Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus typically acquired early in life through salivary exchange. EBV is the causative agent of infectious mononucleosis and is associated with a variety of malignancies of B-cell and epithelial cell origin (
48,
49). Like all herpesviruses, EBV can exist in either a latent or lytic state with respect to viral gene expression. An infection of B cells typically results in latency, in which only a subset of viral genes is expressed and progeny virus is not released. Latently infected B cells occasionally reactivate into the lytic cycle in response to stimuli such as B-cell activation (
57) or differentiation (
9). Differentiated epithelial cells are also permissive for lytic infection (
28,
32,
55,
65). The induction of the viral lytic cycle in either B cells or epithelial cells results in the expression of the majority of viral genes and the release of progeny virus capable of infecting new cells.
Entry into the viral lytic cycle is initiated by expression of the immediate-early (IE) EBV proteins, BZLF1 (Z) and BRLF1 (R) (
7,
8,
45,
50,
58,
68). Z, a bZIP protein with sequence homology to c-Jun and c-Fos, binds and transactivates promoters containing AP-1-like motifs (
6,
13,
59,
61). R can also activate target promoters through direct binding (
20,
21,
44); however, R also activates transcription indirectly through the induction of signaling cascades (
1,
10). Stimuli that induce a lytic infection initially activate the transcription of both IE genes (
57), and the expression of either IE protein in latently infected cells is sufficient to induce the lytic form of EBV infection (
7,
8,
45,
58,
64,
68). Each IE protein activates the promoter of the other IE gene, and together the two IE proteins then activate the viral early genes and lytic viral replication (
2,
14).
The ability of each IE protein to activate transcription of the other IE gene is essential for the disruption of viral latency by either protein (
2,
14,
46,
68). Z transactivates the R promoter (Rp) by directly binding to Rp (
2,
54). In contrast, R activates Z transcription by enhancing the transcriptional functions of cellular factors (ATF-2 and c-Jun) binding to a CREB response element (CRE) motif (ZII) in the Z promoter (Zp) (
1). This effect is mediated through the induction of the stress-associated mitogen-activated protein (MAP) kinases (SAPKs) p38 and c-Jun N-terminal kinase (JNK) (
1), which phosphorylate and activate the transcription factors ATF-2 and c-Jun, respectively (
12,
23,
47,
62).
In addition to the Z and R genes, the IE locus of EBV contains another open reading frame, BRRF1 (also designated Na), which lies within the first intron of the R gene and is oriented in the opposite direction (
38). Na mRNA appears with early kinetics following induction of the viral lytic cycle in several latently infected B-cell lines (
38,
53). The promoter driving expression of Na (Nap) is located within the coding sequence for R, and reporter assays indicate that Nap is activated by Z (
53). This activation may be mediated by the direct binding of Z to Nap, given that Z binds several sites in Nap between nucleotides −469 and +1 in electromobility shift assays (
53). The Na gene product is a 34-kDa protein that localizes to the nucleus in HeLa cells (
53). However, no studies to date have identified a function for Na.
For this paper, we examined the role of Na in the viral life cycle. Utilizing several different cell lines that are latently infected with a recombinant EBV defective for expression of both R and Na, we demonstrate that the Na gene product cooperates with R to induce an efficient lytic EBV infection in certain cell lines. Furthermore, we show that Na by itself activates promoters containing CRE motifs (including the IE Z promoter) in EBV-negative cell lines and that this effect is mediated through an enhanced c-Jun transcriptional function. However, Na by itself cannot activate BZLF1 transcription in the context of the intact viral genome. Our results suggest the existence of a previously unexpected positive auto-regulatory circuit for R-mediated lytic EBV induction, in which R by itself initially activates BZLF1 transcription with a low efficiency, but subsequently activates BZLF1 transcription with a much higher efficiency (in conjunction with Na) after the Z gene product has induced Na transcription.
MATERIALS AND METHODS
Plasmids.
The Na expression vector pRC-FLAG-BRRF1 and the Z expression vector pSG5-Z were described previously (
51,
53). pSG5-R (a gift from Diane Hayward) contains the genomic R sequence beginning with the second exon (including the ATG start codon for R) but lacking the first intron (containing the Na gene) (
24). Appropriate control vectors, pRC (Invitrogen) and pSG5 (Stratagene), were used in all experiments. The pRK5-BALF4 expression vector encodes the EBV gp110 protein and was previously shown to enhance the infection efficiency of EBV virions derived from the B95-8 strain of EBV (
42). Zp-CAT, Zp-CAT ΔZIA/ZIB, Zp-CAT ΔZII, and −65Zp-CAT (all gifts from E. Flemington) were described previously (
17). Briefly, Zp-CAT contains the region of the BZLF1 promoter from −221 to +12 (relative to the transcription start site) linked upstream of the chloramphenicol acetyltransferase (CAT) gene. Zp-CAT ΔZIA/ZIB and Zp-CAT ΔZII represent mutants of Zp in which the ZIA and ZIB sites and the ZII site, respectively, have been mutated via site-directed mutagenesis. The plasmid −65Zp-CAT contains nucleotides −65 to +12 of Zp upstream of CAT. EApBS-CAT (also called BMRF1-CAT) contains the BMRF1 promoter sequence from −331 to +1 upstream of CAT and was described previously (
44). CRE-CAT contains three consensus CRE sites upstream of CAT; CRE(m)-CAT is identical to CRE-CAT except for specific mutations in the CRE site. The SG424 vector encodes the Gal4 DNA binding domain alone, whereas Gal4-CREB, Gal4-ATF1, Gal4-ATF2 (all gifts from M. Green), and Gal4-c-Jun (a gift from A. Baldwin) contain the respective proteins linked in-frame to the Gal4 DNA binding domain. Gal4-EIB-CAT (a gift from M. Green) consists of five copies of the Gal4 DNA binding site linked upstream of the minimal EIB TATA box and CAT gene. The CR2/puro expression vector was a gift from L. Hutt-Fletcher; CR2 is the primary cellular receptor for EBV (
15). CMV-c-Jun expresses full-length c-Jun under the control of the cytomegalovirus (CMV) promoter. Plasmids were purified by use of Qiagen Maxiprep kits (Qiagen) according to the manufacturer's instructions.
EBV wild-type, Z-KO, and R-KO viruses and cell lines.
293 cells infected with the R-KO virus (293 R-KO), Z-KO virus (293 Z-KO), and wild-type virus (293 WT) have been described previously (
11,
14). In the R-KO virus, nucleotides 103,638 to 105,083 (B95.8 coordinates [accession no. V01555 ]) within the BRLF1 gene were removed via the insertion of a tetracycline resistance cassette into the virus (
14). In the Z-KO virus, nucleotides 102,389 to 103,388 (B95.8 coordinates) within the BZLF1 gene were removed via the insertion of a kanamycin resistance cassette. The R-KO, Z-KO, and WT viruses also encode enhanced green fluorescent protein (GFP) and a hygromycin B resistance gene (both cloned into the B95.8 deletion of EBV, where the second copy of oriLyt normally resides). 293 R-KO, 293 Z-KO, and 293 WT cells were maintained in Dulbecco's modified Eagle medium containing 10% fetal bovine serum (FBS), penicillin-streptomycin, and hygromycin B (100 μg/ml; Roche). HeLa cells are a cervical adenocarcinoma line and were maintained in Dulbecco's modified Eagle medium with 10% FBS and penicillin-streptomycin. AGS R-KO cells (see below) were maintained in F-12 medium containing 10% FBS, penicillin-streptomycin, and 100 μg of hygromycin B/ml. BL30 R-KO cells and LCL R-KO cells (see below) were maintained in RPMI 1640 medium supplemented with 10% FBS, penicillin-streptomycin, and 400 μg of hygromycin B/ml. Raji cells, an EBV-positive Burkitt lymphoma cell line, were maintained in RPMI 1640 with 10% FBS and penicillin-streptomycin.
DNA transfection.
Transfections of 293 R-KO and 293 WT cells were done by use of Lipofectamine 2000 (Invitrogen). Cells (6 × 10
5) were seeded the day prior to transfection in 2 ml of medium in a six-well plate. Transfections were performed according to the manufacturer's instructions, except that the reagent/DNA ratio was scaled down to 2 μl/1 μg. Transfections of HeLa cells for reporter assays were performed by use of Fugene 6 (Roche) according to the manufacturer's instructions. AGS R-KO cells were electroporated with 5 μg of total DNA in a Zapper electroporation unit as described previously (
60). BL30 R-KO and LCL R-KO cells were transfected in buffer V (Amaxa Biosystems) with an Amaxa Nucleofector (Amaxa Biosystems). Cells were cultured for 24 h in antibiotic-free medium, followed by two washes in 1× phosphate-buffered saline (PBS). Cells (10
7) were resuspended in 100 μl of buffer V (per condition) and 5 μg of total DNA was added. Settings G-16 and T-20 were used for BL30 R-KO and LCL R-KO cells, respectively. After being transfected, cells were incubated overnight in 1.5 ml of antibiotic-free medium at 37°C in a 12-well plate; 2.5 ml of complete medium was then added.
Generation of R-KO cell lines.
For the production of R-KO viral stocks, 5 × 106 293 R-KO cells were plated in 10 ml of RPMI 1640-10% FBS-penicillin-streptomycin in a 100-mm-diameter dish the day prior to transfection. The cells were transfected with 1.5 μg each of pSG5-R, pSG5-Z, pRC-FLAG-BRRF1, and pRK5-BALF4 expression plasmids by use of Lipofectamine 2000 (12 μl of reagent per condition) according to the manufacturer's instructions. At 72 h posttransfection, supernatants were filtered through 0.45-μm-pore-size filters and viral stocks were stored at 4°C. AGS cells (a gift from L. Hutt-Fletcher) are an EBV-negative gastric adenocarcinoma cell line. In order to create an AGS line infected with the R-KO virus, we first transfected AGS cells with 1 μg of an expression vector encoding CD21-puromycin resistance by using Fugene 6 (Roche) according to the manufacturer's instructions. At 48 h posttransfection, cells were selected with 0.5 μg of puromycin HCl (Roche)/ml; puromycin-resistant colonies were pooled. For infections with R-KO virus, puromycin-resistant cells were seeded at 50% confluence in a 60-mm-diameter dish and were incubated with R-KO viral stocks. Four days after infection, cells were selected with hygromycin B (100 μg/ml); one hygromycin B-resistant colony representing an AGS cell infected with the R-KO virus was isolated. BL30 cells are an EBV-negative Burkitt lymphoma line. For the generation of BL30 R-KO cells, approximately 5 × 106 BL30 cells were pelleted and resuspended in 5 ml of R-KO viral stock for 3 h at 37°C. After the incubation, the cells were diluted into 50 ml of medium. Selection with 400 μg of hygromycin B/ml was begun at 5 days postinfection. For the generation of LCL R-KO cells, 3 × 106 peripheral blood mononuclear cells (provided by the laboratory of Susan Fiscus, University of North Carolina at Chapel Hill) were resuspended in 3 ml of R-KO virus and incubated overnight at 37°C. Seven milliliters of RPMI 1640-20% FBS-penicillin-streptomycin was then added, and cyclosporine A (Sigma) was added to achieve a final concentration of 500 ng/ml. Fifty percent of the medium was changed every 7 days until transformation into LCLs was apparent (at ∼3 to 4 weeks postinfection). LCLs were then maintained in RPMI 1640-10% FBS-penicillin-streptomycin.
Northern blotting.
Total RNAs were prepared by use of RNAeasy kits (Qiagen), including DNase treatment, according to the manufacturer's instructions. Ten micrograms of total RNA was subjected to denaturing agarose gel electrophoresis, and fractionated RNAs were transferred to a membrane by use of the Turboblotter system (Schleicher and Schuell) according to the manufacturer's instructions. After being transferred, RNAs were cross-linked to the membrane via UV irradiation (1,200 J). The following double-stranded DNA fragments were used as probes for Z, Na, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), respectively: a PstI/HindIII fragment from pSG5-Z, a BglII/XhoI fragment of pRC-FLAG-BRRF1, and a mouse GAPDH DECAtemplate (Ambion). Double-stranded DNA fragments were gel purified with a Qiagen gel extraction kit (Qiagen) and radiolabeled with a Prime-A-Gene kit (Promega), and unincorporated radioactivity was removed by the use of Sephadex G-50 columns (Amersham); all steps were performed according to the manufacturer's instructions. Prehybridization and hybridization (2 × 106 cpm of probe) were performed in Quikhyb solution (Stratagene) according to the manufacturer's instructions. Membranes were exposed to film overnight at −80°C.
Immunoblotting.
Immunoblotting was performed as described previously (
1). Briefly, cells were lysed in NP-40 lysis buffer supplemented with protease and phosphatase inhibitors; equivalent amounts of protein were then separated in sodium dodecyl sulfate-10% polyacrylamide gel electrophoresis gels. For the examination of c-Jun phosphorylation status, cells were kept at 4°C at all times during the harvesting procedure. After being transferred and blocked, membranes were incubated overnight at 4°C with the appropriate primary antibodies diluted in 5% milk in 1× PBS and 0.1% Tween 20 (PBS-T). Primary antibody dilutions were as follows: 1:100 anti-BRLF1 (Argene), 1:100 anti-BZLF1 (Argene), 1:100 anti-BMRF1 (Capricorn), 1 μg of M2 anti-FLAG (Sigma)/ml, and 1:5,000 anti-β-actin (clone AC-15; Sigma). Anti-c-Jun (Cell Signaling) and anti-phosphoSer73-c-Jun (Cell Signaling) antibodies were diluted 1:1,000 in 5% bovine serum albumin in 1× PBS-T. The appropriate horseradish peroxidase-conjugated secondary antibodies (Promega) were used at a dilution of 1:10,000 in 5% milk-1× PBS-T for 1 h at room temperature. After being washed, bound antibodies were visualized by use of ECL reagent (Amersham) according to the manufacturer's instructions.
CAT assays.
CAT assays were performed as described previously (
19) with cell extracts harvested at 48 h posttransfection. Chloramphenicol acetylation was determined by thin-layer chromatography followed by PhosphorImager (Molecular Dynamics) quantification.
Viral titration assay.
Supernatants from 293 R-KO cells were harvested at 48 h posttransfection and filtered through a 0.45-μm-pore-size filter. Raji cells (2 × 105) were then incubated in 0.5 ml of virus for 3 h at 37°C in a 12-well plate. One and one-half milliliters of medium was then added to each well and the cells were incubated for an additional 48 h at 37°C. Phorbol-12-myristate-3-acetate (PMA; Sigma) and sodium butyrate (Sigma) were then added to achieve a final concentration of 50 ng of PMA/ml and 3 mM sodium butyrate. The number of GFP-expressing Raji cells was quantitated 24 h later by fluorescence microscopy.
DISCUSSION
The switch from a latent to lytic infection during the EBV life cycle is a tightly regulated process in which the expression of the IE genes Z and R plays a pivotal role in initiating the transition. The expression of either IE protein results in expression of the other, and this ability of Z or R to turn on the promoter of the other corresponding IE gene is critical for the efficient induction of lytic infections (
2,
14,
68). The IE region of EBV expresses a gene within the intronic sequence of R, designated Na or BRRF1, that is transcribed with early kinetics in response to Z (
53). Na encodes a 34-kDa nuclear protein whose function in the viral life cycle was previously unknown. In this report, we demonstrated that Na functions as a viral transactivator that enhances the ability of R to disrupt latency in a 293 cell line carrying a recombinant EBV defective for expression of both R and Na. The expression of R in conjunction with Na results in increased levels of viral lytic genes compared to expression of R alone. In addition, the presence of Na also results in a fivefold increase in the amount of infectious virus produced after an R-induced lytic infection.
The ability of Na to enhance R-induced lytic infection in 293 cells appears to be at least partially mediated through Na's ability to activate the Z promoter. Increased levels of Z were observed in the presence of R and Na in combination compared to R alone, suggesting that the presence of Na increased the transcription of Z from the endogenous genome. Although Na alone could not activate Z transcription in the context of the intact viral genome, Na alone was able to specifically transactivate a Zp reporter construct in EBV-negative HeLa cells. The ability of Na to activate Zp required the CRE (ZII) site, and the consensus CRE site was shown to be sufficient to confer Na responsiveness to a heterologous promoter. These data suggest that the CRE element is the primary target of Na-induced transactivation.
The Zp CRE site binds ATF-1, ATF-2, c-Jun, and CREB (
1,
34,
37,
63). Of these, the phospho-ATF-2/phospho-c-Jun heterodimer may represent the predominant complex responsible for R activation of the Zp CRE site (
1). We showed here that Na specifically activates the transcriptional function of c-Jun while not affecting the transcriptional function of ATF-1, ATF-2, or CREB-1. Thus, Na likely activates Zp by enhancing c-Jun transcriptional functioning. This activation of c-Jun by Na appears to be independent of changes in c-Jun DNA binding, as the activity of a Gal4-c-Jun fusion protein (in which DNA binding was mediated by Gal4) was also activated by Na. Since the phosphorylation of c-Jun residues Ser63 and Ser73 is required for efficient c-Jun transcriptional functioning, we examined the effect of Na on c-Jun phosphorylation. We found that Na increases the levels of a hyperphosphorylated form of c-Jun, suggesting that Na activates c-Jun by increasing its phosphorylation. Since the phosphorylation of c-Jun residues Ser63 and Ser73 is commonly mediated by JNK (
12), these results indicate that Na may activate JNK. However, the exact mechanism by which Na modulates c-Jun phosphorylation has not yet been determined. Given that the ZII element is responsive to transforming growth factor beta and binds SMADs (
33), it is possible that Na modulates SMADs in addition to c-Jun in order to transactivate Zp through ZII; this possibility remains to be examined.
An interesting result of these studies was the finding that while Na efficiently activates Zp in reporter gene assays, it does not by itself activate BZLF1 transcription from the intact viral genome in cells that are latently infected with EBV. Instead, Na primarily enhances the ability of R to activate BZLF1 transcription from the endogenous viral genome in certain settings. We speculate that the Na transactivator function, which is mediated through the c-Jun transcription factor, is inhibited by the epigenetic modification of the EBV genome in latently infected cells, which includes DNA methylation of the BZLF1 promoter region as well as an inactive (deacetylated) state of the chromatin surrounding Zp (
22). R-mediated activation of Zp is also thought to be mediated through cellular factors binding to the CRE motif, suggesting that the relative inability of R to efficiently activate Z transcription in certain cell types (such as lymphoblastoid lines) may likewise reflect the effect of inhibitory epigenetic modifications of the viral genome. Notably, MEF2D binding to the ZI motifs in Zp has been previously shown to modulate the histone acetylation of Zp in the context of the intact viral genome (
22). Zp reporter constructs may not accurately mimic the state of Zp in the context of the intact viral genome, particularly in regard to the effects of chromatin structure and DNA methylation.
The relative contributions of the various cellular transcription factors (ATF-1, CREB-1, ATF-2, and c-Jun) which activate Zp transcription through the CRE motif to reporter gene assays are not currently understood with regard to their importance for inducing BZLF1 transcription in the context of the intact viral genome. It is possible that the state of Zp methylation and/or chromatin acetylation differentially influences the ability of these cellular factors to activate Zp transcription from the endogenous viral genome. In addition, cell-type-dependent factors may influence the relative importance of each transcription factor in activating BZLF1 transcription from the viral genome. In situations in which the combination of ATF-2 and c-Jun is sufficient for activating BZLF1 transcription from the endogenous viral genome, the synergistic effect of R and Na together may reflect the ability of R to activate p38 kinase (which phosphorylates and activates ATF-2) combined with the ability of Na to enhance the level of heavily phosphorylated c-Jun. In cell types in which R and Na together do not efficiently induce BZLF1 transcription (such as lymphoblastoid cells), it remains possible that activated CREB-1 and/or ATF-1 could efficiently induce BZLF1 transcription from the intact viral genome. Our finding that Na enhances R-induced lytic infections in certain epithelial lines, but not in two B-cell lines, suggests that c-Jun activation may be more important for BZLF1 transcription in some cell types than in others. Hence, Na may be more important for viral replication in some cells and/or tissues than in others.
Our results suggest (but do not prove) that Na may activate JNK. Alternatively, Na may inhibit the dephosphorylation of, or degradation of, phosphorylated c-Jun. JNK, along with p38, is a member of the SAPK family, and both of these kinases activate c-Jun and/or ATF-2 in response to cellular stress (
12,
23,
47,
62). SAPK activation is a common theme among herpesviruses, with herpes simplex virus type 1, CMV, Kaposi's sarcoma-associated herpesvirus, and EBV all inducing p38 and/or JNK during infection (
1,
3,
26,
40,
66,
67). Cross-linking of the B-cell receptor, an event that is thought to be a physiologically important stimulus for the in vivo reactivation of EBV (
57), also results in JNK and p38 activation (
56). Inhibition of the SAPKs results in a reduced efficiency of lytic infection for alpha-, beta-, and gammaherpesviruses (
1,
27,
40), suggesting that viruses have evolved to utilize the cellular SAPK pathway to both trigger and facilitate lytic infection. Members of our lab have previously demonstrated that both Z and R activate the SAPKs and that R-mediated SAPK induction is required for the efficient activation of Zp by R (
1). The ability of yet another lytic EBV protein, Na, to increase the levels of hyperphosphorylated c-Jun suggests that activated c-Jun is indeed important for efficient lytic EBV infection and that there may be multiple, redundant mechanisms encoded by the virus to ensure that this activation occurs.
In summary, we demonstrated here for the first time a function of the early Na gene product of EBV. Na enhances the ability of R to induce lytic infection by cooperating with R to activate the Z promoter, and this effect of Na appears to be more important in some cell types than in others. Whether there are additional Na-responsive viral promoters in the EBV genome (in addition to Zp) remains an open question. Our results suggest the existence of a previously unrecognized autostimulatory pathway for lytic EBV induction in which an initial activating stimulus (e.g., differentiation) which induces a low-level activation of either Zp or Rp would subsequently be amplified by the ability of Na to further induce Zp activation. Future insights into the mechanism of the Na function should allow a more thorough understanding of lytic cycle regulation, and given the cell type dependence of Na activity, could shed light on the cellular and viral characteristics that influence viral permissiveness.