Transcriptional regulation of early-response genes during polyomavirus infection

Toxoplasma and Eimeria co-opt the host cFos expression for intracellular development in mammalian cells

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Gene expression profiles differ significantly between Toxoplasma and Eimeria-infected host cells.

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Several distinct and shared host-signaling cascades are regulated by coccidian parasites.

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cFos is one of the few host transcripts mutually regulated during infection by both pathogens.

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Host cFos is required for optimal in vitro development of E. falciformis and T. gondii.

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Transcriptomics of parasitized wild-type and cFos^-/- host cells reveals a perturbation of cFos network.

Successful asexual reproduction of intracellular pathogens depends on their potential to exploit host resources and subvert antimicrobial defense. In this work, we deployed two prevalent apicomplexan parasites of mammalian cells, namely Toxoplasma gondii and Eimeria falciformis, to identify potential host determinants of infection. Expression analyses of the young adult mouse colonic (YAMC) epithelial cells upon infection by either parasite showed regulation of several distinct transcripts, indicating that these two pathogens program their intracellular niches in a tailored manner. Conversely, parasitized mouse embryonic fibroblasts (MEFs) displayed a divergent transcriptome compared to corresponding YAMC epithelial cells, suggesting that individual host cells mount a fairly discrete response when encountering a particular pathogen. Among several host transcripts similarly altered by T. gondii and E. falciformis, we identified cFos, a master transcription factor, that was consistently induced throughout the infection. Indeed, asexual growth of both parasites was strongly impaired in MEF host cells lacking cFos expression. Last but not the least, our differential transcriptomics of the infected MEFs (parental and cFos^-/- mutant) and YAMC epithelial cells disclosed a cFos-centered network, underlying signal cascades, as well as a repertoire of nucleotides- and ion-binding proteins, which presumably act in consort to acclimatize the mammalian cell and thereby facilitate the parasite development.

SV40 Polyomavirus Activates the Ras-MAPK Signaling Pathway for Vacuolization, Cell Death, and Virus Release

Polyomaviruses are a family of small, non-enveloped DNA viruses that can cause severe disease in immunosuppressed individuals. Studies with SV40, a well-studied model polyomavirus, have revealed the role of host proteins in polyomavirus entry and trafficking to the nucleus, in viral transcription and DNA replication, and in cell transformation. In contrast, little is known about host factors or cellular signaling pathways involved in the late steps of productive infection leading to release of progeny polyomaviruses. We previously showed that cytoplasmic vacuolization, a characteristic late cytopathic effect of SV40 infection, depends on the specific interaction between the major viral capsid protein VP1 and its cell surface ganglioside receptor GM1. Here, we show that, late during infection, SV40 activates a signaling cascade in permissive monkey CV-1 cells involving Ras, Rac1, MKK4, and JNK to stimulate SV40-specific cytoplasmic vacuolization and subsequent cell lysis and virus release. Inhibition of individual components of this signaling pathway inhibits vacuolization, lysis, and virus release, even though high-level intracellular virus replication occurs. Identification of this pathway for SV40-induced vacuolization and virus release provides new insights into the late steps of non-enveloped virus infection.

Effect of the Large and Small T-Antigens of Human Polyomaviruses on Signaling Pathways

Viruses are intracellular parasites that require a permissive host cell to express the viral genome and to produce new progeny virus particles. However, not all viral infections are productive and some viruses can induce carcinogenesis. Irrespective of the type of infection (productive or neoplastic), viruses hijack the host cell machinery to permit optimal viral replication or to transform the infected cell into a tumor cell. One mechanism viruses employ to reprogram the host cell is through interference with signaling pathways. Polyomaviruses are naked, double-stranded DNA viruses whose genome encodes the regulatory proteins large T-antigen and small t-antigen, and structural proteins that form the capsid. The large T-antigens and small t-antigens can interfere with several host signaling pathways. In this case, we review the interplay between the large T-antigens and small t-antigens with host signaling pathways and the biological consequences of these interactions.

Murine Polyomavirus Cell Surface Receptors Activate Distinct Signaling Pathways Required for Infection

Virus binding to the cell surface triggers an array of host responses, including activation of specific signaling pathways that facilitate steps in virus entry. Using mouse polyomavirus (MuPyV), we identified host signaling pathways activated upon virus binding to mouse embryonic fibroblasts (MEFs). Pathways activated by MuPyV included the phosphatidylinositol 3-kinase (PI3K), FAK/SRC, and mitogen-activated protein kinase (MAPK) pathways. Gangliosides and α4-integrin are required receptors for MuPyV infection. MuPyV binding to both gangliosides and the α4-integrin receptors was required for activation of the PI3K pathway; however, either receptor interaction alone was sufficient for activation of the MAPK pathway. Using small-molecule inhibitors, we confirmed that the PI3K and FAK/SRC pathways were required for MuPyV infection, while the MAPK pathway was dispensable. Mechanistically, the PI3K pathway was required for MuPyV endocytosis, while the FAK/SRC pathway enabled trafficking of MuPyV along microtubules. Thus, MuPyV interactions with specific cell surface receptors facilitate activation of signaling pathways required for virus entry and trafficking. Understanding how different viruses manipulate cell signaling pathways through interactions with host receptors could lead to the identification of new therapeutic targets for viral infection.

Virus binding to cell surface receptors initiates outside-in signaling that leads to virus endocytosis and subsequent virus trafficking. How different viruses manipulate cell signaling through interactions with host receptors remains unclear, and elucidation of the specific receptors and signaling pathways required for virus infection may lead to new therapeutic targets. In this study, we determined that gangliosides and α4-integrin mediate mouse polyomavirus (MuPyV) activation of host signaling pathways. Of these pathways, the PI3K and FAK/SRC pathways were required for MuPyV infection. Both the PI3K and FAK/SRC pathways have been implicated in human diseases, such as heart disease and cancer, and inhibitors directed against these pathways are currently being investigated as therapies. It is possible that these pathways play a role in human PyV infections and could be targeted to inhibit PyV infection in immunosuppressed patients.

Mechanisms of viral entry: sneaking in the front door

Recent developments in methods to study virus internalisation are providing clearer insights into mechanisms used by viruses to enter host cells. The use of dominant negative constructs, specific inhibitory drugs and RNAi to selectively prevent entry through particular pathways has provided evidence for the clathrin-mediated entry of hepatitis C virus (HCV) as well as the caveolar entry of Simian Virus 40. Moreover, the ability to image and track fluorescent-labelled virus particles in real-time has begun to challenge the classical plasma membrane entry mechanisms described for poliovirus and human immunodeficiency virus. This review will cover both well-documented entry mechanisms as well as more recent discoveries in the entry pathways of enveloped and non-enveloped viruses. This will include viruses which enter the cytosol directly at the plasma membrane and those which enter via endocytosis and traversal of internal membrane barrier(s). Recent developments in imaging and inhibition of entry pathways have provided insights into the ill-defined entry mechanism of HCV, bringing it to the forefront of viral entry research. Finally, as high-affinity receptors often define viral internalisation pathways, and tropism in vivo, host membrane proteins to which viral particles specifically bind will be discussed throughout.

Independent contributions of polyomavirus middle T and small T to the regulation of early and late gene expression and DNA replication

We previously showed that murine polyomavirus mutants that lack both middle T (MT) and small T (ST) functions have a severe pleiotropic defect in early and late viral gene expression as well as genome amplification. The respective contribution of MT and ST to this phenotype was unclear. This work separates the roles of MT and ST in both permissive mouse cells and nonpermissive rat cells. It demonstrates for the first time a role for both proteins. To gain insight into the signaling pathways that might be required, we focused on MT and its mutants. The results show that each of the major MT signaling connections, Shc, phosphatidylinositol 3'-kinase, and phospholipase C gamma1, could contribute in an additive way. Unexpectedly, a mutant lacking all these connections because the three major tyrosines had been converted to phenylalanine retained some activity. A mutant in which all six MT C-terminal tyrosines had been mutated was inactive. This suggests a novel signaling pathway for MT that uses the minor tyrosines. What is common to ST and the individual MT signaling pathways is the ability to signal to the polyomavirus enhancer, in particular to the crucial AP-1 and PEA3/ets binding sites. This connection explains the pleiotropy of MT and ST effects on transcription and DNA replication.

Role of AP-1 in ethanol-induced N-methyl-d-aspartate receptor 2B subunit gene up-regulation in mouse cortical neurons

Activator protein 1 (AP-1) has been reported to regulate the gene expression in a wide variety of cellular processes in response to stimuli. In this study, we investigated the DNA-protein binding activities and promoter activity in the N-methyl-D-aspartate R2B (NR2B) gene AP-1 site in normal and ethanol-treated cultured neurons. The identity of the AP-1 site as the functional binding factor is suggested by the specific binding of nuclear extract derived from cultured cortical neurons to the labeled probes and the specific antibody-induced supershift. Mutations in the core sequence resulted in a significantly reduced promoter activity and the ability to compete for the binding. Moreover, treatment of the cultured neuron with 75 mm ethanol for 5 days caused a significant increase in the AP-1 binding activity and promoter activity. The AP-1 DNA-binding complex in control and ethanol-treated nuclear extract was composed of c-Fos, FosB, c-Jun, JunD, and phosphorylated CREB (p-CREB). Western blot analysis showed that p-CREB and FosB significantly increased, whereas c-Jun decreased. The DNA affinity precipitation assay indicated that FosB, p-CREB, and c-Jun increased in the AP-1 complex following ethanol treatment. These results suggest that AP-1 is an active regulator of the NR2B transcription and ethanol-induced changes may result at multiple levels in the regulation including AP-1 proteins expression, CREB phosphorylation and perhaps reorganization of dimmers.

A role of the TATA box and the general co-activator hTAF(II)130/135 in promoter-specific trans-activation by simian virus 40 small t antigen

The small t antigen (st-ag) of simian virus 40 can exert pleiotropic effects on biological processes such as DNA replication, cell cycle progression and gene expression. One possible mode of achieving these effects is through stimulation of NFkappaB-responsive genes encoding growth factors, cytokines, transcription factors and cell cycle regulatory proteins. Indeed, a previous study has shown that st-ag enhanced NFkappaB-mediated transcription. This study demonstrates that promoters possessing a consensus TATA box (i.e. TATAAAAG) in the context of either NFkappaB- or Sp1-binding sites are trans-activated by st-ag. Overexpressing the general transcription factor hTAF(II)130/135, but not hTAF(II)28 or hTAF(II)80, stimulated the activity of promoters in a consensus TATA box-dependent mode. Converting the consensus TATA motif into a non-consensus TATA box strongly impaired activation by st-ag and hTAF(II)130/135. Conversely, mutating a non-consensus TATA motif into the consensus TATA box rendered the mutated promoter inducible by st-ag and hTAF(II)130/135. Mutation of the TATA box had no effect on TNFalpha- or RelA/p65-mediated induction of NFkappaB-responsive promoters, indicating a specific st-ag effect on hTAF(II)130/135. St-ag stimulated the intrinsic transcriptional activity of hTAF(II)130/135. Substitutions in the conserved HPDKGG motif in the N-terminal region or a mutation that impaired the interaction with protein phosphatase 2A abrogated the ability of st-ag to activate hTAF(II)130/135-mediated transcription. These results indicate that trans-activation of promoters by st-ag may depend on a consensus TATA motif and suggest that such promoters recruit the general transcription factor hTAF(II)130/135.

Members of the AP-1 family, c-Jun and c-Fos, functionally interact with JC virus early regulatory protein large T antigen

The activating protein 1 (AP-1) family of regulatory proteins is characterized as immediate-early inducible transcription factors which were shown to be activated by a variety of stress-related stimuli and to be involved in numerous biological processes, including cellular and viral gene expression, cell proliferation, differentiation, and tumorigenesis. We have recently demonstrated the involvement of the AP-1 family members c-Jun and c-Fos in transcriptional regulation of the human polyomavirus, JC virus (JCV), genome. Here, we further examined their role in JCV gene regulation and replication through their physical and functional interaction with JCV early regulatory protein large T antigen (T-Ag). Transfection and replication studies indicated that c-Jun and c-Fos can significantly diminish T-Ag-mediated JCV gene transcription and replication. Affinity chromatography and coimmunoprecipitation assays demonstrated that c-Jun and T-Ag physically interact with each other. Results from band shift assays showed that the binding efficiency of c-Jun to the AP-1 site was reduced in the presence of T-Ag. In addition, we have mapped, through the use of a series of deletion mutants, the regions of these proteins which are important for their interaction. While the c-Jun interaction domain of T-Ag is localized to the middle portion of the protein, the T-Ag interacting domain of c-Jun maps to its basic-DNA binding region. Results of transient-transfection assays with various c-Jun mutants and T-Ag expression constructs further confirm the specificity of the functional interaction between c-Jun and T-Ag. Taken together, these data demonstrate that immediate-early inducible transcription factors c-Jun and c-Fos physically and functionally interact with JCV major early regulatory protein large T-Ag and that this interaction modulates JCV transcription and replication in glial cells.

Regulation of human polyomavirus JC virus gene transcription by AP-1 in glial cells

The activating transcription factor 1 (AP-1) family of proteins consists of a large number of inducible factors that are implicated in many biological processes, including cellular and viral gene expression, cell proliferation, differentiation, and tumorigenesis. Here, we investigated the role of the AP-1 family members c-Jun and c-Fos in transcriptional regulation of the JC virus (JCV) promoter in glial cells. DNA binding studies demonstrated the specific association of c-Jun with its DNA sequences corresponding to the AP-1 site within the JCV promoter. Functional analysis of the promoter showed that ectopic expression of c-Jun and c-Fos results in an additive activation of the JCV early and late promoters. Further functional assays indicated that the JCV AP-1 binding site is sufficient to confer responsiveness to both c-Jun/c-Fos- and UV-induced activation when transposed to a heterologous promoter. Analysis of c-Jun expression during the viral infection cycle by Western blotting revealed that c-Jun is posttranslationally modified by phosphorylation and its protein level is substantially increased at the late phases of infection cycle. Altogether, our findings indicate that AP-1 family members may play a role in the pathogenesis of JCV-induced disease in the human brain by modulating JCV gene transcription.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

A mutation in the DE loop of the VP1 protein that prevents polyomavirus transcription and replication

Natural mutants of the DE loop of the Polyomavirus (Py) major coat protein VP1 have been previously shown to display an altered host specificity (L. Ricci, R. Maione, C. Passananti, A. Felsani, and P. Amati, 1992, J. Virol. 66, 7153-7158). To better understand the role of this outfacing loop of the VP1 protein in Py infectivity, we constructed and characterized a Py mutant (Py M17) harboring a deletion of 7 AA within the tip of the DE loop. The mutant virions obtained after DNA transfection were unable to replicate and initiate early transcription in fibroblast cells. Complementation experiments performed to rescue the deficient M17 replication by means of wt functions revealed the cis-dominance of the mutation. In situ cell fractionation experiments demonstrated that the Py mutant, like the Py wt, enters the cells, reaches the nucleus and that both the viral DNA and VP1 protein are found tightly bound to the nuclear matrix. These data suggest that the VP1 protein, associated to the viral DNA, conditions early viral gene expression and that the DE loop of the protein must be involved in this process.

How do animal DNA viruses get to the nucleus?

Genome and pre-genome replication in all animal DNA viruses except poxviruses occurs in the cell nucleus (Table 1). In order to reproduce, an infecting virion enters the cell and traverses through the cytoplasm toward the nucleus. Using the cell's own nuclear import machinery, the viral genome then enters the nucleus through the nuclear pore complex. Targeting of the infecting virion or viral genome to the multiplication site is therefore an essential process in productive viral infection as well as in latent infection and transformation. Yet little is known about how infecting genomes of animal DNA viruses reach the nucleus in order to reproduce. Moreover, this nuclear locus for viral multiplication is remarkable in that the sizes and composition of the infectious particles vary enormously. In this article, we discuss virion structure, life cycle to reproduce infectious particles, viral protein's nuclear import signal, and viral genome nuclear targeting.

Signaling from polyomavirus middle T and small T defines different roles for protein phosphatase 2A

Polyomavirus causes a broad spectrum of tumors as the result of the action of its early proteins. This work compares signaling from middle T antigen (MT), the major transforming protein, to that from small T antigen (ST). The abilities of MT mutants to promote cell cycle progression in serum-starved NIH 3T3 cells were compared. Transformation-defective mutants lacking association with SHC or with phosphatidylinositol 3-kinase (PI3-K) retained the ability to induce DNA synthesis as measured by bromodeoxyuridine incorporation. Only when both interactions were lost in the Y250F/Y315F double mutant was MT inactive. ST promoted cell cycle progression in a manner dependent on its binding of protein phosphatase 2A (PP2A). Since the Y250F/Y315F MT mutant was wild type for PP2A binding yet unable to promote cell cycle progression, while ST was capable of promoting cell cycle progression, these experiments revealed a functional difference in MT and ST signaling via PP2A. Assays testing the abilities of MT and ST to induce the c-fos promoter and to activate c-jun kinase led to the same conclusion. ST, but not Y250F/Y315F MT, was able to activate the c-fos promoter through its interaction with PP2A. In contrast, MT, but not ST, was able to activate c-jun kinase by virtue of its interaction with PP2A.

In vitro phosphorylation of the polyomavirus major capsid protein VP1 on serine 66 by casein kinase II

Phosphorylation of the polyomavirus major capsid protein VP1 plays a role in virus assembly and may function in virus-cell recognition. Previous mapping of the in vivo phosphorylation sites on VP1 identified phosphorylation of threonine residues Thr-63 and Thr-156 (Li, M., and Garcea, R. L. (1994) J. Virol. 68, 320-327). Phosphoserine was detected in a tryptic phosphopeptide encompassing residues 58-78. Because of consensus casein kinase II (CK II) sites in this peptide, we examined the in vitro phosphorylation of the purified recombinant VP1 protein by CK II. CK II phosphorylated VP1 on serine, and the resulting tryptic phosphopeptide eluted in a 30-31 min high performance liquid chromatography fraction corresponding to residues 58-78. The VP1 tryptic phosphopeptide also co-migrated in two-dimensional peptide analysis with one of the tryptic peptides obtained from VP1 isolated after in vivo 32P labeling of virus-infected cells. A site-directed mutant VP1 protein, Ser-66 to Ala, was phosphorylated poorly by CK II in vitro. As determined by electron microscopy, all of the mutant proteins were isolated in pentameric form similar to the wild-type protein, although the Ala-66 pentamers had a tendency to self-assemble in vitro into tubular as well as capsid-like structures. These findings identify Ser-66 as a site of VP1 phosphorylation in vitro, and suggest that VP1 may serve as a substrate for CK II in vivo.

Host range and cell cycle activation properties of polyomavirus large T-antigen mutants defective in pRB binding

We have examined the growth properties of polyomavirus large T-antigen mutants that are unable to bind pRB, the product of the retinoblastoma tumor suppressor gene. These mutants grow poorly on primary mouse cells yet grow well on NIH 3T3 and other established mouse cell lines. Preinfection of primary baby mouse kidney (BMK) epithelial cells with wild-type simian virus 40 renders these cells permissive to growth of pRB-binding polyomavirus mutants. Conversely, NIH 3T3 cells transfected by and expressing wild-type human pRB become nonpermissive. Primary fibroblasts from mouse embryos that carry a homozygous knockout of the RB gene are permissive, while those from normal littermates are nonpermissive. The host range of polyomavirus pRB-binding mutants is thus determined by expression or lack of expression of functional pRB by the host. These results demonstrate the importance of pRB binding by large T antigen for productive viral infection in primary cells. Failure of pRB-binding mutants to grow well in BMK cells correlates with their failure to induce progression from G0 or G1 through the S phase of the cell cycle. Time course studies show delayed synthesis and lower levels of accumulation of large T antigen, viral DNA, and VP1 in mutant compared with wild-type virus-infected BMK cells. These results support a model in which productive infection by polyomavirus in normal mouse cells is tightly coupled to the induction and progression of the cell cycle.

DNA content distribution of mouse cells following infection with polyoma virus

Infection of primary to tertiary mouse embryo fibroblasts or mouse kidney cells with polyoma virus leads to stimulation of cellular DNA synthesis. When either confluent or growing mouse cells were infected, the monolayer cells were found to accumulate cells with a DNA content of S and G2/M phases of the cell cycle as assayed by flow cytometry. A similar pattern of DNA content was also observed in cells in the supernatant, which are probably cells replicating virus and dying. When compared with control cells, the infected monolayer and supernatant cells exhibited a population (5-27%) with a > G2 DNA content. The increase in DNA content of these > G2 cells was calculated to be an average of 26.7%, which is probably due to viral DNA. Polyoma contrasts with another papovavirus, SV40, which stimulates cells into DNA synthesis, with the majority of cells attaining a > G2/tetraploid DNA content, suggesting that there are differences in polyploidization between these two viruses.

Induction of the DNA-binding activity of c-jun/c-fos heterodimers by the hepatitis B virus transactivator pX

The hepatitis B virus (HBV) X protein (pX) is capable of activating transcription regulated by viral and cellular promoters containing binding sites for different transcription factors, including AP1. In this study we have analyzed the mechanisms of AP1 induction by pX. The hepatitis B virus transactivator was able to activate TRE (12-O-tetradecanoylphorbol-13-acetate response element)-directed transcription in different cell lines, including HepG2, HeLa, CV1, and PLC/PRF/5 cells. pX-induced AP1 activation in HepG2 cells was associated with an increase in the DNA-binding activity of c-Jun/c-Fos heterodimers, which was not dependent either on an increase in the overall amount of c-Fos and c-Jun proteins in the cells or on formation of dimers between pX and the two proteins, thus suggesting the involvement of posttranslational modifications of the transcription factor. The observation that the overexpression of c-Jun and c-Fos in the cells results in a strong augmentation of the effect of pX on TRE-directed transcription is additional evidence indicating the involvement of posttranscriptional modifications of c-Jun/c-Fos heterodimers. The increased AP1 binding observed in the presence of pX was unaffected by the protein kinase C inhibitors calphostin C and sphingosine and by the protein kinase A inhibitor HA1004, while it was almost completely blocked by staurosporine, a potent and nonspecific protein kinase inhibitor, suggesting that protein kinase C- and A-independent phosphorylation events might play a role in the phenomenon. The ability of pX also to increase TRE-directed transcription in cell lines in which AP1-binding activity is not increased (i.e., HeLa, CV1, and PLC/PRF/5 cells) suggests that pX can activate canonical TRE sites by different mechanisms as well.

Induction of the DNA-binding activity of c-jun/c-fos heterodimers by the hepatitis B virus transactivator pX

The hepatitis B virus (HBV) X protein (pX) is capable of activating transcription regulated by viral and cellular promoters containing binding sites for different transcription factors, including AP1. In this study we have analyzed the mechanisms of AP1 induction by pX. The hepatitis B virus transactivator was able to activate TRE (12-O-tetradecanoylphorbol-13-acetate response element)-directed transcription in different cell lines, including HepG2, HeLa, CV1, and PLC/PRF/5 cells. pX-induced AP1 activation in HepG2 cells was associated with an increase in the DNA-binding activity of c-Jun/c-Fos heterodimers, which was not dependent either on an increase in the overall amount of c-Fos and c-Jun proteins in the cells or on formation of dimers between pX and the two proteins, thus suggesting the involvement of posttranslational modifications of the transcription factor. The observation that the overexpression of c-Jun and c-Fos in the cells results in a strong augmentation of the effect of pX on TRE-directed transcription is additional evidence indicating the involvement of posttranscriptional modifications of c-Jun/c-Fos heterodimers. The increased AP1 binding observed in the presence of pX was unaffected by the protein kinase C inhibitors calphostin C and sphingosine and by the protein kinase A inhibitor HA1004, while it was almost completely blocked by staurosporine, a potent and nonspecific protein kinase inhibitor, suggesting that protein kinase C- and A-independent phosphorylation events might play a role in the phenomenon. The ability of pX also to increase TRE-directed transcription in cell lines in which AP1-binding activity is not increased (i.e., HeLa, CV1, and PLC/PRF/5 cells) suggests that pX can activate canonical TRE sites by different mechanisms as well.

Mutation of a cysteine residue in polyomavirus middle T antigen abolishes interactions with protein phosphatase 2A, pp60c-src, and phosphatidylinositol-3 kinase, activation of c-fos expression, and cellular transformation

Polyomavirus middle T antigen (MT) interacts with several cellular proteins involved in cell proliferation. MT forms complexes with protein phosphatase 2A (PP2A), pp60c-src (and the related kinases c-fyn and c-yes), and phosphatidylinositol-3 kinase. We made a single point mutation in MT, changing a conserved cysteine residue at position 120 to tryptophan, and characterized the biochemical and biological properties of the mutant (C120W) protein. The mutant MT protein does not associate with PP2A, pp60c-src, or phosphatidylinositol-3 kinase as judged by coimmunoprecipitation and associated phosphatase or kinase activity. The C120W mutant is defective in activation of c-fos expression and in morphological transformation of NIH 3T3 cells.

Cell cycle control of polyomavirus-induced transformation

The cell cycle dependence of polyomavirus transformation was analyzed in infections of nonpermissive Fischer rat (FR3T3) cells released from G0. A 5- to 100-fold (average, ca. 20-fold) difference in relative frequency of transformation was found for cells infected in the early G1 phase of the cell cycle compared with cells infected in G2. Differences in the relative level of early viral gene expression in those two cell populations were equivalent to those obtained for transformation frequencies. The difference in transformation potential was accounted for only in part by a cell cycle control of viral adsorption (2- to 15-fold effect). Furthermore, in cells infected in the early G1 phase, viral gene expression was induced as a big synchronous burst of large transcripts of variable sizes, delayed till the G1 phase of the cell cycle after that in which infection took place. Thus, the results demonstrate that the abortive infection cycle of G0-released FR3T3 cells is cell cycle regulated at least at two steps: adsorption and another early step, nuclear transport, decapsidation, up to or including the transcription of the viral early genes. The cell cycle regulation of these steps results in a similar regulation of the abortive and stable transformation processes, although it is more pronounced for the latter. A model implicating c-fos and c-jun is proposed.

Mutations in the VP1 coding region of polyomavirus determine differentiating stage specificity

Polyomavirus mutants capable of replicating in undifferentiated murine C2 myoblasts were selected and characterized. These mutants grow normally in 3T6 mouse fibroblast cells, and they do not complement the wild-type virus in coinfection experiments of C2 myoblasts. Of 12 isolates, 10 possess duplications of the regulatory region including the enhancer A domain. On the bases of the regulatory region structure and the presence and length of the enhancer duplication, the mutant viruses could be grouped into three classes. One mutant class (e.g., PyMB3) possesses an enhancer duplication of 91 bp identical to that of a previously characterized polyomavirus mutant, PyNB11/1. We have demonstrated that this enhancer duplication gives rise at its junction to a novel recognition motif for the transcriptional factor NF-1 (M. Caruso, C. Iacobini, C. Passananti, A. Felsani, and P. Amati, EMBO J. 9:947-955, 1990). The regulatory region PyMB3 virus recombined in a wild-type genome context maintains the mutant phenotype. The other two types of mutants, one with a 30-bp enhancer duplication (e.g., PyMB40) and one with a wild-type enhancer structure (e.g., PyMB27), possess two similar but distinct 6-bp deletions in the same region of the VP1 coding gene. In both cases, the ability to replicate in undifferentiated C2 myoblasts is strictly correlated to the mutation in the VP1 coding region.

Induction of c-jun protooncogene expression and transcription factor AP-1 activity by the polyoma virus middle-sized tumor antigen

Polyoma virus middle-sized tumor (PymT) antigen is required for neoplastic cell transformation by polyoma virus. We studied changes in gene expression accompanying expression of PymT in murine fibroblasts. These experiments showed that PymT differentially affects several growth-related genes. c-jun protooncogene expression was highly increased, whereas the expression of two growth arrest-specific genes (gas) was reduced, in cells transformed by PymT. Cotransfection experiments showed that the increase in c-jun expression resulted from elevated activity of the transcription factor AP-1 and was mediated through the phorbol 12-tetradecanoate 13-acetate response element in the c-jun promoter. The degree of c-Jun/AP-1 activation by different PymT mutants correlated with their transforming capability, suggesting that regulation of c-Jun/AP-1 activity may play a role in cell transformation by polyoma virus.

An AP-1 binding site is the predominant cis-acting regulatory element in the 1.2-kilobase early RNA promoter of human cytomegalovirus

To extend our analysis of the regulation of human cytomegalovirus (HCMV) early gene expression, we examined a transcription unit located in the terminal repeats of the long segment of the viral genome. This region encodes a major 1.2-kb RNA which is induced at early times in infection but undergoes its largest increase in abundance after the onset of viral DNA replication. To identify the important cis-acting regulatory elements for this gene, two constructs were prepared for use in transient expression assays. One contained 413 bp of the upstream sequence and 43 bp of the leader sequence fused to the gene for chloramphenicol acetyltransferase (CAT). The second construct included 1,722 bp upstream of the start site of the 1.2-kb RNA, the entire transcribed region with an additional 166-bp insert derived from the CAT gene as an assayable marker, and 2,393 bp downstream of the polyadenylation signal. Both constructs were individually transfected into human fibroblast cells, and the cells were infected with HCMV. RNA specified by the hybrid construct was initiated at the correct position and accumulated with the same kinetics as the authentic viral transcript at early times in the infection but did not undergo the increase in abundance at late at late times. By 5'-end-deletion analysis, we determined that the promoter for the 1.2-kb RNA contains a number of cis-acting elements, the most significant of which are the TATA-like sequence CATAAA at -30 and a sequence corresponding to the binding site for the transcription factor AP-1 at -75. Using extracts prepared from HeLa cells as well as from infected and uninfected fibroblasts in gel retardation assays, we obtained evidence for the specific interaction of a cellular factor(s) with the AP-1 binding site. The pattern of binding differed in the HeLa and fibroblast cells but did not change as a function of the HCMV infection. However, the functional importance of the AP-1 binding site and its key role in the regulation of the 1.2-kb RNA was supported by analysis of constructs containing specific point mutations at this site in gel retardation and transient expression assays. Site-specific mutations in the AP-1 consensus sequence, which resulted in the complete loss of binding to cellular factors, eliminated the basal activity and reduced the inducible promoter activity by eightfold.

Polyomavirus large and small T antigens cooperate in induction of the S phase in serum-starved 3T3 mouse fibroblasts

The induction of an S phase in the host cell is a prerequisite for the lytic replication cycle of polyomavirus. This function was attributed to proteins coded for by the early region of the viral DNA, the T antigens. A consideration of the role of the T antigens in the initiation of a mitogenic response of the host cell has to take into account the recent discovery that virus adsorption is sufficient to induce the synthesis of proteins which are known to appear early after quiescent cells are stimulated by the addition of serum, namely fos, jun, and myc (J. Zullo, C.D. Stiles, and R.L. Garcea, Proc. Natl. Acad. Sci. USA 84:1210-1214, 1987; G. M. Glenn and W. Eckhart, J. Virol. 64:2193-2201, 1990). This induction is followed by an initiation of DNA synthesis. It is therefore important to dissociate the effects of the T antigens on the host cell from those of virus adsorption. To do so, we used dexamethasone-regulated versions of the large and small T antigens of polyomavirus stably integrated into the genome of Swiss 3T3 cells to study their function in S-phase induction. When the production of the large or small T antigen in serum-starved 3T3 mouse fibroblasts was activated, only a small fraction of cells was able to leave G0/G1 despite the synthesis of considerable amounts of the respective T antigen. Activation of both T antigens within the same cell, on the other hand, resulted in S-phase induction in a notable percentage of cells, suggesting that the two proteins cooperate in this activity. Polyomavirus T antigens appear to bypass the pathway of growth regulation involving the activation of c-fos. These results are discussed in relation to other known functions of the two virally coded proteins.

PEA1 and PEA3 enhancer elements are primary components of the polyomavirus late transcription initiator element

The circular polyomavirus genome is transcribed from divergent promoter regions. Early mRNAs are initiated from a transcription complex formed at a TATA motif, the site of binding of transcription factor TFIID. Early transcription is promoted at a distance by the viral enhancer, which includes DNA motifs bound by cellular proteins of the PEA1 and PEA3 families of transcription activators. In contrast, the predominant viral late mRNAs are initiated within the viral enhancer, which lacks a TATA motif, near the PEA1 and PEA3 DNA motifs. Here, we demonstrate that these PEA1 and PEA3 binding sites are primary components of an autonomous transcription initiator element (Inr). They cause transcription of most polyomavirus late mRNAs and can direct the transcription of heterologous reporter genes. Alternative roles of these DNA motifs as activators of early mRNA transcription and as an initiator element for late mRNA transcription help explain how polyomavirus gene expression is regulated during lytic growth and provides a model for cellular transcription during development.

Transcriptional activation of cellular oncogenes fos, jun, and myc by human cytomegalovirus

The mechanisms responsible for the human cytomegalovirus (HCMV)-induced increase in cellular oncogene RNAs for c-jun, c-fos, and c-myc in human embryo lung cells (I. Boldogh, S. AbuBakar, and T. Albrecht, Science 247:561-564, 1990) were investigated. Results of transcription assays indicated that the rapid increase in RNA levels for the above-noted oncogenes was controlled at the transcriptional level and was related to enhanced transcription. The maximum rates of transcription for c-jun and c-fos genes occurred at 40 min postinfection, while for the c-myc gene the maximum rate occurred at about 60 min. The magnitude of HCMV-induced activation of these cellular genes was similar to the activation induced by serum. The half-lives of the cellular oncogenes showed similar decay rates after either serum or HCMV activation when measured by dactinomycin chase. The half-life for c-fos or c-jun was about 20 min, and that for c-myc was about 40 min. Furthermore, inhibition of the RNA increase by dactinomycin or by alpha-amanitin suggested that the increase in RNA levels was due to an increase in the transcriptional activity of oncogenes triggered by HCMV.