Transactivators Zta and Rta of Epstein-Barr virus promote G0/G1 to S transition in Raji cells: a novel relationship between lytic virus and cell cycle

A new compound, phomaherbarine A, induces cytolytic reactivation in epstein-barr virus-positive B cell lines

Epstein-Barr virus (EBV), the first virus found to induce cancer in humans, has been frequently detected in various types of B cell lymphomas. During its latent phase, EBV expresses a limited set of proteins crucial for its persistence. Induction of the lytic phase of EBV has shown promise in the treatment of EBV-associated malignancies. The present study assessed the ability of phomaherbarine A, a novel compound derived from the endophytic fungus Phoma herbarum DBE-M1, to stimulate lytic replication of EBV in B95-8 cells. Phomaherbarine A was found to efficiently initiate the expression of both early and late EBV lytic genes in B95-8 cells, with this initiation being further heightened by the addition of phorbol myristate acetate and sodium butyrate. Moreover, phomaherbarine A demonstrated notable cytotoxicity against the EBV-associated B cell lymphoma cell lines B95-8 and Raji. Mechanistically, phomaherbarine A induces apoptosis in these cells through the activation of caspase-3/7. When combined with ganciclovir, phomaherbarine A does not interfere with the reduction of viral replication by ganciclovir and sustains its apoptosis induction. In conclusion, these findings indicate that phomaherbarine A may be a promising candidate for therapeutic intervention in patients with EBV-associated B cell lymphomas.

Mechanisms of T cell evasion by Epstein-Barr virus and implications for tumor survival

Epstein-Barr virus (EBV) is a prevalent oncogenic virus estimated to infect greater than 90% of the world's population. Following initial infection, it establishes latency in host B cells. EBV has developed a multitude of techniques to avoid detection by the host immune system and establish lifelong infection. T cells, as important contributors to cell-mediated immunity, make an attractive target for these immunoevasive strategies. Indeed, EBV has evolved numerous mechanisms to modulate T cell responses. For example, it can augment expression of programmed cell death ligand-1 (PD-L1), which inhibits T cell function, and downregulates the interferon response, which has a strong impact on T cell regulation. It also modulates interleukin secretion and can influence major histocompatibility complex (MHC) expression and presentation. In addition to facilitating persistent EBV infection, these immunoregulatory mechanisms have significant implications for evasion of the immune response by tumor cells. This review dissects the mechanisms through which EBV avoids detection by host T cells and discusses how these mechanisms play into tumor survival. It concludes with an overview of cancer treatments targeting T cells in the setting of EBV-associated malignancy.

Epstein-Barr virus immediate-early protein Zta mediates the proliferation and migration of HER2-overexpressing cancer cells

Epstein-Barr virus immediate-early protein Zta plays an active role in altering cellular gene expression, which may be fundamentally linked to the viral life cycle, cell cycle, cell growth, and differentiation. HER2 is associated with a wide variety of human cancers, and its knockdown significantly reverses the malignant features of HER2-positive cancers. The aim of this study was to investigate the potential role of Zta in regulating HER2 expression and phenotype changes of MDA-MB-453 cells. Our results indicate that ectopic expression of Zta resulted in downregulation of the HER2 protein in cancer cells (MDA-MB-453, SKBR-3, BT474, and SKOV-3). The Zta protein significantly decreased HER2 mRNA and protein expression in MDA-MB-453 cells in a dose-dependent manner. Mechanistically, Zta recognized and targeted the promoter of HER2 gene, reducing the transcriptional activity of the HER2 gene. Zta induced G0/G1 arrest of MDA-MB-453 cells, inhibiting their proliferation and migration activity. These data suggest that Zta may act as a transforming suppressor of the HER2 gene.

The lytic phase of Epstein-Barr virus plays an important role in tumorigenesis

Epstein-Barr virus (EBV) is a recognized oncogenic virus that is related to the occurrence of lymphoma, nasopharyngeal carcinoma (NPC), and approximately 10% of gastric cancer (GC). EBV is a herpesvirus, and like other herpesviruses, EBV has a biphasic infection mode made up of latent and lytic infections. It has been established that latent infection promotes tumorigenesis in previous research, but in recent years, there has been new evidence that suggests that the lytic infection mode could also promote tumorigenesis. In this review, we mainly discuss the contribution of the EBV lytic phase to tumorigenesis, and graphically illustrate their relationship in detail. In addition, we described the relationship between the lytic cycle of EBV and autophagy. Finally, we also preliminarily explored the influence of the tumorigenesis effect of the EBV lytic phase on the future treatment of EBV-associated tumors.

Mechanism of cell cycle regulation and cell proliferation during human viral infection

Over the history of the coevolution of Host viral interaction, viruses have customized the host cellular machinery into their use for viral genome replication, causing effective infection and ultimately aiming for survival. They do so by inducing subversions to the host cellular pathways like cell cycle via dysregulation of important cell cycle checkpoints by viral encoded proteins, arresting the cell cycle machinery, blocking cytokinesis as well as targeting subnuclear bodies, thus ultimately disorienting the cell proliferation. Both DNA and RNA viruses have been active participants in such manipulation resulting in serious outcomes of cancer. They achieve this by employing different mechanisms-Protein-protein interaction, protein-phosphorylation, degradation, redistribution, viral homolog, and viral regulation of APC at different stages of cell cycle events. Several DNA viruses cause the quiescent staged cells to undergo cell cycle which increases nucleotide pools logistically significantly persuading viral replication whereas few other viruses arrest a particular stage of cell cycle. This allows the latter group to sustain the infection which allows them to escape host immune response and support viral multiplication. Mechanical study of signaling such viral mediated pathways could give insight into understanding the etiology of tumorigenesis and progression. Overall this chapter highlights the possible strategies employed by DNA/RNA viral families which impact the normal cell cycle but facilitate viral infected cell replication. Such information could contribute to comprehending viral infection-associated disorders to further depth.

Stabilization of p18 by deubiquitylase CYLD is pivotal for cell cycle progression and viral replication

p18 is a key negative regulator of cell cycle progression and mediates cell cycle arrest at the G1/S phase. Ubiquitination is the prime mechanism in regulating p18 protein abundance. However, so far no post- translational regulator, especially DUBs, has been identified to regulate the protein stability of p18. In this paper, we identified CYLD as a deubiquitinase of p18, which binds to and removes the K48-linked polyubiquitylation chains conjugated onto p18, thus stabilizing the p18 protein. Loss of CYLD causes the degradation of p18 and induces the G1/S transition. Epstein-Barr virus (EBV), is the human oncovirus etiologically linked to nasopharyngeal carcinoma (NPC). Here we found that EBV drives a replication passive environment by deregulating the CYLD-p18 axis. Functionally, CYLD inhibits cell proliferation and tumorigenesis through p18 in vivo. Restoring CYLD prevents EBV induced viral replication and tumor growth. Collectively, our results identify CYLD directly stabilizes p18 to regulate the cellular G1/S transition. The reconstitution of CYLD-p18 axis could be a promising approach for EBV-positive cancer therapy.

Epstein-Barr virus-encoded miR-BART6-3p inhibits cancer cell proliferation through the LOC553103-STMN1 axis

Epstein-Barr virus (EBV) is a tumorigenic virus that can cause various human malignancies such as nasopharyngeal carcinoma (NPC) and gastric cancer (GC). EBV encodes 44 mature micro (mi)RNAs, mostly exhibiting oncogenic properties and promoting cancer progression. However, we have previously found that one EBV-encoded miRNA, namely EBV-miR-BART6-3p, acts as a tumor suppressor by inhibiting metastasis and invasion. Here, we report that EBV-miR-BART6-3p inhibits the proliferation of EBV-associated cancers, NPC, and GC, by targeting and downregulating a long non-coding RNA (lncRNA), LOC553103. Through proteomics analysis, we determined that stathmin (STMN1) is affected by EBV-miR-BART6-3p and LOC553103. Further, via RNA immunoprecipitation and luciferase reporter assay, we confirmed that LOC553103 directly binds and stabilizes the 3'UTR region of STMN1 mRNA. These results indicate that the EBV-miR-BART6-3p/LOC553103/STMN1 axis regulates the expression of cell cycle-associated proteins, which then inhibit EBV-associated tumor cell proliferation. These findings provide potential targets or strategies for novel EBV-related cancer treatments, as well as contributes new insights into the understanding of EBV infection-related carcinogenesis.

Viral-Targeted Strategies Against EBV-Associated Lymphoproliferative Diseases

Epstein-Barr virus (EBV) is strongly associated with a spectrum of EBV-associated lymphoproliferative diseases (EBV-LPDs) ranging from post-transplant lymphoproliferative disorder, B cell lymphomas (e.g., endemic Burkitt lymphoma, Hodgkin lymphoma, and diffuse large B cell lymphoma) to NK or T cell lymphoma (e.g., nasal NK/T-cell lymphoma). The virus expresses a number of latent viral proteins which are able to manipulate cell cycle and cell death processes to promote survival of the tumor cells. Several FDA-approved drugs or novel compounds have been shown to induce killing of some of the EBV-LPDs by inhibiting the function of latent viral proteins or activating the viral lytic cycle from latency. Here, we aim to provide an overview on the mechanisms by which EBV employs to drive the pathogenesis of various EBV-LPDs and to maintain the survival of the tumor cells followed by a discussion on the development of viral-targeted strategies based on the understanding of the patho-mechanisms.

Analysis of Epstein-Barr virus and cellular gene expression during the early phases of Epstein-Barr virus lytic induction

In order to develop novel host/pathogen real-time PCR assays for routine diagnostic use, early gene expression patterns from both Epstein-Barr virus (EBV) and Raji cells were examined after inducing the lytic life cycle using 12-O-tetradecanoyl-13-phorbol ester and sodium butyrate. Real-time PCR identified several highly induced (>90-fold) EBV lytic genes over a 48 h time course during the lytic induction phase. Latent genes were induced at low levels during this phase. The cellular response to lytic viral replication is poorly understood. Whole human genome microarray analysis identified 113 cellular genes regulated twofold or more by EBV, including 63 upregulated and 46 downregulated genes, over a 24 h time course post-induction. The most upregulated gene was CHI3L1, a chitinase-3-like 1 protein (18.1-fold; P<0.0084), and the most downregulated gene was TYMS, a thymidylate synthetase (-7.6-fold). Gene Ontology enrichment analysis using MetaCore software revealed cell cycle (core), cell cycle (role of anaphase-promoting complex) in cell cycle regulation) and lymphatic diseases as the most significantly represented biological network processes, canonical pathways and disease biomarkers, respectively. Chemotaxis, DNA damage and inflammation (IL-4 signalling) together with lymphoproliferative disorders and non-Hodgkin's lymphoma were significantly represented biological processes and disease biomarkers.

Epstein-Barr virus-encoded miR-BART6-3p inhibits cancer cell metastasis and invasion by targeting long non-coding RNA LOC553103

Epstein-Barr virus (EBV) infection is causatively related to a variety of human cancers, including nasopharyngeal carcinoma (NPC) and gastric cancer (GC). EBV encodes 44 mature miRNAs, a number of which have been proven to promote carcinogenesis by targeting host genes or self-viral genes. However, in this study, we found that an EBV-encoded microRNA, termed EBV-miR-BART6-3p, inhibited EBV-associated cancer cell migration and invasion including NPC and GC by reversing the epithelial-mesenchymal transition (EMT) process. Using microarray analysis, we identified and validated that a novel long non-coding RNA (lncRNA) LOC553103 was downregulated by EBV-miR-BART6-3p, and LOC553103 knockdown by specific siRNAs phenocopied the effect of EBV-miR-BART6-3p, while LOC553103 overexpression promoted cancer cell migration and invasion to facilitate EMT. In conclusion, we determined that EBV-miR-BART6-3p, a microRNA encoded by oncogenic EBV, inhibited EBV-associated cancer cell migration and invasion by targeting and downregulating a novel lncRNA LOC553103. Thus, our study presents an unreported mechanism underlying EBV infection in EBV-associated cancer carcinogenesis, and provides a potential novel diagnosis and treatment biomarker for NPC and other EBV-related cancers.

Extracellular vesicles during Herpes Simplex Virus type 1 infection: an inquire

Extracellular vesicles are defined as a heterogeneous group of vesicles that are released by prokaryotic to higher eukaryotic cells and by plant cells in an evolutionary conserved manner. The significance of these vesicles lies in their capacity to transfer selected cargo composed of proteins, lipids and nucleic acids to both recipient and parent cells and to influence various physiological and pathological functions. Microorganisms such as parasites, fungi and protozoa and even single cell organisms such as bacteria generate extracellular vesicles. In addition, several viruses have evolved strategies to hijack the extracellular vesicles for egress or to alter the surrounding environment. The thesis of this article is that: a) during HSV-1 infection vesicles are delivered from infected to uninfected cells that influence the infection; b) the cargo of these vesicles consists of viral and host transcripts (mRNAs, miRNAs and non-coding RNAs) and proteins including innate immune components, such as STING; and c) the viral vesicles carry the tetraspanins CD9, CD63 and CD81, which are considered as markers of exosomes. Therefore, we assume that the STING-carrying vesicles, produced during HSV-1 infection, are reminiscent to exosomes. The presumed functions of the exosomes released from HSV-1 infected cells include priming the recipient cells and accelerating antiviral responses to control the dissemination of the virus. This may be one strategy used by the virus to prevent the elimination by the host and establish persistent infection. In conclusion, the modification of the cargo of exosomes appears to be part of the strategy that HSV-1 has evolved to establish lifelong persistent infections into the human body to ensure successful dissemination between individuals.

A tumour necrosis factor receptor-like protein encoded by Singapore grouper iridovirus modulates cell proliferation, apoptosis and viral replication

It has been demonstrated that tumour necrosis factor receptor (TNFR) homologues encoded by viruses are usually involved in virus immune evasion by regulating the host immune response or mediating apoptotic cell death. Here, a novel TNFR-like protein encoded by Singapore grouper iridovirus (SGIV VP51) was cloned and characterized. Amino acid analysis showed that VP51 contained three cysteine-rich domains (CRDs) and a transmembrane domain at its C terminus. The expression of VP51 in vitro enhanced cell proliferation, and affected cell cycle progression via altering the G1/S transition. Furthermore, VP51 overexpression improved cell viability during SGIV infection via inhibiting virus-induced apoptosis, evidenced by the reduction of apoptotic bodies and the decrease of caspase-3 activation. In addition, overexpression of VP51 increased viral titre and the expression of viral structural protein gene MCP and cell proliferation promoting gene ICP-18. In contrast, the expression of the viral apoptosis inducing gene, LITAF, was significantly decreased. Although all three CRDs were essential for the action of VP51, CRD2 and CRD3 exerted more crucial roles on virus-induced apoptosis, viral gene transcription and virus production, while CRD1 was more crucial for cell proliferation. Together, SGIV TNFR-like products not only affected cell cycle progression and enhanced cell growth by increasing the expression of the virus encoded cell proliferation gene, but also inhibited virus-induced apoptotic cell death by decreasing the expression of the viral apoptosis inducing gene. Our results provided new insights into understanding the underlying mechanism by which iridovirus regulated the apoptotic pathway to complete its life cycle.

Uracil DNA glycosylase BKRF3 contributes to Epstein-Barr virus DNA replication through physical interactions with proteins in viral DNA replication complex

Epstein-Barr virus (EBV) BKRF3 shares sequence homology with members of the uracil-N-glycosylase (UNG) protein family and has DNA glycosylase activity. Here, we explored how BKRF3 participates in the DNA replication complex and contributes to viral DNA replication. Exogenously expressed Flag-BKRF3 was distributed mostly in the cytoplasm, whereas BKRF3 was translocated into the nucleus and colocalized with the EBV DNA polymerase BALF5 in the replication compartment during EBV lytic replication. The expression level of BKRF3 increased gradually during viral replication, coupled with a decrease of cellular UNG2, suggesting BKRF3 enzyme activity compensates for UNG2 and ensures the fidelity of viral DNA replication. In immunoprecipitation-Western blotting, BKRF3 was coimmuno-precipitated with BALF5, the polymerase processivity factor BMRF1, and the immediate-early transactivator Rta. Coexpression of BMRF1 appeared to facilitate the nuclear targeting of BKRF3 in immunofluorescence staining. Residues 164 to 255 of BKRF3 were required for interaction with Rta and BALF5, whereas residues 81 to 166 of BKRF3 were critical for BMRF1 interaction in glutathione S-transferase (GST) pulldown experiments. Viral DNA replication was defective in cells harboring BKRF3 knockout EBV bacmids. In complementation assays, the catalytic mutant BKRF3(Q90L,D91N) restored viral DNA replication, whereas the leucine loop mutant BKRF3(H213L) only partially rescued viral DNA replication, coupled with a reduced ability to interact with the viral DNA polymerase and Rta. Our data suggest that BKRF3 plays a critical role in viral DNA synthesis predominantly through its interactions with viral proteins in the DNA replication compartment, while its enzymatic activity may be supplementary for uracil DNA glycosylase (UDG) function during virus replication.

IMPORTANCE

Catalytic activities of both cellular UDG UNG2 and viral UDGs contribute to herpesviral DNA replication. To ensure that the enzyme activity executes at the right time and the right place in DNA replication forks, complex formation with other components in the DNA replication machinery provides an important regulation for UDG function. In this study, we provide the mechanism for EBV UDG BKRF3 nuclear targeting and the interacting domains of BKRF3 with viral DNA replication proteins. Through knockout and complementation approaches, we further demonstrate that in addition to UDG activity, the interaction of BKRF3 with viral proteins in the replication compartment is crucial for efficient viral DNA replication.

Sequence analysis of EBV immediate-early gene BZLF1 and BRLF1 in lymphomas

The immediate-early (IE) genes, BZLF1 and BRLF1, play an important role in switching Epstein-Barr virus from the latent to the lytic state. The functions of the two IE genes and their respective proteins: ZEBRA and Rta have been well studied, but little is known about their DNA coding sequence variations and disease association. In order to investigate the sequence variation patterns and elucidate their association with lymphomas, BZLF1 and BRLF1 were analyzed in 26 and 33 lymphomas using PCR-direct sequencing method respectively. Three sequence variation types of BZLF1 gene were identified. The type BZLF1-A and BZLF1-B was detected in 34.6% (9/26) and 57.7% (15/26) of the tumor specimens, respectively. Among the three functional domains of ZEBRA, the transactivation domain had the most mutations. Three variation types were also identified in BRLF1 gene where type BR1-A and BR1-C were detected in 27.3% (9/33) and 69.7% (23/33) of specimens, respectively. Among the three functional domains of Rta, the dimerization domain was well conserved while multiple mutations were detected in both the DNA binding domain and the transactivation domain. The variation types BZLF1-B and BR1-C were more frequent in the lymphomas, compared with the throat washing samples from the healthy donors. These results suggest that the type BZLF1-B and BR1-C may be associated with the tumorigenesis of lymphoma.

An insulin-like growth factor homologue of Singapore grouper iridovirus modulates cell proliferation, apoptosis and enhances viral replication

Insulin-like growth factors (IGFs) play crucial roles in regulating cell differentiation, proliferation and apoptosis. In this study, a novel IGF homologue gene (IGF-like) encoded by Singapore grouper iridovirus (SGIV) ORF062R (termed SGIV–IGF), was cloned and characterized. The coding region of SGIV–IGF is 771 bp in length, with a variable number of tandem repeats (VNTR) locus at the 3′-end. We cloned one isoform of this novel gene, 582 bp in length, containing the predicted IGF domain and 3.6 copy numbers of the 27 bp repeat unit. SGIV–IGF was an early transcribed gene during viral infection, and SGIV–IGF was distributed predominantly in the cytoplasm with a diffused granular appearance. Intriguingly, overexpression of SGIV–IGF was able to promote the growth of grouper embryonic cells (GP cells) by promoting G1/S phase transition, which was at least partially dependent on its 3′-end VNTR locus. Furthermore, viral titre assay and real-time quantitative PCR (RT-qPCR) analysis proved that SGIV–IGF could promote SGIV replication in grouper cells. In addition, overexpression of SGIV–IGF mildly facilitated apoptosis in SGIV-infected non-host fathead minnow (FHM) cells. Together, our study demonstrated a novel functional gene of SGIV which may regulate viral replication and cellular processes through multiple mechanisms that appear to be cell type-dependent.

Characterization of Epstein-Barr virus reactivation in a modeled spaceflight system

Epstein-Barr virus (EBV) is the causative agent of mononucleosis and is also associated with several malignancies, including Burkitt's lymphoma, Hodgkin's lymphoma, and nasopharyngeal carcinoma, among others. EBV reactivates during spaceflight, with EBV shedding in saliva increasing to levels ten times those observed pre-and post-flight. Although stress has been shown to increase reactivation of EBV, other factors such as radiation and microgravity have been hypothesized to contribute to reactivation in space. We used a modeled spaceflight environment to evaluate the influence of radiation and microgravity on EBV reactivation. BJAB (EBV-negative) and Raji (EBV-positive) cell lines were assessed for viability/apoptosis, viral antigen and reactive oxygen species expression, and DNA damage and repair. EBV-infected cells did not experience decreased viability and increased apoptosis due to modeled spaceflight, whereas an EBV-negative cell line did, suggesting that EBV infection provided protection against apoptosis and cell death. Radiation was the major contributor to EBV ZEBRA upregulation. Combining modeled microgravity and radiation increased DNA damage and reactive oxygen species while modeled microgravity alone decreased DNA repair in Raji cells. Additionally, EBV-infected cells had increased DNA damage compared to EBV-negative cells. Since EBV-infected cells do not undergo apoptosis as readily as uninfected cells, it is possible that virus-infected cells in EBV seropositive individuals may have an increased risk to accumulate DNA damage during spaceflight. More studies are warranted to investigate this possibility.

RNAs in Epstein-Barr virions control early steps of infection

Herpesviruses are dsDNA viruses, but their virions may additionally contain RNAs that can be transduced to recipient cells. The biological functions of herpes virion RNA species are unknown. Here we address this issue for EBV, a widespread human herpesvirus with oncogenic potential. We show that EBV-derived particles that include virions, virus-like particles, and subviral vesicles contain viral mRNAs, microRNAs, and other noncoding RNAs. Viral RNAs were transduced during infection and deployed immediate functions that enhanced EBV's capacity to transform primary B cells. Among these transduced viral RNAs, BZLF1 transcripts transactivated viral promoters triggering the prelatent phase of EBV infection, noncoding EBV-encoded RNA transcripts induced cellular cytokine synthesis, and BNLF2a mRNA led to immune evasion that prevented T-cell responses to newly infected B cells. Hence, transduced viral RNAs govern critical processes immediately after infection of B cells with EBV and likely play important roles in herpesviral infection in general.

Screening and functional analysis of differentially expressed genes in EBV-transformed lymphoblasts

BACKGROUND

Epstain-Barr virus (EBV) can transform human B lymphocytes making them immortalized and inducing tumorigenic ability in vitro, but the molecular mechanisms remain unclear. The aim of the present study is to detect and analyze differentially expressed genes in two types of host cells, normal human lymphocytes and coupled EBV-transformed lymphoblasts in vitro using gene chips, and to screen the key regulatory genes of lymphocyte transformation induced by EB virus.

METHODS

Fresh peripheral blood samples from seven healthy donors were collected. EBV was used to transform lymphocytes in vitro. Total RNA was extracted from 7 cases of the normal lymphocytes and transformed lymphoblasts respectively, marked with dihydroxyfluorane after reverse transcription, then hybridized with 4 × 44 K Agilent human whole genome microarray. LIMMA, String, Cytoscape and other softwares were used to screen and analyze differentially expressed genes. Real-time PCR was applied to verify the result of gene expression microarrays.

RESULTS

There were 1745 differentially expressed genes that had been screened, including 917 up-regulated genes and 828 down-regulated genes. According to the results of Generank, String and Cytoscape analyses, 38 genes may be key controlled genes related to EBV-transformed lymphocytes, including 22 up-regulated genes(PLK1, E2F1, AURKB, CDK2, PLCG2, CD80, PIK3R3, CDC20, CDC6, AURKA, CENPA, BUB1B, NUP37, MAD2L1, BIRC5, CDC25A, CCNB1, RPA3, HJURP, KIF2C, CDK1, CDCA8) and 16 down-regulated genes(FYN, CD3D, CD4, CD3G, ZAP70, FOS, HCK, CD247, PRKCQ, ITK, LCP2, CXCL1, CD8A, ITGB5, VAV3, CXCR4), which primarily control biological processes such as cell cycle, mitosis, cytokine-cytokine pathway, immunity response and so on.

CONCLUSIONS

Human lymphocyte transformation induced by EB virus is a complicated process, involving multiple-genes and -pathways in virus-host interactions. Global gene expression profile analysis showed that EBV may transform human B lymphocytes by promoting cell cycle and mitosis, inhibiting cell apoptosis, hindering host immune function and secretion of cytokines.

Epstein-Barr virus Rta-mediated transactivation of p21 and 14-3-3σ arrests cells at the G1/S transition by reducing cyclin E/CDK2 activity

Many herpesviral immediate-early proteins promote their robust lytic phase replications by hijacking the cell cycle machinery. Previously, lytic replication of Epstein-Barr virus (EBV) was found to be concurrent with host cell cycle arrest. In this study, we showed that ectopic expression of EBV immediate-early protein Rta in HEp-2 cells resulted in increased G1/S population, hypophosphorylation of pRb and decreased incorporation of 5-bromo-2'-deoxyuridine. In addition, EBV Rta transcriptionally upregulates the expressions of p21 and 14-3-3σ in HEp-2 cells, 293 cells and nasopharyngeal carcinoma TW01 cells. Although p21 and 14-3-3σ are known targets for p53, Rta-mediated p21 and 14-3-3σ transactivation can be detected in the absence of p53. In addition, results from luciferase reporter assays indicated that direct binding of Rta to either promoter sequences is not required for activation. On the other hand, a special class of Sp1-responsive elements was involved in Rta-mediated transcriptional activation on both promoters. Finally, Rta-induced p21 expression diminished the activity of CDK2/cyclin E complex, and, Rta-induced 14-3-3σ expression sequestered CDK1 and CDK2 in the cytoplasm. Based on these results, we hypothesize that through the disruption of CDK1 and CDK2 activities, EBV Rta might contribute to cell cycle arrest in EBV-infected epithelial cells during viral reactivation.

Regulation of nucleocytoplasmic trafficking of viral proteins: an integral role in pathogenesis?

Signal-dependent targeting of proteins into and out of the nucleus is mediated by members of the importin (IMP) family of transport receptors, which recognise targeting signals within a cargo protein and mediate passage through the nuclear envelope-embedded nuclear pore complexes. Regulation of this process is paramount to processes such as cell division and differentiation, but is also critically important for viral replication and pathogenesis; phosphorylation appears to play a major role in regulating viral protein nucleocytoplasmic trafficking, along with other posttranslational modifications. This review focuses on viral proteins that utilise the host cell IMP machinery in order to traffic into/out of the nucleus, and in particular those where trafficking is critical to viral replication and/or pathogenesis, such as simian virus SV40 large tumour antigen (T-ag), human papilloma virus E1 protein, human cytomegalovirus processivity factor ppUL44, and various gene products from RNA viruses such as Rabies. Understanding of the mechanisms regulating viral protein nucleocytoplasmic trafficking is paramount to the future development of urgently needed specific and effective anti-viral therapeutics. This article was originally intended for the special issue "Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import". The Publisher apologizes for any inconvenience caused.

Epstein-Barr virus (EBV) Rta-mediated EBV and Kaposi's sarcoma-associated herpesvirus lytic reactivations in 293 cells

Epstein–Barr virus (EBV) Rta belongs to a lytic switch gene family that is evolutionarily conserved in all gamma-herpesviruses. Emerging evidence indicates that cell cycle arrest is a common means by which herpesviral immediate-early protein hijacks the host cell to advance the virus's lytic cycle progression. To examine the role of Rta in cell cycle regulation, we recently established a doxycycline (Dox)-inducible Rta system in 293 cells. In this cell background, inducible Rta modulated the levels of signature G1 arrest proteins, followed by induction of the cellular senescence marker, SA-β-Gal. To delineate the relationship between Rta-induced cell growth arrest and EBV reactivation, recombinant viral genomes were transferred into Rta-inducible 293 cells. Somewhat unexpectedly, we found that Dox-inducible Rta reactivated both EBV and Kaposi's sarcoma-associated herpesvirus (KSHV), to similar efficacy. As a consequence, the Rta-mediated EBV and KSHV lytic replication systems, designated as EREV8 and ERKV, respectively, were homogenous, robust, and concurrent with cell death likely due to permissive lytic replication. In addition, the expression kinetics of EBV lytic genes in Dox-treated EREV8 cells was similar to that of their KSHV counterparts in Dox-induced ERKV cells, suggesting that a common pathway is used to disrupt viral latency in both cell systems. When the time course was compared, cell cycle arrest was achieved between 6 and 48 h, EBV or KSHV reactivation was initiated abruptly at 48 h, and the cellular senescence marker was not detected until 120 h after Dox treatment. These results lead us to hypothesize that in 293 cells, Rta-induced G1 cell cycle arrest could provide (1) an ideal environment for virus reactivation if EBV or KSHV coexists and (2) a preparatory milieu for cell senescence if no viral genome is available. The latter is hypothetical in a transient-lytic situation.

Getting the message direct manipulation of host mRNA accumulation during gammaherpesvirus lytic infection

The Gammaherpesvirinae subfamily of herpesviruses comprises lymphotropic viruses, including the oncogenic human pathogens Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus. During lytic infection, gammaherpesviruses manipulate host gene expression to optimize the cellular environment for viral replication and to evade the immune response. Additionally, although a lytically infected cell will itself be killed in the process of viral replication, lytic infection can contribute to pathogenesis by inducing the secretion of paracrine factors with functions in cell survival and proliferation, and angiogenesis. The mechanisms by which these viruses manipulate host gene expression are varied and target the accumulation of cellular mRNAs and their translation, signaling pathways, and protein stability. Here, we discuss how gammaherpesviral proteins directly influence host mRNA biogenesis and stability, either selectively or globally, in order to fine-tune the cellular environment to the advantage of the virus. Appreciation of the mechanisms by which these viruses interface with and adapt normal cellular processes continues to inform our understanding of gammaherpesviral biology and the regulation of mRNA accumulation and turnover in our own cells.