Epstein-Barr virus immediate-early proteins BZLF1 and BRLF1 activate the ATF2 transcription factor by increasing the levels of phosphorylated p38 and c-Jun N-terminal kinases

Uses for JNK: the many and varied substrates of the c-Jun N-terminal kinases

The c-Jun N-terminal kinases (JNKs) are members of a larger group of serine/threonine (Ser/Thr) protein kinases from the mitogen-activated protein kinase family. JNKs were originally identified as stress-activated protein kinases in the livers of cycloheximide-challenged rats. Their subsequent purification, cloning, and naming as JNKs have emphasized their ability to phosphorylate and activate the transcription factor c-Jun. Studies of c-Jun and related transcription factor substrates have provided clues about both the preferred substrate phosphorylation sequences and additional docking domains recognized by JNK. There are now more than 50 proteins shown to be substrates for JNK. These include a range of nuclear substrates, including transcription factors and nuclear hormone receptors, heterogeneous nuclear ribonucleoprotein K, and the Pol I-specific transcription factor TIF-IA, which regulates ribosome synthesis. Many nonnuclear substrates have also been characterized, and these are involved in protein degradation (e.g., the E3 ligase Itch), signal transduction (e.g., adaptor and scaffold proteins and protein kinases), apoptotic cell death (e.g., mitochondrial Bcl2 family members), and cell movement (e.g., paxillin, DCX, microtubule-associated proteins, the stathmin family member SCG10, and the intermediate filament protein keratin 8). The range of JNK actions in the cell is therefore likely to be complex. Further characterization of the substrates of JNK should provide clearer explanations of the intracellular actions of the JNKs and may allow new avenues for targeting the JNK pathways with therapeutic agents downstream of JNK itself.

Epstein-Barr Virus Induces Cellular Transcription Factors to Allow Active Expression of EBER Genes by RNA Polymerase III

The EBER genes of Epstein-Barr virus (EBV) are transcribed by RNA polymerase (pol) III to produce untranslated RNAs that are implicated in oncogenesis. These EBER transcripts are the most highly expressed viral gene products in EBV-transformed cells. We have identified changes to the cellular transcription machinery that may contribute to the high levels of EBER RNA. These include phosphorylation of ATF2, which interacts with EBER promoters. A second is induction of TFIIIC, a pol III-specific factor that activates EBER genes; all five subunits of TFIIIC are overexpressed in EBV-positive cells. In addition, EBV induces BDP1, a subunit of the pol III-specific factor TFIIIB. Although BDP1 is the only TFIIIB subunit induced by EBV, its induction is sufficient to stimulate EBER expression in vivo, implying a limiting function. The elevated levels of BDP1 and TFIIIC in EBV-positive cells stimulate production of tRNA, 7SL, and 5S rRNA. Abnormally high expression of these cellular pol III products may contribute to the ability of EBV to enhance growth potential.

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

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

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.

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

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

Cells lytically infected with Epstein-Barr virus are detected and separable by immunoglobulins from EBV-seropositive individuals

The role of Epstein-Barr virus (EBV) lytic cycle gene expression in lymphocytes in the pathogenesis of EBV-associated diseases is incompletely understood. The ability to physically separate lytically induced from latently infected cells from the same population and to examine them in parallel would significantly enhance understanding of the viral, cellular, and environmental factors that govern susceptibility of an EBV-infected cell to lytic cycle induction stimuli at the single cell level. This study demonstrates, using a flow cytometry-based system, that human serum immunoglobulins from individuals immune to EBV reproducibly discriminate between and can be used to physically separate lytically induced from latently infected B cells. Using this new quantitative and sensitive technique, two novel observations about lytic cycle activation were made. First, the kinetics of lytic cycle activation by histone deacetylase inhibitors is more rapid than induction by a DNA methyl transferase inhibitor. Second, butyrate-treated cells which are initially refractory to lytic cycle activation can be induced upon subsequent exposure to the inducing agent. Therefore, susceptibility to lytic cycle induction of a latently infected B cell is governed by environmental and physiologic factors and not by permanent cellular or viral genetic changes.

Structural basis of lytic cycle activation by the Epstein-Barr virus ZEBRA protein

Epstein-Barr virus (EBV) causes infectious mononucleosis and is linked to several human malignancies. EBV has a biphasic infection cycle consisting of a latent and a lytic, replicative phase. The switch from latent to lytic infection is triggered by the EBV immediate-early transcription factor ZEBRA (BZLF1, Zta, Z, EB1). We present the crystal structure of ZEBRA's DNA binding domain bound to an EBV lytic gene promoter element. ZEBRA exhibits a variant of the basic-region leucine zipper (bZIP) fold in which a C-terminal moiety stabilizes the coiled coil involved in dimer formation. The structure provides insights into ZEBRA's broad target site specificity, preferential activation of specific EBV promoters in their methylated state, ability to dimerize despite lacking a leucine zipper motif, and failure to heterodimerize with cellular bZIP proteins. The structure will allow for the design of new therapeutic agents that block activation of the EBV lytic cycle.

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.

Epstein-Barr virus lytic infection is required for efficient production of the angiogenesis factor vascular endothelial growth factor in lymphoblastoid cell lines

Although Epstein-Barr virus (EBV)-associated malignancies are primarily composed of cells with one of the latent forms of EBV infection, a small subset of tumor cells containing the lytic form of infection is often observed. Whether the rare lytically infected tumor cells contribute to the growth of the latently infected tumor cells is unclear. Here we have investigated whether the lytically infected subset of early-passage lymphoblastoid cell lines (LCLs) could potentially contribute to tumor growth through the production of angiogenesis factors. We demonstrate that supernatants from early-passage LCLs infected with BZLF1-deleted virus (Z-KO LCLs) are highly impaired in promoting endothelial cell tube formation in vitro compared to wild-type (WT) LCL supernatants. Furthermore, expression of the BZLF1 gene product in trans in Z-KO LCLs restored angiogenic capacity. The supernatants of Z-KO LCLs, as well as supernatants from LCLs derived with a BRLF1-deleted virus (R-KO LCLs), contained much less vascular endothelial growth factor (VEGF) in comparison to WT LCLs. BZLF1 gene expression in Z-KO LCLs restored the VEGF level in the supernatant. However, the cellular level of VEGF mRNA was similar in Z-KO, R-KO, and WT LCLs, suggesting that lytic infection may enhance VEGF translation or secretion. Interestingly, a portion of the vasculature in LCL tumors in SCID mice was derived from the human LCLs. These results suggest that lytically infected cells may contribute to the growth of EBV-associated malignancies by enhancing angiogenesis. In addition, as VEGF is a pleiotropic factor with effects other than angiogenesis, lytically induced VEGF secretion may potentially contribute to viral pathogenesis.

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

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

Activation of Sp1-mediated transcription by Rta of Epstein-Barr virus via an interaction with MCAF1

Rta is a transcription factor encoded by BRLF1 of the Epstein–Barr virus (EBV). This factor is expressed during the immediate-early stage of the lytic cycle to activate the genes required for EBV lytic development. Although transcription activation by Rta is frequently associated with the binding of Rta to the Rta-response element (RRE) in promoters, Rta sometimes activates promoters without an RRE. Here we show that Rta interacts with an Sp1-interacting protein, MBD1-containing chromatin-associated factor 1 (MCAF1). This interaction is critical to the formation of an Sp1–MCAF1–Rta complex at Sp1 sites. Therefore, following lytic induction and the expression of Rta, Rta increases Sp1-mediated transcription. The genes that are thus activated include p16, p21, SNRPN and BRLF1. However, the binding of Rta to RRE prevents the interaction between Rta and MCAF1; therefore, transcription activation by RRE depends only on Rta, and not on MCAF1 or Sp1. Furthermore, this study finds that MCAF1 promotes the expression of Rta and Zta from EBV, indicating that MCAF1 participates EBV lytic activation. Our study documents the critical role of Rta in regulating the transcription of the genes that are mediated by Sp1.

Functional interactions between the Epstein-Barr virus BZLF1 protein and the promyelocytic leukemia protein

The Epstein-Barr virus immediate-early protein BZLF1 (Z) has been shown to alter the cellular localization of the promyelocytic leukemia (PML) protein. PML has important implications for growth control, apoptosis, anti-viral effects and many more processes. Here we further examined the relationship between PML and the Epstein-Barr virus Z protein. We examined the effect of Z expression on PML protein levels, and the effect of increased PML protein levels on Z-mediated dispersion of PML bodies. We found that increased levels of PML protein, such as through interferon treatment, were able to suppress Z-mediated PML body dispersion. We also studied the consequences of PML dispersion by Z, by examining p21 transactivation, A20 transactivation, and MHC Class I presentation levels in Z-expressing cells. We found that, while Z-mediated dispersion of PML did not affect MHC Class I presentation, it did alter p21 and A20 expression. In addition, we found that increased levels of PML were able to prevent Z protein binding to mitotic chromosomes. Our work implies that the balance of PML and Z levels in cells may affect how each protein functions.

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.

Cellular uptake of the EBV transcription factor EB1/Zta

A number of viral proteins have the property to penetrate into the cells when present in the extra-cellular compartment. Here, we report that the Epstein-Barr virus (EBV) transcriptional activator EB1/Zta, which is responsible for the activation of the EBV lytic replication, binds to lymphoid cells surface, is efficiently translocated and accumulates in the nucleus. The internalization of EB1/Zta is energy-dependent and shares common features with endocytosis. As the EB1/Zta was not degraded in the cells and reached the nucleus, the potential effect of its internalisation on viral reactivation was assessed.

Herpes simplex virus ICP27 activation of stress kinases JNK and p38

We previously reported that herpes simplex virus type 1 (HSV-1) can activate the stress-activated protein kinases (SAPKs) p38 and JNK. In the present study, we undertook a comprehensive and comparative analysis of the requirements for viral protein synthesis in the activation of JNK and p38. Infection with the UL36 mutant tsB7 or with UV-irradiated virus indicated that both JNK and p38 activation required viral gene expression. Cycloheximide reversal or phosphonoacetic acid treatment of wild-type virus-infected cells as well as infection with the ICP4 mutant vi13 indicated that only the immediate-early class of viral proteins were required for SAPK activation. Infection with ICP4, ICP27, or ICP0 mutant viruses indicated that only ICP27 was necessary. Additionally, we determined that in the context of virus infection ICP27 was sufficient for SAPK activation and activation of the p38 targets Mnk1 and MK2 by infecting with mutants deleted for various combinations of immediate-early proteins. Specifically, the d100 (0-/4-) and d103 (4-/22-/47-) mutants activated p38 and JNK, while the d106 (4-/22-/27-/47-) and d107 (4-/27-) mutants did not. Finally, infections with a series of ICP27 mutants demonstrated that the functional domain of ICP27 required for activation was located in the region encompassing amino acids 20 to 65 near the N terminus of the protein and that the C-terminal transactivation activity of ICP27 was not necessary.

Interferon regulatory factor 7 is negatively regulated by the Epstein-Barr virus immediate-early gene, BZLF-1

Virus infection stimulates potent antiviral responses; specifically, Epstein-Barr virus (EBV) infection induces and activates interferon regulatory factor 7 (IRF-7), which is essential for production of alpha/beta interferons (IFN-alpha/beta) and upregulates expression of Tap-2. Here we present evidence that during cytolytic viral replication the immediate-early EBV protein BZLF-1 counteracts effects of IRF-7 that are central to host antiviral responses. We initiated these studies by examining IRF-7 protein expression in vivo in lesions of hairy leukoplakia (HLP) in which there is abundant EBV replication but the expected inflammatory infiltrate is absent. This absence might predict that factors involved in the antiviral response are absent or inactive. First, we detected significant levels of IRF-7 in the nucleus, as well as in the cytoplasm, of cells in HLP lesions. IRF-7 activity in cell lines during cytolytic viral replication was examined by assay of the IRF-7-responsive promoters, IFN-alpha4, IFN-beta, and Tap-2, as well as of an IFN-stimulated response element (ISRE)-containing reporter construct. These reporter constructs showed consistent reduction of activity during lytic replication. Both endogenous and transiently expressed IRF-7 and EBV BZLF-1 proteins physically associate in cell culture, although BZLF-1 had no effect on the nuclear localization of IRF-7. However, IRF-7-dependent activity of the IFN-alpha4, IFN-beta, and Tap-2 promoters, as well as an ISRE promoter construct, was inhibited by BZLF-1. This inhibition occurred in the absence of other EBV proteins and was independent of IFN signaling. Expression of BZLF-1 also inhibited activation of IRF-7 by double-stranded RNA, as well as the activity of a constitutively active mutant form of IRF-7. Negative regulation of IRF-7 by BZLF-1 required the activation domain but not the DNA-binding domain of BZLF-1. Thus, EBV may subvert cellular antiviral responses and immune detection by blocking the activation of IFN-alpha4, IFN-beta, and Tap-2 by IRF-7 through the medium of BZLF-1 as a negative regulator.

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.

Modeling early Epstein-Barr virus infection in Drosophila melanogaster: the BZLF1 protein

Epstein-Barr virus (EBV) is the causative agent of infectious mononucleosis and is associated with several forms of cancer, including lymphomas and nasopharyngeal carcinoma. The EBV immediate-early protein BZLF1 functions as a transcriptional activator of EBV early gene expression and is essential for the viral transition between latent and lytic replication. In addition to its role in the EBV life cycle, BZLF1 (Z) also has profound effects upon the host cellular environment, including disruption of cell cycle regulation, signal transduction pathways, and transcription. In an effort to understand the nature of Z interactions with the host cellular environment, we have developed a Drosophila model of early EBV infection, where we have expressed Z in the Drosophila eye. Using this system, we have identified a highly conserved interaction between the Epstein-Barr virus Z protein and shaven, a Drosophila homolog of the human Pax2/5/8 family of genes. Pax5 is a well-characterized human gene involved with B-cell development. The B-cell-specific Pax5 also promotes the transcription of EBV latent genes from the EBV Wp promoter. Our work clearly demonstrates that the Drosophila system is an appropriate and powerful tool for identifying the underlying genetic networks involved in human infectious disease.

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

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

Reactivation of latent Epstein-Barr virus by methotrexate: a potential contributor to methotrexate-associated lymphomas

BACKGROUND

Patients with rheumatoid arthritis or polymyositis treated with methotrexate (MTX) develop Epstein-Barr virus (EBV)-positive lymphomas more frequently than patients treated with other, equally immunosuppressive regimens. Here we determined whether MTX, in contrast to other commonly used medications for rheumatoid arthritis or polymyositis, is unique in its ability to induce the release of infectious EBV from latently infected cells.

METHODS

The effect of MTX and other immunosuppressant drugs on EBV replication in vitro was assessed using latently infected EBV-positive lymphoblastoid and gastric carcinoma cell lines. Inhibitors of signal transduction pathways were used to define requirements for induction of lytic infection. Drug effects on transcription of the two EBV immediate-early promoters (BRLF1 and BZLF1) and on promoter constructs lacking cis-acting sequences required for activation by other effectors was examined using reporter gene assays. EBV viral load in rheumatoid arthritis and polymyositis patients receiving MTX was compared with that in patients receiving other immunosuppressive medications. Statistical tests were two-sided.

RESULTS

MTX activated the release of infectious EBV from latently infected cell lines in vitro, and MTX treatment was associated with activation of the two viral immediate-early promoters in reporter gene assays. Induction of lytic EBV infection by MTX required the p38 MAP kinase, PI3 kinase, and MEK pathways and specific cis-acting motifs in the two viral immediate-early promoters. Patients treated with MTX-containing regimens had statistically significantly higher mean EBV loads in their blood than patients treated with immunosuppressing regimens that did not include MTX (40 EBV copies per 10(6) cellular genomes versus 5.1 copies; geometric mean fold difference in copies = 10.8, 95%, confidence interval = 3.0 to 38; P = .011).

CONCLUSION

MTX may promote EBV-positive lymphomas in rheumatoid arthritis and polymyositis patients by its immunosuppressive properties as well as by reactivating latent EBV.

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.

Analysis of the autoantibody repertoire in Burkitt's lymphoma patients: frequent response against the transcription factor ATF-2

In the last few years, serological identification of tumour-associated antigens (TAAs) by recombinant cDNA expression cloning (SEREX) has enabled the mapping of humoral immune responses against TAAs in various types of cancer. The present paper describes the application of SEREX to Burkitt's lymphoma (BL), a malignancy not previously characterized by SEREX. By using a cDNA library from a BL cell line that does not express IgG, technical difficulties related to background immunoglobulin clones were overcome. Screening with sera from three BL patients revealed immunoreactivity against seven different gene products, six of which represent known human genes. Five proteins had previously been characterized by SEREX in other malignancies or identified as targets of autoantibodies in autoimmune disease. Seroreactivity against ATF-2, a member of the AP-1 transcription factor family, was validated by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis using recombinant ATF-2 protein. Autoantibody responses against ATF-2 were detected by ELISA in 6 of 8 BL patients, compared with 6 of 13 patients with T-cell non-Hodgkin's lymphoma (T-NHL), 5 of 23 patients with follicular lymphoma and 2 of 27 diffuse large B-cell lymphoma patients. In contrast, reactivity was found in only 1 of 50 healthy volunteers. Next, we showed by immunohistochemistry that the activated form of ATF2 (ATF-2pp) was highly expressed in six different BL samples. We conclude that the SEREX approach with a B-cell cDNA source is applicable in NHL. Furthermore, we identified genes with possible involvement in the pathogenesis of BL using this technique.

The C-mer gene is induced by Epstein-Barr virus immediate-early protein BRLF1

BRLF1 (R) is one of two Epstein-Barr virus (EBV) immediate-early proteins that mediate the switch from the latent to the lytic form of viral replication. In this report, we show that R induces expression of the cellular C-mer gene in a variety of cell lines. C-mer expression was detected in lymphoblastoid cells immortalized with wild-type EBV but not in lymphoblastoid cells immortalized with an EBV that had BRLF1 deleted. Oral hairy leukoplakia tongue tissue, which contains the lytic form of EBV replication, also has enhanced C-mer expression. C-mer is a receptor tyrosine kinase activated by the ligand Gas6. C-mer is required for phagocytosis of apoptotic debris by monocytes/macrophages and retinal pigment epithelial cells and is capable of producing an antiapoptotic signal. Modulation of the C-mer signal transduction cascade by a variety of different approaches did not alter the ability of R to induce lytic EBV gene transcription. Therefore, C-mer activation may be important for some other aspect of lytic EBV infection.

Inhibition of Epstein-Barr virus (EBV) reactivation by short interfering RNAs targeting p38 mitogen-activated protein kinase or c-myc in EBV-positive epithelial cells

Latent Epstein-Barr virus (EBV) is reactivated by 12-O-tetradecanoylphorbol-13-acetate (TPA) in EBV-infected cells. In this study, we found that TPA up-regulated phosphorylation of p38, a mitogen-activated protein kinase, and activated c-myc mRNA in EBV-positive epithelial GT38 cells. The EBV immediate-early gene BZLF1 mRNA and its product ZEBRA protein were induced following TPA treatment. Protein kinase C inhibitors, 1-(5-isoquinolinesulphonyl)-2, 5-dimethylpiperazine (H7) and staurosporine, inhibited the induction of p38 phosphorylation and the activation of c-Myc by TPA. The p38 inhibitor SB203580 blocked both p38 phosphorylation and ZEBRA expression by TPA. Pretreatment of GT38 cells with the nitric oxide (NO) donor S-nitroso-N-acetylpenicillamine inhibited p38 phosphorylation and c-Myc activation by TPA, suggesting that NO may inhibit EBV reactivation via both p38 and c-Myc. By using short interfering RNA (siRNA) targeting either p38 or c-myc, we found that p38 or c-myc siRNA specifically inhibited expression of the respective gene and also suppressed the induction of ZEBRA and EBV early antigen. The interferon (IFN)-responsive gene expression tests ruled out the possibility that the antiviral effect of siRNA is dependent on IFN. Our present study demonstrates for the first time that either p38 or c-myc siRNA can efficiently inhibit TPA-induced EBV reactivation in GT38 cells, indicating that p38- and/or c-myc-associated signaling pathways may play critical roles in the disruption of EBV latency by TPA.

The EBV lytic switch protein, Z, preferentially binds to and activates the methylated viral genome

DNA methylation promotes gene silencing, yet the Epstein-Barr virus immediate-early protein, BZLF1 (Z), converts the virus from the latent to the lytic form of infection even when the viral genome is highly methylated. Here we show that methylation of CpG motifs in Z-responsive elements of the viral BRLF1 immediate-early promoter enhances Z binding to, and activation of, this promoter. Demethylation of the viral genome impairs Z activation of lytic viral genes. Z is the first transcription factor that preferentially binds to, and activates, a methylated promoter. These results identify an unexpected mechanism by which Epstein-Barr virus circumvents the inhibitory effects of viral genome methylation.

The Epstein–Barr virus transactivator Zta binds to its own promoter and is required for full promoter activity during anti-Ig- and TGF-beta1-mediated reactivation

Transcription of the immediate early gene BZLF1 is mediated initially through the activation of cellular transcription factors. Reporter-based studies have provided evidence that following this initial activation, the BZLF1 gene product Zta may be involved in an autoactivation loop through binding to its promoter Zp. In contrast, other reports have shown that transfection of a Zta expression vector in latently infected cells does not activate endogenous Zp. Using chromatin immunoprecipitation (ChIP) assays, we show here that Zta binds to endogenous Zp following induction of the lytic cycle by anti-Ig and TGF-beta1 and that binding occurs early enough to play a role in the activation of Zp. We have also generated a dominant-negative Zta and shown that it inhibits activation of endogenous Zp. These data support a two-step model for Zp activation during reactivation involving initial activation by cellular factors followed by an autoactivation step.

ORF36 protein kinase of Kaposi's sarcoma herpesvirus activates the c-Jun N-terminal kinase signaling pathway

alpha-, beta-, and gamma-Herpesviruses encode putative viral protein kinases. The herpes simplex virus UL13, varicella-zoster virus ORF47, and Epstein-Barr virus BGLF4 genes all show protein kinase domains in their protein sequences. Mutational analysis of these herpesviruses demonstrated that the viral kinase is important for optimal virus growth. Previous studies have shown that ORF36 of Kaposi's sarcoma herpesvirus (KSHV) has protein kinase activity and is autophosphorylated on serine. The gene for ORF36 is expressed during lytic growth of the virus and has been classified as a late gene. Inspection of the ORF36 sequence indicated potential motifs that could be involved in activation of cellular transcription factors. To analyze the function of ORF36, the cDNA for this viral gene was tagged with the FLAG epitope and inserted into an expression vector for mammalian cells. Transfection experiments in 293T and SLK cells demonstrated that expression of ORF36 resulted in phosphorylation of the c-Jun N-terminal kinase. Autophosphorylation of ORF36 is important for JNK activation because a mutation in the predicted catalytic domain of ORF36 blocked its ability to phosphorylate JNK. Western blot analysis, using phosphospecific antibodies, revealed that mitogen-activated kinases MKK4 and MKK7 were phosphorylated by ORF36 but not by the kinase-negative mutant. Binding experiments in transfected cells also demonstrated that both the wild type and kinase-negative mutant of ORF36 form a complex with JNK, MKK4, and MKK7. In addition, using a tetracycline-inducible Rta BCBL-1 cell line (TREx BCBL1-Rta), JNK was phosphorylated during lytic replication, and inhibition of JNK activation blocked late viral gene expression but not early viral gene expression. In summary, these studies demonstrate that KSHV ORF36 activates the JNK pathway; thus this cell signaling pathway may function in the KSHV life cycle by regulating viral and/or cellular transcription.

Inhibition of the Epstein–Barr virus lytic cycle by Zta-targeted RNA interference

Epstein–Barr virus (EBV) reactivation into the lytic cycle plays certain roles in the development of EBV-associated diseases, so an effective strategy to block the viral lytic cycle may be of value to reduce the disease risk or to improve the clinical outcome. This study examined whether the EBV lytic cycle could be inhibited using RNA interference (RNAi) directed against the essential viral gene Zta. In cases of EBV reactivation triggered by chemicals or by exogenous Rta, Zta-targeted RNAi prevented the induction of Zta and its downstream genes and further blocked the lytic replication of viral genomes. This antiviral effect of RNAi was not likely to be mediated by activation of the interferon pathway, as phosphorylation of STAT1 was not induced. In addition, novel EBV-infected epithelial cells showing constitutive activation of the lytic cycle were cloned; such established lytic infection was also suppressed by Zta-targeted RNAi. These results indicate that RNAi can be used to inhibit the EBV lytic cycle effectively in vitro and could also be of potential use to develop anti-EBV treatments.

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

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

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

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

Lytic induction therapy for Epstein-Barr virus-positive B-cell lymphomas

A novel therapy for Epstein-Barr virus (EBV)-positive tumors involves the intentional induction of the lytic form of EBV infection combined with ganciclovir (GCV) treatment. Virally encoded kinases (thymidine kinase and BGLF4) which are expressed only during the lytic form of infection convert GCV (a nucleoside analogue) into its active, cytotoxic form. However, tightly latent EBV infection in B cells has made it difficult to identify drugs that can be used clinically to induce lytic viral infection in B-cell lymphomas. Here we demonstrate that gemcitabine and doxorubicin (but not 5-azacytidine, cis-platinum, or 5-fluorouracil) induce lytic EBV infection in EBV-transformed B cells in vitro and in vivo. Gemcitabine and doxorubicin both activated transcription from the promoters of the two viral immediate-early genes, BZLF1 and BRLF1, in EBV-negative B cells. This effect required the EGR-1 motif in the BRLF1 promoter and the CRE (ZII) and MEF-2D (ZI) binding sites in the BZLF1 promoter. GCV enhanced cell killing by gemcitabine or doxorubicin in lymphoblastoid cells transformed with wild-type EBV, but not in lymphoblastoid cells transformed by a mutant virus (with a deletion in the BZLF1 immediate-early gene) that is unable to enter the lytic form of infection. Most importantly, the combination of gemcitabine or doxorubicin and GCV was significantly more effective for the inhibition of EBV-driven lymphoproliferative disease in SCID mice than chemotherapy alone. In contrast, the combination of zidovudine and gemcitabine was no more effective than gemcitabine alone. These results suggest that the addition of GCV to either gemcitabine- or doxorubicin-containing chemotherapy regimens may enhance the therapeutic efficacy of these drugs for EBV-driven lymphoproliferative disease in patients.

Phosphatidylinositol 3-Kinase Is a Determinant of Responsiveness to B Cell Antigen Receptor-Mediated Epstein-Barr Virus Activation

B cell Ag receptor (BCR) cross-linking with anti-Ig Abs efficiently induces activation of latently infected EBV in some B cell lines, but not in others. The present study was aimed at defining the molecular mechanisms that determine the response to BCR-mediated EBV activation. Comparison of Burkitt's lymphoma-derived Akata, Mutu-I, and Daudi cells, which are representative responders and nonresponders to BCR-mediated EBV activation, respectively, indicated that three signaling pathways, phosphatidylinositol 3-kinase (PI3K), extracellular signal-regulated kinase (ERK), and p38 mitogen-activated protein kinase (MAPK), were activated in anti-Ig-treated Akata and Mutu-I cells. However, in anti-Ig-treated Daudi cells PI3K was not activated, ERK was faintly activated, and p38 MAPK was constitutively phosphorylated irrespective of anti-Ig treatment. Restoration of PI3K activity with insulin-like growth factor 1 restored ERK and p38 MAPK pathways, and was accompanied by EBV activation in anti-Ig-treated Daudi cells. In contrast, a specific inhibitor for PI3K, wortmannin, inhibited EBV activation by anti-Ig Abs in Akata and Mutu-I cells. Transfection assays in EBV-negative Daudi cells revealed that PI3K activated a promoter for BZLF1, which is a switch of EBV activation from a latent infection, in the absence of other EBV products suggesting that the BZLF promoter was a target of BCR signaling, and that PI3K was important for BCR-mediated BZLF1 activation. These results indicate that the absence of PI3K impedes the progression of signals through the BCR and becomes a determinant of unresponsiveness to BCR-mediated EBV activation.

Virally targeted therapies for EBV-associated malignancies

In Epstein-Barr virus (EBV)-positive lymphomas, the presence of the EBV genome in virtually all tumor cells, but very few normal cells, suggests that novel, EBV-targeted therapies could be used to treat these malignancies. In this paper, we review a variety of different approaches currently under development that specifically target EBV-infected cells for destruction. EBV-based strategies for treating cancer include prevention of viral oncogene expression, inducing loss of the EBV episome, the purposeful induction of the lytic form of EBV infection, and enhancing the host immune response to virally encoded antigens.

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

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

Potentiation of TRAIL-induced apoptosis in primary effusion lymphoma through azidothymidine-mediated inhibition of NF-kappa B

The survival of viral mediated lymphomas depends upon constitutive nuclear factor kappa B (NF-kappaB) activity. AIDS-related human herpesvirus type 8-associated primary effusion lymphoma (PEL) responds poorly to chemotherapy and is almost invariably fatal. We have previously demonstrated that the antiviral combination of interferon alpha (IFN-alpha) and azidothymidine (AZT) induces apoptosis in PEL cell lines. We therefore used these agents as therapy for an AIDS patient with PEL. The patient had a dramatic response, with complete resolution of his malignant effusion in 5 days. In PEL cells, the death receptor ligand known as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is markedly up-regulated by IFN-alpha; however, signals transduced by death receptors may also activate an antiapoptotic response mediated by NF-kappaB. In both the primary tumor cells from our patient and PEL cell lines, AZT selectively blocked nuclear entry of the NF-kappaB heterodimer p50 and p65, an effect not seen with other nonthymidine antiviral nucleosides. AZT monophosphate, the principal intracellular metabolite, inhibited phosphorylation and degradation of IkappaB by the IkappaB kinase complex. AZT- and IFN-alpha-mediated apoptosis was blocked by expression and nuclear localization of an IkappaB-resistant form of NF-kappaB (the p50 subunit linked to the transactivation domain of herpes simplex virus VP16). The proapoptotic effect of AZT and IFN-alpha in PEL occurs through the concomitant activation of TRAIL and blockade of NF-kappaB and represents a novel antiviral therapy for a virally mediated tumor.

The Epstein-Barr virus immediate-early protein BZLF1 induces expression of E2F-1 and other proteins involved in cell cycle progression in primary keratinocytes and gastric carcinoma cells

The Epstein-Barr virus (EBV) immediate-early protein BZLF1 mediates the switch between the latent and lytic forms of EBV infection and has been previously shown to induce a G(1)/S block in cell cycle progression in some cell types. To examine the effect of BZLF1 on cellular gene expression, we performed microarray analysis on telomerase-immortalized human keratinocytes that were mock infected or infected with a control adenovirus vector (AdLacZ) or a vector expressing the EBV BZLF1 protein (AdBZLF1). Cellular genes activated by BZLF1 expression included E2F-1, cyclin E, Cdc25A, and a number of other genes involved in cell cycle progression. Immunoblot analysis confirmed that BZLF1 induced expression of E2F-1, cyclin E, Cdc25A, and stem loop binding protein (a protein known to be primarily expressed during S phase) in telomerase-immortalized keratinocytes. Similarly, BZLF1 increased expression of E2F-1, cyclin E, and stem loop binding protein (SLBP) in primary tonsil keratinocytes. In contrast, BZLF1 did not induce E2F-1 expression in normal human fibroblasts. Cell cycle analysis revealed that while BZLF1 dramatically blocked G(1)/S progression in normal human fibroblasts, it did not significantly affect cell cycle progression in primary human tonsil keratinocytes. Furthermore, in EBV-infected gastric carcinoma cells, the BZLF1-positive cells had an increased number of cells in S phase compared to the BZLF1-negative cells. Thus, in certain cell types (but not others), BZLF1 enhances expression of cellular proteins associated with cell cycle progression, which suggests that an S-phase-like environment may be advantageous for efficient lytic EBV replication in some cell types.

The Epstein-Barr virus immediate-early protein BZLF1 regulates p53 function through multiple mechanisms

The Epstein-Barr virus (EBV) immediate-early protein BZLF1 is a transcriptional activator that mediates the switch between the latent and the lytic forms of EBV infection. It was previously reported that BZLF1 inhibits p53 transcriptional function in reporter gene assays. Here we further examined the effects of BZLF1 on p53 function by using a BZLF1-expressing adenovirus vector (AdBZLF1). Infection of cells with the AdBZLF1 vector increased the level of cellular p53 but prevented the induction of p53-dependent cellular target genes, such as p21 and MDM2. BZLF1-expressing cells had increased p53-specific DNA binding activity in electrophoretic mobility shift assays, increased p53 phosphorylation at multiple residues (including serines 6, 9, 15, 33, 46, 315, and 392), and increased acetylation at lysine 320 and lysine 382. Thus, the inhibitory effects of BZLF1 on p53 transcriptional function cannot be explained by its effects on p53 phosphorylation, acetylation, or DNA binding activity. BZLF1 substantially reduced the level of cellular TATA binding protein (TBP) in both normal human fibroblasts and A549 cells, and the inhibitory effects of BZLF1 on p53 transcriptional function could be partially rescued by the overexpression of TBP. Thus, BZLF1 has numerous effects on p53 posttranslational modification but may inhibit p53 transcriptional function in part through an indirect mechanism involving the suppression of TBP expression.

Use of adenovirus vectors expressing Epstein-Barr virus (EBV) immediate-early protein BZLF1 or BRLF1 to treat EBV-positive tumors

The Epstein-Barr virus (EBV) genome is present in a variety of tumor types, including virtually all undifferentiated nasopharyngeal carcinomas (NPC) and a portion of gastric carcinomas. The uniform presence of the EBV genome in certain tumors (versus only a very small number of normal B cells) suggests that novel therapies which specifically target EBV-positive cells for destruction might be effective for treating such tumors. Although the great majority of EBV-positive tumor cells are infected with one of the latent forms of EBV infection, expression of either viral immediate-early protein (BZLF1 or BRLF1) is sufficient to convert the virus to the lytic form of infection. Induction of the lytic form of EBV infection could potentially result in death of the tumor cell. Here we have examined the efficacy of adenovirus vectors expressing the BZLF1 or BRLF1 proteins for treatment of EBV-positive epithelial tumors. The BZLF1 and BRLF1 vectors induced preferential killing of EBV-positive, versus EBV-negative, gastric carcinoma cells in vitro. Infection of C18 NPC tumors (grown in nude mice) with either the BZLF1 or BRLF1 vector, but not a control adenovirus vector, induced expression of early lytic EBV genes in tumor cells. Injection of C18 tumors with the BZLF1 or BRLF1 adenovirus vector, but not the control vector, also significantly inhibited growth of the tumors in nude mice. The addition of ganciclovir did not significantly affect the antitumor effect of the BZLF1 and BRLF1 adenovirus vectors. These results suggest a potential cancer therapy against EBV-related tumors.

Activation of cellular and heterologous promoters by the human herpesvirus 8 replication and transcription activator

The key regulator of the switch from latent to lytic replication of the human herpesvirus 8 (HHV-8; KSHV) is the replication and transcription activator (Rta). The ability of Rta to regulate cellular gene expression was examined by transient transfection into cells that were not infected with HHV-8. Rta induced some, but not all, NF-kappa B-responsive reporters through mechanisms that did not involve activation of classic forms of NF-kappa B. Furthermore, transfection of the NF-kappa B subunit Rel A inhibited the ability of Rta to transactivate some but not all reporters. For example, Rel A inhibited the ability of Rta to transactivate the IL-6 promoter, but only when sequences upstream of the NF-kappa B site were present. The ability of Rel A to inhibit Rta-mediated transactivation was not dependent on a functional NF-kappa B site within the promoter, suggesting an indirect mechanism for inhibition. These studies suggest that Rta expression during lytic reactivation of HHV-8 would lead to expression of some cellular genes, including IL-6, whereas activation of NF-kappa B could inhibit some responses to Rta.

The lytic cycle of Epstein-Barr virus is associated with decreased expression of cell surface major histocompatibility complex class I and class II molecules

Human herpesviruses utilize an impressive range of strategies to evade the immune system during their lytic replicative cycle, including reducing the expression of cell surface major histocompatibility complex (MHC) and immunostimulatory molecules required for recognition and lysis by virus-specific cytotoxic T cells. Study of possible immune evasion strategies by Epstein-Barr virus (EBV) in lytically infected cells has been hampered by the lack of an appropriate permissive culture model. Using two-color immunofluorescence staining of cell surface antigens and EBV-encoded lytic cycle antigens, we examined EBV-transformed B-cell lines in which a small subpopulation of cells had spontaneously entered the lytic cycle. Cells in the lytic cycle showed a four- to fivefold decrease in cell surface expression of MHC class I molecules relative to that in latently infected cells. Expression of MHC class II molecules, CD40, and CD54 was reduced by 40 to 50% on cells in the lytic cycle, while no decrease was observed in cell surface expression of CD19, CD80, and CD86. Downregulation of MHC class I expression was found to be an early-lytic-cycle event, since it was observed when progress through late lytic cycle was blocked by treatment with acyclovir. The immediate-early transactivator of the EBV lytic cycle, BZLF1, did not directly affect expression of MHC class I molecules. However, BZLF1 completely inhibited the upregulation of MHC class I expression mediated by the EBV cell-transforming protein, LMP1. This novel function of BZLF1 elucidates the paradox of how MHC class I expression can be downregulated when LMP1, which upregulates MHC class I expression in latent infection, remains expressed in the lytic cycle.

Role of regulatory elements and the MAPK/ERK or p38 MAPK pathways for activation of human cytomegalovirus gene expression

A series of recombinant viruses with either site-specific mutations or various deletions of the early UL4 promoter of human cytomegalovirus were used to determine the roles of regulatory elements and the effects of the mitogen-activated protein kinase (MAPK) pathways. Viral gene expression was regulated by upstream cis-acting sites and by basic promoter elements that respond to the MAPK signal transduction pathways. Inhibitors of either the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway or the p38 MAPK pathway affected expression equally with either wild-type or mutant early UL4 promoters in the viral genome, indicating that the effects of the inhibitors are not exclusive for a single transcription factor. The minimal responsive element is the TATA box-containing early viral promoter.

Open reading frame 50 protein of Kaposi's sarcoma-associated herpesvirus directly activates the viral PAN and K12 genes by binding to related response elements

Open reading frame (ORF) 50 protein is capable of activating the entire lytic cycle of Kaposi's sarcoma-associated herpesvirus (KSHV), but its mechanism of action is not well characterized. Here we demonstrate that ORF 50 protein activates two KSHV lytic cycle genes, PAN (polyadenylated nuclear RNA) and K12, by binding to closely related response elements located approximately 60 to 100 nucleotides (nt) upstream of the start of transcription of the two genes. The 25-nt sequence 5' AAATGGGTGGCTAACCTGTCCAAAA from the PAN promoter (PANp) confers a response to ORF 50 protein in both epithelial cells and B cells in the absence of other KSHV proteins. The responsive region of DNA can be transferred to a heterologous minimal promoter. Extensive point mutagenesis showed that a span of at least 20 nt is essential for a response to ORF 50 protein. However, a minimum of six positions within this region were ambiguous. The related 26-nt responsive element in the K12 promoter (K12p), 5' GGAAATGGGTGGCTAACCCCTACATA, shares 20 nt (underlined) with the comparable region of PANp. The divergence is primarily at the 3' end. The DNA binding domain of ORF 50 protein, encompassing amino acids 1 to 490, fused to a heterologous activation domain from herpes simplex virus VP16 [ORF 50(1-490)+VP] can mediate activation of reporter constructs bearing these response elements. Most importantly, ORF 50(1-490)+VP can induce PAN RNA and K12 transcripts in transfected cells. ORF 50(1-490)+VP expressed in human cells binds specifically to duplex oligonucleotides containing the responsive regions from PANp and K12p. These DNA-protein complexes were supershifted by antibody to VP16. ORF 50(1-490) without a VP16 tag also bound to the response element. There was a strong correlation between DNA binding by ORF 50 and transcriptional activation. Mutations within PANp and K12p that impaired transactivation by ORF 50 or ORF 50(1-490)+VP also abolished DNA binding. Only one of eight related complexes formed on PANp and K12p oligonucleotides was due to ORF 50(1-490)+VP. The other complexes were due to cellular proteins. Two KSHV lytic-cycle promoters are activated by a similar mechanism that involves direct recognition of a homologous response element by the DNA binding domain of ORF 50 protein in the context of related cellular proteins.

The chemopreventive compound curcumin is an efficient inhibitor of Epstein-Barr virus BZLF1 transcription in Raji DR-LUC cells

To characterize the effects of inhibitors of Epstein-Barr virus (EBV) reactivation, we established Raji DR-LUC cells as a new test system. These cells contain the firefly luciferase (LUC) gene under the control of an immediate-early gene promoter (duplicated right region [DR]) of EBV on a self-replicating episome. Luciferase induction thus serves as an intrinsic marker indicative for EBV reactivation from latency. The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induced the viral key activator BamH fragment Z left frame 1 (BZLF1) protein ("ZEBRA") in this system, as demonstrated by induction of the BZLF1 protein-responsive DR promoter upstream of the luciferase gene. Conversely, both BZLF1 protein and luciferase induction were inhibited effectively by the chemopreventive agent curcumin. Semiquantitative reverse transcriptase (RT)-polymerase chain reaction (PCR) further demonstrated that the EBV inducers TPA, sodium butyrate, and transforming growth factor-beta (TGF-beta) increased levels of the mRNA of BZLF1 mRNA at 12, 24, and 48 h after treatment in these cells. TPA treatment also induced luciferase mRNA with similar kinetics. Curcumin was found to be highly effective in decreasing TPA-, butyrate-, and TGF-beta-induced levels of BZLF1 mRNA, and of TPA-induced luciferase mRNA, indicating that three major pathways of EBV are inhibited by curcumin. Electrophoretic mobility shift assays (EMSA) showed that activator protein 1 (AP-1) binding to a cognate AP-1 sequence was detected at 6 h and could be blocked by curcumin. Protein binding to the complete BZLF1 promoter ZIII site (ZIIIA+ZIIIB) demonstrated several specific complexes that gave weak signals at 6 h and 12 h but strong signals at 24 h, all of which were reduced after application of curcumin. Autostimulation of BZLF1 mRNA induction through binding to the ZIII site at 24 h was confirmed by antibody-induced supershift analysis. The present results confirm our previous finding that curcumin is an effective agent for inhibition of EBV reactivation in Raji DR-CAT cells (carrying DR-dependent chloramphenicol acetyltransferase), and they show for the first time that curcumin inhibits EBV reactivation mainly through inhibition of BZLF1 gene transcription.

Inhibition of IFN-gamma signaling by an Epstein-Barr virus immediate-early protein

Viruses have evolved elaborate mechanisms to target many aspects of the host's immune response. The cytokine IFN-gamma plays a central role in resistance of the host to infection via direct antiviral effects as well as modulation of the immune response. In this study, we demonstrate that the Epstein-Barr virus (EBV) immediate-early protein, BZLF1, inhibits the IFN-gamma signaling pathway. BZLF1 decreases the ability of IFN-gamma to activate a variety of important downstream target genes, such as IRF-1, p48, and CIITA, and prevents IFN-gamma-induced class II MHC surface expression. Additionally, BZLF1 inhibits IFN-gamma-induced STAT1 tyrosine phosphorylation and nuclear translocation. Finally, we demonstrate that BZLF1 decreases expression of the IFN-gamma receptor, suggesting a mechanism by which EBV may escape antiviral immune responses during primary infection.

Epstein-Barr virus immediate-early protein BRLF1 induces the lytic form of viral replication through a mechanism involving phosphatidylinositol-3 kinase activation

Expression of the Epstein-Barr virus (EBV) immediate-early (IE) protein BRLF1 induces the lytic form of viral replication in most EBV-positive cell lines. BRLF1 is a transcriptional activator that binds directly to a GC-rich motif present in some EBV lytic gene promoters. However, BRLF1 activates transcription of the other IE protein, BZLF1, through an indirect mechanism which we previously showed to require activation of the stress mitogen-activated protein kinases. Here we demonstrate that BRLF1 activates phosphatidylinositol-3 (PI3) kinase signaling in host cells. We show that the specific PI3 kinase inhibitor, LY294002, completely abrogates the ability of a BRLF1 adenovirus vector to induce the lytic form of EBV infection, while not affecting lytic infection induced by a BZLF1 adenovirus vector. Furthermore, we demonstrate that the requirement for PI3 kinase activation in BRLF1-induced transcriptional activation is promoter dependent. BRLF1 activation of the SM early promoter (which occurs through a direct binding mechanism) does not require PI3 kinase activation, whereas activation of the IE BZLF1 and early BMRF1 promoters requires PI3 kinase activation. Thus, there are clearly two separate mechanisms by which BRLF1 induces transcriptional activation.

Variant chromatin structure of the oriP region of Epstein-Barr virus and regulation of EBER1 expression by upstream sequences and oriP

Most of the Epstein-Barr virus genome in latently infected cells is in a standard nucleosomal structure, but the region encompassing oriP and the Epstein-Barr virus-encoded small RNA (EBER) genes shows a distinctive pattern when digested with micrococcal nuclease. This pattern corresponds to a previously mapped nuclear matrix attachment region. Although the EBER genes are adjacent to oriP, there is only a two- to fourfold effect of oriP on EBER expression. However, sequences containing a consensus ATF site upstream of EBER1 are important for EBER1 expression.

Epstein-Barr virus immediate-early protein BRLF1 interacts with CBP, promoting enhanced BRLF1 transactivation

The Epstein-Barr virus (EBV) immediate-early protein BRLF1 is a transcriptional activator that mediates the switch from latent to lytic viral replication. Many transcriptional activators function, in part, due to an interaction with histone acetylases, such as CREB-binding protein (CBP). Here we demonstrate that BRLF1 interacts with the amino and carboxy termini of CBP and that multiple domains of the BRLF1 protein are necessary for this interaction. Furthermore, we show that the interaction between BRLF1 and CBP is important for BRLF1-induced activation of the early lytic EBV gene SM in Raji cells.

Molecular pathways in virus-induced cytokine production

Virus infections induce a proinflammatory response including expression of cytokines and chemokines. The subsequent leukocyte recruitment and antiviral effector functions contribute to the first line of defense against viruses. The molecular virus-cell interactions initiating these events have been studied intensively, and it appears that viral surface glycoproteins, double-stranded RNA, and intracellular viral proteins all have the capacity to activate signal transduction pathways leading to the expression of cytokines and chemokines. The signaling pathways activated by viral infections include the major proinflammatory pathways, with the transcription factor NF-kappaB having received special attention. These transcription factors in turn promote the expression of specific inducible host proteins and participate in the expression of some viral genes. Here we review the current knowledge of virus-induced signal transduction by seven human pathogenic viruses and the most widely used experimental models for viral infections. The molecular mechanisms of virus-induced expression of cytokines and chemokines is also analyzed.

SUMO, ubiquitin's mysterious cousin

Covalent modification of cellular proteins by the ubiquitin-like modifier SUMO regulates various cellular processes, such as nuclear transport, signal transduction, stress response and cell-cycle progression. But, in contrast to ubiquitylation, sumoylation does not tag proteins for degradation, but seems to enhance their stability or modulate their subcellular compartmentalization.