The Epstein-Barr virus gene BHRF1, a homologue of the cellular oncogene Bcl-2, inhibits apoptosis induced by gamma radiation and chemotherapeutic drugs

Virus-Mediated Inhibition of Apoptosis in the Context of EBV-Associated Diseases: Molecular Mechanisms and Therapeutic Perspectives

Epstein-Barr virus (EBV), the representative of the Herpesviridae family, is a pathogen extensively distributed in the human population. One of its most characteristic features is the capability to establish latent infection in the host. The infected cells serve as a sanctuary for the dormant virus, and therefore their desensitization to apoptotic stimuli is part of the viral strategy for long-term survival. For this reason, EBV encodes a set of anti-apoptotic products. They may increase the viability of infected cells and enhance their resistance to chemotherapy, thereby contributing to the development of EBV-associated diseases, including Burkitt’s lymphoma (BL), Hodgkin’s lymphoma (HL), gastric cancer (GC), nasopharyngeal carcinoma (NPC) and several other malignancies. In this paper, we have described the molecular mechanism of anti-apoptotic actions of a set of EBV proteins. Moreover, we have reviewed the pro-survival role of non-coding viral transcripts: EBV-encoded small RNAs (EBERs) and microRNAs (miRNAs), in EBV-carrying malignant cells. The influence of EBV on the expression, activity and/or intracellular distribution of B-cell lymphoma 2 (Bcl-2) protein family members, has been presented. Finally, we have also discussed therapeutic perspectives of targeting viral anti-apoptotic products or their molecular partners.

B cell-directed therapies: a new era in Multiple Sclerosis treatment

Multiple sclerosis (MS) is a chronic autoimmune demyelinating disease of the central nervous system (CNS) that often progresses to severe disability. Previous studies have highlighted the role of T cells in disease pathophysiology; however, the success of B-cell-targeted therapies has led to an increased interest in how B cells contribute to disease immunopathology. In this review, we summarize evidence of B-cell involvement in MS disease mechanisms, starting with pathology and moving on to review aspects of B cell immunobiology potentially relevant to MS. We describe current theories of critical B cell contributions to the inflammatory CNS milieu in MS, namely (i) production of autoantibodies, (ii) antigen presentation, (iii) production of proinflammatory cytokines (bystander activation), and (iv) EBV involvement. In the second part of the review, we summarize medications that have targeted B cells in patients with MS and their current position in the therapeutic armamentarium based on clinical trials and real-world data. Covered therapeutic strategies include the targeting of surface molecules such as CD20 (rituximab, ocrelizumab, ofatumumab, ublituximab) and CD19 (inebilizumab), and molecules necessary for B-cell activation such as B cell activating factor (BAFF) (belimumab) and Bruton's Tyrosine Kinase (BTK) (evobrutinib). We finally discuss the use of B-cell-targeted therapeutics in pregnancy.

EBV BCL-2 homologue BHRF1 drives chemoresistance and lymphomagenesis by inhibiting multiple cellular pro-apoptotic proteins

Epstein-Barr virus (EBV), which is ubiquitous in the adult population, is causally associated with human malignancies. Like many infectious agents, EBV has evolved strategies to block host cell death, including through expression of viral homologues of cellular BCL-2 pro-survival proteins (vBCL-2s), such as BHRF1. Small molecule inhibitors of the cellular pro-survival BCL-2 family proteins, termed 'BH3-mimetics', have entered clinical trials for blood cancers with the BCL-2 inhibitor venetoclax already approved for treatment of therapy refractory chronic lymphocytic leukaemia and acute myeloid leukaemia in the elderly. The generation of BH3-mimetics that could specifically target vBCL-2 proteins may be an attractive therapeutic option for virus-associated cancers, since these drugs would be expected to only kill virally infected cells with only minimal side effects on normal healthy tissues. To achieve this, a better understanding of the contribution of vBCL-2 proteins to tumorigenesis and insights into their biochemical functions is needed. In the context of Burkitt lymphoma (BL), BHRF1 expression conferred strong resistance to diverse apoptotic stimuli. Furthermore, BHRF1 expression in mouse haematopoietic stem and progenitor cells accelerated MYC-induced lymphoma development in a model of BL. BHRF1 interacts with the cellular pro-apoptotic BCL-2 proteins, BIM, BID, PUMA and BAK, but its capability to inhibit apoptosis could not be mapped solely to one of these interactions, suggesting plasticity is a key feature of BHRF1. Site-directed mutagenesis revealed a site in BHRF1 that was critical for its interaction with PUMA and blocking DNA-damage-induced apoptosis, identifying a potentially therapeutically targetable vulnerability in BHRF1.

EBV and Apoptosis: The Viral Master Regulator of Cell Fate?

Epstein-Barr virus (EBV) was first discovered in cells from a patient with Burkitt lymphoma (BL), and is now known to be a contributory factor in 1-2% of all cancers, for which there are as yet, no EBV-targeted therapies available. Like other herpesviruses, EBV adopts a persistent latent infection in vivo and only rarely reactivates into replicative lytic cycle. Although latency is associated with restricted patterns of gene expression, genes are never expressed in isolation; always in groups. Here, we discuss (1) the ways in which the latent genes of EBV are known to modulate cell death, (2) how these mechanisms relate to growth transformation and lymphomagenesis, and (3) how EBV genes cooperate to coordinately regulate key cell death pathways in BL and lymphoblastoid cell lines (LCLs). Since manipulation of the cell death machinery is critical in EBV pathogenesis, understanding the mechanisms that underpin EBV regulation of apoptosis therefore provides opportunities for novel therapeutic interventions.

Understanding Epstein-Barr Virus Life Cycle with Proteomics: A Temporal Analysis of Ubiquitination During Virus Reactivation

Epstein-Barr virus (EBV) is a human γ-herpesvirus associated with cancer, including Burkitt lymphoma, nasopharyngeal, and gastric carcinoma. EBV reactivation in latently infected B cells is essential for persistent infection whereby B cell receptor (BCR) activation is a physiologically relevant stimulus. Yet, a global view of BCR activation-regulated protein ubiquitination is lacking when EBV is actively replicating. We report here, for the first time, the long-term effects of IgG cross-linking-regulated protein ubiquitination and offer a basis for dissecting the cellular environment during the course of EBV lytic replication. Using the Akata-BX1 (EBV⁺) and Akata-4E3 (EBV^-) Burkitt lymphoma cells, we monitored the dynamic changes in protein ubiquitination using quantitative proteomics. We observed temporal alterations in the level of ubiquitination at ∼150 sites in both EBV⁺ and EBV^- B cells post-IgG cross-linking, compared with controls with no cross-linking. The majority of protein ubiquitination was downregulated. The upregulated ubiquitination events were associated with proteins involved in RNA processing. Among the downregulated ubiquitination events were proteins involved in apoptosis, ubiquitination, and DNA repair. These comparative and quantitative proteomic observations represent the first analysis on the effects of IgG cross-linking at later time points when the majority of EBV genes are expressed and the viral genome is actively being replicated. In all, these data enhance our understanding of mechanistic linkages connecting protein ubiquitination, RNA processing, apoptosis, and the EBV life cycle.

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 and multiple sclerosis: potential opportunities for immunotherapy

Multiple sclerosis (MS) is a common chronic inflammatory demyelinating disease of the central nervous system (CNS) causing progressive disability. Many observations implicate Epstein–Barr virus (EBV) in the pathogenesis of MS, namely universal EBV seropositivity, high anti-EBV antibody levels, alterations in EBV-specific CD8⁺ T-cell immunity, increased spontaneous EBV-induced transformation of peripheral blood B cells, increased shedding of EBV from saliva and accumulation of EBV-infected B cells and plasma cells in the brain. Several mechanisms have been postulated to explain the role of EBV in the development of MS including cross-reactivity between EBV and CNS antigens, bystander damage to the CNS by EBV-specific CD8⁺ T cells, activation of innate immunity by EBV-encoded small RNA molecules in the CNS, expression of αB-crystallin in EBV-infected B cells leading to a CD4⁺ T-cell response against oligodendrocyte-derived αB-crystallin and EBV infection of autoreactive B cells, which produce pathogenic autoantibodies and provide costimulatory survival signals to autoreactive T cells in the CNS. The rapidly accumulating evidence for a pathogenic role of EBV in MS provides ground for optimism that it might be possible to prevent and cure MS by effectively controlling EBV infection through vaccination, antiviral drugs or treatment with EBV-specific cytotoxic CD8⁺ T cells. Adoptive immunotherapy with in vitro-expanded autologous EBV-specific CD8⁺ T cells directed against viral latent proteins was recently used to treat a patient with secondary progressive MS. Following the therapy, there was clinical improvement, decreased disease activity on magnetic resonance imaging and reduced intrathecal immunoglobulin production.

Latent and lytic Epstein-Barr virus gene expression in the peripheral blood of astronauts

Epstein-Barr virus (EBV) latent and replicative gene transcription was analyzed in peripheral blood B-lymphocytes from astronauts who flew on short-duration (∼11 days) Shuttle missions and long-duration (∼180 days) International Space Station (ISS) missions. Latent, immediate-early, and early gene replicative viral transcripts were detected in samples from six astronauts who flew on short-duration Shuttle missions, whereas viral gene transcription was mostly absent in samples from 24 healthy donors. Samples from six astronauts who flew on long-duration ISS missions were characterized by expanded expression of latent, immediate-early, and early gene transcripts and new onset expression of late replicative transcription upon return to Earth. These data indicate that EBV-infected cells are no longer expressing the restricted set of viral genes that characterize latency but are expressing latent and lytic gene transcripts. These data also suggest the possibility of EBV-related complications in future long-duration missions, in particular interplanetary travel.

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).

The essential role of Epstein-Barr virus in the pathogenesis of multiple sclerosis

There is increasing evidence that infection with the Epstein-Barr virus (EBV) plays a role in the development of multiple sclerosis (MS), a chronic inflammatory demyelinating disease of the CNS. This article provides a four-tier hypothesis proposing (1) EBV infection is essential for the development of MS; (2) EBV causes MS in genetically susceptible individuals by infecting autoreactive B cells, which seed the CNS where they produce pathogenic autoantibodies and provide costimulatory survival signals to autoreactive T cells that would otherwise die in the CNS by apoptosis; (3) the susceptibility to develop MS after EBV infection is dependent on a genetically determined quantitative deficiency of the cytotoxic CD8+ T cells that normally keep EBV infection under tight control; and (4) sunlight and vitamin D protect against MS by increasing the number of CD8+ T cells available to control EBV infection. The hypothesis makes predictions that can be tested, including the prevention and successful treatment of MS by controlling EBV infection.

Mitochondria and viruses

Mitochondria are involved in a variety of cellular metabolic processes, and their functions are regulated by extrinsic and intrinsic stimuli including viruses. Recent studies have shown that mitochondria play a central role in the primary host defense mechanisms against viral infections, and a number of novel viral and mitochondrial proteins are involved in these processes. Some viral proteins localize in mitochondria and interact with mitochondrial proteins to regulate cellular responses. This review summarizes recent findings on the functions and roles of these molecules as well as mitochondrial responses to viral infections.

To kill or be killed: how viruses interact with the cell death machinery

A virus (from the Latin virus meaning toxin or poison) is a small infectious agent that can only replicate inside the cells of another organism. Viruses are found wherever there is life and have probably existed since living cells first evolved. Viruses do not have their own metabolism and require a host cell to make new products. The range of structural and biochemical (i.e., cytopathic) effects that viruses have on the host cell is extensive. Most viral infections eventually result in the death of the host cell. The causes of death include cell lysis, alterations to the cell's surface membrane and various modes of programmed cell death. Some viruses cause no apparent changes to the infected cell. Cells in which the virus is latent and inactive show few signs of infection and often function normally. This causes persistent infection and the virus is often dormant for many months or years. Some viruses can cause cells to proliferate without causing malignancy, whereas others are established causes of cancer. Human organisms use a genetically controlled cell death programme that prevents the spreading of viral infection and kills the virus. Between 19 and 21 November 2009, with sponsorship from the Journal of Internal Medicine, the Swedish Research Foundation and the Swedish Cancer Society hosted a conference in Stockholm entitled: 'To kill or to be killed. Viral evasion strategies and interference with cell death machinery'. Four comprehensive reviews from this conference are presented in this issue of the Journal of Internal Medicine. These reviews include descriptions of: the modulation of host innate and adaptive immune defenses by cytomegalovirus; the impact of gamma-chain family cytokines on T cell homoeostasis in HIV-1 infection and the therapeutic implications; approaches to killing tumours by depriving them of the mechanisms for detoxification; and viral strategies for the evasion of immunogenic cell death.

Burkitt's lymphoma: the Rosetta Stone deciphering Epstein-Barr virus biology

Epstein-Barr virus was originally identified in the tumour cells of a Burkitt's lymphoma, and was the first virus to be associated with the pathogenesis of a human cancer. Studies on the relationship of EBV with Burkitt's lymphoma have revealed important general principles that are relevant to other virus-associated cancers. In addition, the impact of such studies on the knowledge of EBV biology has been enormous. Here, we review some of the key historical observations arising from studies on Burkitt's lymphoma that have informed our understanding of EBV, and we summarise the current hypotheses regarding the role of EBV in the pathogenesis of Burkitt's lymphoma.

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.

An Epstein-Barr virus anti-apoptotic protein constitutively expressed in transformed cells and implicated in burkitt lymphomagenesis: the Wp/BHRF1 link

Two factors contribute to Burkitt lymphoma (BL) pathogenesis, a chromosomal translocation leading to c-myc oncogene deregulation and infection with Epstein-Barr virus (EBV). Although the virus has B cell growth–transforming ability, this may not relate to its role in BL since many of the transforming proteins are not expressed in the tumor. Mounting evidence supports an alternative role, whereby EBV counteracts the high apoptotic sensitivity inherent to the c-myc–driven growth program. In that regard, a subset of BLs carry virus mutants in a novel form of latent infection that provides unusually strong resistance to apoptosis. Uniquely, these virus mutants use Wp (a viral promoter normally activated early in B cell transformation) and express a broader-than-usual range of latent antigens. Here, using an inducible system to express the candidate antigens, we show that this marked apoptosis resistance is mediated not by one of the extended range of EBNAs seen in Wp-restricted latency but by Wp-driven expression of the viral bcl2 homologue, BHRF1, a protein usually associated with the virus lytic cycle. Interestingly, this Wp/BHRF1 connection is not confined to Wp-restricted BLs but appears integral to normal B cell transformation by EBV. We find that the BHRF1 gene expression recently reported in newly infected B cells is temporally linked to Wp activation and the presence of W/BHRF1-spliced transcripts. Furthermore, just as Wp activity is never completely eclipsed in in vitro–transformed lines, low-level BHRF1 transcripts remain detectable in these cells long-term. Most importantly, recognition by BHRF1-specific T cells confirms that such lines continue to express the protein independently of any lytic cycle entry. This work therefore provides the first evidence that BHRF1, the EBV bcl2 homologue, is constitutively expressed as a latent protein in growth-transformed cells in vitro and, in the context of Wp-restricted BL, may contribute to virus-associated lymphomagenesis in vivo.

Cancer almost always develops through the cumulative effects of several independent changes in the target cell. For certain tumors, one step in the chain involves infection of the cell with a particular type of virus. The best example is Burkitt lymphoma (BL), a tumor of B lymphocytes which develops through the combined action of a genetic accident leading to uncontrolled expression of the c-myc oncogene and infection with a common herpesvirus, the Epstein-Barr virus (EBV). Recent evidence suggests that, although latent EBV infection can itself drive B cell growth, the virus plays a different role in the context of BL, namely to counteract the naturally poor survival ability of c-myc–expressing cells while leaving their c-myc–driven growth intact. Here we show that EBV achieves this by unexpectedly switching on a viral protein that was thought never to be seen in latent infection; this viral protein resembles one of the cell's own key survival proteins called bcl2. Furthermore, the work has led us to realise that this virally encoded bcl2-like protein is not only important in the context of BL but, contrary to conventional wisdom, is actually part of EBV's natural strategy for B cell growth transformation.

Expression of BHRF1 improves survival of murine hybridoma cultures in batch and continuous modes

Cell death by apoptosis limits growth and productivity in most animal cell cultures. It is therefore desirable to define genetic interventions to generate robust cell lines with superior performance in bioreactors, either by increasing specific productivity, life-span of the cultures or both. In this context, forced expression of BHRF1, an Epstein-Barr virus-encoded early protein with structural and functional homology with the anti-apoptotic protein Bcl-2, effectively protected hybridomas in culture and delayed cell death under conditions of glutamine starvation. In the present study, we explored the potential application of BHRF1 expression in hybridomas for long-term apoptosis protection under different biotechnological process designs (batch and continuous) and compared it to strategies based on Bcl-2 overexpression. Our results confirmed that long-term maintenance of the anti-apoptotic effect of BHRF1 can be obtained using bicistronic configurations conferring enhanced protection compared to Bcl-2, even in the absence of selective pressure. Such protective effect of BHRF1 is demonstrated both in batch and continuous culture. Moreover, a further analysis at high cell densities in semi-continuous perfusion cultures indicated that the mechanism of action of BHRF1 involves cell cycle arrest in G0-G1 state and this is translated in lower numbers of dead cells.

The granzyme B gene is highly polymorphic in wild mice but essentially invariant in common inbred laboratory strains

Granzyme B is a 247 amino acid pro-apoptotic protease secreted by effector lymphocytes for the purpose of killing virus-infected cells. While the capacity of granzyme B to potently induce caspase-dependent apoptosis has long been recognized, it has only recently been found that human and mouse granzyme B activate overlapping but distinct apoptotic pathways. To investigate a possible evolutionary basis for this observation, we sequenced the exons and flanking intronic sequences of the mouse Gzmb gene from a variety of inbred laboratory strains and wild mice. The sequences of 12/13 inbred strains encoded identical proteins, the exception being DBA/2, whose sequence varied at two amino acids. By contrast with the laboratory strains, there was extensive polymorphism in the Gzmb gene of 54 wild mice and 28 wild-derived inbred mice examined, resulting in 2-18 amino acid differences in the predicted proteins, a discrepancy rate of up to 7.3%. Many of these amino acid variations were found in rat and/or human granzyme B. The granzyme B allotype of inbred laboratory strains could be identified in only one of three geographically dispersed clans of wild mice and was absent from all 28 wild-derived inbred strains. The Gzmb gene of Mus musculus castaneus, a close relative of laboratory mice, encoded six amino acid differences compared with the laboratory strains, all of which were also found in corresponding positions in the granzyme B molecules of wild mice. Unlike the protease, the extended granzyme B recognition and cleavage site in Bid, a key pro-apoptotic substrate, was invariant.

Adult Burkitt's Lymphoma

Burkitt's lymphoma is a mature B-cell lymphoma that is characterized by a rapid proliferative rate and propensity for extranodal sites of involvement such as the gastrointestinal tract and central nervous system. This subtype of non-Hodgkin's lymphoma is associated with unique cytogenetic translocations involving the c-MYC oncogene on chromosome 8, which appears to be involved in the pathogenesis of this disease. Although current literature is limited by a lack of randomized trials, Burkitt's lymphoma appears to be curable in a high proportion of cases if treated with aggressive multiagent chemotherapy regimens. The use of autologous stem cell transplantation appears to benefit patients who have had chemotherapy-sensitive relapses. The role of allogeneic stem cell transplantation for this disease remains uncertain. Patients with HIV-associated Burkitt's lymphoma appear to have a better prognosis today, which is likely a result of more effective antiretroviral therapy and the ability to treat selected patients with more aggressive chemotherapeutic regimens than before. This article will review the epidemiologic, biologic, diagnostic, and therapeutic aspects of Burkitt's lymphoma in adults.

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.

Characterization of monoclonal antibody to the Epstein-Barr virus BHRF1 protein, a homologue of Bcl-2

A monoclonal antibody (MAb), designated 3E8, was produced against the Epstein-Barr virus BHRF1 which is a viral homologue of the anti-apoptotic protein Bcl-2. The MAb recognized the BHRF1 protein in extracts from EBV-containing cell lines after activation and EBV-negative cell lines transfected by the BHRF1 gene. Epitope mapping by Western blot analysis revealed that the antibody bound region encompassing amino acid residues 28-33 of the BHRF1. In addition to immunoblotting, the MAb could be applied widely in detection of the BHRF1 in many assays, including immunofluorescence assay, immunohistochemistry, enzyme-linked immunosorbent assay and immunoprecipitation. Most of all, when used in immunoprecipitation experiments, the MAb 3E8 showed a better effect than the existing anti-BHRF1 MAbs since radioactive isotopes were not required to intensify signals of its target antigen. Based on its great use in a variety of immunological reactions, it is a powerful tool to elucidate the biological functions of BHRF1.

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.

Viruses and apoptosis

Successful viral replication requires not only the efficient production and spread of progeny, but also evasion of host defense mechanisms that limit replication by killing infected cells. In addition to inducing immune and inflammatory responses, infection by most viruses triggers apoptosis or programmed cell death of the infected cell. This cell response often results as a compulsory or unavoidable by-product of the action of critical viral replicative functions. In addition, some viruses seem to use apoptosis as a mechanism of cell killing and virus spread. In both cases, successful replication relies on the ability of certain viral products to block or delay apoptosis until sufficient progeny have been produced. Such proteins target a variety of strategic points in the apoptotic pathway. In this review we summarize the great amount of recent information on viruses and apoptosis and offer insights into how this knowledge may be used for future research and novel therapies.

Effects of smokeless tobacco and tumor promoters on cell population growth and apoptosis of B lymphocytes infected with epstein-barr virus types 1 and 2

The effects of smokeless tobacco purified products 4-(N-methyl-N-nitrosamine)-1-3-pyridinyl)-1-butanone (NNK) and N-nitrosonornicotine (NNN), smokeless tobacco extracts (dry snuff, moist snuff, and loose leaf), and the tumor promoters 12-O-tetradecanoyl phorbol-13-acetate (TPA) and n-butyrate on cell population growth, cell death, and apoptosis were studied in B lymphocyte cell lines harboring Epstein-Barr virus (EBV) type 1 (Raji and X50-7) or type 2 (HR-1K and AG876) and in an EBV-uninfected control lymphocyte cell line (Ramos). Spontaneous apoptosis was present in all EBV-infected cell lines, but at varying levels. Spontaneous and induced apoptosis were generally greater by Student-Newman-Keuls tests in cells harboring EBV type 2 compared to EBV type 1. The greatest effects on cell population growth, cell death, and apoptosis on cells harboring lytic EBV type 1 (X50-7) was with each of the three smokeless tobacco extracts. The greatest effects on cells harboring EBV type 2 was with TPA and n-butyrate. There were no effects of smokeless tobacco extracts on the Raji cell line that harbors EBV type 1 incapable of lytic replication. Smokeless tobacco purified products, NNN and NNK, had no discernible effects. At the concentrations used in these experiments, there appears to be an EBV type-specific response to chemical induction, with greater susceptibility of lytic EBV type 1 to smokeless tobacco extracts and lytic EBV type 2 to TPA and n-butyrate. This EBV type-specific susceptibility to the effects of smokeless tobacco extracts is another phenotypic difference between EBV types. The use of smokeless tobacco products may affect B lymphocytes infected with replication-capable EBV in the oropharynx. The absence of significant effects with NNK and NNN suggests that these properties reside with other compounds present in tobacco extracts.

Low rate of apoptosis and overexpression of bcl-2 in Epstein-Barr virus-associated gastric carcinoma

AIMS

Epstein-Barr virus (EBV) has been demonstrated in about 10% of gastric carcinomas. However, the pathogenetic role of EBV in gastric carcinoma is uncertain. We compared the rate of apoptotic cell death, cell proliferation and the expression of apoptosis-related proteins in gastric carcinomas with or without EBV.

METHODS AND RESULTS

Epstein-Barr virus was detected in 40 gastric carcinomas by EBV-encoded small RNA-1 in-situ hybridization. Apoptotic cell death, MIB-1, p53, bcl-2 and bcl-x were examined by the terminal deoxynucleotidyl-mediated dUTP-nick end labelling method and immunohistochemistry. We also included 40 age-, sex- and disease stage-matched EBV-negative cases as a control. The number of apoptotic cells was significantly lower in EBV-positive (20 +/- 15. 1/1000 cells) and bcl-2-positive (17 +/- 12.9/1000 cells) tumours than in EBV-negative (43 +/- 37.1) and bcl-2-negative tumours (38 +/- 32.1, P < 0.001, P < 0.001, respectively). bcl-2 immunostaining was significantly higher in EBV-positive tumours (24 cases) than in EBV-negative tumours (12 cases, P < 0.05). There was no significant difference in bcl-x and p53 expression between EBV-positive and -negative tumours. The number of MIB-1-positive cells in EBV-positive tumours (237 +/- 161/1000) was significantly lower than in EBV-negative tumours (480 +/- 208/1000 cells, P < 0.001).

CONCLUSIONS

A low rate of apoptosis and high bcl-2 expression were recognized in EBV-positive gastric carcinomas, suggesting that bcl-2 protein is the main inhibitor of apoptosis in EBV-positive carcinomas. In addition, the low apoptotic and proliferative activities may reflect a low biological activity in EBV-positive gastric carcinomas.

Resistance to etoposide-induced apoptosis in a Burkitt's lymphoma cell line

Burkitt's lymphoma cells that vary in their phenotypic characteristics show significantly different degrees of susceptibility to radiation-induced apoptosis. Propensity to undergo apoptosis is reflected in the degradation of substrates such as DNA-dependent protein kinase but the status of bcl-2, c-myc and p53 has been uninformative. In this study, we have focused on 2 Epstein-Barr virus (EBV)-associated Burkitt's cell lines, one (WW2) susceptible and the other (BL29) resistant to etoposide-induced apoptosis. Differences in expression of BHRF1, an EBV gene that is homologous to the Bcl-2 proto-oncogene and known to inhibit apoptosis, or changes in apoptosis inhibitory proteins (IAPs), did not appear to account for the difference in susceptibility in the 2 cell lines. Cytoplasmic extracts from etoposide-treated WW2 cells caused apoptotic changes in nuclei isolated from either BL29 or WW2 cells, whereas extracts from BL29 cells failed to do so. In addition, extracts from etoposide-treated WW2 cells degraded the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), an important indicator of apoptosis, but this protein was resistant to degradation by BL29 extracts. It appears likely that caspase 3 (CPP32) is involved in this degradation since it was activated only in the apoptosis susceptible cells and the pattern of cleavage of DNA-PKcs was similar to that reported previously with recombinant caspase 3. As observed previously, addition of caspase 3 to nuclei failed to induce morphological changes indicative of apoptosis, but addition of caspase 3 to nuclei in the presence of extract from the resistant cells led to apoptotic changes. We conclude that resistance to apoptosis in BL29 cells is due to a failure of etoposide to activate upstream effectors of caspase activity.