, a cofactor of myogenic bHLH proteins, overexpression of E proteins and MEF2 failed to rescue the inhibitory effects of M-Twist on MyoD. We report here that M-Twist physically interacts with the myogenic bHLH proteins in vitro and in vivo and that this interaction is required for the inhibition of MyoD by M-Twist. In contrast to the conventional HLH-HLH domain interaction formed in the MyoD/E12 heterodimer, this novel type of interaction uses the basic domains of the two proteins. While the MyoD HLH domain without the basic domain failed to interact with M-Twist, a MyoD peptide containing only the basic and helix 1 regions was sufficient to interact with M-Twist, suggesting that the basic domain contacts M-Twist. The replacement of three arginine residues by alanines in the M-Twist basic domain was sufficient to abolish both the binding and inhibition of MyoD by M-Twist, while the domain retained other M-Twist functions such as heterodimerization with an E protein and inhibition of MEF2 transactivation. These findings demonstrate that M-Twist interacts with MyoD through the basic domains, thereby inhibiting MyoD.

Molecular mechanisms of myogenic coactivation by p300: direct interaction with the activation domain of MyoD and with the MADS box of MEF2C

By searching for molecules that assist MyoD in converting fibroblasts to muscle cells, we have found that p300 and CBP, two related molecules that act as transcriptional adapters, coactivate the myogenic basic-helix-loop-helix (bHLH) proteins. Coactivation by p300 involves novel physical interactions between p300 and the amino-terminal activation domain of MyoD. In particular, disruption of the FYD domain, a group of three amino acids conserved in the activation domains of other myogenic bHLH proteins, drastically diminishes the transactivation potential of MyoD and abolishes both p300-mediated coactivation and the physical interaction between MyoD and p300. Two domains of p300, at its amino and carboxy terminals, independently function to both mediate coactivation and physically interact with MyoD. A truncated segment of p300, unable to bind MyoD, acts as a dominant negative mutation and abrogates both myogenic conversion and transactivation by MyoD, suggesting that endogenous p300 is a required coactivator for MyoD function. The p300 dominant negative peptide forms multimers with intact p300. p300 and CBP serve as coactivators of another class of transcriptional activators critical for myogenesis, myocyte enhancer factor 2 (MEF2). In fact, transactivation mediated by the MEF2C protein is potentiated by the two coactivators, and this phenomenon is associated with the ability of p300 to interact with the MADS domain of MEF2C. Our results suggest that p300 and CBP may positively influence myogenesis by reinforcing the transcriptional autoregulatory loop established between the myogenic bHLH and the MEF2 factors.

Cyclin-mediated inhibition of muscle gene expression via a mechanism that is independent of pRB hyperphosphorylation

It was recently demonstrated that ectopic expression of cyclin D1 inhibits skeletal muscle differentiation and, conversely, that expression of cyclin-dependent kinase (cdk) inhibitors facilitates activation of this differentiation program (S. S. Rao, C. Chu, and D. S. Kohtz, Mol. Cell. Biol. 14:5259-5267, 1994; S. S. Rao and D. S. Kohtz, J. Biol. Chem. 270:4093-4100, 1995; S. X. Skapek, J. Rhee, D. B. Spicer, and A. B. Lassar, Science 267:1022-1024, 1995). Here we demonstrate that cyclin D1 inhibits muscle gene expression without affecting MyoD DNA binding activity. Ectopic expression of cyclin D1 inhibits muscle gene activation by both MyoD and myogenin, including a mutated form of myogenin in which two potential inhibitory cdk phosphorylation sites are absent. Because the retinoblastoma gene product, pRB, is a known target for cyclin D1-cdk phosphorylation, we determined whether cyclin D1-mediated inhibition of myogenesis was due to hyperphosphorylation of pRB. In pRB-deficient fibroblasts, the ability of MyoD to activate the expression of muscle-specific genes requires coexpression of ectopic pRB (B. G. Novitch, G. J. Mulligan, T. Jacks, and A. B. Lassar, J. Cell Biol., 135:441-456, 1996). In these cells, the expression of cyclins A and E can lead to pRB hyperphosphorylation and can inhibit muscle gene expression. The negative effects of cyclins A or E on muscle gene expression are, however, reversed by the presence of a mutated form of pRB which cannot be hyperphosphorylated. In contrast, cyclin D1 can inhibit muscle gene expression in the presence of the nonhyperphosphorylatable form of pRB. On the basis of these results we propose that G1 cyclin-cdk activity blocks the initiation of skeletal muscle differentiation by two distinct mechanisms: one that is dependent on pRB hyperphosphorylation and one that is independent of pRB hyperphosphorylation.

Skeletal muscle cells lacking the retinoblastoma protein display defects in muscle gene expression and accumulate in S and G2 phases of the cell cycle

Viral oncoproteins that inactivate the retinoblastoma tumor suppressor protein (pRb) family both block skeletal muscle differentiation and promote cell cycle progression. To clarify the dependence of terminal differentiation on the presence of the different pRb-related proteins, we have studied myogenesis using isogenic primary fibroblasts derived from mouse embryos individually deficient for pRb, p107, or p130. When ectopically expressed in fibroblasts lacking pRb, MyoD induces an aberrant skeletal muscle differentiation program characterized by normal expression of early differentiation markers such as myogenin and p21, but attenuated expression of late differentiation markers such as myosin heavy chain (MHC). Similar defects in MHC expression were not observed in cells lacking either p107 or p130, indicating that the defect is specific to the loss of pRb. In contrast to wild-type, p107-deficient, or p130-deficient differentiated myocytes that are permanently withdrawn from the cell cycle, differentiated myocytes lacking pRb accumulate in S and G2 phases and express extremely high levels of cyclins A and B, cyclin-dependent kinase (Cdk2), and Cdc2, but fail to readily proceed to mitosis. Administration of caffeine, an agent that removes inhibitory phosphorylations on inactive Cdc2/cyclin B complexes, specifically induced mitotic catastrophe in pRb-deficient myocytes, consistent with the observation that the majority of pRb-deficient myocytes arrest in S and G2. Together, these findings indicate that pRb is required for the expression of late skeletal muscle differentiation markers and for the inhibition of DNA synthesis, but that a pRb-independent mechanism restricts entry of differentiated myocytes into mitosis.

Expression of E1A in terminally differentiated muscle cells reactivates the cell cycle and suppresses tissue-specific genes by separable mechanisms

Terminally differentiated cells are characterized by permanent withdrawal from the cell cycle; they do not enter S phase even when stimulated by growth factors or retroviral oncogenes. We have shown, however, that the adenovirus E1A oncogene can reactivate the cell cycle in terminally differentiated cells. In this report, we describe the molecular events triggered by E1A in terminally differentiated skeletal muscle cells. We found that in myotubes infected with the adenovirus mutant dl520, 12S E1A bypasses the early G1 phase and activates the expression of late-G1 genes, such as the cyclin E and cyclin A genes, cdk2, PCNA, and B-myb. Of these, the cyclin E gene and cdk2 were significantly overexpressed in comparison with levels in proliferating, undifferentiated myoblasts. p130 and pRb were phosphorylated before the infected myotubes entered S phase, despite the high expression of the cyclin-dependent kinase inhibitor p21, and E2F was released. Our results suggest that one of the mechanisms that E1A uses to overcome the proliferative block of terminally differentiated cells involves coordinated overexpression of cyclin E and cdk2. Following E1A expression, the myogenic transcription factors MyoD and myogenin and the muscle-specific structural genes encoding muscle creatine kinase and myosin heavy chain were downregulated. The muscle regulatory factors were also silenced in myotubes infected with adenovirus E1A mutants incapable of reactivating the cell cycle in terminally differentiated muscle cells. Thus, the suppression of the differentiation program is not a consequence of cell cycle reactivation in myotubes, and it is induced by an independent mechanism. Our results show that E1A reactivates the cell cycle and suppresses tissue-specific gene expression in terminally differentiated muscle cells, thus causing dedifferentiation.

A novel E1A domain mediates skeletal-muscle-specific enhancer repression independently of pRB and p300 binding

The adenovirus E1A oncoprotein completely blocks muscle differentiation and specifically inhibits the transactivating function of myogenic basic helix-loop-helix (bHLH) transcription factors. This inhibition is dependent on the conserved region CR1 of E1A, which also constitutes part of the binding sites for the pocket proteins pRB, p107, and p130 and the transcriptional coactivators p300 and CBP. Here we report a detailed mutational analysis of E1A and the identification of a muscle inhibition motif within CR1. This motif encompasses amino acids 38 to 62 and inhibits Myf-5- or MyoD-mediated activation of myogenin and the muscle creatine kinase gene. Overexpression of this E1A region also inhibits the conversion of 10T1/2 fibroblasts to the myogenic lineage. The sequence motif EPDNEE (amino acids 55 to 60) within CR1 appears to be particularly important, because point mutations of this sequence diminish the E1A inhibitory activity. Interactions of E1A with pRB and with p300 do not seem to be necessary for the muscle-specific enhancer repression, because E1A mutants which lack these interactions still inhibit Myf-5- and MyoD-mediated transactivation. Moreover, overexpression of p300 fails to overcome muscle-specific inhibition by wild-type E1A and mutant E1A protein which lacks pRB binding. Since we have no evidence for direct E1A interaction with bHLH proteins, we propose that E1A may target a necessary cofactor of the muscle-specific bHLH transcription complex.

Induction of DNA synthesis and apoptosis in cardiac myocytes by E1A oncoprotein

Beginning during the second half of gestation, increasing numbers of cardiac myocytes withdraw from the cell cycle such that DNA synthesis is no longer detectable in these cells by neonatal day 17 in vivo. The mechanisms that exclude these and other terminally differentiated cells from the cell division cycle are poorly understood. To begin to explore the molecular basis of the barrier to G1/S progression in cardiac myocytes, we used adenoviruses to express wild-type and mutant E1A proteins in primary cultures from embryonic day 20 rats. While most of these cardiac myocytes are ordinarily refractory to DNA synthesis, even in the presence of serum growth factors, expression of wild-type E1A stimulates DNA synthesis in up to 94% or almost all successfully transduced cells. Rather than complete the cell cycle, however, these cells undergo apoptosis. Apoptosis is limited to those cells that engage in DNA synthesis, and the kinetics of the two processes suggest that DNA synthesis precedes apoptosis. Mutations in E1A that disable it from binding Rb and related pocket proteins have little effect on its ability to stimulate DNA synthesis in cardiac myocytes. In contrast, mutants that are defective in binding the cellular protein p300 stimulate DNA synthesis 2.4-4.1-fold less efficiently, even in the context of retained E1A pocket protein binding. In the absence of ElA pocket protein binding, the usual situation in the cell, loss of p300 binding severely decreases the ability of ElA to stimulate DNA synthesis. These results suggest that the barrier to G1/S progression in cardiac myocytes is mediated. at least in part, by the same molecules that gate the G1/S transition in actively cycling cells, and that p300 or related family members play an important role in this process.

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).

The cellular transcription factor USF cooperates with varicella-zoster virus immediate-early protein 62 to symmetrically activate a bidirectional viral promoter

The mechanisms governing the function of cellular USF and herpesvirus immediate-early transcription factors are subjects of considerable interest. In this regard, we identified a novel form of coordinate gene regulation involving a cooperative interplay between cellular USF and the varicella-zoster virus immediate-early protein 62 (IE 62). A single USF-binding site defines the potential level of IE 62-dependent activation of a bidirectional viral early promoter of the DNA polymerase and major DNA-binding protein genes. We also report a dominant negative USF-2 mutant lacking the DNA-binding domain that permits the delineation of the biological role of both USF-1 and USF-2 in this activation process. The symmetrical stimulation of the bidirectional viral promoter by IE 62 is achieved at concentrations of USF-1 (43 kDa) or USF-2 (44 kDa) already existing in cells. Our observations support the notion that cellular USF can intervene in and possibly target promoters for activation by a herpesvirus immediate-early protein.

The cellular transcription factor USF cooperates with varicella-zoster virus immediate-early protein 62 to symmetrically activate a bidirectional viral promoter

The mechanisms governing the function of cellular USF and herpesvirus immediate-early transcription factors are subjects of considerable interest. In this regard, we identified a novel form of coordinate gene regulation involving a cooperative interplay between cellular USF and the varicella-zoster virus immediate-early protein 62 (IE 62). A single USF-binding site defines the potential level of IE 62-dependent activation of a bidirectional viral early promoter of the DNA polymerase and major DNA-binding protein genes. We also report a dominant negative USF-2 mutant lacking the DNA-binding domain that permits the delineation of the biological role of both USF-1 and USF-2 in this activation process. The symmetrical stimulation of the bidirectional viral promoter by IE 62 is achieved at concentrations of USF-1 (43 kDa) or USF-2 (44 kDa) already existing in cells. Our observations support the notion that cellular USF can intervene in and possibly target promoters for activation by a herpesvirus immediate-early protein.

Interaction of the Dr1 inhibitory factor with the TATA binding protein is disrupted by adenovirus E1A

Past experiments have shown that the adenovirus E1A12S product activates the hsp70 promoter, dependent on the TATA element and dependent on N-terminal E1A sequences. Other experiments have identified a factor termed Dr1 that interacts with and inhibits the transcriptional activity of the TATA-binding protein (TBP). We now find that the E1A12S protein can disrupt the interaction of the Dr1 factor with the TATA-specific TBP factor, allowing the productive interaction of TBP with TFIIA. This E1A-mediated disruption is dependent on N-terminal sequences that are also essential for the TATA-dependent trans-activation of the hsp70 promoter. Moreover, we also find that Dr1 expression in transfected cells can inhibit transcription from the hsp70 promoter and that this can be overcome by coexpression of the wild-type E1A protein, dependent on N-terminal sequences. We conclude that the activation of hsp70 through the TATA element may be mechanistically similar to the activation of the E2 promoter via E2F, in each case involving a release of a transcription factor from an inactive complex.