Reovirus-induced sigma1s-dependent G(2)/M phase cell cycle arrest is associated with inhibition of p34(cdc2)

RNA-seq reveals role of cell-cycle regulating genes in the pathogenicity of a field very virulent infectious bursal disease virus

Infectious bursal disease virus (IBDV) infection causes highly contagious and immunosuppressive disease in poultry. The thymus, serving as the primary organ for T cell maturation and differentiation, plays an important role in the pathogenicity of IBDV in the infected chickens. However, there are no reports on the molecular pathogenesis of IBDV in the thymus currently. The aim of the study was to elucidate the molecular mechanisms underlying the pathogenicity of a field very virulent (vv) IBDV strain NN1172 in the thymus of SPF chickens using integrative transcriptomic and proteomic analyses. Our results showed that a total of 4,972 Differentially expressed genes (DEGs) in the thymus of NN1172-infected chickens by transcriptomic analysis, with 2,796 up-regulated and 2,176 down-regulated. Meanwhile, the proteomic analysis identified 726 differentially expressed proteins (DEPs) in the infected thymus, with 289 up-regulated and 437 down-regulated. Overall, a total of 359 genes exhibited differentially expression at both mRNA and protein levels, with 134 consistently up-regulated and 198 genes consistently down-regulated, as confirmed through a comparison of the RNA-seq and the proteomic datasets. The gene ontology (GO) analysis unveiled the involvement of both DEGs and DEPs in diverse categories encompassing cellular components, biological processes, and molecular functions in the pathological changes in IBDV-infected thymus. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that the host mainly displayed severely disruption of cell survival/repair, proliferation and metabolism pathway, meanwhile, the infection triggers antiviral immune activation with a potential emphasis on the MDA5 pathway. Network inference analysis identified seven core hub genes, which include CDK1, TYMS, MCM5, KIF11, CCNB2, MAD2L1, and MCM4. These genes are all associated with cell-cycle regulating pathway and are likely key mediators in the pathogenesis induced by NN1172 infection in the thymus. This study discovered dominant pathways and genes which enhanced our understanding of the molecular mechanisms underlying IBDV pathogenesis in the thymus.

Mechanism of cell cycle regulation and cell proliferation during human viral infection

Over the history of the coevolution of Host viral interaction, viruses have customized the host cellular machinery into their use for viral genome replication, causing effective infection and ultimately aiming for survival. They do so by inducing subversions to the host cellular pathways like cell cycle via dysregulation of important cell cycle checkpoints by viral encoded proteins, arresting the cell cycle machinery, blocking cytokinesis as well as targeting subnuclear bodies, thus ultimately disorienting the cell proliferation. Both DNA and RNA viruses have been active participants in such manipulation resulting in serious outcomes of cancer. They achieve this by employing different mechanisms-Protein-protein interaction, protein-phosphorylation, degradation, redistribution, viral homolog, and viral regulation of APC at different stages of cell cycle events. Several DNA viruses cause the quiescent staged cells to undergo cell cycle which increases nucleotide pools logistically significantly persuading viral replication whereas few other viruses arrest a particular stage of cell cycle. This allows the latter group to sustain the infection which allows them to escape host immune response and support viral multiplication. Mechanical study of signaling such viral mediated pathways could give insight into understanding the etiology of tumorigenesis and progression. Overall this chapter highlights the possible strategies employed by DNA/RNA viral families which impact the normal cell cycle but facilitate viral infected cell replication. Such information could contribute to comprehending viral infection-associated disorders to further depth.

Deregulation of the cyclin-dependent kinase inhibitor p27 as a putative candidate for transformation in Chlamydia trachomatis infected mesenchymal stem cells

Purpose

Several pathological conditions might cause the degradation of the cyclin-dependent kinase inhibitor (CKI) p27 and cell cycle arrest at the G1 phase, including cancers and infections. Chlamydia trachomatis (Ctr), as an obligatory intracellular pathogen, has been found to alter the fate of the cell from different aspects. In this study, we aimed to investigate the effect of Ctr infection on the expression of the important cell cycle regularity protein p27 in mesenchymal stem cells (MSCs).

Methods

Isolation of MSCs from healthy human fallopian tube was confirmed by detection of the stemness markers Sox2, Nanog and Oct4 and the surface markers CD44, CD73 and CD90 by Western blotting and fluorescence-activated cell sorting analysis. The expression of p27 was downregulated at the protein level upon Ctr D infection measured by Real-Time Quantitative Reverse Transcription PCR (qRT-PCR), IF and Western blotting. Recovery of p27 in Ctr D-infected MSCs was achieved by treatment with difluoromethylornithine (DFMO). Ctr D infected MSCs were able to produce colonies in anchorage-independent soft agar assay.

Conclusion

Ctr D infection was able to downregulate the expression of the important cell cycle regulator protein p27, which will be considered a putative candidate for transformation in Ctr D infected MSCs.

Japanese Encephalitis Virus NS1' Protein Interacts with Host CDK1 Protein to Regulate Antiviral Response

Type I interferon (IFN-I) is a key component of the host innate immune system. To establish efficient replication, viruses have developed several strategies to escape from the host IFN response. Japanese encephalitis virus (JEV) NS1', a larger NS1-related protein, is known to inhibit the mitochondrial antiviral signaling (MAVS)-mediated IFN-β induction by increasing the binding of transcription factors (CREB and c-Rel) to the microRNA 22 (miRNA-22) promoter. However, the mechanism by which NS1' induces the recruitment of CREB and c-Rel onto the miRNA-22 promoter is unknown. Here, we found that JEV NS1' protein interacts with the host cyclin-dependent kinase 1 (CDK1) protein. Mechanistically, NS1' interrupts the CDC25C phosphatase-mediated dephosphorylation of CDK1, which prolongs the phosphorylation status of CDK1 and leads to the inhibition of MAVS-mediated IFN-β induction. Furthermore, the CREB phosphorylation and c-Rel activation through the IκBα phosphorylation were observed to be enhanced upon the augmentation of CDK1 phosphorylation by NS1'. The abrogation of CDK1 activity by a small-molecule inhibitor significantly suppressed the JEV replication in vitro and in vivo. Moreover, the administration of CDK1 inhibitor protected the wild-type mice from JEV-induced lethality but showed no effect on the MAVS^-/- mice challenged with JEV. In conclusion, our study provides new insight into the mechanism of JEV immune evasion, which may lead to the development of novel therapeutic options to treat JEV infection. IMPORTANCE Japanese encephalitis virus (JEV) is the main cause of acute human encephalitis in Asia. The unavailability of specific treatment for Japanese encephalitis demands a better understanding of the basic cellular mechanisms that contribute to the onset of disease. The present study identifies a novel interaction between the JEV NS1' protein and the cellular CDK1 protein, which facilitates the JEV replication by dampening the cellular antiviral response. This study sheds light on a novel mechanism of JEV replication, and thus our findings could be employed for developing new therapies against JEV infection.

The reovirus μ2 C-terminal loop inversely regulates NTPase and transcription functions versus binding to factory-forming μNS and promotes replication in tumorigenic cells

Wild type reovirus serotype 3 'Dearing PL strain' (T3wt) is being heavily evaluated as an oncolytic and immunotherapeutic treatment for cancers. Mutations that promote reovirus entry into tumor cells were previously reported to enhance oncolysis; herein we aimed to discover mutations that enhance the post-entry steps of reovirus infection in tumor cells. Using directed evolution, we identified that reovirus variant T3v10^M1 exhibited enhanced replication relative to T3wt on a panel of cancer cells. T3v10^M1 contains an alanine-to-valine substitution (A612V) in the core-associated μ2, which was previously found to have NTPase activities in virions and to facilitate virus factory formation by association with μNS. Paradoxically, the A612V mutation in μ2 from T3v10^M1 was discovered to impair NTPase activities and RNA synthesis, leading to five-fold higher probability of abortive infection for T3v10^M1 relative to T3wt. The A612V mutation resides in a previously uncharacterized C-terminal region that juxtaposes the template entry site of the polymerase μ2; our findings thus support an important role for this domain during virus transcription. Despite crippled onset of infection, T3v10^M1 exhibited greater accumulation of viral proteins and progeny during replication, leading to increased overall virus burst size. Both Far-Western and co-immunoprecipitation approaches corroborated that the A612V mutation in μ2 increased association with the non-structural virus protein μNS and enhances burst size. Altogether the data supports that mutations in the C-terminal loop domain of μ2 inversely regulate NTPase and RNA synthesis versus interactions with μNS, but with a net gain of replication in tumorigenic cells.SIGNIFICANCEReovirus is a model system for understanding virus replication but also a clinically relevant virus for cancer therapy. We identified the first mutation that increases reovirus infection in tumorigenic cells by enhancing post-entry stages of reovirus replication. The mutation is in a previously uncharacterized c-terminal region of the M1-derived μ2 protein, which we demonstrated affects multiple functions of μ2; NTPase, RNA synthesis, inhibition of antiviral immune response and association with the virus replication factory-forming μNS protein. These findings promote a mechanistic understanding of viral protein functions. In the future, the benefits of μ2 mutations may be useful for enhancing reovirus potency in tumors.

Egress of non-enveloped enteric RNA viruses

A long-standing paradigm in virology was that non-enveloped viruses induce cell lysis to release progeny virions. However, emerging evidence indicates that some non-enveloped viruses exit cells without inducing cell lysis, while others engage both lytic and non-lytic egress mechanisms. Enteric viruses are transmitted via the faecal-oral route and are important causes of a wide range of human infections, both gastrointestinal and extra-intestinal. Virus cellular egress, when fully understood, may be a relevant target for antiviral therapies, which could minimize the public health impact of these infections. In this review, we outline lytic and non-lytic cell egress mechanisms of non-enveloped enteric RNA viruses belonging to five families: Picornaviridae, Reoviridae, Caliciviridae, Astroviridae and Hepeviridae. We discuss factors that contribute to egress mechanisms and the relevance of these mechanisms to virion stability, infectivity and transmission. Since most data were obtained in traditional two-dimensional cell cultures, we will further attempt to place them into the context of polarized cultures and in vivo pathogenesis. Throughout the review, we highlight numerous knowledge gaps to stimulate future research into the egress mechanisms of these highly prevalent but largely understudied viruses.

Role of p38 mitogen-activated protein kinase signalling in virus replication and potential for developing broad spectrum antiviral drugs

Mitogen-activated protein kinases (MAPKs) play a key role in complex cellular processes such as proliferation, development, differentiation, transformation and apoptosis. Mammals express at least four distinctly regulated groups of MAPKs which include extracellular signal-related kinases (ERK)-1/2, p38 proteins, Jun amino-terminal kinases (JNK1/2/3) and ERK5. p38 MAPK is activated by a wide range of cellular stresses and modulates activity of several downstream kinases and transcription factors which are involved in regulating cytoskeleton remodeling, cell cycle modulation, inflammation, antiviral response and apoptosis. In viral infections, activation of cell signalling pathways is part of the cellular defense mechanism with the basic aim of inducing an antiviral state. However, viruses can exploit enhanced cell signalling activities to support various stages of their replication cycles. Kinase activity can be inhibited by small molecule chemical inhibitors, so one strategy to develop antiviral drugs is to target these cellular signalling pathways. In this review, we provide an overview on the current understanding of various cellular and viral events regulated by the p38 signalling pathway, with a special emphasis on targeting these events for antiviral drug development which might identify candidates with broad spectrum activity.

Combination Therapy with Reovirus and ATM Inhibitor Enhances Cell Death and Virus Replication in Canine Melanoma

Oncolytic virotherapy using reovirus is a promising new anti-cancer treatment with potential for use in humans and dogs. Because reovirus monotherapy shows limited efficacy in human and canine cancer patients, the clinical development of a combination therapy is necessary. To identify candidate components of such a combination, we screened a 285-compound drug library for those that enhanced reovirus cytotoxicity in a canine melanoma cell line. Here, we show that exposure to an inhibitor of the ataxia telangiectasia mutated protein (ATM) enhances the oncolytic potential of reovirus in five of six tested canine melanoma cell lines. Specifically, the ATM inhibitor potentiated reovirus replication in cancer cells along with promoting the lysosomal activity, resulting in an increased proportion of caspase-dependent apoptosis and cell cycle arrest at G2/M compared to those observed with reovirus alone. Overall, our study suggests that the combination of reovirus and the ATM inhibitor may be an attractive option in cancer therapy.

Cdc20 and molecular chaperone CCT2 and CCT5 are required for the Muscovy duck reovirus p10.8-induced cell cycle arrest and apoptosis

This study demonstrates that the Muscovy duck reovirus (MDRV) p10.8 protein is one of many viral non-structural proteins that induces both cell cycle arrest and apoptosis. The p10.8 but not σC is a nuclear targeting protein that shuttles between the nucleus and the cytoplasm. Our results reveal that p10.8-induced apoptosis in cultured cells occurs by the nucleoporin Tpr/p53-dependent and Fas/caspase 8-mediated pathways. Furthermore, a compelling finding from this study is that the p10.8 and σC proteins of MDRV facilitate CDK2 and CDK4 degradation via the ubiquitin-proteasome pathway. We found that depletion of Cdc20 reversed the p10.8- and σC- mediated CDK4 degradation and p10.8-induced apoptosis, suggesting that Cdc20 plays a critical role in modulating p10.8-mediated cell cycle and apoptosis. Furthermore, we found that depletion of chaperonin-containing tailless complex polypeptide 1 (CCT) 2 and CCT5 reduced the level of Cdc20 and reversed the p10.8- and σC-mediated CDK4 degradation and p10.8-induced apoptosis, indicating that molecular chaperone CCT2 and CCT5 are required for stabilization of Ccd20 for mediating both cell cycle arrest and apoptosis. This study provides mechanistic insights into how p10.8 induces both cell cycle arrest and apoptosis.

Cell Cycle Arrest in G2/M Phase Enhances Replication of Interferon-Sensitive Cytoplasmic RNA Viruses via Inhibition of Antiviral Gene Expression

Vesicular stomatitis virus (VSV) (a rhabdovirus) and its variant VSV-ΔM51 are widely used model systems to study mechanisms of virus-host interactions. Here, we investigated how the cell cycle affects replication of these viruses using an array of cell lines with different levels of impairment of antiviral signaling and a panel of chemical compounds arresting the cell cycle at different phases. We observed that all compounds inducing cell cycle arrest in G₂/M phase strongly enhanced the replication of VSV-ΔM51 in cells with functional antiviral signaling. G₂/M arrest strongly inhibited type I and type III interferon (IFN) production as well as expression of IFN-stimulated genes in response to exogenously added IFN. Moreover, G₂/M arrest enhanced the replication of Sendai virus (a paramyxovirus), which is also highly sensitive to the type I IFN response but did not stimulate the replication of a wild-type VSV that is more effective at evading antiviral responses. In contrast, the positive effect of G₂/M arrest on virus replication was not observed in cells defective in IFN signaling. Altogether, our data show that replication of IFN-sensitive cytoplasmic viruses can be strongly stimulated during G₂/M phase as a result of inhibition of antiviral gene expression, likely due to mitotic inhibition of transcription, a global repression of cellular transcription during G₂/M phase. The G₂/M phase thus could represent an "Achilles' heel" of the infected cell, a phase when the cell is inadequately protected. This model could explain at least one of the reasons why many viruses have been shown to induce G₂/M arrest.IMPORTANCE Vesicular stomatitis virus (VSV) (a rhabdovirus) and its variant VSV-ΔM51 are widely used model systems to study mechanisms of virus-host interactions. Here, we investigated how the cell cycle affects replication of VSV and VSV-ΔM51. We show that G₂/M cell cycle arrest strongly enhances the replication of VSV-ΔM51 (but not of wild-type VSV) and Sendai virus (a paramyxovirus) via inhibition of antiviral gene expression, likely due to mitotic inhibition of transcription, a global repression of cellular transcription during G₂/M phase. Our data suggest that the G₂/M phase could represent an "Achilles' heel" of the infected cell, a phase when the cell is inadequately protected. This model could explain at least one of the reasons why many viruses have been shown to induce G₂/M arrest, and it has important implications for oncolytic virotherapy, suggesting that frequent cell cycle progression in cancer cells could make them more permissive to viruses.

A single mutation in the mammalian orthoreovirus S1 gene is responsible for increased interferon sensitivity in a virus mutant selected in Vero cells

In a previous study, a mammalian orthoreovirus mutant was isolated based on its increased ability to infect interferon-defective Vero cells and was referred to as Vero-cells-adapted virus (VeroAV). This virus exhibits reduced ability to resist the antiviral effect of interferon. In the present study, the complete genome sequence of VeroAV was first determined. Reverse genetics was then used to identify a unique mutation on the S1 gene, overlapping the σ1 and σ1 s reading frame, resulting in increased sensitivity to interferon. A virus lacking σ1 s expression consecutive to mutation of its initiation codon was then shown to exhibit a further increase in sensitivity to interferon, supporting the idea that σ1 s is the viral protein responsible. This identification of a new determinant of reovirus sensitivity to interferon gives credentials to the idea that multiple reovirus genes are responsible for the level of interferon induction and susceptibility to the interferon-induced antiviral activities.

Breaking Bad: How Viruses Subvert the Cell Cycle

Interactions between the host and viruses during the course of their co-evolution have not only shaped cellular function and the immune system, but also the counter measures employed by viruses. Relatively small genomes and high replication rates allow viruses to accumulate mutations and continuously present the host with new challenges. It is therefore, no surprise that they either escape detection or modulate host physiology, often by redirecting normal cellular pathways to their own advantage. Viruses utilize a diverse array of strategies and molecular targets to subvert host cellular processes, while evading detection. These include cell-cycle regulation, major histocompatibility complex-restricted antigen presentation, intracellular protein transport, apoptosis, cytokine-mediated signaling, and humoral immune responses. Moreover, viruses routinely manipulate the host cell cycle to create a favorable environment for replication, largely by deregulating cell cycle checkpoints. This review focuses on our current understanding of the molecular aspects of cell cycle regulation that are often targeted by viruses. Further study of their interactions should provide fundamental insights into cell cycle regulation and improve our ability to exploit these viruses.

Protoparvovirus Interactions with the Cellular DNA Damage Response

Protoparvoviruses are simple single-stranded DNA viruses that infect many animal species. The protoparvovirus minute virus of mice (MVM) infects murine and transformed human cells provoking a sustained DNA damage response (DDR). This DDR is dependent on signaling by the ATM kinase and leads to a prolonged pre-mitotic cell cycle block that features the inactivation of ATR-kinase mediated signaling, proteasome-targeted degradation of p21, and inhibition of cyclin B1 expression. This review explores how protoparvoviruses, and specifically MVM, co-opt the common mechanisms regulating the DDR and cell cycle progression in order to prepare the host nuclear environment for productive infection.

Minute Virus of Mice Inhibits Transcription of the Cyclin B1 Gene during Infection

Replication of minute virus of mice (MVM) induces a sustained cellular DNA damage response (DDR) which the virus then exploits to prepare the nuclear environment for effective parvovirus takeover. An essential aspect of the MVM-induced DDR is the establishment of a potent premitotic block, which we previously found to be independent of activated p21 and ATR/Chk1 signaling. This arrest, unlike others reported previously, depends upon a significant, specific depletion of cyclin B1 and its encoding RNA, which precludes cyclin B1/CDK1 complex function, thus preventing mitotic entry. We show here that while the stability of cyclin B1 RNA was not affected by MVM infection, the production of nascent cyclin B1 RNA was substantially diminished at late times postinfection. Ectopic expression of NS1 alone did not reduce cyclin B1 expression. MVM infection also reduced the levels of cyclin B1 protein, and RNA levels normally increased in response to DNA-damaging reagents. We demonstrated that at times of reduced cyclin B1 expression during infection, there was a significantly reduced occupancy of RNA polymerase II and the essential mitotic transcription factor FoxM1 on the cyclin B1 gene promoter. Additionally, while total FoxM1 levels remained constant, there was a significant decrease of the phosphorylated, likely active, forms of FoxM1. Targeting of a constitutively active FoxM1 construct or the activation domain of FoxM1 to the cyclin B1 gene promoter via clustered regularly interspaced short palindromic repeats (CRISPR)-enzymatically inactive Cas9 in MVM-infected cells increased both cyclin B1 protein and RNA levels, implicating FoxM1 as a critical target for cyclin B1 inhibition during MVM infection.IMPORTANCE Replication of the parvovirus minute virus of mice (MVM) induces a sustained cellular DNA damage response (DDR) which the virus exploits to prepare the nuclear environment for effective takeover. An essential aspect of the MVM-induced DDR is establishment of a potent premitotic block. This block depends upon a significant, specific depletion of cyclin B1 and its encoding RNA that precludes cyclin B1/CDK1 complex functions necessary for mitotic entry. We show that reduced cyclin B1 expression is controlled primarily at the level of transcription initiation. Additionally, the essential mitotic transcription factor FoxM1 and RNA polymerase II were found to occupy the cyclin B1 gene promoter at reduced levels during infection. Recruiting a constitutively active FoxM1 construct or the activation domain of FoxM1 to the cyclin B1 gene promoter via CRISPR-catalytically inactive Cas9 (dCas9) in MVM-infected cells increased expression of both cyclin B1 protein and RNA, implicating FoxM1 as a critical target mediating MVM-induced cyclin B1 inhibition.

Growth Arrest-Specific 6 (GAS6) Promotes Prostate Cancer Survival by G1 Arrest/S Phase Delay and Inhibition of Apoptosis During Chemotherapy in Bone Marrow

Prostate cancer (PCa) is known to develop resistance to chemotherapy. Growth arrest-specific 6 (GAS6), plays a role in tumor progression by regulating growth in many cancers. Here, we explored how GAS6 regulates the cell cycle and apoptosis of PCa cells in response to chemotherapy. We found that GAS6 is sufficient to significantly increase the fraction of cells in G1 and the duration of phase in PCa cells. Importantly, the effect of GAS6 on G1 is potentiated during docetaxel chemotherapy. GAS6 altered the levels of several key cell cycle regulators, including the downregulation of Cyclin B1 (G2 /M phase), CDC25A, Cyclin E1, and CDK2 (S phase entry), while the upregulation of cell cycle inhibitors p27 and p21, Cyclin D1, and CDK4. Importantly, these changes became further accentuated during docetaxel treatment in the presence of GAS6. Moreover, GAS6 alters the apoptotic response of PCa cells during docetaxel chemotherapy. Docetaxel induced PCa cell apoptosis is efficiently suppressed in PCa cell culture in the presence of GAS6 or GAS6 secreted from co-cultured osteoblasts. Similarly, the GAS6-expressing bone environment protects PCa cells from apoptosis within primary tumors in vivo studies. Docetaxel induced significant levels of Caspase-3 and PARP cleavage in PCa cells, while GAS6 protected PCa cells from docetaxel-induced apoptotic signaling. Together, these data suggest that GAS6, expressed by osteoblasts in the bone marrow, plays a significant role in the regulation of PCa cell survival during chemotherapy, which will have important implications for targeting metastatic disease. J. Cell. Biochem. 117: 2815-2824, 2016. © 2016 Wiley Periodicals, Inc.

Suppression of Vimentin Phosphorylation by the Avian Reovirus p17 through Inhibition of CDK1 and Plk1 Impacting the G2/M Phase of the Cell Cycle

The p17 protein of avian reovirus (ARV) causes cell cycle retardation in a variety of cell lines; however, the underlying mechanism(s) by which p17 regulates the cell cycle remains largely unknown. We demonstrate for the first time that p17 interacts with CDK1 and vimentin as revealed by reciprocal co-immunoprecipitation and GST pull-down assays. Both in vitro and in vivo studies indicated that direct interaction of p17 and CDK1/vimentin was mapped within the amino terminus (aa 1-60) of p17 and central region (aa 27-118) of CDK1/vimentin. Furthermore, p17 was found to occupy the Plk1-binding site within the vimentin, thereby blocking Plk1 recruitment to CDK1-induced vimentin phosphorylation at Ser 56. Interaction of p17 to CDK1 or vimentin interferes with CDK1-catalyzed phosphorylation of vimentin at Ser 56 and subsequently vimentin phosphorylation at Ser 82 by Plk1. Furthermore, we have identified upstream signaling pathways and cellular factor(s) targeted by p17 and found that p17 regulates inhibitory phosphorylation of CDK1 and blocks vimentin phosphorylation at Ser 56 and Ser 82. The p17-mediated inactivation of CDK1 is dependent on several mechanisms, which include direct interaction with CDK1, p17-mediated suppression of Plk1 by activating the Tpr/p53 and ATM/Chk1/PP2A pathways, and p17-mediated cdc25C degradation via an ubiquitin- proteasome pathway. Additionally, depletion of p53 with a shRNA as well as inhibition of ATM and vimentin by inhibitors diminished virus yield while Tpr and CDK1 knockdown increased virus yield. Taken together, results demonstrate that p17 suppresses both CDK1 and Plk1functions, disrupts vimentin phosphorylation, causes G2/M cell cycle arrest and thus benefits virus replication.

8-60hIPP5(m)-induced G2/M cell cycle arrest involves activation of ATM/p53/p21(cip1/waf1) pathways and delayed cyclin B1 nuclear translocation

Protein phosphatase 1 (PP1) is a major serine/threonine phosphatase that controls gene expression and cell cycle progression. The active mutant IPP5 (8-60hIPP5(m)), the latest member of the inhibitory molecules for PP1, has been shown to inhibit the growth of human cervix carcinoma cells (HeLa). In order to elucidate the underlying mechanisms, the present study assessed overexpression of 8-60hIPP5(m) in HeLa cells. Flow cytometric and biochemical analyses showed that overexpression of 8-60hIPP5(m) induced G2/M-phase arrest, which was accompanied by the upregulation of cyclin B1 and phosphorylation of G2/M-phase proteins ATM, p53, p21(cip1/waf1) and Cdc2, suggesting that 8-60hIPP5(m) induces G2/M arrest through activation of the ATM/p53/p21(cip1/waf1)/Cdc2/ cyclin B1 pathways. We further showed that overexpression of 8-60hIPP5(m) led to delayed nuclear translocation of cyclin B1. 8-60hIPP5(m) also could translocate to the nucleus in G2/M phase and interact with pp1α and Cdc2 as demonstrated by co-precipitation assay. Taken together, our data demonstrate a novel role for 8-60hIPP5(m) in regulation of cell cycle in HeLa cells, possibly contributing to the development of new therapeutic strategies for cervix carcinoma.

Genome Sequence of the Novel Reassortant Mammalian Orthoreovirus Strain MRV00304/13, Isolated from a Calf with Diarrhea from the United States

Mammalian orthoreovirus (MRV) strain MRV00304/13 was isolated from diarrheic calves. The serotype-specific antigen σ1 was found to be 95% identical to that of bovine MRV1. All predicted viral proteins had >92% identity to those of MRV except µ2 and σ1s (80 and 72% identities, respectively), suggesting that MRV00304/13 is a novel reassortant MRV1.

Cell cycle regulation during viral infection

To replicate their genomes in cells and generate new progeny, viruses typically require factors provided by the cells that they have infected. Subversion of the cellular machinery that controls replication of the infected host cell is a common activity of many viruses. Viruses employ different strategies to deregulate cell cycle checkpoint controls and modulate cell proliferation pathways. A number of DNA and RNA viruses encode proteins that target critical cell cycle regulators to achieve cellular conditions that are beneficial for viral replication. Many DNA viruses induce quiescent cells to enter the cell cycle; this is thought to increase pools of deoxynucleotides and thus, facilitate viral replication. In contrast, some viruses can arrest cells in a particular phase of the cell cycle that is favorable for replication of the specific virus. Cell cycle arrest may inhibit early cell death of infected cells, allow the cells to evade immune defenses, or help promote virus assembly. Although beneficial for the viral life cycle, virus-mediated alterations in normal cell cycle control mechanisms could have detrimental effects on cellular physiology and may ultimately contribute to pathologies associated with the viral infection, including cell transformation and cancer progression and maintenance. In this chapter, we summarize various strategies employed by DNA and RNA viruses to modulate the replication cycle of the virus-infected cell. When known, we describe how these virus-associated effects influence replication of the virus and contribute to diseases associated with infection by that specific virus.

Mechanisms of reovirus bloodstream dissemination

Many viruses cause disease within an infected host after spread from an initial portal of entry to sites of secondary replication. Viruses can disseminate via the bloodstream or through nerves. Mammalian orthoreoviruses (reoviruses) are neurotropic viruses that use both bloodborne and neural pathways to spread systemically within their hosts to cause disease. Using a robust mouse model and a dynamic reverse genetics system, we have identified a viral receptor and a viral nonstructural protein that are essential for hematogenous reovirus dissemination. Junctional adhesion molecule-A (JAM-A) is a member of the immunoglobulin superfamily expressed in tight junctions and on hematopoietic cells that serves as a receptor for all reovirus serotypes. Expression of JAM-A is required for infection of endothelial cells and development of viremia in mice, suggesting that release of virus into the bloodstream from infected endothelial cells requires JAM-A. Nonstructural protein σ1s is implicated in cell cycle arrest and apoptosis in reovirus-infected cells but is completely dispensable for reovirus replication in cultured cells. Surprisingly, a recombinant σ1s-null reovirus strain fails to spread hematogenously in infected mice, suggesting that σ1s facilitates apoptosis of reovirus-infected intestinal epithelial cells. It is possible that apoptotic bodies formed as a consequence of σ1s expression lead to reovirus uptake by dendritic cells for subsequent delivery to the mesenteric lymph node and the blood. Thus, both host and viral factors are required for efficient hematogenous dissemination of reovirus. Understanding mechanisms of reovirus bloodborne spread may shed light on how microbial pathogens invade the bloodstream to disseminate and cause disease in infected hosts.

Nonstructural protein σ1s mediates reovirus-induced cell cycle arrest and apoptosis

Reovirus nonstructural protein σ1s is implicated in cell cycle arrest at the G2/M boundary and induction of apoptosis. However, the contribution of σ1s to these effects in an otherwise isogenic viral background has not been defined. To evaluate the role of σ1s in cell cycle arrest and apoptosis, we used reverse genetics to generate a σ1s-null reovirus. Following infection with wild-type virus, we observed an increase in the percentage of cells in G2/M, whereas the proportion of cells in G2/M following infection with the σ1s-null mutant was unaffected. Similarly, we found that the wild-type virus induced substantially greater levels of apoptosis than the σ1s-null mutant. These data indicate that σ1s is required for both reovirus-induced cell cycle arrest and apoptosis. To define sequences in σ1s that mediate these effects, we engineered viruses encoding C-terminal σ1s truncations by introducing stop codons in the σ1s open reading frame. We also generated viruses in which charged residues near the σ1s amino terminus were replaced individually or as a cluster with nonpolar residues. Analysis of these mutants revealed that amino acids 1 to 59 and the amino-terminal basic cluster are required for induction of both cell cycle arrest and apoptosis. Remarkably, viruses that fail to induce cell cycle arrest and apoptosis also are attenuated in vivo. Thus, identical sequences in σ1s are required for reovirus-induced cell cycle arrest, apoptosis, and pathogenesis. Collectively, these findings provide evidence that the σ1s-mediated properties are genetically linked and suggest that these effects are mechanistically related.

Apoptosis induced by mammalian reovirus is beta interferon (IFN) independent and enhanced by IFN regulatory factor 3- and NF-κB-dependent expression of Noxa

A variety of signal transduction pathways are activated in response to viral infection, which dampen viral replication and transmission. These mechanisms involve both the induction of type I interferons (IFNs), which evoke an antiviral state, and the triggering of apoptosis. Mammalian orthoreoviruses are double-stranded RNA viruses that elicit apoptosis in vitro and in vivo. The transcription factors interferon regulatory factor 3 (IRF-3) and nuclear factor kappa light-chain enhancer of activated B cells (NF-κB) are required for the expression of IFN-β and the efficient induction of apoptosis in reovirus-infected cells. However, it is not known whether IFN-β induction is required for apoptosis, nor have the genes induced by IRF-3 and NF-κB that are responsible for apoptosis been identified. To determine whether IFN-β is required for reovirus-induced apoptosis, we used type I IFN receptor-deficient cells, IFN-specific antibodies, and recombinant IFN-β. We found that IFN synthesis and signaling are dispensable for the apoptosis of reovirus-infected cells. These results indicate that the apoptotic response following reovirus infection is mediated directly by genes responsive to IRF-3 and NF-κB. Noxa is a proapoptotic BH3-domain-only protein of the Bcl-2 family that requires IRF-3 and NF-κB for efficient expression. We found that Noxa is strongly induced at late times (36 to 48 h) following reovirus infection in a manner dependent on IRF-3 and NF-κB. The level of apoptosis induced by reovirus is significantly diminished in cells lacking Noxa, indicating a key prodeath function for this molecule during reovirus infection. These results suggest that prolonged innate immune response signaling induces apoptosis by eliciting Noxa expression in reovirus-infected cells.

The reovirus sigma1s protein is a determinant of hematogenous but not neural virus dissemination in mice

Nonstructural protein σ1s is a critical determinant of hematogenous dissemination by type 1 reoviruses, which reach the central nervous system (CNS) by a strictly blood-borne route. However, it is not known whether σ1s contributes to neuropathogenesis of type 3 reoviruses, which disseminate by both vascular and neural pathways. Using isogenic type 3 viruses that vary only in σ1s expression, we observed that mice survived at a higher frequency following hind-limb inoculation with σ1s-null virus than when inoculated with wild-type virus. This finding suggests that σ1s is essential for reovirus virulence when inoculated at a site that requires systemic spread to cause disease. Wild-type and σ1s-null viruses produced comparable titers in the spinal cord, suggesting that σ1s is dispensable for invasion of the CNS. Although the two viruses ultimately achieved similar peak titers in the brain, loads of wild-type virus were substantially greater than those of the σ1s-null mutant at early times after inoculation. In contrast, wild-type virus produced substantially higher titers than the σ1s-null virus in peripheral organs to which reovirus spreads via the blood, including the heart, intestine, liver, and spleen. Concordantly, viral titers in the blood were higher following infection with wild-type virus than following infection with the σ1s-null mutant. These results suggest that differences in viral brain titers at early time points postinfection are due to limited virus delivery to the brain by hematogenous pathways. Transection of the sciatic nerve prior to hind-limb inoculation diminished viral spread to the spinal cord. However, wild-type virus retained the capacity to disseminate to the brain following sciatic nerve transection, indicating that wild-type reovirus can spread to the brain by the blood. Together, these results indicate that σ1s is not required for reovirus spread by neural mechanisms. Instead, σ1s mediates hematogenous dissemination within the infected host, which is required for full reovirus neurovirulence.

Human herpesvirus 6 suppresses T cell proliferation through induction of cell cycle arrest in infected cells in the G2/M phase

Human herpesvirus 6 (HHV-6) is an important immunosuppressive and immunomodulatory virus that primarily infects immune cells and strongly suppresses the proliferation of infected cells. However, the mechanisms responsible for the regulation and suppression mediated by HHV-6 are still unknown. In this study, we examined the ability of HHV-6A to manipulate cell cycle progression in infected cells and explored the potential molecular mechanisms. We demonstrated that infection with HHV-6A imposed a growth-inhibitory effect on HSB-2 cells by inducing cell cycle arrest at the G(2)/M phase. We then showed that the activity of the Cdc2-cyclin B1 complex was significantly decreased in HHV-6A-infected HSB-2 cells. Furthermore, we found that inactivation of Cdc2-cyclin B1 in HHV-6A-infected cells occurred through the inhibitory Tyr15 phosphorylation resulting from elevated Wee1 expression and inactivated Cdc25C. The reduction of Cdc2-cyclin B1 activity in HHV-6-infected cells was also partly due to the increased expression of the cell cycle-regulatory molecule p21 in a p53-dependent manner. In addition, HHV-6A infection activated the DNA damage checkpoint kinases Chk2 and Chk1. Our data suggest that HHV-6A infection induces G(2)/M arrest in infected T cells via various molecular regulatory mechanisms. These results further demonstrate the potential mechanisms involved in immune suppression and modulation mediated by HHV-6 infection, and they provide new insights relevant to the development of novel vaccines and immunotherapeutic approaches.

The effect of cell cycle synchronization on tumor sensitivity to reovirus oncolysis

The potential for increased sensitivity of tumor cells to oncolytic reovirus by altering the normal cell cycle using clinically available pharmacological agents was investigated. B16.F10 mouse melanoma cells were partially synchronized with hydroxyurea, thymidine, or by mitotic shake-off. Cell survival was determined using MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)- 2-(4-sulfophenyl)-2H-tetrazolium)] survival assay and virus yield in tumors by plaque assay. An enhanced sensitivity to reovirus was observed following the removal of either hydroxyurea or thymidine from the culture medium (P < 0.0001). The greatest survival difference compared to normal cycling cells was noted when the majority of cells were in S and G2/M phases, and was associated with increased viral replication. Cells collected by mitotic shake-off were nearly devoid of cells in S phase and were less susceptible to reovirus-induced cell kill than their nonsynchronized counterparts (P < 0.0001). In vivo combination of hydroxyurea followed by intratumoral reovirus resulted in reduced tumor growth and increased survival compared to monotherapy (P = 0.0041) at 15 days. Increased amounts of virus were retrieved from tumors from mice treated with sequential hydroxyurea/reovirus compared to concomitant treatment or reovirus monotherapy. These data justify clinical evaluation of this approach supported by the extensive experience, low cost, simple administration, and availability of hydroxyurea.

Avian reovirus nonstructural protein p17-induced G(2)/M cell cycle arrest and host cellular protein translation shutoff involve activation of p53-dependent pathways

The effects of avian reovirus (ARV) p17 protein on cell cycle progression and host cellular protein translation were studied. ARV infection and ARV p17 transfection resulted in the accumulation of infected and/or transfected cells in the G(2)/M phase of the cell cycle. The accumulation of cells in the G(2)/M phase was accompanied by upregulation and phosphorylation of the G(2)/M-phase proteins ATM, p53, p21(cip1/waf1), Cdc2, cyclin B1, Chk1, Chk2, and Cdc25C, suggesting that p17 induces a G(2)/M cell cycle arrest through activation of the ATM/p53/p21(cip1/waf1)/Cdc2/cyclin B1 and ATM/Chk1/Chk2/Cdc25C pathways. The G(2)/M cell cycle arrest resulted in increased virus replication. In the present study, we also provide evidence demonstrating that p17 protein is responsible for ARV-induced host cellular protein translation shutoff. Increased phosphorylation levels of the eukaryotic translation elongation factor 2 (eEF2) and initiation factor eIF2alpha and reduced phosphorylation levels of the eukaryotic translation initiation factors eIF4E, eIF4B, and eIF4G, as well as 4E-BP1 and Mnk-1 in p17-transfected cells, demonstrated that ARV p17 suppresses translation initiation factors and translation elongation factors to induce host cellular protein translation shutoff. Inhibition of mTOR by rapamycin resulted in a decrease in the levels of phosphorylated 4E-BP1, eIF4B, and eIF4G and an increase in the levels eEF2 but did not affect ARV replication, suggesting that ARV replication was not hindered by inhibition of cap-dependent translation. Taken together, our data indicate that ARV p17-induced G(2)/M arrest and host cellular translation shutoff resulted in increased ARV replication.

Reovirus nonstructural protein sigma1s is required for establishment of viremia and systemic dissemination

Serotype-specific patterns of reovirus disease in the CNS of newborn mice segregate with the viral S1 gene segment, which encodes attachment protein sigma1 and nonstructural protein sigma1s. The importance of receptor recognition in target cell selection by reovirus implicates the sigma1 protein as the primary effector of disease outcome. However, the contribution of sigma1s to reovirus disease is unknown. To define the function of sigma1s in reovirus pathogenesis, we generated a sigma1s-deficient virus by altering a single nucleotide to disrupt the sigma1s translational start site. Viruses were recovered that contain nine gene segments from strain type 3 Dearing and either the wild-type or sigma1s-null S1 gene segment from strain type 1 Lang. Following peroral inoculation of newborn mice, both viruses replicated in the intestine, although the wild-type virus achieved higher yields than the sigma1s-null virus. However, unlike the wild-type virus, the sigma1s-deficient virus failed to disseminate to sites of secondary viral replication, including the brain, heart, and liver. Within the small intestine, both viruses were detected in Peyer's patches, but only the wild-type virus reached the mesenteric lymph node. Concordantly, wild-type virus, but not sigma1s-deficient virus, was detected in the blood of infected animals. Wild-type and sigma1s-null viruses produced equivalent titers following intracranial inoculation, indicating that sigma1s is dispensable for viral growth in the murine CNS. These results suggest a key role for sigma1s in virus spread from intestinal lymphatics to the bloodstream, thereby allowing the establishment of viremia and dissemination to sites of secondary replication within the infected host.

Apoptin induces chromatin condensation in normal cells

Apoptin, a chicken anemia virus protein, was reported to induce tumor specific apoptosis, which was correlated with the nuclear localization of the protein in tumor cells. While in normal human cells, Apoptin was detected mainly in the cytoplasm and did not induce apoptosis. Using a recombinant adenovirus expressing Apoptin, we have found that Apoptin induced G(2)-M cell cycle arrest and chromatin condensation in cancer cells. Here we report that adenovirus mediated Apoptin expression also induces G(2)-M arrest in normal cells. In normal cells Apoptin is localized mainly in the cytoplasm but is also found in the nucleus of a subset of cells. Apoptin induces chromatin condensation not only when it is expressed in the nucleus but also when it is expressed in the cytoplasm. Our results indicate that Apoptin-induced chromatin condensation in the normal cells may not correlate with its nuclear localization and the mechanism of regulating the G(2)-M transition might be a target for Apoptin.

G2/M cell cycle arrest in the life cycle of viruses

There is increasing evidence that viral infection, expression of viral protein or the presence of viral DNA causes the host cell cycle to arrest during G2/M. The mechanisms used by viruses to cause arrest vary widely; some involve the activation of the cellular pathways that induce arrest in response to DNA damage, while others use completely novel means. The analysis of virus-mediated arrest has not been proven easy, and in most cases the consequences of arrest for the virus life cycle are not well defined. However, a number of effects of arrest are being investigated and it will be interesting to see to what extent perturbation of the G2/M transition is involved in viral infections.

Chlamydia trachomatis infection induces cleavage of the mitotic cyclin B1

The obligate intracellular pathogen Chlamydia trachomatis interferes with a number of host cell processes, including cytoskeletal organization, vesicular trafficking, and apoptosis. In this study we report that C. trachomatis-infected cells proliferate more slowly than uninfected cells, suggesting that C. trachomatis may also manipulate the eukaryotic cell cycle. We further demonstrate that C. trachomatis infection destabilizes specific cell cycle proteins involved in the G2/M transition. C. trachomatis-infected cells, compared to uninfected cells, have lower levels of cyclin-dependent kinase 1. Additionally, C. trachomatis infection induces an N-terminal truncation of the mitotic cyclin B1. Manipulation of the host cell cycle may represent a strategy used by C. trachomatis to ensure a stable environment conducive to bacterial growth and replication.

Role for Wee1 in inhibition of G2-to-M transition through the cooperation of distinct human papillomavirus type 1 E4 proteins

The infectious cycle of human papillomavirus type 1 (HPV1) is accompanied by abundant expression of the full-length E1;E4 protein (17-kDa) and smaller E4 polypeptides (16-, 11-, and 10-kDa) that arise by sequential loss of N-terminal E1;E4 sequences. HPV1 E4 inhibits G(2)-to-M transition of the cell cycle. Here, we show that HPV1 E4 proteins mediate inhibition of cell division by more than one mechanism. Cells arrested by coexpression of E1;E4 (E4-17K) and a truncated protein equivalent to the 16-kDa species (E4-16K) contain inactive cyclin B1-cdk1 complexes. Inactivation of cdk1 is through inhibitory Tyr(15) phosphorylation, with cells containing elevated levels of Wee1, the kinase responsible for inhibitory cdk1 phosphorylation. Consistent with these findings, overexpression of Wee1 enhanced the extent to which E4-17K/16K-expressing cells arrest in G(2), indicating that maintenance of Wee1 activity is necessary for inhibition of cell division induced by coexpression of the two E4 proteins. Moreover, we have determined that depletion of Wee1 by small interfering RNA (siRNA) alleviates the G(2) block imposed by E4-17K/16K. In contrast however, maintenance of Wee1 activity is not necessary for G(2)-to-M inhibition mediated by E4-16K alone, as overexpression or depletion of Wee1 does not influence the G(2) arrest function of E4-16K. Cells arrested by E4-16K expression contain low levels of active cyclin B1-cdk1 complexes. We hypothesize that differential expression of HPV1 E4 proteins during the viral life cycle determines the host cell cycle status. Different mechanisms of inhibition of G(2)-to-M transition reinforce the supposition that distinct E4 functions are important for HPV replication.

Gene expression profiling revealed survivin as a target of 3,3'-diindolylmethane-induced cell growth inhibition and apoptosis in breast cancer cells

The phytochemical indole-3-carbinol (I3C), found in cruciferous vegetables, and its major acid-catalyzed reaction product 3,3'-diindolylmethane (DIM) showed anticancer activity mediated by its pleiotropic effects on cell cycle progression, apoptosis, carcinogen bioactivation, and DNA repair. To further elucidate the molecular mechanism(s) by which 3,3'-diindolylmethane exerts its effects on breast cancer cells, we have used microarray gene expression profiling analysis. We found a total of 1,238 genes altered in 3,3'-diindolylmethane-treated cells, among which 550 genes were down-regulated and 688 genes were up-regulated. Clustering analysis showed significant alterations in some genes that are critically involved in the regulation of cell growth, cell cycle, apoptosis, and signal transduction, including down-regulation of survivin. Previous studies have shown that antiapoptotic protein survivin is overexpressed in many human cancers, including breast cancer. However, very little or no information is available regarding the consequence of down-regulation of survivin for cancer therapy. We, therefore, hypothesized that down-regulation of survivin as observed by 3,3'-diindolylmethane could be an important approach for the treatment of breast cancer. We have tested our hypothesis using multiple molecular approaches and found that 3,3'-diindolylmethane inhibited cell growth and induced apoptosis in MDA-MB-231 breast cancer cells by down-regulating survivin, Bcl-2, and cdc25A expression and also caused up-regulation of p21(WAF1) expression, which could be responsible for cell cycle arrest. Down-regulation of survivin by small interfering RNA before 3,3'-diindolylmethane treatment resulted in enhanced cell growth inhibition and apoptosis, whereas overexpression of survivin by cDNA transfection abrogated 3,3'-diindolylmethane-induced cell growth inhibition and apoptosis. These results suggest that targeting survivin by 3,3'-diindolylmethane could be a new and novel approach for the prevention and/or treatment of breast cancer.

Not all viruses are bad guys: the case for reovirus in cancer therapy

Efforts to improve on cancer therapy have begun to capitalize on recent advances in our understanding of tumorigenesis. Tumor-specific characteristics are being exploited to develop selective antibodies and pharmacological inhibitors that specifically target cancer cells, and these agents are already showing clinical promise. None of these approaches, however, has captured our imagination as much as the use of replication-competent viruses to kill cancer cells. Whereas normal cells resist replication, tumor cells have an impaired antiviral response that sensitizes them to oncolytic viruses. One such virus is reovirus, a benign, naturally occurring virus that can effect tumor regression in animal models. Reovirus is demonstrating much promise in pre-clinical studies of cancer therapy and in clinical trials, where a lack of toxicity and signs of efficacy are generating excitement for this novel potential cancer therapeutic.

Inhibition of NF-kappa B activity and cFLIP expression contribute to viral-induced apoptosis

Virus-induced activation of nuclear factor-kappa B (NF-kappaB) is required for Type 3 (T3) reovirus-induced apoptosis. We now show that NF-kappaB is also activated by the prototypic Type 1 reovirus strain Lang (T1L), which induces significantly less apoptosis than T3 viruses, indicating that NF-kappaB activation alone is not sufficient for apoptosis in reovirus-infected cells. A second phase of virus-induced NF-kappaB regulation, where NF-kappaB activation is inhibited at later times following infection with T3 Abney (T3A), is absent in T1L-infected cells. This suggests that inhibition of NF-kappaB activation at later times post infection also contributes to reovirus-induced apoptosis. Reovirus-induced inhibition of stimulus-induced activation of NF-kappaB is significantly associated with apoptosis following infection of HEK293 cells with reassortant reoviruses and is determined by the T3 S1 gene segment, which is also the primary determinant of reovirus-induced apoptosis. Inhibition of stimulus-induced activation of NF-kappaB also occurs following infection of primary cardiac myocytes with apoptotic (8B) but not non-apoptotic (T1L) reoviruses. Expression levels of the NF-kappaB-regulated cellular FLICE inhibitory protein (cFLIP) reflect NF-kappaB activation in reovirus-infected cells. Further, inhibition of NF-kappaB activity and cFLIP expression promote T1L-induced apoptosis. These results demonstrate that inhibition of stimulus-induced activation of NF-kappaB and the resulting decrease in cFLIP expression promote reovirus-induced apoptosis.

Human papillomavirus type 16 E1 E4-induced G2 arrest is associated with cytoplasmic retention of active Cdk1/cyclin B1 complexes

Human papillomavirus type 16 (HPV16) can cause cervical cancer. Expression of the viral E1 E4 protein is lost during malignant progression, but in premalignant lesions, E1 E4 is abundant in cells supporting viral DNA amplification. Expression of 16E1 E4 in cell culture causes G2 cell cycle arrest. Here we show that unlike many other G2 arrest mechanisms, 16E1 E4 does not inhibit the kinase activity of the Cdk1/cyclin B1 complex. Instead, 16E1 E4 uses a novel mechanism in which it sequesters Cdk1/cyclin B1 onto the cytokeratin network. This prevents the accumulation of active Cdk1/cyclin B1 complexes in the nucleus and hence prevents mitosis. A mutant 16E1 E4 (T22A, T23A) which does not bind cyclin B1 or alter its intracellular location fails to induce G2 arrest. The significance of these results is highlighted by the observation that in lesions induced by HPV16, there is evidence for Cdk1/cyclin B1 activity on the keratins of 16E1 E4-expressing cells. We hypothesize that E1 E4-induced G2 arrest may play a role in creating an environment optimal for viral DNA replication and that loss of E1 E4 expression may contribute to malignant progression.

Mechanisms of apoptosis during reovirus infection

Reovirus infection has proven to be an excellent experimental system for studying mechanisms of virus-induced pathogenesis. Reoviruses induce apoptosis in a wide variety of cultured cells in vitro and in target tissues in vivo, including the heart and central nervous system. In vivo, viral infection, tissue injury, and apoptosis colocalize, suggesting that apoptosis is a critical mechanism by which disease is triggered in the host. This review examines the mechanisms of reovirus-induced apoptosis and investigates the possibility that inhibition of apoptosis may provide a novel strategy for limiting virus-induced tissue damage following infection.

Heliocoverpa armigera single nucleocapsid nucleopolyhedrovirus induces Hz-AM1 cell cycle arrest at the G2 phase with accumulation of cyclin B1

The cell cycle phase distributions of Hz-AM1 cells grown in monolayer culture were G1 = 49.7 +/- 3.3%, S = 22.7 +/- 3.8% and G2/M = 27.8 +/- 4.2% without >4N DNA content. The culture doubling time was about 40 h and the duration of the G1, S and G2/M phases was estimated to be 10, 14 and 16 h, respectively. HaSNPV infection of Hz-AM1 cells resulted in both unsynchronized and synchronized G1 phase arrest at G2/M phase. HaSNPV infection also resulted in the appearance of more than 4N DNA content, which accumulated to the highest levels 72 h post-infection. We also found that the expression level of cyclin B1 increased significantly after 16 h post-infection, while cyclin A did not show any change. This observation supports the Hz-AM1-infected arrest at the G2/M phage. Cytoplasm location of cyclin B1 indicated that the Hz-AM1 cell cycle arrest was at the G2 phase.

Nonstructural protein sigma1s is a determinant of reovirus virulence and influences the kinetics and severity of apoptosis induction in the heart and central nervous system

The mechanisms by which viruses kill susceptible cells in target organs and ultimately produce disease in the infected host remain poorly understood. Dependent upon the site of inoculation and strain of virus, experimental infection of neonatal mice with reoviruses can induce fatal encephalitis or myocarditis. Reovirus-induced apoptosis is a major mechanism of tissue injury, leading to disease development in both the brain and heart. In cultured cells, differences in the capacity of reovirus strains to induce apoptosis are determined by the S1 gene segment, which also plays a major role as a determinant of viral pathogenesis in both the heart and the central nervous system (CNS) in vivo. The S1 gene is bicistronic, encoding both the viral attachment protein sigma-1 and the nonstructural protein sigma-1-small (sigma1s). Although sigma1s is dispensable for viral replication in vitro, we wished to investigate the expression of sigma1s in the infected heart and brain and its potential role in reovirus pathogenesis in vivo. Two-day-old mice were inoculated intramuscularly or intracerebrally with either sigma1s(-) or sigma1s(+) reovirus strains. While viral replication in target organs did not differ between sigma1s(-) and sigma1s(+) viral strains, virus-induced caspase-3 activation and resultant histological tissue injury in both the heart and brain were significantly reduced in sigma1s(-) reovirus-infected animals. These results demonstrate that sigma1s is a determinant of the magnitude and extent of reovirus-induced apoptosis in both the heart and CNS and thereby contributes to reovirus pathogenesis and virulence.

Regulation of microtubule, apoptosis, and cell cycle-related genes by taxotere in prostate cancer cells analyzed by microarray

Taxotere showed antitumor activity against solid tumors including prostate cancer. However, the molecular mechanism(s) of action of Taxotere has not been fully elucidated. In order to establish such molecular mechanism(s) in both hormone-insensitive (PC3) and hormone-sensitive (LNCaP) prostate cancer cells, comprehensive gene expression profiles were obtained by Affymetrix Human Genome U133A Array. The RNA from the cells treated with 2 nM Taxotere was subjected to microarray analysis. We found that a total of 166, 365, and 1785 genes showed greater than two-fold change in PC3 cells after 6, 36, and 72 hours of treatment, respectively compared to 57, 823, and 964 genes in LNCaP cells. The expression of tubulin was decreased, whereas the expression of microtubule-associated proteins was increased in Taxotere-treated prostate cancer cells, confirming the microtubule-targeting effect of Taxotere. Clustering analysis showed downregulation of some genes for cell proliferation and cell cycle. In contrast, Taxotere upregulated some genes that are related to induction of apoptosis and cell cycle arrest. From these results, we conclude that Taxotere caused alterations of a large number of genes, many of which may contribute to the molecular mechanism(s) by which Taxotere affects prostate cancer cells. Further molecular studies are needed in order to determine the cause and effect relationships between these genes altered by Taxotere. Nevertheless, our results could be further exploited for devising strategies to optimize therapeutic effects of Taxotere for the treatment of prostate cancer.

Murine coronavirus nonstructural protein p28 arrests cell cycle in G0/G1 phase

Murine coronavirus mouse hepatitis virus (MHV) gene 1 encodes several nonstructural proteins. The functions are unknown for most of these nonstructural proteins, including p28, which is encoded at the 5' end of the MHV genome. Transient expression of cloned p28 in several different cultured cells inhibited cell growth, indicating that p28 expression suppressed cell proliferation. Expressed p28 was exclusively localized in the cytoplasm. Cell cycle analysis by flow cytometry demonstrated that p28 expression induced G(0)/G(1) cell cycle arrest. Characterization of various cellular proteins that are involved in regulating cell cycle progression demonstrated that p28 expression resulted in an accumulation of hypophosphorylated retinoblastoma protein (pRb), tumor suppressor p53, and cyclin-dependent kinase (Cdk) inhibitor p21(Cip1). Expression of p28 did not alter the amount of p53 transcripts yet increased the amount of p21(Cip1) transcripts, suggesting that p28 expression increased p53 stability and that p21(Cip1) was transcriptionally activated in a p53-dependent manner. Our present data suggest the following model of p28-induced G(0)/G(1) cell cycle arrest. Expressed cytoplasmic p28 induces the stabilization of p53, and accumulated p53 causes transcriptional upregulation of p21(Cip1). The increased amount of p21(Cip1) suppresses cyclin E/Cdk2 activity, resulting in the inhibition of pRb hyperphosphorylation. Accumulation of hypophosphorylated pRb thus prevents cell cycle progression from G(0)/G(1) to S phase.

Novel nuclear herniations induced by nuclear localization of a viral protein

A common consequence of viral infection is perturbation of host cell nuclear functions. For cytoplasmically replicating viruses, this process may require regulated transport of specific viral proteins into the nucleus. Here, we describe a novel form of virus-induced perturbation of host cell nuclear structures. Active signal-mediated nuclear import of the reovirus sigma1s protein results in redistribution of nuclear pore complexes and nuclear lamins and formation of nuclear herniations. These herniations represent a previously undescribed mechanism by which cytoplasmic viral infection can perturb nuclear architecture and induce cytopathic effects, which ultimately lead to disease pathogenesis in the infected host.

The human immunodeficiency virus Vpr protein binds Cdc25C: implications for G2 arrest

The human immunodeficiency virus (HIV) encodes a gene product, Vpr, which causes infected cells to arrest or delay in the G2 phase of the cell cycle. The arrest in G2 is characterized by low levels of Cyclin B1-p34Cdc2 activity and corresponding inhibitory phosphorylation of p34Cdc2. We find that Vpr directly inhibits the in vitro activity of a phosphatase, Cdc25C, which normally activates Cyclin B1-p34Cdc2. Vpr binds to Cdc25C both in vitro and in mammalian cells. Vpr is also able to inhibit Cdc25C phosphatase activity in vitro although it binds to a site on Cdc25C that is distinct from the catalytic site of the enzyme. Expression of both a catalytically active mutant of Cdc25C that has reduced binding to Vpr as well as a catalytically inactive mutant of Cdc25C that retains binding to Vpr is able to largely overcome Vpr-mediated G2 arrest. Finally, depletion of Cdc25C from cells also renders them partially resistant to the effects of Vpr.

Borna disease virus nucleoprotein interacts with the CDC2-cyclin B1 complex

Transition from G(2) to M phase, a cell cycle checkpoint, is regulated by the Cdc2-cyclin B1 complex. Here, we report that persistent infection with Borna disease virus (BDV), a noncytolytic RNA virus infecting the central nervous system, results in decelerated proliferation of infected host cells due to a delayed G(2)-to-M transition. Persistent BDV-infected rat fibroblast cells showed reduced proliferation compared to uninfected cells. In pull-down assays we observed an interaction of the viral nucleoprotein with the Cdc2-cyclin B1 complex. Transfection of the viral nucleoprotein but not of the phosphoprotein also results in decelerated proliferation. This phenomenon was found in BDV-susceptible primary rat fibroblast cells and also in primary mouse cells, which are not susceptible to BDV infection. This is the first evidence that the noncytolytic Borna disease virus can manipulate host cell functions via interaction of the viral nucleoprotein with mitotic entry regulators. BDV preferentially infects and persists in nondividing neurons. The present report could give an explanation for this selective choice of host cell by BDV.

The hepatitis C virus non-structural NS5A protein inhibits activating protein-1 function by perturbing ras-ERK pathway signaling

The hepatitis C virus nonstructural 5A (NS5A) protein is a pleiotropic phosphoprotein that has been shown to associate with a wide variety of cellular signaling proteins. Of particular interest is the observation that a highly conserved C-terminal Class II polyproline motif within NS5A mediated association with the Src homology 3 domains of members of the Src family of tyrosine kinases and the mitogenic adaptor protein Grb2 (A. Macdonald, K. Crowder, A. Street, C. McCormick, and M. Harris, submitted for publication). In this study, we analyzed the consequences of NS5A expression on mitogenic signaling pathways within a variety of cell lines. Utilizing a transient luciferase reporter system, we observed that NS5A inhibited the activity of the mitogenic and stress-activated transcription factor activating protein-1 (AP1). This inhibition was dependent upon a Class II polyproline motif within NS5A. Using a combination of dominant active and negative mutants of components of the MAPK signaling pathways, selective inhibitors, together with immunoblotting with phospho-specific and phosphorylation-independent antibodies, we determined the signaling pathways targeted by NS5A to inhibit AP1. These studies demonstrated that in both stable NS5A-expressing cells and Huh-7-derived cells harboring subgenomic hepatitis C virus (HCV) replicons, this inhibition was mediated through the ERK signaling pathway. Importantly, a comparable inhibition of AP1 reporter activity was observed in hepatocyte-derived cell lines transduced with a baculovirus vector driving expression of full-length HCV polyprotein. In conclusion, these data strongly suggest a role for the NS5A protein in the perturbation of mitogenic signaling pathways in HCV-infected hepatocytes.

Negative regulation of mitotic promoting factor by the checkpoint kinase chk1 in simian virus 40 lytic infection

Lytic infection of African green monkey kidney (CV-1) cells by simian virus 40 (SV40) is characterized by stimulation of DNA synthesis leading to bypass of mitosis and replication of cellular and viral DNA beyond a 4C DNA content. To define mechanisms underlying the absence of mitosis, the expression levels of upstream regulatory molecules of mitosis-promoting factor (MPF) were compared in parallel synchronized cultures of SV40-infected and uninfected CV-1 cells. The DNA replication/damage checkpoint kinase Chk1 was phosphorylated in both uninfected and SV40-infected cultures arrested at G(1)/S by mimosine, consistent with checkpoint activation. Following release of uninfected cultures from G(1)/S, Chk1 phosphorylation was lost even though Chk1 protein levels were retained. In contrast, G(1)/S-released SV40-infected cultures exhibited dephosphorylation of Chk1 in S phase, followed by an increase in Chk1 phosphorylation coinciding with entry of infected cells into >G(2). Inhibitors of Chk1, UCN-01 and caffeine, induced mitosis and abnormal nuclear condensation and increased the protein kinase activity of MPF in SV40-infected CV-1 cells. These results demonstrate that SV40 lytic infection triggers components of a DNA damage checkpoint pathway. In addition, chemical inhibition of Chk1 activity suggests that Chk1 contributes to the absence of mitosis during SV40 lytic infection.

Identification of a G(2) arrest domain in the E1 wedge E4 protein of human papillomavirus type 16

Human papillomavirus type 16 (HPV16) is the most common cause of cervical carcinoma. Cervical cancer develops from low-grade lesions that support the productive stages of the virus life cycle. The 16E1 wedge E4 protein is abundantly expressed in such lesions and can be detected in cells supporting vegetative viral genome amplification. Using an inducible mammalian expression system, we have shown that 16E1 wedge E4 arrests HeLa cervical epithelial cells in G(2). 16E1 wedge E4 also caused a G(2) arrest in SiHa, Saos-2 and Saccharomyces pombe cells and, as with HeLa cells, was found in the cytoplasm. However, whereas 16E1 wedge E4 is found on the keratin networks in HeLa and SiHa cells, in Saos-2 and S. pombe cells that lack keratins, 16E1 wedge E4 had a punctate distribution. Mutagenesis studies revealed a proline-rich region between amino acids 17 and 45 of 16E1 wedge E4 to be important for arrest. This region, which we have termed the "arrest domain," contains a putative nuclear localization signal, a cyclin-binding motif, and a single cyclin-dependent kinase (Cdk) phosphorylation site. A single point mutation in the putative Cdk phosphorylation site (T23A) abolished 16E1 wedge E4-mediated G(2) arrest. Arrest did not involve proteins regulating the phosphorylation state of Cdc2 and does not appear to involve the activation of the DNA damage or incomplete replication checkpoint. G(2) arrest was also mediated by the E1 wedge E4 protein of HPV11, a low-risk mucosal HPV type that also causes cervical lesions. The E1 wedge E4 protein of HPV1, which is more distantly related to that of HPV16, did not cause G(2) arrest. We conclude that, like other papillomavirus proteins, 16E1 wedge E4 affects cell cycle progression and that it targets a conserved component of the cell cycle machinery.

Reovirus-induced alterations in gene expression related to cell cycle regulation

Mammalian reovirus infection results in perturbation of host cell cycle progression. Since reovirus infection is known to activate cellular transcription factors, we investigated alterations in cell cycle-related gene expression following HEK293 cell infection by using the Affymetrix U95A microarray. Serotype 3 reovirus infection results in differential expression of 10 genes classified as encoding proteins that function at the G(1)-to-S transition, 11 genes classified as encoding proteins that function at G(2)-to-M transition, and 4 genes classified as encoding proteins that function at the mitotic spindle checkpoint. Serotype 1 reovirus infection results in differential expression of four genes classified as encoding proteins that function at the G(1)-to-S transition and three genes classified as encoding proteins that function at G(2)-to-M transition but does not alter any genes classified as encoding proteins that function at the mitotic spindle checkpoint. We have previously shown that serotype 3, but not serotype 1, reovirus infection induces a G(2)-to-M transition arrest resulting from an inhibition of cdc2 kinase activity. Of the differentially expressed genes encoding proteins regulating the G(2)-to-M transition, chk1, wee1, and GADD45 are known to inhibit cdc2 kinase activity. A hypothetical model describing serotype 3 reovirus-induced inhibition of cdc2 kinase is presented, and reovirus-induced perturbations of the G(1)-to-S, G(2)-to-M, and mitotic spindle checkpoints are discussed.

Reoviruses and the host cell

Reovirus infection of target cells can perturb cell cycle regulation and induce apoptosis. Differences in the capacity of reovirus strains to induce cell cycle arrest at G1 and G2/M have been mapped to the viral S1 genome segment, which also determines differences in the ability of reovirus strains to induce apoptosis and to activate specific mitogen-activated protein kinase (MAPK) cascades selectively. Reovirus-induced apoptosis involves members of the tumor necrosis factor (TNF) superfamily of death receptors and is associated with activation of both death receptor- and mitochondrial-associated caspases. Reovirus infection is also associated with the activation of a variety of transcription factors, including nuclear factor (NF)-kappaB. Junctional adhesion molecule (JAM) has recently been identified as a novel reovirus receptor. Reovirus binding to JAM appears to be required for induction of apoptosis and activation of NF-kappaB, although the precise cellular pathways involved have not yet been identified.