An integral membrane protein (LMP2) blocks reactivation of Epstein-Barr virus from latency following surface immunoglobulin crosslinking

Role of the Kaposi's sarcoma-associated herpesvirus K15 SH3 binding site in inflammatory signaling and B-cell activation

The Kaposi's sarcoma-associated herpesvirus (KSHV) contains several open reading frames (ORFs) that encode proteins capable of initiating and modulating cellular signaling pathways. Among them is ORF K15, encoding a 12-transmembrane-spanning protein with a cytoplasmic C-terminal domain. Through conserved binding motifs, such as Src homology 2 (SH2) and SH3 binding sites, K15 interacts with cellular proteins, activates the NF-kappaB, MEK/Erk, and Jun N-terminal protein kinase (JNK) pathways, and induces the expression of several inflammatory and angiogenic genes. In this study, we investigated the role of an SH3 domain binding site centered on a PPLP motif in K15. We screened libraries of cellular SH3 domains to identify signaling molecules interacting with the KSHV PPLP motif. We found its affinities for two Src kinase family members, Lyn and Hck, to exceed those of other viral proteins. While the SH2 binding motif YEEV is essential for the inflammatory response induced by KSHV K15, recruitment of Lyn and Hck to the K15 PPLP motif seems to be dispensable for this inflammatory response. However, the PPLP motif is essential for the decrease in B-cell receptor-mediated signaling induced by K15, as measured by calcium mobilization assays.

Epstein-Barr virus-encoded LMP2A induces an epithelial-mesenchymal transition and increases the number of side population stem-like cancer cells in nasopharyngeal carcinoma

It has been recently reported that a side population of cells in nasopharyngeal carcinoma (NPC) displayed characteristics of stem-like cancer cells. However, the molecular mechanisms underlying the modulation of such stem-like cell populations in NPC remain unclear. Epstein-Barr virus was the first identified human tumor virus to be associated with various malignancies, most notably NPC. LMP2A, the Epstein-Barr virus encoded latent protein, has been reported to play roles in oncogenic processes. We report by immunostaining in our current study that LMP2A is overexpressed in 57.6% of the nasopharyngeal carcinoma tumors sampled and is mainly localized at the tumor invasive front. We found also in NPC cells that the exogenous expression of LMP2A greatly increases their invasive/migratory ability, induces epithelial-mesenchymal transition (EMT)-like cellular marker alterations, and stimulates stem cell side populations and the expression of stem cell markers. In addition, LMP2A enhances the transforming ability of cancer cells in both colony formation and soft agar assays, as well as the self-renewal ability of stem-like cancer cells in a spherical culture assay. Additionally, LMP2A increases the number of cancer initiating cells in a xenograft tumor formation assay. More importantly, the endogenous expression of LMP2A positively correlates with the expression of ABCG2 in NPC samples. Finally, we demonstrate that Akt inhibitor (V) greatly decreases the size of the stem cell side populations in LMP2A-expressing cells. Taken together, our data indicate that LMP2A induces EMT and stem-like cell self-renewal in NPC, suggesting a novel mechanism by which Epstein-Barr virus induces the initiation, metastasis and recurrence of NPC.

MicroRNA miR-155 inhibits bone morphogenetic protein (BMP) signaling and BMP-mediated Epstein-Barr virus reactivation

MicroRNA miR-155 is expressed at elevated levels in human cancers including cancers of the lung, breast, colon, and a subset of lymphoid malignancies. In B cells, miR-155 is induced by the oncogenic latency gene expression program of the human herpesvirus Epstein-Barr virus (EBV). Two other oncogenic herpesviruses, Kaposi's sarcoma-associated herpesvirus and Marek's disease virus, encode functional homologues of miR-155, suggesting a role for this microRNA in the biology and pathogenesis of these viruses. Bone morphogenetic protein (BMP) signaling is involved in an array of cellular processes, including differentiation, growth inhibition, and senescence, through context-dependent interactions with multiple signaling pathways. Alteration of this pathway contributes to a number of disease states including cancer. Here, we show that miR-155 targets the 3' untranslated region of multiple components of the BMP signaling cascade, including SMAD1, SMAD5, HIVEP2, CEBPB, RUNX2, and MYO10. Targeting of these mediators results in the inhibition of BMP2-, BMP6-, and BMP7-induced ID3 expression as well as BMP-mediated EBV reactivation in the EBV-positive B-cell line, Mutu I. Further, miR-155 inhibits SMAD1 and SMAD5 expression in the lung epithelial cell line A549, it inhibits BMP-mediated induction of the cyclin-dependent kinase inhibitor p21, and it reverses BMP-mediated cell growth inhibition. These results suggest a role for miR-155 in controlling BMP-mediated cellular processes, in regulating BMP-induced EBV reactivation, and in the inhibition of antitumor effects of BMP signaling in normal and virus-infected cells.

Epstein-Barr virus in Burkitt's lymphoma: a role for latent membrane protein 2A

Burkitt's lymphoma (BL) is characterized by translocation of the MYC gene to an immunoglobulin locus. Transgenic mouse models have been used to study the molecular changes that are necessary to bypass tumor suppression in the presence of translocated MYC. Inactivation of the p53 pathway is a major step to tumor formation in mouse models that is also seen in human disease. Human BL is often highly associated with Epstein-Barr virus (EBV). The EBV latency protein latent membrane protein 2A (LMP2A) is known to promote B cell survival by affecting levels of pro-survival factors. Using LMP2A transgenic mouse models, we have identified a novel mechanism that permits lymphomagenesis in the presence of an intact p53 pathway. This work uncovers a contribution of EBV to molecular events that have documented importance in BL pathogenesis, and may underlie the poorly understood link between EBV and BL.

Regulation of angiogenesis in malignancies associated with Epstein–Barr virus and Kaposi’s sarcoma-associated herpes virus

Tumor angiogenesis is the process by which new blood vessels are formed within emerging or progressing malignancies. The human Epstein–Barr virus and Kaposi’s sarcoma-associated herpesvirus critically contribute to the pathogenesis of selected tumor types, including nasopharyngeal carcinoma and Kaposi’s sarcoma, respectively, where angiogenesis is robust and often disrupted. Lymphangiogenesis, the process by which new lymphatic vessels are formed, is also induced in Epstein–Barr virus and Kaposi’s sarcoma-associated herpesvirus-associated malignancies and in some cases may contribute to metastasis. Recent studies have identified a number of molecules and signaling pathways that underlie angiogenesis and lymphangiogenesis, and clarified the pivotal role of the VEGF family of proteins and their receptors. New treatment modalities that target members of this family have gained approval for clinical use in cancer. Pathogenetic steps are often difficult to dissect in many cancer types, but virus-induced malignancies provide a unique opportunity for understanding the molecular regulation of cancer progression, including angiogenesis. Dissection of viral gene contribution to tumor angiogenesis could result in a better understanding of the angiogenic process, its contribution to cancer and help in the design of rational therapies that target tumor growth and vascularization.

The dynamics of EBV shedding implicate a central role for epithelial cells in amplifying viral output

To develop more detailed models of EBV persistence we have studied the dynamics of virus shedding in healthy carriers. We demonstrate that EBV shedding into saliva is continuous and rapid such that the virus level is replaced in ≤2 minutes, the average time that a normal individual swallows. Thus, the mouth is not a reservoir of virus but a conduit through which a continuous flow stream of virus passes in saliva. Consequently, virus is being shed at a much higher rate than previously thought, a level too high to be accounted for by replication in B cells in Waldeyer's ring alone. Virus shedding is relatively stable over short periods (hours-days) but varies through 3.5 to 5.5 logs over longer periods, a degree of variation that also cannot be accounted for solely by replication in B cells. This variation means, contrary to what is generally believed, that the definition of high and low shedder is not so much a function of variation between individuals but within individuals over time. The dynamics of shedding describe a process governing virus production that is occurring independently ≤3 times at any moment. This process grows exponentially and is then randomly terminated. We propose that these dynamics are best explained by a model where single B cells sporadically release virus that infects anywhere from 1 to 5 epithelial cells. This infection spreads at a constant exponential rate and is terminated randomly, resulting in infected plaques of epithelial cells ranging in size from 1 to 10⁵ cells. At any one time there are a very small number (≤3) of plaques. We suggest that the final size of these plaques is a function of the rate of infectious spread within the lymphoepithelium which may be governed by the structural complexity of the tissue but is ultimately limited by the immune response.

Epstein-Barr virus is a human pathogen associated with several human cancers that nevertheless persists benignly as a latent infection in the majority of adults. EBV persistence is characterized by the presence of latently infected cells in the blood and the shedding of virus into saliva. We present the first systematic quantitative analysis of virus shedding. We show, contrary to what was previously thought, that shedding is continuous and at a high level for all subjects tested. This constant presence of infectious virus may be a crucial risk factor in the development of the EBV-associated tumor nasopharyngeal carcinoma. Unlike infected cells in the blood, which are maintained at very stable levels for years, we show that virus shedding is highly variable such that at any time any individual may be a relatively high or low shedder. We have analyzed these dynamics mathematically and with a simple simulation model. We find that they can be explained by a simple exponential function which we hypothesize is the expansion of 1–3 plaques of epithelial cells.

Role of latent membrane protein 2 isoforms in Epstein-Barr virus latency

The oncogenic Epstein-Barr virus (EBV) infects the majority of the human population without doing harm and establishes a latent infection in the memory B-cell compartment. To accomplish this, EBV hijacks B-cell differentiation pathways and uses its own viral genes to interfere with B-cell signalling to achieve life-long persistence. EBV latent membrane protein 2A (LMP2A) provides a surrogate B-cell receptor signal essential for cell survival and is believed to have a crucial role in the maintenance of latency by blocking B-cell activation which would otherwise lead to lytic EBV infection. These two functions demand tight control of LMP2A activity and expression levels. Based on recent insights in the function of LMP2B, an isoform of LMP2A, we propose a model for how LMP2B modulates the activity of LMP2A contributing to maintenance of EBV latency.

EBV LMP2A provides a surrogate pre-B cell receptor signal through constitutive activation of the ERK/MAPK pathway

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) provides developmental and survival signals that mimic those of a B-cell receptor (BCR). Expression of LMP2A during B-cell development results in the ability of B cells to exit the bone marrow in the absence of a BCR and persist in the periphery, where they would normally undergo apoptosis. This study extends the current knowledge of LMP2A function by examining the growth properties of bone marrow B cells from TgE LMP2A mice. Despite the lack of pre-BCR expression, bone marrow B cells from TgE LMP2A mice proliferate and survive in low concentrations of interleukin 7, similar to wild-type cells. Constitutive phosphorylation of ERK/MAPK and PI3K/Akt in TgE LMP2A bone marrow B cells is also reminiscent of signalling through the pre-BCR, altogether demonstrating that LMP2A provides a pre-BCR-like signal to developing B cells.

Nasopharyngeal carcinoma--review of the molecular mechanisms of tumorigenesis

Nasopharyngeal carcinoma (NPC) is a head and neck cancer rare throughout most of the world but common in certain geographic areas, such as southern Asia. While environmental factors and genetic susceptibility play important roles in NPC pathogenesis, the Epstein-Barr virus in particular has been implicated in the molecular abnormalities leading to NPC. There is upregulation of cellular proliferation pathways such as the Akt pathway, mitogen-activated protein kinases, and the Wnt pathway. Cell adhesion is compromised due to abnormal E-cadherin and beta-catenin function. Aberrations in cell cycle are due to dysregulation of factors such as p16, cyclin D1, and cyclin E. Anti-apoptotic mechanisms are also upregulated. There are multiple abnormalities unique to NPC that are potential targets for novel treatments.

Epstein-Barr virus latent membrane protein 2A preferentially signals through the Src family kinase Lyn

Latent membrane protein 2A (LMP2A) is a viral protein expressed during Epstein-Barr virus (EBV) latency in EBV-infected B cells both in cell culture and in vivo. LMP2A has important roles in modulating B-cell receptor signal transduction and provides survival and developmental signals to B cells in vivo. Although Lyn has been shown to be important in mediating LMP2A signaling, it is still unclear if Lyn is used preferentially or if LMP2A associates promiscuously with other Src family kinase (SFK) members. To investigate the role of various SFKs in LMP2A signaling, we crossed LMP2A transgenic mice (TgE) with Lyn(-/-), Fyn(-/-), or Blk(-/-) mice. TgE Lyn(-/-) mice had a larger immunoglobulin M (IgM)-positive B-cell population than TgE mice, suggesting that the absence of Lyn prevents LMP2A from delivering survival and developmental signals to the B cells. Both TgE Fyn(-/-) and TgE Blk(-/-) mice have an IgM-negative population of splenic B cells, similar to the TgE mice. LMP2A was also transiently transfected into the human EBV-negative B-cell line BJAB to determine which SFK members associate with LMP2A. Lyn was detected in LMP2A immunoprecipitates, whereas Fyn was not. Both Lyn and Fyn were able to bind to an LMP2A mutant which contained a sequence shown previously to bind tightly to the SH2 domain of multiple SFK members. From these results, we conclude that LMP2A preferentially associates with and signals through Lyn compared to its association with other SFKs. This preferential association is due in part to the SH2 domain of Lyn associating with LMP2A.

The ID proteins contribute to the growth of rodent fibroblasts during LMP1-mediated transformation

LMP1 induces the expression of two members of the family of Id proteins, Id1 and Id3, and affects cell cycle regulation by decreasing the expression of the cyclin dependent kinase inhibitor, p27, and increasing levels and phosphorylation of cdk2 and Rb. In the present study, the contribution of the Id proteins to LMP1-mediated transformation was determined. Although LMP1 effectively inhibited p27 expression, the Id proteins alone did not affect expression of p27, cdk2, and Rb. Neither Id1 nor Id3 was sufficient to transform Rat-1 cells and inhibition of Id1 expression did not affect LMP1-induced morphologic transformation of Rat-1 cells or reduction of p27. However, reduced Id expression resulted in smaller foci and impaired the growth rate of Rat-1 cells. These data indicate that overexpression of the Id proteins is not sufficient for the effects of LMP1 on the cell cycle but that inhibition of Id expression does affect the growth of LMP1-transformed and parental Rat1 cells.

Latent membrane protein 2B regulates susceptibility to induction of lytic Epstein-Barr virus infection

The B-lymphotropic Epstein-Barr virus (EBV) encodes two isoforms of latent membrane protein 2 (LMP2), LMP2A and LMP2B, which are expressed during latency in B cells. The function of LMP2B is largely unknown, whereas LMP2A blocks B-cell receptor (BCR) signaling transduction and induction of lytic EBV infection, thereby promoting B-cell survival. Transfection experiments on LMP2B in EBV-negative B cells and the silencing of LMP2B in EBV-harboring Burkitt's lymphoma-derived Akata cells suggest that LMP2B interferes with the function of LMP2A, but the role of LMP2B in the presence of functional EBV has not been established. Here, LMP2B, LMP2A, or both were overexpressed in EBV-harboring Akata cells to study the function of LMP2B. The overexpression of LMP2B increased the magnitude of EBV switching from its latent to its lytic form upon BCR cross-linking, as indicated by a more-enhanced upregulation and expression of EBV lytic genes and significantly increased production of transforming EBV compared to Akata vector control cells or LMP2A-overexpressing cells. Moreover, LMP2B lowered the degree of BCR cross-linking required to induce lytic EBV infection. Finally, LMP2B colocalized with LMP2A as demonstrated by immunoprecipitation and immunofluorescence and restored calcium mobilization upon BCR cross-linking, a signaling process inhibited by LMP2A. Thus, our findings suggest that LMP2B negatively regulates the function of LMP2A in preventing the switch from latent to lytic EBV replication.

Aberrant Epstein-Barr virus persistence in HIV carriers is characterized by anti-Epstein-Barr virus IgA and high cellular viral loads with restricted transcription

OBJECTIVE

Epstein-Barr virus (EBV)-positive lymphomas in HIV carriers are paralleled by elevated EBV-DNA loads in the circulation. Approximately 20% of asymptomatic HIV carriers also show elevated circulating EBV-DNA loads. We aimed to characterize the nature of this EBV DNA and to determine the transcriptional phenotype of EBV in blood, in relation to serological parameters.

DESIGN

A total of 197 random asymptomatic HIV carriers, representing 2% of the Dutch HIV-positive population, were sampled for blood, peripheral blood mononuclear cells and plasma. In addition, 39 EBV-DNA carriers were sampled twice, with a 5-year interval.

METHODS

EBV-DNA loads were determined by LightCycler-based real-time polymerase chain reaction (PCR). EBV transcription was studied by nucleic acid sequence-based amplification and reverse transcriptase PCR. IgA and IgG antibodies to EBV antigens EBNA1 and VCA-p18 were quantified by synthetic peptide-based enzyme-linked immunosorbent assay.

RESULTS

: Elevated EBV-DNA loads were found in whole blood of 19.3% of the tested HIV population, which were persistent in 82%. Plasma samples were EBV-DNA negative and circulating EBV DNA could be attributed to the B-cell compartment. Transcription of only LMP2 and (non-translated) transcripts from the BamHI-A region of the EBV genome was found, whereas EBNA1, LMP1 and lytic EBV transcripts were absent despite high cellular EBV-DNA loads. IgA-reactivity to VCA-p18 was seen in 69%. IgG to VCA-p18 was significantly higher in high EBV-DNA load carriers.

CONCLUSION

Asymptomatic HIV carriers show aberrant EBV persistence in the circulation, characterized by elevated, B-cell-associated EBV-DNA loads. EBV transcription is restricted, arguing for EBV gene shutdown in circulating EBV-carrying B cells. Increased IgA and IgG reactive to VCA-p18 is indicative of increased lytic EBV replication, possibly occurring at mucosal lymphoid sites but not in the circulation.

Epstein-Barr virus latent membrane protein 2A is a B-cell receptor mimic and essential for B-cell survival

Many cells latently infected with Epstein-Barr virus (EBV), including certain virus-associated tumors, express latent membrane protein 2A (LMP2A), suggesting an important role for this protein in viral latency and oncogenesis. LMP2A mimics B-cell receptor signaling but can also act as a decoy receptor blocking B-cell receptor (BCR) activation. Studies of peripheral B cells have not resolved this apparent contradiction because LMP2A seems to be dispensable for EBV-induced transformation of these B cells in vitro. We show here that LMP2A is essential for growth transformation of germinal center B cells, which do not express the genuine BCR because of deleterious somatic hypermutations in their immunoglobulin genes. BCR-positive (BCR(+)) and BCR-negative (BCR(-)) B cells are readily transformed with a recombinant EBV encoding a conditional, floxed LMP2A allele, but the survival and continued proliferation of both BCR(+) and BCR(-) B cells is strictly dependent on LMP2A. These findings indicate that LMP2A has potent, distinct antiapoptotic and/or transforming characteristics and point to its role as an indispensable BCR mimic in certain B cells from which human B-cell tumors such as Hodgkin lymphoma originate.

Silencing of latent membrane protein 2B reduces susceptibility to activation of lytic Epstein-Barr virus in Burkitt's lymphoma Akata cells

Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) blocks B-cell receptor (BCR) signalling after BCR cross-linking to inhibit activation of lytic EBV, and ectopically expressed LMP2B negatively regulates LMP2A. Here, it is demonstrated that silencing of LMP2B in EBV-harbouring Burkitt's lymphoma Akata cells results in reduced expression of EBV immediate-early lytic BZLF1 gene mRNA and late lytic gp350/220 protein upon BCR cross-linking. Similarly, reduction of lytic EBV activation was observed in Akata cells overexpressing LMP2A. In contrast, silencing of LMP2A expression resulted in higher lytic EBV mRNA and protein expression in BCR cross-linked Akata cells. These observations indicate a role for LMP2B distinct from that of LMP2A in regulation of lytic EBV activation in the host cell and support the hypothesis that LMP2B exhibits a negative-regulatory effect on the ability of LMP2A to maintain EBV latency by preventing the switch to lytic replication.

Epstein-Barr virus latent membrane protein 2A mediates transformation through constitutive activation of the Ras/PI3-K/Akt Pathway

Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) is widely expressed in EBV-infected cells within the infected human host and EBV-associated malignancies, suggesting that LMP2A is important for EBV latency, persistence, and EBV-associated tumorigenesis. Previously, we demonstrated that LMP2A provides an antiapoptotic signal through the activation of phosphatidylinositol 3-kinase (PI3-K)/Akt pathway in vitro. However, the exact function of LMP2A in tumor progression is not well understood. In this study, we found that LMP2A did not induce anchorage-independent cell growth in a human keratinocyte cell line, HaCaT, but did in a human gastric carcinoma cell line, HSC-39. In addition, LMP2A activated the PI3-K/Akt pathway in both HaCaT and HSC-39 cells; however, LMP2A did not activate Ras in HaCaT cells but did in HSC-39 cells. Furthermore, the Ras inhibitors manumycin A and a dominant-negative form of Ras (RasN17) and the PI3-K inhibitor LY294002 blocked LMP2A-mediated Akt phosphorylation and anchorage-independent cell growth in HSC-39 cells. These results suggest that constitutive activation of the Ras/PI3-K/Akt pathway by LMP2A is a key factor for LMP2A-mediated transformation.

[Mechanisms of EBV-mediated oncogenesis]

Epstein-Barr virus (EBV) is the DNA tumor virus, which is known to be relevant to various cancers. EBV maintains latent infection in cancer cells, and there are three types of latent infection (type I-III) according to the patterns of viral latent genes expression. EBV has the ability to transform B cells into immortalized lymphoblastoid cell lines (LCL) showing type III latency, in which all latent genes are expressed. The mechanism of B-cell transformation has provided a model of EBV-associated lymphomas in immunosuppressed individuals. In type I and II latency, the limited numbers of latent genes are expressed. Previous studies have demonstrated the oncogenic functions of latent EBV genes including nuclear antigen EBNA1, membrane protein LMP1 and LMP2A. In addition, we have demonstrated that EBV-encoded small RNA EBERs play a significant role in oncogenesis. Here we summarize recent progresses in the studies on molecular mechanisms of EBV-mediated oncogenesis.

Epstein-barr virus latent membrane protein 2B (LMP2B) modulates LMP2A activity

Latent membrane protein 2A (LMP2A) and LMP2B are viral proteins expressed during Epstein-Barr virus (EBV) latency in EBV-infected B cells both in cell culture and in vivo. LMP2A has important roles in modulating B-cell receptor (BCR) signal transduction by associating with the cellular tyrosine kinases Lyn and Syk via specific phosphotyrosine motifs found within the LMP2A N-terminal tail domain. LMP2A has been shown to alter normal BCR signal transduction in B cells by reducing levels of Lyn and by blocking tyrosine phosphorylation and calcium mobilization following BCR cross-linking. Although little is currently known about the function of LMP2B in B cells, the similarity in structure between LMP2A and LMP2B suggests that they may localize to the same cellular compartments. To investigate the function of LMP2B, B-cell lines expressing LMP2A, LMP2B, LMP2A/LMP2B, and the relevant vector controls were analyzed. As was previously shown, cells expressing LMP2A had a dramatic block in normal BCR signal transduction as measured by calcium mobilization and tyrosine phosphorylation. There was no effect on BCR signal transduction in cells expressing LMP2B. Interestingly, when LMP2B was expressed in conjunction with LMP2A, there was a restoration of normal BCR signal transduction upon BCR cross-linking. The expression of LMP2B did not alter the cellular localization of LMP2A but did bind to and prevent the phosphorylation of LMP2A. A restoration of Lyn levels, but not a change in LMP2A levels, was also observed in cells coexpressing LMP2B with LMP2A. From these results, we conclude that LMP2B modulates LMP2A activity.

Identification of the WW domain-interaction sites in the unstructured N-terminal domain of EBV LMP 2A

Epstein-Barr virus latency is maintained by the latent membrane protein (LMP) 2A, which mimics the B-cell receptor (BCR) and perturbs BCR signaling. The cytoplasmic N-terminal domain of LMP2A is composed of 119 amino acids. The N-terminal domain of LMP2A (LMP2A NTD) contains two PY motifs (PPPPY) that interact with the WW domains of Nedd4 family ubiquitin-protein ligases. Based on our analysis of NMR data, we found that the LMP2A NTD adopts an overall random-coil structure in its native state. However, the region between residues 60 and 90 was relatively ordered, and seemed to form the hydrophobic core of the LMP2A NTD. This region resides between two PY motifs and is important for WW domain binding. Mapping of the residues involved in the interaction between the LMP2A NTD and WW domains was achieved by chemical shift perturbation, by the addition of WW2 and WW3 peptides. Interestingly, the binding of the WW domains mainly occurred in the hydrophobic core of the LMP2A NTD. In addition, we detected a difference in the binding modes of the two PY motifs against the two WW peptides. The binding of the WW3 peptide caused the resonances of five residues (Tyr(60), Glu(61), Asp(62), Trp(65), and Gly(66)) just behind the N-terminal PY motif of the LMP2A NTD to disappear. A similar result was obtained with WW2 binding. However, near the C-terminal PY motif, the chemical shift perturbation caused by WW2 binding was different from that due to WW3 binding, indicating that the residues near the PY motifs are involved in selective binding of WW domains. The present work represents the first structural study of the LMP2A NTD and provides fundamental structural information about its interaction with ubiquitin-protein ligase.

Involvement of caspase activation and mitochondrial stress in taxol-induced apoptosis of Epstein–Barr virus-infected Akata cells

Taxol (paclitaxel) is one of the most potent antimicrotubule agents currently used in cancer chemoprevention and treatment. However, the effects of taxol on the induction of apoptosis in Epstein-Barr virus (EBV)-infected cells are unknown. This study investigated the mechanisms of taxol on cell cycle arrest and apoptosis induction using the EBV-infected cell line, Akata. Taxol treatment sensitively and dose-independently induced growth inhibition, cytotoxicity, and apoptosis in the cells, which was demonstrated by the decreased level of tritium incorporation and cell viability, the increased number of positively stained cells in the trypan blue staining and TUNEL assay, the increased population of cells in the sub-G(0)/G(1) phase in flow cytometric analysis, and ladder formation of the genomic DNA. Treatment with z-VAD-fmk almost completely protected the cells from taxol-induced apoptosis indicating that the taxol-induced apoptosis of Akata cells is caspase-dependent. In addition, taxol-induced apoptosis is proposed to be associated with a lower mitochondrial membrane potential and G(2)/M arrest. However, the tubulin expression level doses not appear to be a direct mediator of taxol-induced apoptosis in cells. The presence of EBV in these cells was not related to the sensitivity of the cells to the induction of apoptosis by taxol. Overall, these results demonstrate that taxol induces apoptosis in EBV-infected Akata cells in a dose-independent manner, and that caspase activation and mitochondrial stress are involved in the induction.

DNA tumor viruses and human cancer

There is a strong association between viruses and the development of human malignancies. A group of oncogenic DNA viruses exists in the human population today, members of which serve as infectious agents of cancer worldwide. The group includes the Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, human papillomaviruses and human polyomaviruses. Globally, it is estimated that 20% of all cancers are linked to infectious agents. Studies of DNA viruses have contributed to our current understanding of the key molecular players in the transformation process. Research has also shed light on the molecular mechanisms of tumorigenesis that are employed by these viruses and there are indications that cofactors could be required for viral oncogenicity in some cases.

A mutation in Epstein-Barr virus LMP2A reveals a role for phospholipase D in B-Cell antigen receptor trafficking

Epstein-Barr virus (EBV) latent infection of B cells blocks the interrelated signaling and antigen-trafficking functions of the BCR through the activity of its latent membrane protein 2A (LMP2A). At present, the molecular mechanisms by which LMP2A exerts its control of BCR functions are only poorly understood. Earlier studies showed that in B cells expressing LMP2A containing a tyrosine mutation at position 112 in its cytoplasmic domain (Y112-LMP2A), the BCR could initiate signaling but could not properly traffic antigen for processing. Here, we show that BCR signaling in Y112-LMP2A-expressing cells is attenuated with a reduction in both the degree and duration of phosphorylation of key components of the BCR signaling cascade including Syk, BLNK, PI3K, and Btk. Notably, Y112-LMP2A expression completely blocked the BCR-induced activation of phospholipase D (PLD), a lipase implicated in the intracellular trafficking of a variety of surface receptors. We show that blocking PLD activity, by expressing Y112-LMP2A, treating cells with the PLD inhibitor 1-butanol or reducing PLD expression by siRNA, blocked BCR trafficking to class II-containing compartments. Moreover, Y112-LMP2A expression blocked the recruitment of phosphorylated forms of the downstream BCR signaling components, Erk and JNK, through both PLD-dependent and PLD-independent mechanisms. Thus, the investigation of the mechanism by which Y112-LMP2A blocks BCR function revealed an essential role for PLD in BCR trafficking for antigen processing.

EBV latent membrane protein 2A induces autoreactive B cell activation and TLR hypersensitivity

EBV is associated with systemic lupus erythematosus (SLE), but how it might contribute to the etiology is not clear. Since EBV-encoded latent membrane protein 2A (LMP2A) interferes with normal B cell differentiation and function, we sought to determine its effect on B cell tolerance. Mice transgenic for both LMP2A and the Ig transgene 2-12H specific for the ribonucleoprotein Smith (Sm), a target of the immune system in SLE, develop a spontaneous anti-Sm response. LMP2A allows anti-Sm B cells to overcome the regulatory checkpoint at the early preplasma cell stage by a self-Ag-dependent mechanism. LMP2A induces a heightened sensitivity to TLR ligand stimulation, resulting in increased proliferation or Ab-secreting cell differentiation or both. Thus, we propose a model whereby LMP2A induces hypersensitivity to TLR stimulation, leading to activation of anti-Sm B cells through the BCR/TLR pathway. These data further implicate TLRs in the etiology of SLE and suggest a mechanistic link between EBV infection and SLE.

Cytobiological consequences of calcium-signaling alterations induced by human viral proteins

Since calcium-signaling regulates specific and fundamental cellular processes, it represents the ideal target of viral proteins, in order for the virus to control cellular functions and favour its persistence, multiplication and spread. A detailed analysis of reports focused on the impact of viral proteins on calcium-signaling has shown that virus-related elevations of cytosolic calcium levels allow increased viral protein expression (HIV-1, HSV-1/2), viral replication (HBx, enterovirus 2B, HTLV-1 p12(I), HHV-8, EBV), viral maturation (rotavirus), viral release (enterovirus 2B) and cell immortalization (EBV). Interestingly, virus-induced decreased cytosolic calcium levels have been found to be associated with inhibition of immune cells functions (HIV-1 Tat, HHV-8 K15, EBV LMP2A). Finally, several viral proteins are able to modulate intracellular calcium-signaling to control cell viability (HIV-1 Tat, HTLV-1 p13(II), HCV core, HBx, enterovirus 2B, HHV-8 K7). These data point out calcium-signaling as a key cellular target for viral infection and should stimulate further studies exploring new calcium-related therapeutic strategies.

Epstein-Barr virus LMP2A enhances B-cell responses in vivo and in vitro

Epstein-Barr virus (EBV) establishes latent infections in a significant percentage of the population. Latent membrane protein 2A (LMP2A) is an EBV protein expressed during latency that inhibits B-cell receptor signaling in lymphoblastoid cell lines. In the present study, we have utilized a transgenic mouse system in which LMP2A is expressed in B cells that are specific for hen egg lysozyme (E/HEL-Tg). To determine if LMP2A allows B cells to respond to antigen, E/HEL-Tg mice were immunized with hen egg lysozyme. E/HEL-Tg mice produced antibody in response to antigen, indicating that LMP2A allows B cells to respond to antigen. In addition, E/HEL-Tg mice produced more antibody and an increased percentage of plasma cells after immunization compared to HEL-Tg littermates, suggesting that LMP2A increased the antibody response in vivo. Finally, in vitro studies determined that LMP2A acts directly on the B cell to increase antibody production by augmenting the expansion and survival of the activated B cells, as well as increasing the percentage of plasma cells generated. Taken together, these data suggest that LMP2A enhances, not diminishes, B-cell-specific antibody responses in vivo and in vitro in the E/HEL-Tg system.

Quantitative profiling of housekeeping and Epstein-Barr virus gene transcription in Burkitt lymphoma cell lines using an oligonucleotide microarray

Background

The Epstein-Barr virus (EBV) is associated with lymphoid malignancies, including Burkitt's lymphoma (BL), and can transform human B cells in vitro. EBV-harboring cell lines are widely used to investigate lymphocyte transformation and oncogenesis. Qualitative EBV gene expression has been extensively described, but knowledge of quantitative transcription is lacking. We hypothesized that transcription levels of EBNA1, the gene essential for EBV persistence within an infected cell, are similar in BL cell lines.

Results

To compare quantitative gene transcription in the BL cell lines Namalwa, Raji, Akata, Jijoye, and P3HR1, we developed an oligonucleotide microarray chip, including 17 housekeeping genes, six latent EBV genes (EBNA1, EBNA2, EBNA3A, EBNA3C, LMP1, LMP2), and four lytic EBV genes (BZLF1, BXLF2, BKRF2, BZLF2), and used the cell line B95.8 as a reference for EBV gene transcription. Quantitative polymerase chain reaction assays were used to validate microarray results. We found that transcription levels of housekeeping genes differed considerably among BL cell lines. Using a selection of housekeeping genes with similar quantitative transcription in the tested cell lines to normalize EBV gene transcription data, we showed that transcription levels of EBNA1 were quite similar in very different BL cell lines, in contrast to transcription levels of other EBV genes. As demonstrated with Akata cells, the chip allowed us to accurately measure EBV gene transcription changes triggered by treatment interventions.

Conclusion

Our results suggest uniform EBNA1 transcription levels in BL and that microarray profiling can reveal novel insights on quantitative EBV gene transcription and its impact on lymphocyte biology.

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.

Inhibition of Immune Responses by ITAM-Bearing Receptors

Cells of the immune system possess many multisubunit receptors that are composed of a ligand-binding subunit associated with distinct signaling adaptors containing one or more immunoreceptor tyrosine-based activation motifs (ITAMs). These receptors include the T cell receptor, the B cell receptor, and many Fc receptors, as well as families of activating receptors on myeloid and natural killer cells. Receptors that associate with ITAM-containing adaptors classically have been viewed as transducing activating signals involving phosphorylation of the tyrosines within the ITAM and recruitment of Syk family tyrosine kinases. Receptors associated with ITAM-containing adaptors in myeloid cells have also been implicated in inhibition of cellular activation. Here, we discuss these new negative roles for signaling by receptors that associate with ITAM-bearing adaptors in myeloid and other cell types within the immune system.

Syk tyrosine kinase mediates Epstein-Barr virus latent membrane protein 2A-induced cell migration in epithelial cells

Although spleen tyrosine kinase (Syk) is known to be important in hematopoietic cell development, the roles of Syk in epithelial cells have not been well studied. Limited data suggest that Syk plays alternate roles in carcinogenesis under different circumstances. In breast cancer, Syk has been suggested to be a tumor suppressor. In contrast, Syk is essential for murine mammary tumor virus-mediated transformation. However, the roles of Syk in tumor migration are still largely unknown. Nasopharyngeal carcinoma, an unusually highly metastatic tumor, expresses Epstein-Barr virus LMP2A (latent membrane protein 2A) in most clinical specimens. Previously, we demonstrated LMP2A triggers epithelial cell migration. LMP2A contains an immunoreceptor tyrosine-based activation motif, which is important for Syk kinase activation in B cells. In this study, we explored whether Syk is important for LMP2A-mediated epithelial cell migration. We demonstrate that LMP2A expression can activate endogenous Syk activity. The activation requires the tyrosine residues in LMP2A ITAM but not YEEA motif, which is important for Syk activation by Lyn in B cells. LMP2A interacts with Syk as demonstrated by coimmunoprecipitation and confocal microscopy. Furthermore, LMP2A-induced cell migration is inhibited by a Syk inhibitor and short interfering RNA. Tyrosines 74 and 85 in the LMP2A immunoreceptor tyrosine-based activation motif are essential for both Syk activation and LMP2A-mediated cell migration, indicating the involvement of Syk in LMP2A-triggered cell migration. The LMP2A-Syk pathway may provide suitable drug targets for treatment of nasopharyngeal carcinoma.

Involvement of caspase activation and mitochondrial stress in trichostatin A-induced apoptosis of Burkitt's lymphoma cell line, Akata

Epstein-Barr virus (EBV) infects more than 90% of the human population and has a potential oncogenic nature. Trichostatin A (TSA) has potent antitumor activity, but its exact mechanism on EBV-infected cells is unclear. This study examined the effects of TSA on proliferation and apoptosis of the Burkitt's lymphoma cell line, Akata. TSA treatment inhibited cell growth and induced cytotoxicity in both the EBV-negative and -positive Akata cells. TSA sensitively induced apoptosis in both cells, as demonstrated by the increased number of positively stained cells in the TUNEL assay, the migration of many cells to sub-G1 phase by flow cytometric analysis, and the formation of DNA ladders. This suggests that EBV has no effect on the sensitivity to TSA. Western blot analysis showed that the cleavage of PARP and Bid and the activation of caspases are closely related to the TSA-induced apoptosis of the cells. The reduction in mitochondrial transition potential and the release of apoptosis-inducing factor from mitochondria to cytosol was also observed after the TSA treatment, but was suppressed by treating the cells with a cathepsin B inhibitor. Overall, these findings suggest that besides the caspase-dependent pathway, mitochondrial events are also associated with the TSA-induced apoptosis of Akata cells.

Expression and characterization of N-terminal domain of Epstein-Barr virus latent membrane protein 2A in Escherichia coli

Latency of Epstein-Barr virus (EBV) is maintained by the transmembrane protein latent membrane protein (LMP) 2A, which mimics the B-cell receptor (BCR) and perturbs BCR signaling. LMP2A contains a cytoplasmic N-terminal domain composed of 119 amino acids, which provides signals that are responsible for the association with various signal molecules, resulting in negative regulation of B-cell signaling and the EBV lytic cycle. In the present study, to obtain N-terminal domain of LMP2A (LMP2A NTD, 13 kDa) in Escherichia coli for structural analysis, a strategy for obtaining the unfused form of LMP2A NTD without any fusion partners was proposed. Recombinant LMP2A NTD has previously been expressed using the GST fusion system in E. coli [Virology 268 (2000) 178, J. Virol. 71 (1997) 4752, Mol. Cell. Biol. 20 (2000) 8526]. However, we were unable to obtain untagged LMP2A NTD from this construct because of rapid proteolysis by thrombin. To overcome the proteolysis by thrombin, C-terminal His-tagged LMP2A NTD and intein-fused LMP2A NTD were prepared. As a result, LMP2A NTD without a fusion partner could be successfully obtained using non-enzymatic cleavage. The secondary structure of the recombinant LMP2A NTD was analyzed using circular dichroism. In aqueous solution, LMP2A NTD adopts an unordered structure, which was not affected by varying pH and salt concentration. In addition, any secondary structural components of LMP2A NTD were not induced in the membrane-mimicking environments, suggesting that LMP2A NTD may intrinsically have a random coil-like structure. The biological activity of recombinant LMP2A NTD was monitored by chemical shift perturbation in HSQC spectra of LMP2A NTD with or without WW domains, which result supports that the structural change induced by WW domains is restricted within narrow region.

Epstein-Barr virus (EBV) latent membrane protein 2A regulates B-cell receptor-induced apoptosis and EBV reactivation through tyrosine phosphorylation

Epstein-Barr virus (EBV) is a human herpesvirus that establishes a lifelong latent infection of B cells. Within the immune system, apoptosis is a central mechanism in normal lymphocyte homeostasis both during early lymphocyte development and in response to antigenic stimuli. In this study, we found that latent membrane protein 2A (LMP2A) inhibited B-cell receptor (BCR)-induced apoptosis in Burkitt's lymphoma cell lines. Genistein, a specific inhibitor of tyrosine-specific protein kinases, blocked BCR-induced apoptosis and EBV reactivation in the cells. These findings indicate that LMP2A blocks BCR-induced cell apoptosis and EBV reactivation through the inhibition of activation of tyrosine kinases by BCR cross-linking.

Roles of the ITAM and PY motifs of Epstein-Barr virus latent membrane protein 2A in the inhibition of epithelial cell differentiation and activation of {beta}-catenin signaling

Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) is important for maintenance of latency in infected B lymphocytes. Through its immunoreceptor tyrosine-based activation motif (ITAM) and PY motifs, LMP2A is able to block B-cell receptor (BCR) signaling, bind BCR-associated kinases, and manipulate the turnover of itself and these kinases via a PY-mediated interaction with the Nedd4 family of ubiquitin ligases. In epithelial cells, LMP2A has been shown to activate the phosphatidylinositol 3'-OH kinase/Akt and beta-catenin signaling pathways. In the present study, the biological consequences of LMP2A expression in the normal human foreskin keratinocyte (HFK) cell line were investigated and the importance of the ITAM and PY motifs for LMP2A signaling effects in HFK cells was ascertained. The ITAM was essential for the activation of Akt by LMP2A in HFK cells, while both the ITAM and PY motifs contributed to LMP2A-mediated accumulation and nuclear translocation of the oncoprotein beta-catenin. LMP2A inhibited induction of differentiation in an assay conducted with semisolid methylcellulose medium, and the PY motifs were critical for this inhibition. LMP2A is expressed in the EBV-associated epithelial malignancies nasopharyngeal carcinoma and gastric carcinoma, and these data indicate that LMP2A affects cellular processes that likely contribute to carcinogenesis.

The Epstein-Barr virus protein, latent membrane protein 2A, co-opts tyrosine kinases used by the T cell receptor

Epstein-Barr virus (EBV) is the causative agent of infectious mononucleosis and is associated with several human malignancies. The EBV protein latent membrane protein 2A (LMP2A) promotes viral latency in memory B cells by interfering with B cell receptor signaling and provides a survival signal for mature B cells that have lost expression of surface immunoglobulin. The latter function has suggested that LMP2A may enhance the survival of EBV-positive tumors. EBV is associated with several T cell malignancies and, since LMP2A has been detected in several of these disorders, we examined the ability of LMP2A to transmit signals and interfere with T cell receptor signaling in T cells. We show that LMP2A is tyrosine-phosphorylated in Jurkat TAg T cells, which requires expression of the Src family tyrosine kinases, Lck and Fyn. Lck and Fyn are recruited to the tyrosine-phosphorylated Tyr112 site in LMP2A, whereas phosphorylation of an ITAM motif in LMP2A creates a binding site for the ZAP-70/Syk tyrosine kinases. LMP2A also associates through its two PPPPY motifs with AIP4, a NEDD4 family E3 ubiquitin ligase; this interaction results in ubiquitylation of LMP2A and serves to regulate the stability of LMP2A and LMP2A-kinase complexes. Furthermore, stable expression of LMP2A in Jurkat T cells down-regulated T cell receptor levels and attenuated T cell receptor signaling. Thus, through recruiting tyrosine kinases involved in T cell receptor activation, LMP2A may provide a survival signal for EBV-positive T cell tumors, whereas LMP2A-associated NEDD4 E3 ligases probably titer the strength of this signal.

Epstein-Barr virus LMP2A alters in vivo and in vitro models of B-cell anergy, but not deletion, in response to autoantigen

A significant percentage of the population latently harbors Epstein-Barr virus (EBV) in B cells. One EBV-encoded protein, latent membrane protein 2A (LMP2A), is expressed in tissue culture models of EBV latent infection, in human infections, and in many of the EBV-associated proliferative disorders. LMP2A constitutively activates proteins involved in the B-cell receptor (BCR) signal transduction cascade and inhibits the antigen-induced activation of these proteins. In the present study, we investigated whether LMP2A alters B-cell receptor signaling in primary B cells in vivo and in vitro. LMP2A does not inhibit antigen-induced tolerance in response to strong stimuli in an in vivo tolerance model in which B cells are reactive to self-antigen. In contrast, LMP2A bypasses anergy induction in response to low levels of soluble hen egg lysozyme (HEL) both in vivo and in vitro as determined by the ability of LMP2A-expressing HEL-specific B cells to proliferate and induce NF-kappaB nuclear translocation after exposure to low levels of antigen. Furthermore, LMP2A induces NF-kappaB nuclear translocation independent of BCR cross-linking. Since NF-kappaB is required to bypass tolerance induction, this LMP2A-dependent NF-kappaB activation may complete the tolerogenic signal induced by low levels of soluble HEL. Overall, the findings suggest that LMP2A may not inhibit BCR-induced signals under all conditions as previously suggested by studies with EBV immortalized B cells.

Reactivation of Epstein-Barr virus from latency

The general problem in cancer treatment centres on finding agents that specifically affect cancer cells without damaging normal cells. The differences between cancer cells and normal cells are usually very subtle but about 15% of all human cancers involve a virus infection, for example the Epstein-Barr virus associated cancers. In these cancers, every tumour cell carries the virus in a latent infection but the number of normal cells infected is very low. So a treatment that could somehow cause the elimination of EBV infected cells would be very specific for the cancer in such cases. One potential approach could involve finding ways to reactivate the latent virus in cancer cells into the early part of the lytic cycle, impeding cell proliferation, targeting chemotherapeutic agents to the cancer and causing the cancer cells to become targets for immune surveillance. This review considers the mechanisms by which EBV reactivation is controlled and discusses possible therapeutic approaches.

T-cell transformation and oncogenesis by gamma2-herpesviruses

gamma2-Herpesviruses, also termed rhadinoviruses, have long been known as animal pathogens causing lymphoproliferative diseases such as malignant catarrhal fever in cattle or T-cell lymphoma in certain Neotropical primates. The rhadinovirus prototype is Herpesvirus saimiri (HVS), a T-lymphotropic agent of squirrel monkeys (Saimiri sciureus); Herpesvirus ateles (HVA) is closely related to HVS. The first human rhadinovirus, human herpesvirus type 8 (HHV-8), was discovered a decade ago in Kaposi's sarcoma (KS) biopsies. It was found to be strongly associated with all forms of KS, as well as with multicentric Castleman's disease and primary effusion lymphoma (PEL). Since DNA of this virus is regularly found in all KS forms, and specifically in the spindle cells of KS, it was also termed KS-associated herpesvirus (KSHV). Several simian rhadinoviruses related to KSHV have been discovered in various Old World primates, though they seem only loosely associated with pathogenicity or tumor induction. In contrast, HVS and HVA cause T-cell lymphoma in numerous non-natural primate hosts; HVS strains of the subgroup C are capable of transforming human and simian T-lymphocytes to continuous growth in cell culture and can provide useful tools for T-cell immunology or gene transfer. Here, we describe their natural history, genome structure, biology, and pathogenesis in T-cell transformation and oncogenesis.

Infectious agents and cancer: criteria for a causal relation

Infectious agents, mainly viruses, are among the few known causes of cancer and contribute to a variety of malignancies worldwide. The agents and cancers considered here are human papillomaviruses (cervical carcinoma); human polyomaviruses (mesotheliomas, brain tumors); Epstein-Barr virus (B-cell lymphoproliferative diseases and nasopharyngeal carcinoma); Kaposi's Sarcoma Herpesvirus (Kaposi's Sarcoma and primary effusion lymphomas); hepatitis B and hepatitis C viruses (hepatocellular carcinoma); Human T-cell Leukemia Virus-1 (T-cell leukemias); and helicobacter pylori (gastric carcinoma), which account for up to 20% of malignancies around the globe. The criteria most often used in determining causality are consistency of the association, either epidemiologic or on the molecular level, and oncogenicity of the agent in animal models or cell cultures. However use of these generally applied criteria in deciding on causality is selective, and the criteria may be weighted differently. Whereas for most of the tumor viruses the viral genome persists in an integrated or episomal form with a subset of viral genes expressed in the tumor cells, some agents (HBV, HCV, helicobacter) are not inherently oncogenic, but infection leads to transformation of cells by indirect means. For some malignancies the viral agent appears to serve as a cofactor (Burkitt's lymphoma-EBV; mesothelioma - SV(40)). For others the association is inconsistent (Hodgkin's Disease, gastric carcinomas, breast cancer-EBV) and may either define subsets of these malignancies, or the virus may act to modify phenotype of an established tumor, contributing to tumor progression rather than causing the tumor. In these cases and for the human polyomaviruses the association with malignancy is less consistent or still emerging. In contrast despite the potent oncogenic properties of some strains of human adenovirus in tissue culture and animals the virus has not been linked with any human cancers. Finally it is likely that more agents, most likely viruses, both known and unidentified, have yet to be implicated in human cancer. In the meantime study of tumorigenic infectious agents will continue to illuminate molecular oncogenic processes.

Epstein-Barr virus latent membrane protein 2A mimics B-cell receptor-dependent virus reactivation

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) shares protein motifs with the B-cell receptor that play a role in B-cell receptor signalling and has been shown to mimic an activated B-cell receptor by providing a survival signal for mature B cells in transgenic mice. Conversely, LMP2A has been reported not to support but to inhibit B-cell receptor signalling with respect to virus reactivation and to block lytic virus induction after anti-Ig treatment of EBV-infected B cells. To solve this apparent paradox, the role of LMP2A in lytic-cycle induction was re-examined in B cells conditionally immortalized by EBV. It was shown that, in the absence of other stimuli, LMP2A expression alone could lead to induction of the virus lytic cycle. Similarly to B-cell receptor stimulation by anti-Ig treatment, this LMP2A-mediated reactivation was dependent on the mitogen-activated protein kinase pathway and could be inhibited by the viral LMP1. Our data reinforce the notion that LMP2A is a functional homologue of the B-cell receptor, not only with respect to B-cell survival but also with respect to regulation of the lytic cycle.

Expression of Epstein-Barr virus genes in EBV-associated gastric carcinomas

AIM: To understand the expression of latent and lytic genes of Epstein-Barr virus (EBV) in EBV-associated gastric carcinoma (EBVaGC) and to explore the relationship between EBV-encoded genes and development of EBVaGC at molecular level.

METHODS: One hundred and seventy-two gastric carcinoma tissues and 172 corresponding para-carcinoma tissues were tested for EBV genome by polymerase chain reaction (PCR)-Southern blotting. EBV-encoded small RNA (EBER) 1 of the PCR positive specimens was detected by in situ hybridization (ISH). Gastric carcinomas with positive EBER1 signals were classified as EBVaGCs. RT-PCR and Southern hybridization were applied to the detection of expression of nuclear antigen (EBNA) promoters (Qp, Wp and Cp), EBNA 1 and EBNA 2, latent membrane proteins (LMP) 1, 2A and 2B and lytic genes (immediate early genes BZLF1 and BRLF1, early genes BARF1 and BHRF1, late genes BcLF1 and BLLF1) in EBVaGCs.

RESULTS: Eleven EBV positive samples existed in gastric carcinoma tissues (6.39%). No EBV positive sample was found in corresponding para-carcinoma tissues. The difference between EBV positivity in carcinoma tissues and corresponding para-carcinoma tissues was significant (χ² = 9.0909, P = 0.0026). Transcripts of Qp and EBNA1 were detected in all the 11 EBVaGCs, while both Wp and Cp were silent. EBNA2, LMP1 and LMP2B mRNA were absent in all the cases, while LMP2A mRNA was detected in 4 of the 11 cases. Of the 11 EBVaGCs, 7 exhibited BcLF1 transcripts and 2 exhibited BHRF1 transcripts. The transcripts of BZLF1 and BARF1 were detected in 5 cases, respectively. No BLLF1 and BRLF mRNA were detected.

CONCLUSION: The latent pattern of EBV in gastric carcinoma corresponds to the latency I/II. Some lytic infection genes are expressed in EBVaGCs tissues. BARF1 and BHRF1 genes may play an important role in tumorigenesis of gastric carcinoma.

LMP2A does not require palmitoylation to localize to buoyant complexes or for function

Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) is expressed constitutively in lipid rafts in latently infected B lymphocytes. Lipid rafts are membrane microdomains enriched in cholesterol and sphingolipids selective for specific protein association. Lipid rafts have been shown to be necessary for B-cell receptor (BCR) signal transduction. LMP2A prevents BCR recruitment to lipid rafts, thereby abrogating BCR function. As LMP2A is palmitoylated, whether this fatty acid modification is necessary for LMP2A to localize to lipid rafts and for protein function was investigated. LMP2A palmitoylation was confirmed in latently infected B cells. LMP2A was found to be palmitoylated on multiple cysteines only by S acylation. An LMP2A mutant that was not palmitoylated was identified and functioned similar to wild-type LMP2A; unmodified LMP2A localized to lipid rafts, was tyrosine phosphorylated, was associated with LMP2A-associated proteins, was ubiquitinated, and was able to block calcium mobilization following BCR cross-linking. Therefore, palmitoylation of LMP2A is not required for LMP2A targeting to buoyant complexes or for function.

Epstein–Barr virus-specific cytotoxic T-lymphocytes for adoptive immunotherapy of post-transplant lymphoproliferative disease

Post-transplant lymphoproliferative disease (PTLD) refers to a collection of clinically and pathologically diverse tumours associated with iatrogenic immunosuppression following transplantation. In most cases, tumourigenesis results from a deficit in Epstein-Barr virus (EBV)-specific cytotoxic T-lymphocyte (CTL) activity that leads to uncontrolled EBV-driven outgrowth of latently infected B-lymphocytes. Conventional treatment for PTLD typically involves a reduction in immunosuppression, but this approach is frequently unsuccessful and mortality remains high. An alternative, adoptive immunotherapy, involving the administration of EBV-specific CTLs cultured in vitro has been developed with the aim of selectively reconstituting EBV-directed immunity and effecting targeted tumour destruction. This approach has been the subject of several clinical studies, and these provide encouraging evidence of its clinical efficacy. This review presents an overview of the pathogenesis of PTLD and examines current progress in the use of adoptive immunotherapy for its treatment.

Rheumatoid factors induce signaling from B cells, leading to Epstein-Barr virus and B-cell activation

B-cell antigen receptor signaling is initiated upon binding of the antigen to membrane-bound immunoblobulin (Ig), and the anti-Ig antibody (Ab) mimics this signaling. In B cells latently infected with Epstein-Barr virus (EBV), the same signals induce virus activation. We examine here whether rheumatoid factors (RFs), autoantibodies directed against the Fc portion of IgG, induce EBV and B-cell activation. As a source of RFs, RF-producing lymphoblastoid cell line (LCL) clones were isolated from peripheral blood mononuclear cells (PBMC) and synovial cells from patients with rheumatoid arthritis (RA) by EBV transformation. Burkitt's lymphoma-derived Akata cells, which are highly responsive to EBV activation by anti-Ig Abs, were used for the assay of EBV activation. Akata cells expressed IgG3 as membrane-bound Ig. RFs from a synovium-derived LCL were directed to IgG3 and induced EBV activation in 16 to 18% of Akata cells, whereas RFs from another synovium-derived LCL were directed to IgG1 and did not induce EBV activation. Pretreatment of RFs with the purified Fc fragment of human IgG completely abolished EBV activation. Furthermore, B-cell activation was assessed by incorporation of [3H]thymidine. RFs from synovium-derived LCLs efficiently induced B-cell activation, and the addition of CD40 ligand had a synergistic effect. On the other hand, RFs from PBMC-derived LCLs were polyreactive, had a lower affinity to IgG, and did not induce EBV and B-cell activation. The present findings imply a possible role for RFs as EBV and B-cell activators.

Epstein-Barr virus (EBV) LMP2A alters normal transcriptional regulation following B-cell receptor activation

The latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) is an important mediator of viral latency in infected B-lymphocytes. LMP2A inhibits B-cell receptor (BCR) signaling in vitro and allows for the survival of BCR-negative B cells in vivo. In this study, we compared gene transcription in BCR-activated B cells from non-transgenic and LMP2A Tg6 transgenic mice. We found that the transcriptional induction and down-regulation of many genes that normally occurs in B cells following BCR activation did not occur in B cells from LMP2A Tg6 transgenic mice. Furthermore, LMP2A induced the expression of various transcription factors and genes associated with DNA/RNA metabolism, which may allow for the altered transcriptional regulation observed in BCR-activated B cells from LMP2A Tg6 mice. These results suggest that LMP2A may inhibit the downstream effects of BCR signaling by directly or indirectly altering gene transcription to ensure EBV persistence in infected B cells.

The Nedd4 family of E3 ubiquitin ligases: functional diversity within a common modular architecture

Neuronal precursor cell-expressed developmentally downregulated 4 (Nedd4) is the prototypical protein in a family of E3 ubiquitin ligases that have a common domain architecture. They are comprised of a catalytic C-terminal HECT domain and N-terminal C2 domain and WW domains responsible for cellular localization and substrate recognition. These proteins are found throughout eukaryotes and regulate diverse biological processes through the targeted degradation of proteins that generally have a PPxY motif for WW domain recognition, and are found in the nucleus and at the plasma membrane. Whereas the yeast Saccharomyces cerevisiae uses a single protein, Rsp5p, to carry out these functions, evolution has provided higher eukaryotes with several related Nedd4 proteins that appear to have specialized roles. In this review we discuss how knowledge of individual domain function has provided insight into the physiological roles of the Nedd4 proteins and describe recent results that suggest discrete functions for individual family members.

Epstein–Barr virus oncogenesis and the ubiquitin–proteasome system

Epstein-Barr virus (EBV), a human herpesvirus associated with lymphoid and epithelial cell tumors, encodes several proteins that exploit the ubiquitin-proteasome system to regulate latency and allow the persistence of infected cells in immunocompetent hosts. Further modifications of ubiquitin-dependent proteolysis by activated cellular oncogenes contribute to malignant transformation. A detailed understanding of these processes may lead to the development of new therapeutic strategies for EBV-associated cancers.

Phosphatidylinositol 3-Kinase Is a Determinant of Responsiveness to B Cell Antigen Receptor-Mediated Epstein-Barr Virus Activation

B cell Ag receptor (BCR) cross-linking with anti-Ig Abs efficiently induces activation of latently infected EBV in some B cell lines, but not in others. The present study was aimed at defining the molecular mechanisms that determine the response to BCR-mediated EBV activation. Comparison of Burkitt's lymphoma-derived Akata, Mutu-I, and Daudi cells, which are representative responders and nonresponders to BCR-mediated EBV activation, respectively, indicated that three signaling pathways, phosphatidylinositol 3-kinase (PI3K), extracellular signal-regulated kinase (ERK), and p38 mitogen-activated protein kinase (MAPK), were activated in anti-Ig-treated Akata and Mutu-I cells. However, in anti-Ig-treated Daudi cells PI3K was not activated, ERK was faintly activated, and p38 MAPK was constitutively phosphorylated irrespective of anti-Ig treatment. Restoration of PI3K activity with insulin-like growth factor 1 restored ERK and p38 MAPK pathways, and was accompanied by EBV activation in anti-Ig-treated Daudi cells. In contrast, a specific inhibitor for PI3K, wortmannin, inhibited EBV activation by anti-Ig Abs in Akata and Mutu-I cells. Transfection assays in EBV-negative Daudi cells revealed that PI3K activated a promoter for BZLF1, which is a switch of EBV activation from a latent infection, in the absence of other EBV products suggesting that the BZLF promoter was a target of BCR signaling, and that PI3K was important for BCR-mediated BZLF1 activation. These results indicate that the absence of PI3K impedes the progression of signals through the BCR and becomes a determinant of unresponsiveness to BCR-mediated EBV activation.