Epstein-Barr virus nuclear protein 2 transactivation of the latent membrane protein 1 promoter is mediated by J kappa and PU.1

Identification and characterization of CCAAT enhancer-binding protein (C/EBP) as a transcriptional activator for Epstein-Barr virus oncogene latent membrane protein 1

Epstein-Barr virus LMP1, a major oncoprotein expressed in latent infection, is critical for primary B cell transformation, functioning as a TNFR family member by aggregation in the plasma membrane resulting in constitutive activation of cellular signals, such as NF-κB, MAPK, JAK/STAT, and AKT. Although transcription of LMP1 in latent type III cells is generally under the control of the viral coactivator EBNA2, little is known about EBNA2-independent LMP1 expression in type II latency. We thus screened a cDNA library for factors that can activate the LMP1 promoter in an EBNA2-independent manner, using a reporter assay system. So far, we have screened >20,000 clones, and here identified C/EBPε as a new transcriptional activator. Exogenous expression of C/EBPα, -β, or -ε efficiently augmented LMP1 mRNA and protein levels in EBV-positive cell lines, whereas other members of the C/EBP family exhibited modest or little activity. It has been demonstrated that LMP1 gene transcription depends on two promoter regions: proximal (ED-L1) and distal (TR-L1). Interestingly, although we first used the proximal promoter for screening, we found that C/EBP increased transcription from both promoters in latent EBV-positive cells. Mutagenesis in reporter assays and EMSA identified only one functional C/EBP binding site, through which activation of both proximal and distal promoters is mediated. Introduction of point mutations into the identified C/EBP site in EBV-BAC caused reduced LMP1 transcription from both LMP1 promoters in epithelial cells. In conclusion, C/EBP is a newly identified transcriptional activator of the LMP1 gene, independent of the EBNA2 coactivator.

RNA polymerase II stalling promotes nucleosome occlusion and pTEFb recruitment to drive immortalization by Epstein-Barr virus

Epstein-Barr virus (EBV) immortalizes resting B-cells and is a key etiologic agent in the development of numerous cancers. The essential EBV-encoded protein EBNA 2 activates the viral C promoter (Cp) producing a message of ~120 kb that is differentially spliced to encode all EBNAs required for immortalization. We have previously shown that EBNA 2-activated transcription is dependent on the activity of the RNA polymerase II (pol II) C-terminal domain (CTD) kinase pTEFb (CDK9/cyclin T1). We now demonstrate that Cp, in contrast to two shorter EBNA 2-activated viral genes (LMP 1 and 2A), displays high levels of promoter-proximally stalled pol II despite being constitutively active. Consistent with pol II stalling, we detect considerable pausing complex (NELF/DSIF) association with Cp. Significantly, we observe substantial Cp-specific pTEFb recruitment that stimulates high-level pol II CTD serine 2 phosphorylation at distal regions (up to +75 kb), promoting elongation. We reveal that Cp-specific pol II accumulation is directed by DNA sequences unfavourable for nucleosome assembly that increase TBP access and pol II recruitment. Stalled pol II then maintains Cp nucleosome depletion. Our data indicate that pTEFb is recruited to Cp by the bromodomain protein Brd4, with polymerase stalling facilitating stable association of pTEFb. The Brd4 inhibitor JQ1 and the pTEFb inhibitors DRB and Flavopiridol significantly reduce Cp, but not LMP1 transcript production indicating that Brd4 and pTEFb are required for Cp transcription. Taken together our data indicate that pol II stalling at Cp promotes transcription of essential immortalizing genes during EBV infection by (i) preventing promoter-proximal nucleosome assembly and ii) necessitating the recruitment of pTEFb thereby maintaining serine 2 CTD phosphorylation at distal regions.

C-terminal region of EBNA-2 determines the superior transforming ability of type 1 Epstein-Barr virus by enhanced gene regulation of LMP-1 and CXCR7

Type 1 Epstein-Barr virus (EBV) strains immortalize B lymphocytes in vitro much more efficiently than type 2 EBV, a difference previously mapped to the EBNA-2 locus. Here we demonstrate that the greater transforming activity of type 1 EBV correlates with a stronger and more rapid induction of the viral oncogene LMP-1 and the cell gene CXCR7 (which are both required for proliferation of EBV-LCLs) during infection of primary B cells with recombinant viruses. Surprisingly, although the major sequence differences between type 1 and type 2 EBNA-2 lie in N-terminal parts of the protein, the superior ability of type 1 EBNA-2 to induce proliferation of EBV-infected lymphoblasts is mostly determined by the C-terminus of EBNA-2. Substitution of the C-terminus of type 1 EBNA-2 into the type 2 protein is sufficient to confer a type 1 growth phenotype and type 1 expression levels of LMP-1 and CXCR7 in an EREB2.5 cell growth assay. Within this region, the RG, CR7 and TAD domains are the minimum type 1 sequences required. Sequencing the C-terminus of EBNA-2 from additional EBV isolates showed high sequence identity within type 1 isolates or within type 2 isolates, indicating that the functional differences mapped are typical of EBV type sequences. The results indicate that the C-terminus of EBNA-2 accounts for the greater ability of type 1 EBV to promote B cell proliferation, through mechanisms that include higher induction of genes (LMP-1 and CXCR7) required for proliferation and survival of EBV-LCLs.

Epstein-Barr virus (EBV) is a common human virus that is involved in several types of cancer and directly causes human B lymphocytes to proliferate when they become infected. EBV occurs naturally as two different viral types (type 1 and type 2). The genomes of these viruses are mostly very similar but they differ in a few genes, particularly the EBNA-2 gene. For many years it has been known that type 1 EBV is much more effective than type 2 EBV at causing B lymphocyte proliferation and this difference is mediated by the EBNA-2 gene. Here we have shown that the greater ability of type 1 EBNA-2 to cause B cell proliferation is due to superior induction of the EBV LMP-1 and the cell CXCR7 genes, both of which are required for growth of EBV-infected lymphocytes. We mapped the section of type 1 EBNA-2 responsible for this to the C-terminus of the protein, including the transactivation and EBNA-LP interaction domains. The results provide a mechanism for the long-standing question of the functional difference between these two major types of EBV and will be important in understanding the significance of the EBV types in human infection.

Epstein-Barr virus exploits intrinsic B-lymphocyte transcription programs to achieve immortal cell growth

Epstein-Barr virus nuclear antigen 2 (EBNA2) regulation of transcription through the cell transcription factor RBPJ is essential for resting B-lymphocyte (RBL) conversion to immortal lymphoblast cell lines (LCLs). ChIP-seq of EBNA2 and RBPJ sites in LCL DNA found EBNA2 at 5,151 and RBPJ at 10,529 sites. EBNA2 sites were enriched for RBPJ (78%), early B-cell factor (EBF, 39%), RUNX (43%), ETS (39%), NFκB (22%), and PU.1 (22%) motifs. These motif associations were confirmed by LCL RBPJ ChIP-seq finding 72% RBPJ occupancy and Encyclopedia Of DNA Elements LCL ChIP-seq finding EBF, NFκB RELA, and PU.1 at 54%, 31%, and 17% of EBNA2 sites. EBNA2 and RBPJ were predominantly at intergene and intron sites and only 14% at promoter sites. K-means clustering of EBNA2 site transcription factors identified RELA-ETS, EBF-RUNX, EBF, ETS, RBPJ, and repressive RUNX clusters, which ranked from highest to lowest in H3K4me1 signals and nucleosome depletion, indicative of active chromatin. Surprisingly, although quantitatively less, the same genome sites in RBLs exhibited similar high-level H3K4me1 signals and nucleosome depletion. The EBV genome also had an LMP1 promoter EBF site, which proved critical for EBNA2 activation. LCL HiC data mapped intergenic EBNA2 sites to EBNA2 up-regulated genes. FISH and chromatin conformation capture linked EBNA2/RBPJ enhancers 428 kb 5' of MYC to MYC. These data indicate that EBNA2 evolved to target RBL H3K4me1 modified, nucleosome-depleted, nonpromoter sites to drive B-lymphocyte proliferation in primary human infection. The primed RBL program likely supports antigen-induced proliferation.

Cellular transcription factor Oct-1 interacts with the Epstein-Barr virus BRLF1 protein to promote disruption of viral latency

The Epstein-Barr virus (EBV) latent-to-lytic switch is an essential part of the viral life cycle, but the cellular factors that promote viral reactivation are not well defined. In this report, we demonstrate that the cellular transcription factor Oct-1 cooperates with the EBV immediate-early protein BRLF1 (R, Rta) to induce lytic viral reactivation. We show that cotransfected Oct-1 enhances the ability of BRLF1 to activate lytic gene expression in 293 cells stably infected with a BRLF1-defective EBV mutant (BRLF1-stop) and that Oct-1 increases BRLF1-mediated activation of lytic EBV promoters in reporter gene assays. We find that Oct-1 interacts directly with BRLF1 in vitro and that a mutant BRLF1 protein (the M140A mutant) attenuated for the ability to interact with Oct-1 in vitro is also resistant to Oct-1-mediated transcriptional enhancement in 293 BRLF1-stop cells. Furthermore, we show that cotransfected Oct-1 augments BRLF1 binding to a variety of lytic EBV promoters in chromatin immunoprecipitation (ChIP) assays (including the BZLF1, BMRF1, and SM promoters) and that BRLF1 tethers Oct-1 to lytic EBV promoters. In addition, we demonstrate that an Oct-1 mutant defective in DNA binding (the S335D mutant) still retains the ability to enhance BRLF1 transcriptional effects. Finally, we show that knockdown of endogenous Oct-1 expression reduces the level of constitutive lytic EBV gene expression in both EBV-positive B-cell and EBV-positive epithelial cell lines. These results suggest that Oct-1 acts as a positive regulator of EBV lytic gene expression and that this effect is at least partially mediated through its interaction with the viral protein BRLF1.

Identification of a sub-population of B cells that proliferates after infection with Epstein-Barr virus

Background

Epstein-Barr virus (EBV)-driven B cell proliferation is critical to its subsequent persistence in the host and is a key event in the development of EBV-associated B cell diseases. Thus, inquiry into early cellular events that precede EBV-driven proliferation of B cells is essential for understanding the processes that can lead to EBV-associated B cell diseases.

Methods

Infection with high titers of EBV of mixed, primary B cells in different stages of differentiation occurs during primary EBV infection and in the setting of T cell-immunocompromise that predisposes to development of EBV-lymphoproliferative diseases. Using an ex vivo system that recapitulates these conditions of infection, we correlated expression of selected B cell-surface markers and intracellular cytokines with expression of EBV latency genes and cell proliferation.

Results

We identified CD23, CD58, and IL6, as molecules expressed at early times after EBV-infection. EBV differentially infected B cells into two distinct sub-populations of latently infected CD23⁺ cells: one fraction, marked as CD23^hiCD58⁺IL6^- by day 3, subsequently proliferated; another fraction, marked as CD23^loCD58⁺, expressed IL6, a B cell growth factor, but failed to proliferate. High levels of LMP1, a critical viral oncoprotein, were expressed in individual CD23^hiCD58⁺ and CD23^loCD58⁺ cells, demonstrating that reduced levels of LMP1 did not explain the lack of proliferation of CD23^loCD58⁺ cells. Differentiation stage of B cells did not appear to govern this dichotomy in outcome either. Memory or naïve B cells did not exclusively give rise to either CD23^hi or IL6-expressing cells; rather memory B cells gave rise to both sub-populations of cells.

Conclusions

B cells are differentially susceptible to EBV-mediated proliferation despite expression of viral gene products known to be critical for continuous B cell growth. Cellular events, in addition to viral gene expression, likely play a critical role in determining the outcome of EBV infection. By indentifying cells predicted to undergo EBV-mediated proliferation, our study provides new avenues of investigation into EBV pathogenesis.

Development of drugs for Epstein-Barr virus using high-throughput in silico virtual screening

IMPORTANCE OF THE FIELD

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that is causally associated with endemic forms of Burkitt's lymphoma, nasopharyngeal carcinoma and lymphoproliferative disease in immunosuppressed individuals. On a global scale, EBV infects > 90% of the adult population and is responsible for ∼ 1% of all human cancers. To date, there is no efficacious drug or therapy for the treatment of EBV infection and EBV-related diseases.

AREAS COVERED IN THIS REVIEW

In this review, we discuss the existing anti-EBV inhibitors and those under development. We discuss the value of different molecular targets, including EBV lytic DNA replication enzymes as well as proteins that are expressed exclusively during latent infection, such as EBV nuclear antigen 1 (EBNA-1) and latent membrane protein 1. As the atomic structure of the EBNA-1 DNA binding domain has been described, it is an attractive target for in silico methods of drug design and small molecule screening. We discuss the use of computational methods that can greatly facilitate the development of novel inhibitors and how in silico screening methods can be applied to target proteins with known structures, such as EBNA-1, to treat EBV infection and disease.

WHAT THE READER WILL GAIN

The reader is familiarized with the problems in targeting of EBV for inhibition by small molecules and how computational methods can greatly facilitate this process.

TAKE HOME MESSAGE

Despite the impressive efficacy of nucleoside analogs for the treatment of herpesvirus lytic infection, there remain few effective treatments for latent infections. As EBV latent infection persists within and contributes to the formation of EBV-associated cancers, targeting EBV latent proteins is an unmet medical need. High-throughput in silico screening can accelerate the process of drug discovery for novel and selective agents that inhibit EBV latent infection and associated disease.

Roles of TRAF2 and TRAF3 in Epstein-Barr virus latent membrane protein 1-induced alternative NF-kappaB activation

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1)-induced NF-kappaB activation is essential for EBV-transformed B cell survival. LMP1 has two C-terminal cytoplasmic domains referred to as C-Terminal Activation Regions (CTAR) 1 and 2 that activate the alternative and canonical NF-kappaB pathways, respectively. While CTAR2 activates TRAF6, IKKbeta and IKKgamma-dependent canonical NF-kappaB pathway, CTAR1 interacts with TRAF2 and TRAF3 and activates NIK and IKKalpha-dependent alternative NF-kappaB pathway involving p100 processing into functional p52. Using IKKalpha(-/-), IKKbeta(-/-), IKKgamma(-/-), TRAF2(-/-), TRAF3(-/-), TRAF6(-/-), and NIK(aly/aly) mouse embryonic fibroblasts (MEFs), potential roles of these proteins in LMP1-induced alternative NF-kappaB activation were investigated. Deficiency in IKKalpha or functional NIK, but not in IKKbeta, IKKgamma, or TRAF6, severely impaired LMP1-induced p100 processing. Notably, p100 was constitutively processed in TRAF2(-/-) or TRAF3(-/-) MEFs independently of LMP1 suggesting that TRAF2 or TRAF3 may play a regulatory role in p100 processing. Subsequently, TRAF2 or TRAF3 over-expression in HEK293 cells significantly blocked LMP1-induced p100 processing. The LMP1 CTAR1 expression in 293HEK cells activated the alternative p65/p52 complex while CTAR2 failed to do so. Taken together, LMP1 activates alternative NF-kappaB pathway through functional NIK and IKKalpha that is regulated by TRAF2 or TRAF3.

Interaction of Epstein-Barr virus (EBV) with human B-lymphocytes

Epstein-Barr virus, EBV, and humans have a common history that reaches back to our primate ancestors. The virus co-evolved with man and has established a largely harmless and highly complex co-existence. It is carried as silent infection by almost all human adults. A serendipitous discovery established that it is the causative agent of infectious mononucleosis. Still, EBV became known first in 1964, in a rare, geographically prevalent malignant lymphoma of B-cell origin, Burkitt lymphoma BL. Its association with a malignancy prompted intensive studies and its capacity to immortalize B-lymphocytes in vitro was soon demonstrated. Consequently EBV was classified therefore as a potentially tumorigenic virus. Despite of this property however, the virus carrier state itself does not lead to malignancies because the transformed cells are recognized by the immune response. Consequently the EBV induced proliferation of EBV carrying B-lymphocytes is manifested only under immunosuppressive conditions. The expression of EBV encoded genes is regulated by the cell phenotype. The virus genome can be found in malignancies originating from cell types other than the B-lymphocyte. Even in the EBV infected B-cell, the direct transforming capacity is restricted to a defined window of differentiation. A complex interaction between virally encoded proteins and B-cell specific cellular proteins constitute the proliferation inducing program. In this short review we touch upon aspects which are the subject of our present work. We describe the mechanisms of some of the functional interactions between EBV encoded and cellular proteins that determine the phenotype of latently infected B-cells. The growth promoting EBV encoded genes are not expressed in the virus carrying BL cells. Still, EBV seems to contribute to the etiology of this tumor by modifying events that influence cell survival and proliferation. We describe a possible growth promoting mechanism in the genesis of Burkitt lymphoma that depends on the presence of EBV.

The p38 signaling pathway upregulates expression of the Epstein-Barr virus LMP1 oncogene

The Epstein-Barr virus (EBV)-encoded LMP1 oncogene has a role in transformation, proliferation, and metastasis of several EBV-associated tumors. Furthermore, LMP1 is critically involved in transformation and growth of EBV-immortalized B cells in vitro. The oncogenic properties of LMP1 are attributed to its ability to upregulate anti-apoptotic proteins and growth signals. The transcriptional regulation of LMP1 is dependent on the context of cellular and viral proteins present in the cell. Here, we investigated the effect of several signaling pathways on the regulation of LMP1 expression. Inhibition of p38 signaling, using p38-specific inhibitors SB203580 and SB202190, downregulated LMP1 in estrogen-induced EREB2.5 cells. Similarly, p38 inhibition decreased trichostatin A-induced LMP1 expression in P3HR1 cells. Exogenous expression of p38 in lymphoblastoid cell lines (LCLs) led to an increase in LMP1 promoter activity in reporter assays, and this activation was mediated by the previously identified CRE site in the promoter. Inhibition of p38 by SB203580 and p38-specific small interfering RNA (siRNA) also led to a modest decrease in endogenous LMP1 expression in LCLs. Chromatin immunoprecipitation indicated decreased binding of CREB-ATF1 to the CRE site in the LMP1 promoter after inhibition of the p38 pathway in EREB2.5 cells. Taken together, our results suggest that an increase in p38 activation upregulates LMP1 expression. Since p38 is activated in response to stimuli such as stress or possibly primary infection, a transient upregulation of LMP1 in response to p38 may allow the cells to escape apoptosis. Since the p38 pathway itself is activated by LMP1, our results also suggest the presence of an autoregulatory loop in LMP1 upregulation.

Augmented latent membrane protein 1 expression from Epstein-Barr virus episomes with minimal terminal repeats

The major oncogene of the Epstein-Barr virus (EBV), latent membrane protein 1 (LMP1), can be expressed from either of two promoters, ED-L1 or L1-TR, producing mRNAs of 2.8 kb or 3.5 kb, respectively. L1-TR, active in nasopharyngeal carcinoma and Hodgkin's lymphoma, is located within the first of a highly variable reiteration of terminal repeat (TR) sequences that are joined by random recombination upon circularization of the linear genome at entry into cells. To determine whether the resultant TR number affects LMP1 promoter activity, we isolated single-cell clones bearing episomes of distinct TR numbers (6TR to 12TR) from epithelial cells newly infected with EBV. LMP1 mRNA levels correlated directly with the quantity of LMP1 protein expressed but varied inversely to TR number. Unexpectedly, the 3.5-kb transcript predominated only at lower TR reiterations. Diminished L1-TR activity in the context of a higher TR count was confirmed with a green fluorescent protein (GFP) reporter construct driven by L1-TR. Various levels of LMP1, expressed from virus isogenic in all but TR number, produced divergent morphological and growth phenotypes in each cell clone. Abundant LMP1 in 6TR cells yielded a relatively cytostatic state compared to the proliferative one produced by intermediate and smaller amounts in 8TR and 12TR clones. These findings suggest that the diversification of TR number, inherent in a round of EBV reactivation and reinfection, may itself be a component of the oncogenic process. The replicative burst preceding onset of many EBV-linked cancers may increase the likelihood that LMP1 levels compatible with clonal outgrowth are achieved in a subset of infected cells.

Burkitt's lymphoma: the Rosetta Stone deciphering Epstein-Barr virus biology

Epstein-Barr virus was originally identified in the tumour cells of a Burkitt's lymphoma, and was the first virus to be associated with the pathogenesis of a human cancer. Studies on the relationship of EBV with Burkitt's lymphoma have revealed important general principles that are relevant to other virus-associated cancers. In addition, the impact of such studies on the knowledge of EBV biology has been enormous. Here, we review some of the key historical observations arising from studies on Burkitt's lymphoma that have informed our understanding of EBV, and we summarise the current hypotheses regarding the role of EBV in the pathogenesis of Burkitt's lymphoma.

Wide-scale use of Notch signaling factor CSL/RBP-Jkappa in RTA-mediated activation of Kaposi's sarcoma-associated herpesvirus lytic genes

For Kaposi's sarcoma-associated herpesvirus (KSHV; also called human herpesvirus 8 [HHV8]), the switch from latency to active lytic replication requires RTA, the product of open reading frame 50 (ORF50). RTA activates transcription from nearly 40 early and delayed-early viral promoters, mainly through interactions with cellular DNA binding proteins, such as CSL/RBP-Jkappa, Oct-1, C/EBPalpha, and c-Jun. Reliance on cellular coregulators may allow KSHV to adjust its lytic program to suit different cellular contexts or interpret signals from the outside. CSL is a key component of the Notch signaling pathway and is targeted by several viruses. A search with known CSL binding sequences from cellular genes found at least 260 matches in the KSHV genome, many from regions containing known or suspected lytic promoters. Analysis of clustered sites located immediately upstream of ORF70 (thymidylate synthase), ORF19 (tegument protein), and ORF47 (glycoprotein L) uncovered RTA-responsive promoters that were validated using mRNAs isolated from KSHV-infected cells undergoing lytic reactivation. Notably, ORF19 behaves as a true late gene, indicating that RTA regulates all three phases of the lytic program. For each new promoter, the response to RTA was dependent on CSL, and 5 of the 10 candidate sites were shown to bind CSL in vitro. Analysis of individual sites highlighted the importance of a cytosine residue flanking the core CSL binding sequence. These findings broaden the role for CSL in coordinating the KSHV lytic gene expression program and help to define a signature motif for functional CSL sites within the viral genome.

Silencing mediator of retinoic acid and thyroid hormone receptor regulates enhanced activation of signal transducer and activator of transcription 3 by epstein-barr virus-derived epstein-barr nuclear antigen 2

The Epstein-Barr virus (EBV)-encoded latency protein Epstein-Barr nuclear antigen 2 (EBNA2) is a nuclear transcriptional activator that is essential for EBV-induced cellular transformation. In a previous study, we demonstrated that EBNA2 interacts with signal transducer and activator of transcription 3 (STAT3), a signal transducer for an interleukin (IL)-6 family cytokine, and enhances its transcriptional activity. Here, we show that overexpression of a corepressor, silencing mediator of retinoic acid and thyroid hormone receptor (SMRT), decreases the EBNA2-mediated enhanced STAT3 activation. Furthermore, small-interfering RNA-mediated reduction of endogenous SMRT expression augments the EBNA2-mediated enhanced STAT3 activation. Importantly, EBNA2 reduces interactions between STAT3 and SMRT. These data demonstrate that EBNA2 acts as a transcriptional coactivator of STAT3 by influencing the SMRT corepressor complex.

Endoplasmic reticulum stress triggers XBP-1-mediated up-regulation of an EBV oncoprotein in nasopharyngeal carcinoma

Endoplasmic reticulum (ER) stress-activated unfolded protein response (UPR) plays multiple roles in cancer development, but its specific roles for virus-associated cancers have not been fully understood. It is still unknown whether ER stress/UPR occurs in and contributes to nasopharyngeal carcinoma (NPC), an epithelial malignancy closely associated with EBV. Here, we report that UPR proteins are frequently detected in NPC biopsies. In addition, we reveal that, in EBV-infected NPC cells, ER stress inducers up-regulate a potent EBV oncoprotein latent membrane protein 1 (LMP1), and the ER stress-induced LMP1 enhances production of interleukin-8. ER stress triggers LMP1 expression at a transcriptional level, activating a distal LMP1 promoter TR-L1. TR-L1 contains an ER stress-responsive element, which is targeted by an UPR protein XBP-1. Ectopic expression of XBP-1 induces LMP1 expression, and knockdown of XBP-1 blocks ER stress-triggered up-regulation of LMP1 in NPC cells. Furthermore, XBP-1 significantly correlates with LMP1 expression in NPC tumor biopsies. Therefore, this study not only provides a novel clue linking ER stress/UPR to EBV-associated NPC but also suggests that ER stress/UPR can promote virus-associated cancer in a unique way by driving expression of a viral oncogene.

The LMP1 promoter can be transactivated directly by NF-kappaB

A bioinformatic analysis identified two putative NF-kappaB binding sites in the Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) promoter. The ability of p65RelA to interact with the LMP1 promoter was shown by in vitro and in vivo assays. Using an EBV-transformed lymphoblastoid cell line as a reporter system for the activity of the +40/-328 LMP1 promoter region, the functional importance of NF-kappaB and other transcription factor binding sites was demonstrated. p65RelA could also induce LMP1 expression from the EBV genome in Daudi and P3HR1 Burkitt's lymphoma cell lines. Finally, it was shown that p65RelA could cooperate with EBNA2 or the aryl hydrocarbon receptor in the transactivation of the LMP1 promoter. Our study established the importance of NF-kappaB and several cis-acting elements in the regulation of the LMP1 promoter in a latency III environment and highlighted a complex interplay between NF-kappaB and other transcription factors in this process.

Sustained viral activity of epstein-Barr virus contributes to cellular immortalization of lymphoblastoid cell lines

EBV-transformed lymphoblastoid cell lines (LCLs) are used as a resource for human genetic, immunological, and pharmacogenomic studies. We investigated the biological activity of 20 LCL strains during continuous long-term subculture up to a passage number of 160. Out of 20 LCL strains, 17 proliferated up to a passage number of 160, at which point LCLs are generally considered as "immortalized". The other three LCL strains lost the ability to proliferate at an average passage number of 41, during which these LCLs may have undergone cellular crisis. These non-immortal LCL strains exhibited no telomerase activity, decreased EBV gene expression, and a lower copy number of the EBV genome and mitochondrial DNA when compared with immortal LCLs. Thus, this study suggests that sustained EBV viral activity as well as telomerase activity may be required for complete LCL immortalization.

Epstein-Barr virus nuclear protein EBNA3C residues critical for maintaining lymphoblastoid cell growth

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is essential for efficient conversion of primary human B lymphocytes to lymphoblastoid cell lines (LCLs) and for continued LCL growth. We used a transcomplementation assay in the context of LCLs transformed by an EBV with a conditional EBNA3C to identify the EBNA3C amino acids (aa) necessary for maintaining LCL growth. Surprisingly, we found that most EBNA3C aa were essential for continued LCL growth. Only EBNA3C mutants deleted for residues within aa 507-515, 516-620, 637-675, or 676-727 maintained full LCL growth, and EBNA3C mutants deleted for residues within aa 728-732 or 910-992 maintained slow LCL growth. In contrast, EBNA3C lacking aa 180-231, which mediate RBP-Jkappa association and are necessary for EBNA3C abrogation of EBNA2-induced transcription through RBP-Jkappa, could not support LCL growth. Furthermore, 2 EBNA3C alanine substitution mutants within aa 180-231, which were wild-type (wt) in abrogating EBNA2-mediated transcription through RBP-Jkappa, maintained LCL growth, and 2 alanine substitution mutants within aa 180-231, which were null in abrogating EBNA2-mediated transcription through RBP-Jkappa, did not maintain LCL growth. This indicates that EBNA3C regulation of transcription through RBP-Jkappa is critical to maintaining LCL growth. Several other EBNA3C functions also are critical for LCL growth, because EBNA3C mutants deleted for residues within aa 130-159, 251-506, or 733-909 were wt in abrogating transcription through RBP-Jkappa and expression level, but did not maintain LCL growth.

Current understanding of the role of Epstein-Barr virus in lymphomagenesis and therapeutic approaches to EBV-associated lymphomas

A heterogeneous group of malignancies are associated with Epstein-Barr virus (EBV) infection. These malignancies arise in both immunosuppressed and immunocompetent individuals and can be divided into three patterns of latency depending on the viral genes that are expressed. In Type III latency malignancies, such as post-transplant lymphoproliferative disorder (PTLD), EBV has a direct role and the activated B-cell phenotype is characterised by high-level expression of all the immunodominant EBV latency proteins. Thus, EBV-infected B cells are good targets for EBV-specific cytotoxic T lymphocytes (CTLs). New immune-based treatments for PTLD include transfer of ex vivo generated autologous EBV-specific CTLs or, in the case of bone marrow transplant recipients, donor-derived EBV-specific T cells. This strategy could, perhaps, also work in Type II latency malignancies, where EBV acts like a cofactor rather than having a direct role. In initial studies, T cells specific for the weakly immunogenic latent membrane protein 2 have been expanded ex vivo and have promoted tumor regression in a subset of patients. Another potential therapeutic strategy could be to try to induce lytic EBV infection in the tumor cells. This could be done by targeting genes that switch the EBV-infected B cells from the latent to the lytic cycle.

Epstein-Barr virus-derived EBNA2 regulates STAT3 activation

The Epstein-Barr virus (EBV)-encoded latency protein EBNA2 is a nuclear transcriptional activator that is essential for EBV-induced cellular transformation. Here, we show that EBNA2 interacts with STAT3, a signal transducer for an interleukin-6 family cytokine, and enhances the transcriptional activity of STAT3 by influencing its DNA-binding activity. Furthermore, EBNA2 cooperatively acts on STAT3 activation with LMP1. These data demonstrate that EBNA2 acts as a transcriptional coactivator of STAT3.

Nuclear factor-kappaB binds to the Epstein-Barr Virus LMP1 promoter and upregulates its expression

The latent membrane protein 1 (LMP1) oncogene carried by Epstein-Barr virus (EBV) is essential for transformation and maintenance of EBV-immortalized B cells in vitro, and it is expressed in most EBV-associated tumor types. The activation of the NF-kappaB pathway by LMP1 plays a critical role in the upregulation of antiapoptotic proteins. The EBV-encoded EBNA2 transactivator is required for LMP1 activation in latency III, while LMP1 itself appears to be critical for its activation in the latency II gene expression program. In both cases, additional viral and cellular transcription factors are required in mediating transcription activation of the LMP1 promoter. Using DNA affinity purification and chromatin immunoprecipitation assay, we showed here that members of the NF-kappaB transcription factor family bound to the LMP1 promoter in vitro and in vivo. Electrophoretic mobility shift assay analyses indicated the binding of the p50-p50 homodimer and the p65-p50 heterodimer to an NF-kappaB site in the LMP1 promoter. Transient transfections and reporter assays showed that the LMP1 promoter is activated by exogenous expression of NF-kappaB factors in both B cells and epithelial cells. Exogenous expression of NF-kappaB factors in the EBNA2-deficient P3HR1 cell line induced LMP1 protein expression. Overall, our data are consistent with the presence of a positive regulatory circuit between NF-kappaB activation and LMP1 expression.

Regulation of transcription by the Epstein-Barr virus nuclear antigen EBNA 2

The EBNA 2 (Epstein-Barr nuclear antigen 2) transcription factor is essential for B-cell transformation by the cancer-associated EBV (Epstein-Barr virus) and for the continuous proliferation of infected cells. EBNA 2 activates transcription from the viral Cp (C promoter) during infection to generate the 120 kb transcript that encodes all nuclear antigens required for immortalization by EBV. EBNA 2 contains an acidic activation domain and can interact with a number of general transcription factors and co-activators. It is now becoming clear, however, that the regulation of transcription elongation in addition to initiation by EBNA 2, at least in part through CDK9 (cyclin-dependent kinase 9)-dependent phosphorylation of the RNA polymerase C-terminal domain, is likely to play a crucial role in the mechanism of action of this key viral protein.

Association of Epstein-Barr Virus with Nasopharyngeal Carcinoma and Current Status of Development of Cancer-derived Cell Lines

It is well known that the Epstein-Barr virus (EBV) contributes directly to tumourigenesis in nasopharyngeal carcinoma (NPC), primarily in the undifferentiated form of NPC (WHO type III; UNPC or UC), which is commonly found in South East Asia. Unfortunately, research in NPC has been severely hampered by the lack of authentic EBV-positive (EBV+) human NPC cell lines for study. Since 1975, there have been more than 20 reported NPC cell lines. However, many of these NPC-derived cell lines do not express EBV transcripts in long-term culture, and therefore that finding may dispute the fundamental theory of NPC carcinogenesis. In fact, currently only one EBV+ human NPC cell line (C-666) in long-term culture has been reported. Hence, most of the NPC cell lines may not be representative of the disease itself. In order to better understand and treat NPC, there is an urgent need to develop more EBV+ human NPC cell lines. In this review, we discuss the authenticity of existing NPC cell lines and the impact of our understanding of NPC biology on the treatment of the disease and the relationship of EBV to NPC in the context of cell lines. Key words: Carcinogenesis, Cell culture, Epstein-Barr virus, Hayflick’s limit

The Epstein-Barr virus LF2 protein inhibits viral replication

The switch from Epstein-Barr virus (EBV) latent infection to lytic replication is governed by two transcriptional regulators, Zta and Rta. We previously reported that the EBV protein encoded by the LF2 gene binds to Rta and can inhibit Rta activity in reporter gene assays. We now report that LF2 associates with Rta in the context of EBV-infected cells induced for lytic replication. LF2 inhibition of Rta occurs in both epithelial and B cells, and this downregulation is promoter specific: LF2 decreases Rta activation of the BALF2, BMLF1, and BMRF1 promoters by 60 to 90% but does not significantly decrease Rta activation of its own promoter (Rp). LF2 decreases Rta activation by at least two mechanisms: decreased DNA binding and interference with transcriptional activation by the Rta acidic activation domain. Coexpression of LF2 also specifically induces modification of Rta by the small ubiquitin-like modifiers SUMO2 and SUMO3. We further demonstrate that LF2 overexpression blocks lytic activation in EBV-infected cells induced with Rta or Zta. Our results demonstrate that LF2, a gene deleted from the EBV reference strain B95-8, encodes a potent inhibitor of EBV replication, and they suggest that future studies of EBV replication need to account for the potential effects of LF2 on Rta activity.

Differentiation of EBV-induced post-transplant Hodgkin lymphoma from Hodgkin-like post-transplant lymphoproliferative disease

The development of lymphomas after SOT is a well-known complication of the immunosuppressive therapy necessary to prevent graft rejection. Epstein-Barr virus plays a central role in the pathogenesis of lymphomas because of its ability to transform infected cells. Differentiating PTLD from malignant lymphomas, especially HL can be challenging. We report on two patients, who developed EBV-associated lymphomas several years after SOT. A histological examination of lymph nodes led to a diagnosis of HL in both patients, who were started on chemotherapy according to current treatment protocols. A rapid and complete remission in one patient prompted us to analyze the expression pattern of EBV-latency genes. In this patient, the EBV expression profile revealed a latency type III suggesting the diagnosis of Hodgkin-like PTLD. The other patient required six courses of chemotherapy plus radiotherapy to reach a complete remission. In his tumor cells, a restricted EBV-latency type II pattern was found, suggesting a diagnosis of classical HL. These two cases demonstrate that in post-transplant lymphomas with histological features of HL, an analysis of the expression pattern of EBV proteins might aid in the differentiation between PTLD and HL.

Differential gene regulation by Epstein-Barr virus type 1 and type 2 EBNA2

A transfection assay with a lymphoblastoid cell line infected with Epstein-Barr virus was used to compare the abilities of type 1 and type 2 EBNA2 to sustain cell proliferation. The reduced proliferation in cells expressing type 2 EBNA2 correlated with loss of expression of some cell genes that are known to be targets of type 1 EBNA2. Microarray analysis of EBNA2 target genes identified a small number of genes that are more strongly induced by type 1 than by type 2 EBNA2, and one of these genes (CXCR7) was shown to be required for proliferation of lymphoblastoid cell lines. The Epstein-Barr virus LMP1 gene was also more strongly induced by type 1 EBNA2 than by type 2, but this effect was transient. Type 1 and type 2 EBNA2 were equally effective at arresting cell proliferation of Burkitt's lymphoma cell lines lacking Epstein-Barr virus and were also shown to cause apoptosis in these cells. The results indicate that differential gene regulation by Epstein-Barr virus type 1 and type 2 EBNA2 may be the basis for the much weaker B-cell transformation activity of type 2 Epstein-Barr virus strains compared to type 1 strains.

Interleukin-21 regulates expression of key Epstein-Barr virus oncoproteins, EBNA2 and LMP1, in infected human B cells

Epstein-Barr virus (EBV) persists for the life of the host by accessing the long-lived memory B cell pool. It has been proposed that EBV uses different combinations of viral proteins, known as latency types, to drive infected B cells to make the transition from resting B cells to memory cells. This process is normally antigen-driven. A major unresolved question is what factors coordinate expression of EBV latency proteins. We have recently described novel type III latency EBV+ B cell lines (OCI-BCLs) that were induced to differentiate into late plasmablasts/early plasma cells in culture with interleukin-21 (IL-21), mimicking normal B cell development. The objective of this study was to determine whether IL-21-mediated signals also regulate the expression of key EBV latent proteins during this window of development. Here we show that IL-21-reduced gene and protein expression of growth-transforming EBV nuclear antigen 2 (EBNA2) in OCI-BCLs. By contrast, the expression of CD40-like, latent membrane protein 1 (LMP1) strongly increased in these cells suggesting an EBNA2-independent mode of regulation. Same results were also observed in Burkitt's lymphoma line Jijoye and B95-8 transformed lymphoblastoid cell lines. The effect of IL-21 on EBNA2 and LMP1 expression was attenuated by a pharmacological JAK inhibitor indicating involvement of JAK/STAT signalling in this process. Our study also shows that IL-21 induced transcription of ebna1 from the viral Q promoter (Qp).

The LMP1 oncogene of EBV activates PERK and the unfolded protein response to drive its own synthesis

The oncogene latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) without a ligand drives proliferation of EBV-infected B cells. Its levels vary in cells of clonal populations by more than 100-fold, which leads to multiple distinct activities of the oncogene. At intermediate levels it drives proliferation, and at high levels it inhibits general protein synthesis by inducing phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha). We have found that LMP1 activates PERK to induce phosphorylation of eIF2alpha, which upregulates activating transcription factor 4 (ATF4) expression. ATF4, in turn, transactivates LMP1's own promoter. LMP1 activates not only PERK but also inositol requiring kinase 1 (IRE1) and ATF6, 3 pathways of the unfolded protein response (UPR). Increasing expression levels of LMP1 induced a dose-dependent increase in IRE1 activity, as measured by its "splicing" of XBP-1. These infected B cells secrete immunoglobins independent of the levels of LMP1, indicating that only a threshold level of XBP-1 is required for the secretion. These findings indicate that LMP1's activation of the UPR is a normal event in a continuum of LMP1's expression that leads both to stimulatory and inhibitory functions and regulates the physiology of EBV-infected B cells in multiple, unexpected modes.

Role of a consensus AP-2 regulatory sequence within the Epstein-Barr virus LMP1 promoter in EBNA2 mediated transactivation

The Epstein-Barr virus (EBV) tumor-associated latent membrane protein 1 (LMP1) gene expression is transactivated by EBV nuclear antigen 2 (EBNA2) in human B cells. We previously reported that an E-box element at the LMP1 regulatory sequence (LRS) represses transcription of the LMP1 gene through the recruitment of a Max-Mad1-mSin3A complex. In the present study, using deletion/mutation analysis, and electrophoretic mobility shift assays, we show that the promoter region adjacent to the E-box (-59/-67) is required for the full repression conferred by E-box binding proteins. The repressive effect of these factors was overcome by an inhibitor of histone deacetylation, Trichostatin A (TSA), concurring with the reports that histone deacetylation plays an important role in repression mediated by Max-Mad1-mSin3A complex. Furthermore, ChIP analyses showed that histones at the transcriptionally active LMP1 promoter were hyperacetylated, whereas in the absence of transcription they were hypoacetylated. EBNA2 activation of the promoter required a consensus AP-2 sequence in the -103/-95 LRS region. While EMSA results and the low level of AP-2 factors expression in B cells argue against known AP-2 factors binding to this site, several pieces of evidence point to a similar mechanism of promoter activation as seen by AP-2 factors. We conclude that an AP-2 site-binding factor and EBNA2 act in concert to overcome the repression of the LMP1 promoter via the consensus AP-2 site. This activation showed strong correlation with histone hyperacetylation at the promoter, indicating this to be a major mechanism for the EBNA2 mediated LMP1 transactivation.

Hsp72 up-regulates Epstein-Barr virus EBNALP coactivation with EBNA2

The Epstein-Barr virus (EBV) transcriptional coactivator EBNALP specifically associates and colocalizes with Hsp72 in lymphoblastoid cell lines. We now find that overexpression of Hsp72 more than doubled EBNALP coactivation with EBNA2 of a transfected EBV LMP1 promoter in B lymphoblasts, did not affect EBNA2 or EBNALP protein levels, and strongly up-regulated EBNA2 and EBNALP coactivation of LMP1 protein expression from the endogenous EBV genome in latency I infected Akata cells. The Hsp72 ATP, protein binding, and the C-terminal regulatory domains were required for full activity. An EBNALP deletion mutant, EBNALPd45, which does not associate with Hsp72, coactivated with EBNA2, but was not affected by Hsp72 overexpression, despite Hsp72 up-regulation of wild-type EBNALP coactivation with EBNA2 at all levels of EBNALP expression, indicating the importance of Hsp72 association with EBNALP for Hsp72 up-regulation of coactivation. Of importance, a 90% RNAi knockdown of Hsp72 reduced EBNALP coactivation with EBNA2 of transfected EBV LMP1 and Cp promoters by approximately 50%. Overexpression of the Hsp72 C-terminal interacting and regulatory protein, CHIP, strongly down-regulated EBNALP coactivation, independently of CHIP ubiquitin ligase activity. CHIP effects were Hsp72 dependent, indicating a background downmodulating role for CHIP in Hsp72 augmentation of EBNA2 and EBNALP coactivation. Based on these and other cited data, we favor a model in which Hsp72 chaperones EBNALP shuttling of repressors from EBNA2-enhanced promoters.

EBV-associated diseases in the AIDS patient

EBV-associated malignancies remain a considerable problem in HIV-infected individuals, even in the era of HAART. Although EBV is a common factor, each disease has a unique pathogenesis. Study of these diseases reveals the viral proteins expressed in the malignancies that might contribute to the development of the disease as well as the molecular basis for pathogenesis. It is likely that this knowledge will contribute to the development of novel therapeutics that will result in more favorable outcomes in the future.

Epstein-Barr nuclear antigen leader protein coactivates transcription through interaction with histone deacetylase 4

Epstein-Barr nuclear antigen (EBNA) leader protein (EBNALP) coactivates promoters with EBNA2 and is important for Epstein-Barr virus immortalization of B cells. Investigation of the role of histone deacetylases (HDACs) in EBNALP and EBNA2 promoter regulation has now identified EBNALP and EBNA2 to be associated with HDAC4 in a lymphoblastoid cell line. Furthermore, a transcription-deficient EBNALP point mutant did not associate with HDAC4. HDAC4 and 5 overexpression repressed EBNA2 activation and EBNALP coactivation, whereas other HDACs had little effect. Moreover, EBNALP expression decreased nuclear HDAC4. Expression of 14-3-3 anchors HDAC4 in the cytoplasm, increased EBNALP effects, and reversed HDAC4 or 5 repression. HDAC4 reversal depended on the HDAC4 nuclear export sequence. Consistent with EBNALP coactivation being mediated by nuclear HDAC4 depletion, HDAC4 overexpression increased nuclear HDAC4 and specifically repressed EBNA2-dependent activation as well as EBNALP-dependent coactivation. Also, EBNALP, HDAC4, and 14-3-3 could be immunoprecipitated in a single complex. Thus, these data strongly support a model in which EBNALP coactivates transcription by relocalizing HDAC4 and 5 from EBNA2 activated promoters to the cytoplasm. The observed EBNALP effects are likely also in part through HDAC5, which is highly homologous to HDAC4.

Epstein-Barr virus EBNA-3C is targeted to and regulates expression from the bidirectional LMP-1/2B promoter

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA-3C) is essential for EBV-mediated immortalization of human B lymphocytes and regulates both the cell cycle and transcription. Transient reporter gene assays have implicated a pivotal role for EBNA-3C in the regulation of transcription of the majority of latency-associated genes expressed during the EBV growth program, including the viral oncoprotein LMP-1. To examine the regulation of latency gene expression by EBNA-3C, we generated an EBV-positive cell line that inducibly expresses EBNA-3C. This cell line allowed us to examine expression from the endogenous latency gene promoters in the context of an actual latent infection and the presence of other EBNA proteins, in particular EBNA-2, which is presumed to coregulate transcription with EBNA-3C. EBNA-3C induced the expression of both LMP-1 and LMP-2B mRNAs from the bidirectional LMP-1/LMP-2B promoter. In contrast, no effect was seen on expression from the common EBNA promoter Cp, which is responsive to EBNA-3C in reporter assays. Activation of LMP expression was not the consequence of increases in EBNA-2, PU.1 or Spi-B transcription factors, all of which are believed to be critical for activation of LMP-1. Chromatin immunoprecipitation assays furthermore indicated that EBNA-3C is present at the bidirectional LMP-1/LMP-2B promoter. These results indicate that EBNA-3C directly activates the expression of LMP-1 and LMP-2B but is unlikely to significantly regulate EBNA expression via Cp under normal growth conditions.

Kaposi's Sarcoma-associated herpesvirus lytic switch protein stimulates DNA binding of RBP-Jk/CSL to activate the Notch pathway

Kaposi's sarcoma-associated herpesvirus (KSHV) lytic switch protein, Rta, is a ligand-independent inducer of the Notch signal transduction pathway, and KSHV cannot reactivate from latency in cells null for the Notch target protein RBP-Jk. Here we show that Rta promotes DNA binding of RBP-Jk, a mechanism that is fundamentally different from that established for the RBP-Jk-activating proteins, Notch intracellular domain (NICD) and Epstein-Barr virus EBNA2. Although constitutively active RBP-Jk and NICD do not transactivate KSHV promoters independently, cotransfection of an Rta mutant lacking its transactivation domain robustly restores transcriptional activation. Cooperation requires intact DNA binding sites for Rta and RBP-Jk and trimeric complex formation between the three molecules in vitro. In infected cells, RBP-Jk is virtually undetectable on a series of viral and cellular promoters during KSHV latency but is significantly enriched following Rta expression during viral reactivation. Accordingly, Rta, but not EBNA2 and NICD, reactivates the complete viral lytic cycle.

Epstein-Barr virus nuclear antigen 2 trans-activates the cellular antiapoptotic bfl-1 gene by a CBF1/RBPJ kappa-dependent pathway

The human herpesvirus Epstein-Barr virus (EBV) establishes latency and promotes the long-term survival of its host B cell by targeting the molecular machinery controlling cell fate decisions. The cellular antiapoptotic bfl-1 gene confers protection from apoptosis under conditions of growth factor deprivation when expressed ectopically in an EBV-negative Burkitt's lymphoma-derived cell line (B. D'Souza, M. Rowe, and D. Walls, J. Virol. 74:6652-6658, 2000), and the EBV latent membrane protein 1 (LMP1) and its cellular functional homologue CD40 can both drive bfl-1 via an NF-kappaB-dependent enhancer element in the bfl-1 promoter (B. N. D'Souza, L. C. Edelstein, P. M. Pegman, S. M. Smith, S. T. Loughran, A. Clarke, A. Mehl, M. Rowe, C. Gélinas, and D. Walls, J. Virol. 78:1800-1816, 2004). Here we show that the EBV nuclear antigen 2 (EBNA2) also upregulates bfl-1. EBNA2 trans-activation of bfl-1 requires CBF1 (or RBP-J kappa), a nuclear component of the Notch signaling pathway, and there is an essential role for a core consensus CBF1-binding site on the bfl-1 promoter. trans-activation is dependent on the EBNA2-CBF1 interaction, is modulated by other EBV gene products known to interact with the CBF1 corepressor complex, and does not involve activation of NF-kappaB. bfl-1 expression is induced and maintained at high levels by the EBV growth program in a lymphoblastoid cell line, and withdrawal of either EBNA2 or LMP1 does not lead to a reduction in bfl-1 mRNA levels in this context, whereas the simultaneous loss of both EBV proteins results in a major decrease in bfl-1 expression. These findings are relevant to our understanding of EBV persistence, its role in malignant disease, and the B-cell developmental process.

Autoactivation of the Epstein-Barr virus oncogenic protein LMP1 during type II latency through opposite roles of the NF-kappaB and JNK signaling pathways

Epstein-Barr virus (EBV) is associated with several human malignancies where it expresses limited subsets of latent proteins. Of the latent proteins, latent membrane protein 1 (LMP1) is a potent transforming protein that constitutively induces multiple cell signaling pathways and contributes to EBV-associated oncogenesis. Regulation of LMP1 expression has been extensively described during the type III latency of EBV. Nevertheless, in the majority of EBV-associated tumors, the virus is commonly found to display a type II latency program in which it is still unknown which viral or cellular protein is really involved in maintaining LMP1 expression. Here, we demonstrate that LMP1 activates its own promoter pLMP1 through the JNK signaling pathway emerging from the TES2 domain. Our results also reveal that this activation is tightly controlled by LMP1, since pLMP1 is inhibited by LMP1-activated NF-kappaB signaling pathway. By using our physiological models of EBV-infected cells displaying type II latency as well as lymphoblastoid cell lines expressing a type III latency, we also demonstrate that this balanced autoregulation of LMP1 is shared by both latency programs. Finally, we show that this autoactivation is the most important mechanism to maintain LMP1 expression during the type II latency program of EBV.

Epstein-Barr virus nuclear antigen 2 induces FcRH5 expression through CBF1

Fc-receptor homolog 5 (FcRH5) is a recently identified B-cell membrane protein of unknown function. In Burkitt lymphoma cell lines with chromosome 1q21 abnormalities, FcRH5 expression is deregulated, implicating FcRH5 in lymphomagenesis. Epstein-Barr virus infects and immortalizes B cells, and is implicated in the etiology of several tumors of B-cell origin. Overexpression of genes located on 1q21-25 has been proposed as a surrogate for Epstein-Barr virus in Burkitt lymphoma. We now report that Epstein-Barr virus nuclear antigen 2 (EBNA2) markedly induces the expression of the FcRH5 gene, encoded on chromosome 1q21. Induction occurred in the absence of other viral proteins and did not require de novo protein synthesis. EBNA2 lacks a DNA-binding domain and can target responsive genes through the host DNA binding protein CBF1. We show that induction of FcRH5 by EBNA2 is strictly CBF1 dependent, as it was abolished in CBF1-deficient cells. Accordingly, EBNA2 targeted CBF1 binding sites present in the FcRH5 promoter in vivo, as detected by chromatin immunoprecipitation. These results identify FcRH5 as a novel, direct target of EBNA2 that may contribute to the development of Epstein-Barr virus–associated tumors.

EBV EBNA 2 stimulates CDK9-dependent transcription and RNA polymerase II phosphorylation on serine 5

EBNA 2 is one of only five viral genes essential for the infection and immortalization of human B cells by the cancer-associated virus Epstein-Barr virus (EBV). EBNA 2 activates cellular and viral transcription and associates with components of the basal transcription apparatus and a number of coactivators. We provide the first evidence to show that the mechanism of transcriptional activation by EBNA 2 also involves phosphorylation of the C-terminal domain (CTD) of RNA polymerase II (pol II). We found that transcriptional activation by EBNA 2 was inhibited by a dominant-negative mutant of the pol II CTD kinase, CDK9, and by low concentrations of the CDK9 inhibitor 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole. Moreover, using chromatin immunoprecipitation assays we demonstrated that EBNA 2 stimulates both pol II recruitment and pol II phosphorylation on serine 5 of the CTD in vivo. These results identify a new step in the transcription cycle that is subject to regulation by a key EBV-encoded transcription factor and highlight CDK9 inhibitors as potential anti-EBV agents.

cdc2/cyclin B1-dependent phosphorylation of EBNA2 at Ser243 regulates its function in mitosis

Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA2) transactivates EBV genes in latently infected B cells. We have shown that mitotic hyperphosphorylation of EBNA2 suppresses its ability to transactivate the latent membrane protein 1 (LMP1) promoter. In this follow-up study, we identify EBNA2 Ser243 as a phosphorylation site for mitotic cdc2/cyclin B1 kinase. Mutation at Ser243, which mimics constitutive phosphorylation of the protein, decreases endogenous levels of both LMP1 and EBNA2. Moreover, mutation at Ser243 reduces the ability of EBNA2 to transactivate Cp, the promoter for all six EBV EBNA genes. Our data implicate EBNA2 Ser243 as a cdc2/cyclin B1 site of phosphorylation important for EBNA2's cotranscriptional function in mitosis.

Interferon regulatory factor 5 represses expression of the Epstein-Barr virus oncoprotein LMP1: braking of the IRF7/LMP1 regulatory circuit

We have reported evidence for a positive regulatory circuit between interferon regulatory factor 7 (IRF7) and the Epstein-Barr virus (EBV) oncoprotein 1 (LMP1) (S. Ning, A. M. Hahn, and J. S. Pagano, J. Virol. 77:9359-9368, 2003). To explore a possible braking mechanism for this circuit, several type II EBV-infected cell lines that express different levels of LMP1 and IRF7 proteins and therefore are convenient for studying modulation of expression of LMP1 were analyzed. Endogenous levels of IRF7 and LMP1 were directly correlated. Transient expression of an IRF7 dominant-negative mutant decreased LMP1 levels. Endogenous IRF5 and IRF7 proteins were shown to physically associate in EBV-positive cells. Transient expression of IRF5 decreased activation of the LMP1 promoter by IRF7 in a dose-dependent manner. Finally, transfection of either an IRF5 dominant-negative construct or IRF5 small interfering RNA in these cells resulted in increases in endogenous levels of LMP1. These results indicate that IRF5 can downregulate IRF7's induction of expression of LMP1 most likely by interacting with IRF7 and provide a means of modulating a regulatory circuit between IRF7 and LMP1.

Epstein-Barr virus nuclear protein 3A domains essential for growth of lymphoblasts: transcriptional regulation through RBP-Jkappa/CBF1 is critical

Experimental reverse genetic replacement of Epstein-Barr virus nuclear antigen 3A (EBNA3A) with a conditional mutant EBNA3A revealed that EBNA3A is critical for continued lymphoblastoid cell (LCL) growth. Wild-type (wt) EBNA3A expressed in the LCLs specifically sustained growth under nonpermissive conditions, whereas EBNA3B or EBNA3C expression had no effect (S. Mauro, E. Johannsen, D. Illanes, A. Cooper, and E. Kieff, J. Virol. 77:10437-10447, 2003). This genetic system and related biochemical assays have now been used to discover that EBNA3A lacking amino acid residues 170 to 240 (delta170-240), TLGC202 to AAGA202, or delta300-386, which are deficient in repression of EBNA2 activation of an RBP-Jkappa/CBF1-dependent EBV Cp enhancer, are null mutations for LCL growth, whereas EBNA3A delta2-124, delta410-439, delta440-470, delta470-500, delta500-523, delta523-612, and delta620-820, which are wt in repression are wt for LCL growth. Thus, EBNA3A regulation of transcription through RBP-Jkappa/CBF1 is critical for LCL growth. EBNA3A mutants deleted of amino acid residues 240 to 300, 386 to 410, or 827 to 944 were intermediate, null, or intermediate, respectively, for LCL growth despite being wt for RBP-Jkappa association and repression. Amino acid residues 240 to 300, 386 to 410, and, particularly, C-terminal residues 827 to 944 are therefore likely to contribute to LCL growth through RBP-Jkappa-independent mechanisms.

Hyperphosphorylation of EBNA2 by Epstein-Barr virus protein kinase suppresses transactivation of the LMP1 promoter

The Epstein-Barr virus (EBV) BGLF4 gene encodes a serine/threonine protein kinase (PK) that is expressed in the cytolytic cycle. EBV nuclear antigen 2 (EBNA2) is a key latency gene essential for immortalization of B lymphocytes and transactivation of viral and cellular promoters. Here we report that EBV PK phosphorylates EBNA2 at Ser-243 and that these two proteins physically associate. PK suppresses EBNA2's ability to transactivate the LMP1 promoter, and Ser-243 of EBNA2 is involved in this suppression. Moreover, EBNA2 is hyperphosphorylated during EBV reactivation in latently infected B cells, which is associated with decreased LMP1 protein levels. This is the first report about the effect of EBV PK on the function of one of its target proteins and regulation of EBNA2 phosphorylation during the EBV lytic cycle.

Regulation of expression of the Epstein-Barr virus BamHI-A rightward transcripts

The Epstein-Barr virus (EBV) BamHI-A rightward transcripts, or BARTs, are a family of mRNAs expressed in all EBV latency programs, including EBV-infected B cells in healthy carriers. Despite their ubiquitous expression, the regulation and biological function of BARTs are still unclear. In this study, the BART 5' termini were characterized by using a procedure that selects capped, full-length mRNAs. Two TATA-less promoter regions, designated P1 and P2, were mapped. P1 had relatively high basal activity in both epithelial and B cells, whereas P2 exhibited higher activity in epithelial cells. Upon EBV infection of B cells, transcription from P1 was detected soon after infection, while expression from P2 was delayed. Promoter-reporter assays in transiently transfected cells revealed that P1 and P2 were differentially regulated. Interferon regulatory factor 7 (IRF7) and IRF5 negatively regulated P1 activity. c-Myc and C/EBP family members positively regulated P2. Regulation of P2 by C/EBPs was characterized by electrophoretic mobility shift assay, chromatin immunoprecipitation, and reporter assays. More-abundant BART expression in epithelial cells correlated with the relative expression of positive and negative regulators in these cells.

Epstein-Barr virus sustains Burkitt's lymphomas and Hodgkin's disease

Two proteins of Epstein-Barr Virus make formerly unrecognized contributions to maintaining the tumors of Burkitt's lymphomas and Hodgkin's disease. The Epstein-Barr nuclear antigen 1 (EBNA1) protein can support the synthesis and maintenance of the viral genome. New data show that inhibiting EBNA1 in Burkitt's lymphoma cells induces cell death by apoptosis. Therefore, EBNA1 inhibits apoptosis and, according to recent findings, does so independently of other viral genes. The latent membrane protein 2a (LMP2a) binds to signaling molecules that are engaged by the B-cell receptor and inhibits the signaling that is mediated by antigen binding. New findings have revealed how LMP2a overcomes the apoptosis that normally results from the absence of functional B-cell receptors, and explain how Hodgkin's disease tumor cells, which are B cells, survive but lack functional antibodies.

Induction of Epstein-Barr virus latent membrane protein 1 by a lytic transactivator Rta

Latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is a transforming protein that affects multiple cell signaling pathways and contributes to EBV-associated oncogenesis. LMP1 can be expressed in some states of EBV latency, and significant induction of full-length LMP1 is also observed frequently during virus reactivation into the lytic cycle. It is still unknown how LMP1 expression is regulated during the lytic stage and whether any EBV lytic protein is involved in the induction of LMP1. In this study, we first identified that LMP1 expression is associated with the spontaneous virus reactivation in EBV-infected 293 cells and that its expression is a downstream event of the lytic cycle. We further found that LMP1 can be induced by ectopic expression of Rta, an EBV immediate-early lytic protein. The Rta-mediated LMP1 induction is independent of another immediate-early protein, Zta. Northern blotting and reverse transcription-PCR analysis revealed that Rta upregulates LMP1 at the RNA level. Reporter gene assays further demonstrated that Rta activates both the proximal and distal promoters of the LMP1 gene in EBV-negative cells. Both the amino and carboxyl termini of the Rta protein are required for the induction of LMP1. In addition, Rta transactivates LMP1 not only in epithelial cells but also in B-lymphoid cells. This study reveals a new mechanism to upregulate LMP1 expression, expanding the knowledge of LMP1 regulation in the EBV life cycle. Considering an equivalent case of Kaposi's sarcoma-associated herpesvirus, induction of a transforming membrane protein by a key lytic transactivator during virus reactivation is likely to be a conserved event for gammaherpesviruses.

Viral interactions with the Notch pathway

The Notch signaling pathway influences cell fate decisions, proliferation versus differentiation and cell survival. Viruses both utilize and manipulate the differentiation state of infected cells, promote or block cell cycling and employ a variety of mechanisms to evade innate cellular anti-viral responses and promote cell survival. In light of these commonalities, it is perhaps not surprising that several viruses have tapped into the Notch pathway to advance their own life cycles. This first became apparent from studies showing targeting of Epstein-Barr virus proteins to the nuclear effector of Notch signaling CSL (CBF1/RBPJk). More recently the Kaposi's sarcoma-associated herpesvirus RTA protein has been found to bind CSL. Notch pathway interactions have also been described for adenovirus SV40 and human papilloma virus. This review focuses on the herpesvirus protein interactions with the Notch pathway and the insights that these interactions have provided.

Biophysical and mutational analysis of the putative bZIP domain of Epstein-Barr virus EBNA 3C

Epstein-Barr virus nuclear antigen 3C (EBNA 3C) is essential for B-cell immortalization and functions as a regulator of viral and cellular transcription. EBNA 3C contains glutamine-rich and proline-rich domains and a region in the N terminus consisting of a stretch of basic residues followed by a run of leucine residues spaced seven amino acids apart. This N-terminal domain is widely believed to represent a leucine zipper dimerization motif (bZIP). We have performed the first structural and functional analysis of this motif and demonstrated that this domain is not capable of forming stable homodimers. Peptides encompassing the EBNA 3C zipper domain are approximately 54 to 67% alpha-helical in solution but cannot form dimers at physiologically relevant concentrations. Moreover, the EBNA 3C leucine zipper cannot functionally substitute for another homodimerizing zipper domain in domain-swapping experiments. Our data indicate, however, that the EBNA 3C zipper domain behaves as an atypical bZIP domain and is capable of self-associating to form higher-order alpha-helical oligomers. Using directed mutagenesis, we also identified a new role for the bZIP domain in maintaining the interaction between EBNA 3C and RBP-Jkappa in vivo. Disruption of the helical nature of the zipper domain by the introduction of proline residues reduces the ability of EBNA 3C to inhibit EBNA 2 activation and interact with RBP-Jkappa in vivo by 50%, and perturbation of the charge on the basic region completely abolishes this function of EBNA 3C.