Induction of the cell cycle in baby rat kidney cells by adenovirus type 5 E1A in the absence of E1B and a possible influence of p53

The biology of the adenovirus E1B 55K protein

The adenovirus E1B 55K (E1B) protein plays major roles in productive adenoviral infection and cellular transformation. Interest in E1B increased because of the potential of adenoviruses as therapeutic vectors, and the E1B gene is commonly deleted from adenovirus vectors for anticancer therapy. E1B activities are spatiotemporally regulated through SUMOylation and phosphorylation, and through interactions with multiple partners that occur presumably at different intracellular sites and times postinfection. E1B is implicated in the formation of viral replication compartments and regulates viral genome replication and transcription, transcriptional repression, degradation of cellular proteins, and several intranuclear steps of viral late mRNA biogenesis. Here, we review advances in our understanding of E1B during productive adenovirus replication and discuss fundamental aspects that remain unresolved.

Effects of the deletion of early region 4 (E4) open reading frame 1 (orf1), orf1-2, orf1-3 and orf1-4 on virus-host cell interaction, transgene expression, and immunogenicity of replicating adenovirus HIV vaccine vectors

The global health burden engendered by human immunodeficiency virus (HIV)-induced acquired immunodeficiency syndrome (AIDS) is a sobering reminder of the pressing need for a preventative vaccine. In non-human primate models replicating adenovirus (Ad)-HIV/SIV recombinant vaccine vectors have been shown to stimulate potent immune responses culminating in protection against challenge exposures. Nonetheless, an increase in the transgene carrying capacity of these Ad vectors, currently limited to approximately 3000 base pairs, would greatly enhance their utility. Using a replicating, E3-deleted Ad type 5 host range mutant (Ad5 hr) encoding full-length single-chain HIVBaLgp120 linked to the D1 and D2 domains of rhesus macaque CD4 (rhFLSC) we systematically deleted the genes encoding early region 4 open reading frame 1 (E4orf1) through E4orf4. All the Ad-rhFLSC vectors produced similar levels of viral progeny. Cell cycle analysis of infected human and monkey cells revealed no differences in virus-host interaction. The parental and E4-deleted viruses expressed comparable levels of the transgene with kinetics similar to Ad late proteins. Similar levels of cellular immune responses and transgene-specific antibodies were elicited in vaccinated mice. However, differences in recognition of Ad proteins and induced antibody subtypes were observed, suggesting that the E4 gene products might modulate antibody responses by as yet unknown mechanisms. In short, we have improved the transgene carrying capacity by one thousand base pairs while preserving the replicability, levels of transgene expression, and immunogenicity critical to these vaccine vectors. This additional space allows for flexibility in vaccine design that could not be obtained with the current vector and as such should facilitate the goal of improving vaccine efficacy. To the best of our knowledge, this is the first report describing the effects of these E4 deletions on transgene expression and immunogenicity in a replicating Ad vector.

Infection with E1B-mutant adenovirus stabilizes p53 but blocks p53 acetylation and activity through E1A

Wild-type adenovirus type 5 eliminates p53 through the E1B-55kDa and E4-34kDa gene products. Deletion or mutation of E1B-55kDa has long been thought to confer p53-selective replication of oncolytic viruses. We show here that infection with E1B-defective adenovirus mutants induces massive accumulation of p53, without obvious defects in p53 localization, phosphorylation, conformation and oligomerization. Nonetheless, p53 completely failed to induce its target genes in this scenario, for example, p21/CDKN1A, Mdm2 and PUMA. Two regions of the E1A gene products independently contributed to the suppression of p21 transcription. Depending on the E1A conserved region 3, E1B-defective adenovirus impaired the ability of the transcription factor Sp1 to bind the p21 promoter. Moreover, the amino terminal region of E1A, binding the acetyl transferases p300 and CREB-binding protein, blocked p53 K382 acetylation in infected cells. Mutating either of these E1A regions, in addition to E1B, partially restored p21 mRNA levels. Our findings argue that adenovirus attenuates p53-mediated p21 induction, through at least two E1B-independent mechanisms. Other virus species and cancer cells may employ analogous strategies to impair p53 activity.

Adenovirus E1B55K region is required to enhance cyclin E expression for efficient viral DNA replication

Adenoviruses (Ads) with E1B55K mutations can selectively replicate in and destroy cancer cells. However, the mechanism of Ad-selective replication in tumor cells is not well characterized. We have shown previously that expression of several cell cycle-regulating genes is markedly affected by the Ad E1b gene in WI-38 human lung fibroblast cells (X. Rao, et al., Virology 350:418-428, 2006). In the current study, we show that the Ad E1B55K region is required to enhance cyclin E expression and that the failure to induce cyclin E overexpression due to E1B55K mutations prevents viral DNA from undergoing efficient replication in WI-38 cells, especially when the cells are arrested in the G(0) phase of the cell cycle by serum starvation. In contrast, cyclin E induction is less dependent on the function encoded in the E1B55K region in A549 and other cancer cells that are permissive for replication of E1B55K-mutated viruses, whether the cells are in the S phase or G(0) phase. The small interfering RNA that specifically inhibits cyclin E expression partially decreased viral replication. Our study provides evidence suggesting that E1B55K may be involved in cell cycle regulation that is important for efficient viral DNA replication and that cyclin E overexpression in cancer cells may be associated with the oncolytic replication of E1B55K-mutated viruses.

p400 function is required for the adenovirus E1A-mediated suppression of EGFR and tumour cell killing

We have recently shown that E1A protein of human adenovirus downregulates epidermal growth factor receptor (EGFR) expression and induces apoptosis in head and neck (HNSCC) and lung cancer cells independently of their p53 status. E1A has five isoforms of which the major ones E1A12S and E1A13S regulate transcription of cellular genes by binding to transcriptional modulators such as pRB, CtBP, p300 and p400. In this study, we have identified E1A12S isoform to have the highest effect on EGFR suppression and induction of apoptosis in HNSCC cells. Similar to Ad5, E1A12S from human adenovirus types 2, 3, 9 and 12 suppressed EGFR, whereas E1A12S of adenovirus types 4 and 40 had no effect on EGFR expression. Using deletion mutants of E1A12S we have shown that interaction of E1A with p400, but not p300 or pRB, is required for EGFR suppression and apoptosis. Inhibition of p400 by short hairpin RNA confirmed that HNSCC cells with reduced p400 expression were less sensitive to E1A-induced suppression of EGFR and apoptosis. p300 function was shown to be dispensable, as cells expressing E1A mutants that are unable to bind p300, or p300 knockout cells, remained sensitive to E1A-induced apoptosis. In summary, this study identifies p400 as an important mediator of E1A-induced downregulation of EGFR and apoptosis.

Accumulation of p53 in response to adenovirus early region 1A sensitizes human cells to tumor necrosis factor alpha-induced apoptosis

Many tumor cells are resistant to tumor necrosis factor alpha (TNFalpha)-induced apoptosis. Adenovirus early region 1A (AdE1A) sensitizes the otherwise resistant cells to TNFalpha. AdE1A also stabilizes the p53 protein. The present study demonstrates a correlation between AdE1A-induced sensitization and stabilization of p53 in TNFalpha-induced apoptosis since the N-terminal and CR2 regions, the binding sites for CBP/p300, Rb and 26S proteasome regulatory components, are required for both these actions of AdE1A. TNFalpha does not induce apoptosis and AdE1A fails to sensitize TNFalpha cytotoxicity in p53-negative cells. However, introduction of exogenous p53 overcomes the cellular resistance to TNFalpha toxicity and enhances AdE1A sensitization, demonstrating that AdE1A sensitizes TNFalpha-induced apoptosis by its stabilization of p53. A proteasome inhibitor, lactacystin, enhances TNFalpha cytotoxicity in p53-positive and -negative cells, suggesting that accumulation of cellular proteins other than p53 might also regulate the cellular response to TNFalpha signaling.

BH3-only BIK regulates BAX,BAK-dependent release of Ca2+ from endoplasmic reticulum stores and mitochondrial apoptosis during stress-induced cell death

BIK, a pro-apoptotic BH3-only member of the BCL-2 family, targets the membrane of the endoplasmic reticulum (ER). It is induced in human cells in response to several stress stimuli, including genotoxic stress (radiation, doxorubicin) and overexpression of E1A or p53 but not by ER stress pathways resulting from protein malfolding. BIK initiates an early release of Ca2+ from ER upstream of the activation of effector caspases. Release of the mobile ER Ca2+ stores in baby mouse kidney cells doubly deficient in BAX and BAK, on the other hand, is resistant to BIK but is sensitive to ectopic BAK. Over-expression of p53 stimulates recruitment of BAK to the ER, and both its recruitment and assembly into higher order structures is inhibited by BIK small interfering RNA. Employing small interfering RNA knockdowns, we also demonstrated that release of ER Ca2+ and mitochondrial apoptosis in human epithelial cells requires BIK and that a Ca2+-regulated target, the dynamin-related GTPase DRP1, is involved in p53-induced mitochondrial fission and release of cytochrome c to the cytosol. Endogenous cellular BIK, therefore, regulates a BAX,BAK-dependent ER pathway that contributes to mitochondrial apoptosis.

Cell transformation by human adenoviruses

The last 40 years of molecular biological investigations into human adenoviruses have contributed enormously to our understanding of the basic principles of normal and malignant cell growth. Much of this knowledge stems from analyses of their productive infection cycle in permissive host cells. Also, initial observations concerning the carcinogenic potential of human adenoviruses subsequently revealed decisive insights into the molecular mechanisms of the origins of cancer, and established adenoviruses as a model system for explaining virus-mediated transformation processes. Today it is well established that cell transformation by human adenoviruses is a multistep process involving several gene products encoded in early transcription units 1A (E1A) and 1B (E1B). Moreover, a large body of evidence now indicates that alternative or additional mechanisms are engaged in adenovirus-mediated oncogenic transformation involving gene products encoded in early region 4 (E4) as well as epigenetic changes resulting from viral DNA integration. In particular, detailed studies on the tumorigenic potential of subgroup D adenovirus type 9 (Ad9) E4 have now revealed a new pathway that points to a novel, general mechanism of virus-mediated oncogenesis. In this chapter, we summarize the current state of knowledge about the oncogenes and oncogene products of human adenoviruses, focusing particularly on recent findings concerning the transforming and oncogenic properties of viral proteins encoded in the E1B and E4 transcription units.

Regulation of the p53 homolog p73 by adenoviral oncogene E1A

p73 is a p53 homolog, as they are similar structurally and functionally. Unlike p53, p73 is not inactivated by the products of viral oncogenes such as SV40 T antigen and human papilloma virus E6. Here we show that the product of adenoviral oncogene E1A inhibits the transcriptional activation by both p73alpha and p73beta. Electrophoretic mobility shift assays revealed that E1A does not inhibit the sequence-specific DNA binding by p73. Transcriptional activation by a fusion protein containing the Gal4 DNA-binding domain and either of the activation domains of p73 was inhibited by wild-type (WT) E1A, but not by the N-terminal deletion mutant E1A(Delta2-36). E1A(Delta2-36), which does not bind to the p300/CBP family of coactivators, failed to inhibit p73-mediated transcription, whereas E1A(DeltaCR2), a deletion mutant that does not bind to the pRb family of proteins, inhibited p73-mediated transcription as efficiently as WT E1A. Consistent with these observations, growth arrest induced by p73 expressed from a recombinant adenovirus was abrogated by WT E1A, which correlated with inhibition of p73-mediated induction of p21(WAF1/CIP1) by E1A. However, p73 was able to induce p21(WAF1/CIP1) and to mediate growth arrest in the presence of E1A(Delta2-36). Furthermore, the expression of either wild-type E1A or E1A(Delta2-36) resulted in the stabilization of endogenous p73. However, p73 stabilized in response to the expression of E1A(Delta2-36), but not WT E1A, was able to activate the expression of p21(WAF1/CIP1). These results suggest that the transcriptional activation function of p73 is specifically targeted by E1A through a mechanism involving p300/CBP proteins during the process of transformation and that p73 may have a role to play as a tumor suppressor.

Induction and endoplasmic reticulum location of BIK/NBK in response to apoptotic signaling by E1A and p53

A DNA microarray analysis identified the BH3-only BCL-2 family member, BIK/NBK, as a transcript that is upregulated during induction of apoptosis by oncogenic E1A. E1A depended on wild-type p53 to induce BIK and activate the death program. Further, p53 independently induced BIK RNA and protein, and BIK alone stimulated cell death in p53-null cells, dependent on the activation of caspases. BIK function, however, was abrogated by a disabling point mutation within the BH3 domain. Collectively, these results argue that BIK is a downstream apoptotic effector of p53 in response to a physiological p53-mediated death stimulus provided by E1A. Elevated BCL-2 functioned downstream of p53 and BIK induction to inhibit the E1A death pathway, with the ratio of anti-apoptotic BCL-2 and pro-apoptotic BIK determining cell death or survival in E1A-expressing cells. Cells expressing BCL-2 or treated with the pan caspase inhibitor, zVAD-fmk, allowed accumulation of high levels of cytotoxic BIK compared to control cells. Of note, a significant fraction of either ectopic or endogenous BIK was found associated with the endoplasmic reticulum, suggesting that this organelle, in addition to mitochondria, may be a target of BIK function.

The procaspase-8 isoform, procaspase-8L, recruited to the BAP31 complex at the endoplasmic reticulum

BAP31 is an integral protein of the endoplasmic reticulum membrane and a substrate of caspase-8. Here, we describe the procaspase-8 isoform, procaspase-8L, which is ubiquitously expressed and selectively recruited to the BAP31 complex in response to apoptotic signaling by E1A. Procaspase-8L is characterized by the N-terminal extension (Nex) domain, which extends procaspase-8/a at the N terminus and is required for selective association of procaspase-8L with the BAP31 complex. Gene deletion identified BAP31 and related BAP29 as required for processing of procaspase-8L in response to E1A, by a FADD-independent mechanism that was blocked by BCL-2. Further, Bap29,31 deletion, as well as a Nex-domain dominant-negative mutant, curtailed the activation of downstream caspases (IETDase and DEVDase) and cell death in response to E1A. Preferential recruitment of procaspase-8L by the BAP31 complex at the endoplasmic reticulum suggests an additional pathway for regulating initiator caspase-8 during apoptosis.

E1A-induced processing of procaspase-8 can occur independently of FADD and is inhibited by Bcl-2

Expression of the 243-residue form of the adenovirus E1A protein in the absence of other viral proteins triggers apoptosis by a pathway that requires p53. This pathway includes processing and activation of initiator procaspase-8, redistribution of cytochrome c, and activation of procaspase-3. Bcl-2 functions at or upstream of procaspase-8 processing to inhibit all of these events and prevent cell death. This contrasts with the anti-apoptotic influence of Bcl-2 family proteins in the cell death pathway induced by Fas ligand or tumor necrosis factor (TNF), in which Bcl-2 typically acts downstream of Fas/TNFR1-mediated activation of caspase-8. Moreover, E1A induces procaspase-8 processing and cell death in cells deleted of FADD, an adaptor protein critical for Fas/TNFR1 activation of caspase-8. The results indicate that E1A is capable of activating caspase-8 by a Bcl-2-inhibitable pathway that does not involve autocrine stimulation of FADD-dependent death receptor pathways.

p53 status does not determine outcome of E1B 55-kilodalton mutant adenovirus lytic infection

The ability of the adenovirus type 5 E1B 55-kDa mutants dl1520 and dl338 to replicate efficiently and independently of the cell cycle, to synthesis viral DNA, and to lyse infected cells did not correlate with the status of p53 in seven cell lines examined. Rather, cell cycle-independent replication and virus-induced cell killing correlated with permissivity to viral replication. This correlation extended to S-phase HeLa cells, which were more susceptible to virus-induced cell killing by the E1B 55-kDa mutant virus than HeLa cells infected during G1. Wild-type p53 had only a modest effect on E1B mutant virus yields in H1299 cells expressing a temperature-sensitive p53 allele. The defect in E1B 55-kDa mutant virus replication resulting from reduced temperature was as much as 10-fold greater than the defect due to p53 function. At 39 degreesC, the E1B 55-kDa mutant viruses produced wild-type yields of virus and replicated independently of the cell cycle. In addition, the E1B 55-kDa mutant viruses directed the synthesis of late viral proteins to levels equivalent to the wild-type virus level at 39 degreesC. We have previously shown that the defect in mutant virus replication can also be overcome by infecting HeLa cells during S phase. Taken together, these results indicate that the capacity of the E1B 55-kDa mutant virus to replicate independently of the cell cycle does not correlate with the status of p53 but is determined by yet unidentified mechanisms. The cold-sensitive nature of the defect of the E1B 55-kDa mutant virus in both late gene expression and cell cycle-independent replication leads us to speculate that these functions of the E1B 55-kDa protein may be linked.

E1B 55-kilodalton-associated protein: a cellular protein with RNA-binding activity implicated in nucleocytoplasmic transport of adenovirus and cellular mRNAs

The adenovirus type 5 (Ad5) early 1B 55-kDa protein (E1B-55kDa) is a multifunctional phosphoprotein that regulates viral DNA replication and nucleocytoplasmic RNA transport in lytically infected cells. In addition, E1B-55kDa provides functions required for complete oncogenic transformation of rodent cells in cooperation with the E1A proteins. Using the far-Western technique, we have isolated human genes encoding E1B-55kDa-associated proteins (E1B-APs). The E1B-AP5 gene encodes a novel nuclear RNA-binding protein of the heterogeneous nuclear ribonucleoprotein (hnRNP) family that is highly related to hnRNP-U/SAF-A. Immunoprecipitation experiments indicate that two distinct segments in the 55-kDa polypeptide which partly overlap regions responsible for p53 binding are required for complex formation with E1B-AP5 in Ad-infected cells and that this protein interaction is modulated by the adenovirus E4orf6 protein. Expression of E1B-AP5 efficiently interferes with Ad5 E1A/E1B-mediated transformation of primary rat cells. Furthermore, stable expression of E1B-AP5 in Ad-infected cells overcomes the E1B-dependent inhibition of cytoplasmic host mRNA accumulation. These data suggest that E1B-AP5 might play a role in RNA transport and that this function is modulated by E1B-55kDa in Ad-infected cells.

Role of baculovirus IE2 and its RING finger in cell cycle arrest

The ie2 gene of Autographa californica nuclear polyhedrosis virus (AcMNPV) is known to transactivate transient expression from viral promoters in a host cell-specific manner. We report that transfection of Spodoptera frugiperda (SF-21) cells with ie2 was sufficient to arrest the cell cycle, resulting in the accumulation of enlarged cells with abnormally high DNA contents. By 72 h posttransfection, more than 50% of ie2-transfected cells had DNA contents greater than 4N. There was no evidence of mitotic spindle formation in these cells, and expression of ie2 appeared to block cell cycle progression in S phase. Several ie2 mutants were analyzed to further define the region of IE2 responsible for arresting the cell cycle. Analysis of these mutants showed that deletion of the RING finger motif eliminated the ability of IE2 to arrest the cell cycle but did not affect its ability to transactivate the ie1 promoter. Moreover, mutation of a single conserved cysteine (C251) of the RING finger motif abolished the ability of IE2 to block cell cycle progression but had no apparent effect on its transregulatory activity. In contrast, a mutant of IE2 containing a deletion of residues 94 to 173 was able to block cell division but lacked trans-regulatory activity. Thus, the ability of IE2 to arrest the cell cycle depended on the integrity of the RING finger motif and was distinct from and independent of its ability to trans-activate the ie1 promoter. IE2 also arrested the division of cells derived from other insect species, Trichoplusia ni (TN-368 and BTI-TN-5B1-4) and Helicoverpa zea (Hz-AM1).

p28 Bap31, a Bcl-2/Bcl-XL- and procaspase-8-associated protein in the endoplasmic reticulum

We have identified a human Bcl-2-interacting protein, p28 Bap31. It is a 28-kD (p28) polytopic integral protein of the endoplasmic reticulum whose COOH-terminal cytosolic region contains overlapping predicted leucine zipper and weak death effector homology domains, flanked on either side by identical caspase recognition sites. In cotransfected 293T cells, p28 is part of a complex that includes Bcl-2/Bcl-XL and procaspase-8 (pro-FLICE). Bax, a pro-apoptotic member of the Bcl-2 family, does not associate with the complex; however, it prevents Bcl-2 from doing so. In the absence (but not presence) of elevated Bcl-2 levels, apoptotic signaling by adenovirus E1A oncoproteins promote cleavage of p28 at the two caspase recognition sites. Purified caspase-8 (FLICE/MACH/Mch5) and caspase-1(ICE), but not caspase-3 (CPP32/apopain/ Yama), efficiently catalyze this reaction in vitro. The resulting NH2-terminal p20 fragment induces apoptosis when expressed ectopically in otherwise normal cells. Taken together, the results suggest that p28 Bap31 is part of a complex in the endoplasmic reticulum that mechanically bridges an apoptosis-initiating caspase, like procaspase-8, with the anti-apoptotic regulator Bcl-2 or Bcl-XL. This raises the possibility that the p28 complex contributes to the regulation of procaspase-8 or a related caspase in response to E1A, dependent on the status of the Bcl-2 setpoint within the complex.

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-dependent apoptosis, transcriptional repression, and cell transformation by phosphorylation of the 55-kilodalton E1B protein of human adenovirus type 5

The adenovirus type 5 55-kDa E1B protein (E1B-55kDa) cooperates with E1A gene products to induce cell transformation. E1A proteins stimulate DNA synthesis and cell proliferation; however, they also cause rapid cell death by p53-dependent and p53-independent apoptosis. It is believed that the role of the E1B-55kDa protein in transformation is to protect against p53-dependent apoptosis by binding to and inactivating p53. It has been shown previously that the 55-kDa polypeptide abrogates p53-mediated transactivation and that mutants defective in p53 binding are unable to cooperate with E1A in transformation. We have previously mapped phosphorylation sites near the carboxy terminus of the E1B-55kDa protein at Ser-490 and Ser-491, which lie within casein kinase II consensus sequences. Conversion of these sites to alanine residues greatly reduced transforming activity, and although the mutant 55-kDa protein was found to interact with p53 at normal levels, it was somewhat defective for suppression of p53 transactivation activity. We now report that a nearby residue, Thr-495, also appears to be phosphorylated. We demonstrate directly that the wild-type 55-kDa protein is able to block E1A-induced p53-dependent apoptosis, whereas cells infected by mutant pm490/1/5A, which contains alanine residues at all three phosphorylation sites, exhibited extensive DNA fragmentation and classic apoptotic cell death. The E1B-55kDa product has been shown to exhibit intrinsic transcriptional repression activity when localized to promoters, such as by fusion with the GAL4 DNA-binding domain, even in the absence of p53. Such repression activity was totally absent with mutant pm490/1/5A. These data suggested that inhibition of p53-dependent apoptosis may depend on the transcriptional repression function of the 55-kDa protein, which appears to be regulated be phosphorylation at the carboxy terminus.

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.

The early region 1B 55-kilodalton oncoprotein of adenovirus relieves growth restrictions imposed on viral replication by the cell cycle

The E1B 55-kDa oncoprotein of adenovirus enables the virus to overcome restrictions imposed on viral replication by the cell cycle. Approximately 20% of HeLa cells infected with an E1B 55-kDa mutant adenovirus produced virus when evaluated by electron microscopy or by assays for infectious centers. By contrast, all HeLa cells infected with a wild-type adenovirus produced virus. The yield of E1B mutant virus from randomly cycling HeLa cells correlated with the fraction of cells in S phase at the time of infection. In synchronously growing HeLa cells, approximately 75% of the cells infected during S phase with the E1B mutant virus produced virus, whereas only 10% of the cells infected during G1 produced virus. The yield of E1B mutant virus from HeLa cells infected during S phase was sevenfold greater than that of cells infected during G1 and threefold greater than that of cells infected during asynchronous growth. Cells infected during S phase with the E1B mutant virus exhibited severe cytopathic effects, whereas cells infected with the E1B mutant virus during G1 exhibited a mild cytopathic effect. Viral DNA synthesis appeared independent of the cell cycle because equivalent amounts of viral DNA were synthesized in cells infected with either wild-type or E1B mutant virus. The inability of the E1B mutant virus to replicate was not mediated by the status of p53. These results define a novel property of the large tumor antigen of adenovirus in relieving growth restrictions imposed on viral replication by the cell cycle.

Adenovirus type 5 early region 4 is responsible for E1A-induced p53-independent apoptosis

In the absence of E1B, the 289- and 243-residue E1A products of human adenovirus type 5 induce p53-dependent apoptosis. However, our group has shown recently that the 289-residue E1A protein is also able to induce apoptosis by a p53-independent mechanism (J. G. Teodoro, G. C. Shore, and P. E. Branton, Oncogene 11:467-474, 1995). Preliminary results suggested that p53-independent cell death required expression of one or more additional adenovirus early gene products. Here we show that both the E1B 19-kDa protein and cellular Bcl-2 inhibit or significantly delay p53-independent apoptosis. Neither early region E2 or E3 appeared to be necessary for such cell death. Analysis of a series of E1A mutants indicated that mutations in the transactivation domain and other regions of E1A correlated with E1A-mediated transactivation of E4 gene expression. Furthermore, p53-deficient human SAOS-2 cells infected with a mutant which expresses E1B but none of the E4 gene products remained viable for considerably longer times than those infected with wild-type adenovirus type 5. In addition, an adenovirus vector lacking both E1 and E4 was unable to induce DNA degradation and cell killing in E1A-expressing cell lines. These data showed that an E4 product is essential for E1A-induced p53-independent apoptosis.

Importance of the Ser-132 phosphorylation site in cell transformation and apoptosis induced by the adenovirus type 5 E1A protein

The 289-residue (289R) and 243R early region 1A (E1A) proteins of human adenovirus type 5 induce cell transformation in cooperation with either E1B or activated ras. Here we report that Ser-132 in both E1A products is a site of phosphorylation in vivo and is the only site phosphorylated in vitro by purified casein kinase II. Ser-132 is located in conserved region 2 near the primary binding site for the pRB tumor suppressor and, in 289R, just upstream of the conserved region 3 transactivation domain involved in regulation of early viral gene expression. Mutants containing alanine or glycine in place of Ser-132 interacted with pRB-related proteins at somewhat reduced efficiency; however, all Ser-132 mutants transformed primary rat cells in cooperation with E1B as well as or better than the wild type when both major E1A proteins were expressed. Such was not the case with mutants expressing only 289R. In cooperation with E1B, the Asp-132 and Gly-132 mutants yielded reduced numbers of smaller transformed foci. With activated ras, all Ser-132 mutants were significantly defective for transformation and the rare foci produced were small and contained extensive areas populated by low densities of flat cells. In the absence of E1B, all Ser-132 mutants induced p53-independent cell death more readily than virus expressing wild-type 289R. These results suggested that phosphorylation at Ser-132 may enhance the binding of pRB and related proteins and also reduce the toxicity of E1A 289R, thus increasing transforming activity.

Transduction with recombinant adeno-associated virus for gene therapy is limited by leading-strand synthesis

Adeno-associated virus is an integrating DNA parvovirus with the potential to be an important vehicle for somatic gene therapy. A potential barrier, however, is the low transduction efficiencies of recombinant adeno-associated virus (rAAV) vectors. We show in this report that adenovirus dramatically enhances rAAV transduction in vitro in a way that is dependent on expression of early region 1 and 4 (E1 and E4, respectively) genes and directly proportional to the appearance of double-stranded replicative forms of the rAAV genome. Expression of the open reading frame 6 protein from E4 in the absence of E1 accomplished a similar but attenuated effect. The helper activity of adenovirus E1 and E4 for rAAV gene transfer was similarly demonstrated in vivo by using murine models of liver- and lung-directed gene therapy. Our data indicate that conversion of a single-stranded rAAV genome to a duplex intermediate limits transduction and usefulness for gene therapy.

Transfected lymphocyte extracts of patients with urological tumours: complement temperature-sensitive adenovirus mutants in vitro

Patients with renal or bladder cancers exhibit a unique association with adenovirus (Ad) infections. About 60% of them contain antibodies to Ad early antigens. Both in their tumour cells and peripheral blood lymphocytes (PBL) they have detectable early Ad antigens known to be involved in malignant cell transformation. Transfection of tumour cell extracts resulted in complementing temperature-sensitive (ts) Ad mutants at nonpermissive temperatures (39 degrees C) indicating that some cells of the tumour mass possess active functions for Ad. Only 4 to 18% of control subjects were positive in these tests. Here we studied whether lymphocytes might be involved in tumourigenesis by Ad. PBL extracts of patients were transfected into HEp-2 culture cells, which were subsequently superinfected with Ad-5 ts18 and ts19 mutants at 39 degrees C. Titration of virus yields indicated complementation in 76% of patients with renal and bladder cancers in contrast to 20% of control individuals. Complementing ability of lymphocytes which had been prestimulated with phytohaemagglutinin (PHA) approached that of tumour extracts. It means that both specimens contain advanced functions in contrast to resting lymphocytes. Lymphocytes are nonpermissive for latently carried Ad infections. Expression, possible transfer of early Ad gene products via frequent contacts with tissue cells can result in removal of tumour suppressor gene products from complexes regulating cell cycle negatively. Further interaction with hormone-sensitive protooncogenes explains tissue, age and gender specificity of urological malignancies. These phenomena suggest an important cofactorial role for Ad in kidney and bladder tumours.

The kinetics of simian virus 40-induced progression of quiescent cells into S phase depend on four independent functions of large T antigen

Microinjection of purified simian virus 40 large-T-antigen protein or DNA encoding T antigen into serum-starved cells stimulates them to re-enter the cell cycle and progress through G1 into the S phase. Genetic analysis of T antigen indicated that neither its Rb/p107-binding activity nor its p53-binding activity is essential to induce DNA synthesis in CV1P cells. However, T antigens bearing missense mutations that inactivate either activity induced slower progression of the cells into the S phase than did wild-type T antigen. Inactivation of both activities resulted in a T antigen essentially unable to induce DNA synthesis. Missense mutations in either the DNA-binding region of the N terminus also impaired the ability of full-length T antigen to stimulate DNA synthesis in CV1P cells. The wild-type kinetics of cell cycle progression were restored by genetic complementation after coinjection of plasmid DNAs encoding different mutant T antigens or coinjection of purified mutant T-antigen proteins, suggesting that the four mitogenic functions of T antigen are independent. The maximal rate of induction of DNA synthesis in secondary primate cells and established rodent cell lines required the same four functions of T antigen. A model to explain how four independent activities could cooperate to stimulate cell cycle progression is presented.