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

Activated Notch1 modulates gene expression in B cells similarly to Epstein-Barr viral nuclear antigen 2

Both Epstein-Barr viral nuclear antigen 2 (EBNA2) and activated Notch transactivate genes by interacting with the transcription factor RBP-Jkappa. The viral protein EBNA2 may hence be regarded as a functional equivalent of an activated Notch receptor. Until now, nothing has been known about the physiological role of Notch signaling in B cells. Here we investigated whether activated Notch can induce the same phenotypic changes as EBNA2 in Burkitt's lymphoma cells. An estrogen receptor fusion protein of the intracellular part of mouse Notch 1 (mNotch1-IC), mimicking in the presence of estrogen a constitutively active Notch receptor, was stably transfected into the Burkitt's lymphoma cell lines BL41-P3HR1 and HH514. Northern blot analysis revealed that the LMP2A gene is induced by Notch-IC in the presence of estrogen, whereas increased expression of LMP1 could be detected only if cycloheximide was simultaneously added. Concerning the cellular genes regulated by EBNA2, Notch-IC was able to upregulate CD21 but not CD23 expression. Immunoglobulin mu (Igmu) expression, which is downregulated by EBNA2, was also negatively regulated by Notch-IC. Similarly to EBNA2, Notch-IC was able to repress c-myc expression, which is under the control of the immunoglobulin heavy-chain locus in Burkitt's lymphoma cells with a t(8;14) translocation. The data show that Notch-IC is able to participate in gene regulation in B cells.

Intracellular forms of human NOTCH1 functionally activate essential Epstein-Barr virus major latent promoters in the Burkitt's lymphoma BJAB cell line but repress these promoters in Jurkat cells

We have demonstrated that intracellular forms of NOTCH1 transactivate two major Epstein-Barr virus (EBV) latent promoters, the LMP1 and Cp1 promoters in an EBV-negative B-cell line, BJAB. Truncated intracellular NOTCH1 associated with the nuclear membrane (DeltaE) transactivates the LMP1 promoter fivefold; however, the intranucleus localized form of NOTCH1 (ICN) transactivates this promoter approximately twofold in chloroamphenicol acetyltransferase (CAT) reporter assays in BJAB cells. Additionally, DeltaE activated the major Cp1 promoter 12-fold, whereas the ICN form of NOTCH1 activates at only about half that level when compared to that of DeltaE membrane-bound NOTCH1. This result differs from previously observed data, where intracellular NOTCH1 bound to the nuclear membrane, DeltaE, and nucleus-localized NOTCH1, ICN, all had similar levels of activation in 293 cells. This suggests distinct transcriptional activities in different cell types. Moreover, in Jurkat cells, a T-cell line, intranucleus localized NOTCH1 molecules demonstrated a repressive activity against the two EBV major latent promoters. Only DeltaE activated the Cp1 and LMP1 promoters at a level slightly above background, whereas intranucleus localized NOTCH1 ICN, or the form of NOTCH1 lacking the ankyrin repeats, DeltaE(TAR), surprisingly resulted in the repression of these promoters in Jurkat cells. Similarly, another truncated form of NOTCH1, referred to as ICNW, which contains the tryptophan residue W(1767) within one of the RBP-Jkappa interacting domains, repressed the LMP1 promoter approximately twofold. Further analysis of the truncated NOTCH1 molecules on the LMP1 promoter element, lacking the two RBP-Jkappa binding sites, suggests that repression in Jurkat cells may be affected by the presence of the two RBP-Jkappa binding sites. These studies indicate that intracellular NOTCH1 can activate the EBV major latent promoters in BJAB cells. However, in Jurkat cells, intracellular truncated forms of NOTCH1 lacking the RBP-Jkappa binding sites repress these EBV latent promoters. Only the membrane-bound form of NOTCH1, DeltaE, activated the EBV major latent promoters in Jurkat cells, albeit at a lower level than that seen in BJAB cells. Our data suggest that EBNA2 and truncated intracellular nuclear localized forms of NOTCH1 may be functionally similar in their interactions with RBP-Jkappa; however, these molecules may have distinctly different transcriptional partners in BJAB and Jurkat cells. Moreover, these truncated NOTCH1 molecules may not represent the normal processed forms of NOTCH1 in cells and may exhibit dominant negative phenotypes in the absence of the required posttranslational modifications. Further investigations are necessary to determine the similarity and differences occurring with intracellular NOTCH1 in other B- and T-cell lines.

Epstein-Barr virus nuclear protein 2 interacts with p300, CBP, and PCAF histone acetyltransferases in activation of the LMP1 promoter

The Epstein-Barr virus (EBV) nuclear protein 2 (EBNA2) and herpes simplex virion protein 16 (VP16) acidic domains that mediate transcriptional activation now are found to have affinity for p300, CBP, and PCAF histone acetyltransferases (HATs). Transcriptionally inactive point mutations in these domains lack affinity for p300, CBP, or PCAF. P300 and CBP copurify with the principal HAT activities that bind to EBNA2 or VP16 acidic domains through velocity sedimentation and anion-exchange chromatography. EBNA2 binds to both the N- and C-terminal domains of p300 and coimmune-precipitates from transfected 293T cells with p300. In EBV-infected Akata Burkitt's tumor cells that do not express the EBV encoded oncoproteins EBNA2 or LMP1, p300 expression enhances the ability of EBNA2 to up-regulate LMP1 expression. Through its intrinsic HAT activity, PCAF can further potentiate the p300 effect. In 293 T cells, P300 and CBP (but not PCAF) can also coactivate transcription mediated by the EBNA2 or VP16 acidic domains and HAT-negative mutants of p300 have partial activity. Thus, the EBNA2 and VP16 acidic domains can utilize the intrinsic HAT or scaffolding properties of p300 to activate transcription.

Intracellular forms of human NOTCH1 interact at distinctly different levels with RBP-jkappa in human B and T cells

The cellular transcriptional repressor RBP-Jkappa associates with the Epstein-Barr virus nuclear antigens (EBNAs) determined to be essential for transformation of human primary B lymphocytes. It was demonstrated through genetic analysis that interaction between the viral transactivator EBNA2 and RBP-Jkappa is essential for EBV immortalization of primary B lymphocytes. We have shown that the association of RBP-Jkappa with intracellular NOTCH1 differs significantly in B and T cells. Immunoprecipitation analyses with antibodies to both the intracellular forms of NOTCH1 and to RBP-Jkappa demonstrated that little or no RBP-Jkappa is associated with NOTCH1 in B cell lines compared to the RBP-Jkappa associated with NOTCH1 in T cell lines and was further demonstrated in human primary lymphocytes. Additionally, EBNA2 can compete with intracellular NOTCH1 for binding to GST-RBP-Jkappa in vitro. Northern blot for the cellular gene hairy enhancer of split (HES1) demonstrated that HES1 is upregulated in the EBV transformed lymphoblastoid cells expressing high levels of EBNA2 and in a T cell line SupT1 overexpressing intracellular activated NOTCH1. Hence, EBNA2 may be able to compete for the available pool of RBP-Jkappa more effectively in human B cells than in T cells and provides a possible explanation for the ability of EBV to potently and efficiently infect and immortalize human B cells. Leukemia (2000) 14, 84-92.

An Epstein-Barr virus that expresses only the first 231 LMP1 amino acids efficiently initiates primary B-lymphocyte growth transformation

An Epstein-Barr virus (EBV) recombinant (MS231) that expresses the first 231 amino acids (aa) of LMP1 and is truncated 155 aa before the carboxyl terminus transformed resting B lymphocytes into lymphoblastoid cell lines (LCLs) only when the infected cells were grown on fibroblast feeder cells (K. M. Kaye et al., J. Virol. 69:675-683, 1995). Higher-titer MS231 virus has now been compared to wild-type (WT) EBV recombinants for the ability to cause resting primary B-lymphocyte transformation. Unexpectedly, MS231 is as potent as WT EBV recombinants in causing infected B lymphocytes to proliferate in culture for up to 5 weeks. When more than one transforming event is initiated in a microwell, the MS231 recombinant supports efficient long-term LCL outgrowth and fibroblast feeder cells are not required. However, with limited virus input, MS231-infected cells differed in their growth from WT virus-infected cells as early as 6 weeks after infection. In contrast to WT virus-infected cells, most MS231-infected cells could not be grown into long-term LCLs. Thus, the LMP1 amino-terminal 231 aa are sufficient for initial growth transformation but the carboxyl-terminal 155 aa are necessary for efficient long-term outgrowth. Despite the absence of the carboxyl-terminal 155 aa, MS231- and WT-transformed LCLs are similar in latent EBV gene expression, in ICAM-1 and CD23 expression, and in NF-kappaB and c-jun N-terminal kinase activation. MS231 recombinant-infected LCLs, however, require 16- to 64-fold higher cell density than WT-infected LCLs for regrowth after limiting dilution. These data indicate that the LMP1 carboxyl-terminal 155 aa are important for growth at lower cell density and appear to reduce dependence on paracrine growth factors.

Cytomegalovirus IE2 protein stimulates interleukin 1beta gene transcription via tethering to Spi-1/PU.1

Potent induction of the gene coding for human prointerleukin 1beta (il1b) normally requires a far-upstream inducible enhancer in addition to a minimal promoter located between positions -131 and +12. The transcription factor Spi-1 (also called PU.1) is necessary for expression and binds to the minimal promoter, thus providing an essential transcription activation domain (TAD). In contrast, infection by human cytomegalovirus (HCMV) can strongly activate il1b via the expression of immediate early (IE) viral proteins and eliminates the requirement for the upstream enhancer. Spi-1 has been circumstantially implicated as a host factor in this process. We report here the molecular basis for the direct involvement of Spi-1 in HCMV activation of il1b. Transfection of Spi-1-deficient HeLa cells demonstrated both the requirement of Spi-1 for IE activity and the need for a shorter promoter (-59 to +12) than that required in the absence of IE proteins. Furthermore, in contrast to normal, enhancer-dependent il1b expression, which absolutely requires both the Spi-1 winged helix-turn-helix (wHTH) DNA-binding domain and the majority of the Spi-1 TAD, il1b expression in the presence of IE proteins does not require the Spi-1 TAD, which plays a synergistic role. In addition, we demonstrate that a single IE protein, IE2, is critical for the induction of il1b. Protein-protein interaction experiments revealed that the wing motif within the Spi-1 wHTH domain directly recruits IE2. In turn, IE2 physically associates with the Spi-1 wing and requires the integrity of at least one region of IE2. Functional analysis demonstrates that both this region and a carboxy-terminal acidic TAD are required for IE2 function. Therefore, we propose a protein-tethered transactivation mechanism in which the il1b promoter-bound Spi-1 wHTH tethers IE2, which provides a TAD, resulting in the transactivation of il1b.

Strong selective pressure for evolution of an Epstein-Barr virus LMP2B homologue in the rhesus lymphocryptovirus

Latent membrane protein 2B (LMP2B) is expressed during latent Epstein-Barr virus (EBV) infection, but little is known about its role. The goal of this study was to determine whether an LMP2B homologue is conserved in the rhesus monkey lymphocryptovirus (LCV). Both rhesus LCV LMP2A and LMP2B genes were cloned and sequenced. The rhesus LCV LMP2B gene is positionally conserved, and the EBNA-2 responsiveness and the bidirectional nature of the LMP1-LMP2B promoter have also been functionally conserved. However, this region of the genome encoding the LMP1, LMP1-LMP2B promoter, and LMP2B first exon demonstrates the most dramatic nucleotide sequence divergence between human and nonhuman LCV observed to date. Evolution of the rhesus LCV LMP2B promoter and transcript despite the dynamic nature of this genomic region reflects strong selective pressure for a yet-to-be-identified LMP2B function.

Characterization of DP103, a novel DEAD box protein that binds to the Epstein-Barr virus nuclear proteins EBNA2 and EBNA3C

The Epstein-Barr virus-encoded nuclear antigens EBNA2 and EBNA3C both interact with the cellular transcription factor RBP-Jkappa and modulate the expression of several shared target genes, suggesting a tight cooperation in latently infected cells. In a survey for additional cellular factors that bind to EBNA2 as well as EBNA3C, we have isolated and characterized DP103, a novel human member of the DEAD box family of putative ATP-dependent RNA helicases. The interaction with DP103 is mediated by amino acids (aa) 121-213 of EBNA2 and aa 534-778 of EBNA3C, regions that are not involved in binding of the viral proteins to RBP-Jkappa. The DP103-cDNA encodes a protein of 824 aa that harbors all of the common DEAD box motifs. Monoclonal antibodies raised against DP103 detect a protein of 103 kDa in mammalian cells that resides in high molecular weight complexes in vivo. We have detected an ATPase activity intrinsic to or closely associated with DP103. By subcellular fractionation, we find DP103 in both a soluble nuclear fraction as well as in the insoluble skeletal fraction. Whereas the protein and its mRNA are uniformly expressed in all tested cell lines, we observed differential expression of the mRNA in normal human tissues.

Epstein-barr virus (EBV) nuclear protein 2-induced disruption of EBV latency in the Burkitt's lymphoma cell line Akata: analysis by tetracycline-regulated expression

The Burkitt's lymphoma (BL) cell line Akata retains the latency I program of Epstein-Barr virus (EBV) gene expression and cross-linking of its surface immunoglobulin G (IgG) by antibodies results in activation of viral replication. When EBV nuclear antigen 2 (EBNA2) was artificially expressed by a constitutive expression vector, the Cp EBNA promoter remained inactive and accordingly the latency III program was not induced. In contrast, expression of LMP2A and activity of the Fp lytic promoter were activated. Consistent with this Fp activity, the rate of spontaneous activation of the EBV replicative cycle was increased significantly, suggesting the possibility that EBNA2 can induce EBV replication. The efficiency of anti-IgG-induced activation of the viral replication was reduced in Akata cells expressing EBNA2. To obtain more direct evidence for EBNA2-induced activation of the EBV replicative cycle, this protein was next expressed by a tetracycline-regulated expression system. EBNA2 was undetectable with low doses (<0.5 microgram/ml) of tetracycline, while its expression was rapidly induced after removal of the antibiotic. This induced expression of EBNA2 was immediately followed by expression of EBV replicative cycle proteins in up to 50% of the cells, as shown by indirect immunofluorescence and immunoblot analysis. These results suggest an unexpected potential of EBNA2 to disrupt EBV latency and to activate viral replication.

LMP-1, the Epstein-Barr virus-encoded oncogene with a B cell activating mechanism similar to CD40

Many details in the expression pattern of Epstein-Barr virus (EBV)-encoded proteins, their role in blast and growth transformation in infected B lymphocytes are known, but 'black holes' still exist. The two main types of virus-B lymphocyte interactions are denoted as Type I and Type III. These are characterized by the difference in the EBV protein expression which is related to the phenotype of the cell. The difference was first detected in comparisons between Burkitt lymphoma cells (BL) and lymphoblastoid cell lines, LCLs, which arise from normal B lymphocytes after experimental infection and are growth transformed by EBV. A third type of interaction can be seen in B-CLL cells which carry the EBV receptor CD21 and can be thus infected with the virus in vitro. The infected cells express the EBV-encoded proteins with a pattern which is different from the above mentioned two types, in that they express the nuclear proteins but not the membrane localized LMP-1. Importantly, the infected B-CLL cells do not enter DNA synthesis and they do not growth transform. Normal B lymphocytes with similar expression pattern have been seen in analysis of the lymphoreticular tissues of patients which responded to the primary EBV infection with development of the infectious mononucleosis symptoms.

The proto-oncogene c-myc is a direct target gene of Epstein-Barr virus nuclear antigen 2

Epstein-Barr virus (EBV) infects and transforms primary B lymphocytes in vitro. Viral infection initiates the cell cycle entry of the resting B lymphocytes. The maintenance of proliferation in the infected cells is strictly dependent on functional EBNA2. We have recently developed a conditional immortalization system for EBV by rendering the function of EBNA2, and thus proliferation of the immortalized cells, dependent on estrogen. This cellular system was used to identify early events preceding induction of proliferation. We show that LMP1 and c-myc are directly activated by EBNA2, indicating that all cellular factors essential for induction of these genes by EBNA2 are present in the resting cells. In contrast, induction of the cell cycle regulators cyclin D2 and cdk4 are secondary events, which require de novo protein synthesis.

Activated mouse Notch1 transactivates Epstein-Barr virus nuclear antigen 2-regulated viral promoters

Epstein-Barr virus nuclear antigen 2 (EBNA2) is essential for B-cell immortalization by EBV, most probably by its ability to transactivate a number of cellular and viral genes. EBNA2-responsive elements (EBNA2REs) have been identified in several EBNA2-regulated viral promoters, each of them carrying at least one RBP-Jkappa recognition site. RBP-Jkappa recruits EBNA2 to the EBNA2RE and, once complexed to EBNA2, is converted from a repressor into an activator. An activated form of the cellular receptor Notch also interacts with RBP-Jkappa, providing a link between EBNA2 and Notch signalling. To determine whether activated Notch is able to transactivate EBNA2-responsive viral promoters, we performed cotransfection experiments with activated mouse Notch1 (mNotch1-IC) and luciferase constructs of the BamHI C, LMP1, and LMP2A promoters. We present here evidence that mNotch1-IC transactivates viral promoters known to be regulated by EBNA2. As shown for EBNA2, mutations or deletions of the RBP-Jkappa sites diminish or eliminate mNotch1-IC-mediated transactivation of the promoters, pointing to an essential role for Notch-RBP-Jkappa interaction. In addition to RBP-Jkappa, other cellular factors may bind within the EBNA2REs of viral promoters. While some factors appear to play an important role in both EBNA2- and mNotch1-IC-mediated transactivation, others are only important for the activity of either EBNA2 or mNotch1-IC. We could observe specific mNotch1-IC-responsive regions, thereby throwing more light upon which cofactors interact with EBNA2 and mNotch1-IC, thus enabling them to become functionally transactivators in vivo.

Silencing of the Epstein-Barr virus latent membrane protein 1 gene by the Max-Mad1-mSin3A modulator of chromatin structure

The tumor-associated latent membrane protein 1 (LMP1) gene in the Epstein-Barr virus (EBV) genome is activated by EBV-encoded proteins and cellular factors that are part of general signal transduction pathways. As previously demonstrated, the proximal region of the LMP1 promoter regulatory sequence (LRS) contains a negative cis element with a major role in EBNA2-mediated regulation of LMP1 gene expression in B cells. Here, we show that this silencing activity overlaps with a transcriptional enhancer in an LRS sequence that contains an E-box-homologous motif. Mutation of the putative repressor binding site relieved the repression both in a promoter-proximal context and in a complete LRS context, indicating a functional role of the repressor. Gel retardation assays showed that members of the basic helix-loop-helix transcription factor family, including Max, Mad1, USF, E12, and E47, and the corepressor mSin3A bound to the E-box-containing sequence. The enhancer activity correlated with the binding of USF. Moreover, the activity of the LMP1 promoter in reporter constructs was upregulated by overexpression of USF1 and USF2a, and the transactivation was inhibited by the concurrent expression of Max and Mad1. This suggests that Max-Mad1-mediated anchorage of a multiprotein complex including mSin3A and histone deacetylases to the E-box site constitutes the basis for the repression. Removal of acetyl moieties from histones H3 and H4 should result in a chromatin structure that is inaccessible to transcription factors. Accordingly, inhibition of deacetylase activity with trichostatin A induced expression of the endogenous LMP1 gene in EBV-transformed cells.

Residues 231 to 280 of the Epstein-Barr virus nuclear protein 2 are not essential for primary B-lymphocyte growth transformation

Epstein-Barr virus (EBV) nuclear protein 2 (EBNA-2) is a transcriptional transactivator of cellular and viral gene expression and is essential for the transformation of resting human B lymphocytes into long-term lymphoblastoid cell lines (LCLs). Previous molecular genetic analyses identified three domains that are critical for transformation and showed that the rest of EBNA-2 is not critical. We now find that codons 231 to 280 that were part of one of the critical domains (J. I. Cohen, F. Wang, and E. Kieff, J. Virol. 65:2545-2554, 1991) can be deleted with only a small effect on the ability of EBNA-2 to transactivate gene expression. In transient transfection assays, EBNA-2 deleted for codons 231 to 280 accumulated to higher levels and was similar to wild-type EBNA-2 in activation of the BamC promoter and in association with RBPJk, a cellular transcription factor that is important for EBNA-2 interaction with promoter regulatory elements. However, EBNA-2 d231-280 activated the viral latent membrane protein 1 (LMP1) promoter with only 60% of wild-type efficiency. Recombinant EBVs specifically deleted for EBNA-2 codons 231 to 280 were efficient in initiating the transformation of resting primary human B lymphocytes into LCLs. However, these LCLs grew less well than wild-type EBV-transformed LCLs, and 4- to 10-fold more cells were required for outgrowth following limit dilution. EBNA-2 d231-280 accumulated to unusually high levels in the recombinant transformed LCLs, and this was associated with somewhat higher EBNA-1 and lower LMP1 expression, consistent with the near-wild-type activation of the BamC EBNA promoter and the abnormally low activation of the LMP1 promoter in transient transfection assays. Thus, EBNA-2 d231-280 modestly perturbed the regulation of viral gene expression and resulted in less LMP1, while having surprisingly subtle effects on LCL outgrowth. Deletion of EBNA-2 codons 292 to 310, which are closer to the site that specifies interaction with RBPJk, was more disruptive of RBPJk association and of the ability to transform B lymphocytes.

An ATF/CRE element mediates both EBNA2-dependent and EBNA2-independent activation of the Epstein-Barr virus LMP1 gene promoter

The Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) is a viral oncogene whose expression is regulated by both viral and cellular factors. EBV nuclear antigen 2 (EBNA2) is a potent transactivator of LMP1 expression in human B cells, and several EBNA2 response elements have been identified in the promoter regulatory sequence (LRS). We have previously shown that an activating transcription factor/cyclic AMP response element (ATF/CRE) site in LRS is involved in EBNA2 responsiveness. We now establish the importance of the ATF/CRE element by mutational analysis and show that both EBNA2-dependent activation and EBNA2-independent activation of the promoter occur via this site but are mediated by separate sets of factors. An electrophoretic mobility shift assay (EMSA) with specific antibodies showed that the ATF-1, CREB-1, ATF-2 and c-Jun factors bind to the site as ATF-1/CREB-1 and ATF-2/c-Jun heterodimers whereas the Sp1 and Sp3 factors bind to an adjacent Sp site. Overexpression of ATF-1 and CREB-1 in the cells by expression vectors demonstrated that homodimeric as well as heterodimeric forms of the factors transactivate the LMP1 promoter in an EBNA2-independent manner. The homodimers of ATF-2 and c-Jun did not significantly stimulate promoter activity. In contrast, the ATF-2/c-Jun heterodimer had only a minor stimulatory effect in the absence of EBNA2 but induced a strong transactivation of the LMP1 promoter when coexpressed with this protein. Evidence for a direct interaction between the ATF-2/c-Jun heterodimeric complex and EBNA2 was obtained by EMSA and coimmunoprecipitation experiments. Thus, our results suggest that EBNA2-induced transactivation via the ATF/CRE site occurs through a direct contact between EBNA2 and an ATF-2/c-Jun heterodimer. EBNA2-independent promoter activation via this site, on the other hand, is mediated by a heterodimeric complex between the ATF-1 and CREB-1 factors.

An Intronic Silencer Regulates B Lymphocyte Cell- and Stage-Specific Expression of the Human Complement Receptor Type 2 (CR2, CD21) Gene

Human CR2 (CD21) is a B lymphocyte protein whose surface expression is restricted primarily to the mature cell stage during development. To study the transcriptional mechanisms that govern cell- and stage-restricted CR2 expression, we first performed transient transfection analysis using constructs extending from −5 kb to +75 bp (−5 kb/+75) in the CR2 promoter. The promoter was found to be broadly active, with no evidence of cell- or stage-specific reporter gene expression. However, the addition of a 2.5-kb intronic gene segment (containing a DNase I hypersensitive site) to the (−5-kb/+75) construct resulted in appropriate reporter gene expression, defined as the silencing of the (−5-kb/+75) promoter activity only in non-CR2-expressing cells. Interestingly, appropriate reporter gene expression required stable transfection of the constructs in cell lines, suggesting nuclear matrix or chromatin interactions may be important for appropriate CR2 gene expression. Importantly, transgenic mice also required the intronic silencer to generate lymphoid tissue-specific reporter gene expression. Some transgenic founder lines did not demonstrate reporter gene expression, however, indicating that additional transcriptional regulatory elements are present in other regions of the CR2 gene. In summary, these data support the hypothesis that human CR2 expression is regulated primarily by an intronic silencer with lineage- and B cell stage-specific activity.

Characterization of the CBF2 binding site within the Epstein-Barr virus latency C promoter and its role in modulating EBNA2-mediated transactivation

The Epstein-Barr virus (EBV) EBNA2 protein is a transcriptional activator that regulates viral and cellular gene expression and is also essential for EBV-driven immortalization of B lymphocytes. The EBNA2-responsive enhancer in the viral latency C promoter (Cp) binds two cellular factors, CBF1 and CBF2. The precise role of the CBF2 protein for Cp enhancer function is presently unclear. CBF2 does not appear to interact with EBNA2 and binds just downstream of CBF1 between positions -339 and -368 in the Cp EBNA2 enhancer. Within this region an 8-bp sequence, CAGTGCGT, can be found, and a similar sequence is also located downstream of CBF1 binding sites in other EBNA2-responsive promoters. Previous studies have indicated that mutations and methylation in this sequence affect EBNA2 responsiveness. To investigate the requirements for CBF2 binding, we synthesized a series of oligonucleotides carrying double transversion mutations spanning both the conserved core sequence and outside flanking sequences. Surprisingly, mutations outside of the conserved core sequence in 4 bases immediately flanking the 5' end, GGTT, had the most deleterious effect on CBF2 binding. Mutations in the conserved core had a gradient effect, with those near the 5' end having the most deleterious effects on CBF2 binding. In addition, the affinities of CBF2 for binding to the LMP-1, LMP-2, and CD23 promoters were also measured. These promoters contain the conserved core but lack the 5' flanking GGTT motif and bound CBF2 weakly or not at all. Using Cp reporter plasmids containing CBF2 mutant binding sites, we were also able to show that at lower doses of EBNA2, Cp transactivation required a functional CBF2 binding site but that higher doses of EBNA2 transactivated CBF2 mutant promoters to 40% of wild-type levels. These assays indicate that CBF2 is important for EBNA2-mediated transactivation of the viral latency Cp. In addition, CBF2 activity was found to be associated with two polypeptides of 27 and 33 kDa.

B-cell lines immortalized with an Epstein-Barr virus mutant lacking the Cp EBNA2 enhancer are biased toward utilization of the oriP-proximal EBNA gene promoter Wp1

During Epstein-Barr virus (EBV) latent infection of B lymphocytes in vitro, six viral nuclear antigens (EBNAs) are expressed from one of two promoters, Cp or Wp, whose activities have previously been shown to be mutually exclusive in established lymphoblastoid cell lines. Initially after infection, the EBNA genes are transcribed from Wp, which is present in multiples copies within the major internal repeat of EBV. Approximately 48 to 72 h postinfection, Wp is downregulated, with a corresponding increase in transcription from Cp. An EBNA2-responsive enhancer exists upstream of Cp, and a role for EBNA2 in the induction of Cp activity during the establishment of viral latency has previously been proposed (Woisetschlaeger et al., Proc. Natl. Acad. Sci. USA 87:1725-1729, 1991). To critically assess the potential role for this enhancer region in determining relative usage of Cp and Wp, an EBNA2 enhancer deletion mutant virus was generated. Lymphoblastoid cell lines were screened by PCR and Southern blotting for the presence of mutant virus harboring the EBNA2 enhancer deletion. A quantitative S1 nuclease protection assay was developed to allow comparison of relative Cp and Wp activities for the cell lines containing mutant virus and those of the wild-type recombinants which lacked the enhancer deletion. In general, the wild-type recombinants had higher levels of Cp-initiated transcripts than Wp-initiated transcripts. In contrast, the Cp EBNA2 enhancer deletion mutants exhibited a strong bias toward Wp activity. Notably, only the first Wp (oriP-proximal Wp; Wp1) appears active in these mutants. S1 nuclease protection assays using a probe which hybridizes to the W2 exon, contained in both Cp- and Wp-initiated transcripts, indicated that the total level of transcription from Cp and Wp remained the same in wild-type and EBNA2 enhancer mutant cell lines. The presence of both Cp and Wp activity in the wild-type recombinants, as well as in newly derived lymphoblastoid cell lines established with the prototype B95.8 virus, demonstrated that Cp and Wp activities are not always mutually exclusive.

Epstein-Barr virus EBNA3C represses Cp, the major promoter for EBNA expression, but has no effect on the promoter of the cell gene CD21

EBNA3C is a potent repressor of transcription when bound to DNA as a fusion with the DNA binding domain (DBD) of GALA. A survey of promoters has revealed that the wild-type, unfused EBNA3C can specifically repress expression from reporter plasmids containing the Epstein-Barr virus Cp latency-associated promoter. Repression of Cp activity required amino acids 207 to 368, which encompasses a region resembling a basic DBD adjacent to a leucine zipper DNA binding motif and a site which binds to the cellular factor CBF1/RBP-Jkappa. However, amino acids 207 to 368 are dispensable when the protein is bound to DNA as a fusion with the GAL4 DBD, thus implicating this region in DNA binding. Mutation of the CBF1/RBP-Jkappa binding site in EBNA3C abrogated repression, strongly suggesting that CBF1/RBP-Jkappa is necessary for targeting the viral protein to Cp. Consistent with this result, mutation of the EBNA2 response element (a CBF1/RBP-Jkappa binding site) in Cp also prevented significant repression. In addition, amino acids 346 to 543, which were previously defined as important for the repressor activity of the GAL4-EBNA3C fusion proteins, also appear to be necessary for the repression of Cp. Since repression by these fusions was not observed in all cell types, it seems likely that EBNA3C either depends on a corepressor which may interact with amino acids 346 to 543 or is modified in a cell-specific manner in order to repress. These data are consistent with EBNA3C contributing to the regulation of EBNA expression in latently infected B cells through CBF1/RBP-Jkappa and another factor, but this need not directly involve EBNA2. Finally, although it has been reported that EBNA3C can upregulate CD21 in some B cells, we were unable to demonstrate any effect of EBNA3C on reporter plasmids which contain the CD21 promoter.

Epstein-Barr virus genes and cancer cells

Human B lymphocytes infected with Epstein-Barr virus (EBV) express 11 viral genes, of which six are essential for efficient transformation. The protein products of these genes appear to cause cell growth by modifying cell signal transduction pathways. For example, EBNA-2 mimics the Notch 1 pathway and LMP-1 interacts with the signalling from CD40/CD40-L, which promotes growth in normal B cells. In the human cancers linked to EBV, most of the viral transforming genes are not expressed. It is likely that growth of these cells is controlled by a combination of the EBV genes whose expression continues with altered cell proto-oncogenes and tumour suppressor genes, but other explanations of the role of EBV in cancer cells are also possible. The presence of the virus in the tumour cells of EBV-associated cancers constitutes a potentially useful tumour specific marker that might be used to direct therapy to the tumour cells.

Epstein-Barr virus leader protein enhances EBNA-2-mediated transactivation of latent membrane protein 1 expression: a role for the W1W2 repeat domain

The Epstein-Barr virus (EBV)-encoded leader protein EBNA-LP is made up of several 66-amino-acid repeats (the W1W2 domains) linked to a unique 45-amino-acid C-terminal sequence (the Y1Y2 domain). This protein is highly expressed along with a second nuclear antigen, EBNA-2, during the initial stages of virus-induced B-cell transformation. While EBNA-2's essential role in transformation as a transcriptional activatory is well documented, very little is known about EBNA-LP function except that recombinant viruses lacking the EBNA-LP Y1Y2 exons show reduced, but still detectable, transforming ability. This was taken as evidence that EBNA-LP plays an auxiliary role but is not essential for transformation. A recent study showed that EBNA-LP could cooperate with EBNA-2 in activating cyclin D2 transcription in resting B cells (A.J. Sinclair, L Palmero, G. Peters, and P.J. Farrell, EMBO J. 13:3321-3328, 1994). Here we report that EBNA-LP can also cooperate with EBNA-2 in up-regulating expression of the major EBV effector protein of B-cell transformation, latent membrane protein 1 (LMP1). In transient-transfection assays, EBNA-LP enhanced the level of EBNA-2-induced LMP1 expression by 5- to 10-fold in one Latency I Burkitt's lymphoma cell line, Eli-BL, and was absolutely required, along with EBNA-2, to induce LMP1 in a second line, Akata-BL. These changes in LMP1 protein expression appeared to be reflected at the transcriptional level. A study of EBNA-LP mutants showed that this cooperative function mapped to the W1W2 repeat domain rather than to Y1Y2. Because a Y1Y2-deleted form of EBNA-LP may therefore retain some aspects of wild-type function, the original data from virus recombinants leave open the possibility that EBNA-LP is actually an essential transforming gene.

Epstein-Barr virus nuclear protein LP stimulates EBNA-2 acidic domain-mediated transcriptional activation

Epstein-Barr virus (EBV) nuclear proteins EBNA-LP and EBNA-2 are the first two proteins expressed in latent infection of primary B lymphocytes. EBNA-2 is essential for lymphocyte transformation, and EBNA-LP is at least critical. While EBNA-2 activates specific viral and cellular promoters, EBNA-LP's role has been obscure. We now show that EBNA-LP stimulates EBNA-2 activation of the LMP1 promoter and of the LMP1/LMP2B bidirectional transcriptional regulatory element. EBNA-LP alone has only a negative effect. EBNA-LP also stimulates EBNA-2 activation of a multimerized regulatory element from the BamC EBNA promoter. Since both viral regulatory elements can bind the EBNA-2-associated cell protein RBPJ kappa, consensus RBPJ kappa binding sites were positioned upstream of the herpes simplex virus type 1 thymidine kinase promoter and were found to be sufficient for EBNA-LP and EBNA-2 coactivation. EBNA-LP strongly stimulated activation of an adenovirus E1b promoter with upstream Gal4 binding sites by a Gal4 DNA binding domain/ EBNA-2 acidic domain fusion protein, indicating that EBNA-LP coactivation requires only the EBNA-2 acidic domain to be localized near a promoter. The EBNA-LP stimulatory activity resides in the amino-terminal 66-amino-acid repeat domain. The carboxyl-terminal unique 45 amino acids appear to regulate EBNA-LP's effects. The first 11 amino acids of the 45 have a strong negative effect, while the last 10 are critical for the ability of the last 34 to relieve the negative effect. These results indicate that EBNA-LP's critical role in EBV-mediated cell growth transformation is in stimulating (and probably regulating) EBNA-2-mediated transcriptional activation.

Sequence Analysis of the Epstein-Barr Virus (EBV) Latent Membrane Protein-1 Gene and Promoter Region: Identification of Four Variants Among Wild-Type EBV Isolates

Sequence variations in the Epstein-Barr virus (EBV) encoded latent membrane protein-1 (LMP-1) gene have been described in a Chinese nasopharyngeal carcinoma-derived isolate (CAO), and in viral isolates from various EBV-associated tumors. It has been suggested that these genetic changes, which include loss of a Xho I restriction site (position 169425) and a C-terminal 30-base pair (bp) deletion (position 168287-168256), define EBV genotypes associated with increased tumorigenicity or with disease among particular geographic populations. To determine the frequency of LMP-1 variations in European wild-type virus isolates, we sequenced the LMP1 promoter and gene in EBV from lymphoblastoid cell lines from healthy carriers and patients without EBV-associated disease. Sequence changes were often present, and defined at least four main groups of viral isolates, which we designate Groups A through D. The widespread prevalence of LMP-1 sequence variations, particularly the Xho I polymorphism and the 30-bp deletion, indicate that they cannot be used as simple markers for oncogenic viruses related to particular forms of EBV-associated tumor. Several of the structural changes detected occur, however, at sites where they may affect transcription, translation, or function of LMP-1. Future in vitro studies should aim to establish the functional importance of variations at these sites.

Sequence Analysis of the Epstein-Barr Virus (EBV) Latent Membrane Protein-1 Gene and Promoter Region: Identification of Four Variants Among Wild-Type EBV Isolates

Sequence variations in the Epstein-Barr virus (EBV) encoded latent membrane protein-1 (LMP-1) gene have been described in a Chinese nasopharyngeal carcinoma-derived isolate (CAO), and in viral isolates from various EBV-associated tumors. It has been suggested that these genetic changes, which include loss of a Xho I restriction site (position 169425) and a C-terminal 30-base pair (bp) deletion (position 168287-168256), define EBV genotypes associated with increased tumorigenicity or with disease among particular geographic populations. To determine the frequency of LMP-1 variations in European wild-type virus isolates, we sequenced the LMP1 promoter and gene in EBV from lymphoblastoid cell lines from healthy carriers and patients without EBV-associated disease. Sequence changes were often present, and defined at least four main groups of viral isolates, which we designate Groups A through D. The widespread prevalence of LMP-1 sequence variations, particularly the Xho I polymorphism and the 30-bp deletion, indicate that they cannot be used as simple markers for oncogenic viruses related to particular forms of EBV-associated tumor. Several of the structural changes detected occur, however, at sites where they may affect transcription, translation, or function of LMP-1. Future in vitro studies should aim to establish the functional importance of variations at these sites.

Identification of a promoter for the latent membrane protein 1 gene of Epstein-Barr virus that is specifically activated in human epithelial cells

Latent membrane protein 1 (LMP 1) is one of two Epstein-Barr virus (EBV)-encoded proteins that expressed in nasopharyngeal carcinoma (NPC) cells. Previous studies showed that a 3.5-kb transcript of the LMP 1 gene, in addition to the 2.8-kb transcript, was detected in a B95-8-EBV-containing, nude mice-passaged NPC tumor, C15. This indicated that a transcript was initiated from a region 5' to the putative promoter, ED-L1. We have isolated an EBV variant from a NPC tissue, and this virus strain contained a more pathogenic LMP 1 gene. DNA sequence analysis of the 5'-upstream region showed distinct variations as compared to that of B95-8 strain. To test if the LMP 1 gene of the NPC strain also contained an upstream promoter, we generated a series of deletion plasmids encompassing positions -1,030 to +20 of the LMP 1 promoter and tested for their abilities to drive the expression of the reporter gene in human epithelial cell lines, C-33A and NPC-TW076. We found that the region between -643 and -496 contained a promoter activity that was approximately five-fold higher than the putative promoter, ED-L1. This region between -643 and -496 was designated as ED-L1E. C-33A cells containing the genomic clone pT7(E) or the clone that had deleted a 94-bp ED-L1 sequence (delta94) was used to determine the transcription initiation sites by RNase protection assay. Results showed that a transcription initiation site was located at nucleotide 170,099 ("A") of EBV genome. The transcript was expressed in NPC biopsies and in human primary normal epithelial cells transfected with pT7(E) and delta94, respectively, as examined by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Furthermore, the ED-L1E was not regulated by the EBV-encoded nuclear antigen 1-mediated transcriptional enhancer family of repeats (FR) in C-33A cells. Our results suggested that the ED-L1E was specifically activated in epithelial cells. The biological significance of the selective usage of the ED-L1E promoter was discussed.

Oncogenic forms of NOTCH1 lacking either the primary binding site for RBP-Jkappa or nuclear localization sequences retain the ability to associate with RBP-Jkappa and activate transcription

Truncated forms of the NOTCH1 transmembrane receptor engineered to resemble mutant forms of NOTCH1 found in certain cases of human T cell leukemia/lymphoma (T-ALL) efficiently induce T-ALL when expressed in the bone marrow of mice. Unlike full-sized NOTCH1, two such truncated forms of the protein either lacking a major portion of the extracellular domain (DeltaE) or consisting only of the intracellular domain (ICN) were found to activate transcription in cultured cells, presumably through RBP-Jkappa response elements within DNA. Both truncated forms also bound to the transcription factor RBP-Jkappa in extracts prepared from human and murine T-ALL cell lines. Transcriptional activation required the presence of a weak RBP-Jkappa-binding site within the NOTCH1 ankyrin repeat region of the intracellular domain. Unexpectedly, a second, stronger RBP-Jkappa-binding site, which lies within the intracellular domain close to the transmembrane region and significantly augments association with RBP-Jkappa, was not needed for oncogenesis or for transcriptional activation. While ICN appeared primarily in the nucleus, DeltaE localized to cytoplasmic and nuclear membranes, suggesting that intranuclear localization is not essential for oncogenesis or transcriptional activation. In support of this interpretation, mutation of putative nuclear localization sequences decreased nuclear localization and increased transcriptional activation by membrane-bound DeltaE. Transcriptional activation by this mutant form of membrane-bound DeltaE was approximately equivalent to that produced by intranuclear ICN. These data are most consistent with NOTCH1 oncogenesis and transcriptional activation being independent of association with RBP-Jkappa at promoter sites.

Epstein-Barr virus immortalization: Notch2 interacts with CBF1 and blocks differentiation

EBNA2 is essential for immortalization of B cells by Epstein-Barr virus. EBNA2 is tethered to responsive promoters through a cellular factor, CBF1. CBF1 also binds to the activated form of mammalian Notch1, providing a linkage between EBNA2 function and Notch signalling. However, Notch2 is the predominant form expressed in spleen. The degree to which these Notch homologs are functionally convergent is not known. We present evidence that Notch2 also signals through CBF1. As is the case for Notch1, Notch2 interacted with the minimal repression domain of CBF1 and was targeted to CBF1 through the intracellular, subtransmembrane domain. Additional characterization suggested that the interaction domain of Notch may be bipartite. The intracellular domain of Notch2 (Notch2IC) located to the nucleus. This activated form of Notch2 transactivated expression of a target gene containing upstream CBF1 binding sites. The use of CBF1 mutants carrying amino acid substitutions in the transcriptional repression domain revealed that activation of gene expression by Notch2 is also based on masking of CBF1-mediated repression. Targeting of Notch1 and targeting of Notch2 were found to be identical and distinguishable from targeting by EBNA2. Mutation of CBF1 at codons 249 to 251 abolished interaction with both Notch proteins but not with EBNA2. In a biological examination of Notch2 function in muscle cells, Notch2IC activated endogenous HES-1 gene expression and blocked muscle cell differentiation. Overall, the data imply that at least a subset of the intracellular events following signalling in cells expressing Notch2 are common to those in Notch1-expressing cells. The concept that EBNA2 functions by mimicking Notch signalling is therefore viable whether cells are expressing Notch1 or Notch2.

oriP is essential for EBNA gene promoter activity in Epstein-Barr virus-immortalized lymphoblastoid cell lines

During Epstein-Barr virus latent infection of B lymphocytes in vitro, six viral nuclear antigens (EBNAs) are expressed from one of two promoters, Cp or Wp, whose activities are mutually exclusive. Upon infection, Wp is initially active, followed by a switch to Cp for the duration of latency. In this study, the region upstream of Cp was analyzed for the presence of cis elements involved in regulating the activities of the EBNA gene promoters in established in vitro immortalized lymphoblastoid cell lines (LCLs). It was determined that oriP, the origin for episomal maintenance during latency, is essential for efficient transcription initiation from either Cp or Wp in LCLs, as well as in some Burkitt's lymphoma cell lines. Deletion of the EBNA2-dependent enhancer located upstream of Cp resulted in a ca. two- to fivefold reduction in Cp activity in the LCLs assayed. More extensive deletion of sequences upstream of Cp, including the EBNA2-dependent enhancer, resulted in nearly complete loss of Cp activity. This loss of activity was shown to correlate with deletion of two CCAAT boxes, a proximal CCAAT box located at bp -61 to -65 and a distal CCAAT box located at bp -253 to -257, upstream of Cp. Site-directed mutagenesis of these cis elements demonstrated that Cp activity is highly dependent on the presence of a properly positioned CCAAT box, with the dependence on the distal CCAAT box apparent only when the proximal CCAAT box was deleted or mutated. Deletion of the glucocorticoid response elements located at ca. bp -850 upstream of Cp did not result in a significant loss in activity. In general, deletions which diminished Cp activity resulted in induction of Wp activity, consistent with suppression of Wp activity by transcriptional interference from Cp. The identification of oriP and the EBNA2-dependent enhancer as the major positive cis elements involved in regulating Cp activity in LCL suggests that EBNA gene transcription is largely autoregulated by EBNA 1 and EBNA 2.

Epstein-Barr virus EBNA3A and EBNA3C proteins both repress RBP-J kappa-EBNA2-activated transcription by inhibiting the binding of RBP-J kappa to DNA

Following infection by Epstein-Barr virus (EBV), the production of viral nuclear proteins EBNA1, EBNA2, EBNA3A, and EBNA3C and the viral membrane protein LMP1 is essential for the permanent proliferation of primary B lymphocytes to occur. Among these, the transcription factor EBNA2 is central to the immortalizing process, since it activates not only the transcription of all the EBNA proteins and LMP1, TP1, and TP2 but also certain cellular genes. EBNA2 is targeted to its DNA-responsive elements through direct interaction with the DNA-binding cellular repressor RBP-J kappa. In a transient-expression assay, the EBNA2-activated transcription was found to be downregulated by EBNA3A, EBNA3B, and EBNA3C. However, since it has been reported that EBNA3C, but not EBNA3A, directly contacts RBP-J kappa in vitro, these proteins appear to repress through different mechanisms. Here, we report for the first time that EBNA3A and EBNA3C both stably interact with RBP-J kappa and most probably repress EBNA2-activated transcription by destabilizing the binding of RBP-J kappa to DNA.

Epstein-Barr virus nuclear protein 2 (EBNA2) binds to a component of the human SNF-SWI complex, hSNF5/Ini1

Epstein-Barr nuclear antigen 2 (EBNA2), one of the six viral nuclear proteins expressed in latently infected B lymphocytes, is essential to the immortalization of B cells by Epstein-Barr virus (EBV). EBNA2 promotes transcriptional transactivation of viral and cellular genes by acting as an adapter molecule that binds to cellular sequence-specific DNA-binding proteins, JK recombination signal-binding protein (RBP-JK), and PU.1 and engages multiple members of the RNA polymerase II transcription complex. In the present study, we show that EBNA2 also interacts with hSNF5/Ini1, the human homolog of the yeast transcription factor SNF5. Gel filtration fractionation of partially purified EBV-positive lymphocyte nuclear extracts shows that a fraction of EBNA2 coelutes with both hSNF5/Ini1 and BRG1, a human homolog of SWI/SNF2, in the high-molecular-mass region (1.5 to 2.0 MDa) of a Superose 6 chromatogram. An affinity-purified rabbit antibody directed against hSNF5/Ini1 coimmunoprecipitates EBNA2 from this high-molecular-mass nuclear protein fraction, demonstrating that EBNA2 and hSNF5/Ini1 interact in vivo. This interaction is restricted to a subpopulation of phosphorylated viral EBNA2. Deletion mutation analysis of EBNA2 shows that the proline-rich aminoterminal end and a domain within the divergent region of EBNA2 mediate EBNA2-hSNF5/Ini1 interaction. Since the SNF-SWI complex participates in gene regulation through the alteration of nucleosome configuration and may be a component of the RNA polymerase II holoenzyme, the EBNA2-hSNF5/Ini1 interaction supports the hypothesis that EBNA2 facilitates transcriptional transactivation by acting as a transcription adapter molecule. We postulate that EBNA2 engages the hSNF-SWI complex to generate an open chromatin conformation at the EBNA2-responsive target genes, thereby potentiating the function of the RBP-JK-EBNA2-polymerase II transcription complex.

A conserved domain of the Epstein-Barr virus nuclear antigens 3A and 3C binds to a discrete domain of Jkappa

EBNA-3C can affect the LMP-1 promoter in both a positive and a negative manner through distinct DNA sequence elements. The viral transactivator EBNA-2 normally binds DNA indirectly via Jkappa to activate transcription, but this activation is prevented in the presence of EBNA-3C. The DNA element recognized by Jkappa is both required and sufficient for this inhibition. Jkappa clones isolated in a yeast two-hybrid screen using EBNA-3C as bait allowed us to delineate the sequences of both proteins mediating the interaction. Two isoforms of Jkappa that differ in exon 1, Jkappa-1 and RBP-2N, interact with EBNA-3C, suggesting that exon 1 is not required for this interaction; indeed, clones with deletion of the N-terminal third of Jkappa interacted as efficiently with EBNA-3C as full-length Jkappa clones. A Jkappa domain as small as 56 amino acids was sufficient to bind to EBNA-3C. A 74-amino-acid domain of EBNA-3C, conserved in all three EBNA-3 family members, was sufficient to interact with Jkappa. A specific mutation in this conserved domain suppressed the ability of EBNA-3C to downregulate transcription. Accordingly, EBNA-3A was also able to interact with Jkappa and downregulate Jkappa-mediated transcription as efficiently as EBNA-3C. The ability of the EBNA-3 proteins to prevent Jkappa from binding to DNA in vitro and suppress transactivation via Jkappa DNA elements suggests that the EBNA-3 proteins act analogously to the Drosophila protein Hairless.

Octamer binding factors and their coactivator can activate the murine PU.1 (spi-1) promoter

PU.1 (spi-1), a member of the Ets transcription factor family, is predominantly expressed in myeloid and B cells, activates many B cell and myeloid genes, and is critical for development of both of these lineages. Our previous studies (Chen, H. M., Ray-Gallet, D., Zhang, P., Hetherington, C. J., Gonzalez, D. A., Zhang, D.-E., Moreau-Gachelin, F., and Tenen, D. G. (1995) Oncogene 11, 1549-1560) demonstrate that the PU.1 promoter directs cell type-specific reporter gene expression in myeloid cell lines, and that PU.1 activates its own promoter in an autoregulatory loop. Here we show that the murine PU.1 promoter is also specifically and highly functional in B cell lines as well. Oct-1 and Oct-2 can bind specifically to a site at base pair -55 in vitro, and this site is specifically protected in B cells in vivo. We also demonstrate that two other sites contribute to promoter activity in B cells; an Sp1 binding site adjacent to the octamer site, and the PU.1 autoregulatory site. Finally, we show that the B cell coactivator OBF-1/Bob1/OCA-B is only expressed in B cells and not in myeloid cells, and that OBF-1/Bob1/OCA-B can transactivate the PU.1 promoter in HeLa and myeloid cells. This B cell restricted coactivator may be responsible for the B cell specific expression of PU.1 mediated by the octamer site.

EBNA-2 and EBNA-3C extensively and mutually exclusively associate with RBPJkappa in Epstein-Barr virus-transformed B lymphocytes

Although genetic and biochemical data indicate that the cell protein RBPJkappa is a mediator of EBNA-2 and EBNA-3C effects on transcriptional regulatory elements, the extent of association of these Epstein-Barr virus nuclear proteins with RBPJkappa in transformed B lymphocytes has not been determined. We now report that most of the EBNA-2 and at least 20% of the EBNA-3C coimmunoprecipitated with RBPJkappa from extracts of transformed B lymphocytes that contained most of the cellular EBNA-2 and EBNA3C. Both proteins are associated preferentially with the smaller of the two RBPJkappa isoforms. EBNA-2-RBPJkappa complexes do not contain EBNA-3C, and EBNA-3C-RBPJkappa complexes do not contain EBNA-2. Although EBNA-2 and EBNA-3C are extensively associated with RBPJkappa, a fraction of RBPJkappa appears to be free of EBNAs after repeated immunoprecipitations with anti-EBNA, Epstein-Barr virus-immune, human antibody. Promoters with RBPJkappa sites in their regulatory elements are likely to be differentially regulated by these RBPJkappa-EBNA-2 and RBPJkappa-EBNA-3 complexes.

The amino-terminal domains of Epstein-Barr virus nuclear proteins 3A, 3B, and 3C interact with RBPJ(kappa)

The ability of Epstein-Barr virus (EBV) latent infection nuclear protein EBNA3C to activate transcription of two EBNA2-responsive genes and to inhibit EBNA2 activation of transcription in transient-transfection assays appears to be due to its ability to interact with RBPJkappa, a cell protein that links EBNA2 to its response elements. We now show that EBNA3A and EBNA3B expressed in non-EBV-infected Burkitt tumor lymphoblasts are similar to EBNA3C in binding to glutathione S-transferase-RBPJkappa in vitro and in coimmunoprecipitating from cell lysates with antibody to RBPJkappa. EBNA3A and EBNA3B can also inhibit the interaction of RBPJkappa with cognate DNA in vitro. Although EBNA3 open reading frames are each close to 1,000 codons long, EBNA3A amino acids 1 to 138, EBNA3B amino acids 1 to 311, and EBNA3C amino acids 1 to 183 are sufficient for RBPJkappa interaction, while EBNA3B amino acids I to 109 have less or no binding. The RBPJkappa interacting domains overlap with the most highly conserved domain (amino acids 90 to 320) among the EBNA3 proteins. Thus, the EBNA3 gene family appears to have evolved to differentially regulate promoters with RBPJkappa binding sites. EBNA2, EBNA3A, and EBNA3C are important in EBV transformation of primary human B lymphocytes. Their interaction with RBPJkappa links EBV transformation to the notch signaling pathway and the effects of activated notch in T-cell leukemogenesis.

The N-terminal half of EBNA2, except for seven prolines, is not essential for primary B-lymphocyte growth transformation

Previous molecular genetic analyses of Epstein-Barr virus nuclear protein 2 (EBNA2) identified a negative effect of deletion of codons 19 to 33 on transformation and gene transactivation, while deletion of codons 19 to 110 was a null mutation for transformation and gene transactivation. We here report the surprising finding that codons 2 to 88, which encode the highly conserved unique N terminus (amino acids 1 to 58) and most of the polyproline repeat (amino acids 59 to 95), can be deleted with only minimal effects on transformation. Codons 97 to 122 can also be deleted with only minimal effects on transformation. However, deletion of 35 of the 37 prolines (amino acids 59 to 93) or deletion of codons 2 to 95 results in a null transforming phenotype. Although EBNA2 from which codons 59 to 93 were deleted was a null mutation for transformation, it was similar to some transforming mutants of EBNA2 in abundance, in interaction with RBPJK, and in transactivation of the LMP1 promoter in transient transfection assays. These data indicate that between three and seven prolines are critical for EBNA2 structure or for intermolecular interaction. Aside from these seven prolines, codons encoding the rest of the N-terminal half (amino acids 2 to 230) of EBNA2 are nonessential for primary B-lymphocyte growth transformation.

Molecular genetic analysis of Epstein-Barr virus Cp promoter function

The Cp promoter of Epstein-Barr virus (EBV) directs most transcription of the EBNA genes in lymphoblastoid cell lines. The functions of two control regions in the Cp promoter have been studied by construction of recombinant EBV strains containing specific mutations in these elements. Mutation of the RBP-Jk (CBF1) binding site reduced but did not completely abolish EBNA-2-dependent Cp activity in transient transfection assays. The same mutation in recombinant virus gave only a modest average reduction in Cp function, ranging from full activity to almost no activity in different isolates. Separate deletion of a 262-bp region containing glucocorticoid response elements had little effect in a transient assay but caused a fivefold increase in the steady-state level of Cp RNA in recombinant virus. The results indicate that other elements in addition to the intensively studied RBP-Jk site are important in determining Cp activity in the whole virus. Clonal EBV-infected cell lines expressed RNA from both the Cp and Wp promoters, but the level of Wp RNA did not simply compensate for changes in the level of Cp RNA. The levels of EBNA proteins varied much less than the levels of Cp and Wp RNA, suggesting other types of control in addition to initiation of transcription. A survey of RNAs derived from the internal repeat region of the virus indicated that gene expression from this region of EBV in lymphoblastoid cell lines is accounted for by the known transcripts.

Epstein-Barr virus nuclear antigen 2 is a transcriptional suppressor of the immunoglobulin mu gene: implications for the expression of the translocated c-myc gene in Burkitt's lymphoma cells

A conditional mutant of Epstein-Barr virus nuclear antigen 2 (EBNA2) regulated by estrogen was employed to study the effect of EBNA2 on the cellular phenotype. Activation of EBNA2 in lymphoblastoid cell lines (LCLs) and in B cell lymphoma lines resulted in down-regulation of cell surface IgM and Ig-mu steady-state RNA expression. In LCLs, activation of EBNA2 is required for maintaining proliferation, whereas in Burkitt's lymphoma (BL) cell lines with t(8;14) translocations, activation of EBNA2 induces growth arrest. In these cells, Northern and nuclear run-on analyses revealed rapid simultaneous repression of Ig-mu and c-myc transcription as early as 30 min after activation of EBNA2. Since c-myc expression is under the control of the Ig heavy chain locus in BL cell lines with a t(8;14) translocation, we propose that Ig-mu and c-myc are down-regulated by EBNA2 through a common mechanism.

The Epstein-Barr virus nuclear protein 2 acidic domain forms a complex with a novel cellular coactivator that can interact with TFIIE

Epstein-Barr virus nuclear antigen 2 (EBNA 2) activates transcription of specific genes and is essential for B-lymphocyte transformation. EBNA 2 has an acidic activation domain which interacts with general transcription factors TFIIB, TFIIH, and TAF40. We now show that EBNA 2 is specifically bound to a novel nuclear protein, p100, and that p100 can coactivate gene expression mediated by the EBNA 2 acidic domain. The EBNA 2 acidic domain was used to affinity purify p100. cDNA clones encoding the p100 open reading frame were identified on the basis of peptide sequences of the purified protein. Antibody against p100 coimmunoprecipitated p100 and EBNA 2 from Epstein-Barr virus-transformed lymphocyte extracts, indicating that EBNA 2 and p100 are complexed in vivo. p100 overexpression in cells specifically augmented EBNA 2 acidic domain-mediated activation. The coactivating effect is probably mediated by p100 interaction with TFIIE. Bacterially expressed p100 specifically adsorbs TFIIE from nuclear extracts, and in vitro-translated p56 or p34 TFIIE subunit can independently bind to p100. p100 also appears to be essential for normal cell growth, since cell viability was reduced by antisense p100 RNA and restored by sense p100 RNA expression.

The Epstein-Barr virus 3.5-kilobase latent membrane protein 1 mRNA initiates from a TATA-Less promoter within the first terminal repeat

The complete 5' sequence of the Epstein-Barr virus 3.5-kb latent membrane protein 1 (LMP1) mRNA, expressed in nasopharyngeal carcinoma, has been determined. The transcript initiates from heterogeneous start sites within the first terminal repeat (TR) of the viral genome. This region is TATA-less, consistent with heterogeneous starting, but contains multiple GC-rich elements which potentially interact with the Sp1 transcription factor. Expression of the 3.5-kb mRNA was consistently detected in nasopharyngeal carcinoma samples and in additional cell types, including a Burkitt's lymphoma. This is the first identification of an Epstein-Barr virus mRNA containing TR sequence and the first report of the ability of the TR to function as a transcriptional promoter.

Epstein-Barr virus nuclear protein 3C modulates transcription through interaction with the sequence-specific DNA-binding protein J kappa

The Epstein-Barr virus (EBV) nuclear protein 3C (EBNA 3C) is essential for EBV-mediated transformation of primary B lymphocytes, is turned on by EBNA 2, and regulates transcription of some of the viral and cellular genes which are regulated by EBNA 2. EBNA 2 is targeted to response elements by binding to the DNA sequence-specific, transcriptional repressor protein J kappa. We now show that EBNA 3C also binds to J kappa. EBNA 3C causes J kappa to not bind DNA or EBNA 2. J kappa DNA binding activity in EBV-transformed lymphoblastoid cells is consequently reduced. More than 10% of the EBNA 3C coimmunoprecipitated with J kappa from extracts of non-EBV-infected B lymphoblasts that had been stably converted to EBNA 3C expression. EBNA 3C in nuclear extracts from these cells (or in vitro-translated EBNA 3C) prevented J kappa from interacting with a high-affinity DNA binding site. Under conditions of transient overexpression in B lymphoblasts, EBNA 2 and EBNA 3C associated with J kappa and less EBNA 2 associated with J kappa when EBNA 3C was coexpressed in the same cell. EBNA 3C had no effect on the activity of a -512/+40 LMP1 promoter-CAT reporter construct that has two upstream J kappa sites, but it did inhibit EBNA 2 transactivation of this promoter. These data are compatible with a role for EBNA 3C as a "feedback" down modulator of EBNA 2-mediated transactivation. EBNA 3C could, in theory, also activate transcription by inhibiting the interaction of the J kappa repressor with its cognate DNA. The interaction of two viral transcriptional regulators with the same cell protein may reflect an unusually high level of complexity or stringency in target gene regulation.

The 62- and 80-kDa subunits of transcription factor IIH mediate the interaction with Epstein-Barr virus nuclear protein 2

EBNA 2 (Epstein-Barr virus nuclear antigen 2) is an acidic transactivator essential for EBV transformation of B lymphocytes. We show that EBNA 2 directly interacts with general transcription factor IIH. Glutathione S-transferase (GST)-EBNA 2 acidic domain fusion protein depleted transcription factor IIH activity from a TFIIH nuclear fraction. The p89 (ERCC3), p80 (ERCC2), and p62 subunits of TFIIH were among the proteins retained by GST-EBNA 2. Eluates from the GST-EBNA 2 beads reconstituted activity in a TFIIH-dependent in vitro transcription assay. The p62 and p80 subunits of TFIIH independently bound to GST-EBNA 2, whereas the p34 subunit of TFIIH only bound in the presence of p62. A Trp-->Thr mutation in the EBNA 2 acidic domain abolishes EBNA 2 transactivation in vivo and greatly compromised EBNA 2 association with TFIIH activity and with the p62 and p80 subunits, providing a link between EBNA 2 transactivation and these interactions. Antibodies directed against the p62 subunit of TFIIH coimmunoprecipitated EBNA 2 from EBV-transformed B lymphocytes, indicating that EBNA 2 associates with TFIIH in vivo.