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

Bombyx mori Nucleopolyhedrovirus (BmNPV) Induces G2/M Arrest to Promote Viral Multiplication by Depleting BmCDK1

Simple Summary

Baculoviruses arrest the cell cycle in the S or G2/M phase in insect cells, but the exact mechanism of this process still remains obscure. Bombyx mori nucleopolyhedrovirus (BmNPV), one of the best characterized baculoviruses, is an important pathogen in silkworms. In the present study, we determined that downregulation of BmCDK1 and BmCyclin B expression was required for BmNPV-mediated G2/M phase arrest, which plays an essential role in facilitating BmNPV replication. Further investigations showed that BmNPV IAP1 interacted with BmCDK1. The overexpression of the BmNPV iap1 gene led to the accumulation of cells in the G2/M phase, and BmNPV iap1 gene knockdown attenuated the effect of BmNPV-mediated G2/M phase arrest. These findings enhance the understanding of BmNPV pathogenesis, and indicate a novel mechanism through which baculoviruses impact the cell cycle progression.

Abstract

Understanding virus–host interaction is very important for delineating the mechanism involved in viral replication and host resistance. Baculovirus, an insect virus, can cause S or G2/M phase arrest in insect cells. However, the roles and mechanism of Baculovirus-mediated S or G2/M phase arrest are not fully understood. Our results, obtained using flow cytometry (FCM), tubulin-labeling, BrdU-labeling, and CellTiter 96^® AQueous One Solution Cell Proliferation Assay (MTS), showed that Bombyx mori nucleopolyhedrovirus (BmNPV) induced G2/M phase arrest and inhibited cellular DNA replication as well as cell proliferation in BmN-SWU1 cells. We found that BmNPV induced G2/M arrest to support its replication and proliferation by reducing the expression of BmCDK1 and BmCyclin B. Co-immunoprecipitation assays confirmed that BmNPV IAP1 interacted with BmCDK1. BmNPV iap1 was involved in the process of BmNPV-induced G2/M arrest by reducing the content of BmCDK1. Taken together, our results improve the understanding of the virus–host interaction network, and provide a potential target gene that connects apoptosis and the cell cycle.

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.

Interplay between p300 and HDAC1 regulate acetylation and stability of Api5 to regulate cell proliferation

Api5, is a known anti-apoptotic and nuclear protein that is responsible for inhibiting cell death in serum-starved conditions. The only known post-translational modification of Api5 is acetylation at lysine 251 (K251). K251 acetylation of Api5 is responsible for maintaining its stability while the de-acetylated form of Api5 is unstable. This study aimed to find out the enzymes regulating acetylation and deacetylation of Api5 and the effect of acetylation on its function. Our studies suggest that acetylation of Api5 at lysine 251 is mediated by p300 histone acetyltransferase while de-acetylation is carried out by HDAC1. Inhibition of acetylation by p300 leads to a reduction in Api5 levels while inhibition of deacetylation by HDAC1 results in increased levels of Api5. This dynamic switch between acetylation and deacetylation regulates the localisation of Api5 in the cell. This study also demonstrates that the regulation of acetylation and deacetylation of Api5 is an essential factor for the progression of the cell cycle.

The Adenovirus Death Protein - a small membrane protein controls cell lysis and disease

Human adenoviruses (HAdVs) cause widespread acute and persistent infections. Infections are usually mild and controlled by humoral and cell-based immunity. Reactivation of persistently infected immune cells can lead to a life-threatening disease in immunocompromised individuals, especially children and transplant recipients. To date, no effective therapy or vaccine against HAdV disease is available to the public. HAdV-C2 and C5 are the best-studied of more than 100 HAdV types. They persist in infected cells and release their progeny by host cell lysis to neighbouring cells and fluids, a process facilitated by the adenovirus death protein (ADP). ADP consists of about 100 amino acids and harbours a single membrane-spanning domain. It undergoes post-translational processing in endoplasmic reticulum and Golgi compartments, before localizing to the inner nuclear membrane. Here, we discuss the current knowledge on how ADP induces membrane rupture. Membrane rupture is essential for both progression of disease and efficacy of therapeutic viruses in clinical applications, in particular oncolytic therapy.

Cell transformation by the adenovirus oncogenes E1 and E4

Extensive studies on viral-mediated oncogenic transformation by human adenoviruses have revealed much of our current understanding on the molecular mechanisms that are involved in the process. To date, these studies have shown that cell transformation is a multistep process regulated by the cooperation of several adenoviral gene products encoded in the early regions 1 (E1) and 4 (E4). Early region 1A immortalizes primary rodent cells, whereas co-expression of early region protein 1B induces full manifestation of the transformed phenotype. Beside E1 proteins, also some E4 proteins have partial transforming activities through regulating many cellular pathways. Here, we summarize recent data of how adenoviral oncoproteins may contribute to viral transformation and discuss the challenge of pinpointing the underlying mechanisms.

Epigenetics and the dynamics of chromatin during adenovirus infections

The DNA genome of eukaryotic cells is compacted by histone proteins within the nucleus to form chromatin. Nuclear-replicating viruses such as adenovirus have evolved mechanisms of chromatin manipulation to promote infection and subvert host defenses. Epigenetic factors may also regulate persistent adenovirus infection and reactivation in lymphoid tissues. In this review, we discuss the viral proteins E1A and protein VII that interact with and alter host chromatin, as well as E4orf3, which separates host chromatin from sites of viral replication. We also highlight recent advances in chromatin technologies that offer new insights into virus-directed chromatin manipulation. Beyond the role of chromatin in the viral replication cycle, we discuss the nature of persistent viral genomes in lymphoid tissue and cell lines, and the potential contribution of epigenetic signals in maintaining adenovirus in a quiescent state. By understanding the mechanisms through which adenovirus manipulates host chromatin, we will understand new aspects of this ubiquitous virus and shed light on previously unknown aspects of chromatin biology.

Coxsackievirus A6 Induces Cell Cycle Arrest in G0/G1 Phase for Viral Production

Recent epidemiological data indicate that outbreaks of hand, foot, and mouth disease (HFMD), which can be categorized according to its clinical symptoms as typical or atypical, have markedly increased worldwide. A primary causative agent for typical HFMD outbreaks, enterovirus 71 (EV71), has been shown to manipulate the cell cycle in S phase for own replication; however, it is not clear whether coxsackievirus (CVA6), the main agent for atypical HFMD, also regulates the host cell cycle. In this study, we demonstrate for the first time that CVA6 infection arrests the host cell cycle in G0/G1-phase. Furthermore, synchronization in G0/G1 phase, but not S phase or G2/M phase, promotes viral production. To investigate the mechanism of cell cycle arrest induced by CVA6 infection, we analyzed cell cycle progression after cell cycle synchronization at G0/G1 or G2/M. Our results demonstrate that CVA6 infection promotes G0/G1 phase entry from G2/M phase, and inhibits G0/G1 exit into S phase. In line with its role to arrest cells in G0/G1 phase, the expression of cyclinD1, CDK4, cyclinE1, CDK2, cyclinB1, CDK1, P53, P21, and P16 is regulated by CVA6. Finally, the non-structural proteins of CVA6, RNA-dependent RNA polymerase 3D and protease 3C , are demonstrated to be responsible for the G0/G1-phase arrest. These findings suggest that CVA6 infection arrested cell cycle in G0/G1-phase via non-structural proteins 3D and 3C, which may provide favorable environments for virus production.

Nuclear targeting of the betanodavirus B1 protein via two arginine-rich domains induces G1/S cell cycle arrest mediated by upregulation of p53/p21

The molecular functions of betanodavirus non-structural protein B and its role in host cell survival remain unclear. In the present study, we examined the roles of specific nuclear targeting domains in B1 localization as well as the effect of B1 nuclear localization on the cell cycle and host cell survival. The B1 protein of the Red spotted grouper nervous necrosis virus (RGNNV) was detected in GF-1 grouper cells as early as 24 hours post-infection (hpi). Using an EYFP-B1 fusion construct, we observed nuclear localization of the B1 protein (up to 99%) in GF-1 cells at 48 hpi. The nuclear localization of B1 was mediated by two arginine-rich nuclear targeting domains (B domain: ⁴⁶RRSRR⁵¹; C domain: ⁶³RDKRPRR⁷⁰) and domain C was more important than domain B in this process. B1 nuclear localization correlated with upregulation of p53 and p21^(wef1/cip1); downregulation of Cyclin D1, CDK4 and Mdm2; and G1/S cell cycle arrest in GF-1 cells. In conclusion, nuclear targeting of the RGNNV B1 protein via two targeting domains causes cell cycle arrest by up-regulating p53/p21 and down-regulating Mdm2, thereby regulating host cell survival.

The Adenovirus E1A C Terminus Suppresses a Delayed Antiviral Response and Modulates RAS Signaling

The N-terminal half of adenovirus e1a assembles multimeric complexes with host proteins that repress innate immune responses and force host cells into S-phase. In contrast, the functions of e1a's C-terminal interactions with FOXK, DCAF7, and CtBP are unknown. We found that these interactions modulate RAS signaling, and that a single e1a molecule must bind all three of these host proteins to suppress activation of a subset of IFN-stimulated genes (ISGs). These ISGs were otherwise induced in primary respiratory epithelial cells at 12 hr p.i. This delayed activation of ISGs required IRF3 and coincided with an ∼10-fold increase in IRF3 from protein stabilization. The induced IRF3 bound to chromatin and localized to the promoters of activated ISGs. While IRF3, STAT1/2, and IRF9 all greatly increased in concentration, there were no corresponding mRNA increases, suggesting that e1a regulates the stabilities of these key activators of innate immune responses, as shown directly for IRF3.

Exploitation of EP300 and CREBBP Lysine Acetyltransferases by Cancer

p300 and CREB-binding protein (CBP), two homologous lysine acetyltransferases in metazoans, have a myriad of cellular functions. They exert their influence mainly through their roles as transcriptional regulators but also via nontranscriptional effects inside and outside of the nucleus on processes such as DNA replication and metabolism. The versatility of p300/CBP as molecular tools has led to their exploitation by viral oncogenes for cellular transformation and by cancer cells to achieve and maintain an oncogenic phenotype. How cancer cells use p300/CBP in their favor varies depending on the cellular context and is evident by the growing list of loss- and gain-of-function genetic alterations in p300 and CBP in solid tumors and hematological malignancies. Here, we discuss the biological functions of p300/CBP and how disruption of these functions by mutations and alterations in expression or subcellular localization contributes to the cancer phenotype.

Human Enterovirus 68 Interferes with the Host Cell Cycle to Facilitate Viral Production

Enterovirus D68 (EV-D68) is an emerging pathogen that recently caused a large outbreak of severe respiratory disease in the United States and other countries. Little is known about the relationship between EV-D68 virus and host cells. In this study, we assessed the effect of the host cell cycle on EV-D68 viral production, as well as the ability of EV-D68 to manipulate host cell cycle progression. The results suggest that synchronization in G0/G1 phase, but not S phase, promotes viral production, while synchronization in G2/M inhibits viral production. Both an early EV-D68 isolate and currently circulating strains of EV-D68 can manipulate the host cell cycle to arrest cells in the G0/G1 phase, thus providing favorable conditions for virus production. Cell cycle regulation by EV-D68 was associated with corresponding effects on the expression of cyclins and CDKs, which were observed at the level of the protein and/or mRNA. Furthermore, the viral non-structural protein 3D of EV-D68 prevents progression from G0/G1 to S. Interestingly, another member of the Picornaviridae family, EV-A71, differs from EV-D68 in that G0/G1 synchronization inhibits, rather than promotes, EV-A71 viral replication. However, these viruses are similar in that G2/M synchronization inhibits the production and activity of both viruses, which is suggestive of a common therapeutic target for both types of enterovirus. These results further clarify the pathogenic mechanisms of enteroviruses and provide a potential strategy for the treatment and prevention of EV-D68-related disease.

Pseudolaric acid B inhibits the secretion of hepatitis B virus

High hepatitis B virus (HBV) load and chronic hepatitis B infection increase the risk of developing hepatocellular carcinoma (HCC), and is also associated with recurrence of HBV-related HCC. The aim of the present study was to investigate whether pseudolaric acid B (PAB), a diterpene acid isolated from the root and trunk bark of Pseudolarix kaempferi Gordon (Pinaceae), has an inhibitory role on the HBV secretion in HBV-related HCC. By detecting HBV surface antigen (HBsAg) by ELISA it was found that PAB inhibited HBV secretion in HepG2215 compared to control group, but did not decrease the intracellular HBV level, and the results were repeated in HepG2 cell transfect with HBV gene. Therefore, our results proved that PAB had the ability to inhibit HBV secretion. Moreover, it was shown that HepG2215 cells with HBV gene accumulated more in G0/G1 phase than HepG2 cells without HBV gene through detecting cell cycle distribution by flow cytometry, which indicated that HBV replication might favor the cell cycle environment of G0/G1 phase. However, HepG2 cells entered G2/M phase earlier than HepG2215 when PAB treatment induced G2/M arrest, therefore, HBV retarded the entry of G2/M to sustain the status of G0/G1 phase, while PAB finally changed the cell cycle environment favored by HBV virus. In addition, PAB also induced HepG2215 cell apoptosis, which would be helpful to kill the cells infected by HBV and help for devouring HBV by macrophage. Therefore, PAB inhibited HBV secretion through apoptosis and cell cycle arrest. The present findings contribute to a future potential chemotherapeutic drug in the treatment of HBV-related HCC.

Combinatorial strategies based on CRAd-IL24 and CRAd-ING4 virotherapy with anti-angiogenesis treatment for ovarian cancer

Background

A major hurdle incurrent to the human clinical application of conditionally replicative adenovirus (CRAd)-based virotherapy agents is their limited therapeutic efficacy. In this study we evaluated whether arming our previously reported Ad5/3Δ24 CRAd vector containing a 24-base pair deletion in the E1A conserved region 2, which allows selective replication within Rb-p16-deficient tumor cells, to express therapeutic genes could improve oncolytic virus potency in ovarian cancer cells. We choose to assess the therapeutic benefits achieved by virus-mediated expression of interleukin 24 (IL-24), a cytokine-like protein of the IL-10 family, and the inhibitor of growth 4 (ING4) tumor suppressor protein.

Results

The generated CRAd-IL24 and CRAd-ING4 vectors were tested in ovarian cancer cell lines in vitro to compare their replication, yield, and cytotoxic effects with control CRAd Ad5/3∆24 lacking the therapeutic gene. These studies showed that CRAd-IL24 infection resulted in significantly increased yield of infectious particles, which translated to a marked enhancement of virus-induced cytotoxic effects as compared to CRAd-ING4 and non-armed CRAd. Testing CRAd-IL24 and CRAd-ING4 vectors combined together did not revealed synergistic effects exceeding oncolytic potency of single CRAD-IL24 vector. Both CRAds were also tested along with anti-VEGF monoclonal antibody Avastin and showed no significant augmentation of viral cytolysis by anti-angiogenesis treatment in vitro.

Conclusions

Our studies validated that arming with these key immunomodulatory genes was not deleterious to virus-mediated oncolysis. These findings thus, warrant further preclinical studies of CRAd-IL24 tumoricidal efficacy in murine ovarian cancer models to establish its potential utility for the virotherapy of primary and advanced neoplastic diseases.

Enterovirus 71 mediates cell cycle arrest in S phase through non-structural protein 3D

Many viruses disrupt the host cell cycle to facilitate their own growth. We assessed the mechanism and function of enterovirus 71 (EV71), a primary causative agent for recent hand, foot, and mouth disease outbreaks, in manipulating cell cycle progression. Our results suggest that EV71 infection induces S-phase arrest in diverse cell types by preventing the cell cycle transition from the S phase into the G2/M phase. Similar results were observed for an alternate picornavirus, Coxsackievirus A16. Synchronization in S phase, but not G0/G1 phase or G2/M phase, promotes viral replication. Consistent with its ability to arrest cells in S phase, the expression of cyclin A2, CDK 2, cyclin E1, and cyclin B1 was regulated by EV71 through increasing transcription of cyclin E1, promoting proteasome-mediated degradation of cyclin A2 and regulating the phosphorylation of CDK 2. Finally, a non-structural protein of EV71, the RNA-dependent RNA polymerase 3D, was demonstrated to mediate S-phase cell cycle arrest. These findings suggest that EV71 induces S-phase cell cycle arrest in infected cells via non-structural protein 3D, which may provide favorable conditions for virus production.

Murine norovirus replication induces G0/G1 cell cycle arrest in asynchronously growing cells

Many viruses replicate most efficiently in specific phases of the cell cycle, establishing or exploiting favorable conditions for viral replication, although little is known about the relationship between caliciviruses and the cell cycle. Microarray and Western blot analysis of murine norovirus 1 (MNV-1)-infected cells showed changes in cyclin transcript and protein levels indicative of a G1 phase arrest. Cell cycle analysis confirmed that MNV-1 infection caused a prolonging of the G1 phase and an accumulation of cells in the G0/G1 phase. The accumulation in G0/G1 phase was caused by a reduction in cell cycle progression through the G1/S restriction point, with MNV-1-infected cells released from a G1 arrest showing reduced cell cycle progression compared to mock-infected cells. MNV-1 replication was compared in populations of cells synchronized into specific cell cycle phases and in asynchronously growing cells. Cells actively progressing through the G1 phase had a 2-fold or higher increase in virus progeny and capsid protein expression over cells in other phases of the cell cycle or in unsynchronized populations. These findings suggest that MNV-1 infection leads to prolonging of the G1 phase and a reduction in S phase entry in host cells, establishing favorable conditions for viral protein production and viral replication. There is limited information on the interactions between noroviruses and the cell cycle, and this observation of increased replication in the G1 phase may be representative of other members of the Caliciviridae.

IMPORTANCE

Noroviruses have proven recalcitrant to growth in cell culture, limiting our understanding of the interaction between these viruses and the infected cell. In this study, we used the cell-culturable MNV-1 to show that infection of murine macrophages affects the G1/S cell cycle phase transition, leading to an arrest in cell cycle progression and an accumulation of cells in the G0/G1 phase. Furthermore, we show that MNV replication is enhanced in the G1 phase compared to other stages of the cell cycle. Manipulating the cell cycle or adapting to cell cycle responses of the host cell is a mechanism to enhance virus replication. To the best of our knowledge, this is the first report of a norovirus interacting with the host cell cycle and exploiting the favorable conditions of the G0/G1 phase for RNA virus replication.

Extended stability of cyclin D1 contributes to limited cell cycle arrest at G1-phase in BHK-21 cells with Japanese encephalitis virus persistent infection

There is increasing evidence that many RNA viruses manipulate cell cycle control to achieve favorable cellular environments for their efficient replication during infection. Although virus-induced G0/G1 arrest often delays early apoptosis temporarily, a prolonged replication of the infected virus leads host cells to eventual death. In contrast, most mammalian cells with RNA virus persistent infection often escape cytolysis in the presence of productive viral replication. In this study, we demonstrated that the extended endurance of cyclin D1 was clearly associated with the suppression of glycogen synthase kinase-3ß (GSK-3ß) expression in BHK-21 cells that are persistently infected with Japanese encephalitis virus (JEV). The G0/G1 arrest of these cells turned much loose compared to the normal BHK-21 cells with JEV acute infection. After cycloheximide treatment, cyclin D1 in the persistently infected cells lasted several hours longer than those in acutely infected cells. Furthermore, both p21^Cip1 and p27^Kip1, positive regulators for cyclin D1 accumulation in the nucleus, were suppressed in their expression, which contrasts with those in JEV acute infection. Inhibition of the GSK-3ß by lithium chloride treatment rescued a significant number of cells from cytolysis in JEV acute infection, which coincided with the levels of cyclin D1 that escaped from proteolysis. Therefore, the limitation of G1/S arrest in the BHK-21 cells with JEV persistent infection is associated with the suppression of GSK-3ß expression, resulting in the extended duration of cyclin D1.

The high-risk HPV16 E7 oncoprotein mediates interaction between the transcriptional coactivator CBP and the retinoblastoma protein pRb

The oncoprotein E7 from human papillomavirus (HPV) strains that confer high cancer risk mediates cell transformation by deregulating host cellular processes and activating viral gene expression through recruitment of cellular proteins such as the retinoblastoma protein (pRb) and the cyclic-AMP response element binding binding protein (CBP) and its paralog p300. Here we show that the intrinsically disordered N-terminal region of E7 from high-risk HPV16 binds the TAZ2 domain of CBP with greater affinity than E7 from low-risk HPV6b. HPV E7 and the tumor suppressor p53 compete for binding to TAZ2. The TAZ2 binding site in E7 overlaps the LxCxE motif that is crucial for interaction with pRb. While TAZ2 and pRb compete for binding to a monomeric E7 polypeptide, the full-length E7 dimer mediates an interaction between TAZ2 and pRb by promoting formation of a ternary complex. Cell-based assays show that expression of full-length HPV16 E7 promotes increased pRb acetylation and that this response depends both on the presence of CBP/p300 and on the ability of E7 to form a dimer. These observations suggest a model for the oncogenic effect of high-risk HPV16 E7. The disordered region of one E7 molecule in the homodimer interacts with the pocket domain of pRb, while the same region of the other E7 molecule binds the TAZ2 domain of CBP/p300. Through its ability to dimerize, E7 recruits CBP/p300 and pRb into a ternary complex, bringing the histone acetyltransferase domain of CBP/p300 into proximity to pRb and promoting acetylation, leading to disruption of cell cycle control.

Rotavirus infection induces G1 to S phase transition in MA104 cells via Ca⁺²/Calmodulin pathway

Viruses, obligate cellular parasites rely on host cellular functions and target the host cell cycle for their own benefit. In this study, effect of rotavirus infection on cell cycle machinery was explored. We found that rotavirus (RV) infection in MA104 cells induces the expression of cyclins and cyclin dependent kinases and down-regulates expression of CDK inhibitors, resulting in G1 to S phase transition. The rotavirus induced S phase accumulation was found to be concurrent with induction in expression of calmodulin and activation of CaMKI which is reported as inducer of G1-S phase transition. This cell cycle manipulation was found to be Ca(+2)/Calmodulin pathway dependent. The physiological relevance of G1 to S phase transition was established when viral gene expressions as well as viral titers were found to be increased in S phase synchronized cells and decreased in G0/G1 phase synchronized cells compared to unsynchronized cells during rotavirus infection.

A potential molecular pathogenesis of cardiac/laterality defects in Oculo-Facio-Cardio-Dental syndrome

Pitx2 is the last effector of the left-right (LR) cascade known to date and plays a crucial role in the patterning of LR asymmetry. In Xenopus embryos, the expression of Pitx2 gene in the left lateral plate mesoderm (LPM) is directly regulated by Xnr1 signaling, which is mediated by Smads and FoxH1. Previous studies suggest that the suppression of Pitx2 gene in the left LPM is a potential cause of cardiac/laterality defects in Oculo-Facio-Cardio-Dental (OFCD) syndrome, which is known to be caused by mutations in BCL6 co-repressor (BCOR) gene. Recently, our work has revealed that the BCL6/BCOR complex blocks Notch-dependent transcriptional activity to protect the expression of Pitx2 in the left LPM from the inhibitory activity of Notch signaling. These studies indicated that uncontrolled Notch activity in the left LPM caused by dysfunction of BCOR may result in cardiac/laterality defects of OFCD syndrome. However, this Notch-dependent inhibitory mechanism of Pitx2 gene transcription still remains unknown. Here we report that transcriptional repressor ESR1, which acts downstream of Notch signaling, inhibits the expression of Pitx2 gene by binding to a left side-specific enhancer (ASE) region in Pitx2 gene and recruiting histone deacetylase 1 (HDAC1) to this region. Once HDAC1 is tethered, histone acetyltransferase p300 is no longer recruited to the Xnr1-dependent transcriptional complex on the ASE region, leading to the suppression of Pitx2 gene in the left LPM. The study presented here uncovers the regulatory mechanism of Pitx2 gene transcription which may contribute to an understanding of pathogenesis of OFCD syndrome.

Switching a replication-defective adenoviral vector into a replication-competent, oncolytic adenovirus

The adenovirus immediate early gene E1A initiates the program of viral gene transcription and reprograms multiple aspects of cell function and behavior. For adenoviral (Ad) vector-mediated gene transfer and therapy approaches, where replication-defective (RD) gene transfer is required, E1A has thus been the primary target for deletions. For oncolytic gene therapy for cancer, where replication-competent (RC) Ad viral gene expression is needed, E1A has been either mutated or placed under tumor-specific transcriptional control. A novel Ad vector that initially infected target tumor cells in an RD manner for transgene expression but that could be "switched" into an RC, oncolytic state when needed might represent an advance in vector technology. Here, we report that we designed such an Ad vector (proAdΔ24.GFP), where initial Ad replication is silenced by a green fluorescent protein (GFP) transgene that blocks cytomegalovirus (CMV)-mediated transcription of E1A. This vector functions as a bona fide E1A-deleted RD vector in infected tumor cells. However, because the silencing GFP transgene is flanked by FLP recombination target (FRT) sites, we show that it can be efficiently excised by Flp recombinase site-specific recombination, either when Flp is expressed constitutively in cells or when it is provided in trans by coinfection with a second RD herpes simplex virus (HSV) amplicon vector. This switches the RD Ad, proAdΔ24.GFP, into a fully RC, oncolytic Ad (rAdΔ24) that lyses tumor cells in culture and generates oncolytic progeny virions. In vivo, coinfection of established flank tumors with the RD proAdΔ24.GFP and the RD Flp-bearing HSV1 amplicon leads to generation of RC, oncolytic rAdΔ24. In an orthotopic human glioma xenograft tumor model, coinjection of the RD proAdΔ24.GFP and the RD Flp-bearing HSV1 amplicon also led to a significant increase in animal survival, compared to controls. Therefore, Flp-FRT site-specific recombination can be applied to switch RD Ad into fully oncolytic RC Ad for tumor therapy and is potentially applicable to a variety of gene therapy approaches.

Adenovirus-mediated sensitization to the cytotoxic drugs docetaxel and mitoxantrone is dependent on regulatory domains in the E1ACR1 gene-region

Oncolytic adenoviruses have shown promising efficacy in clinical trials targeting prostate cancers that frequently develop resistance to all current therapies. The replication-selective mutants AdΔΔ and dl922–947, defective in pRb-binding, have been demonstrated to synergise with the current standard of care, mitoxantrone and docetaxel, in prostate cancer models. While expression of the early viral E1A gene is essential for the enhanced cell killing, the specific E1A-regions required for the effects are unknown. Here, we demonstrate that replicating mutants deleted in small E1A-domains, binding pRb (dl1108), p300/CBP (dl1104) and p400/TRRAP or p21 (dl1102) sensitize human prostate cancer cells (PC-3, DU145, 22Rv1) to mitoxantrone and docetaxel. Through generation of non-replicating mutants, we demonstrate that the small E1A12S protein is sufficient to potently sensitize all prostate cancer cells to the drugs even in the absence of viral replication and the E1A transactivating domain, conserved region (CR) 3. Furthermore, the p300/CBP-binding domain in E1ACR1 is essential for drug-sensitisation in the absence (AdE1A1104) but not in the presence of the E1ACR3 (dl1104) domain. AdE1A1104 also failed to increase apoptosis and accumulation of cells in G2/M. All E1AΔCR2 mutants (AdE1A1108, dl922–947) and AdE1A1102 or dl1102 enhance cell killing to the same degree as wild type virus. In PC-3 xenografts in vivo the dl1102 mutant significantly prolongs time to tumor progression that is further enhanced in combination with docetaxel. Neither dl1102 nor dl1104 replicates in normal human epithelial cells (NHBE). These findings suggest that additional E1A-deletions might be included when developing more potent replication-selective oncolytic viruses, such as the AdΔCR2-mutants, to further enhance potency through synergistic cell killing in combination with current chemotherapeutics.

Expression of the Adenovirus Early Gene 1A Transcription-Repression Domain Alone Downregulates HER2 and Results in the Death of Human Breast Cancer Cells Upregulated for the HER2 Proto-Oncogene

Adenovirus (Ad) early gene 1A 243 residue protein (E1A 243R) possesses a potent transcription-repression function within the N-terminal 80 amino acids (E1A 1-80). We examined the ability of E1A 243R and E1A 1-80 to repress transcription of both an exogenous and the endogenous HER2 promoter in a human breast cancer cell line upregulated for the HER2 proto-oncogene (SK-BR-3). Both moieties repressed HER2 expression by over 90%. When E1A 1-80 was expressed from a nonreplicative Ad vector, levels of expression were lower than anticipated. Addition of nonspecific sequences to the E1A 1-80 C-terminus (E1A 1-80 C+) enhanced its expression 10- to 20-fold. Because "oncogene addiction" suggests that repression of HER2 could kill HER2 upregulated cells, we examined the ability of full-length E1A 243R and E1A 1-80 C+ delivered by an Ad vector to kill HER2 upregulated SK-BR-3 cells. Expression of both E1A 243R and E1A 1-80 C+ killed SK-BR-3 cells but not normal breast cells. E1A 1-80 C+ is a particularly effective killer of SK-BR-3 cells. At 144 h post infection, over 85% of SK-BR-3 cells were killed by a 100 moi of the Ad vector expressing E1A 1-80 C+. As controls, Ad vectors expressing E1A 243R with deletion of all known functional domains or expressing unrelated β-galactosidase had no effect. Three additional human breast cancer cells lines reported to be upregulated for HER2 or another EGF family member (EGFR) were found to be efficiently killed by expression of E1A 1-80 C+, whereas three additional "normal" cell lines (two derived from breast and one from foreskin) were not. The ability of the E1A transcription-repression domain alone to kill HER2 upregulated breast cancer cells has potential for development of therapies for treatment of aggressive human breast cancers and potentially other human cancers that overexpress HER2.

The virtue of just enough stress: a molecular model

Molecular biology emphasizes the study of all-or-nothing phenomena and molecular events with a large dynamic range. However, many important physiologic parameters in the clinical setting are tightly constrained (e.g., serum sodium concentration, body mass, venous oxygen saturation, sleep duration). Stress responses exhibit both a wide dynamic range and a potential for important effects at a "just-enough" threshold activation level. Stress responses occur in a number of body systems (e.g., neuropsychiatric, immune, cardiovascular) and are essential for short-term damage control, but also must be tightly constrained in range and duration to permit the organism to walk the narrow homeostatic path to long-term survival. Using an example of a newly appreciated stress-responsive molecule in the heart, acetyltransferase p300, as well as examples from the literature, this article discusses the advantages of self-limited stress, the adverse effects of sustained stress, and the built-in mechanisms that feed back on and terminate stress signals, and advances a hypothesis regarding stress as a pharmacological target in the heart.

Adenovirus E1A directly targets the E2F/DP-1 complex

Deregulation of the cell cycle is of paramount importance during adenovirus infection. Adenovirus normally infects quiescent cells and must initiate the cell cycle in order to propagate itself. The pRb family of proteins controls entry into the cell cycle by interacting with and repressing transcriptional activation by the E2F transcription factors. The viral E1A proteins indirectly activate E2F-dependent transcription and cell cycle entry, in part, by interacting with pRb and family members to free the E2Fs. We report here that an E1A 13S isoform can unexpectedly activate E2F-responsive gene expression independently of binding to the pRb family of proteins. We demonstrate that E1A binds to E2F/DP-1 complexes through a direct interaction with DP-1. E1A appears to utilize this binding to recruit itself to E2F-regulated promoters, and this allows the E1A 13S protein, but not the E1A 12S protein, to activate transcription independently of interaction with pRb. Importantly, expression of E1A 13S, but not E1A 12S, led to significant enhancement of E2F4 occupancy of E2F sites of two E2F-regulated promoters. These observations identify a novel mechanism by which adenovirus deregulates the cell cycle and suggest that E1A 13S may selectively activate a subset of E2F-regulated cellular genes during infection.

Influenza A virus replication induces cell cycle arrest in G0/G1 phase

Many viruses interact with the host cell division cycle to favor their own growth. In this study, we examined the ability of influenza A virus to manipulate cell cycle progression. Our results show that influenza A virus A/WSN/33 (H1N1) replication results in G(0)/G(1)-phase accumulation of infected cells and that this accumulation is caused by the prevention of cell cycle entry from G(0)/G(1) phase into S phase. Consistent with the G(0)/G(1)-phase accumulation, the amount of hyperphosphorylated retinoblastoma protein, a necessary active form for cell cycle progression through late G(1) into S phase, decreased after infection with A/WSN/33 (H1N1) virus. In addition, other key molecules in the regulation of the cell cycle, such as p21, cyclin E, and cyclin D1, were also changed and showed a pattern of G(0)/G(1)-phase cell cycle arrest. It is interesting that increased viral protein expression and progeny virus production in cells synchronized in the G(0)/G(1) phase were observed compared to those in either unsynchronized cells or cells synchronized in the G(2)/M phase. G(0)/G(1)-phase cell cycle arrest is likely a common strategy, since the effect was also observed in other strains, such as H3N2, H9N2, PR8 H1N1, and pandemic swine H1N1 viruses. These findings, in all, suggest that influenza A virus may provide favorable conditions for viral protein accumulation and virus production by inducing a G(0)/G(1)-phase cell cycle arrest in infected cells.

Update on human polyomaviruses and cancer

Over 50 years of polyomavirus research has produced a wealth of insights into not only general biologic processes in mammalian cells, but also, how conditions can be altered and signaling systems tweaked to produce transformation phenotypes. In the past few years three new members (KIV, WUV, and MCV) have joined two previously known (JCV and BKV) human polyomaviruses. In this review, we present updated information on general virologic features of these polyomaviruses in their natural host, concentrating on the association of MCV with human Merkel cell carcinoma. We further present a discussion on advances made in SV40 as the prototypic model, which has and will continue to inform our understanding about viruses and cancer.

E1A interacts with two opposing transcriptional pathways to induce quiescent cells into S phase

Despite data suggesting that the adenovirus E1A protein of 243 amino acids creates an S-phase environment in quiescent cells by overcoming the nucleosomal repression of E2F-regulated genes, the precise mechanisms underlying E1A's ability in this process have not yet been defined at the biochemical level. In this study, we show by kinetic analysis that E1A, as opposed to an E1A mutant failing to bind p130, can temporally eliminate corepressor complexes consisting of p130-E2F4 and HDAC1/2-mSin3B from the promoters of E2F-regulated genes in quiescent cells. Once the complexes are removed, the di-methylation of H3K9 at these promoters becomes dramatically diminished, and this in turn allows for the acetylation of H3K9/14 and the recruitment of activating E2F family members, which is then followed by the transcriptional activity of the E2F-regulated genes. Remarkably, although an E1A mutant that can no longer bind to a histone acetyltransferase (PCAF) is as capable as wild-type E1A in eliminating corepressor complexes and methyl groups from the promoters of these genes, it cannot mediate the acetylation of H3K9/14 or induce their transcription. These findings suggest that corepressors as well as coactivators are acted upon by E1A to derepress E2F-regulated genes in quiescent cells. Thus, our results highlight for the first time a functional relationship between E1A and two transcriptional pathways of differing functions for transitioning cells out of quiescence and into S phase.

How the Rb tumor suppressor structure and function was revealed by the study of Adenovirus and SV40

The review recounts the history of how the study of the DNA tumor viruses including polyoma, SV40 and Adenovirus brought key insights into the structure and function of the Retinoblastoma protein (Rb). Knudsen's model of the two-hit hypothesis to explain patterns of hereditary and sporadic retinoblastoma provided the foundation for the tumor suppressor hypothesis that ultimately led to the cloning of the Rb gene. The discovery that SV40 and Adenovirus could cause tumors when inoculated into animals was startling not only because SV40 had contaminated the poliovirus vaccine and Adenovirus was a common cause of viral induced pneumonia but also because they provided an opportunity to study the genetics and biochemistry of cancer. Studies of mutant forms of these viruses led to the identification of the E1A and Large T antigen (LT) oncogenes and their small transforming elements including the Adenovirus Conserved Regions (CR), the SV40 J domain and the LxCxE motif. The immunoprecipitation studies that initially revealed the size and ultimately the identity of cellular proteins that could bind to these transforming elements were enabled by the widespread development of highly specific monoclonal antibodies against E1A and LT. The identification of Rb as an E1A and LT interacting protein quickly led to the cloning of p107, p130, p300, CBP, p400 and TRRAP and the concept that viral transformation was due, at least in part, to the perturbation of the function of normal cellular proteins. In addition, studies on the ability of E1A to transactivate the Adenovirus E2 promoter led to the cloning of the heterodimeric E2F and DP transcription factor and recognition that Rb repressed transcription of cellular genes required for cell cycle entry and progression. More recent studies have revealed how E1A and LT combine the activity of Rb and the other cellular associated proteins to perturb expression of many genes during viral infection and tumor formation.

Enhanced expression of 70-kilodalton heat shock protein limits cell division in a sepsis-induced model of acute respiratory distress syndrome

OBJECTIVE

Fibrotic changes are initiated early in acute respiratory distress syndrome. This may involve overproliferation of alveolar type II cells. In an animal model of acute respiratory distress syndrome, we have shown that the administration of an adenoviral vector overexpressing the 70-kd heat shock protein (AdHSP) limited pathophysiological changes. We hypothesized that this improvement may be modulated, in part, by an early AdHSP-induced attenuation of alveolar type II cell proliferation.

DESIGN

Laboratory investigation.

SETTING

Hadassah-Hebrew University and University of Pennsylvania animal laboratories.

SUBJECTS

Sprague-Dawley Rats (250 g).

INTERVENTIONS

Lung injury was induced in male Sprague-Dawley rats via cecal ligation and double puncture. At the time of cecal ligation and double puncture, we injected phosphate-buffered saline, AdHSP, or AdGFP (an adenoviral vector expressing the marker green fluorescent protein) into the trachea. Rats then received subcutaneous bromodeoxyuridine. In separate experiments, A549 cells were incubated with medium, AdHSP, or AdGFP. Some cells were also stimulated with tumor necrosis factor-alpha. After 48 hrs, cytosolic and nuclear proteins from rat lungs or cell cultures were isolated. These were subjected to immunoblotting, immunoprecipitation, electrophoretic mobility shift assay, fluorescent immunohistochemistry, and Northern blot analysis.

MEASUREMENTS AND MAIN RESULTS

Alveolar type I cells were lost within 48 hrs of inducing acute respiratory distress syndrome. This was accompanied by alveolar type II cell proliferation. Treatment with AdHSP preserved alveolar type I cells and limited alveolar type II cell proliferation. Heat shock protein 70 prevented overexuberant cell division, in part, by inhibiting hyperphosphorylation of the regulatory retinoblastoma protein. This prevented retinoblastoma protein ubiquitination and degradation and, thus, stabilized the interaction of retinoblastoma protein with E2F1, a key cell division transcription factor.

CONCLUSIONS

: Heat shock protein 70-induced attenuation of cell proliferation may be a useful strategy for limiting lung injury when treating acute respiratory distress syndrome if consistent in later time points.

Adenovirus E1A proteins are closely associated with chromatin in productively infected and transformed cells

The adenovirus E1A 243R oncoprotein encodes a potent transcription-repression function within the N-terminal 80 amino acids. Our proposed model of E1A repression predicts that E1A interacts with important cellular proteins on chromatin. Consistent with this idea, we report here that E1A proteins from in vivo formaldehyde cross-linked 293 cells are closely associated with chromatin even after several stringent purification steps including double isopycnic CsCl density gradient centrifugation and size exclusion chromatography. Likewise, E1A proteins expressed from virus during productive infection of HeLa cells are closely associated with chromatin starting at early times after infection. No other adenoviral proteins are necessary for E1A 243R protein to associate with chromatin. Analyses of chromatin from HeLa cells infected with adenovirus vectors expressing E1A 243R protein with deletions in different E1A functional domains indicate that sequences within the E1A N-terminal repression domain are needed for the majority of E1A's interactions with chromatin.

Adenovirus type 5 exerts genome-wide control over cellular programs governing proliferation, quiescence, and survival

The effects of the adenovirus Ad5 on basic host cell programs, such as cell-cycle regulation, were studied in a microarray analysis of human fibroblasts. About 2,000 genes were up- or down-regulated after Ad5 infection and Ad5 infection was shown to induce reversal of the quiescence program and recapitulation of the core serum response.

Background

Human adenoviruses, such as serotype 5 (Ad5), encode several proteins that can perturb cellular mechanisms that regulate cell cycle progression and apoptosis, as well as those that mediate mRNA production and translation. However, a global view of the effects of Ad5 infection on such programs in normal human cells is not available, despite widespread efforts to develop adenoviruses for therapeutic applications.

Results

We used two-color hybridization and oligonucleotide microarrays to monitor changes in cellular RNA concentrations as a function of time after Ad5 infection of quiescent, normal human fibroblasts. We observed that the expression of some 2,000 genes, about 10% of those examined, increased or decreased by a factor of two or greater following Ad5 infection, but were not altered in mock-infected cells. Consensus k-means clustering established that the temporal patterns of these changes were unexpectedly complex. Gene Ontology terms associated with cell proliferation were significantly over-represented in several clusters. The results of comparative analyses demonstrate that Ad5 infection induces reversal of the quiescence program and recapitulation of the core serum response, and that only a small subset of the observed changes in cellular gene expression can be ascribed to well characterized functions of the viral E1A and E1B proteins.

Conclusion

These findings establish that the impact of adenovirus infection on host cell programs is far greater than appreciated hitherto. Furthermore, they provide a new framework for investigating the molecular functions of viral early proteins and information relevant to the design of conditionally replicating adenoviral vectors.

C-terminal-binding protein interacting protein binds directly to adenovirus early region 1A through its N-terminal region and conserved region 3

C-terminal-binding protein interacting protein (CtIP) was first isolated as a binding partner of C-terminal-binding protein (CtBP). It is considered to contribute to the transcriptional repression and cell cycle regulatory properties of the retinoblastoma (Rb) family of proteins and to have a role in the cellular response to DNA damage. Here, we have shown that CtIP is a novel target for the adenovirus oncoprotein early region 1A (AdE1A). AdE1A associates with CtIP in both Ad5E1-transformed cells and Ad5-infected cells and binds directly in glutathione-S-transferase pull-down assays. Two binding sites have been mapped on Ad5E1A - the N-terminal alpha-helical region (residues 1-30) and conserved region 3 (CR3) - the transcriptional activation domain. CtIP can bind AdE1A and CtBP independently, raising the possibility that ternary complexes exist in Ad-transformed and -infected cells. Significantly, reduction of CtIP expression with small interfering RNAs results in reduction of the ability of a Gal4 DNA-binding domain-CR3 construct to transactivate a Gal 4-responsive luciferase reporter and this effect is reversed by reduction of CtBP expression. Therefore, in this model, CtIP acts as a transcriptional co-activator of AdE1A when dissociated from CtBP, through the action of AdE1A. These data are consistent with observations that CtIP expression is induced by AdE1A during viral infection and that reduction of CtIP expression with RNA interference can retard virus replication. In addition, AdE1A causes disruption of the CtIP/Rb complex during viral infection by its interaction with CtIP, possibly contributing to transcriptional derepression.

Development of oncolytic adenovirus armed with a fusion of soluble transforming growth factor-beta receptor II and human immunoglobulin Fc for breast cancer therapy

We have developed an approach to cancer gene therapy in which the oncolytic effects of an adenoviral vector have been combined with selective expression of a soluble form of transforming growth factor (TGF)-beta receptor II fused with Fc (sTGFbetaRIIFc). We chose to use adenoviral dl01/07 mutant because it can replicate in all cancer cells regardless of their genetic defects. An oncolytic adenovirus expressing sTGFbetaRIIFc (Ad.sT- betaRFc) was constructed by homologous recombination. Infection of MDA-MB-231 and MCF-7 human breast cancer cells with Ad.sTbetaRFc produced sTGFbetaRIIFc, which was released into the media. The conditioned media containing sTGFbetaRIIFc could bind with TGF-beta 1 and inhibited TGF-beta-dependent transcription in target cells. Infection of MDA-MB-231, MCF-7, and 76NE human breast cancer cells with Ad.sTbetaRFc resulted in high levels of viral replication, comparable to that of a wild-type dl309 virus. Although some viral replication was observed in actively dividing normal human lung fibroblasts, there was no replication in nonproliferating normal cells. Direct injection of Ad.sTbetaRFc into MDA-MB-231 human breast xenograft tumors grown in nude mice resulted in a significant inhibition of tumor growth, causing tumor regression in more than 85% of the animals. These results indicate that it is possible to construct an oncolytic virus expressing sTGFbetaRIIFc in which both viral replication and transgene expression remain intact, and the recombinant adenovirus is oncolytic in a human tumor xenograft model. On the basis of these results we believe that it may be feasible to develop a cancer gene therapy approach using Ad.sTbetaRFc as an antitumor agent.

The effect of CtBP1 binding on the structure of the C-terminal region of adenovirus 12 early region 1A

Adenovirus early region 1A (AdE1A) binds to the C-terminal binding protein 1 (CtBP1) primarily through a highly conserved PXDLS motif located close to its C-terminus. Purified synthetic peptides equivalent to this region of AdE1A have been shown to form a series of beta-turns. In this present study the effect of CtBP1 binding on the conformation of C-terminal region of Ad12E1A has been investigated. Using one- and two-dimensional (1)H NMR spectroscopy, the conformation of 20-residue peptides equivalent to amino acids I(241)-V(260) and E(247)-N(266) of Ad12E1A were examined in the absence of CtBP1. Whilst the latter peptide forms a series of beta-turns in its C-terminal half as reported previously, the former peptide is alpha-helical over the region D(243)-Q(253). Upon interaction with CtBP1 the conformation of the backbone in the region (255)PVDLCVK(261) of the Ad12E1A E(247)-N(266) peptide reorganises from a predominately beta-turn to an alpha-helical conformation. This structural isomerisation is characterised by a shift upfield of 0.318 ppm for the delta-CH(3) proton resonance of V(256). 2-D NOESY experiments showed new signals in the amide-alpha region which correlate to transferred NOEs from the protein to the peptide residues E(251), V(256) and K(261). In further analyses the contribution of individual amino acids within the sequence (254)VPVDLS(259) was assessed for their importance in determining structure and consequently affinity of the peptide for CtBP. It has been concluded that Ad12E1A residues (255)P-V(260) serve initially as a recognition site for CtBP and then as an anchor through a beta-turns-->alpha-helix conformational rearrangement. In addition it has been predicted that regions N-terminal to the PXDLS motif in AdE1As from different virus serotypes and from mammalian proteins form alpha-helices.

Acetylation at a lysine residue adjacent to the CtBP binding motif within adenovirus 12 E1A causes structural disruption and limited reduction of CtBP binding

C-terminal binding protein (CtBP) has been shown to bind to a highly conserved five-amino-acid motif (PXDLS) located very close to the C-terminus of adenovirus early region 1A proteins. It has also been demonstrated that amino acids C-terminal and N-terminal to this original proposed binding site contribute to the interaction. However, conflicting evidence has been presented to show that acetylation of an adjacent lysine residue in Ad5E1A may or may not influence binding. It has now been demonstrated here that acetylation of a lysine, equivalent to position 261 in Ad12 E1A and position 285 in Ad5E1A, in a synthetic peptide disrupts the binding to CtBP1 and CtBP2 and alters the K(i) of the peptide, indicative of a reduction in the affinity of the peptide for CtBP1 and CtBP2, but only to a rather limited extent (less than 2-fold). The solution structures of synthetic peptides equivalent to wild-type and acetylated forms of the Ad12 E1A peptide have been determined by proton NMR spectroscopy. The wild-type form of the peptide adopts a series of beta-turns over the region Val(254)-Arg(262). Within the acetylated isoform, the beta-turn conformation is less extensive, Val(260)-Arg(262) adopting a random confirmation. We conclude that secondary structure (beta-turns) and an appropriate series of amino acid side chains over an extended binding site (PXDLSXK) are necessary for recognition by CtBP, acetylation of lysine interfering with both of these features, but not to such an extent as to totally inhibit interaction. Moreover, it is possible that the beta-turn conformation at the C-terminus of AdE1A contributes to binding to alpha importin and nuclear import. Acetylation of lysine (261) could disrupt interaction through structural destabilization as well as charge neutralization and subsequent nuclear localization.

Changes in C-terminal binding protein 2 (CtBP2) corepressor complex induced by E1A and modulation of E1A transcriptional activity by CtBP2

The N-terminal region of adenovirus E1A interacts with histone acetyl transferases (HATs) such as p300, P/CAF, and GCN5. The C-terminal region interacts with the transcriptional corepressors CtBP1 and CtBP2. The functional significance of co-recruitment of HATs and CtBPs by E1A is not well understood. In this study, we have shown that E1A enhanced acetylation of CtBP2 by recruitment of p300 to the CtBP2 complex. Additionally, E1A also displaced the histone methyltransferase G9a and the E-box repressor ZEB from the CtBP2 complex through the C-terminal CtBP-binding domain. A transcriptional activation function encoded by the E1A N-terminal region was efficiently inhibited by CtBP2 but not by a mutant with an N-terminal deletion or by a mutant deficient in interaction with E1A. Two isoforms of CtBP1 (CtBP1-L and CtBP1-S) poorly inhibited transcriptional activity of the E1A N-terminal region. Thus, the N-terminal domain of CtBP2 may contribute a unique transcriptional regulatory activity of CtBP2. Our results provide new insights by which CtBP might modulate the biochemical activities of E1A.

Roles for the coactivators CBP and p300 and the APC/C E3 ubiquitin ligase in E1A-dependent cell transformation

Adenovirus early region 1A (E1A) possesses potent transforming activity when expressed in concert with activated ras or E1B genes in in vitro tissue culture systems such as embryonic human retinal neuroepithelial cells or embryonic rodent epithelial and fibroblast cells. Early region 1A has thus been used extensively and very effectively as a tool to determine the molecular mechanisms that underlie the basis of cellular transformation. In this regard, roles for the E1A-binding proteins pRb, p107, p130, cyclic AMP response element-binding protein (CBP)/p300, p400, TRRAP and CtBP in cellular transformation have been established. However, the mechanisms by which E1A promotes transformation through interaction with these partner proteins are not fully delineated. In this review, we focus on recent advances in our understanding of CBP/p300 function, particularly with regard to its relationship to the anaphase-promoting complex/cyclosome E3 ubiquitin ligase, which has recently been shown to interact and affect the activity of CBP/p300 through interaction domains that are evolutionarily conserved in E1A.

Adenovirus subversion of immune surveillance, apoptotic and growth regulatory pathways: a model for tumorigenesis

The adenovirus system provides a novel model for evaluating the roles of multiple factors involved in tumour progression. In common with other DNA tumour viruses, adenovirus employs a variety of strategies to evade immune surveillance and perturbs cellular apoptotic and growth regulatory pathways to ensure efficient replication of progeny virions. Such subversion of cellular networks is also found in tumour cells. The mechanism behind the avoidance of immune surveillance and the extent of cellular network interference achieved by adenovirus is still being uncovered and is predicted to have ramifications for the design of cancer therapeutics.

Mutational and functional analysis of an essential subdomain of the adenovirus E1A N-terminal transcription repression domain

Adenovirus early gene 1A (E1A) possesses a potent transcriptional repression function within the first 80 amino acids (E1A 1-80). Our previous analysis of subdomain 1 (residues 1 to 30) revealed strong correlations between residues required for repression and for disruption of TBP-TATA complexes. Here, we report a functional analysis of subdomain 2 (48 to 60) by alanine-scanning mutagenesis. 53Ala, 54Pro, 55Glu, and 56Asp are required for repression in vitro and in vivo and for efficient interaction with p300 but not for disruption of TBP-TATA. These combined results suggest a model for E1A transcription repression. E1A through subdomains 1 and 2 uses coactivators like p300 as scaffolds to access E1A repressible promoters. At the promoter, subdomain 1 interacts with TBP to disrupt TBP-TATA and abort transcription initiation. In further support of this model, we show that E1A 1-80 bound to the p300-binding site retains the ability to interact with TBP.

Restriction of adenoviral replication to the transcriptional intersection of two different promoters for colorectal and pancreatic cancer treatment

In our current study, we developed oncolytic adenoviruses which preferentially lyse pancreatic and colon cancer cells by replacing viral E1 and/or E4 promoter with the tumor/tissue-specific promoters, cyclooxygenase-2 (COX-2), midkine (MK), or the cell cycle-dependent promoter, E2F1. We generated three sets of recombinant adenoviral vectors. In the first set, only the native E1A promoter was replaced by the COX-2, MK, or E2F1 promoter, respectively. In the second set, the viral E4 promoter was substituted by these heterologous promoters and the viral E1A promoter was substituted by the ubiquitously active cytomegalovirus-IE promoter. In the third set, we substituted the viral E1A and E4 promoters with the COX-2, MK, or E2F1 promoter, respectively. In our system, transcriptional targeting of solitary viral E1A resulted in 50% enhanced restricted vector replication when compared with an unrestricted replication-competent adenovirus. Furthermore, a targeted expression of the viral E1A gene products had a greater effect on restricted adenoviral replication than that of the E4 region. With our vectors, Ad.COX.MK and Ad.MK.COX, using two different heterologous promoters to control E1A and E4 expression, we showed enhanced viral replication specificity when compared with Ad.COX.COX or Ad.MK.MK, respectively. In a s.c. xenograft tumor model, there was no significant difference in the antineoplastic efficacy of the double heterologous promoter-controlled vectors when compared with our unrestricted replication-competent control adenovirus or vectors with only E1A transcriptionally driven by a heterologous promoter.

An oncolytic adenovirus expressing soluble transforming growth factor-β type II receptor for targeting breast cancer: in vitro evaluation

In recent years, adenoviruses that selectively replicate in tumor cells have been developed. However, there is a tremendous need to improve their anticancer efficacy. We wish to investigate whether a strategy that combines the oncolytic effects of an adenoviral vector with simultaneous expression of soluble form of transforming growth factor-beta type II receptor (sTGFbetaRII) offers a therapeutic advantage. We chose to target TGF-betas because they play a pivotal role in late-stage tumorigenesis by enhancing tumor invasion and metastasis. A sTGFbetaRII cDNA was cloned in conditionally replicating adenoviral vector rAd-sTRII and in a replication-deficient adenovirus Ad-sTRII. Infection of MDA-MB-231 breast cancer cells with rAd-sTRII or Ad-sTRII followed by Western blot analysis indicated the expression of diffused glycosylated forms of sTGFbetaRII that were also secreted into the extracellular medium. The secreted proteins were shown to bind with TGF-beta and antagonize TGF-beta-induced p38 mitogen-activated protein kinase activity. However, marked differences in the replication potential of rAd-sTRII and Ad-sTRII were observed in breast tumor cells. Infection of MDA-MB-231 cells with rAd-sTRII resulted in cytotoxicity and significant increase in the adenoviral titers that were comparable with a wild-type adenovirus dl309. However, Ad-sTRII was much less toxic to the tumor cells, and the viral titers of Ad-sTRII remained relatively unchanged. These results suggest that the infection of breast tumor cells with conditionally replicating adenoviral vector rAd-sTRII produced sTGFbetaRII that can abrogate TGF-beta signaling while maintaining the replication potential of the virus, indicating that rAd-sTRII could be a potential anticancer agent.

Employment of liver tissue slice analysis to assay hepatotoxicity linked to replicative and nonreplicative adenoviral agents

Whereas virotherapy has emerged as a novel and promising approach for neoplastic diseases, appropriate model systems have hampered preclinical evaluation of candidate conditionally replicative adenovirus agents (CRAds) with respect to liver toxicity. This is due to the inability of human viral agents to cross species. We have recently shown the human liver tissue slice model to be a facile means to validate adenoviral replication. On this basis, we sought to determine whether our ex vivo liver tissue slice model could be used to assess CRAd-mediated liver toxicity. We analyzed and compared the toxicity of a conditionally replicative adenovirus (AdDelta24) to that of a replication incompetent adenovirus (Adnull [E1-]) in mouse and human liver tissue slices. To accomplish this, we examined the hepatic apoptosis expression profile by DNA microarray analyses, and compared these results to extracellular release of aminotransferase enzymes, along with direct evidence of apoptosis by caspase-3 immunhistochemical staining and TUNEL assays. Human and mouse liver tissue slices demonstrated a marked increase in extracellular release of aminotransferase enzymes on infection with AdDelta24 compared to Adnull. AdDelta24-mediated liver toxicity was further demonstrated by apoptosis induction, as detected by caspase-3 immunohistochemical staining, TUNEL assay and microarray analysis. In conclusion, concordance of CRAd-mediated apoptosis in both the human and the mouse liver tissue slice models was demonstrated, despite the limited replication ability of CRAds in mouse liver slices. The results of this study, defining the CRAd-mediated apoptosis gene expression profiles in human and mouse liver, may lay a foundation for preclinical liver toxicity analysis of CRAd agents.

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

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

Viruses – seeking and destroying the tumor program

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

Matching complementing functions of transformed cells with stable expression of selected viral genes for production of E1-deleted adenovirus vectors

Production of E1-deleted adenovirus (rAd) vectors requires complementation by E1A and E1B functions provided by the production cell line. The two cell lines most commonly used for production of rAd vectors, 293 and Per.C6, were derived from human primary cells and contain contiguous E1A and E1B sequences from the Ad genome. As an alternative system, we tested complementation of rAd vectors using sequential transfection of individual E1A and E1B expression cassettes into A549 human lung tumor cells, which support highly efficient replication of wild type adenovirus. We found that E1A function could be complemented in A549 cells by the mutant E1Adl01/07, and that E1B function could be provided in such cells using only the 55K E1B gene. Production yields in the resulting producer cell line, designated SL0003, were similar to those obtained from 293 cells without generation of detectable recombinant replication competent adenovirus.

A cancer-specific transcriptional signature in human neoplasia

The molecular anatomy of cancer cells is being explored through unbiased approaches aimed at the identification of cancer-specific transcriptional signatures. An alternative biased approach is exploitation of molecular tools capable of inducing cellular transformation. Transcriptional signatures thus identified can be readily validated in real cancers and more easily reverse-engineered into signaling pathways, given preexisting molecular knowledge. We exploited the ability of the adenovirus early region 1 A protein (E1A) oncogene to force the reentry into the cell cycle of terminally differentiated cells in order to identify and characterize genes whose expression is upregulated in this process. A subset of these genes was activated through a retinoblastoma protein/E2 viral promoter required factor-independent (pRb/E2F-independent) mechanism and was overexpressed in a fraction of human cancers. Furthermore, this overexpression correlated with tumor progression in colon cancer, and 2 of these genes predicted unfavorable prognosis in breast cancer. A proof of principle biological validation was performed on one of the genes of the signature, skeletal muscle cell reentry-induced (SKIN) gene, a previously undescribed gene. SKIN was found overexpressed in some primary tumors and tumor cell lines and was amplified in a fraction of colon adenocarcinomas. Furthermore, knockdown of SKIN caused selective growth suppression in overexpressing tumor cell lines but not in tumor lines expressing physiological levels of the transcript. Thus, SKIN is a candidate oncogene in human cancer.

Hypoxia reduces adenoviral replication in cancer cells by downregulation of viral protein expression

Successful cancer therapy using replicating viral vectors relies on the spread of virus from infected to uninfected cells. To date, there has been limited clinical success in the use of replicating adenoviruses. In animal models, established xenograft tumors are rarely eliminated despite the persistence of high viral titers within the tumor. Hypoxia is a prevalent characteristic of solid tumors, whereas adenovirus naturally infects tissues exposed to ambient oxygen concentrations. Here, we report that hypoxia (1% oxygen) reduces adenoviral replication in H1299 and A549 lung cancer cells, BxPC-3 pancreatic cancer cells, LNCaP prostate cancer cells and HCT116 colon cancer cells. However, hypoxia does not reduce cell viability or restrict S-phase entry. Importantly, the production of E1a and fiber proteins under hypoxic conditions was substantially decreased at 24 and 48 h compared to room air controls. In contrast, Northern analysis showed similar levels of E1a mRNA in room air and hypoxic conditions. In conclusion, a level of hypoxia similar to that found within solid tumors reduces the replication of adenoviral vectors by reduction of viral protein expression without a reduction in mRNA levels. To further improve oncolytic therapy using a replicating adenovirus, it is important to understand the mechanism through which hypoxia and the virus interact to control expression of viral and cellular proteins, and consequently to develop means to overcome decreased viral production in hypoxic conditions.

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.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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