Identification of separate domains in the adenovirus E1A gene for immortalization activity and the activation of virus early genes

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

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

Kaposi's sarcoma-associated herpesvirus lana2 protein interacts with the pocket proteins and inhibits their sumoylation

The pocket proteins retinoblastoma protein (pRb), p107 and p130 are the key targets of oncoproteins expressed by DNA tumor viruses. Some of these viral proteins contain an LXCXE motif that mediates the interaction with the three pocket proteins and the inhibition of the pRb SUMOylation. Kaposi's sarcoma herpesvirus (KSHV) contains at least two proteins that can regulate pRb function but, so far, a KSHV-encoded protein targeting p107 and p130 has not been identified. Here, we show that the KSHV latent protein LANA2 binds to pRb, p107 and p130. LANA2 contains an LXCXE motif that is required for bypassing pRb-mediated cell-cycle arrest and for inhibiting pRb SUMOylation. Finally, we demonstrate that, in addition to pRb, both p107 and p130 can be SUMOylated, and this modification is also inhibited by LANA2 in an LXCXE-dependent manner. These results demonstrate, for the first time, the SUMOylation of p107 or p130 and, so far, they represent the first example of a KSHV protein able to interact with the three pocket proteins and to inhibit their conjugation to SUMO.

Identification of a second independent binding site for the pCAF acetyltransferase in adenovirus E1A

The conserved region 3 (CR3) portion of the human adenovirus (HAdV) 5 E1A protein functions as a potent transcriptional activator that induces expression of viral early genes during infection. Expression of HAdV-5 CR3 in the yeast Saccharomyces cerevisiae inhibits growth, as do the corresponding regions of the HAdV-3, 4, 9, 12 and 40 E1A proteins, which represent the remaining five HAdV subgroups. Growth inhibition is alleviated by disruption of the SAGA transcriptional regulatory complex, suggesting that CR3 targets the yeast SAGA complex. In yeast, transcriptional activation by several, but not all, of the CR3 regions requires the Gcn5 acetyltransferase component of SAGA. The CR3 regions of HAdV-3, 5, 9 and 40, but not HAdV-4 and 12 interact with the pCAF acetyltransferase, a mammalian ortholog of yeast Gcn5. Disruption of the previously described N-terminal pCAF binding site abrogates binding by the HAdV-5 243R E1A protein, but not the larger 289R E1A protein, which is otherwise identical except for the presence of CR3. RNA interference directed against pCAF decreased HAdV-5 CR3 dependent transcriptional activation in mammalian cells. Our results identify a second independent binding site for pCAF in E1A and suggest that it contributes to CR3 dependent transcriptional activation.

Requirements for E1A dependent transcription in the yeast Saccharomyces cerevisiae

BACKGROUND

The human adenovirus type 5 early region 1A (E1A) gene encodes proteins that are potent regulators of transcription. E1A does not bind DNA directly, but is recruited to target promoters by the interaction with sequence specific DNA binding proteins. In mammalian systems, E1A has been shown to contain two regions that can independently induce transcription when fused to a heterologous DNA binding domain. When expressed in Saccharomyces cerevisiae, each of these regions of E1A also acts as a strong transcriptional activator. This allows yeast to be used as a model system to study mechanisms by which E1A stimulates transcription.

RESULTS

Using 81 mutant yeast strains, we have evaluated the effect of deleting components of the ADA, COMPASS, CSR, INO80, ISW1, NuA3, NuA4, Mediator, PAF, RSC, SAGA, SAS, SLIK, SWI/SNF and SWR1 transcriptional regulatory complexes on E1A dependent transcription. In addition, we examined the role of histone H2B ubiquitylation by Rad6/Bre1 on transcriptional activation.

CONCLUSION

Our analysis indicates that the two activation domains of E1A function via distinct mechanisms, identify new factors regulating E1A dependent transcription and suggest that yeast can serve as a valid model system for at least some aspects of E1A function.

How the Rb tumor suppressor structure and function was revealed by the study of Adenovirus and SV40

The review recounts the history of how the study of the DNA tumor viruses including polyoma, SV40 and Adenovirus brought key insights into the structure and function of the Retinoblastoma protein (Rb). Knudsen's model of the two-hit hypothesis to explain patterns of hereditary and sporadic retinoblastoma provided the foundation for the tumor suppressor hypothesis that ultimately led to the cloning of the Rb gene. The discovery that SV40 and Adenovirus could cause tumors when inoculated into animals was startling not only because SV40 had contaminated the poliovirus vaccine and Adenovirus was a common cause of viral induced pneumonia but also because they provided an opportunity to study the genetics and biochemistry of cancer. Studies of mutant forms of these viruses led to the identification of the E1A and Large T antigen (LT) oncogenes and their small transforming elements including the Adenovirus Conserved Regions (CR), the SV40 J domain and the LxCxE motif. The immunoprecipitation studies that initially revealed the size and ultimately the identity of cellular proteins that could bind to these transforming elements were enabled by the widespread development of highly specific monoclonal antibodies against E1A and LT. The identification of Rb as an E1A and LT interacting protein quickly led to the cloning of p107, p130, p300, CBP, p400 and TRRAP and the concept that viral transformation was due, at least in part, to the perturbation of the function of normal cellular proteins. In addition, studies on the ability of E1A to transactivate the Adenovirus E2 promoter led to the cloning of the heterodimeric E2F and DP transcription factor and recognition that Rb repressed transcription of cellular genes required for cell cycle entry and progression. More recent studies have revealed how E1A and LT combine the activity of Rb and the other cellular associated proteins to perturb expression of many genes during viral infection and tumor formation.

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

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

E1A- and E1B-Double mutant replicating adenovirus elicits enhanced oncolytic and antitumor effects

Gene-modified replication-competent adenoviruses (Ads) are emerging as a promising new modality for the treatment of cancer. We have previously shown that E1B 19kDa and E1B 55kDa gene-deleted Ad (Ad-DeltaE1B19/55) exhibits improved tumor-specific replication and cell lysis, leading to an enhanced antitumor effect. In an effort to increase cancer cell selectivity of a replicating adenovirus, we first generated 11 E1A mutant Ads (Ad-E1mt1 to Ad-E1mt11) with deletion or substitution in retinoblastoma (pRb)-binding sites of E1A. Of these, Ad-E1mt7 demonstrated significant improvement in cytopathic effect (CPE) and viral replication in a cancer cell-specific manner. To further enhance the cancer cell specificity of Ad-E1mt7, Ad-DeltaE1Bmt7 was generated, in which both the E1B 19kDa and E1B 55kDa genes were deleted. As assessed in CPE assay and immunoblot analysis for Ad fiber expression, Ad-DeltaE1Bmt7 exerted marked enhancement in cancer cell-specific killing as well as viral replication in comparison with its comparative controls (Ad-E1mt7, Ad-DeltaE1B55). Furthermore, the growth of established human cervical carcinoma in nude mice was significantly suppressed by intratumoral injection of Ad-DeltaE1Bmt7. In summary, we have developed an oncolytic adenovirus with a significantly improved therapeutic profile for cancer treatment.

Cellular proteins that are targets for transformation by DNA tumour viruses

Small DNA tumour viruses produce proteins that redirect cellular gene expression and growth control. The E1A polypeptides of adenovirus perform the functions of transcriptional activation and cellular transformation. These two functions are carried out by different domains within the E1A protein. The E1A protein associates with several cellular proteins, including the product of the retinoblastoma gene, pRb-1. Mutational analysis correlates transformation with the sites required for binding pRb and two other cellular proteins, p107 and a 300 kDa polypeptide. This correlation suggests that these proteins are targets for E1A-mediated transformation. Transforming proteins from other small DNA tumour viruses interact with pRb, raising the possibility that a common event in viral transformation is the inactivation of proteins that inhibit cellular proliferation. The role of the E1A-associated 60 kDa protein, p60, in transformation is being investigated. In the absence of E1A, p60 binds to the human homologue of the Schizosaccharomyces pombe cdc2 gene product, p34, to form a complex that has kinase activity that oscillates during the cell cycle. Ongoing studies of the effect of adenovirus infection, and specifically E1A expression, on this cellular kinase may provide clues to how E1A overcomes cell cycle controls and transforms cells.

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

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

A Novel NF-κB-Regulated Site within the Human Iγ1 Promoter Requires p300 for Optimal Transcriptional Activity

Transcriptional activation of germline (GL) promoters occurs through binding of NF-kappaB to three evolutionarily conserved sites within a CD40 response region in the human and mouse GL Igamma and Iepsilon promoters. Here we identify and characterize a novel NF-kappaB binding site (kappaB6) within the human GL Igamma1 promoter that plays an essential role in basal- and CD40-induced transcription. This site is adjacent to identified CREB/activating transcription factor (ATF) sites, present in the Igamma1 but not the Igamma3 promoter, which are important for the amplification of transcription. Our data suggest a cohesive protein complex regulating Igamma1 promoter activity because disruption of any individual NF-kappaB or CREB/ATF site markedly lowers the overall inducible activity of the promoter. In addition, alteration of helical phasing within the promoter indicates spatial orientation of CREB/ATF and NF-kappaB, proteins contributes directly to promoter activity. We found that CREB and p50 transactivators, as well as coactivator p300, interact in vivo with the Igamma1 promoter in the presence and absence of CD40 signaling in Ramos and primary B cells. However, the level of CREB and p300 binding differs as a consequence of activation in primary B cells. Furthermore, overexpression of p300, and not a mutant lacking acetyltransferase activity, significantly increases Igamma1 construct-specific transcription. Together these data support a model whereby CREB and multiple NF-kappaB complexes bind to the Igamma1 promoter and recruit p300. CD40 signals induce p300-dependent changes that result in optimal Igamma1 promoter activity.

Regulation of DNA methyltransferase 1 by the pRb/E2F1 pathway

Tumor suppressor gene silencing by DNA hypermethylation contributes to tumorigenesis in many tumor types. This aberrant methylation may be due to increased expression and activity of DNA methyltransferases, which catalyze the transfer of methyl groups from S-adenosylmethionine to cytosines in CpG dinucleotides. Elevated expression of the maintenance DNA methyltransferase, DNA methyltransferase 1 (DNMT-1), has been shown in carcinomas of the colon, lung, liver, and prostate. Based on the nearly ubiquitous alterations of both DNA methylation and the retinoblastoma protein (pRb) pathway found in human cancer, we investigated a potential regulatory pathway linking the two alterations in murine and human prostate epithelial cells. Analysis of DNA methyltransferase levels in Rb-/- murine prostate epithelial cell lines revealed elevated Dnmt-1 levels. Genomic DNA sequence analysis identified conserved E2F consensus binding sites in proximity to the transcription initiation points of murine and human Dnmt-1. Furthermore, the Dnmt-1 promoter was shown to be regulated by the pRb/E2F pathway in murine and human cell lines of epithelial and fibroblast origin. In the absence of pRb, Dnmt-1 transcripts exhibited aberrant cell cycle regulation and Rb-/- cells showed aberrant methylation of the paternally expressed gene 3 (Peg3) tumor suppressor gene. These findings show a link between inactivation of the pRb pathway and induction of DNA hypermethylation of CpG island-containing genes in tumorigenesis.

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

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

Functional genetics and experimental models of human cancer

Abundant evidence supports the hypothesis that cancer arises from normal cells through the stepwise accumulation of genetic mutations. The study of cells obtained from patients with cancer has identified numerous molecules and pathways that fundamentally contribute to malignant transformation; however, cancer cell lines are often difficult to isolate or maintain, and the cell lines that are available for experimentation represent only a small subset of late-stage human cancers. Recent work has elucidated the role of telomerase in regulating human cell lifespan and has enabled the development of new experimental systems to study human cancer. This review highlights the recent progress in combining genetic methods and primary human cells to understand the role of specific genes and pathways in cancer pathogenesis.

Tumor necrosis factor-alpha activation by adenovirus E1A 13S CR3 occurs in a cell-dependent and cell-independent manner

The adenovirus (Ad) early gene product 13S transactivates the tumor necrosis factor (TNF)-alpha promoter in inflammatory cells. We examined both the subdomains of E1A and the upstream TNF promoter elements involved. In both Jurkat and U-937 cells, zinc finger or carboxyl region mutation of Ad E1A 13S conserved region 3 resulted in a significant loss of transactivation of the TNF promoter (> or =69%). For both cell types there was a TNF-negative regulatory element in the -242 to -199 region and a positive regulatory element between -199 and -118. In contrast, an upstream positive regulatory element was detected in different regions in both cell types. In U-937 cells the positive regulatory unit was between -600 and -576, whereas in Jurkat cells it was between -576 and -242. The U-937 upstream element was dependent on a site previously designated epsilon in cooperation with an adjacent nuclear factor-kappaB-2a site. Therefore, transactivation of the TNF promoter by Ad 13S in lymphocyte and monocyte cell types involves similar subdomains of the E1A protein, but cell-specific TNF promoter elements.

E1A: tumor suppressor or oncogene? Preclinical and clinical investigations of E1A gene therapy

In the late 1980s, we have shown that the E1A gene can downregulate HER-2/neu overexpression, thus reversing the tumorigenic and metastatic phenotype. Further, E1A can function as a tumor suppressor gene by inducing apoptosis and inhibiting metastasis. At The University of Texas M. D. Anderson Cancer Center, we have been investigating the adenovirus type 5 E1A gene as a potential therapeutic gene in breast and ovarian cancer since 1995 by using cationic liposome as gene delivery system. In this chapter, we recount our development of E1A as a therapeutic gene.

Identification of a region on the adenovirus E1A gene responsible for induction by phorbol ester tumor promoter

12-O-Tetradecanoylphorbol-13-acetate (TPA) treatment induces human adenovirus (Ad) early region 1A (E1A) messenger ribonucleic acid expression in infected or Ad-transformed cells. Here, we report that deletion analysis has identified a TPA-responsive element (TRE) in the E1A enhancer region. Deletion analysis indicates that the TRE is located upstream of the E1A cap site between nucleotides -237 and -47. Incubation of extracts from TPA-treated cells with radioactively labeled deoxyribonucleic acid (DNA) fragments containing the TRE (-237 to -47) form specific DNA-protein complexes as demonstrated by gel shift analysis and Southwestern blotting. These experiments provide evidence that novel protein-DNA complexes are formed on a region of the E1A promoter required for TPA-enhanced expression. We speculate that these DNA-binding proteins may interact with the TRE and play a critical role in the mechanism through which TPA upregulates transcription from the Ad E1A gene.

Collective inhibition of pRB family proteins by phosphorylation in cells with p16INK4a loss or cyclin E overexpression

The activity of the retinoblastoma protein pRB is regulated by phosphorylation that is mediated by G(1) cyclin-associated cyclin-dependent kinases (CDKs). Since the pRB-related pocket proteins p107 and p130 share general structures and biological functions with pRB, their activity is also considered to be regulated by phosphorylation. In this work, we generated phosphorylation-resistant p107 and p130 molecules by replacing potential cyclin-CDK phosphorylation sites with non-phosphorylatable alanine residues. These phosphorylation-resistant mutants retained the ability to bind E2F and cyclin. Upon introduction into p16(INK4a)-deficient U2-OS osteosarcoma cells, in which cyclin D-CDK4/6 is dysregulated, the phosphorylation-resistant mutants, but not wild-type p107 or p130, were capable of inhibiting cell proliferation. Furthermore, when ectopically expressed in pRB-deficient SAOS-2 osteosarcoma cells, the wild-type as well as the phosphorylation-resistant pRB family proteins were capable of inducing large flat cells. The flat cell-inducing activity of the wild-type proteins, but not that of the phosphorylation-resistant mutants, was abolished by coexpressing cyclin E. Our results indicate that the elevated cyclin D- or cyclin E-associated kinase leads to systemic inactivation of the pRB family proteins and suggest that dysregulation of the pRB kinase provokes an aberrant cell cycle in a broader range of cell types than those induced by genetic inactivation of the RB gene.

Interaction of the retinoblastoma protein (pRb) with the catalytic subunit of DNA polymerase delta (p125)

The retinoblastoma gene product (pRb) interacts with many cellular proteins to function in the control of cell division, differentiation, and apoptosis. Several pRb binding proteins complex with pRb through an amino acid sequence called the LXCXE motif. The catalytic subunit of DNA polymerase delta (p125) contains a LXCXE motif. To further study the biochemical function of this polymerase, we sought to determine if p125 interacts with pRb. Experiments using GST-pRb fusion proteins showed that p125 from breast epithelial (MCF10A) cell extracts associates with pRb. In addition, GST-p125 fusion proteins bound pRb from the same cell extracts. The pRb that associated with GST-p125 was largely unphosphorylated. Coimmunoprecipitation experiments using cell cycle synchronized cells revealed that p125 and pRb form a complex predominantly during G1 phase, the phase during which pRb is mostly unphosphorylated. In vitro phosphorylation of GST-pRb by the cyclin dependent kinases reduced the ability of p125 to associate with GST-pRh. Addition of the LXCXE containing protein SV40 large T antigen to GST-pRb blocks the ability of p125 to associate with pRb, suggesting that it may be through a LXCXE sequence by which p125 interacts with pRb. Finally, in vitro polymerase assays demonstrate that GST-pRb fusion protein stimulates DNA polymerase delta activity.

An adenovirus E1A mutant that demonstrates potent and selective systemic anti-tumoral efficacy

Replication-selective oncolytic viruses constitute a rapidly evolving and new treatment platform for cancer. Gene-deleted viruses have been engineered for tumor selectivity, but these gene deletions also reduce the anti-cancer potency of the viruses. We have identified an E1A mutant adenovirus, dl922-947, that replicates in and lyses a broad range of cancer cells with abnormalities in cell-cycle checkpoints. This mutant demonstrated reduced S-phase induction and replication in non-proliferating normal cells, and superior in vivo potency relative to other gene-deleted adenoviruses. In some cancers, its potency was superior to even wild-type adenovirus. Intravenous administration reduced the incidence of metastases in a breast tumor xenograft model. dl922-947 holds promise as a potent, replication-selective virus for the local and systemic treatment of cancer.

Granulocytic differentiation of myeloid progenitor cells by p130, the retinoblastoma tumor suppressor homologue

The retinoblastoma protein (pRB) and the related pocket proteins, p107 and p130, play crucial roles in mammalian cell cycle control. Recent studies indicate that these pocket proteins are also involved in cellular differentiation processes. We demonstrate in this work that the pRB-related p130 selectively accumulates during the in vitro differentiation of the myeloid progenitor cell, 32Dcl3, into granulocyte in response to granulocyte-colony stimulating factor (G-CSF). This G-CSF-dependent granulocytic differentiation is blocked by the adenovirus E1A oncoprotein, which binds to and inactivates the pRB family of pocket proteins including p130. Furthermore, enforced overexpression of p130 but not pRB inhibits the myeloid cell proliferation that is concomitantly associated with granulocytic differentiation morphologically characterized by nuclear segmentation. However, simple G1-cell cycle arrest induced by cytokine deprivation or ectopic overexpression of the p27 cyclin-dependent kinase inhibitor, or inhibition of E2F activities by dominant negative DP-1 is not sufficient to trigger granulocytic differentiation. The differentiation-promoting activity of p130 in myeloid cells requires both the pocket domain and the spacer domain. Our results indicate that the pRB-related p130 plays a critical role in myeloid cell differentiation and suggest that coupling of cell cycle exit with the cellular differentiation program may be specifically achieved by p130.

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

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

E1A oncogene induction of cellular susceptibility to killing by cytolytic lymphocytes through target cell sensitization to apoptotic injury

E1A oncogene expression increases mammalian cell susceptibility to lysis by cytolytic lymphocytes (CLs) at a stage in this intercellular interaction that is independent of cell surface recognition events. Since CLs can induce either apoptotic or necrotic cell death, we asked whether E1A sensitization to injury-induced apoptosis is sufficient to explain E1A-induced cytolytic susceptibility. Mouse, rat, hamster, and human cells that were rendered cytolytic susceptible by E1A were also sensitized to CL-induced and chemically induced apoptosis. In contrast, E1A-positive cells were no more susceptible to injury-induced necrosis than E1A-negative cells. Similar to induction of cytolytic susceptibility and in contrast to other E1A activities, cellular sensitization to chemically induced apoptosis depended on high-level E1A oncoprotein expression. Loss of both cytolytic susceptibility and sensitization to chemically induced apoptosis was coselected during in vivo selection of E1A-positive sarcoma cells for increased tumorigenicity. Furthermore, E1A mutant proteins that cannot bind the cellular transcriptional coactivator, p300, and that fail to induce cytolytic susceptibility also failed to sensitize cells to injury-induced apoptosis. These data indicate that E1A induces susceptibility to killer cell-induced lysis through sensitization of cells to injury-induced apoptosis.

Regulation of CBP-mediated transcription by neuronal calcium signaling

The transcription factor CREB is involved in mediating many of the long-term effects of activity-dependent plasticity at glutamatergic synapses. Here, we show that activation of NMDA receptors and voltage-sensitive calcium channels leads to CREB-mediated transcription in cortical neurons via a mechanism regulated by CREB-binding protein (CBP). Recruitment of CBP to the promoter is not sufficient for transactivation, but calcium influx can induce CBP-mediated transcription via two distinct transactivation domains. CBP-mediated transcription is stimulus strength-dependent and can be induced by activation of CaM kinase II, CaM kinase IV, and protein kinase A, but not by activation of the Ras-MAP kinase pathway. These observations indicate that CBP can function as a calcium-sensitive transcriptional coactivator that may act as a regulatory switch for glutamate-induced CREB-mediated transcription.

Adenovirus mediated-gene transfer into cardiomyocytes

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

Mutations in CR1 of E1A 12S yield dominant negative suppressors of immortalization and the lytic cycle

The Adenovirus 5 E1A 12S gene is responsible for the establishment of immortalization of primary cells by Adenovirus. We have identified two mutants of 12S (30K and NTdl814), which, when coexpressed with wild-type 12S in primary baby rat kidney cells, were capable of suppressing the immortalizing function of the wild-type 12S gene, even when the mutant proteins were expressed at levels lower than wild type. 30K and NTdl814 did not affect the ability of the coexpressed 12S to activate the cell cycle, but have a suppressive effect on 12S-induced DNA synthesis and proliferation at late times in the immortalization pathway. Both the dominant negative mutants have a deletion in conserved region (CR)1 in the first exon of E1A, which encompasses one of the pRb-family binding regions. However, the mutants did not effect the binding of cellular proteins to full-length 12S. A suppressive effect on wild-type 12S was not observed with mutants that have lost any other region or function. In addition, expression of 30K, which is equivalent to the protein encoded by the 10S mRNA of E1A, inhibited E1A function in lytic cycle. Thus, loss of the CR1 seems to be a prerequisite for a mutant to have a dominant negative effect on E1A functions.

Polyomavirus large T-antigen binds the "pRb related' protein p130 through sequences in conserved region 2

The transforming potential of DNA tumor viruses derives mainly from the ability of their encoded oncogene products to interact with cellular proteins. Many of these viral oncoproteins share regions of sequence similarity, designated conserved region 1 and 2, which have been implicated in complex formation with pRb, the product of the retinoblastoma tumor suppressor gene, and related p107 and p130 species. It has now been shown that the EIA protein of adenovirus is able to bind to all three pRb-related proteins through sequences in conserved region 1 and 2. We have shown previously that polyomavirus large T-antigen also interacts with pRb and p107 in vitro. The pRb and p107 binding domains reside between residues 141, 158 which include conserved region 2. In the present study, we demonstrate that polyomavirus large T-antigen also interacted with p130 in vitro through the same sequences in conserved region 2.

mUBC9, a novel adenovirus E1A-interacting protein that complements a yeast cell cycle defect

Adenovirus E1A encodes two nuclear phosphoproteins that can transform primary rodent fibroblasts in culture. Transformation by E1A is mediated at least in part through binding to several cellular proteins, including the three members of the retinoblastoma family of growth inhibitory proteins. We report here the cloning of a novel murine cDNA whose encoded protein interacts with both adenovirus type 5 and type 12 E1A proteins. The novel E1A-interacting protein shares significant sequence homology with ubiquitin-conjugating enzymes, a family of related proteins that is involved in the proteasome-mediated proteolysis of short-lived proteins. Highest homology was seen with a Saccharomyces cerevisiae protein named UBC9. Importantly, the murine E1A-interacting protein complements a cell cycle defect of a S. cerevisiae mutant which harbors a temperature-sensitive mutation in UBC9. We therefore named this novel E1A-interacting protein mUBC9. We mapped the region of E1A that is required for mUBC9 binding and found that the transformation-relevant conserved region 2 of E1A is required for interaction.

Plant cells contain a novel member of the retinoblastoma family of growth regulatory proteins

The product of the retinoblastoma susceptibility gene (Rb) controls the passage of mammalian cells through G1 phase. Animal virus oncoproteins interact with the Rb protein via an LXCXE motif and disrupt Rb-E2F complexes, driving cells into S-phase. Recently, we found that the RepA protein of a plant geminivirus contains an LXCXE motif that is essential for its function, a finding that predicts the existence of Rb-related proteins in plant cells. Here we report the isolation of a maize cDNA clone encoding a protein (ZmRb1) which, based on structural and functional studies, is closely related to the mammalian Rb family of growth regulatory proteins. ZmRb1 shows a high degree of amino acid conservation when compared with animal Rb members, particularly in the A/B 'pocket' domain, but ZmRb1 has a shorter N-terminal domain. ZmRb1 forms stable complexes with plant LXCXE-containing proteins, e.g. geminivirus RepA protein. Geminivirus DNA replication is reduced in plant cells transfected with plasmids encoding either ZmRb1 or human p130, a member of the Rb family. This suggests that ZmRb1 controls the G1/S transit in plant cells and is consistent with the fact that geminiviruses need an S-phase environment for DNA replication, as animal DNA tumor viruses do. Our results allow the extension of the Rb family of tumor suppressor proteins to plants and have implications on animal and plant strategies for cell growth control.

E1A promotes association between p300 and pRB in multimeric complexes required for normal biological activity

The oncogenes of the small DNA tumor viruses encode transforming proteins with multiple domains that influence the cell cycle and aspects of the transformed phenotype. Like other gene products of this type, the adenovirus E1A proteins influence the cell by binding to specific cell growth control proteins. These include members of the retinoblastoma gene product (pRB) family, which are bound by the E1A region 2-specific site, and p300, which is bound at the E1A amino terminus. Binding at these two sites is largely independent, and discrete transcription-regulating functions remain intact in E1A products when only one or the other binding site is functional. In this report, immunoprecipitation with p300 antibodies reveals the presence of the pRB family proteins in p300 complexes when E1A is expressed in host cells, indicating that E1A can mediate physical contact between p300 and the pRB-related proteins. The ability of E1A to induce proliferation efficiently in quiescent primary cells correlates closely with the ability to bind p300 and individual members of the pRB family simultaneously in multimeric complexes, even though the E1A active sites can bind their target proteins efficiently when separated on different molecules. Conservation of a spacer region between the two binding sites that is required for simultaneous binding and efficient induction of proliferation supports the concept that the E1A protein structure has evolved to facilitate simultaneous binding. These results indicate that the E1A proteins are designed not merely to sequester these cellular products, but also to bring them into proximal association with each other in biologically significant complexes.

Efficient nuclear localization and immortalizing ability, two functions dependent on the adenovirus type 5 (Ad5) E1A second exon, are necessary for cotransformation with Ad5 E1B but not with T24ras

Expression of adenovirus type 5 E1A 12S is sufficient to immortalize primary baby rat kidney cells, but another viral or cellular oncogene, such as E1B or T24ras, is necessary for complete transformation. The regions of 12S sufficient for T24ras cotransformation have been well characterized and are located in the first exon. The second exon is dispensable for ras cotransformation, although it contains a region which appears to modulate the transforming phenotype. The same 12S first exon regions important in ras transformation are also necessary for E1B transformation. Analysis of an extensive series of second exon deletion and amino acid point mutations demonstrated that mutations affecting either the efficient nuclear localization and/or the immortalizing ability of the 12S protein also prevented cooperation with E1B. In general, the entire C-terminal half of 12S, including the nuclear localization signal, was necessary for efficient cotransformation with E1B. In addition to the differences between T24ras and E1B regarding 12S regions necessary for cotransformation, the characteristics of E1B-cotransformed foci differed from those of T24ras. The E1B foci took longer to appear and had a much slower growth rate. No hypertransformed foci were produced with E1B cotransfections, and established E1A-E1B lines exhibited minimal growth in soft agar compared with that of E1A-T24ras lines.

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

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

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

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

Derivation and partial analysis of two highly active myeloma cell transfectants

Vectors have been designed to optimise the expression of heterologous proteins in transfected mouse myeloma cells. The over-ridingly important DNA element contained in these constructs is the classical mouse immunoglobulin heavy chain enhancer. It is shown that even in the absence of a well-known promoter element, the enhancer can drive gene expression in stable cell transfectants and the main transcriptional start site utilized in such situations has been mapped to within the previously defined enhancer region. Using chicken lysozyme as a reporter function in these vectors, two transfected myeloma cell clones have been isolated which secrete this protein at levels 50-100-times as high as those usually obtained with the same vectors and it is shown that in molar terms this is at least as high as endogenous immunoglobulin produced by a related line. Analysis of these lines show that in one case only a single copy, and in the other two to three copies, of the apparently unrearranged vector have integrated at a single locus within the genome. Possible explanations for the high-level expression are discussed.

The retinoblastoma protein and BRG1 form a complex and cooperate to induce cell cycle arrest

The retinoblastoma tumor suppressor protein (RB) binds several cellular proteins involved in cell cycle progression. Using the yeast two-hybrid system, we found that RB bound specifically to the protein BRG1. BRG1 shares extensive sequence similarity to Drosophila brahma, an activator of homeotic gene expression, and the yeast transcriptional activator SNF2/SW12. BRG1 contains an RB-binding motif found in viral oncoproteins and bound to the A/B pocket and the hypophosphorylated form of RB. BRG1 did not bind RB in viral oncoprotein-transformed cells. Coimmunoprecipitation experiments suggested BRG1 associates with the RB family in vivo. In the human carcinoma cell line SW13, BRG1 exhibited tumor suppressor activity by inducing formation of flat, growth-arrested cells. This activity depended on the ability of BRG1 to cooperate and complex with RB, as both an RB-nonbinding mutant of BRG1 and the sequestration of RB by adenovirus E1A protein abolished flat cell formation.

The cell cycle and the retinoblastoma protein family

Tumor formation results from alterations in the control of normal cell proliferation. To further our understanding of the molecular mechanisms underlying the deregulation of cell proliferation much attention, over the past decade, has been focused on the function of proto-oncogenes. Cellular oncogenes are thought to be growth promoting. More recently, a class of genes known as tumor suppressors have come under intense study. Tumor suppressors are largely thought to restrain cell proliferation. The retinoblastoma protein (Rb) is one of a growing list of tumor suppressors. Concurrent with the study of tumor suppressor genes has been a rapid increase in our understanding of the cell cycle at the molecular level. Rb and a related protein p107 are involved in the processes of cell proliferation and differentiation. Each functionally interacts with and affects the activity of the transcription factor E2F as well as other transcription factors involved in cell proliferation and differentiation. Additionally, Rb and p107 are modified by, and/or form specific complexes with, several elements of the basic cell cycle machinery. Specifically, Rb and p107 interact with and are modified by various cyclins and cyclin dependent kinases (cdk), some of which have been shown to be essential for cell cycle progression and in some cases their deregulation has been implicated in the development of cancer. This review will attempt to convey our current functional and mechanistic understanding of the biological roles Rb and p107 play in proliferation, development and differentiation. A knowledge of the interplay between these positive and negative regulators of cell proliferation and differentiation, noted above, is central to our understanding of human cancer.

Constructing chimeric type 12/type 5 adenovirus E1A genes and using them to identify an oncogenic determinant of adenovirus type 12

The E1A gene of highly oncogenic type 12 adenovirus (Ad12) possesses a segment unique to this serotype and comprising 60 base pairs contiguous with and separating conserved regions 2 and 3 in the gene. A similar but slightly longer segment is also present in the E1A gene of highly oncogenic simian adenovirus type 7 (D. Kimelman, J. S. Miller, D. Porter, and B. E. Roberts, J. Virol. 53:399-409, 1985). This segment is missing entirely from the E1A gene of type 5 adenovirus, which is nononcogenic. To test the hypothesis that this unique separating or "spacer" region influences the oncogenicity of Ad12, we constructed ClaI and SmaI restriction sites on either side of it, which allowed reciprocal exchange between this and the equivalent cassette from type 5 adenovirus E1A, bounded by the same restriction sites intrinsic to that gene. The resultant Ad12-based chimeric viruses, ch702 and ch704, in which the spacer region is replaced with (in-frame) type 5 sequence, grow normally on human A549 cells and display wild-type transformation frequencies on baby rat and mouse kidney cells. In contrast, the oncogenic capacity of these chimeric viruses, as measured by tumor induction following virus inoculation in Hooded Lister rats, is greatly reduced. Likewise, cells transformed by ch702 and ch704 display reduced tumorigenicity compared with wild-type transformants in syngeneic rats. These results, coupled with recent preliminary tests using a mutant with a point mutation in this region, support the view that the unique spacer region of type 12 is an oncogenic determinant of this virus.

Proliferation and differentiation properties of bipotent glial progenitor cell lines immortalized with the adenovirus E1A gene

Bipotent glial progenitors have been immortalized by the transfer of the adenovirus E1A gene into primary cultured cells from embryonic rat brain. The lines obtained are phenotypically untransformed, retain growth contact-inhibition, and are able to differentiate, unless they are surtransfected with transforming oncogenes. Depending on the growth conditions, these immortalized cells express differentially either oligodendrocyte or astrocyte-specific markers and genes. After being seeded in serum-free medium, they display gangliosides recognized by A2B5 monoclonal antibody, and then they express sequentially O4 epitopes, galactocerebroside, and the myelin protein DM20. When grown in serum-supplemented medium, the cells express at first A2B5 epitopes, and then transiently O4 and galactocerebroside; after reaching confluence, O4 and galactocerebroside become undetectable, whereas the cells begin to coexpress glial fibrillary acidic protein and glutamine synthetase. These results indicate that the cell lines can undergo a differentiation reminiscent both of O-2A progenitors and of plastic process-bearing glial subpopulations. The cells were also genetically marked by the stable introduction of the nlslacZ reporter gene. Thus, the lines could be useful for studying direct interactions in vitro, or for post-grafting investigations. They should also provide a model for studying the mechanisms involved in the commitment and in the control of proliferation and differentiation of this cell lineage. This suggestion is consistent with the data indicating a growth arrest-dependent differential expression of a novel gene encoding a protein with a helix-loop-helix domain.

Deletion of the beta-turn/alpha-helix motif at the exon 2/3 boundary of human c-Myc leads to the loss of its immortalizing function

The protein product (c-Myc) of the human c-myc proto-oncogene carries a beta-turn/alpha-helix motif at the exon2/exon3 boundary. The amino acid (aa) sequence and secondary structure of this motif are highly conserved among several nuclearly localized oncogene products, c-Myc, N-Myc, c-Fos, SV40 large T and adenovirus (Ad) Ela. Removal of this region from Ad E1a results in the loss of the transforming properties of the virus without destroying its known transregulatory functions. In order to analyse whether deletion of the above-mentioned region from c-Myc has a similar effect on its transformation activity, we constructed a deletion mutant (c-myc delta) lacking the respective aa at the exon2/exon3 boundary. In contrast to the c-myc wild-type gene product, constitutive expression of c-myc delta does not lead to the immortalization of primary mouse embryo fibroblast cells (MEF cells). This result indicates that c-Myc and Ad El a share a common domain which is involved in the transformation process by both oncogenes.

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

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

Physical interaction of the retinoblastoma protein with human D cyclins

The retinoblastoma protein (pRb) functions as a regulator of cell proliferation and in turn is regulated by cyclin-dependent kinases. Cyclins D1 and D3 can form complexes with pRb that resemble those formed by several viral oncoproteins and are disrupted by the adenovirus E1A oncoprotein and derived peptides. These cyclins contain a sequence motif similar to the pRb-binding conserved region II motif of the viral oncoproteins. Alteration of this motif in cyclin D1 prevents formation of cyclin D1-pRb complexes while enhancing the biological activity of cyclin D1 assayed in vivo. We conclude that cyclins D1 and D3 interact with pRb in a fashion distinct from cyclins A and E, which can induce pRb hyperphosphorylation, and that cyclin D1 activity may be regulated by its association with pRb.

Adenovirus-E1A proteins transform cells by sequestering regulatory proteins

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

Specific phosphorylation of the acidic central region of the N-myc protein by casein kinase II

The central region of the N-myc protein has a characteristic amino acid sequence EDTLSDSDDEDD, which is very similar to those of particular domains of adenovirus E1A, human papilloma virus E7, Simian virus 40 large T, c-myc and L-myc proteins. Domains of these three viral oncoproteins have recently been shown to be specific binding sites for the tumor-suppressor gene retinoblastoma protein. We have noted that the sequence of serine followed by a cluster of acidic amino acids is exactly the same as that of a typical substrate of casein kinase II (CKII). Therefore, we investigated whether these nuclear oncoproteins are phosphorylated by CKII. For this purpose, we fused the beta-galactosidase and N-myc genes including this domain and expressed it in Escherichia coli cells. Several mutant N-myc genes, containing single amino acid substitutions in this domain, were also used to produce fused proteins. Strong phosphorylation by CKII was detected with the fused protein of wild-type N-myc. However, no phosphorylation of beta-galactosidase itself was observed and the phosphorylations of fused mutant proteins were low. Another fused N-myc protein containing most of the C-terminal region downstream of this acidic region was not phosphorylated by CKII. Analysis of phosphorylation sites in synthetic peptides of this acidic region identified the major sites phosphorylated by CKII as Ser261 and Ser263. On two-dimensional tryptic mapping of phosphorylated N-myc proteins, major spots of in vitro-labeled and in-vivo-labeled N-myc proteins were detected in the same positions. These results suggest that two serine residues of the acidic central region of the N-myc protein are phosphorylated by CKII in vivo as well as in vitro. The functional significance of this acidic domain is discussed.

Expression, purification, and functional characterization of adenovirus 5 and 12 E1A proteins produced in insect cells

The 12 S and 13 S E1A cDNAs from both the Adenovirus (Ad) nononcogenic type 5 and the oncogenic type 12 were overexpressed in an insect cell/baculovirus system. Upon infection of Spodoptera frugiperda cells, the production of E1A proteins reached a level of about 15 micrograms/10(6) cells. The E1A proteins are highly soluble and apparently are processed authentically. They are readily recognized by various antibodies and display phosphorylation patterns similar to those of E1A proteins synthesized in mammalian cells. Single-step immunoaffinity chromatography was used to purify the Ad5 E1A proteins to near homogeneity under nondenaturing conditions. The Ad5 and Ad12 E1A proteins are able to form complexes with the retinoblastoma susceptibility gene product (Rb) and other cellular proteins. Interestingly, the presence of a cellular extract seems to be a prerequisite for association between highly purified E1A and Rb polypeptides.

Regulation of retinoblastoma protein functions by ectopic expression of human cyclins

The retinoblastoma susceptibility gene (RB) product, the retinoblastoma protein (pRb), functions as a regulator of cell proliferation. Introduction of the RB gene into SAOS-2 osteosarcoma cells, which lack functional pRb, prevents cell cycle progression. Such growth-suppressive functions can be modulated by phosphorylation of pRb, which occurs via cell cycle-regulated kinases. We show that constitutively expressed cyclins A and E can overcome pRb-mediated suppression of proliferation. pRb becomes hyperphosphorylated in cells overexpressing these cyclins, and this phosphorylation is essential for cyclin A- and cyclin E-mediated rescue of pRb-blocked cells. This suggests that G1 and S phase cyclins can act as regulators of pRb function in the cell cycle by promoting pRb phosphorylation.