Tyrosines 60, 64, and 101 of Epstein-Barr virus LMP2A are not essential for blocking B cell signal transduction

Epstein-Barr Virus Latent Membrane Protein 2A (LMP2A) Enhances ATP Production in B Cell Tumors through mTOR and HIF-1α

Epstein-Barr Virus (EBV) exists in a latent state in 90% of the world's population and is linked to numerous cancers, such as Burkitt's Lymphoma, Hodgkin's, and non-Hodgkin's Lymphoma. One EBV latency protein, latency membrane protein 2A (LMP2A), is expressed in multiple latency phenotypes. LMP2A signaling has been extensively studied and one target of LMP2A is the mammalian target of rapamycin (mTOR). Since mTOR has been linked to reprogramming tumor metabolism and increasing levels of hypoxia-inducible factor 1 α (HIF-1α), we hypothesized that LMP2A would increase HIF-1α levels to enhance ATP generation in B lymphoma cell lines. Our data indicate that LMP2A increases ATP generation in multiple Burkitt lymphoma cell lines that were dependent on HIF-1α. Subsequent studies indicate that the addition of the mTOR inhibitor, rapamycin, blocked the LMP2A-dependent increase in HIF-1α. Further studies demonstrate that LMP2A does not increase HIF-1α levels by increasing HIF-1α RNA or STAT3 activation. In contrast, LMP2A and mTOR-dependent increase in HIF-1α required mTOR-dependent phosphorylation of p70 S6 Kinase and 4E-BP1. These findings implicate the importance of LMP2A in promoting B cell lymphoma survival by increasing ATP generation and identifying potential pharmaceutical targets to treat EBV-associated tumors.

EBV LMP-2A employs a novel mechanism to transactivate the HERV-K18 superantigen through its ITAM

EBV encodes latent membrane protein (LMP)-2A that functions as a homologue of the activated BCR. We have previously shown that LMP-2A transactivates a human endogenous retrovirus, HERV-K18, in infected B-lymphocytes. The Env protein of HERV-K18 encodes a superantigen that strongly stimulates a large number of T cells. To delineate the mechanism through which LMP-2A transactivates HERV-K18 env, we tested a panel of tyrosine mutants of LMP-2A in a murine B lymphoma that stably harbors HERV-K18. Our analysis revealed that the immunoreceptor tyrosine-based activation motif (ITAM) of LMP-2A is important for HERV-K18 env transactivation. ITAM contains 2 tyrosines that initiate signaling cascades when both residues are phosphorylated. However, in our study, single-tyrosine mutants of ITAM still retained full induction of HERV-K18 env. After truncating 25 kb of genomic sequence downstream of HERV-K18, LMP-2A failed to transactivate HERV-K18 env. Thus, an LMP-2A-inducible element is located downstream of HERV-K18.

The Shb signalling scaffold binds to and regulates constitutive signals from the Epstein–Barr virus LMP2A membrane protein

The Epstein-Barr virus latency-associated membrane protein LMP2A has been shown to activate the survival kinase Akt in epithelial and B cells in a phosphoinositide 3-kinase-dependent fashion. In this study, we demonstrate that the signalling scaffold Shb associates through SH2 and PTB domain interactions with phosphorylated tyrosine motifs in the LMP2A N-terminal tail. Additionally, we show that mutation of tyrosines in these motifs as well as shRNA-mediated downregulation of Shb leads to a loss of constitutive Akt-activation in LMP2A-expressing cells. Furthermore, utilization by Shb of the LMP2A ITAM motif regulates stability of the Syk tyrosine kinase in LMP2A-expressing cells. Our data set the precedent for viral utilization of the Shb signalling scaffold and implicate Shb as a regulator of LMP2A-dependent Akt activation.

The Epstein-Barr virus-encoded LMP2A and LMP2B proteins promote epithelial cell spreading and motility

The frequent expression of latent membrane proteins LMP2A and LMP2B in Epstein Barr virus (EBV)-associated tumors suggests that these proteins play a role in EBV-induced epithelial cell growth transformation. Expression of LMP2A and LMP2B had no effect on the morphology of squamous epithelial cells in monolayer culture, but their expression was associated with an increased capacity to spread and migrate on extracellular matrix. Although the mechanisms by which LMP2A and LMP2B promote cell spreading and motility are unclear, the use of selective pharmacological inhibitors has established a role for tyrosine kinases in this phenotype but ruled out contributions of phosphatidylinositol 3-kinase, extracellular signal-regulated kinase/mitogen-activated protein kinase, and protein kinase C. The ability of LMP2B to induce a phenotype that is virtually indistinguishable from that of LMP2A suggests that regions of the LMP2 protein in addition to the cytosolic amino terminus are capable of inducing phenotypic effects in epithelial cells. Thus, rather than serving to modulate the activity of LMP2A, LMP2B may directly engage signaling pathways to influence epithelial cell behavior such as cell adhesion and motility.

The Epstein-Barr virus encoded latent membrane protein 2A augments signaling from latent membrane protein 1

The frequent coexpression of the EBV-encoded latent membrane proteins LMP1 and LMP2A/B in virus-associated tumors suggests that these two proteins may cooperate in the transformation process. While LMP2A is unable to directly activate the NF-kappaB and AP-1 pathways, we found that coexpression of LMP2A with LMP1 resulted in a significant enhancement of LMP1-mediated activation of these pathways. This enhancement was found to be critically dependent on the tyrosine residues present within the ITAM motif (Y74/Y85) and, to a lesser extent, the tyrosine at position 112 (Y112). Subsequent analysis revealed that LMP2A is able to stabilize and modulate the turnover of LMP1 by extending its half-life. This ability does not require a direct physical interaction between the two proteins but rather, results from an indirect effect of LMP2A on the turnover of the LMP1 protein. This study highlights an important role for LMP2A as a modulator of LMP1 activity in epithelial cells.

Epstein-Barr virus infection and human malignancies

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

PY motifs of Epstein-Barr virus LMP2A regulate protein stability and phosphorylation of LMP2A-associated proteins

Latent membrane protein 2A (LMP2A) is expressed in latent Epstein-Barr virus (EBV) infection. We have demonstrated that Nedd4 family ubiquitin-protein ligases (E3s), AIP4, WWP2/AIP2, and Nedd4, bind specifically to two PY motifs present within the LMP2A amino-terminal domain. In this study, LMP2A PY motif mutant viruses were constructed to investigate the role of the LMP2A PY motifs. AIP4 was found to specifically associate with the LMP2A PY motifs in EBV-transformed lymphoblastoid cell lines (LCLs), extending our original observation to EBV-infected cells. Mutation of both of the LMP2A PY motifs resulted in an absence of binding of AIP4 to LMP2A, which resulted in an increase in the expression of Lyn and the constitutive hyperphosphorylation of LMP2A and an unknown 120-kDa protein. In addition, there was a modest increase in the constitutive phosphorylation of Syk and an unidentified 60-kDa protein. These results indicate that the PY motifs contained within LMP2A are important in regulating phosphorylation in EBV-infected LCLs, likely through the regulation of Lyn activity by specifically targeting the degradation of Lyn by ubiquination by Nedd4 family E3s. Despite differences between PY motif mutant LCLs and wild-type LCLs, the PY motif mutants still exhibited a block in B-cell receptor (BCR) signal transduction as measured by the induction of tyrosine phosphorylation and BZLF1 expression following BCR activation. EBV-transformed LCLs with mutations in the PY motifs were not different from wild-type LCLs in serum-dependent cell growth. Protein stability of LMP1, which colocalizes with LMP2A, was not affected by the LMP2A-associated E3s.

Epstein-Barr virus infection and human malignancies

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

Epstein-Barr virus infection and human malignancies

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

The effects of the Epstein-Barr virus latent membrane protein 2A on B cell function

Epstein-Barr Virus (EBV) infects B-lymphocytes circulating through the oral epithelium and establishes a lifelong latent infection in a subset of mature-memory B cells. In these latently infected B cells, EBV exhibits limited gene expression with the latent membrane protein 2A (LMP2A) being the most consistently detected transcript. This persistent expression, coupled with many studies ofthe function of LMP2A in vitro and invivo, indicates that LMP2A is functioning to control some aspect of viral latency. Establishment and maintenance of viral latency requires exquisite manipulation of normal B cell signaling and function. LMP2A is capable of blocking normal B cell signal transduction in vitro, suggesting that LMP2A may act to regulate lytic activation from latency in vivo. Furthermore, LMP2A is capable of providing B cells with survival signals in the absence of normal BCR signaling. These data show that LMP2A may help EBV-infected cells to persist in vivo. This review discusses the advances that have been made in our understanding of LMP2A and the effects it has on B cell development, activation, and viral latency.

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

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

Epstein-Barr virus latency: LMP2, a regulator or means for Epstein-Barr virus persistence?

Like other herpesviruses, Epstein-Barr virus (EBV) persists in its host through an ability to establish a latent infection that periodically reactivates, producing infectious virus that infects naïve hosts. Disease syndromes in humans caused by EBV reflect the cell types that EBV infects, being primarily of lymphoid or epithelial origin. The most notable lymphoid disease, infectious mononucleosis, is a self-limiting lymphoproliferative disease that occurs in normal adolescents on primary infection. Children are normally able to resolve primary EBV infection with few or no symptoms. By the age of 25 most individuals are EBV seropositive. EBV is associated with a variety of hematopoietic cancers such as African Burkitt's lymphoma, Hodgkin's, and adult T-cell leukemia. EBV-associated lymphoproliferative disease occurs in individuals with congenital or acquired cellular immune deficiencies. The two notable epithelial diseases associated with EBV infection are nasopharyngeal cancer, a malignancy endemic to southern China, and oral hairy leukoplakia, an epithelial hyperplasia of the lingual squamous epithelium in AIDS patients. Latent membrane protein 2 (LMP2) is expressed both in normal EBV latency and EBV-associated pathologies. LMP2 may regulate reactivation from latency by interfering with normal B-cell signal transduction processes and in doing so may also provide a survival signal that could be important for viral persistence. Current knowledge about the function of LMP2 is described, defining a new class of regulators of herpesvirus latency.