Retinoblastoma growth suppressor and a 300-kDa protein appear to regulate cellular DNA synthesis

Effects of the Ad5 upstream E1 region and gene products on heterologous promoters

BACKGROUND

All recombinant adenovirus vectors contain the upstream region of the E1A gene comprising the viral origin of replication, encapsidation signal, and cis-acting regulatory elements for transcription of the E1A and other early genes. Using different reporter genes, some previous studies demonstrated the maintenance of heterologous promoter specificity in the adenoviral context, while others reported that adenoviral sequences interfere with promoter activity.

METHODS

Plasmid DNA-based luciferase reporter gene assays and adenovirus type 5 (Ad5) infection were combined to examine the effect of the Ad5 (nt 1-353) element and/or adenoviral gene products on tissue-specific (Midkine (MK) and COX-2), cell cycle associated (Ki-67 and E2F1) and viral promoters (Ad5 E1, Ad5 E4 and SV40). As a proof of concept, data were verified in the setting of recombinant replication-defective and replication-competent adenoviral vectors.

RESULTS

Viral and E2F1 promoter activities were enhanced by the Ad5 (nt 1-353) segment by approximately 100% and 145%, respectively, regardless of its position. A polyadenylation sequence (polyA) upstream of the promoter had no effect, confirming an enhancer element within the Ad5 (nt 1-353) segment. Ad5 (nt 1-353) increased COX-2 promoter activity by 146% but was blocked by an upstream polyA, indicating a cryptic transcription start site. When placing the reporter gene cassette in a replication-defective adenovirus, similar data were obtained. In the plasmid vector-based system, adenoviral gene products transactivated the E2F1 and viral promoters by 194%, 19%, 67%, and 16%, respectively. Tissue-specific promoter activities were not significantly affected by the Ad5 (nt 1-353) segment, nor adenoviral gene products. In concert with these data, we were able to target replication-competent adenoviral vectors with the COX-2 promoter, but not with the cell cycle associated promotor.

CONCLUSIONS

The adenovirus E1A upstream regulatory region and gene products interact with some but not all heterologous promoters. Often, the basal promoter activity can be reduced with an upstream polyA. Since the data obtained in our plasmid vector-based assay with internal control and infection with adenovirus could be confirmed in the adenoviral setting, our system might be suitable to speed up the identification of promoters which maintain their specificity in the adenoviral context and circumvent the problems associated with determining infectious adenovirus titers.

Acute hepatotoxicity of oncolytic adenoviruses in mouse models is associated with expression of wild-type E1a and induction of TNF-alpha

Replication competent adenoviruses with various E1 modifications designed to restrict their replication to tumor cells are being evaluated as oncolytic agents in clinical trials. In mouse models, we observed that such oncolytic adenoviruses showed greater hepatotoxicity than E1-deleted adenovirus vectors following intravenous administration. Additional studies in congenic BALB/c, nude, and beige/Scid mice demonstrated dose-dependent hepatotoxicity and indicated that beige/Scid was the most sensitive strain. Comparison of E1-containing viruses showed that hepatotoxicity correlated with expression of wild-type E1a in the liver. Pharmacokinetic analysis showed rapid increases in viral DNA levels in the liver with a virus containing wild-type E1a. This was correlated with rapid induction of TNF-alpha to high levels and with rapid elevation of serum ALT. Hepatotoxicity was significantly reduced for an adenovirus with deletions in the region E1a (dl01/07) or a virus lacking E1a. The results suggest a mechanism for hepatotoxicity involving virus-induced production of local TNF-alpha release and E1a-mediated sensitization of hepatocyte killing.

Effects of depletion of CREB-binding protein on c-Myc regulation and cell cycle G1-S transition

We recently reported that the transcriptional coactivator and histone acetyltransferase p300 plays an important role in the G(1) phase of the cell cycle by negatively regulating c-myc and thereby preventing premature G(1) exit (Kolli, et al. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 4646-4651; Baluchamy, et al. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 9524-9529). Because p300 does not substitute for all CREB-binding protein (CBP) functions, we investigated whether CBP also negatively regulates c-myc and prevents premature DNA synthesis. Here, we show that antisense-mediated depletion of CBP in serum-deprived human cells leads to induction of c-myc and that such cells emerge from quiescence without growth factors at a rate comparable with that of p300-depleted cells. The CBP-depleted cells contained significantly reduced levels of the cyclin-dependent kinase inhibitor p21 and low levels of p107 and p130 (but not pRb) phosphorylation, suggesting that these factors, along with elevated levels of c-Myc, contribute to induction of DNA synthesis. Antisense c-Myc reversed the phosphorylation of p107 and p130 and the induction of S phase in CBP-depleted cells, indicating that up-regulation of c-myc is directly responsible for the induction of S phase. Furthermore, the serum-stimulated p300/CBP-depleted cells did not traverse beyond S phase, and a significant number of these cells died of apoptosis, which was not related to p53 levels. These cells also contained significantly higher levels of c-Myc compared with normal cells. When c-myc expression was blocked by antisense c-Myc, the apoptosis of the serum-stimulated CBP-depleted cells was reversed, indicating that high levels of c-Myc contribute to apoptosis. Thus, despite their high degree of structural and functional similarities, normal levels of both p300 and CBP are essential for keeping c-myc in a repressed state in G(1) and thereby preventing inappropriate entry of cells into S phase. In addition, both these proteins also provide important functions in coordinated cell cycle progression.

Murine coronavirus nonstructural protein p28 arrests cell cycle in G0/G1 phase

Murine coronavirus mouse hepatitis virus (MHV) gene 1 encodes several nonstructural proteins. The functions are unknown for most of these nonstructural proteins, including p28, which is encoded at the 5' end of the MHV genome. Transient expression of cloned p28 in several different cultured cells inhibited cell growth, indicating that p28 expression suppressed cell proliferation. Expressed p28 was exclusively localized in the cytoplasm. Cell cycle analysis by flow cytometry demonstrated that p28 expression induced G(0)/G(1) cell cycle arrest. Characterization of various cellular proteins that are involved in regulating cell cycle progression demonstrated that p28 expression resulted in an accumulation of hypophosphorylated retinoblastoma protein (pRb), tumor suppressor p53, and cyclin-dependent kinase (Cdk) inhibitor p21(Cip1). Expression of p28 did not alter the amount of p53 transcripts yet increased the amount of p21(Cip1) transcripts, suggesting that p28 expression increased p53 stability and that p21(Cip1) was transcriptionally activated in a p53-dependent manner. Our present data suggest the following model of p28-induced G(0)/G(1) cell cycle arrest. Expressed cytoplasmic p28 induces the stabilization of p53, and accumulated p53 causes transcriptional upregulation of p21(Cip1). The increased amount of p21(Cip1) suppresses cyclin E/Cdk2 activity, resulting in the inhibition of pRb hyperphosphorylation. Accumulation of hypophosphorylated pRb thus prevents cell cycle progression from G(0)/G(1) to S phase.

Impact of E1a Modifications on Tumor-Selective Adenoviral Replication and Toxicity

Replicating adenoviral vectors are capable of multiplying up to a thousandfold in the target cell, a property that might prove to be of tremendous potential for cancer therapy. However, restricting viral replication and toxicity to cancer cells is essential to optimize safety. It has been proposed that modifications of the E1a protein that impair binding to Rb or p300 will prevent S-phase induction in normal cells, resulting in selective viral replication in tumor cells. However, it remains uncertain which of the several possible E1a modifications would be most effective at protecting normal cells without compromising the oncolytic effect of the vector. In this study, we have expressed several E1a-deletion mutants at high levels using the CMV promoter and tested them for their ability to facilitate S-phase induction, viral replication, and cytotoxicity in both normal and cancer cells. Deletion of the Rb-binding domain within E1a only slightly decreased the ability of the virus to induce S phase in growth-arrested cells. The effect of this deletion on viral replication and cytotoxicity was variable. There was reduced cytotoxicity in normal bronchial epithelial cells; however, in some normal cell types there was equal viral replication and cytotoxicity compared with wild type. Deletions in both the N-terminus and the Rb-binding domain were required to block S-phase induction effectively in growth-arrested normal cells; in addition, this virus demonstrated reduced viral replication and cytotoxicity in normal cells. An equally favorable replication and cytotoxicity profile was induced by a virus expressing E1a that is incapable of binding to the transcriptional adapter motif (TRAM) of p300. All viruses were equally cytotoxic to cancer cells compared with wild-type virus. In conclusion, deletion of the Rb-binding site alone within E1a may not be the most efficacious means of targeting viral replication and toxicity. However, deletion within the N-terminus in conjunction with a deletion within the Rb-binding domain, or deletion of the p300-TRAM binding domain, induces a more favorable cytotoxicity profile.

Selective replicating viral vectors : potential for use in cancer gene therapy

Treatment of cancer is limited by toxicity to normal tissue with standard approaches (chemotherapy, surgery and radiotherapy). The use of selective replicating viral vectors may enable the targeting of gene-modified viruses to malignant tissue without toxic effect. Studies of these vectors have demonstrated tumour-selective replication and minimal evidence of replication in normal tissue. The most advanced clinical results reported involve gene-modified adenoviral vectors. Several completed, histologically confirmed responses to local/regional injection have been induced, particularly in recurrent squamous cell carcinoma involving the head and neck region. Dose limiting toxicity above 10(13) viral particles per injection has been observed. Anti-tumour effect is demonstrable in animal models without evidence of significant toxicity when these vectors are used alone or in combination with chemotherapy, radiation therapy or as gene delivery vehicles. Preliminary clinical trials, particularly with E1B-deleted adenoviruses, report evidence of clinical activity in comparison with expected historical responses. Enhancement in replication selectivity to malignant tissue is also demonstrated preclinically and clinically with an E1B-deleted adenovirus utilising a prostate-specific antigen promoter. Other selective replicating viral vectors such as herpes simplex virus and vaccinia virus have also been explored clinically and suggest evidence of activity in patients with cancer. Modifications may one day enable more aggressive use of these new and exciting therapeutics as systemic gene delivery vehicles.

Murine coronavirus replication induces cell cycle arrest in G0/G1 phase

Mouse hepatitis virus (MHV) replication in actively growing DBT and 17Cl-1 cells resulted in the inhibition of host cellular DNA synthesis and the accumulation of infected cells in the G(0)/G(1) phase of the cell cycle. UV-irradiated MHV failed to inhibit host cellular DNA synthesis. MHV infection in quiescent 17Cl-1 cells that had been synchronized in the G(0) phase by serum deprivation prevented infected cells from entering the S phase after serum stimulation. MHV replication inhibited hyperphosphorylation of the retinoblastoma protein (pRb), the event that is necessary for cell cycle progression through late G(1) and into the S phase. While the amounts of the cellular cyclin-dependent kinase (Cdk) inhibitors p21(Cip1), p27(Kip1), and p16(INK4a) did not change in infected cells, MHV infection in asynchronous cultures induced a clear reduction in the amounts of Cdk4 and G(1) cyclins (cyclins D1, D2, D3, and E) in both DBT and 17Cl-1 cells and a reduction in Cdk6 levels in 17Cl-1 cells. Infection also resulted in a decrease in Cdk2 activity in both cell lines. MHV infection in quiescent 17Cl-1 cells prevented normal increases in Cdk4, Cdk6, cyclin D1, and cyclin D3 levels after serum stimulation. The amounts of cyclin D2 and cyclin E were not increased significantly after serum stimulation in mock-infected cells, whereas they were decreased in MHV-infected cells, suggesting the possibility that MHV infection may induce cyclin D2 and cyclin E degradation. Our data suggested that a reduction in the amounts of G(1) cyclin-Cdk complexes in MHV-infected cells led to a reduction in Cdk activities and insufficient hyperphosphorylation of pRb, resulting in inhibition of the cell cycle in the G(0)/G(1) phase.

Expression of an E1A/E7 chimeric protein sensitizes tumor cells to killing by activated macrophages but not NK cells

Adenovirus (Ad) E1A and human papillomavirus (HPV) E7 express homologous conserved regions (CRs) that mediate their shared biological functions. Despite their similarities, the expression of E1A sensitizes tumor cells to killing by NK cells and macrophages but the expression of E7 does not, a factor that may contribute to the dissimilar oncogenicities of Ad and HPV. This study was undertaken to define molecular differences between E1A and E7 that are responsible for the ability of E1A and the inability of E7 to sensitize cells to killing by NK cells and macrophages. Genetic mapping studies using human fibrosarcoma cells (H4) that stably expressed mutant forms of E1A showed that only those forms of E1A that interacted with the transcriptional coadaptor protein p300 sensitized cells to killing by NK cells and macrophages. E7 lacks the N-terminal p300-binding region present in E1A. Therefore, a chimeric E1A/E7 gene was constructed that included the N terminus and the CR1 (p300-binding) domain of E1A fused to CR2 and the C-terminal sequences of E7. The E1A/E7 protein interacted with p300 and pRb and immortalized primary mouse embryo fibroblasts (MEF). The expression of E1A/E7 sensitized H4 and MEF cells to killing by activated macrophages but not to killing by NK cells. Therefore, N-terminal differences between E1A and E7 that map to the E1A-p300 binding region accounted for differences in their abilities to sensitize cells to killing by macrophages. However, regions in addition to the E1A-p300 binding region are required to sensitize cells to killing by NK cells.

Replicating Adenoviruses in Cancer Therapy

The potential use of adenoviruses in therapy against cancer has evoked a rapidly moving field of research. Unlike conventional gene therapy vectors, oncolytic adenoviruses retain the ability to replicate. However, replication is restricted as much as possible to tumor cells, with the aim of eliminating these cells through viral cytotoxicity. The two key issues are to improve the efficiency of virus replication and cell killing while ensuring the specificity of these activities for tumor cells. Wild-type adenoviruses as such may already be usable for cancer therapy. Strategies to further improve efficiency and specificity include the partial or complete removal of viral genes. The idea is that functions carried out by the corresponding gene products are not required for replication in tumor cells, but are needed in normal cells. Accordingly, the removal of genes encoding E1B-55 kDa or E1B-19 kDa, or the mutation of E1A may improve the selective killing of tumor cells. On the other hand, the overexpression of the adenovirus death protein (ADP) may enhance viral spread and oncolytic efficiency. Other strategies to improve the specific oncolytic activity of replicating adenoviruses have been pursued. For instance, some promoters are active specifically in tumor cells, and these promoters were introduced into the viral genome, to regulate essential viral genes. Moreover, replicating viruses were engineered to express toxic proteins or drug converters. A number of these viruses have been tested successfully using tumor xenografts in nude mice as a model system. An oncolytic adenovirus lacking the E1B-55 kDa gene product, termed dl1520 or ONYX015, was injected into squamous cell carcinomas of head and neck in phase II clinical trials, and the results were encouraging when chemotherapy was applied in parallel. In the future, further progress might be achieved on the level of virus constructs, but also by refining and adjusting simultaneous conventional therapies, and by standardizing the assessment of the clinical outcome. Recent progress has been made towards the use of replicating virus constructs in cancer therapy. The goal of these developments is to remove cancerous cells from patients with the help of viruses that selectively replicate in these cells. These viruses are generally termed oncolytic viruses. Some convenient properties of adenovirus make this virus particularly useful for this purpose. It infects a large number of human cell types, especially epithelial cells, which give rise to the vast majority of human malignancies. It can be grown easily and to high titers, and the creation of virus recombinants is well established. Finally, a large body of basic research has already been carried out on this virus, facilitating its manipulation. Various approaches to use adenovirus as a cancer drug have been reviewed (Alemany et al. 1999a, 2000; Curiel 2000; Galanis et al. 2001b; Gromeier 2001; Heise and Kirn 2000; Kirn 2000a; Kirn et al. 2001; Kirn and McCormick 1996; Smith and Chiocca 2000; Sunamura 2000; Wells 2000; Wodarz 2001). The aim of this chapter is to provide an integrated overview of these strategies.

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

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

Repression of c-Myc and inhibition of G1 exit in cells conditionally overexpressing p300 that is not dependent on its histone acetyltransferase activity

p300 and cAMP response element binding protein (CREB)-binding protein (CBP) are two highly homologous, conserved transcriptional coactivators, and histone acetyltransferases (HATs) that link chromatin remodeling with transcription. Cell transformation by viral oncogene products such as adenovirus E1A and SV40 large T antigen depends on their ability to inactivate p300 and CBP. To investigate the role of p300 in cell-cycle progression, we constructed stable rat cell lines, which conditionally overexpress p300 from a tetracycline-responsive promoter. When p300 was induced in these cells, serum-stimulated S-phase entry was significantly inhibited. The inhibition of S-phase induction was associated with down-regulation of c-Myc, but not of c-Fos or c-Jun. Simultaneous overexpression of c-Myc and p300 before serum stimulation reversed the inhibition of S-phase induction to a significant level, indicating that the inhibition of c-Myc to a large extent is responsible for the p300 inhibition of G1 exit. Similar studies with stable rat cell lines that overexpress a mutant p300, which lacks the HAT activity, showed that the intrinsic HAT activity of p300 is not required for the negative regulation of c-Myc or G1. These findings, and our previously published results (Kolli, S., Buchmann, A. M., Williams, J., Weitzman, S. & Thimmapaya, B. (2001) Proc. Natl. Acad. Sci. USA 98, 4646-4651), establish an important negative regulatory role for p300 in c-Myc expression that may be important in maintaining the cells in the G0/G1 phase of the cell cycle.

Transactivation of human cdc2 promoter by adenovirus E1A

Expression of the adenovirus oncoprotein E1A 12S induces the heterotrimeric transcription factor, NF-Y. NF-Y binds to the two CCAAT motifs upstream of the transcriptional start site of the human cdc2 promoter and is required for activation of the promoter by E1A 12S in cycling cells. The observations that a number of eukaryotic cell cycle regulatory genes also contain the CCAAT motifs and NF-Y binds to them support the notion that E1A 12S could play an important role in deregulated expression of these genes through activation of NF-Y gene in cycling cells.

Caspase-mediated cleavage of adenovirus early region 1A proteins

Adenovirus 2 and 12 early region 1A (Ad2 and Ad12 E1A) proteins were cleaved during cisplatin-induced apoptosis of Ad-transformed rat and human cells. Cleavage was inhibited in the presence of caspase inhibitors such as Z-VAD-FMK. In Ad12 transformants both 13S and 12S E1A proteins were cleaved at a similar rate. In Ad2 transformants the E1A 13S component was appreciably less stable than the 12S component. In in vitro studies Ad2 and Ad12 E1A 13S and Ad2 12S proteins were rapidly cleaved by caspase 3 whereas Ad12 12S E1A and Ad12 13S E1A were rapidly degraded by caspase 7. Cleavage sites in Ad12 13S proteins for caspase 3 have been determined. Initial cleavage occurred at D24 and D150; this was followed by cleavage at D204 and D242. Caspase-3-mediated cleavage of Ad12 13S E1A destroyed its ability to bind to CBP and TBP but interaction between C terminal E1A polypeptides and CtBP was observed. During viral infection Ad5 and Ad12 E1A 12S proteins were markedly more stable than 13S proteins but no difference was observed in Ad E1A levels in the absence or presence of the caspase inhibitors Z-VAD-FMK or Z-D(OMe)-E(OMe)-V-D(OMe)-CH(2)F. Limited caspase 3 and 10 activation occurred during infection with the E1B 19K(-) virus Ad2 pm1722 but little or no activation of caspase 3 was observed during wt virus infection. Examination of protein cleavage during viral infection of A549 cells showed proteolysis of lamin B and PARP in response to Ad5 wt and Ad2 pm1722. Protein degradation in response to both viruses was partially inhibited by Z-VAD-FMK. Following infection of human skin fibroblasts lamin B was degraded, although only limited changes in PARP levels were observed. We have concluded that Ad E1A is cleaved by caspases during apoptosis but not during viral infection. However, some of the processes commonly associated with apoptosis occur during viral infection, particularly with E1B 19K(-) mutants, although apoptosis per se is not evident.

Upregulation of the Golgi protein GP73 by adenovirus infection requires the E1A CtBP interaction domain

GP73 is a novel type II Golgi transmembrane protein that is expressed at high levels in the hepatocytes of patients with viral hepatitis (R. D. Kladney, G. A. Bulla, L. Guo, A. L. Mason, A. E. Tollefson, D. J. Simon, Z. Koutoubi, and C. J. Fimmel, 2000, Gene 249, 53-65) and is induced in cultured cells by infection with viruses including adenoviruses. Its biological function and the mechanisms by which its expression may be regulated by viral infection are unknown. Here we report that GP73 is induced at the RNA and protein level in human Hep3B hepatoma cells infected by human Ad5 and Ad2. Hep3B cells were infected with wild-type or mutant adenoviruses. GP73 expression was measured by RNase protection assay, immunoblotting, or immunofluorescence microscopy. GP73 RNA and protein levels were strikingly induced following infection. The rise in GP73 expression coincided with the appearance of the adenovirus E1A and DBP proteins and preceded the expression of the fiber protein, a marker of the late phase of infection. Infection did not affect the expression of giantin, GPP130, or golgin-84, three integral Golgi membrane proteins with structural similarities to GP73. Mapping studies using a panel of mutant adenoviruses demonstrated that the E1A C-terminus, specifically its CtBP interaction domain (CID), is required for GP73 expression. Subsequently, Hep3B cells were transiently transfected with plasmids expressing wild-type or mutant E1A proteins. These studies confirmed that E1A induced GP73 expression via the CID. Our studies establish GP73 as a novel adenovirus-induced cellular protein whose expression is regulated through the CID of the E1A protein.

Adenoviruses in oncology: a viable option?

The feasibility of using adenoviruses for gene therapy has been under close scrutiny recently, as it has become clear that significant toxicity can result from the strong immune response created by intravenous administration of large doses of first generation adenovirus vectors. This suggests that other vectors could be more useful for treatment of metabolic and hereditary disease, where widespread transduction is often necessary for effective gene replacement, and the viability of target cells is important. However, promising recent results in human cancer trials have confirmed that adenoviruses can be very useful in oncology. For cancer treatment, the unparalleled transduction efficacy of adenovirus in dividing and dormant cells is a major benefit. As the goal in cancer gene therapy is to kill infected tumour cells, long-term transgene expression is not necessary. In addition, the immune response generated against infected cells could be useful for eradicating uninfected tumour. Importantly, more than 670 cancer patients have been treated with adenovirus intratumorally, intra-arterially, intraperitoneally and intravenously with very manageable adverse effects and no unexpected severe or lethal toxicity. Currently, the most promising approaches are based on replication-competent agents that allow efficient tumour penetration because of their capacity for tissue-specific replication. In addition to transcriptional control, it is becoming clear that targeting is necessary for efficient tumour transduction and less infection of normal tissues. Exciting results are anticipated when the first selectively replicating targeted adenoviruses go to clinical trials. In conclusion, intense gene therapy and virological research have suggested that while other vectors could be more useful for treatment of hereditary disease, adenoviruses are highly promising and safe agents for oncology, as suggested in a number of early phase clinical trials.

Adenovirus replication-competent vectors (KD1, KD3) complement the cytotoxicity and transgene expression from replication-defective vectors (Ad-GFP, Ad-Luc)

The successful clinical application of adenovirus (Ad) in cancer control has been of limited success because of the current inability to infect the majority of cancer cells with a large amount of vector. In this study, we show that when human lung tumors growing in immunodeficient nude mice were coinfected with a replication-defective (RD) Ad vector expressing green fluorescent protein and a replication-competent (RC) Ad vector named KD3, KD3 enhanced the expression of green fluorescent protein throughout the tumor. Also, KD3 and another RC vector named KD1 complemented the expression of luciferase from a RD vector in a human liver tumor xenotransplant in nude mice. Altogether, these results suggest that the combination of a RD vector with a RC vector might be a more effective treatment for cancer than either vector alone due to more widespread dissemination of the virus.

Adenovirus-5 E1A: paradox and paradigm

The adenovirus early region 1A (E1A) proteins were described originally as immortalizing oncoproteins that altered transcription in rodent cells. Surprisingly, the 243-amino-acid form of adenovirus-5 E1A was found subsequently to reverse-transform many human tumour cells. Tumour suppression apparently results from the ability of E1A to re-programme transcription in tumour cells, and the molecular basis of this intriguing effect is now beginning to emerge. These discoveries have provided a tool with which to study the regulation of fundamental cellular processes.

Selectively replicating adenoviruses targeting deregulated E2F activity are potent, systemic antitumor agents

We have engineered a human adenovirus, ONYX-411, that selectively replicates in human tumor cells, but not normal cells, depending upon the status of their retinoblastoma tumor suppressor protein (pRB) pathway. Early and late viral gene expression as well as DNA replication were significantly reduced in a functional pRB-pathway-dependent manner, resulting in a restricted replication profile similar to that of nonreplicating adenoviruses in normal cells both in vitro and in vivo. In contrast, the viral life cycle and tumor cell killing activity of ONYX-411 was comparable to that of wild-type adenovirus following infection of human tumor cells in vitro as well as after systemic administration in tumor-bearing animals.

Adenovirus E1A N-terminal amino acid sequence requirements for repression of transcription in vitro and in vivo correlate with those required for E1A interference with TBP-TATA complex formation

The adenovirus (Ad) E1A 243R oncoprotein encodes an N-terminal transcription repression domain that is essential for early viral functions, cell immortalization, and cell transformation. The transcription repression function requires sequences within amino acids 1 to 30 and 48 to 60. To elucidate the roles of the TATA-binding protein (TBP), p300, and the CREB-binding protein (CBP) in the mechanism(s) of E1A repression, we have constructed 29 amino acid substitution mutants and 5 deletion mutants spanning the first 30 amino acids within the E1A 1-80 polypeptide backbone. These mutant E1A polypeptides were characterized with regard to six parameters: the ability to repress transcription in vitro and in vivo, to disrupt TBP-TATA box interaction, and to bind TBP, p300, and CBP. Two regions within E1A residues 1 to 30, amino acids 2 to 6 and amino acid 20, are critical for E1A transcription repression in vitro and in vivo and for the ability to interfere with TBP-TATA interaction. Replacement of 6Cys with Ala in the first region yields the most defective mutant. Replacement of 20Leu with Ala, but not substitutions in flanking residues, yields a substantially defective phenotype. Protein binding assays demonstrate that replacement of 6Cys with Ala yields a mutant completely defective in interaction with TBP, p300, and CBP. Our findings are consistent with a model in which the E1A repression function involves interaction of E1A with p300/CBP and interference with the formation of a TBP-TATA box complex.

Re-engineering adenovirus regulatory pathways to enhance oncolytic specificity and efficacy

Replicating adenoviruses may prove to be effective anticancer agents if they can be engineered to selectively destroy tumor cells. We have constructed a virus (01/PEME) containing a novel regulatory circuit in which p53-dependent expression of an antagonist of the E2F transcription factor inhibits viral replication in normal cells. In tumor cells, however, the combination of p53 pathway defects and deregulated E2F allows replication of 01/PEME at near wild-type levels. The re-engineered virus also showed significantly enhanced efficacy compared with extensively studied E1b-deleted viruses such as dl1520 in human xenograft tumor models.

Induction of p53-independent apoptosis by simian virus 40 small t antigen

Simian virus 40 small t antigen (st) is required for optimal transformation and replication properties of the virus. We find that in certain cell types, such as the human osteosarcoma cell line U2OS, st is capable of inducing apoptosis, as evidenced by a fragmented nuclear morphology and positive terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling staining of transfected cells. The cell death can be p53 independent, since it also occurs in p53-deficient H1299 cells. Genetic analysis indicates that two specific mutants affect apoptosis induction. One of these (C103S) has been frequently used as a PP2A binding mutant. The second mutant (TR4) lacks the final four amino acids of st, which have been reported to be unimportant for PP2A binding in vitro. However, TR4 unexpectedly fails to bind PP2A in vivo. Furthermore, a long-term colony assay reveals a potent colony inhibition upon st expression, and the behavior of st mutants in this assay reflects the relative frequency of nuclear fragmentation observed in transfections using the same mutants. Notably, either Bcl-2 coexpression or broad caspase inhibitor treatment could restore normal nuclear morphology. Finally, fluorescence-activated cell sorting analysis suggests a correlation between the ability of st to modulate cell cycle progression and apoptosis. Taken together, these observations underscore that st does not always promote proliferation but may, depending on conditions and cell type, effect a cell death response.

Bovine adenovirus type 3 E1B(small) protein is essential for growth in bovine fibroblast cells

In order to study the function of bovine adenovirus type 3 (BAV-3) E1A and E1B(small) proteins, we constructed two mutants: (a) BAV102A carries an in-frame deletion in the coding region for the E1A protein (nt 831-1080); (b) BAV102B carries an insertion of triple stop codons in the E1B region (nt 1654, 178 bp downstream of the E1B(small) start codon), which stops the translation of the E1B(small) gene. BAV102A virus could grow to the wild-type BAV-3 titer in transformed cell line VIDO R2 (HAV-5 E1 transformed) cells, but no progeny virus could be found in fetal bovine retina cells (FBRC). RT-PCR and Western blot analysis showed that neither mRNA transcripts nor protein expression of early genes [E1B(small) and DNA binding protein (DBP)] could be detected in BAV102A infected FBRC. The BAV102B grew 1.5 log less than wild-type BAV-3 in FBRC; however, no BAV102B progeny virus could be observed in bovine fibroblast (BFB) cells. No appreciable difference was observed in DBP transcript synthesis between wild-type BAV-3- or BAV102B-infected FBRC. However, compared to wild-type BAV-3, BAV102B viral DNA synthesis and fiber gene expression were found to be slightly reduced in FBRC. In contrast, compared to wild-type BAV-3, DBP transcripts and viral DNA synthesis were drastically reduced in BAV102B-infected BFB cells. In addition, no fiber gene expression could be detected in BAV102B-infected BFB cells. These results suggest that BAV-3 E1A is essential for virus replication and is required for activating the transcription of other BAV-3 early genes. However, the requirement for E1B(small) protein for BAV-3 replication appears to be cell type-dependent.

Tissue-specific, tumor-selective, replication-competent adenovirus vector for cancer gene therapy

We have previously described two replication-competent adenovirus vectors, named KD1 and KD3, for potential use in cancer gene therapy. KD1 and KD3 have two small deletions in the E1A gene that restrict efficient replication of these vectors to human cancer cell lines. These vectors also have increased capacity to lyse cells and spread from cell to cell because they overexpress the adenovirus death protein, an adenovirus protein required for efficient cell lysis and release of adenovirus from the cell. We now describe a new vector, named KD1-SPB, which is the KD1 vector with the E4 promoter replaced by the promoter for surfactant protein B (SPB). SPB promoter activity is restricted in the adult to type II alveolar epithelial cells and bronchial epithelial cells. Because KD1-SPB has the E1A mutations, it should replicate within and destroy only alveolar and bronchial cancer cells. We show that KD1-SPB replicates, lyses cells, and spreads from cell to cell as well as does KD1 in H441 cells, a human cancer cell line where the SPB promoter is active. KD1-SPB replicates, lyses cells, and spreads only poorly in Hep3B liver cancer cells. Replication was determined by expression of the E4ORF3 protein, viral DNA accumulation, fiber synthesis, and virus yield. Cell lysis and vector spread were measured by lactate dehydrogenase release and a "vector spread" assay. In addition to Hep3B cells, KD1-SPB also did not express E4ORF3 in HT29.14S (colon), HeLa (cervix), KB (nasopharynx), or LNCaP (prostate) cancer cell lines, in which the SPB promoter is not expected to be active. Following injection into H441 or Hep3B tumors growing in nude mice, KD1-SPB caused a three- to fourfold suppression of growth of H441 tumors, similar to that seen with KD1. KD1-SPB had only a minimal effect on the growth of Hep3B tumors, whereas KD1 again caused a three- to fourfold suppression. These results establish that the adenovirus E4 promoter can be replaced by a tissue-specific promoter in a replication-competent vector. The vector has three engineered safety features: the tissue-specific promoter, the mutations in E1A that preclude efficient replication in nondividing cells, and a deletion of the E3 genes which shield the virus from attack by the immune system. KD1-SPB may have use in treating human lung cancers in which the SPB promoter is active.

Plant orthologs of p300/CBP: conservation of a core domain in metazoan p300/CBP acetyltransferase-related proteins

p300 and CBP participate as transcriptional coregulators in the execution of a wide spectrum of cellular gene expression programs controlling cell differentiation, growth and homeostasis. Both proteins act together with sequence-specific transcription factors to modify chromatin structure of target genes via their intrinsic acetyltransferase activity directed towards core histones and some transcription factors. So far, p300-related proteins have been described in animals ranging from Drosophila and Caenorhabditis elegans to humans. In this report, we describe p300/CBP-like polypeptides in the plant Arabidopsis thaliana. Interestingly, homology between animal and plant p300/CBP is largely restricted to a C-terminal segment, about 600 amino acids in length, which encompasses acetyltransferase and E1A-binding domains. We have examined whether this conservation in sequence is paralleled by a conservation in function. The same amino acid residues critical for acetyltransferase activity in human p300 are also critical for the function of one of the plant orthologs. Remarkably, plant proteins bind to the adenovirus E1A protein in a manner recapitulating the binding specificity of mammalian p300/CBP. The striking conservation of an extended segment of p300/CBP suggests that it may constitute a functional entity fulfilling functions that may be essential for all metazoan organisms.

Identification of three functions of the adenovirus e4orf6 protein that mediate p53 degradation by the E4orf6-E1B55K complex

Complexes containing adenovirus E4orf6 and E1B55K proteins play critical roles in productive infection. Both proteins interact directly with the cellular tumor suppressor p53, and in combination they promote its rapid degradation. To examine the mechanism of this process, degradation of exogenously expressed p53 was analyzed in p53-null human cells infected with adenovirus vectors encoding E4orf6 and/or E1B55K. Coexpression of E4orf6 and E1B55K greatly reduced both the level and the half-life of wild-type p53. No effect was observed with the p53-related p73 proteins, which did not appear to interact with E4orf6 or E1B55K. Mutant forms of p53 were not degraded if they could not efficiently bind E1B55K, suggesting that direct interaction between p53 and E1B55K may be required. Degradation of p53 was independent of both MDM2 and p19ARF, regulators of p53 stability in mammalian cells, but required an extended region of E4orf6 from residues 44 to 274, which appeared to possess three separate biological functions. First, residues 39 to 107 were necessary to interact with E1B55K. Second, an overlapping region from about residues 44 to 218 corresponded to the ability of E4orf6 to form complexes with cellular proteins of 19 and 14 kDa. Third, the nuclear retention signal/amphipathic arginine-rich alpha-helical region from residues 239 to 253 was required. Interestingly, neither the E4orf6 nuclear localization signal nor the nuclear export signal was essential. These results suggested that if nuclear-cytoplasmic shuttling is involved in this process, it must involve another export signal. Degradation was significantly blocked by the 26S proteasome inhibitor MG132, but unlike the HPV E6 protein, E4orf6 and E1B55K were unable to induce p53 degradation in vitro in reticulocyte lysates. Thus, this study implies that the E4orf6-E1B55K complex may direct p53 for degradation by a novel mechanism.

Evaluation of E1-mutant adenoviruses as conditionally replicating agents for cancer therapy

The oncolytic effect of adenoviruses may provide an efficient means to destroy tumor tissue if viruses could be developed with sufficient selectivity and efficacy. In this report we have characterized several adenoviruses, each with different mutations in the E1 region, for selective cytopathic effect in tumor cells in vitro and for their ability to inhibit tumor growth in vivo. Of the E1 mutants tested, we have identified one, E1Adl01/07, which preferentially induces cytopathic effects in a range of tumor cells versus primary cells. In addition, E1Adl01/07 significantly inhibited tumor growth and increased survival of mice in several models of human cancer. These results suggest that E1Adl01/07 might serve as an effective cancer therapeutic, combining both selectivity and efficacy.

Aryl hydrocarbon receptor is required for p300-mediated induction of DNA synthesis by adenovirus E1A

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the biological responses to environmental contaminants such as 2,3,7, 8-tetrachlorodibenzo-p-dioxin. Embryonic fibroblast (EF) isolated from AHR-null mice exhibited slow cell growth compared with wild-type EF. Reintroduction of AHR into AHR-null EF increased cell growth, suggesting that AHR is involved in cell cycle control. The role of the AHR in cell cycle control was examined using the adenovirus oncoprotein E1A. EF, derived from wild-type and AHR-null mice, were transfected with two mutant E1A expression plasmids that inactivate either p300/CBP or retinoblastoma protein (pRb). Although DNA synthesis of wild-type EF was induced by both E1A mutants, DNA synthesis in the AHR-null EF was induced only by the mutant that binds pRb, not by the mutant to p300/CBP. These data show that both pRb and p300/CBP were the target of E1A-induced DNA synthesis in wild-type EF. In AHR-null mice, however, only pRb was the target of E1A-induced DNA synthesis and p300/CBP cannot be inactivated by E1A in the absence of AHR. Immunoprecipitation revealed that AHR directly bound to p300, thus suggesting the intriguing possibility that AHR is involved in control of the cell cycle via interaction with p300.

Tumor-specific, replication-competent adenovirus vectors overexpressing the adenovirus death protein

We have constructed two novel adenovirus (Ad) replication-competent vectors, named KD1 and KD3, that may have use in anticancer therapy. The vectors have two key features. First, they markedly overexpress the Ad death protein (ADP), an Ad nuclear membrane glycoprotein required at late stages of infection for efficient cell lysis and release of Ad from cells. Overexpression of ADP was achieved by deleting the E3 region and reinserting the adp gene. Because ADP is overexpressed, KD1 and KD3 are expected to spread more rapidly and effectively through tumors. Second, KD1 and KD3 have two E1A mutations (from the mutant dl1101/1107) that prevent efficient replication in nondividing cells but allow replication in dividing cancer cells. These E1A mutations preclude binding of E1A proteins to p300 and pRB. As a result, the virus should not be able to drive cells from G(0) to S phase and therefore should not be able to replicate in normal tissues. We show that KD1 and KD3 do not replicate well in quiescent HEL-299 cells or in primary human bronchial epithelial cells, small airway epithelial cells, or endothelial cells; however, they replicate well in proliferating HEL-299 cells and human A549 lung carcinoma cells. In cultured A549 cells, KD1 and KD3 lyse cells and spread from cell to cell more rapidly than their control virus, dl1101/1107, or wild-type Ad. They are also more efficient than dl1101/1107 or wild-type Ad in complementing the spread from cell to cell of an E1(-) E3(-) replication-defective vector expressing beta-galactosidase. A549 cells form rapidly growing solid tumors when injected into the hind flanks of immunodeficient nude mice; however, when A549 cells were infected with 10(-4) PFU of KD3/cell prior to injection into mice, tumor formation was nearly completely suppressed. When established A549 tumors in nude mice were examined, tumors injected with buffer grew 13.3-fold over 5 weeks, tumors injected with dl1101/1107 grew 8-fold, and tumors injected with KD1 or KD3 grew 2.6-fold. Hep 3B tumors injected with buffer grew 12-fold over 3.5 weeks, whereas tumors injected with KD1 or KD3 grew 4-fold. We conclude that KD1 and KD3 show promise as anticancer therapeutics.

Acetyltransferases and tumour suppression

The acetyltransferase p300 was first identified associated with the adenoviral transforming protein E1A, suggesting a potential role for p300 in the regulation of cell proliferation. Direct evidence demonstrating a role for p300 in human tumours was lacking until the recently publication by Gayther et al, which strongly supports a role for p300 as a tumour suppressor. The authors identify truncating mutations associated with the loss or mutation of the second allele in both tumour samples and cell lines, suggesting that loss of p300 may play a role in the development of a subset of human cancers.

Induction of transformation and p53-dependent apoptosis by adenovirus type 5 E4orf6/7 cDNA

Adenovirus (Ad) E4orf6/7, one of the early gene products of human Ads, forms a stable complex with the cellular transcription factor E2F to activate transcription from the Ad E2 promoter. E2F cDNAs have growth-promoting and apoptosis-inducing activities when overexpressed in cells. We cloned Ad5 E4orf6/7 cDNA in both simian virus 40- and human cytomegalovirus-based expression vectors to examine its transforming and apoptotic activities. The cloned E4orf6/7 collaborated with a retinoblastoma protein (RB)-nonbinding and therefore E2F-nonreleasing mutant of Ad5 E1A (dl922/947) to morphologically transform primary rat cells, suggesting that E2F is an important cellular protein functioning downstream of E1A for transformation. In a G418 colony formation assay, E4orf6/7 was shown to suppress growth of untransformed rat cells. Moreover, a recombinant Ad expressing Ad5 E4orf6/7 induced apoptosis in rat cells when coinfected with wild-type p53-expressing Ad. Mutational analysis of E4orf6/7 revealed that both of the domains required for growth inhibition and transformation by E4orf6/7 lay in the C-terminal region, which is essential for transactivation from the upstream sequence of an E2a promoter containing E2F-binding sites. However, the smallest mutant of E4orf6/7, encoding the C-terminal 59 amino acids, failed to complement the RB-nonbinding dl922/947 mutant despite showing growth inhibition and E2F transactivation activities. Thus, it is suggested that a subregion of E4orf6/7 which is required for growth inhibition and transformation in collaboration with dl922/947 overlaps the transactivation domain of E4orf6/7.

Transactivation of the human cdc2 promoter by adenovirus E1A. E1A induces the expression and assembly of a heteromeric complex consisting of the CCAAT box binding factor, CBF/NF-Y, and a 110-kDa DNA-binding protein

Cyclin-dependent kinases (CDKs) play an important role in the eukaryotic cell cycle progression. Cdc2 (CDK1) is expressed in late G(1)/S phase and required for G(2) to M phase transition in higher eukaryotes. The oncoproteins, SV40 large T antigen and adenovirus E1A, induce a 110-kDa protein which specifically recognizes the two inverted CCAAT motifs of the cdc2 promoter in cycling cells and plays an essential role in transactivation of the human cdc2 promoter. Since these CCAAT motifs also conform to the consensus binding sites for the ubiquitous heterotrimeric transcription factor, CBF/NF-Y, the role of CBF/NF-Y in the transactivation of the cdc2 promoter was examined in this study. Our results indicate that CBF/NF-Y and the 110-kDa protein interact with the CCAAT box motif to form a heteromeric complex. However, mutagenesis of the pentanucleotide CCAAT motif or in the presence of urea greater than 2.5 M, no heteromeric complex was formed. In contrast, the 110-kDa protein could still bind the mutant CCAAT motif or with the wild type motif in the presence of 2.5 M urea. Furthermore, E1A.12S induced the gene expression of all three subunits of CBF/NF-Y. Coexpression of E1A and a dominant negative mutant NF-YA subunit significantly reduced the E1A-mediated transactivation of the cdc2 promoter in a dose-dependent manner. These results support the conclusion that E1A protein mediates optimal transactivation of the human cdc2 promoter by inducing the expression and assembly of a heteromeric complex consisting of the 110-kDa protein and the CBF/NF-Y which interacts with the two CCAAT motifs of the cdc2 promoter.

The mitogen-activated protein kinase pathway mediates growth arrest or E1A-dependent apoptosis in SKBR3 human breast cancer cells

Previously, we have shown that phorbol ester (PMA) induces p21(WAF1/CIP1)-dependent growth arrest in SKBr3 breast cancer and LNCaP prostate cancer cells. Here, I demonstrate that inhibition of Raf-1 kinase by dominant-negative Raf-1 or pharmacological depletion of Raf-1 prevented PMA-mediated induction of p21(WAF1/CIP1). Similarly, PD98059, a specific inhibitor of MEK, abolished p21(WAF1/CIP1) induction and PMA-induced growth arrest. Like PMA, the H-ras oncogene, another activator of the Raf-1/MEK/MAPK pathway, transactivated p21(WAF1/CIP1) in SKBr3 cells. I further investigated PMA-induced growth arrest following infection of SKBr3 cells with 12S E1A-expressing adenovirus. Although high levels of E1A oncoprotein prevented both PMA-induced p21(WAF1/CIP1) and growth arrest, smaller amounts of E1A abrogated growth arrest without down-regulation of p21(WAF1/CIP1). Therefore, E1A can stimulate proliferation downstream of p21(WAF1/CIP1). Albeit less effective than full activity, either Rb- or p300-binding activity of E1A was sufficient for the abrogation of PMA-mediated growth arrest. E1A-driven proliferation of PMA-treated SKBr3 cells was accompanied by apoptosis. New therapeutic approaches can be envisioned that would utilize stimulation of the Raf-1/MEK/MAPK pathway to inhibit growth of PMA-sensitive cancer cells.

A novel function of adenovirus E1A is required to overcome growth arrest by the CDK2 inhibitor p27(Kip1)

We show here that the adenovirus E1A oncoprotein prevents growth arrest by the CDK2 inhibitor p27(Kip1) (p27) in rodent fibroblasts. However, E1A neither binds p27 nor prevents inhibition of CDK2 complexes in vivo. In contrast, the amount of free p27 available to inhibit cyclin E/CDK2 is increased in E1A-expressing cells, owing to reduced expression of cyclins D1 and D3. Moreover, E1A allows cell proliferation in the presence of supraphysiological p27 levels, while c-Myc, known to induce a cellular p27-inhibitory activity, is only effective against physiological p27 concentrations. E1A also bypasses G1 arrest by roscovitine, a chemical inhibitor of CDK2. Altogether, these findings imply that E1A can act downstream of p27 and CDK2. Retinoblastoma (pRb)-family proteins are known CDK substrates; as expected, association of E1A with these proteins (but not with p300/CBP) is required for E1A to prevent growth arrest by either p27 or the CDK4/6 inhibitor p16(INK4a). Bypassing CDK2 inhibition requires an additional function of E1A: the mutant E1A Delta26-35 does not overcome p27-induced arrest, while it binds pRb-family proteins, prevents p16-induced arrest, and alleviates pRb-mediated repression of E2F-1 transcriptional activity (although E1A Delta26-35 fails to restore expression of E2F-regulated genes in p27-arrested cells). We propose that besides the pRb family, E1A targets specific effector(s) of CDK2 in G1-S control.

Genetic analysis of adenovirus E1A: induction of genetic instability and altered cell morphologic and growth characteristics are segregatable functions

Single multifunctional oncoproteins contribute to genomic instability development, but relationships between one or more oncoprotein-associated activities and genetic changes accompanying tumor cell progression are uncertain. Using NIH 3T3 derivative EN/NIH 2-20 containing transcriptionally silent neomycin phosphotransferase gene (neo) integrants with undetectable spontaneous reactivations, we studied wild-type (WT) and mutant adenovirus E1A-induced neo reactivation by neo-allelic rearrangement. WT E1A expression, yielding differential splice transcripts 12S and 13S and resulting in altered cell morphologic and growth characteristics, produced neo reactivations in 9 of 21 subclones (median rate per cell, 35 x 10(-6); range, 0.33 x 10(-6) to 936 x 10(-6)). Only 3 of 17 cell lines expressing CTdl976, a '12S' functional equivalent inducing altered cell morphologic and growth characteristics while lacking the 13S trans activation domain, yielded neo reactivations (range, 0.33 x 10(-6) to 0.67 x 10(-6)). One of 21 subclones expressing NTdl646, an E1A mutant retaining the trans domain but lacking p300 binding activity and the ability to alter cell morphologic and growth characteristics, produced neo reactivations (8.7 x 10(-6)). Other E1A mutants, all lacking the ability to alter cell morphologic and growth characteristics while binding pRb but variously lacking the trans domain and binding for p107 and/or p300, displayed undetectable neo-reactivations. 98 EN/NIH 2-20 derivatives coexpressing complementary mutant E1As exhibited altered morphologic and growth features, but only 10 of these produced neo reactivations, and maximum rates (14 x 10(-6)) were substantially lower than those in comparably derived, morphologically altered E1AWT-expressing counterparts (497 x 10(-6)). These findings suggest that maximum rates of gene reactivations by genomic rearrangement require the collective activities of functional domains assembled in single multifunctional proteins (or complexes) while altered cell morphologic and growth characteristics may arise through comparable sets of functional domains distributed across more than one protein (or complex).

The early region 4 orf4 protein of human adenovirus type 5 induces p53-independent cell death by apoptosis

Previous studies by our group showed that infection of human and rodent cells by human adenovirus type 5 (Ad5) results in the induction of p53-independent apoptosis and cell death that are dependent upon transactivation of early region 4 (E4). To identify which E4 products are involved, studies were conducted with p53-deficient human SAOS-2 cells infected with various Ad5 E4 mutants. An E4orf6-deficient mutant was defective in cell killing, whereas another that expressed only E4orf6 and E4orf4 killed like wild-type virus, suggesting that E4orf6 may be responsible for cytotoxicity; however, a mutant expressing only E4orf4 induced high levels of cell death, indicating that this E4 product may also be able to induce cytotoxicity. To define the E4 cell death-inducing functions more precisely, cDNAs encoding individual E4 products were introduced into cells by DNA transfection in the absence of other Ad5 proteins. In cotransfections with a cDNA encoding firefly luciferase, enzymatic activity was high in all cases except with E4orf4, where luciferase levels were less than 20% of those in controls. In addition, drug selection of several cell types following transfection with retroviral vector DNA encoding individual E4 products as well as puromycin resistance yielded a large number of cell colonies except when E4orf4 was expressed. These data demonstrated that E4orf4 is the only E4 product capable of independent cell killing. Cell death induced by E4orf4 was due to apoptosis, as evidenced by 4',6-diamidino-2-phenylindole (DAPI) staining of cell nuclei in E4orf4-expressing cells. Thus, although E4orf6 may play some role, these results suggested that E4orf4 may be the major E4 product responsible for induction of p53-independent apoptosis.

p300/CREB binding protein-related protein p270 is a component of mammalian SWI/SNF complexes

p300 and the closely related CREB binding protein (CBP) are transcriptional adaptors that are present in intracellular complexes with TATA binding protein (TBP) and bind to upstream activators including p53 and nuclear hormone receptors. They have intrinsic and associated histone acetyltransferase activity, suggesting that chromatin modification is an essential part of their role in regulating transcription. Detailed characterization of a panel of antibodies raised against p300/CBP has revealed the existence of a 270-kDa cellular protein, p270, distinct from p300 and CBP but sharing at least two independent epitopes with p300. The subset of p300/CBP-derived antibodies that cross-reacts with p270 consistently coprecipitates a series a cellular proteins with relative molecular masses ranging from 44 to 190 kDa. Purification and analysis of various proteins in this group reveals that they are components of the human SWI/SNF complex and that p270 is an integral member of this complex.

Human cells arrest in S phase in response to adenovirus 12 E1A

It has previously been shown that following viral infection, Ad5 E1A induces cell cycle progression of quiescent rodent cells, leading to DNA synthesis and mitosis. Here we have examined the effect of Ad12 E1A on the cell cycle characteristics of human cells. Human tumor (A549, KB, and HeLa) cells were infected with Ad12 d/620, a mutant virus which has a lesion in the E1B gene and essentially expresses only E1A. These infected cells progressed from being largely in G1 into S phase, where they arrested. Even up to 96 h postinfection (p.i.) the cells remained blocked in S phase. DNA synthesis did, however, proceed in Ad12 d/620-infected cells, giving rise to multiple copies of cellular DNA. Similar results were obtained when primary human skin fibroblasts were infected, although the polyploidy was less marked. The expression of cyclins A, B1, and E in the tumor cells increased appreciably in response to E1A. In contrast, there was a dramatic reduction in the levels of cyclin D1 and D3. Increases in cyclin D1 expression could be detected at very late times p.i. In those cell lines expressing low levels of cdc2 and cdk2 an appreciable increase in expression was seen soon after Ad12 E1A could be detected. The elevated levels of cyclins A, B1, and E were associated with increased protein kinase activity directed against histone H1. An increase in cyclin D1-associated kinase activity against Rb1 was also observed at late times. This deregulation of the cell cycle was not solely dependent on E1A inactivation of Rb, since similar effects were seen in Ad12 d/620-infected retinoblastoma (Y-79) cells, implicating p107 and p130 in E1A-mediated changes in cell cycle progression. We propose that the E1A-induced levels of cyclins A, B1, and E by Ad12 E1A in human cells may lead to an uncoupling of S phase from cell cycle progression.

Conjunction dysfunction: CBP/p300 in human disease

CBP and its homolog p300 are large nuclear molecules that coordinate a variety of transcriptional pathways with chromatin remodeling. They interact with transcriptional activators as well as repressors, direct chromatin-mediated transcription, function in TP53-mediated apoptosis, and participate in terminal differentiation of certain tissue types. Recent evidence suggests that the demand for CBP/p300 is greater than the supply, and that competition for CBP/p300 might play an important role in cell growth regulation. Alterations of the human CBP gene have been implicated in hematological malignancies as well as in congenital malformation and mental retardation. Likewise, the p300 gene has been recently implicated in leukemia and mutations in both alleles have been observed in gastric and colorectal carcinomas. The role of these proteins in human disease coupled with biochemical evidence suggests that CBP and p300 are tumor suppressor proteins essential in cell-cycle control, cellular differentiation and human development.

Gene dosage-dependent embryonic development and proliferation defects in mice lacking the transcriptional integrator p300

The transcriptional coactivator and integrator p300 and its closely related family member CBP mediate multiple, signal-dependent transcriptional events. We have generated mice lacking a functional p300 gene. Animals nullizygous for p300 died between days 9 and 11.5 of gestation, exhibiting defects in neurulation, cell proliferation, and heart development. Cells derived from p300-deficient embryos displayed specific transcriptional defects and proliferated poorly. Surprisingly, p300 heterozygotes also manifested considerable embryonic lethality. Moreover, double heterozygosity for p300 and cbp was invariably associated with embryonic death. Thus, mouse development is exquisitely sensitive to the overall gene dosage of p300 and cbp. Our results provide genetic evidence that a coactivator endowed with histone acetyltransferase activity is essential for mammalian cell proliferation and development.

Two regions of simian virus 40 large T-antigen independently extend the life span of primary C57BL/6 mouse embryo fibroblasts and cooperate in immortalization

Expression of the SV40 large T-antigen allows primary cells to escape senescence and thereby become immortalized. Immortalization occurs in two steps, extension of life span and acquisition of unlimited cell division potential. By following the increase in expression of a senescence-associated marker with increased cell passage, we show that C57Bl/6 mouse embryo fibroblast (B6MEF) cultures senesce by passage 4. Thus, the development of colonies from cultures transfected with T-antigen expressing constructs indicates extension of life span. Two T-antigen regions independently extended the life span of B6MEF. Expression of either a T-antigen consisting of amino acids 1-147 (T1-147) or a T-antigen consisting of amino acids 251-708 (T251-708) resulted in colony development. However, the colonies expressing these truncated T-antigens could not be expanded into cell lines efficiently. In contrast, coexpression of T1-147 and T251-708 produced colonies that could be expanded into cell lines as efficiently as could colonies expressing full-length T-antigen. Thus, the two regions of T-antigen contain analogous activities that are sufficient to extend cell life span; they cooperate to immortalize primary B6MEF; and they act in trans, indicating that the functions involved are independent.

Expression of the pRb-binding regions of E1A enables efficient transformation of primary epithelial cells by v-src

Primary cultures of rat embryo fibroblasts have been shown to be resistant to transformation by dominant oncogenes such as v-src. We sought to determine if similar resistance is displayed by primary epithelial cells, and, if so, whether an immortalizing oncogene such as E1A could enhance transformation of primary epithelial cells by v-src. Transformation of primary rat epithelial cells by v-src was synergistically enhanced when E1A expression plasmids were cotransfected with a v-src expression plasmid. Foci were more numerous and observed earlier (9 to 14 days) with E1A plus v-src than with v-src alone (18 to 28 days). This cotransformation ability was abrogated by deletions in CR1 or CR2 of E1A, which encode the binding regions for the pRb family and are responsible for E1A-mediated cell cycle activation. Mutations in the p300 binding site or the second exon, which abolish immortalization, did not affect v-src cooperation, in contrast to ras and adenovirus E1B. While kinase activation was required for growth in soft agar, differential activation of Src kinase did not correlate with transformation efficiency. Cell morphology and actin structures were not dramatically impacted by E1A expression; thus, hypertransformation, as previously described for ras cotransformation, was not observed with v-src and second-exon mutants of E1A. However, growth rates for cells expressing both E1A and v-Src were higher than those for cells expressing only v-Src. These results suggest that functions involved in cell cycle activation encoded by E1A first exon can enhance v-src transformation of primary epithelial cells.

Identification of specific amino acid residues of adenovirus 12 E1A involved in transformation and p300 binding

The early region 1A (E1A) gene of highly oncogenic adenovirus type 12 (Ad12) was analyzed for transforming activity and protein binding using specific mutations. The Ad12 E1A proteins were found to bind p300 protein mainly within the CR1 region, although mutations that affect both p300 binding and transformation were identified in both the CR1 and the N-terminal region. The most critical mutation dlf89 located in the CR1 region was further dissected by point mutations and the results identified 68S as the most critical for transformation and 67E as the most critical for p300 binding. Specific mutations that retain p300 binding but impair transcriptional repression of a viral enhancer were also identified in both the N-terminal and CR1 regions.

Adenovirus E1A inhibits cardiac myocyte-specific gene expression through its amino terminus

Adenovirus E1A oncoproteins inhibit muscle-specific gene expression and myogenic differentiation by suppressing the transcriptional activating functions of basic helix-loop-helix proteins. As one approach to identifying cardiac-specific gene regulatory proteins, we analyzed the functional regions of E1A proteins that are required for muscle gene repression in cardiac cells. Myocyte-specific promoters, including the alpha-actins and alpha-myosin heavy chain, were selectively and potently inhibited (>90%) by E1A, while the ubiquitously expressed beta-actin promoter was only partially ( approximately 30%) repressed; endogenous gene expression was also affected. Distinct E1A protein binding sites mediated repression of muscle-specific and ubiquitous actin promoters. E1A-mediated inhibition of beta-actin required both an intact binding site for the tumor repressor proteins pRb and p107 and a second E1A domain (residues 15-35). In contrast, cardiac-specific promoter repression required the E1A amino-terminal residues 2-36. The proximal skeletal actin promoter (3' to base pair -153) was a target for repression by E1A. Although E1A binding to p300 was not required for inhibition of either promoter, co-expression of p300 partially reversed E1A-mediated transcriptional repression. We conclude that cardiac-specific and general promoter inhibition by E1A occurs by distinct mechanisms and that cardiac-specific gene expression is modulated by cellular factors interacting with the E1A p300/CBP-binding domain.

Transcriptional coactivator p300 stimulates cell type-specific gene expression in cardiac myocytes

Terminal differentiation is characterized by cell cycle arrest and the expression of cell type-specific genes. Previous work has suggested that the p300 family of transcriptional coactivators plays an important role in preventing the re-initiation of DNA synthesis in terminally differentiated cardiac myocytes. In this study, we investigated whether p300 proteins are also involved in the transcriptional activation of cell type-specific genes in these cells. Since p300 function can be abrogated through direct binding by the adenovirus E1A protein, we overexpressed E1A in cardiac myocytes using recombinant adenoviral vectors. The expression of transfected reporter genes driven by alpha- or beta-myosin heavy chain promoters was markedly diminished by expression of the 12 S E1A protein. In contrast, the activity of a promoter derived from the ubiquitously expressed beta-actin gene was affected only modestly. While an E1A mutant unable to bind members of the retinoblastoma family of pocket proteins decreased the activity of alpha- and beta-myosin heavy chain promoters to nearly the same extent as wild type 12 S E1A, transcriptional repression by a mutant defective for p300 binding was severely impaired. Furthermore, overexpression of p300 and, to an even greater extent, p300del33, a mutant lacking residues required for binding by E1A, relieved E1A's repression of beta-myosin heavy chain promoter activity while having no effect on the activity of the beta-actin promoter. Thus, E1A's transcriptional repression of cell type-specific genes in cardiac myocytes is mediated through its binding of p300 proteins, and these proteins appear to be involved in maintaining both cell type-specific gene expression and cell cycle arrest in cardiac myocytes.

Monoclonal antibody NM11 recognizes a C-terminal epitope shared by p300 and CBP

The epitope recognized by the monoclonal antibody NM11, previously shown to recognize both CBP and p300, has been mapped here to the C-terminal third of p300 and CBP by Western analysis of p300 and CBP prokaryotic fusion proteins. More precise epitope mapping, carried out by screening a plasmid expression library derived from small randomly generated CBP cDNA fragments localizes the NM11 epitope to a 21 amino acid stretch spanning amino acids 2071-2091 near the CBP C-terminus. CBP and p300 differ by three noncontiguous residues within this 21 amino acid region, a difference that does not detectably affect the reactivity of NM11.

Accumulation of p53 induced by the adenovirus E1A protein requires regions involved in the stimulation of DNA synthesis

It has been known for some time that expression of the 243-residue (243R) human adenovirus type 5 (Ad5) early region 1A (E1A) protein causes an increase in the level of the cellular tumor suppressor p53 and induction of p53-dependent apoptosis. Deletion of a portion of conserved region 1 (CR1) had been shown to prevent apoptosis, suggesting that binding of p300 and/or the pRB retinoblastoma tumor suppressor and related proteins might be implicated. To examine the mechanism of the E1A-induced accumulation of p53, cells were infected with viruses expressing E1A-243R containing various deletions which have well-characterized effects on p300 and pRB binding. It was found that in human HeLa cells and rodent cells, complex formation with p300 but not pRB was required for the rise in p53 levels. However, in other human cell lines, including MRC-5 cells, E1A proteins which were able to form complexes with either p300 or pRB induced a significant increase in p53 levels. Only E1A mutants defective in binding both classes of proteins were unable to stimulate p53 accumulation. This same pattern was also apparent in p53-null mouse cells coinfected by Ad5 mutants and an adenovirus vector expressing either wild-type or mutant human p53 under a cytomegalovirus promoter, indicating that the difference in importance of pRB binding may relate to differences between rodent and human p53 expression. The increase in p53 levels correlated well with the induction of apoptosis and, as shown previously, with the stimulation of cellular DNA synthesis. Thus, it is possible that the accumulation of p53 is induced by the induction of unscheduled DNA synthesis by E1A proteins and that increased levels of p53 then activate cell death pathways.

Regulation of p53 levels by the E1B 55-kilodalton protein and E4orf6 in adenovirus-infected cells

The adenovirus type 5 243R E1A protein induces p53-dependent apoptosis in the absence of the 19- and 55-kDa E1B polypeptides. This effect appears to result from an accumulation of p53 protein and is unrelated to expression of E1B products. We now report that in the presence of the E1B 55-kDa polypeptide, the 289R E1A protein does not induce such p53 accumulation and, in fact, is able to block that induced by E1A 243R. This inhibition also requires the 289R-dependent transactivation of E4orf6 expression. E4orf6 is known to form complexes with the E1B 55-kDa protein and to function both in the transport and stabilization of viral mRNA and in shutoff of host cell protein synthesis. We demonstrated that the block in p53 accumulation is not due to the generalized shutoff of host cell metabolism. Rather, it appears to result from a mechanism targeted specifically to p53, most likely involving a decrease in the stability of p53 protein. The E1B 55-kDa protein is known to interact with both E4orf6 and p53, and as demonstrated recently by others, we showed that E4orf6 also binds directly to p53. Thus, multiple interactions between all three proteins may regulate p53 stability, resulting in the maintenance of low levels of p53 following virus infection.