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

DNA damage response(DDR): a link between cellular senescence and human cytomegalovirus

The DNA damage response (DDR) is a signaling cascade that is triggered by DNA damage, involving the halting of cell cycle progression and repair. It is a key event leading to senescence, which is characterized by irreversible cell cycle arrest and the senescence-associated secretory phenotype (SASP) that includes the expression of inflammatory cytokines. Human cytomegalovirus (HCMV) is a ubiquitous pathogen that plays an important role in the senescence process. It has been established that DDR is necessary for HCMV to replicate effectively. This paper reviews the relationship between DDR, cellular senescence, and HCMV, providing new sights for virus-induced senescence (VIS).

Intercellular communication within the virus microenvironment affects the susceptibility of cells to secondary viral infections

Communication between infected cells and cells in the surrounding tissue is a determinant of viral spread. However, it remains unclear how cells in close or distant proximity to an infected cell respond to primary or secondary infections. We establish a cell-based system to characterize a virus microenvironment, distinguishing infected, neighboring, and distal cells. Cell sorting, microscopy, proteomics, and cell cycle assays allow resolving cellular features and functional consequences of proximity to infection. We show that human cytomegalovirus (HCMV) infection primes neighboring cells for both subsequent HCMV infections and secondary infections with herpes simplex virus 1 and influenza A. Neighboring cells exhibit mitotic arrest, dampened innate immunity, and altered extracellular matrix. Conversely, distal cells are poised to slow viral spread due to enhanced antiviral responses. These findings demonstrate how infection reshapes the microenvironment through intercellular signaling to facilitate spread and how spatial proximity to an infection guides cell fate.

Suppressive effects of pterostilbene on human cytomegalovirus (HCMV) infection and HCMV-induced cellular senescence

BACKGROUND

Human cytomegalovirus (HCMV), a member of the β-herpesvirus family, causes the establishment of a latent infection that persists throughout the life of the host and can be reactivated when immunity is weakened. To date, there is no vaccine to prevent HCMV infection, and clinically approved drugs target the stage of viral replication and have obvious adverse reactions. Thus, development of novel therapeutics is urgently needed.

METHODS

In the current study, we identified a naturally occurring pterostilbene that inhibits HCMV Towne strain replication in human diploid fibroblast WI-38 cells through Western blotting, qPCR, indirect immunofluorescence assay, tissue culture infective dose assays. The time-of-addition experiment was carried out to identify the stage at which pterostilbene acted. Finally, the changes of cellular senescence biomarkers and reactive oxygen species production brought by pterostilbene supplementation were used to partly elucidate the mechanism of anti-HCMV activity.

RESULTS

Our findings revealed that pterostilbene prevented lytic cytopathic changes, inhibited the expression of viral proteins, suppressed the replication of HCMV DNA, and significantly reduced the viral titre in WI-38 cells. Furthermore, our data showed that pterostilbene predominantly acted after virus cell entry and membrane fusion. The half-maximal inhibitory concentration was determined to be 1.315 μM and the selectivity index of pterostilbene was calculated as 26.73. Moreover, cell senescence induced by HCMV infection was suppressed by pterostilbene supplementation, as shown by a decline in senescence-associated β-galactosidase activity, decreased production of reactive oxygen species and reduced expression of p16, p21 and p53, which are considered biomarkers of cellular senescence.

CONCLUSION

Together, our findings identify pterostilbene as a novel anti-HCMV agent that may prove useful in the treatment of HCMV replication.

Porcine Circovirus 2 Activates the PERK-Reactive Oxygen Species Axis To Induce p53 Phosphorylation with Subsequent Cell Cycle Arrest at S Phase in Favor of Its Replication

Coinfections or noninfectious triggers have long been considered to potentiate PCV2 infection, leading to manifestation of PCVAD. The triggering mechanisms remain largely unknown.

Association of Epstein–Barr Virus and Cytomegalovirus Infections with Esophageal Carcinoma

Background: Given the fact that viral infections play an important role, either directly or indirectly, in around 20 percent of human cancers, this study aimed at investigating the potential association of Epstein–Barr virus (EBV) and cytomegalovirus (CMV) infections in esophageal cancer that is the sixth most common cause of cancer-related deaths. Methods: In this case-control study, a total of 200 paraffin-embedded biopsies of cancerous and benign esophageal tissues were gathered from the biopsy bank of Imam Reza Hospital, Tabriz, Iran in 2017. All samples were first deparaffinized, and then subjected to commercial DNA extraction. The quality of extracted DNA was evaluated by amplification of the beta globulin gene. Identification of EBV and CMV DNA was performed using primers designed for the EBER region of EBV and the immediate early (IE) region of the CMV genome, respectively. Results: The mean age of the subjects in the test and control groups was 52.2 (17.1) and 59.9 (18.9), respectively. The distribution of gender (male/female) in patient and control groups was 54/46 and 53/47, respectively. Our results showed that the frequency of EBV (P < 0.001) and CMV (P < 0.001) in cancerous samples was statistically higher than control group. Moreover, in the cancerous group the rate of EBV was significantly higher in the esophageal adenocarcinomas (EAC) sample (12 out of 70) than esophageal squamous cell carcinomas (ESCC) (0 out of 30) (P = 0.016) but, in the ESCC group, 17 out of 30 subjects were positive for CMV which was significantly higher in comparison with EAC patients (1 out of 70) (P < 0.001). Conclusions: Findings indicated that EBV and CMV might be contributed to the pathogenesis of EAC and ESCC types of esophageal carcinoma, respectively, although further studies are warranted.

Inhibition of Osteoclastogenesis by the RNA-Binding Protein QKI5: a Novel Approach to Protect from Bone Resorption

Increased osteoclastogenesis is a common feature of bone erosion, notably in osteoporosis but also in inflammatory diseases such as rheumatoid arthritis (RA) and osteoarticular infections. Human cytomegalovirus (HCMV) infection has been described to impair monocyte differentiation into macrophages and dendritic cells. However, its effect on monocyte-derived osteoclasts is yet to be determined. We showed here that in vitro HCMV infection is associated with an inhibition of osteoclastogenesis through decreased expression of colony stimulating factor 1 receptor (CSF-1R) and RANK in monocytes, which was mediated by an upregulation of quaking I-5 protein (QKI-5), a cellular RNA-interacting protein. We found that deliberate QKI5 overexpression in the absence of HCMV infection is able to decrease CSF-1R and RANK expression, leading to osteoclastogenesis inhibition. Finally, by using lentiviral vectors in a calvarial bone erosion mouse model, we showed that QKI5 inhibits bone degradation. This work identifies QKI5 as a strong inhibitor of bone resorption. Future research will point out whether QKI5 could be a target for bone pathologies. © 2019 American Society for Bone and Mineral Research.

Chronic inflammation is a key to inducing liver injury in blunt snout bream (Megalobrama amblycephala) fed with high-fat diet

The aim of this article is to investigate the mechanism of lipotoxicity induced by high-fat diets (HFD) in Megalobrama amblycephala. In the present study, fish (average initial weight 40.0 ± 0.35 g) were fed with two fat levels (6% and 11%) diets with four replicates for 60 days. At the end of the feeding trial, fish were challenged by thioacetamide (TAA) and survival rate was recorded for the next 96 h. The result showed that long-term HFD feeding induced a significant increase (P < 0.05) in the levels of aspartate aminotransferase (GOT) and alanine aminotransferase (GPT) in plasma. In addition, liver histopathological analysis showed an increased dilation of the blood vessels, erythrocytes outside of the blood vessels and vacuolization in fish fed with high-fat diet. After TAA challenge, compared with group fed with normal-fat diets (NFD), fish fed with HFD showed a significantly (P < 0.05) low survival rate. After feeding Megalobrama amblycephala with HFD for 60 days, the protein content and gene expression of pro-inflammatory factors were significantly elevated (P < 0.05). The protein and gene relative expressions of a Caspase-3, Caspase-9 and CD68 were significantly increased (P < 0.05), while antioxidant-related enzyme activities were significantly reduced (P < 0.05) in the liver of fish fed with HFD. In addition, HFD feeding also induced genotoxicity. Comet assay showed a significantly (P < 0.05) elevated DNA damage in blunt snout bream fed with HFD. Compared with normal-fat diets (NFD) group, the protein expression of γH2AX and gene expressions involved in cell cycle arrest were significantly increased (P < 0.05) in fish fed with HFD. Data in this research showed that lipotoxicity induced by HFD was likely mediated by chronic inflammation regulating macrophage recruitment, apoptosis and DNA damage. The study was valuable to understand the mechanism by which liver injury is induced in fish fed with HFD.

Analysis of potential key genes in very early hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is the major pathological type of primary liver cancer, one of the leading causes of cancer death worldwide. In addition, the long-term survival rates of HCC still remain low. Therefore, we attempted to identify the potential key genes in the occurrence of HCC by comparing the expression profiles of very early HCC tissue samples with that of chronic cirrhotic tissue samples by integrating the bioinformatics analysis in this study.

Methods

Gene expression profiles of 19 very early HCC and 19 cirrhotic tissue samples were selected from GSE63898. Differentially expressed genes (DEGs) were also identified by using online tool GEO2R. Furthermore, the GO and KEGG enrichment analysis of the DGEs were conducted on DAVID datasets. Then a protein–protein interaction (PPI) network was constructed and the modules were analyzed based on STRING database and Cytoscape software. The hub genes were screened by applying the cytoHubba plugin and then analyzed with the Kaplan Meier plotter.

Results

A total of 118 DEGs were identified between very early HCC and cirrhotic tissue samples. These DGEs were strongly associated with several biological processes, such as negative regulation of growth and p53 signaling pathway. A PPI network was constructed and top eight hub genes, including CDKN3, CDK1, CCNB1, TOP2A, CCNA2, CCNB2, PRC1, and RRM2, were determined. High expressions of CDK1, CCNB1, TOP2A, CCNA2, PRC1, RRM2, CDKN3, and CCNB2 were associated with poorer overall survivals (OS) in HCC patients.

Conclusion

We had compared the expression profiles between the very early HCC and cirrhotic tissue samples by using bioinformatics analysis tools, which might help us better to understand the molecular mechanism of the initiation of HCC and even to find novel targets for HCC therapy.

Impact of Human Cytomegalovirus Infection and its Immune Response on Survival of Patients with Ovarian Cancer

Human cytomegalovirus (HCMV) has been detected in various types of tumors. We studied the prevalence of HCMV in ovarian cancer and its relation to clinical outcome. Paraffin-embedded tissues obtained prospectively from 45 patients with ovarian cancer and 30 patients with benign ovarian cystadenoma were analyzed for expression of HCMV immediate-early protein (IE) and HCMV tegument protein (pp65) by immunohistochemistry. Plasma was analyzed for HCMV serology. HCMV-IgG levels were higher in patients with ovarian cancer or benign cystadenoma than in age-matched controls (P = .002, P < .0001, respectively). HCMV IgM was detected in 12% of ovarian cancer patients and 3% of patients with benign tumors but was absent in controls. In patients with ovarian cancer, higher IgG levels were associated with better outcomes (P = .04). Extensive HCMV-IE protein expression was detected in 75% of ovarian cancers and 26% of benign tumors; pp65 was detected in 67% of ovarian cancers and 14% of benign tumors. A higher grade of HCMV infection was associated with higher stage of disease. Extensive HCMV-pp65 expression was associated with shorter median overall survival than focal expression (39 versus 42.5 months, P = .03). At study closure, 58% of ovarian cancer patients with focal pp65 expression were alive versus 27% of patients with extensive pp65 expression (P = .03). Thus, HCMV proteins are detected at different levels in ovarian tumors and benign cystadenomas. Ovarian cancer patients with focal HCMV-pp65 expression in their tumors and high IgG levels against HCMV lived longer, highlighting a need for in-depth studies of the oncomodulatory role of HCMV in ovarian cancer.

Almost Complete Lack of Human Cytomegalovirus and Human papillomaviruses Genome in Benign and Malignant Breast Lesions in Shiraz, Southwest of Iran

Breast cancer ranks as the most common cancer among women worldwide. There have been controversial reports regarding contributions of human papillomaviruses (HPVs) and human cytomegalovirus (HCMV) to its development. The aim of this study was to determine the frequency of HPV and HCMV positivity in benign and malignant breast tumors.

An intein-mediated modulation of protein stability system and its application to study human cytomegalovirus essential gene function

Functional analysis of the essential proteins encoded by human cytomegalovirus (HCMV) is hindered by the lack of complementing systems. To overcome this difficulty, we have established a novel approach, termed the intein-mediated modulation of protein stability (imPS), in which a destabilizing domain and part of a split intein are fused to the essential protein. The growth of the mutant virus can then be regulated by the degradation and splicing of the protein. We found that an ultrafast gp41-1 split intein was able to rescue or degrade the protein of interest (POI) by removing or adding a strong degron through protein splicing. As a result, the function of the POI was turned on or off during the process. Using HCMV essential gene IE1/IE2, we confirmed that imPS worked remarkably well in conditionally regulating protein stability during viral infection. This conditional approach is likely to be applicable for dissecting the gene functions of HCMV or other viruses.

Human cytomegalovirus riding the cell cycle

Human cytomegalovirus (HCMV) infection modulates the host cell cycle to create an environment that is optimal for viral gene expression, DNA replication, and production of infectious virus. The virus mostly infects quiescent cells and thus must push the cell into G1 phase of the cell cycle to co-opt the cellular mechanisms that could be used for DNA synthesis. However, at the same time, cellular functions must be subverted such that synthesis of viral DNA is favored over that of the host. The molecular mechanisms by which this is accomplished include altered RNA transcription, changes in the levels and activity of cyclin-dependent kinases, and other proteins involved in cell cycle control, posttranslational modifications of proteins, modulation of protein stability through targeted effects on the ubiquitin-proteasome degradation pathway, and movement of proteins to different cellular locations. When the cell is in the optimal G0/G1 phase, multiple signaling pathways are altered to allow rapid induction of viral gene expression once negative factors have been eliminated. For the most part, the cell cycle will stop prior to initiation of host cell DNA synthesis (S phase), although many cell cycle proteins characteristic of the S/G2/M phase accumulate. The environment of a cell progressing through the cell cycle and dividing is not favorable for viral replication, and HCMV has evolved ways to sense whether cells are in S/G2 phase, and if so, to prevent initiation of viral gene expression until the cells cycle back to G1. A major target of HCMV is the anaphase-promoting complex E3 ubiquitin ligase, which is responsible for the ubiquitination and subsequent degradation of cyclins A and B and other cell cycle proteins at specific phases in the cell cycle. This review will discuss the effects of HCMV infection on cell cycle regulatory pathways, with the focus on selected viral proteins that are responsible for these effects.

UL84-independent replication of human cytomegalovirus strains conferred by a single codon change in UL122

The UL84 gene of human cytomegalovirus (HCMV) is thought to be involved in the initiation of viral DNA replication, and is essential for replication of strains AD169 and Towne. Hence, discovery that strain TB40-BAC4 is viable in the absence of UL84 presented an enigma requiring an explanation. Data reported here show that strain TR also tolerated loss of UL84, whereas strains FIX, Merlin, Ph, and Toledo did not. UL84-independent growth required the viral replication origin. The genetic locus in TB40 that controls UL84 dependence was mapped to codon 388 of the UL122 gene, which encodes the immediate early 2 (IE2) 86kD protein. Introduction of this TB40-BAC4 variant (H388D) into FIX and Toledo clones converted these strains to UL84 independence. These results provide genetic evidence in virus-infected cells that supports the hypothesis that UL122 participates in the initiation of viral DNA replication by a mechanism involving transcription-mediated activation of the origin.

In vitro cytotoxicity of SiO2 or ZnO nanoparticles with different sizes and surface charges on U373MG human glioblastoma cells

Silicon dioxide (SiO₂) and zinc oxide (ZnO) nanoparticles are widely used in various applications, raising issues regarding the possible adverse effects of these metal oxide nanoparticles on human cells. In this study, we determined the cytotoxic effects of differently charged SiO₂ and ZnO nanoparticles, with mean sizes of either 100 or 20 nm, on the U373MG human glioblastoma cell line. The overall cytotoxicity of ZnO nanoparticles against U373MG cells was significantly higher than that of SiO₂ nanoparticles. Neither the size nor the surface charge of the ZnO nanoparticles affected their cytotoxicity against U373MG cells. The 20 nm SiO₂ nanoparticles were more toxic than the 100 nm nanoparticles against U373MG cells, but the surface charge had little or no effect on their cytotoxicity. Both SiO₂ and ZnO nanoparticles activated caspase-3 and induced DNA fragmentation in U373MG cells, suggesting the induction of apoptosis. Thus, SiO₂ and ZnO nanoparticles appear to exert cytotoxic effects against U373MG cells, possibly via apoptosis.

History of the molecular biology of cytomegaloviruses

The history of the molecular biology of cytomegaloviruses from the purification of the virus and the viral DNA to the cloning and expression of the viral genes is reviewed. A key genetic element of cytomegalovirus (the CMV promoter) contributed to our understanding of eukaryotic cell molecular biology and to the development of lifesaving therapeutic proteins. The study of the molecular biology of cytomegaloviruses also contributed to the development of antivirals to control the viral infection.

Interaction network of proteins associated with human cytomegalovirus IE2-p86 protein during infection: a proteomic analysis

Human cytomegalovirus protein IE2-p86 exerts its functions through interaction with other viral and cellular proteins. To further delineate its protein interaction network, we generated a recombinant virus expressing SG-tagged IE2-p86 and used tandem affinity purification coupled with mass spectrometry. A total of 9 viral proteins and 75 cellular proteins were found to associate with IE2-p86 protein during the first 48 hours of infection. The protein profile at 8, 24, and 48 h post infection revealed that UL84 tightly associated with IE2-p86, and more viral and cellular proteins came into association with IE2-p86 with the progression of virus infection. A computational analysis of the protein-protein interaction network indicated that all of the 9 viral proteins and most of the cellular proteins identified in the study are interconnected to varying degrees. Of the cellular proteins that were confirmed to associate with IE2-p86 by immunoprecipitation, C1QBP was further shown to be upregulated by HCMV infection and colocalized with IE2-p86, UL84 and UL44 in the virus replication compartment of the nucleus. The IE2-p86 interactome network demonstrated the temporal development of stable and abundant protein complexes that associate with IE2-p86 and provided a framework to benefit future studies of various protein complexes during HCMV infection.

Lupeol is one of active components in the extract of Chrysanthemum indicum Linne that inhibits LMP1-induced NF-κB activation

We have previously reported that seventy percent ethanol extract of Chrysanthemum indicum Linne (CIE) strongly reduces Epstein-Barr virus (EBV)-transformed lymphoblastoid cell line (LCL) survival by inhibiting virus-encoded latent infection membrane protein 1 (LMP1)-induced NF-κB activation. To identify an active compound(s) in CIE that inhibits LMP1-induced NF-κB activation, activity-guided fractionation was employed. The CH₂Cl₂ fraction of CIE strongly reduced LMP1-induced NF-κB activation and LCL viability with relatively low cytotoxic effects on primary human foreskin fibroblast (HFF), HeLa or Burkitt’s lymphoma (BL41) cells. Furthermore, lupeol, a pentacyclic triterpene, was identified in the CH₂Cl₂ fraction of CIE to attenuate LMP1-induced NF-κB activation and LCL viability. This study demonstrates that lupeol is one of active compounds in the CH₂Cl₂ fraction of CIE that inhibits LMP1-induced NF-κB activation and reduces NF-κB-dependent LCL viability.

The extract of Elaeocarpus sylvestris inhibits human cytomegalovirus immediate early gene expression and replication in vitro

Human cytomegalovirus (HCMV) is a major cause of morbidity and mortality in newborn infants, immunocompromised individuals with HIV/AIDS and organ transplant recipients. In order to identify a novel antiviral candidate for HCMV-related diseases, crude ethanol extracts from plants were screened for their potential inhibitory activity on HCMV replication in vitro. Ethanol (70%) extract of Elaeocarpus sylvestris leaves (ESE) markedly inhibited the replication of the HCMV Towne strain without exhibiting any significant adverse effects on the viability of human foreskin fibroblasts (HFF). In addition, ESE significantly downregulated HCMV immediate early (IE) gene expression. Taken together, this is the first study, to the best of our knowledge, demonstrating that ESE has a potent antiviral activity against HCMV by downregulating HCMV IE gene expression and replication.

The p53-microRNA-34a axis regulates cellular entry receptors for tumor-associated human herpes viruses

A growing number of reports indicate the frequent presence of DNA sequences and gene products of human cytomegalovirus in various tumors as compared to adjacent normal tissues, the brain tumors being studied most intensely. The mechanisms underlying the tropism of human cytomegalovirus to the tumor cells or to the cells of tumor origin, as well as the role of the host's genetic background in virus-associated oncogenesis are not well understood. It is also not clear why cytomegalovirus can be detected in many but not in all tumor specimens. Our in silico prediction results indicate that microRNA-34a may be involved in replication of some human DNA viruses by targeting and downregulating the genes encoding a diverse group of proteins, such as platelet-derived growth factor receptor-alpha, complement component receptor 2, herpes simplex virus entry mediators A, B, and C, and CD46. Notably, while their functions vary, these surface molecules have one feature in common: they serve as cellular entry receptors for human DNA viruses (cytomegalovirus, Epstein-Barr virus, human herpes virus 6, herpes simplex viruses 1 and 2, and adenoviruses) that are either proven or suspected to be linked with malignancies. MicroRNA-34a is strictly dependent on its transcriptional activator tumor suppressor protein p53, and both p53 and microRNA-34a are frequently mutated or downregulated in various cancers. We hypothesize that p53-microRNA-34a axis may alter susceptibility of cells to infection with some viruses that are detected in tumors and either proven or suspected to be associated with tumor initiation and progression.

Toll-like receptor 4 is involved in the cell cycle modulation and required for effective human cytomegalovirus infection in THP-1 macrophages

Suitable host cell metabolic conditions are fundamental for the effective development of the human cytomegalovirus (HCMV) lytic cycle. Indeed, several studies have demonstrated the ability of this virus to interfere with cell cycle regulation, mainly by blocking proliferating cells in G1 or G1/S. In the present study, we demonstrate that HCMV deregulates the cell cycle of THP-1 macrophages (a cell line irreversibly arrested in G0) by pushing them into S and G2 phases. Moreover, we show that HCMV infection of THP-1 macrophages leads to Toll-like receptor 4 (TLR4) activation. Since various studies have indicated TLR4 to be involved in promoting cell proliferation, here we investigate the possible role of TLR4 in the observed HCMV-induced cell cycle perturbation. Our data strongly support TLR4 as a mediator of HCMV-triggered cell cycle activation in THP-1 macrophages favouring, in turn, the development of an efficient viral lytic cycle.

High prevalence of human cytomegalovirus proteins and nucleic acids in primary breast cancer and metastatic sentinel lymph nodes

Background

Breast cancer is a leading cause of death among women worldwide. Increasing evidence implies that human cytomegalovirus (HCMV) infection is associated with several malignancies. We aimed to examine whether HCMV is present in breast cancer and sentinel lymph node (SLN) metastases.

Materials and Methods

Formalin-fixed paraffin-embedded tissue specimens from breast cancer and paired sentinel lymph node (SLN) samples were obtained from patients with (n = 35) and without SLN metastasis (n = 38). HCMV immediate early (IE) and late (LA) proteins were detected using a sensitive immunohistochemistry (IHC) technique and HCMV DNA by real-time PCR.

Results

HCMV IE and LA proteins were abundantly expressed in 100% of breast cancer specimens. In SLN specimens, 94% of samples with metastases (n = 34) were positive for HCMV IE and LA proteins, mostly confined to neoplastic cells while some inflammatory cells were HCMV positive in 60% of lymph nodes without metastases (n = 35). The presence of HCMV DNA was confirmed in 12/12 (100%) of breast cancer and 10/11 (91%) SLN specimens from the metastatic group, but was not detected in 5/5 HCMV-negative, SLN-negative specimens. There was no statistically significant association between HCMV infection grades and progesterone receptor, estrogen receptor alpha and Elston grade status.

Conclusions

The role of HCMV in the pathogenesis of breast cancer is unclear. As HCMV proteins were mainly confined to neoplastic cells in primary breast cancer and SLN samples, our observations raise the question whether HCMV contributes to the tumorigenesis of breast cancer and its metastases.

The cellular protein MCM3AP is required for inhibition of cellular DNA synthesis by the IE86 protein of human cytomegalovirus

Like all DNA viruses, human cytomegalovirus (HCMV) infection is known to result in profound effects on host cell cycle. Infection of fibroblasts with HCMV is known to induce an advance in cell cycle through the G₀-G₁ phase and then a subsequent arrest of cell cycle in early S-phase, presumably resulting in a cellular environment optimum for high levels of viral DNA replication whilst precluding replication of cellular DNA. Although the exact mechanisms used to arrest cell cycle by HCMV are unclear, they likely involve a number of viral gene products and evidence points to the ability of the virus to prevent licensing of cellular DNA synthesis. One viral protein known to profoundly alter cell cycle is the viral immediate early 86 (IE86) protein - an established function of which is to initially drive cells into early S phase but then inhibit cellular DNA synthesis. Here we show that, although IE86 interacts with the cellular licensing factor Cdt1, it does not inhibit licensing of cellular origins. Instead, IE86-mediated inhibition of cellular DNA synthesis requires mini-chromosome-maintenance 3 (MCM3) associated protein (MCM3AP), which can cause subsequent inhibition of initiation of cellular DNA synthesis in a licensing-independent manner.

Growth inhibition of human hepatocellular carcinoma cells by overexpression of G-protein-coupled receptor kinase 2

Hepatocellular carcinoma (HCC) is one of the deadliest forms of human liver cancer and does not respond well to conventional therapies. Novel effective treatments are urgently in need. G-protein-coupled kinase 2 (GRK2) is unique serine/threonine kinase that involves in many signaling pathways and regulates various essential cellular processes. Altered levels of GRK2 have been linked with several human diseases including cancer. In this study, we investigated a novel approach for HCC treatment by inducing overexpression of GRK2 in human HCC cells. We found that overexpression of GRK2 through recombinant adenovirus transduction inhibits the growth of human HCC cells. BrdU incorporation assay showed that the growth inhibition caused by elevated GRK2 level was due to reduced cell proliferation but not apoptosis. To examine the anti-proliferative function of increased GRK2 level, we performed cell cycle analysis using propidium iodide staining. We found that the proliferation suppression was associated with G2/M phase cell cycle arrest by the wild-type GRK2 but not its kinase-dead K220R mutant. Furthermore, increased levels of wild-type GRK2 induced upregulation of phosphor-Ser(15) p53 and cyclin B1 in a dose-dependent manner. Our data indicate that the anti-proliferative function of elevated GRK2 is associated with delayed cell cycle progression and is GRK2 kinase activity-dependent. Enforced expression of GRK2 in human HCC by molecular delivery may offer a potential therapeutic approach for the treatment of human liver cancer.

The roles of viruses in brain tumor initiation and oncomodulation

While some avian retroviruses have been shown to induce gliomas in animal models, human herpesviruses, specifically, the most extensively studied cytomegalovirus, and the much less studied roseolovirus HHV-6, and Herpes simplex viruses 1 and 2, currently attract more and more attention as possible contributing or initiating factors in the development of human brain tumors. The aim of this review is to summarize and highlight the most provoking findings indicating a potential causative link between brain tumors, specifically malignant gliomas, and viruses in the context of the concepts of viral oncomodulation and the tumor stem cell origin.

Mutation of glutamine to arginine at position 548 of IE2 86 in human cytomegalovirus leads to decreased expression of IE2 40, IE2 60, UL83, and UL84 and increased transcription of US8-9 and US29-32

The IE2 86 protein of human cytomegalovirus (HCMV) is essential for productive infection. The mutation of glutamine to arginine at position 548 of IE2 86 causes the virus to grow both slowly and to very low titers, making it difficult to study this mutant via infection. In this study, Q548R IE2 86 HCMV was produced on the complementing cell line 86F/40HA, which allowed faster and higher-titer production of mutant virus. The main defects observed in this mutant were greatly decreased expression of IE2 40, IE2 60, UL83, and UL84. Genome replication and the induction of cell cycle arrest were found to proceed at or near wild-type levels, and there was no defect in transitioning to early or late protein expression. Q548R IE2 86 was still able to interact with UL84. Furthermore, Q548R IE2 40 maintained the ability to enhance UL84 expression in a cotransfection assay. Microarray analysis of Q548R IE2 HCMV revealed that the US8, US9, and US29-32 transcripts were all significantly upregulated. These results further confirm the importance of IE2 in UL83 and UL84 expression as well as pointing to several previously unknown regions of the HCMV genome that may be regulated by IE2.

Interference with p53 functions in human viral infections, a target for novel antiviral strategies?

Viral infections cause a major stress in host cells. The cellular responses to stress are mediated by p53, which by deregulation of cell cycle and apoptosis, may also be part of the host cell reaction to fight infections. Therefore, during evolutionary viral adaptation to host organisms, viruses have developed strategies to manipulate host cell p53 dependent pathways to facilitate their viral life cycles. Thus, interference with p53 function is an important component in viral pathogenesis. Many viruses have proteins that directly affect p53, whereas others alter the regulation of p53 in an indirect manner, mediated by Hdm2 or Akt, or induction of interferon. Rescue of p53 activity is becoming an area of therapeutic development in oncology. It might be feasible that manipulation of p53 mediated responses can become a therapeutic option to limit viral replication or dissemination. In this report, the mechanisms by which viral proteins manipulate p53 responses are reviewed, and it is proposed that a pharmacological rescue of p53 functions might help to control viral infections.

HIV-1 viral infectivity factor interacts with TP53 to induce G2 cell cycle arrest and positively regulate viral replication

Viral infectivity factor, an accessory protein encoded in the HIV-1 genome, induces G2 cell cycle arrest; however, the biological significance and mechanism(s) remain totally unclear. Here we demonstrate that the TP53 pathway is involved in Vif-mediated G2 cell cycle arrest. Vif enhances the stability and transcriptional activity of TP53 by blocking the MDM2-mediated ubiquitination and nuclear export of TP53. Furthermore, Vif causes G2 cell cycle arrest in a TP53-dependent manner. HXB2 Vif lacks these activities toward TP53 and cannot induce G2 cell cycle arrest. Using mutagenesis, we demonstrate that the critical residues for this function are located in the N-terminal region of Vif. Finally, we construct a mutant NL4-3 virus with an NL4-3/HXB2 chimeric Vif defective for the ability to induce cell cycle arrest and show that the mutant virus replicates less effectively than the wild-type NL4-3 virus in T cells expressing TP53. These data imply that Vif induces G2 cell cycle arrest through functional interaction with the TP53/MDM2 axis and that the G2 cell cycle arrest induced by Vif has a positive effect on HIV-1 replication. This report demonstrates the molecular mechanisms and the biological significance of Vif-mediated G2 cell cycle arrest for HIV-1 infection.

Alternative splicing of the human cytomegalovirus major immediate-early genes affects infectious-virus replication and control of cellular cyclin-dependent kinase

The major immediate-early (MIE) gene locus of human cytomegalovirus (HCMV) is the master switch that determines the outcomes of both lytic and latent infections. Here, we provide evidence that alteration in the splicing of HCMV (Towne strain) MIE genes affects infectious-virus replication, movement through the cell cycle, and cyclin-dependent kinase activity. Mutation of a conserved 24-nucleotide region in MIE exon 4 increased the abundance of IE1-p38 mRNA and decreased the abundance of IE1-p72 and IE2-p86 mRNAs. An increase in IE1-p38 protein was accompanied by a slight decrease in IE1-p72 protein and a significant decrease in IE2-p86 protein. The mutant virus had growth defects, which could not be complemented by wild-type IE1-p72 protein in trans. The phenotype of the mutant virus could not be explained by an increase in IE1-p38 protein, but prevention of the alternate splice returned the recombinant virus to the wild-type phenotype. The lower levels of IE1-p72 and IE2-p86 proteins correlated with a delay in early and late viral gene expression and movement into the S phase of the cell cycle. Mutant virus-infected cells had significantly higher levels of cdk-1 expression and enzymatic activity than cells infected with wild-type virus. The mutant virus induced a round-cell phenotype that accumulated in the G(2)/M compartment of the cell cycle with condensation and fragmentation of the chromatin. An inhibitor of viral DNA synthesis increased the round-cell phenotype. The round cells were characteristic of an abortive viral infection.

The human cytomegalovirus major immediate-early proteins as antagonists of intrinsic and innate antiviral host responses

The major immediate-early (IE) gene of human cytomegalovirus (CMV) is believed to have a decisive role in acute infection and its activity is an important indicator of viral reactivation from latency. Although a variety of gene products are expressed from this region, the 72-kDa IE1 and the 86-kDa IE2 nuclear phosphoproteins are the most abundant and important. Both proteins have long been recognized as promiscuous transcriptional regulators. More recently, a critical role of the IE1 and IE2 proteins in counteracting non-adaptive host cell defense mechanisms has been revealed. In this review we will briefly summarize the available literature on IE1- and IE2-dependent mechanisms contributing to CMV evasion from intrinsic and innate immune responses.

Human cytomegalovirus IE1-72 protein interacts with p53 and inhibits p53-dependent transactivation by a mechanism different from that of IE2-86 protein

Infection of host cells with human cytomegalovirus (HCMV) induces cell cycle dysregulation. Two HCMV immediate-early (IE) proteins, IE1-72 and IE2-86, are promiscuous transactivators that have been implicated in the dysregulatory events. Cellular p53 protein is accumulated to high levels in HCMV-infected cells, but the indicative marker of p53 transcriptional activity, p21, is markedly decreased. Both IE1-72 and IE2-86 were able to transactivate the p53 promoter and interact with p53 protein in DNA-transfected or HCMV-infected cells. HCMV UL84, a multiregulatory protein expressed in early periods of HCMV infection, also interacted with p53. HCMV IE1-72 prevented or disrupted p53 binding to p53-specific DNA sequences, while IE2-86 and/or UL84 enhanced p53 binding and induced supershift of this DNA-protein complex. Both HCMV IE1-72 and IE2-86 were able to inhibit p53-dependent transcriptional activation in plasmid-transfected cells. IE1-72, rather than IE2-86, was found to be responsible for p21 downregulation in HCMV-infected HEL cells. DNA transfection analysis using IE1-72 mutants revealed that exon 2/3 and the zinc finger region of IE1-72 are essential for IE1-72's effect on the repression of p53-dependent transcriptional activation. These data suggest that HCMV IE1-72 and/or IE2-86 transactivates the p53 promoter and induces p53 accumulation, but HCMV IE1-72 represses the p53 transactivation activity by a unique binding hindrance mechanism different from that of IE2-86. Thus, various modes of viral IE proteins and p53 interactions might result in multiple outcomes, such as stimulation of cellular DNA synthesis, cell cycle progression and cell cycle arrest, and prevention of program cell death.

The story of human cytomegalovirus and cancer: increasing evidence and open questions

Although human cytomegalovirus (HCMV) is generally not regarded to be an oncogenic virus, HCMV infection has been implicated in malignant diseases from different cancer entities. On the basis of our experimental findings, we developed the concept of "oncomodulation" to better explain the role of HCMV in cancer. Oncomodulation means that HCMV infects tumor cells and increases their malignancy. By this concept, HCMV was proposed to be a therapeutic target in a fraction of cancer patients. However, the clinical relevance of HCMV-induced oncomodulation remains to be clarified. One central question that has to be definitively answered is if HCMV establishes persistent virus replication in tumor cells or not. In our eyes, recent clinical findings from different groups in glioblastoma patients and especially the detection of a correlation between the numbers of HCMV-infected glioblastoma cells and tumor stage (malignancy) strongly increase the evidence that HCMV may exert oncomodulatory effects. Here, we summarize the currently available knowledge about the molecular mechanisms that may contribute to oncomodulation by HCMV as well as the clinical findings that suggest that a fraction of tumors from different entities is indeed infected with HCMV.

Identification of a novel transcriptional repressor (HEPIS) that interacts with nsp-10 of SARS coronavirus

A novel gene was previously isolated from a cDNA library of human embryo lung tissue by its encoded protein, which interacts with non-structural protein 10 (nsp-10) of the severe acute respiratory syndrome coronavirus (SARS-CoV). The protein was named human embryo lung cellular protein interacting with SARS-CoV nsp-10 (HEPIS), and it is composed of 147 amino acids with several CK II phosphorylation sites. In the present study, we demonstrated that HEPIS was capable of suppressing chloramphenicol acetyltransferase (CAT) gene expression controlled by different enhancerelements in a transcription assay. HEPIS interacted specifically with the HSP70 TATA sequence, and not with various other enhancer elements in a binding test. Furthermore, we co-immunoprecipitated HEPIS with BTF3, a component of the RNA pol II initiation complex, and observed reduced proliferation of HeLa cells transfected with the HEPIS gene. Taken together, our results suggest that HEPIS may function as a potential transcriptional repressor.

Cell cycle-independent expression of immediate-early gene 3 results in G1 and G2 arrest in murine cytomegalovirus-infected cells

The infectious cycle of human cytomegalovirus (HCMV) is intricately linked to the host's cell cycle. Viral gene expression can be initiated only in G(0)/G(1) phase. Once expressed, the immediate-early gene product IE2 prevents cellular DNA synthesis, arresting infected cells with a G(1) DNA content. This function is required for efficient viral replication in vitro. A prerequisite for addressing its in vivo relevance is the characterization of cell cycle-regulatory activities of CMV species for which animal models have been established. Here, we show that murine CMV (MCMV), like HCMV, has a strong antiproliferative capacity and arrests cells in G(1). Unexpectedly, and in contrast to HCMV, MCMV can also block cells that have passed through S phase by arresting them in G(2). Moreover, MCMV can also replicate in G(2) cells. This is made possible by the cell cycle-independent expression of MCMV immediate-early genes. Transfection experiments show that of several MCMV candidate genes, only immediate-early gene 3 (ie3), the homologue of HCMV IE2, exhibits cell cycle arrest activity. Accordingly, an MCMV ie3 deletion mutant has lost the ability to arrest cells in either G(1) or G(2). Thus, despite interspecies variations in the cell cycle dependence of viral gene expression, the central theme of HCMV IE2-induced cell cycle arrest is conserved in the murine counterpart, raising the possibility of studying its physiological relevance at the level of the whole organism.

The S phase of the cell cycle and its perturbation by human cytomegalovirus

Human cytomegalovirus (HCMV) is a complex human herpesvirus that is known to productively infect a wide range of cell types. In addition, it has been suggested to contribute to some proliferative disorders, particularly atherosclerosis. Consistent with this, a number of studies have shown that HCMV profoundly affects normal cell cycle control. Specifically, the virus can stimulate early entry into S phase thus ensuring adequate resources for viral DNA replication. Importantly, however, the virus concomitantly inhibits potentially competing cellular DNA synthesis allowing cellular precursors to be used for viral but not cellular DNA replication. The mechanisms by which HCMV perturbs S phase entry involve interactions between the virus and the cellular replication machinery such that formation of competent pre-replication complexes (Pre-RC) at cellular origins of replication is restricted in infected cells.

Functional roles of the human cytomegalovirus essential IE86 protein

The IE86 protein of human cytomegalovirus (HCMV) is unique among viral and cellular proteins because it negatively autoregulates its own expression, activates the viral early and late promoters, and both activates and inhibits cellular promoters. It promotes cell cycle progression from Go/G1 to G1/S and arrests cell cycle progression at the G1/S interface or at G2/M. The IE86 protein is essential because it creates a cellular environment favorable for viral replication. The multiple functions of the IE86 protein during the replication of HCMV are reviewed.

Subversion of cell cycle regulatory pathways

Human cytomegalovirus (HCMV) has evolved numerous strategies to commandeer the host cell for producing viral progeny. The virus manipulates host cell cycle pathways from the early stages of infection to stimulate viral DNA replication at the expense of cellular DNA synthesis. At the same time, cell cycle checkpoints are by-passed, preventing apoptosis and allowing sufficient time for the assembly of infectious virus.

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.

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.

The autoregulatory and transactivating functions of the human cytomegalovirus IE86 protein use independent mechanisms for promoter binding

The functions of the human cytomegalovirus (HCMV) IE86 protein are paradoxical, as it can both activate and repress viral gene expression through interaction with the promoter region. Although the mechanism for these functions is not clearly defined, it appears that a combination of direct DNA binding and protein-protein interactions is involved. Multiple sequence alignment of several HCMV IE86 homologs reveals that the amino acids (534)LPIYE(538) are conserved between all primate and nonprimate CMVs. In the context of a bacterial artificial chromosome (BAC), mutation of both P535 and Y537 to alanines (P535A/Y537A) results in a nonviable BAC. The defective HCMV BAC does not undergo DNA replication, although the P535A/Y537A mutant IE86 protein appears to be stably expressed. The P535A/Y537A mutant IE86 protein is able to negatively autoregulate transcription from the major immediate-early (MIE) promoter and was recruited to the MIE promoter in a chromatin immunoprecipitation (ChIP) assay. However, the P535A/Y537A mutant IE86 protein was unable to transactivate early viral genes and was not recruited to the early viral UL4 and UL112 promoters in a ChIP assay. From these data, we conclude that the transactivation and repressive functions of the HCMV IE86 protein can be separated and must occur through independent mechanisms.

Ectopic expression of HCMV IE72 and IE86 proteins is sufficient to induce early gene expression but not production of infectious virus in undifferentiated promonocytic THP-1 cells

Human cytomegalovirus (HCMV) reactivation from latency causes disease in individuals who are immunocompromised or immunosuppressed. Activation of the major immediate-early (MIE) promoter is thought to be an initial step for reactivation. We determined whether expression of the MIE gene products in trans was sufficient to circumvent an HCMV latent-like state in an undifferentiated transformed human promonocytic (THP)-1 cell model system. Expression of the functional MIE proteins was achieved with a replication-defective adenovirus vector, Ad-IE1/2, which contains the MIE gene locus. Expression of the MIE proteins by Ad-IE1/2 prior to HCMV infection induced viral early gene expression accompanied by an increase in active chromatin signals. Expression of the anti-apoptotic protein encoded by UL37x1 increased viral early gene expression. However, viral DNA replication and production of infectious virus was not detected. As expected, cellular differentiation with phorbol 12-myristate 13-acetate and hydrocortisone induced virus production. Cellular differentiation is required for efficient viral reactivation.

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.

Evidence that the human cytomegalovirus IE2-86 protein binds mdm2 and facilitates mdm2 degradation

Levels of the p53 tumor suppressor protein are increased in human cytomegalovirus (HCMV)-infected cells and may be important for HCMV pathogenesis. In normal cells p53 levels are kept low due to an autoregulatory feedback loop where p53 activates the transcription of mdm2 and mdm2 binds and ubiquitinates p53, targeting p53 for proteasomal degradation. Here we report that, in contrast to uninfected cells, mdm2 was undetectable upon treatment of infected fibroblasts with the proteasome inhibitor MG132. Cellular depletion of mdm2 was reproducible in p53-null cells transfected with the HCMV IE2-86 protein, but not with IE172, independently of the endogenous mdm2 promoter. IE2-86 also prevented the emergence of presumably ubiquitinated species of p53. The regions of IE2-86 important for mdm2 depletion were those containing the sequences corresponding to the putative zinc finger and C-terminal acidic motifs. mdm2 and IE2-86 coimmunoprecipitated in transfected and infected cell lysates and in a cell-free system. IE2-86 blocked mdm2's p53-independent transactivation of the cyclin A promoter in transient-transfection experiments. Pulse-chase experiments revealed that IE2-86 but not IE1-72 or several loss-of-function IE2-86 mutants increased the half-life of p53 and reduced the half-life of mdm2. Short interfering RNA-mediated depletion of IE2-86 restored the ability of HCMV-infected cells to accumulate mdm2 in response to proteasome inhibition. Taken together, the data suggest that specific interactions between IE2-86 and mdm2 cause proteasome-independent degradation of mdm2 and that this may be important for the accumulation of p53 in HCMV-infected cells.

Inhibition of cellular DNA synthesis by the human cytomegalovirus IE86 protein is necessary for efficient virus replication

Human cytomegalovirus (HCMV) expresses several proteins that manipulate normal cellular functions, including cellular transcription, apoptosis, immune response, and cell cycle control. The IE2 gene, which is expressed from the HCMV major immediate-early (MIE) promoter, encodes the IE86 protein. IE86 is a multifunctional protein that is essential for viral replication. The functions of IE86 include transactivation of cellular and viral early genes, negative autoregulation of the MIE promoter, induction of cell cycle progression from G0/G1 to G1/S, and arresting cell cycle progression at the G1/S transition in p53-positive human foreskin fibroblast (HFF) cells. Mutations were introduced into the IE2 gene in the context of the viral genome using bacterial artificial chromosomes (BACs). From these HCMV BACs, a recombinant virus (RV) with a single amino acid substitution in the IE86 protein was isolated that replicates slower and to lower titers than wild-type HCMV. HFF cells infected with the Q548R RV undergo cellular DNA synthesis and do not arrest at any point in the cell cycle. The Q548R RV is able to negatively autoregulate the MIE promoter, transactivate viral early genes, activate cellular E2F-responsive genes, and produce infectious virus. This is the first report of a viable recombinant HCMV that is unable to inhibit cellular DNA synthesis in infected HFF cells.

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.

Human cytomegalovirus IE1-72 activates ataxia telangiectasia mutated kinase and a p53/p21-mediated growth arrest response

Human cytomegalovirus (HCMV) encodes several proteins that can modulate components of the cell cycle machinery. The UL123 gene product, IE1-72, binds the Rb-related, p107 protein and relieves its repression of E2F-responsive promoters; however, it is unable to induce quiescent cells to enter S phase in wild-type (p53(+/+)) cells. IE1-72 also induces p53 accumulation through an unknown mechanism. We present here evidence suggesting that IE1-72 may activate the p53 pathway by increasing the levels of p19(Arf) and by inducing the phosphorylation of p53 at Ser15. Phosphorylation of this residue by IE1-72 expression alone or HCMV infection is found to be dependent on the ataxia-telangiectasia mutated kinase. IE2-86 expression leads to p53 phosphorylation and may contribute to this phenotype in HCMV-infected cells. We also found that IE1-72 promotes p53 nuclear accumulation by abrogating p53 nuclear shuttling. These events result in the stimulation of p53 activity, leading to a p53- and p21-dependent inhibition of cell cycle progression from G(1) to S phase in cells transiently expressing IE1-72. Thus, like many of the small DNA tumor viruses, the first protein expressed upon HCMV infection activates a p53 response by the host cell.