p53 and RPA are sequestered in viral replication centers in the nuclei of cells infected with human cytomegalovirus

Human herpesvirus 6B inhibits cell proliferation by a p53-independent pathway

BACKGROUND

Various forms of cellular stress can activate the tumour suppressor protein p53, an important regulator of cell cycle arrest, apoptosis, and cellular senescence. Cells infected by human herpesvirus 6B (HHV-6B) accumulate aberrant amounts of p53.

OBJECTIVES

The aim of this study was to investigate the role of p53 accumulation in the HHV-6B-induced cell cycle arrest.

STUDY DESIGN

The role of p53 was studied using the p53 inhibitor pifithrin-a, and cells genetically deficient in functional p53 by homologous recombination.

RESULTS

In response to HHV-6B infection, epithelial cells were arrested in the G1/S phase of the cell cycle concomitant with an aberrant accumulation of p53. However, the known p53-induced mediator of cell cycle arrest, p21, was not upregulated. Approximately 90% of the cells expressed HHV-6B p41, indicative of viral infection. The presence of pifithrin-a, a p53 inhibitor, did not reverse the HHV-6B-induced cell cycle block. In support of this, HHV-6B infection of p53(-/-) cells induced a cell cycle block before S-phase with kinetics similar to or faster than that observed by infection in wt cells.

CONCLUSIONS

HHV-6B infection inhibited host cell proliferation concomitantly with p53 accumulation, but importantly the block in cell cycle occurred by a pathway independent of p53.

ORF73-null murine gammaherpesvirus 68 reveals roles for mLANA and p53 in virus replication

Gammaherpesviruses establish lifelong, latent infections in host lymphocytes, during which a limited subset of viral gene products facilitates maintenance of the viral episome. Among the gamma-2-herpesvirus (rhadinovirus) subfamily, this includes expression of the conserved ORF73-encoded LANA proteins. We previously demonstrated by loss-of-function mutagenesis that the murine gammaherpesvirus 68 (MHV68) ORF73 gene product, mLANA, is required for the establishment of latency following intranasal inoculation of mice (N. J. Moorman, D. O. Willer, and S. H. Speck, J. Virol. 77:10295-10303, 2003). mLANA-deficient viruses also exhibited a defect in acute virus replication in the lungs of infected mice. The latter observation led us to examine the role of mLANA in productive viral replication. We assessed the capacity of mLANA-deficient virus (73.Stop) to replicate in cell culture at low multiplicities of infection (MOIs) and found that 73.Stop growth was impaired in murine fibroblasts but not in Vero cells. A recombinant virus expressing an mLANA-green fluorescent protein (GFP) fusion revealed that mLANA is expressed throughout the virus replication cycle. In addition, 73.Stop infection of murine fibroblasts at high MOIs was substantially more cytotoxic than infection with a genetically repaired marker rescue virus (73.MR), a phenotype that correlated with enhanced kinetics of viral gene expression and increased activation of p53. Notably, augmented cell death, viral gene expression, and p53 induction were independent of viral DNA replication. Expression of a mLANA-GFP fusion protein in fibroblasts correlated with both reduced p53 stabilization and reduced cell death following treatment with p53-inducing agonists. In agreement, accentuated cell death associated with 73.Stop infection was reduced in p53-deficient murine embryonic fibroblasts. Additionally, replication of 73.Stop in p53-deficient cells was restored to levels comparable to those of 73.MR. More remarkably, the absence of p53 led to an overall delay in replication for both 73.Stop and 73.MR viruses, which correlated with delayed viral gene expression, indicating a role for p53 in MHV68 replication. Consistent with these findings, the expression of replication-promoting viral genes was positively influenced by p53 overexpression or treatment with the p53 agonist etoposide. Overall, these data demonstrate the importance of mLANA in MHV68 replication and suggest that LANA proteins limit the induction of cellular stress responses to regulate the viral gene expression cascade and limit host cell injury.

Stabilization of p53 in human cytomegalovirus-initiated cells is associated with sequestration of HDM2 and decreased p53 ubiquitination

Human cytomegalovirus (HCMV) induces serum- or density-arrested human lung (LU) cells to traverse the cell cycle, providing it with a strategy to replicate in post-mitotic cells that are its cellular substrate in vivo. HCMV infection also induces high cellular levels of p53, seemingly in contradiction to the observed cell cycle progression. This study was undertaken to examine the mechanism(s) of the increased p53 abundance. HCMV infection caused a 4-fold increase in p53 that preceded a substantial increase in p53 transcripts by more than 24 h. p53 was stabilized in HCMV-infected cells (from a half-life of less than 30 min to about 8 h) and was less sensitive to proteasome-mediated degradation. Ubiquitination of p53 in mock-infected LU cells was sensitive to inhibition by trans-4-iodo, 4'-boranyl-chalcone, consistent with HDM2-catalyzing ubiquitination of p53. In HCMV-infected cells, ubiquitination of p53 was essentially undetectable. Although HDM2 had a nuclear distribution in mock-infected LU cells, in HCMV-infected cells HDM2 was translocated to the cytoplasm beginning at 12 h and demonstrated decreased cellular abundance thereafter. HDM2 was stabilized in the HCMV-infected cells by MG132, indicating a shift from p53 to HDM2 ubiquitination. p53 demonstrated a predominantly nuclear distribution in HCMV-infected cells through 48 h, resulting in p53 and HDM2 in distinct subcellular compartments. The principal mechanism responsible for increased p53 stabilization was nuclear export and degradation of HDM2. Thus, HCMV uses a shift from p53 to HDM2 ubiquitination and destabilization to obtain protracted high levels of p53, while promoting cell cycle traverse.

Human cytomegalovirus (HCMV) and hearing impairment: infection of fibroblast cells with HCMV induces chromosome breaks at 1q23.3, between loci DFNA7 and DFNA49 -- both involved in dominantly inherited, sensorineural, hearing impairment

Human cytomegalovirus (HCMV) infection is the most common congenital infection in developed countries and is responsible for a substantial fraction of sensorineural hearing impairment (SNHI) in children. The risk of hearing impairment is associated with viral load in urine and blood collected during the first postnatal month. However, although inner ear abnormalities are observed in some children with HCMV-induced SNHI, the exact mechanism whereby congenital HCMV infection causes hearing impairment is unknown. Earlier studies using standard cytogenetic mapping techniques showed that infection of S-phase human fibroblast cells with HCMV resulted in two specific, site-directed, chromosome breaks at band positions 1q21 and 1q42 which include loci involved in dominantly and recessively inherited hearing impairment, respectively. These findings suggested that cells infected with HCMV might provide a reservoir for genetic damage and, in a clinical perspective, a scenario could be envisioned whereby hearing impairment could result from early DNA damage of dividing fetal cells rather than viral replication and cell lysis. In this work we demonstrate, using fine mapping techniques, that HCMV infection in S-phase fibroblast cells induces genetic damage at 1q23.3, within a maximal region of 37 kb, containing five low copy repeat (LCR) elements. The breakpoint is situated between two hearing impairment (HI) loci, DFNA49 and DFNA7, and in close proximity to the MPZ gene previously shown to be involved in autosomal dominant Charcot-Marie-Tooth syndrome (CMT1B) with auditory neuropathy.

Long-term infection and shedding of human cytomegalovirus in T98G glioblastoma cells

Human cytomegalovirus (HCMV) is the leading viral cause of birth defects, affecting primarily the central nervous system (CNS). To further understand this CNS pathology, cells from glioblastoma cell lines T98G and A172, the astrocytic glioblastoma cell line CCF-STTG1 (CCF), and the neuroblastoma cell line SH-SY5Y (SY5Y) were infected with HCMV. CCF and SY5Y cells were fully permissive for infection, while A172 cells were nonpermissive. In T98G cells, the majority of cells showed viral deposition into the nucleus by 6 h postinfection (hpi); however, viral immediate-early gene expression was observed in only approximately 30% of cells in the first 72 h. In viral antigen (Ag)-positive cells, although the development of complete viral replication centers was delayed, fully developed centers formed by 96 hpi. Interestingly, even at very late times postinfection, a mixture of multiple small, bipolar, and large foci was always present. The initial trafficking of input pp65 into the nucleus was also delayed. Titer and infectious-center assays showed a small number of T98G cells shedding virus at very low levels. Surprisingly, both Ag-positive and Ag-negative cells continued to divide; because of this continuous division, we adopted a protocol for passaging the T98G cells every third day to prevent overcrowding. Under this protocol, detectable infectious-virus shedding continued until passage 5 and viral gene expression continued through eight passages. This evidence points to T98G cells as a promising model for long-term infections.

Human cytomegalovirus disrupts both ataxia telangiectasia mutated protein (ATM)- and ATM-Rad3-related kinase-mediated DNA damage responses during lytic infection

Many viruses (herpes simplex virus type 1, polyomavirus, and human immunodeficiency virus type 1) require the activation of ataxia telangiectasia mutated protein (ATM) and/or Mre11 for a fully permissive infection. However, the longer life cycle of human cytomegalovirus (HCMV) may require more specific interactions with the DNA repair machinery to maximize viral replication. A prototypical damage response to the double-stranded ends of the incoming linear viral DNA was not observed in fibroblasts at early times postinfection (p.i.). Apparently, a constant low level of phosphorylated ATM was enough to phosphorylate its downstream targets, p53 and Nbs1. p53 was the only cellular protein observed to relocate at early times, forming foci in infected cell nuclei between 3.5 and 5.5 h p.i. Approximately half of these foci localized with input viral DNA, and all localized with viral UL112/113 prereplication site foci. No other DNA repair proteins localized with the virus or prereplication foci in the first 24 h p.i. When viral replication began in earnest, between 24 and 48 h p.i., there were large increases in steady-state levels and phosphorylation of many proteins involved in the damage response, presumably triggered by ATM-Rad3-related kinase activation. However, a sieving process occurred in which only certain proteins were specifically sequestered into viral replication centers and others were particularly excluded. In contrast to other viruses, activation of a damage response is neither necessary nor detrimental to infection, as neither ATM nor Mre11 was required for full virus replication and production. Thus, by preventing simultaneous relocalization of all the necessary repair components to the replication centers, HCMV subverts full activation and completion of both double-stranded break and S-phase checkpoints that should arrest all replication within the cell and likely lead to apoptosis.

Repression of HMGA2 gene expression by human cytomegalovirus involves the IE2 86-kilodalton protein and is necessary for efficient viral replication and inhibition of cyclin A transcription

Human cytomegalovirus (HCMV) infection results in dysregulation of several cell cycle genes, including inhibition of cyclin A transcription. In this work, we examine the effect of the HCMV infection on expression of the high-mobility group A2 (HMGA2) gene, which encodes an architectural transcription factor that is involved in cyclin A promoter activation. We find that expression of HMGA2 RNA is repressed in infected cells. To determine whether repression of HMGA2 is directly related to the inhibition of cyclin A expression and impacts on the progression of the infection, we constructed an HCMV recombinant that expressed HMGA2. In cells infected with the recombinant virus, cyclin A mRNA and protein are induced, and there is a significant delay in viral early gene expression and DNA replication. To determine the mechanism of HMGA2 repression, we used recombinant viruses that expressed either no IE1 72-kDa protein (CR208) or greatly reduced levels of IE2 86-kDa (IE2 86) protein (IE2 86DeltaSX-EGFP). At a high multiplicity of infection, the IE1 deletion mutant is comparable to the wild type with respect to inhibition of HMGA2. In contrast, the IE2 86DeltaSX-EGFP mutant does not significantly repress HMGA2 expression, suggesting that IE2 86 is involved in the regulation of this gene. Cyclin A expression is also induced in cells infected with this mutant virus. Since HMGA2 is important for cell proliferation and differentiation, particularly during embryogenesis, it is possible that the repression of HMGA2 expression during fetal development could contribute to the specific birth defects in HCMV-infected neonates.

Potential role for p53 in the permissive life cycle of human cytomegalovirus

Infection of primary fibroblasts with human cytomegalovirus (HCMV) causes a rapid stabilization of the cellular protein p53. p53 is a major effector of the cellular damage response, and activation of this transcription factor can lead either to cell cycle arrest or to apoptosis. Viruses employ many tactics to avoid p53-mediated effects. One method HCMV uses to counteract p53 is sequestration into its viral replication centers. In order to determine whether or not HCMV benefits from this sequestration, we infected a p53(-/-) fibroblast line. We find that although these cells are permissive for viral infection, several parameters are substantially altered compared to wild-type (wt) fibroblasts. p53(-/-) cells show delayed and decreased accumulation of infectious viral particles compared to control fibroblasts, with the largest difference of 100-fold at 72 h post infection (p.i.) and peak titers decreased by approximately 10- to 20-fold at 144 h p.i. Viral DNA accumulation is also delayed and somewhat decreased in p53(-/-) cells; however, on average, levels of DNA are not more than fivefold lower than wt at any time p.i. and thus cannot account entirely for the observed differences in titers. In addition, there are delays in the expression of several key viral proteins, including the early replication protein UL44 and some of the late structural proteins, pp28 (UL99) and MCP (UL86). UL44 localization also indicates delayed formation and maturation of the replication centers throughout the course of infection. Localization of the major tegument protein pp65 (UL83) is also altered in these p53(-/-) cells. Partial reconstitution of the p53(-/-) cells with a wt copy of p53 returns all parameters toward wt, while reconstitution with mutant p53 does not. Taken together, our data suggest that wt p53 enhances the ability of HCMV to replicate and produce high concentrations of infectious virions in permissive cells.

Mechanisms for human cytomegalovirus-induced cytoplasmic p53 sequestration in endothelial cells

Human cytomegalovirus (HCMV) infection results in endothelial dysfunction, typically known as dysregulated apoptosis, and aberrant expression and sub-cellular localization of p53, a tumor suppressor that accumulates at the late stage of infection. In this study, we examined three hypotheses that could be responsible for HCMV-induced cytoplasmic p53 accumulation at the later stage of infection: hyperactive nuclear export, cytoplasmic p53 tethering and delayed p53 degradation. Leptomycin B treatment, a nuclear export inhibitor, was unable to reduce cytoplasmic p53, thereby eliminating the hyperactive nuclear export mechanism. The findings that nascent p53 still entered nuclei after the nuclear export inhibition indicated that cytoplasmic tethering may play a minor role. Cytoplasmic p53 was still observed after the translation activities were blocked by cycloheximide. There was more than an eight-fold increase in the cytoplasmic p53 half-life with abnormal p53 ubiquitination. Taken together, these results suggest that delayed degradation could be responsible for the cytoplasmic p53 accumulation. The general slow-down of the proteasomal activity and the dysregulated p53 ubiquitination process at the later stage of infection could contribute to the reduced cytoplasmic p53 degradation and might be relevant to dysregulated endothelial apoptosis. The HCMV-induced changes in p53 dynamics could contribute to endothelial dysfunction.

An intact sequence-specific DNA-binding domain is required for human cytomegalovirus-mediated sequestration of p53 and may promote in vivo binding to the viral genome during infection

The p53 protein is stabilized during infection of primary human fibroblasts with human cytomegalovirus (HCMV). However, the p53 in HCMV-infected cells is unable to activate its downstream targets. HCMV accomplishes this inactivation, at least in part, by sequestering p53 into viral replication centers within the cell's nucleus soon after they are established. In order to better understand the interplay between HCMV and p53 and the mechanism of sequestration, we constructed a panel of mutant p53-GFP fusion constructs for use in transfection/infection experiments. These mutants affected several post-translational modification sites and several sites within the central sequence-specific DNA-binding domain of the protein. Two categories of p53 sequestration were observed when the mutant constructs were transfected into primary fibroblasts and then infected at either high or low multiplicity. The first category, including all of the post-translational modification mutants, showed sequestration comparable to a wild-type (wt) control, while the second category, mutants affecting the DNA-binding core, were not specifically sequestered above control GFP levels. This suggested that the DNA-binding ability of the protein was required for sequestration. When the HCMV genome was analyzed for p53 consensus binding sites, 21 matches were found, which localized either to the promoters or the coding regions of viral proteins involved in DNA replication and processing as well as structural proteins. An analysis of in vivo binding to these identified sites via chromatin immunoprecipitation assays revealed differential binding to several of the sites over the course of infection.

Mitochondrial cell death suppressors carried by human and murine cytomegalovirus confer resistance to proteasome inhibitor-induced apoptosis

Human cytomegalovirus carries a mitochondria-localized inhibitor of apoptosis (vMIA) that is conserved in primate cytomegaloviruses. We find that inactivating mutations within UL37x1, which encodes vMIA, do not substantially affect replication in TownevarATCC (Towne-BAC), a virus that carries a functional copy of the betaherpesvirus-conserved viral inhibitor of caspase 8 activation, the UL36 gene product. In Towne-BAC infection, vMIA reduces susceptibility of infected cells to intrinsic death induced by proteasome inhibition. vMIA is sufficient to confer resistance to proteasome inhibition when expressed independent of viral infection. Murine cytomegalovirus m38.5, whose position in the viral genome is analogous to UL37x1, exhibits mitochondrial association and functions in much the same manner as vMIA in inhibiting intrinsic cell death. This work suggests a common role for vMIA in rodent and primate cytomegaloviruses, modulating the threshold of virus-infected cells to intrinsic cell death.

Human herpesvirus 6 open reading frame U14 protein and cellular p53 interact with each other and are contained in the virion

A mass spectroscopic analysis of proteins from human herpesvirus 6 (HHV-6)-infected cells showed that the HHV-6 U14 protein coimmunoprecipitated with the tumor suppressor p53. The binding of U14 to p53 was verified by coimmunoprecipitation experiments in both Molt-3 cells infected with HHV-6 and 293 cells cotransfected with U14 and p53 expression vectors. Indirect immunofluorescence assays (IFAs) showed that by 18 h postinfection (hpi) U14 localized to the dot-like structures observed in both the nucleus and cytoplasm where p53 was partly accumulated. Despite Northern blotting evidence that U14 follows late kinetics, the U14 protein was detected immediately after infection (at 3 hpi) by IFA. In addition, by Western blotting, U14 was detected at 0 hpi or in the presence of cycloheximide which completely abolished the expression of IE1 protein. In addition to U14, p53 was detected at 0 hpi although it was not detected in mock-infected cells. Furthermore, both U14 and p53 were clearly detected in the viral particles by Western blotting and immunoelectron microscopy, supporting the idea that U14 and p53 are incorporated into virions. Our study provides the first evidence of the incorporation of cellular p53 into viral particles and suggests that p53 may play an important role in viral infection.

Inhibition of cell division by the human cytomegalovirus IE86 protein: role of the p53 pathway or cyclin-dependent kinase 1/cyclin B1

The human cytomegalovirus (HCMV) IE86 protein induces the human fibroblast cell cycle from G(0)/G(1) to G(1)/S, where cell cycle progression stops. Cells with a wild-type, mutated, or null p53 or cells with null p21 protein were transduced with replication-deficient adenoviruses expressing HCMV IE86 protein or cellular p53 or p21. Even though S-phase genes were activated in a p53 wild-type cell, IE86 protein also induced phospho-Ser(15) p53 and p21 independent of p14ARF but dependent on ATM kinase. These cells did not enter the S phase. In human p53 mutant, p53 null, or p21 null cells, IE86 protein did not up-regulate p21, cellular DNA synthesis was not inhibited, but cell division was inhibited. Cells accumulated in the G(2)/M phase, and there was increased cyclin-dependent kinase 1/cyclin B1 activity. Although the HCMV IE86 protein increases cellular E2F activity, it also blocks cell division in both p53(+/+) and p53(-/-) cells.

Cell death suppression by cytomegaloviruses

Cytomegaloviruses (CMVs), a subset of betaherpesviruses, employ multiple strategies to suppress apoptosis in infected cells and thus to delay their death. Human cytomegalovirus (HCMV) encodes at least two proteins that directly interfere with the apoptotic signaling pathways, viral inhibitor of caspase-8-induced apoptosis vICA (pUL36), and mitochondria-localized inhibitor of apoptosis vMIA (pUL37 x 1). vICA associates with pro-caspase-8 and appears to block its recruitment to the death-inducing signaling complex (DISC), a step preceding caspase-8 activation. vMIA binds and sequesters Bax at mitochondria, and interferes with BH3-only-death-factor/Bax-complex-mediated permeabilization of mitochondria. vMIA does not seem to either interact with Bak, a close structural and functional homologue of Bax, or to suppress Bak-mediated permeabilization of mitochondria and Bak-mediated apoptosis. All sequenced betaherpesviruses, including CMVs, encode close homologues of vICA, and those vICA homologues that have been tested, were found to be functional cell death suppressors. Overt sequence homologues of vMIA were found only in the genomes of primate CMVs, but recent observations made with murine CMV (MCMV) indicate that non-primate CMVs may also encode a cell death suppressor functionally resembling vMIA. The exact physiological roles and relative contributions of vMIA and vICA in suppressing death of CMV-infected cells in vivo have not been elucidated. There is strong evidence that the cell death suppressing function of vMIA is indispensable, and that vICA is dispensable for replication of HCMV. In addition to suppressed caspase-8 activation and sequestered Bax, CMV-infected cells display several other phenomena, less well characterized, that may diminish, directly or indirectly the extent of cell death.

Cellular stress and signal transduction responses to human cytomegalovirus infection

Human cytomegalovirus (HCMV) receptor-ligand interactions and viral entry excite cellular responses such as receptor tyrosine kinase and mitogen-activated protein kinase signaling, cytoskeletal rearrangement, and the induction of transcription factors, prostaglandins, and cytokines. Bi-phasic stimulation of these pathways, excepting interferon, facilitates productive viral infection and likely contributes to viral pathogenesis.

Cyclin-dependent kinase activity is required at early times for accurate processing and accumulation of the human cytomegalovirus UL122-123 and UL37 immediate-early transcripts and at later times for virus production

Human cytomegalovirus (HCMV) infection leads to dysregulation of multiple cell cycle-regulatory proteins. In this study, we examined the effects of inhibition of cyclin-dependent kinase (cdk) activity on viral replication. With the drug Roscovitine, a specific inhibitor of cyclin-dependent kinases 1, 2, 5, 7, and 9, we have shown that during the first 6 h of infection, cyclin-dependent kinase-dependent events occurred that included the regulated processing and accumulation of the immediate-early (IE) UL122-123 transcripts and UL36-37 transcripts. Altered processing of UL122-123 led to a loss of IE1-72 and an increase in IE2-86. The ratio of spliced to unspliced UL37 transcripts also changed. These effects did not require de novo protein synthesis or degradation of proteins by the proteasome. Addition of Roscovitine at the beginning of the infection was also associated with inhibition of expression of selected viral early gene products, viral DNA replication, and late viral gene expression. When Roscovitine was added after the first 6 h of infection, the effects on IE gene expression were no longer observed and viral replication proceeded through the late phase, but viral titers were reduced. The reduction in viral titer was observed even when Roscovitine was first added at 48 h postinfection, indicating that cyclin-dependent kinase activity is required at both IE and late times. Flavopiridol, another specific inhibitor of cyclin-dependent kinases, had similar effects on IE and early gene expression. These results underscore the importance of accurate RNA processing and reiterate the significant role of cell cycle-regulatory factors in HCMV infection.

Sumoylation of the major immediate-early IE2 protein of human cytomegalovirus Towne strain is not required for virus growth in cultured human fibroblasts

Sumoylation of the major immediate-early IE2 protein of human cytomegalovirus has been shown to increase transactivation activity in target reporter gene assays. This study examined the role of IE2 sumoylation in viral infection. A Towne strain-based bacterial artificial chromosome clone was generated encoding a mutated form of the IE2 protein with Lys-->Arg substitutions at positions 175 and 180, the two major sumoylation sites. When human fibroblast (HF) cells were infected with the reconstituted mutant virus, (i) viral growth kinetics, (ii) the accumulation of IE1 (UL123), IE2 (UL122), p52 (UL44) and pp65 (UL83) proteins and (iii) the relocalization of the cellular small ubiquitin-like modifier (SUMO)-1, p53 and proliferating cell nuclear antigen proteins into viral DNA replication compartments were comparable with those of the wild-type and the revertant virus. The data demonstrate that sumoylation of IE2 is not essential for virus growth in cultured HF cells.

Herpes simplex virus type 1 infection induces the stabilization of p53 in a USP7- and ATM-independent manner

The major oncoprotein p53 regulates several cellular antiproliferation pathways that can be triggered in response to a variety of cellular stresses, including viral infection. The stabilization of p53 is a key factor in the ability of cells to initiate an efficient transcriptional response after cellular stress. Here we present data demonstrating that herpes simplex virus type 1 (HSV-1) infection of HFFF-2 cells, a low-passage-number nontransformed human primary cell line, results in the stabilization of p53. This process required viral immediate-early gene expression but occurred independently of the viral regulatory protein ICP0 and viral DNA replication. No specific viral protein could be identified as being solely responsible for the effect, which appears to be a cellular response to developing HSV-1 infections. HSV-1 infection also induced the phosphorylation of p53 at residues Ser15 and Ser20, which have previously been implicated in its stabilization in response to DNA damage. However, an HSV-1 infection of ATM(-/-) cells, which lack a kinase implicated in these phosphorylation events, did not lead to the phosphorylation of p53 at these residues, but nonetheless p53 was stabilized. We also show that the wild-type p53 expressed by osteosarcoma U2OS cells can be stabilized in response to DNA damage induced by UV irradiation, but not in response to HSV-1 infection. These data suggest that multiple cellular mechanisms are initiated to stabilize p53 during an HSV-1 infection. These mechanisms occur independently of ICP0 and its ability to sequester USP7 and may differ from those initiated in response to DNA damage.

Bromodeoxyuridine-labeled viral particles as a tool for visualization of the immediate-early events of human cytomegalovirus infection

We describe here a simple method for labeling the genome of human cytomegalovirus, a large double-stranded DNA virus, with bromodeoxyuridine (BrdU). The labeled DNA was incorporated into viral particles, which were then collected in cell supernatant. To demonstrate the versatility and effectiveness of this method, labeled virions were used to study the immediate-early events of virus-host cell interaction via indirect immunofluorescence microscopy. It is our hope that this new methodology will prove useful in the study of binding, entry and viral genome deposition in diverse virus systems.

Productive human herpesvirus 6 infection causes aberrant accumulation of p53 and prevents apoptosis

p53 plays an important role in tumour suppression in cells exposed to some genotoxic stresses. We found that the p53 protein level was increased in a variety of cell lines infected with human herpesvirus 6 (HHV-6). Because the elevation in p53 began very soon after infection (4 h) and did not occur with UV-inactivated virus infection, it appeared to require the expression of one or more viral immediate-early (IE) genes. To elucidate the mechanism of p53 induction, we investigated its regulation at the protein level. Pulse-chase analysis showed that the stability of p53 increased in HHV-6-infected cells. In addition, the ubiquitination of p53 decreased after infection, indicating that the stability of p53 was increased through deubiquitination. We showed by confocal microscopy that the additional p53 mainly localized to the cytoplasm and that p53 was retained in the cytoplasm even after UV irradiation, but that it translocated into the nucleus in mock-infected cells. Furthermore, DNA fragmentation analysis, a terminal deoxynucleotidyl transferase-mediated dUTP nick end-labelling (TUNEL) assay and annexin V staining showed that infected cells were resistant to UV-induced apoptosis. These results lead us to propose that HHV-6 has a mechanism for retaining p53 within the cytoplasm and protects the infected cells from apoptosis.

Human herpesvirus 6 infection arrests cord blood mononuclear cells in G(2) phase of the cell cycle

We here report that after infection with human herpesvirus 6A, human cord blood mononuclear cells accumulate in G(2)/M phase of the cell cycle. Experiments with foscarnet or ultraviolet (UV)-irradiated virus stocks pointed at an (immediate-)early, newly formed viral protein to be responsible for the arrest. At the molecular level, p53, cyclin B(1), cyclin A and tyrosine(15)-phosphorylated cdk1 accumulated after HHV-6A infection, indicating an arrest in G(2). However, no change was observed in the levels of downstream effectors of p53 in establishing a G(2) arrest, i.e. p21 and 14-3-3sigma. We thus conclude that the HHV-6A-induced G(2) arrest occurs independently of p53 accumulation.

The Rep protein of adeno-associated virus type 2 interacts with single-stranded DNA-binding proteins that enhance viral replication

Adeno-associated virus (AAV) type 2 is a human parvovirus whose replication is dependent upon cellular proteins as well as functions supplied by helper viruses. The minimal herpes simplex virus type 1 (HSV-1) proteins that support AAV replication in cell culture are the helicase-primase complex of UL5, UL8, and UL52, together with the UL29 gene product ICP8. We show that AAV and HSV-1 replication proteins colocalize at discrete intranuclear sites. Transfections with mutant genes demonstrate that enzymatic functions of the helicase-primase are not essential. The ICP8 protein alone enhances AAV replication in an in vitro assay. We also show localization of the cellular replication protein A (RPA) at AAV centers under a variety of conditions that support replication. In vitro assays demonstrate that the AAV Rep68 and Rep78 proteins interact with the single-stranded DNA-binding proteins (ssDBPs) of Ad (Ad-DBP), HSV-1 (ICP8), and the cell (RPA) and that these proteins enhance binding and nicking of Rep proteins at the origin. These results highlight the importance of intranuclear localization and suggest that Rep interaction with multiple ssDBPs allows AAV to replicate under a diverse set of conditions.

Mechanisms governing maintenance of Cdk1/cyclin B1 kinase activity in cells infected with human cytomegalovirus

Previous work has demonstrated dysregulation of key cell cycle components in human cytomegalovirus (HCMV)-infected human fibroblasts, resulting in cell cycle arrest (F. M. Jault, J.-M. Jault, F. Ruchti, E. A. Fortunato, C. L. Clark, J. Corbeil, D. D. Richman, and D. H. Spector, J. Virol. 69:6697-6704, 1995). The activation of the mitotic kinase Cdk1/cyclin B, which was detected as early as 8 h postinfection (p.i.) and maintained throughout the time course, was particularly interesting. To understand the mechanisms underlying the induction of this kinase activity, we have examined the pathways that regulate the activation of Cdk1/cyclin B1 complexes. The accumulation of the cyclin B1 subunit in HCMV-infected cells is the result of increased synthesis and reduced degradation of the protein. In addition, the catalytic subunit, Cdk1, accumulates in its active form in virus-infected cells. The decreased level of the Tyr15-phosphorylated form of Cdk1 in virus-infected fibroblasts is due in part to the down-regulation of the expression and activity of the Cdk1 inhibitory kinases Myt1 and Wee1. Increased degradation of Wee1 via the proteasome also accounts for its absence at 24 h p.i. At late times, we observed accumulation of the Cdc25 phosphatases that remove the inhibitory phosphates from Cdk1. Interestingly, biochemical fractionation studies revealed that the active form of Cdk1, a fraction of total cyclin B1, and the Cdc25 phosphatases reside predominantly in the cytoplasm of infected cells. Collectively, these data suggest that the maintenance of Cdk1/cyclin B1 activity observed in HCMV-infected cells can be explained by three mechanisms: the accumulation of cyclin B1, the inactivation of negative regulatory pathways for Cdk1, and the accumulation of positive factors that promote Cdk1 activity.

Redistribution of cyclophilin A to viral factories during vaccinia virus infection and its incorporation into mature particles

Cyclophilins are peptidyl-prolyl cis-trans isomerases involved in catalyzing conformational changes and accelerating the rate of protein folding and refolding in several cellular systems. In the present study, we analyzed the expression pattern and intracellular distribution of the cellular isomerase cyclophilin A (CypA) during vaccinia virus (VV) infection. An impressive increase in CypA stability was observed, leading to a practically unchanged accumulation of CypA during infection, although its synthesis was completely inhibited at late times. By confocal microscopy, we observed that CypA went through an intense reorganization in the cell cytoplasm and colocalized with the virosomes late in infection. CypA relocation to viral factories required the synthesis of viral postreplicative proteins, and treatment of infected cells with cyclosporine (CsA) prevented CypA relocation, clearly excluding the virosomes from CypA staining. Immunoelectron microscopy of VV-infected cells showed that CypA was incorporated into VV particles during morphogenesis. Biochemical and electron microscopic assays with purified virions confirmed that CypA was encapsidated within the virus particle and localized specifically in the core. This work suggests that CypA may develop an important role in VV replication.

Kaposi's sarcoma-associated herpesvirus K8 protein interacts with hSNF5

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human gammaherpesvirus related to Epstein-Barr virus (EBV) and herpesvirus saimiri. KSHV open reading frame K8 encodes a basic region-leucine zipper protein of 237 aa that homodimerizes. K8 shows significant similarity to the EBV immediate-early protein Zta, a key regulator of EBV reactivation and replication. In this study, a carboxyl-terminal deletion mutant of K8, K8(1-115), that had strong transactivating properties was found. Screening using transcriptionally inactive K8(1-75) showed that K8 interacts and co-localizes with hSNF5, a cellular chromatin-remodelling factor, both in vivo and in vitro. This interaction requires aa 48-183 of hSNF5 and 1-75 of K8. In a yeast expression system, the ability of K8 and K8(1-115) to activate transcription requires the presence of SNF5, the yeast homologue of hSNF5. These data suggest a mechanism by which the SWI-SNF complex is recruited to specific genes. They also suggest that K8 functions as a transcriptional activator under specific conditions and that its transactivation activity requires its interaction with the cellular chromatin remodelling factor hSNF5.

The spatio-temporal organization of DNA replication sites is identical in primary, immortalized and transformed mammalian cells

We investigated the organization of DNA replication sites in primary (young or presenescent), immortalized and transformed mammalian cells. Four different methods were used to visualize replication sites: in vivo pulse-labeling with 5-bromo-2'-deoxyuridine (BrdU), followed by either acid depurination, or incubation in nuclease cocktail to expose single-stranded BrdU-substituted DNA regions for immunolabeling; biotin-dUTP labeling of nascent DNA by run-on replication within intact nuclei and staining with fluorescent streptavidin; and, finally, immunolabeling of the replication fork proteins PCNA and RPA. All methods produced identical results, demonstrating no fundamental differences in the spatio-temporal organization of replication patterns between primary, immortal or transformed mammalian cells. In addition, we did not detect a spatial coincidence between the early firing replicons and nuclear lamin proteins, the retinoblastoma protein or the nucleolus in primary human and rodent cells. The retinoblastoma protein does not colocalize in vivo with members of the Mcm family of proteins (Mcm2, 3 and 7) at any point of the cell cycle and neither in the chromatin-bound nor in the soluble nucleoplasmic fraction. These results argue against a direct role for the retinoblastoma or nuclear lamin proteins in mammalian DNA synthesis under normal physiological conditions.

Human cytomegalovirus induces drug resistance and alteration of programmed cell death by accumulation of deltaN-p73alpha

Intrauterine transmission of human cytomegalovirus (HCMV) to the fetus following primary infection in early and late pregnancy usually results in severe neurological handicaps and sensorineural hearing loss with typical migrational anomalies, optic atrophy, disturbed myelination, cerebella hypoplasia, microcephaly, hydrocephaly, and lissencephaly. Recently, evidences raised from the phenotype of p73-deficient mice show that an association may exist between the expression of the TP53 homologous gene and HCMV tropism in the brain, suggesting an implication of p73 in viral persistence. In this study, we demonstrated that HCMV-mediated inhibition of apoptosis only occurs in p73-expressing cells. Upon infection, an accumulation of deltaN-p73alpha isoforms was observed in HCMV-infected p73-positive cells. This phenomenon was shown to be responsible for the subsequent acquired resistance to apoptosis of infected cells. Inhibition of apoptosis in p73-positive cells by HCMV may thus contribute both to virus persistency and abnormal nervous cell survival. This finding provides the first molecular basis for HCMV-associated abnormal embryonic development and neurological defects in newborns.

The human papillomavirus E6 protein and its contribution to malignant progression

The human papillomavirus (HPV) E6 protein is one of three oncoproteins encoded by the virus. It has long been recognized as a potent oncogene and is intimately associated with the events that result in the malignant conversion of virally infected cells. In order to understand the mechanisms by which E6 contributes to the development of human malignancy many laboratories have focused their attention on identifying the cellular proteins with which E6 interacts. In this review we discuss these interactions in the light of their respective contributions to the malignant progression of HPV transformed cells.

Human-herpesvirus-8-encoded K8 protein colocalizes with the promyelocytic leukemia protein (PML) bodies and recruits p53 to the PML bodies

Promyelocytic leukemia protein (PML) bodies are nuclear sites for both input viral genome deposition and immediate-early (IE) gene transcription during infection with certain human DNA viruses, such as human cytomegalovirus (HCMV), herpes simplex virus type 1, and adenovirus. In this study, we showed that the K8 (K-bZIP) protein, an early protein encoded by the human herpesvirus 8 (HHV-8), colocalized with the PML bodies in HHV-8-infected primary effusion lymphoma cells. Cotransfection of two plasmids expressing the K8 protein and green-fluorescence protein (GFP)-PML fusion protein into 293T cells revealed that the K8 protein colocalized with PML in cells with high PML expression. Overexpression of the K8 protein in Chinese hamster ovary (CHO) cells with stable GFP-PML expression did not induce the dispersion of the PML bodies, unlike the IE1 protein of HCMV. Transfection of a truncated K8 gene revealed that the leucine zipper domain of the K8 protein was required for the colocalization with PML. We also demonstrated that the K8 protein bound to p53 in vivo and in vitro, and that high expression of the K8 protein caused the accumulation of p53 to the PML bodies in CHO cells, suggesting that the K8 protein functions in the recruitment of p53 to the PML bodies. These data suggest that the K8 protein may be associated with the functional modulation of p53 in the nucleus during the lytic phase of HHV-8.

Microbes, inflammation and atherosclerosis: will old pathology lessons guide new therapies?

Although attractive, the microbial pathogenesis theory for atherosclerosis remains unproven. Over the last century, microbiologists have invoked fulfillment of Koch's postulates to determine pathogen causality. Certainly a multifactorial disease process such as atherosclerosis unlikely will be due to a single microbial agent, an agent when transferred to another host, will always induce atherosclerosis. Conflicting epidemiological data also do not support a single causative agent. However, as presented here, considerable in vitro, animal, and human epidemiological data support the plausibility that infectious agents can promote a proinflammatory, procoagulant and proatherogenic environment in the vessel wall. Microbial genes and molecules can catalyze these processes and foil normal cellular events. But, must intact microbes enter the vessel wall or can microbial molecules incite immune responses from afar? A new focus on pathogen-induced auto-immunity toward vasculature has been presented. For example, microbes contain molecules that mimic host cellular components (55). An immune response to a pathogen may cross react with vessel wall cellular structures. This immune response enhanced by infection may lead to high levels of cross reacting auto-antibodies or auto-aggressive T-cells. Epstein has championed the concept of pathogen burden in support of this auto-immune theory (56). Individuals infected with multiple pathogens such as HSV-1, HSV-2, CMV, Helicobacter pylori, and Hepatitis A, have high C-reative protein levels (markers of inflammation) and the greatest relative risk for coronary artery disease (57). Thus, pathogens might contribute to the atherosclerotic process by promoting inflammatory responses. It is this author's view that microbes and inflammation do play a role in the pathogenesis of atherosclerosis (58). Infection may contribute to the process promoting vessel wall injury initiated by oxidized lipids, smoking derived oxidants, hypertensive shear or diabetes glyoxidized molecules. Inflammation and immune reactions in response to infection can exacerbate and act synergistically with all of the aforementioned vasculotoxic moieties. Continued investigations in the 21st century will determine if vaccines, antibiotics, anti-inflammatory agents or immunosuppressants will alter the picture the early 19th century pathologists observed under their monocular microscopes.

Role of human cytomegalovirus immediate-early proteins in cell growth control

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus that has been implicated in several disorders, including an association between HCMV reactivation and the overproliferation of arterial smooth muscle cells observed in restenosis. Although HCMV can mediate a growth-arrest phenotype in infected cells, the virus can also promote an environment conducive to proliferation. Here, we present evidence that the HCMV immediate-early (IE) proteins, IE1-72 and IE2-86, may be responsible for inducing this proliferative environment by altering cell cycle control. We find that expression of either of these IE proteins can alter the cell cycle distribution of randomly cycling cells towards S and G(2)/M phases. Additionally, we find that expression of IE2-86, but not IE1-72, induces quiescent cells into S phase and delays cell cycle exit. In the absence of p53, IE1-72 expression can induce S phase and delay cell cycle exit. We also demonstrate that p53 protein levels increase in fibroblasts following the expression of IE1-72. The observed accumulation of p53 protein in IE1-72-expressing cells may account for the inability of IE1-72 to induce S phase and delay cell cycle exit. Our data suggest that expression of HCMV IE1-72 and IE2-86 is sufficient to alter the cell cycle to generate an environment conducive to proliferation.

Disruption of PML-associated nuclear bodies by IE1 correlates with efficient early stages of viral gene expression and DNA replication in human cytomegalovirus infection

In human cytomegalovirus (HCMV) infection, both of the major immediate-early proteins IE1(IE68, UL123) and IE2(IE86, UL122) target to PML protein-associated nuclear bodies known as PODs or ND10 at very early times after infection. IE1 causes a redistribution of both PML and IE1 from the PODs into a nuclear diffuse form, whereas IE2 initially localizes adjacent to PODs but later associates with viral DNA replication compartments. The peripheries of PODs are also believed to be sites for initiation of both viral IE transcription and DNA replication. However, because IE1 is nonessential at high multiplicity of infection (m.o.i.) in HF cells, the exact role of these processes in viral infection has been enigmatic. Therefore, we investigated the effects of overexpression of PML in the presence or absence of IE1 on the intranuclear distribution of IE2 and formation of viral DNA replication compartments, as well as on the levels of delayed-early and late viral transcription and protein accumulation. Infection with wild-type HCMV(Towne) and the IE1-deleted derivative HCMV(CR208), which fails to disrupt PODs, was compared in a pair of related astrocytoma/glioblastoma cell lines, the U373-Neo control and a variant U373-PML that constitutively overexpresses PML(560) in much larger than normal PODs. IFA studies on the localization patterns for IE1, IE2, and PML showed that, although the numbers of IE2-positive cells were not significantly reduced in either the wild-type virus-infected U373-PML cell line or in DeltaIE1-infected control cells, POD disruption by IE1 in wild-type virus infection was delayed by up to 6 h in U373-PML cells compared to control cells. Furthermore, there was considerable enhancement of IE2 colocalization with PODs in Delta IE1-infected U373-PML cells. Formation of viral DNA replication compartments in the U373-PML cell line was also greatly delayed, measured at fivefold lower after wild-type virus infection and 12-fold lower after infection with Delta IE1 than in the control cell line at 48 h at an m.o.i. of 1.0. The levels of representative early and late viral proteins detected by Western blotting were suppressed by fivefold and 22-fold at 24 and 72 h, respectively, in the U373-PML cell line, even with high m. o.i. wild-type HCMV infection. Decreased viral protein levels also occurred when control cells were infected with the Delta IE1 virus and these two effects were additive in the U373-PML cell line. Similarly, when U373-PML cells were infected with recombinant HCMV expressing an extragenic luciferase reporter gene under the control of viral early (Pol) or late (pp28) promoters, their transcriptional activation was reduced up to fivefold at both high and low m.o.i. compared to that of the control cells. Overall, these results suggest that POD disruption by IE1 and subsequent redistribution of both PML and IE1 at very early times after infection may play an important role in the efficient utilization of cellular transcription and replication machinery by HCMV and contribute to rapid progression of the HCMV lytic cycle.

Stable expression of Epstein-Barr virus BZLF-1–encoded ZEBRA protein activates p53-dependent transcription in human Jurkat T-lymphoblastoid cells

Interaction between viral proteins and tumor suppressor p53 is a common mechanism of viral pathogenesis. The Epstein-Barr virus (EBV) BZLF-1 ORF-encoded ZEBRA protein (also denoted EB1, Z, Zta) binds to p53 in vitro and has been associated with the altered transcription of p53-regulated genes in B lymphocytes and epithelial cells. In this work, Jurkat T-lymphoblastoid cells that express ZEBRA were characterized by the use of transiently transfected p53 and p53 reporter genes. Stable expression of ZEBRA was associated with the activation of p53-dependent transcription and increased p53 dependent apoptotic cell death. In Jurkat cell lines, stably expressed ZEBRA protein was apparently localized to the cell cytoplasm, in contrast to the typical nuclear localization of this protein in other cell types. Previous studies have suggested that EBV infection of T lymphocytes may contribute to the malignant transformation of T cells and the increased replication of human immunodeficiency virus. Our observations suggest a mechanism through which ZEBRA protein expressed in human T lymphocytes could alter T-cell proliferation and apoptosis during EBV infection.

Stable expression of Epstein-Barr virus BZLF-1–encoded ZEBRA protein activates p53-dependent transcription in human Jurkat T-lymphoblastoid cells

Interaction between viral proteins and tumor suppressor p53 is a common mechanism of viral pathogenesis. The Epstein-Barr virus (EBV) BZLF-1 ORF-encoded ZEBRA protein (also denoted EB1, Z, Zta) binds to p53 in vitro and has been associated with the altered transcription of p53-regulated genes in B lymphocytes and epithelial cells. In this work, Jurkat T-lymphoblastoid cells that express ZEBRA were characterized by the use of transiently transfected p53 and p53 reporter genes. Stable expression of ZEBRA was associated with the activation of p53-dependent transcription and increased p53 dependent apoptotic cell death. In Jurkat cell lines, stably expressed ZEBRA protein was apparently localized to the cell cytoplasm, in contrast to the typical nuclear localization of this protein in other cell types. Previous studies have suggested that EBV infection of T lymphocytes may contribute to the malignant transformation of T cells and the increased replication of human immunodeficiency virus. Our observations suggest a mechanism through which ZEBRA protein expressed in human T lymphocytes could alter T-cell proliferation and apoptosis during EBV infection.

Human cytomegalovirus immediate early proteins upregulate endothelial p53 function

Infected endothelial cells are found to be resistant to apoptosis possibly mediated by p53 cytoplasmic sequestration. We explored whether the immediate early 84 kDa protein (IE84) of cytomegalovirus (CMV) is responsible for p53 cytoplasmic sequestration. The endothelial cells were transfected with plasmids containing IE1 and 2 coding regions which are known to synthesize IE84 and 72 proteins. Our study found that p53 expression was significantly elevated in endothelial cells transfected with IE1 and 2 plasmids. However, p53 was only found in the nucleus rather than sequestered in the cytoplasm. We have demonstrated that IE84 and 72 are not responsible for p53 dysfunction caused by CMV infection, rather they upregulate p53 function and promote endothelial apoptosis.

Dysregulation of cyclin E gene expression in human cytomegalovirus-infected cells requires viral early gene expression and is associated with changes in the Rb-related protein p130

We have previously shown that many cell cycle regulatory gene products are markedly affected by infection of primary fibroblasts with human cytomegalovirus (HCMV) (F. M. Jault, J. M. Jault, F. Ruchti, E. A. Fortunato, C. Clark, J. Corbeil, D. D. Richman, and D. H. Spector, J. Virol. 69:6697-6704, 1995). One of these proteins, cyclin E, is a key determinant of cell cycle progression during G(1), and its mRNA levels are significantly increased in HCMV-infected fibroblasts (B. S. Salvant, E. A. Fortunato, and D. H. Spector, J. Virol. 72:3729-3741, 1998). To determine the molecular basis of this effect, we have examined the events that occur at the endogenous cyclin E promoter during the course of infection. In vivo dimethyl sulfate footprinting of the cyclin E promoter revealed several regions of protection and hypersensitivity that were unique to infected cells. In accord with this observation, we find that the virus-induced cyclin E transcripts initiate downstream of the start site identified in mock-infected cells, in regions where these newly appearing protected and hypersensitive sites occur. Viral gene expression is required for this induction. However, the viral immediate-early proteins IE1-72 and IE2-86, either alone or in combination, cannot induce expression of the endogenous cyclin E. The virus must progress past the immediate-early phase and express an early gene product(s) for activation of cyclin E expression. Moreover, IE1-72 does not appear to be required, as infection of cells with an HCMV mutant containing a deletion in the IE1-72 gene leads to full upregulation of cyclin E expression. Using electrophoretic mobility shift assays with infected cell extracts and a region of the cyclin E promoter that includes two previously defined E2F sites as the probe, we detected the appearance of an infection-specific banding pattern. One of the infection-specific bands contained the proteins E2F-4, DP-1, and p130, which were maintained in the infected cells as uniquely phosphorylated species. These results suggest that an altered E2F-4-DP-1-p130 complex along with viral early gene expression may play a role in the transcriptional regulation of cyclin E mRNA during HCMV infection.

Exploitation of cellular signaling and regulatory pathways by human cytomegalovirus

Human cytomegalovirus is a ubiquitous human pathogen that is the leading viral cause of birth defects. It also causes significant morbidity and mortality in both chemically and virally immunosuppressed individuals. Recent studies have begun to elucidate the interplay between this virus and its host cell on a molecular level. The interactions begin upon contact with the cell membrane, involve multiple processes including cell signaling, cell-cycle control and immune response mechanisms, and culminate in a productive infection.

Specific chromosome 1 breaks induced by human cytomegalovirus

Human cytomegalovirus (HCMV) is the major viral cause of birth defects and a serious problem for immunocompromised individuals. Here we show that infection of cells with HCMV during the S-phase of the cell cycle results in two specific chromosome 1 breaks at positions 1q42 and 1q21. We demonstrate that purified virions, and not infected cell supernatant alone, are responsible for the damage. In addition, we show that the specific breaks occur when different sources of fibroblasts and strains of HCMV are used. Incubation of the virus with neutralizing antibody prevents the induction of breaks. However, UV-inactivated virus is as efficient as untreated virus in inducing specific damage to chromosome 1. Thus, there is a requirement for viral adsorption/penetration, but not new viral gene expression. This HCMV-mediated induction of site-specific damage in actively dividing cells may provide clues for the development of neurological defects in the congenitally infected infant.

Interaction between the HPV-16 E2 transcriptional activator and p53

The HPV-16 E2 protein is a major regulator of viral DNA replication and gene expression. Through interactions with the viral origin binding protein, E1, it localizes E1 to the origin of replication and stimulates the initiation of viral DNA replication. However, several recent reports have described a number of diverse activities of E2 relating to the induction of apoptosis through both p53 dependent and independent mechanisms, and to induction of growth arrest in both the G1 and G2M phases of the cell cycle. Recent studies have also shown that p53 can specifically inhibit HPV DNA replication, albeit through an unknown mechanism. Since p53 has been described in the replication centres of Herpes Viruses, Adenovirus and SV40 we decided to investigate whether any of the above activities of E2 may be related to an association with p53. We show, in a series of in vitro assays, specific interaction between p53 and HPV-16 E2 via residues in the carboxy terminal half of the E2 protein. Mutational analysis of p53 indicates that sequences in both the DNA binding and oligomerization domains are essential for the interaction, and a mutant of p53 which is unable to bind E2 is also unable to inhibit HPV DNA replication. Finally, using an inducible system of p53 expression we also show that E2 will complex with p53 in vivo. These results raise the intriguing possibility that p53 may also be involved in HPV DNA replication centres, and also provides explanations for some of the diverse activities reported for the HPV E2 proteins.

The role of the E6-p53 interaction in the molecular pathogenesis of HPV

Human papillomaviruses (HPVs) are associated with a number of clinical conditions, of which the most serious is cervical carcinoma. The E6 protein of the oncogenic, mucosal-specific HPV types has been shown to complex with p53 and, as a result, target it for rapid proteasome-mediated degradation. As a consequence, p53's growth-arrest and apoptosis-inducing activities are abrogated. Since p53 is frequently wild type in cervical cancers, unlike other cancers in which it is often mutated, the notion has arisen that E6's activity with respect to p53 is equivalent to an inactivating mutation of p53. In addition, several studies have shown that the pathways both upstream and downstream of p53 are intact in cervical cancers; this suggests the potential importance of the E6 - p53 interaction for therapeutic intervention. However, like all viral oncoproteins, E6 is a multifunctional protein and a plethora of other cellular targets has been identified. Indeed, E6's interactions with some of these additional targets appear to be equally important in the pathogenesis of HPV, and may also represent valid targets for therapeutic intervention.

The human cytomegalovirus IE2 and UL112-113 proteins accumulate in viral DNA replication compartments that initiate from the periphery of promyelocytic leukemia protein-associated nuclear bodies (PODs or ND10)

During human cytomegalovirus (HCMV) infection, the periphery of promyelocytic leukemia protein (PML)-associated nuclear bodies (also known as PML oncogenic domains [PODs] or ND10) are sites for both input viral genome deposition and immediate-early (IE) gene transcription. At very early times after infection, the IE1 protein localizes to and subsequently disrupts PODs, whereas the IE2 protein localizes within or adjacent to PODs. This process appears to be required for efficient viral gene expression and DNA replication. We have investigated the initiation of viral DNA replication compartment formation by studying the localization of viral IE proteins, DNA replication proteins, and the PML protein during productive infection. Localization of IE2 adjacent to PODs between 2 and 6 h after infection was confirmed by confocal microscopy of human fibroblasts (HF cells) infected with both wild-type HCMV(Towne) and with an IE1-deletion mutant HCMV(CR208) that fails to disrupt PODs. In HCMV(Towne)-infected HF cells at 24 to 48 h, IE2 also accumulated in newly formed viral DNA replication compartments containing the polymerase processivity factor (UL44), the single-stranded DNA binding protein (SSB; UL57), the UL112-113 accessory protein, and newly incorporated bromodeoxyuridine (BrdU). Double labeling of the HCMV(CR208)-infected HF cells demonstrated that formation of viral DNA replication compartments initiates within granular structures that bud from the periphery of some of the PODs and subsequently coalesce into larger structures that are flanked by PODs. In transient DNA transfection assays, both the N terminus (codons 136 to 290) and the C terminus (codons 379 to 579) of IE2 exon 5, but not the central region between them, were found to be necessary for both the punctate distribution of IE2 and its association with PODs. Like IE2, the UL112-113 accessory replication protein was also distributed in a POD-associated pattern in both DNA-transfected and virus-infected cells beginning at 6 h. Furthermore, when all six replication core machinery proteins (polymerase complex, SSB, and helicase-primase complex) were expressed together in the presence of UL112-113, they also accumulated at POD-associated sites, suggesting that the UL112-113 protein (but not IE2) may play a role in recruitment of viral replication fork proteins into the periphery of PODs. These results show that (i) subsequent to accumulating at the periphery of PODs, IE2 is incorporated together with the core proteins into viral DNA replication compartments that initiate from the periphery of PODs and then grow to fill the space between groups of PODs, and (ii) the UL112-113 protein appears to have a key role in assembling and recruiting the core replication machinery proteins in the initial stages of viral replication compartment formation.

The interaction between p53 and papillomaviruses

The p53 tumour suppressor is one of the host's principal defences against viral replication and subsequent cell transformation. The human papillomaviruses have evolved an elaborate strategy whereby the viral E6 proteins directly target p53 for ubiquitin mediated degradation and thus overcome the inhibitory effects of p53. However, a more detailed picture of the HPV*b1p53 interaction is now emerging in which there is a complex interplay between both positive and negative effectors of these interactions. This demonstrates the existence of a finely balanced virus*b1host relationship which, on rare occasion, fails and initiates the processes that ultimately lead to malignancy.

Human cytomegalovirus infection decreases expression of thrombospondin-1 independent of the tumor suppressor protein p53

Thrombospondin-1 (TSP-1) is a potent inhibitor of angiogenesis. It has been shown that promoter sequences of the TSP-1 gene can be transactivated by the wild-type tumor suppressor protein p53. As human cytomegalovirus (HCMV) infection inactivates wild-type p53 of various cell types, we investigated whether HCMV infection is associated with reduced TSP-1 production. We found, in conjunction with accumulated p53, that TSP-1 mRNA and protein expression was significantly reduced in HCMV-infected cultured human fibroblasts. To determine whether the observed TSP-1 suppression depends on p53 inactivation, the p53-defective astrocytoma cell line U373MG was infected with HCMV. In these cells TSP-1 expression was also significantly reduced by HCMV infection whereas expression of the p53 mutant variant remained unaltered. In both cell lines the decreased expression of TSP-1 mRNA occurred early after infection (4 hours), indicating that HCMV inhibits TSP-1 transcription during the immediate-early phase of infection before HCMV DNA replication. Inhibition of HCMV DNA synthesis by ganciclovir did not influence TSP-1 reduction whereas the antisense oligonucleotide ISIS 2922, complementary to HCMV immediate-early mRNA, completely prevented the HCMV-mediated TSP-1 suppression. These findings strongly suggest a novel role for HCMV in the modulation of angiogenesis due to p53-independent down-regulation of TSP-1 expression.

Adenovirus type 5 E4orf3 protein relieves p53 inhibition by E1B-55-kilodalton protein

The E1B-55-kDa protein of adenovirus type 5 and the p53 tumor suppressor gene product form a complex that localizes to the cytoplasm, thereby downregulating p53's transcriptional activity. The E4orf6 protein binds and relocalizes E1B-55-kDa, and the proteins act synergistically to inactivate p53. We show that another adenovirus E4 gene product, E4orf3, is also sufficient to relocalize E1B-55-kDa from the cytoplasm to the nucleus. Both proteins are then found in discrete nuclear structures (tracks) that are known to contain components of the promyelocytic leukemia-associated nuclear structure. Simultaneously, p53 is dissociated from E1B-55-kDa and is found evenly distributed over the nucleoplasm. In the presence of E4orf3, p53-dependent transcriptional activity is no longer repressed by E1B-55-kDa. When E1B-55-kDa is coexpressed with E4orf3 and E4orf6, E1B-55-kDa is found to colocalize with E4orf6 rather than E4orf3. In parallel, p53 is inhibited and degraded by the combination of E1B-55-kDa and E4orf6, regardless of coexpressed E4orf3. This suggests that the effects of E4orf6 on E1B-55-kDa overrule the actions of E4orf3. When cells are infected with virus expressing E4orf3 but not E4orf6, E1B is found in the cell nucleus and p53 enters the virus replication centers. After infection with wild-type adenovirus, E4orf3 is expressed before E4orf6 and E1B temporarily colocalizes with E4orf3 in nuclear tracks before associating with E4orf6. We propose that during adenovirus infection, the E4orf3 protein transiently liberates p53 from its association with E1B-55-kDa. Subsequently, p53 is inactivated and degraded by the combination of E1B-55-kDa and E4orf6.

Human papillomavirus DNA replication compartments in a transient DNA replication system

Many DNA viruses replicate their genomes at nuclear foci in infected cells. Using indirect immunofluorescence in combination with fluorescence in situ hybridization, we colocalized the human papillomavirus (HPV) replicating proteins E1 and E2 and the replicating origin-containing plasmid to nuclear foci in transiently transfected cells. The host replication protein A (RP-A) was also colocalized to these foci. These nuclear structures were identified as active sites of viral DNA synthesis by bromodeoxyuridine (BrdU) pulse-labeling. Unexpectedly, the great majority of RP-A and BrdU incorporation was found in these HPV replication domains. Furthermore, E1, E2, and RP-A were also colocalized to nuclear foci in the absence of an origin-containing plasmid. These observations suggest a spatial reorganization of the host DNA replication machinery upon HPV DNA replication or E1 and E2 expression. Alternatively, viral DNA replication might be targeted to host nuclear domains that are active during the late S phase, when such domains are limited in number. In a fraction of cells expressing E1 and E2, the promyelocytic leukemia protein, a component of nuclear domain 10 (ND10), was either partially or completely colocalized with E1 and E2. Since ND10 structures were recently hypothesized to be sites of bovine papillomavirus virion assembly, our observation suggests that HPV DNA amplification might be partially coupled to virion assembly.

Surface plasmon resonance measurements reveal stable complex formation between p53 and DNA polymerase alpha

Surface plasmon resonance measurements were used for detecting and quantifying protein-protein interactions between the tumor suppressor protein p53, the SV40 large T antigen (T-ag), the cellular DNA polymerase alpha-primase complex (pol-prim), and the cellular single-strand DNA binding protein RPA. Highly purified p53 protein bound to immobilized T-ag with an apparent binding constant of 2 x 10(8) M(-1). Binding of p53 to RPA was in the same order of magnitude with a binding constant of 4 x 10(8) M(-1), when RPA was coupled to the sensor chip via its smallest subunit, and 1 x 10(8) M(-1), when RPA was coupled via its p70 subunit. Furthermore, p53 bound human DNA polymerase alpha-primase complex (pol-prim) with a K(A) value of 1 x 10(10) m(-1). Both the p68 subunit and the p180 subunit of pol-prim could interact with p53 displaying binding constants of 2 x 10(10) m1(-1) and 5 X 10(9) M(-1), respectively. Complex formation was also observed with a p180/p68 heterodimer, and again with a binding constant similar. Hence, there was no synergistic effect when p53 bound to higher order complexes of pol-prim. A truncated form of p53, consisting of amino acids 1-320, bound pol-prim by four orders of magnitude less efficiently. Therefore, an intact C-terminus of p53 seems to be important for efficient binding to pol-prim. It was also tried to measure complex formation between p53, pol-prim, and T-ag. However there was no evidence for the existence of a ternary complex consisting of T-ag, pol-prim, and p53.