Mutual interference of adenovirus infection and myc expression

Almost famous: Human adenoviruses (and what they have taught us about cancer)

Papillomaviruses, polyomaviruses and adenoviruses are collectively categorized as the small DNA tumour viruses. Notably, human adenoviruses were the first human viruses demonstrated to be able to cause cancer, albeit in non-human animal models. Despite their long history, no human adenovirus is a known causative agent of human cancers, unlike a subset of their more famous cousins, including human papillomaviruses and human Merkel cell polyomavirus. Nevertheless, seminal research using human adenoviruses has been highly informative in understanding the basics of cell cycle control, gene expression, apoptosis and cell differentiation. This review highlights the contributions of human adenovirus research in advancing our knowledge of the molecular basis of cancer.

Metabolic Control by DNA Tumor Virus-Encoded Proteins

Viruses co-opt a multitude of host cell metabolic processes in order to meet the energy and substrate requirements for successful viral replication. However, due to their limited coding capacity, viruses must enact most, if not all, of these metabolic changes by influencing the function of available host cell regulatory proteins. Typically, certain viral proteins, some of which can function as viral oncoproteins, interact with these cellular regulatory proteins directly in order to effect changes in downstream metabolic pathways. This review highlights recent research into how four different DNA tumor viruses, namely human adenovirus, human papillomavirus, Epstein-Barr virus and Kaposi's associated-sarcoma herpesvirus, can influence host cell metabolism through their interactions with either MYC, p53 or the pRb/E2F complex. Interestingly, some of these host cell regulators can be activated or inhibited by the same virus, depending on which viral oncoprotein is interacting with the regulatory protein. This review highlights how MYC, p53 and pRb/E2F regulate host cell metabolism, followed by an outline of how each of these DNA tumor viruses control their activities. Understanding how DNA tumor viruses regulate metabolism through viral oncoproteins could assist in the discovery or repurposing of metabolic inhibitors for antiviral therapy or treatment of virus-dependent cancers.

Time-resolved proteomics of adenovirus infected cells

Viral infections cause large problems in the world and deeper understanding of the disease mechanisms is needed. Here we present an analytical strategy to investigate the host cell protein changes during human adenovirus type 2 (HAdV-C2 or Ad2) infection of lung fibroblasts by stable isotope labelling of amino acids in cell culture (SILAC) and nanoLC-MS/MS. This work focuses on early phase of infection (6 and 12 h post-infection (hpi)) but the data is combined with previously published late phase (24 and 36 hpi) proteomics data to produce a time series covering the complete infection. As many as 2169 proteins were quantitatively monitored from 6 to 36 hpi, while some proteins were time-specific. After applying different filter criteria, 2027 and 2150 proteins were quantified at 6 and 12 hpi and among them, 431 and 544 were significantly altered at the two time points. Pathway analysis showed that the De novo purine and pyrimidine biosynthesis, Glycolysis and Cytoskeletal regulation by Rho GTPase pathways were activated early during infection while inactivation of the Integrin signalling pathway started between 6 and 12 hpi. Moreover, upstream regulator analysis predicted MYC to be activated with time of infection and protein and RNA data for genes controlled by this transcription factor showed good correlation, which validated the use of protein data for this prediction. Among the identified phosphorylation sites, a group related to glycolysis and cytoskeletal reorganization were up-regulated during infection. The results show specific aspects on how the host cell proteins, the final products in the genetic information flow, are influenced by Ad2 infection, which would be overlooked if only knowledge derived from mRNA data is considered.

Retinoic acid receptor β, a potential therapeutic target in the inhibition of adenovirus replication

Human adenoviruses (HAdVs) usually cause mild respiratory infections, but they can also lead to fatal outcomes for immunosuppressive patients. Unfortunately, there has been no specific anti-HAdV drug approved for medical use. A better understanding of the nature of virus-host interactions during infection is beneficial to the discovery of potential antiviral targets and new antiviral drugs. In this study, a time-course transcriptome analysis of HAdV-infected human lung epithelial cells (A549 cells) was performed to investigate virus-host interactions, and several key host molecules involved in the HAdV infection process were identified. The RARβ (retinoic acid receptor β) molecule, one of the upstream regulatory factors of differentially expressed genes (DEGs), played important roles in HAdV replication. The results of reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting showed that RARβ mRNA and protein were downregulated by HAdV infection in the A549 cells. The knockdown of RARβ by RARβ siRNA increased the HAdV production and the overexpression of RARβ decreased the HAdV production. Furthermore, FDA-approved Tazarotene, which is an RAR selective agonist with relatively more selectivity for RARβ, was found to inhibit HAdV replication in vitro. Taken together, our study presents a key host molecule in adenovirus infection, which could be developed as a potential host target to an anti-adenovirus drug. In addition, this study provides evidence for the re-exploitation of an FDA-approved small molecule for therapeutic applications in adenovirus replication.

The Cellular Protein Complex Associated with a Transforming Region of E1A Contains c-MYC

The cell-transforming activity of human adenovirus 5 (hAd5) E1A is mediated by the N-terminal half of E1A, which interacts with three different major cellular protein complexes, p300/CBP, TRRAP/p400, and pRb family members. Among these protein interactions, the interaction of pRb family proteins with conserved region 2 (CR2) of E1A is known to promote cell proliferation by deregulating the activities of E2F family transcription factors. The functional consequences of interaction with the other two protein complexes in regulating the transforming activity of E1A are not well defined. Here, we report that the E1A N-terminal region also interacted with the cellular proto-oncoprotein c-MYC and the homolog of enhancer of yellow 2 (ENY2). Our results suggested that these proteins interacted with an essential E1A transforming domain spanning amino acid residues 26 to 35 which also interacted with TRRAP and p400. Small interfering RNA (siRNA)-mediated depletion of TRRAP reduced c-MYC interaction with E1A, while p400 depletion did not. In contrast, depletion of TRRAP enhanced ENY2 interaction with E1A, suggesting that ENY2 and TRRAP may interact with E1A in a competitive manner. The same E1A region additionally interacted with the constituents of a deubiquitinase complex consisting of USP22, ATXN7, and ATXN7L3 via TRRAP. Acute short hairpin RNA (shRNA)-mediated depletion of c-MYC reduced the E1A transforming activity, while depletion of ENY2 and MAX did not. These results suggested that the association of c-MYC with E1A may, at least partially, play a role in the E1A transformation activity, independently of MAX.

IMPORTANCE

The transforming region of adenovirus E1A consists of three short modules which complex with different cellular protein complexes. The mechanism by which one of the transforming modules, CR2, promotes cell proliferation, through inactivating the activities of the pRb family proteins, is better understood than the activities of the other domains. Our analysis of the E1A proteome revealed the presence of the proto-oncoprotein c-MYC and of ENY2. We mapped these interactions to a critical transforming module of E1A that was previously known to interact with the scaffolding molecule TRRAP and the E1A-binding protein p400. We showed that c-MYC interacted with E1A through TRRAP, while ENY2 interacted with it independently. The data reported here indicated that depletion of c-MYC in normal human cells reduced the transforming activity of E1A. Our result raises a novel paradigm in oncogenic transformation by a DNA viral oncogene, the E1A gene, that may exploit the activity of a cellular oncogene, the c-MYC gene, in addition to inactivation of the tumor suppressors, such as pRb.

Replication and virus-induced transcriptome of HAdV-5 in normal host cells versus cancer cells--differences of relevance for adenoviral oncolysis

Adenoviruses (Ads), especially HAdV-5, have been genetically equipped with tumor-restricted replication potential to enable applications in oncolytic cancer therapy. Such oncolytic adenoviruses have been well tolerated in cancer patients, but their anti-tumor efficacy needs to be enhanced. In this regard, it should be considered that cancer cells, dependent on their tissue of origin, can differ substantially from the normal host cells to which Ads are adapted by complex virus-host interactions. Consequently, viral replication efficiency, a key determinant of oncolytic activity, might be suboptimal in cancer cells. Therefore, we have analyzed both the replication kinetics of HAdV-5 and the virus-induced transcriptome in human bronchial epithelial cells (HBEC) in comparison to cancer cells. This is the first report on genome-wide expression profiling of Ads in their native host cells. We found that E1A expression and onset of viral genome replication are most rapid in HBEC and considerably delayed in melanoma cells. In squamous cell lung carcinoma cells, we observed intermediate HAdV-5 replication kinetics. Infectious particle production, viral spread and lytic activity of HAdV-5 were attenuated in melanoma cells versus HBEC. Expression profiling at the onset of viral genome replication revealed that HAdV-5 induced the strongest changes in the cellular transcriptome in HBEC, followed by lung cancer and melanoma cells. We identified prominent regulation of genes involved in cell cycle and DNA metabolism, replication and packaging in HBEC, which is in accord with the necessity to induce S phase for viral replication. Strikingly, in melanoma cells HAdV-5 triggered opposing regulation of said genes and, in contrast to lung cancer cells, no weak S phase induction was detected when using the E2F promoter as reporter. Our results provide a rationale for improving oncolytic adenoviruses either by adaptation of viral infection to target tumor cells or by modulating tumor cell functions to better support viral replication.

Opposing oncogenic activities of small DNA tumor virus transforming proteins

The E1A gene of species C human adenovirus is an intensely investigated model viral oncogene that immortalizes primary cells and mediates oncogenic cell transformation in cooperation with other viral or cellular oncogenes. Investigations using E1A proteins have illuminated important paradigms in cell proliferation and about the functions of cellular proteins such as the retinoblastoma protein. Studies with E1A have led to the unexpected discovery that E1A also suppresses cell transformation and oncogenesis. Here, I review our current understanding of the transforming and tumor-suppressive functions of E1A, and how E1A studies led to the discovery of a related tumor-suppressive function in benign human papillomaviruses. The potential role of these opposing functions in viral replication in epithelial cells is also discussed.

Adenoviral E1A function through Myc

The study of DNA tumor viruses has been invaluable in uncovering the cellular nodes and pathways that contribute to oncogenesis. Perhaps one of the best-studied oncoproteins encoded by a DNA tumor virus is adenovirus E1A, which modifies the function of key regulatory proteins such as retinoblastoma (Rb) and the chromatin remodeling protein p400. Although the interaction of E1A with Rb has long been known to target regulation of the E2F transcription factors, the downstream target of the E1A-p400 interaction has remained elusive. We have recently reported that a critical downstream link of the E1A-p400 nexus is the oncoprotein transcription factor c-Myc. Through its interaction with p400, E1A stabilizes Myc and promotes formation of Myc-p400 complexes on chromatin, leading to activation of Myc target genes. These findings point to an important role for p400 in Myc function and reveal that E1A drives oncogenesis by tapping into two important transcriptional networks: those of E2F and Myc.

Adenovirus E4orf4 protein downregulates MYC expression through interaction with the PP2A-B55 subunit

The adenovirus E4 open reading frame 4 (E4orf4) protein is a multifunctional viral regulator that is involved in the temporal regulation of viral gene expression by modulating cellular and viral genes at the transcription and translation levels and by controlling alternative splicing of adenoviral late mRNAs. When expressed individually, E4orf4 induces apoptosis in transformed cells. Using oligonucleotide microarray analysis, validated by quantitative real time PCR, we found that MYC (also known as c-Myc) is downregulated early after the induction of E4orf4 expression. As a result, Myc protein levels are reduced in E4orf4-expressing cells. MYC downregulation is observed both when E4orf4 is expressed individually and within the context of viral infection. E4orf4 reduces MYC transcription but does not affect transcriptional elongation or RNA stability. An interaction with the PP2A-B55 subunit is required for the downregulation of MYC by E4orf4. Since Myc overexpression was previously shown to inhibit adenovirus replication, the downregulation of Myc by E4orf4 would contribute to efficient virus infection.

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.

Viral mRNA expression but not DNA replication is required for lipogenic effect of human adenovirus Ad-36 in preadipocytes

OBJECTIVE

Human adenovirus Ad-36 causes adiposity in animal models and shows association with human obesity. Ad-36 enhances differentiation of 3T3-L1 and human preadipocytes, without cell lysis, a characteristic that may contribute to its adipogenic effect observed in vivo. Ad-2, another human adenovirus is nonadipogenic in animals and in 3T3-L1 cells and shows no correlation with human obesity. The objective of this study was to determine the adipogenic roles of viral mRNA and DNA, which may explain the differential effects of Ad-36 and Ad-2 on preadipocyte differentiation.

METHODS

This study determined the duration of selected Ad-36 gene expression in 3T3-L1 cells, and the effect on preadipocytes differentiation, when Ad-36 gene expression was attenuated by Cidofovir, an antiadenoviral agent.

RESULTS

The results showed that Ad-36, but not Ad-2, expresses viral mRNA. Ad-36 gene expression peaked at 2-4 days postinoculation and very low levels persisted after day 7. Despite the viral mRNA expression, Ad-36 infection of 3T3-L1 cells was abortive as indicated by a progressive decrease in viral DNA quantity. Attenuation of Ad-36 mRNA expression by Cidofovir reduced the adipogenic effect of the virus.

CONCLUSION

In conclusion, viral mRNA expression, although transient, is a prerequisite for enhancing differentiation of preadipocytes by Ad-36. Viral DNA replication was not required for the effect. This is the first evidence for the role of gene expression of an adipogenic human virus in enhancing preadipocytes differentiation. This study provides the basis for further understanding novel regulatory modulators of preadipocytes differentiation.

Modulation of host cell gene expression during onset of the late phase of an adenovirus infection is focused on growth inhibition and cell architecture

Microarray analysis of host cell gene expression during an adenovirus type 2 infection showed that the number of regulated genes, as well as the magnitude of change, was increased as the infection proceeded into the late phase. In contrast to the early phase of infection when the majority of differentially expressed genes were upregulated, expression of most of the regulated genes (82 out of 112) declined during the late phase. In particular, numerous TGF-beta inducible genes and several TGF-beta-independent growth-arrest-inducing genes were targeted. Of the 30 genes upregulated more than 2-fold at 20 h post-infection, nearly two-thirds of encoded proteins are involved in cell metabolism. The data indicate that adenovirus primarily targets cellular genes involved in antiviral defense, cell growth arrest and apoptosis, as well as cell metabolism, to ensure sufficient production of viral progeny.

A real-time RT-PCR assay for the quantitative determination of adenoviral gene expression in tumor cells

Oncolytic adenoviruses are exploited as possible anticancer agents in clinical trails. To monitor adenoviral gene expression, a real-time RT-PCR method with a LightCycler was developed that allows the rapid and easy quantification of a number of early and late adenoviral genes in infected tumor cells. Primers were designed that can amplify the spliced forms of the genes encoding E1A13S, DNA polymerase (Pol), pre-terminal protein (pTP), adenoviral death protein (ADP), Hexon (Hex) and Penton (Pent) genes. Standard curves were generated using two-fold serial dilutions of cDNAs derived from non-small cell lung cancer (NSCLC) H460 cells infected for 24h with wild-type adenovirus serotype 5. For all genes correlation coefficients of the standard curves of 0.984 or higher were obtained. The dynamic range of the assay was sufficient to allow the quantitative determination of adenoviral gene expression during a lytic cycle. This RT-PCR assay could be used as a research tool to study the effect of host-cell factors or exogenous treatments on adenoviral gene expression. As example, it is shown that the procedure is suitable to detect changes in adenoviral gene expression in infected H460 cells treated with paclitaxel that is known to enhance the antitumor effect of oncolytic adenoviruses.

Identification of specific cellular genes up-regulated late in adenovirus type 12 infection

The infection of human cells by adenoviruses leads to a gradual reduction in the activity of host cell functions while viral gene expression progresses in a regulated way. We used the DNA microarray technique to determine the transcriptional activity profiles of cellular genes upon infection with adenovirus type 12 (Ad12). The microarray data were validated by quantitative real-time PCR for genes which showed significant alterations after Ad12 infection. At 12 h postinfection, there is a striking up-regulation between 10- and 30-fold in the expression of the G1P2, IFIT1, and IFIT2 cellular immune response genes compared to mock-infected cells. At later stages of infection, when the majority of regulated cellular genes has been turned down, a limited number of cellular genes exhibit increased activities by factors of 3 or less. These genes belong to the signal transduction or transcriptional regulator classes or are active in protein degradation, like ANPEP, an aminopeptidase. The SCD and CYP2S1 genes function in lipid metabolism. The eucaryotic translation initiation factor 4 is up-regulated, and one of the major histocompatibility complex genes is diminished in activity. For two of the genes, one up-regulated (CTSF gene) and one down-regulated (CYR61 gene), alterations in gene activity were confirmed at the protein level by Western blotting experiments. Increased genetic activity of cellular genes late in adenovirus infection has not been reported previously and demonstrates that Ad12 has a sustained control of host cell gene expression well into the late phase of infection.

Programmed cell death protein 4 suppresses CDK1/cdc2 via induction of p21(Waf1/Cip1)

We show that the recently discovered tumor suppressor pdcd4 represses the transcription of the mitosis-promoting factor cyclin-dependent kinase (CDK)1/cdc2 via upregulation of p21(Waf1/Cip1). p21(Waf1/Cip1) inhibits CDK4/6 and CDK2. Decrease of CDK4/6 and CDK2 enhances the binding of pRb to E2F/DP, which in turn together bind to and repress the cdc2 promoter. Upregulation of CDK1/cdc2 accompanied by a malignant change was previously reported in colon cancer. We show that expression of pdcd4 as an indirect suppressor of CDK1/cdc2 is lost in progressed carcinomas of lung, breast, colon, and prostate. Furthermore, it seems that localization and expression of pdcd4 directly correlate with tumor progression. Finally, the CDK1/cdc2 inhibitor roscovitine reduces the proliferation of several tumor cell lines, suggesting that inhibition of CDK1/cdc2 may be a useful strategy against malignant transformation. Therefore, pdcd4 might serve as a novel target for antineoplastic therapies.

E1B-55-kilodalton protein is not required to block p53-induced transcription during adenovirus infection

The adenovirus E1B-55-kDa protein binds and inactivates the tumor suppressor protein p53. However, the role of this interaction during infection is still poorly understood and was therefore examined here. Infection with a virus carrying the E1B-55-kDa mutation R239A, preventing the interaction with p53, led to the accumulation of p53. However, p53 target genes were not activated in the infected cells, although p53 phosphorylation did occur and the p53 antagonists Mdm2 and deltaNp73 did not accumulate. Deletion of E4orf6, alone or in combination with E1B-55-kDa, did not allow the induction of p53-responsive genes either. In transient reporter assays, the viral E1A-13S protein antagonized p53 activity; mutational analysis suggested that this depends partially on p300 binding, but it depends even more strongly on the interaction of E1A with the p400/TRRAP protein complex. However, viruses expressing E1A mutants lacking these binding activities, in combination with E1B-55-kDa R239A, still abolished p53 activity. In contrast, when the mutation of E1B-55-kDa at R239A was combined with a deletion of the apoptosis inhibitor E1B-19-kDa, infected cells showed more extensive apoptosis than after infection with single mutants, suggesting that accumulated p53, albeit transcriptionally inactive, might nonetheless enhance apoptosis. Despite extensive apoptosis of the infected cells, the deletion of E1B-19-kDa, in combination with the E1B-55-kDa mutation or in the presence of the constitutively active p53 mutant p53mt24-28, reduced virus replication less than fivefold. In conclusion, adenovirus does not need direct binding of E1B-55-kDa to inactivate p53, and forced p53 activity with consecutive apoptosis does not severely impair virus replication.