Adenovirus E1B proteins are required for accumulation of late viral mRNA and for effects on cellular mRNA translation and transport

Adipocyte commitment of 3T3-L1 cells is required to support human adenovirus 36 productive replication concurrent with altered lipid and glucose metabolism

Human adenovirus 36 (HAdV-D36) can cause obesity in animal models, induces an adipogenic effect and increased adipocyte differentiation in cell culture. HAdV-D36 infection alters gene expression and the metabolism of the infected cells resulting in increased glucose internalization and triglyceride accumulation. Although HAdV-D36 prevalence correlates with obesity in humans, whether human preadipocytes may be targeted in vivo has not been determined and metabolic reprogramming of preadipocytes has not been explored in the context of the viral replication cycle. HAdV-D36 infection of the mouse fibroblasts, 3T3-L1 cells, which can differentiate into adipocytes, promotes proliferation and differentiation, but replication of the virus in these cells is abortive as indicated by short-lived transient expression of viral mRNA and a progressive loss of viral DNA. Therefore, we have evaluated whether a productive viral replication cycle can be established in the 3T3-L1 preadipocyte model under conditions that drive the cell differentiation process. For this purpose, viral mRNA levels and viral DNA replication were measured by RT-qPCR and qPCR, respectively, and viral progeny production was determined by plaque assay. The lipogenic effect of infection was evaluated with Oil Red O (ORO) staining, and expression of genes that control lipid and glucose metabolism was measured by RT-qPCR. In the context of a viral productive cycle, HAdV-D36 modulated the expression of the adipogenic genes, C/EBPα, C/EBPβ and PPARγ, as well as intracellular lipid accumulation, and the infection was accompanied by altered expression of glucolytic genes. The results show that only adipocyte-committed 3T3-L1 cells are permissive for the expression of early and late viral mRNAs, as well as viral DNA replication and progeny production, supporting productive HAdV-D36 viral replication, indicating that a greater effect on adipogenesis occurs in adipocytes that support productive viral replication.

Adenovirus-mediated ubiquitination alters protein-RNA binding and aids viral RNA processing

Viruses promote infection by hijacking the ubiquitin machinery of the host to counteract or redirect cellular processes. Adenovirus encodes two early proteins, E1B55K and E4orf6, that together co-opt a cellular ubiquitin ligase complex to overcome host defences and promote virus production. Adenovirus mutants lacking E1B55K or E4orf6 display defects in viral RNA processing and protein production, but previously identified substrates of the redirected ligase do not explain these phenotypes. Here, we used a quantitative proteomics approach to identify substrates of E1B55K/E4orf6-mediated ubiquitination that facilitate RNA processing. While all currently known cellular substrates of E1B55K and E4orf6 are degraded by the proteasome, we uncovered RNA-binding proteins as high-confidence substrates that are not decreased in overall abundance. We focused on two RNA-binding proteins, RALY and hnRNP-C, which we confirm are ubiquitinated without degradation. Knockdown of RALY and hnRNP-C increased levels of viral RNA splicing, protein abundance and progeny production during infection with E1B55K-deleted virus. Furthermore, infection with E1B55K-deleted virus resulted in an increased interaction of hnRNP-C with viral RNA and attenuation of viral RNA processing. These data suggest that viral-mediated ubiquitination of RALY and hnRNP-C relieves a restriction on viral RNA processing and reveal an unexpected role for non-degradative ubiquitination in the manipulation of cellular processes during virus infection.

Characterization of Adenovirus 5 E1A Exon 1 Deletion Mutants in the Viral Replicative Cycle

Human adenovirus infection is driven by Early region 1A (E1A) proteins, which are the first proteins expressed following the delivery of the viral genome to the cellular nucleus. E1A is responsible for reprogramming the infected cell to support virus replication alongside the activation of expression of all viral transcriptional units during the course of the infection. Although E1A has been extensively studied, most of these studies have focused on understanding the conserved region functions outside of a full infection. Here, we investigated the effects of small deletions in E1A exon 1 on the viral replicative cycle. Almost all deletions were found to have a negative impact on viral replication with the exception of one deletion found in the mutant dl1106, which replicated better than the wild-type E1A expressing dl309. In addition to growth, we assessed the virus mutants for genome replication, induction of the cytopathic effect, gene and protein expression, sub-cellular localization of E1A mutant proteins, induction of cellular S-phase, and activation of S-phase specific cellular genes. Importantly, our study found that virus replication is likely limited by host-specific factors, rather than specific viral aspects such as the ability to replicate genomes or express late proteins, after a certain level of these has been expressed. Furthermore, we show that mutants outside of the conserved regions have significant influence on viral fitness. Overall, our study is the first comprehensive evaluation of the dl1100 series of exon 1 E1A deletion mutants in viral fitness and provides important insights into the contribution that E1A makes to viral replication in normal human cells.

Adenovirus E1B 55-Kilodalton Protein Targets SMARCAL1 for Degradation during Infection and Modulates Cellular DNA Replication

Here, we show that the cellular DNA replication protein and ATR substrate SMARCAL1 is recruited to viral replication centers early during adenovirus infection and is then targeted in an E1B-55K/E4orf6- and cullin RING ligase-dependent manner for proteasomal degradation. In this regard, we have determined that SMARCAL1 is phosphorylated at S123, S129, and S173 early during infection in an ATR- and CDK-dependent manner, and that pharmacological inhibition of ATR and CDK activities attenuates SMARCAL1 degradation. SMARCAL1 recruitment to viral replication centers was shown to be largely dependent upon SMARCAL1 association with the RPA complex, while Ad-induced SMARCAL1 phosphorylation also contributed to SMARCAL1 recruitment to viral replication centers, albeit to a limited extent. SMARCAL1 was found associated with E1B-55K in adenovirus E1-transformed cells. Consistent with its ability to target SMARCAL1, we determined that E1B-55K modulates cellular DNA replication. As such, E1B-55K expression initially enhances cellular DNA replication fork speed but ultimately leads to increased replication fork stalling and the attenuation of cellular DNA replication. Therefore, we propose that adenovirus targets SMARCAL1 for degradation during infection to inhibit cellular DNA replication and promote viral replication.IMPORTANCE Viruses have evolved to inhibit cellular DNA damage response pathways that possess antiviral activities and utilize DNA damage response pathways that possess proviral activities. Adenovirus has evolved, primarily, to inhibit DNA damage response pathways by engaging with the ubiquitin-proteasome system and promoting the degradation of key cellular proteins. Adenovirus differentially regulates ATR DNA damage response signaling pathways during infection. The cellular adenovirus E1B-55K binding protein E1B-AP5 participates in ATR signaling pathways activated during infection, while adenovirus 12 E4orf6 negates Chk1 activation by promoting the proteasome-dependent degradation of the ATR activator TOPBP1. The studies detailed here indicate that adenovirus utilizes ATR kinase and CDKs during infection to promote the degradation of SMARCAL1 to attenuate normal cellular DNA replication. These studies further our understanding of the relationship between adenovirus and DNA damage and cell cycle signaling pathways during infection and establish new roles for E1B-55K in the modulation of cellular DNA replication.

Effect of adenovirus infection on transgene expression under the adenoviral MLP/TPL and the CMVie promoter/enhancer in CHO cells

The adenovirus major late promoter (MLP) and its translational regulator - the tripartite leader (TPL) sequence - can actively drive efficient gene expression during adenoviral infection. However, both elements have not been widely tested in transgene expression outside of the adenovirus genome context. In this study, we tested whether the combination of MLP and TPL would enhance transgene expression beyond that of the most widely used promoter in transgene expression in mammalian cells, the cytomegalovirus immediate early (CMVie) promoter/enhancer. The activity of these two regulatory elements was compared in Chinese hamster ovary (CHO) cells. Although transient expression was significantly higher under the control of the CMVie promoter/enhance compared to the MLP/TPL, this difference was greater at the level of transcription (30 folds) than translation (11 folds). Even with adenovirus infection to provide additional elements (in trans), CMVie promoter/enhancer exhibited significantly higher activity relative to MLP/TPL. Interestingly, the CMVie promoter/enhancer was 1.9 folds more active in adenovirus-infected cells than in non-infected cells. Our study shows that the MLP-TPL drives lower transgene expression than the CMVie promoter/enhancer particularly at the transcription level. The data also highlight the utility of the TPL sequence at the translation level and/or possible overwhelming of the cellular translational machinery by the high transcription activity of the CMVie promoter/enhancer. In addition, here we present data that show stimulation of the CMVie promoter/enhancer by adenovirus infection, which may prove interesting in future work to test the combination of CMVie/TPL sequence, and additional adenovirus elements, for transgene expression.

Normal human cell proteins that interact with the adenovirus type 5 E1B 55kDa protein

Several of the functions of the human adenovirus type 5 E1B 55kDa protein are fulfilled via the virus-specific E3 ubiquitin ligase it forms with the viral E4 Orf6 protein and several cellular proteins. Important substrates of this enzyme have not been identified, and other functions, including repression of transcription of interferon-sensitive genes, do not require the ligase. We therefore used immunoaffinity purification and liquid chromatography-mass spectrometry of lysates of normal human cells infected in parallel with HAdV-C5 and E1B 55kDa protein-null mutant viruses to identify specifically E1B 55kDa-associated proteins. The resulting set of >90 E1B-associated proteins contained the great majority identified previously, and was enriched for those associated with the ubiquitin-proteasome system, RNA metabolism and the cell cycle. We also report very severe inhibition of viral genome replication when cells were exposed to both specific or non-specific siRNAs and interferon prior to infection.

Automatic detection and measurement of viral replication compartments by ellipse adjustment

Viruses employ a variety of strategies to hijack cellular activities through the orchestrated recruitment of macromolecules to specific virus-induced cellular micro-environments. Adenoviruses (Ad) and other DNA viruses induce extensive reorganization of the cell nucleus and formation of nuclear Replication Compartments (RCs), where the viral genome is replicated and expressed. In this work an automatic algorithm designed for detection and segmentation of RCs using ellipses is presented. Unlike algorithms available in the literature, this approach is deterministic, automatic, and can adjust multiple RCs using ellipses. The proposed algorithm is non iterative, computationally efficient and is invariant to affine transformations. The method was validated over both synthetic images and more than 400 real images of Ad-infected cells at various timepoints of the viral replication cycle obtaining relevant information about the biogenesis of adenoviral RCs. As proof of concept the algorithm was then used to quantitatively compare RCs in cells infected with the adenovirus wild type or an adenovirus mutant that is null for expression of a viral protein that is known to affect activities associated with RCs that result in deficient viral progeny production.

Beyond Oncolytics: E1B55K-Deleted Adenovirus as a Vaccine Delivery Vector

Type 5 human adenoviruses (Ad5) deleted of genes encoding the early region 1B 55-kDa (E1B55K) protein including Onyx-015 (dl1520) and H101 are best known for their oncolytic potential. As a vaccine vector the E1B55K deletion may allow for the insertion of a transgene nearly 1,000 base pairs larger than now possible. This has the potential of extending the application for which the vectors are clinically known. However, the immune priming ability of E1B55K-deleted vectors is unknown, undermining our ability to gauge their usefulness in vaccine applications. For this reason, we created an E1B55K-deleted Ad5 vector expressing full-length single chain HIV_BaLgp120 attached to a flexible linker and the first two domains of rhesus CD4 (rhFLSC) in exchange for the E3 region. In cell-based experiments the E1B55K-deleted vector promoted higher levels of innate immune signals including chemokines, cytokines, and the NKG2D ligands MIC A/B compared to an E1B55K wild-type vector expressing the same immunogen. Based on these results we evaluated the immune priming ability of the E1B55K-deleted vector in mice. The E1B55K-deleted vector promoted similar levels of Ad5-, HIVgp120, and rhFLSC-specific cellular and humoral immune responses as the E1B55K wild-type vector. In pre-clinical HIV-vaccine studies the wild-type vector has been employed as part of a very effective prime-boost strategy. This study demonstrates that E1B55K-deleted adenoviruses may serve as effective vaccine delivery vectors.

The Human Adenovirus Type 5 E4orf6/E1B55K E3 Ubiquitin Ligase Complex Can Mimic E1A Effects on E2F

During the course of work on the adenovirus E3 ubiquitin ligase formed by the viral E4orf6 and E1B55K proteins, we found, very surprisingly, that expression of these species was sufficient to permit low levels of replication of an adenovirus vector lacking E1A, the central regulator of infection. E1A products uncouple E2F transcription factors from Rb repression complexes, thus stimulating viral gene expression and cell and viral DNA synthesis. We found that the E4orf6/E1B55K ligase mimics these functions. This finding is of significance because it represents an entirely new function for the ligase in regulating adenovirus replication.

The human adenovirus E4orf6/E1B55K E3 ubiquitin ligase is well known to promote viral replication by degrading an increasing number of cellular proteins that inhibit the efficient production of viral progeny. We report here a new function of the adenovirus 5 (Ad5) viral ligase complex that, although at lower levels, mimics effects of E1A products on E2F transcription factors. When expressed in the absence of E1A, the E4orf6 protein in complex with E1B55K binds E2F, disrupts E2F/retinoblastoma protein (Rb) complexes, and induces hyperphosphorylation of Rb, leading to induction of viral and cellular DNA synthesis as well as stimulation of early and late viral gene expression and production of viral progeny of E1/E3-defective adenovirus vectors. These new and previously undescribed functions of the E4orf6/E1B55K E3 ubiquitin ligase could play an important role in promoting the replication of wild-type viruses.

IMPORTANCE During the course of work on the adenovirus E3 ubiquitin ligase formed by the viral E4orf6 and E1B55K proteins, we found, very surprisingly, that expression of these species was sufficient to permit low levels of replication of an adenovirus vector lacking E1A, the central regulator of infection. E1A products uncouple E2F transcription factors from Rb repression complexes, thus stimulating viral gene expression and cell and viral DNA synthesis. We found that the E4orf6/E1B55K ligase mimics these functions. This finding is of significance because it represents an entirely new function for the ligase in regulating adenovirus replication.

Oncolytic adenovirus research evolution: from cell-cycle checkpoints to immune checkpoints

Oncolytic adenoviruses are modified to exploit the aberrant expression of proteins in cancer cells to obtain cancer-selective replication. Moreover, the natural tropism of oncolytic adenoviruses can be redirected to tumor cells. Clinical trials revealed that oncolytic viruses showed poor replication in the tumor that is due in part to the immune response against the virus. More recent data demonstrated that tumor infection might subvert the tumor immune system and lead to an anti-tumor immune response. In the next few years, combination of adenoviruses with immune checkpoint antibodies and other immune modulators will be tested in clinical trials.

Insight into the mechanism of inhibition of adeno-associated virus by the Mre11/Rad50/Nbs1 complex

Adeno-associated virus (AAV) is a dependent virus of the family Parvoviridae. The gene expression and replication of AAV and derived recombinant AAV (rAAV) vectors are severely limited (>10-fold) by the cellular DNA damage-sensing complex made up of Mre11, Rad50, and Nbs1 (MRN). The AAV genome does not encode the means to circumvent this block to productive infection but relies on coinfecting helper virus to do so. Using adenovirus helper proteins E1B55k and E4orf6, which enhance the transduction of AAV via degradation of MRN, we investigated the mechanism through which this DNA damage complex inhibits gene expression from rAAV. We tested the substrate specificity of inhibition and the contribution of different functions of the MRN complex. Our results demonstrate that both single- and double-stranded rAAV vectors are inhibited by MRN, which is in contrast to the predominant model that inhibition is the result of a block to second-strand synthesis. Exploring the contribution of known functions of MRN, we found that inhibition of rAAV does not require downstream DNA damage response factors, including signaling kinases ATM and ATR. The nuclease domain of Mre11 appears to play only a minor role in inhibition, while the DNA binding domain makes a greater contribution. Additionally, mutation of the inverted terminal repeat of the rAAV genome, which has been proposed to be the signal for interaction with MRN, is tolerated by the mechanism of inhibition. These results articulate a model of inhibition of gene expression in which physical interaction is more important than enzymatic activity and several key downstream damage repair factors are dispensable.

IMPORTANCE

Many viruses modulate the host DNA damage response (DDR) in order to create a cellular environment permissive for infection. The MRN complex is a primary sensor of damage in the cell but also responds to invading viral genomes, often posing a block to infection. AAV is greatly inhibited by MRN and dependent on coinfecting helper virus, such as adenovirus, to remove this factor. Currently, the mechanism through which MRN inhibits AAV and other viruses is poorly understood. Our results reform the predominant model that inhibition of rAAV by MRN is due to limiting second-strand DNA synthesis. Instead, a novel mechanism of inhibition of gene expression independent of a block in rAAV DNA synthesis or downstream damage factors is indicated. These findings have clear implications for understanding this restriction to transduction of AAV and rAAV vectors, which have high therapeutic relevance and likely translate to other viruses that must navigate the DDR.

DNA virus replication compartments

Viruses employ a variety of strategies to usurp and control cellular activities through the orchestrated recruitment of macromolecules to specific cytoplasmic or nuclear compartments. Formation of such specialized virus-induced cellular microenvironments, which have been termed viroplasms, virus factories, or virus replication centers, complexes, or compartments, depends on molecular interactions between viral and cellular factors that participate in viral genome expression and replication and are in some cases associated with sites of virion assembly. These virus-induced compartments function not only to recruit and concentrate factors required for essential steps of the viral replication cycle but also to control the cellular mechanisms of antiviral defense. In this review, we summarize characteristic features of viral replication compartments from different virus families and discuss similarities in the viral and cellular activities that are associated with their assembly and the functions they facilitate for viral replication.

A ubiquitin-specific protease possesses a decisive role for adenovirus replication and oncogene-mediated transformation

Adenoviral replication depends on viral as well as cellular proteins. However, little is known about cellular proteins promoting adenoviral replication. In our screens to identify such proteins, we discovered a cellular component of the ubiquitin proteasome pathway interacting with the central regulator of adenoviral replication. Our binding assays mapped a specific interaction between the N-terminal domains of both viral E1B-55K and USP7, a deubiquitinating enzyme. RNA interference-mediated downregulation of USP7 severely reduced E1B-55K protein levels, but more importantly negatively affected adenoviral replication. We also succeeded in resynthesizing an inhibitor of USP7, which like the knockdown background reduced adenoviral replication. Further assays revealed that not only adenoviral growth, but also adenoviral oncogene-driven cellular transformation relies on the functions of USP7. Our data provide insights into an intricate mechanistic pathway usurped by an adenovirus to promote its replication and oncogenic functions, and at the same time open up possibilities for new antiviral strategies.

Adenoviral infections can result in severe outcomes leading to mortality especially in children undergoing immunosuppressive therapies. Unfortunately, no specific anti-adenoviral treatments are available to treat disseminated adenoviral infections. We have set out to identify host factors promoting adenoviral growth and could identify the cellular protein Ubiquitin-specific protease 7 (USP7) being central to adenoviral infection. Here we show that USP7 interacts with the viral protein E1B-55K, a central regulator of adenoviral replication and adenoviral oncogene-mediated cellular transformation. We demonstrate that USP7 ensures stability and/or proper expression levels of adenoviral proteins at early and late time points of infection. Consistent with this, small-molecule inhibitors of USP7 showed efficient reduction of capsid protein levels and viral progeny numbers. Thus, USP7 inhibition might be a useful treatment option in the context of disseminated adenoviral infections. Moreover, we were also able to show that adenoviral oncogene-mediated cellular transformation can be hampered by USP7 disruption. In summary, this study shows that two different adenoviral disease mechanisms can be inhibited by targeting one host cellular factor.

Aggresome formation by the adenoviral protein E1B55K is not conserved among adenovirus species and is not required for efficient degradation of nuclear substrates

Much of the work on the basic molecular biology of human adenoviruses has been carried out on a very limited number of the more than 60 serotypes, primarily the highly related species C viruses adenovirus type 5 (Ad5) and Ad2 and, to some extent, Ad12 of species A. Until recently, it has been widely assumed that insights obtained with these model viruses were representative of all human adenoviruses. Recent studies on the E3 ubiquitin ligase formed by the viral E1B55K and E4orf6 proteins with a cellular Cullin-based complex indicated that although all species form such a functional complex, significant variations exist in terms of complex composition and the substrates that are degraded. In the present report we conducted a comprehensive analysis of the localization of E1B55K products from representatives of six of the seven adenovirus species in the presence and the absence of the corresponding E4orf6 protein. We found that although in some species E1B55K localized in aggresomes, such was not always the case, suggesting that these structures are not necessary for the efficient degradation of substrates. In addition, differences were evident in the localization of E1B55K, although all forms readily associated with PML. Finally, Ad5 E1B55K was seen to localize in close proximity to Rab11, a marker for the endosomal recycling compartment, and both focused at the microtubule organizing center. These findings suggest that E1B55K from some species may employ the transport system utilized by the membrane recycling pathway to assemble aggresomes and the possibility that this structure might then affect recycling of cell surface components.

The human adenovirus type 5 E1B 55-kilodalton protein is phosphorylated by protein kinase CK2

The human adenovirus type 5 (HAdV5) early region 1B 55-kDa protein (E1B-55K) is a multifunctional phosphoprotein playing several critical roles during adenoviral productive infection, e.g., degradation of host cell proteins, viral late mRNA export, and inhibition of p53-mediated transcription. Many of these functions are apparently regulated at least in part by the phosphorylation of E1B-55K occurring at a stretch of amino acids resembling a potential CK2 consensus phosphorylation motif. We therefore investigated the potential role of CK2 phosphorylation upon E1B-55K during adenoviral infection. A phosphonegative E1B-55K mutant showed severely reduced virus progeny production, although viral early, late, and structural protein levels and viral DNA replication were not obviously affected. Binding studies revealed an interaction between the CK2α catalytic subunit and wild-type E1B-55K, which is severely impaired in the phosphonegative E1B mutant. In addition, in situ the α-catalytic subunit is redistributed into ring-like structures surrounding E1B-55K nuclear areas and distinct cytoplasmic accumulations, where a significant amount of CK2α colocalizes with E1B-55K. Furthermore, in in vitro phosphorylation assays, wild-type E1B-55K glutathione S-transferase fusion proteins were readily phosphorylated by the CK2α subunit but inefficiently phosphorylated by the CK2 holoenzyme. Addition of the CK2-specific inhibitors TBB (4,5,6,7-tetrabromobenzotriazole) and DMAT (2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole) to infected cells confirmed that CK2α binding to E1B-55K is necessary for efficient phosphorylation of E1B-55K. In summary, our data show that CK2α interacts with and phosphorylates HAdV5 E1B-55K at residues S490/491 and T495 and that these posttranslational modifications are essential for E1B-55K lytic functions.

CRM1-dependent transport supports cytoplasmic accumulation of adenoviral early transcripts

The life cycle of adenoviruses is divided by convention into early and late phases, separated by the onset of viral genome replication. Early events include virus adsorption, transport of the genome into the nucleus, and the expression of early genes. After the onset of viral DNA replication, transcription of the major late transcription unit (MLTU) and thereby synthesis of late proteins is induced. These steps are controlled by an orchestra of regulatory processes and require import of the genome and numerous viral proteins into the nucleus, as well as active transport of viral transcripts and proteins from the nucleus to the cytoplasm. The latter is achieved by exploiting the shuttling functions of cellular transport receptors, which normally stimulate the nuclear export of cellular mRNA and protein cargos. A set of adenoviral early and late proteins contains a leucine-rich nuclear export signal of the HIV-1 Rev type, known to be recognized by the cellular export receptor CRM1. However, a role for CRM1-dependent export in supporting adenoviral replication has not been established. To address this issue in detail, we investigated the impact of two different CRM1 inhibitors on several steps of the adenoviral life cycle. Inhibition of CRM1 led to a reduction in viral early and late gene expression, viral genome replication, and progeny virus production. For the first time, our findings indicate that CRM1-dependent shuttling is required for the efficient export of adenoviral early mRNA.

Adenovirus E4orf3 targets transcriptional intermediary factor 1γ for proteasome-dependent degradation during infection

The ability of adenovirus early region proteins, E1B-55K and E4orf6, to usurp control of cellular ubiquitin ligases and target proteins for proteasome-dependent degradation during infection is well established. Here we show that the E4 gene product, E4orf3 can, independently of E1B-55K and E4orf6, target the transcriptional corepressor transcriptional intermediary factor 1γ (TIF1γ) for proteasome-mediated degradation during infection. Initial mass spectrometric studies identified TIF1 family members-TIF1α, TIF1β, and TIF1γ-as E1B-55K-binding proteins in both transformed and infected cells, but analyses revealed that, akin to TIF1α, TIF1γ is reorganized in an E4orf3-dependent manner to promyelocytic leukemia protein-containing nuclear tracks during infection. The use of a number of different adenovirus early region mutants identified the specific and sole requirement for E4orf3 in mediating TIF1γ degradation. Further analyses revealed that TIF1γ is targeted for degradation by a number of divergent human adenoviruses, suggesting that the ability of E4orf3 to regulate TIF1γ expression is evolutionarily conserved. We also determined that E4orf3 does not utilize the Cullin-based ubiquitin ligases, CRL2 and CRL5, or the TIF1α ubiquitin ligase in order to promote TIF1γ degradation. Further studies suggested that TIF1γ possesses antiviral activity and limits adenovirus early and late gene product expression during infection. Indeed, TIF1γ knockdown accelerates the adenovirus-mediated degradation of MRE11, while TIF1γ overexpression delays the adenovirus-mediated degradation of MRE11. Taken together, these studies have identified novel adenovirus targets and have established a new role for the E4orf3 protein during infection.

The E3 ubiquitin ligase activity associated with the adenoviral E1B-55K-E4orf6 complex does not require CRM1-dependent export

The adenovirus type 5 (Ad5) E1B-55K and E4orf6 (E1B-55K/E4orf6) proteins are multifunctional regulators of Ad5 replication, participating in many processes required for virus growth. A complex containing the two proteins mediates the degradation of cellular proteins through assembly of an E3 ubiquitin ligase and induces shutoff of host cell protein synthesis through selective nucleocytoplasmic viral late mRNA export. Both proteins shuttle between the nuclear and cytoplasmic compartments via leucine-rich nuclear export signals (NES). However, the role of their NES-dependent export in viral replication has not been established. It was initially shown that mutations in the E4orf6 NES negatively affect viral late gene expression in transfection/infection complementation assays, suggesting that E1B-55K/E4orf6-dependent viral late mRNA export involves a CRM1 export pathway. However, a different conclusion was drawn from similar studies showing that E1B-55K/E4orf6 promote late gene expression without active CRM1 or functional NES. To evaluate the role of the E1B-55K/E4orf6 NES in viral replication in the context of Ad-infected cells and in the presence of functional CRM1, we generated virus mutants carrying amino acid exchanges in the NES of either or both proteins. Phenotypic analyses revealed that mutations in the NES of E1B-55K and/or E4orf6 had no or only moderate effects on viral DNA replication, viral late protein synthesis, or viral late mRNA export. Significantly, such mutations also did not interfere with the degradation of cellular substrates, indicating that the NES of E1B-55K or E4orf6 is dispensable both for late gene expression and for the activity associated with the E3 ubiquitin ligase.

The E4orf6/E1B55K E3 ubiquitin ligase complexes of human adenoviruses exhibit heterogeneity in composition and substrate specificity

Although human adenovirus type 5 (Ad5) has been widely studied, relatively little work has been done with other human adenovirus serotypes. The Ad5 E4orf6 and E1B55K proteins form Cul5-based E3 ubiquitin ligase complexes to degrade p53, Mre11, DNA ligase IV, integrin α3, and almost certainly other targets, presumably to optimize the cellular environment for viral replication and perhaps to facilitate persistence or latency. As this complex is essential for the efficient replication of Ad5, we undertook a systematic analysis of the structure and function of corresponding E4orf6/E1B55K complexes from other serotypes to determine the importance of this E3 ligase throughout adenovirus evolution. E4orf6 and E1B55K coding sequences from serotypes representing all subgroups were cloned, and each pair was expressed and analyzed for their capacity to assemble the Cullin-based ligase complex and to degrade substrates following plasmid DNA transfection. The results indicated that all formed Cullin-based E3 ligase complexes but that heterogeneity in both structure and function existed. Whereas Cul5 was present in the complexes of some serotypes, others recruited primarily Cul2, and the Ad16 complex clearly bound both Cul2 and Cul5. There was also heterogeneity in substrate specificity. Whereas all serotypes tested appeared to degrade DNA ligase IV, complexes from some serotypes failed to degrade Mre11, p53, or integrin α3. Thus, a major evolutionary pressure for formation of the adenovirus ligase complex may lie in the degradation of DNA ligase IV; however, it seems possible that the degradation of as-yet-unidentified critical targets or, perhaps even more likely, appropriate combinations of substrates plays a central role for these adenoviruses.

The adenovirus E1B 55-kilodalton and E4 open reading frame 6 proteins limit phosphorylation of eIF2alpha during the late phase of infection

During a productive infection, species C adenovirus reprograms the host cell to promote viral translation at the expense of cellular translation. The E1B 55-kilodalton (E1B-55K) and E4 open reading frame 6 (E4orf6) proteins are important in this control of gene expression. As part of a ubiquitin-protein ligase, these viral proteins stimulate viral mRNA export, inhibit cellular mRNA export, promote viral gene expression, and direct the degradation of certain host proteins. We report here that the E1B-55K and E4orf6 proteins limited phosphorylation of eIF2alpha and the activation of the eIF2alpha kinase PKR. Phospho-eIF2alpha levels were observed to rise and fall at least twice during infection. The E1B-55K and E4orf6 proteins prevented a third increase at late times of infection. PKR appeared to phosphorylate eIF2alpha only in the absence of E1B-55K/E4orf6 function. PKR activation and eIF2alpha phosphorylation was unrelated to the cytoplasmic levels of the adenovirus inhibitor of PKR, VA-I RNA. Nonetheless, expression of a PKR inhibitor, the reovirus double-stranded RNA-binding protein sigma 3, prevented PKR activation and eIF2alpha phosphorylation. The sigma 3 protein largely corrected the defect in viral late protein synthesis associated with the E1B-55K and E4orf6 mutant viruses without affecting cytoplasmic levels of the late viral mRNA. The ubiquitin-protein ligase activity associated with the E1B-55K/E4orf6 complex was necessary to prevent activation of PKR and phosphorylation of eIF2alpha. These findings reveal a new contribution of the E1B-55K/E4orf6 complex to viral late protein synthesis and the existence of multiple layers of regulation imposed on eIF2alpha phosphorylation and PKR activation in adenovirus-infected cells.

Evaluation of E1A double mutant oncolytic adenovectors in anti-glioma gene therapy

Malignant glioma, in particular glioblastoma multiforme (GBM), represents one of the most devastating cancers currently known and existing treatment regimens do little to change patient prognosis. Conditionally replicating adenoviral vectors (CRAds) represent attractive experimental anti-cancer agents with potential for clinical application. However, early protein products of the wild type adenovirus backbone--such as E1A--limit CRAds' replicative specificity. In this study, we evaluated the oncolytic potency and specificity of CRAds in which p300/CPB and/or pRb binding capacities of E1A were ablated to reduce non-specific replicative cytolysis. In vitro cytopathic assays, quantitative PCR analysis, Western blot, and flow cytometry studies demonstrate the superior anti-glioma efficacy of a double-mutated CRAd, Ad2/24CMV, which harbors mutations that reduce E1A binding to p300/CPB and pRb. When compared to its single-mutated and wild type counterparts, Ad2/24CMV demonstrated attenuated replication and cytotoxicity in representative normal human brain while displaying enhanced replicative cytotoxicity in malignant glioma. These results have implications for the development of double-mutated CRAd vectors for enhanced GBM therapy.

Control of mRNA export by adenovirus E4orf6 and E1B55K proteins during productive infection requires E4orf6 ubiquitin ligase activity

During the adenovirus infectious cycle, the early proteins E4orf6 and E1B55K are known to perform several functions. These include nuclear export of late viral mRNAs, a block of nuclear export of the bulk of cellular mRNAs, and the ubiquitin-mediated degradation of selected proteins, including p53 and Mre11. Degradation of these proteins occurs via a cellular E3 ubiquitin ligase complex that is assembled through interactions between elongins B and C and BC boxes present in E4orf6 to form a cullin 5-based ligase complex. E1B55K, which has been known for some time to associate with the E4orf6 protein, is thought to bind to specific substrate proteins to bring them to the complex for ubiquitination. Earlier studies with E4orf6 mutants indicated that the interaction between the E4orf6 and E1B55K proteins is optimal only when E4orf6 is able to form the ligase complex. These and other observations suggested that most if not all of the functions ascribed to E4orf6 and E1B55K during infection, including the control of mRNA export, are achieved through the degradation of specific substrates by the E4orf6 ubiquitin ligase activity. We have tested this hypothesis through the generation of a virus mutant in which the E4orf6 product is unable to form a ligase complex and indeed have found that this mutant behaves identically to an E4orf6(-) virus in production of late viral proteins, growth, and export of the late viral L5 mRNA.

Adenovirus ubiquitin-protein ligase stimulates viral late mRNA nuclear export

Theadenovirus type 5 (Ad5) E1B-55K and E4orf6 proteins are required together to stimulate viral late nuclear mRNA export to the cytoplasm and to restrict host cell nuclear mRNA export during the late phase of infection. Previous studies have shown that these two viral proteins interact with the cellular proteins elongins B and C, cullin 5, RBX1, and additional cellular proteins to form an E3 ubiquitin-protein ligase that polyubiquitinates p53 and probably one or more subunits of the MRE11-RAD50-NBS1 (MRN) complex, directing their proteasomal degradation. The MRN complex is required for cellular DNA double-strand break repair and induction of the DNA damage response by adenovirus infection. To determine if the ability of E1B-55K and E4orf6 to stimulate viral late mRNA nuclear export requires the ubiquitin-protein ligase activity of this viral ubiquitin-protein ligase complex, we designed and expressed a dominant-negative mutant form of cullin 5 in HeLa cells before infection with wild-type Ad5 or the E1B-55K null mutant dl1520. The dominant-negative cullin 5 protein stabilized p53 and the MRN complex, indicating that it inhibited the viral ubiquitin-protein ligase but had no effect on viral early mRNA synthesis, early protein synthesis, or viral DNA replication. However, expression of the dominant-negative cullin 5 protein caused a decrease in viral late protein synthesis and viral nuclear mRNA export similar to the phenotype produced by mutations in E1B-55K. We conclude that the stimulation of adenovirus late mRNA nuclear export by E1B-55K and E4orf6 results from the ubiquitin-protein ligase activity of the adenovirus ubiquitin-protein ligase complex.

Characterization of the recombinant adenovirus vector AdYB-1: implications for oncolytic vector development

Conditionally replicating adenoviruses are a promising new modality for the treatment of cancer. However, early clinical trials demonstrate that the efficacy of current vectors is limited. Interestingly, DNA replication and production of viral particles do not always correlate with virus-mediated cell lysis and virus release depending on the vector utilized for infection. However, we have previously reported that nuclear accumulation of the human transcription factor YB-1 by regulating the adenoviral E2 late promoter facilitates viral DNA replication of E1-deleted adenovirus vectors which are widely used for cancer gene therapy. Here we report the promotion of virus-mediated cell killing as a new function of the human transcription factor YB-1. In contrast to the E1A-deleted vector dl312 the first-generation adenovirus vector AdYB-1, which overexpresses YB-1 under cytomegalovirus promoter control, led to necrosis-like cell death, virus production, and viral release after infection of A549 and U2OS tumor cell lines. Our data suggest that the integration of YB-1 in oncolytic adenoviruses is a promising strategy for developing oncolytic vectors with enhanced potency against different malignancies.

Genetic identification of adenovirus type 5 genes that influence viral spread

The mechanisms that control cell-to-cell spread of human adenoviruses (Ad) are not well understood. Two early viral proteins, E1B-19K and E3-ADP, appear to have opposing effects since viral mutants that are individually deficient in E1B-19K produce large plaques (G. Chinnadurai, Cell 33:759-766, 1983), while mutants deficient in E3-ADP produce small plaques (A. E. Tollefson et al., J. Virol. 70:2296-2306, 1996) on infected cell monolayers. We have used a genetic strategy to identify different viral genes that influence adenovirus type 5 (Ad5) spread in an epithelial cancer cell line. An Ad5 mutant (dl327; lacking most of the E3 region) with the restricted-spread (small-plaque) phenotype was randomly mutagenized with UV, and 27 large-plaque (lp) mutants were isolated. A combination of analyses of viral proteins and genomic DNA sequences have indicated that 23 mutants contained lesions in the E1B region affecting either 19K or both 19K and 55K proteins. Four other lp mutants contained lesions in early regions E1A and E4, in the early L1 region that codes for the i-leader protein, and in late regions that code for the viral structural proteins, penton base, and fiber. Our results suggest that the requirement of E3-ADP for Ad spread could be readily compensated for by abrogation of the functions of E1B-19K and provide genetic evidence that these two viral proteins influence viral spread in opposing manners. In addition to E1B and E3 proteins, other early and late proteins that regulate viral replication and infectivity also influence lateral viral spread. Our studies have identified novel mutations that could be exploited in designing efficient oncolytic Ad vectors.

Recent lessons in gene expression, cell cycle control, and cell biology from adenovirus

Adenovirus continues to be an important model system for investigating basic aspects of cell biology. Interactions of several cellular proteins with E1A conserved regions (CR) 1 and 2, and inhibition of apoptosis by E1B proteins are required for oncogenic transformation. CR2 binds RB family members, de-repressing E2F transcription factors, thus activating genes required for cell cycling. E1B-19K is a BCL2 homolog that binds and inactivates proapoptotic BAK and BAX. E1B-55K binds p53, inhibiting its transcriptional activation function. In productively infected cells, E1B-55K and E4orf6 assemble a ubiquitin ligase with cellular proteins Elongins B and C, Cullin 5 and RBX1 that polyubiquitinates p53 and one or more subunits of the MRN complex involved in DNA double-strand break repair, directing them to proteosomal degradation. E1A CR3 activates viral transcription by interacting with the MED23 Mediator subunit, stimulating preinitiation complex assembly on early viral promoters and probably also the rate at which they initiate transcription. The viral E1B-55K/E4orf6 ubiquitin ligase is also required for efficient viral late protein synthesis in many cell types, but the mechanism is not understood. E1A CR1 binds several chromatin-modifying complexes, but how this contributes to stimulation of cellular DNA synthesis and transformation is not clear. E1A CR4 binds the CtBP corepressor, but the mechanism by which this modulates the frequency of transformation remains to be determined. Clearly, adenovirus has much left to teach us about fundamental cellular processes.

Viruses – seeking and destroying the tumor program

DNA viruses have enormous utility in cancer research, both as tools for tumor target discovery as well as agents for lytic cancer therapies. This is because there is a profound functional overlap between the DNA viral and tumor cell programs. DNA viruses encode proteins that elicit growth deregulation in infected cells similar to that engendered by mutations in tumor cells. Evolution has refined viral proteins to target the critical cellular hubs that regulate growth. Thus, viral proteins are discriminating biochemical probes that can be used to identify and characterize novel tumor targets. Moreover, the overlap between the DNA viral and tumor programs can also be exploited for the development of lytic cancer therapies. Discovering whether tumor cells selectively complement the replication of viral mutants can reveal novel oncolytic viral therapies, as well as unexpected tumor properties. For example, altered RNA export was recently uncovered as a novel tumor cell property that underlies ONYX-015 replication, a promising oncolytic adenoviral therapy. A perspective is provided on how adenovirus could be systematically exploited to map the requisite role, or indeed the redundancy, of cellular pathways that act in an integrated program to elicit pathological replication. This knowledge has important applications for the rational design of the next generation of oncolytic viruses, as well as the discovery of efficacious combination cancer therapies.

Diverse roles for E4orf3 at late times of infection revealed in an E1B 55-kilodalton protein mutant background

Species C human adenovirus mutants that fail to express open reading frame 3 of early region 4 (E4orf3) are phenotypically indistinguishable from the wild-type virus when evaluated in cells cultured in vitro. However, E4orf3 gene function has been productively studied in the context of additional viral mutations. This study identifies diverse roles for the E4orf3 protein that are evident in the absence of early region 1B 55-kDa protein (E1B-55K) function. In an E1B-55K-deficient background, the E4orf3 protein promotes viral replication by increasing both the burst size and the probability that an infected cell will produce virus. Early viral gene expression is not impaired in E1B-55K/E4orf3 double mutant virus-infected cells. Cells infected with the double mutant virus accumulated concatemers of viral DNA. However, the E1B-55K/E4orf3 double mutant virus did not replicate any better in MO59J cells, in which viral DNA concatemers did not accumulate, than in MO59K cells, in which viral DNA concatemers were produced, suggesting that viral DNA concatenation is not the primary growth defect of the E1B-55K/E4orf3 double mutant virus. Accumulation of viral mRNA in the nucleus and cytoplasm of E1B-55K/E4orf3 double mutant virus-infected cells was severely reduced compared to that on wild-type virus-infected cells. Thus, in an E1B-55K mutant background, the E4orf3 protein promotes the accumulation of late viral RNA and enhances late gene expression. Finally, within the context of an E1B-55K mutant virus, the E4orf3 protein acts to suppress host cell translation and preserve the viability of cells at moderately late times of infection.

A novel E1A-E1B mutant adenovirus induces glioma regression in vivo

Malignant gliomas are the most frequently occurring primary brain tumors and are resistant to conventional therapy. Conditionally replicating adenoviruses are a novel strategy in glioma treatment. Clinical trials using E1B mutant adenoviruses have been reported recently and E1A mutant replication-competent adenoviruses are in advanced preclinical testing. Here we constructed a novel replication-selective adenovirus (CB1) incorporating a double deletion of a 24 bp Rb-binding region in the E1a gene, and a 903 bp deleted region in the E1b gene that abrogates the expression of a p53-binding E1B-55 kDa protein. CB1 exerted a potent anticancer effect in vitro in U-251 MG, U-373 MG, and D-54 MG human glioma cell lines, as assessed by qualitative and quantitative viability assays. Replication analyses demonstrated that CB1 replicates in vitro in human glioma cells. Importantly, CB1 acquired a highly attenuated replicative phenotype in both serum-starved and proliferating normal human astrocytes. In vivo experiments using intracranially implanted D-54 MG glioma xenografts in nude mice showed that a single dose of CB1 (1.5 x 10(8) PFU/tumor) significantly improved survival. Immunohistochemical analyses of expressed adenoviral proteins confirmed adenoviral replication within the tumors. The CB1 oncolytic adenovirus induces a potent antiglioma effect and could ultimately demonstrate clinical relevance and therapeutic utility.

Inhibition of nucleo-cytoplasmic trafficking by RNA viruses: targeting the nuclear pore complex

Analysis of virus-host interactions has revealed a variety of ways in which viruses utilize and/or alter host functions in an effort to facilitate efficient replication. Recent work has suggested that certain RNA viruses that replicate in the cytoplasm disrupt the normal trafficking of cellular RNAs and proteins within the host cell. This review will examine the recent evidence showing that poliovirus and vesicular stomatitis virus (VSV) can inhibit nucleo-cytoplasmic transport within cells. Interestingly, the data indicate that inhibition by both viruses involves targeting components of the nuclear pore complex (NPC). Following this, several possible explanations for why viruses might disrupt nucleo-cytoplasmic transport are discussed. Finally, the possibility that disruption of nucleo-cytoplasmic trafficking may be a more common feature of RNA virus-host interactions than previously thought is examined.

Replication-deficient human adenovirus type 35 vectors for gene transfer and vaccination: efficient human cell infection and bypass of preexisting adenovirus immunity

Replication-deficient human adenovirus type 5 (Ad5) can be produced to high titers in complementing cell lines, such as PER.C6, and is widely used as a vaccine and gene therapy vector. However, preexisting immunity against Ad5 hampers consistency of gene transfer, immunological responses, and vector-mediated toxicities. We report the identification of human Ad35 as a virus with low global prevalence and the generation of an Ad35 vector plasmid system for easy insertion of heterologous genes. In addition, we have identified the minimal sequence of the Ad35-E1B region (molecular weight, 55,000 [55K]), pivotal for complementation of fully E1-lacking Ad35 vector on PER.C6 cells. After stable insertion of the 55K sequence into PER.C6 cells a cell line was obtained (PER.C6/55K) that efficiently transcomplements both Ad5 and Ad35 vectors. We further demonstrate that transduction with Ad35 is not hampered by preexisting Ad5 immunity and that Ad35 efficiently infects dendritic cells, smooth muscle cells, and synoviocytes, in contrast to Ad5.

An activity associated with human chromosome 21 permits nuclear colocalization of the adenovirus E1B-55K and E4orf6 proteins and promotes viral late gene expression

The adenovirus E1B-55K and E4orf6 proteins cooperate during virus infection while performing several tasks that contribute to a productive infection, including the selective nucleocytoplasmic transport of late viral mRNA. Previous studies have shown that the E4orf6 protein retains the E1B-55K protein in the nucleus of human and monkey cells, but not in those of rodents, suggesting that primate-specific cellular factors contribute to the E4orf6-mediated retention of the E1B-55K protein in the nucleus. In an effort to identify these proposed primate-specific cellular factors, the interaction of the E1B-55K and E4orf6 proteins was studied in a panel of stable human-rodent monochromosomal somatic cell hybrids. Analysis of this panel of cell lines has demonstrated the existence of an activity associated with human chromosome 21 that permits the E1B-55K and E4orf6 proteins to colocalize in the nucleus of a rodent cell. Additional hybrid cells bearing portions of human chromosome 21 were used to map this activity to a 10-megabase-pair segment of the chromosome, extending from 21q22.12 to a region near the q terminus. Strikingly, this region also facilitates the expression of adenovirus late genes in a rodent cell background while having little impact on the expression of early viral genes.

Mutual interference of adenovirus infection and myc expression

During infection with adenovirus, massive changes in the transcription of virus genes are observed, suggesting that the expression of cellular genes may also be modulated. To characterize the levels of cellular RNA species in infected cells, cDNA arrays were screened 24 h after infection of HeLa cells with wild-type adenovirus type 5, strain dl309. Despite complete transduction of the cells, fewer than 20 cellular genes (out of 4,600 analyzed and 1,200 found detectable and expressed above background) were altered more than threefold in their corresponding RNA levels compared to mock-infected cells. In particular, the expression of the myc oncogene was reduced at the mRNA level. This reduction was dependent on the replication of virus DNA and partially dependent on the presence of the adenovirus gene products E1B-55 kDa and E4orf6, but not E4orf3. On the other hand, MYC protein had an increased half-life in infected cells, resulting in roughly constant steady-state protein levels. The adenovirus E1A gene product is necessary and sufficient to stabilize MYC. Overexpressed MYC inhibited adenovirus replication and the proper formation of the virus replication centers. We conclude that adenovirus infection leads to the stabilization of MYC, perhaps as a side effect of E1A activities. On the other hand, myc mRNA levels are negatively regulated during adenovirus infection, and this may avoid the detrimental effect of excessive MYC on adenovirus replication.

Evidence that replication of the antitumor adenovirus ONYX-015 is not controlled by the p53 and p14(ARF) tumor suppressor genes

The adenovirus mutant ONYX-015 is in phase III clinical trials as a novel antitumor therapy. Its apparent efficacy is thought to be due to its ability to replicate selectively in tumor cells defective in the signaling pathway for p53. Recent data have shown that p14(ARF), a positive regulator of p53, inhibits ONYX-015 replication in cells with a wild-type p53, a phenotype that characterizes normal cells. We, however, found that ONYX-015 activates p53 in tumor cells and in normal cells and that this can occur without p14(ARF) induction. We also show that ONYX-015 is not attenuated in cells with functional p53, whether or not p14(ARF) is expressed, and that where attenuation does occur, it is cell type specific.

Analysis of the adenovirus E1B-55K-anchored proteome reveals its link to ubiquitination machinery

During the early phase of infection, the E1B-55K protein of adenovirus type 5 (Ad5) counters the E1A-induced stabilization of p53, whereas in the late phase, E1B-55K modulates the preferential nucleocytoplasmic transport and translation of the late viral mRNAs. The mechanism(s) by which E1B-55K performs these functions has not yet been clearly elucidated. In this study, we have taken a proteomics-based approach to identify and characterize novel E1B-55K-associated proteins. A multiprotein E1B-55K-containing complex was immunopurified from Ad5-infected HeLa cells and found to contain E4-orf6, as well as several cellular factors previously implicated in the ubiquitin-proteasome-mediated destruction of proteins, including Cullin-5, Rbx1/ROC1/Hrt1, and Elongins B and C. We further demonstrate that a complex containing these as well as other proteins is capable of directing the polyubiquitination of p53 in vitro. These ubiquitin ligase components were found in a high-molecular-mass complex of 800 to 900 kDa. We propose that these newly identified binding partners (Cullin-5, Elongins B and C, and Rbx1) complex with E1B-55K and E4-orf6 during Ad infection to form part of an E3 ubiquitin ligase that targets specific protein substrates for degradation. We further suggest that E1B-55K functions as the principal substrate recognition component of this SCF-type ubiquitin ligase, whereas E4-orf6 may serve to nucleate the assembly of the complex. Lastly, we describe the identification and characterization of two novel E1B-55K interacting factors, importin-alpha 1 and pp32, that may also participate in the functions previously ascribed to E1B-55K and E4-orf6.

Bak and Bax function to limit adenovirus replication through apoptosis induction

Adenovirus infection and expression of E1A induces both proliferation and apoptosis, the latter of which is blocked by the adenovirus Bcl-2 homologue E1B 19K. The mechanism of apoptosis induction and the role that it plays in productive infection are not known. Unlike apoptosis mediated by death receptors, infection with proapoptotic E1B 19K mutant viruses did not induce cleavage of Bid but nonetheless induced changes in Bak and Bax conformation, Bak-Bax interaction, caspase 9 and 3 activation, and apoptosis. In wild-type-adenovirus-infected cells, in which E1B 19K inhibits apoptosis, E1B 19K was bound to Bak, precluding Bak-Bax interaction and changes in Bax conformation. Infection with E1B 19K mutant viruses induced apoptosis in wild-type and Bax- or Bak-deficient baby mouse kidney cells but not in those deficient for both Bax and Bak. Furthermore, Bax and Bak deficiency dramatically increased E1A expression and virus replication. Thus, Bax- and Bak-mediated apoptosis severely limits adenoviral replication, demonstrating that Bax and Bak function as an antiviral response at the cellular level.

Effects of mutations in the adenoviral E1B 55-kilodalton protein coding sequence on viral late mRNA metabolism

The human subgroup C adenoviral E1B 55-kDa protein cooperates with the viral E4 Orf6 protein to induce selective export of viral, late mRNAs from the nucleus to the cytoplasm. Previous studies have suggested that such preferential transport of viral mRNA and the concomitant inhibition of export of cellular mRNAs are the result of viral colonization of specialized microenvironments within the nucleus. However, neither the molecular basis of this phenomenon nor the mechanism by which the E1B 55-kDa protein acts has been elucidated. We therefore examined viral late mRNA metabolism in HeLa cells infected with a series of mutant viruses that carry insertions at various positions in the E1B protein coding sequence (P. R. Yew, C. C. Kao, and A. J. Berk, Virology 179:795-805, 1990). All the mutations examined impaired cytoplasmic accumulation of viral L2 mRNAs and reduced L2 mRNA export efficiency. However, in most cases these defects could be ascribed to reduced E1B 55-kDa protein concentration or the unexpected failure of the altered E1B proteins to enter the nucleus efficiently. The latter property, the pleiotropic defects associated with all the mutations that impaired nuclear entry of the E1B protein, and consideration of its primary sequence suggest that these insertions result in misfolding of the protein. Insertion of four amino acids at residue 143 also inhibited viral mRNA export but resulted in increased rather than decreased accumulation of the E1B 55-kDa protein in the nucleus. This mutation specifically impaired the previously described association of the E1B protein with intranuclear structures that correspond to sites of adenoviral DNA replication and transcription (D. Ornelles and T. Shenk, J. Virol. 65:424-439, 1991) and the colocalization of the E1B and E4 Orf6 proteins. As this insertion has been shown to inhibit the interaction of the E1B with the E4 Orf6 protein in infected cell extracts (S. Rubenwolf, H. Schütt, M. Nevels, H. Wolf, and T. Dobner, J. Virol. 71:1115-1123, 1997), these phenotypes provide direct support for the hypothesis that selective viral mRNA export is determined by the functional organization of the infected cell nucleus.

ONYX-015: mechanisms of action and clinical potential of a replication-selective adenovirus

Accumulated knowledge in the molecular processes of tumour development combined with the availability of genetically modified viruses resemble the basis for new promising cancer therapeutics. The main advantages of employing replication-competent viruses are achievement of tumour selective killing and amplification of their oncolytic potential within the tumour mass. In this review, we describe the development of ONYX-015, one of the first and most advanced replication-competent viruses for cancer therapy. We discuss the molecular biology of this therapeutic approach and the interesting results obtained with this virus in clinical trials.

Analyses of single-amino-acid substitution mutants of adenovirus type 5 E1B-55K protein

The E1B-55K protein plays an important role during human adenovirus type 5 productive infection. In the early phase of the viral infection, E1B-55K binds to and inactivates the tumor suppressor protein p53, allowing efficient replication of the virus. During the late phase of infection, E1B-55K is required for efficient nucleocytoplasmic transport and translation of late viral mRNAs, as well as for host cell shutoff. In an effort to separate the p53 binding and inactivation function and the late functions of the E1B-55K protein, we have generated 26 single-amino-acid mutations in the E1B-55K protein. These mutants were characterized for their ability to modulate the p53 level, interact with the E4orf6 protein, mediate viral late-gene expression, and support virus replication in human cancer cells. Of the 26 mutants, 24 can mediate p53 degradation as efficiently as the wild-type protein. Two mutants, R240A (ONYX-051) and H260A (ONYX-053), failed to degrade p53 in the infected cells. In vitro binding assays indicated that R240A and H260A bound p53 poorly compared to the wild-type protein. When interaction with another viral protein, E4orf6, was examined, H260A significantly lost its ability to bind E4orf6, while R240A was fully functional in this interaction. Another mutant, T255A, lost the ability to bind E4orf6, but unexpectedly, viral late-gene expression was not affected. This raised the possibility that the interaction between E1B-55K and E4orf6 was not required for efficient viral mRNA transport. Both R240A and H260A have retained, at least partially, the late functions of wild-type E1B-55K, as determined by the expression of viral late proteins, host cell shutoff, and lack of a cold-sensitive phenotype. Virus expressing R240A (ONYX-051) replicated very efficiently in human cancer cells, while virus expressing H260A (ONYX-053) was attenuated compared to wild-type virus dl309 but was more active than ONYX-015. The ability to separate the p53-inactivation activity and the late functions of E1B-55K raises the possibility of generating adenovirus variants that retain the tumor selectivity of ONYX-015 but can replicate more efficiently than ONYX-015 in a broad spectrum of cell types.

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.

ONYX-015 selectivity and the p14ARF pathway

ONYX-015, dl1520, is an adenovirus that lacks the E1B 55K gene and therefore lacks the capacity to neutralize p53 during infection. This virus induces high levels of p53 and fails to grow efficiently in primary epithelial cells. However, it does replicate in many tumor cells, including those expressing wild-type p53. In these cells, ONYX-015 fails to induce active p53. This is because the pathway from Ela to p53 is disrupted through loss of p14ARF. We propose that high levels of Mdm2 activity resulting from loss of p14ARF, and high levels of Mdm2 protein resulting from activated Ras prevent accumulation of functional p53 during infection of tumor cells that retain wild-type p53.

The nuclear export signal within the E4orf6 protein of adenovirus type 5 supports virus replication and cytoplasmic accumulation of viral mRNA

During the late phase of adenovirus infection, viral mRNA is efficiently transported from the nucleus to the cytoplasm while most cellular mRNA species are retained in the nucleus. Two viral proteins, E1B-55 kDa and E4orf6, are both necessary for these effects. The E4orf6 protein of adenovirus type 5 binds and relocalizes E1B-55 kDa, and the complex of the two proteins was previously shown to shuttle continuously between the nucleus and cytoplasm. Nucleocytoplasmic transport of the complex is achieved by a nuclear export signal (NES) within E4orf6. Mutation of this signal sequence severely reduces the ability of the E1B-55 kDa-E4orf6 complex to leave the nucleus. Here, we examined the role of functional domains within E4orf6 during virus infection. E4orf6 or mutants derived from it were transiently expressed, followed by infection with recombinant adenovirus lacking the E4 region and determination of virus yield. An arginine-rich putative alpha helix near the carboxy terminus of E4orf6 contributes to E1B-55 kDa binding and relocalization as well as to the synthesis of viral DNA, mRNA, and proteins. Further mutational analysis revealed that mutation of the NES within E4orf6 considerably reduces its ability to support virus production. The same effect was observed when nuclear export was blocked with a competitor. Further, a functional NES within E4orf6 contributed to the efficiency of late virus protein synthesis and viral DNA replication, as well as total and cytoplasmic accumulation of viral late mRNA. Our data support the view that NES-mediated nucleocytoplasmic shuttling strongly enhances most, if not all, intracellular activities of E4orf6 during the late phase of adenovirus infection.

Roles for the E4 orf6, orf3, and E1B 55-kilodalton proteins in cell cycle-independent adenovirus replication

Adenoviruses bearing lesions in the E1B 55-kDa protein (E1B 55-kDa) gene are restricted by the cell cycle such that mutant virus growth is most impaired in cells infected during G(1) and least restricted in cells infected during S phase (F. D. Goodrum and D. A. Ornelles, J. Virol. 71:548-561, 1997). A similar defect is reported here for E4 orf6-mutant viruses. An E4 orf3-mutant virus was not restricted for growth by the cell cycle. However, orf3 was required for enhanced growth of an E4 orf6-mutant virus in cells infected during S phase. The cell cycle restriction may be linked to virus-mediated mRNA transport because both E1B 55-kDa- and E4 orf6-mutant viruses are defective at regulating mRNA transport at late times of infection. Accordingly, the cytoplasmic-to-nuclear ratio of late viral mRNA was reduced in G(1) cells infected with the mutant viruses compared to that in G(1) cells infected with the wild-type virus. By contrast, this ratio was equivalent among cells infected during S phase with the wild-type or mutant viruses. Furthermore, cells infected during S phase with the E1B 55-kDa- or E4 orf6-mutant viruses synthesized more late viral protein than did cells infected during G(1). However, the total amount of cytoplasmic late viral mRNA was greater in cells infected during G(1) than in cells infected during S phase with either the wild-type or mutant viruses, indicating that enhanced transport of viral mRNA in cells infected during S phase cannot account for the difference in yields in cells infected during S phase and in cells infected during G(1). Thus, additional factors affect the cell cycle restriction. These results indicate that the E4 orf6 and orf3 proteins, in addition to the E1B 55-kDa protein, may cooperate to promote cell cycle-independent adenovirus growth.

Role of the type 5 adenovirus gene encoding the early region 1B 55-kDa protein in pulmonary pathogenesis

Comparison of the inflammatory response of Sigmodon hispidus cotton rats to pulmonary infection with wild-type 5 adenovirus (Ad5) or with a viral mutant, in which the early region 1B gene encoding a 55-kDa protein, Ad5dl110 (dl110), was deleted, indicated that the inflammation in animals infected with dl110 was markedly reduced compared with the inflammation in animals infected with wild-type Ad5, although both viruses replicated to the same extent. Comparable experiments done with C57BL/6 mice yielded identical results, even though only the early phase of gene expression essential for viral replication occurs in mice. Cytokine analysis of infected mouse lungs indicated that tumor necrosis factor-alpha and IL-6 were produced in relatively large quantities in wild-type Ad5-infected mice and at significantly lower levels in dl110-infected mice during the early stages of infection.

p53-Independent and -dependent requirements for E1B-55K in adenovirus type 5 replication

The adenovirus type 5 mutant dl1520 was engineered previously to be completely defective for E1B-55K functions. Recently, this mutant (also known as ONYX-015) has been suggested to replicate preferentially in p53(-) and some p53(+) tumor cell lines but to be attenuated in primary cultured cells (C. Heise, A. Sampson-Johannes, A. Williams, F. McCormick, D. D. F. Hoff, and D. H. Kirn, Nat. Med. 3:639-645, 1997). It has been suggested that dl1520 might be used as a "magic bullet" that could selectively lyse tumor cells without harm to normal tissues. However, we report here that dl1520 replication is independent of p53 genotype and occurs efficiently in some primary cultured human cells, indicating that the mutant virus does not possess a tumor selectivity. Although it was not the sole host range determinant, p53 function did reduce dl1520 replication when analyzed in a cell line expressing temperature-sensitive p53 (H1299-tsp53) (K. L. Fries, W. E. Miller, and N. Raab-Traub, J. Virol. 70:8653-8659, 1996). As found earlier for other E1B-55K mutants in HeLa cells (Y. Ho, R. Galos, and J. Williams, Virology 122:109-124, 1982), dl1520 replication was temperature dependent in H1299 cells. When p53 function was restored at low temperature in H1299-tsp53 cells, it imposed a modest defect in viral DNA replication and accumulation of late viral cytoplasmic mRNA. However, in both H1299 and H1299-tsp53 cells, the defect in late viral protein synthesis appeared to be much greater than could be accounted for by the modest defects in late viral mRNA levels. We therefore propose that in addition to countering p53 function and modulating viral and cellular mRNA nuclear transport, E1B-55K also stimulates late viral mRNA translation.

Adenovirus type 5 E4orf3 protein relieves p53 inhibition by E1B-55-kilodalton protein

The E1B-55-kDa protein of adenovirus type 5 and the p53 tumor suppressor gene product form a complex that localizes to the cytoplasm, thereby downregulating p53's transcriptional activity. The E4orf6 protein binds and relocalizes E1B-55-kDa, and the proteins act synergistically to inactivate p53. We show that another adenovirus E4 gene product, E4orf3, is also sufficient to relocalize E1B-55-kDa from the cytoplasm to the nucleus. Both proteins are then found in discrete nuclear structures (tracks) that are known to contain components of the promyelocytic leukemia-associated nuclear structure. Simultaneously, p53 is dissociated from E1B-55-kDa and is found evenly distributed over the nucleoplasm. In the presence of E4orf3, p53-dependent transcriptional activity is no longer repressed by E1B-55-kDa. When E1B-55-kDa is coexpressed with E4orf3 and E4orf6, E1B-55-kDa is found to colocalize with E4orf6 rather than E4orf3. In parallel, p53 is inhibited and degraded by the combination of E1B-55-kDa and E4orf6, regardless of coexpressed E4orf3. This suggests that the effects of E4orf6 on E1B-55-kDa overrule the actions of E4orf3. When cells are infected with virus expressing E4orf3 but not E4orf6, E1B is found in the cell nucleus and p53 enters the virus replication centers. After infection with wild-type adenovirus, E4orf3 is expressed before E4orf6 and E1B temporarily colocalizes with E4orf3 in nuclear tracks before associating with E4orf6. We propose that during adenovirus infection, the E4orf3 protein transiently liberates p53 from its association with E1B-55-kDa. Subsequently, p53 is inactivated and degraded by the combination of E1B-55-kDa and E4orf6.

Analysis of synthesis, stability, phosphorylation, and interacting polypeptides of the 34-kilodalton product of open reading frame 6 of the early region 4 protein of human adenovirus type 5

The 34-kDa early-region 4 open reading frame 6 (E4orf6) product of human adenovirus type 5 forms complexes with both the cellular tumor suppressor p53 and the viral E1B 55-kDa protein (E1B-55kDa). E4orf6 can inhibit p53 transactivation activity, as can E1B-55kDa, and in combination these viral proteins cause the rapid turnover of p53. In addition, E4orf6-55kDa complexes play a critical role at later times in the regulation of viral mRNA transport and shutoff of host cell protein synthesis. In the present study, we have further characterized some of the biological properties of E4orf6. Analysis of extracts from infected cells by Western blotting indicated that E4orf6, like E1A and E1B products, is present at high levels until very late times, suggesting that it is available to act throughout the infectious cycle. This pattern is similar to that of E4orf4 but differs markedly from that of another E4 product, E4orf6/7, which is present only transiently. Synthesis of E4orf6 is maximal at early stages but ceases completely with the onset of shutoff of host protein synthesis; however, it was found that unlike E4orf6/7, E4orf6 is very stable, thus allowing high levels to be maintained even at late times. E4orf6 was shown to be phosphorylated at low levels. Coimmunoprecipitation studies in cells lacking p53 indicated that E4orf6 interacts with a number of other proteins. Five of these were shown to be viral or virally induced proteins ranging in size from 102 to 27 kDa, including E1B-55kDa. One such species, of 72 kDa, was shown not to represent the E2 DNA-binding protein and thus remains to be identified. Another appeared to be the L4 100-kDa nonstructural adenovirus late product, but it appeared to be present nonspecifically and not as part of an E4orf6 complex. Apart from p53, three additional cellular proteins, of 84, 19, and 14 kDa were detected by using an adenovirus vector that expresses only E4orf6. The 19-kDa species and a 16-kDa cellular protein were also shown to interact with E4orf6/7. It is possible that complex formation with these viral and cellular proteins plays a role in one or more of the biological activities associated with E4orf6 and E4orf6/7.

Definition of a major p53 binding site on Ad2E1B58K protein and a possible nuclear localization signal on the Ad12E1B54K protein

Previous studies have established that adenovirus 2/5 early region 1B (Ad E1B) 58K protein binds p53 strongly and co-localizes with it to cytoplasmic dense bodies whilst the homologous Ad12E1B54K protein binds only weakly and co-localizes primarily to the nucleus in Ad12E1 transformed cells. We have used these properties of the E1B proteins from different viral serotypes to map the p53 binding site on the Ad2/5 protein. A set of chimaeric genes was constructed containing different proportions of the Ad12 and Ad2E1B DNA. These, together with Ad12E1A and E1B19K DNA, were transfected into baby rat kidney cells and transformed lines isolated. From an examination of the properties of these Ad12/Ad2E1B fusion proteins in co-immunoprecipitation and subcellular localization experiments it has been concluded that the p53 binding site on Ad2E1B58K protein lies between amino acids 216 and 235 and that the homologous region on Ad12E1B54K protein also binds p53. In addition, a unique nuclear localization signal is located on Ad12E1B54K between residues 228 and 239. We suggest that primary structure differences in these regions of the Ad2 and Ad12E1B proteins are responsible for the different subcellular localizations in AdE1 transformants.

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.