A membrane protein encoded by Epstein-Barr virus in latent growth-transforming infection

The Epstein-Barr virus LMP1 interactome: biological implications and therapeutic targets

The oncogenic potential of Epstein-Barr virus (EBV) is mostly attributed to latent membrane protein 1 (LMP1), which is essential and sufficient for transformation of fibroblast and primary lymphocytes. LMP1 expression results in the activation of multiple signaling cascades like NF-ΚB and MAP kinases that trigger cell survival and proliferative pathways. LMP1 specific signaling events are mediated through the recruitment of a number of interacting proteins to various signaling domains. Based on these properties, LMP1 is an attractive target to develop effective therapeutics to treat EBV-related malignancies. In this review, we focus on LMP1 interacting proteins, associated signaling events, and potential targets that could be exploited for therapeutic strategies.

Phage-display screening identifies LMP1-binding peptides targeting the C-terminus region of the EBV oncoprotein

Latent membrane protein 1 (LMP1), a major oncoprotein of Epstein Barr Virus (EBV) is responsible for transforming B lymphocytes in vitro. LMP1 is overexpressed in several EBV-associated malignancies, and different approaches have been developed to reduce its level and accordingly its oncogenic function in tumor tissues. This study aimed to use phage display peptide library to obtain peptides which could specifically bind to the cytoplasmic region of LMP1 to prevent its interaction with signaling proteins. The LMP1 C-terminus region was produced in bacterial E. coli and used as target for the phage library panning. After 3 rounds, 20 phage clones were randomly selected and 8 showed high binding affinity to the recombinant C-terminus LMP1 protein. The most interesting candidates are the FO5 "QPTKDSSPPLRV" and NO4 "STTSPPAVPHNN" peptides since both bind the C-terminus LMP1 as showed by molecular docking. Furthermore, sequence alignment revealed that the FO5 peptide shared sequence similarity with the Death Receptor 4 which belongs to the tumor necrosis factor-related apoptosis-inducing receptor which plays key role in anti-tumor immunity.

The Latent Membrane Protein 1 (LMP1)

Almost exactly twenty years after the discovery of Epstein-Barr virus (EBV), the latent membrane protein 1 (LMP1) entered the EBV stage, and soon thereafter, it was recognized as the primary transforming gene product of the virus. LMP1 is expressed in most EBV-associated lymphoproliferative diseases and malignancies, and it critically contributes to pathogenesis and disease phenotypes. Thirty years of LMP1 research revealed its high potential as a deregulator of cellular signal transduction pathways leading to target cell proliferation and the simultaneous subversion of cell death programs. However, LMP1 has multiple roles beyond cell transformation and immortalization, ranging from cytokine and chemokine induction, immune modulation, the global alteration of gene and microRNA expression patterns to the regulation of tumor angiogenesis, cell-cell contact, cell migration, and invasive growth of tumor cells. By acting like a constitutively active receptor, LMP1 recruits cellular signaling molecules associated with tumor necrosis factor receptors such as tumor necrosis factor receptor-associated factor (TRAF) proteins and TRADD to mimic signals of the costimulatory CD40 receptor in the EBV-infected B lymphocyte. LMP1 activates NF-κB, mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3-K), IRF7, and STAT pathways. Here, we review LMP1's molecular and biological functions, highlighting the interface between LMP1 and the cellular signal transduction network as an important factor of virus-host interaction and a potential therapeutic target.

Characterization of the rapamycin-inducible EBV LMP1 activation system

Epstein-Barr virus (EBV) latent infection membrane protein 1 (LMP1) is required for EBV-mediated B lymphocyte transformation into proliferating lymphoblastoid cell lines (LCL). LMP1 oligomerizes spontaneously in membrane lipid rafts via its transmembrane domain and constitutively activates signal transduction pathways, including NF-κB, p38 Mitogen-Activated Protein Kinase (MAPK), and c-Jun N-terminal Kinase (JNK). Since LMP1 mimics the tumor necrosis factor receptor (TNFR), CD40, it may be effectively utilized to study the effects of constitutive activation of signal transduction pathways on cellular physiology. On the other hand, LMP1 presents a disadvantage in terms of determining the sequential events and factors involved in signaling pathways. A CD40-LMP1 chimeric molecule has been generated to overcome this limitation but does not represent the authentic and physiological nature of LMP1. In the current study, a ligand-dependent activation system for LMP1 using rapamycin-inducible dimerization was generated to delineate the LMP1 signaling pathway.

Regulation of Latent Membrane Protein 1 Signaling through Interaction with Cytoskeletal Proteins

Latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) induces constitutive signaling in EBV-infected cells to ensure the survival of the latently infected cells. LMP1 is localized to lipid raft domains to induce signaling. In the present study, a genome-wide screen based on bimolecular fluorescence complementation (BiFC) was performed to identify LMP1-binding proteins. Several actin cytoskeleton-associated proteins were identified in the screen. Overexpression of these proteins affected LMP1-induced signaling. BiFC between the identified proteins and LMP1 was localized to lipid raft domains and was dependent on LMP1-induced signaling. Proximity biotinylation assays with LMP1 induced biotinylation of the actin-associated proteins, which were shifted in molecular mass. Together, the findings of this study suggest that the association of LMP1 with lipid rafts is mediated at least in part through interactions with the actin cytoskeleton.

IMPORTANCE

LMP1 signaling requires oligomerization, lipid raft partitioning, and binding to cellular adaptors. The current study utilized a genome-wide screen to identify several actin-associated proteins as candidate LMP1-binding proteins. The interaction between LMP1 and these proteins was localized to lipid rafts and dependent on LMP1 signaling. This suggests that the association of LMP1 with lipid rafts is mediated through interactions with actin-associated proteins.

High molecular weight complex analysis of Epstein-Barr virus Latent Membrane Protein 1 (LMP-1): structural insights into LMP-1's homo-oligomerization and lipid raft association

LMP-1 is a constitutively active Tumor Necrosis Factor Receptor analog encoded by Epstein-Barr virus. LMP-1 activation correlates with oligomerization and raft localization, but direct evidence of LMP-1 oligomers is limited. We report that LMP-1 forms multiple high molecular weight native LMP-1 complexes when analyzed by BN-PAGE, the largest of which are enriched in detergent resistant membranes. The largest of these high molecular weight complexes are not formed by purified LMP-1 or by loss of function LMP-1 mutants. Consistent with these results we find a dimeric form of LMP-1 that can be stabilized by disulfide crosslinking. We identify cysteine 238 in the C-terminus of LMP-1 as the crosslinked cysteine. Disulfide crosslinking occurs post-lysis but the dimer can be crosslinked in intact cells with membrane permeable crosslinkers. LMP-1/C238A retains wild type LMP-1 NF-κB activity. LMP-1's TRAF binding, raft association and oligomerization are associated with the dimeric form of LMP-1. Our results suggest the possibility that the observed dimeric species results from inter-oligomeric crosslinking of LMP-1 molecules in adjacent core LMP-1 oligomers.

Identification of transmembrane protein 134 as a novel LMP1-binding protein by using bimolecular fluorescence complementation and an enhanced retroviral mutagen

Latent membrane protein 1 (LMP1) of Epstein-Barr virus induces constitutive signaling in infected cells. LMP1 signaling requires oligomerization of LMP1 via its transmembrane domain, localization to lipid rafts in the membrane, and association of the LMP1 cytoplasmic domain to adaptor proteins, such as the tumor necrosis factor receptor-associated factors (TRAFs). Protein complementation is a novel technique to examine protein-protein interaction through the assembly of functional fluorescent proteins or enzymes from inactive fragments. A previous study in our lab demonstrated the use of bimolecular fluorescence complementation (BiFC) to study the assembly of the LMP1 signaling complexes within the plasma membrane of mammalian cells. In the present study, LMP1 was used as bait in a genome-wide BiFC screen with an enhanced retroviral mutagen to identify new LMP1-binding proteins. Our screen identified a novel LMP1-binding protein, transmembrane protein 134 (Tmem134). Tmem134 is a candidate oncogene that is amplified in breast cancer cell lines. Binding, colocalization, and cofractionation between LMP1 and Tmem134 were confirmed. Finally, Tmem134 affected LMP1-induced NF-κB induction. Together, these data suggest that BiFC is a unique and novel platform to identify proteins recruited to the LMP1-signaling complex.

Characterization of the latent membrane protein 1 signaling complex of Epstein-Barr virus in the membrane of mammalian cells with bimolecular fluorescence complementation

Background

Bimolecular fluorescence complementation (BiFC) is a novel technique to examine protein-protein interaction through the assembly of fluorescent proteins. In the present study, BiFC was used to study the assembly of the Epstein-Barr virus latent membrane protein 1 (LMP1) signaling complex within the membrane of mammalian cells. LMP1 signaling requires oligomerization, localization to lipid rafts, and association of the cytoplasmic domain to adaptor proteins, such as the tumor necrosis factor receptor associated factors (TRAFs).

Methods

LMP1-TRAF and LMP1-LMP1 interactions were assayed by BiFC using fluorescence microscopy and flow cytometry. Function of LMP1 BiFC contructs were confirmed by transformation assays and nuclear factor- κB (NF-κB) reporter assays.

Results

BiFC was observed between LMP1 and TRAF2 or TRAF3 and mutation of the LMP1 signaling domains reduced complementation. Fluorescence was observed in previously described LMP1 signaling locations. Oligomerization of LMP1 with itself induced complementation and BiFC. LMP1-BiFC constructs were fully functional in rodent fibroblast transformation assays and activation of NF-κB reporter activity. The BiFC domain partially suppressed some LMP1 mutant phenotypes.

Conclusions

Together these data suggest that BiFC is a unique and novel platform to identify and characterize proteins recruited to the LMP1-signaling complex.

LMP1 TRAFficking Activates Growth and Survival pathways

Epstein-Barr Virus (EBV) Latent Infection Membrane Protein 1 (LMP1) is expressed in all the EBV related malignancies. LMP1 expression is critical for transformation of human B-cells by EBV. LMP1 expression in human B cells induces activation and adhesion molecule expression and cell dumping, which are characteristic of CD40 activated B lymphocytes. In immortalized fibroblasts, LMP1 mimics aspects of activated ras in enabling serum, contact, and anchorage independent growth. Reverse genetic analyses implicate six transmembrane domains (TM), TM1-6, and two C-terminal cytosolic domains, transformation effector sites 1 and 2 (TES1 and 2) or C-terminal activation regions 1 and 2 (CTAR1 and 2) as the essential domains for LMP1 effects. The 6 transmembrane domains cause intermolecular interaction, whereas the C-terminal domains signal through tumor necrosis factor receptor (TNFR) associated factors (TRAFs) or TNFR associated death domain proteins (TRADD) and activate NF-kappaB, JNK, and p38. LMP1 TES1/CTAR1 directly recruits TRAFs 1, 2, 3 and 5 whereas LMP1 TES2/CTAR2 indirectly recruits TRAF6 via BS69. LMP1 TES1/CTAR1 activates TRAF2, NIK, IKKalpha and p52 mediated noncanonical NF-KB pathway and LMP1 TES2/CTAR2 activates TRAF6, TAB1, TAK1, IKKalpha/ IKKbeta/ IKKgamma mediated canonical NF-KB pathway. Interestingly, TRAF3 is a negative regulator of noncanonical NF-kappaB activation, although a positive role in LMP1 signaling has also been described. LMP1 mediated JNK activation is predominantly TES2/CTAR2 dependent and requires TRAF6. LMP1 specifically increases TRAF3 partitioning into lipid rafts and interestingly does not induce degradation of any of the TRAFs upon NF-kappaB activation. Studies of the chemistry and biology of LMP1-TRAF interaction mediated activation of signaling pathways are important for controlling EBV infected cell survival and growth.

Blockade of AP-1 activity by dominant-negative TAM67 can abrogate the oncogenic phenotype in latent membrane protein 1-positive human nasopharyngeal carcinoma

Although activating protein-1 (AP-1) transcription factors play an important role in mediating metastasis for nasopharyngeal carcinoma (NPC), the biological and physiological functions of AP-1, in relation to the oncogenic phenotype of NPC, are not fully understood. Our previous study showed that the latent membrane protein 1 (LMP1) mediated a primary dimer form of c-jun and jun B. In this study, we used a NPC cell line that express a specific inhibitor of AP-1, a dominant-negative c-jun mutant (TAM67), to investigate the role of AP-1 in regulating the NPC oncogenic phenotype. First, we observed that TAM67 inhibited cell growth in vitro and in vivo. Next, with Western blotting, we discovered that TAM67 impaired the cyclin D1/cdk4 complex but had little effect on the cyclin E/cdk2 complex, concomitantly with inhibiting Rb phosphorylation. RT-PCR and luciferase assay results demonstrated that the levels of cyclin D1 mRNA and the promoter activity in TAM67 transfectants were reduced as compared with control cells. Thereby, we show that blockade of AP-1 transcriptional activity has a negative impact on cyclin D1 transcription. We obtained the first evidence that TAM67 prevented NPC growth both in vitro and in vivo. AP-1 appears to be a novel target for treating or preventing LMP1-positive NPC effectively.

LMP1 transmembrane domain 1 and 2 (TM1-2) FWLY mediates intermolecular interactions with TM3-6 to activate NF-kappaB

The Epstein-Barr virus oncoprotein LMP1 has six transmembrane domains (TMs) that enable intermolecular aggregation and constitutive signaling through two C-terminal cytosolic domains. Expression of both TMs 1 and 2 without the C terminus (TM1-2DeltaC) and TMs 3 to 6 fused to the C terminus (TM3-6) results in partial association, which is substantially decreased by TM1 F38WLY41 mutation to A38ALA41. We now investigate whether TM1-2DeltaC can functionally interact with TM3-6. TM1-2DeltaC induced TM3-6 to mediate NF-kappaB activation at 59% of LMP1 levels, and the effect was dependent on TM1-2 F38WLY41. TM1-2DeltaC even induced TM3-4 C terminus-mediated NF-kappaB activation to 44% of LMP1 levels. Surprisingly, this effect was TM1 F38WLY41 independent, indicative of a role for TMs 5 and 6 in TM1 F38WLY41 effects. TM3 W98 was also important for TM1-2DeltaC induction of TM3-6-mediated NF-kappaB activation, for association, and for TM1 F38WLY41 dependence on C-terminal NF-kappaB activation. These data support models in which the TM1 F38WLY41 effects are at least partially dependent on TM3 W98 and a residue(s) in TMs 5 and 6.

Colocalization of interferon regulatory factor 7 (IRF7) with latent membrane protein 1 (LMP1) of Epstein-Barr virus

Interferon regulatory factor 7 (IRF7) is one of the transcriptional factors for the activation of type I Interferon (IFN) genes. It is known that IRF7 and the latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) are highly expressed in EBV type III latency cells, and LMP1 induces mRNA expression of IRF7. In this study, the expression pattern of endogenous IRF7 was observed in several B cell lines with or without EBV infection by immunofluorescence staining. IRF7 was localized in the cytoplasm of EBV-negative B cells and EBV type I latency B cell lines. However, IRF7 was located both in the cytoplasm and nucleus of EBV type III latency cell lines. In the Jijoye cell (type III latency cell), IRF7 was colocalized with LMP1 in the cytoplasm in a capping configuration, and their interaction was confirmed by co-immunoprecipitation of LMP1 and IRF7. This colocalization was confirmed by co-transfection of IRF7 and LMP1 plasmids in EBV-negative B cells. These results suggest that the IRF7 and LMP1 interact with each other, and this may relate to the mechanism whereby LMP1 exerts functional effects in B-lymphocytes.

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.

The C-terminal tail of the gp41 transmembrane envelope glycoprotein of HIV-1 clades A, B, C, and D may exist in two conformations: an analysis of sequence, structure, and function

In addition to the major ectodomain, the gp41 transmembrane glycoprotein of HIV-1 is now known to have a minor ectodomain that is part of the long C-terminal tail. Both ectodomains are highly antigenic, carry neutralizing and non-neutralizing epitopes, and are involved in virus-mediated fusion activity. However, data have so far been biologically based, and derived solely from T cell line-adapted (TCLA), B clade viruses. Here we have carried out sequence and theoretically based structural analyses of 357 gp41 C-terminal sequences of mainly primary isolates of HIV-1 clades A, B, C, and D. Data show that all these viruses have the potential to form a tail loop structure (the minor ectodomain) supported by three, β-sheet, membrane-spanning domains (MSDs). This means that the first (N-terminal) tyrosine-based sorting signal of the gp41 tail is situated outside the cell membrane and is non-functional, and that gp41 that reaches the cell surface may be recycled back into the cytoplasm through the activity of the second tyrosine-sorting signal. However, we suggest that only a minority of cell-associated gp41 molecules – those destined for incorporation into virions – has 3 MSDs and the minor ectodomain. Most intracellular gp41 has the conventional single MSD, no minor ectodomain, a functional first tyrosine-based sorting signal, and in line with current thinking is degraded intracellularly. The gp41 structural diversity suggested here can be viewed as an evolutionary strategy to minimize HIV-1 envelope glycoprotein expression on the cell surface, and hence possible cytotoxicity and immune attack on the infected cell.

Epstein-Barr virus latent membrane protein 1 modulates epidermal growth factor receptor promoter activity in a nuclear factor kappa B-dependent manner

The Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) oncoprotein may cause multiple cellular changes including the induction of epidermal growth factor receptor (EGFR) expression and activation of the NFkappaB transcription factor. LMP1 increases the levels of both EGFR protein and mRNA, but does not stabilize EGFR mRNA. Thus, the effects of LMP1 are likely to be mediated by the direct activation of the EGFR promoter. In this study, induction of LMP1 increased the EGFR in both protein and promoter levels in a dose-dependent manner using tetracycline-regulated LMP1 expression in nasopharyngeal carcinoma (NPC) cell line. Mutational analysis of the LMP1 protein indicated that the C-terminal activation region-1 (CTAR1) domain was mainly involved in the EGFR promoter induction, while CTAR2 was necessary but not sufficient to induce EGFR promoter. Inhibition of LMP1-mediated NFkappaB activation by constitutive repressive IkappaBalpha marginally decreased EGFR promoter activity using transiently transfected IkappaBalpha dominant negative mutant. Promoter mutagenesis analysis demonstrated that two putative NFkappaB binding sites of EGFR promoter were very necessary for the transcriptional activity of EGFR induced by LMP1, the proximal NFkappaB binding site was more important than the distal NFkappaB binding site, and both NFkappaB binding sites played a cooperative role. Taken together, Epstein-Barr virus latent membrane protein 1 modulated the EGFR promoter activity in a NFkappaB-dependent manner.

Epstein-Barr virus signal transduction and B-lymphocyte growth transformation

Latent EBV growth transformation of resting B-cells into indefinitely proliferating cell lines is a successful viral strategy for survival in its host and the basis of several human malignancies. EBV transforms cell growth through viral proteins that modify cell gene expression at the level of transcription or by appropriating signaling pathways. Analyses of the EBV-transforming protein LMP1 have begun to reveal that this receptor transduces critical signals by appropriating the TNF receptor signal transduction pathway to activate NF-kappaB and MAPK. While this has brought an important aspect into clearer focus, future progress in delineating the underlying mechanism of transformation, which will be essential to devising effective therapies to treat EBV-associated malignancies, will depend on resolving the intricacies of TRAF signal transduction. Since expression of cytokines, receptors, and anti-apoptotic proteins are regulated by TRAF signaling, another critical issue is delineating the genes that are specifically targeted by LMP1 in order to transform B-lymphocyte growth.

Latent infection membrane protein transmembrane FWLY is critical for intermolecular interaction, raft localization, and signaling

Relatively little is known about the biochemical mechanisms through which the Epstein-Barr virus latent infection integral membrane protein 1 (LMP1) transmembrane domains cause constitutive LMP1 aggregation and continuous cytoplasmic C terminus-mediated signal transduction. We now evaluate the role of the three consecutive LMP1 hydrophobic transmembrane pairs, transmembrane domains (TM)1-2, TM3-4, and TM5-6, in intermolecular aggregation and NF-kappaB activation. LMP1TM1-2 enabled approximately 40% of wild-type LMP1 cytoplasmic domain-mediated NF-kappaB activation, whereas TM3-4 or TM5-6 assayed in parallel had almost no effect independent of LMP1TM1-2. Alanine mutagenesis of conserved residues in LMP1TM1-2 identified FWLY(38-41) to be critical for LMP1TM1-2 intermolecular association with LMP1TM3-6. Further, in contrast to wild-type LMP1, LMP1 with FWLY(38-41) mutated to AALA(38-41) did not (i). significantly partition to lipid Rafts or Barges and effectively intermolecularly associate, (ii). enable cytoplasmic C terminus engagement of tumor necrosis factor receptor-associated factor 3, (iii). activate NF-kappaB, and thereby (iv). induce tumor necrosis factor receptor-associated factor 1 expression. Other LMP1 intermolecular associations were observed that involved LMP1TM1-2/LMP1TM1-2 or LMP1TM3-4/LMP1TM3-6 interactions; these probably also contribute to LMP1 aggregation. Because FWLY(38-41) was essential for LMP1-mediated signal transduction, and LMP1 activation of NF-kappaB is essential for proliferating B lymphocyte survival, inhibition of LMP1FWLY(41)-mediated LMP1/LMP1 intermolecular interactions is an attractive therapeutic target.

CD40:CD40L interactions in X-linked and non-X-linked hyper-IgM syndromes

Hyper-IgM (HIM) syndrome is a rare immunodeficiency characterized by low or absent IgG, IgA, and IgE with normal or elevated levels of IgM. This disorder can be acquired or familial with either X-linked or autosomal patterns of inheritance. The X-linked form of the disease is a consequence of mutations in the CD40 ligand (CD40L) gene that encodes a protein expressed primarily on activated CD4+ T cells. The cognate interaction between CD40L on T cells and CD40 on antigen-stimulated B cells, macrophage, and dendritic cells is critical for the development of a comprehensive immune response. The non-X-linked form of HIM syndrome is heterogeneous and appears in some cases to be a consequence of mutations in the AlD gene which encodes a B cell specific protein required for class switch recombination, somatic mutation, and germinal center formation. However, mutations in other unidentified genes are clearly the basis of the disease in a subset of patients. In this article, we review the essential features of the X-linked and non-X-linked forms of HIM syndrome and discuss the critical role the CD40:CD40L receptor-ligand pair plays in the pathogenesis of these immune deficiencies.

Antisense oligodeoxynucleotides to latent membrane protein 1 induce growth inhibition, apoptosis and Bcl-2 suppression in Epstein-Barr virus (EBV)-transformed B-lymphoblastoid cells, but not in EBV-positive natural killer cell lymphoma cells

Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP-1) is essential for immortalization of B cells by EBV, protects the infected cells from apoptotic cell death and induces Bcl-2 expression. Suppression of LMP-1 expression by antisense oligodeoxynucleotides (AS-oligo) to LMP-1 inhibits proliferation, promotes apoptosis and suppresses Bcl-2 expression in EBV-transformed B cells. However, the function of LMP-1 expression in EBV-positive natural killer (NK) cell lymphoma cells has not been reported previously. We examined the function of LMP-1 in two EBV-positive NK cell lymphoma cell lines (NK-YS and YT) through suppressing LMP-1 expression by AS-oligo to LMP-1. The AS-oligo to LMP-1 suppressed LMP-1 mRNA and protein expression in two EBV-positive NK cell lymphoma cell lines, as well as in an EBV-transformed B-cell line (CMG-1). Proliferation was inhibited, apoptosis was induced and Bcl-2 expression was suppressed in CMG-1 cells, but none of these events were observed in NK-YS or YT cells. These results suggest that proliferation, inhibition of apoptosis and Bcl-2 expression in EBV-positive NK cell lymphoma cells are not directly regulated by LMP-1 as in EBV-transformed B-cell lines, but are probably mediated through other signal transducing systems.

Expression of the tumour necrosis factor receptor-associated factors 1 and 2 in Hodgkin's disease

The tumour necrosis factor receptor-associated factors (TRAFs) 1 and 2 participate in the signal transduction of various members of the tumour necrosis factor receptor (TNFR) family, including TNFR1, TNFR2, CD40, CD30, and the Epstein-Barr virus (EBV)-encoded latent membrane protein-1 (LMP1). Previous in situ hybridization studies have demonstrated TRAF1 transcripts in the malignant cells of the majority of Hodgkin's disease (HD) tumours, where the expression of TRAF1 was higher in EBV-associated tumours than in their EBV-negative counterparts. In order to determine whether TRAF1 and also TRAF2 were expressed at the protein level in HD and whether there was any relationship to EBV status, immunohistochemistry has been used to detect these proteins in a series of HD specimens. TRAF1 protein was detected more frequently in Hodgkin/Reed-Sternberg (HRS) cells from EBV-positive tumours than in their EBV-negative counterparts. This difference was statistically significant (p=0.01). In contrast, TRAF2 expression by HRS cells appeared to be independent of EBV status. Using a sequential labelling approach, co-localization of LMP1 with either TRAF1 or TRAF2 was also demonstrated in HRS cells from EBV-positive tumours.

Degradation of the epstein-barr virus latent membrane protein 1 (LMP1) by the ubiquitin-proteasome pathway. Targeting via ubiquitination of the N-terminal residue

The latent membrane protein 1 (LMP1) of the Epstein-Barr virus is a constitutively active receptor essential for B lymphocyte transformation by the Epstein-Barr virus. It is a short-lived protein, but the proteolytic pathway involved in its degradation is not known. The ubiquitin pathway is a major system for specific protein degradation in eukaryotes. Most plasma membrane substrates of the pathway are internalized upon ubiquitination and delivered for degradation in the lysosome/vacuole. Here we show that LMP1 is a substrate of the ubiquitin pathway and is ubiquitinated both in vitro and in vivo. However, in contrast to other plasma membrane substrates of the ubiquitin system, it is degraded mostly by the proteasome and not by lysosomes. Degradation is independent of the single Lys residue of the protein; a lysine-less mutant LMP1 is degraded in a ubiquitin- and proteasome-dependent manner similar to the wild type protein. Degradation of both wild type and lysine-less protein is sensitive to fusion of a Myc tag to the N terminus of LMP1. In addition, deletion of as few as 12 N-terminal amino acid residues stabilizes the protein. These findings suggest that the first event in LMP1 degradation is attachment of ubiquitin to the N-terminal residue of the protein. We present evidence suggesting that phosphorylation is also required for degradation of LMP1.

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.

Epstein-Barr virus-encoded latent membrane protein 1 co-expresses with epidermal growth factor receptor in nasopharyngeal carcinoma

Latent membrane protein 1 (LMP-1) is the only Epstein-Barr virus (EBV)-encoded oncogenic protein that has been detected in nasopharyngeal carcinoma (NPC), a cancer that is closely associated with EBV. Previous in-vitro studies have demonstrated that LMP-1 can upregulate epidermal growth factor receptor (EGFR) in epithelial cells. It was not established whether this cellular effect exists in NPC. To assess the association between LMP-1 and EGFR in NPC tissues, 60 NPC specimens were examined by immunohistochemistry using anti-LMP-1 antibody (CS 1-4) and anti-EGFR antibodies (EGFR 1, EGFR 1005). The results revealed that 41 (68.3%) specimens were immunopositive for LMP-1 and 44 (73.3%) specimens over-expressed EGFR. Morphologically, the expressions of LMP-1 and EGFR were homogeneously distributed in the tumor nests. In addition, the correlation between LMP-1 and EGFR was statistically significant (P<0.001, chi2 test, d.f. = 1). To elucidate further the correlation between LMP-1 and EGFR in vivo and in situ, an indirect dual immunofluorescence assay was conducted, using secondary antibodies conjugated with fluorescein isothiocyanate (FITC) or indocarbocyanine (Cy3). The results disclosed an intimate co-expression of LMP-1 and EGFR. In summary, the data indicate that over-expression of EGFR is a common phenomenon in NPC, and that EGFR is co-expressed with LMP-1 in NPC. Thus, EBV may play a role in the tumorigenesis of NPC through the effects of LMP-1 and EGFR.

Epstein-barr virus transformation: involvement of latent membrane protein 1-mediated activation of NF-kappaB

Epstein-Barr virus (EBV) transforms resting primary human B lymphocytes into indefinitely proliferating lymphoblastoid cell lines in vitro and is associated with several human malignancies in vivo. Recombinant EBV genetic analyses combined with in vitro B lymphocyte transformation assays demonstrate that latent infection membrane protein 1 (LMP1) is essential for EBV-mediated lymphocyte transformation. LMP1 has no intrinsic enzymatic activity but instead aggregates cellular proteins of the tumor necrosis factor receptor signaling pathway to activate transcription factor NF-kappaB. Mutants rendering LMP1 defective in these protein interactions are impaired in their abilities to activate NF-kappaB in reporter gene assays. Concordantly, EBV recombinants with LMP1 mutations that are compromised for NF-kappaB activation are impaired for growth transformation. Thus, EBV-mediated growth transformation is genetically and biochemically linked to LMP1-mediated activation of NF-kappaB.

Involvement of Interleukin-10 (IL-10) and Viral IL-6 in the Spontaneous Growth of Kaposi’s Sarcoma Herpesvirus-Associated Infected Primary Effusion Lymphoma Cells

Primary effusion lymphoma (PEL) is a distinct type of lymphoma associated with Kaposi’s sarcoma-associated herpesvirus (KSHV) infection. To determine the factors responsible for the unrestrained proliferation of PEL, we have studied the growth factor requirements of the PEL-derived BCBL-1 and BC-1 cell lines. Both cell lines were found to be autocrine growth factor dependent and to release human interleukin-6 (IL-6), viral IL-6 (vIL-6), and human IL-10 in the culture supernatant. To establish whether these cytokines contribute to autocrine growth, neutralizing antibodies against human IL-6, vIL-6, human IL-10, and soluble IL-10 receptor were used. These experiments showed that human IL-10 and, to a lesser degree, vIL-6 serve as autocrine growth factors for BCBL-1 and BC-1 cells. Thus, human IL-10 and vIL-6 are growth factors released and used by PEL cells for autonomous proliferation and may be critical to the development and progression of PEL.

Involvement of Interleukin-10 (IL-10) and Viral IL-6 in the Spontaneous Growth of Kaposi’s Sarcoma Herpesvirus-Associated Infected Primary Effusion Lymphoma Cells

Primary effusion lymphoma (PEL) is a distinct type of lymphoma associated with Kaposi’s sarcoma-associated herpesvirus (KSHV) infection. To determine the factors responsible for the unrestrained proliferation of PEL, we have studied the growth factor requirements of the PEL-derived BCBL-1 and BC-1 cell lines. Both cell lines were found to be autocrine growth factor dependent and to release human interleukin-6 (IL-6), viral IL-6 (vIL-6), and human IL-10 in the culture supernatant. To establish whether these cytokines contribute to autocrine growth, neutralizing antibodies against human IL-6, vIL-6, human IL-10, and soluble IL-10 receptor were used. These experiments showed that human IL-10 and, to a lesser degree, vIL-6 serve as autocrine growth factors for BCBL-1 and BC-1 cells. Thus, human IL-10 and vIL-6 are growth factors released and used by PEL cells for autonomous proliferation and may be critical to the development and progression of PEL.

Expression of Epstein-Barr virus latent membrane proteins leads to changes in keratinocyte cell adhesion

Epstein-Barr virus (EBV) has 3 latent membrane proteins (LMPs)--LMP1, LMP2a, and LMP2b--which are expressed in nasopharyngeal carcinoma. Using keratinocyte cell lines expressing LMP2a and LMP2b and coexpressing LMP1/LMP2a, we grew organotypic raft cultures to analyze changes in morphology and expression of the cell adhesion molecule ICAM-1; alpha2, alpha3, alpha5, beta1, and alpha6beta4 integrins; laminin 5; E-cadherin; and desmoplakin. Cells expressing LMP2a or LMP2b were defective in their ability to mature and progress through normal squamous stratification when compared to the parental cell lines. Cells coexpressing LMP1/LMP2a additionally demonstrated "pseudoinvasion" into the raft dermal equivalent. There was a consistent and dramatic up-regulation in the suprabasal expression of laminin 5 and alpha6beta4 and beta1 integrins in the LMP-expressing cell lines. ICAM-1, not expressed in the control cell lines, was up-regulated in the LMP-expressing cell lines. Expression of alpha3 and alpha5 integrins was also up-regulated in the LMP-expressing cell lines, while alpha2 demonstrated a loss of the normal basal layer expression. E-cadherin and desmoplakin expression patterns were essentially unchanged. We conclude that LMP2a and LMP2b singly, and LMP1/LMP2a coexpressed, are capable of altering keratinocyte cell adhesion molecule expression consistent with nasopharyngeal carcinoma.

TNF receptor associated factors in cytokine signaling

Just four years ago the first two members of a new family of molecules involved in signal transduction by members of the TNF receptor superfamily were described and designated TNF Receptor Associated Factors (TRAFs). In the meantime six human and murine TRAFs as well as a TRAF protein from C. elegans have been molecularly cloned. From our current point of view, TRAF proteins appear to represent multifunctional signal adaptors, tightly embedded in a network of signals culminating in the activation of kinase cascades that finally lead to the activation of c-Jun N-terminal kinase. p38 mitogen activated protein kinase, and the transcription factor NF-kappaB, thereby also affecting the balance between survival and cell death. Some of the activities of the individual TRAF family members may be redundant although transgenic knockout animal models have already shown that crucial signaling pathways for single TRAF molecules in vivo can be defined.

Expression of the Epstein-Barr virus latent membrane protein 1 induces B cell lymphoma in transgenic mice

The latent membrane protein 1 (LMP1) of the Epstein-Barr virus has transforming properties in rodent fibroblasts and is expressed in most of the cancers associated with Epstein-Barr virus (EBV) infection including posttransplant lymphomas, Hodgkin's disease, nasopharyngeal carcinoma, and AIDS-related lymphomas. In this study, three lineages of LMP1 transgenic mice were established with LMP1 expressed under the control of the Ig heavy chain promoter and enhancer. Lymphoma developed in all three lineages, and the incidence of lymphoma increased significantly with age with lymphomas developing in 42% of transgenic mice over 18 months. The expression of LMP1 was detected at high levels in the lymphoma tissues but only at trace levels in normal lymphoid tissues. Gene rearrangement of the Ig heavy chain indicated monoclonality or oligoclonality in all lymphomas, some of the lymphoid hyperplastic spleens, and some histologically normal spleens. These data reveal that LMP1, without the expression of other EBV genes, is oncogenic in vivo and indicate that LMP1 is a major contributing factor to the development of EBV-associated lymphomas.

Interaction of tumor necrosis factor receptor-associated factor signaling proteins with the latent membrane protein 1 PXQXT motif is essential for induction of epidermal growth factor receptor expression

The Epstein-Barr virus latent membrane protein 1 (LMP1) oncoprotein causes multiple cellular changes, including induction of epidermal growth factor receptor (EGFR) expression and activation of the NF-kappaB transcription factor. LMP1 and the cellular protein CD40, which also induces EGFR expression, interact with the tumor necrosis factor receptor-associated factor (TRAF) proteins. The LMP1 carboxy-terminal activation region 1 signaling domain interacts specifically with the TRAFs and is essential for EGFR induction through a mechanism independent of NF-kappaB alone. LMP1 and CD40 share a common TRAF binding motif, PXQXT. In this study, the PXQXT motifs in both LMP1 and CD40 were altered and mutant proteins were analyzed for induction of EGFR expression. Replacement of the T residue with A in CD40 completely blocked induction of the EGFR, while the same mutation in LMP1 did not affect EGFR induction. Replacement of both P and Q residues with A's in LMP1 reduced EGFR induction by >75%, while deletion of PXQXT blocked EGFR induction. These results genetically link EGFR induction by LMP1 to the TRAF signaling pathway. Overexpression of TRAF2 potently activates NF-kappaB, although TRAF2 did not induce expression of the EGFR either alone or in combination with TRAF1 and TRAF3. In vivo analyses of the interaction of the TRAFs with LMP1 variants mutated in the PXQXT domain indicate that high-level induction of EGFR expression requires interaction with TRAF1, -2, and -3. However, exogenous expression of TRAF3 decreased EGFR induction mediated by either LMP1 or CD40. These data suggest that TRAF-mediated activation of EGFR expression requires assembly of a complex containing the appropriate stoichiometry of TRAF proteins clustered at the cell membrane with LMP1.

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-mediated B-cell proliferation is dependent upon latent membrane protein 1, which simulates an activated CD40 receptor

The Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) is essential for the immortalization of human B cells and is linked etiologically to several human tumors. LMP1 is an integral membrane protein which acts like a constitutively active receptor. It binds tumor necrosis factor (TNF)-receptor-associated factors (TRAFs), activates NF-kappaB and triggers the transcription factor AP-1 via the c-Jun N-terminal kinase (JNK) cascade, but its specific contribution to B-cell immortalization has not been elucidated fully. To address the function of LMP1, we established B cell lines with a novel mini-EBV plasmid in which the LMP1 gene can be regulated at will without affecting the expression of other latent EBV genes. We demonstrate here that continuous expression of LMP1 is essential for the proliferation of EBV-immortalized B cells in vitro. Re-induction of LMP1 expression or activation of the cellular CD40 receptor both induce the JNK signaling cascade, activate the transcription factor NF-kappaB and stimulate proliferation of these B cells. Our findings strongly suggest that LMP1 mimics B-cell activation processes which are physiologically triggered by CD40-CD40 ligand signals. Since LMP1 acts in a ligand-independent manner, it replaces the T cell-derived activation signal to sustain indefinite B-cell proliferation.

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.

Latent membrane protein 1 of Epstein-Barr virus mimics a constitutively active receptor molecule

Latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is an integral membrane protein which has transforming potential and is necessary but not sufficient for B-cell immortalization by EBV. LMP1 molecules aggregate in the plasma membrane and recruit tumour necrosis factor receptor (TNF-R) -associated factors (TRAFs) which are presumably involved in the signalling cascade leading to NF-kappaB activation by LMP1. Comparable activities are mediated by CD40 and other members of the TNF-R family, which implies that LMP1 could function as a receptor. LMP1 lacks extended extracellular domains similar to beta-adrenergic receptors but, in contrast, it also lacks any motifs involved in ligand binding. By using LMP1 mutants which can be oligomerized at will, we show that the function of LMP1 in 293 cells and B cells is solely dependent on oligomerization of its carboxy-terminus. Biochemically, oligomerization is an intrinsic property of the transmembrane domain of wild-type LMP1 and causes a constitutive phenotype which can be conferred to the signalling domains of CD40 or the TNF-2 receptor. In EBV, immortalized B cells cross-linking in conjunction with membrane targeting of the carboxy-terminal signalling domain of LMP1 is sufficient for its biological activities. Thus, LMP1 acts like a constitutively activated receptor whose biological activities are ligand-independent.

Latent membrane protein 1 of Epstein-Barr virus mimics a constitutively active receptor molecule

Latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is an integral membrane protein which has transforming potential and is necessary but not sufficient for B-cell immortalization by EBV. LMP1 molecules aggregate in the plasma membrane and recruit tumour necrosis factor receptor (TNF-R) -associated factors (TRAFs) which are presumably involved in the signalling cascade leading to NF-kappaB activation by LMP1. Comparable activities are mediated by CD40 and other members of the TNF-R family, which implies that LMP1 could function as a receptor. LMP1 lacks extended extracellular domains similar to beta-adrenergic receptors but, in contrast, it also lacks any motifs involved in ligand binding. By using LMP1 mutants which can be oligomerized at will, we show that the function of LMP1 in 293 cells and B cells is solely dependent on oligomerization of its carboxy-terminus. Biochemically, oligomerization is an intrinsic property of the transmembrane domain of wild-type LMP1 and causes a constitutive phenotype which can be conferred to the signalling domains of CD40 or the TNF-2 receptor. In EBV, immortalized B cells cross-linking in conjunction with membrane targeting of the carboxy-terminal signalling domain of LMP1 is sufficient for its biological activities. Thus, LMP1 acts like a constitutively activated receptor whose biological activities are ligand-independent.

Early detection of the lytic LMP-1 protein in EBV-infected B-cells suggests its presence in the virion

The Epstein-Barr virus (EBV)-positive B958 cell line expresses two related membrane proteins encoded by BNLF-1 open reading frames. One protein (LMP-1) has been shown to be essential for the growth transforming properties of EBV. The second protein (the lytic LMP-1) is an amino-terminally truncated form of LMP-1 whose expression is associated with induction of EBV's lytic cycle. We have investigated the expression of full-length and lytic forms of LMP-1 immediately after infection of the EBV-negative, B-lymphoma cell line BJAB. Only the lytic LMP-1 protein is present in BJAB cells early (within minutes following addition of virus) after infection with virus derived from either uninduced or tetradecanoyl phorbol acetate and sodium butyrate-induced B958 cells. Lytic LMP-1 protein levels begin to decline by 48 hr after infection, whereas levels of full-length LMP-1 increase between 24 and 48 hr after infection and then remain constant. The presence of the lytic LMP-1 protein in infected cells is independent of both protein synthesis and virus internalization. We also find the lytic LMP-1 protein in BJAB cells early after infection (within 3 hr of addition of virus) with the HH514 strain of EBV, and HH514 cells treated with tetradecanoyl phorbol acetate and sodium butyrate, express high levels of both the lytic LMP-1 and full-length LMP-1 proteins. The lytic LMP-1 protein is enriched in purified virion preparations, and immunoelectron microscopic analysis indicates that EBV virions can be specifically labeled with anti-LMP-1 antisera. Together, these results are consistent with a model in which the lytic LMP-1 protein is present in the EBV virion and is carried into the B-cell upon infection and suggest a role for this protein in early infection events and/or in EBV's lytic cycle.

Deletion variants within the NF-kappaB activation domain of the LMP1 oncogene in acquired immunodeficiency syndrome-related large cell lymphomas, in prelymphomas and atypical lymphoproliferations

The latent membrane protein 1 (LMP1) oncogene of Epstein Barr virus (EBV) is expressed in tumor cells of acquired immunodeficiency syndrome (AIDS) related lymphomas, HIV-negative, EBV-associated malignant lymphoproliferations, nasopharyngeal carcinoma, as well as in reactive immunoblasts of infectious mononucleosis. Naturally occurring LMP1 deletion variants (LMP1-del), characterized by clustered mutations and a distinct 30 base pair deletion within the carboxy terminal domain of LMP1, essential for maximal NF-kappaB stimulation, have been identified in the same conditions. These variants prevail in AIDS-related lymphomas, and are associated with clinically aggressive behaviour in HIV-negative lymphomas, and are frequent in prelymphomatous and reactive states. Functional studies showing a growth advantage of cells infected by these variants may explain the accumulation of LMP1-del in these entities. In the carboxy terminal NF-kappaB activation domain of LMP1, evidence of a hypervariable region close to the highly conserved 23 outermost amino acids essential for malignant transformation, may reflect the natural selection of growth promoting variants involved in signalling pathways. The prevalence of the same mutational pattern in AIDS-related lymphoma as well as in hyperplastic reactive states and prelymphomas supports the hypothesis that these variants confer a growth advantage manifested under impaired cellular immunity.

Preventive effect of IgG from EBV-seropositive donors on the development of human lympho-proliferative disease in SCID mice

The effect of weekly treatments with various gammaglobulin preparations on the development of human B-cell tumors was studied in severe combined immunodeficient (SCID) mice. SCID mice were injected i.p. with human peripheral blood mononuclear cells (PBMCs) from an Epstein-Barr virus (EBV)-seropositive healthy blood donor. Repopulated SCID mice were divided into 7 treatment groups receiving either PBS, 2 commercial gammaglobulin preparations, purified IgG prepared from pooled plasma from EBV-seronegative or -seropositive blood donors, a rabbit anti-serum against EBV envelope glycoprotein gp340 or interferon (IFN)-alpha. All treatments started 1 day after injection of PBMC and continued for 8 weeks. In the PBS-treated control group, 85% of mice developed tumors in the abdominal cavity, mostly with liver metastasis within 150 days. Tumor formation was prevented by treatment with the 2 commercial gammaglobulin preparations as well as by purified IgG from EBV-seropositive donors. In contrast, purified IgG from EBV-seronegative donors, rabbit anti-gp340 anti-serum or IFN-alpha had no effect. Our results indicate that the effect of gammaglobulin is due to the presence of specific antibodies against EBV antigens. Further experiments showed that both the time of onset and the duration of treatment, as well as the dose of Ig, are important factors for prevention of tumor formation. Studies aiming at identification of target antigens for antibodies which prevent lymphoma development may be clinically relevant for prevention and possibly treatment of lympho-proliferative disease in severely immuno-compromised patients.

Aggressive natural killer cell lymphoproliferative disorder associated with Epstein-Barr viral RNA

Lymphoproliferative disorder of natural killer cells is a heterogeneous disorder, and an association with Epstein-Barr virus (EBV) is suggested in some cases. A Japanese male presenting with recurrent nasopharyngeal problems developed fever, generalized lymphadenopathy, and hepatosplenomegaly. Separated cells from lymph nodes were shown to have a natural killer (NK) cell, CD2(+), CD3(-), CD16(+), CD56(+), HLA-DR(+) phenotype. A progressive abnormality of hepatic function was associated with hepatorenal failure and death. A serologic study suggested reactivated EBV infection. In situ hybridization (ISH) studies showed Epstein-Barr virus-encoded RNA (EBER)-1 in lymph nodes, with lymphocytes infiltrating the liver and tissue from ethmoid sinus surgery 3 years prior to development of obvious lymphoproliferative disease. Polymerase chain reaction performed on lymph node DNA, using oligonucleotide primers specific for the EBV lymphocyte-determined membrane antigen (LYDMA) gene, revealed a single band, suggesting monoclonal proliferation of the tumor. NK activities of the lymphocytes from the lymph node and peripheral blood were markedly decreased. These findings suggest a close relationship between EBV infection and development of NK cell lymphoproliferative disorder.

Epstein-Barr virus LMP1 induction of the epidermal growth factor receptor is mediated through a TRAF signaling pathway distinct from NF-kappaB activation

The Epstein-Barr virus (EBV)-encoded LMP1 protein induces several cellular changes including induction of epidermal growth factor receptor (EGFR) expression and activation of the NF-kappaB transcription factor. Two domains within the carboxy terminus have been identified that activate NF-kappaB. In this study, mutational analysis of the LMP1 protein indicated that the proximal NF-kappaB activation domain, which is identical to the TRAF interaction domain (amino acids 187 to 231), is essential for induction of the EGFR. The distal NF-kappaB activation domain (amino acids 352 to 386) did not induce expression of the EGFR. In contrast, the two domains both independently activated a kappaB-CAT reporter gene and induced expression of the NF-kappaB-regulated A20 gene in C33A epithelial cells. These results indicate that induction of the EGFR by LMP1 involves the TRAF interaction domain and that activation of NF-kappaB alone is not sufficient. Northern blot analysis revealed that induction of EGFR and A20 expression is likely to be at the transcriptional level. Interestingly expression of CD40 in the C33A cells also induced expression of the EGFR. Overexpression of either TRAF3 or an amino-terminal-truncated form of TRAF3 (TRAF3-C) inhibited signaling from the LMP1 TRAF interaction domain but did not affect signaling from the distal NF-kappaB activation domain. These data further define the mechanism by which LMP1 induces expression of the EGFR and indicate that TRAF signaling from LMP1 and CD40 activates a downstream transcription pathway distinct from NF-kappaB that induces expression of the EGFR.

Association of TRAF1, TRAF2, and TRAF3 with an Epstein-Barr virus LMP1 domain important for B-lymphocyte transformation: role in NF-kappaB activation

The Epstein-Barr virus (EBV) transforming protein LMP1 appears to be a constitutively activated tumor necrosis factor receptor (TNFR) on the basis of an intrinsic ability to aggregate in the plasma membrane and an association of its cytoplasmic carboxyl terminus (CT) with TNFR-associated factors (TRAFs). We now show that in EBV-transformed B lymphocytes most of TRAF1 or TRAF3 and 5% of TRAF2 are associated with LMP1 and that most of LMP1 is associated with TRAF1 or TRAF3. TRAF1, TRAF2, and TRAF3 bind to a single site in the LMP1 CT corresponding to amino acids (aa) 199 to 214, within a domain which is important for B-lymphocyte growth transformation (aa 187 to 231). Further deletional and alanine mutagenesis analyses and comparison with TRAF binding sequences in CD40, in CD30, and in the LMP1 of other lymphycryptoviruses provide the first evidence that PXQXT/S is a core TRAF binding motif. The negative effects of point mutations in the LMP1(1-231) core TRAF binding motif on TRAF binding and NF-kappaB activation genetically link the TRAFs to LMP1(1-231)-mediated NF-kappaB activation. NF-kappaB activation by LMP1(1-231) is likely to be mediated by TRAF1/TRAF2 heteroaggregates since TRAF1 is unique among the TRAFs in coactivating NF-kappaB with LMP1(1-231), a TRAF2 dominant-negative mutant can block LMP1(1-231)-mediated NF-kappaB activation as well as TRAF1 coactivation, and 30% of TRAF2 is associated with TRAF1 in EBV-transformed B cells. TRAF3 is a negative modulator of LMP1(1-231)-mediated NF-kappaB activation. Surprisingly, TRAF1, -2, or -3 does not interact with the terminal LMP1 CT aa 333 to 386 which can independently mediate NF-kappaB activation. The constitutive association of TRAFs with LMP1 through the aa 187 to 231 domain which is important in NF-kappaB activation and primary B-lymphocyte growth transformation implicates TRAF aggregation in LMP1 signaling.

Tumor necrosis factor receptor associated factor 2 is a mediator of NF-kappa B activation by latent infection membrane protein 1, the Epstein-Barr virus transforming protein

Latent infection membrane protein 1 (LMP1), the Epstein-Barr virus transforming protein, associates with tumor necrosis factor receptor (TNFR) associated factor 1 (TRAF1) and TRAF3. Since TRAF2 has been implicated in TNFR-mediated NF-kappa B activation, we have evaluated the role of TRAF2 in LMP1-mediated NF-kappa B activation. TRAF2 binds in vitro to the LMP1 carboxyl-terminal cytoplasmic domain (CT), coprecipitates with LMP1 in B lymphoblasts, and relocalizes to LMP1 plasma membrane patches. A dominant negative TRAF2 deletion mutant that lacks amino acids 6-86 (TRAF/ delta 6-86) inhibits NF-kappa B activation from the LMP1 CT and competes with TRAF2 for LMP1 binding. TRAF2 delta 6-86 inhibits NF-kappa B activation mediated by the first 45 amino acids of the LMP1 CT by more than 75% but inhibits NF-kappa B activation through the last 55 amino acids of the CT by less than 40%. A TRAF interacting protein, TANK, inhibits NF-kappa B activation by more than 70% from both LMP1 CT domains. These data implicate TRAF2 aggregation in NF-kappa B activation by the first 45 amino acids of the LMP1 CT and suggest that a different TRAF-related pathway may be involved in NF-kappa B activation by the last 55 amino acids of the LMP1 CT.

Identification and characterization of murine gammaherpesvirus 68 gp150: a virion membrane glycoprotein

Murine gammaherpesvirus 68 (MHV-68) is a naturally occurring virus of murid rodents which displays pathobiological characteristics similar to those of other gammaherpesviruses, including Epstein-Barr virus (EBV). However, unlike EBV and many other gammaherpesviruses, MHV-68 replicates in epithelial cells in vitro and infects laboratory strains of mice and therefore provides a good model for the study of gammaherpesviruses. Studies of sequences around the center of the MHV-68 genome identified a gene (designated BPRF1 for BamHI P fragment rightward open reading frame 1) whose putative product had motifs reminiscent of a transmembrane glycoprotein. All other gammaherpesviruses have a glycoprotein in this genomic position, but the BPRF1 gene showed sequence homology with only the EBV membrane antigen gp340/220. Biochemical analysis showed that the product of BPRF1 was a glycoprotein present on the surface of infected cells, and immunoelectron microscopy showed that it was present in the virus particle. In addition, antibodies to the BPRF1 product raised by using a bacterial fusion protein neutralized the virus in the absence of complement. The predominant molecular weights of the protein were 150,000 and 130,000. Pulse-chase analysis and endoglycosidase-H digestion showed that the 130,000-molecular-weight form was a precursor of the 150,000-molecular-weight form, and cell surface labelling showed that the 150,000-molecular-weight form alone was on the cell surface. We therefore named the protein gp150. Since gp150 is the first virion-associated glycoprotein and neutralizing determinant of MHV-68 to be characterized, it provides a valuable tool for the future study of virus-host interactions.

Identification and characterization of murine gammaherpesvirus 68 gp150: a virion membrane glycoprotein

Murine gammaherpesvirus 68 (MHV-68) is a naturally occurring virus of murid rodents which displays pathobiological characteristics similar to those of other gammaherpesviruses, including Epstein-Barr virus (EBV). However, unlike EBV and many other gammaherpesviruses, MHV-68 replicates in epithelial cells in vitro and infects laboratory strains of mice and therefore provides a good model for the study of gammaherpesviruses. Studies of sequences around the center of the MHV-68 genome identified a gene (designated BPRF1 for BamHI P fragment rightward open reading frame 1) whose putative product had motifs reminiscent of a transmembrane glycoprotein. All other gammaherpesviruses have a glycoprotein in this genomic position, but the BPRF1 gene showed sequence homology with only the EBV membrane antigen gp340/220. Biochemical analysis showed that the product of BPRF1 was a glycoprotein present on the surface of infected cells, and immunoelectron microscopy showed that it was present in the virus particle. In addition, antibodies to the BPRF1 product raised by using a bacterial fusion protein neutralized the virus in the absence of complement. The predominant molecular weights of the protein were 150,000 and 130,000. Pulse-chase analysis and endoglycosidase-H digestion showed that the 130,000-molecular-weight form was a precursor of the 150,000-molecular-weight form, and cell surface labelling showed that the 150,000-molecular-weight form alone was on the cell surface. We therefore named the protein gp150. Since gp150 is the first virion-associated glycoprotein and neutralizing determinant of MHV-68 to be characterized, it provides a valuable tool for the future study of virus-host interactions.

Analysis of the Epstein-Barr viral genome in so-called malignant histiocytosis syndrome

Malignant histiocytosis has been described as a proliferation of morphologically atypical histiocytes, but it is difficult to determine whether or not malignant proliferation is present based on morphology alone. Recently the disorder has been thought to be heterogeneous, and therefore a true histiocytic origin is considered to be rare. The Epstein-Barr virus (EBV) is thought to have the ability to transform human cells. Therefore, eight cases of malignant histiocytic (MH) syndrome and five cases of virus-associated hemophagocytic syndrome (VAHS) were analyzed using a polymerase chain reaction (PCR) and the in situ hybridization (ISH) method in order to determine their relationship to EBV infection. At the same time, the cellular origin of these syndromes was also studied. The results indicated that three of the MH cases were derived from T cells while four the MH cases were from histiocytes. The amplification of the EBV-LYDMA region, which was used to determine the monoclonality, was detected in two MH cases and one VAHS case, and all these cases showed only one band. An ISH study also demonstrated the presence of an EBV in these three cases. One of the EBV-positive cases revealed an amplification of the EBV-LYDMA region by the PCR method before showing any sign of MH clinically. In the VAHS cases, the EBV genome was detected in hemophagocytic cells. The EBV-positive cases all demonstrated a rapid clinical course. Based on these results it is possible that EBV infection causes similar rapid clinical features in some cases of both MH and VAHS by the same mechanism.