BHRF1, the Epstein-Barr virus gene with homology to Bc12, is dispensable for B-lymphocyte transformation and virus replication

Epstein-Barr virus and host cell 3D genome organization

The human genome is organized in an extremely complexed yet ordered way within the nucleus. Genome organization plays a critical role in the regulation of gene expression. Viruses manipulate the host machinery to influence host genome organization to favor their survival and promote disease development. Epstein-Barr virus (EBV) is a common human virus, whose infection is associated with various diseases, including infectious mononucleosis, cancer, and autoimmune disorders. This review summarizes our current knowledge of how EBV uses different strategies to control the cellular 3D genome organization to affect cell gene expression to transform normal cells into lymphoblasts.

The interaction between Vav1 and EBNA1 promotes survival of Burkitt's lymphoma cells by down-regulating the expression of Bim

Vav1 is a guanine nucleotide exchange factor (GEF) predominantly expressed in hematopoietic cells, and functions in the development and antigen-stimulated response of lymphocytes. Burkitt's lymphoma (BL) is characterized as transformed B cell lymphoma, and is highly associated with Epstein-Barr virus (EBV). EBV nuclear antigen 1 (EBNA1) is the only viral protein expressed across all three types of latency and essential for the persistence of EBV genome. It is not clear yet how EBNA1 contributes to the growth advantage of latently infected cells such as in EBV⁺ lymphoma B cells. Here, we reported that Vav1 interacts with EBNA1 via its C-terminal SH3 domain. This interaction suppresses the expression of a pro-apoptotic Bcl-2 family member, Bim, resulting in the resistance of the BL cells to apoptotic inductions. Our data uncovered Vav1 as a novel target for EBNA1, and suggested a pro-survival role of Vav1 in the pathogenesis of EBV associated BLs.

The Methylation Status and Expression of Epstein-Barr Virus Early Genes BARF1 and BHRF1 in Epstein-Barr Virus-Associated Gastric Carcinomas

Epstein-Barr virus (EBV) is an important DNA virus which establishes latent infection in human malignancies. Expression of EBV-encoded genes in the associated tumors is strongly modulated by promoter CpG methylation of EBV genome. This study aimed to explore the methylation status of the promoters of EBV BamHI-A rightward frame 1 (BARF1) and BamHI-H rightward open reading frame 1 (BHRF1) and their influence on transcriptional expression, to further understand the roles of BARF1 and BHRF1 in the occurrence of EBV-associated cancer. We evaluated the methylation status of BARF1 and BHRF1 promoters in 43 EBV-associated gastric carcinoma (EBVaGC) tissues and EBV-positive cell lines. Their expressions were evaluated by real-time quantitative PCR. We found that the promoters of BARF1 and BHRF1 were methylated by varying degrees in different EBV-positive cell lines and were almost hypermethylated in all EBVaGC tissues. The methylation status of BARF1 and BHRF1 promoters were significantly reduced by 5-Aza-CdR along with the increasing gene expressions. Hypermethylation of Ap and Hp mediates the frequent silencing of BARF1 and BHRF1 in EBV-associated tumors, which could be reactivated by a demethylation agent, suggesting that promoter demethylation and activation is important for BARF1 and BHRF1 transcription and their further action.

REVIEW ■ : Keeping Neurons Alive: The Molecular Control of Apoptosis (Part I

Apoptosis is a form of cellular suicide in which the cell activates an intrinsic program to bring about its own demise. Recognized for years as the mechanism by which developing cells are lost naturally, it has become apparent recently that this same process may play an important role in many acute and chronic diseases in which neural cell death occurs, such as stroke and Alzheimer's disease. This growing recognition suggests that a knowledge of the gene products controlling this process may lead to improved treatments for some disease states, as well as to improved understanding of neuronal development, physiology, and pathophysiology. Some controls with important roles in neural apoptosis have been identified, and these controls, as well as their putative mechanisms of action, are described in this article. NEUROSCIENTIST 2:181-190, 1996

The N Terminus of the Vaccinia Virus Protein F1L Is an Intrinsically Unstructured Region That Is Not Involved in Apoptosis Regulation

Subversion of host cell apoptotic responses is a prominent feature of viral immune evasion strategies to prevent premature clearance of infected cells. Numerous poxviruses encode structural and functional homologs of the Bcl-2 family of proteins, and vaccinia virus harbors antiapoptotic F1L that potently inhibits the mitochondrial apoptotic checkpoint. Recently F1L has been assigned a caspase-9 inhibitory function attributed to an N-terminal α helical region of F1L spanning residues 1-15 (1) preceding the domain-swapped Bcl-2-like domains. Using a reconstituted caspase inhibition assay in yeast we found that unlike AcP35, a well characterized caspase-9 inhibitor from the insect virus Autographa californica multiple nucleopolyhedrovirus, F1L does not prevent caspase-9-mediated yeast cell death. Furthermore, we found that deletion of the F1L N-terminal region does not impede F1L antiapoptotic activity in the context of a viral infection. Solution analysis of the F1L N-terminal regions using small angle x-ray scattering indicates that the region of F1L spanning residues 1-50 located N-terminally from the Bcl-2 fold is an intrinsically unstructured region. We conclude that the N terminus of F1L is not involved in apoptosis inhibition and may act as a regulatory element in other signaling pathways in a manner reminiscent of other unstructured regulatory elements commonly found in mammalian prosurvival Bcl-2 members including Bcl-xL and Mcl-1.

BHRF1 exerts an antiapoptotic effect and cell cycle arrest via Bcl-2 in murine hybridomas

Apoptosis has been widely studied in order to find methods to increase the life-span and production performance in large-scale animal cell cultures. The use of anti-apoptotic genes has emerged as an efficient method to reduce apoptosis in a variety of biotechnological relevant cell lines, including CHO and hybridomas, alternatively to small molecule inhibitors. It is already known that expression of BHRF1, an Epstein-Barr virus-encoded early protein homologous to the anti-apoptotic protein Bcl-2, protects hybridoma cells from apoptosis in batch and continuous operation modes resulting in a delay in the cell death process under glutamine starvation conditions. In the present study, the mechanism of action of BHRF1 was investigated in a murine hybridoma cell line. BHRF1 protein was found in the mitochondrial cell fraction both under normal growing conditions and apoptosis-inducing conditions. Remarkably, the expression of the anti-apoptotic gene bcl2 in BHRF1-expressing cells was up-regulated 25-fold compared to mock-transfected controls under apoptosis triggering conditions and its expression correlated with survival of transgenic cultures and cell cycle arrest in G1. Bcl-2 activity was revealed to be crucial for the BHRF1-mediated effect since the addition of specific inhibitors of Bcl-2 (namely HA14-1 and YC-137) resulted in a loss of function of BHRF1-expressing cells under glutamine starvation conditions. Moreover, the interaction of BHRF1 with the pro-apoptotic BH3-only Bim conferred mitochondrial stability to BHRF1 expressing cells under apoptosis-triggering conditions.

Epstein-Barr virus latent genes

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

Epstein-Barr virus interactions with the Bcl-2 protein family and apoptosis in human tumor cells

Epstein-Barr virus (EBV), a human gammaherpesvirus carried by more than 90% of the world's population, is associated with malignant tumors such as Burkitt's lymphoma (BL), Hodgkin lymphoma, post-transplant lymphoma, extra-nodal natural killer/T cell lymphoma, and nasopharyngeal and gastric carcinomas in immune-compromised patients. In the process of infection, EBV faces challenges: the host cell environment is harsh, and the survival and apoptosis of host cells are precisely regulated. Only when host cells receive sufficient survival signals may they immortalize. To establish efficiently a lytic or long-term latent infection, EBV must escape the host cell immunologic mechanism and resist host cell apoptosis by interfering with multiple signaling pathways. This review details the apoptotic pathway disrupted by EBV in EBV-infected cells and describes the interactions of EBV gene products with host cellular factors as well as the function of these factors, which decide the fate of the host cell. The relationships between other EBV-encoded genes and proteins of the B-cell leukemia/lymphoma (Bcl) family are unknown. Still, EBV seems to contribute to establishing its own latency and the formation of tumors by modifying events that impact cell survival and proliferation as well as the immune response of the infected host. We discuss potential therapeutic drugs to provide a foundation for further studies of tumor pathogenesis aimed at exploiting novel therapeutic strategies for EBV-associated diseases.

The essential role of evasion from cell death in cancer

The link between evasion of apoptosis and the development of cellular hyperplasia and ultimately cancer is implicitly clear if one considers how many cells are produced each day and, hence, how many cells must die to make room for the new ones (reviewed in Raff, 1996). Furthermore, cells are frequently experiencing noxious stimuli that can cause lesions in their DNA and faults in DNA replication can occur during cellular proliferation. Such DNA damage needs to be repaired efficiently or cells with irreparable damage must be killed to prevent subsequent division of aberrant cells that may fuel tumorigenesis (reviewed in Weinberg, 2007). The detection of genetic lesions in human cancers that activate prosurvival genes or disable proapoptotic genes have provided the first evidence that defects in programmed cell death can cause cancer (Tagawa et al., 2005; Tsujimoto et al., 1984; Vaux, Cory, and Adams, 1988) and this concept was proven by studies with genetically modified mice (Egle et al., 2004b; Strasser et al., 1990a). It is therefore now widely accepted that evasion of apoptosis is a requirement for both neoplastic transformation and sustained growth of cancer cells (reviewed in Cory and Adams, 2002; Hanahan and Weinberg, 2000; Weinberg, 2007). Importantly, apoptosis is also a major contributor to anticancer therapy-induced killing of tumor cells (reviewed in Cory and Adams, 2002; Cragg et al., 2009). Consequently, a detailed understanding of apoptotic cell death will help to better comprehend the complexities of tumorigenesis and should assist with the development of improved targeted therapies for cancer based on the direct activation of the apoptotic machinery (reviewed in Lessene, Czabotar, and Colman, 2008).

Structural basis for apoptosis inhibition by Epstein-Barr virus BHRF1

Epstein-Barr virus (EBV) is associated with human malignancies, especially those affecting the B cell compartment such as Burkitt lymphoma. The virally encoded homolog of the mammalian pro-survival protein Bcl-2, BHRF1 contributes to viral infectivity and lymphomagenesis. In addition to the pro-apoptotic BH3-only protein Bim, its key target in lymphoid cells, BHRF1 also binds a selective sub-set of pro-apoptotic proteins (Bid, Puma, Bak) expressed by host cells. A consequence of BHRF1 expression is marked resistance to a range of cytotoxic agents and in particular, we show that its expression renders a mouse model of Burkitt lymphoma untreatable. As current small organic antagonists of Bcl-2 do not target BHRF1, the structures of it in complex with Bim or Bak shown here will be useful to guide efforts to target BHRF1 in EBV-associated malignancies, which are usually associated with poor clinical outcomes.

Altruistic suicide of infected host cells is a key defense mechanism to combat viral infection. To ensure their own survival and proliferation, certain viruses, including Epstein-Barr virus (EBV), have mechanisms to subvert apoptosis, including the expression of homologs of the mammalian pro-survival protein Bcl-2. EBV was first identified in association with Burkitt lymphoma and it is also linked to certain Hodgkin's lymphomas and nasopharyngeal carcinoma. Whereas increased expression of Bcl-2 promotes malignancies such as human follicular lymphoma, the precise role of the EBV encoded Bcl-2 homolog BHRF1 in EBV-associated malignancies is less well defined. BHRF1 is known to bind the pro-apoptotic BH3-only protein Bim, and here we demonstrate that it also binds other pro-apoptotic proteins (Bid, Puma, Bak) expressed by host cells. Crystal structures of BHRF1 with the BH3 regions of Bim and Bak illustrate these interactions in atomic detail. A consequence of BHRF1 expression is marked resistance to a range of cytotoxic agents, and we show that its expression renders a mouse model of Burkitt lymphoma untreatable. As current antagonists of Bcl-2 do not target BHRF1, our crystal structures will be useful to guide efforts to target BHRF1 in EBV-associated malignancies, which are usually associated with poor clinical outcomes.

Negative autoregulation of Epstein-Barr virus (EBV) replicative gene expression by EBV SM protein

The Epstein-Barr virus (EBV) SM protein is essential for lytic EBV DNA replication and virion production. When EBV replication is induced in cells infected with an SM-deleted recombinant EBV, approximately 50% of EBV genes are expressed inefficiently. When EBV replication is rescued by transfection of SM, SM enhances expression of these genes by direct and indirect mechanisms. While expression of most EBV genes is either unaffected or enhanced by SM, expression of several genes is decreased in the presence of SM. Expression of BHRF1, a homolog of cellular bcl-2, is particularly decreased in the presence of SM. Investigation of the mechanism of BHRF1 downregulation revealed that SM downregulates expression of the immediate-early EBV transactivator R. In EBV-infected cells, R-responsive promoters, including the BHRF1 and SM promoters, were less active in the presence of SM, consistent with SM inhibition of R expression. SM decreased spliced R mRNA levels, supporting a posttranscriptional mechanism of R inhibition. R and BHRF1 expression were also found to decrease during later stages of EBV lytic replication in EBV-infected lymphoma cells. These data indicate that feedback regulation of immediate-early and early genes occurs during the lytic cycle of EBV regulation.

BH3 domains define selective inhibitory interactions with BHRF-1 and KSHV BCL-2

The Epstein-Barr and Kaposi's sarcoma gamma-herpesviruses (KSHVs) are associated with certain cancers, and encode B-cell leukemia/lymphoma 2 (BCL-2) homologs, BHRF-1 and KSHV BCL-2, respectively. Little is known, however, about the molecular interactions allowing viral BCL-2 homologs to mediate their anti-apoptotic function. Cellular anti-apoptotic proteins, such as BCL-2 and MCL-1, prevent death via selective interactions with pro-death BH3-only proteins. To investigate whether BHRF-1 and KSHV BCL-2 function similarly, we made recombinant BHRF-1 and KSHV BCL-2 proteins. We identified the individual binding patterns for BHRF-1 and KSHV BCL-2 to BH3 domains. These studies surprisingly showed that KSHV BCL-2 is more closely related to MCL-1 than to BCL-2, a result confirmed by sequence analysis. GST-BHRF-1 and GST-KSHV BCL-2 bound BH3-only family proteins from human cells. BHRF-1 protected mammalian cells from growth factor withdrawal, etoposide and adriamycin. We found that both BCL-2 and BHRF-1 sequestered pro-death BH3-only proteins under growth factor-deficient conditions. Finally, we tested the ability of a panel of BH3 peptides to inhibit BHRF-1 and KSHV BCL-2 function in a mitochondrial model of apoptosis. We found that each could be inhibited by the select group of BH3 peptides identified in our binding assay. Our studies define the biochemical interactions underlying BHRF-1 and KSHV BCL-2 anti-apoptotic function, and identify peptides that are prototypic inhibitors of this function.

DNA viruses in human cancer: An integrated overview on fundamental mechanisms of viral carcinogenesis

The first experimental data suggesting that neoplasm development in animals might be influenced by infectious agents were published in the early 1900s. However, conclusive evidence that DNA viruses play a role in the pathogenesis of some human cancers only emerged in the 1950s, when Epstein-Barr virus (EBV) was discovered within Burkitt lymphoma cells. Besides EBV, other DNA viruses consistently associated with human cancers are the hepatitis B virus (HBV), human papillomavirus (HPV), and Kaposi sarcoma herpesvirus (KSHV). Although each virus has unique features, it is becoming clearer that all these oncogenic agents target multiple cellular pathways to support malignant transformation and tumor development.

Human cellular protein VRK2 interacts specifically with Epstein-Barr virus BHRF1, a homologue of Bcl-2, and enhances cell survival

BHRF1, an early gene product of Epstein-Barr virus (EBV), is structurally and functionally homologous to Bcl-2, a cellular anti-apoptotic protein. BHRF1 has been shown to protect cells from apoptosis induced by numerous external stimuli. Nasopharyngeal carcinoma is an epithelial cancer associated closely with EBV infection. Specific proteins that might interact with and modulate the BHRF1 anti-apoptotic activity in normal epithelial cells are of interest. Therefore, a cDNA library derived from normal human foreskin keratinocytes was screened by the yeast two-hybrid system and a cellular gene encoding human vaccinia virus B1R kinase-related kinase 2 (VRK2) was isolated. Interaction between the cellular VRK2 and viral BHRF1 proteins was further demonstrated by glutathione S-transferase pull-down assays, confocal laser-scanning microscopy and co-immunoprecipitation. Analyses of VRK2-deletion mutants revealed that a 108 aa fragment at the C terminus was important for VRK2 to interact with BHRF1. For BHRF1, aa 1-18 and 89-142 were crucial in interacting with VRK2 and these two regions are counterparts of Bcl-2 homology domains 4 and 1. Overexpressed VRK2 alone showed a modest effect in anti-apoptosis and appeared to enhance cell survival in the presence of BHRF1. However, this enhancement was not observed when VRK2 was co-expressed with Bcl-2. The results indicate that human VRK2 interacts specifically with EBV BHRF1 and that the interaction is involved in protecting cells from apoptosis.

Epstein-Barr virus lytic infection contributes to lymphoproliferative disease in a SCID mouse model

Most Epstein-Barr virus (EBV)-positive tumor cells contain one of the latent forms of viral infection. The role of lytic viral gene expression in EBV-associated malignancies is unknown. Here we show that EBV mutants that cannot undergo lytic viral replication are defective in promoting EBV-mediated lymphoproliferative disease (LPD). Early-passage lymphoblastoid cell lines (LCLs) derived from EBV mutants with a deletion of either viral immediate-early gene grew similarly to wild-type (WT) virus LCLs in vitro but were deficient in producing LPD when inoculated into SCID mice. Restoration of lytic EBV gene expression enhanced growth in SCID mice. Acyclovir, which prevents lytic viral replication but not expression of early lytic viral genes, did not inhibit the growth of WT LCLs in SCID mice. Early-passage LCLs derived from the lytic-defective viruses had substantially decreased expression of the cytokine interleukin-6 (IL-6), and restoration of lytic gene expression reversed this defect. Expression of cellular IL-10 and viral IL-10 was also diminished in lytic-defective LCLs. These results suggest that lytic EBV gene expression contributes to EBV-associated lymphoproliferative disease, potentially through induction of paracrine B-cell growth factors.

Epstein-Barr virus provides a new paradigm: a requirement for the immediate inhibition of apoptosis

DNA viruses such as herpesviruses are known to encode homologs of cellular antiapoptotic viral Bcl-2 proteins (vBcl-2s), which protect the virus from apoptosis in its host cell during virus synthesis. Epstein-Barr virus (EBV), a human tumor virus and a prominent member of γ-herpesviruses, infects primary resting B lymphocytes to establish a latent infection and yield proliferating, growth-transformed B cells in vitro. In these cells, 11 viral genes that contribute to cellular transformation are consistently expressed. EBV also encodes two vBcl-2 genes whose roles are unclear. Here we show that the genetic inactivation of both vBcl-2 genes disabled EBV's ability to transform primary resting B lymphocytes. Primary B cells infected with a vBcl-2-negative virus did not enter the cell cycle and died of immediate apoptosis. Apoptosis was abrogated in infected cells in which vBcl-2 genes were maximally expressed within the first 24 h postinfection. During latent infection, however, the expression of vBcl-2 genes became undetectable. Thus, both vBcl-2 homologs are essential for initial cellular transformation but become dispensable once a latent infection is established. Because long-lived, latently infected memory B cells and EBV-associated B-cell lymphomas are derived from EBV-infected proapoptotic germinal center B cells, we conclude that vBcl-2 genes are essential for the initial evasion of apoptosis in cells in vivo in which the virus establishes a latent infection or causes cellular transformation or both.

The transformation of resting B-lymphocytes by a human tumor virus is shown to require vBcl-2 genes, which abrogate host cell apoptosis. These genes are not required once latent infection is established.

Herpesvirus pan encodes a functional homologue of BHRF1, the Epstein-Barr virus v-Bcl-2

Background

Epstein-Barr virus (EBV) latently infects about 90% of the human population and is associated with benign and malignant diseases of lymphoid and epithelial origin. BHRF1, an early lytic cycle antigen, is an apoptosis suppressing member of the Bcl-2 family. In vitro studies imply that BHRF1 is dispensable for both virus replication and transformation. However, the fact that BHRF1 is highly conserved not only in all EBV isolates studied to date but also in the analogous viruses Herpesvirus papio and Herpesvirus pan that infect baboons and chimpanzees respectively, suggests BHRF1 may play an important role in vivo.

Results

Herpesvirus papio BHRF1 has been shown to function in an analogous manner to EBV BHRF1 in response to DNA damaging agents in human keratinocytes. In this study we show that the heterologous expression of the previously uncharacterised Herpesvirus pan BHRF1 in the human Burkitt's lymphoma cell line Ramos-BL provides similar anti-apoptotic functions to that of EBV BHRF1 in response to apoptosis triggered by serum withdrawal, etoposide treatment and ultraviolet (UV) radiation. We also map the amino acid changes onto the recently solved structure of the EBV BHRF1 and reveal that these changes are unlikely to alter the 3D structure of the protein.

Conclusions

These findings show that the functional conservation of BHRF1 extends to a lymphoid background, suggesting that the primate virus proteins interact with cellular proteins that are themselves highly conserved across the higher primates. Further weight is added to this suggestion when we show that the difference in amino acid sequences map to regions on the 3D structure of EBV BHRF1 that are unlikely to change the conformation of the protein.

Mitochondria as Functional Targets of Proteins Coded by Human Tumor Viruses

Molecular analyses of tumor virus-host cell interactions have provided key insights into the genes and pathways involved in neoplastic transformation. Recent studies have revealed that the human tumor viruses Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), human papillomavirus (HPV), hepatitis B virus (HBV), hepatitis C virus (HCV), and human T-cell leukemia virus type 1 (HTLV-1) express proteins that are targeted to mitochondria. The list of these viral proteins includes BCL-2 homologues (BHRF1 of EBV; KSBCL-2 of KSHV), an inhibitor of apoptosis (IAP) resembling Survivin (KSHV K7), proteins that alter mitochondrial ion permeability and/or membrane potential (HBV HBx, HPV E[wedge]14, HCV p7, and HTLV-1 p13(II)), and K15 of KSHV, a protein with undefined function. Consistent with the central role of mitochondria in energy production, cell death, calcium homeostasis, and redox balance, experimental evidence indicates that these proteins have profound effects on host cell physiology. In particular, the viral BCL-2 homologues BHRF1 and KSBCL-2 inhibit apoptosis triggered by a variety of stimuli. HBx, p7, E1[wedge]4, and p13(II) exert powerful effects on mitochondria either directly due to their channel-forming activity or indirectly through interactions with endogenous channels. Further investigation of these proteins and their interactions with mitochondria will provide important insights into the mechanisms of viral replication and tumorigenesis and could aid in the discovery of new targets for anti-tumor therapy.

Human cytomegalovirus proteins encoded by UL37 exon 1 protect infected fibroblasts against virus-induced apoptosis and are required for efficient virus replication

Human cytomegalovirus (HCMV) strain AD169 mutants carrying transposon insertions or large deletions in UL37 exon 1 (UL37x1) were recovered from modified bacterial artificial chromosomes by reconstitution in human fibroblasts expressing the adenovirus anti-apoptotic protein E1B19K. UL37x1 mutant growth was severely compromised in normal fibroblasts, with minimal release of infectious progeny. Growth in E1B19K-expressing cells was restored, but did not reach wild-type levels. Normal fibroblasts infected by UL37x1 mutants underwent apoptosis spontaneously between 48 and 96 h after infection. Apoptosis was inhibited by treatment of cells with the broad-spectrum caspase inhibitor z-Val-Ala-Asp(OMe)-fluoromethylketone, resulting in substantially increased release of virus. Inhibition of viral DNA replication by phosphonoformate or ganciclovir also inhibited apoptosis, implying that death was triggered by late viral functions or by replication and packaging of the viral genome. Immunofluorescent staining showed that although viral proteins accumulated normally during delayed-early phase and viral DNA replication compartments formed, viral late proteins were detected only rarely, suggesting that spontaneous apoptosis occurs early in late phase. These results demonstrate that anti-apoptotic proteins encoded by HCMV UL37x1 [pUL37x1 (vMIA), gpUL37 and gpUL37(M)] prevent apoptosis that would otherwise be initiated by the replication programme of the virus and are required for efficient and sustainable virus replication.

Viral Bcl-2 homologs and their role in virus replication and associated diseases

Cellular Bcl-2 family proteins regulate a critical step in the mammalian programmed cell death pathway by modulating mitochondrial permeability and function. Bcl-2 family proteins are also encoded by several large DNA viruses, including all known gamma herpesviruses, adenoviruses, and several other unrelated viruses. Viral Bcl-2 proteins can prevent cell death but often escape cellular regulatory mechanisms that govern their cellular counterparts. By evading the "altruistic" suicide of infected cells, viruses can ensure replication and propagation in the infected host, but sometimes in surprising ways. Many human cancers and other disorders are associated with viruses that encode Bcl-2 homologs. Here we consider the available mechanistic data for viral compared to cellular Bcl-2 protein function along with relevance to the virus life cycle and human disease states.

Solution structure of the BHRF1 protein from Epstein-Barr virus, a homolog of human Bcl-2

The three-dimensional structure of BHRF1, the Bcl-2 homolog from Epstein-Barr virus (EBV), has been determined by NMR spectroscopy. Although the overall structure is similar to other Bcl-2 family members, there are important structural differences. Unlike some of the other Bcl-2 family members, BHRF1 does not contain the prominent hydrophobic groove that mediates binding to pro-apoptotic family members. In addition, in contrast to the anti-apoptotic Bcl-2 proteins, BHRF1 does not bind tightly to peptides derived from the pro-apoptotic proteins Bak, Bax, Bik, and Bad. The lack of an exposed, pre-formed binding groove in BHRF1 and the lack of significant binding to peptides derived from pro-apoptotic family members that bind to other anti-apoptotic family members, suggest that the mechanism of the BHRF1 anti-apoptotic activity does not parallel that of cellular Bcl-x(L) or Bcl-2.

Inhibition of apoptosis by the gamma-herpesviruses

All members of the gamma-herpesvirus family encode genes capable of inhibiting apoptosis. Inhibition of a variety of types of apoptotic stimuli have been demonstrated for specific viral genes, including pathways induced by the immune system as well as internal pathways. Virally encoded genes inhibit the activation of caspase-8 by the TNF receptor and Fas; activate NF-kappaB to increase expression of antiapoptotic genes; inhibit interferon response; bind to p53, thereby blocking p53 dependent apoptosis; and interact with other pro- and antiapoptotic cellular genes. All gamma-herpesviruses also express viral homologues of cellular antiapoptotic genes, including one or two Bcl-2 homologues. The human gamma-herpesviruses encode genes that can inhibit apoptosis during both latent and lytic infection. During latent phase infection inhibition of apoptosis is likely important for persistence of the gamma-herpesviruses in the face of immune attack, but it is also required for maintenance of infected cells in culture. During lytic replication the virus inhibits apoptosis to prevent cell death before viral replication and spread occurs.

Viral versus cellular BCL-2 proteins

All gamma herpesviruses and a few other viruses encode at least one homologue of the mammalian cell death inhibitor BCL-2. Gamma herpesviruses are associated with human and animal lymphoid and epithelial tumours. However, the role of these viral BCL-2 homologues in the virus replication cycle or in human disease is not known, though recent developments show progress in this area. The structure of viral BCL-2 family protein, KSBcl-2, is similar to that of cellular family members, but viral BCL-2 proteins differ functionally from the cellular proteins, apparently escaping the regulatory mechanisms to which their cellular counterparts are subjected. Thus, exploring the biochemical and biological functions of the viral BCL-2 family proteins will increase our understanding of their role in virus infections and will undoubtedly teach us something about their cellular kin.

Herpesviral proteins regulating apoptosis

The induction of apoptosis of virus-infected cells is an important defense mechanism of the host. Apoptosis of an infected cell can be induced cell autonomously as a consequence of viral replication or can be mediated by CTLs attacking the infected cells. Herpesviruses have developed different strategies to interfere with cell-autonomous apoptosis and to block CTL-induced apoptosis mediated by death receptors such as Fas and TRAIL. Herpesviruses, which establish a lifelong persistence in the infected host, can be found principally in two different conditions, episomal persistence with a limited number of genes expressed and lytic replication with expression of almost all genes. To meet the need of the virus to enhance survival of the infected cell, herpesviruses have evolved different strategies that function during both episomal persistence and lytic replication. Herpesviruses, which encode 70 to more than 200 genes have incorporated cell homologous antiapoptotic genes, they code for multifunctional genes that can also regulate apoptosis, and, finally, they modulate the expression of cellular apoptosis-regulating genes to favor survival of the infected cells. Viral interference with host cell apoptosis enhances viral replication, facilitates virus spread and persistence, and may promote the development of virus-induced cancer.

Manipulation of immune responses by Epstein–Barr virus

Epstein-Barr virus (EBV) infects and persists for life in the majority of the human population. Persistence is achieved through a combination of strictly regulated programs of latent infection in B-cells and chronic reactivation of virus replication in lymphoid tissue and mucosal surfaces. The resulting multiple patterns of virus-host interaction have selected unique strategies of immune escape. T-cell mediated immunity plays a central role in the control of EBV latency and several immune escape mechanism that protect the virus at this stage of its life circle have been characterized in details. In contrast, the contribution of innate immunity and the immune regulation of productive infection are largely unexplored areas that may yield important clues on the establishment and maintenance of EBV persistence. This review summarizes well known and emerging mechanisms of EBV immune escape that may reveal new strategies of immunoregulation and promote new approaches to the prophylaxis and treatment of EBV associated diseases.

Identification of the in vivo role of a viral bcl-2

Many gamma-herpesviruses encode candidate oncogenes including homologues of host bcl-2 and cyclin proteins (v-bcl-2, v-cyclin), but the physiologic roles of these genes during infection are not known. We show for the first time in any virus system the physiologic role of v-bcl-2. A gamma-herpesvirus v-bcl-2 was essential for efficient ex vivo reactivation from latent infection, and for both persistent replication and virulence during chronic infection of immunocompromised (interferon [IFN]-gamma(-/-)) mice. The v-cyclin was also critical for the same stages in pathogenesis. Strikingly, while the v-bcl-2 and v-cyclin were important for chronic infection, these genes were not essential for viral replication in cell culture, viral replication during acute infection in vivo, establishment of latent infection, or virulence during acute infection. We conclude that v-bcl-2 and v-cyclin have important roles during latent and persistent gamma-herpesvirus infection and that herpesviruses encode genes with specific roles during chronic infection and disease, but not acute infection and disease. As gamma-herpesviruses primarily cause human disease during chronic infection, these chronic disease genes may be important targets for therapeutic intervention.

Epstein-Barr virus BALF1 is a BCL-2-like antagonist of the herpesvirus antiapoptotic BCL-2 proteins

Cellular BCL-2 family proteins can inhibit or induce programmed cell death in part by counteracting the activity of other BCL-2 family members. All sequenced gammaherpesviruses encode a BCL-2 homologue that potently inhibits apoptosis and apparently escapes some of the regulatory mechanisms that govern the functions of their cellular counterparts. Examples of these protective proteins include BHRF1 of Epstein-Barr virus (EBV) and KSBcl-2 of Kaposi's sarcoma-associated herpesvirus, also known as human herpesvirus 8. The gamma-1 subgroup of these viruses, such as EBV, encodes a second BCL-2 homologue. We have now found that this second BCL-2 homologue encoded by EBV, BALF1, inhibits the antiapoptotic activity of EBV BHRF1 and of KSBcl-2 in several transfected cell lines. However, BALF1 failed to inhibit the cellular BCL-2 family member, BCL-x(L). Thus, BALF1 acts as a negative regulator of the survival function of BHRF1, similar to the counterbalance observed between cellular BCL-2 family members. Unlike the cellular BCL-2 family antagonists, BALF1 lacked proapoptotic activity and could not be converted into a proapoptotic factor in a manner similar to cellular BCL-2 proteins by caspase cleavage or truncation of the N terminus. Coimmunoprecipitation experiments and immunofluorescence assays suggest that a minimal amount, if any, of the BHRF1 and BALF1 proteins colocalizes inside cells, suggesting that mechanisms other than direct interaction explain the suppressive function of BALF1.

Complete nucleotide sequence of the rhesus lymphocryptovirus: genetic validation for an Epstein-Barr virus animal model

We sequenced the rhesus lymphocryptovirus (LCV) genome in order to determine its genetic similarity to Epstein-Barr virus (EBV). The rhesus LCV encodes a repertoire identical to that of EBV, with 80 open reading frames, including cellular interleukin-10, bcl-2, and colony-stimulating factor 1 receptor homologues and an equivalent set of viral glycoproteins. The highly conserved rhesus LCV gene repertoire provides a unique animal model for the study of EBV pathogenesis.

Identification of EBV transforming genes by recombinant EBV technology

Epstein-Barr virus (EBV) is able to infect primary B-lymphocytes but usually does not proceed to replicate more virions. Instead, EBV persists as an incomplete virus and expresses 12 gene products that transform the growth of these cells into continuously proliferating lymphoblastoid cell lines. Because EBV is associated with several human malignancies, there is intense interest in delineating the molecular functions of these EBV gene products in transformation. This review focuses on the recombinant EBV technologies that have been developed to introduce specific mutations into EBV and test the functions of these EBV genes in primary B-lymphocyte growth transformation.

The Cellular Protein PRA1 Modulates the Anti-apoptotic Activity of Epstein-Barr Virus BHRF1, a Homologue of Bcl-2, through Direct Interaction

The Epstein-Barr virus-encoded early protein, BHRF1, is a structural and functional homologue of the anti-apoptotic protein, Bcl-2. There is accumulating evidence that BHRF1 protects a variety of cell types from apoptosis induced by various external stimuli. To identify specific proteins from normal epithelial cells that interact with BHRF1 and that might promote or inhibit its anti-apoptotic activity, we screened a yeast two-hybrid cDNA library derived from human normal foreskin keratinocytes and identified a cellular gene encoding human prenylated rab acceptor 1 (hPRA1). The interaction of hPRA1 with BHRF1 was confirmed using glutathione S-transferase pull-down assays, confocal laser scanning microscopy, and co-immunoprecipitation. Two regions of PRA1, amino acids 30-53 and the carboxyl-terminal 21 residues, are important for BHRF1 interactions and two regions of BHRF1, amino acids 1-18 and 89-142, including the Bcl-2 homology domains BH4 and BH1, respectively, are crucial for PRA1 interactions. PRA1 expression interferes with the anti-apoptotic activity of BHRF1, although not of Bcl-2. These results indicate that the PRA1 interacts selectively with BHRF1 to reduce its anti-apoptotic activity and might play a role in the impeding completion of virus maturation.

Molecular virology of Epstein-Barr virus

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

Comparative analysis of the transforming mechanisms of Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, and Herpesvirus saimiri

Members of the gamma herpesvirus family include the lymphocryptoviruses (gamma-1 herpesviruses) and the rhadinoviruses (gamma-2 herpesviruses). Gammaherpesvirinae uniformly establish long-term, latent, reactivatable infection of lymphocytes, and several members of the gamma herpesviruses are associated with lymphoproliferative diseases. Epstein-Barr virus is a lymphocryptovirus, whereas Kaposi sarcoma-associated herpesvirus and Herpesvirus saimiri are members of the rhadinovirus family. Genes encoded by these viruses are involved in a diverse array of cellular signaling pathways. This review attempts to cover our understanding of how viral proteins deregulate cellular signaling pathways that ultimately contribute to the conversion of normal cells to cancerous cells.

Activators of the Epstein-Barr virus lytic program concomitantly induce apoptosis, but lytic gene expression protects from cell death

Expression of the lytic cycle genes of Epstain-Barr virus (EBV) is induced in type I Burkitt's lymphoma-derived cells by treatment with phorbol esters (e.g., phorbol myristate acetate [PMA]), anti-immunoglobulin, or the cytokine transforming growth factor beta (TGF-beta). Concomitantly, all these agents induce apoptosis as judged by a sub-G1 fluorescence-activated cell sorter (FACS) profile, proteolytic cleavage of poly(ADP-ribose) polymerase (PARP) and terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) staining. However, caspase activation is not required for induction of the lytic cycle since the latter is not blocked by the caspase inhibitor ZVAD. Furthermore, not all agents that induce apoptosis in these cultures (for example, cisplatin and ceramide) induce the EBV lytic programme. Although it is closely associated with the lytic cycle, apoptosis is neither necessary nor sufficient for its activation. Multiparameter FACS analysis of cultures treated with PMA, anti-Ig, or TGF-beta revealed BZLF1-expressing cells distributed in different phases of the cell cycle according to which inducer was used. However, BZLF1-positive cells did not appear to undergo apoptosis and accumulate with a sub-G1 DNA content, irrespective of the inducer used. This result, which suggests that lytic gene expression is protective, was confirmed and extended by immunofluorescence staining doubled with TUNEL analysis. BZLF1- and also gp350-expressing cells were almost always shown to be negative for TUNEL staining. Similar experiments using EBV-positive and -negative subclones of Akata BL cells carrying an episomal BZLF1 reporter plasmid confirmed that protection from apoptosis was associated with the presence of the EBV genome. Finally, treatment with phosphonoacetic acid or acyclovir prior to induction with PMA, anti-Ig, or TGF-beta blocked the protective effect in Mutu-I cells. These data suggest that a late gene product(s) may be particularly important for protection against caspase activity and cell death.

EBNA-LP associates with cellular proteins including DNA-PK and HA95

EBNA-LP-associated proteins were identified by sequencing proteins that immunoprecipitated with Flag epitope-tagged EBNA-LP (FLP) from lymphoblasts in which FLP was stably expressed. The association of EBNA-LP with Hsp70 (72/73) was confirmed, and sequences of DNA-PK catalytic subunit (DNA-PKcs), HA95, Hsp27, prolyl 4-hydroxylase alpha-1 subunit, alpha-tubulin, and beta-tubulin were identified. The fraction of total cellular HA95 that associated with FLP was very high, while progressively lower fractions of the total DNA-PKcs, Hsp70, Hsp 27, alpha-tubulin, and beta-tubulin specifically associated with EBNA-LP as determined by immunoblotting with antibodies to these proteins. EBNA-LP bound to two domains in the DNA-PKcs C terminus and DNA-PKcs associated with the EBNA-LP repeat domain. DNA-PKcs that was bound to EBNA-LP phosphorylated p53 or EBNA-LP in vitro, and the phosphorylation of EBNA-LP was inhibited by Wortmannin, a specific in vitro inhibitor of DNA-PKcs.

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.

Viral subversion of the immune system

This review describes the diverse array of pathways and molecular targets that are used by viruses to elude immune detection and destruction. These include targeting of pathways for major histocompatibility complex-restricted antigen presentation, apoptosis, cytokine-mediated signaling, and humoral immune responses. The continuous interactions between host and pathogens during their coevolution have shaped the immune system, but also the counter measures used by pathogens. Further study of their interactions should improve our ability to manipulate and exploit the various pathogens.

Herpesvirus papio encodes a functional homologue of the Epstein-Barr virus apoptosis suppressor, BHRF1

The human tumour virus Epstein-Barr virus (EBV) encodes a 17 kDa protein, BHRF1, which is a member of the BCL:-2 family and has been shown to suppress apoptosis. The role of this gene in the life-cycle of EBV has not been fully elucidated. In order to identify motifs conserved in herpesviruses and possibly shed light on its function we isolated a BHRF1 homologue from herpesvirus papio (cercopithecine herpesvirus-12) a closely related gammaherpesvirus of baboons. The gene, hvpBHRF1, also encodes a 17 kDa protein which shares 64% identity and 79% similarity with EBV BHRF1 at the amino acid level. In biological assays, hvpBHRF1 and BHRF1 conferred similar levels of protection on human keratinocytes induced to apoptose with cis-platin.

Antiapoptotic herpesvirus Bcl-2 homologs escape caspase-mediated conversion to proapoptotic proteins

The antiapoptotic Bcl-2 and Bcl-x(L) proteins of mammals are converted into potent proapoptotic factors when they are cleaved by caspases, a family of apoptosis-inducing proteases (E. H.-Y. Cheng, D. G. Kirsch, R. J. Clem, R. Ravi, M. B. Kastan, A. Bedi, K. Ueno, and J. M. Hardwick, Science 278:1966-1968, 1997; R. J. Clem, E. H.-Y. Cheng, C. L. Karp, D. G. Kirsch, K. Ueno, A. Takahashi, M. B. Kastan, D. E. Griffin, W. C. Earnshaw, M. A. Veliuona, and J. M. Hardwick, Proc. Natl. Acad. Sci. USA 95:554-559, 1998). Gamma herpesviruses also encode homologs of the Bcl-2 family. All tested herpesvirus Bcl-2 homologs possess antiapoptotic activity, including the more distantly related homologs encoded by murine gammaherpesvirus 68 (gammaHV68) and bovine herpesvirus 4 (BHV4), as described here. To determine if viral Bcl-2 proteins can be converted into death factors, similar to their cellular counterparts, five herpesvirus Bcl-2 homologs from five different viruses were tested for their susceptibility to caspases. Only the viral Bcl-2 protein encoded by gammaHV68 was susceptible to caspase digestion. However, unlike the caspase cleavage products of cellular Bcl-2, Bcl-x(L), and Bid, which are potent inducers of apoptosis, the cleavage product of gammaHV68 Bcl-2 lacked proapoptotic activity. KSBcl-2, encoded by the Kaposi's sarcoma-associated herpesvirus, was the only viral Bcl-2 homolog that was capable of killing cells when expressed as an N-terminal truncation. However, because KSBcl-2 was not cleavable by caspases, the latent proapoptotic activity of KSBcl-2 apparently cannot be released. The Bcl-2 homologs encoded by herpesvirus saimiri, Epstein-Barr virus, and BHV4 were not cleaved by apoptotic cell extracts and did not possess latent proapoptotic activities. Thus, herpesvirus Bcl-2 homologs escape negative regulation by retaining their antiapoptotic activities and/or failing to be converted into proapoptotic proteins by caspases during programmed cell death.

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

Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) is expressed on the membrane of B-lymphocytes and blocks B cell receptor (BCR) signaling in EBV-transformed B-lymphocytes in vitro. The LMP2A amino-terminal domain, which is essential for the LMP2A-mediated block of B cell signal transduction, contains eight tyrosine residues. Three of these tyrosine residues (Y74, Y85, and Y112) have been demonstrated to be essential for the LMP2A-mediated block on protein tyrosine phosphorylation, calcium mobilization, and induction of BZLF1 expression after BCR activation. To investigate the importance of tyrosines at positions 60, 64, and 101 on B cell signaling, EBV recombinants were constructed containing a tyrosine-to-phenylalanine point mutation at amino acid 60, 64, or 101 within LMP2A. Tyrosine phosphorylation, calcium mobilization, and induction of BZLF1 expression were not affected by any of the tyrosine point mutations after BCR activation. In addition, constitutive phosphorylation of LMP2A was unaffected by any of the tyrosine point mutations. These data indicate that tyrosines 60, 64, and 101 are not essential for the LMP2A-mediated block of B cell signal transduction in transformed cell lines.

Epstein-Barr virus lacking latent membrane protein 2 immortalizes B cells with efficiency indistinguishable from that of wild-type virus

Epstein-Barr virus (EBV) is a human herpesvirus that efficiently transforms and immortalizes human primary B lymphocytes. In this study, the role of latent membrane protein 2 (LMP2) in EBV growth transformation was investigated. LMP2 is a virally encoded membrane protein expressed in EBV-immortalized B cells previously shown to be nonessential for EBV transformation. However, a recent study reported that LMP2 may be an important determinant for efficient B cell transformation (Brielmeier et al., Journal of General Virology 77, 2807-2818, 1996). In this study a deletion mutation was introduced into the LMP2 gene using an E. coli mini-EBV construct containing sufficient EBV DNA to result in growth transformation of primary B cells. In an alternative approach, the introduction of the gene encoding the enhanced green fluorescent protein (EGFP) by homologous recombination into the LMP2 gene of EBV strain B95-8, generating the same LMP2 deletion mutation is reported. Careful quantification of B cell transformation using the EGFP+ LMP2- recombinant virus determined that in liquid culture medium or in culture medium containing soft agarose there was no difference in the ability of LMP2- virus to immortalize primary human B cells when compared to that of wild-type virus.

Epstein-Barr virus encodes a novel homolog of the bcl-2 oncogene that inhibits apoptosis and associates with Bax and Bak

The sequenced gammaherpesviruses each contain a single viral bcl-2 homolog (v-bcl-2) which may encode a protein that functions in preventing the apoptotic death of virus-infected cells. Epstein-Barr virus (EBV), a gammaherpesvirus associated with several lymphoid and epithelial malignancies, encodes the v-Bcl-2 homolog BHRF1. In this report the previously uncharacterized BALF1 open reading frame in EBV is identified as having significant sequence similarity to other v-bcl-2 homologs and cellular bcl-2. Transfection of cells with a BALF1 cDNA conferred apoptosis resistance. Furthermore, a recombinant green fluorescent protein-BALF1 fusion protein suppressed apoptosis and associated with Bax and Bak. These results indicate that EBV encodes a second functional v-bcl-2.

Three distinct regions of the murine gammaherpesvirus 68 genome are transcriptionally active in latently infected mice

The program(s) of gene expression operating during murine gammaherpesvirus 68 (gammaHV68) latency is undefined, as is the relationship between gammaHV68 latency and latency of primate gammaherpesviruses. We used a nested reverse transcriptase PCR strategy (sensitive to approximately one copy of gammaHV68 genome for each genomic region tested) to screen for the presence of viral transcripts in latently infected mice. Based on the positions of known latency-associated genes in other gammaherpesviruses, we screened for the presence of transcripts corresponding to 11 open reading frames (ORFs) in the gammaHV68 genome in RNA from spleens and peritoneal cells of latently infected B-cell-deficient (MuMT) mice which have been shown contain high levels of reactivable latent gammaHV68 (K. E. Weck, M. L. Barkon, L. I. Yoo, S. H. Speck, and H. W. Virgin, J. Virol. 70:6775-6780, 1996). To control for the possible presence of viral lytic activity, we determined that RNA from latently infected peritoneal and spleen cells contained few or no detectable transcripts corresponding to seven ORFs known to encode viral gene products associated with lytic replication. However, we did detect low-level expression of transcripts arising from the region of gene 50 (encoding the putative homolog of the Epstein-Barr virus BRLF1 transactivator) in peritoneal but not spleen cells. Latently infected peritoneal cells consistently scored for expression of RNA derived from 4 of the 11 candidate latency-associated ORFs examined, including the regions of ORF M2, ORF M11 (encoding v-bcl-2), gene 73 (a homolog of the Kaposi's sarcoma-associated herpesvirus [human herpesvirus 8] gene encoding latency-associated nuclear antigen), and gene 74 (encoding a G-protein coupled receptor homolog, v-GCR). Latently infected spleen cells consistently scored positive for RNA derived from 3 of the 11 candidate latency-associated ORFs examined, including ORF M2, ORF M3, and ORF M9. To further characterize transcription of these candidate latency-associated ORFs, we examined their transcription in lytically infected fibroblasts by Northern analysis. We detected abundant transcription from regions of the genome containing ORF M3 and ORF M9, as well as the known lytic-cycle genes. However, transcription of ORF M2, ORF M11, gene 73, and gene 74 was barely detectable in lytically infected fibroblasts, consistent with a role of these viral genes during latent infection. We conclude that (i) we have identified several candidate latency genes of murine gammaHV68, (ii) expression of genes during latency may be different in different organs, consistent with multiple latency programs and/or multiple cellular sites of latency, and (iii) regions of the viral genome (v-bcl-2 gene, v-GCR gene, and gene 73) are transcribed during latency with both gammaHV68 and primate gammaherpesviruses. The implications of these findings for replacing previous operational definitions of gammaHV68 latency with a molecular definition are discussed.

Functional differences between BHRF1, the Epstein-Barr virus-encoded Bcl-2 homologue, and Bcl-2 in human epithelial cells

BHRF1, a component of the restricted early antigen complex of the Epstein-Barr virus lytic cycle, encodes a 17-kDa protein with both sequence and functional homology to the antiapoptotic Bcl-2 oncogene. Recent work has suggested that BHRF1 behaves like Bcl-2 in protecting cells from apoptosis induced by a range of stimuli. In this study, the effect of BHRF1 and Bcl-2 on the growth and differentiation of the SCC12F human epithelial cell line was examined. The levels of stable transfected BHRF1 expression achievable in SCC12F cells was consistently lower than that obtained with Bcl-2. While both BHRF1 and Bcl-2 inhibited epithelial differentiation, the effect of Bcl-2 was more pronounced, resulting in an almost complete blockade of differentiation in organotypic raft cultures. However, BHRF1-expressing SCC12F cells proliferated at a much higher rate than SCC12F cells expressing Bcl-2, and this effect was supported by cell cycle analysis which demonstrated that BHRF1, but not Bcl-2, promotes rapid transit through the cell cycle. These data highlight important differences between BHRF1 and Bcl-2 and suggest that BHRF1 may function to promote the survival and proliferation of lytically infected cells. The proliferative properties of BHRF1 described in this study, together with the demonstration that other oncogenic gamma herpesviruses encode Bcl-2 homologues, suggests that these proteins may serve to increase the susceptibility of virus-infected cells to oncogenic transformation, thereby contributing to the development of virus-associated tumors.

Tyrosine 112 of latent membrane protein 2A is essential for protein tyrosine kinase loading and regulation of Epstein-Barr virus latency

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) is expressed on the plasma membrane of B lymphocytes latently infected with EBV and blocks B-cell receptor (BCR) signal transduction in EBV-immortalized B cells in vitro. The LMP2A amino-terminal domain that is essential for the LMP2A-mediated block on BCR signal transduction contains eight tyrosine residues. Association of Syk protein tyrosine kinase (PTK) with LMP2A occurs at the two tyrosines of the LMP2A immunoreceptor tyrosine-based activation motif, and it is hypothesized that Lyn PTK associates with the YEEA amino acid motif at LMP2A tyrosine 112 (Y112). To examine the specific association of Lyn PTK to LMP2A, a panel of LMP2A cDNA expression vectors containing LMP2A mutations were transfected into an EBV-negative B-cell line and analyzed for Lyn and LMP2A coimmunoprecipitation. Lyn associates with wild-type LMP2A and other LMP2A mutant constructs, but Lyn association is lost in the LMP2A construct containing a tyrosine (Y)-to-phenylalanine (F) mutation at LMP2A residue Y112 (LMP2AY112F). Next, the LMP2AY112F mutation was recombined into the EBV genome to generate stable lymphoblastoid cell lines (LCLs) transformed with the LMP2AY112F mutant virus. Analysis of BCR-mediated signal transduction in the LMP2AY112F LCLs revealed loss of the LMP2A-mediated block in BCR signal transduction. In addition, LMP2A was not tyrosine phosphorylated in LMP2AY112F LCLs. Together these data indicate the importance of the LMP2A Y112 residue in the ability of LMP2A to block BCR-mediated signal transduction and place the role of this residue and its interaction with Lyn PTK as essential to LMP2A phosphorylation, PTK loading, and down-modulation of PTKs involved in BCR-mediated signal transduction.

Anti-apoptotic strategies of lymphotropic viruses

Induction of apoptosis of virus-infected cells is an important host cell defence mechanism. However, some viruses have incorporated genes that encode anti-apoptotic proteins or modulate the expression of cellular regulators of apoptosis. Here, Edgar Meinl and colleagues discuss recent evidence that viral interference with host cell apoptosis leads to enhanced viral replication, and to evasion of cytotoxic T-cell effects.

Suppression of c-Myc-induced apoptosis by the Epstein-Barr virus gene product BHRF1

Constitutive expression of the c-myc proto-oncogene in growth factor-deprived fibroblasts promotes proliferation and induces apoptosis. In these cells, apoptosis can be inhibited by survival factors such as insulin-like growth factor I or the bcl-2 proto-oncogene product. Deregulated c-Myc expression is a common feature in Epstein-Barr virus-positive Burkitt's lymphoma in which the c-myc gene is reciprocally translocated and placed under the control of one of the immunoglobulin loci. BHRF1 is an Epstein-Barr virus protein expressed early in the lytic cycle. BHRF1 is a member of the Bcl-2 family and has been shown to suppress apoptosis and to increase cell survival in different settings. In the present study, we report that BHRF1 inhibits c-Myc-induced apoptosis which occurs in the absence of survival factors. It does not, however, affect the capacity of c-Myc to promote cell growth. These findings demonstrate that BHRF1 has not only structural but also functional similarities to Bcl-2.