Identification of distinct roles for separate E1A domains in disruption of E2F complexes

Breaking Bad: How Viruses Subvert the Cell Cycle

Interactions between the host and viruses during the course of their co-evolution have not only shaped cellular function and the immune system, but also the counter measures employed by viruses. Relatively small genomes and high replication rates allow viruses to accumulate mutations and continuously present the host with new challenges. It is therefore, no surprise that they either escape detection or modulate host physiology, often by redirecting normal cellular pathways to their own advantage. Viruses utilize a diverse array of strategies and molecular targets to subvert host cellular processes, while evading detection. These include cell-cycle regulation, major histocompatibility complex-restricted antigen presentation, intracellular protein transport, apoptosis, cytokine-mediated signaling, and humoral immune responses. Moreover, viruses routinely manipulate the host cell cycle to create a favorable environment for replication, largely by deregulating cell cycle checkpoints. This review focuses on our current understanding of the molecular aspects of cell cycle regulation that are often targeted by viruses. Further study of their interactions should provide fundamental insights into cell cycle regulation and improve our ability to exploit these viruses.

Adenovirus 5 E1A-Mediated Suppression of p53 via FUBP1

Far-upstream element (FUSE) binding protein 1 (FUBP1) was originally identified as a regulator of the oncogene c-Myc via binding to the FUSE within the c-Myc promoter and activating the expression of the gene. Recent studies have identified FUBP1 as a regulator of transcription, translation, and splicing via its DNA and RNA binding activities. Here we report the identification of FUBP1 as a novel binding partner of E1A. FUBP1 binds directly to E1A via the N terminus (residues 1 to 82) and conserved region 3 (residues 139 to 204) of adenovirus 5 E1A. The depletion of FUBP1 via short interfering RNAs (siRNA) reduces virus growth and drives the upregulation of the cellular stress response by activating the expression of p53-regulated genes. During infection, FUBP1 is relocalized within the nucleus, and it is recruited to viral promoters together with E1A while at the same time being lost from the FUSE upstream of the c-Myc promoter. The depletion of FUBP1 affects viral and cellular gene expression. Importantly, in FUBP1-depleted cells, p53-responsive genes are upregulated, p53 occupancy on target promoters is enhanced, and histone H3 lysine 9 is hyperacetylated. This is likely due to the loss of the FUBP1-mediated suppression of p53 DNA binding. We also observed that E1A stabilizes the FUBP1-p53 complex, preventing p53 promoter binding. Together, our results identify, for the first time, FUBP1 as a novel E1A binding protein that participates in aspects of viral replication and is involved in the E1A-mediated suppression of p53 function.IMPORTANCE Viral infection triggers innate cellular defense mechanisms that have evolved to block virus replication. To overcome this, viruses have counterevolved mechanisms that ensure that cellular defenses are either disarmed or not activated to guarantee successful replication. One of the key regulators of cellular stress is the tumor suppressor p53 that responds to a variety of cellular stress stimuli and safeguards the integrity of the genome. During infection, many viruses target the p53 pathway in order to deactivate it. Here we report that human adenovirus 5 coopts the cellular protein FUBP1 to prevent the activation of the p53 stress response pathway that would block viral replication. This finding adds to our understanding of p53 deactivation by adenovirus and highlights its importance in infection and innate immunity.

The Influence of E1A C-Terminus on Adenovirus Replicative Cycle

Adenovirus Early 1A proteins (E1A) are crucial for initiation of the viral life cycle after infection. The E1A gene is encoded at the left end of the viral genome and consists of two exons, the first encoding 185 amino acids in the 289 residues adenovirus 5 E1A, while the second exon encodes 104 residues. The second exon-encoded region of E1A is conserved across all E1A isoforms except for the 55 residues protein, which has a unique C-terminus due to a frame shift following splicing into the second exon. This region of E1A contributes to a variety of processes including the regulation of viral and cellular gene expression, immortalization and transformation. Here we evaluated the contributions that different regions of the second exon of E1A make to the viral life cycle using deletion mutants. The region of E1A encoded by the second exon was found to be important for overall virus growth, induction of viral and cellular gene expression, viral genome replication and deregulation of the cell cycle. Efficient viral replication was found to require exon 2 and the nuclear localization signal, as loss of either resulted in severe growth deficiency. Induction of cellular DNA synthesis was also deficient with any deletion of E1A within the C-terminus even if these deletions were outside of conserved region 4. Overall, our study provides the first comprehensive insight into the contributions of the C-terminus of E1A to the replicative fitness of human adenovirus 5 in arrested lung fibroblasts.

Adenovirus small E1A employs the lysine acetylases p300/CBP and tumor suppressor Rb to repress select host genes and promote productive virus infection

Oncogenic transformation by adenovirus small e1a depends on simultaneous interactions with the host lysine acetylases p300/CBP and the tumor suppressor RB. How these interactions influence cellular gene expression remains unclear. We find that e1a displaces RBs from E2F transcription factors and promotes p300 acetylation of RB1 K873/K874 to lock it into a repressing conformation that interacts with repressive chromatin-modifying enzymes. These repressing p300-e1a-RB1 complexes specifically interact with host genes that have unusually high p300 association within the gene body. The TGF-β, TNF-, and interleukin-signaling pathway components are enriched among such p300-targeted genes. The p300-e1a-RB1 complex condenses chromatin in a manner dependent on HDAC activity, p300 lysine acetylase activity, the p300 bromodomain, and RB K873/K874 and e1a K239 acetylation to repress host genes that would otherwise inhibit productive virus infection. Thus, adenovirus employs e1a to repress host genes that interfere with viral replication.

Proto-oncogene activity of melanoma antigen-A11 (MAGE-A11) regulates retinoblastoma-related p107 and E2F1 proteins

Melanoma antigen-A11 (MAGE-A11) is a low-abundance, primate-specific steroid receptor coregulator in normal tissues of the human reproductive tract that is expressed at higher levels in prostate cancer. Increased expression of MAGE-A11 enhances androgen receptor transcriptional activity and promotes prostate cancer cell growth. Further investigation into the mechanisms of MAGE-A11 function in prostate cancer demonstrated interactions with the retinoblastoma-related protein p107 and Rb tumor suppressor but no interaction with p130 of the Rb family. MAGE-A11 interaction with p107 was associated with transcriptional repression in cells with low MAGE-A11 and transcriptional activation in cells with higher MAGE-A11. Selective interaction of MAGE-A11 with retinoblastoma family members suggested the regulation of E2F transcription factors. MAGE-A11 stabilized p107 by inhibition of ubiquitination and linked p107 to hypophosphorylated E2F1 in association with the stabilization and activation of E2F1. The androgen receptor and MAGE-A11 modulated endogenous expression of the E2F1-regulated cyclin-dependent kinase inhibitor p27(Kip1). The ability of MAGE-A11 to increase E2F1 transcriptional activity was similar to the activity of adenovirus early oncoprotein E1A and depended on MAGE-A11 interactions with p107 and p300. The immunoreactivity of p107 and MAGE-A11 was greater in advanced prostate cancer than in benign prostate, and knockdown with small inhibitory RNA showed that p107 is a transcriptional activator in prostate cancer cells. These results suggest that MAGE-A11 is a proto-oncogene whose increased expression in prostate cancer reverses retinoblastoma-related protein p107 from a transcriptional repressor to a transcriptional activator of the androgen receptor and E2F1.

Different regions of the HPV-E7 and Ad-E1A viral oncoproteins bind competitively but through distinct mechanisms to the CH1 transactivation domain of p300

p300 is a transcriptional coactivator that participates in many important processes in the cell, including proliferation, differentiation, and apoptosis. The viral oncoproteins, adenovirus (Ad) E1A and human papillomavirus (HPV) E7, have been implicated in binding to p300. The Ad-E1A-p300 interaction has been shown to result in the induction of cellular proliferation, epigenetic reprogramming, and cellular transformation and cancer. The HPV-E7-p300 interaction, on the other hand, is not well understood. p300 contains three zinc-binding domains, CH1-CH3, and studies have shown that Ad-E1A can bind to the p300 CH1 and CH3 domains whereas E7 can bind to the CH1 domain and to a lesser extent to the CH2 and CH3 domains. Here we address how high-risk HPV16-E7 and Ad5-E1A, which have different structures, can both bind the p300 CH1 domain. Using pull-down, gel filtration, and analytical ultracentrifugation studies, we show that the N-terminus and CR1 domains of Ad5-E1A and the CR1 and CR2 domains of HPV16-E7 bind to the p300 CH1 domain competitively and with midnanomolar and low micromolar dissociation constants, respectively. We also show that Ad5-E1A can form a ternary complex with the p300 CH1 domain and the retinoblastoma pRb transcriptional repressor, whereas HPV16-E7 cannot. These studies suggest that the HPV16-E7 and Ad5-E1A viral oncoproteins bind to the same p300 CH1 domain to disrupt p300 function by distinct mechanisms.

Regulation of transcription and chromatin structure by pRB: here, there and everywhere

Commitment to divide is one of the most crucial steps in the mammalian cell division cycle. It is critical for tissue and organismal homeostasis, and consequently is highly regulated. The vast majority of cancers evade proliferative control, further emphasizing the importance of the commitment step in cell cycle regulation. The Retinoblastoma (RB) tumor suppressor pathway regulates this decision-making step. Since being the subject of Knudson's 'two hit hypothesis', there has been considerable interest in understanding pRB's role in cancer. It is best known for repressing E2F dependent transcription of cell cycle genes. However, pRB's role in controlling chromatin structure is expanding and bringing it into new regulatory paradigms. In this review we discuss pRB function through protein-protein interactions, at the level of transcriptional regulation of individual promoters and in organizing higher order chromatin domains.

Replication and virus-induced transcriptome of HAdV-5 in normal host cells versus cancer cells--differences of relevance for adenoviral oncolysis

Adenoviruses (Ads), especially HAdV-5, have been genetically equipped with tumor-restricted replication potential to enable applications in oncolytic cancer therapy. Such oncolytic adenoviruses have been well tolerated in cancer patients, but their anti-tumor efficacy needs to be enhanced. In this regard, it should be considered that cancer cells, dependent on their tissue of origin, can differ substantially from the normal host cells to which Ads are adapted by complex virus-host interactions. Consequently, viral replication efficiency, a key determinant of oncolytic activity, might be suboptimal in cancer cells. Therefore, we have analyzed both the replication kinetics of HAdV-5 and the virus-induced transcriptome in human bronchial epithelial cells (HBEC) in comparison to cancer cells. This is the first report on genome-wide expression profiling of Ads in their native host cells. We found that E1A expression and onset of viral genome replication are most rapid in HBEC and considerably delayed in melanoma cells. In squamous cell lung carcinoma cells, we observed intermediate HAdV-5 replication kinetics. Infectious particle production, viral spread and lytic activity of HAdV-5 were attenuated in melanoma cells versus HBEC. Expression profiling at the onset of viral genome replication revealed that HAdV-5 induced the strongest changes in the cellular transcriptome in HBEC, followed by lung cancer and melanoma cells. We identified prominent regulation of genes involved in cell cycle and DNA metabolism, replication and packaging in HBEC, which is in accord with the necessity to induce S phase for viral replication. Strikingly, in melanoma cells HAdV-5 triggered opposing regulation of said genes and, in contrast to lung cancer cells, no weak S phase induction was detected when using the E2F promoter as reporter. Our results provide a rationale for improving oncolytic adenoviruses either by adaptation of viral infection to target tumor cells or by modulating tumor cell functions to better support viral replication.

p21 Promotes oncolytic adenoviral activity in ovarian cancer and is a potential biomarker

The oncolytic adenovirus dl922-947 replicates selectively within and lyses cells with a dysregulated Rb pathway, a finding seen in > 90% human cancers. dl922-947 is more potent than wild type adenovirus and the E1B-deletion mutant dl1520 (Onyx-015). We wished to determine which host cell factors influence cytotoxicity. SV40 large T-transformed MRC5-VA cells are 3-logs more sensitive to dl922-947 than isogenic parental MRC5 cells, confirming that an abnormal G1/S checkpoint increases viral efficacy. The sensitivity of ovarian cancer cells to dl922-947 varied widely: IC₅₀ values ranged from 51 (SKOV3ip1) to 0.03 pfu/cell (TOV21G). Cells sensitive to dl922-947 had higher S phase populations and supported earlier E1A expression. Cytotoxicity correlated poorly with both infectivity and replication, but well with expression of p21 by microarray and western blot analyses. Matched p21+/+ and -/- Hct116 cells confirmed that p21 influences dl922-947 activity in vitro and in vivo. siRNA-mediated p21 knockdown in sensitive TOV21G cells decreases E1A expression and viral cytotoxicity, whilst expression of p21 in resistant A2780CP cells increases virus activity in vitro and in intraperitoneal xenografts. These results highlight that host cell factors beyond simple infectivity can influence the efficacy of oncolytic adenoviruses. p21 expression may be an important biomarker of response in clinical trials.

Regulation of the retinoblastoma proteins by the human herpesviruses

Viruses are obligate intracellular parasites that alter the environment of infected cells in order to replicate more efficiently. One way viruses achieve this is by modulating cell cycle progression. The main regulators of progression out of G0, through G1, and into S phase are the members of the retinoblastoma (Rb) family of tumor suppressors. Rb proteins repress the transcription of genes controlled by the E2F transcription factors. Because the expression of E2F-responsive genes is required for cell cycle progression into the S phase, Rb arrests the cell cycle in G0/G1. A number of viral proteins directly target Rb family members for inactivation, presumably to create an environment more hospitable for viral replication. Such viral proteins include the extensively studied oncoproteins E7 (from human papillomavirus), E1A (from adenovirus), and the large T (tumor) antigen (from simian virus 40). Elucidating how these three viral proteins target and inactivate Rb has proven to be an invaluable approach to augment our understanding of both normal cell cycle progression and carcinogenesis. In addition to these proteins, a number of other virally-encoded inactivators of the Rb family have subsequently been identified including a surprising number encoded by human herpesviruses. Here we review how the human herpesviruses modulate Rb function during infection, introduce the individual viral proteins that directly or indirectly target Rb, and speculate about what roles Rb modulation by these proteins may play in viral replication, pathogenesis, and oncogenesis.

Structure of the retinoblastoma protein bound to adenovirus E1A reveals the molecular basis for viral oncoprotein inactivation of a tumor suppressor

The adenovirus (Ad) E1A (Ad-E1A) oncoprotein mediates cell transformation, in part, by displacing E2F transcription factors from the retinoblastoma protein (pRb) tumor suppressor. In this study we determined the crystal structure of the pRb pocket domain in complex with conserved region 1 (CR1) of Ad5-E1A. The structure and accompanying biochemical studies reveal that E1A-CR1 binds at the interface of the A and B cyclin folds of the pRb pocket domain, and that both E1A-CR1 and the E2F transactivation domain use similar conserved nonpolar residues to engage overlapping sites on pRb, implicating a novel molecular mechanism for pRb inactivation by a viral oncoprotein.

A mechanism of COOH-terminal binding protein-mediated repression

The E2F4 and E2F5 proteins specifically associate with the Rb-related p130 protein in quiescent cells to repress transcription of various genes encoding proteins important for cell growth. A series of reports has provided evidence that Rb-mediated repression involves both histone deacetylase (HDAC)-dependent and HDAC-independent events. Our previous results suggest that one such mechanism for Rb-mediated repression, independent of recruitment of HDAC, involves the recruitment of the COOH-terminal binding protein (CtBP) corepressor, a protein now recognized to play a widespread role in transcriptional repression. We now find that CtBP can interact with the histone acetyltransferase, cyclic AMP--responsive element--binding protein (CREB) binding protein, and inhibit its ability to acetylate histone. This inhibition is dependent on a NH2-terminal region of CtBP that is also required for transcription repression. These results thus suggest two complementary mechanisms for E2F/p130-mediated repression that have in common the control of histone acetylation at target promoters.

The E7 proteins of low- and high-risk human papillomaviruses share the ability to target the pRB family member p130 for degradation

High-risk human papillomaviruses (HPVs) (e.g., HPV-16) cause anogenital and head and neck cancers, and low-risk HPVs (e.g., HPV-6) cause benign hyperproliferative disease. The E7 protein of HPV-16 binds all retinoblastoma tumor suppressor protein (pRB) family members with higher affinity than HPV-6E7. The HPV-16 E7 protein has been reported to target pRB family members for degradation and to immortalize cells. Here we tested the hypothesis that the low-risk E7 protein has an intrinsic ability to decrease expression of pRB family members. First, we introduced a high-affinity pRB-binding site into HPV-6 E7 (6E7G22D) and showed that, in human foreskin keratinocytes, HPV-6 E7G22D decreased the level of pRB protein but not pRB mRNA. Second, we analyzed the ability of wild-type HPV-6 E7 to destabilize the other pRB family members, p107 and p130. HPV-6 E7, like HPV-16 E7, decreased the level of p130 protein. This decrease was inhibited by MG132, a proteasome inhibitor. Binding of HPV-6 E7 to p130 was necessary but not sufficient to decrease the level of p130. Furthermore, the destabilization of p130 correlated with a decrease in the expression of involucrin, a differentiation marker. We suggest that the shared activity of HPV-16 E7 and HPV-6 E7 to destabilize p130 and decrease or delay differentiation may be related to the role of E7 in the HPV life cycle. The added ability of HPV-16 E7 to regulate pRB and p107 may be related to oncogenic activity.

Downmodulation of E1A protein expression as a novel strategy to design cancer-selective adenoviruses

Oncolytic adenoviruses are being tested as potential therapies for human malignant tumors, including gliomas. Here we report for the first time that a mutation in the E1A gene results in low levels of E1A protein, conditioning the replication of mutant adenoviruses specifically to cancer cells. In this study, we compared the oncolytic potencies of three mutant adenoviruses encompassing deletions within the CR1 (Delta-39), CR2 (Delta-24) regions, or both regions (Delta-24/39) of the E1A protein. Delta-39 and Delta-24 induced a cytopathic effect with similar efficiency in glioma cells and a comparable capacity for replication. Importantly, the activity of Delta-39 was significantly attenuated compared to Delta-24 in proliferating normal human astrocytes. Direct analyses of the activation of E2F-1 promoter demonstrated the inability of Delta-39 to induce S-phase-related transcriptional activity in normal cells. Interestingly, E1A protein levels in cells infected with Delta-39 were remarkably downmodulated. Furthermore, protein stability studies revealed enhanced degradation of CR1 mutant E1A proteins, and inhibition of the proteasome activity resulted in the striking rescue of E1A levels. We conclude that the level of E1A protein is a critical determinant of oncolytic phenotype and we propose a completely novel strategy for the design and construction of conditionally replicative adenoviruses.

Transcription of mouse DNA methyltransferase 1 (Dnmt1) is regulated by both E2F-Rb-HDAC-dependent and -independent pathways

Abnormal expression of Dnmt1 in vivo induces cellular alterations such as transformation, and an increase in Dnmt1 mRNA plays a causal role in c-fos-, ras- and SV40 large T antigen-induced transformation of fibroblasts in vitro. Here, we have investigated the regulation of Dnmt1 transcription. We identified the promoter region and major transcription start sites of mouse Dnmt1 and found two important cis-elements within the core promoter region. One is an E2F binding site, and the other is a binding site for an as yet unidentified factor. Point mutations in the two cis-elements decreased promoter activity in both non-transformed and transformed cells. Thus, both sites play a critical role in regulation of Dnmt1 transcription in proliferating cells. Treatment with trichostatin A, a specific inhibitor of histone deacetylase, increased Dnmt1 promoter activity in G0/G1-arrested NIH 3T3 cells. Furthermore, the decrease in promoter activity induced by expression of E2F-1 and Rb was reversed by trichostatin A treatment of Saos-2 cells. Taken together, these data indicate that transcription of Dnmt1 is regulated in a complex fashion by E2F and other transcription factors through E2F-Rb-HDAC-dependent and -independent pathways. These findings suggest that Dnmt1 is a target gene of these pathways in cell proliferation, cell transformation and tumorigenesis.

Human cytomegalovirus pp71 stimulates cell cycle progression by inducing the proteasome-dependent degradation of the retinoblastoma family of tumor suppressors

The oncoproteins of the DNA tumor viruses, adenovirus E1A, simian virus 40 T antigen, and papillomavirus E7, each interact with the retinoblastoma family of tumor suppressors, leading to cell cycle stimulation, apoptosis induction, and cellular transformation. These proteins utilize a conserved LXCXE motif, which is also found in cellular proteins, to target the retinoblastoma family. Here, we describe a herpesvirus protein that shares a subset of the properties of the DNA tumor virus oncoproteins but maintains important differences as well. The human cytomegalovirus pp71 protein employs an LXCXD motif to attack the retinoblastoma family members and induce DNA synthesis in quiescent cells. pp71 binds to and induces the degradation of the hypophosphorylated forms of the retinoblastoma protein and its family members p107 and p130 in a proteasome-dependent manner. However, pp71 does not induce apoptosis and fails to transform cells. Thus, the similarities and differences in comparison to E1A, T antigen, and E7 make pp71 an interesting new tool with which to further dissect the role of the retinoblastoma/E2F pathway in cellular growth control and carcinogenesis.

Interaction of YY1 with E2Fs, mediated by RYBP, provides a mechanism for specificity of E2F function

To explore mechanisms for specificity of function within the family of E2F transcription factors, we have identified proteins that interact with individual E2F proteins. A two-hybrid screen identified RYBP (Ring1- and YY1-binding protein) as a protein that interacts specifically with the E2F2 and E2F3 family members, dependent on the marked box domain in these proteins. The Cdc6 promoter contains adjacent E2F- and YY1-binding sites, and both are required for promoter activity. In addition, YY1 and RYBP, in combination with either E2F2 or E2F3, can stimulate Cdc6 promoter activity synergistically, dependent on the marked box domain of E2F3. Using chromatin immunoprecipitation assays, we show that both E2F2 and E2F3, as well as YY1 and RYBP, associate with the Cdc6 promoter at G(1)/S of the cell cycle. In contrast, we detect no interaction of E2F1 with the Cdc6 promoter. We suggest that the ability of RYBP to mediate an interaction between E2F2 or E2F3 and YY1 is an important component of Cdc6 activation and provides a basis for specificity of E2F function.

T antigens of simian virus 40: molecular chaperones for viral replication and tumorigenesis

Simian virus 40 (SV40) is a small DNA tumor virus that has been extensively characterized due to its relatively simple genetic organization and the ease with which its genome is manipulated. The large and small tumor antigens (T antigens) are the major regulatory proteins encoded by SV40. Large T antigen is responsible for both viral and cellular transcriptional regulation, virion assembly, viral DNA replication, and alteration of the cell cycle. Deciphering how a single protein can perform such numerous and diverse functions has remained elusive. Recently it was established that the SV40 T antigens, including large T antigen, are molecular chaperones, each with a functioning DnaJ domain. The molecular chaperones were originally identified as bacterial genes essential for bacteriophage growth and have since been shown to be conserved in eukaryotes, participating in an array of both viral and cellular processes. This review discusses the mechanisms of DnaJ/Hsc70 interactions and how they are used by T antigen to control viral replication and tumorigenesis. The use of the DnaJ/Hsc70 system by SV40 and other viruses suggests an important role for these molecular chaperones in the regulation of the mammalian cell cycle and sheds light on the enigmatic SV40 T antigen-a most amazing molecule.

Destabilization of the retinoblastoma tumor suppressor by human papillomavirus type 16 E7 is not sufficient to overcome cell cycle arrest in human keratinocytes

The E7 oncoprotein of human papillomavirus type 16 promotes cell proliferation in the presence of antiproliferative signals. Mutagenesis of E7 has revealed that this activity requires three regions, conserved regions 1 and 2 and a C-terminal zinc finger. Binding to the retinoblastoma tumor repressor (Rb) through an LxCxE motif in conserved region 2 is necessary, but not sufficient, for E7 to induce proliferation. We tested the hypothesis that binding to Rb is not sufficient because conserved region 1 and/or the C terminus are required for E7 to functionally inactivate Rb and thus induce proliferation. One mechanism proposed for how E7 inactivates Rb is by blocking Rb-E2F binding. Either conserved region 1 or the C terminus was necessary, in combination with the LxCxE motif, for E7 to block Rb-E2F binding in vitro. While all full-length E7 proteins with mutations outside of the LxCxE motif inhibited Rb-E2F binding, some failed to abrogate cell cycle arrest, demonstrating that blocking Rb-E2F binding is not sufficient for abrogating antiproliferative signals. Another mechanism proposed for how E7 inactivates Rb is by promoting the destabilization of Rb protein. Mutations in conserved region 1 or the LxCxE motif prevented E7 from reducing the half-life of Rb. Though no specific C-terminal residues of E7 were essential for destabilizing Rb, a novel class of mutations that uncouple the destabilization of Rb from the deregulation of keratinocyte proliferation was discovered. Destabilization of Rb correlated with the abrogation of Rb-induced quiescence but was not sufficient for overriding DNA damage-induced cell cycle arrest or for increasing keratinocyte life span. Finally, the same regions of E7 required for destabilizing Rb were required for reducing p107 and p130 levels. Together, these results suggest that inactivation of all three Rb family members is not sufficient to deregulate keratinocyte cell cycle control.

RBP1 recruits the mSIN3-histone deacetylase complex to the pocket of retinoblastoma tumor suppressor family proteins found in limited discrete regions of the nucleus at growth arrest

Retinoblastoma (RB) tumor suppressor family pocket proteins induce cell cycle arrest by repressing transcription of E2F-regulated genes through both histone deacetylase (HDAC)-dependent and -independent mechanisms. In this study we have identified a stable complex that accounts for the recruitment of both repression activities to the pocket. One component of this complex is RBP1, a known pocket-binding protein that exhibits both HDAC-dependent and -independent repression functions. RB family proteins were shown to associate via the pocket with previously identified mSIN3-SAP30-HDAC complexes containing exclusively class I HDACs. Such enzymes do not interact directly with RB family proteins but rather utilize RBP1 to target the pocket. This mechanism was shown to account for the majority of RB-associated HDAC activity. We also show that in quiescent normal human cells this entire RBP1-mSIN3-SAP30-HDAC complex colocalizes with both RB family members and E2F4 in a limited number of discrete regions of the nucleus that in other studies have been shown to represent the initial origins of DNA replication following growth stimulation. These results suggest that RB family members, at least in part, drive exit from the cell cycle by recruitment of this HDAC complex via RBP1 to repress transcription from E2F-dependent promoters and possibly to alter chromatin structure at DNA origins.

The molecular chaperone activity of simian virus 40 large T antigen is required to disrupt Rb-E2F family complexes by an ATP-dependent mechanism

The simian virus 40 large T antigen (T antigen) inactivates tumor suppressor proteins and therefore has been used in numerous studies to probe the mechanisms that control cellular growth and to generate immortalized cell lines. Binding of T antigen to the Rb family of growth-regulatory proteins is necessary but not sufficient to cause transformation. The molecular mechanism underlying T-antigen inactivation of Rb function is poorly understood. In this study we show that T antigen associates with pRb and p130-E2F complexes in a stable manner. T antigen dissociates from a p130-E2F-4-DP-1 complex, coincident with the release of p130 from E2F-4-DP-1. The dissociation of this complex requires Hsc70, ATP, and a functional T-antigen J domain. We also report that the "released" E2F-DP-1 complex is competent to bind DNA containing an E2F consensus binding site. We propose that T antigen disrupts Rb-E2F family complexes through the action of its J domain and Hsc70. These findings indicate how Hsc70 supports T-antigen action and help to explain the cis requirement for a J domain and Rb binding motif in T-antigen-induced transformation. Furthermore, this is the first demonstration linking Hsc70 ATP hydrolysis to the release of E2F bound by Rb family members.

Role of the LXCXE binding site in Rb function

Oncoproteins from DNA tumor viruses such as adenovirus E1a, simian virus 40 T antigen, and human papillomavirus E7 contain an LXCXE sequence, which they use to bind the retinoblastoma protein (Rb) and inhibit its function. Cellular proteins such as histone deacetylases 1 and 2 (HDAC1 and -2) also contain an LXCXE-like sequence, which they use to interact with Rb. The LXCXE binding site in Rb was mutated to assess its role in Rb function. These mutations inhibited binding to HDAC1 and -2, which each contain an LXCXE-like sequence, but had no effect on binding to HDAC3, which lacks an LXCXE-like sequence. Mutation of the LXCXE binding site inhibited active transcriptional repression by Rb and prevented it from effectively repressing the cyclin E and A gene promoters. In contrast, mutations in the LXCXE binding site did not prevent Rb from binding and inactivating E2F. Thus, the LXCXE mutations appear to separate Rb's ability to bind and inactivate E2F from its ability to efficiently recruit HDAC1 and -2 and actively repress transcription. In transient assays, several of the LXCXE binding site mutants caused an increase in the percentage of cells in G(1) by flow cytometry, suggesting that they can arrest cells. However, this effect was transient, as none of the mutants affected cell proliferation in longer-term assays examining bromodeoxyuridine incorporation or colony formation. Our results then suggest that the LXCXE binding site is important for full Rb function. Mutation of the LXCXE binding site does not inhibit binding of the BRG1 ATPase component of the SWI/SNF nucleosome remodeling complex, which has been shown previously to be important for Rb function. Indeed, overexpression of BRG1 and Rb in cells deficient for the proteins led to stable growth inhibition, suggesting a cooperative role for SWI/SNF and the LXCXE binding site in efficient Rb function.

Establishment of irreversible growth arrest in myogenic differentiation requires the RB LXCXE-binding function

The crystal structure of the A-B domain of RB has defined the binding pocket for the LXCXE peptide motif. Using the crystal structure as a guide, we have inactivated the LXCXE-binding pocket by replacing N757 with Phe [to obtain RB(N757F)]. RB(N757F) does not bind to viral oncoproteins but retains the ability to bind and inhibit E2F. RB(N757F) is less effective than the wild-type RB [RB(WT)] in repressing E2F-regulated transcription, and its repression activity is not affected by trichostatin A, an inhibitor of histone deacetylases. However, RB(N757F) is as effective as RB(WT) in suppressing cell growth. Interestingly, RB(N757F) cannot establish an irreversible growth arrest in differentiated myocytes. Differentiated myocytes with RB(WT) become refractory to serum. By contrast, differentiated myocytes with RB(N757F) undergo DNA synthesis and phosphorylate RB(N757F) in response to serum, despite a high level of p21Cip1 expression. Mutation of the phosphorylation sites in RB(N757F) rescued its defect and allowed myocytes to permanently withdraw from the cell cycle. These results demonstrate that it is possible to inactivate the LXCXE-binding pocket without compromising the overall integrity of RB. Moreover, the LXCXE-binding pocket is dispensable for the intrinsic growth suppression function of RB. However, the LXCXE-binding function is essential for RB to establish the serum-refractory state in differentiated myocytes.

Synergism with germ line transcription factor Oct-4: viral oncoproteins share the ability to mimic a stem cell-specific activity

Activation of transcription by Oct-4 from remote binding sites requires a cofactor that is restricted to embryonal stem cells. The adenovirus E1A protein can mimic the activity of this stem cell-specific factor and stimulates Oct-4 activity in differentiated cells. Here we characterize the Oct-4-E1A interaction and show that the E1A 289R protein harbors two independent Oct-4 binding sites, both of which specifically interact with the POU domain of Oct-4. Furthermore, we demonstrate that, like E1A, the human papillomavirus E7 oncoprotein also specifically binds to the Oct-4 POU domain. E7 and Oct-4 can form a complex both in vitro and in vivo. Expression of E7 in differentiated cells stimulates Oct-4-mediated transactivation from distal binding sites. Moreover, Oct-4, but not other Oct factors, is active when expressed in cells transformed by human papillomavirus. Our results suggest that different viruses have evolved oncoproteins that share the ability to target Oct-4 and to mimic a stem cell-specific activity.

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.

Novel mechanisms of E2F induction by BK virus large-T antigen: requirement of both the pRb-binding and the J domains

E2F activity is regulated in part by the retinoblastoma family of tumor suppressor proteins. Viral oncoproteins, such as simian virus 40 (SV40) large-T antigen (TAg), adenovirus E1A, and human papillomavirus E7, can disrupt the regulation of cellular proliferation by binding to pRb family members and dissociating E2F-pRb family protein complexes. BK virus (BKV), which infects a large percentage of the human population and has been associated with a variety of human tumors, encodes a TAg homologous to SV40 TAg. It has been shown that BKV TAg, when expressed at low levels, does not detectably bind to pRb family members, yet it induces a serum-independent phenotype and causes a decrease in the overall levels of pRb family proteins. The experiments presented in this report show that, despite the lack of TAg-pRb interactions, BKV TAg can induce transcriptionally active E2F and that this induction does in fact require an intact pRb-binding domain as well as an intact J domain. In addition, E2F-pRb family member complexes can be detected in both BKV and SV40 TAg-expressing cells. These results suggest the presence of alternate cellular mechanisms for the release of E2F in addition to the well-established model for TAg-pRb interactions. These results also emphasize a role for BKV TAg in the deregulation of cellular proliferation, which may ultimately contribute to neoplasia.

The J domain of simian virus 40 large T antigen is required to functionally inactivate RB family proteins

Transformation by simian virus 40 large T antigen (TAg) is dependent on the inactivation of cellular tumor suppressors. Transformation minimally requires the following three domains: (i) a C-terminal domain that mediates binding to p53; (ii) the LXCXE domain (residues 103 to 107), necessary for binding to the retinoblastoma tumor suppressor protein, pRB, and the related p107 and p130; and (iii) an N-terminal domain that is homologous to the J domain of DnaJ molecular chaperone proteins. We have previously demonstrated that the N-terminal J domain of TAg affects the RB-related proteins by perturbing the phosphorylation status of p107 and p130 and promoting the degradation of p130 and that this domain is required for transformation of cells that express either p107 or p130. In this work, we demonstrate that the J domain of TAg is required to inactivate the ability of each member of the pRB family to induce a G1 arrest in Saos-2 cells. Furthermore, the J domain is required to override the repression of E2F activity mediated by p130 and pRB and to disrupt p130-E2F DNA binding complexes. These results imply that while the LXCXE domain serves as a binding site for the RB-related proteins, the J domain plays an important role in inactivating their function.

Dual mechanisms for the inhibition of E2F binding to RB by cyclin-dependent kinase-mediated RB phosphorylation

The growth suppression function of RB is dependent on its protein binding activity. RB contains at least three distinct protein binding functions: (i) the A/B pocket, which binds proteins with the LXCXE motif; (ii) the C pocket, which binds the c-Abl tyrosine kinase; and (iii) the large A/B pocket, which binds the E2F family of transcription factors. Phosphorylation of RB, which is catalyzed by cyclin-dependent protein kinases, inhibits all three protein binding activities. We have previously shown that LXCXE binding is inactivated by the phosphorylation of two threonines (Thr821 and Thr826), while the C pocket is inhibited by the phosphorylation of two serines (Ser807 and Ser811). In this report, we show that the E2F binding activity of RB is inhibited by two sets of phosphorylation sites acting through distinct mechanisms. Phosphorylation at several of the seven C-terminal sites can inhibit E2F binding. Additionally, phosphorylation of two serine sites in the insert domain can inhibit E2F binding, but this inhibition requires the presence of the RB N-terminal region. RB mutant proteins lacking all seven C-terminal sites and two insert domain serines can block Rat-1 cells in G1. These RB mutants can bind LXCXE proteins, c-Abl, and E2F even after they become phosphorylated at the remaining nonmutated sites. Thus, multiple phosphorylation sites regulate the protein binding activities of RB through different mechanisms, and a constitutive growth suppressor can be generated through the combined mutation of the relevant phosphorylation sites in RB.

Identification of positively and negatively acting elements regulating expression of the E2F2 gene in response to cell growth signals

Mammalian cell growth is governed by regulatory activities that include the products of genes such as c-myc and ras that act early in G1, as well as the E2F family of transcription factors that accumulate later in G1 to regulate the expression of genes involved in DNA replication. Previous work has shown that the expression of the E2F1, E2F2, and E2F3 gene products is tightly regulated by cell growth. To further explore the mechanisms controlling accumulation of E2F activity, we have isolated genomic sequences flanking the 5' region of the E2F2 coding sequence. Various assays demonstrate promoter activity in this sequence that reproduces the normal control of E2F2 expression during a growth stimulation. Sequence comparison reveals the presence of a variety of known transcription factor binding sites, including E-box elements that are consensus Myc binding sites, as well as E2F binding sites. We demonstrate that the E-box elements, which we show can function as Myc-responsive sites, contribute in a positive fashion to promoter function. We also find that E2F-dependent negative regulation in quiescent cells plays a significant role in the cell growth-dependent control of the promoter, similar to the regulation of the E2F1 gene promoter.

The viral oncoprotein E1A blocks transforming growth factor beta-mediated induction of p21/WAF1/Cip1 and p15/INK4B

The adenovirus early gene product E1A is a potent stimulator of cellular proliferation, which when overexpressed can overcome the growth-inhibitory effects of the polypeptide hormone transforming growth factor beta (TGF-beta). The ability of TGF-beta to arrest cell growth in G1 correlates with the transcriptional induction of the cyclin-dependent kinase inhibitors, p15/INK4B and p21/WAF1/Cip1; an inhibition of the G1 cyclin-Cdk complexes; and a maintenance of the retinoblastoma susceptibility gene product, Rb, in a hypophosphorylated state. The ability of E1A to overcome TGF-beta-mediated growth inhibition derives, in part, from its ability to sequester Rb and Rb family members. We report here that E1A also acts upstream of Rb by blocking the TGF-beta-mediated induction of p15 and p21. Consistent with these findings, E1A expression also blocks the ability of TGF-beta to inhibit Cdk2 kinase activity, as well as its ability to hold Rb in a hypophosphorylated state. The effect of E1A on the induction of p15 and p21 is independent of E1A's Rb binding activity. The E1A-mediated decrease in p15 levels is primarily the result of a block at the level of transcriptional activation by TGF-beta. This effect is dependent on E1A's ability to bind p300, one of E1A's target proteins. Thus, the ability of E1A to affect p15 and p21 expression represents an additional possible mechanism by which E1A can circumvent the negative regulation of cell cycle progression.

The Rb family contains a conserved cyclin-dependent-kinase-regulated transcriptional repressor motif

Progression through the cell cycle is dependent on the sequential expression of cyclins, which combine with cyclin-dependent kinases (cdks) to form active kinases. The transition from G1 to S phase is dependent on D cyclins in complex with cdk4 or cdk6 and cyclin E complexed with cdk2. One target of G1 cyclins is the retinoblastoma susceptibility protein (Rb). Rb is a transcriptional repressor that is selectively targeted to genes through interaction with the E2F family of cell cycle transcription factors. Rb is a member of a family of proteins that include p107 and p130. The three proteins share a region known as the pocket that is important for binding E2F and is also the binding site for oncoproteins from DNA tumor viruses that inactivate Rb. We have found that two conserved domains within the Rb pocket (A and B) interact to form a transcriptional repressor motif (K. N. B. Chow and D. C. Dean, Mol. Cell. Biol. 16:4862-4868, 1996). Here we demonstrate that p107 also has an A-B repressor motif, and using domain swapping and coimmunoprecipitation assays, we compare A and B from Rb and p107. Finally and most importantly, we demonstrate that the A-B interaction which forms the repressor motif is blocked by G1 cdk phosphorylation, thereby blocking repressor activity. This A-B repressor motif is then the first example of a cdk-regulated transcriptional repressor.

Role of p300-family proteins in E1A oncogene induction of cytolytic susceptibility and tumor cell rejection

The mechanism by which the adenoviral (Ad) E1A oncogene induces cellular susceptibility to lysis by killer lymphocytes involves interactions between its first exon and different second-exon accessory regions. Mutational analysis showed that two first-exon regions--one in the N terminus and one in the conserved region 1 (CR1) domain--are necessary for this activity. E1A complex formation with cellular p300 protein through these first-exon-encoded regions correlated with induction of the cytolytic susceptible phenotype but was only effective in the context of E1A second-exon expression. An E1A first-exon deletion that prevented p300 binding eliminated both oncoprotein-induced cytolytic susceptibility and rejection of transfected sarcoma cells by immunocompetent animals. These results suggest that the E1A oncogene induces cytolytic susceptibility and tumor rejection by interactions with cellular proteins of the p300 family that affect transcription of genes involved in the cellular response to injury inflicted by host killer cells.

Abrogation of growth arrest signals by human papillomavirus type 16 E7 is mediated by sequences required for transformation

Cells arrest in the G1 or G0 phase of the cell cycle in response to a variety of negative growth signals that induce arrest by different molecular pathways. The ability of human papillomavirus (HPV) oncogenes to bypass these signals and allow cells to progress into the S phase probably contributes to the neoplastic potential of the virus. The E7 protein of HPV-16 was able to disrupt the response of epithelial cells to three different negative growth arrest signals: quiescence imposed upon suprabasal epithelial cells, G1 arrest induced by DNA damage, and inhibition of DNA synthesis caused by treatment with transforming growth factor beta. The same set of mutated E7 proteins was able to abrogate all three growth arrest signals. Mutant proteins that failed to abrogate growth arrest signals were transformation deficient and included E7 proteins that bound retinoblastoma protein in vitro. In contrast, HPV-16 E6 was able to bypass only DNA damage-induced G1 arrest, not suprabasal quiescence or transforming growth factor beta-induced arrest. The E6 and E7 proteins from the low-risk virus HPV-6 were not able to bypass any of the growth arrest signals.

Transcriptional repression and growth suppression by the p107 pocket protein

p107 is a member of the pocket family of proteins that includes the retinoblastoma tumor suppressor. Overexpression of p107 arrests cells in G1, suggesting that it is important for cell cycle control. This growth suppression is mediated at least in part through the interaction of p107 with a member of the E2F family of cell cycle transcription factors, and this interaction can be disrupted by oncoproteins from DNA tumor viruses such as adenovirus E1a that bind p107. Not only does the binding of p107 to E2F inactivate E2F, but also we show that when p107 is tethered to the promoter through binding to E2F it functions as a general transcriptional repressor. This general repressor activity was also evident when p107 was fused to the DNA binding domain of Gal4 so that it could be directly targeted to the promoter in an E2F-independent fashion. Using p107 mutants, we compared the regions of the protein required for transcriptional repression and cell growth suppression. We found that the pocket domain is sufficient for inactivation of E2F, general repressor activity, and most of the growth suppressor activity. Binding of conserved region 1 from Ela to p107 blocked interaction with E2F, but it did not affect general repressor activity, demonstrating that binding and inactivation of E2F and general repressor activity are distinguishable properties of p107. Within the pocket, two conserved domains, A and B, were sufficient for growth suppression and transcriptional repressor activity. Surprisingly, we found that these two domains were fully functional when they were coexpressed as separate proteins, and we present results suggesting that the domains may interact at the promoter to form an active pocket.

Cyclin-dependent kinases phosphorylate the adenovirus E1A protein, enhancing its ability to bind pRb and disrupt pRb-E2F complexes

The adenovirus E1A protein of 243 amino acids has been shown to affect a variety of cellular functions, most notably the immortalization of primary cells and the promotion of quiescent cells into S phase. The activity of E1A is derived, in part, from its association with various cellular proteins, many of which play important roles in regulating cell cycle progression. E1A is known to have multiple sites of phosphorylation. It has been suggested that cell cycle-dependent phosphorylation may also control some of E1A's functions. We find now that immune complexes of cyclin-dependent kinases such as cdk4, cdk2, and cdc2 are all capable of phosphorylating E1A in vitro. Additionally, the sites on E1A phosphorylated by these kinases in vitro are similar to the E1A sites phosphorylated in vivo. We have also found that a phosphorylated E1A is far more efficient than an unphosphorylated E1A in associating with pRB and in disrupting E2F/DP-pRB complexes as well. On the basis of our findings and the differences in timing and expression levels of the various cyclins regulating cdks, we suggest that E1A functions at different control points in the cell cycle and that phosphorylation controls, to some extent, its biological functions.

A unique role for the Rb protein in controlling E2F accumulation during cell growth and differentiation

Examination of the interactions involving transcription factor E2F activity during cell growth and terminal differentiation suggests distinct roles for Rb family members in the regulation of E2F accumulation. The major species of E2F in quiescent cells is a complex containing the E2F4 product in association with the Rb-related p130 protein. As cells enter the cell cycle, this complex disappears, and there is a concomitant accumulation of free E2F activity of which E2F4 is a major component. E2F4 then associates with the Rb-related p107 protein as cells enter S phase. Rb can be found in interactions with each E2F species, including E2F4, during G1, but there appears to be a limited amount of Rb with respect to E2F, likely due to the maintenance of most Rb protein in an inactive state by phosphorylation. A contrasting circumstance can be found during the induction of HL60 cell differentiation. As these cells exit the cell cycle, active Rb protein appears to exceed E2F, as there is a marked accumulation of E2F-Rb interactions, involving all E2F species, including E2F4, which is paralleled by the conversion of Rb from a hyperphosphorylated state to a hypophosphorylated state. These results suggest that the specific ability of Rb protein to interact with each E2F species, dependent on concentration of active Rb relative to accumulation of E2F, may be critical in cell-growth decisions.

Adenovirus E1A activates cyclin A gene transcription in the absence of growth factors through interaction with p107

Using the infection of quiescent human fibroblasts with adenovirus type 5 and various deletion mutants, we show that E1A can stimulate transcription of the cyclin A gene in the absence of exogenous growth factors. Required for this activity is conserved region 2 (CR2), while both the N-terminal part of E1A and CR1 are dispensable. This indicates that activation of cyclin A gene expression requires the binding of E1A to p107, while binding to either pRB or p300 is not involved in transcriptional activation. We demonstrate that p107 represses the cyclin A promoter through its cell cycle-regulatory E2F binding site and that 12S E1A can activate the cyclin A promoter, essentially by counteracting its repression by p107. Since Cr2 is required for cell transformation, transcriptional activation of the cyclin A gene by E1A appears to be important for its capacity to override control of cellular growth.

E2F1 and E1A(12S) have a homologous activation domain regulated by RB and CBP

The E2F1 transcription factor has a well-characterized activation domain at its C terminus and the E1A protein has a recently defined activation domain at its N terminus. Here we show that these activation domains are highly related in sequence. The sequence homology reflects, at least partly, the conservation of common binding sites for the RB and CBP/p300 proteins, which are preserved in the same relative order along E2F1 and E1A. Furthermore, the interaction of RB and CBP with these two activation domains results in the same functional consequences: RB represses both activation domains, whereas CBP stimulates them. We conclude that the activation domains of E1A(12s) and E2F1 belong to a novel functional class, characterized by specific protein binding sites. The implication of this conservation with respect to E1A-induced stimulation of E2F activity is discussed.

A potent transrepression domain in the retinoblastoma protein induces a cell cycle arrest when bound to E2F sites

An intact T/E1A-binding domain (the pocket) is necessary, but not sufficient, for the retinoblastoma protein (RB) to bind to DNA-protein complexes containing E2F and for RB to induce a G1/S block. Indirect evidence suggests that the binding of RB to E2F may, in addition to inhibiting E2F transactivation function, generate a complex capable of functioning as a transrepressor. Here we show that a chimera in which the E2F1 transactivation domain was replaced with the RB pocket could, in a DNA-binding and pocket-dependent manner, mimic the ability of RB to repress transcription and induce a cell cycle arrest. In contrast, a transdominant negative E2F1 mutant that is capable of blocking E2F-dependent transactivation did not. Fusion of the RB pocket to a heterologous DNA-binding domain unrelated to E2F likewise generated a transrepressor protein when scored against a suitable reporter. These results suggest that growth suppression by RB is due, at least in part, to transrepression mediated by the pocket domain bound to certain promoters via E2F.

Direct transcriptional repression by pRB and its reversal by specific cyclins

It was recently shown that the E2F-pRB complex is a negative transcriptional regulator. However, it was not determined whether the whole complex or pRB alone is required for repression. Here we show that pRB and the related protein p107 are capable of direct transcriptional repression independent of E2F. When fused to the DNA binding domain of GAL4, pRB or p107 represses transcription of promoters with GAL4 binding sites. Thus, E2F acts as a tether for pRB or p107 but is not actively involved in repression of other enhancers. This function of pRB maps to the pocket and is abrogated by mutation of this domain. This result suggests an intriguing model in which the pocket has a dual function, first to bind E2F and second to repress transcription directly, possibly through interaction with other proteins. We also show that direct transcriptional repression by pRB is regulated by phosphorylation. Mutations which render pRB constitutively hypophosphorylated potentiate repression, while phosphorylation induced by cyclin A or E reduces repression ninefold.

E2F1 overexpression in quiescent fibroblasts leads to induction of cellular DNA synthesis and apoptosis

Various experiments have demonstrated a role for the E2F transcription factor in the regulation of cell growth during the G0/G1/S phase transition. Indeed, overexpression of the E2F1 product, a component of the cellular E2F activity, induces DNA synthesis in quiescent fibroblasts. To provide an approach to a more detailed biochemical analysis of these events, we have made use of a recombinant adenovirus containing the E2F1 cDNA in order to efficiently express the E2F1 product in an entire population of cells. We demonstrate an induction of DNA synthesis when quiescent cells are infected with the E2F1 recombinant virus. However, we also find that the induction does not lead to a complete replication of the cellular genome, as revealed by flow cytometry. The incomplete nature of cellular DNA replication is due, at least in part, to the fact that E2F1 overexpression leads to massive cell death that is characteristic of apoptosis. This E2F1-mediated induction of apoptosis is largely dependent on endogenous wild-type p53 activity and can be subverted by introducing mutant forms of p53 into these cells or by overexpressing E2F1 in fibroblasts derived from p53-null mouse embryos. We conclude that E2F1 can induce events leading to S phase but that the process is not normal and appears to result from the activation of a cell death pathway.

The Rb-related p107 protein can suppress E2F function independently of binding to cyclin A/cdk2

The interaction of the retinoblastoma susceptibility gene product (Rb)-related p107 protein with the E2F transcription factor in S-phase cells facilitates the formation of a multicomponent complex also containing cyclin A and the p33cdk2 kinase. We have created a series of p107 mutants to assess the ability of p107 to inhibit E2F function and the role of the cyclin A/cdk2 complex in this process. We find that p107 mutants that do not bind to E2F also fail to repress E2F-dependent transcription. Moreover, we find that the ability of p107 to suppress E2F-dependent transcription is not dependent on the ability of p107 to associate with cyclin A/cdk2. Finally, an analysis of the ability of the p107 mutant proteins to suppress cell growth suggests that both E2F-dependent and E2F-independent events correlate with this activity.

Functional properties of a Drosophila homolog of the E2F1 gene

A variety of studies have now implicated the cellular transcription factor E2F as a key participant in transcription control during the cell growth cycle. Although the recent isolation of molecular clones encoding proteins that are components of the E2F activity (E2F1 and DP-1) provides an approach to defining the specific involvement of E2F in these events, definitive experiments remain difficult in the absence of appropriate genetic systems. We have now identified a Drosophila equivalent of E2F1 that we hope will allow an eventual genetic approach to the role of E2F in cellular regulatory events. A cDNA clone was isolated from a Drosophila cDNA library by using a probe containing sequence from the E2F1 DNA binding domain. The sequence of the clone, which we term drosE2F1, demonstrates considerable homology to the human E2F1 sequence, with over 65% identity in the DNA binding region and 50% identity in the region of E2F1 known to interact with the retinoblastoma gene product. A glutathione S-transferase-drosE2F1 fusion protein was capable of binding specifically to an E2F recognition site, and transfection assays demonstrated that the drosE2F1 product was capable of transcription activation, dependent on functional E2F sites as well as sequences within the C terminus of the protein. Finally, we have also identified E2F recognition sequences within the promoter of the Drosophila DNA polymerase alpha gene, and we demonstrate that the drosE2F1 product activates transcription of a test gene under the control of this promoter. We conclude that the drosE2F1 cDNA encodes an activity with extensive structural and functional similarity to the human E2F1 protein.

Functional properties of a Drosophila homolog of the E2F1 gene

A variety of studies have now implicated the cellular transcription factor E2F as a key participant in transcription control during the cell growth cycle. Although the recent isolation of molecular clones encoding proteins that are components of the E2F activity (E2F1 and DP-1) provides an approach to defining the specific involvement of E2F in these events, definitive experiments remain difficult in the absence of appropriate genetic systems. We have now identified a Drosophila equivalent of E2F1 that we hope will allow an eventual genetic approach to the role of E2F in cellular regulatory events. A cDNA clone was isolated from a Drosophila cDNA library by using a probe containing sequence from the E2F1 DNA binding domain. The sequence of the clone, which we term drosE2F1, demonstrates considerable homology to the human E2F1 sequence, with over 65% identity in the DNA binding region and 50% identity in the region of E2F1 known to interact with the retinoblastoma gene product. A glutathione S-transferase-drosE2F1 fusion protein was capable of binding specifically to an E2F recognition site, and transfection assays demonstrated that the drosE2F1 product was capable of transcription activation, dependent on functional E2F sites as well as sequences within the C terminus of the protein. Finally, we have also identified E2F recognition sequences within the promoter of the Drosophila DNA polymerase alpha gene, and we demonstrate that the drosE2F1 product activates transcription of a test gene under the control of this promoter. We conclude that the drosE2F1 cDNA encodes an activity with extensive structural and functional similarity to the human E2F1 protein.