The cytotoxicity of the autonomous parvovirus minute virus of mice nonstructural proteins in FR3T3 rat cells depends on oncogene expression

Non-viral gene delivery of the oncotoxic protein NS1 for treatment of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is related to increasing incidence rates and poor clinical outcomes due to lack of efficient treatment options and emerging resistance mechanisms. The aim of the present study is to exploit a non-viral gene therapy enabling the expression of the parvovirus-derived oncotoxic protein NS1 in HCC. This anticancer protein interacts with different cellular kinases mediating a multimodal host-cell death. Lipoplexes (LPX) designed to deliver a DNA expression plasmid encoding NS1 are characterized using a comprehensive set of in vitro assays. The mechanisms of cell death induction are assessed and phosphoinositide-dependent kinase 1 (PDK1) is identified as a potential predictive biomarker for a NS1-LPX-based gene therapy. In an HCC xenograft mouse model, NS1-LPX therapeutic approach results in a significant reduction in tumor growth and extended survival. Data provide convincing evidence for future studies using a targeted NS1 gene therapy for PDK1 overexpressing HCC.

Cancer Treatment Goes Viral: Using Viral Proteins to Induce Tumour-Specific Cell Death

Cell death is a tightly regulated process which can be exploited in cancer treatment to drive the killing of the tumour. Several conventional cancer therapies including chemotherapeutic agents target pathways involved in cell death, yet they often fail due to the lack of selectivity they have for tumour cells over healthy cells. Over the past decade, research has demonstrated the existence of numerous proteins which have an intrinsic tumour-specific toxicity, several of which originate from viruses. These tumour-selective viral proteins, although from distinct backgrounds, have several similar and interesting properties. Though the mechanism(s) of action of these proteins are not fully understood, it is possible that they can manipulate several cell death modes in cancer exemplifying the intricate interplay between these pathways. This review will discuss our current knowledge on the topic and outstanding questions, as well as deliberate the potential for viral proteins to progress into the clinic as successful cancer therapeutics.

Implications of infectious agents on results of animal experiments

Report of the Working Group on Hygiene of the Gesellschaft für Versuchstierkunde–Society for Laboratory Animal Science (GV-SOLAS)

GV-SOLAS Working Group on Hygiene: Werner Nicklas (Chairman), Felix R. Homberger, Brunhilde Illgen-Wilcke, Karin Jacobi, Volker Kraft, Ivo Kunstyr, Michael Mähler, Herbert Meyer & Gabi Pohlmeyer-Esch

Profiling of Host Cell Response to Successive Canine Parvovirus Infection Based on Kinetic Proteomic Change Identification

Canine parvovirus (CPV) reproduces by co-opting the resources of host cells, inevitably causing cytotoxic effects to the host cells. Feline kidney F81 cells are sensitive to CPV infection and show disparate growing statuses at different time points post-infection. This study analysed the response of F81 cells to CPV infection at successive infection time points by iTRAQ-based quantitative proteomics. Differentially expressed proteins (DEPs) during 60 h of infection and at selected time points post-infection were identified by an analysis of variance test and a two-tailed unpaired t test, respectively. DEPs with similar quantitative changes were clustered by hierarchical clustering and analysed by gene ontology enrichment, revealing that 12 h and 60 h post-infection were the optimal times to analyse the autonomous parvovirus replication and apoptosis processes, respectively. Using the Metacore^TM database, 29 DEPs were enriched in a network involved in p53 regulation. Besides, a significantly enriched pathway suggests that the CPV-induced cytopathic effect was probably due to the deficiency of functional CFTR caused by CPV infection. This study uncovered the systemic changes in key cellular factors involved in CPV infection and help to understand the molecular mechanisms of the anti-cancer activity of CPV and the cytopathic effects induced by CPV infection.

Regulation of BmBDV NS1 by phosphorylation: Impact of mutagenesis at consensus phosphorylation sites on ATPase activity and cytopathic effects

Bombyx mori bidensovirus (BmBDV) is a single-stranded DNA virus belonging to the Bidensovirus genus, Bidnaviridae family. Previous studies showed that parvovirus nonstructural protein 1 (NS1) contains endonuclease, helicase, and ATPase activities and that these activities are regulated by serine/threonine phosphorylation. We have reported that residue Thr-184 site of BmBDV NS1 is phosphorylated, and that residues of Thr-181 and Thr-191 are potentially phosphorylated. However, whether phosphorylation affects BmBDV NS1 activities remains unclear. In this study, the substitution of threonine with Glycine at positions 181, 184 and 191 of BmBDV NS1 reduced its ATPase activity. After wild-type NS1 was treated with calf intestinal alkaline phosphatase (CIP), ATPase activity decreased significantly. Moreover, silkworms that were injected with recombinant viruses carrying these NS1 mutations exhibited significant increases in the median lethal time to death compared with silkworms that were injected with a virus that expressed wild-type NS1. In conclusion, these results showed that the ATPase activity and virulence of BmBDV NS1 are regulated via phosphorylation.

Double-faceted mechanism of parvoviral oncosuppression

The H-1 parvovirus (H-1PV) exerts oncosuppressive action that has two components: oncotoxicity and immunostimulation. While many human tumor cells, including conventional drug-resistant ones, can be killed by H-1PV, some fail to support progeny virus production, necessary for infection propagation in neoplastic tissues. This limitation can be overcome through forced selection of H-1PV variants capable of enhanced multiplication and spreading in human tumor cells. In the context of further developing H-1PV for use in cancer therapy, arming it with immunostimulatory CpG motifs under conditions preserving replication and oncolysis enhances its action as an anticancer vaccine adjuvant. A first clinical study of H-1PV treatment in glioma patients has yielded evidence of intratumoral synthesis of the viral oncotoxic protein NS1 and immune cell infiltration.

Viral genes as oncolytic agents for cancer therapy

Many viruses have the ability to modulate the apoptosis, and to accomplish it; viruses encode proteins which specifically interact with the cellular signaling pathways. While some viruses encode proteins, which inhibit the apoptosis or death of the infected cells, there are viruses whose encoded proteins can kill the infected cells by multiple mechanisms, including apoptosis. A particular class of these viruses has specific gene(s) in their genomes which, upon ectopic expression, can kill the tumor cells selectively without affecting the normal cells. These genes and their encoded products have demonstrated great potential to be developed as novel anticancer therapeutic agents which can specifically target and kill the cancer cells leaving the normal cells unharmed. In this review, we will discuss about the viral genes having specific cancer cell killing properties, what is known about their functioning, signaling pathways and their therapeutic applications as anticancer agents.

Oncolytic parvoviruses: from basic virology to clinical applications

Accumulated evidence gathered over recent decades demonstrated that some members of the Parvoviridae family, in particular the rodent protoparvoviruses H-1PV, the minute virus of mice and LuIII have natural anticancer activity while being nonpathogenic to humans. These studies have laid the foundations for the launch of a first phase I/IIa clinical trial, in which the rat H-1 parvovirus is presently undergoing evaluation for its safety and first signs of efficacy in patients with glioblastoma multiforme. After a brief overview of the biology of parvoviruses, this review focuses on the studies which unraveled the antineoplastic properties of these agents and supported their clinical use as anticancer therapeutics. Furthermore, the development of novel parvovirus-based anticancer strategies with enhanced specificity and efficacy is discussed, in particular the development of second and third generation vectors and the combination of parvoviruses with other anticancer agents. Lastly, we address the key challenges that remain towards a more rational and efficient use of oncolytic parvoviruses in clinical settings, and discuss how a better understanding of the virus life-cycle and of the cellular factors involved in virus infection, replication and cytotoxicity may promote the further development of parvovirus-based anticancer therapies, open new prospects for treatment and hopefully improve clinical outcome.

Tumor suppressing properties of rodent parvovirus NS1 proteins and their derivatives

Cancer chemotherapy with monospecific agents is often hampered by the rapid development of tumor resistance to the drug used. Therefore, combination treatments aiming at several different targets are sought. Viral regulatory proteins, modified or not, appear ideal for this purpose because of their multimodal killing action against neoplastically transformed cells. The large nonstructural protein NS1 of rodent parvoviruses is an excellent candidate for an anticancer agent, shown to interfere specifically with cancer cell growth and survival. The present review describes the structure, functions, and regulation of the multifunctional protein NS1, its specific interference with cell processes and cell protein activities, and what is known so far about the mechanisms underlying NS1 interference with cancer growth. It further outlines prospects for the development of new, multimodal cancer toxins and their potential applications.

Periplaneta fuliginosa densovirus nonstructural protein NS1 contains an endonuclease activity that is regulated by its phosphorylation

Periplaneta fuliginosa densovirus (PfDNV) is a single-stranded DNA virus, belonging to Densovirinae subfamily, Parvoviridae family. Parvovirus nonstructural protein 1 (NS1) contains various activities required for parvoviral DNA replication, like endonuclease, helicase and ATPase, which are regulated by serine/threonine phosphorylation. However, for PfDNV, NS1 endonuclease activity has not been determined. Moreover, for densoviruses, whether NS1 is phosphorylated, and if so, phosphorylation pattern and impact on NS1 activities have not been investigated. Here, we demonstrated that PfDNV NS1 possesses endonuclease activity, covalently attaches to 5'-end of nicking site, and includes an active-site tyrosine (Y178). Moreover, using different phosphatases, we uncovered that both serine/threonine and tyrosine phosphorylations are critical for NS1 endonuclease and helicase activities. Further mass-spec and mutational analyses revealed that Y345 is phosphorylated and functions as a critical regulatory site for NS1 activities. This study should foster our understanding of NS1 activities and regulations in PfDNV and other densoviruses.

Molecular pathways: rodent parvoviruses--mechanisms of oncolysis and prospects for clinical cancer treatment

Rodent parvoviruses (PV) are recognized for their intrinsic oncotropism and oncolytic activity, which contribute to their natural oncosuppressive effects. Although PV uptake occurs in most host cells, some of the subsequent steps leading to expression and amplification of the viral genome and production of progeny particles are upregulated in malignantly transformed cells. By usurping cellular processes such as DNA replication, DNA damage response, and gene expression, and/or by interfering with cellular signaling cascades involved in cytoskeleton dynamics, vesicular integrity, cell survival, and death, PVs can induce cytostasis and cytotoxicity. Although productive PV infections normally culminate in cytolysis, virus spread to neighboring cells and secondary rounds of infection, even abortive infection or the sole expression of the PV nonstructural protein NS1, is sufficient to cause significant tumor cell death, either directly or indirectly (through activation of host immune responses). This review highlights the molecular pathways involved in tumor cell targeting by PVs and in PV-induced cell death. It concludes with a discussion of the relevance of these pathways to the application of PVs in cancer therapy, linking basic knowledge of PV-host cell interactions to preclinical assessment of PV oncosuppression.

The parvoviral capsid controls an intracellular phase of infection essential for efficient killing of stepwise-transformed human fibroblasts

Members of the rodent subgroup of the genus Parvovirus exhibit lytic replication and spread in many human tumor cells and are therefore attractive candidates for oncolytic virotherapy. However, the significant variation in tumor tropism observed for these viruses remains largely unexplained. We report here that LuIII kills BJ-ELR 'stepwise-transformed' human fibroblasts efficiently, while MVM does not. Using viral chimeras, we mapped this property to the LuIII capsid gene, VP2, which is necessary and sufficient to confer the killer phenotype on MVM. LuIII VP2 facilitates a post-entry, pre-DNA-amplification step early in the life cycle, suggesting the existence of an intracellular moiety whose efficient interaction with the incoming capsid shell is critical to infection. Thus targeting of human cancers of different tissue-type origins will require use of parvoviruses with capsids that effectively make this critical interaction.

Parvovirus B19 infection in Hashimoto's thyroiditis, papillary thyroid carcinoma, and anaplastic thyroid carcinoma

BACKGROUND

The human pathogenic parvovirus B19 (B19) has recently been detected in papillary thyroid carcinoma (PTC) and Hashimoto's thyroiditis (HT) tissues at a high frequency in two studies of a Chinese cohort. We wanted to extend these data to include another cohort and expand the thyroid tumor tissue types assessed. In particular, we were interested to find whether B19 also infects anaplastic thyroid carcinoma (ATC), one of the most aggressive human cancers.

METHODS

Commercially available thyroid tumor tissue arrays were used to detect B19 capsid protein by immunohistochemistry in various types of thyroid tumors and disorders. The arrays were representative of the four main types of thyroid tumors, as well as other thyroid autoimmune disorders such as HT and Graves' disease, and adenomas, goiters, lymphomas, and normal thyroid tissue. In total, at least 12 different types of thyroid conditions as well as normal tissue were represented, many with multiple subjects.

RESULTS

Twenty-one of the 24 (88%) PTC tumors, 3 of the 3 ATC/undifferentiated tumors, and 3 of the 3 HT tissue samples were positive for B19 capsid protein by immunohistochemistry. The localization of the protein differed based on pathological disease type, with a nuclear to cytoplasmic shift seen from unaffected to tumor tissue.

CONCLUSIONS

We extend the data available on B19 detection in the thyroid to show a high correlation of virus in another cohort of PTC and HT at the protein level. We also show, for the first time, B19 infection of much more highly aggressive ATC/undifferentiated tumors. Nuclear to cytoplasmic shift in B19 protein in cancer tissue suggests a possible link between B19 and thyroid cancer pathogenesis/progression.

Parvovirus infection-induced cell death and cell cycle arrest

The cytopathic effects induced during parvovirus infection have been widely documented. Parvovirus infection-induced cell death is often directly associated with disease outcomes (e.g., anemia resulting from loss of erythroid progenitors during parvovirus B19 infection). Apoptosis is the major form of cell death induced by parvovirus infection. However, nonapoptotic cell death, namely necrosis, has also been reported during infection of the minute virus of mice, parvovirus H-1 and bovine parvovirus. Recent studies have revealed multiple mechanisms underlying the cell death during parvovirus infection. These mechanisms vary in different parvoviruses, although the large nonstructural protein (NS)1 and the small NS proteins (e.g., the 11 kDa of parvovirus B19), as well as replication of the viral genome, are responsible for causing infection-induced cell death. Cell cycle arrest is also common, and contributes to the cytopathic effects induced during parvovirus infection. While viral NS proteins have been indicated to induce cell cycle arrest, increasing evidence suggests that a cellular DNA damage response triggered by an invading single-stranded parvoviral genome is the major inducer of cell cycle arrest in parvovirus-infected cells. Apparently, in response to infection, cell death and cell cycle arrest of parvovirus-infected cells are beneficial to the viral cell lifecycle (e.g., viral DNA replication and virus egress). In this article, we will discuss recent advances in the understanding of the mechanisms underlying parvovirus infection-induced cell death and cell cycle arrest.

Advances in human B19 erythrovirus biology

Since its discovery, human parvovirus B19 (B19V), now termed erythrovirus, has been associated with many clinical situations (neurological and myocardium infections, persistent B19V DNAemia) in addition to the prototype clinical manifestations, i.e., erythema infectiosum and erythroblastopenia crisis. In 2002, the use of new molecular tools led to the characterization of three different genotypes of human B19 erythrovirus. Although the genomic organization is conserved, the geographic distribution of the different genotypes varies worldwide, and the nucleotidic divergences can impact the molecular diagnosis of B19 virus infection. The cell cycle of the virus remains partially unresolved; however, recent studies have shed light on the mechanism of cell entry and the interactions of B19V proteins with apoptosis pathways.

Ectopic expression of H-1 parvovirus NS1 protein induces alterations in actin filaments and cell death in human normal MRC-5 and transformed MRC-5 SV2 cells

When grown in human cell lines, oncolytic H-1 parvovirus (H-1PV) replication preferentially occurs in transformed cells, which ultimately die upon infection. H-1PV-induced cytotoxicity is mainly due to P4 promoter-driven NS1 protein expression. Infection of untransformed cells generally does not induce deleterious effects because the P4 promoter is not activated. Here, we show that ectopic CMV-driven NS1 protein expression in normal human MRC-5 cells results in alterations of actin filaments and cell death, and both effects are prevented by a serine 473 mutation. The same substitution preserves actin filaments of transfected MRC-5 SV2 cells, that are MRC-5 transformed counterparts, but does not impair NS1-induced cytotoxicity.

Translation control by protein kinase R restricts minute virus of mice infection: role in parvovirus oncolysis

The relevance of translational control in the gene expression and oncotropism of the autonomous parvoviruses was investigated with MVMp, the prototype strain of minute virus of mice (MVM), infecting normal and transformed rodent and human cells of different tissue origins. Mouse embryo fibroblasts (MEFs) and NIH 3T3 fibroblasts were resistant to MVMp infection, but 3T3 fibroblasts derived from double-stranded RNA (dsRNA)-dependent protein kinase R (PKR) knockout mice (PKR(o/o)) behaved in a manner that was highly permissive to productive MVMp replication. NIH 3T3 resistance correlated with significant phosphorylation of eukaryotic translation initiation factor 2 (eIF2) occurring at early time points after infection. Permissive PKR(o/o) cells were converted to MVMp-restrictive cells after reintroduction of the PKR gene by transfection. Conversely, regulated expression of the vaccinia virus E3 protein, a PKR inhibitor, in MEFs prevented eIF2alpha phosphorylation and increased MVMp protein synthesis. In vitro-synthesized genome-length R1 mRNA of MVMp was a potent activator of PKR. Virus-resistant primary MEFs and NIH 3T3 cells responded to MVMp infection with significant increases in eIF2alpha phosphorylation. In contrast, virus-permissive mouse (PKR(o/o), BHK21, and A9) and human transformed (NB324K fibroblast, U373 glioma, and HepG2 hepatoma) cells consistently showed no significant increase in the level of eIF2alpha phosphorylation following MVMp infection. The synthesis of the viral NS1 protein was inversely correlated with the steady-state PKR levels. Our results show that the PKR-mediated antiviral response is an important mechanism for control of productive MVMp infection, and its impairment in human transformed cells allowed efficient MVMp gene expression. PKR translational control may therefore contribute to the oncolysis of MVMp and other autonomous parvoviruses.

Oncolytic rat parvovirus H-1PV, a candidate for the treatment of human lymphoma: In vitro and in vivo studies

The incidence of lymphomas developing in both immunocompetent and immunosuppressed patients continues to steadily increase worldwide. Current chemotherapy and immunotherapy approaches have several limitations, such as severe side toxicity and selection of resistant cell variants. Autonomous parvoviruses (PVs), in particular the rat parvovirus H-1PV, have emerged as promising anticancer agents. Although it is apathogenic in humans, H-1PV has been shown to infect and suppress various rat and human tumors in animal models. In this study, we demonstrate the capacity of H-1PV for efficiently killing, through necrosis, cell cultures originating from Burkitt's lymphoma (BL), while sparing normal B lymphocytes. The cytotoxic effect was generally accompanied by a productive H-1PV infection. Remarkably, parvovirus-based monotherapy efficiently suppressed established BL at an advanced stage in a severe combined immunodeficient (SCID) mouse model of the disease. The data show for the first time that an oncolytic parvovirus deserves further consideration as a potential tool for the treatment of some non-Hodgkin B-cell lymphomas, including those resistant to apoptosis induction by rituximab.

Non-structural proteins of Periplaneta fuliginosa densovirus inhibit cellular gene expression and induce necrosis in Sf9 cell cultures

The non-structural protein NS1 of Periplaneta fuliginosa densovirus (PfDNV) is a multifunctional protein that has previously been shown to possess ATP-binding, ATPase, site-specific DNA-binding, helicase, and transcription activation activities. We report here an investigation of the cytopathogenicity of this viral non-structural (NS) protein, as well as other two NSs, NS2, and NS3, in cultured insect cells. The expression of NS1 alone potently inhibited cellular gene expression, whereas NS2 and NS3 did not produce a similar effect. The inhibition of gene expression by NS1 was confirmed to be specific and not a simple manifestation of toxicity. For example, NS1 inhibited expression of several reporter genes under the control of different RNA polymerase II promoters, whereas it did not inhibit expression from a T7 RNA polymerase promoter construct. Mapping analysis identified the carboxy-terminal peptide of this protein as the region important for the inhibition of cellular gene expression, suggesting that this inhibition is independent of its DNA-binding activity. Next, the mutagenesis assay showed that ATP-binding was essential for the unique function of this protein. Furthermore, we found that NS2 and NS3 cooperatively enhanced the NS1-induced transcription inhibition. Co-expression of all the three NS proteins in Sf9 cells also led to necrotic cell death by ATP depletion.

Killing of p53-deficient hepatoma cells by parvovirus H-1 and chemotherapeutics requires promyelocytic leukemia protein

AIM: To evaluate the synergistic targeting and killing of human hepatocellular carcinoma (HCC) cells lacking p53 by the oncolytic autonomous parvovirus (PV) H-1 and chemotherapeutic agents and its dependence on functional promyelocytic leukemia protein (PML).

METHODS: The role of p53 and PML in regulating cytotoxicity and gene transfer mediated by wild-type (wt) PV H-1 were explored in two pairs of isogenic human hepatoma cell lines with different p53 status. Furthermore, H-1 PV infection was combined with cytostatic drug treatment.

RESULTS: While the HCC cells with different p53 status studied were all susceptible to H-1 PV-induced apoptosis, the cytotoxicity of H-1 PV was more pronounced in p53-negative than in p53-positive cells. Apoptosis rates in p53-negative cell lines treated by genotoxic drugs were further enhanced by a treatment with H-1 PV. In flow cytometric analyses, H-1 PV infection resulted in a reduction of the mitochondrial transmembrane potential. In addition, H-1 PV cells showed a significant increase in PML expression. Knocking down PML expression resulted in a striking reduction of the level of H-1 PV infected tumor cell death.

CONCLUSION: H-1 PV is a suitable agent to circumvent the resistance of p53-negative HCC cells to genotoxic agents, and it enhances the apoptotic process which is dependent on functional PML. Thus, H-1 PV and its oncolytic vector derivatives may be considered as therapeutic options for HCC, particularly for p53-negative tumors.

Augmented transgene expression in transformed cells using a parvoviral hybrid vector

Autonomous parvoviruses possess an intrinsic oncotropism based on viral genetic elements controlling gene expression and genome replication. We constructed a hybrid vector consisting of the H1 parvovirus-derived expression cassette comprising the p4 promoter, the ns1 gene and the p38 promoter flanked by the adeno-associated viruses 2 (AAV2) inverted terminal repeats and packaged into AAV2 capsids. Gene transduction using this vector could be stimulated by coinfection with adenovirus, by irradiation or treatment with genotoxic agents, similar to standard AAV2 vectors. However, the latter were in most cases less efficient in gene transduction than the hybrid vector. With the new vector, tumor cell-selective increase in transgene expression was observed in pairs of transformed and non-transformed cells, leading to selective killing of the transformed cells after expression of a prodrug-converting enzyme. Preferential gene expression in tumor versus normal liver tissue was also observed in vivo in a syngeneic rat model. Comparative transduction of a panel of different tumor cell lines with the H1 and the H1/AAV hybrid vector showed a preference of each vector for distinct cell types, probably reflecting the dependence of the viral tropism on capsid determinants.

Parvovirus interference with intracellular signalling: mechanism of PKCeta activation in MVM-infected A9 fibroblasts

Autonomous parvoviruses are strongly dependent on the phosphorylation of the major non-structural protein NS1 by members of the protein kinase C (PKC) family. Besides being accompanied with changes in the overall phosphorylation pattern of NS1 and acquiring new modifications at consensus PKC sites, ongoing minute virus of mice (MVM) infections lead to the appearance of new phosphorylated cellular protein species. This prompted us to investigate whether MVM actively interferes with phosphoinositol-dependent kinase (PDK)/PKC signalling. The activity, subcellular localization and phosphorylation status of the protein kinases PDK1, PKCeta and PKClambda were measured in A9 cells in the presence or absence of MVM infection. Parvovirus infection was found to result in activation of both PDK1 and PKCeta, as evidenced by changes in their subcellular distribution and overall (auto)phosphorylation. We show evidence that activation of PKCeta by PDK1 is driven by atypical PKClambda. By modifying the hydrophobic motif of PKCeta, PKClambda appeared to control docking and consecutive phosphorylation of PKCeta's activation-loop by PDK1, a process that was inhibited in vivo in the presence of a dominant-negative PKClambda mutant.

A viral adaptor protein modulating casein kinase II activity induces cytopathic effects in permissive cells

Autonomous parvoviruses induce severe morphological and physiological alterations in permissive host cells, eventually leading to cell lysis and release of progeny virions. Viral cytopathic effects (CPE) result from specific rearrangements and destruction of cytoskeletal micro- and intermediate filaments. We recently reported that inhibition of endogenous casein kinase II (CKII) protects target cells from parvovirus minute virus of mice (MVM)-induced CPE, pointing to this kinase as an effector of MVM toxicity. The present work shows that the parvoviral NS1 protein mediates CKII-dependent cytoskeletal alterations and cell death. NS1 can act as an adaptor molecule, linking the cellular protein kinase CKIIalpha to tropomyosin and thus modulating the substrate specificity of the kinase. This action results in an altered tropomyosin phosphorylation pattern both in vitro and in living cells. The capacity of NS1 to induce CPE was impaired by mutations abolishing binding with either CKIIalpha or tropomyosin. The cytotoxic adaptor function of NS1 was confirmed with fusion peptides, where the tropomyosin-binding domain of NS1 and CKIIalpha are physically linked. These adaptor peptides were able to mimic NS1 in its ability to induce death of transformed MVM-permissive cells.

NS1 interaction with CKII alpha: novel protein complex mediating parvovirus-induced cytotoxicity

During a productive infection, the prototype strain of the parvovirus minute virus of mice (MVMp) induces dramatic morphological alterations in permissive A9 fibroblasts, culminating in cell lysis at the end of infection. These cytopathic effects (CPE) result from rearrangements and destruction of the cytoskeletal micro- and intermediate filaments, while other structures such as the nuclear lamina and particularly the microtubule network remain protected throughout the infection (J. P. F. Nüesch et al., Virology 331:159-174, 2005). In order to unravel the mechanism(s) by which parvoviruses trigger CPE, we searched for NS1 interaction partners by differential affinity chromatography, using distinct NS1 mutants debilitated specifically for this function. Thereby, we isolated an NS1 partner polypeptide, whose interaction with NS1 correlated with the competence of the viral product for CPE induction, and further identified it by tandem mass spectrometry and Western blotting analyses to consist of the catalytic subunit of casein kinase II, CKIIalpha. This interaction of NS1 with CKIIalpha suggested interference by the viral protein with intracellular signaling. Using permanent cell lines expressing dominant-negative CKIIalpha mutants, we were able to show that this kinase activity was indeed specifically involved in parvoviral CPE and progeny particle release. Furthermore, the NS1/CKIIalpha complex proved to be able to specifically phosphorylate viral capsids, indicating a mediator function of NS1 for CKII activity and specificity, at least in vitro. Altogether our data suggest that parvovirus-induced CPE is mediated by NS1 interference with intracellular CKII signaling.

Oncolytic potential of rodent parvoviruses for cancer therapy in humans: a brief review

Summary Rodent parvoviruses are promising candidates for oncolytic virotherapy of cancer in humans because of their oncotropism (preferential killing of transformed cells) in the absence of pathogenicity. Here, we give an overview concerning the possible application of parvovirus H-1 for cancer therapy, with specific emphasis on malignant brain tumours in humans.

Expression profiling of human hepatoma cells reveals global repression of genes involved in cell proliferation, growth, and apoptosis upon infection with parvovirus H-1

Autonomous parvoviruses are characterized by their stringent dependency on host cell S phase and their cytopathic effects on neoplastic cells. To better understand the interactions between the virus and its host cell, we used oligonucleotide arrays that carry more than 19,000 unique human gene sequences to profile the gene expression of the human hepatocellular carcinoma cell line QGY-7703 at two time points after parvovirus H-1 infection. At the 6-h time point, a single gene was differentially expressed with a >2.5-fold change. At 12 h, 105 distinct genes were differentially expressed in virus-infected cells compared to mock-treated cells, with 93% of these genes being down-regulated. These repressed genes clustered mainly into classes involved in transcriptional regulation, signal transduction, immune and stress response, and apoptosis, as exemplified by genes encoding the transcription factors Myc, Jun, Fos, Ids, and CEBPs. Quantitative real-time reverse transcription-PCR analysis on selected genes validated the array data and allowed the changes in cellular gene expression to be correlated with the accumulation of viral transcripts and NS1 protein. Western blot analysis of several cellular proteins supported the array results and substantiated the evidence given by these and other data to suggest that the H-1 virus kills QGY-7703 cells by a nonapoptotic process. The promoter regions of most of the differentially expressed genes analyzed fail to harbor any motif for sequence-specific binding of NS1, suggesting that direct binding of NS1 to cellular promoters may not participate in the modulation of cellular gene expression in H-1 virus-infected cells.

Selective alterations of the host cell architecture upon infection with parvovirus minute virus of mice

During a productive infection, the prototype strain of parvovirus minute virus of mice (MVMp) induces dramatic morphological alterations to the fibroblast host cell A9, resulting in cell lysis and progeny virus release. In order to understand the mechanisms underlying these changes, we characterized the fate of various cytoskeletal filaments and investigated the nuclear/cytoplasmic compartmentalization of infected cells. While most pronounced effects could be seen on micro- and intermediate filaments, manifest in dramatic rearrangements and degradation of filamentous (F-)actin and vimentin structures, only little impact could be seen on microtubules or the nuclear envelope during the entire monitored time of infection. To further analyze the disruption of the cytoskeletal structures, we investigated the viral impact on selective regulatory pathways. Thereby, we found a correlation between microtubule stability and MVM-induced phosphorylation of alpha/beta tubulin. In contrast, disassembly of actin filaments late in infection could be traced back to the disregulation of two F-actin associated proteins gelsolin and Wiscott-Aldrich Syndrome Protein (WASP). Thereby, an increase in the amount of gelsolin, an F-actin severing protein was observed during infection, accounting for the disruption of stress fibers upon infection. Concomitantly, the actin polymerization activity also diminished due to a loss of WASP, the activator protein of the actin polymerization machinery the Arp2/3 complex. No effects could be seen in amount and distribution of other F-actin regulatory factors such as cortactin, cofilin, and profilin. In summary, the selective attack of MVM towards distinct host cell cytoskeletal structures argues for a regulatory feature during infection, rather than a collapse of the host cell as a mere side effect of virus production.

Investigation of the sensitivities of distinct gastric cancer cells to parvovirus H-1 induced cytotoxicity

OBJECTIVE

To investigate the sensitivities of distinct gastric cancer cells to parvovirus H-1 induced cytotoxicity and the possible mechanism(s).

METHODS

There were six distinct differentiated gastric cancer cell lines: HGC27 (undifferentiated), BGC823 (undifferentiated), MKN45 (poorly differentiated), AGS (poorly differentiated), SGC7901 (moderately differentiated) and MKN28 (well differentiated). The cell cycle distributions were measured by flow cytometry and the differential sensitivities of the six distinct gastric cancer cells after H-1 virus infection were detected by MTT assay. RT-PCR was used to detect viral NS1 gene expression in all six gastric cancer cell lines.

RESULTS

The S phase ratios of HGC27, BGC823, MKN45, AGS, SGC7901 and MKN28 were 24.72%, 30.15%, 27.10%, 29.03%, 31.82% and 33.73%, respectively. HGC27 cells were sensitive to H-1 virus induced cytotoxicity, followed by SGC7901 cells. MKN45 and AGS cells were moderately sensitive and MKN28 cells were insensitive. However, BGC823 cells were resistant to H-1 virus induced cytotoxicity. The expressions of viral NS1 were higher in HGC27, BGC823, MKN45 and SGC7901 cells, and lower in AGS and MKN28 cells.

CONCLUSIONS

The sensitivities of the distinct gastric cancer cells to H-1 virus induced cytotoxicity were markedly different. In general, the poorly differentiated cells showed an enhanced sensitivity to H-1 virus attack compared with well-differentiated ones. The enhanced sensitivity of poorly versus well-differentiated gastric cancer cells to H-1 virus is related in part to the enhanced capacity of the former for NS1 protein production and accumulation. The undifferentiated BGC823 cells were resistant to H-1 virus triggered cytotoxicity. It may further verify that not all tumor cells are sensitive to H-1 virus lytic effects.

Modulation of minute virus of mice cytotoxic activities through site-directed mutagenesis within the NS coding region

Late in infection, parvovirus minute virus of mice (MVMp) induces the lysis of mouse A9 fibroblasts. This effect depends on the large nonstructural phosphoprotein NS1, which plays in addition a major role in viral DNA replication and progeny particle production. Since the NS1 C-terminal region is subjected to late phosphorylation events and protein kinase C (PKC) family members regulate NS1 replicative activities, the present study was conducted to determine the impact of PKCs on NS1 cytotoxic functions. To this end, we performed site-directed mutagenesis, substituting alanine residues for two consensus PKC-phosphorylation sites located within the NS1 C-terminal region, T585 and S588. Although these substitutions had no detectable effect on virus multiplication in a single-round infection, the NS1-585A mutant virus was significantly less toxic to A9 cells than wild-type MVMp, whereas the NS1-588A mutant virus was endowed with a higher killing potential. These alterations correlated with specific changes in the late phosphorylation pattern of the mutant NS1 proteins compared to the wild-type polypeptide. Since the mutations introduced in this region of the viral genome also made changes in the minor nonstructural protein NS2, a contribution of this polypeptide to the above-mentioned phenotypes of mutant viruses cannot be excluded at present. However, the involvement of NS1 in these phenotypes was directly supported by the respective reduced and enhanced capacity of NS1-585A and NS1-588A recombinant proteins for inducing morphological alterations and cell detachment in transfected A9 cultures. Altogether, these data suggest that late-occurring phosphorylation of NS1 specifically regulates the cytotoxic functions of the viral product and that residues T585 and S588 contribute to this control in an antagonistic way.

Reverse genetic system for the analysis of parvovirus telomeres reveals interactions between transcription factor binding sites in the hairpin stem

The left-hand or 3'-terminal hairpin of minute virus of mice (MVM) contains sequence elements essential for both viral DNA replication at the left-hand origin (oriL) and for the modulation of the P4 promoter, from which the viral nonstructural gene cassette is transcribed. This hairpin sequence has proven difficult to manipulate in the context of the viral genome. Here we describe a system for generating mutant viruses using synthetic hairpin oligonucleotides and a truncated form of the infectious clone. This allows manipulation of the sequence of the left-hand hairpin and examination of the effects in the context of the viral life cycle. We have confirmed the requirement for a functional parvovirus initiation factor (PIF) binding site and determined that an optimized PIF binding site, with 6 bases between the half-sites, was actually detrimental to viral growth. The distal PIF half-site overlaps a cyclic AMP-responsive element (CRE), which was shown to play an important role in initiating infection, particularly in 324K simian virus 40-transformed human fibroblasts. Interestingly, reducing the spacing of the PIF half-sites, and thus the affinity of the binding site for PIF, increased viral fitness relative to wild type in 324K cells, but not in murine A9 cells. These results indicate that the relative importance of factor binding to the CRE and PIF sites during the establishment of an infection differs markedly between these two host cells and suggest that the suboptimal spacing of PIF half-sites found in wild-type virus represents a necessary reduction in the affinity of the PIF interaction in favor of CRE function.

Transient suppression of transgene expression by means of antisense oligonucleotides: a method for the production of toxin-transducing recombinant viruses

Some of the therapeutic genes to be delivered by means of recombinant adenoviruses code for toxic compounds. Expression of these sequences can be deleterious to the complementation cells used for vector production, making it often difficult to generate high-titer stocks of toxin-transducing recombinant adenoviruses. In this work, we present a novel strategy for the transient post-transcriptional down-regulation of toxic transgene expression during the vector production phase, through the administration of phosphorothioate-modified antisense oligodeoxyribonucleotides. This method was successfully applied to the production of hybrid adenoviruses that contain the gene encoding the cytotoxic parvoviral protein NS1. The generation of recombinant adenoviruses in 293T cells was found to be fully suppressed as a result of adding of the NS gene to the vector genome. Yet, the production of NS-harboring hybrid adenoviruses could be rescued by treating the producer cells with antisense oligonucleotides specific for the translation initiation region of the NS transcript. This rescue correlated with a striking reduction of NS RNA and protein levels in the complementation cells. These data provide proof of principle of the suitability of the antisense oligonucleotides strategy for overcoming the interference of harmful transgenes with the production of adenoviral and other vectors.

Caspase activation is required for permissive replication of Aleutian mink disease parvovirus in vitro

Aleutian mink disease parvovirus (ADV) is distinct among the parvoviruses as infection in vivo is persistent, restricted, and noncytopathic. In contrast, infections with other more prototypic parvoviruses, like mink enteritis virus (MEV), are acute, cytopathic, and characterized by permissive replication in vivo. Although apoptosis results in the death of cells acutely infected by parvoviruses, the role of apoptosis in ADV infections is unknown. Permissive infection of ADV resulted in apoptosis of Crandell feline kidney (CrFK) cells as indicated by TUNEL staining, Annexin-V staining, and characteristic changes in cell morphology. Pretreatment of infected cells with caspase 3 or broad-spectrum caspase inhibitors prevented apoptosis. In addition, treatment of infected cells with these inhibitors caused a 2 log(10) reduction in the yield of infectious virus compared to untreated cultures. This block in replication preceded substantial viral DNA amplification and gene expression. However, inhibitors of caspases 1, 6, and 8 did not have this effect. MEV also induced caspase-dependent apoptosis following infection of CrFK cells, although production of infectious progeny was not affected by inhibition of apoptosis. Thus, permissive replication of ADV in vitro depended upon activation of specific caspases. If ADV infection of cells in vivo fails to initiate caspase activation, the requirement of caspase activity for replication may not be met, thus providing a possible mechanism for persistent, restricted infection.

Genome replication and postencapsidation functions mapping to the nonstructural gene restrict the host range of a murine parvovirus in human cells

The infection outcome of the Parvoviridae largely relies on poorly characterized intracellular factors modulated by proliferation, differentiation, and transformation of host cells. We have studied the interactions displayed by the highly homologous p and i strains of the murine parvovirus minute virus of mice (MVM), with a series of transformed cells of rat (C6) and human (U373, U87, SW1088, SK-N-SH) nervous system origin, seeking for molecular mechanisms governing parvovirus host range. The MVMp infection of C6 and U373 cells was cytotoxic and productive, whereas the other nervous cells behaved essentially as resistant to this virus. In contrast, MVMi did not complete its life cycle in any of the human nervous cells, though it efficiently killed the astrocytic tumor cells by two types of nonproductive infections: (i) normal synthesis of all viral macromolecules with a late defect in infectious virion maturation and release to the medium in U373; and (ii) high levels of accumulation of the full set of viral messenger RNAs and of both nonstructural (NS-1) and structural (VP-1 and VP-2) proteins, under a very low viral DNA amplification, in U87 and SW1088 cells. Further analyses showed that U87 was permissive for nuclear transport of MVMi proteins, leading to efficient assembly of empty viral capsids with a normal phosphorylation and VP1-to-VP2 ratio. The DNA amplification blockade in U87 occurred after conversion of the incoming MVMi genome to the monomeric replicative form, and it operated independently of the delivery pathway used by the viral particle, since it could not be overcome by transfection with cloned infectious viral DNA. Significantly, a chimeric MVMi virus harboring the coding region of the nonstructural (NS) gene replaced with that of MVMp showed a similar pattern of restriction in U87 cells as the parental MVMi virus, and it attained in U373 cultures an infectious titer above 100-fold higher under equal levels of DNA amplification and genome encapsidation. The results suggest that the activity of complexes formed by the NS polypeptides and recruited cellular factors restrict parvovirus DNA amplification in a cell type-dependent manner and that NS functions may in addition determine MVM host range acting at postencapsidation steps of viral maturation. These data are relevant for understanding the increased multiplication of autonomous parvovirus in some transformed cells and the transduction efficacy of nonreplicative parvoviral vectors, as well as a general remark on the mechanisms by which NS genes may regulate viral tropism and pathogenesis.

NS1- and minute virus of mice-induced cell cycle arrest: involvement of p53 and p21(cip1)

The nonstructural protein NS1 of the autonomous parvovirus minute virus of mice (MVMp) is cytolytic when expressed in transformed cells. Before causing extensive cell lysis, NS1 induces a multistep cell cycle arrest in G(1), S, and G(2), well reproducing the arrest in S and G(2) observed upon MVMp infection. In this work we investigated the molecular mechanisms of growth inhibition mediated by NS1 and MVMp. We show that NS1-mediated cell cycle arrest correlates with the accumulation of the cyclin-dependent kinase (Cdk) inhibitor p21(cip1) associated with both the cyclin A/Cdk and cyclin E/Cdk2 complexes but in the absence of accumulation of p53, a potent transcriptional activator of p21(cip1). By comparison, MVMp infection induced the accumulation of both p53 and p21(cip1). We demonstrate that p53 plays an essential role in the MVMp-induced cell cycle arrest in both S and G(2) by using p53 wild-type (+/+) and null (-/-) cells. Furthermore, only the G(2) arrest was abrogated in p21(cip1) null (-/-) cells. Together these results show that the MVMp-induced cell cycle arrest in S is p53 dependent but p21(cip1) independent, whereas the arrest in G(2) depends on both p53 and its downstream effector p21(cip1). They also suggest that induction of p21(cip1) by the viral protein NS1 arrests cells in G(2) through inhibition of cyclin A-dependent kinase activity.

Propagation of rat parvovirus in thymic lymphoma cell line C58(NT)d and subsequent appearance of a resistant cell clone after lytic infection

Rat parvovirus (RPV) is nonpathogenic in rats but causes persistent lymphocytotropic infection. We found that RPV was propagated in rat thymic lymphoma cell line C58(NT)D and induced apoptosis. Interestingly, a resistant subclone, C58(NT)D/R, from surviving cells after lytic infection had differentiated phenotypic modifications, such as increased cell adherence, resistance to apoptosis, and suppressed tumorigenicity.

Regulation of MVM NS1 by protein kinase C: impact of mutagenesis at consensus phosphorylation sites on replicative functions and cytopathic effects

Minute virus of mice NS1, an 83-kDa mainly nuclear phosphoprotein, is the only viral nonstructural protein required in all cell types and it is involved in multiple processes necessary for virus propagation. The diversity of functions assigned to NS1, together with the variation of its complex phosphorylation pattern during infection, suggested that the various activities of NS1 could be regulated by distinct phosphorylation events. So far, it has been demonstrated that NS1 replicative functions, in particular, DNA-unwinding activities, are regulated by protein kinase C (PKC), as exemplified by the modulation of NS1 helicase activity by PKClambda phosphorylation. In order to determine further impact of phosphorylation on NS1 functions, including the induction of cytopathic effects, a mutational approach was pursued in order to produce NS1 variants harboring amino acid substitutions at candidate PKC target residues. Besides the determination of two additional in vivo phosphorylation sites in NS1, this mutagenesis allowed the segregation of distinct NS1 functions from one another, generating NS1 variants with a distinct activity profile. Thus, we obtained NS1 mutants that were fully proficient for trans activation of the viral P38 promoter, while being impaired in their replicative functions. Moreover, the alterations of specific PKC phosphorylation sites gave rise to NS1 polypeptides that exerted reduced cytotoxicity, leading to sustained gene expression, while keeping functions necessary for progeny virus production, i.e., viral DNA replication and activation of the capsid gene promoter. These data suggested that in the course of a viral infection, NS1 may undergo a shift from productive to cytotoxic functions as a result of a phosphorylation-dependent regulation.

Viruses and apoptosis

Successful viral replication requires not only the efficient production and spread of progeny, but also evasion of host defense mechanisms that limit replication by killing infected cells. In addition to inducing immune and inflammatory responses, infection by most viruses triggers apoptosis or programmed cell death of the infected cell. This cell response often results as a compulsory or unavoidable by-product of the action of critical viral replicative functions. In addition, some viruses seem to use apoptosis as a mechanism of cell killing and virus spread. In both cases, successful replication relies on the ability of certain viral products to block or delay apoptosis until sufficient progeny have been produced. Such proteins target a variety of strategic points in the apoptotic pathway. In this review we summarize the great amount of recent information on viruses and apoptosis and offer insights into how this knowledge may be used for future research and novel therapies.

Nuclear export factor CRM1 interacts with nonstructural proteins NS2 from parvovirus minute virus of mice

The nonstructural NS2 proteins of autonomous parvoviruses are known to act in a host cell-dependent manner and to play a role in viral DNA replication, efficient translation of viral mRNA, and/or encapsidation. Their exact function during the parvovirus life cycle remains, however, still obscure. We report here the characterization of the interaction with the NS2 proteins from the parvovirus minute virus of mice (MVM) and rat as well as mouse homologues of the human CRM1 protein, a member of the importin-beta family recently identified as an essential nuclear export factor. Using the two-hybrid system, we could detect the interaction between the carboxy-terminal region of rat CRM1 and each of the three isoforms of NS2 (P [or major], Y [or minor], and L [or rare]). NS2 proteins were further shown to interact with the full-length CRM1 by coimmunoprecipitation experiments using extracts from both mouse and rat cell lines. Our data show that CRM1 preferentially binds to the nonphosphorylated isoforms of NS2. Moreover, we observed that the treatment of MVM-infected cells with leptomycin B, a drug that specifically inhibits the CRM1-dependent nuclear export pathway, leads to a drastic accumulation of NS2 proteins in the nucleus. Both NS2 interaction with CRM1 and nuclear accumulation upon leptomycin B treatment strongly suggest that these nonstructural viral proteins are actively exported out of the nuclei of infected cells via a CRM1-mediated nuclear export pathway.

DNA unwinding functions of minute virus of mice NS1 protein are modulated specifically by the lambda isoform of protein kinase C

The parvovirus minute virus of mice NS1 protein is a multifunctional protein involved in a variety of processes during virus propagation, ranging from viral DNA replication to promoter regulation and cytotoxic action to the host cell. Since NS1 becomes phosphorylated during infection, it was proposed that the different tasks of this protein might be regulated in a coordinated manner by phosphorylation. Indeed, comparing biochemical functions of native NS1 with its dephosphorylated counterpart showed that site-specific nicking of the origin and the helicase and ATPase activities are remarkably reduced upon NS1 dephosphorylation while site-specific affinity of the protein to the origin became enhanced. As a consequence, the dephosphorylated polypeptide is deficient for initiation of DNA replication. By adding fractionated cell extracts to a kinase-free in vitro replication system, the combination of two protein components containing members of the protein kinase C (PKC) family was found to rescue the replication activity of the dephosphorylated NS1 protein upon addition of PKC cofactors. One of these components, termed HA-1, also stimulated NS1 helicase function in response to acidic lipids but not phorbol esters, indicating the involvement of atypical PKC isoforms in the modulation of this NS1 function (J. P. F. Nüesch, S. Dettwiler, R. Corbau, and J. Rommelaere, J. Virol. 72:9966-9977, 1998). The present study led to the identification of atypical PKClambda/iota as the active component of HA-1 responsible for the regulation of NS1 DNA unwinding and replicative functions. Moreover, a target PKClambda phosphorylation site was localized at S473 of NS1. By site-directed mutagenesis, we showed that this residue is essential for NS1 helicase activity but not promoter regulation, suggesting a possible modulation of NS1 functions by PKClambda phosphorylation at residue S473.

Phosphorylation of the viral nonstructural protein NS1 during MVMp infection of A9 cells

The major nonstructural protein of parvovirus MVMp, NS1, is an 83-kDa nuclear phosphoprotein which exerts a variety of functions during a viral infection. These multiple tasks range from its major involvement in viral DNA amplification and promoter regulation to the cytotoxic action on the host cell. Since these most divergent functions are exerted in an orderly fashion, it has been proposed that NS1 is regulated by posttranslational modifications, in particular phosphorylation. So far it has been shown that the capacity of NS1 for initiation of replication is regulated in vitro by phosphorylation through members of the protein kinase C family, most likely as a result of control of the DNA unwinding activity (J. P. F. Nüesch et al., 1998, J. Virol. 72, 9966-9977). To substantiate these in vitro findings in vivo, we investigated NS1 phosphorylation during an MVMp infection in a natural host cell, A9 fibroblasts, with reference to characteristic features of the virus cycle. The NS1 phosphorylation pattern was found to change throughout the infection, raising the possibility that distinct tasks of NS1 might be achieved through differential phosphorylation of the polypeptide. In addition, we present in vivo evidence that a phosphorylated form of NS1 is able to initiate viral DNA replication and becomes covalently attached to replicated DNA. Moreover, NS1 was found to be phosphorylated in vivo within the helicase domain, showing alignment with at least one phosphopeptide generated by an "activating" kinase in vitro. These data suggest that phosphorylation-mediated regulation of NS1 for replicative functions as observed in vitro may also take place during a natural virus infection.

cis requirements for the efficient production of recombinant DNA vectors based on autonomous parvoviruses

The replication of viral genomes and the production of recombinant viral vectors from infectious molecular clones of parvoviruses MVMp and H1 were greatly improved by the introduction of a consensus NS-1 nick site at the junction between the left-hand viral terminus and the plasmid DNA. Progressive deletions of up to 1600 bp in the region encoding the structural genes as well as insertions of foreign DNA in replacement of those sequences did not appreciably affect the replication ability of the recombinant H1 virus genomes. In contrast, the incorporation of these genomes into recombinant particles appeared to depend on in cis-provided structural gene sequences. Indeed, the production of H1 viral vectors by cotransfection of recombinant clones and helper plasmids providing the structural proteins (VPs) in trans, drastically decreased when more than 800 bp was removed from the VP transcription unit. Furthermore, titers of viral vectors, in which most of the VP-coding region was replaced by an equivalent-length sequence consisting of reporter cDNA and stuffer DNA, were reduced more than 50 times in comparison with recombinant vectors in which stuffer DNA was not substituted for the residual VP sequence. In addition, viral vector production was restricted by the overall size of the genome, with a mere 6% increase in DNA length leading to an approximately 10 times lower encapsidation yield. Under conditions fulfilling the above-mentioned requirements for efficient packaging, titers of virus vectors from improved recombinant molecular DNA clones amounted to 5 x 10(7) infectious units per milliliter of crude extract. These titers should allow the assessment of the therapeutic effect of recombinant parvoviruses expressing small transgenes in laboratory animals.

Replicative functions of minute virus of mice NS1 protein are regulated in vitro by phosphorylation through protein kinase C

NS1, the major nonstructural protein of the parvovirus minute virus of mice, is a multifunctional phosphoprotein which is involved in cytotoxicity, transcriptional regulation, and initiation of viral DNA replication. For coordination of these various functions during virus propagation, NS1 has been proposed to be regulated by posttranslational modifications, in particular phosphorylation. Recent in vitro studies (J. P. F. Nüesch, R. Corbau, P. Tattersall, and J. Rommelaere, J. Virol. 72:8002-8012, 1998) provided evidence that distinct NS1 activities, notably the intrinsic helicase function, are modulated by the phosphorylation state of the protein. In order to study the dependence of the initiation of viral DNA replication on NS1 phosphorylation and to identify the protein kinases involved, we established an in vitro replication system that is devoid of endogenous protein kinases and is based on plasmid substrates containing the minimal left-end origins of replication. Cellular components necessary to drive NS1-dependent rolling-circle replication (RCR) were freed from endogenous serine/threonine protein kinases by affinity chromatography, and the eukaryotic DNA polymerases were replaced by the bacteriophage T4 DNA polymerase. While native NS1 (NS1(P)) supported RCR under these conditions, dephosphorylated NS1 (NS1(O)) was impaired. Using fractionated HeLa cell extracts, we identified two essential protein components which are able to phosphorylate NS1(O), are enriched in protein kinase C (PKC), and, when present together, reactivate NS1(O) for replication. One of these components, containing atypical PKC, was sufficient to restore NS1(O) helicase activity. The requirement of NS1(O) reactivation for characteristic PKC cofactors such as Ca2+/phosphatidylserine or phorbol esters strongly suggests the involvement of this protein kinase family in regulation of NS1 replicative functions in vitro.

Induction of programmed cell death by parvovirus H-1 in U937 cells: connection with the tumor necrosis factor alpha signalling pathway

The human promonocytic cell line U937 undergoes apoptosis upon treatment with tumor necrosis factor alpha (TNF-alpha). This cell line has previously been shown to be very sensitive to the lytic effect of the autonomous parvovirus H-1. Parvovirus infection leads to the activation of the CPP32 ICE-like cysteine protease which cleaves the enzyme poly(ADP-ribose)polymerase and induces morphologic changes that are characteristic of apoptosis in a way that is similar to TNF-alpha treatment. This effect is also observed when the U937 cells are infected with a recombinant H-1 virus which expresses the nonstructural (NS) proteins but in which the capsid genes are replaced by a reporter gene, indicating that the induction of apoptosis can be assigned to the cytotoxic nonstructural proteins in this cell system. The c-Myc protein, which is overexpressed in U937 cells, is rapidly downregulated during infection, in keeping with a possible role of this product in mediating the apoptotic cell death induced by H-1 virus infection. Interestingly, four clones (designated RU) derived from the U937 cell line and selected for their resistance to H-1 virus (J. A. Lopez-Guerrero et al., Blood 89:1642-1653, 1997) failed to decrease c-Myc expression upon treatment with differentiation agents and also resisted the induction of cell death after TNF-alpha treatment. Our data suggest that the RU clones have developed defense strategies against apoptosis, either by their failure to downregulate c-Myc and/or by activating antiapoptotic factors.

Biochemical activities of minute virus of mice nonstructural protein NS1 are modulated In vitro by the phosphorylation state of the polypeptide

NS1, the 83-kDa major nonstructural protein of minute virus of mice (MVM), is a multifunctional nuclear phosphoprotein which is required in a variety of steps during progeny virus production, early as well as late during infection. NS1 is the initiator protein for viral DNA replication. It binds specifically to target DNA motifs; has site-specific single-strand nickase, intrinsic ATPase, and helicase activities; trans regulates viral and cellular promoters; and exerts cytotoxic stress on the host cell. To investigate whether these multiple activities of NS1 depend on posttranslational modifications, in particular phosphorylation, we expressed His-tagged NS1 in HeLa cells by using recombinant vaccinia viruses, dephosphorylated it at serine and threonine residues with calf intestine alkaline phosphatase, and compared the biochemical activities of the purified un(der)phosphorylated (NS1(O)) and the native (NS1(P)) polypeptides. Biochemical analyses of replicative functions of NS1(O) revealed a severe reduction of intrinsic helicase activity and, to a minor extent, of ATPase and nickase activities, whereas its affinity for the target DNA sequence [ACCA]2-3 was enhanced compared to that of NS1(P). In the presence of endogenous protein kinases found in replication extracts, NS1(O) showed all functions necessary for resolution and replication of the 3' dimer bridge, indicating reactivation of NS1(O) by rephosphorylation. Partial reactivation of the helicase activity was found as well when NS1(O) was incubated with protein kinase C.

Viruses and the cell cycle

Viruses depend on the host's machineries to replicate and express their genome. Actively replicating cells have large pools of deoxynucleotides and high levels of key enzyme activities that viruses exploit to their own needs. Some viruses have developed strategies for driving quiescent cells into the S phase of the cell cycle, e.g. adenovirus, others, such as parvovirus, wait until the host itself begins to replicate. Viruses may also force the host cell to stay in a favourable phase, e.g. Epstein-Barr virus, or, if necessary, they may inhibit apoptotic cell death, e.g. human cytomegalovirus. In this review, we focus on the different strategies that viruses use to create in infected cells an environment favourable to the accomplishment of the viral life cycle through acting on cell cycle regulators.

Dose-dependent regression of HeLa cell-derived tumours in SCID mice after parvovirus H-1 infection

Parvoviruses of rodents are endowed with oncosuppressive properties. In particular, parvoviral infections protect host animals from spontaneous and chemical- or virus-induced tumour initiation in laboratory animals. The present study was undertaken to substantiate the capacity of parvovirus H-1 to inhibit therapeutically the growth of established tumours originating from human carcinoma cells implanted in recipient mice. To this end, quickly growing s.c. carcinomas were established by injection of human cervical carcinoma cells (HeLa) into immunodeficient (SCID) mice. Tumour-bearing mice subsequently were inoculated with H-1 at various multiplicities of infection. H-1 virus infection led to regression of tumours, the onset and efficiency of which were dose-dependent.

Inhibition of parvovirus minute virus of mice replication by a peptide involved in the oligomerization of nonstructural protein NS1

The large nonstructural protein NS1 of the minute virus of mice and other parvoviruses is involved in essential steps of the viral life cycle, such as DNA replication and transcriptional regulation, and is a major contributor to the toxic effect on host cells. Various biochemical functions, such as ATP binding, ATPase, site-specific DNA binding and nicking, and helicase activities, have been assigned to NS1. Homo-oligomerization is a prerequisite for a number of proteins to be fully functional. In particular, helicases generally act as homo-oligomers. Indirect evidence of NS1 self-association has been recently obtained by a nuclear cotransport assay (J. P. Nüesch and P. Tattersall, Virology 196:637-651, 1993). In order to demonstrate the oligomerizing property of NS1 in a direct way and localize the protein region(s) involved, the yeast two-hybrid system was used in combination with deletion mutagenesis across the whole NS1 molecule, followed by high-resolution mapping of the homo-oligomerization domain by a peptide enzyme-linked immunosorbent assay method. This study led to the identification of a distinct NS1 peptide that contains a bipartite domain involved in NS1 oligomerization. Furthermore, this isolated peptide was found to act as a specific competitive inhibitor and suppress NS1 helicase activity in vitro and parvovirus DNA replication in vivo, arguing for the involvement of NS1 oligomerization in these processes. Our results point to drug targeting of oligomerization motifs of viral regulatory proteins as a potentially useful antiviral strategy.

The NS1 protein of the autonomous parvovirus minute virus of mice blocks cellular DNA replication: a consequence of lesions to the chromatin?

The nonstructural protein NS1 of the autonomous parvovirus minute virus of mice interferes with cell division and can cause cell death, depending on the cell transformation state. Upon infection, the synthesis of NS1 protein is massively initiated during S phase. In this article, we show that minute virus of mice-infected cells accumulate in this phase. To investigate the link between NS1 accumulation and S-phase arrest, we have used stably transfected cells in which NS1 expression is under the control of a glucocorticoid-inducible promoter (the long terminal repeat of mouse mammary tumor virus). NS1 expression interferes with cell DNA replication, and consequently, the cell cycle stops in S phase. NS1 expression also induces nicks in the cell chromatin, as detected by an in situ nick translation assay. The nicks are observed several hours before any cell cycle perturbation. As cell cycle arrest is a common consequence of DNA damage, we propose that NS1 exerts its cytostatic activity by inducing lesions in cell chromatin.

The cytotoxicity of the parvovirus minute virus of mice nonstructural protein NS1 is related to changes in the synthesis and phosphorylation of cell proteins

Autonomous parvoviruses exert lytic and cytostatic effects believed to contribute to their antineoplastic activity. Studies with inducible clones have demonstrated a direct involvement of parvovirus nonstructural proteins (NS) in oncolysis. Human and rat fibroblasts have been stably transfected with MVM(p) (minute virus of mice prototype strain) NS genes cloned under the control of a hormone-inducible promoter. Dexamethasone-induced synthesis of the NS proteins in sensitive transformed cells results in cell killing within a few days. From these sensitive cell lines have been isolated some NS-resistant clones that also prove resistant to MVM(p) infection, suggesting that cell factors modulate NS cytotoxicity. We have previously reported that factors involved in cell cycle regulation may contribute to this modulation, since NS toxicity requires cell proliferation and correlates with a cell cycle perturbation leading to an arrest in phase S/G2. In addition to its role in cytotoxicity, NS1 can regulate transcription driven by parvovirus and nonparvovirus promoters. Since phosphorylation is a critical event in controlling the activity of many proteins, notably transcription factors and cell cycle-regulated proteins, we have examined the effect of NS1 on the synthesis and phosphorylation of cell proteins. Our results indicate that NS1 interferes, within 7 h of induction, with phosphorylation of a protein of about 14 kDa (p14). Cell synchronization has enabled us to show that phosphorylation of this protein occurs in early S phase and is prevented when NS1 is induced. This early effect of NS1 on p14 phosphorylation may be directly linked to cytotoxicity and is probably related to the previously reported inhibition of cell DNA synthesis. Late in the induction period (24 h), NS1 also alters the synthesis of a 50-kDa protein and a 35-kDa protein (p50 and p35, respectively). Microsequencing of p35 reveals sequence homology with beta-tubulin. These effects of NS1, observed only in NS1-sensitive cell lines, may be related to the protein's cytotoxicity.