Regulation of the 26S proteasome by adenovirus E1A

A comparative review of adenovirus A12 and C5 oncogenes

Oncogenic viruses contribute to 15% of global human cancers. To achieve that, virus-encoded oncoproteins deregulate cellular transcription, antagonize common cellular pathways, and thus drive cell transformation. Notably, adenoviruses were the first human viruses proven to induce cancers in diverse animal models. Over the past decades, human adenovirus (HAdV)-mediated oncogenic transformation has been pivotal in deciphering underlying molecular mechanisms. Key adenovirus oncoproteins, encoded in early regions 1 (E1) and 4 (E4), co-ordinate these processes. Among the different adenovirus species, the most extensively studied HAdV-C5 displays lower oncogenicity than HAdV-A12. A complete understanding of the different HAdV-A12 and HAdV-C5 oncoproteins in virus-mediated cell transformation, as summarized here, is relevant for adenovirus research and offers broader insights into viral transformation and oncogenesis.

Ras Participates in the Regulation of the Stability of Adenoviral Protein E1A via MAP-kinase ERK

The E1A adenoviral protein required for the initiation of the viral life cycle is being actively studied as a sensitizing agent in the combination therapy of cancer, and tumors with activated Ras in particular. We investigated the role played by the Ras signaling pathway in the regulation of E1A protein stability and showed that overexpression of activated Ras increases the basal level of E1A, but enhances the degradation of the E1A protein under treatment with histone deacetylase inhibitors (HDIs). It has been found that the MAP kinase ERK is the key factor in E1A stabilization, and ERK inactivation upon HDI treatment reduces the E1A protein level. Our results indicate that the combination treatment of tumors with activated Ras using adenoviral E1A and HDI has limitations attributed to intense HDI-dependent degradation of E1A. Nevertheless, the established contribution of ERK kinase to the regulation of E1A stability can be used to search for new effective drug combinations based on the adenoviral E1A protein.

Expression of Adenoviral E1A in Transformed Cells as an Additional Factor of HDACi-Dependent FoxO Regulation

The adenoviral early region 1A (E1A) protein has proapoptotic and angiogenic activity, along with its chemosensitizing effect, making it the focus of increased interest in the context of cancer therapy. It was previously shown that E1A-induced chemosensitization to different drugs, including histone deacetylases inhibitors (HDACi), appears to be mediated by Forkhead box O (FoxO) transcription factors. In this study, we explore the relationship between E1A expression and the modulation of FoxO activity with HDACi sodium butyrate (NaBut). We show here that the basal FoxO level is elevated in E1A-expressing cells. Prolonged NaBut treatment leads to the inhibition of the FoxO expression and activity in E1A-expressing cells. However, in E1A-negative cells, NaBut promotes the transactivation ability of FoxO over time. A more detailed investigation revealed that the NaBut-induced decrease of FoxO activity in E1A-expressing cells is due to the NaBut-dependent decrease in E1A expression. Therefore, NaBut-induced inhibition of FoxO in E1A-positive cells can be overcome under unregulated overexpression of E1A. Remarkably, the CBP/p300-binding domain of E1Aad5 is responsible for stabilization of the FoxO protein. Collectively, these data show that the expression of E1A increases the FoxO stability but makes the FoxO level more sensitive to HDACi treatment.

Aberrant activation of cyclin A-CDK induces G2/M-phase checkpoint in human cells

Cyclin A-cyclin dependent kinase (CDK) activity is regulated by cyclin A proteolysis and CDK inhibitors (CKIs) during M and G1 phases. Our previous work has shown that constitutive activation of cyclin A-CDK in mouse somatic cells, by ectopic expression of stabilized human cyclin A2 (lacking the destruction box: CycAΔ80) in triple CKI (p21, p27, and p107)-knocked-out mouse embryonic fibroblasts, induces rapid tetraploidization. However, effects of such cyclin A-CDK hyperactivation in human cells have been unknown. Here, we show hyperactivity of cyclin A-CDK induces G2/M-phase arrest in human cell lines with relatively low expression of p21 and p27. Moreover, adenovirus E1A protein promoted CycAΔ80-derived G2/M-phase arrest by increasing the amount of cyclin A and cyclin A-CDK2 complex. This response was suppressed by an addition of ATR or Chk1 inhibitor. The amount of repressive phosphorylation of CDK1 at tyrosine 15 (Y15) was decreased by Chk1 inhibitor treatment. Moreover, we observed that co-expressing CDK1AF mutant, which is resistant to the repressive phosphorylation at threonine 14 and Y15, or cdc25A, which dephosphorylates CDK1 at Y15, suppressed the G2/M-phase arrest by CycAΔ80 with E1A. These results suggest that G2/M-phase arrest in human cells by hyperactivity of cyclin A-CDK2 is caused by repression of CDK1 via the cell cycle checkpoint ATR-Chk1 pathway.

Proteasomal Protein Degradation: Adaptation of Cellular Proteolysis With Impact on Virus-and Cytokine-Mediated Damage of Heart Tissue During Myocarditis

Viral myocarditis is an inflammation of the heart muscle triggered by direct virus-induced cytolysis and immune response mechanisms with most severe consequences during early childhood. Acute and long-term manifestation of damaged heart tissue and disturbances of cardiac performance involve virus-triggered adverse activation of the immune response and both immunopathology, as well as, autoimmunity account for such immune-destructive processes. It is a matter of ongoing debate to what extent subclinical virus infection contributes to the debilitating sequela of the acute disease. In this review, we conceptualize the many functions of the proteasome in viral myocarditis and discuss the adaptation of this multi-catalytic protease complex together with its implications on the course of disease. Inhibition of proteasome function is already highly relevant as a strategy in treating various malignancies. However, cardiotoxicity and immune-related adverse effects have proven significant hurdles, representative of the target's wide-ranging functions. Thus, we further discuss the molecular details of proteasome-mediated activity of the immune response for virus-mediated inflammatory heart disease. We summarize how the spatiotemporal flexibility of the proteasome might be tackled for therapeutic purposes aiming to mitigate virus-mediated adverse activation of the immune response in the heart.

Overexpression of Adenoviral E1A Sensitizes E1A+Ras-Transformed Cells to the Action of Histone Deacetylase Inhibitors

The adenoviral E1A protein induces cell proliferation, transformation, and tumor formation in rodents, on the one hand. On the other hand, E1A expression increases cell sensitivity to a number of cytotoxic agents. Therefore, E1A is a candidate for use as a component of combination therapy for malignant tumors. The highest augmentation in the cytotoxic effect was achieved by a combined use of E1A expression and histone deacetylases (HDAC) inhibitors. However, HDAC inhibitors do not induce apoptosis in cells transformed with E1A and cHa-ras oncogenes. In this study, it was shown that HDAC inhibitors reduce the expression of adenoviral E1A. However, under unregulated E1A overexpression, these cells undergo apoptosis in the presence of HDAC inhibitors. Treatment with a HDAC inhibitor, sodium butyrate (NaBut), was shown to activate the anti-apoptotic factor NF-kB in control cells. However, NaBut was unable to modulate the NF-kB activity in E1A overexpressed cells. Therefore, it is fair to postulate that cells transformed with E1A and cHa-ras oncogenes avoid the apoptosis induced by HDAC inhibitors thanks to a NaBut-dependent decrease in E1A expression.

Hacking the Cell: Network Intrusion and Exploitation by Adenovirus E1A

As obligate intracellular parasites, viruses are dependent on their infected hosts for survival. Consequently, viruses are under enormous selective pressure to utilize available cellular components and processes to their own advantage. As most, if not all, cellular activities are regulated at some level via protein interactions, host protein interaction networks are particularly vulnerable to viral exploitation. Indeed, viral proteins frequently target highly connected “hub” proteins to “hack” the cellular network, defining the molecular basis for viral control over the host. This widespread and successful strategy of network intrusion and exploitation has evolved convergently among numerous genetically distinct viruses as a result of the endless evolutionary arms race between pathogens and hosts. Here we examine the means by which a particularly well-connected viral hub protein, human adenovirus E1A, compromises and exploits the vulnerabilities of eukaryotic protein interaction networks. Importantly, these interactions identify critical regulatory hubs in the human proteome and help define the molecular basis of their function.

The Human Adenovirus Type 5 E4orf6/E1B55K E3 Ubiquitin Ligase Complex Enhances E1A Functional Activity

Following our demonstration that adenovirus E3 ubiquitin ligase formed by the viral E4orf6 and E1B55K proteins is able to mimic the activation of E2F by E1A, we conducted a series of studies to determine if this complex might also promote the ability of E1A to do so. We found that the complex both significantly stabilizes E1A proteins and also enhances their ability to activate E2F. This finding is of significance because it represents an entirely new function for the ligase in regulating adenovirus replication by enhancing the action of E1A products.

Human adenovirus (Ad) E1A proteins have long been known as the central regulators of virus infection as well as the major source of adenovirus oncogenic potential. Not only do they activate expression of other early viral genes, they make viral replication possible in terminally differentiated cells, at least in part, by binding to the retinoblastoma (Rb) tumor suppressor family of proteins to activate E2F transcription factors and thus viral and cellular DNA synthesis. We demonstrate in an accompanying article (F. Dallaire et al., mSphere 1:00014-15, 2016) that the human adenovirus E3 ubiquitin ligase complex formed by the E4orf6 and E1B55K proteins is able to mimic E1A activation of E2F transactivation factors. Acting alone in the absence of E1A, the Ad5 E4orf6 protein in complex with E1B55K was shown to bind E2F, disrupt E2F/Rb complexes, and induce 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. While these activities were significantly lower than those exhibited by E1A, we report here that this ligase complex appeared to enhance E1A activity in two ways. First, the E4orf6/E1B55K complex was shown to stabilize E1A proteins, leading to higher levels in infected cells. Second, the complex was demonstrated to enhance the activation of E2F by E1A products. These findings indicated a new role of the E4orf6/E1B55K ligase complex in promoting adenovirus replication.

IMPORTANCE Following our demonstration that adenovirus E3 ubiquitin ligase formed by the viral E4orf6 and E1B55K proteins is able to mimic the activation of E2F by E1A, we conducted a series of studies to determine if this complex might also promote the ability of E1A to do so. We found that the complex both significantly stabilizes E1A proteins and also enhances their ability to activate E2F. This finding is of significance because it represents an entirely new function for the ligase in regulating adenovirus replication by enhancing the action of E1A products.

KAP1 Is a Host Restriction Factor That Promotes Human Adenovirus E1B-55K SUMO Modification

Once transported to the replication sites, human adenoviruses (HAdVs) need to ensure decondensation and transcriptional activation of their viral genomes to synthesize viral proteins and initiate steps to reprogram the host cell for viral replication. These early stages during adenoviral infection are poorly characterized but represent a decisive moment in the establishment of a productive infection. Here, we identify a novel host viral restriction factor, KAP1. This heterochromatin-associated transcription factor regulates the dynamic organization of the host chromatin structure via its ability to influence epigenetic marks and chromatin compaction. In response to DNA damage, KAP1 is phosphorylated and functionally inactive, resulting in chromatin relaxation. We discovered that KAP1 posttranslational modification is dramatically altered during HAdV infection to limit the antiviral capacity of this host restriction factor, which represents an essential step required for efficient viral replication. Conversely, we also observed during infection an HAdV-mediated decrease of KAP1 SUMO moieties, known to promote chromatin decondensation events. Based on our findings, we provide evidence that HAdV induces KAP1 deSUMOylation to minimize epigenetic gene silencing and to promote SUMO modification of E1B-55K by a so far unknown mechanism.

IMPORTANCE

Here we describe a novel cellular restriction factor for human adenovirus (HAdV) that sheds light on very early modulation processes in viral infection. We reported that chromatin formation and cellular SWI/SNF chromatin remodeling play key roles in HAdV transcriptional regulation. We observed that the cellular chromatin-associated factor and epigenetic reader SPOC1 represses HAdV infection and gene expression. Here, we illustrate the role of the SPOC1-interacting factor KAP1 during productive HAdV growth. KAP1 binds to the viral E1B-55K protein, promoting its SUMO modification, therefore illustrating a crucial step for efficient viral replication. Simultaneously, KAP1 posttranslational modification is dramatically altered during infection. We observed an HAdV-mediated decrease in KAP1 SUMOylation, known to promote chromatin decondensation events. These findings indicate that HAdV induces the loss of KAP1 SUMOylation to minimize epigenetic gene silencing and to promote the SUMO modification of E1B-55K by a so far unknown mechanism.

PML isoforms IV and V contribute to adenovirus-mediated oncogenic transformation by functionally inhibiting the tumor-suppressor p53

Although modulation of the cellular tumor-suppressor p53 is considered to have the major role in E1A/E1B-55K-mediated tumorigenesis, other promyelocytic leukemia nuclear body (PML-NB)/PML oncogenic domain (POD)-associated factors including SUMO, Mre11, Daxx, as well as the integrity of these nuclear bodies contribute to the transformation process. However, the biochemical consequences and oncogenic alterations of PML-associated E1B-55K by SUMO-dependent PML-IV and PML-V interaction have so far remained elusive. We performed mutational analysis to define a PML interaction motif within the E1B-55K polypeptide. Our results showed that E1B-55K/PML binding is not required for p53, Mre11 and Daxx interaction. We also observed that E1B-55K lacking subnuclear PML localization because of either PML-IV or PML-V-binding deficiency was no longer capable of mediating E1B-55K-dependent SUMOylation of p53, inhibition of p53-mediated transactivation or efficiently transforming primary rodent cells. These results together with the observation that E1B-55K-dependent SUMOylation of p53 is required for efficient cell transformation, provides evidence for the idea that the SUMO ligase activity of the E1B-55K viral oncoprotein is intimately linked to its growth-promoting oncogenic activities.

Regulation of human adenovirus alternative RNA splicing by the adenoviral L4-33K and L4-22K proteins

Adenovirus makes extensive use of alternative RNA splicing to produce a complex set of spliced viral mRNAs. Studies aimed at characterizing the interactions between the virus and the host cell RNA splicing machinery have identified three viral proteins of special significance for the control of late viral gene expression: L4-33K, L4-22K, and E4-ORF4. L4-33K is a viral alternative RNA splicing factor that controls L1 alternative splicing via an interaction with the cellular protein kinases Protein Kinase A (PKA) and DNA-dependent protein kinase (DNA-PK). L4-22K is a viral transcription factor that also has been implicated in the splicing of a subset of late viral mRNAs. E4-ORF4 is a viral protein that binds the cellular protein phosphatase IIA (PP2A) and controls Serine/Arginine (SR)-rich protein activity by inducing SR protein dephosphorylation. The L4-33K, and most likely also the L4-22K protein, are highly phosphorylated in vivo. Here we will review the function of these viral proteins in the post-transcriptional control of adenoviral gene expression and further discuss the significance of potential protein kinases phosphorylating the L4-33K and/or L4-22K proteins.

Novel mechanism of JNK pathway activation by adenoviral E1A

The adenoviral oncoprotein E1A influences cellular regulation by interacting with a number of cellular proteins. In collaboration with complementary oncogenes, E1A fully transforms primary cells. As part of this action, E1A inhibits transcription of c-Jun:Fos target genes while promoting that of c-Jun:ATF2-dependent genes including jun. Both c-Jun and ATF2 are hyperphosphorylated in response to E1A. In the current study, E1A was fused with the ligand binding domain of the estrogen receptor (E1A-ER) to monitor the immediate effect of E1A activation. With this approach we now show that E1A activates c-Jun N-terminal kinase (JNK), the upstream kinases MKK4 and MKK7, as well as the small GTPase Rac1. Activation of the JNK pathway requires the N-terminal domain of E1A, and, importantly, is independent of transcription. In addition, it requires the presence of ERM proteins. Downregulation of signaling components upstream of JNK inhibits E1A-dependent JNK/c-Jun activation. Taking these findings together, we show that E1A activates the JNK/c-Jun signaling pathway upstream of Rac1 in a transcription-independent manner, demonstrating a novel mechanism of E1A action.

The proto-oncogene PBF binds p53 and is associated with prognostic features in colorectal cancer

The PTTG1-binding factor (PBF) is a transforming gene capable of eliciting tumor formation in xenograft models. However, the precise role of PBF in tumorigenesis and its prognostic value as a cancer biomarker remain largely uncharacterised, particularly in malignancies outside the thyroid. Here, we provide the first evidence that PBF represents a promising prognostic marker in colorectal cancer. Examination of a total of 39 patients demonstrated higher PBF expression at both the mRNA (P = 0.009) and protein (P < 0.0001) level in colorectal tumors compared to matched normal tissue. Critically, PBF was most abundant in colorectal tumors associated with Extramural Vascular Invasion (EMVI), increased genetic instability (GI) and somatic TP53 mutations, all features linked with recurrence and poorer patient survival. We further demonstrate by glutathione-S-transferase (GST) pull-down and coimmunoprecipitation that PBF binds to the tumor suppressor protein p53, as well as to p53 mutants (Δ126-132, M133K, V197E, G245D, I255F and R273C) identified in the colorectal tumors. Importantly, overexpression of PBF in colorectal HCT116 cells interfered with the transcriptional activity of p53-responsive genes such as mdm2, p21 and sfn. Diminished p53 stability (> 90%; P < 0.01) was also evident with a concurrent increase in ubiquitinated p53. Human colorectal tumors with wild-type TP53 and high PBF expression also had low p53 protein levels (P < 0.05), further emphasizing a putative interaction between these genes in vivo. Overall, these results demonstrate an emerging role for PBF in colorectal tumorigenesis through regulating p53 activity, with implications for PBF as a prognostic indicator for invasive tumors.

Transcriptome and comparative gene expression analysis of Sogatella furcifera (Horváth) in response to southern rice black-streaked dwarf virus

Background

The white backed planthopper (WBPH), Sogatella furcifera (Horváth), causes great damage to many crops by direct feeding or transmitting plant viruses. Southern rice black-streaked dwarf virus (SRBSDV), transmitted by WBPH, has become a great threat to rice production in East Asia.

Methodology/Principal Findings

By de novo transcriptome assembling and massive parallel pyrosequencing, we constructed two transcriptomes of WBPH and profiled the alternation of gene expression in response to SRBSDV infection in transcriptional level. Over 25 million reads of high-quality DNA sequences and 81388 different unigenes were generated using Illumina technology from both viruliferous and non-viruliferous WBPH. WBPH has a very similar gene ontological distribution to other two closely related rice planthoppers, Nilaparvata lugens and Laodelphax striatellus. 7291 microsatellite loci were also predicted which could be useful for further evolutionary analysis. Furthermore, comparative analysis of the two transcriptomes generated from viruliferous and non-viruliferous WBPH provided a list of candidate transcripts that potentially were elicited as a response to viral infection. Pathway analyses of a subset of these transcripts indicated that SRBSDV infection may perturb primary metabolism and the ubiquitin-proteasome pathways. In addition, 5.5% (181 out of 3315) of the genes in cell cytoskeleton organization pathway showed obvious changes. Our data also demonstrated that SRBSDV infection activated the immunity regulatory systems of WBPH, such as RNA interference, autophagy and antimicrobial peptide production.

Conclusions/Significance

We employed massively parallel pyrosequencing to collect ESTs from viruliferous and non-viruliferous samples of WBPH. 81388 different unigenes have been obtained. We for the first time described the direct effects of a Reoviridae family plant virus on global gene expression profiles of its insect vector using high-throughput sequencing. Our study will provide a road map for future investigations of the fascinating interactions between Reoviridae viruses and their insect vectors, and provide new strategies for crop protection.

The C-terminal region of E1A: a molecular tool for cellular cartography

The adenovirus E1A proteins function via protein–protein interactions. By making many connections with the cellular protein network, individual modules of this virally encoded hub reprogram numerous aspects of cell function and behavior. Although many of these interactions have been thoroughly studied, those mediated by the C-terminal region of E1A are less well understood. This review focuses on how this region of E1A affects cell cycle progression, apoptosis, senescence, transformation, and conversion of cells to an epithelial state through interactions with CTBP1/2, DYRK1A/B, FOXK1/2, and importin-α. Furthermore, novel potential pathways that the C-terminus of E1A influences through these connections with the cellular interaction network are discussed.

Transformation by E1A oncoprotein involves ubiquitin-mediated proteolysis of the neuronal and tumor repressor REST in the nucleus

The adenovirus early region 1A (E1A) protein promotes cell immortalization and transformation by mediating the activities of key cellular regulators. The repressor element 1-silencing transcription factor (REST), which is a major neuronal and tumor suppressor, was previously found mainly in the cytoplasm rather than in the nuclei of adenovirus-transformed rodent cells (22). We now demonstrate that the loss of REST in the nucleus is due to its rapid degradation by the ubiquitin-proteasome system. Only nuclear REST, but not its cytoplasmic counterpart, was ubiquitinated and degraded. REST degradation was blocked by the ubiquitination inhibitor PYR-41 and the proteasome inhibitor MG-132 but not by the nuclear export inhibitor leptomycin B. REST degradation required both of its two C-terminal degrons that are recognized by the ubiquitin ligase SCF(β-TrCP), since deletion or mutation of either degron eliminated degradation. Importantly, E1A was shown to mediate REST ubiquitination and degradation by upregulating β-TrCP. Knockdown of E1A in virus-transformed cells reduced both β-TrCP and ubiquitination of nuclear REST. In contrast, when expressed in HeLa cells, E1A enhanced the degradation of nuclear REST. Reconstitution of REST in virus-transformed cells negatively affected E1A-mediated cell proliferation and anchorage-independent growth. These data strongly indicate that E1A stimulates ubiquitination and proteolysis of REST in the nucleus, thereby abolishing the tumor suppressor functions of REST.

Characterization of the 55-residue protein encoded by the 9S E1A mRNA of species C adenovirus

Early region 1A (E1A) of human adenovirus (HAdV) has been the focus of over 30 years of investigation and is required for the oncogenic capacity of HAdV in rodents. Alternative splicing of the E1A transcript generates mRNAs encoding multiple E1A proteins. The 55-residue (55R) E1A protein, which is encoded by the 9S mRNA, is particularly interesting due to the unique properties it displays relative to all other E1A isoforms. 55R E1A does not contain any of the conserved regions (CRs) present in the other E1A isoforms. The C-terminal region of the 55R E1A protein contains a unique sequence compared to all other E1A isoforms, which results from a frameshift generated by alternative splicing. The 55R E1A protein is thought to be produced preferentially at the late stages of infection. Here we report the first study to directly investigate the function of the species C HAdV 55R E1A protein during infection. Polyclonal rabbit antibodies (Abs) have been generated that are capable of immunoprecipitating HAdV-2 55R E1A. These Abs can also detect HAdV-2 55R E1A by immunoblotting and indirect immunofluorescence assay. These studies indicate that 55R E1A is expressed late and is localized to the cytoplasm and to the nucleus. 55R E1A was able to activate the expression of viral genes during infection and could also promote productive replication of species C HAdV. 55R E1A was also found to interact with the S8 component of the proteasome, and knockdown of S8 was detrimental to viral replication dependent on 55R E1A.

Adenovirus E1A interacts directly with, and regulates the level of expression of, the immunoproteasome component MECL1

Proteasomes represent the major non-lysosomal mechanism responsible for the degradation of proteins. Following interferon γ treatment 3 proteasome subunits are replaced producing immunoproteasomes. Adenovirus E1A interacts with components of the 20S and 26S proteasome and can affect presentation of peptides. In light of these observations we investigated the relationship of AdE1A to the immunoproteasome. AdE1A interacts with the immunoproteasome subunit, MECL1. In contrast, AdE1A binds poorly to the proteasome β2 subunit which is replaced by MECL1 in the conversion of proteasomes to immunoproteasomes. Binding sites on E1A for MECL1 correspond to the N-terminal region and conserved region 3. Furthermore, AdE1A causes down-regulation of MECL1 expression, as well as LMP2 and LMP7, induced by interferon γ treatment during Ad infections or following transient transfection. Consistent with previous reports AdE1A reduced IFNγ-stimulated STAT1 phosphorylation which appeared to be responsible for its ability to reduce expression of immunoproteasome subunits.

Sprouty is a cytoplasmic target of adenoviral E1A oncoproteins to regulate the receptor tyrosine kinase signalling pathway

Background

Oncoproteins encoded by the early region of adenoviruses have been shown to be powerful tools to study gene regulatory mechanisms, which affect major cellular events such as proliferation, differentiation, apoptosis and oncogenic transformation. They are possesing a key role to favor viral replication via their interaction with multiple cellular proteins. In a yeast two-hybrid screen we have identified Sprouty1 (Spry1) as a target of adenoviral E1A Oncoproteins. Spry proteins are central and complex regulators of the receptor tyrosine kinase (RTK) signalling pathway. The deregulation of Spry family members is often associated with alterations of the RTK signalling and its downstream effectors, leading to the ERK pathway.

Results

Here, we confirm our yeast two-hybrid data, showing the interaction between Spry1 and E1A in GST pull-down and immunoprecipitation assays. We also demonstrated the interaction of E1A with two further Spry isoforms. Using deletion mutants we identified the N-terminus and the CR conserved region (CR) 3 of E1A- and the C-terminal half of Spry1, which contains the highly conserved Spry domain, as the essential sites for direct interaction between Spry and E1A. Immunofluorescent microscopy data revealed a co-localization of E1A_13S with Spry1 in the cytoplasm. SRE and TRE reporter assays demonstrated that co-expression of Spry1 with E1A_13S abolishes the inhibitory function of Spry1 in RTK signalling, which is consequently accompanied with a decrease of E1A_13S-induced gene expression.

Conclusions

These results establish Spry1 as a cytoplasmic localized cellular target for E1A oncoproteins to regulate the RTK signalling pathway, and consequently cellular events downstream of RTK that are essential for viral replication and transformation.

The critical protein interactions and structures that elicit growth deregulation in cancer and viral replication

One of the greatest challenges in biomedicine is to define the critical targets and network interactions that are subverted to elicit growth deregulation in human cells. Understanding and developing rational treatments for cancer requires a definition of the key molecular targets and how they interact to elicit the complex growth deregulation phenotype. Viral proteins provide discerning and powerful probes to understand both how cells work and how they can be manipulated using a minimal number of components. The small DNA viruses have evolved to target inherent weaknesses in cellular protein interaction networks to hijack the cellular DNA and protein replication machinery. In the battle to escape the inevitability of senescence and programmed cell death, cancers have converged on similar mechanisms, through the acquisition and selection of somatic mutations that drive unchecked cellular replication in tumors. Understanding the dynamic mechanisms through which a minimal number of viral proteins promote host cells to undergo unscheduled and pathological replication is a powerful strategy to identify critical targets that are also disrupted in cancer. Viruses can therefore be used as tools to probe the system-wide protein-protein interactions and structures that drive growth deregulation in human cells. Ultimately this can provide a path for developing system context-dependent therapeutics. This review will describe ongoing experimental approaches using viruses to study pathways deregulated in cancer, with a particular focus on viral cellular protein-protein interactions and structures.

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.

Activating transcription factor 3 activates p53 by preventing E6-associated protein from binding to E6

Genomic integration of human papillomavirus (HPV) DNA accounts for more than 90% of cervical cancers. High-risk genital HPVs encode E6 proteins that can interact with a cellular ubiquitin ligase E6-associated protein (E6AP) and target the tumor suppressor p53 for ubiquitin-mediated proteolysis. Currently, how this critical event is regulated is largely unknown. Here we report that activating transcription factor 3 (ATF3), a broad DNA damage sensor whose expression is frequently downregulated in cervical cancer, interacted with E6 and prevented p53 from ubiquitination and degradation mediated by the viral protein. Consistent with its role as a potent E6 antagonist, ATF3 expressed enforcedly in HPV-positive SiHa cells activated p53, leading to expression of p53-target genes (e.g. p21 and PUMA), cell cycle arrest and apoptotic cell death. The leucine zipper domain of ATF3 appears indispensable for these effects as an ATF3 mutant lacking this domain failed to interact with E6 and activate p53 in the cervical cancer cells. The prevention of p53 degradation was unlikely caused by binding of ATF3 to the tumor suppressor, but rather was a consequence of disruption of the E6-E6AP interaction by ATF3. These results indicate that ATF3 plays a key role in a mechanism defending against HPV-induced carcinogenesis, and could serve as a novel therapeutic target for HPV-positive cancers.

Adenovirus E1A and E1B-19K proteins protect human hepatoma cells from transforming growth factor beta1-induced apoptosis

Primary and some transformed hepatocytes undergo apoptosis in response to transforming growth factor beta1 (TGFbeta). We report that infection with species C human adenovirus conferred resistance to TGFbeta-induced apoptosis in human hepatocellular carcinoma cells (Huh-7). Protection against TGFbeta-mediated cell death in adenovirus-infected cells correlated with the maintenance of normal nuclear morphology, lack of pro-caspases 8 and 3 processing, maintenance of the mitochondrial membrane potential, and lack of cellular DNA degradation. The TGFbeta pro-apoptotic signaling pathway was blocked upstream of mitochondria in adenovirus-infected cells. Both the N-terminal sequences of the E1A proteins and the E1B-19K protein were necessary to protect infected cells against TGFbeta-induced apoptosis.

A novel mechanism of sodium iodide symporter repression in differentiated thyroid cancer

Differentiated thyroid cancers and their metastases frequently exhibit reduced iodide uptake, impacting on the efficacy of radioiodine ablation therapy. PTTG binding factor (PBF) is a proto-oncogene implicated in the pathogenesis of thyroid cancer. We recently reported that PBF inhibits iodide uptake, and have now elucidated a mechanism by which PBF directly modulates sodium iodide symporter (NIS) activity in vitro. In subcellular localisation studies, PBF overexpression resulted in the redistribution of NIS from the plasma membrane into intracellular vesicles, where it colocalised with the tetraspanin CD63. Cell-surface biotinylation assays confirmed a reduction in plasma membrane NIS expression following PBF transfection compared with vector-only treatment. Coimmunoprecipitation and GST-pull-down experiments demonstrated a direct interaction between NIS and PBF, the functional consequence of which was assessed using iodide-uptake studies in rat thyroid FRTL-5 cells. PBF repressed iodide uptake, whereas three deletion mutants, which did not localise within intracellular vesicles, lost the ability to inhibit NIS activity. In summary, we present an entirely novel mechanism by which the proto-oncogene PBF binds NIS and alters its subcellular localisation, thereby regulating its ability to uptake iodide. Given that PBF is overexpressed in thyroid cancer, these findings have profound implications for thyroid cancer ablation using radioiodine.

Rb function is required for E1A-induced S-phase checkpoint activation

It is widely accepted that adenoviral E1A exerts its influence on recipient cells through binding to the retinoblastoma (Rb) family proteins, followed by a global release of E2F factors from pocket-protein control. Our study challenges this simple paradigm by demonstrating previously unappreciated complexity. We show that E1A-expressing primary and transformed cells are characterized by the persistence of Rb-E2F1 complexes. We provide evidence that E1A causes Rb stabilization by interfering with its proteasomal degradation. Functional experiments supported by biochemical data reveal not only a dramatic increase in Rb and E2F1 protein levels in E1A-expressing cells but also demonstrate their activation throughout the cell cycle. We further show that E1A activates an Rb- and E2F1-dependent S-phase checkpoint that attenuates the growth of cells that became hyperploid through errors in mitosis and supports the fidelity DNA replication even in the absence of E2F complexes with other Rb family proteins, thereby functionally substituting for the loss of p53. Our results support the essential role of Rb and E2F1 in the regulation of genomic stability and DNA damage checkpoints.

Adenovirus E1A targets p400 to induce the cellular oncoprotein Myc

Adenovirus E1A drives oncogenesis by targeting key regulatory pathways that are critical for cellular growth control. The interaction of E1A with p400 is essential for many E1A activities, but the downstream target of this interaction is unknown. Here, we present evidence that the oncoprotein transcription factor Myc is the target of this interaction. We show that E1A stabilizes Myc protein via p400 and promotes the coassociation of Myc and p400 at Myc target genes, leading to their transcriptional induction. We also show that E1A requires Myc for its ability to activate Myc-dependent gene expression and induce apoptosis, and that forced expression of Myc is sufficient to rescue the activity of an E1A-mutant defective in p400 binding. Together, these findings establish that Myc, via p400, is an essential downstream target of E1A.

Adenovirus E1A proteins direct subcellular redistribution of Nek9, a NimA-related kinase

A monoclonal antibody raised against adenovirus E1A-associated cellular proteins recognized Nek9, a NimA-related protein kinase. Subcellular fractionation and immunofluorescence indicated that Nek9 was primarily cytoplasmic with a small portion located in the nucleus whereas E1A was primarily nuclear. Although co-immunoprecipitation experiments indicated that nuclear Nek9 interacted, directly or indirectly, with E1A, the major effect of E1A was to diminish the amount of Nek9 in the nucleus suggesting that E1A alters the subcellular distribution of Nek9 and that the interaction is transient. A Nek9 deletion mutant lacking a central RCC1-like domain interacted stably with E1A and accumulated in the nucleus in the presence of E1A, possibly representing an intermediate stage of the normally transient Nek9/E1A interaction. The interaction of Nek9 with E1A was dependent on the N-terminal sequences of E1A. Attempts to stably overexpress either Nek9 or the kinase-inactive mutant in various cell lines were unsuccessful; however, the presence of E1A allowed stable overexpression of both proteins. These results suggest that E1A disrupts a nuclear function of Nek9.

Inhibition of MITF transcriptional activity independent of targeting p300/CBP coactivators

Microphthalmia-associated transcription factor (MITF) activates the expression of melanocyte-specific markers and promotes the survival of embryonic, adult and malignant melanocytes. Although numerous MITF-dependent downstream genes have been identified, the mechanisms by which the MITF activity is coregulated remain elusive. Here we used a non-melanocytic cell line U2-OS as a model in which MITF evokes transcription of a paradigmatic MITF target tyrosinase and show that the adenoviral E1A protein represses the MITF-driven transcription in these cells. The E1A CR1 domain (which alone is insufficient to bind p300) was sufficient for repression, while the N-terminus, through which E1A binds the p300/CBP proteins and other coactivators, was unable to repress. Correspondingly, CR1 inhibited colony formation of MITF-positive, but not MITF-negative, melanoma cells. The repression by CR1 was largely independent of the PCAF-binding motif, previously recognized to be necessary for suppression of muscle-specific enhancer. Interestingly, CR1 conferred transcriptional competence to the MITF-CR1 chimera in which the MITF portion was rendered transcription-deficient. Moreover, MITF mutants defective in binding to p300/CBP in vivo still activated transcription, further supporting a p300/CBP-independent coactivation of MITF targets. MITF is amplified in a subset of melanomas and is thought to be required for sustained proliferation of malignant melanocytes. Our results suggest that understanding how CR1 represses Mitf activity may reveal a route to melanoma therapy.

Roles for APIS and the 20S proteasome in adenovirus E1A-dependent transcription

We have determined distinct roles for different proteasome complexes in adenovirus (Ad) E1A-dependent transcription. We show that the 19S ATPase, S8, as a component of 19S ATPase proteins independent of 20S (APIS), binds specifically to the E1A transactivation domain, conserved region 3 (CR3). Recruitment of APIS to CR3 enhances the ability of E1A to stimulate transcription from viral early gene promoters during Ad infection of human cells. The ability of CR3 to stimulate transcription in yeast is similarly dependent on the functional integrity of yeast APIS components, Sug1 and Sug2. The 20S proteasome is also recruited to CR3 independently of APIS and the 26S proteasome. Chromatin immunoprecipitation reveals that E1A, S8 and the 20S proteasome are recruited to both Ad early region gene promoters and early region gene sequences during Ad infection, suggesting their requirement in both transcriptional initiation and elongation. We also demonstrate that E1A CR3 transactivation and degradation sequences functionally overlap and that proteasome inhibitors repress E1A transcription. Taken together, these data demonstrate distinct roles for APIS and the 20S proteasome in E1A-dependent transactivation.

Mechanisms for human cytomegalovirus-induced cytoplasmic p53 sequestration in endothelial cells

Human cytomegalovirus (HCMV) infection results in endothelial dysfunction, typically known as dysregulated apoptosis, and aberrant expression and sub-cellular localization of p53, a tumor suppressor that accumulates at the late stage of infection. In this study, we examined three hypotheses that could be responsible for HCMV-induced cytoplasmic p53 accumulation at the later stage of infection: hyperactive nuclear export, cytoplasmic p53 tethering and delayed p53 degradation. Leptomycin B treatment, a nuclear export inhibitor, was unable to reduce cytoplasmic p53, thereby eliminating the hyperactive nuclear export mechanism. The findings that nascent p53 still entered nuclei after the nuclear export inhibition indicated that cytoplasmic tethering may play a minor role. Cytoplasmic p53 was still observed after the translation activities were blocked by cycloheximide. There was more than an eight-fold increase in the cytoplasmic p53 half-life with abnormal p53 ubiquitination. Taken together, these results suggest that delayed degradation could be responsible for the cytoplasmic p53 accumulation. The general slow-down of the proteasomal activity and the dysregulated p53 ubiquitination process at the later stage of infection could contribute to the reduced cytoplasmic p53 degradation and might be relevant to dysregulated endothelial apoptosis. The HCMV-induced changes in p53 dynamics could contribute to endothelial dysfunction.

Intracellular HIV-1 Tat protein represses constitutive LMP2 transcription increasing proteasome activity by interfering with the binding of IRF-1 to STAT1

The Tat protein is the transcriptional activator of HIV-1 gene expression, which is not only essential for viral replication, but also important in the complex HIV-induced pathogenesis of AIDS, as both an intracellular and an extracellular released protein. Accordingly, Tat is able to profoundly affect cellular gene expression, regulating several cellular functions, also in non-infected cells. We showed recently that Tat induces modification of immunoproteasomes in that it up-regulates LMP7 (low-molecular-mass polypeptide 7) and MECL1 (multicatalytic endopeptidase complex-like 1) subunits and down-modulates the LMP2 subunit, resulting in a change in the generation and presentation of epitopes in the context of MHC class I. In particular, Tat increases presentation of subdominant and cryptic epitopes. In the present study, we investigated the molecular mechanism responsible for the Tat-induced LMP2 down-regulation and show that intracellular Tat represses transcription of the LMP2 gene by competing with STAT1 (signal transducer and activator of transcription 1) for binding to IRF-1 (interferon-regulatory factor-1) on the overlapping ICS-2 (interferon consensus sequence-2)-GAS (gamma-interferon-activated sequence) present in the LMP2 promoter. This element is constitutively occupied in vivo by the unphosphorylated STAT1-IRF-1 complex, which is responsible for the basal transcription of the gene. Sequestration of IRF-1 by intracellular Tat impairs the formation of the complex resulting in lower LMP2 gene transcription and LMP2 protein expression, which is associated with increased proteolytic activity. On the other hand, extracellular Tat induces the expression of LMP2. These effects of Tat provide another effective mechanism by which HIV-1 affects antigen presentation in the context of the MHC class I complex and may have important implications in the use of Tat for vaccination strategies.

The APC/C and CBP/p300 cooperate to regulate transcription and cell-cycle progression

The anaphase-promoting complex/cyclosome (APC/C) is a multicomponent E3 ubiquitin ligase that, by targeting protein substrates for 26S proteasome-mediated degradation through ubiquitination, coordinates the temporal progression of eukaryotic cells through mitosis and the subsequent G1 phase of the cell cycle. Other functions of the APC/C are, however, less well defined. Here we show that two APC/C components, APC5 and APC7, interact directly with the coactivators CBP and p300 through protein-protein interaction domains that are evolutionarily conserved in adenovirus E1A. This interaction stimulates intrinsic CBP/p300 acetyltransferase activity and potentiates CBP/p300-dependent transcription. We also show that APC5 and APC7 suppress E1A-mediated transformation in a CBP/p300-dependent manner, indicating that these components of the APC/C may be targeted during cellular transformation. Furthermore, we establish that CBP is required in APC/C function; specifically, gene ablation of CBP by RNA-mediated interference markedly reduces the E3 ubiquitin ligase activity of the APC/C and the progression of cells through mitosis. Taken together, our results define discrete roles for the APC/C-CBP/p300 complexes in growth regulation.

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

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

Ubiquitin-dependent degradation of adenovirus E1A protein is inhibited by BS69

Adenovirus E1A protein perturbs the cell cycle and promotes cell transformation. Although E1A is relatively unstable, regulation of E1A stability has not been fully elucidated. Here, we showed that E1A was ubiquitinated and degraded using a proteasome in vivo system. Interestingly, we found that BS69, one of the E1A-binding proteins, inhibited ubiquitination of E1A. BS69 mutants lacking the MYND domain could not bind to E1A and did not inhibit ubiquitination of E1A. Moreover, we demonstrated that overexpression of BS69 stabilized E1A in vivo. These results suggest that BS69 controls E1A stability via inhibition of ubiquitination.

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.

Mitochondrial localization of p53 during adenovirus infection and regulation of its activity by E1B-19K

Recent results have revealed that the p53 tumor suppressor protein possesses a direct transcription-independent apoptotic activity. During apoptosis induced by genotoxic stress, a small fraction of p53 is targeted to mitochondria where it initiates apoptosis by causing mitochondrial dysfunction. In adenovirus-infected cells, the expression of E1A protein enhances the accumulation of p53 during early phases of infection and during late times after infection, it is targeted for degradation by the combined action of E1B-55K and E4-orf6 proteins. The functional significance of E1A-mediated accumulation of p53 during early phases of viral replication is not known. Our studies with isogenic epithelial cell lines that differ only on the status of p53 indicate that Ad infection induces apoptosis by p53-dependent and -independent pathways and both pathways are suppressed by E1B-19K. We show that during early phase of Ad infection, a fraction of p53 is targeted to the mitochondria. In virus infected cells, a large fraction of the viral antiapoptosis protein E1B-19K is also localized in mitochondria during early and late phases of infection. Coimmunoprecipitation analysis has revealed that p53 and E1B-19K form a complex in mitochondria. The interaction of 19K involves two noncontiguous regions located around amino-acid residues 14-15 and 123-124. On p53, the mutations within the DNA-binding domain reduce interaction with E1B-19K. Our studies also suggest that 19K may additionally complex with the multidomain mitochondrial proapoptotic protein BAK, thereby reducing the level of p53 interaction with BAK. We suggest that p53-induced apoptosis may be important for efficient cell lysis and viral spread and that E1B-19K may neutralize the apoptotic activity of p53 at multiple levels.

Recruitment of CBP/p300, TATA-binding protein, and S8 to distinct regions at the N terminus of adenovirus E1A

The N-terminal region of the adenovirus (Ad) 12S E1A gene product targets several cellular proteins that are essential for the induction of S phase, cellular immortalization, cellular transformation, transcriptional repression, and transcriptional activation. The precise binding sites for these proteins, however, remain to be resolved. We therefore undertook an extensive site-directed mutagenesis approach to generate specific point mutants and to precisely map the binding sites for CBP, p300, TATA-binding protein (TBP), S4, S8, hGcn5, P/CAF, and Ran within the first 30 amino acids of the Ad5 12S E1A protein. We determined that although common residues within the N-terminal region can form partial binding sites for these proteins, point mutants were also generated that could discriminate between binding sites. These data indicate that AdE1A can target each of these proteins individually through distinct binding sites. It was evident, however, that the mutation of specific hydrophobic residues typically had the greatest effect upon AdE1A's ability to bind individual partners. Indeed, the mutation of L at positions 19 and 20 eliminated the ability of AdE1A to interact with any of the N-terminal binding proteins studied here. Interestingly, although TBP and S8 or CBP/p300 can exist as functional complexes, RNA interference revealed that the recruitment of either TBP, S8, or CBP/p300 to AdE1A was not dependent upon the expression of the other proteins. These data further indicate that AdE1A can target individual partner proteins in vivo and that it does not necessarily recruit these proteins indirectly as components of larger macromolecular complexes. Finally, we took advantage of the fine-mapping data to ascertain which proteins were targeted during the transformation process. Consistent with previous studies, CBP/p300 was found to be targeted by AdE1A during this process, although our data suggest that binding to other N-terminal proteins is also important for transformation.

The proteasome and MHC class I antigen processing

By generating peptides from intracellular antigens, which are then presented to T cells, the ubiquitin/26S proteasome system plays a central role in the cellular immune response. Under the control of interferon-gamma the proteolytic properties of the proteasome are adapted to the requirements of the immune system. Interferon-gamma induces the formation of immunoproteasomes and the synthesis of the proteasome activator PA28. Both alter the proteolytic properties of the proteasome complex and enhance proteasomal function in antigen presentation. Thus, a combination of several of regulatory events tunes the proteasome system for maximal efficiency in the generation of MHC class I antigens.

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

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

The targeting of the proteasomal regulatory subunit S2 by adenovirus E1A causes inhibition of proteasomal activity and increased p53 expression

Although adenovirus early region 1A (AdE1A) can modulate protein expression through its interaction with transcriptional regulators it can also influence the ability of the cell to degrade proteins by binding to components of the 26 S proteasome. We demonstrate here that AdE1A interacts with the S2 subunit of the 19 S regulatory complex in addition to the ATPase subunits S4 and S8 previously identified. S2 forms complexes with both the 13 and 12 S AdE1A proteins both in vivo and in vitro. Mutational analysis has shown direct binding through a short sequence toward the N terminus of conserved region 2 of AdE1A, which encompasses the LXCXE motif, involved in interaction with the pRb family of proteins. In vivo, additional contacts are made between AdE1A and proteasomal components, as well as within the proteasome, such that deletion of the N-terminal region of E1A as well as part of conserved region 2 is required to completely disrupt S2 binding. Mutation of AdE1A, which disrupts complex formation with S2, results in the loss of its ability to stabilize the p53 protein. Similarly down-regulation of S2 expression using small interfering RNAs leads to the inhibition of p53 degradation. These effects were observed in normally growing cells and those subjected to UV irradiation. Furthermore, AdE1A had no effect on the Mdm2-mediated ubiquitination of p53. We suggest therefore that interaction of AdE1A with S2, as well as with the ATPases S4 and S8, directly causes inhibition of proteasomal activity and consequent increase in the protein levels of p53.

The components of the proteasome system and their role in MHC class I antigen processing

By generating peptides from intracellular antigens which are then presented to T cells, the ubiquitin/26S proteasome system plays a central role in the cellular immune response. The proteolytic properties of the proteasome are adapted to the requirements of the immune system by proteasome components whose synthesis is under the control of interferon-gamma. Among these are three subunits with catalytic sites that are incorporated into the enzyme complex during its de novo synthesis. Thus, the proteasome assembly pathway and the formation of immunoproteasomes play a critical regulatory role in the regulation of the proteasome's catalytic properties. In addition, interferon-gamma also induces the synthesis of the proteasome activator PA28 which, as part of the so-called hybrid proteasome, exerts a more selective function in antigen presentation. Consequently, the combination of a number of regulatory events tunes the proteasome system to gain maximal efficiency in the generation of peptides with regard to their quality and quantity.

Evasion of the immune system by adenoviruses

Human adenoviruses (Ads) have the ability to transform primary cells, and certain Ads, the subgenus A adenoviruses such as Ad12, induce tumours in immunocompetent rodents. The oncogenic phenotype of the subgenus A adenoviruses is determined by the viral E1A oncogene. In order to generate tumours, Ad12-transformed cells must evade the cellular immune system of the host. Ad12 E1A gene products mediate transcriptional repression of several genes in the major histocompatibility complex (MHC) involved in antigen processing and presentation, resulting in evasion of cytotoxic T lymphocyte (CTL) killing of transformed cells. In this review, the molecular mechanisms of E1A-mediated transcriptional repression of MHC gene expression are described. In addition, evasion of natural killer (NK) cell killing by Ad-transformed cells is also considered.

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.

Phosphorylation influences the translation state of FMRP-associated polyribosomes

Fragile X mental retardation protein, FMRP, is absent in patients with fragile X syndrome, a common form of mental retardation. FMRP is a nucleocytoplasmic RNA binding protein that is primarily associated with polyribosomes. FMRP is believed to be a translational repressor and may regulate the translation of certain mRNAs at the base of dendritic spines in neurons. However, little is known about the regulation of FMRP. Using mass spectrometry and site-directed mutagenesis, we show that FMRP is phosphorylated between residues 483 and 521, N-terminal to the RGG box, both in murine brain and in cultured cells. Primary phosphorylation occurs on the highly conserved serine 499, which triggers hierarchical phosphorylation of nearby serines. FMRP is phosphorylated within 2-4 h of synthesis, however, phosphorylation has no effect on the half-life of the protein. In contrast to the Drosophila ortholog dFxr, the phosphorylation status of mammalian FMRP does not influence its association with specific mRNAs in vivo. However, we find unphosphorylated FMRP associated with actively translating polyribosomes while a fraction of phosphorylated FMRP is associated with apparently stalled polyribosomes. Our data suggest that the phosphorylation may regulate FMRP and that the release of FMRP-induced translational suppression may involve a dephosphorylation signal.

Transforming growth factor beta1 receptor II is downregulated by E1A in adenovirus-infected cells

Transforming growth factor beta1 (TGF-beta1) signaling is compromised in many tumors, thereby allowing the tumor to escape the growth-inhibitory and proapoptotic activities of the cytokine. Human adenoviruses interfere with a number of cellular pathways involved in cell cycle regulation and apoptosis, initially placing the cell in a "tumor-like" state by forcing quiescent cells into the cell cycle and also inhibiting apoptosis. We report that adenovirus-infected cells resemble tumor cells in that TGF-beta1 signaling is inhibited. The levels of TGF-beta1 receptor II (TbetaRII) in adenovirus-infected cells were decreased, and this decrease was mapped, by using virus mutants, to the E1A gene and to amino acids 2 to 36 and the C-terminal binding protein binding site in the E1A protein. The decrease in the TbetaRII protein was accompanied by a decrease in TbetaRII mRNA. The decrease in TbetaRII protein levels in adenovirus-infected cells was greater than the decrease in TbetaRII mRNA, suggesting that downregulation of the TbetaRII protein may occur through more than one mechanism. Surprisingly in this context, the half-lives of the TbetaRII protein in infected and uninfected cells were similar. TGF-beta1 signaling was compromised in cells infected with wild-type adenovirus, as measured with 3TP-lux, a TGF-beta-sensitive reporter plasmid expressing luciferase. Adenovirus mutants deficient in TbetaRII downregulation did not inhibit TGF-beta1 signaling. TGF-beta1 pretreatment reduced the relative abundance of adenovirus structural proteins in infected cells, an effect that was potentiated when cells were infected with mutants incapable of modulating the TGF-beta signaling pathway. These results raise the possibility that inhibition of TGF-beta signaling by E1A is a means by which adenovirus counters the antiviral defenses of the host.

The multifunctional role of E1A in the transcriptional regulation of CREB/CBP-dependent target genes

Oncoproteins encoded by the early region 1A (E1A) of adenoviruses (Ads) have been shown to be powerful tools to study gene regulatory mechanisms. As E1A proteins lack a sequence-specific DNA-binding activity, they modulate viral and cellular gene expression by interacting directly with a diverse array of cellular factors, among them sequence-specific transcription factors, proteins of the general transcription machinery, co-activators and chromatin-modifying enzymes. By making use of these factors, E1A affects major cellular events such as cell cycle control, differentiation, apoptosis, and oncogenic transformation. In this review we will focus on the interaction of E1A with cellular components involved in the cAMP/PKA signal transduction pathway and we will discuss the consequences of these interactions in respect to the activation of CREB/CBP-dependent target genes.

A point mutation in the first splice donor leads to reduced oncogenic properties of the adenovirus serotype 12 E1A gene

Cells transformed by proteins of early regions 1A (E1A) and 1B (E1B) of oncogenic adenovirus serotype 12 (Ad12) grow to tumours in syngeneic, immunocompetent rodents. To gain insight into the mechanisms of oncogenic transformation, we point mutated the first splice donor in the Ad12-E1A gene, leading to the loss of the Ad12-E1A(9.5S) and Ad12-E1A(11S/10S) proteins and to a conservative amino acid (aa) exchange at position aa 30 (valine vs. leucine) in the Ad12-E1A(13S) and Ad12-E1A(12S ) proteins. BMK cells transformed by mutant Ad12-E1A (Ad12-E1Am) plus Ad12-E1B via retrovirus-mediated gene transfer showed features comparable to wild-type Ad12-E1A (Ad12-E1Awt) plus Ad12-E1B-transformed cells: they formed foci in soft agar and produced tumours in immunodeficient nude mice, although after a prolonged latency period. These results suggest that Ad12-E1A(9.5S) and Ad12-E1A(11S/10S) are dispensable for cellular transformation. However, in contrast to Ad12-E1Awt cells, Ad12-E1Am cells failed to grow to tumours in syngeneic, immunocompetent rodents, with the exception of one cell line, which produced tumours in about 50% of the immunocompetent animals. Interestingly, the concentration of the putative tumour suppressor and co-activator p300 was elevated in cell lines expressing high levels of Ad12-E1A and Ad12-E1B due to an increased half-life. These results indicate that p300 is stabilized in Ad12-E1-transformed BMK cells, probably by a mechanism linked to high expression of Ad12-E1A/E1B.

Radiation increases the activity of oncolytic adenovirus cancer gene therapy vectors that overexpress the ADP (E3-11.6K) protein

We have described three potential adenovirus type 5 (Ad5)-based replication-competent cancer gene therapy vectors named KD1, KD3, and VRX-007. All three vectors overexpress an Ad5 protein named Adenovirus Death Protein (ADP, also named E3-11.6 K protein). ADP is required for efficient lysis of Ad5-infected cells and spread of virus from cell to cell, and thus its overexpression increases the oncolytic activity of the vectors. KD1 and KD3 contain mutations in the Ad5 E1A gene that knock out binding of the E1A proteins to cellular p300/CBP and pRB; these mutations allow KD1 and KD3 to grow well in cancer cells but not in normal cells. VRX-007 has wild-type E1A. Here we report that radiation increases the oncolytic activity of KD1, KD3, and VRX-007. This increased activity was observed in cultured cells, and it was not because of radiation-induced replication of the vectors. The combination of radiation plus KD3 suppressed the growth of A549 lung adenocarcinoma xenografts in nude mice more efficiently than radiation alone or KD3 alone. The combination of ADP-overexpressing vectors and radiation may have potential in treating cancer.