Pathways of activation of the Epstein-Barr virus productive cycle

The Functional Interaction Between Epstein-Barr Virus and MYC in the Pathogenesis of Burkitt Lymphoma

The Epstein-Barr virus (EBV) is associated with a wide range of diseases, malignant and non-malignant. EBV was, in fact, the first virus described with cell transformation capacity, discovered by Epstein in 1964 in lymphoma samples from African children. Since then, EBV has been associated with several human tumors including nasopharyngeal carcinoma, gastric carcinoma, T-cell lymphoma, Hodgkin lymphoma, diffuse large B cell lymphoma, and Burkitt lymphoma among others. The molecular hallmark of Burkitt lymphoma (BL) is a chromosomal translocation that involves the MYC gene and immunoglobulin loci, resulting in the deregulated expression of MYC, an oncogenic transcription factor that appears deregulated in about half of human tumors. The role of MYC in lymphoma is well established, as MYC overexpression drives B cell proliferation through multiple mechanisms, foremost, the stimulation of the cell cycle. Indeed, MYC is found overexpressed or deregulated in several non-Hodgkin lymphomas. Most endemic and many sporadic BLs are associated with EBV infection. While some mechanisms by which EBV can contribute to BL have been reported, the mechanism that links MYC translocation and EBV infection in BL is still under debate. Here, we review the main EBV-associated diseases, with a special focus on BL, and we discuss the interaction of EBV and MYC translocation during B cell malignant transformation in BL.

Orbivirus NS4 Proteins Play Multiple Roles to Dampen Cellular Responses

Non-structural protein 4 (NS4) of insect-borne and tick-borne orbiviruses is encoded by genome segment 9, from a secondary open reading frame. Though a protein dispensable for bluetongue virus (BTV) replication, it has been shown to counter the interferon response in cells infected with BTV or African horse sickness virus. We further explored the functional role(s) of NS4 proteins of BTV and the tick-borne Great Island virus (GIV). We show that NS4 of BTV or GIV helps an E3L deletion mutant of vaccinia virus to replicate efficiently in interferon-treated cells, further confirming the role of NS4 as an interferon antagonist. Our results indicate that ectopically expressed NS4 of BTV localised with caspase 3 within the nucleus and was found in a protein complex with active caspase 3 in a pull-down assay. Previous studies have shown that pro-apoptotic caspases (including caspase 3) suppress type I interferon response by cleaving mediators involved in interferon signalling. Our data suggest that orbivirus NS4 plays a role in modulating the apoptotic process and/or regulating the interferon response in mammalian cells, thus acting as a virulence factor in pathogenesis.

Malignancy and viral infections in Sub-Saharan Africa: A review

The burden of malignancy related to viral infection is increasing in Sub-Saharan Africa (SSA). In 2018, approximately 2 million new cancer cases worldwide were attributable to infection. Prevention or treatment of these infections could reduce cancer cases by 23% in less developed regions and about 7% in developed regions. Contemporaneous increases in longevity and changes in lifestyle have contributed to the cancer burden in SSA. African hospitals are reporting more cases of cancer related to infection (e.g., cervical cancer in women and stomach and liver cancer in men). SSA populations also have elevated underlying prevalence of viral infections compared to other regions. Of 10 infectious agents identified as carcinogenic by the International Agency for Research on Cancer, six are viruses: hepatitis B and C viruses (HBV and HCV, respectively), Epstein-Barr virus (EBV), high-risk types of human papillomavirus (HPV), Human T-cell lymphotropic virus type 1 (HTLV-1), and Kaposi's sarcoma herpesvirus (KSHV, also known as human herpesvirus type 8, HHV-8). Human immunodeficiency virus type 1 (HIV) also facilitates oncogenesis. EBV is associated with lymphomas and nasopharyngeal carcinoma; HBV and HCV are associated with hepatocellular carcinoma; KSHV causes Kaposi's sarcoma; HTLV-1 causes T-cell leukemia and lymphoma; HPV causes carcinoma of the oropharynx and anogenital squamous cell cancer. HIV-1, for which SSA has the greatest global burden, has been linked to increasing risk of malignancy through immunologic dysregulation and clonal hematopoiesis. Public health approaches to prevent infection, such as vaccination, safer injection techniques, screening of blood products, antimicrobial treatments and safer sexual practices could reduce the burden of cancer in Africa. In SSA, inequalities in access to cancer screening and treatment are exacerbated by the perception of cancer as taboo. National level cancer registries, new screening strategies for detection of viral infection and public health messaging should be prioritized in SSA's battle against malignancy. In this review, we discuss the impact of carcinogenic viruses in SSA with a focus on regional epidemiology.

Epstein-Barr Virus-Encoded Products Promote Circulating Tumor Cell Generation: A Novel Mechanism of Nasopharyngeal Carcinoma Metastasis

Epstein–Barr virus (EBV) is a specific tumorigenic factor in the pathogenesis of nasopharyngeal carcinoma (NPC). Viral products encoded by EBV (LMP1, LMP2A, EBNA1, and miRNAs) have been shown to promote NPC metastasis. EBV-encoded oncoproteins and miRNAs have been shown to induce epithelial–mesenchymal transition (EMT) indirectly by inducing EMT transcription factors (EMT-TFs). These EBV-encoded products also promote the expression of EMT-TFs through post-transcriptional regulation. EMT contributes to generation of circulating tumor cells (CTCs) in epithelial cancers. CTCs exhibit stem cell characteristics, including increased invasiveness, enhanced cell intravasation, and improved cell survival in the peripheral system. EBV may contribute NPC metastasis through promoting generation of CTCs. Furthermore, CTC karyotypes are associated with NPC staging, therapeutic sensitivity, and resistance. We summarized studies showing that EBV-encoded virus-proteins and miRNAs promote generation of NPC CTCs, and highlighted the associated mechanism. This synthesis indicated that EBV mediates NPC metastasis through generation of CTCs.

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.

Epstein-Barr virus BRLF1 induces genomic instability and progressive malignancy in nasopharyngeal carcinoma cells

Nasopharyngeal carcinoma (NPC) is a serious health problem in China and Southeast Asia. Relapse is the major cause of mortality, but mechanisms of relapse are mysterious. Epstein-Barr virus (EBV) reactivation and host genomic instability (GI) have correlated with NPC development. Previously, we reported that lytic early genes DNase and BALF3 induce genetic alterations and progressive malignancy in NPC cells, implying lytic proteins may be required for NPC relapse. In this study, we show that immediate early gene BRLF1 induces chromosome mis-segregation and genomic instability in the NPC cells. Similar phenomenon was also demonstrated in 293 and zebrafish embryonic cells. BRLF1 nuclear localization signal (NLS) mutant still induced genomic instability and inhibitor experiments revealed that BRLF1 interferes with chromosome segregation and induces genomic instability by activating Erk signaling. Furthermore, the chromosome aberrations and tumorigenic features of NPC cells were significantly increased with the rounds of BRLF1 expression, and these cells developed into larger tumor nodules in mice. Therefore, BRLF1 may be the important factor contributing to NPC relapse and targeting BRLF1 may benefit patients.

Radiation-induced Epstein-Barr virus reactivation in gastric cancer cells with latent EBV infection

Epstein-Barr virus, a ubiquitous human herpes virus with oncogenic activity, can be found in 6%-16% of gastric carcinomas worldwide. In Epstein-Barr virus-associated gastric carcinoma, only a few latent genes of the virus are expressed. Ionizing irradiation was shown to induce lytic Epstein-Barr virus infection in lymphoblastoid cell lines with latent Epstein-Barr virus infection. In this study, we examined the effect of ionizing radiation on the Epstein-Barr virus reactivation in a gastric epithelial cancer cell line (SNU-719, an Epstein-Barr virus-associated gastric carcinoma cell line). Irradiation with X-ray (dose = 5 and 10 Gy; dose rate = 0.5398 Gy/min) killed approximately 25% and 50% of cultured SNU-719 cells, respectively, in 48 h. Ionizing radiation increased the messenger RNA expression of immediate early Epstein-Barr virus lytic genes (BZLF1 and BRLF1), determined by real-time reverse transcription polymerase chain reaction, in a dose-dependent manner at 48 h and, to a slightly lesser extent, at 72 h after irradiation. Similar findings were observed for other Epstein-Barr virus lytic genes (BMRF1, BLLF1, and BcLF1). After radiation, the expression of transforming growth factor beta 1 messenger RNA increased and reached a peak in 12-24 h, and the high-level expression of the Epstein-Barr virus immediate early genes can convert latent Epstein-Barr virus infection into the lytic form and result in the release of infectious Epstein-Barr virus. To conclude, Ionizing radiation activates lytic Epstein-Barr virus gene expression in the SNU-719 cell line mainly through nuclear factor kappaB activation. We made a brief review of literature to explore underlying mechanism involved in transforming growth factor beta-induced Epstein-Barr virus reactivation. A possible involvement of nuclear factor kappaB was hypothesized.

The role of cullin 5-containing ubiquitin ligases

The suppressor of cytokine signaling (SOCS) box consists of the BC box and the cullin 5 (Cul5) box, which interact with Elongin BC and Cul5, respectively. SOCS box-containing proteins have ubiquitin ligase activity mediated by the formation of a complex with the scaffold protein Cul5 and the RING domain protein Rbx2, and are thereby members of the cullin RING ligase superfamily. Cul5-type ubiquitin ligases have a variety of substrates that are targeted for polyubiquitination and proteasomal degradation. Here, we review the current knowledge on the identification of Cul5 and the regulation of its expression, as well as the signaling pathways regulated by Cul5 and how viruses highjack the Cul5 system to overcome antiviral responses.

Identification of Novel Small Organic Compounds with Diverse Structures for the Induction of Epstein-Barr Virus (EBV) Lytic Cycle in EBV-Positive Epithelial Malignancies

Phorbol esters, which are protein kinase C (PKC) activators, and histone deacetylase (HDAC) inhibitors, which cause enhanced acetylation of cellular proteins, are the main classes of chemical inducers of Epstein-Barr virus (EBV) lytic cycle in latently EBV-infected cells acting through the PKC pathway. Chemical inducers which induce EBV lytic cycle through alternative cellular pathways may aid in defining the mechanisms leading to lytic cycle reactivation and improve cells' responsiveness towards lytic induction. We performed a phenotypic screening on a chemical library of 50,240 novel small organic compounds to identify novel class(es) of strong inducer(s) of EBV lytic cycle in gastric carcinoma (GC) and nasopharyngeal carcinoma (NPC) cells. Five hit compounds were selected after three successive rounds of increasingly stringent screening. All five compounds are structurally diverse from each other and distinct from phorbol esters or HDAC inhibitors. They neither cause hyperacetylation of histone proteins nor significant PKC activation at their working concentrations, suggesting that their biological mode of action are distinct from that of the known chemical inducers. Two of the five compounds with rapid lytic-inducing action were further studied for their mechanisms of induction of EBV lytic cycle. Unlike HDAC inhibitors, lytic induction by both compounds was not inhibited by rottlerin, a specific inhibitor of PKCδ. Interestingly, both compounds could cooperate with HDAC inhibitors to enhance EBV lytic cycle induction in EBV-positive epithelial cancer cells, paving way for the development of strategies to increase cells' responsiveness towards lytic reactivation. One of the two compounds bears structural resemblance to iron chelators and the other strongly activates the MAPK pathways. These structurally diverse novel organic compounds may represent potential new classes of chemicals that can be used to investigate any alternative mechanism(s) leading to EBV lytic cycle reactivation from latency.

Repression of CIITA by the Epstein-Barr virus transcription factor Zta is independent of its dimerization and DNA binding

Repression of the cellular CIITA gene is part of the immune evasion strategy of the γherpes virus Epstein-Barr virus (EBV) during its lytic replication cycle in B-cells. In part, this is mediated through downregulation of MHC class II gene expression via the targeted repression of CIITA, the cellular master regulator of MHC class II gene expression. This repression is achieved through a reduction in CIITA promoter activity, initiated by the EBV transcription and replication factor, Zta (BZLF1, EB1, ZEBRA). Zta is the earliest gene expressed during the lytic replication cycle. Zta interacts with sequence-specific elements in promoters, enhancers and the replication origin (ZREs), and also modulates gene expression through interaction with cellular transcription factors and co-activators. Here, we explore the requirements for Zta-mediated repression of the CIITA promoter. We find that repression by Zta is specific for the CIITA promoter and can be achieved in the absence of other EBV genes. Surprisingly, we find that the dimerization region of Zta is not required to mediate repression. This contrasts with an obligate requirement of this region to correctly orientate the DNA contact regions of Zta to mediate activation of gene expression through ZREs. Additional support for the model that Zta represses the CIITA promoter without direct DNA binding comes from promoter mapping that shows that repression does not require the presence of a ZRE in the CIITA promoter.

Cellular differentiation regulator BLIMP1 induces Epstein-Barr virus lytic reactivation in epithelial and B cells by activating transcription from both the R and Z promoters

Epstein-Barr virus (EBV) maintains a lifelong latent infection within a subset of its host's memory B cells, while lytic EBV replication takes place in plasma cells and differentiated epithelial cells. Therefore, cellular transcription factors, such as BLIMP1, that are key mediators of differentiation likely contribute to the EBV latent-to-lytic switch. Previous reports showed that ectopic BLIMP1 expression induces reactivation in some EBV-positive (EBV(+)) B-cell lines and transcription from Zp, with all Z(+) cells in oral hairy leukoplakia being BLIMP1(+). Here, we examined BLIMP1's role in inducing EBV lytic gene expression in numerous EBV(+) epithelial and B-cell lines and activating transcription from Rp. BLIMP1 addition was sufficient to induce reactivation in latently infected epithelial cells derived from gastric cancers, nasopharyngeal carcinomas, and normal oral keratinocytes (NOK) as well as some, but not all B-cell lines. BLIMP1 strongly induced transcription from Rp as well as Zp, with there being three or more synergistically acting BLIMP1-responsive elements (BRE) within Rp. BLIMP1's DNA-binding domain was required for reactivation, but BLIMP1 did not directly bind the nucleotide (nt) -660 Rp BRE. siRNA knockdown of BLIMP1 inhibited 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced lytic reactivation in NOK-Akata cells, cells that can be reactivated by R, but not Z. Thus, we conclude that BLIMP1 expression is both necessary and sufficient to induce EBV lytic replication in many (possibly all) EBV(+) epithelial-cell types, but in only a subset of EBV(+) B-cell types; it does so, at least in part, by strongly activating expression of both EBV immediately early genes, BZLF1 and BRLF1.

IMPORTANCE

This study is the first one to show that the cellular transcription factor BLIMP1, a key player in both epithelial and B-cell differentiation, induces reactivation of the oncogenic herpesvirus Epstein-Barr virus (EBV) out of latency into lytic replication in a variety of cancerous epithelial cell types as well as in some, but not all, B-cell types that contain this virus in a dormant state. The mechanism by which BLIMP1 does so involves strongly turning on expression of both of the immediate early genes of the virus, probably by directly acting upon the promoters as part of protein complexes or indirectly by altering the expression or activities of some cellular transcription factors and signaling pathways. The fact that EBV(+) cancers usually contain mostly undifferentiated cells may be due in part to these cells dying from lytic EBV infection when they differentiate and express wild-type BLIMP1.

MicroRNA miR-BART20-5p stabilizes Epstein-Barr virus latency by directly targeting BZLF1 and BRLF1

Epstein-Barr virus (EBV) is a human herpesvirus associated with various tumors. Rather than going through the lytic cycle, EBV maintains latency by limiting the expression of viral genes in tumors. Viral microRNAs (miRNAs) of some herpesviruses have been reported to directly target immediate early genes and suppress lytic induction. In this study, we investigated whether BamHI-A rightward transcript (BART) miRNAs targeted two EBV immediate early genes, BZLF1 and BRLF1. Bioinformatic analysis predicted that 12 different BART miRNAs would target BRLF1. Of these, the results of a luciferase reporter assay indicated that only one interacted with the 3' untranslated region (UTR) of BRLF1: miR-BART20-5p. miR-BART20-5p's effect on gene expression involved two putative seed match sites in the BRLF1 3' UTR, but a mutant version of the miRNA, miR-BART20-5pm, had no effect on expression. As expected from the fact that the entire 3' UTR of BZLF1 resides within the 3' UTR of BRLF1, miR-BART20-5p interacted with the 3' UTR of BZLF1 as well. BZLF1 and BRLF1 mRNA and protein expression was suppressed in cells of an AGS cell line infected with the recombinant Akata strain of EBV (AGS-EBV) transfected with a miR-BART20-5p mimic. The expression of various EBV early proteins was also suppressed by the miR-BART20-5p mimic. In contrast, BZLF1 and BRLF1 expression in AGS-EBV cells transfected with a miR-BART20-5p inhibitor was enhanced. Furthermore, progeny virus production was suppressed by the miR-BART20-5p mimic and enhanced by the miR-BART20-5p inhibitor in AGS-EBV cells induced for the lytic cycle. Our data suggest that miR-BART20-5p plays a key role in latency maintenance in EBV-associated tumors by directly targeting immediate early genes.

IMPORTANCE

Herpesviruses maintain latency using various mechanisms and establish lifelong infection in the host. From time to time, herpesviruses are reactivated and express immediate early genes which trigger a lytic cascade, leading to the production of progeny viruses. Recently, some herpesviruses have been shown to use their own microRNAs (miRNAs) to downregulate immediate early genes to inhibit the lytic cycle. This study presents evidence that EBV also downregulates two immediate early genes by miR-BART20-5p to suppress the lytic cycle and progeny virus production. Overall, this is the first study to report the direct regulation of EBV immediate early genes by an EBV miRNA, implying its likely importance in latency maintenance in EBV-associated tumors.

Reactive oxygen species mediate Epstein-Barr virus reactivation by N-methyl-N'-nitro-N-nitrosoguanidine

N-nitroso compounds (NOCs) and Epstein-Barr virus (EBV) reactivation have been suggested to play a role in the development of nasopharyngeal carcinoma (NPC). Although chemicals have been shown to be a risk factor contributing to the carcinogenesis of NPC, the underlying mechanism is not fully understood. We demonstrated recently that N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) enhances the genomic instability and tumorigenicity of NPC cells via induction of EBV reactivation. However, the mechanisms that trigger EBV reactivation from latency remain unclear. Here, we address the role of ROS in induction of EBV reactivation under MNNG treatment. EBV reactivation was induced in over 70% of EBV-positive NA cells and the promoter of Rta (Rp) was activated after MNNG treatment. Inhibitor experiments revealed ATM, p38 MAPK and JNK were activated by ROS and involved in MNNG-induced EBV reactivation. Significantly, ROS scavengers N-acetyl-L-cysteine (NAC), catalase and reduced glutathione inhibited EBV reactivation under MNNG and H₂O₂ treatment, suggesting ROS mediate EBV reactivation. The p53 was essential for EBV reactivation and the Rp activation by MNNG. Moreover, the p53 was phosphorylated, translocated into nucleus, and bound to Rp following ROS stimulation. The results suggest ROS play an important role in initiation of EBV reactivation by MNNG through a p53-dependent mechanism. Our findings demonstrate novel signaling mechanisms used by NOCs to induce EBV reactivation and provide a novel insight into NOCs link the EBV reactivation in the contribution to the development of NPC. Notably, this study indicates that antioxidants might be effective for inhibiting N-nitroso compound-induced EBV reactivation and therefore could be promising preventive and therapeutic agents for EBV reactivation-associated malignancies.

The Role of Elongin BC-Containing Ubiquitin Ligases

The Elongin complex was originally identified as a positive regulator of RNA polymerase II and is composed of a transcriptionally active subunit (A) and two regulatory subunits (B and C). The Elongin BC complex enhances the transcriptional activity of Elongin A. “Classical” SOCS box-containing proteins interact with the Elongin BC complex and have ubiquitin ligase activity. They also interact with the scaffold protein Cullin (Cul) and the RING domain protein Rbx and thereby are members of the Cullin RING ligase (CRL) superfamily. The Elongin BC complex acts as an adaptor connecting Cul and SOCS box proteins. Recently, it was demonstrated that classical SOCS box proteins can be further divided into two groups, Cul2- and Cul5-type proteins. The classical SOCS box-containing protein pVHL is now classified as a Cul2-type protein. The Elongin BC complex containing CRL family is now considered two distinct protein assemblies, which play an important role in regulating a variety of cellular processes such as tumorigenesis, signal transduction, cell motility, and differentiation.

Dynamic chromatin environment of key lytic cycle regulatory regions of the Epstein-Barr virus genome

The ability of Epstein-Barr virus (EBV) to establish latency allows it to evade the immune system and to persist for the lifetime of its host; one distinguishing characteristic is the lack of transcription of the majority of viral genes. Entry into the lytic cycle is coordinated by the viral transcription factor, Zta (BZLF1, ZEBRA, and EB1), and downstream effectors, while viral genome replication requires the concerted action of Zta and six other viral proteins at the origins of lytic replication. We explored the chromatin context at key EBV lytic cycle promoters (BZLF1, BRLF1, BMRF1, and BALF5) and the origins of lytic replication during latency and lytic replication. We show that a repressive heterochromatin-like environment (trimethylation of histone H3 at lysine 9 [H3K9me3] and lysine 27 [H3K27me3]), which blocks the interaction of some transcription factors with DNA, encompasses the key early lytic regulatory regions. Epigenetic silencing of the EBV genome is also imposed by DNA methylation during latency. The chromatin environment changes during the lytic cycle with activation of histones H3, H4, and H2AX occurring at both the origins of replication and at the key lytic regulatory elements. We propose that Zta is able to reverse the effects of latency-associated repressive chromatin at EBV early lytic promoters by interacting with Zta response elements within the H3K9me3-associated chromatin and demonstrate that these interactions occur in vivo. Since the interaction of Zta with DNA is not inhibited by DNA methylation, it is clear that Zta uses two routes to overcome epigenetic silencing of its genome.

Herpes simplex virus type 1 infection activates the Epstein-Barr virus replicative cycle via a CREB-dependent mechanism

The reactivation of latent Epstein-Barr virus (EBV) to lytic replication is important in pathogenesis and requires virus-host cellular interactions. However, the mechanism underlying the reactivation of EBV is not yet fully understood. In the present study, herpes simplex virus type 1 (HSV-1) was shown to induce the reactivation of latent EBV by triggering BZLF1 expression. The BZLF1 promoter (Zp) was not activated by HSV-1 essential glycoprotein-induced membrane fusion. Nevertheless, Zp was activated within 6 h post HSV-1 infection in virus entry-dependent and replication-independent manners. Using a panel of Zp deletion mutants, HSV-1 was shown to promote Zp through a cyclic adenosine monophosphate (cAMP) response element (CRE) located in ZII. The phosphorylated cAMP response element-binding (phos-CREB) protein, the cellular transactivator that binds to CRE, also increased after HSV-1 infection. By transient transfection, cAMP-dependent protein kinase A and HSV-1 US3 protein were found to be capable of activating Zp in CREB- and CRE-dependent manners. The relationship between EBV activation and HSV-1 infection revealed a possible common mechanism that stimulated latent EBV into lytic cycles in vivo.

Epigenetic control of viral life-cycle by a DNA-methylation dependent transcription factor

Epstein-Barr virus (EBV) encoded transcription factor Zta (BZLF1, ZEBRA, EB1) is the prototype of a class of transcription factor (including C/EBPalpha) that interact with CpG-containing DNA response elements in a methylation-dependent manner. The EBV genome undergoes a biphasic methylation cycle; it is extensively methylated during viral latency but is reset to an unmethylated state following viral lytic replication. Zta is expressed transiently following infection and again during the switch between latency and lytic replication. The requirement for CpG-methylation at critical Zta response elements (ZREs) has been proposed to regulate EBV replication, specifically it could aid the activation of viral lytic gene expression from silenced promoters on the methylated genome during latency in addition to preventing full lytic reactivation from the non-methylated EBV genome immediately following infection. We developed a computational approach to predict the location of ZREs which we experimentally assessed using in vitro and in vivo DNA association assays. A remarkably different binding motif is apparent for the CpG and non-CpG ZREs. Computational prediction of the location of these binding motifs in EBV revealed that the majority of lytic cycle genes have at least one and many have multiple copies of methylation-dependent CpG ZREs within their promoters. This suggests that the abundance of Zta protein coupled with the methylation status of the EBV genome act together to co-ordinate the expression of lytic cycle genes at the majority of EBV promoters.

Epigenetic dysregulation of epstein-barr virus latency and development of autoimmune disease

Epstein-Barr virus (EBV) is ahumanherpesvirus thatpersists in the memory B-cells of the majority of the world population in a latent form. Primary EBV infection is asymptomatic or causes a self-limiting disease, infectious mononucleosis. Virus latency is associated with a wide variety of neoplasms whereof some occur in immune suppressed individuals. Virus production does not occur in strict latency. The expression of latent viral oncoproteins and nontranslated RNAs is under epigenetic control via DNA methylation and histone modifications that results either in a complete silencing of the EBV genome in memory B cells, or in a cell-type dependent usage of a couple of latency promoters in tumor cells, germinal center B cells and lymphoblastoid cells (LCL, transformed by EBV in vitro). Both, latent and lytic EBV proteins elicit a strong immune response. In immune suppressed and infectious mononucleosis patients, an increased viral load can be detected in the blood. Enhanced lytic replication may result in new infection- and transformation-events and thus is a risk factor both for malignant transformation and the development of autoimmune diseases. An increased viral load or a changed presentation of a subset of lytic or latent EBV proteins that cross-react with cellular antigens may trigger pathogenic processes through molecular mimicry that result in multiple sclerosis (MS), systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA).

A subset of replication proteins enhances origin recognition and lytic replication by the Epstein-Barr virus ZEBRA protein

ZEBRA is a site-specific DNA binding protein that functions as a transcriptional activator and as an origin binding protein. Both activities require that ZEBRA recognizes DNA motifs that are scattered along the viral genome. The mechanism by which ZEBRA discriminates between the origin of lytic replication and promoters of EBV early genes is not well understood. We explored the hypothesis that activation of replication requires stronger association between ZEBRA and DNA than does transcription. A ZEBRA mutant, Z(S173A), at a phosphorylation site and three point mutants in the DNA recognition domain of ZEBRA, namely Z(Y180E), Z(R187K) and Z(K188A), were similarly deficient at activating lytic DNA replication and expression of late gene expression but were competent to activate transcription of viral early lytic genes. These mutants all exhibited reduced capacity to interact with DNA as assessed by EMSA, ChIP and an in vivo biotinylated DNA pull-down assay. Over-expression of three virally encoded replication proteins, namely the primase (BSLF1), the single-stranded DNA-binding protein (BALF2) and the DNA polymerase processivity factor (BMRF1), partially rescued the replication defect in these mutants and enhanced ZEBRA's interaction with oriLyt. The findings demonstrate a functional role of replication proteins in stabilizing the association of ZEBRA with viral DNA. Enhanced binding of ZEBRA to oriLyt is crucial for lytic viral DNA replication.

Epstein-Barr virus encodes a protein, ZEBRA, which plays an essential role in the switch between viral latency and the viral lytic cycle. ZEBRA activates transcription of early viral genes and also promotes lytic viral DNA replication. It is not understood how these two functions are discriminated. We studied five ZEBRA mutants that are impaired in activation of replication but are wild-type in the capacity to induce transcription of early viral genes. We demonstrate that these five mutants are impaired in binding to viral DNA regulatory sites. Therefore, replication required stronger interactions between ZEBRA and viral DNA than did transcription. Three components of the EBV-encoded replication machinery, including the single-stranded DNA binding protein, the polymerase processivity factor and the primase markedly enhanced the interaction of ZEBRA with viral DNA. These three components partially rescued the defect in ZEBRA mutants that were impaired in replication. The results suggest that through protein-protein interaction, replication proteins play a role in enhancing ZEBRA's association with the origin of DNA replication and other regulatory sites.

Cellular microRNAs 200b and 429 regulate the Epstein-Barr virus switch between latency and lytic replication

We previously showed that the cellular proteins ZEB1 and ZEB2/SIP1 both play key roles in regulating the latent-lytic switch of Epstein-Barr Virus (EBV) by repressing BZLF1 gene expression. We investigated here the effects of cellular microRNA (miRNA) 200 (miR200) family members on the EBV infection status of cells. We show that miR200b and miR429, but not miR200a, can induce EBV-positive cells into lytic replication by downregulating expression of ZEB1 and ZEB2, leading to production of infectious virus. The levels of miR200 family members in EBV-infected cells strongly negatively correlated with the levels of the ZEBs (e.g., -0.89 [P < 0.001] for miR429 versus ZEB1) and positively correlated with the degree of EBV lytic gene expression (e.g., 0.73 [P < 0.01] for miR429 versus BZLF1). The addition of either miR200b or miR429 to EBV-positive cells led to EBV lytic reactivation in a ZEB-dependent manner; inhibition of these miRNAs led to decreased EBV lytic gene expression. The degree of latent infection by an EBV mutant defective in the primary ZEB-binding site of the EBV BZLF1 promoter was not affected by the addition of these miRNAs. Furthermore, EBV infection of primary blood B cells led to downregulation of these miRNAs and upregulation of ZEB levels. Thus, we conclude that miRNAs 200b and 429 are key regulators via their effects on expression of ZEB1 and ZEB2 of the switch between latent and lytic infection by EBV and, therefore, potential targets for development of new lytic induction therapeutics with which to treat patients with EBV-associated malignancies.

Either ZEB1 or ZEB2/SIP1 can play a central role in regulating the Epstein-Barr virus latent-lytic switch in a cell-type-specific manner

We previously reported that the cellular protein ZEB1 can repress expression of the Epstein-Barr virus (EBV) BZLF1 gene in transient transfection assays by directly binding its promoter, Zp. We also reported that EBV containing a 2-bp substitution mutation in the ZEB-binding ZV element of Zp spontaneously reactivated out of latency into lytic replication at a higher frequency than did wild-type EBV. Here, using small interfering RNA (siRNA) and short hairpin RNA (shRNA) technologies, we definitively show that ZEB1 is, indeed, a key player in maintaining EBV latency in some epithelial and B-lymphocytic cell lines. However, in other EBV-positive epithelial and B-cell lines, another zinc finger E-box-binding protein, ZEB2/SIP1, is the key player. Both ZEB1 and ZEB2 can bind Zp via the ZV element. In EBV-positive cells containing only ZEB1, knockdown of ZEB1 led to viral reactivation out of latency, with synthesis of EBV immediate-early and early lytic gene products. However, in EBV-positive cells containing both ZEBs, ZEB2, not ZEB1, was the primary ZEB family member bound to Zp. Knockdown of ZEB2, but not ZEB1, led to EBV lytic reactivation. Thus, we conclude that either ZEB1 or ZEB2 can play a central role in the maintenance of EBV latency, doing so in a cell-type-dependent manner.

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

In the present study, we show that the treatment of Epstein-Barr virus (EBV) latently infected Raji cells with TPA/SB caused the cell growth arrest. The Zta-positive cells were predominantly enriched in G0/G1 phase of cell cycle. When Zta expression reached a maximal level, a fraction of Zta expressing cell population reentered S phase. Analysis of the expression pattern of a key set of cell cycle regulators revealed that the expression of Zta and Rta substantially interfered with the cell cycle regulatory machinery in Raji cells, strongly inhibiting the expression of Rb and p53 and inducing the expression of E2F1. Down-regulation of Rb was further demonstrated to be mediated by proteasomal degradation, and p53 and p21 affected at transcription level. The data indicate that both Zta and Rta promote entry into S phase of Raji cells. The important roles of Zta and Rta in EBV lytic reactivation were also demonstrated. Our finding suggests that these two transcriptional activators may act synergistically to govern the expression of downstream early and late genes as well as cellular genes and initiation of lytic cycle and manipulation of cell cycle regulatory mechanisms require the joint and interactive contributions of Rta and Zta.

Evaluation of a prediction protocol to identify potential targets of epigenetic reprogramming by the cancer associated Epstein Barr virus

Background

Epstein Barr virus (EBV) infects the majority of the human population, causing fatal diseases in a small proportion in conjunction with environmental factors. Following primary infection, EBV remains latent in the memory B cell population for life. Recurrent reactivation of the virus occurs, probably due to activation of the memory B-lymphocytes, resulting in viral replication and re-infection of B-lymphocytes. Methylation of the viral DNA at CpG motifs leads to silencing of viral gene expression during latency. Zta, the key viral protein that mediates the latency/reactivation balance, interacts with methylated DNA. Zta is a transcription factor for both viral and host genes. A sub-set of its DNA binding sites (ZREs) contains a CpG motif, which is recognised in its methylated form. Detailed analysis of the promoter of the viral gene BRLF1 revealed that interaction with a methylated CpG ZRE (RpZRE3) is key to overturning the epigenetic silencing of the gene.

Methodology and Principal Findings

Here we question whether we can use this information to identify which host genes contain promoters with similar response elements. A computational search of human gene promoters identified 274 targets containing the 7-nucleotide RpZRE3 core element. DNA binding analysis of Zta with 17 of these targets revealed that the flanking context of the core element does not have a profound effect on the ability of Zta to interact with the methylated sites. A second juxtaposed ZRE was observed for one promoter. Zta was able to interact with this site, although co-occupancy with the RpZRE3 core element was not observed.

Conclusions/Significance

This research demonstrates 274 human promoters have the potential to be regulated by Zta to overturn epigenetic silencing of gene expression during viral reactivation from latency.

Functional interaction between Epstein-Barr virus replication protein Zta and host DNA damage response protein 53BP1

Epstein-Barr virus (EBV; human herpesvirus 4) poses major clinical problems worldwide. Following primary infection, EBV enters a form of long-lived latency in B lymphocytes, expressing few viral genes, and it persists for the lifetime of the host with sporadic bursts of viral replication. The switch between latency and replication is governed by the action of a multifunctional viral protein Zta (also called BZLF1, ZEBRA, and Z). Using a global proteomic approach, we identified a host DNA damage repair protein that specifically interacts with Zta: 53BP1. 53BP1 is intimately connected with the ATM signal transduction pathway, which is activated during EBV replication. The interaction of 53BP1 with Zta requires the C-terminal ends of both proteins. A series of Zta mutants that show a wild-type ability to perform basic functions of Zta, such as dimer formation, interaction with DNA, and the transactivation of viral genes, were shown to have lost the ability to induce the viral lytic cycle. Each of these mutants also is compromised in the C-terminal region for interaction with 53BP1. In addition, the knockdown of 53BP1 expression reduced viral replication, suggesting that the association between Zta and 53BP1 is involved in the viral replication cycle.

Negative autoregulation of Epstein-Barr virus (EBV) replicative gene expression by EBV SM protein

The Epstein-Barr virus (EBV) SM protein is essential for lytic EBV DNA replication and virion production. When EBV replication is induced in cells infected with an SM-deleted recombinant EBV, approximately 50% of EBV genes are expressed inefficiently. When EBV replication is rescued by transfection of SM, SM enhances expression of these genes by direct and indirect mechanisms. While expression of most EBV genes is either unaffected or enhanced by SM, expression of several genes is decreased in the presence of SM. Expression of BHRF1, a homolog of cellular bcl-2, is particularly decreased in the presence of SM. Investigation of the mechanism of BHRF1 downregulation revealed that SM downregulates expression of the immediate-early EBV transactivator R. In EBV-infected cells, R-responsive promoters, including the BHRF1 and SM promoters, were less active in the presence of SM, consistent with SM inhibition of R expression. SM decreased spliced R mRNA levels, supporting a posttranscriptional mechanism of R inhibition. R and BHRF1 expression were also found to decrease during later stages of EBV lytic replication in EBV-infected lymphoma cells. These data indicate that feedback regulation of immediate-early and early genes occurs during the lytic cycle of EBV regulation.

Functional Epstein-Barr virus reservoir in plasma cells derived from infected peripheral blood memory B cells

The Epstein-Barr virus (EBV) causes infectious mononucleosis, establishes latency in resting memory B lymphocytes, and is involved in oncogenesis through poorly understood mechanisms. The EBV lytic cycle is initiated during plasma cell differentiation by mRNAs transcripts encoded by BZLF1, which induce the synthesis of EBV proteins such as the immediate-early antigen ZEBRA and the late membrane antigen gp350. Therefore, we assessed the capacity of circulating EBV-infected B lymphocytes from healthy EBV-seropositive subjects to enter and complete the EBV lytic cycle. Purified B lymphocytes were polyclonally stimulated and BZLF1- or gp350-secreting cells (BZLF1-SCs or gp350-SCs) were enumerated by ELISpot assays. The number of BZLF1-SCs ranged from 50 to 480/107 lymphocytes (median, 80; 25th-75th percentiles, 70-150) and gp350-SCs from 10 to 40/107 lymphocytes (median, 17; 25th-75th percentiles, 10-20). gp350-SCs represented only 7.7% to 28.6% of BZLF1-SCs (median, 15%; 25th-75th percentiles, 10.5%-20%). This EBV functional reservoir was preferentially restricted to plasma cells derived from CD27(+) IgD(-) memory B lymphocytes. In 9 of 13 subjects, EBV DNA quantification in B-cell culture supernatants gave evidence of completion of EBV lytic cycle. These results demonstrate that EBV proteins can be secreted by EBV-infected B lymphocytes from healthy carriers, a majority generating an abortive EBV lytic cycle and a minority completing the cycle.

The reversal of epigenetic silencing of the EBV genome is regulated by viral bZIP protein

EBV (Epstein-Barr virus) alternates between latency and lytic replication. During latency, the viral genome is largely silenced by host-driven methylation of CpG motifs and in the switch to the lytic cycle this epigenetic silencing is overturned. A key event is the activation of the viral protein Zta with three ZREs (Zta-response elements) from the BRLF1 promoter (referred to as Rp). Two of these ZREs contain CpG motifs and are methylated in the latent genome. Biochemical analyses and molecular modelling of Zta bound to methylated RpZRE3 indicate the precise contacts made between a serine and a cysteine residue of Zta with methyl cytosines. A single point mutant of Zta, C189S, is defective in binding to the methylated ZREs both in vitro and in vivo. This was used to probe the functional relevance of the interaction. ZtaC189S was not able to activate Rp in a B-cell line, demonstrating the relevance of the interaction with methylated ZREs. This demonstrates that Zta plays a role in overturning the epigenetic control of viral latency.

Methylated DNA recognition during the reversal of epigenetic silencing is regulated by cysteine and serine residues in the Epstein-Barr virus lytic switch protein

Epstein-Barr virus (EBV) causes infectious mononucleosis and is associated with various malignancies, including Burkitt's lymphoma and nasopharyngeal carcinoma. Like all herpesviruses, the EBV life cycle alternates between latency and lytic replication. During latency, the viral genome is largely silenced by host-driven methylation of CpG motifs and, in the switch to the lytic cycle, this epigenetic silencing is overturned. A key event is the activation of the viral BRLF1 gene by the immediate-early protein Zta. Zta is a bZIP transcription factor that preferentially binds to specific response elements (ZREs) in the BRLF1 promoter (Rp) when these elements are methylated. Zta's ability to trigger lytic cycle activation is severely compromised when a cysteine residue in its bZIP domain is mutated to serine (C189S), but the molecular basis for this effect is unknown. Here we show that the C189S mutant is defective for activating Rp in a Burkitt's lymphoma cell line. The mutant is compromised both in vitro and in vivo for binding two methylated ZREs in Rp (ZRE2 and ZRE3), although the effect is striking only for ZRE3. Molecular modeling of Zta bound to methylated ZRE3, together with biochemical data, indicate that C189 directly contacts one of the two methyl cytosines within a specific CpG motif. The motif's second methyl cytosine (on the complementary DNA strand) is predicted to contact S186, a residue known to regulate methyl-ZRE recognition. Our results suggest that C189 regulates the enhanced interaction of Zta with methylated DNA in overturning the epigenetic control of viral latency. As C189 is conserved in many bZIP proteins, the selectivity of Zta for methylated DNA may be a paradigm for a more general phenomenon.

A comprehensive library of mutations of Epstein Barr virus

A mutant library of 249 mutants with mutations that span the entire Epstein-Barr virus (EBV) genome was generated by transposition with EZ : : TN <KAN-2> and insertion with an apramycin resistance gene by a PCR-targeting method. This study also demonstrates the feasibility of generating deletions and site-specific mutations in the BRLF1 promoter on the EBV genome to determine the regions in the promoter that are crucial to transcription. Analysing BZLF1 and BRLF1 mutants by microarray analysis revealed that these two genes regulate the transcription of EBV lytic genes differently. A BZLF1 mutation affects global expression of EBV lytic genes; almost no lytic gene is expressed by the mutant after lytic induction. However, although a BRLF1 mutant still transcribes most lytic genes, the expression of these lytic genes is inefficient. Furthermore, this study shows that the proximal Zta-response element in the BRLF1 promoter is crucial to BRLF1 transcription from the EBV genome, despite the fact that another work demonstrated that this site was unimportant in transient transfection analysis. Furthermore, mutants with a mutation in BDLF1 and BORF1 cannot assemble viral capsids. Results of this study demonstrate the usefulness of a comprehensive mutant library in genetic analyses of EBV.

Epstein-Barr virus transcription activator Rta upregulates decoy receptor 3 expression by binding to its promoter

Decoy receptor 3 (DcR3) is a soluble decoy receptor belonging to the tumor necrosis factor receptor superfamily that is overexpressed in various malignant tumor types. DcR3 has been implicated in tumor cell survival by inhibiting apoptosis and by interfering with immune surveillance. A previous study showed that DcR3 expression is associated with Epstein-Barr virus (EBV)-positive lymphomas but rarely with non-EBV-positive B-cell lymphomas, suggesting that the presence of EBV may affect DcR3 expression. Here, we demonstrated enhanced DcR3 expression upon EBV reactivation in P3HR1 cells and in EBV-infected 293 cells. This enhancement, however, could not be detected in 293 cells infected with EBV with BRLF1 deleted. We found that EBV transactivator, Rta, could upregulate DcR3 expression by direct binding to an Rta-responsive element (RRE) located in the DcR3 promoter region and that this RRE is important for Rta-mediated DcR3 expression. Overexpressing CREB-binding protein (CBP) further enhanced Rta-dependent DcR3 expression, suggesting Rta-dependent DcR3 transcription activity is mediated by CBP. Previously, Rta was shown to enhance phosphatidylinositol-3 kinase (PI3-K) activity. However, Rta-transduced PI 3-K activity plays a minor role in DcR3 expression. This is the first report to demonstrate that Rta upregulates a cellular gene by direct binding to an RRE.

Phosphoacceptor site S173 in the regulatory domain of Epstein-Barr Virus ZEBRA protein is required for lytic DNA replication but not for activation of viral early genes

The Epstein-Barr virus ZEBRA protein controls the viral lytic cycle. ZEBRA activates the transcription of viral genes required for replication. ZEBRA also binds to oriLyt and interacts with components of the viral replication machinery. The mechanism that differentiates the roles of ZEBRA in regulation of transcription and initiation of lytic replication is unknown. Here we show that S173, a residue in the regulatory domain, is obligatory for ZEBRA to function as an origin binding protein but is dispensable for its role as a transcriptional activator of early genes. Serine-to-alanine substitution of this residue, which prevents phosphorylation of S173, resulted in a threefold reduction in the DNA binding affinity of ZEBRA for oriLyt, as assessed by chromatin immunoprecipitation. An independent assay based on ZEBRA solubility demonstrated a marked defect in DNA binding by the Z(S173A) mutant. The phenotype of a phosphomimetic mutant, the Z(S173D) mutant, was similar to that of wild-type ZEBRA. Our findings suggest that phosphorylation of S173 promotes viral replication by enhancing ZEBRA's affinity for DNA. The results imply that stronger DNA binding is required for ZEBRA to activate replication than that required to activate transcription.

Role of the TSG101 gene in Epstein-Barr virus late gene transcription

Rta, an Epstein-Barr virus (EBV)-encoded immediate-early protein, governs the reactivation of the viral lytic program by transactivating a cascade of lytic gene expression. Cellular transcription factors such as Sp1, ATF2, E2F, and Akt have been demonstrated to mediate Rta transactivation of lytic genes. We report herein that Rta associates with another potent transcription factor, tumor susceptibility gene 101 (TSG101), to promote the activation of EBV late genes. Results from an EBV cDNA array reveal that depletion of TSG101 by siRNA potently inhibits the transcription of five Rta-responsive EBV late genes, BcLF1, BDLF3, BILF2, BLLF1, and BLRF2. Depletion of TSG101 impairs the Rta transactivation of these late promoters severely. Moreover, a concordant augmentation of Rta transactivating activity is observed when TSG101 is overexpressed following ectopic transfection. Mechanistically, Rta interaction with TSG101 causes the latter to accumulate principally in the nuclei, wherein the proteins colocalize and are recruited to the viral promoters. Of note, TSG101 is crucial for the efficient binding of Rta to these late promoters. As a result, cells with defective TSG101 fail to express late viral proteins, leading to a decrease in the yield of virus particles. Thus, the contribution of TSG101 to Rta-mediated late gene activation is of great importance for completion of the EBV productive lytic cycle. These observations consolidate a role for TSG101 in the replication of EBV, a DNA virus, that differs from what is observed for RNA viruses, where TSG101 aids mainly in the endosomal sorting of enveloped late viral proteins for assembly at the plasma membrane.

Epstein-Barr virus BZLF1 gene, a switch from latency to lytic infection, is expressed as an immediate-early gene after primary infection of B lymphocytes

We demonstrate here that the Epstein-Barr virus (EBV) BZLF1 gene, a switch from latent infection to lytic infection, is expressed as early as 1.5 h after EBV infection in Burkitt's lymphoma-derived, EBV-negative Akata and Daudi cells and primary B lymphocytes. Since BZLF1 mRNA is expressed even when the cells are infected with EBV in the presence of anisomycin, an inhibitor of protein synthesis, its expression does not require prerequisite protein synthesis, indicating that BZLF1 is expressed as an immediate-early gene following primary EBV infection of B lymphocytes.

Amino acids in the basic domain of Epstein-Barr virus ZEBRA protein play distinct roles in DNA binding, activation of early lytic gene expression, and promotion of viral DNA replication

The ZEBRA protein of Epstein-Barr virus (EBV) drives the viral lytic cycle cascade. The capacity of ZEBRA to recognize specific DNA sequences resides in amino acids 178 to 194, a region in which 9 of 17 residues are either lysine or arginine. To define the basic domain residues essential for activity, a series of 46 single-amino-acid-substitution mutants were examined for their ability to bind ZIIIB DNA, a high-affinity ZEBRA binding site, and for their capacity to activate early and late EBV lytic cycle gene expression. DNA binding was obligatory for the protein to activate the lytic cascade. Nineteen mutants that failed to bind DNA were unable to disrupt latency. A single acidic replacement of a basic amino acid destroyed DNA binding and the biologic activity of the protein. Four mutants that bound weakly to DNA were defective at stimulating the expression of Rta, the essential first target of ZEBRA in lytic cycle activation. Four amino acids, R183, A185, C189, and R190, are likely to contact ZIIIB DNA specifically, since alanine or valine substitutions at these positions drastically weakened or eliminated DNA binding. Twenty-three mutants were proficient in binding to ZIIIB DNA. Some DNA binding-proficient mutants were refractory to supershift by BZ-1 monoclonal antibody (epitope amino acids 214 to 230), likely as the result of the increased solubility of the mutants. Mutants competent to bind DNA could be separated into four functional groups: the wild-type group (eight mutants), a group defective at activating Rta (five mutants, all with mutations at the S186 site), a group defective at activating EA-D (three mutants with the R179A, S186T, and K192A mutations), and a group specifically defective at activating late gene expression (seven mutants). Three late mutants, with a Y180A, Y180E, or K188A mutation, were defective at stimulating EBV DNA replication. This catalogue of point mutants reveals that basic domain amino acids play distinct functions in binding to DNA, in activating Rta, in stimulating early lytic gene expression, and in promoting viral DNA replication and viral late gene expression. These results are discussed in relationship to the recently solved crystal structure of ZEBRA bound to an AP-1 site.

Induction of the early growth response 1 gene by Epstein-Barr virus lytic transactivator Zta

Early growth response 1 (Egr-1) is a cellular transcription factor involved in diverse biologic functions. Egr-1 has been associated with Epstein-Barr virus (EBV) infection, but it is still unknown whether any EBV protein regulates Egr-1 expression. In this study, we first showed that EBV reactivation is involved in upregulation of Egr-1 and that Egr-1 can be induced by Zta, an EBV lytic transactivator. Zta not only binds to the Egr-1 promoter but also activates the ERK signaling pathway to trigger binding of Elk-1 to the Egr-1 promoter. In addition, knockdown of Egr-1 significantly reduces the spontaneous expression of Zta and Rta in EBV-infected 293 cells, suggesting that a positive-feedback network involving Egr-1 is required for EBV reactivation. This study also implies that Zta has the potential to affect expression of certain genes through Egr-1.

Expression, purification, crystallization and preliminary X-ray analysis of a C-terminal fragment of the Epstein-Barr virus ZEBRA protein

A C-terminal fragment of the Epstein-Barr virus immediate-early transcription factor ZEBRA has been expressed as a recombinant protein in Escherichia coli and purified to homogeneity. The fragment behaves as a dimer in solution, consistent with the presence of a basic region leucine-zipper (bZIP) domain. Crystals of the fragment in complex with a DNA duplex were grown by the hanging-drop vapour-diffusion technique using polyethylene glycol 4000 and magnesium acetate as crystallization agents. Crystals diffract to better than 2.5 A resolution using synchrotron radiation (lambda = 0.976 A). Crystals belong to space group C2, with unit-cell parameters a = 94.2, b = 26.5, c = 98.1 A, beta = 103.9 degrees.

Identification of constitutive phosphorylation sites on the Epstein-Barr virus ZEBRA protein

ZEBRA, the product of the Epstein-Barr virus gene bzlf1, and a member of the AP-1 subfamily of basic zipper (bZIP) transcription factors, is necessary and sufficient to disrupt viral latency and to initiate the viral lytic cycle. Two serine residues of ZEBRA, Ser167 and Ser173, are substrates for casein kinase 2 (CK2) and are constitutively phosphorylated in vivo. Phosphorylation of ZEBRA at its CK2 sites is required for proper temporal regulation of viral gene expression. Phosphopeptide analysis indicated that ZEBRA contains additional constitutive phosphorylation sites. Here we employed a co-migration strategy to map these sites in vivo. The cornerstone of this strategy was to correlate the migration of 32P- and 35S-labeled tryptic peptides of ZEBRA. The identity of the peptides was revealed by mutagenesis of methionine and cysteine residues present in each peptide. Phosphorylation sites within the peptide were identified by mutagenesis of serines and threonines. ZEBRA was shown to be phosphorylated at serine and threonine residues, but not tyrosine. Two previously unrecognized phosphorylation sites of ZEBRA were identified in the NH2-terminal region of the transactivation domain: a cluster of weak phosphorylation sites at Ser6, Thr7, and Ser8 and a strong phosphorylation site at Thr14. Thr14 was embedded in a MAP kinase consensus sequence and could be phosphorylated in vitro by JNK, despite the absence of a canonical JNK docking site. Thus ZEBRA is now known to be constitutively phosphorylated at three distinct sites.

Mutation of a single amino acid residue in the basic region of the Epstein-Barr virus (EBV) lytic cycle switch protein Zta (BZLF1) prevents reactivation of EBV from latency

Zta, the product of the BZLF1 gene carried by Epstein-Barr virus (EBV), is crucial for reactivation of EBV from latency. Zta is a member of the bZIP family of transcription factors, and in common with many of these, Zta possesses a conserved cysteine residue in its basic region (C189) and a further cysteine residue in its ZIP region (C222). We demonstrate that C189 is required to reactivate EBV from latency but C222 is not and that this single amino acid affects two independent functions of Zta, (i) binding to a Zta-responsive site and (ii) manipulating the cell cycle.

Reactivation of Epstein–Barr virus can be triggered by an Rta protein mutated at the nuclear localization signal

Rta, an immediate-early protein of Epstein–Barr virus (EBV), is a transcriptional activator that induces lytic gene expression and triggers virus reactivation. Being located predominantly in the nucleus, Rta can exert its transactivation function through either direct DNA binding or certain indirect mechanisms mediated by cellular signalling and other transcriptional factors. This study examined whether the subcellular localization of Rta was critical for the induction of target genes. First, 410KRKK413 was identified as a nuclear localization signal (NLS) of Rta. An Rta mutant with the NLS converted to 410AAAA413 showed cytoplasmic localization and failed to activate the promoter of BGLF5. Interestingly, ectopic expression of the Rta mutant still disrupted EBV latency in an epithelial cell line. Reporter gene assays revealed that the NLS-mutated Rta retained the ability to activate two lytic promoters, Zp and Rp, at a considerable level. Thus, the cytoplasmic Rta mutant could induce expression of endogenous Zta and Rta, triggering reactivation of EBV.

Identification of protein tyrosine kinases required for B-cell- receptor-mediated activation of an Epstein-Barr Virus immediate-early gene promoter

Epstein-Barr Virus (EBV) is a potentially oncogenic herpesvirus that infects >90% of the world's population. EBV exists predominantly as a latent infection in B lymphocytes, with periodic lytic-cycle reactivation essential for cellular and host transmission. Viral reactivation can be stimulated by ligand-induced activation of B-cell-receptor (BCR)-coupled signaling pathways. The critical first step in the transition from latency to the lytic cycle is the expression of the viral immediate-early gene BZLF1 through the transcription activation of its promoter, Zp. However, the BCR-coupled signal transduction cascade(s) leading to the induction of Zp and the expression of the BZLF1 gene product, Zta, is currently unclear. A major obstacle to delineating the relevant signal transduction events has been the lack of a model of EBV infection that is amenable to genetic manipulation. The use of the avian B-cell line DT40 has proven to be a powerful tool for delineating BCR-mediated signal transduction pathways that appear to be highly conserved between avian and mammalian systems. We demonstrate that the DT40 cell line is a robust and genetically tractable system for the study of BCR-mediated signaling pathways leading to transcriptional activation of BZLF1. Using this system, we demonstrate that activation of Zp requires the BCR-coupled protein tyrosine kinases Syk and Btk and that it is positively regulated by Lyn. Thus, the use of DT40 cells has allowed us to delineate the early signaling components required for BCR-dependent reactivation of latent EBV, and this system is likely to prove useful for further dissection of the downstream signaling cascades involved.

Phosphorylation of Epstein-Barr virus ZEBRA protein at its casein kinase 2 sites mediates its ability to repress activation of a viral lytic cycle late gene by Rta

ZEBRA, a member of the bZIP family, serves as a master switch between latent and lytic cycle Epstein-Barr virus (EBV) gene expression. ZEBRA influences the activity of another viral transactivator, Rta, in a gene-specific manner. Some early lytic cycle genes, such as BMRF1, are activated in synergy by ZEBRA and Rta. However, ZEBRA suppresses Rta's ability to activate a late gene, BLRF2. Here we show that this repressive activity is dependent on the phosphorylation state of ZEBRA. We find that two residues of ZEBRA, S167 and S173, that are phosphorylated by casein kinase 2 (CK2) in vitro are also phosphorylated in vivo. Inhibition of ZEBRA phosphorylation at the CK2 substrate motif, either by serine-to-alanine substitutions or by use of a specific inhibitor of CK2, abolished ZEBRA's capacity to repress Rta activation of the BLRF2 gene, but did not alter its ability to initiate the lytic cycle or to synergize with Rta in activation of the BMRF1 early-lytic-cycle gene. These studies illustrate how the phosphorylation state of a transcriptional activator can modulate its behavior as an activator or repressor of gene expression. Phosphorylation of ZEBRA at its CK2 sites is likely to play an essential role in proper temporal control of the EBV lytic life cycle.

The BRRF1 early gene of Epstein-Barr virus encodes a transcription factor that enhances induction of lytic infection by BRLF1

The switch from the latent to the lytic form of Epstein-Barr virus (EBV) infection is mediated by expression of the viral immediate-early (IE) proteins, BZLF1 (Z) and BRLF1 (R). An EBV early protein, BRRF1 (Na), is encoded by the opposite strand of the BRLF1 intron, but the function of this nuclear protein in the viral life cycle is unknown. Here we demonstrate that Na enhances the R-mediated induction of lytic EBV infection in 293 cells latently infected with a recombinant EBV (R-KO) defective for the expression of both R and Na. Na also enhances R-induced lytic infections in a gastric carcinoma line (AGS) carrying the R-KO virus, although it has no effect in a Burkitt lymphoma line (BL-30) stably infected with the same mutant virus. We show that Na is a transcription factor that increases the ability of R to activate Z expression from the R-KO viral genome in 293 cells and that Na by itself activates the Z promoter (Zp) in EBV-negative cells. Na activation of Zp requires a CRE motif (ZII), and a consensus CRE motif is sufficient to transfer Na responsiveness to the heterologous E1b promoter. Furthermore, we show that Na enhances the transactivator function of a Gal4-c-Jun fusion protein but does not increase the transactivator function of other transcription factors (including ATF-1, ATF-2, and CREB) known to bind CRE motifs. Na expression in cells results in increased levels of a hyperphosphorylated form of c-Jun, suggesting a mechanism by which Na activates c-Jun. Our results indicate that Na is a transcription factor that activates the EBV Zp IE promoter through its effects on c-Jun and suggest that Na cooperates with BRLF1 to induce the lytic form of EBV infection in certain cell types.

Fatty acid synthase expression is induced by the Epstein-Barr virus immediate-early protein BRLF1 and is required for lytic viral gene expression

The Epstein-Barr virus (EBV) immediate-early (IE) protein BRLF1 (R) is a transcription factor that induces the lytic form of EBV infection. R activates certain early viral promoters through a direct binding mechanism but induces transcription of the other EBV IE gene, BZLF1 (Z), indirectly through cellular factors binding to a CRE motif in the Z promoter (Zp). Here we demonstrate that R activates expression of the fatty acid synthase (FAS) cellular gene through a p38 stress mitogen-activated protein kinase-dependent mechanism. B-cell receptor engagement of Akata cells also increases FAS expression. The FAS gene product is required for de novo synthesis of the palmitate fatty acid, and high-level FAS expression is normally limited to liver, brain, lung, and adipose tissue. We show that human epithelial tongue cells lytically infected with EBV (from oral hairy leukoplakia lesions) express much more FAS than uninfected cells. Two specific FAS inhibitors, cerulenin and C75, prevent R activation of IE (Z) and early (BMRF1) lytic EBV proteins in Jijoye cells. In addition, cerulenin and C75 dramatically attenuate IE and early lytic gene expression after B-cell receptor engagement in Akata cells and constitutive lytic viral gene expression in EBV-positive AGS cells. However, FAS inhibitors do not reduce lytic viral gene expression induced by a vector in which the Z gene product is driven by a strong heterologous promoter. In addition, FAS inhibitors do not reduce R activation of a naked DNA reporter gene construct driven by the Z promoter (Zp). These results suggest that cellular FAS activity is important for induction of Z transcription from the intact latent EBV genome, perhaps reflecting the involvement of lipid-derived signaling pathways or palmitoylated proteins. Furthermore, using FAS inhibitors may be a completely novel approach for blocking the lytic form of EBV replication.

Stealth technology: how Epstein-Barr virus utilizes DNA methylation to cloak itself from immune detection

Epstein-Barr virus (EBV) is a large lymphotrophic DNA virus that establishes life-long residency in the infected host and is associated with a number of human tumors. The EBV genome encodes proteins essential for persistence, an oncoprotein, and proteins that render it vulnerable to the host's immune system; therefore, EBV gene transcription is tightly regulated. One critically important regulatory mechanism utilized by EBV is DNA methylation. Methylation of cytosines within CpG dinucleotides at promoter regions is important for gene silencing and genome integrity. Although most parasitic elements are methylated in mammalian cells never to be reactivated again, EBV has evolved to utilize DNA methylation to maximize persistence and cloak itself from immune detection. EBV's reliance on DNA methylation also provides a unique therapeutic strategy for the treatment of EBV-associated tumors. DNA demethylating agents are capable of reactivating transcription of highly immunogenic viral proteins, rendering tumor cells susceptible to killing by the host immune system, and inducing the viral lytic cycle which culminates in cell lysis.

NF-kappaB inhibits gammaherpesvirus lytic replication

Nasopharyngeal carcinoma, Kaposi's sarcoma, and B-cell lymphomas are human malignancies associated with gammaherpesvirus infections. Members of this virus family are characterized by their ability to establish latent infections in lymphocytes. The latent viral genome expresses very few gene products. The infected cells are therefore poorly recognized by the host immune system, allowing the virus to persist for long periods of time. We sought to identify the cell-specific factors that allow these viruses to redirect their life cycle from productive replication to latency. We find that the cellular transcription factor NF-kappaB can regulate this process. Epithelial cells and fibroblasts support active (lytic) gammaherpesvirus replication and have low NF-kappaB activity. However, overexpression of NF-kappaB in these cells inhibits the replication of the gammaherpesvirus murine herpesvirus 68 (MHV68). In addition, overexpression of NF-kappaB inhibits the activation of lytic promoters from MHV68 and human gammaherpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV). In lymphocytes latently infected with KSHV or EBV, the level of NF-kappaB activity is high, and treatment of these cells with an NF-kappaB inhibitor leads to lytic protein synthesis consistent with virus reactivation. These results suggest that high levels of NF-kappaB can inhibit gammaherpesvirus lytic replication and may therefore contribute to the establishment and maintenance of viral latency in lymphocytes. They also suggest that NF-kappaB may be a novel target for the disruption of virus latency and therefore the treatment of gammaherpesvirus-related malignancies.

Lytic switch protein (ORF50) response element in the Kaposi's sarcoma-associated herpesvirus K8 promoter is located within but does not require a palindromic structure

Kaposi's sarcoma-associated virus (KSHV) ORF50 protein induces lytic replication and activates the K8 promoter. We show that ORF50-induced and tetradecanoyl phorbol acetate (TPA) induced K8 transcripts initiated from the same start site. A newly identified palindrome (PAL2), containing a 12-bp response region required for ORF50-induced activation in lymphoid cells, was identified in the K8 promoter. Specific DNA binding of bacterially expressed ORF50 was not seen with the K8 promoter despite specific binding to the PAN promoter. The new palindrome shared homology with a previously described ORF50 response element (50RE(K8) and 50RE(57)). We demonstrate that the new 50RE(K8) (50RE(K8-PAL2)) is not the palindrome per se. Instead, the response element is buried within the right arm of the palindrome. We propose that the complexity of the K8 response elements reflects the complexity of mechanisms used by ORF50 during viral reactivation.

The latency-associated nuclear antigen homolog of herpesvirus saimiri inhibits lytic virus replication

Herpesvirus saimiri (HVS), a T-lymphotropic tumor virus of neotropical primates, and the Kaposi's sarcoma-associated human herpesvirus 8 (KSHV) belong to the gamma-(2)-herpesvirus (Rhadinovirus) subfamily and share numerous features of genome structure and organization. The KSHV latency-associated nuclear antigen (LANA) protein appears to be relevant for viral persistence, latency, and transformation. It binds to DNA, colocalizes with viral episomal DNA, and presumably mediates efficient persistence of viral genomes. LANA further represses the transcriptional and proapoptotic activities of the p53 tumor suppressor protein. Here we report on the ORF73 gene of HVS strain C488, which is the positional and structural homolog of KSHV LANA. The ORF73 gene in OMK cells can encode a 62-kDa protein that localizes to the nucleus in a pattern similar to that of LANA. We show that the ORF73 gene product can regulate viral gene expression by acting as a transcriptional modulator of latent and lytic viral promoters. To define the HVS ORF73 function in the background of a replication-competent virus, we constructed a viral mutant that expresses ORF73 under the transcriptional control of a mifepristone (RU-486)-inducible promoter. The HVS ORF73 gene product efficiently suppresses lytic viral replication in permissive cells, indicating that it defines a critical control point between viral persistence and lytic replication.