Human cytomegalovirus IE1 and IE2 proteins block apoptosis

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.

The pathogenicity of cytomegalovirus

Human cytomegalovirus is ubiquitous, yet causes little illness in immunocompetent individuals. Disease is evident in immunodeficient groups such as neonates, transplant recipients and AIDS patients either following a primary infection or reactivation of a latent infection. Little is known of the mechanisms underlying the pathogenicity of the virus. The recent determination of the nucleotide sequence of both human cytomegalovirus (strain AD169) and murine cytomegalovirus (murine cytomegalovirus strain Smith) has allowed an analysis of the biological importance of several virus genes. Studies with human cytomegalovirus have indicated that many viral genes are non-essential for replication in vitro which are thus assumed to be important in the pathogenesis of the virus. This is being examined in the murine model where the role of the gene and its product in disease can be directly examined in vivo using viral mutants in which the relevant gene has been interrupted or deleted. Current information on the role of cytomegalovirus genes in tissue tropism, immune evasion, latency, reactivation from latency and damage is described.

Human cytomegalovirus and human herpesvirus 6 genes that transform and transactivate

This review is an update on the transforming genes of human cytomegalovirus (HCMV) and human herpesvirus 6 (HHV-6). Both viruses have been implicated in the etiology of several human cancers. In particular, HCMV has been associated with cervical carcinoma and adenocarcinomas of the prostate and colon. In vitro transformation studies have established three HCMV morphologic transforming regions (mtr), i.e., mtrI, mtrII, and mtrIII. Of these, only mtrII (UL111A) is retained and expressed in both transformed and tumor-derived cells. The transforming and tumorigenic activities of the mtrII oncogene were localized to an open reading frame (ORF) encoding a 79-amino-acid (aa) protein. Furthermore, mtrII protein bound to the tumor suppressor protein p53 and inhibited its ability to transactivate a p53-responsive promoter. In additional studies, the HCMV immediate-early protein IE86 (IE2; UL122) was found to interact with cell cycle-regulatory proteins such as p53 and Rb. However, IE86 exhibited transforming activity in vitro only in cooperation with adenovirus E1A. HHV-6 is a T-cell-tropic virus associated with AIDS-related and other lymphoid malignancies. In vitro studies identified three transforming fragments, i.e., SalI-L, ZVB70, and ZVH14. Of these, only SalI-L (DR7) was retained in transformed and tumor-derived cells. The transforming and tumorigenic activities of SalI-L have been localized to a 357-aa ORF-1 protein. The ORF-1 protein was expressed in transformed cells and, like HCMV mtrII, bound to p53 and inhibited its ability to transactivate a p53-responsive promoter. HHV-6 has also been proposed to be a cofactor in AIDS because both HHV-6 and human immunodeficiency virus type 1 (HIV-1) have been demonstrated to coinfect human CD4(+) T cells, causing accelerated cytopathic effects. Interestingly, like the transforming proteins of DNA tumor viruses such as simian virus 40 and adenovirus, ORF-1 was also a transactivator and specifically up-regulated the HIV-1 long terminal repeat when cotransfected into CD4(+) T cells. Finally, based on the interactions of HCMV and HHV-6 transforming proteins with tumor suppressor proteins, a scheme is proposed for their role in oncogenesis.

The regulation of apoptosis by microbial pathogens

In the past few years, there has been remarkable progress unraveling the mechanism and significance of eukaryotic programmed cell death (PCD), or apoptosis. Not surprisingly, it has been discovered that numerous, unrelated microbial pathogens engage or circumvent the host's apoptotic program. In this chapter, we briefly summarize apoptosis, emphasizing those studies which assist the reader in understanding the subsequent discussion on PCD and pathogens. We then examine the relationship between virulent bacteria and apoptosis. This section is organized to reflect both common and diverse mechanisms employed by bacteria to induce PCD. A short discussion of parasites and fungi is followed by a detailed description of the interaction of viral pathogens with the apoptotic machinery. Throughout the review, apoptosis is considered within the broader contexts of pathogenesis, virulence, and host defense. Our goals are to update the reader on this rapidly expanding field and identify topics in the current literature which demand further investigation.

Effects of human cytomegalovirus major immediate-early proteins in controlling the cell cycle and inhibiting apoptosis: studies with ts13 cells

The major immediate-early (MIE) gene of human cytomegalovirus (HCMV) encodes several MIE proteins (MIEPs) produced as a result of alternative splicing and polyadenylation of the primary transcript. Previously we demonstrated that the HCMV MIEPs expressed from the entire MIE gene could rescue the temperature-sensitive (ts) transcriptional defect in the ts13 cell line. This defect is caused by a ts mutation in TAFII250, the 250-kDa TATA binding protein-associated factor (TAF). These and other data suggested that the MIEPs perform a TAF-like function in complex with the basal transcription factor TFIID. In addition to the transcriptional defect, the ts mutation in ts13 cells results in a defect in cell cycle progression which ultimately leads to apoptosis. Since all of these defects can be rescued by wild-type TAFII250, we asked whether the MIEPs could rescue the cell cycle defect and/or affect the progression to apoptosis. We have found that the MIEPs, expressed from the entire MIE gene, do not rescue the cell cycle block in ts13 cells grown at the nonpermissive temperature. However, despite the maintenance of the cell cycle block, the ts13 cells which express the MIEPs are resistant to apoptosis. MIEP mutants, which have previously been shown to be defective in rescuing the ts transcriptional defect, maintained the ability to inhibit apoptosis. Hence, the MIEP functions which affect transcription appear to be separable from the functions which inhibit apoptosis. We discuss these data in the light of the HCMV life cycle and the possibility that the MIEPs promote cellular transformation by a "hit-and-run" mechanism.

Humoral immune response to human cytomegalovirus in patients undergoing percutaneous transluminal coronary angioplasty

Possible causal relations between prior human cytomegalovirus (HCMV) infection and atherosclerosis and between HCMV reactivation and restenosis after coronary angioplasty have been suggested. We investigated patterns of antibodies directed to HCMV in 112 patients undergoing percutaneous transluminal coronary angioplasty (PTCA) and in a group of sex- and age-matched controls (blood donors without evidence of atherosclerosis). Levels of antibodies to HCMV were measured by enzyme-linked immunosorbent assay (ELISA) of serum samples drawn before and 5 weeks after PTCA. To further differentiate the humoral immune response, we specifically tested antibody reactivity towards four single HCMV proteins (IE2, p52, pp150, and pp65) by recombinant ELISAs. We found that 73% of PTCA patients and 69% of sex- and age-matched controls were seropositive for HCMV (odds ratio, 1.2 [not significant]). The corresponding odds ratios for matched pairs ranged in the recombinant ELISAs from 1.2 to 1.4. Patients had more often high titers of anti-HCMV antibodies (11 versus 4%; odds ratio = 3.3 [0.9 to 15.2]; P = 0.052) and high titers of anti-pp150 antibodies (13 versus 4%; odds ratio = 6.0 [1.3 to 38.8]; P = 0.008). Anti-HCMV immunoglobulin M antibodies were not detected in any patient. There was no evidence of acute HCMV reactivation after PTCA, since the titers of antibodies to the investigated recombinant proteins did not increase at 5 weeks after PTCA. Our results show a limited association between prior HCMV infection and coronary artery disease. We infer that positive anti-HCMV titers are not a major risk factor at the time of disease manifestation. However, this study cannot rule out a possible role of HCMV at earlier stages of the atherosclerotic process. Recombinant ELISAs provide a valuable tool for investigating the antiviral immune response.

Anti-apoptotic strategies of lymphotropic viruses

Induction of apoptosis of virus-infected cells is an important host cell defence mechanism. However, some viruses have incorporated genes that encode anti-apoptotic proteins or modulate the expression of cellular regulators of apoptosis. Here, Edgar Meinl and colleagues discuss recent evidence that viral interference with host cell apoptosis leads to enhanced viral replication, and to evasion of cytotoxic T-cell effects.

Apoptosis mediated by Fas but not tumor necrosis factor receptor 1 prevents chronic disease in mice infected with murine cytomegalovirus

The role of Fas- and TNF-receptor 1 (TNF-R1)-mediated apoptosis in the clearance of virally infected cells and in the regulation of the immune response was analyzed after murine cytomegalovirus (MCMV) infection of C57BL/6 (B6)-+/+ mice, Fas-mutant B6-lpr/lpr mice, TNF-R1 knockout B6-tnfr0/0 mice, and double-deficient B6-tnfr0/0 lpr/lpr mice. There was approximately equivalent clearance of MCMV in B6-+/+, B6-tnfr0/0, and B6-lpr/lpr mice, and by day 28 no infectious virus could be detected in the liver, kidney, lung, or peritoneal exudate. However, delayed virus clearance was observed in B6-tnfr0/0 lpr/lpr mice. An acute inflammatory response occurred in the liver, lung, and kidney of all mice, which was most severe 7 d after MCMV infection, but resolved by day 28 in B6-+/+ and B6-tnfr0/0 mice, but not in B6-lpr/lpr or B6-tnfr0/0 lpr/lpr mice. These results indicate that apoptosis mediated by either Fas or TNF-R1 is sufficient for rapid clearance of the virus. However, apoptosis induced by Fas, but not TNF-R1, is required for the downmodulation of the immune response to the virus and prevention of a chronic inflammatory reaction.

Cytolytic activity against allogeneic human endothelia: resistance of cytomegalovirus-infected cells and virally activated lysis of uninfected cells

BACKGROUND

Cytomegalovirus (CMV) has been implicated as an exacerbating agent in the development of transplant vascular sclerosis; however, specific etiologic mechanisms remain unresolved. Based upon our previous observations that CMV-infected endothelial cells (ECs) stimulate proliferation and cytokine production by allogeneic T cells, we now test the hypothesis that CMV-driven cytolytic activity may contribute to graft endothelial injury.

METHODS

Limiting dilutions of CMV-seropositive or -seronegative donor-derived T cells were stimulated with CMV-infected or uninfected allogeneic ECs in the presence of interleukin-2. T-cell proliferation was monitored by assay of [3H]thymidine incorporation and stimulated T cells were tested for lytic activity against CMV-infected or uninfected radiolabeled EC targets by 51Cr release assay. Natural killer (NK) cell activity was examined by incubating freshly isolated peripheral blood mononuclear cells with 51Cr-labeled targets, followed by assay of radiolabel release.

RESULTS

CMV-infected ECs were resistant to T cell- and NK-mediated cytolysis regardless of donor serostatus, nature of stimulation, or level of T-cell proliferation. In contrast, although uninfected ECs were unharmed by NK cells, these targets experienced significant lysis by T cells stimulated with either uninfected or CMV-infected ECs.

CONCLUSIONS

These results implicate CMV-infected graft endothelium as a persistent source of infectious virus, a chronic stimulus for potentially destructive host inflammatory activity, and a potential trigger for the generation of lytic injury to uninfected bystander endothelia, suggesting multiple mechanisms by which this virus might perturb equilibrium at the graft/host interface.

[Cytomegalovirus and herpes simplex virus in pathogenesis and progression of native arteriosclerosis and recurrent stenosis after intervention]

An increasing number of clinical and experimental studies point to a contribution of various infectious organisms like chlamydia pneumoniae or herpesviruses to atherosclerosis in man. Cytomegalovirus induces atherosclerotic lesions in animals. In vitro studies reveal functional changes of endothelial cells after infection with cytomegalovirus. Infection with this virus renders endothelial cells immunogenic for cellular and humoral immune reactions. In man a significant association of infections with herpesviruses and atherosclerosis could be established in several studies. Cytomegalovirus infection has been incriminated as an independent risk factor in restenosis after coronary angioplasty.

Apoptosis of vascular smooth muscle cells in atherosclerosis

Apoptosis (programmed cell death) of vascular smooth muscle cells and macrophages has recently been demonstrated in the following: (a) human atherosclerotic plaques; (b) physiological remodelling of the vessel; and (c) a variety of disease states. Apoptosis is a highly regulated mechanism of death, controlled by the interactions between the following: (i) cellular receptors; (ii) cytoplasmic and nuclear gene products; and (iii) the local cytokine and cellular environment within the plaque. The knowledge of the key regulators of apoptosis does, however, offer novel therapeutic targets in both the prevention and treatment of atherosclerosis.

Viruses and the cell cycle

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

Herpes simplex virus 1 induces and blocks apoptosis at multiple steps during infection and protects cells from exogenous inducers in a cell-type-dependent manner

Several publications have attested to the ability of herpes simplex viruses to protect cells against apoptosis. We investigated the ability of the virus to protect cells in continuous cultivation from apoptosis induced by the virus itself, and by other known inducers such as exposure to the tumor necrosis factor alpha (TNFalpha), antibody to Fas, C2-ceramide, osmotic shock (sorbitol), and thermal shock. The salient features of the results were that the virus was able to protect cells against apoptosis by all of the agents tested, and that apoptosis induced by the virus was a very early event that did not require de novo expression of viral genes. However, these events were cell-type specific. Thus: (i) The cell lines tested exhibited fragmented chromosomal DNA following infection with a virus lacking functional alpha4 and US3 genes encoding the major regulatory protein and a viral protein kinase, respectively, but not by wild-type virus. (ii) Wild-type virus protected subconfluent SK-N-SH but not HeLa cells against induction of apoptosis by anti-Fas antibody, TNFalpha, C2-ceramide, and thermal shock. Confluent SK-N-SH cells were not protected from osmotic shock-induced apoptosis by wild-type infection. (iii) Wild-type virus protected SK-N-SH but not HeLa cells against induction of apoptosis by sorbitol, anti-Fas antibody, or TNFalpha and C2-ceramide. (iv) Mutant HSV-1(HFEM)tsB7 at the nonpermissive temperature infects cells but the DNA is not released from capsids, and therefore viral gene expression is restricted to the function of viral proteins introduced into the cell along with the capsid containing the viral DNA. HSV-1(HFEM)tsB7 induced apoptosis in Vero cells but not in SK-N-SH cells infected and maintained at 39.5 degrees C. (v) Tests of two caspase inhibitors showed that they blocked apoptosis induced by C2-ceramide and sorbitol, but were not able to block apoptosis induced by the virus lacking functional alpha4 and US3 genes. We conclude that HSV-1 triggers apoptosis at multiple metabolic checkpoints and in turn has evolved mechanisms to block apoptosis at each point and that some of the pathways of induction are shared with exogenous inducers tested in this study whereas others are not.

Evaluation and mapping of the DNA binding and oligomerization domains of the IE2 regulatory protein of human cytomegalovirus using yeast one and two hybrid interaction assays

The 86-kDa IE2 nuclear phosphoprotein encoded by the human cytomegalovirus (HCMV) major immediate-early (MIE) gene behaves as both a non-specific transactivator of viral and cellular gene expression and as a specific DNA-binding protein targeted to the cis-repression sequence (CRS) at the cap site of its own promoter/enhancer region. Although the IE2 protein produced in bacteria has been shown to bind to the 14-bp palindromic CRS motif and IE2 synthesized in vitro forms stable dimers in solution through the conserved C-terminus of the protein, there is no direct evidence as yet that the intracellular mammalian forms of IE2 do so. Here, we show that the intact HCMV IE2 protein both binds to CRS DNA and dimerizes in yeast cells. In a one-hybrid assay system, a GAL4/IE2 fusion protein expressed in yeast cells activated target HIS3 expression only when CRS sites were located upstream of the GAL1 minimal promoter, but failed to do so on mutant CRS sites, demonstrating a requirement for sequence-specific DNA-binding by IE2. Examination of a series of deletion and triple amino acid point mutations in the C-terminal half of IE2 mapped the domains required for DNA-binding in yeast to the entire region between codons 313 and 579, whereas in the previous in-vitro study with truncated bacterial GST fusion proteins, it was mapped to between codons 346 and 579. Transient co-transfection assays with deleted IE2 effector genes in Vero cells showed that the extra segment of IE2 between codons 313 and 346 is also required for both autoregulation and transactivation activity in mammalian cells. In a two-hybrid assay to study IE2 self-interations, we generated both GAL4 DNA-binding (DB) and activation domain (A)/IE2 fusion proteins and showed that IE2 could also dimerize or oligomerize through the C-terminus of the protein in yeast cells. Domains required for this interaction were all mapped to within the region between codons 388 and 542, which is coincident with the domain mapped previously for dimerization by co-translation and immunoprecipitation in vitro. Comparison of the domains of the IE2 protein required for CRS binding and dimerization in yeast suggests that these activities correlate precisely with requirements for the negative autoregulation function of the IE2 protein in mammalian cells.

Herpesvirus in atherosclerosis and thrombosis: etiologic agents or ubiquitous bystanders?

The role of herpesvirus infections in the pathogenesis of vascular diseases remains an enigma. Although there is abundant circumstantial evidence of a role for herpesviruses in atherosclerosis and related processes, a cause-and-effect relationship has yet to be definitively established. This article will review the pathological, molecular, and biochemical evidence supporting the hypothesis that herpesviruses are involved in the development of atherosclerosis, restenosis after coronary angioplasty, accelerated atherosclerosis in recipients of heart transplants, and the induction of a prothrombotic phenotype in vascular endothelial cells.

Identification of positive and negative regulatory regions involved in regulating expression of the human cytomegalovirus UL94 late promoter: role of IE2-86 and cellular p53 in mediating negative regulatory function

The human cytomegalovirus (HCMV) UL94 gene product is a herpesvirus-common virion protein that is expressed with true late kinetics. To identify the important cis- and trans-acting factors which contribute to UL94 transcriptional regulation, we have cloned, sequenced, and analyzed UL94 promoter function by transient transfection analysis. Transfection of UL94 promoter-reporter gene constructs into permissive human fibroblasts or U373(MG) cells indicated that promoter activity was detected following infection with HCMV. Point mutations within a TATA-like element located upstream of the RNA start site significantly reduced UL94 promoter activity. Deletion mutagenesis of the promoter indicated that a positive regulatory element (PRE) was likely to exist downstream of the UL94 mRNA start site, while a negative regulatory element (NRE) was present upstream of the TATA box. At late times of infection, the PRE appeared to have a dominant effect over the NRE to stimulate maximum levels of UL94 promoter activity, while at earlier times of infection, no activity associated with the PRE could be detected. The NRE, however, appeared to cause constitutive down-regulation of UL94 promoter activity. Binding sites for the cellular p53 protein located within the NRE appeared to contribute to NRE function, and NRE function could be recapitulated in cotransfection assays by concomitant expression of p53 and HCMV IE2-86 protein. Our results suggest a novel mechanism by which the cellular protein p53, which is involved in both transcriptional regulation and progression of cellular DNA synthesis, plays a central role in the regulation of a viral promoter which is not activated prior the onset of viral DNA replication.

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

Previously, we reported that human cytomegalovirus (HCMV) infection of fibroblasts markedly affects p53 and other regulatory proteins and inhibits transit through the cell cycle (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). Although the p53 steady-state levels are elevated throughout the infection, evidence suggests that the ability of p53 to transactivate some of its downstream targets is compromised. To elucidate the mechanisms governing the accumulation of p53, we examined the synthesis, stability, and localization of the protein in HCMV-infected fibroblasts. Synthesis of p53 was not increased in the infected cells during the first 24 h postinfection. In fact, pulse-chase experiments revealed that synthesis of p53 in infected fibroblasts was lower than in mock-infected cells. However, after an initial decay, the p53 was stabilized. In addition, beginning at approximately 30 h postinfection, p53 was localized to discrete foci within the nuclei of infected cells. The morphology of these foci suggested that they were replication centers. We confirmed that these are sites of DNA replication by demonstrating both incorporation of bromodeoxyuridine and localization of UL44 (the viral polymerase processivity factor) into these centers. The single-stranded DNA binding protein RPA was also sequestered. In contrast, Rb and HCMV IE1 72 remained distributed throughout the infected cell nuclei, indicating specific targeting of certain proteins. Taken together, our results provide two alternative mechanisms to account for the increased steady-state levels of p53 observed in HCMV-infected fibroblasts.

The HIV-1 vpr protein acts as a negative regulator of apoptosis in a human lymphoblastoid T cell line: possible implications for the pathogenesis of AIDS

Although apoptosis is considered one of the major mechanisms of CD4(+) T cell depletion in HIV-infected patients, the virus-infected cells somehow appear to be protected from apoptosis, which generally occurs in bystander cells. Vpr is an auxiliary HIV-1 protein, which, unlike the other regulatory gene products, is present at high copy number in virus particles. We established stable transfectants of CD4+ T Jurkat cells constitutively expressing low levels of vpr. These clones exhibited cell cycle characteristics similar to those of control-transfected cells. Treatment of control clones with apoptotic stimuli (i.e., cycloheximide/tumor necrosis factor alpha (TNF-alpha), anti-Fas antibody, or serum starvation) resulted in a massive cell death by apoptosis. In contrast, all the vpr-expressing clones showed an impressive protection from apoptosis independently of the inducer. Notably, vpr antisense phosphorothioate oligodeoxynucleotides render vpr-expressing cells as susceptible to apoptosis induced by cycloheximide and TNF-alpha as the control clones. Moreover, the constitutive expression of HIV-1 vpr resulted in the upregulation of bcl-2, an oncogene endowed with antiapoptotic activities, and in the downmodulation of bax, a proapoptotic factor of the bcl-2 family. Altogether, these results suggest that low levels of the endogenous vpr protein can interfere with the physiological turnover of T lymphocytes at early stages of virus infection, thus facilitating HIV persistence and, subsequently, viral spread. This might explain why apoptosis mostly occurs in bystander uninfected cells in AIDS patients.

The fight of viruses against apoptosis

The induction of apoptosis of virus-infected cells is an important host defense mechanism against invading pathogens. Some viruses express anti-apoptotic proteins that efficiently block apoptosis induced by death receptors or in response to stress signaled through mitochondria. Viral interference with host cell apoptosis leads to enhanced viral replication and may promote cancer.

Nucleo-cytoplasmic shuttling of the hdm2 oncoprotein regulates the levels of the p53 protein via a pathway used by the human immunodeficiency virus rev protein

The hdm2 gene is overexpressed in a variety of human tumors. Its gene product localizes predominantly to the nucleus, where it acts as an inhibitor of the p53 tumor suppressor gene product. It is shown here that the hdm2 oncoprotein constantly shuttles between the nucleus and the cytoplasm. Shuttling of hdm2 does not depend on its interaction with p53. Nuclear export of hdm2 is mediated by a signal sequence similar to the nuclear export signal of the rev protein from human immunodeficiency virus and other lentiviruses. Mutation of this signal sequence abolishes detectable nucleo-cytoplasmic shuttling. When fused to a carrier protein, the hdm2 signal sequence can mediate nuclear export after intranuclear microinjection into HeLa cells. The export of hdm2 can be blocked by a competitive inhibitor of rev export, arguing that the export pathways for hdm2 and rev are either overlapping or identical. Inhibition of its export modifies the ability of hdm2 to block p53-mediated transcriptional activation, and hdm2's export function is required to accelerate the degradation of p53. Thus the rev nuclear export pathway may be used to regulate an oncogene product's activity and modulate cellular growth.

X-gene product of hepatitis B virus induces apoptosis in liver cells

Hepatitis B virus is a causative agent of hepatocellular carcinoma, and in the course of tumorigenesis, the X-gene product (HBx) is known to play important roles. Here, we investigated the transforming potential of HBx by conventional focus formation assay in NIH3T3 cells. Cells were cotransfected with the HBx expression plasmid along with other oncogenes including Ha-ras, v-src, v-myc, v-fos, and E1a. Unexpectedly, the introduction of HBx completely abrogated the focus-forming ability of all five tested oncogenes. In addition, the cotransfection of Bcl-2, an apoptosis inhibitor, reversed the HBx-mediated inhibition of focus formation, suggesting that the observed repression of focus formation by HBx is through the induction of apoptosis. Next, to test unequivocally whether HBx induces apoptosis in liver cells, we established stable Chang liver cell lines expressing HBx under the control of a tetracycline-inducible promoter. Induction of HBx in these cells in the presence of 1% calf serum resulted in typical apoptosis phenomena such as DNA fragmentation, nuclear condensation, and fragmentation. Based on these results, we propose that HBx sensitizes liver cells to apoptosis upon hepatitis B virus infection, contributing to the development of hepatitis and the subsequent generation of hepatocellular carcinoma.

Transforming potential of the herpesvirus oncoprotein MEQ: morphological transformation, serum-independent growth, and inhibition of apoptosis

Marek's disease virus (MDV) induces the rapid development of overwhelming T-cell lymphomas in chickens. One of its candidate oncogenes, meq (MDV Eco Q) which encodes a bZIP protein, has been biochemically characterized as a transcription factor. Interestingly, MEQ proteins are expressed not only in the nucleoplasm but also in the coiled bodies and the nucleolus. Its novel subcellular localization suggests that MEQ may be involved in other functions beyond its transcriptional potential. In this report we show that MEQ proteins are expressed ubiquitously and abundantly in MDV tumor cell lines. Overexpression of MEQ results in transformation of a rodent fibroblast cell line, Rat-2. The criteria of transformation are based on morphological transfiguration, anchorage-independent growth, and serum-independent growth. Furthermore, MEQ is able to distend the transforming capacity of MEQ-transformed Rat-2 cells through inhibition of apoptosis. Specifically, MEQ can efficiently protect Rat-2 cells from cell death induced by multiple modes including tumor necrosis factor alpha, C2-ceramide, UV irradiation, and serum deprivation. Its antiapoptotic function requires new protein synthesis, as treatment with a protein synthesis inhibitor, cycloheximide, partially reversed MEQ's antiapoptotic effect. Coincidentally, transcriptional induction of bcl-2 and suppression of bax are also observed in MEQ-transformed Rat-2 cells. Taken together, our results suggest that MEQ antagonizes apoptosis through regulation of its downstream target genes involved in apoptotic and/or antiapoptotic pathways.

Defective growth correlates with reduced accumulation of a viral DNA replication protein after low-multiplicity infection by a human cytomegalovirus ie1 mutant

To investigate the importance of the IE1 p72 regulatory protein during human cytomegalovirus replication, a recombinant virus unable to synthesize IE1 p72 was constructed. The Towne strain mutant CR208 lacked exon 4 of the major immediate-early gene and was isolated and complemented in an IE1-expressing immortalized human fibroblast line (ihfie1.3). Replication of CR208 in primary human fibroblasts was completed after an input multiplicity of 10 PFU/cell but was severely-impaired at 0.1 PFU/cell. CR208 formed plaques with lower efficiency on primary fibroblasts than on ihfiel.3 cells, and the relationship between the CR208 inoculum size and the resulting number of undersized plaques was nonlinear, indicating that multiple particles of CR208 were required to initiate lytic replication in a single primary fibroblast. After infection of primary fibroblasts with CR208 at 5 PFU/cell, a normal pattern of viral antigens was detected, although IE1 p72 was absent. During lower-multiplicity infections, IE2 protein was consistently detected at similar levels in a similar proportion of CR208-infected cells relative to the case for a Towne infection, but many fewer CR208-infected cells contained the ppUL44 polymerase accessory protein when evaluated at 24 or 48 h after infection. Furthermore, fibroblasts infected with CR208 at a low multiplicity failed to form viral DNA replication compartments, despite having expressed IE2 p86. These low-multiplicity growth and expression defects were corrected in two rescued derivatives of CR208 able to synthesize IE1 p72. One rescued virus (CR249) carried a deletion removing the large intron between exons 1 and 2 of the ie1-ie2 locus, revealing that this intron was dispensable for growth in cell culture.

Apoptosis and host restriction of vaccinia virus in RK13 cells

Poxviruses express a number of host range (hr) genes that control virus growth in distinctive cell types. Inactivation of hr gene expression in several reported cases has led to apoptosis of virus-infected cells. In RK13 cells, the K1L gene serves as a hr gene for vaccinia virus. We therefore investigated the effect of K1L expression in apoptosis of RK13 cells. In contrast to other hr genes, no significant increase of apoptosis was detected in RK13 cells infected with a K1L- mutant virus. Also, expression of a CHO hr gene CP77 rescues K1L- mutant virus in RK13 cells with little effect on apoptosis. We then set out an experimental approach to investigate the relationship between apoptosis and host restriction in CHO and RK13 cells. A recombinant vaccinia virus expressing a human bcl-2 gene, bcl2-VV, was constructed. Expression of bcl-2 suppressed apoptosis of virus-infected CHO cells as expected. However, bcl-2 expression did not allow virus growth in CHO cells, suggesting apoptosis suppression is not sufficient to rescue host restriction. Moreover, infection of bcl-2VV in RK13 cells induced significant apoptosis with no reduction on virus production, indicating that apoptosis does not contribute to host restriction. In consideration of this data, we conclude that host restriction of vaccinia virus in CHO and RK13 cells is mediated by a pathway distinct from apoptosis.

Negative regulation of a heterologous promoter by human cytomegalovirus immediate-early protein IE2

The HCMV IE2 protein promiscuously activates transcription of many viral and cellular genes. IE2 also negatively autoregulates its own expression by binding to a strategically positioned IE2 binding site, called CRS, located immediately downstream of the TATA box of the HCMV major IE promoter. Here we show that IE2 is able to repress transcription driven by a heterologous promoter, RSV LTR. Repression of RSV LTR by IE2 is completely dependent of DNA sequences downstream of the TATA box of RSV LTR. A DNA sequence, 5'-CGATACAATAAACG-3', evidently matching the proposed CRS consensus sequence, is located between nucleotides -13 and +1 (relative to the transcription start site) of RSV LTR. Three lines of evidence support the notion that this RSV CRS element is involved in the IE2-mediated repression of RSV LTR. First, introduction of mutation to the RSV CRS element renders to the mutant RSV LTR resistance to IE2-mediated repression. Second, a mutant IE2 defective in DNA binding cannot downregulate transcription from RSV LTR. Third, IE2 specifically binds to the wild-type, but not the mutant, RSV CRS element in vitro. These data, in conjunction with previous works, demonstrate that IE2 can passively repress transcription of homologous and heterologous promoters that contain a CRS element.

Identification of domains within the human cytomegalovirus major immediate-early 86-kilodalton protein and the retinoblastoma protein required for physical and functional interaction with each other

The human cytomegalovirus major immediate-early 86-kDa protein (IE2 86) plays an important role in the trans activation and regulation of HCMV gene expression. Previously, we demonstrated that IE2 86 contains three regions (amino acids [aa] 86 to 135, 136 to 290, and 291 to 364) that can independently bind to in vitro-translated Rb when IE2 86 is produced as a glutathione S-transferase fusion protein (M. H. Sommer, A. L. Scully, and D. H. Spector, J. Virol. 68:6223-6231, 1994). In this report, we have elucidated the regions of Rb involved in binding to IE2 86 and have further analyzed the functional nature of the interaction between these two proteins. We find that two domains on Rb, the A/B pocket and the carboxy terminus, can each independently form a complex with IE2 86. In functional assays, we demonstrate that IE2 86 and another IE protein, IE1 72, can counter the enlarged flat cell phenotype, but not the G1/S block, which results from expression of wild-type Rb in the human osteosarcoma cell line Saos-2. Mutational analysis reveals that there are two domains on IE2 86 that can independently affect Rb function. One region (aa 241 to 369) includes the major Rb-binding domain, while the second maps to the amino-terminal region (aa 1 to 85) common to both IE2 86 and IE1 72. These data show that Rb and IE2 86 physically and functionally interact with each other via at least two separate domains and provide further support for the hypothesis that IE2 86 may exert its pleiotropic effects through the formation of multimeric protein complexes.

Export of hepatitis B virus RNA on a Rev-like pathway: inhibition by the regenerating liver inhibitory factor IkappaB alpha

Nuclear export of hepatitis B virus (HBV) RNA is mediated by a specific RNA element but, in contrast to lentivirus genomic RNA, does not depend on viral proteins. We show that nonetheless, the export of HBV RNA can be blocked by competitive inhibitors of Rev-mediated lentivirus RNA export, suggesting that the export pathways of both viral species share components and might be driven by the same nuclear export machinery. HBV RNA export is also inhibited by overexpression of IkappaB alpha, as reported previously for the export of human immunodeficiency virus RNA. Since IkappaB alpha is strongly overexpressed during liver regeneration, its inhibition of HBV RNA export might contribute to elimination or silent persistence of HBV.

Human cytomegalovirus IE2 86-kilodalton protein binds p53 but does not abrogate G1 checkpoint function

Physical interactions between human cytomegalovirus (HCMV) immediate-early (IE) proteins and key cell cycle regulatory proteins have been suggested as a mechanism whereby this herpesvirus modifies cellular control of proliferation. Observed similarities to interactions of other DNA virus proteins (human papillomavirus type 16 E6 and E7, simian virus 40 large T antigen, and adenovirus type 5 E1A and E1B) with cell cycle modulatory proteins such as p53 and Rb have suggested that HCMV IE proteins may likewise alter the G1-to-S phase transition. The IE2 region gene product IE86 has been shown to specifically bind p53, potentially modifying p53 G1 checkpoint function. To examine this possibility, p53-mediated G1 arrest in the presence of IE86 was assessed. Retroviral constructs were created to facilitate the stable expression of IE86 and IE72, another IE protein implicated in HCMV-mediated alteration of cell cycle progression. Western analysis and immunoprecipitation confirmed IE protein expression and binding of IE86 to p53, respectively. Chloramphenicol acetyltransferase assays examining the ability of IE86 to repress activity from the HCMV major IE promoter or activate the HCMV early promoter for the 2.2-kb class of RNAs demonstrated the functional integrity of the IE86 protein. Induction of DNA damage in normal, uninfected fibroblasts (FB) or FB expressing IE86 by actinomycin D (Act D) resulted in increased p53 levels, a predominance of the hypophosphorylated form of Rb, and increased expression of both p21(CIP1/WAF1) and mdm-2. Fluorescence-activated cell sorting revealed that both uninfected and IE86-expressing FB experienced dramatic G1 arrest following exposure to Act D. The clear demonstration of these p53-dependent responses in the presence of IE86 indicates that binding to this viral protein does not compromise the ability of p53 to elicit growth arrest following DNA damage.

Nuclear export of the E1B 55-kDa and E4 34-kDa adenoviral oncoproteins mediated by a rev-like signal sequence

The E1B 55-kDa and E4 34-kDa oncoproteins of adenovirus type 5 (abbreviated here as E1B-55kD and E4-34kD) promote the export of viral mRNA and inhibit the export of most cellular mRNA species. We show that the intracellular complex containing E1B-55kD and E4-34kD continuously shuttles between the nucleus and the cytoplasm, and may thus serve as a nucleocytoplasmic transporter for viral mRNA. We present evidence that within this complex, it is the E4-34kD protein that directs both nuclear import and nuclear export. E4-34kD contains a functional nuclear export signal similar to corresponding sequences found in the retroviral proteins rev and rex. This sequence element is required for nuclear export of the complex, and it can function autonomously when fused to a carrier protein and microinjected in HeLa cell nuclei. When E4-34kD is expressed alone, a portion of the protein that contains a predicted arginine-rich amphipathic alpha-helical structure mediates nuclear retention of the protein. This retention, however, can be abolished by the association with E1B-55kD or by a specific point mutation within the arginine-rich motif. The export of E4-34kD can be blocked by an HTLV-rex derived competitive inhibitor and overexpressed E4-34kD inhibits rev-mediated transport, suggesting that the export pathways accessed by the adenoviral and retroviral proteins share components. The interplay between two polypeptides as well as the involvement of a dominant nuclear retention domain are novel features that might contribute to the efficiency and regulation of the adenovirus export system.

Monocytes harboring cytomegalovirus: interactions with endothelial cells, smooth muscle cells, and oxidized low-density lipoprotein. Possible mechanisms for activating virus delivered by monocytes to sites of vascular injury

Cytomegalovirus (CMV) infection and its periodic reactivation from latency may contribute to atherogenesis and restenosis. It is unknown how CMV is delivered to the vessel wall and is reactivated. We examined the following hypothesis: CMV, present in monocytes recruited to sites of vascular injury, is activated by endothelial cell (EC) or smooth muscle cell (SMC) contact and by oxidized low-density lipoproteins (oxLDLs). The CMV major immediate-early promoter (MIEP) controls immediate-early (IE) gene expression, and thereby viral replication. To determine whether elements of the vessel wall can activate CMV present in monocytes, we transiently transfected the promonocytic cell line HL-60 with a chloramphenicol acetyltransferase reporter gene construct driven by MIEP. MIEP activity increased 1.7 +/- 0.5-fold (P = .02) when the transfected HL-60 cells were cocultured with ECs, 4.5 +/- 1.5-fold when cocultured with SMCs (P = .03), and 2.0 +/- 0.5-fold (P = .01) when exposed to oxLDL. The combination of oxLDL and EC coculture increased MIEP activity over 7-fold. We also found that freshly isolated human monocytes, infected with endothelium-passaged CMV, were capable of transmitting infectious virus to cocultured ECs or SMCs. CMV-related progression of atherosclerosis or restenosis may, at least in part, involve monocyte delivery of the virus to the site of vascular injury, where the vascular milieu, ie, contact with ECs, SMCs, and oxLDL, can contribute to viral reactivation and/or replication by enhancing CMV IE gene expression. The virus may then infect neighboring ECs or SMCs, initiating a cascade of events predisposing to the development of atherogenesis-related processes.

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

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

Apoptosis induced in the spinal cord and dorsal root ganglion by infection of herpes simplex virus type 2 in the mouse

After inoculation of a highly neuro-invasive strain of herpes simplex virus (HSV) type 2 (186) into the mouse hind-paw planter skin, many virus-positive neurons and glial cells were detected in the dorsal root ganglia (DRGs) and lumbar spinal cord by immunohistochemistry for HSV-2 antigen. A number of apoptotic cells were also observed in the spinal cord and DRGs by the terminal dUTP nick-end-labeling (TUNEL) method. Double labeling with the immunohistochemistry for HSV-2 and the TUNEL method revealed further that some glial and neuronal cells in the spinal cord, either infected or non-infected by HSV-2, showed apoptotic signs. In DRGs, however, apoptosis was detected in no neuronal cells, although some of HSV-2 -infected glial cells were apoptotic.

The major immediate-early proteins IE1 and IE2 of human cytomegalovirus colocalize with and disrupt PML-associated nuclear bodies at very early times in infected permissive cells

The major immediate-early (MIE) gene products of human cytomegalovirus (HCMV) are nuclear phosphoproteins that are thought to play key roles in initiating lytic cycle gene regulation pathways. We have examined the intranuclear localization pattern of both the IE1 and IE2 proteins in virus-infected and DNA-transfected cells. When HCMV-infected human diploid fibroblast (HF) cells were stained with specific monoclonal antibodies, IE1 localized as a mixture of nuclear diffuse and punctate patterns at very early times (2 h) but changed to an exclusively nuclear diffuse pattern at later times. In contrast, IE2 was distributed predominantly in nuclear punctate structures continuously from 2 to at least 12 h after infection. These punctate structures resembled the preexisting PML-associated nuclear bodies (ND10 or PML oncogenic domains [PODs]) that are disrupted and dispersed by the IE110 protein as a very early event in herpes simplex virus (HSV) infection. However, HCMV differed from HSV by leading instead to a change in both the PML and SP100 protein distribution from punctate bodies to uniform diffuse patterns, a process that was complete in 50% of the cells at 2 h and in 90% of the cells by 4 h after infection. Confocal double-label indirect immunofluorescence assay analysis confirmed that both IE1 and IE2 colocalized transiently with PML in punctate bodies at very early times after infection. In transient expression assays, introduction of IE1-encoding plasmid DNA alone into Vero or HF cells produced the typical total redistribution of PML into a uniform nuclear diffuse pattern together with the IE1 protein, whereas introduction of IE2-encoding plasmid DNA alone resulted in stable colocalization of the IE2 protein with PML in the PODs. A truncated mutant form of IE1 gave large nuclear aggregates and failed to redistribute PML, and similarly a deleted mutant form of IE2 failed to colocalize with the punctate PML bodies, confirming the specificity of these effects. Furthermore, both Vero and U373 cell lines constitutively expressing IE1 also showed total PML relocalization together with the IE1 protein into a nuclear diffuse pattern, although a very small percentage of the cells which failed to express IE1 reverted to a punctate PML pattern. Finally, the PML redistribution activity of IE1 and the direct association of IE2 with PML punctate bodies were both confirmed by infection with E1A-negative recombinant adenovirus vectors expressing either IE1 or IE2 alone. These results confirm that transient colocalization with and disruption of PML-associated nuclear bodies by IE1 and continuous targeting to PML-associated nuclear bodies by IE2 are intrinsic properties of these two MIE regulatory proteins, which we suggest may represent critical initial events for efficient lytic cycle infection by HCMV.

Human cytomegalovirus tegument protein pp71 (ppUL82) enhances the infectivity of viral DNA and accelerates the infectious cycle

Three tegument proteins of human cytomegalovirus (HCMV), ppUL82 (pp71), pUL69, and ppUL83 (pp65), were examined for the ability to stimulate the production of infectious virus from human diploid fibroblasts transfected with viral DNA. Although viral DNA alone had a low intrinsic infectivity of 3 to 8 plaques/microg of viral DNA, cotransfection of a plasmid expressing pp71 increased the infectivity of HCMV DNA 30- to 80-fold. The increase in infectivity produced by pp71 was reflected in an increased number of nuclei observed to express high levels of the major immediate-early proteins IE1 and IE2. Cotransfection of viral DNA with plasmids directing expression of IE1 and IE2 also resulted in extensive IE1 and IE2 expression in the transfected cells; however, the infectivity of viral DNA was only marginally increased. pp71 also facilitated late gene expression, virus transmission to adjacent cells, and plaque formation. In contrast, expression of pUL69 reduced the pp71- and IE1/IE2-mediated enhancement of HCMV DNA infectivity and also failed to produce any increase in the number of cells expressing IE1 and IE2 over that seen with viral DNA alone. Expression of pp65 did not alter the infectivity of HCMV DNA, nor did it modify the effects of pp71 or pUL69. These results imply that pp71 plays a critical role in the initiation of infection apart from its function as a transactivator of IE1 and IE2.

Identification of a novel antiapoptotic protein, GAM-1, encoded by the CELO adenovirus

We have developed a simple screening method to identify genes that mimic bcl-2 or adenovirus E1B 19K in enhancing cell survival after transfection and have used this method to identify such a gene in the avian adenovirus CELO. The gene encodes a novel 30-kDa nuclear protein, which we have named GAM-1, that functions comparably to Bcl-2 and adenovirus E1B 19K in blocking apoptosis. However, GAM-1 has no sequence homology to Bcl-2, E1B 19K, or any other known antiapoptotic proteins and thus defines a novel antiapoptotic function.

Human cytomagalovirus IE1 and IE2 proteins are mutagenic and mediate "hit-and-run" oncogenic transformation in cooperation with the adenovirus E1A proteins

Some epidemiological studies have suggested a possible link between human cytomegalovirus (HCMV) infection and various malignancies, and HCMV has been shown to transform cultured cells. However, viral DNA is not detected in most transformants, and the mechanism by which HCMV might contribute to oncogenesis has remained obscure. Here we show that the HCMV immediate early 1 and 2 genes can cooperate with the adenovirus E1A gene to generate transformed foci of primary baby rat kidney cells. HCMV gene expression is transient and viral DNA is not present in clonal cell lines derived from the transformed foci. We find that the HCMV immediate early proteins are mutagenic, and we propose that HCMV has the potential to contribute to oncogenesis through a "hit-and-run" mechanism, by inducing mutations in cellular genes.

Human cytomegalovirus infection inhibits G1/S transition

Cell cycle progression during cytomegalovirus infection was investigated by fluorescence-activated cell sorter (FACS) analysis of the DNA content in growth-arrested as well as serum-stimulated human fibroblasts. Virus-infected cells maintained in either low (0.2%) or high (10%) serum failed to progress into S phase and failed to divide. DNA content analysis in the presence of G1/S (hydroxyurea and mimosine) and G2/M (nocodazole and colcemid) inhibitors demonstrated that upon virus infection of quiescent (G0) cells, the cell cycle did not progress beyond the G1/S border even after serum stimulation. Proteins which normally indicate G1/S transition (proliferating cell nuclear antigen [PCNA]) or G2/M transition (cyclin B1) were elevated by virus infection. PCNA levels were induced in infected cells and exhibited a punctate pattern of nuclear staining instead of the diffuse pattern observed in mock-infected cells. Cyclin B1 was induced in infected cells which exhibited a G1/S DNA content by FACS analysis, suggesting that expression of this key cell cycle function was dramatically altered by viral functions. These data demonstrate that contrary to expectations, cytomegalovirus inhibits normal cell cycle progression. The host cell is blocked prior to S phase to provide a favorable environment for viral replication.

Characterization of the human cytomegalovirus irs1 and trs1 genes: a second immediate-early transcription unit within irs1 whose product antagonizes transcriptional activation

We have characterized the irs1 and trs1 genes of human cytomegalovirus. The previously identified mRNAs as well as their corresponding protein products pIRS1 and pTRS1 could be detected during all phases of the viral replication cycle. The proteins were present in the nucleus and cytoplasm during the immediate-early and early phases of the viral growth cycle but were predominantly cytoplasmic late after infection. Although pIRS1 and pTRS1 exhibited little transcriptional activation potential on their own, both cooperated with the IE1 and IE2 proteins to enhance expression from a variety of viral promoters. We have also identified a previously undescribed immediate-early gene product encoded within the irs1 gene that we have termed pIRS1(263). This new protein is encoded within the 3' end of the irs1 gene and is in the same reading frame as the large pIRS1 protein. Expression of the irs1(263) gene is controlled by a promoter that resides within the irs1 open reading frame in the unique short region of the viral genome. pIRS1(263) resides in the nucleus and antagonizes transcriptional activation by cytomegalovirus immediate-early proteins. We propose that pIRS1(263), whose promoter responds to immediate-early transcriptional activators, serves as part of a regulatory loop, antagonizing the function of the viral proteins that are responsible for its synthesis.

Human cytomegalovirus infection inhibits cell cycle progression at multiple points, including the transition from G1 to S

Human cytomegalovirus inhibits the growth of human foreskin fibroblast cells by 12 h after infection. Analysis of the cellular DNA content of infected cells by flow cytometry demonstrated that cytomegalovirus does not arrest cell cycle progression at a single point. At least two blockages occur, one of which is in the G1 phase of the cell cycle. The G1 arrest introduced by cytomegalovirus infection blocks S-phase entry after serum stimulation.

The activity of cytosolic phospholipase A2 is required for the lysis of adenovirus-infected cells by tumor necrosis factor

Most cell types are resistant to apoptosis induced by tumor necrosis factor (TNF) unless the cells are treated with a sensitizing agent. Inhibitors of transcription or translation act as sensitizing agents, as do adenoviruses lacking one or more resistance genes. We have reported recently that the activity of cytosolic phospholipase A2 (cPLA2) is necessary for the TNF-induced lysis of cells that are sensitized by inhibitors of transcription or translation (C. Voelkel-Johnson, T. E. Thorne, and S. M. Laster, J. Immunol. 156:201-207, 1996). In this report we have asked whether the lysis of cells infected by the adenovirus dl758 (which lacks the E3 14.7-kDa resistance gene product) also involves the activity of cPLA2. We report that a phosphorothioate-modified antisense oligonucleotide specific for cPLA2, but not the control oligonucleotide, inhibited the TNF-induced release of both [3H]arachidonic acid and 51Cr from infected cells. Arachidonyltrifluoromethyl ketone (AA COCF3), an inhibitor of cPLA2, also inhibited the release of 51Cr, and we found that the release of [3H]arachidonic acid was highly selective and was preferred over the release of [3H]palmitic acid. Taken together, these results suggest strongly that cPLA2 is indeed the phospholipase responsible for the release of [3H]arachidonic acid during the lysis of infected cells and that its activity is necessary for cell death. Finally, since arachidonic acid serves as the substrate for the synthesis of inflammatory lipids, our results suggest a possible link between the TNF-induced lysis of infected cells and inflammation. The E3 14.7-kDa resistance protein may, therefore, play two roles: preventing TNF-induced cell death and, as our results show, preventing the TNF-induced release of arachidonic acid.

The human cytomegalovirus IE1-72 protein interacts with the cellular p107 protein and relieves p107-mediated transcriptional repression of an E2F-responsive promoter

The Rb-related p107 protein has been implicated as an important control element in proper cell cycle progression. The p107 protein is thought to restrict cellular proliferation in part through its interaction with the E2F family of transcription factors and is, therefore, a specific target for regulation by several DNA viruses. Here, we demonstrate that p107 protein levels are induced in a biphasic manner in human fibroblasts during productive infection by the human cytomegalovirus (HCMV). Expression patterns of p107 protein levels during HCMV infection of human embryonic lung cells (HELs) demonstrate a sustained induction from early to late times of infection. We also demonstrate that the HCMV immediate-early protein IE1-72 complexes in vivo with the p107 protein and that this interaction can be reconstituted in an in vitro system by using reticulocyte-translated protein. Our data demonstrate that the interaction between p107 and the IE1-72 protein occurs at times of infection that temporally match the second tier of p107 protein induction and the phosphorylation pattern of the IE1-72 protein. Furthermore, we show here that the ability of p107 to transcriptionally repress E2F-responsive promoters can be overcome by expression of the IE1-72 protein. This effect appears to be specific, since the IE1-72 protein is not capable of relieving Rb-mediated repression of an E2F-responsive promoter. Finally, our data demonstrate that HCMV infection can induce cellular proliferation in quiescent cells and that IE1-72 expression alone can, to a degree, drive a similar progression through the cell cycle. These data suggest that IE1-72-mediated transactivation of E2F-responsive promoters through alleviation of p107 transcriptional repression may play a key role in the cell cycle progression stimulated by HCMV infection.

Cytoplasmic sequestration of p53 in cytomegalovirus-infected human endothelial cells

Human umbilical vein endothelial cells were infected with human cytomegalovirus (HCMV) at a multiplicity of infection of 0.1 plaque-forming unit/cell and remained > 95% viable even after 10 days of infection. To induce apoptosis, control human umbilical vein endothelial cells and cells infected with HCMV for 3, 5, and 7 days were serum starved for 48 hours. Almost one-half of the uninfected cells lost viability after 48 hours of serum starvation whereas HCMV-infected cells were virtually unaffected (< 20% death, P < 0.05). Uninfected cells showed typical hallmarks of apoptosis, including unique morphological changes and DNA laddering. HCMV-infected cells, concomitant with their resistance to serum-starvation-induced death, displayed almost none of these characteristics. Active replication of HCMV was necessary for the anti-apoptotic effect, as cells treated with ultraviolet light-inactivated virus were not protected. p53, the G1/S phase cell cycle brake protein, was elevated in HCMV-infected cells. However, rather than accumulating in the nucleus, immunofluorescent and Western blot studies demonstrated remarkable and predominant cytoplasmic sequestration of p53 in HCMV-infected endothelial cells. Although HCMV proteins have already been shown to block apoptosis, we suggest that the aberrant subcellular pattern of p53 is the disturbed cellular mechanism that may be responsible for the anti-apototic properties of HCMV-infected cells. The selective resistance to apoptosis can be important during HCMV replication and may explain the oncogenic potential of HCMV as well as its pathogenic role in intimal-proliferation-mediated vascular diseases.

Evidence that the UL84 gene product of human cytomegalovirus is essential for promoting oriLyt-dependent DNA replication and formation of replication compartments in cotransfection assays

The protein products of 11 viral genomic loci cooperate in a transient cotransfection assay to mediate lytic-phase DNA replication of oriLyt, the human cytomegalovirus (HCMV) origin of replication. Six of these genes have homology with the well-characterized herpes simplex virus replication genes and encode core replication machinery proteins that are typically essential for DNA synthesis. The remaining five HCMV gene loci, initially referred to as auxiliary components, include several known immediate-early (IE) transcriptional regulatory proteins as well as genes encoding functionally uncharacterized polypeptides. Some or all of the auxiliary components may be necessary in trans to replicate the HCMV oriLyt only because they are required for efficient expression or transactivation of the native early promoters and 3' processing elements included in the genomic clones. Therefore, we reassessed the requirements for the auxiliary components by adding constitutive heterologous promoters and control signals to the coding regions and carrying out transient DpnI replication assays in cotransfected Vero cells. The results revealed that in the presence of the UL69 posttranscriptional activator and the remaining auxiliary polypeptides, UL84 was the only auxiliary component that could not be omitted to obtain oriLyt-dependent DNA replication. Nevertheless, in human diploid fibroblasts, some additional auxiliary loci as well as UL84 were critical. There was also an obligatory requirement for UL84, in cooperation with two other auxiliary factors, UL112-113 and IE2, and the core machinery, to constitute the minimal HCMV proteins necessary to direct oriLyt-dependent DNA amplification. However, the Epstein-Barr virus core replication genes could substitute for the HCMV core genes, and in these circumstances, UL84 alone directed amplification of HCMV oriLyt. Moreover, there was also an absolute requirement for UL84 along with the core and other auxiliary factors for the formation of intranuclear replication compartments as assayed by immunofluorescence in transient DNA cotransfection assays. These compartments were typical of those associated with active viral DNA replication in HCMV-infected cells, they incorporated pulse-labeled bromodeoxyuridine, and their formation was both phosphonoacetic acid sensitive and oriLyt dependent. These results demonstrate that UL84 is obligatory for both intranuclear replication compartment formation and origin-dependent DNA amplification and suggest that it is a key viral component in promoting the initiation of HCMV oriLyt-directed DNA replication.

Oncogenic potential of the adenovirus E4orf6 protein

The group C adenovirus E4orf6 protein has previously been shown to bind to the p53 cellular tumor suppressor protein and block its ability to activate transcription. Here we show that the E4orf6 protein blocks the induction of p53-mediated apoptosis when AT6 cells, which harbor a temperature-sensitive p53, are shifted to the permissive temperature. The E4orf6 protein does not, however, prevent the induction of apoptosis in p53-deficient H1299 cells by treatment with tumor necrosis factor alpha and cycloheximide. The E4orf6 protein also cooperates with the adenovirus E1A protein to transform primary baby rat kidney cells, and it cooperates with the adenovirus E1A plus E1B 19-kDa and E1B 55-kDa proteins to increase the number of baby rat kidney cell transformants and enhance the rate at which they arise. The level of p53 is substantially reduced in transformed cells expressing the E4orf6 protein in comparison to adenovirus transformants lacking it. The E4orf6 gene also accelerates tumor formation when transformed baby rat kidney cells are injected subcutaneously into the nude mouse, and it converts human 293 cells from nontumorigenic to tumorigenic in nude mice. In addition to the well-studied E1A and E1B oncogenes, group C adenoviruses harbor a third oncogene, E4orf6, which functions in some respects similarly to the E1B oncogene.