Epstein-Barr viral latency is disrupted by the immediate-early BRLF1 protein through a cell-specific mechanism

Conserved herpesvirus kinases target the DNA damage response pathway and TIP60 histone acetyltransferase to promote virus replication

Herpesviruses, which are major human pathogens, establish life-long persistent infections. Although the α, β, and γ herpesviruses infect different tissues and cause distinct diseases, they each encode a conserved serine/threonine kinase that is critical for virus replication and spread. The extent of substrate conservation and the key common cell-signaling pathways targeted by these kinases are unknown. Using a human protein microarray high-throughput approach, we identify shared substrates of the conserved kinases from herpes simplex virus, human cytomegalovirus, Epstein-Barr virus (EBV), and Kaposi's sarcoma-associated herpesvirus. DNA damage response (DDR) proteins were statistically enriched, and the histone acetyltransferase TIP60, an upstream regulator of the DDR pathway, was required for efficient herpesvirus replication. During EBV replication, TIP60 activation by the BGLF4 kinase triggers EBV-induced DDR and also mediates induction of viral lytic gene expression. Identification of key cellular targets of the conserved herpesvirus kinases will facilitate the development of broadly effective antiviral strategies.

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

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

Endoplasmic reticulum stress causes EBV lytic replication

Endoplasmic reticulum (ER) stress triggers a homeostatic cellular response in mammalian cells to ensure efficient folding, sorting, and processing of client proteins. In lytic-permissive lymphoblastoid cell lines (LCLs), pulse exposure to the chemical ER-stress inducer thapsigargin (TG) followed by recovery resulted in the activation of the EBV immediate-early (BRLF1, BZLF1), early (BMRF1), and late (gp350) genes, gp350 surface expression, and virus release. The protein phosphatase 1 a (PP1a)-specific phosphatase inhibitor Salubrinal (SAL) synergized with TG to induce EBV lytic genes; however, TG treatment alone was sufficient to activate EBV lytic replication. SAL showed ER-stress-dependent and -independent antiviral effects, preventing virus release in human LCLs and abrogating gp350 expression in 12-O-tetradecanoylphorbol-13-acetate (TPA)-treated B95-8 cells. TG resulted in sustained BCL6 but not BLIMP1 or CD138 expression, which is consistent with maintenance of a germinal center B-cell, rather than plasma-cell, phenotype. Microarray analysis identified candidate genes governing lytic replication in LCLs undergoing ER stress.

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.

NF-kappaB-mediated modulation of inducible nitric oxide synthase activity controls induction of the Epstein-Barr virus productive cycle by transforming growth factor beta 1

Transforming growth factor beta 1 (TGF-β1) signal transduction has been implicated in many second-messenger pathways, including the NF-κB pathway. We provide evidence of a novel TGF-β1-mediated pathway that leads to extracellular signal-regulated kinase (ERK) 1/2 phosphorylation, which in turn induces expression of an Epstein-Barr virus (EBV) protein, ZEBRA, that is responsible for the induction of the viral lytic cycle. This pathway includes two unexpected steps, both of which are required to control ERK 1/2 phosphorylation: first, a quick and transient activation of NF-κB, and second, downregulation of inducible nitric oxide synthase (iNOS) activity that requires the participation of NF-κB activity. Although necessary, NF-κB alone is not sufficient to produce downregulation of iNOS, suggesting that another uncharacterized event(s) is involved in this pathway. Dissection of the steps involved in the switch from the EBV latent cycle to the lytic cycle will be important to understand how virus-host relationships modulate the innate immune system.

Valproic acid antagonizes the capacity of other histone deacetylase inhibitors to activate the Epstein-barr virus lytic cycle

Diverse stimuli reactivate the Epstein-Barr virus (EBV) lytic cycle in Burkitt lymphoma (BL) cells. In HH514-16 BL cells, two histone deacetylase (HDAC) inhibitors, sodium butyrate (NaB) and trichostatin A (TSA), and the DNA methyltransferase inhibitor azacytidine (AzaCdR) promote lytic reactivation. Valproic acid (VPA), which, like NaB, belongs to the short-chain fatty acid class of HDAC inhibitors, fails to induce the EBV lytic cycle in these cells. Nonetheless, VPA behaves as an HDAC inhibitor; it causes hyperacetylation of histone H3 (J. K. Countryman, L. Gradoville, and G. Miller, J. Virol. 82:4706-4719, 2008). Here we show that VPA blocked the induction of EBV early lytic proteins ZEBRA and EA-D in response to NaB, TSA, or AzaCdR. The block in lytic activation occurred prior to the accumulation of BZLF1 transcripts. Reactivation of EBV in Akata cells, in response to anti-IgG, and in Raji cells, in response to tetradecanoyl phorbol acetate (TPA), was also inhibited by VPA. MS-275 and apicidin, representing two additional classes of HDAC inhibitors, and suberoylanilide hydroxamic acid (SAHA) reactivated EBV in HH514-16 cells; this activity was also inhibited by VPA. Although VPA potently blocked the expression of viral lytic-cycle transcripts, it did not generally block the transcription of cellular genes and was not toxic. The levels and kinetics of specific cellular transcripts, such as Stat3, Frmd6, Mad1, Sepp1, c-fos, c-jun, and egr1, which were activated by NaB and TSA, were similar in HH514-16 cells treated with VPA. When combined with NaB or TSA, VPA did not inhibit the activation of these cellular genes. Changes in cellular gene expression in response to VPA, NaB, or TSA were globally similar as assessed by human genome arrays; however, VPA selectively stimulated the expression of some cellular genes, such as MEF2D, YY1, and ZEB1, that could repress the EBV lytic cycle. We describe a novel example of functional antagonism between HDAC inhibitors.

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

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

The Epstein-Barr virus BRRF1 protein, Na, induces lytic infection in a TRAF2- and p53-dependent manner

The Epstein-Barr virus (EBV) BRRF1 lytic gene product (Na) is encoded within the same immediate-early region as the BZLF1 (Z) and BRLF1(R) gene products, but its role during EBV infection has not been well defined. We previously showed that Na cooperates with the R protein to induce lytic gene expression in latently infected EBV-positive 293 cells, and in some EBV-negative cell lines it can activate the Z promoter in reporter gene assays. Here we show that overexpression of Na alone is sufficient to induce lytic gene expression in several different latently infected epithelial cell lines (Hone-Akata, CNE2-Akata, and AGS-Akata), while knockdown of endogenous Na expression reduces lytic gene expression. Consistent with its ability to interact with tumor necrosis factor receptor-associated factor 2 (TRAF2) in a yeast two-hybrid assay, we demonstrate that Na interacts with TRAF2 in cells. Furthermore, we show that TRAF2 is required for Na induction of lytic gene expression, that Na induces Jun N-terminal protein kinase (JNK) activation in a TRAF2-dependent manner, and that a JNK inhibitor abolishes the ability of Na to disrupt viral latency. Additionally, we show that Na and the tumor suppressor protein p53 cooperate to induce lytic gene expression in epithelial cells (including the C666-1 nasopharyngeal carcinoma cell line), although Na does not appear to affect p53 function. Together these data suggest that Na plays an important role in regulating the switch between latent and lytic infection in epithelial cells and that this effect requires both the TRAF2 and p53 cellular proteins.

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

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

Prospects of a novel vaccination strategy for human gamma-herpesviruses

Due to the oncogenic potential associated with persistent infection of human gamma-herpesviruses, including Epstein-Barr virus (EBV or HHV-4) and Kaposi's sarcoma-associated herpesvirus (KSHV or HHV-8), vaccine development has focused on subunit vaccines. However, the results using an animal model of mouse infection with a related rodent virus, murine gamma-herpesvirus 68 (MHV-68, γHV-68, or MuHV-4), have shown that the only effective vaccination strategy is based on live attenuated viruses, including viruses engineered to be incapable of establishing persistence. Vaccination with a virus lacking persistence would eliminate many potential complications. Progress in understanding persistent infections of EBV and KSHV raises the possibility of engineering a live attenuated virus without persistence. Therefore, we should keep the option open for developing a live EBV or KSHV vaccine.

Sequence analysis of the Epstein-Barr virus (EBV) BRLF1 gene in nasopharyngeal and gastric carcinomas

Background

Epstein-Barr virus (EBV) has a biphasic infection cycle consisting of a latent and a lytic replicative phase. The product of immediate-early gene BRLF1, Rta, is able to disrupt the latency phase in epithelial cells and certain B-cell lines. The protein Rta is a frequent target of the EBV-induced cytotoxic T cell response. In spite of our good understanding of this protein, little is known for the gene polymorphism of BRLF1.

Results

BRLF1 gene was successfully amplified in 34 EBV-associated gastric carcinomas (EBVaGCs), 57 nasopharyngeal carcinomas (NPCs) and 28 throat washings (TWs) samples from healthy donors followed by PCR-direct sequencing. Fourteen loci were found to be affected by amino acid changes, 17 loci by silent nucleotide changes. According to the phylogenetic tree, 5 distinct subtypes of BRLF1 were identified, and 2 subtypes BR1-A and BR1-C were detected in 42.9% (51/119), 42.0% (50/119) of samples, respectively. The distribution of these 2 subtypes among 3 types of specimens was significantly different. The subtype BR1-A preferentially existed in healthy donors, while BR1-C was seen more in biopsies of NPC. A silent mutation A/G was detected in all the isolates. Among 3 functional domains, the dimerization domain of Rta showed a stably conserved sequence, while DNA binding and transactivation domains were detected to have multiple mutations. Three of 16 CTL epitopes, NAA, QKE and ERP, were affected by amino acid changes. Epitope ERP was relatively conserved; epitopes NAA and QKE harbored more mutations.

Conclusions

This first detailed investigation of sequence variations in BRLF1 gene has identified 5 distinct subtypes. Two subtypes BR1-A and BR1-C are the dominant genotypes of BRLF1. The subtype BR1-C is more frequent in NPCs, while BR1-A preferentially presents in healthy donors. BR1-C may be associated with the tumorigenesis of NPC.

The nuclear and adherent junction complex component protein ubinuclein negatively regulates the productive cycle of Epstein-Barr virus in epithelial cells

The Epstein-Barr Virus (EBV) productive cycle is initiated by the expression of the viral trans-activator EB1 (also called Zebra, Zta, or BZLF1), which belongs to the basic leucine zipper transcription factor family. We have previously identified the cellular NACos (nuclear and adherent junction complex components) protein ubinuclein (Ubn-1) as a partner for EB1, but the function of this complex has never been studied. Here, we have evaluated the consequences of this interaction on the EBV productive cycle and find that Ubn-1 overexpression represses the EBV productive cycle whereas Ubn-1 downregulation by short hairpin RNA (shRNA) increases virus production. By a chromatin immunoprecipitation (ChIP) assay, we show that Ubn-1 blocks EB1-DNA interaction. We also show that in epithelial cells, relocalization and sequestration of Ubn-1 to the tight junctions of nondividing cells allow increased activation of the productive cycle. We propose a model in which Ubn-1 is a modulator of the EBV productive cycle: in proliferating epithelial cells, Ubn-1 is nuclear and inhibits activation of the productive cycle, whereas in differentiated cells, Ubn-1 is sequestrated to tight junctions, thereby allowing EB1 to fully function in the nucleus.

Cellular immediate-early gene expression occurs kinetically upstream of Epstein-Barr virus bzlf1 and brlf1 following cross-linking of the B cell antigen receptor in the Akata Burkitt lymphoma cell line

The Epstein-Barr virus (EBV) lytic activator genes bzlf1 and brlf1 are conventionally referred to as immediate-early (IE) genes. However, previous studies showed that the earliest expression of these genes was blocked by cycloheximide when the EBV lytic cycle was induced by histone deacetylase (HDAC) inhibitors and protein kinase C agonists. Anti-IgG activates a complex signal transduction pathway that leads to EBV lytic activation in the Akata cell line. Here we demonstrate that in Akata cells, where lytic cycle activation occurs very rapidly after anti-IgG treatment, de novo protein synthesis is also required for induction of bzlf1 and brlf1 expression. New protein synthesis is required up to 1.25 h after application of anti-IgG; bzlf1 and brlf1 mRNAs can be detected 1.5 h after anti-IgG. Five cellular IE genes were shown to be expressed by 1 h after addition of anti-IgG, and their expression preceded that of bzlf1 and brlf1. These include early growth response genes (egr1, egr2, and egr3) and nuclear orphan receptors (nr4a1 and nr4a3). These genes were activated by anti-IgG treatment of Akata cells with and without the EBV genome; therefore, their expression was not dependent on expression of any EBV gene product. EGR1, EGR2, and EGR3 proteins were kinetically upstream of ZEBRA and Rta proteins. Expression of EGR1, ZEBRA, and Rta proteins were inhibited by bisindolylmaleimide X, a selective inhibitor of PKC. The findings suggest a revised model in which the signal transduction cascade activated by cross-linking of the B cell receptor induces expression of cellular IE genes, such as early growth response and nuclear orphan receptor genes, whose products, in turn, regulate bzlf1 and brlf1 expression.

The Epstein-Barr virus BZLF1 protein inhibits tumor necrosis factor receptor 1 expression through effects on cellular C/EBP proteins

The Epstein-Barr virus immediate-early protein, BZLF1 (Z), initiates the switch between latent and lytic infection and plays an essential role in mediating viral replication. Z also inhibits expression of the major receptor for tumor necrosis factor (TNF), TNFR1, thus repressing TNF cytokine signaling, but the mechanism for this effect is unknown. Here, we demonstrate that Z prevents both C/EBPα- and C/EBPβ-mediated activation of the TNFR1 promoter (TNFR1p) by interacting directly with both C/EBP family members. We show that Z interacts directly with C/EBPα and C/EBPβ in vivo and that a Z mutant altered at alanine residue 204 in the bZIP domain is impaired for the ability to interact with both C/EBP proteins. Furthermore, we find that the Z(A204D) mutant is attenuated in the ability to inhibit the TNFR1p but mediates lytic viral reactivation and replication in vitro in 293 cells as well as wild-type Z. Although Z does not bind directly to the TNFR1p in EMSA studies, chromatin immunoprecipitation studies indicate that Z is complexed with this promoter in vivo. The Z(A204D) mutant has reduced interaction with the TNFR1p in vivo but is similar to wild-type Z in its ability to complex with the IL-8 promoter. Finally, we show that the effect of Z on C/EBPα- and C/EBPβ-mediated activation is promoter dependent. These results indicate that Z modulates the effects of C/EBPα and C/EBPβ in a promoter-specific manner and that in some cases (including that of the TNFR1p), Z inhibits C/EBPα- and C/EBPβ-mediated activation.

Viral latency and its regulation: lessons from the gamma-herpesviruses

Latency is a state of cryptic viral infection associated with genomic persistence and highly restricted gene expression. Its hallmark is reversibility: under appropriate circumstances, expression of the entire viral genome can be induced, resulting in the production of infectious progeny. Among the small number of virus families capable of authentic latency, the herpesviruses stand out for their ability to produce such infections in every infected individual and for being completely dependent upon latency as a mode of persistence. Here, we review the molecular basis of latency, with special attention to the gamma-herpesviruses, in which the understanding of this process is most advanced.

Epstein-Barr virus LF2 protein regulates viral replication by altering Rta subcellular localization

The switch from Epstein-Barr virus (EBV) latent infection to lytic replication is governed by two viral transactivators, Zta and Rta. We previously reported that the EBV protein LF2 binds Rta, inhibits Rta promoter activation, and blocks EBV replication in cells. In addition, LF2 induces SUMO2/3 modification of Rta. We now show that this modification occurs at four lysines within the Rta activation domain (426, 446, 517, and 530) and that sumoylation of Rta is not essential for its repression. Coexpression studies demonstrated that Rta is sequestered to the extranuclear cytoskeleton in the presence of LF2. We mapped the LF2 binding site to Rta amino acids (aa) 476 to 519 and showed that LF2 binding is critical for Rta relocalization and repression. The core of this binding site, Rta aa 500 to 526, confers LF2-mediated relocalization and repression onto the artificial transcription factor GAL4-VP16. Mutational analysis of LF2 provided further evidence that Rta redistribution is essential for repression. Rta localization changes during replication of the LF2-positive P3HR1 genome, but not during replication of the LF2-negative B95-8 genome. BLRF2 protein expression was decreased and delayed in P3HR1 cells compared with B95-8 cells, consistent with reduced Rta activity. By contrast, BMRF1 expression, regulated primarily by Zta, did not differ significantly between the two cell lines. Our results support a model in which LF2 regulates EBV replication by binding to Rta and redistributing it out of the nucleus.

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.

Viral response to chemotherapy in endemic burkitt lymphoma

PURPOSE

Some EBV-directed therapies are predicted to be effective only when lytic viral replication occurs. We studied whether cyclophosphamide chemotherapy induces EBV to switch from latent to lytic phases of infection in a series of EBV-associated Burkitt lymphomas.

EXPERIMENTAL DESIGN

Children with first presentation of an expanding, solid maxillary or mandibular mass consistent with Burkitt lymphoma underwent fine-needle aspiration just prior to the initiation of cyclophosphamide therapy and again 1 to 5 days later. Aspirated cells were examined for latent and lytic EBV infection using in situ hybridization to EBV-encoded RNA (EBER), immunohistochemical analysis of the lytic EBV proteins BZLF1 and BMRF1, reverse transcription PCR targeting BZLF1 transcripts, and EBV viral load measurement by quantitative PCR.

RESULTS

Among 21 lymphomas expressing EBER prior to chemotherapy, 9 of 10 still expressed EBER on day 1 after therapy whereas only 2 of 11 (18%) specimens still expressed EBER at days 3 to 5, implying that chemotherapy was fairly effective at eliminating latently infected cells. Neither of the lytic products, BZLF1 or BMRF1, were significantly upregulated at the posttherapy time points examined. However, EBV genomic copy number increased in 5 of 10 samples 1 day after treatment began, suggesting that viral replication occurs within the first 24 hours.

CONCLUSION

Cyclophosphamide may induce the lytic phase of EBV infection and is fairly effective in diminishing EBER-expressing tumor cells within 5 days. These findings provide the rationale for a trial testing synergistic tumor cell killing using cyclophosphamide with a drug like ganciclovir targeting lytically infected cells.

Sumoylation of the Epstein-Barr virus BZLF1 protein inhibits its transcriptional activity and is regulated by the virus-encoded protein kinase

The Epstein-Barr virus (EBV) immediate-early protein BZLF1 (Z) mediates the switch between latent and lytic EBV infection. Z not only activates early lytic viral gene transcription but also plays a direct role in lytic viral genome replication. Although a small fraction of Z is known to be sumoylated, the effects of this posttranslational modification on various different Z functions have not been well defined. In this report, we show that only the lysine at amino acid residue 12 is required for the sumoylation of Z, and that Z can be sumoylated by SUMO isoforms 1, 2, and 3. We also demonstrate that the sumo-defective Z mutants ZK12A and ZK12R have enhanced transcriptional activity. The sumoylated and nonsumoylated forms of Z were found to have a similar cellular location, both being localized primarily within the nuclear matrix. The Z sumo-defective mutants were, however, partially defective for disrupting promyelocytic leukemia (PML) bodies compared to the ability of wild-type Z. In addition, we show that lytic viral genome replication does not require the sumoylation of Z, although a Z mutant altered at both amino acids 12 and 13 is replication defective. Furthermore, we show that the sumoylation of Z is greatly increased (from less than 1 to about 11%) in lytically induced 293 cells infected with an EBV mutant virus deleted for the EBV-encoded protein kinase (EBV-PK) compared to that of 293 cells infected with wild-type EBV, and that the overexpression of EBV-PK leads to the reduced sumoylation of Z in EBV-negative cells. Our results suggest that the sumoylation of Z helps to promote viral latency, and that EBV-PK inhibits Z sumoylation during viral reactivation.

Phosphatidylinositol 3-kinase/Akt pathway targets acetylation of Smad3 through Smad3/CREB-binding protein interaction: contribution to transforming growth factor beta1-induced Epstein-Barr virus reactivation

Epstein-Barr virus, a ubiquitous human herpesvirus, is associated with the development of carcinomas and lymphomas. We previously showed that transforming growth factor beta1 (TGF-beta1) mediated the virus to enter the lytic cycle, which is triggered by expression of Z Epstein-Barr virus replication activator (ZEBRA), through the ERK 1/2 MAPK signaling pathway. We report here that Akt, activated downstream from ERK 1/2, was required for TGF-beta1-induced ZEBRA expression and enabled Smad3, a mediator of TGF-beta1 signaling, to be acetylated by direct interaction with the co-activator CREB-binding protein and then to regulate TGF-beta1-induced ZEBRA expression.

Methylation-dependent binding of the epstein-barr virus BZLF1 protein to viral promoters

The switch between latent and lytic Epstein-Barr virus (EBV) infection is mediated by the viral immediate-early (IE) protein, BZLF1 (Z). Z, a homologue of c-jun that binds to AP1-like motifs (ZREs), induces expression of the BRLF1 (R) and BRRF1 (Na) viral proteins, which cooperatively activate transcription of the Z promoter and thereby establish a positive autoregulatory loop. A unique feature of Z is its ability to preferentially bind to, and activate, the methylated form of the BRLF1 promoter (Rp). To date, however, Rp is the only EBV promoter known to be regulated in this unusual manner. We now demonstrate that the promoter driving transcription of the early BRRF1 gene (Nap) has two CpG-containing ZREs (ACGCTCA and TCGCCCG) that are only bound by Z in the methylated state. Both Nap ZREs are highly methylated in cells with latent EBV infection. Z efficiently activates the methylated, but not unmethylated, form of Nap in reporter gene assays, and both ZREs are required. Z serine residue 186, which was previously shown to be required for Z binding to methylated ZREs in Rp, but not for Z binding to the AP1 site, is required for Z binding to methylated Nap ZREs. The Z(S186A) mutant cannot activate methylated Nap in reporter gene assays and does not induce Na expression in cells with latent EBV infection. Molecular modeling studies of Z bound to the methylated Nap ZREs help to explain why methylation is required for Z binding, and the role of the Z Ser186 residue. Methylation-dependent Z binding to critical viral promoters may enhance lytic reactivation in latently infected cells, where the viral genome is heavily methylated. Conversely, since the incoming viral genome is initially unmethylated, methylation-dependent Z activation may also help the virus to establish latency following infection.

In cells with long-term latent Epstein-Barr virus (EBV) infection, the majority of the EBV genome becomes highly methylated. Methylation of cytosines plays a critical role in inhibiting the expression of cellular genes. In contrast, our laboratory previously showed that the EBV protein, BZLF1 (Z), which mediates viral reactivation and replication, preferentially binds to, and activates, the methylated form of the viral BRLF1 promoter. To date, however, BRLF1 is the only EBV promoter known to be activated by Z in this unusual manner. Here, we show that another EBV promoter (Nap, driving transcription of the BRRF1 gene) likewise has two methylation-dependent Z binding sites, and that Z only activates the Nap efficiently in the methylated form. Molecular modeling studies suggest why methylation of the Nap enhances Z binding. Since the BRLF1 and BRRF1 genes encode essential viral transcription factors that work cooperatively with Z to induce the lytic form of viral infection, our results indicate that methylation of the EBV genome enhances Z-mediated disruption of viral latency.

RTA promoter demethylation and histone acetylation regulation of murine gammaherpesvirus 68 reactivation

Gammaherpesviruses have a common biological characteristic, latency and lytic replication. The balance between these two phases in murine gammaherpesvirus 68 (MHV-68) is controlled by the replication and transcription activator (RTA) gene. In this report, we investigated the effect of DNA demethylation and histone acetylation on MHV-68 replication. We showed that distinctive methylation patterns were associated with MHV-68 at the RTA promoter during latency or lytic replication. Treatment of MHV-68 latently-infected S11E cells with a DNA methyltransferases (DNMTs) inhibitor 5-azacytidine (5-AzaC), only weakly reactivated MHV-68, despite resulted in demethylation of the viral RTA promoter. In contrast, treatment with a histone deacetylase (HDAC) inhibitor trichostatin A (TSA) strongly reactivated MHV-68 from latency, and this was associated with significant change in histone H3 and H4 acetylation levels at the RTA promoter. We further showed that HDAC3 was recruited to the RTA promoter and inhibited RTA transcription during viral latency. However, TSA treatment caused rapid removal of HDAC3 and also induced passive demethylation at the RTA promoter. In vivo, we found that the RTA promoter was hypomethylated during lytic infection in the lung and that methylation level increased with virus latent infection in the spleen. Collectively, our data showed that histone acetylation, but not DNA demethylation, is sufficient for effective reactivation of MHV-68 from latency in S11E cells.

Two phenylalanines in the C-terminus of Epstein-Barr virus Rta protein reciprocally modulate its DNA binding and transactivation function

The Rta (R transactivator) protein plays an essential role in the Epstein-Barr viral (EBV) lytic cascade. Rta activates viral gene expression by several mechanisms including direct and indirect binding to target viral promoters, synergy with EBV ZEBRA protein, and stimulation of cellular signaling pathways. We previously found that Rta proteins with C-terminal truncations of 30 aa were markedly enhanced in their capacity to bind DNA (Chen, L.W., Chang, P.J., Delecluse, H.J., and Miller, G., (2005). Marked variation in response of consensus binding elements for the Rta protein of Epstein-Barr virus. J. Virol. 79(15), 9635-9650.). Here we show that two phenylalanines (F600 and F605) in the C-terminus of Rta play a crucial role in mediating this DNA binding inhibitory function. Amino acids 555 to 605 of Rta constitute a functional DNA binding inhibitory sequence (DBIS) that markedly decreased DNA binding when transferred to a minimal DNA binding domain of Rta (aa 1-350). Alanine substitution mutants, F600A/F605A, abolished activity of the DBIS. F600 and F605 are located in the transcriptional activation domain of Rta. Alanine substitutions, F600A/F605A, decreased transcriptional activation by Rta protein, whereas aromatic substitutions, such as F600Y/F605Y or F600W/F605W, partially restored transcriptional activation. Full-length Rta protein with F600A/F605A mutations were enhanced in DNA binding compared to wild-type, whereas Rta proteins with F600Y/F605Y or F600W/F605W substitutions were, like wild-type Rta, relatively poor DNA binders. GAL4 (1-147)/Rta (416-605) fusion proteins with F600A/F605A mutations were diminished in transcriptional activation, relative to GAL4/Rta chimeras without such mutations. The results suggest that, in the context of a larger DBIS, F600 and F605 play a role in the reciprocal regulation of DNA binding and transcriptional activation by Rta. Regulation of DNA binding by Rta is likely to be important in controlling its different modes of action.

Alternatively initiated gene 50/RTA transcripts expressed during murine and human gammaherpesvirus reactivation from latency

In the process of characterizing the requirements for expression of the essential immediate-early transcriptional activator (RTA) encoded by gene 50 of murine gammaherpesvirus 68 (MHV68), a recombinant virus was generated in which the known gene 50 promoter was deleted (G50pKO). Surprisingly, the G50pKO mutant retained the ability to replicate in permissive murine fibroblasts, albeit with slower kinetics than wild-type MHV68. 5'-rapid amplification of cDNA ends analyses of RNA prepared from G50pKO-infected fibroblasts revealed a novel upstream transcription initiation site, which was also utilized during wild-type MHV68 infection of permissive cells. Furthermore, the region upstream of the distal gene 50/RTA transcription initiation site exhibited promoter activity in both permissive NIH 3T12 fibroblasts as well as in the murine macrophage cell line RAW 264.7. In addition, in RAW 264.7 cells the activity of the distal gene 50/RTA promoter was strongly upregulated (>20-fold) by treatment of the cells with lipopolysaccharide. Reverse transcriptase PCR analyses of RNA prepared from Kaposi's sarcoma-associated herpesvirus- and Epstein-Barr virus-infected B-cell lines, following induction of virus reactivation, also revealed the presence of gene 50/RTA transcripts initiating upstream of the known transcription initiation site. The latter argues that alternative initiation of gene 50/RTA transcription is a strategy conserved among murine and human gammaherpesviruses. Infection of mice with the MHV68 G50pKO demonstrated the ability of this mutant virus to establish latency in the spleen and peritoneal exudate cells (PECs). However, the G50pKO mutant was unable to reactivate from latently infected splenocytes and also exhibited a significant reactivation defect from latently infected PECs, arguing in favor of a model where the proximal gene 50/RTA promoter plays a critical role in virus reactivation from latency, particularly from B cells. Finally, analyses of viral genome methylation in the regions upstream of the proximal and distal gene 50/RTA transcription initiation sites revealed that the distal promoter is partially methylated in vivo and heavily methylated in MHV68 latently infected B-cell lines, suggesting that DNA methylation may serve to silence the activity of this promoter during virus latency.

Activation of the ERK signal transduction pathway by Epstein-Barr virus immediate-early protein Rta

BRCA1-associated protein 2 (BRAP2) is known to interact with the kinase suppressor of Ras 1 (KSR1), inhibiting the ERK signal transduction cascade. This study found that an Epstein-Barr virus (EBV) immediate-early protein, Rta, is a binding partner of BRAP2 in yeast and confirmed the binding in vitro by a glutathione S-transferase pull-down assay and in vivo by co-immunoprecipitation in 293(maxi-EBV) cells. Binding studies also showed that Rta and KSR1 interacted with the C-terminal 202 aa region in BRAP2. Additionally, Rta appeared to prevent the binding of KSR1 to BRAP2, activating the ERK signal transduction pathway and the transcription of an EBV immediate-early gene, BZLF1. Activation of the ERK signal transduction pathway by Rta may be critical for the maintenance of the lytic state of EBV.

Alveolar epithelial cell injury with Epstein-Barr virus upregulates TGFbeta1 expression

Idiopathic pulmonary fibrosis (IPF) is a refractory and lethal interstitial lung disease characterized by alveolar epithelial cells apoptosis, fibroblast proliferation, and ECM protein deposition. Epstein-Barr virus (EBV) has previously been localized to alveolar epithelial cells of IPF patients and is associated with a poor prognosis. In this study, we utilized a microarray-based differential gene expression analysis strategy to identify molecular drivers of EBV-associated lung fibrosis. Two cell lines, primary human alveolar epithelial cells type 2 and A549 cells, were infected with EBV. EBV lytic phase induction increased active and total transforming growth factor-beta1 (TGFbeta1) transcript expression in association with reduced cell proliferation and increased caspase 3/7 activity. Exposing EBV-infected cells to ganciclovir resulted in TGFbeta1 deregulation and reduced expression of EBV early response genes, BRLF1 and BZLF1. We targeted the BRLF1 and BZLF1 gene products, Rta and Zta, by silencing RNA, and this resulted in the normalization of TGFbeta1 transcript and cell proliferation levels. Our study using a viral cell line model complements existing human and animal model data and further provides evidence to suggest that viral epithelial cell injury may play a role in IPF.

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.

Histone hyperacetylation occurs on promoters of lytic cycle regulatory genes in Epstein-Barr virus-infected cell lines which are refractory to disruption of latency by histone deacetylase inhibitors

Activation of the Epstein-Barr virus (EBV) lytic cycle is mediated through the combined actions of ZEBRA and Rta, the products of the viral BZLF1 and BRLF1 genes. During latency, these two genes are tightly repressed. Histone deacetylase inhibitors (HDACi) can activate viral lytic gene expression. Therefore, a widely held hypothesis is that Zp and Rp, the promoters for BZLF1 and BRLF1, are repressed by chromatin and that hyperacetylation of histone tails, by allowing the access of positively acting factors, leads to transcription of BZLF1 and BRLF1. To investigate this hypothesis, we used chromatin immunoprecipitation (ChIP) to examine the acetylation and phosphorylation states of histones H3 and H4 on Zp and Rp in three cell lines, Raji, B95-8, and HH514-16, which differ in their response to EBV lytic induction by HDACi. We studied the effects of three HDACi, sodium butyrate (NaB), trichostatin A (TSA), and valproic acid (VPA). We also examined the effects of tetradecanoyl phorbol acetate (TPA) and 5-aza-2'-deoxycytidine, a DNA methyltransferase inhibitor, on histone modification. In Raji cells, TPA and NaB act synergistically to activate the EBV lytic cycle and promote an increase in histone H3 and H4 acetylation and phosphorylation at Zp and Rp. Surprisingly, however, when Raji cells were treated with NaB or TSA, neither of which is sufficient to activate the lytic cycle, an increase of comparable magnitude of hyperacetylated and phosphorylated histone H3 at Zp and Rp was observed. In B95-8 cells, NaB inhibited lytic induction by TPA, yet NaB promoted hyperacetylation of H3 and H4. In HH514-16 cells, NaB and TSA strongly activated the EBV lytic cycle and caused hyperacetylation of histone H3 on Zp and Rp. However, when HH514-16 cells were treated with VPA, lytic cycle mRNAs or proteins were not induced, although histone H3 was hyperacetylated as measured by immunoblotting or by ChIP on Zp and Rp. Taken together, our data suggest that open chromatin at EBV BZLF1 and BRLF1 promoters is not sufficient to activate EBV lytic cycle gene expression.

ZEB1 regulates the latent-lytic switch in infection by Epstein-Barr virus

The immediate-early (IE) BZLF1 gene of Epstein-Barr virus (EBV) regulates the switch between latent and lytic infection by EBV. We previously showed that the cellular transcription factor ZEB1 binds to a sequence element, ZV, located at nt -17 to -12 relative to the transcription initiation site of the BZLF1 promoter, Zp, repressing transcription from Zp in a transient transfection assay. Here, we report the phenotype in the context of a whole EBV genome of a variant of EBV strain B95.8 containing a 2-bp substitution mutation in the ZV element of Zp that reduced, but did not eliminate, ZEB1 binding to Zp. Strikingly, epithelial 293 cells latently infected with the EBV ZV mutant spontaneously produced IE-, early-, and late-gene products and infectious virus, while wild-type (WT)-infected 293 cells did not and have never been reported to do so. Furthermore, treatment with the chemical inducers sodium butyrate and 12-O-tetradecanoyl-phorbol-13-acetate (TPA) led to an additional order-of-magnitude production of infectious virus in the ZV mutant-infected 293 cells, but still no virus in the WT-infected 293 cells. Similarly, ZV mutant-infected Burkitt's lymphoma BJAB cells accumulated at least 10-fold more EBV IE mRNAs than did WT-infected BJAB cells, with TPA or sodium butyrate treatment leading to an additional 5- to 10-fold accumulation of EBV IE mRNAs in the ZV mutant-infected cells. Thus, we conclude that ZEB1 binding to Zp plays a central role in regulating the latent-lytic switch in EBV-infected epithelial and B cells, suggesting ZEB1 as a target for lytic-induction therapies in EBV-associated malignancies.

The ORF49 protein of murine gammaherpesvirus 68 cooperates with RTA in regulating virus replication

Our functional mapping study of murine gammaherpesvirus 68 (MHV-68, or gammaHV-68) revealed that a mutant harboring a transposon at the ORF49 locus (ORF49(null)) evidenced a highly attenuated in vitro growth. ORF49 resides adjacent to and in an opposite direction from RTA, the primary switch of the gammaherpesvirus life cycle. A FLAG-tagged ORF49 protein was able to transcomplement ORF49(null), and a revertant of ORF49(null) restored its attenuated growth to a level comparable to that of the wild type. The FLAG-tagged ORF49 protein promoted the ability of RTA to activate downstream target promoters and enhanced virus replication from the ORF50(null) virus in the presence of RTA. Furthermore, ORF49 enhanced wild-type virus replication by increasing the RTA transcript levels. Our data indicate that ORF49 may perform an important function in MHV-68 replication in cooperation with RTA.

ZEB1 and c-Jun levels contribute to the establishment of highly lytic Epstein-Barr virus infection in gastric AGS cells

The induction of lytic infection has been proposed as a therapeutic strategy for treating Epstein-Barr virus (EBV)-positive malignancies. To succeed, efficient methods are needed for activating the EBV immediate-early (IE) promoters, Zp and Rp. Here we compared factors which regulate Zp and Rp in AGS gastric carcinoma cells that support a remarkably high level of persistently lytic EBV infection with HeLa cervical cells that permit only tightly latent infection. We found that the level of Zp activity assayed by transient transfection assays with reporter plasmids was high in AGS cells but low in HeLa cells. The level of Rp activity was low in both cell types. Mutational analysis indicated that sequences within Zp located between -70 and +27 relative to the transcription initiation site were sufficient to confer a high level of Zp activity in AGS cells. The Zp CRE motif was necessary for this constitutive activity, while the ZIA and ZIB MEF2D motifs were not. Consistent with these findings, immunoblot analysis indicated that phosphorylated c-Jun, which activates Zp through the CRE motif, was expressed at a much higher level in EBV-infected AGS cells than in EBV-infected HeLa cells. In contrast, ZEB1, which represses Zp via the ZV motif located near the transcription initiation site, was abundant in HeLa cells, while it was absent from AGS cells. Exogenous addition of ZEB1 led to the repression of Zp in AGS cells. We conclude that the unusually high Zp activity level in AGS cells is due to the high abundance of positively acting transcription factors such as c-Jun combined with the low abundance of negatively acting factors such as ZEB1.

De novo protein synthesis is required for lytic cycle reactivation of Epstein-Barr virus, but not Kaposi's sarcoma-associated herpesvirus, in response to histone deacetylase inhibitors and protein kinase C agonists

The oncogenic human gammaherpesviruses, Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV), are latent in cultured lymphoma cells. We asked whether reactivation from latency of either virus requires de novo protein synthesis. Using Northern blotting and quantitative reverse transcriptase PCR, we measured the kinetics of expression of the lytic cycle activator genes and determined whether abundance of mRNAs encoding these genes from either virus was reduced by treatment with cycloheximide (CHX), an inhibitor of protein synthesis. CHX blocked expression of mRNAs of EBV BZLF1 and BRLF1, the two EBV lytic cycle activator genes, when HH514-16 Burkitt lymphoma cells were treated with histone deacetylase (HDAC) inhibitors, sodium butyrate or trichostatin A, or a DNA methyltransferase inhibitor, 5-Aza-2'-deoxycytidine. CHX also inhibited EBV lytic cycle activation in B95-8 marmoset lymphoblastoid cells by phorbol ester phorbol-12-myristate-13-acetate (TPA). EBV lytic cycle induction became resistant to CHX between 4 and 6 h after application of the inducing stimulus. KSHV lytic cycle activation, as assessed by ORF50 mRNA expression, was rapidly induced by the HDAC inhibitors, sodium butyrate and trichostatin A, in HH-B2 primary effusion lymphoma cells. In HH-B2 cells, CHX did not inhibit, but enhanced, expression of the KSHV lytic cycle activator gene, ORF50. In BC-1, a primary effusion lymphoma cell line that is dually infected with EBV and KSHV, CHX blocked EBV BRLF1 lytic gene expression induced by TPA and sodium butyrate; KSHV ORF50 mRNA induced simultaneously in the same cells by the same inducing stimuli was resistant to CHX. The experiments show, for the cell lines and inducing agents studied, that the EBV BZLF1 and BRLF1 genes do not behave with "immediate-early" kinetics upon reactivation from latency. KSHV ORF50 is a true "immediate-early" gene. Our results indicate that the mechanism by which HDAC inhibitors and TPA induce lytic cycle gene expression of the two viruses differs and suggest that EBV but not KSHV requires one or more proteins to be newly synthesized between 4 and 6 h after application of an inducing stimulus.

Construction and characterization of monoclonal antibodies specific for the R transactivator 185 of Epstein-Barr virus

Epstein-Barr virus (EBV) has been implicated in the pathogenesis of several human malignancies including B lymphomas and nasopharyngeal carcinoma. The EBV R transactivator (Rta) has been found to play essential roles in stimulating a lytic cycle and viral gene expression. Recently, it was shown that ELISA detecting serum IgG-Rta(150+185) (two internal fragments of Rta) levels may be useful as a serological parameter to assist in the diagnosis of nasopharyngeal carcinoma. The present studies were to prepare monoclonal antibodies specific for the Rta185 and provide a useful tool for the detection of Rta. For this purpose, two monoclonal antibodies (Mabs) specific for the Rta185 were generated. They were identified by Western blot, enzyme-linked immunosorbent assay (ELISA) and immunofluorescence analysis. The results revealed two different immunofluorescence patterns in EBV-positive B cells and epithelial cells, and suggested that there might be a difference in EBV replication mode between B cells and epithelial cells. The Mabs obtained in this study have a potential for the diagnosis of EBV associated diseases.

X-box-binding protein 1 activates lytic Epstein-Barr virus gene expression in combination with protein kinase D

Epstein-Barr virus (EBV) establishes a latent form of infection in memory B cells, while antibody-secreting plasma cells often harbor the lytic form of infection. The switch between latent and lytic EBV infection is mediated by the two viral immediate-early proteins BZLF1 (Z) and BRLF1 (R), which are not expressed in latently infected B cells. Here we demonstrate that a cellular transcription factor that plays an essential role in plasma cell differentiation, X-box-binding protein 1 (XBP-1), also activates the transcription of the two EBV immediate-early gene promoters. In reporter gene assays, XBP-1 alone was sufficient to activate the R promoter, whereas the combination of XBP-1 and protein kinase D (PKD) was required for efficient activation of the Z promoter. Most importantly, the expression of XBP-1 and activated PKD was sufficient to induce lytic viral gene expression in EBV-positive nasopharyngeal carcinoma cells and lymphoblastoid cells, while an XBP-1 small interfering RNA inhibited constitutive lytic EBV gene expression in lymphoblastoid cells. These results suggest that the plasma cell differentiation factor XBP-1, in combination with activated PKD, can mediate the reactivation of EBV, thereby allowing the viral life cycle to be intimately linked to plasma cell differentiation.

The Rta/Orf50 transactivator proteins of the gamma-herpesviridae

The replication and transcription activator protein, Rta, is encoded by Orf50 in Kaposi's sarcoma-associated herpesvirus (KSHV) and other known gammaherpesviruses including Epstein-Barr virus (EBV), rhesus rhadinovirus (RRV), herpesvirus saimiri (HVS), and murine herpesvirus 68 (MHV-68). Each Rta/Orf50 homologue of each gammaherpesvirus plays a pivotal role in the initiation of viral lytic gene expression and lytic reactivation from latency. Here we discuss the Rta/Orf50 of KSHV in comparison to the Rta/Orf50s of other gammaherpesviruses in an effort to identify structural motifs, mechanisms of action, and modulating host factors.

The porcine lymphotropic herpesvirus 1 encodes functional regulators of gene expression

The porcine lymphotropic herpesviruses (PLHV) are discussed as possible risk factors in xenotransplantation because of the high prevalence of PLHV-1, PLHV-2 and PLHV-3 in pig populations world-wide and the fact that PLHV-1 has been found to be associated with porcine post-transplant lymphoproliferative disease. To provide structural and functional knowledge on the PLHV immediate-early (IE) transactivator genes, the central regions of the PLHV genomes were characterized by genome walking, sequence and splicing analysis. Three spliced genes were identified (ORF50, ORFA6/BZLF1(h), ORF57) encoding putative IE transactivators, homologous to (i) ORF50 and BRLF1/Rta, (ii) K8/K-bZIP and BZLF1/Zta and (iii) ORF57 and BMLF1 of HHV-8 and EBV, respectively. Expressed as myc-tag or HA-tag fusion proteins, they were located to the cellular nucleus. In reporter gene assays, several PLHV-promoters were mainly activated by PLHV-1 ORF50, to a lower level by PLHV-1 ORFA6/BZLF1(h) and not by PLHV-1 ORF57. However, the ORF57-encoded protein acted synergistically on ORF50-mediated activation.

A gammaherpesvirus 68 gene 50 null mutant establishes long-term latency in the lung but fails to vaccinate against a wild-type virus challenge

The gammaherpesvirus immediate-early genes are critical regulators of virus replication and reactivation from latency. Rta, encoded by gene 50, serves as the major transactivator of the lytic program and is highly conserved among all the gammaherpesviruses, including Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, and murine gammaherpesvirus 68 (gammaHV68). Introduction of a translation stop codon in gammaHV68 gene 50 (gene 50.stop gammaHV68) demonstrated that Rta is essential for virus replication in vitro. To investigate the role that virus replication plays in the establishment and maintenance of latency, we infected mice with gene 50.stop gammaHV68. Notably, the gene 50.stop virus established a long-term infection in lung B cells following intranasal infection of mice but was unable to establish latency in the spleen. This complete block in the establishment of latency in the spleen was also seen when lytic virus production was inhibited by treating mice infected with wild-type virus with the antiviral drug cidofovir, implicating virus replication and not an independent function of Rta in the establishment of splenic latency. Furthermore, we showed that gene 50.stop gammaHV68 was unable to prime the immune system and was unable to protect against a challenge with wild-type gammaHV68, despite its ability to chronically infect lung B cells. These data indicate gammaherpesviruses that are unable to undergo lytic replication in vivo may not be viable vaccine candidates despite the detection of cells harboring viral genome at late times postinfection.

Contribution of C/EBP proteins to Epstein-Barr virus lytic gene expression and replication in epithelial cells

The contribution of C/EBP proteins to Epstein-Barr virus (EBV) lytic gene expression and replication in epithelial cells was examined. Nasopharyngeal carcinoma cell lines constitutively expressed C/EBPbeta and had limited C/EBPalpha expression, while the AGS gastric cancer cell line expressed significant levels of both C/EBPalpha and C/EBPbeta. Induction of the lytic cycle in EBV-positive AGS/BX1 cells with phorbol ester and sodium butyrate treatment led to a transient stimulation of C/EBPbeta expression and a prolonged increase in C/EBPalpha expression. In AGS/BX1 cells, endogenous C/EBPalpha and C/EBPbeta proteins were detected associated with the ZTA and oriLyt promoters but not the RTA promoter. Electrophoretic mobility shift assays confirmed binding of C/EBP proteins to multiple sites in the ZTA and oriLyt promoters. The response of these promoters in reporter assays to transfected C/EBPalpha and C/EBPbeta proteins was consistent with the promoter binding assays and emphasized the relative importance of C/EBPs for activation of the ZTA promoter. Mutation of the oriLyt promoter proximal C/EBP site had little effect on ZTA activation of the promoter in a reporter assay. However, this mutation impaired oriLyt DNA replication, suggesting a separate replication-specific contribution for C/EBP proteins. Finally, the overall importance of C/EBP proteins for lytic gene expression was demonstrated using CHOP10 to antagonize C/EBP DNA binding activity. Introduction of CHOP10 significantly impaired induction of the ZTA, RTA, and BMRF1 proteins in chemically treated AGS/BX1 cells. Thus, C/EBPbeta and C/EBPalpha expression are associated with lytic induction in AGS cells, and expression of C/EBP proteins in epithelial cells may contribute to the tendency of these cells to exhibit constitutive low-level ZTA promoter activity.

Effects of SUMO-1 upon Epstein-Barr virus BZLF1 function and BMRF1 expression

Epstein-Barr virus (EBV) is a human herpesvirus that has infected at least 90% of the world population. This very successful virus causes infectious mononucleosis and is associated with many different types of cancer. The EBV BZLF1 protein is a transcription factor that has also been shown to interact with many host cell proteins and pathways. BZLF1 (Z) is tagged by the small ubiquitin-related modifier-1 (SUMO-1) protein. Here, we present studies of the functional consequences of SUMO-1 modification of Z. We found that SUMO-1 modification of Z has no apparent effect upon the stability and localization of the Z protein. We did find, however, that SUMO-1 modification decreases the transactivation activity of Z on specific promoters. In addition, when SUMO-1 is supplied to cells when lytic replication is induced, EBV BMRF1 levels greatly increase, suggesting that SUMO-1 enhances EBV lytic replication. Therefore, SUMO-1 modification of proteins appears to have an important role in EBV lytic replication.

Marked variation in response of consensus binding elements for the Rta protein of Epstein-Barr virus

The R transactivator (Rta) protein activates Epstein-Barr virus (EBV) lytic-cycle genes by several distinct mechanisms that include direct binding to viral promoters, synergy with BamHI Z EBV replication activator (ZEBRA), and activation of cellular signaling pathways. In the direct and synergistic mechanisms of action, Rta binds to specific DNA sequences that are present in the promoters of responsive genes. It has been difficult to demonstrate the capacity of Rta expressed in mammalian cells to bind DNA in vitro in order to study the relative affinities of Rta binding elements. We discovered that a short C-terminal region of Rta inhibits the ability of Rta to bind DNA in vitro. C-terminally truncated versions of Rta bind DNA efficiently and thus facilitate a comparison of consensus Rta binding elements (CRBEs) found in promoters of five Rta-responsive genes: BMLF1, BHLF1, BMRF1, BaRF1, and BLRF2. All CRBEs in the promoters of the five genes conform to the proposed recognition sequence GNCCN9GGNG, where N is any nucleotide and N9 represents a sequence of nine nucleotides. Nonetheless, CRBEs varied markedly in their abilities to bind Rta in electrophoretic mobility shift assays. Not all CRBEs bound or responded to Rta. Binding affinities of the CRBEs and the capacity to be activated by Rta in reporter assays were strongly correlated. The CRBEs from the BMLF1 and BHLF1 promoters conferred the greatest response. The response of the BMRF1, BaRF1, and BLRF2 CRBEs was less robust. By creation of chimeras, inversions, and point mutations, differences in binding affinities and transcriptional activation levels could be attributed to N9 sequence variation. The length of N9 was also critical for a maximal response. In Raji and BZLF1-knockout cells, the mRNAs of the five Rta-responsive lytic-cycle genes differed dramatically in kinetics of expression, abundance, and synergistic responses to ZEBRA and Rta. Affinities of Rta response elements for Rta are likely to play an important role in temporal regulation and the level of lytic-cycle EBV gene expression.

BZLF1 activation of the methylated form of the BRLF1 immediate-early promoter is regulated by BZLF1 residue 186

The Epstein-Barr virus (EBV) genome is highly methylated in latently infected cells. We recently reported that the EBV immediate-early (IE) protein BZLF1 (Z) preferentially binds to and activates transcription from the methylated form of the BRLF1 IE gene promoter (Rp). We now report that serine residue 186 in the Z DNA-binding domain plays an important role in the ability of Z to bind to and activate methylated Rp. A Z mutant containing an alanine residue at position 186 [Z(S186A)] was significantly defective in binding to methylated, as well as unmethylated, ZREs (Z-responsive elements) in Rp and was unable to activate lytic EBV gene transcription from the methylated or demethylated form of the viral genome. A Z mutant containing threonine at residue 186 [Z(S186T)] bound only to the methylated form of the ZRE-2 site in Rp and induced lytic EBV gene transcription from the methylated, but not demethylated, form of the viral genome. The defect in both of these mutants was primarily due to an inability to activate the Rp in the context of the viral genome. Finally, a Z mutant containing an aspartic acid at position 186 [Z(S186D)] did not bind to either the consensus AP-1 site or to the methylated or unmethylated Rp ZRE-2 site and did not induce lytic gene transcription. These results indicate that replacement of serine with threonine at residue 186 in the Z DNA-binding domain differentially affects its ability to reactivate the unmethylated, versus methylated, viral genome.

Epstein-Barr virus BZLF1 protein binds to mitotic chromosomes

Epstein-Barr virus (EBV) is a human herpesvirus that causes infectious mononucleosis and is associated with several types of cancers, including nasopharyngeal carcinoma and Burkitt's lymphoma. An EBV protein that plays an integral role during lytic replication is the immediate-early protein BZLF1. Our laboratory has found that BZLF1 (Z) localizes to host chromosomes during mitosis. Two Z-interacting proteins are also found localized to mitotic chromosomes in the presence of Z. The association between Z and mitotic chromosomes may lead to the sequestering of Z-interacting proteins within the cell and potentially cause an alteration of chromosome compaction or chromatin structure.

A11 Tetramer-assisted characterization of Rta-specific CD8+ T-cell responses in healthy virus carriers

HLA Class I-restricted CD8(+) T-cell responses are believed to play an important role in controlling Epstein-Barr virus (EBV) infection, which has been consistently associated with nasopharyngeal carcinoma (NPC). Immediate early transactivator Rta of EBV has been shown to be associated with the reactivation of EBV from latency and drive the lytic cascade of EBV and comprise an important target for EBV-specific cellular cytotoxicity. Furthermore, BRLF1 is specifically expressed in NPC tumor cells. The protein product of BRLF1, Rta, could then be considered as a NPC tumor antigen. Therefore, cellular immunity against Rta represents a very important part of the immunity against NPC, as they should prevent the replication of EBV. In the present study, Rta-specific CD8(+) T-cell responses in healthy virus carriers were characterized by using A1101 tetramer containing the known Rta epitope ATIGTAMYK (134-142). We clearly showed A1101/ATIGTAMYK tetramer-reactive CD8(+) T cells in the circulation of healthy virus carriers, ranging from 2.13 to 9.03%. We then studied the expression of perforin and interferon-gamma (IFN-gamma) secretion in these Rta-specific T cells. Our study demonstrated that Rta-specific T cells are capable of IFN-gamma production and nearly 90% of the Rta-specific CD8(+) T cells expressed perforin. Presumably, these are the cells that play an important role in determining the initiation of the lytic cycle or the clearance of EBV.

Cell transformation by Herpesvirus saimiri

Herpesvirus saimiri (Saimiriine herpesvirus-2), a gamma2-herpesvirus (rhadinovirus) of non-human primates, causes T-lymphoproliferative diseases in susceptible organisms and transforms human and non-human T lymphocytes to continuous growth in vitro in the absence of stimulation. T cells transformed by H. saimiri retain many characteristics of intact T lymphocytes, such as the sensitivity to interleukin-2 and the ability to recognize the corresponding antigens. As a result, H. saimiri is widely used in immunobiology for immortalization of various difficult-to-obtain and/or -to-maintain T cells in order to obtain useful experimental models. In particular, H. saimiri-transformed human T cells are highly susceptible to infection with HIV-1 and -2. This makes them a convenient tool for propagation of poorly replicating strains of HIV, including primary clinical isolates. Therefore, the mechanisms mediating transformation of T cells by H. saimiri are of considerable interest. A single transformation-associated protein, StpA or StpB, mediates cell transformation by H. saimiri strains of group A or B, respectively. Strains of group C, which exhibit the highest oncogenic potential, have two proteins involved in transformation-StpC and Tip. Both proteins have been shown to dramatically affect signal transduction pathways leading to the activation of crucial transcription factors. This review is focused on the biological effects and molecular mechanisms of action of proteins involved in H. saimiri-dependent transformation.

Ex vivo stimulation of B cells latently infected with gammaherpesvirus 68 triggers reactivation from latency

Murine gammaherpesvirus 68 (gammaHV68) infection of mice results in the establishment of a chronic infection, which is largely maintained through latent infection of B lymphocytes. Acute virus replication is almost entirely cleared by 2 weeks postinfection. Spontaneous reactivation of gammaHV68 from latently infected splenocytes upon ex vivo culture can readily be detected at the early stages of infection (e.g., day 16). However, by 6 weeks postinfection, very little spontaneous reactivation is detected upon explant into tissue culture. Here we report that stimulation of latently infected splenic B cells harvested at late times postinfection with cross-linking surface immunoglobulin (Ig), in conjunction with anti-CD40 antibody treatment, triggers virus reactivation. As expected, this treatment resulted in B-cell activation, as assessed by upregulation of CD69 on B cells, and ultimately B-cell proliferation. Since anti-Ig/anti-CD40 stimulation resulted in splenic B-cell proliferation, we assessed whether this reactivation stimulus could overcome the previously characterized defect in virus reactivation of a v-cyclin null gammaHV68 mutant. This analysis demonstrated that anti-Ig/anti-CD40 stimulation could drive reactivation of the v-cyclin null mutant virus in latently infected splenocytes, but not to the levels observed with wild-type gammaHV68. Thus, there appears to be a role for the v-cyclin in B cells following anti-Ig/anti-CD40 stimulation independent of the induction of the cell cycle. Finally, to assess signals that are not mediated through the B-cell receptor, we demonstrate that addition of lipopolysaccharide to explanted splenocyte cultures also enhanced virus reactivation. These studies complement and extend previous analyses of Epstein-Barr virus and Kaposi's sarcoma-associated virus reactivation from latently infected cell lines by investigating reactivation of gammaHV68 from latently infected primary B cells recovered from infected hosts.

Identification of CD8+ T-Cell Epitopes Specific for Immediate-Early Transactivator Rta of Epstein-Barr Virus

Nasopharyngeal carcinoma (NPC) is a human epithelial tumor with a high incidence in Southern Chinese population, with contributions from Epstein-Barr virus (EBV), human leukocyte antigen (HLA), and environmental factors to its etiology. It has been shown previously that the recognition of immediate-early transactivator Rta of EBV by CD8+ T cells may have a significant impact on controlling EBV and, indirectly, NPC. The current study used two computer-aided prediction methods and competition-based HLA-peptide binding assays to screen for HLA B2704/B4601/B5801 restricted T-cell epitopes derived from Rta. HLA tetrameric complexes containing these potential T-cell epitopes were synthesized. Rta-specific CD8+ T-cell responses in healthy virus carriers were then defined by these tetramers and IFN-gamma ELISPOT assays. We clearly demonstrated that healthy virus carriers have detectable Rta-specific CD8+ T cells restricted by B2704 in the circulation. However, there were no B4601/B5801 tetramer-reactive T cells specific for Rta in the peripheral blood of matched/mismatched donors. On the other hand, B4601 tetramers containing the computer-predicted B4601 binder EBNA3A (318-326) showed detectable tetramer-reactive T cells in the circulation of healthy virus carriers. topes also elicited IFN-gamma responses as detected by ELISPOT.

Murine gammaherpesvirus 68 Rta-dependent activation of the gene 57 promoter

The Rta homolog encoded by murine gammaherpesvirus 68 (gammaHV68) gene 50 is essential for virus replication and is capable of driving virus reactivation from the S11 latently infected B lymphoma cell line. Here we characterize Rta activation of gammaHV68 gene 57, which is abundantly transcribed during the early phase of virus replication. Infection of murine fibroblasts with an Rta null virus demonstrated that transcription of gene 57 is dependent on Rta expression. Analysis of the gene 57 promoter identified 2 distinct regions that are Rta responsive, either in the context of the gene 57 promoter or when cloned upstream of a heterologous promoter. Sequence analysis of these regions revealed homology to known Rta-responsive cis-elements in the closely related Kaposi's sarcoma-associated viral (KSHV) genome. In addition, two candidate binding sites for the cellular transcription factor RBP-Jkappa/CBF1 were also identified in one of the Rta-responsive regions, which may play a role in mediating Rta transactivation similar to that observed in some KSHV Rta-responsive genes. Overall, analysis of the gammaHV68 gene 57 promoter suggests that mechanisms of Rta activation are conserved among gamma2-herpesviruses.

Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 RTA reactivates murine gammaherpesvirus 68 from latency

Murine gammaherpesvirus 68 (MHV-68), Kaposi's sarcoma-associated herpesvirus (HHV-8), and Epstein-Barr virus (EBV) are all members of the gammaherpesvirus family, characterized by their ability to establish latency in lymphocytes. The RTA protein, conserved in all gammaherpesviruses, is known to play a critical role in reactivation from latency. Here we report that HHV-8 RTA, not EBV RTA, was able to induce MHV-68 lytic viral proteins and DNA replication and processing and produce viable MHV-68 virions from latently infected cells at levels similar to those for MHV-68 RTA. HHV-8 RTA was also able to activate two MHV-68 lytic promoters, whereas EBV RTA was not. In order to define the domains of RTA responsible for their functional differences in viral promoter activation and initiation of the MHV-68 lytic cycle, chimeric RTA proteins were constructed by exchanging the N-terminal and C-terminal domains of the RTA proteins. Our data suggest that the species specificity of MHV-68 RTA resides in the N-terminal DNA binding domain.

The herpesvirus saimiri Rta gene autostimulates via binding to a non-consensus response element

Herpesvirus saimiri ORF 50a protein expression is sufficient to reactivate the entire lytic-replication cycle. ORF 50a functions as a sequence-specific transactivator that is capable of activating delayed-early gene expression via direct binding to an ORF 50 response element (RE) within the respective promoter. Here, it is shown that ORF 50a is capable of transactivating its own promoter. Deletion analysis of the ORF 50a promoter showed that the ORF 50-responsive element is contained within an 80 bp fragment, situated 293–373 bp from the transcription initiation site. Gel-retardation analysis further mapped the RE to a 34 bp fragment that was able to confer ORF 50 responsiveness to an enhancerless SV40 minimal promoter. Sequence analysis showed that this RE has no direct similarity to previously identified ORF 50 REs. Therefore, it is concluded that ORF 50a is capable of stimulating its own promoter via a novel RE.