Latent membrane protein of Epstein-Barr virus induces cellular phenotypes independently of expression of Bcl-2

Epstein-Barr virus latent genes

Latent Epstein–Barr virus (EBV) infection has a substantial role in causing many human disorders. The persistence of these viral genomes in all malignant cells, yet with the expression of limited latent genes, is consistent with the notion that EBV latent genes are important for malignant cell growth. While the EBV-encoded nuclear antigen-1 (EBNA-1) and latent membrane protein-2A (LMP-2A) are critical, the EBNA-leader proteins, EBNA-2, EBNA-3A, EBNA-3C and LMP-1, are individually essential for in vitro transformation of primary B cells to lymphoblastoid cell lines. EBV-encoded RNAs and EBNA-3Bs are dispensable. In this review, the roles of EBV latent genes are summarized.

The latent membrane protein 1 (LMP1) oncogene of Epstein-Barr virus can simultaneously induce and inhibit apoptosis in B cells

The latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) regulates its own expression and the expression of human genes via its two functional moieties; the transmembrane domains of LMP1 are required to regulate its expression via the unfolded protein response (UPR) and autophagy in B cells, and the carboxy-terminal domain of LMP1 activates cellular signaling pathways that affect cellular proliferation and survival. An apparent anomaly in the complex regulation of the UPR and autophagy by LMP1 is that the induction of either pathway can lead to cellular death, yet neither EBV-infected B cells nor B cells expressing only LMP1 die. Thus, we sought to understand how B cells that express LMP1 survive. The transmembrane domains of LMP1 activated apoptosis in B cells, the apoptosis required the UPR, and the carboxy-terminal domain of LMP1 blocked this apoptosis. The expression of the mRNA of Bcl2a1, encoding an antiapoptotic homolog of BCL2, correlated directly with the expression of LMP1 in EBV-positive B-cell strains, and its expression inhibited the apoptosis induced by the transmembrane domains of LMP1. These findings illustrate how the carboxy-terminal domain of LMP1 supports survival of B cells in the presence of the deleterious effects of the complex regulation of this viral oncogene.

Regulation of intracellular signalling by the terminal membrane proteins of members of the Gammaherpesvirinae

The human gamma(1)-herpesvirus Epstein-Barr virus (EBV) and the gamma(2)-herpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV), rhesus rhadinovirus (RRV), herpesvirus saimiri (HVS) and herpesvirus ateles (HVA) all contain genes located adjacent to the terminal-repeat region of their genomes, encoding membrane proteins involved in signal transduction. Designated 'terminal membrane proteins' (TMPs) because of their localization in the viral genome, they interact with a variety of cellular signalling molecules, such as non-receptor protein tyrosine kinases, tumour-necrosis factor receptor-associated factors, Ras and Janus kinase (JAK), thereby initiating further downstream signalling cascades, such as the MAPK, PI3K/Akt, NF-kappaB and JAK/STAT pathways. In the case of TMPs expressed during latent persistence of EBV and HVS (LMP1, LMP2A, Stp and Tip), their modulation of intracellular signalling pathways has been linked to the provision of survival signals to latently infected cells and, hence, a contribution to occasional cellular transformation. In contrast, activation of similar pathways by TMPs of KSHV (K1 and K15) and RRV (R1), expressed during lytic replication, may extend the lifespan of virus-producing cells, alter their migration and/or modulate antiviral immune responses. Whether R1 and K1 contribute to the oncogenic properties of KSHV and RRV has not been established satisfactorily, despite their transforming qualities in experimental settings.

IL-10 can induce the expression of EBV-encoded latent membrane protein-1 (LMP-1) in the absence of EBNA-2 in B lymphocytes and in Burkitt lymphoma- and NK lymphoma-derived cell lines

EBV-positive nasopharyngeal carcinoma and Hodgkin, T, and natural killer (NK) lymphomas express EBNA-1 and the latent membrane proteins (LMP1-2; type II latency). In contrast to type III EBV-transformed lymphoblastoid cell lines, in these cells the LMPs are expressed in the absence of EBNA-2. We have previously reported that exposure to CD40 ligand and IL-4 could induce LMP-1 in an in vitro EBV-infected Hodgkin lymphoma-derived cell line, which expressed only EBNA-1. We show now that both human and EBV-encoded IL-10 can induce LMP-1 in the absence of EBNA-2 in the Daudi, P3HR1, and other BL cell lines. Interestingly, induction of LMP-1 was not accompanied by the downregulation of BCL-6. IL-10 could also induce LMP-1 in the conditional lymphoblastoid cell line ER/EB2-5 where EBNA-2 was downregulated in the absence of estrogen. Moreover, IL-10 could induce the expression of LMP-1 in tonsillar B cells infected with the nontransforming, EBNA-2-deficient EBV strain P3HR1 and enhance LMP-1 expression in 2 EBV-positive NK lymphoma lines. The demonstration that IL-10 can induce the expression of LMP-1 in an EBNA-2-independent manner shows that the major transforming EBV gene LMP-1 can be induced by extracellular signals in lymphoid cells, and IL-10 might contribute to the establishment of type II EBV latency.

Rescue of "crippled" germinal center B cells from apoptosis by Epstein-Barr virus

Epstein-Barr virus (EBV) is associated with B-cell lymphomas such as Hodgkin lymphoma, Burkitt lymphoma, and post-transplantation lymphoma, which originate from clonal germinal center (GC) B cells. During the process of somatic hypermutation, GC B cells can acquire deleterious or nonsense mutations in the heavy and light immunoglobulin genes. Such mutations abrogate the cell surface expression of the B-cell receptor (BCR), which results in the elimination of these nonfunctional B cells by immediate apoptosis. EBV encodes several latent genes, among them latent membrane protein 1 (LMP1) and LMP2A, which are regularly expressed in EBV-positive Hodgkin lymphoma and posttransplantation lymphomas. Since LMP1 and LMP2A mimic the function of 2 key receptors on B cells, CD40 and BCR, respectively, we wanted to learn whether EBV infection can rescue proapoptotic GC B cells with crippling mutations in the heavy chain immunoglobulin locus from apoptosis. We show here that BCR-negative GC B cells readily enter the cell cycle upon infection with EBV in vitro and yield clonal lymphoblastoid cell lines that are incapable of expressing a functional BCR because the rearranged and formerly functional heavy chain immunoglobulin alleles carry deleterious mutations. Our findings imply an important role for EBV in the process of lymphomagenesis in certain cases of Hodgkin lymphoma and posttransplantation lymphomas.

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

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

Interferon regulatory factor 7 is associated with Epstein-Barr virus-transformed central nervous system lymphoma and has oncogenic properties

Interferon regulatory factor 7 (IRF-7) is implicated in the regulation of Epstein-Barr virus (EBV) latency. EBV transforms primary B cells, and the major EBV oncoprotein, latent membrane protein 1 (LMP-1), is required for the process. LMP-1 both induces the expression of IRF-7 and activates the IRF-7 protein by phosphorylation and nuclear translocation. Here we report that the expression of IRF-7 is increased in EBV-immortalized B lymphocytes compared with that in primary B cells. IRF-7 was phosphorylated and predominantly localized in the nucleus in the immortalized cells. The expression of IRF-7 was detected in 19 of 27 specimens of primary lymphomas of the human central nervous system by immunohistochemical analysis. The association between LMP-1 and IRF-7 was statistically highly significant for these specimens. An appreciable amount of the IRF-7 expressed in lymphoma cells was localized in the nucleus. Furthermore, IRF-7 promoted the anchorage-independent growth of NIH 3T3 cells. LMP-1 and IRF-7 showed additive effects on the growth transformation of NIH 3T3 cells. IRF-7-expressing NIH 3T3 cells formed tumors in athymic mice. Thus, IRF-7 has oncogenic properties and, along with LMP-1, may mediate or potentiate the EBV transformation process in the pathogenesis of EBV-associated lymphomas.

Epstein-Barr virus and cancer

EBV was the first human virus to be directly implicated in carcinogenesis. It infects >90% of the world's population. Although most humans coexist with the virus without serious sequelae, a small proportion will develop tumors. Normal host populations can have vastly different susceptibility to EBV-related tumors as demonstrated by geographical and immunological variations in the prevalence of these cancers. EBV has been implicated in the pathogenesis of Burkitt's lymphoma, Hodgkin's disease, non-Hodgkin's lymphoma, nasopharyngeal carcinoma, and lymphomas, as well as leiomyosarcomas arising in immunocompromised individuals. The presence of this virus has also been associated with epithelial malignancies arising in the gastric region and the breast, although some of this work remains in dispute. EBV uses its viral proteins, the actions of which mimic several growth factors, transcription factors, and antiapoptotic factors, to usurp control of the cellular pathways that regulate diverse homeostatic cellular functions. Recent advances in antiviral therapeutics, application of monoclonal antibodies, and generation of EBV-specific CTLs are beginning to show promise in the treatment of EBV-related disorders.

Multiple signal transducers and activators of transcription are induced by EBV LMP-1

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP-1) is required for EBV immortalization of primary B cells in vitro. Signal transducers and activators of transcription (STATs) play a pivotal role in the initiation and maintenance of certain cancers. STAT proteins, especially STAT-1, -3, and -5, are persistently tyrosine phosphorylated or activated in many cancers. We show here that EBV-infected type III latency cells, in which the EBV oncoprotein, LMP-1 is expressed, express high levels of four STATs (STAT-1, -2, -3, and -5A) and that LMP-1 is responsible for the induction of three (STAT-1, -2, and -3). In addition, the C-terminal activator region 1 (CTAR-1) and CTAR-2 of LMP-1 cooperatively induced the expression of STAT-1. The cooperativity was evident when CTAR-1 and CTAR-2 were present in cis, but not in trans. Furthermore, NF-kappaB is an essential factor involved in the induction of STAT-1. Most of the induced STATs were not phosphorylated at the critical tyrosine residue activated by many cytokines. However, the induced STATs, at least STAT-1, were functional because it could be activated by interferon (IFN) and could upregulate an IFN-inducible gene. Finally, expression of STAT-1, but not STAT-2 and -3, is associated with EBV transformation. The association of the expression of STAT-1, -2, -3, and -5A with EBV type III latency and the expression of STAT-1 in the EBV transformation process may be part of the viral programming that regulates viral latency and cellular transformation.

Nuclear factor kappa B-dependent activation of the antiapoptotic bfl-1 gene by the Epstein-Barr virus latent membrane protein 1 and activated CD40 receptor

Suppression of the cellular apoptotic program by the oncogenic herpesvirus Epstein-Barr virus (EBV) is central to both the establishment of latent infection and the development of EBV-associated malignancies. We have previously shown that expression of the EBV latent membrane protein 1 (LMP1) in Burkitt's lymphoma cell lines leads to increased mRNA levels from the cellular antiapoptotic bfl-1 gene (also known as A1). Furthermore, ectopic expression of Bfl-1 in an EBV-positive cell line exhibiting a latency type 1 infection protects against apoptosis induced by growth factor deprivation (B. N. D'Souza, M. Rowe, and D. Walls, J. Virol. 74:6652-6658, 2000). We now report that LMP1 drives bfl-1 promoter activity through interactions with components of the tumor necrosis factor receptor (TNFR)/CD40 signaling pathway. We present evidence that this process is NF-kappa B dependent, involves the recruitment of TNFR-associated factor 2, and is mediated to a greater extent by the carboxyl-terminal activating region 2 (CTAR2) relative to the CTAR1 domain of LMP1. Activation of CD40 receptor also led to increased bfl-1 mRNA levels and an NF-kappa B-dependent increase in bfl-1 promoter activity in Burkitt's lymphoma-derived cell lines. We have delineated a 95-bp region of the promoter that functions as an LMP1-dependent transcriptional enhancer in this cellular context. This sequence contains a novel NF-kappa B-like binding motif that is essential for transactivation of bfl-1 by LMP1, CD40, and the NF-kappa B subunit protein p65. These findings highlight the role of LMP1 as a mediator of EBV-host cell interactions and may indicate an important route by which it exerts its cellular growth transforming properties.

The significance of LMP1 expression in nasopharyngeal carcinoma

The Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP1) is a key effector of EBV-mediated B cell transformation. LMP1 displays potent oncogenic properties in rodent fibroblasts, and induces a wide range of effects in B cells and epithelial cells. LMP1 functions as a constitutively active tumor necrosis factor receptor (TNFR) engaging a multitude of signaling pathways that include NF-kappaB, the mitogen-activated protein kinases (MAPKs), JNK, p38, the JAK/STAT pathway and, more recently, the small Rho GTPases. The constitutive activation of these signaling cascades explains LMP1's ability to induce such a diverse array of morphological and phenotypic effects in cells and provides an insight into how LMP1 may induce cell transformation. The frequent expression of LMP1 in undifferentiated nasopharyngeal carcinoma (NPC) points to a role for this viral oncoprotein as a key effector molecule in NPC pathogenesis.

Characterization of the Cyno-EBV LMP1 homologue and comparison with LMP1s of EBV and other EBV-like viruses

EBV latent membrane protein 1 (LMP1) is essential for EBV-mediated transformation and has been associated with several cases of malignancies. EBV-like viruses in Cynomolgus monkeys (Macaca fascicularis) have been associated with high lymphoma rates in immunosuppressed monkeys. In the study, the entire coding region of the Cyno-EBV LMP1 gene was cloned, sequenced and expressed in human embryonic kidney (HEK) cells 293. The Cyno-EBV LMP1 homologue sequence predicted a 588 amino acid (a.a.) protein with a short 19 a.a. N-terminus, six transmembrane domains and a long carboxy tail of 404 a.a. The protein contained a series of seven 9 a.a.-tandem repeats and two 20 a.a.-repeats, which harbored two potential TRAF binding motifs, PxQxT/S. These repeats shared no homology with the repeats in any other LMP1. However, the proline-rich sequence GPxxPx(6) found within the 11 a.a.-repeats of EBV LMP1 was conserved in Cyno-EBV carboxy tail and contained two consensus JAK/STAT sequences PxxPxP. A cluster of eight histidine residues was found in proximity to the last transmembrane domain of Cyno-EBV LMP1 and was exploited as a natural protein tag in expression studies. Western blot analysis revealed a major polypeptide of 110 kDa. Comparative functional studies showed that Cyno-EBV LMP1 expressed in HEK 293 cells shares the same ability as EBV LMP1 to induce NFkappaB driven CAT activity.

Antisense to Epstein Barr Virus-encoded LMP1 does not affect the transcription of viral and cellular proliferation-related genes, but induces phenotypic effects on EBV-transformed B lymphocytes

It is generally accepted that Epstein-Barr virus (EBV) latent genes EBNA-2, EBNA-3A, -3C, EBNA-LP and LMP1 are essential for growth transformation and immortalization of B lymphocytes. Among these genes, LMP1 plays a key role in the survival and dissemination of the infected B cells by inducing anti-apoptotic genes and surface expression of several activation antigens and adhesion molecules. We have previously shown that antisense oligodeoxynucleotides directed to LMP1 mRNA, effectively suppress LMP1 gene expression and substantially reduce B95.8 cell proliferation. In this study, we have used antisense LMP1 oligomers to investigate whether LMP1 suppression might influence the expression of latent EBV genes with oncogenic potential, anti-apoptotic genes, or affect the phenotype of EBV-infected B95.8 cells. Our data show that LMP1 suppression does not affect the transcription of EBNA-2, EBNA-3A, -3B and -3C genes, or that of bcl-2 and mcl-1 anti-apoptotic genes. In contrast, consistent modifications in the expression of CD39, CD54, CD23, CD11 and CD10 molecules were observed in B95.8 cells after treatment with antisense LMP1. Our findings support the possibility for using LMP1 antisense oligomers as therapeutics in EBV-associated tumors.

The cytoplasmic amino-terminus of the Latent Membrane Protein-1 of Epstein-Barr Virus: relationship between transmembrane orientation and effector functions of the carboxy-terminus and transmembrane domain

The Latent Membrane Protein 1 (LMP-1) protein of Epstein-Barr virus (EBV) is localized in the plasma membrane of the infected cell. LMP-1 possesses a hydrophobic membrane spanning domain, and charged, intracellular amino- and carboxy-termini. Two models have been proposed for the contribution of the amino-terminus to LMP-1's function: (i) as an effector domain, interacting with cellular proteins, or (ii) as a structural domain dictating the correct orientation of transmembrane domains and thereby positioning LMP-1's critical effector domains (i.e. the carboxy-terminus). However, no studies to date have addressed directly the structural contributions of LMP-1's cytoplasmic amino-terminus to function. This study was designed to determine if LMP-1's cytoplasmic amino-terminus (N-terminus) encodes information required solely for maintenance of proper topological orientation. We have constructed LMP-1 chimeras in which the cytoplasmic N-terminus of LMP-1 is replaced with an unrelated domain of similar size and charge, but of different primary sequence. Retention of the charged amino-terminal (N-terminal) cytoplasmic domain and first predicted transmembrane domain was required for correct transmembrane topology. The absolute primary sequence of the cytoplasmic N-terminus was not critical for LMP-1's cytoskeletal association, turnover, plasma membrane patching, oligomerization, Tumor Necrosis Factor Receptor-associated factor (TRAF) binding, NF-kappaB activation, rodent cell transformation and cytostatic activity. Furthermore, our results point to the hydrophobic transmembrane domain, independent of the cytoplasmic domains, as the primary LMP-1 domain mediating oligomerization, patching and cytoskeletal association. The cytoplasmic amino-terminus provides the structural information whereby proper transmembrane orientation is achieved.

N-terminal domain of BARF1 gene encoded by Epstein-Barr virus is essential for malignant transformation of rodent fibroblasts and activation of BCL-2

The BARF1 gene encoded by the Epstein-Barr virus induces morphological changes, loss of contact inhibition and anchorage independence in established rodent Balb/c3T3 fibroblast. BARF1 gene was also capable of inducing malignant transformation in a human Louckes B cell line. Our recent study showed that BARF1 gene had an ability to immortalize primary epithelial cells. However we do not know which region(s) of BARF1 protein is(are) responsible for inducing malignant transformation in established rodent cells. Using the deletion mutants, we now localized a malignant transforming region in N-terminal of BARF1 protein. The mutants lacking this region were unable to transform the cells in malignant state. Furthermore, we demonstrated that only the mutants containing this region rendered the cells resistant to apoptosis induced by serum deprivation. Surprisingly, the BARF1 gene was capable of activating anti-apoptotic Bcl-2 expression and this activation was due to the N-terminal transforming region. These data suggest that the cooperation of BARF1 with Bcl-2 is essential for the induction of malignant transformation.

The bfl-1 gene is transcriptionally upregulated by the Epstein-Barr virus LMP1, and its expression promotes the survival of a Burkitt's lymphoma cell line

The recently identified bfl-1 gene (also known as A1 or GRS), a homologue of bcl-2, encodes an antiapoptotic protein that suppresses apoptosis induced by the p53 tumor suppressor protein and exhibits proliferative and potent cooperative transforming activities. We show that elevated levels of bfl-1 mRNA are a feature of Epstein-Barr virus (EBV)-immortalized B-cell lines and Burkitt's lymphoma cell lines expressing the full spectrum of EBV latent proteins. Using an EBV-negative Burkitt's lymphoma cell line in which the expression of EBV latent membrane protein 1 (LMP1) is inducibly regulated by tetracycline, we demonstrate that LMP1 expression coincides with a dramatic increase in the level of bfl-1 mRNA. Also in this system, an increase in the level of Bcl-2 protein was seen to occur earlier than that of bcl-2 mRNA, suggesting that both transcriptional and translational mechanisms are involved in the control of Bcl-2 expression by LMP-1. We show that elevated bfl-1 mRNA stability can contribute to this effect of LMP-1, thus providing evidence of a novel mechanism of gene regulation by this EBV protein. Upregulation of bfl-1 by LMP1 was not observed in the T-cell line Jurkat or the epithelial cell line C33A. Ectopic expression of Bfl-1 in an EBV-positive cell line exhibiting a latency type I infection protects against apoptosis induced by growth factor deprivation, thereby providing a functional role for Bfl-1 in this cellular context and adding Bfl-1 to the list of antiapoptotic proteins whose expression is modulated by EBV. This is the first report of the regulation of bfl-1 expression by a viral protein, and this novel finding may thus represent an important link between the EBV oncoprotein LMP1 and its cellular growth-transforming properties.

Epstein-barr virus nuclear antigen 3C activates the latent membrane protein 1 promoter in the presence of Epstein-Barr virus nuclear antigen 2 through sequences encompassing an spi-1/Spi-B binding site

The Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA-3C) protein is a transcriptional regulator of viral and cellular genes that is essential for EBV-mediated immortalization of B lymphocytes in vitro. EBNA-3C can inhibit transcription through an association with the cellular DNA-binding protein Jkappa, a function shared by EBNA-3A and EBNA-3B. Here, we report a mechanism by which EBNA-3C can activate transcription from the EBV latent membrane protein 1 (LMP-1) promoter in conjunction with EBNA-2. Jkappa DNA-binding sites were not required for this activation, and a mutant EBNA-3C protein unable to bind Jkappa activated transcription as efficiently as wild-type EBNA-3C, indicating that EBNA-3C can regulate transcription through a mechanism that is independent of Jkappa. Furthermore, activation of the LMP-1 promoter is a unique function of EBNA-3C, not shared by EBNA-3A and EBNA-3B. The DNA element through which EBNA-3C activates the LMP-1 promoter includes a Spi-1/Spi-B binding site, previously characterized as an important EBNA-2 response element. Although this element has considerable homology to mouse immunoglobulin light chain promoter sequences to which the mouse homologue of Spi-1 binds with its dimerization partner IRF4, we demonstrate that the IRF4-like binding sites in the LMP-1 promoter do not play a role in EBNA-3C-mediated activation. Both EBNA-2 and EBNA-3C were required for transcription mediated through a 41-bp region of the LMP-1 promoter encompassing the Spi binding site. However, EBNA-3C had no effect on transcription mediated in conjunction with the EBNA-2 activation domain fused to the GAL4 DNA-binding domain, suggesting that it does not function as an adapter between EBNA-2 and the cellular transcriptional machinery. Like EBNA-2, EBNA-3C bound directly to both Spi-1 and Spi-B in vitro. This interaction was mediated by a region of EBNA-3C encompassing a likely basic leucine zipper (bZIP) domain and the ets domain of Spi-1 or Spi-B, reminiscent of interactions between bZIP and ets domains of other transcription factors that result in their targeting to DNA. There are many examples of regulation of the hematopoietic-specific Spi transcription factors through protein-protein interactions, and a similar regulation by EBNA-3C, in conjunction with EBNA-2, is likely to be an important and unique contribution of EBNA-3C to EBV-mediated immortalization.

Virocrine transformation: the intersection between viral transforming proteins and cellular signal transduction pathways

This review describes a mechanism of viral transformation involving activation of cellular signaling pathways. We focus on four viral oncoproteins: the E5 protein of bovine papillomavirus, which activates the platelet-derived growth factor beta receptor; gp55 of spleen focus forming virus, which activates the erythropoietin receptor; polyoma virus middle T antigen, which resembles an activated receptor tyrosine kinase; and LMP-1 of Epstein-Barr virus, which mimics an activated tumor necrosis factor receptor. These examples indicate that diverse viruses induce cell transformation by activating cellular signal transduction pathways. Study of this mechanism of viral transformation will provide new insights into viral tumorigenesis and cellular signal transduction.

Interferon regulatory factor 7 is induced by Epstein-Barr virus latent membrane protein 1

Infection by Epstein-Barr virus (EBV) generates several types of latency with different profiles of gene expression but with expression of Epstein-Barr nuclear antigen 1 (EBNA-1) in common. The BamHI Q promoter (Qp) is used for the transcription of EBNA-1 mRNA in type I latency, which is an EBV infection state exemplified by Burkitt's lymphoma (BL). However, Qp is inactive in type III latency, and other promoters (C/Wp) are used for transcription of EBNA-1, which raises the question of how usage of these promoters is governed. Interferon (IFN) regulatory factor 7 (IRF-7) was identified first as a negative regulator of Qp. Expression of IRF-7 is associated with EBV type III latency, where Qp is inactive, but not with type I latency, raising the possibility that a viral gene product(s) expressed in type III latency might induce IRF-7 and repress Qp. Here, detailed analysis of the expression of IRF-7 revealed that it is associated with the expression of EBV latent membrane protein 1 (LMP-1) and that LMP-1 stimulates the expression of IRF-7 in type III latency in which Qp is inactive. In contrast, LMP-1 is not expressed in type I latency cells in which Qp is active. LMP-1 represses the constitutive activity of Qp reporter constructs. Mutational analysis of Qp reporter constructs revealed that the Qp IFN-stimulated response element (ISRE) is essential for the repression by LMP-1. Furthermore, LMP-1 reduced EBNA-1 mRNA derived from Qp only in type I cells in which IRF-7 could be induced. Finally, IFN-alpha, but not IFN-gamma, repressed endogenous Qp activity, which is consistent with the ability of IFN-alpha to induce IRF-7. Thus, IRF-7 may mediate repression of Qp by LMP-1. The induction of IRF-7 by LMP-1 may be relevant to the silencing of Qp in EBV type III latency.

Latent membrane protein 1 associated signaling pathways are important in tumor cells of Epstein-Barr virus negative Hodgkin's disease

The latent membrane protein 1 (LMP1) oncogene of Epstein-Barr virus (EBV) is selectively expressed in the Reed-Sternberg (RS) cells of EBV-associated Hodgkin's disease (HD). However, no differences in clinical presentation and course are found between EBV positive and EBV negative forms of HD suggesting a common pathogenetic mechanism. We have studied the LMP1 associated signaling pathways and their dominant negative inhibition in the myelomonocytic HD-MyZ and the B-lymphoid L-428 HD cell lines. In both EBV negative cell lines expression of LMP1 is associated with the formation of multinuclear RS cells. Dominant negative inhibition of NF-kappa B mediated signaling at the step of I kappa B-alpha phosphorylation results in increased cell death with only a few typical RS cells resistant to overexpression of the dominant negative inhibitor I kappa B-alpha-N delta 54. However, dominant negative inhibition of NF-kappa B mediated signaling at the early step of TRAF2 interaction results in the formation of multinuclear cells in both cell lines and, in addition, in clusters of small mononuclear cells in the HD-MyZ cell line. In HD-MyZ cells overexpression of the powerful JBD-inhibitor of the JNK signal transduction pathway is restricted to small cells and never observed in RS cells. These small cells undergo apoptosis as shown by the TUNEL technique. Apoptosis of small cells is still observed after co-transfection of JBD and LMP1 but in addition a few apoptotic HD-MyZ cells with large fused nuclear masses are identified suggesting that specific inhibition of JNK leads also to apoptosis of LMP1 induced RS cells. Thus, activation of the JNK signaling pathway is also important in the formation of Reed-Sternberg cells. Our findings are consistent with a model where all three LMP1 associated functions, i.e. NF-kappa B mediated transcription, TRAF2 dependent signaling, and c-Jun activation act as a common pathogenetic denominator of both EBV negative and EBV positive HD.

LMP1 signal transduction differs substantially from TNF receptor 1 signaling in the molecular functions of TRADD and TRAF2

The Epstein-Barr virus latent membrane protein 1 (LMP1) binds tumor necrosis factor receptor (TNFR)-associated factors (TRAFs) and the TNFR-associated death domain protein (TRADD). Moreover, it induces NF-kappaB and the c-Jun N-terminal kinase 1 (JNK1) pathway. Thus, LMP1 appears to mimick the molecular functions of TNFR1. However, TNFR1 elicits a wide range of cellular responses including apoptosis, whereas LMP1 constitutes a transforming protein. Here we mapped the JNK1 activator region (JAR) of the LMP1 molecule. JAR overlaps with the TRADD-binding domain of LMP1. In contrast to TNFR1, LMP1 recruits TRADD via the TRADD N-terminus but not the TRADD death domain. Consequently, the molecular function of TRADD in LMP1 signaling differs from its role in TNFR1 signal transduction. Whereas NF-kappaB activation by LMP1 was blocked by a dominant-negative TRADD mutant, LMP1 induces JNK1 independently of the TRADD death domain and TRAF2, which binds to TRADD. Further downstream, JNK1 activation by TNFR1 involves Cdc42, whereas LMP1 signaling to JNK1 is independent of p21 Rho-like GTPases. Although both LMP1 and TNFR1 interact with TRADD and TRAF2, the different topologies of the signaling complexes correlate with substantial differences between LMP1 and TNFR1 signal transduction to JNK1.

Functional analysis of different LMP1 proteins isolated from Epstein-Barr virus-positive carriers

The Epstein-Barr virus (EBV) is the causative agent of infectious mononucleosis and is implicated in the development of several human malignancies. Latent membrane protein 1 (LMP1), an EBV protein with known oncogenic properties, may be important in the pathogenesis of EBV-associated tumors, particularly nasopharyngeal carcinoma (NPC) and Hodgkin's disease (HD). Several reports suggested that sequence variations in the LMP1 gene may define a more aggressive, geographically restricted EBV-genotype. Most mutations in the LMP1 gene described are located within the C-terminus of the protein. However, the effect of these mutations on the biological function of the protein remains widely unknown. Therefore, this study aimed in investigating whether mutations detected in LMP1 genes isolated from different EBV-positive carriers have an effect on the biological function of the protein. For this purpose the LMP1 genes were amplified by nested PCR from DNA out of bone marrow and peripheral blood lymphocytes and sequenced. Three functional assays were performed in order to evaluate the biological activity of the different isolates: activation of the transcription factors NF-kappaB and AP-1 as well as the anchorage independent growth of LMP1 transfected ratl cells in soft agar. The results suggested that whereas differences in the activation of NF-kappaB through the various LMP1 isolates correlated tightly with their different expression levels, the outgrowth of transfected cells in soft agar did not and the transcription factor NF-kappaB therefore appeared not to be the major effector for the transformation of the rodent cell line ratl by LMP1. The various LMP1-isolates also differed in their capacity in activating the transcription factor AP-1. We found no correlation between the transforming ability of the LMPI isolates and activation of AP-1 suggesting that other so far uncharacterized domains also influence the transforming ability of the protein.

Epstein-Barr virus-associated nonsmall cell lung carcinoma: undifferentiated "lymphoepithelioma-like" carcinoma as a distinct entity with better prognosis

BACKGROUND

Epstein-Barr virus (EBV) infection in nonsmall cell lung carcinoma (NSCLC) has been demonstrated in some ethnic groups. The pathobiology and the role of EBV and oncoprotein expression in these tumors have not been studied extensively. In this study, the authors investigated EBV-encoded RNA-1 (EBER1) transcripts by in situ hybridization and the expression of latent membrane protein-1 (LMP-1) and bcl-2 protein by immunohistochemistry in NSCLC patients from Taiwan, where nasopharyngeal carcinoma is endemic.

METHODS

A total of 127 cases of NSCLC (43 cases of squamous cell carcinoma [SCC], 67 cases of adenocarcinoma [AD], 12 cases of large cell carcinoma [LCC], and 5 cases of lymphoepithelioma-like carcinoma [LE]) were included. A sensitive polymerase chain reaction-derived, digoxigenin-labeled DNA probe for in situ detection of EBER1 transcripts was performed for the detection of EBV. Immunohistochemistry using the avidin-biotin-immunoperoxidase method was also performed to evaluate the expression of bcl-2 and LMP-1.

RESULTS

EBER1 was detected in 11 of the 127 NSCLC cases (8.7%; 6 SCC cases and 5 LE cases). All 5 LE cases were EBV-positive, whereas only 6 of the 43 SCC cases (14%), 0 of 67 AD cases, and 12 LCC cases were EBV-positive (P < 0.05). All five LE cases showed diffuse, strong, positive staining of tumor cells; five of the six SCC cases showed diffuse but weak staining. Among the nontumor epithelial cells, there was no EBER1 staining of any of the 11 EBER1-positive cases. The mean age of the LE patients was 10 years younger than that of the patients with other histological types. All 5 LE patients were nonsmokers, whereas 3 of the 6 patients with EBER1-positive SCC (50%) were smokers. EBER1 expression did not correlate with the 2-year survival rate of overall cases, but all 5 LE patients were alive without clinical evidence of disease at last follow-up. Gender, lymph node or distant metastasis, and clinical stage were not found to have any correlation with EBER1 expression (P > 0.05). All LE cases had bcl-2 oncoprotein expression (100%). This frequency was significantly different from other histologic types (P < 0.05). The LMP-1 detection rate was low and demonstrated no correlation with bcl-2 expression.

CONCLUSIONS

In this study, the authors found that the primary LE of the lung is associated with young age, a history of not smoking, high bcl-2 immunoreactivity, and better survival rate. These characteristics demonstrate that EBV-associated LE of the lung is a unique entity. The findings of the current study suggest that EBV infection may play a different role in the tumorigenesis of primary LE of the lung than it does in other EBER1-positive NSCLCs.

Herpesvirus saimiri transforms human T-cell clones to stable growth without inducing resistance to apoptosis

Herpesvirus saimiri (HVS) transforms human T cells to stable growth in vitro. Since HVS codes for two different antiapoptotic proteins, growth transformation by HVS might be expected to confer resistance to apoptosis. We found that the expression of both viral antiapoptotic genes was restricted to cultures with viral replication and absent in growth-transformed human T cells. A comparative examination of HVS-transformed T-cell clones and their native parental clones revealed that the expression of Bcl-2, Bcl-X(L), Bax, and members of the tumor necrosis factor receptor (TNF-R) superfamily with a death domain, namely, TNF-RI, CD95, and TRAMP, were not modulated by HVS. Expression of CD30 was induced in HVS-transformed T cells, and these cells also expressed the CD30 ligand. Uninfected and transformed T cells were sensitive to CD95 ligation but resistant to apoptosis mediated by TRAIL or soluble TNF-alpha. CD95 ligand was constitutively expressed on transformed but not uninfected parental T cells. Both cell types showed similar sensitivity to cell death induction or inhibition of T-cell activation mediated by irradiation, oxygen radicals, dexamethasone, cyclosporine, and prostaglandin E2. Altogether, this study strongly suggests that growth transformation by HVS is based not on resistance to apoptosis but, rather, on utilization of normal cellular activation pathways.

Expression of the Epstein-Barr Virus Protein LMP1 Mediates Tumor Regression In Vivo

By stimulating the expression of murine IP-10 and Mig, CXC chemokines that inhibit neovascularization and cause damage to established tumor vasculature, human B cells immortalized with Epstein-Barr virus (EBV) can promote an effective antitumor response in athymic mice. In the present study, we examined the potential role of EBV in the induction of this antitumor response. Using a panel of EBV+ and EBV− Burkitt lymphoma (BL) cell lines, a significant correlation was detected between the expression of the EBV latency gene LMP1 and the occurrence of spontaneous tumor regression in athymic mice. Inoculation of LMP1+ and LMP1− BL cells in the same subcutaneous site resulted in tumors that completely regressed in a manner indistinguishable from that induced by EBV-immortalized B cells. EBV-converted BL30 and BL41 sublines infected with B95-8 virus expressed LMP1, generated tumors that frequently regressed spontaneously, and promoted an effective antitumor response against progressively growing tumors. In contrast, the EBV− BL30 and BL41 cell lines and the EBV-converted BL30 and BL41 infected with P3HR-1 virus did not express LMP1 protein, and generated progressively growing tumors in nude mice. When transfected with the LMP1 gene, BL41 cells produced tumors that regressed spontaneously in most cases, and could induce the regression of tumors derived from BL41 cells transfected with vector alone. Tumors induced by LMP1-expressing cells expressed murine IP-10 and Mig and displayed histological evidence of extensive tumor tissue necrosis and vascular damage. We conclude that the EBV protein LMP1 is likely responsible for the antitumor response elicited by EBV-immortalized cells in athymic mice.

Expression of the Epstein-Barr Virus Protein LMP1 Mediates Tumor Regression In Vivo

By stimulating the expression of murine IP-10 and Mig, CXC chemokines that inhibit neovascularization and cause damage to established tumor vasculature, human B cells immortalized with Epstein-Barr virus (EBV) can promote an effective antitumor response in athymic mice. In the present study, we examined the potential role of EBV in the induction of this antitumor response. Using a panel of EBV+ and EBV− Burkitt lymphoma (BL) cell lines, a significant correlation was detected between the expression of the EBV latency gene LMP1 and the occurrence of spontaneous tumor regression in athymic mice. Inoculation of LMP1+ and LMP1− BL cells in the same subcutaneous site resulted in tumors that completely regressed in a manner indistinguishable from that induced by EBV-immortalized B cells. EBV-converted BL30 and BL41 sublines infected with B95-8 virus expressed LMP1, generated tumors that frequently regressed spontaneously, and promoted an effective antitumor response against progressively growing tumors. In contrast, the EBV− BL30 and BL41 cell lines and the EBV-converted BL30 and BL41 infected with P3HR-1 virus did not express LMP1 protein, and generated progressively growing tumors in nude mice. When transfected with the LMP1 gene, BL41 cells produced tumors that regressed spontaneously in most cases, and could induce the regression of tumors derived from BL41 cells transfected with vector alone. Tumors induced by LMP1-expressing cells expressed murine IP-10 and Mig and displayed histological evidence of extensive tumor tissue necrosis and vascular damage. We conclude that the EBV protein LMP1 is likely responsible for the antitumor response elicited by EBV-immortalized cells in athymic mice.

Epstein-Barr virus LMP1 modulates the malignant potential of gastric carcinoma cells involving apoptosis

About 10% of gastric carcinomas including lymphoepithelioma-like carcinoma and adenocarcinoma are associated with Epstein-Barr virus (EBV) infection. In EBV-associated gastric carcinomas, the tumor cells express Epstein-Barr nuclear antigen 1 (EBNA-1) but not EBNA-2, -3A, -3B, or -3C, leader protein, or latent membrane proteins (LMPs) because of gene methylation. Only a few exceptional cases have LMP1 expression in tumor cells as demonstrated by immunohistochemical studies. To elucidate the biological effects of LMP1 and the significance of its restricted expression in EBV-associated gastric carcinomas, the LMP1 gene was transferred into EBV-negative gastric carcinoma cell lines (SCM1 and TMC1) and into EBV-negative nasopharyngeal carcinoma (NPC) cells (HONE-1) as a control. The biological effects of LMP1 in gastric carcinoma cells were monitored in vitro and in vivo. These results showed that the consequence of LMP1 expression is a growth enhancement in NPC cells, but it is a growth suppression in gastric carcinoma cells. The LMP1-expressing gastric carcinoma cells had a reduced growth rate, colony-forming efficiency, mean colony size, and tumorigenicity and a lower malignant cytological grade. The reduced growth rate, colony-forming efficiency, and mean colony size were partially reversible in vitro with treatment with LMP1 antisense oligonucleotide. In addition, enhanced apoptosis was found in the LMP1-expressing gastric carcinoma cells. This suggests that LMP1 may negatively modulate the malignant potential of gastric carcinoma cells via an enhancement of apoptosis. We concluded that the restriction of LMP1 expression in EBV-associated gastric carcinomas may lead to a growth advantage for tumor cells by avoiding LMP1 apoptotic effects and immunologically mediated elimination.

Epstein-Barr virus latent membrane protein-1 triggers AP-1 activity via the c-Jun N-terminal kinase cascade

The Epstein-Barr virus latent membrane protein-1 (LMP-1) is an integral membrane protein which transforms fibroblasts and is essential for EBV-mediated B-cell immortalization. LMP-1 has been shown to trigger cellular NF-kappa B activity which, however, cannot fully explain the oncogenic potential of LMP-1. Here we show that LMP-1 induces the activity of the AP-1 transcription factor, a dimer of Jun/Jun or Jun/Fos proteins. LMP-1 effects on AP-1 are mediated through activation of the c-Jun N-terminal kinase (JNK) cascade, but not the extracellular signal-regulated kinase (Erk) pathway. Consequently, LMP-1 triggers the activity of the c-Jun N-terminal transactivation domain which is known to be activated upon JNK-mediated phosphorylation. Deletion analysis indicates that the 55 C-terminal amino acids of the LMP-1 molecule, but not its TRAF interaction domain, are essential for AP-1 activation. JNK-mediated transcriptional activation of AP-1 is the direct output of LMP-1-triggered signaling, as shown by an inducible LMP-1 mutant. Using a tetracycline-regulated LMP-1 allele, we demonstrate that JNK is also an effector of non-cytotoxic LMP-1 signaling in B cells, the physiological target cells of EBV. In summary, our data reveal a novel effector of LMP-1, the SEK/JNK/c-Jun/AP-1 pathway, which contributes to our understanding of the immortalizing and transforming potential of LMP-1.

Reversal of EBV immortalization precedes apoptosis in IL-6-induced human B cell terminal differentiation

Cell death in B cell terminal differentiation rapidly follows cell cycle arrest in IL-6 differentiation of EBV-immortalized, IgG-bearing human lymphoblastoid cells in vitro. G1 arrest is now found to coincide with repression of EBNA2 and LMP1, two EBV genes essential for B cell transformation, without activation of the viral lytic cycle. IL-6-differentiated B cells die by apoptosis, as evidenced by increases in Annexin V binding activity, PARP cleavage, and chromatin disorganization. Expression of Mcl-1, a Bcl-2 family member, was specifically induced during IL-6 differentiation and down-regulated during apoptosis. Thus, IL-6 reverses EBV immortalization and activates the terminal differentiation program in IgG-bearing human B lymphoblastoid cells, including regulation of an anti-apoptotic gene to coordinate differentiation, cell cycle arrest, and cell death.

Localization of the major NF-kappaB-activating site and the sole TRAF3 binding site of LMP-1 defines two distinct signaling motifs

The TRAF3 molecule interacts with the cytoplasmic carboxyl terminus (COOH terminus) of the Epstein-Barr virus-encoded oncogene LMP-1. NF-kappaB activation is a downstream signaling event of tumor necrosis factor receptor-associated factor (TRAF) molecules in other signaling systems (CD40 for example) and is an event caused by LMP-1 expression. One region capable of TRAF3 interaction in LMP-1 is the membrane-proximal 45 amino acids (188-242) of the COOH terminus. We show that this region contains the only site for binding of TRAF3 in the 200-amino acid COOH terminus of LMP-1. The site also binds TRAF2 and TRAF5, but not TRAF6. TRAF3 binds to critical residues localized between amino acids 196 and 212 (HHDDSLPHPQQATDDSG), including the PXQX(T/S) motif, that share limited identity to the CD40 receptor TRAF binding site (TAAPVQETL). Mutation of critical residues in the TRAF3 binding site of LMP-1 that prevents binding of TRAF2, TRAF3, and TRAF5 does not affect NF-kappaB-activating potential. Deletion mapping localized the major NF-kappaB activating region of LMP-1 to critical residues in the distal 4 amino acids of the COOH terminus (383-386). Therefore, TRAF3 binding and NF-kappaB activation occur through two separate motifs at opposite ends of the LMP-1 COOH-terminal sequence.

Expression of Epstein-Barr virus latent membrane protein 1 protects Jurkat T cells from apoptosis induced by serum deprivation

It has been generally accepted that inhibition of apoptosis is important in the development of malignancy. To determine whether Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1), the virus-coded transforming oncogene product, has an anti-apoptotic function in non-B-cells, Jurkat T cells were transfected with the LMP1-expression vector pSV2gptMTLM consisting of the human metallothionein promoter and were selected for mycophonolic acid resistance. LMP1-expressing clones of Jurkat cells showed resistance to apoptosis induced by serum deprivation. In LMP1-expressing clones, although the levels of Bcl-2 and Bax were similar to those in the clones of vector transfectants or parental cells, c-Myc expression was significantly depressed. Down-regulation of c-Myc by LMP1 was confirmed by using LMP1-expressing clones treated with CdCl2. Addition of c-myc antisense oligonucleotides to Jurkat cells specifically inhibited apoptosis induced by serum deprivation at the concentrations which suppressed c-Myc expression. These results suggest that LMP1 expression and subsequent down-regulation of c-Myc protect Jurkat T cells from apoptosis induced by serum deprivation. The significance of the anti-apoptotic function of LMP1 in non-B, Jurkat T cells is discussed in relation to the pathogenesis of EBV malignancy.

Epstein-Barr virus latent membrane protein (LMP1) is not sufficient to maintain proliferation of B cells but both it and activated CD40 can prolong their survival

Epstein-Barr virus (EBV) infects human primary B lymphocytes and induces and maintains proliferation of these cells efficiently in vitro. Mutants of Epstein-Barr virus which express EBV nuclear antigen 2 (EBNA2) in a conditional fashion allow dissection of individual contributions of viral genes to B cell immortalization. EBNA2 is a transcriptional activator of cellular and viral genes, including the viral latent membrane protein 1 (LMP1), which is essential for B cell immortalization and has oncogenic effects in non-lymphoid cells. To analyze the role of this gene in B cell immortalization, LMP1 was constitutively expressed in B cells infected with EBV carrying a conditional EBNA2 allele. In the absence of functional EBNA2, LMP1 was incapable of sustaining B cell proliferation in two independent assays but induced a phenotype consistent with prolonged cell viability. Activation of CD40 displayed a comparable phenotype. These data indicate that both CD40 activation and LMP1 expression may use a common pathway for B cell activation. Proliferation of human B cells, however, requires one or more additional signals triggered by EBNA2.

Comparative analysis identifies conserved tumor necrosis factor receptor-associated factor 3 binding sites in the human and simian Epstein-Barr virus oncogene LMP1

Nonhuman primates are naturally infected with a B-lymphotropic herpesvirus closely related to Epstein-Barr virus (EBV). These simian EBV share considerable genetic, biologic, and epidemiologic features with human EBV, including virus-induced tumorigenesis. However, latent, transformation-associated viral genes demonstrate marked sequence divergence among species despite the conserved functions. We have cloned the latent membrane protein 1 (LMP1) homologs from the simian EBV naturally infecting baboons (cercopithicine herpesvirus 12, herpesvirus papio) and rhesus monkeys (cercopithicine herpesvirus 15) for a comparative study with the human EBV oncogene. The transmembrane domains are well conserved, but there is striking sequence divergence of the carboxy-terminal cytoplasmic domain essential for B-cell immortalization and interaction with the tumor necrosis factor receptor signaling pathway. Nevertheless, the simian EBV LMP1s retain most functions in common with EBV LMP1, including the ability to induce NF-(kappa)B activity in human cells, to bind the tumor necrosis factor-associated factor 3 (TRAF3) in vitro, and to induce expression of tumor necrosis factor-responsive genes, such as ICAM1, in human B lymphocytes. Multiple TRAF3 binding sites containing a PXQXT/S core sequence can be identified in the simian EBV LMP1s by an in vitro binding assay. A PXQXT/S-containing sequence is also present in the cytoplasmic domain of the Hodgkin's disease marker, CD30, and binds TRAF3 in vitro. The last 13 amino acids containing a PXQXT/S sequence are highly conserved in human and simian EBV LMP1 but do not bind TRAF3, suggesting a distinct role for this conserved region of LMP1. The conserved TRAF3 binding sites in LMP1 and the CD30 Hodgkin's disease marker provides further evidence that a TRAF3-mediated signal transduction pathway may be important in malignant transformation.

c-myc activation renders proliferation of Epstein-Barr virus (EBV)-transformed cells independent of EBV nuclear antigen 2 and latent membrane protein 1

Two genetic events contribute to the development of endemic Burkitt lymphoma (BL) infection of B lymphocytes with Epstein-Barr virus (EBV) and the activation of the protooncogene c-myc through chromosomal translocation. The viral genes EBV nuclear antigen 2 (EBNA2) and latent membrane protein 1 (LMP1) are essential for transformation of primary human B cells by EBV in vitro; however, these genes are not expressed in BL cells in vivo. To address the question whether c-myc activation might abrogate the requirement of the EBNA2 and LMP1 function, we have introduced an activated c-myc gene into an EBV-transformed cell line in which EBNA2 was rendered estrogen-dependent through fusion with the hormone binding domain of the estrogen receptor. The c-myc gene was placed under the control of regulatory elements of the immunoglobulin kappa locus composed a matrix attachment region, the intron enhancer, and the 3' enhancer. We show here that transfection of a c-myc expression plasmid followed by selection for high MYC expression is capable of inducing continuous proliferation of these cells in the absence of functional EBNA2 and LMP1. c-myc-induced hormone-independent proliferation was associated with a dramatic change in the growth behavior as well as cell surface marker expression of these cells. The typical lymphoblastoid morphology and phenotype of EBV-transformed cells completely changed into that of BL cells in vivo. We conclude that the phenotype of BL cells reflects the expression pattern of viral and cellular genes rather than its germinal center origin.

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.

bcl-2: role in epithelial differentiation and oncogenesis

The precise regulation and maintenance of balance between cell proliferation and cell death in multicellular organisms is critical for tissue homeostasis. bcl-2 initiates a new gene family involved in the regulation of cell death and survival without affecting cell proliferation. Expression of Bcl-2 has been reported in a wide range of hematopoietic cells, nonneoplastic epithelia (both hormone-responsive and nonresponsive), and epithelial malignancies. Although the major group of epithelial cells expressing Bcl-2 protein are in the proliferating zones, expression is not directly related to cell proliferation. Bcl-2 is also associated with stem cells committed to differentiation and morphogenesis. The survival advantage provided by Bcl-2 prolongs the life span of epithelial cells with differentiation potential and allows proliferation, differentiation, and morphogenesis to proceed. The gene expression in hormone-responsive organs may contribute to the sustained life of those terminally differentiated epithelial cells and a decrease in Bcl-2 levels leads to cell death by apoptosis. Overexpression of bcl-2 protects epithelial cells from death, but it is neither able to immortalize normal cells, nor to cause tumorigenic transformation of immortalized epithelial cells. Heterogeneous expression of Bcl-2 in epithelial malignancies suggests that the gene is differentially regulated. Furthermore, its expression in association with precancerous lesions suggests a role in the early stage of tumorigenesis. The effects of Bcl-2 expression on sensitivity of epithelial cells to drug, radiation, and hormone therapies vary depending on the typed of tumor. Expression of Bcl-2 is associated with resistance to hormone therapy and recurrence in prostate carcinomas, whereas in lung and breast carcinomas it is associated with a better prognosis. Studies now being performed should clarify the underlying mechanisms of differential gene regulation in different tissues and show the clinical significance of the expression of bcl-2 and other members of the bcl-2 gene family.

The ecology and pathology of Epstein-Barr virus

Epstein-Barr virus achieves its ubiquitous and uniform epidemiological distribution by a dual strategy of latency to guarantee lifelong persistence and intermittent replication to guarantee transmission. These two functions appear to dictate residence in different cell types: latency in B lymphocytes and replication in epithelial cells. Both of these cell compartments are potential sites for EBV-associated malignancies.

Epstein-Barr virus nuclear antigen 3C is a transcriptional regulator

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA-3C) is one of five viral nuclear proteins that are essential for EBV-induced immortalization of primary human B lymphocytes in vitro. Previous studies have implied that EBNA-3C acts as a transcription factor. Using transient transfection assays, we demonstrate that EBNA-3C has two effects on reporter genes that are linked to the latent membrane protein 1 promoter, (i) low-level activation by EBNA-3C alone, as well as potentiation of EBNA-2-mediated transactivation, and (ii) inhibition of the normally strong activation mediated by EBNA-2. These two disparate effects seem to be mediated at different stages following cell feeding. The inhibitory effect of EBNA-3C was localized to a known EBNA-2 response element that had previously been shown to be recognized by the DNA-binding protein RBP-J kappa. In addition, direct interaction between RBP-J kappa and EBNA-3C was observed by coimmunoprecipitation. Activation by EBNA-3C, however, seems to be achieved via sequences that are distinct from RBP-J kappa sites, since activation remained even after these sites had been mutated. Consistent with its ability to activate transcription, a region of EBNA-3C which has homology to the glutamine-rich activation domain of Sp1 can function as a transcription activation domain when it is fused to the heterologous DNA-binding domain of Gal4 and can partially restore the activity of a mutant EBNA-2 protein with a deletion in the transactivation domain. Collectively, these data strongly support the role of EBNA-3C as a transcriptional regulator.

Stimulation of NF-kappa B-mediated transcription by mutant derivatives of the latent membrane protein of Epstein-Barr virus

The latent membrane protein (LMP) of Epstein-Barr virus contributes to the immortalizing activity of the virus in primary, human B lymphocytes, but its mechanism of function is unknown. LMP is expressed at the plasma membrane and may act by influencing the signalling pathways of infected cells. LMP increases transcription of reporter plasmids that are responsive to members of the NF-kappa B/Rel family of transcription factors (M.-L. Hammarskjold and M. C. Simurda, J. Virol. 66:6496-6501, 1992, and A. Krikos, C. D. Laherty, and V. M. Dixit, J. Biol. Chem. 267:17971-17976, 1992). We measured the stimulation of the activity of a reporter plasmid by LMP in Jurkat and 293 cells in transfection experiments. Expression of LMP stimulated plasmids that contained kappa B enhancer elements but not plasmids that lacked the elements. In 293 cells, expression of the NF-kappa B inhibitor, I kappa B-alpha, reduced the stimulatory activity of LMP. We used deletional analysis to map the domains of LMP that are required for its activity in 293 cells. Wild-type LMP stimulated NF-kappa B by a factor of 20 to 30, while mutant derivatives of LMP that lack oncogenic activity stimulated NF-kappa B by a factor of 3. The multiple membrane-spanning segments together with the carboxy-terminal 55 amino acid residues of LMP were required for its maximal stimulatory function. Residues within its cytoplasmic amino terminus were not required for LMP's stimulation of NF-kappa B. We tested also for stimulation of NF-kappa B activity in cell lines known to support phenotypic changes mediated by expression of LMP. LMP stimulated little NF-kappa B activity in HEp2 cells and no detectable NF-kappa B activity in BALB/3T3 cells. The LMP stimulation of NF-kappa B factors that occurs in some cell lines provides a useful and biochemically tractable assay for determining the function of LMP.

Epstein-Barr virus efficiently immortalizes human B cells without neutralizing the function of p53

Epstein-Barr virus (EBV) efficiently converts resting human B cells into actively cycling, immortal, lymphoblastoid cell lines (LCLs). Here we show that LCLs expressing the full complement of latent viral genes are very sensitive to DNA-damaging agents such as cisplatin. The response includes a rapid accumulation of the tumour suppressor protein p53 and induction of the cellular genes mdm2 and WAF1/p21. Although the levels of Bcl2 protein and Bax mRNA appear unaltered by the activation of p53, within 24 h the majority of cells undergo apoptosis. Over-expression of wild-type p53 in an LCL also resulted in apoptosis; this was preceded by the dephosphorylation of the retinoblastoma gene product, pRb. Primary resting B cells showed no response to cisplatin and even after drug treatment, p53 remained undetectable. However, after infection with EBV, p53 gene expression was induced to a similar level to that found in mitogen-activated B cells. When the physiologically activated primary B cells were exposed to cisplatin, although p53 accumulated as in LCLs, the outcome was growth-arrest rather than gross cell death. We conclude that, in contrast to the transformation of fibroblasts by adenovirus, SV40 or HPV, when B cells become activated and immortalized by EBV they are sensitized to the p53-mediated damage response. When the resulting LCLs are treated with genotoxic agents such as cisplatin, they are unable to arrest like normal cells because they are driven to proliferate by EBV and consequently undergo apoptosis.

Delay of vaccinia virus-induced apoptosis in nonpermissive Chinese hamster ovary cells by the cowpox virus CHOhr and adenovirus E1B 19K genes

The infection of vaccinia virus in Chinese hamster ovary (CHO) cells produces a rapid shutdown in protein synthesis, and the infection is abortive (R.R. Drillien, D. Spehner, and A. Kirn, Virology 111:488-499, 1978; D.E. Hruby, D.L. Lynn, R. Condit, and J.R. Kates, J. Gen. Virol. 47:485-488, 1980). Cowpox virus, which can productively infect CHO cells, had previously been shown to contain a host range gene, CHOhr, which confers on vaccinia virus the ability to replicate in CHO cells (D. Spehner, S. Gillard, R. Drillien, and A. Kirn, J. Virol. 62:1297-1304, 1988). We found that CHO cells underwent apoptosis when infected with vaccinia virus. The expression of the CHOhr gene in vaccinia virus allowed for the expression of late virus genes. CHOhr also delayed or prevented vaccinia virus-induced apoptosis in CHO cells such that there was sufficient time for replication of the virus before the cell died. The E1B 19K gene from adenovirus also delayed vaccinia virus-induced apoptosis; however, there was no detectable expression of late virus genes. Furthermore, E1B 19K also delayed cell death in CHO cells which had been productively infected with vaccinia virus. This study identifies a new antiapoptotic gene from cowpox virus, CHOhr, for which the protein contains an ankyrin-like repeat and shows no significant homology to other proteins. This work also indicates that an antiapoptotic gene from one virus family can delay cell death in an infection of a virus from a different family.

The genetic regulation of apoptosis

Dramatic advances, most of them within the past two years, have provided a picture of the genetic regulation of apoptosis in mammalian cells. Although much detail remains to be filled in, the general structure--concordant with programmed death in invertebrates--includes signalling systems, genetic determination of susceptibility, critical proteins capable of reversing or re-affirming the death sentence, and a common effector pathway driven by specific proteases.

Burkitt's lymphoma cells are resistant to programmed cell death in the presence of the Epstein-Barr virus latent antigen EBNA-4

Group I Epstein-Barr virus (EBV)-positive Burkitt's lymphoma (BL) cells display a surface phenotype characteristic of germinal centre B cells and readily undergo apoptosis in response to a variety of stimuli, including serum deprivation. Activation of EBV latent gene expression has been shown to increase the survival of these tumour cells by blocking programmed cell death. To investigate the nature of this protection, we assessed the function of the EBV latent EBNA-4 gene in a group I lymphoma line, dG75. Group I BL cells induced to undergo apoptosis in response to serum starvation were protected in the presence of EBNA-4 protein. A possible factor underlying this EBNA-4-associated survival was increased expression of the oncoprotein bcl-2, a known repressor of cell death. Together these data suggest that EBNA-4 plays an important role in the regulation of programmed cell death in BL tumour cells.

Uses of flow cytometry in virology

This article reviews some of the published applications of flow cytometry for in vitro and in vivo detection and enumeration of virus-infected cells. Sample preparation, fixation, and permeabilization techniques for a number of virus-cell systems are evaluated. The use of flow cytometry for multiparameter analysis of virus-cell interactions for simian virus 40, herpes simplex viruses, human cytomegalovirus, and human immunodeficiency virus and its use for determining the effect of antiviral compounds on these virus-infected cells are reviewed. This is followed by a brief description of the use of flow cytometry for the analysis of several virus-infected cell systems, including blue tongue virus, hepatitis C virus, avian reticuloendotheliosis virus, African swine fever virus, woodchuck hepatitis virus, bovine viral diarrhea virus, feline leukemia virus, Epstein-Barr virus, Autographa californica nuclear polyhedrosis virus, and Friend murine leukemia virus. Finally, the use of flow cytometry for the rapid diagnosis of human cytomegalovirus and human immunodeficiency virus in peripheral blood cells of acutely infected patients and the use of this technology to monitor patients on antiviral therapy are reviewed. Future prospects for the rapid diagnosis of in vivo viral and bacterial infections by flow cytometry are discussed.

Upregulation of bcl-2 by the Epstein-Barr virus latent membrane protein LMP1: a B-cell-specific response that is delayed relative to NF-kappa B activation and to induction of cell surface markers

An ability of the Epstein-Barr virus latent membrane protein LMP1 to enhance the survival of infected B cells through upregulation of the bcl-2 oncogene was first suggested by experiments involving gene transfection and the selection of stable LMP1+ clones (S. Henderson, M. Rowe, C. Gregory, F. Wang, E. Kieff, and A. Rickinson, Cell 65:1107-1115, 1991). However, it was not possible to ascertain whether Bcl-2 upregulation was a specific consequence of LMP1 expression or an artifact of the selection procedure whereby rare Bcl-2+ cells already present in the starting population might best be able to tolerate the potentially toxic effects of LMP1. We therefore reexamined this issue by using two different experimental approaches that allowed LMP1-induced effects to be monitored immediately following expression of the viral protein and in the absence of selective pressures; activation of the NF-kappa B transcription factor and upregulation of the cell adhesion molecule ICAM-1 were used as early indices of LMP1 function. In the first approach, stable clones of two B-cell lines carrying an LMP1 gene under the control of an inducible metallothionein promoter were induced to express LMP1 in all cells. Activation of NK-kappa B and upregulation of ICAM-1 occurred within 24 h and were followed at 48 to 72 h by upregulation of Bcl-2. In the second approach, we tested the generality of this phenomenon by transiently expressing LMP1 from a strong constitutively active promoter in a range of different cell types. All six B-cell lines tested showed NF-kappa B activation in response to LMP1 expression, and this was followed in five of six lines by expression of ICAM-1 and Bcl-2. In the same experiments, all three non-B-cell lines showed NF-kappa B activation and ICAM-1 upregulation but never any effect upon Bcl-2. We therefore conclude that Bcl-2 upregulation is part of the panoply of cellular changes induced by LMP1 but that the effect is cell type specific. Our data also suggest that whilst NF-kappa B may be an essential component of LMP1 signal transduction, other cell-specific factors may be required to effect some functions of the viral protein.

Control of programmed cell death by the baculovirus genes p35 and iap

The SF-21 insect cell line undergoes rapid and widespread apoptosis when treated with actinomycin D or when infected with a mutant of the baculovirus Autographa californica nuclear polyhedrosis virus lacking a p35 gene or a functionally active iap (inhibitor of apoptosis) gene. Here we provide evidence that the basis for the induction of apoptosis by these two different stimuli is the cessation of RNA synthesis. We also show that expression of either p35 or two different functional iap homologs blocks apoptosis independently of other viral genes, indicating that these gene products act directly on the cellular apoptotic pathway. The iap genes encode a C3HC4 (or RING) finger motif found in a number of transcriptional regulatory proteins, as well as two additional Cys/His motifs (baculovirus iap repeats). We show that specific amino acids within both the C3HC4 finger and the N-terminal baculovirus iap repeat are critical for anti-apoptosis function. Overexpression of either mammalian bcl-2 or adenovirus E1B-19K, genes which block apoptosis when overexpressed in a number of mammalian cells, does not block actinomycin D-induced apoptosis in SF-21 cells.