Opposite transcriptional effects of cyclic AMP-responsive elements in confluent or p27KIP-overexpressing cells versus serum-starved or growing cells

Mink enteritis virus infection induced cell cycle arrest and autophagy for its replication

Mink enteritis virus (MEV) is an important pathogen causing mink viral enteritis. The mechanisms of cell cycle arrest induced by MEV infection and the roles of autophagy in MEV replication remain unclear. In this study, the roles of MEV NS1 protein in inducing cell cycle arrest were investigated, using the in vitro CRFK cell models. As a result, MEV infection increased the proportion of the cells in S phase, inducing S phase arrest. MEV NS1 protein also led to cycle arrest in S phase. And the deletions of NLS and TAD significantly weakened the ability of NS1 protein to cause cycle arrest in S phase, and NLS and TAD were the indispensable domains of NS1 protein. Furthermore, proteome profiling of the cells infected with MEV at the early stage demonstrated that the autophagy-related protein TRIM23 was significantly up-regulated during MEV infection. To investigate the effects of TRIM23 on MEV replication, the cell models were established, using siRNAs targeting TRIM23. The knockdown of TRIM23 resulted in the decreases in the levels of TBK1 protein and the phosphorylated p62 protein, and an increase in the level of p62 protein in the cells infected with MEV, indirectly influencing virus replication. The findings implied that S phase arrest and the up-regulated TRIM23 induced by MEV infection played the important roles in MEV replication.

The MVMp P4 promoter is a host cell-type range determinant in vivo

The protoparvovirus early promoters, e.g. P4 of Minute Virus of Mice (MVM), play a critical role during infection. Initial P4 activity depends on the host transcription machinery only. Since this is cell-type dependent, it is hypothesized that P4 is a host cell-type range determinant. Yet host range determinants have mapped mostly to capsid, never P4. Here we test the hypothesis using the mouse embryo as a model system. Disruption of the CRE element of P4 drastically decreased infection levels without altering range. However, when we swapped promoter elements of MVM P4 with those from equivalent regions of the closely related H1 virus, we observed elimination of infection in fibroblasts and chondrocytes and the acquisition of infection in skeletal muscle. We conclude that P4 is a host range determinant and a target for modifying the productive infection potential of the virus - an important consideration in adapting these viruses for oncotherapy.

The Mammalian Cell Cycle Regulates Parvovirus Nuclear Capsid Assembly

It is unknown whether the mammalian cell cycle could impact the assembly of viruses maturing in the nucleus. We addressed this question using MVM, a reference member of the icosahedral ssDNA nuclear parvoviruses, which requires cell proliferation to infect by mechanisms partly understood. Constitutively expressed MVM capsid subunits (VPs) accumulated in the cytoplasm of mouse and human fibroblasts synchronized at G0, G1, and G1/S transition. Upon arrest release, VPs translocated to the nucleus as cells entered S phase, at efficiencies relying on cell origin and arrest method, and immediately assembled into capsids. In synchronously infected cells, the consecutive virus life cycle steps (gene expression, proteins nuclear translocation, capsid assembly, genome replication and encapsidation) proceeded tightly coupled to cell cycle progression from G0/G1 through S into G2 phase. However, a DNA synthesis stress caused by thymidine irreversibly disrupted virus life cycle, as VPs became increasingly retained in the cytoplasm hours post-stress, forming empty capsids in mouse fibroblasts, thereby impairing encapsidation of the nuclear viral DNA replicative intermediates. Synchronously infected cells subjected to density-arrest signals while traversing early S phase also blocked VPs transport, resulting in a similar misplaced cytoplasmic capsid assembly in mouse fibroblasts. In contrast, thymidine and density arrest signals deregulating virus assembly neither perturbed nuclear translocation of the NS1 protein nor viral genome replication occurring under S/G2 cycle arrest. An underlying mechanism of cell cycle control was identified in the nuclear translocation of phosphorylated VPs trimeric assembly intermediates, which accessed a non-conserved route distinct from the importin α2/β1 and transportin pathways. The exquisite cell cycle-dependence of parvovirus nuclear capsid assembly conforms a novel paradigm of time and functional coupling between cellular and virus life cycles. This junction may determine the characteristic parvovirus tropism for proliferative and cancer cells, and its disturbance could critically contribute to persistence in host tissues.

Cellular and viral life cycles are connected through multiple, though poorly understood, mechanisms. Parvoviruses infect humans and a broad spectrum of animals, causing a variety of diseases, but they are also used in experimental cancer therapy and serve as vectors for gene therapy. Parvoviruses can only multiply in proliferating cells providing essential replicative and transcriptional functions. However, it is unknown whether the cell cycle regulatory machinery may also control parvovirus assembly. We found that the nuclear translocation of parvovirus MVM capsid subunits (VPs) was highly dependent on physiological cell cycle regulations in mammalian fibroblasts, including: quiescence, progression through G1/S boundary, DNA synthesis, and cell to cell contacts. VPs nuclear translocation was significantly more sensitive to cell cycle controls than viral genome replication and gene expression. The results support nuclear capsid assembly as the major driving process of parvoviruses biological hallmarks, such as pathogenesis in proliferative tissues and tropism for cancer cells. In addition, disturbing the tight coupling of parvovirus assembly with the cell cycle may determine viral persistence in quiescent and post-mitotic host tissues. These findings may contribute to understand cellular regulations on the assembly of other nuclear eukaryotic viruses, and to develop cell cycle-based avenues for antiviral therapy.

Possible involvement of NEDD4 in keloid formation; its critical role in fibroblast proliferation and collagen production

Keloid represents overgrowth of granulation tissue, which is characterized by collection of atypical fibroblasts with excessive deposition of extracellular matrix components, after skin injury, but its etiology is still largely unknown. We recently performed genome-wide association study (GWAS) of keloid and identified NEDD4 to be one of candidate molecules associated with keloid susceptibility. Here we demonstrate a possible mechanism of NEDD4 involvement in keloid formation through enhancement of the proliferation and invasiveness of fibroblasts as well as upregulation of type 1 collagen expression. Activation of NEDD4 affected subcellular localization and protein stability of p27 which was implied its critical role in contact inhibition. It also induced accumulation of β-catenin in the cytoplasm and activated the TCF/β-catenin transcriptional activity. Furthermore, NEDD4 upregulated expressions of fibronectin and type 1 collagen and contributed to the excessive accumulation of extracellular matrix. Our findings provide new insights into mechanism developing keloid and can be applied for development of a novel treatment for keloid.

Through its nonstructural protein NS1, parvovirus H-1 induces apoptosis via accumulation of reactive oxygen species

The rat parvovirus H-1 (H-1PV) attracts high attention as an anticancer agent, because it is not pathogenic for humans and has oncotropic and oncosuppressive properties. The viral nonstructural NS1 protein is thought to mediate H-1PV cytotoxicity, but its exact contribution to this process remains undefined. In this study, we analyzed the effects of the H-1PV NS1 protein on human cell proliferation and cell viability. We show that NS1 expression is sufficient to induce the accumulation of cells in G(2) phase, apoptosis via caspase 9 and 3 activation, and cell lysis. Similarly, cells infected with wild-type H-1PV arrest in G(2) phase and undergo apoptosis. Furthermore, we also show that both expression of NS1 and H-1PV infection lead to higher levels of intracellular reactive oxygen species (ROS), associated with DNA double-strand breaks. Antioxidant treatment reduces ROS levels and strongly decreases NS1- and virus-induced DNA damage, cell cycle arrest, and apoptosis, indicating that NS1-induced ROS are important mediators of H-1PV cytotoxicity.

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.

A cytoskeleton-associated protein, TMAP/CKAP2, is involved in the proliferation of human foreskin fibroblasts

Previously, we reported the cloning of a cytoskeleton-associated protein, TMAP/CKAP2, which was up-regulated in primary human gastric cancers. Although TMAP/CKAP2 has been found to be expressed in most cancer cell lines examined, the function of CKAP2 is not known. In this study, we found that TMAP/CKAP2 was not expressed in G0/G1 arrested HFFs, but that it was expressed in actively dividing cells. After initiating the cell cycle, TMAP/CKAP2 levels remained low throughout most of the G1 phase, but gradually increased between late G1 and G2/M. Knockdown of TMAP/CKAP2 reduced pRB phosphorylation and increased p27 expression, and consequently reduced HFF proliferation, whereas constitutive TMAP/CKAP2 expression increased pRB phosphorylation and enhanced proliferation. Our results show that this novel cytoskeleton-associated protein is expressed cell cycle dependently and that it is involved in cell proliferation.

Targeting of autonomous parvoviruses to colon cancer by insertion of Tcf sites in the P4 promoter

The Wnt signaling pathway is activated by mutations in the adenomatous polyposis coli (APC) or beta-catenin genes in most colon cancers, leading to the transactivation of promoters containing binding sites for the Tcf/LEF family of transcription factors. We have previously shown that it is possible to confer colon cancer specificity on autonomous parvoviruses by inserting Tcf sites into the viral P4 promoter. The mutant Tcf promoters were responsive to activation of the Wnt pathway but the viruses replicated poorly. We show here that reduction of the number of Tcf sites from four to two leads to an increase in the efficiency of replication and toxicity of the viruses in Co115 colon cancer cells, with only a small reduction in selectivity for cells with an active Wnt signaling pathway. Despite this improvement, virus production by most colon cancer cells remained low. Analysis of parental phH1 virus infection of SW480 colon cancer cells showed that the nonstructural and capsid proteins were expressed, but single stranded DNA and progeny virus were not produced. This defect reflects the dependence of autonomous parvoviruses on host functions for many steps in their replication cycle and represents a major limitation to the use of selectively replicating parvoviruses for colon cancer therapy.

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.

Reduced E-cadherin expression contributes to the loss of p27kip1-mediated mechanism of contact inhibition in thyroid anaplastic carcinomas

In the present study, we have characterized several human thyroid cancer cell lines of different histotypes for their responsiveness to contact inhibition. We found that cells derived from differentiated carcinoma (TPC-1, WRO) arrest in G(1) phase at confluence, whereas cells derived from anaplastic carcinoma (ARO, FRO and FB1) continue to grow after reaching confluence. Furthermore, we provide experimental evidence that the axis, E-cadherin/beta-catenin/p27(Kip1), represents an integral part of the regulatory mechanism that controls proliferation at a high cell density, whose disruption may play a key role in determining the clinical behaviour of thyroid cancer. This conclusion derives from the finding that: (i) the expression of p27(Kip1) is enhanced at high cell density only in cells responsive to contact inhibition (TPC-1, WRO), but not in contact-inhibition resistant cells (ARO, FRO or FB1 cells); (ii) the increase in p27(Kip1) also resulted in increased levels of p27(Kip1) bound to cyclin E-Cdk2 complex, a reduction in cyclin E-Cdk2 activity and dephosphorylation of the retinoblastoma protein; (iii) antisense inhibition of p27(Kip1) upregulation at high cell density in confluent-sensitive cells completely prevents the confluence-induced growth arrest; (iv) proper expression and/or membrane localization of E-cadherin is observed only in cells responsive to contact inhibition (TPC-1, NPA, WRO) but not in unresponsive cells (ARO, FRO or FB1); (v) disruption of E-cadherin-mediated cell-cell contacts at high cell density induced by an anti-E-cadherin neutralizing antibody, inhibits the induction of p27(kip1) and restores proliferation in contact-inhibited cells; (vi) re-expression of E-cadherin into cells unresponsive to contact inhibition (ARO, FB1) induces a p27(kip1) expression and growth arrest. In summary, our data indicate that the altered response to contact inhibition exhibited by thyroid anaplastic cancer cells is due to the failure to upregulate p27(Kip1) in response to cell-cell interactions.

p53 mediates density-dependent growth arrest

While the stress-response-associated importance of the p53 tumor suppressor is well established, recent studies have also linked p53 with several basic parameters in the normal behavior of cells. Here, we present evidence that basal p53 expression in WI38 human embryonic lung fibroblasts restricts growth rate and mediates density-dependent inhibition of growth and the associated G1 phase arrest of the cell cycle by affecting the density-dependent regulation of p16/INK4a. Additionally, we show that prolonged culturing of hTert-immortalized WI38 cells leads to a loss of density-dependent growth inhibition that correlates with p27/KIP deregulation as well as the previously shown INK4a locus silencing, and to an onset of contact-induced, p53-dependent cell death.

The Sp1 family of transcription factors is involved in p27(Kip1)-mediated activation of myelin basic protein gene expression

p27(Kip1) levels increase in many cells as they leave the cell cycle and begin to differentiate. The increase in p27(Kip1) levels generally precedes the expression of differentiation-specific genes. Previous studies from our laboratory showed that the overexpression of p27(Kip1) enhances myelin basic protein (MBP) promoter activity. This activation is specific to p27(Kip1). Additionally, inhibition of cyclin-dependent kinase activity alone is not sufficient to increase MBP expression. In this study, we focused on understanding how p27(Kip1) can activate gene transcription by using the MBP gene in oligodendrocytes as a model. We show that the enhancement of MBP promoter activity by p27(Kip1) is mediated by a proximal region of the MBP promoter that contains a conserved GC box binding sequence. This sequence binds transcription factors Sp1 and Sp3. Increased expression of p27(Kip1) increases the level of Sp1 promoter binding to the GC box but does not change the level of Sp3 binding. The binding of Sp1 to this element activates the MBP promoter. p27(Kip1) leads to increased Sp1 binding through a decrease in Sp1 protein turnover. Enhancement of MBP promoter activity by an increase in the level of p27(Kip1) involves a novel mechanism that is mediated through the stabilization and binding of transcription factor Sp1.

Replicating parvoviruses that target colon cancer cells

The wnt signaling pathway is constitutively activated in colon tumors by mutations in the adenomatous polyposis coli and beta-catenin genes. We have modified the minute virus of mice (MVM) P4 promoter to make it responsive to wnt signaling by inserting binding sites for the heterodimeric beta-catenin/Tcf transcription factor. In luciferase assays we can see up to 20-fold selectivity of Tcf mutant P4 promoters for cells with activated wnt signaling. Hybrid MVM/H-1 viruses containing Tcf mutant promoters were tested for NS1 expression, viral DNA replication, virus replication, and cytopathic effect on colon, lung, kidney, and cervical cancer cell lines. Activation of the wnt pathway by expression of Delta N-beta-catenin increased NS1 expression and viral burst size in 293T and H1299 lung cancer cells, showing that the Tcf mutant P4 promoter can respond to wnt signals in the context of the virus. Compared to the parental virus, the burst size of the Tcf mutant viruses was reduced at least 1,000-fold in H1299, 293T, NB324K, and HeLa cells, which have inactive wnt signaling pathways. The burst size and cytopathic effect of the Tcf viruses was near wild-type levels in SW480 and Isreco1 colon cancer cell lines, which have high Tcf activity. The high specificity of these viruses should permit the development of H-1 virus-based vectors which combine high safety and greater efficacy in cancer therapy.

The appearance of truncated cyclin A2 correlates with differentiation of mouse embryonic stem cells

The presence of a form of cyclin A2 with an N-terminal truncation has recently been reported in various murine cell lines and tissues. The truncated cyclin A2 binds to and activates the cyclin-dependent kinase 2 (CDK2). However, CDK2 bound by the truncated cyclin A2 is located in the cytoplasm in contrast to CDK2 bound to full-length cyclin A2, which is in the nucleus. Here, we show that proliferating mouse embryonic stem cells (ES cells) contain very little truncated cyclin A2 but as the cells are induced to differentiate the amount of truncated cyclin A2 increases. The expression pattern of truncated cyclin A2 was the same in p27(Kip1) -/- differentiating ES cells as in the differentiating wild-type cells. We conclude that p27(Kip1) is not necessary for the proteolytic cleavage that gives rise to the truncated form of cyclin A2 in differentiating ES cells and that this post-translational modification is not a function of the cell density but is correlated with differentiation.

The P4 promoter of the parvovirus minute virus of mice is developmentally regulated in transgenic P4-LacZ mice

Activation of the minute virus of mice (MVM) P4 promoter is a key step in the life cycle of the virus and is completely dependent on host transcription factors. Since transcription-factor composition varies widely in different cell types, there is the possibility that only some cell types in the host organism have the capacity to initiate expression from the P4 promoter and therefore that the promoter may be a factor in determining the tropism of MVM. In this study, the ability of various cell types to activate P4, independent of the other virus-host interactions, was examined in transgenic mouse lines bearing a beta-galactosidase reporter sequence driven by the P4 promoter. It was found that lacZ was expressed during embryogenesis and in the adult in a cell-type-specific and differentiation-dependent pattern. The data are consistent with cell-type and stage-specific activation of the P4 promoter having a role in determining the host cell-type range of MVM. The ability of some parvoviruses to replicate in, and kill oncogenically transformed cells, and to destroy induced tumors in laboratory animals is the basis of recent approaches to use MVM-based vectors in cancer gene therapy. Since these vectors rely on the activation of the P4 promoter by the target tissues, understanding the promoter dependence on cell-type and differentiation status is important for their design and potential use.

Autonomous parvovirus vectors

Parvoviruses are small, icosahedral viruses (approximately 25 nm) containing a single-strand DNA genome (approximately 5 kb) with hairpin termini. Autonomous parvoviruses (APVs) are found in many species; they do not require a helper virus for replication but they do require proliferating cells (S-phase functions) and, in some cases, tissue-specific factors. APVs can protect animals from spontaneous or experimental tumors, leading to consideration of these viruses, and vectors derived from them, as anticancer agents. Vector development has focused on three rodent APVs that can infect human cells, namely, LuIII, MVM, and H1. LuIII-based vectors with complete replacement of the viral coding sequences can direct transient or persistent expression of transgenes in cell culture. MVM-based and H1-based vectors with substitution of transgenes for the viral capsid sequences retain viral nonstructural (NS) coding sequences and express the NS1 protein. The latter serves to amplify the vector genome in target cells, potentially contributing to antitumor activity. APV vectors have packaging capacity for foreign DNA of approximately 4.8 kb, a limit that probably cannot be exceeded by more than a few percent. LuIII vectors can be pseudotyped with capsid proteins from related APVs, a promising strategy for controlling tissue tropism and circumventing immune responses to repeated administration. Initial success has been achieved in targeting such a pseudotyped vector by genetic modification of the capsid. Subject to advances in production and purification methods, APV vectors have potential as gene transfer agents for experimental and therapeutic use, particularly for cancer therapy.

p27(Kip1) enhances myelin basic protein gene promoter activity

The process of oligodendrocyte differentiation is a complex event that requires cell cycle withdrawal, followed by the activation of a specific transcriptional program responsible for the synthesis of myelin genes. Because growth arrest precedes differentiation, we sought to investigate the role of cell cycle molecules in the activation of myelin gene promoters. We hypothesized that the cell cycle inhibitor p27(Kip1), which is primarily responsible for arresting proliferating oligodendrocyte progenitors, may be involved in the transcriptional regulation of myelin genes. In agreement with this hypothesis, overexpression of p27(Kip1) in the CG4 cell line, but not in 3T3 fibroblasts, enhances the expression of luciferase driven by the myelin basic protein (MBP) promoter. Interestingly, this effect is specific for p27(Kip1); overexpression of other cell cycle inhibitors had no effect. Additionally, this effect is independent of halting the cell cycle; treatment with the cell cycle blocker roscovitine did not affect MBP promoter usage. We conclude that p27(Kip1) contributes to oligodendrocyte differentiation by regulating transcription of the MBP gene.

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.

Transduction of human MCP-3 by a parvoviral vector induces leukocyte infiltration and reduces growth of human cervical carcinoma cell xenografts

BACKGROUND

The oncosuppressive properties of some autonomous parvoviruses such as H-1 virus, together with their low pathogenicity, make them attractive vectors for tumor-directed gene therapy. Indeed, it was recently shown that these viruses became endowed with an enhanced oncosuppressive activity after they had been engineered to deliver a recognized therapeutic transgene. This prompted us to use a parvoviral vector to analyse the antineoplastic capacity of MCP-3 (monocyte chemotactic protein-3), a CC chemokine which has a broad spectrum of target cells, and can thus be considered to be a promising candidate for cancer treatment.

METHODS

We explored the use of a parvovirus H-1-based vector encoding human MCP-3 for its antitumor potential on human cervical carcinoma cells. HeLa cells were infected in vitro with the recombinant virus hH1/MCP-3 at a low multiplicity [1 replication unit (RU)/cell] and we investigated the effect of parvovirus-mediated MCP-3 transduction on tumor formation and growth upon implantation of HeLa cells in nude mice.

RESULTS

Infection of HeLa cells with hH1/MCP-3 led to secretion of high levels of MCP-3 and to significant retardation of tumor growth in recipient mice, as compared with HeLa cells that were either buffer-treated or infected with a MCP-3-free vector. Tumors from hH1/MCP-3-infected HeLa cells were heavily infiltrated with activated macrophages and showed increased numbers of dendritic cells. In addition, activated natural killer (NK) cells were also recruited into MCP-3-transduced tumors.

CONCLUSION

These observations indicate that parvovirus H-1-transduced MCP-3 is able to exert a significant antitumor activity which is mediated, at least in part, through macrophages and NK cells, under conditions in which activated T cells are lacking.

CREB/ATF-dependent repression of cyclin a by human T-cell leukemia virus type 1 Tax protein

Expression of the human T-cell leukemia virus type 1 (HTLV-1) oncoprotein Tax is correlated with cellular transformation contributing to the development of adult T-cell leukemia. Tax has been shown to modulate the activities of several cellular promoters. Existing evidence suggests that Tax need not directly bind to DNA to accomplish these effects but rather that it can act through binding to cellular factors, including members of the CREB/ATF family. Exact mechanisms of HTLV-1 transformation of cells have yet to be fully defined, but the process is likely to include both activation of cellular-growth-promoting factors and repression of cellular tumor-suppressing functions. While transcriptional activation has been well studied, transcriptional repression by Tax, reported recently from several studies, remains less well understood. Here, we show that Tax represses the TATA-less cyclin A promoter. Repression of the cyclin A promoter was seen in both ts13 adherent cells and Jurkat T lymphocytes. Two other TATA-less promoters, cyclin D3 and DNA polymerase alpha, were also found to be repressed by Tax. Interestingly, all three promoters share a common feature of at least one conserved upstream CREB/ATF binding site. In electrophoretic mobility shift assays, we observed that Tax altered the formation of a complex(es) at the cyclin A promoter-derived ATF site. Functionally, we correlated removal of the CREB/ATF site from the promoter with loss of repression by Tax. Furthermore, since a Tax mutant protein which binds CREB repressed the cyclin A promoter while another mutant protein which does not bind CREB did not, we propose that this Tax repression occurs through protein-protein contact with CREB/ATF.

Cyclin A activates the DNA polymerase delta -dependent elongation machinery in vitro: A parvovirus DNA replication model

Replication of the single-stranded linear DNA genome of parvovirus minute virus of mice (MVM) starts with complementary strand synthesis from the 3'-terminal snap-back telomere, which serves as a primer for the formation of double-stranded replicative form (RF) DNA. This DNA elongation reaction, designated conversion, is exclusively dependent on cellular factors. In cell extracts, we found that complementary strand synthesis was inhibited by the cyclin-dependent kinase inhibitor p21(WAF1/CIP1) and rescued by the addition of proliferating cell nuclear antigen, arguing for the involvement of DNA polymerase (Pol) delta in the conversion reaction. In vivo time course analyses using synchronized MVM-infected A9 cells allowed initial detection of MVM RF DNA at the G(1)/S phase transition, coinciding with the onset of cyclin A expression and cyclin A-associated kinase activity. Under in vitro conditions, formation of RF DNA was efficiently supported by A9 S cell extracts, but only marginally by G(1) cell extracts. Addition of recombinant cyclin A stimulated DNA conversion in G(1) cell extracts, and correlated with a concomitant increase in cyclin A-associated kinase activity. Conversely, a specific antibody neutralizing cyclin A-dependent kinase activity, abolished the capacity of S cell extracts for DNA conversion. We found no evidence for the involvement of cyclin E in the regulation of the conversion reaction. We conclude that cyclin A is necessary for activation of complementary strand synthesis, which we propose as a model reaction to study the cell cycle regulation of the Pol delta-dependent elongation machinery.

Tumor-selective gene transduction and cell killing with an oncotropic autonomous parvovirus-based vector

A recombinant MVMp of the fibrotropic strain of minute virus of mice (MVMp) expressing the chloramphenicol acetyltransferase reporter gene was used to infect a series of biologically relevant cultured cells, normal or tumor-derived, including normal melanocytes versus melanoma cells, normal mammary epithelial cells versus breast adenocarcinoma cells, and normal neurons or astrocytes versus glioma cells. As a reference cell system we used normal human fibroblasts versus the SV40-transformed fibroblast cell line NB324K. After infection, we observed good expression of the reporter gene in the different tumor cell types, but only poor expression if any in the corresponding normal cells. We also constructed a recombinant MVMp expressing the green fluorescent protein reporter gene and assessed by flow cytometry the efficiency of gene transduction into the different target cells. At a multiplicity of infection of 30, we observed substantial transduction of the gene into most of the tumor cell types tested, but only marginal transduction into normal cells under the same experimental conditions. Finally, we demonstrated that a recombinant MVMp expressing the herpes simplex virus thymidine kinase gene can, in vitro, cause efficient killing of most tumor cell types in the presence of ganciclovir, whilst affecting normal proliferating cells only marginally if at all. However, in the same experimental condition, breast tumor cells appeared to be resistant to GCV-mediated cytotoxicity, possibly because these cells are not susceptible to the bystander effect. Our data suggest that MVMp-based vectors could prove useful as selective vehicles for anticancer gene therapy, particularly for in vivo delivery of cytotoxic effector genes into tumor cells.

The E2F transcription factors: key regulators of cell proliferation

Ever since its discovery, the RB-1 gene and the corresponding protein, pRB, have been a focal point of cancer research. The isolation of E2F transcription factors provided the key to our current understanding of RB-1 function in the regulation of the cell cycle and in tumor suppression. It is becoming more and more evident that the regulatory circuits governing the cell cycle are very complex and highly interlinked. Certain aspects of RB-1 function, for instance its role in differentiation, cannot be easily explained by the current models of pRB-E2F interaction. One reason is that pRB has targets different from E2F, molecules like MyoD for instance. Another reason may be that we have not completely understood the full complexity of E2F function, itself. In this review, we will try to illuminate the role of E2F in pRB- and p53-mediated tumor suppression pathways with particular emphasis on the aspect of E2F-mediated transcriptional regulation. We conclude that E2F can mediate transcriptional activation as well as transcriptional repression of E2F target genes. The net effect of E2F on the transcriptional activity of a particular gene may be the result of as yet poorly understood protein-protein interactions of E2F with other components of the transcriptional machinery, as well as it may reflect the readout of the different ways of regulating E2F activity, itself. We will discuss the relevance of a thorough understanding of E2F function for cancer therapy.

Parvovirus H-1-induced cell death: influence of intracellular NAD consumption on the regulation of necrosis and apoptosis

The autonomous parvovirus H-1 exerts tumor-suppressive effects in living organisms and has been shown to specifically interfere with the survival of transformed cells in culture. The mechanism(s) by which H-1 virus induces death of transformed cells is not yet well understood. It has recently been reported that H-1 virus induces apoptotic cell death in the human monocytic U937 cell line, as assessed by biochemical and morphological changes of infected cells (Rayet, B., Lopez-Guerrero, J.-A., Rommelaere, J., Dinsart, C., 1998. Induction of programmed cell death by parvovirus H-1 in U937 cells: connection with the TNFalpha signalling pathway. J. Virol. 72, 8893-8903). Here we show that parvovirus H-1 infection induced early biochemical changes pointing to apoptotic events also in the transformed human keratinocyte cell line, HeLa, and the transformed rat fibroblast cell line, P1. Morphologic changes, however, and in particular the early breakdown of plasma membrane integrity, suggested that apoptosis did not go to completion, leading to necrotic cell death as the major result of parvovirus infection of HeLa and P1 cells. Parvovirus infection of these, and to a significantly lesser extent of U937 cells, was accompanied by rapid depletion of intracellular NAD stores. Inhibition of NAD-consuming enzymes interfered with parvovirus-induced NAD depletion and increased the proportion of H-1 virus-infected cells displaying apoptotic features of cell death. In contrast, a similar prevention of NAD depletion through stimulation of NAD production had little influence on the cell death pathway, suggesting that NAD-consuming enzymes may promote necrosis in a direct way rather than through inducing the overall drop of intracellular NAD.

Atypical nucleoprotein complexes mediate CRE-dependent regulation of the early promoter of minute virus of mice

The P4 promoter of the parvovirus minute virus of mice (MVMp) directs transcription of the genes encoding non-structural proteins. We have previously shown that functional upstream CRE elements contribute to both the ras oncogene-dependent activation of promoter P4 and its down-modulation by known activators of cyclic AMP-dependent protein kinase A (PKA). In the present work, the nucleoprotein complexes formed with the P4 CRE elements were characterized with regard to their polypeptide constituents and the nucleotides taking part in the interaction. Atypical interactions, both at the protein-protein and protein-DNA level, were observed, which may be a reflection of the divergence of the parvoviral CREs from the usual consensus. The CRE-mediated regulation of promoter P4 by PKA and Ras is discussed in light of these findings.

Two new members of the emerging KDWK family of combinatorial transcription modulators bind as a heterodimer to flexibly spaced PuCGPy half-sites

Initially recognized as a HeLa factor essential for parvovirus DNA replication, parvovirus initiation factor (PIF) is a site-specific DNA-binding complex consisting of p96 and p79 subunits. We have cloned and sequenced the human cDNAs encoding each subunit and characterized their products expressed from recombinant baculoviruses. The p96 and p79 polypeptides have 40% amino acid identity, focused particularly within a 94-residue region containing the sequence KDWK. This motif, first described for the Drosophila homeobox activator DEAF-1, identifies an emerging group of metazoan transcriptional modulators. During viral replication, PIF critically regulates the viral nickase, but in the host cell it probably modulates transcription, since each subunit is active in promoter activation assays and the complex binds to previously described regulatory elements in the tyrosine aminotransferase and transferrin receptor promoters. Within its recognition site, PIF binds coordinately to two copies of the tetranucleotide PuCGPy, which, remarkably, can be spaced from 1 to 15 nucleotides apart, a novel flexibility that we suggest may be characteristic of the KDWK family. Such tetranucleotides are common in promoter regions, particularly in activating transcription factor/cyclic AMP response element-binding protein (ATF/CREB) and E-box motifs, suggesting that PIF may modulate the transcription of many genes.

Regulation of E2F: a family of transcription factors involved in proliferation control

Members of the E2F family of transcription factors are key participants in orchestration of the cell cycle, cell growth arrest and apoptosis. Therefore, an understanding of the regulation of E2F activity is essential for an understanding of the control of cellular proliferation. E2F activity is regulated by the retinoblastoma family of tumor suppressors and by multiple other mechanisms. This review will describe our current knowledge of these mechanisms which together constitute a highly complex network by which the cell cycle and cellular proliferation can be controlled.

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.

Activation of promoter P4 of the autonomous parvovirus minute virus of mice at early S phase is required for productive infection

Autonomous parvoviruses are tightly dependent on host cell factors for various steps of their life cycle. In particular, DNA replication and gene expression of the prototype strain of the minute virus of mice (MVMp) are closely linked to the onset of host cell DNA replication, pointing to the involvement of an S-phase-specific cellular factor(s) in parvovirus multiplication. The viral nonstructural protein NS-1 is absolutely required for parvovirus DNA replication and is able to transcriptionally regulate parvoviral and heterologous promoters. We previously showed that the promoter P4, which directs the transcription unit encoding the NS proteins, is activated at the onset of S phase. This activation is dependent on an E2F motif in the proximal region of promoter P4. An infectious MVM DNA clone was mutated in the E2F motif of P4. The wild type and the E2F mutant derivative were tested for their ability to produce progeny viruses after transfection of permissive cells. In the context of the whole MVMp genome, the E2F mutation abolished P4 induction in S phase and inactivated the infectious molecular clone, which failed to become amplified and generate progeny particles. The virus could be rescued when NS proteins were supplied in trans, showing that P4 hyperactivity in S is needed to reach a level of NS-1 expression that is sufficient to drive the viral replication cycle. These data show that E2F-mediated P4 activation at the early S phase is a limiting factor for parvovirus production. The primary barrier to parvovirus gene expression in G1 is thought to be promoter formation rather than activation, due to the poor conversion of the parental single-strand genome to a duplex form. The S dependence of P4 activation may therefore be a sign of the virus adaptation to life in the S-phase host cell. If the conversion block in G1 were to be leaky, the S induction of promoter P4 could be envisioned as a safeguard against the production of toxic NS proteins until cells reach the S phase and provide the full machinery for parvovirus replication.

p27 is involved in N-cadherin-mediated contact inhibition of cell growth and S-phase entry

In this study the direct involvement of cadherins in adhesion-mediated growth inhibition was investigated. It is shown here that overexpression of N-cadherin in CHO cells significantly suppresses their growth rate. Interaction of these cells and two additional fibroblastic lines with synthetic beads coated with N-cadherin ligands (recombinant N-cadherin ectodomain or specific antibodies) leads to growth arrest at the G1 phase of the cell cycle. The cadherin-reactive beads inhibit the entry into S phase and the reduction in the levels of cyclin-dependent kinase (cdk) inhibitors p21 and p27, following serum-stimulation of starved cells. In exponentially growing cells these beads induce G1 arrest accompanied by elevation in p27 only. We propose that cadherin-mediated signaling is involved in contact inhibition of growth by inducing cell cycle arrest at the G1 phase and elevation of p27 levels.

The cyclin-dependent kinase inhibitor p27(Kip1) induces N-terminal proteolytic cleavage of cyclin A

Progression through the cell cycle is regulated in part by the sequential activation and inactivation of cyclin-dependent kinases (CDKs). Many signals arrest the cell cycle through inhibition of CDKs by CDK inhibitors (CKIs). p27(Kip1) (p27) was first identified as a CKI that binds and inhibits cyclin A/CDK2 and cyclin E/CDK2 complexes in G1. Here we report that p27 has an additional property, the ability to induce a proteolytic activity that cleaves cyclin A, yielding a truncated cyclin A lacking the mitotic destruction box. Other CKIs (p15(Ink4b), p16(Ink4a), p21(Cip1), and p57(Kip2)) do not induce cleavage of cyclin A; other cyclins (cyclin B, D1, and E) are not cleaved by the p27-induced protease activity. The C-terminal half of p27, which is dispensable for its kinase inhibitory activity, is required to induce cleavage. Mechanistically, p27 does not appear to cause cleavage through direct interaction with cyclin/CDK complexes. Instead, it activates a latent protease that, once activated, does not require the continuing presence of p27. Mutation of cyclin A at R70 or R71, residues at or very close to the cleavage site, blocks cleavage. Noncleavable mutants are still recognized by the anaphase-promoting complex/cyclosome pathway responsible for ubiquitin-dependent proteolysis of mitotic cyclins, indicating that the p27-induced cleavage of cyclin A is part of a separate pathway. We refer to this protease as Tsap (pTwenty-seven- activated protease).

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.