The residues between the two transformation effector sites of Epstein-Barr virus latent membrane protein 1 are not critical for B-lymphocyte growth transformation

Inhibition of transforming growth factor beta signaling and Smad-dependent activation of transcription by the Latent Membrane Protein 1 of Epstein-Barr virus

Inhibition of transforming growth factor beta (TGFbeta) signaling by the Epstein-Barr virus Latent Membrane Protein 1 (LMP1) may account, at least in part, for the oncogenic activity of LMP1. We found that LMP1 is a potent inhibitor of TGFbeta signaling and Smad-dependent activation of transcription in 293 epithelial cells and COS-7 fibroblasts. LMP1 strongly inhibited the uninduced and the Smad-inducible activity of the promoters of the human p21/WAF1/Cip1 gene and the mouse Smad7 gene. Inhibition of TGFbeta signaling and Smad-dependent activation of transcription by LMP1 was greatly reduced by deletion of both C-terminal activating regions 1 and 2 of LMP1 as well as by overexpression of a non-degradable form of IkappaB. In contrast, specific inhibitors of p38 kinase or MEK kinase did not reverse the inhibitory activity of LMP1. TGFbeta signaling was enhanced by overexpression of dominant negative forms of the LMP1 effectors TRAF2, NIK, and IKKbeta and was abolished by overexpression of p65/RelA or a p50/p65 fusion protein. Deletion of the transactivation domain of p65 abolished its inhibitory activity. Immunoblotting and immunofluorescence microscopy indicated that suppression of TGFbeta signaling and Smad transcriptional activity by LMP1 was not due to Smad degradation or cytoplasmic retention suggesting that LMP1 affects the nuclear function of Smad proteins. Our data are consistent with an essential role of NF-kappaB activation by LMP1 in the inhibition of TGFbeta signaling and Smad-mediated transcriptional responses.

The Epstein-Barr virus latent membrane protein 1 putative Janus kinase 3 (JAK3) binding domain does not mediate JAK3 association or activation in B-lymphoma or lymphoblastoid cell lines

Epstein-Barr virus (EBV) latent infection membrane protein 1 (LMP1) has an intermediate domain between the two cytoplasmic carboxyl-terminal domains that are critical for transforming B-lymphocytes into lymphoblastoid cell lines (LCLs). The intermediate domain has been implicated in Janus kinase 3 (JAK3) association and activation. We now find that LCLs transformed by EBV recombinants that express Flag-LMP1 with the putative JAK3 binding and activating intermediate domain deleted and LCLs transformed by Flag-LMP1 EBV recombinants have similar levels of phosphotyrosine-activated JAK3, signal transducer and activator of transcription 3 (STAT3), or STAT5 and similar very low levels of JAK3 associated with LMP1. Further, transient Flag-LMP1 expression in a B-lymphoma cell line transduces signals that upregulate TRAF1 levels but does not alter JAK3 levels or activation state. Although these data indicate that the LMP1 putative JAK3 binding and activating intermediate domain does not mediate JAK3 association or activation in B-lymphocytes, JAK3 association with LMP1 could be significant, particularly in cells in which LMP1, JAK3, or a JAK3-associated protein is expressed at high levels.

LMP1 structure and signal transduction

The oncogenic Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP1) has structural features and functions reminiscent of a constitutively active TNF family receptor. LMP1 aggregates at the plasma membrane and initiates the activation of signalling pathways, such as NF- kappa B, the mitogen-activated protein kinases JNK and p38, the small GTPase Cdc42 and the JAK/STAT cascade. The constitutive engagement of these signals and the characteristic molecular interactions that regulate them provide the basis for the molecular explanation of the transforming properties of this key EBV protein.

Multiple carboxyl-terminal regions of the EBV oncoprotein, latent membrane protein 1, cooperatively regulate signaling to B lymphocytes via TNF receptor-associated factor (TRAF)-dependent and TRAF-independent mechanisms

Latent membrane protein 1 (LMP1) is an EBV-encoded transforming protein that strongly mimics the B cell-activating properties of a normal cellular membrane protein, CD40. LMP1 and CD40 both associate with the cytoplasmic adapter proteins called TNFR-associated factors (TRAFs). TRAFs 1, 2, and 3 bind to a region of LMP1 that is essential for EBV to transform B lymphocytes, carboxyl-terminal activating region (CTAR) 1. However, studies of transiently overexpressed LMP1 molecules, primarily in epithelial cells, indicated that a second region, CTAR2, is largely responsible for LMP1-mediated activation of NF-kappaB and c-Jun N-terminal kinase. To better understand LMP1 signaling in B lymphocytes, we performed a structure-function analysis of the LMP1 C-terminal cytoplasmic domain stably expressed in B cell lines. Our results demonstrate that LMP1-stimulated Ig production, surface molecule up-regulation, and NF-kappaB and c-Jun N-terminal kinase activation require both CTAR1 and CTAR2, and that these two regions may interact to mediate LMP1 signaling. Furthermore, we find that the function of CTAR1, but not CTAR2, correlates with TRAF binding and present evidence that as yet unidentified cytoplasmic proteins may associate with LMP1 to mediate some of its signaling activities.

Epstein-Barr virus LMP-1 natural sequence variants differ in their potential to activate cellular signaling pathways

The latent membrane protein 1 (LMP-1) oncogene of Epstein-Barr virus (EBV) is believed to contribute to the development of many EBV-associated tumors, and there is evidence that sequence variation can affect some functions of LMP-1. Most studies have been restricted to the prototype B95.8 LMP-1 gene and genes isolated from EBV of nasopharyngeal carcinoma (NPC) patients. Here, we analyzed the signaling functions of LMP-1 from a panel of nine EBV isolates, including representatives of four defined groups of European LMP-1 variants (groups A to D [K. Sandvej, J. W. Gratama, M. Munch, X. G. Zhou, R. L. Bolhuis, B. S. Andresen, N. Gregersen, and S. Hamilton-Dutoit, Blood 90:323-330, 1997]) and Chinese NPC-derived LMP-1. Chinese and group D variants activated the transcription factor NF-kappa B two- to threefold more efficiently than B95.8 LMP-1, while Chinese, group B, and group D variants similarly activated activator protein 1 (AP-1) transcription more efficiently than did B95.8 LMP-1. However, there were no amino acid substitutions in the core binding regions for tumor necrosis factor receptor-associated adapter proteins known to mediate NF-kappa B and AP-1 activation. In contrast, despite sequence variation in the proposed Janus kinase 3 binding region, STAT activation was remarkably constant among the panel of LMP-1 variants. Analysis of the induction of CD54 (intercellular adhesion molecule 1) protein expression by the LMP-1 variants showed differences that did not correlate with either NF-kappa B or AP-1. Therefore, while the defined sequence variant groups do correlate with LMP-1 function, the results highlight the fact that the relationship between sequence variation and signaling function is extremely complex. It appears unlikely that one particular amino acid substitution or deletion will define a disease-associated variant of LMP-1.

Tumor necrosis factor receptor-associated factors (TRAFs)

Tumor necrosis factor receptor-associated factors (TRAFS) were initially discovered as adaptor proteins that couple the tumor necrosis factor receptor family to signaling pathways. More recently they have also been shown to be signal transducers of Toll/interleukin-1 family members. Six members of the TRAF family have been identified. All TRAF proteins share a C-terminal homology region termed the TRAF domain that is capable of binding to the cytoplasmic domain of receptors, and to other TRAF proteins. In addition, TRAFs 2-6 have RING and zinc finger motifs that are important for signaling downstream events. TRAF proteins are thought to be important regulators of cell death and cellular responses to stress, and TRAF2, TRAF5 and TRAF6 have been demonstrated to mediate activation of NF-kappaB and JNK. TRAF proteins are expressed in normal and diseased tissue in a regulated fashion, suggesting that they play an important role in physiological and pathological processes.

An Epstein-Barr virus isolated from a lymphoblastoid cell line has a 16-kilobase-pair deletion which includes gp350 and the Epstein-Barr virus nuclear antigen 3A

Epstein-Barr virus (EBV) is associated with human cancers, including nasopharyngeal carcinoma, Burkitt's lymphoma, gastric carcinoma and, somewhat controversially, breast carcinoma. EBV infects and efficiently transforms human primary B lymphocytes in vitro. A number of EBV-encoded genes are critical for EBV-mediated transformation of human B lymphocytes. In this study we show that an EBV-infected lymphoblastoid cell line obtained from the spontaneous outgrowth of B cells from a leukemia patient contains a deletion, which involves a region of approximately 16 kbp. This deletion encodes major EBV genes involved in both infection and transformation of human primary B lymphocytes and includes the glycoprotein gp350, the entire open reading frame of EBNA3A, and the amino-terminal region of EBNA3B. A fusion protein created by this deletion, which lies between the BMRF1 early antigen and the EBNA3B latent antigen, is truncated immediately downstream of the junction 21 amino acids into the region of the EBNA3B sequence, which is out of frame with respect to the EBNA3B protein sequence, and indicates that EBNA3B is not expressed. The fusion is from EBV coordinate 80299 within the BMRF1 sequence to coordinate 90998 in the EBNA3B sequence. Additionally, we have shown that there is no detectable induction in viral replication observed when SNU-265 is treated with phorbol esters, and no transformants were detected when supernatant is used to infect primary B lymphocytes after 8 weeks in culture. Therefore, we have identified an EBV genome with a major deletion in critical genes involved in mediating EBV infection and the transformation of human primary B lymphocytes that is incompetent for replication of this naturally occurring EBV isolate.

Epstein-Barr virus infection and human malignancies

The Epstein-Barr virus (EBV) is a herpes virus which establishes a life-long persistent infection in over 90% of the human adult population world-wide. Based on its association with a variety of lymphoid and epithelial malignancies, EBV has been classified as a group 1 carcinogen by the International Agency for Research on Cancer. In this article we discuss the evidence supporting an aetiological role for EBV in the pathogenesis of human tumours. The biology of EBV infection will be described with special emphasis on viral transforming gene products. A brief survey of EBV-associated tumours is followed by a discussion of specific problems. Evidence is presented which suggests that failures of the EBV-specific immunity may play a role in the pathogenesis of EBV-associated tumours also in patients without clinically manifest immunodeficiencies. Finally, the timing of EBV infection in the pathogenesis of virus-associated malignancies is discussed. There is good evidence that EBV infection precedes expansion of the malignant cell populations in some virus-associated tumours. However, this is clearly not always the case and for some of these tumours there are indications that clonal genetic alterations may occur prior to EBV infection. Thus, whilst there is good evidence to suggest that EBV is a human carcinogen, its precise role(s) in the development of virus-associated human tumours requires clarification.

Epstein-Barr virus infection and human malignancies

The Epstein-Barr virus (EBV) is a herpes virus which establishes a life-long persistent infection in over 90% of the human adult population world-wide. Based on its association with a variety of lymphoid and epithelial malignancies, EBV has been classified as a group 1 carcinogen by the International Agency for Research on Cancer. In this article we discuss the evidence supporting an aetiological role for EBV in the pathogenesis of human tumours. The biology of EBV infection will be described with special emphasis on viral transforming gene products. A brief survey of EBV-associated tumours is followed by a discussion of specific problems. Evidence is presented which suggests that failures of the EBV-specific immunity may play a role in the pathogenesis of EBV-associated tumours also in patients without clinically manifest immunodeficiencies. Finally, the timing of EBV infection in the pathogenesis of virus-associated malignancies is discussed. There is good evidence that EBV infection precedes expansion of the malignant cell populations in some virus-associated tumours. However, this is clearly not always the case and for some of these tumours there are indications that clonal genetic alterations may occur prior to EBV infection. Thus, whilst there is good evidence to suggest that EBV is a human carcinogen, its precise role(s) in the development of virus-associated human tumours requires clarification.

Epstein-Barr virus infection and human malignancies

The Epstein-Barr virus (EBV) is a herpes virus which establishes a life-long persistent infection in over 90% of the human adult population world-wide. Based on its association with a variety of lymphoid and epithelial malignancies, EBV has been classified as a group 1 carcinogen by the International Agency for Research on Cancer. In this article we discuss the evidence supporting an aetiological role for EBV in the pathogenesis of human tumours. The biology of EBV infection will be described with special emphasis on viral transforming gene products. A brief survey of EBV-associated tumours is followed by a discussion of specific problems. Evidence is presented which suggests that failures of the EBV-specific immunity may play a role in the pathogenesis of EBV-associated tumours also in patients without clinically manifest immunodeficiencies. Finally, the timing of EBV infection in the pathogenesis of virus-associated malignancies is discussed. There is good evidence that EBV infection precedes expansion of the malignant cell populations in some virus-associated tumours. However, this is clearly not always the case and for some of these tumours there are indications that clonal genetic alterations may occur prior to EBV infection. Thus, whilst there is good evidence to suggest that EBV is a human carcinogen, its precise role(s) in the development of virus-associated human tumours requires clarification.

Cytokine signaling and Epstein-Barr virus-mediated cell transformation

Epstein-Barr virus (EBV) latent infection is tightly associated with the development of lymphoid and epithelial human malignancies. The disruption of cell-growth checkpoints is mediated by a limited number of viral proteins that interfere with signal transduction mechanisms and transcription control in the infected cell. Genetic and biochemical evidence supports the notion that EBV-mediated transformation relies extensively on interference with cytokine signaling networks. This is achieved through direct modulation of cytokine receptor signaling mechanisms as well as alterations in the expression levels of various cytokines. The principal effector of these interventions is the EBV latent membrane protein 1 (LMP1) which plays a central role in the transformation process. This viral protein mimics activated receptors of the tumor necrosis factor receptor superfamily to promote cell growth and antiapoptotic mechanisms. LMP1 and other EBV latent proteins upregulate cytokines and growth factors which participate in autocrine and paracrine loops that are likely to promote cell transformation and modulate immune responses. This report will review the molecular mechanisms that underlie the disruption of cytokine signaling mechanisms in EBV-mediated transformation with a particular emphasis on the LMP1 mechanism of function.

Simian homologues of human herpesvirus 8

Gamma-herpesviruses can be found in most primates including Old World an New World monkeys. The gamma-herpesvirinae are grouped into two classes: lymphocryptoviruses (gamma1) and rhadinoviruses (gamma2). The lymphocryptoviruses include Epstein-Barr virus, lymphocryptovirus of rhesus monkeys, and Herpesvirus papio of baboons. Rhadinoviruses that infect New World monkeys include Herpesvirus saimiri, whose natural host is the squirrel monkey, and Herpesvirus ateles, which infects spider monkeys. Rhadinoviruses that infect hominoids and Old World monkeys include Kaposi's sarcoma-associated herpesvirus, also known as HHV-8, and rhesus monkey rhadinovirus.

Molecular virology of Epstein-Barr virus

Epstein-Barr virus (EBV) interacts with its host in three distinct ways in a highly regulated fashion: (i) EBV infects human B lymphocytes and induces proliferation of the infected cells, (ii) it enters into a latent phase in vivo that follows the proliferative phase, and (iii) it can be reactivated giving rise to the production of infectious progeny for reinfection of cells of the same type or transmission of the virus to another individual. In healthy people, these processes take place simultaneously in different anatomical and functional compartments and are linked to each other in a highly dynamic steady-state equilibrium. The development of a genetic system has paved the way for the dissection of those processes at a molecular level that can be studied in vitro, i.e. B-cell immortalization and the lytic cycle leading to production of infectious progeny. Polymerase chain reaction analyses coupled to fluorescent-activated cell sorting has on the other hand allowed a descriptive analysis of the virus-host interaction in peripheral blood cells as well as in tonsillar B cells in vivo. This paper is aimed at compiling our present knowledge on the process of B-cell immortalization in vitro as well as in vivo latency, and attempts to integrate this knowledge into the framework of the viral life cycle in vivo.

The transmembrane domains of the EBV-encoded latent membrane protein 1 (LMP1) variant CAO regulate enhanced signalling activity

Sequence variants of the Epstein-Barr virus (EBV)-encoded latent membrane protein-1 (LMP1) have been reported in association with EBV-linked malignancies but little is known about their effects on signalling pathways and phenotype. We have examined the ability of the nasopharyngeal carcinoma (NPC)-derived variant, CAO-LMP1 to activate the transcription factors NF-kappaB and AP-1 in epithelial cells. In this study, transient expression of CAO-LMP1 was found to activate higher levels of NF-kappaB and AP-1 than the prototype B95.8-LMP1 in human embryonic kidney (HEK) 293 cells and SV40-transformed keratinocytes (SVK). In addition, pulse-chase analysis revealed that CAO-LMP1 has a longer half-life than B95.8-LMP1. Chimera studies localised these phenomena to the transmembrane domains of CAO-LMP1, suggesting that this enhanced signalling capacity may be a consequence of its prolonged half-life. The ability of CAO-LMP1 to activate higher levels of NF-kappaB and AP-1 may contribute to its potent transforming properties.

The LMP1 gene isolated from Russian nasopharyngeal carcinoma has no 30-bp deletion

The Epstein-Barr virus (EBV) is tightly linked to the induction of undifferentiated nasopharyngeal carcinoma (NPC), a tumour endemic in certain areas of southeast Asia. The LMP1 gene encoded by EBV is a classical oncogene due to its ability to transform rodent fibroblasts. LMP1 is absolutely essential for transformation of B cells by the virus and is one of the few EBV genes found to be expressed in NPC. It was originally shown that the LMP1 gene from NPC harbours a deletion of 30 bp in the 3' part of the gene. However, this deletion is also present in the virus spread in healthy people of the areas endemic for NPC and also in other EBV-positive tumours as well as in healthy carriers. We isolated and sequenced the LMP1 gene obtained from tissue of 7 Russian patients with NPC and 1 German patient with an NPC-like tumour of the parotid gland (PG) and compared them with the LMP1 gene isolated from peripheral blood lymphocytes (PBLs) of 6 Russian and 4 German healthy EBV-positive carriers. Neither the Russian NPC cases nor the German NPC-like tumour harboured an LMP1 gene with the 30-bp deletion, while 1 Russian and 2 German carriers contained the LMP1 gene with the 30-bp deletion. In addition, the LMP1 gene isolated from PBLs of the German patient was virtually identical to the gene isolated from the primary tumour. Functional analysis showed no correlation between the presence or absence of the 30-bp deletion and the level of induction of the transcription factors NFkappaB and jun/AP-1 caused by LMP1. These data indicate that the 30-bp deletion is not a factor predisposing for NPC. Comparison of the DNA sequences revealed that the LMP1 genes present in the NPCs most likely represent the "strain" persisting in the general population.

Mechanism of action of a novel latent membrane protein-1 dominant negative

Latent membrane protein-1 (LMP1) is a signaling molecule expressed by Epstein-Barr virus during latency. LMP1 is essential for B-cell immortalization by Epstein-Barr virus and transforms rodent fibroblasts. It activates many distinct signaling pathways including the transcription factors NFkappaB and AP1. We have generated a mutant of LMP1 with four point mutations; amino acids 204, 206, and 208 were mutated to alanine, and amino acid 384 was mutated to glycine. This mutant, termed LMP1(AAAG), is not only unable to activate nuclear signaling pathways, but also inhibits signaling from wild type LMP1. We have demonstrated the effectiveness, selectivity, and mechanism of this inhibitory molecule. It inhibits LMP1-stimulated NFkappaB, STAT, and Jun transcriptional activity. It is selective, as it does not inhibit TNF or interleukin-2 signaling. We have demonstrated that it does not sequester the downstream signaling molecule, TRAF2, but instead binds LMP1 and interferes with its ability to bind TRAF2. This demonstrates the importance of the interplay between the signaling domains of LMP1 and the oligomeric structure of LMP1 for effective signaling. It identifies a tool that will be useful to probe LMP1 function in disease.

Identification and characterization of two CD40-inducible enhancers in the mouse TRAF1 gene locus

We have shown that CD40 engagement induces TRAF1 gene expression in B lymphocytes. Here we report that CD40-dependent TRAF1 gene transcription in murine B cells is controlled by two enhancer regions. One region is located approximately 2 kb upstream of the transcription start site and the other lies in the intron between exons 5 and 6. The upstream enhancer contains a single NF-kappaB site in addition to sites that bind constitutive transcription factors. Mutation of this NF-kappaB site completely abrogates CD40-driven TRAFl transcription. The intronic enhancer contains two sites that strongly bind the CD40-inducible factors NF-kappaB and AP-1. Simultaneous mutation of the AP-1 site and of the NF-kappaB site abolishes transcription driven by this enhancer. When cloned together into reporter constructs, the two TRAF1 enhancers do not synergize, suggesting that each enhancer may separately participate in the induction of TRAF1 transcription in B cells following CD40 activation.

The genetic approach to the Epstein-Barr virus: from basic virology to gene therapy

The Epstein-Barr virus (EBV) infects humans and the genome of this infectious agent has been detected in several tumour types, ranging from lymphomas to carcinomas. The analysis of the functions of the numerous viral proteins encoded by EBV has been impeded by the large size of the viral genome, which renders the construction of viral mutants difficult. To overcome these limitations, several genetic systems have been developed that allow the modification of the viral genome. Two different approaches, depending on the host cell type in which the viral mutants are generated, have been used in the past. Traditionally, mutants were constructed in EBV infected eukaryotic cells, but more recently, approaches that make use of a recombinant EBV cloned in Escherichia coli have been proposed. The phenotype associated with the inactivation or modification of nearly 20 of the 100 EBV viral genes has been reported in the literature. In most of the reported cases, the EBV latent genes that mediate the ability of EBV to immortalize infected cells were the targets of the genetic analysis, but some virus mutants in which genes involved in DNA lytic replication or infection were disrupted have also been reported. The ability to modify the viral genome also opens the way to the construction of viral strains with medical relevance. A cell line infected by a virus that lacks the EBV packaging sequences can be used as a helper cell line for the encapsidation of EBV based viral vectors. This cell line will allow the evaluation of EBV as a gene transfer system with applications in gene therapy. Finally, genetically modified non-pathogenic strains will provide a basis for the design of an attenuated EBV live vaccine.