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

Epstein-Barr virus BALF0/1 subverts the Caveolin and ERAD pathways to target B cell receptor complexes for degradation

Epstein-Barr virus (EBV) establishes persistent infection, causes infectious mononucleosis, is a major trigger for multiple sclerosis and contributes to multiple cancers. Yet, knowledge remains incomplete about how the virus remodels host B cells to support lytic replication. We previously identified that EBV lytic replication results in selective depletion of plasma membrane (PM) B cell receptor (BCR) complexes, composed of immunoglobulin and the CD79A and CD79B signaling chains. Here, we used proteomic and biochemical approaches to identify that the EBV early lytic protein BALF0/1 is responsible for EBV lytic cycle BCR degradation. Mechanistically, an immunoglobulin heavy chain (HC) cytoplasmic tail KVK motif was required for ubiquitin-mediated BCR degradation, while CD79A and CD79B were dispensable. BALF0/1 subverted caveolin-mediated endocytosis to internalize PM BCR complexes and to deliver them to the endoplasmic reticulum. BALF0/1 stimulated immunoglobulin HC cytoplasmic tail ubiquitination, which together with the ATPase valosin-containing protein/p97 drove ER-associated degradation of BCR complexes by cytoplasmic proteasomes. BALF0/1 knockout reduced the viral load of secreted EBV particles from B cells that expressed a monoclonal antibody against EBV glycoprotein 350 but not a control anti-influenza hemagglutinin antibody and increased viral particle immunoglobulin incorporation. Consistent with downmodulation of PM BCR, BALF0/1 overexpression reduced viability of a diffuse large B cell lymphoma cell line whose survival is dependent upon BCR signaling. Collectively, our results suggest that EBV BALF0/1 downmodulates immunoglobulin upon lytic reactivation to block BCR signaling and support virion release, but await the development of suitable models to test its roles in EBV reactivation in vivo.

Mastering Death: The Roles of Viral Bcl-2 in dsDNA Viruses

Proteins of the Bcl-2 family regulate cellular fate via multiple mechanisms including apoptosis, autophagy, senescence, metabolism, inflammation, redox homeostasis, and calcium flux. There are several regulated cell death (RCD) pathways, including apoptosis and autophagy, that use distinct molecular mechanisms to elicit the death response. However, the same proteins/genes may be deployed in multiple biochemical pathways. In apoptosis, Bcl-2 proteins control the integrity of the mitochondrial outer membrane (MOM) by regulating the formation of pores in the MOM and apoptotic cell death. A number of prosurvival genes populate the genomes of viruses including those of the pro-survival Bcl-2 family. Viral Bcl-2 proteins are sequence and structural homologs of their cellular counterparts and interact with cellular proteins in apoptotic and autophagic pathways, potentially allowing them to modulate these pathways and determine cellular fate.

Epstein-Barr virus lytic gene BNRF1 promotes B-cell lymphomagenesis via IFI27 upregulation

Epstein-Barr virus (EBV) is a ubiquitous human lymphotropic herpesvirus that is causally associated with several malignancies. In addition to latent factors, lytic replication contributes to cancer development. In this study, we examined whether the lytic gene BNRF1, which is conserved among gamma-herpesviruses, has an important role in lymphomagenesis. We found that lymphoblastoid cell lines (LCLs) established by BNRF1-knockout EBV exhibited remarkably lower pathogenicity in a mice xenograft model than LCLs produced by wild-type EBV (LCLs-WT). RNA-seq analyses revealed that BNRF1 elicited the expression of interferon-inducible protein 27 (IFI27), which promotes cell proliferation. IFI27 knockdown in LCLs-WT resulted in excessive production of reactive oxygen species, leading to cell death and significantly decreased their pathogenicity in vivo. We also confirmed that IFI27 was upregulated during primary infection in B-cells. Our findings revealed that BNRF1 promoted robust proliferation of the B-cells that were transformed by EBV latent infection via IFI27 upregulation both in vitro and in vivo.

An Epstein-Barr virus protein interaction map reveals NLRP3 inflammasome evasion via MAVS UFMylation

Epstein-Barr virus (EBV) causes infectious mononucleosis, triggers multiple sclerosis, and is associated with 200,000 cancers/year. EBV colonizes the human B cell compartment and periodically reactivates, inducing expression of 80 viral proteins. However, much remains unknown about how EBV remodels host cells and dismantles key antiviral responses. We therefore created a map of EBV-host and EBV-EBV interactions in B cells undergoing EBV replication, uncovering conserved herpesvirus versus EBV-specific host cell targets. The EBV-encoded G-protein-coupled receptor BILF1 associated with MAVS and the UFM1 E3 ligase UFL1. Although UFMylation of 14-3-3 proteins drives RIG-I/MAVS signaling, BILF1-directed MAVS UFMylation instead triggered MAVS packaging into mitochondrial-derived vesicles and lysosomal proteolysis. In the absence of BILF1, EBV replication activated the NLRP3 inflammasome, which impaired viral replication and triggered pyroptosis. Our results provide a viral protein interaction network resource, reveal a UFM1-dependent pathway for selective degradation of mitochondrial cargo, and highlight BILF1 as a novel therapeutic target.

Contribution of Epstein–Barr Virus Lytic Proteins to Cancer Hallmarks and Implications from Other Oncoviruses

Epstein–Barr virus (EBV) is a prevalent human gamma-herpesvirus that infects the majority of the adult population worldwide and is associated with several lymphoid and epithelial malignancies. EBV displays a biphasic life cycle, namely, latent and lytic replication cycles, expressing a diversity of viral proteins. Among the EBV proteins being expressed during both latent and lytic cycles, the oncogenic roles of EBV lytic proteins are largely uncharacterized. In this review, the established contributions of EBV lytic proteins in tumorigenesis are summarized according to the cancer hallmarks displayed. We further postulate the oncogenic properties of several EBV lytic proteins by comparing the evolutionary conserved oncogenic mechanisms in other herpesviruses and oncoviruses.

Wildebeest-Derived Malignant Catarrhal Fever: A Bovine Peripheral T Cell Lymphoma Caused by Cross-Species Transmission of Alcelaphine Gammaherpesvirus 1

Gammaherpesviruses (γHVs) include viruses that can induce lymphoproliferative diseases and tumors. These viruses can persist in the long term in the absence of any pathological manifestation in their natural host. Alcelaphine gammaherpesvirus 1 (AlHV-1) belongs to the genus Macavirus and asymptomatically infects its natural host, the wildebeest (Connochaetes spp.). However, when transmitted to several susceptible species belonging to the order Artiodactyla, AlHV-1 is responsible for the induction of a lethal lymphoproliferative disease, named wildebeest-derived malignant catarrhal fever (WD-MCF). Understanding the pathogenic mechanisms responsible for the induction of WD-MCF is important to better control the risks of transmission and disease development in susceptible species. The aim of this review is to synthesize the current knowledge on WD-MCF with a particular focus on the mechanisms by which AlHV-1 induces the disease. We discuss the potential mechanisms of pathogenesis from viral entry into the host to the maintenance of viral genomes in infected CD8⁺ T lymphocytes, and we present current hypotheses to explain how AlHV-1 infection induces a peripheral T cell lymphoma-like disease.

Functional Implications of Epstein-Barr Virus Lytic Genes in Carcinogenesis

Epstein-Barr virus (EBV) is associated with a diverse range of tumors of both lymphoid and epithelial origin. Similar to other herpesviruses, EBV displays a bipartite life cycle consisting of latent and lytic phases. Current dogma indicates that the latent genes are key drivers in the pathogenesis of EBV-associated cancers, while the lytic genes are primarily responsible for viral transmission. In recent years, evidence has emerged to show that the EBV lytic phase also plays an important role in EBV tumorigenesis, and the expression of EBV lytic genes is frequently detected in tumor tissues and cell lines. The advent of next generation sequencing has allowed the comprehensive profiling of EBV gene expression, and this has revealed the consistent expression of several lytic genes across various types of EBV-associated cancers. In this review, we provide an overview of the functional implications of EBV lytic gene expression to the oncogenic process and discuss possible avenues for future investigations.

Structural Insight into KsBcl-2 Mediated Apoptosis Inhibition by Kaposi Sarcoma Associated Herpes Virus

Numerous large DNA viruses have evolved sophisticated countermeasures to hijack the premature programmed cell death of host cells post-infection, including the expression of proteins homologous in sequence, structure, or function to cellular Bcl-2 proteins. Kaposi sarcoma herpes virus (KSHV), a member of the gammaherpesvirinae, has been shown to encode for KsBcl-2, a potent inhibitor of Bcl-2 mediated apoptosis. KsBcl-2 acts by directly engaging host pro-apoptotic Bcl-2 proteins including Bak, Bax and Bok, the BH3-only proteins; Bim, Bid, Bik, Hrk, Noxa and Puma. Here we determined the crystal structures of KsBcl-2 bound to the BH3 motif of pro-apoptotic proteins Bid and Puma. The structures reveal that KsBcl-2 engages pro-apoptotic BH3 motif peptides using the canonical ligand binding groove. Thus, the presence of the readily identifiable conserved BH1 motif sequence “NWGR” of KsBcl-2, as well as highly conserved Arg residue (R86) forms an ionic interaction with the conserved Asp in the BH3 motif in a manner that mimics the canonical ionic interaction seen in host Bcl-2:BH3 motif complexes. These findings provide a structural basis for KSHV mediated inhibition of host cell apoptosis and reveal the flexibility of virus encoded Bcl-2 proteins to mimic key interactions from endogenous host signalling pathways.

Autophagy in Viral Development and Progression of Cancer

Autophagy is a complex degradative process by which eukaryotic cells capture cytoplasmic components for subsequent degradation through lysosomal hydrolases. Although this catabolic process can be triggered by a great variety of stimuli, action in cells varies according to cellular context. Autophagy has been previously linked to disease development modulation, including cancer. Autophagy helps suppress cancer cell advancement in tumor transformation early stages, while promoting proliferation and metastasis in advanced settings. Oncoviruses are a particular type of virus that directly contribute to cell transformation and tumor development. Extensive molecular studies have revealed complex ways in which autophagy can suppress or improve oncovirus fitness while still regulating viral replication and determining host cell fate. This review includes recent advances in autophagic cellular function and emphasizes its antagonistic role in cancer cells.

Autophagy in Viral Development and Progression of Cancer

Autophagy is a complex degradative process by which eukaryotic cells capture cytoplasmic components for subsequent degradation through lysosomal hydrolases. Although this catabolic process can be triggered by a great variety of stimuli, action in cells varies according to cellular context. Autophagy has been previously linked to disease development modulation, including cancer. Autophagy helps suppress cancer cell advancement in tumor transformation early stages, while promoting proliferation and metastasis in advanced settings. Oncoviruses are a particular type of virus that directly contribute to cell transformation and tumor development. Extensive molecular studies have revealed complex ways in which autophagy can suppress or improve oncovirus fitness while still regulating viral replication and determining host cell fate. This review includes recent advances in autophagic cellular function and emphasizes its antagonistic role in cancer cells.

Epstein-Barr Virus: How Its Lytic Phase Contributes to Oncogenesis

Epstein–Barr Virus (EBV) contributes to the development of lymphoid and epithelial malignancies. While EBV’s latent phase is more commonly associated with EBV-associated malignancies, there is increasing evidence that EBV’s lytic phase plays a role in EBV-mediated oncogenesis. The lytic phase contributes to oncogenesis primarily in two ways: (1) the production of infectious particles to infect more cells, and (2) the regulation of cellular oncogenic pathways, both cell autonomously and non-cell autonomously. The production of infectious particles requires the completion of the lytic phase. However, the regulation of cellular oncogenic pathways can be mediated by an incomplete (abortive) lytic phase, in which early lytic gene products contribute substantially, whereas late lytic products are largely dispensable. In this review, we discuss the evidence of EBV’s lytic phase contributing to oncogenesis and the role it plays in tumor formation and progression, as well as summarize known mechanisms by which EBV lytic products regulate oncogenic pathways. Understanding the contribution of EBV’s lytic phase to oncogenesis will help design ways to target it to treat EBV-associated malignancies.

Immunology of EBV-Related Lymphoproliferative Disease in HIV-Positive Individuals

Epstein-Bar virus (EBV) can directly cause lymphoproliferative disease (LPD), including AIDS-defining lymphomas such as Burkitt’s lymphoma and other non-Hodgkin lymphomas (NHL), as well as human immunodeficiency virus (HIV)-related Hodgkin lymphoma (HL). The prevalence of EBV in HL and NHL is elevated in HIV-positive individuals compared with the general population. Rates of incidence of AIDS-defining cancers have been declining in HIV-infected individuals since initiation of combination anti-retroviral therapy (cART) use in 1996. However, HIV-infected persons remain at an increased risk of cancers related to infections with oncogenic viruses. Proposed pathogenic mechanisms of HIV-related cancers include decreased immune surveillance, decreased ability to suppress infection-related oncogenic processes and a state of chronic inflammation marked by alteration of the cytokine profile and expanded numbers of cytotoxic T lymphocytes with down-regulated co-stimulatory molecules and increased expression of markers of senescence in the setting of treated HIV infection. Here we discuss the cooperation of EBV-infected B cell- and environment-associated factors that may contribute to EBV-related lymphomagenesis in HIV-infected individuals. Environment-derived lymphomagenic factors include impaired host adaptive and innate immune surveillance, cytokine dysregulation and a pro-inflammatory state observed in the setting of chronic, cART-treated HIV infection. B cell factors include distinctive EBV latency patterns and host protein expression in HIV-associated LPD, as well as B cell-stimulating factors derived from HIV infection. We review the future directions for expanding therapeutic approaches in targeting the viral and immune components of EBV LPD pathogenesis.

BHRF1, a BCL2 viral homolog, disturbs mitochondrial dynamics and stimulates mitophagy to dampen type I IFN induction

Mitochondria respond to many cellular functions and act as central hubs in innate immunity against viruses. This response is notably due to their role in the activation of interferon (IFN) signaling pathways through the activity of MAVS (mitochondrial antiviral signaling protein) present at the mitochondrial surface. Here, we report that the BHRF1 protein, a BCL2 homolog encoded by Epstein-Barr virus (EBV), inhibits IFNB/IFN-β induction by targeting the mitochondria. Indeed, we have demonstrated that BHRF1 expression modifies mitochondrial dynamics and stimulates DNM1L/Drp1-mediated mitochondrial fission. Concomitantly, we have shown that BHRF1 is pro-autophagic because it stimulates the autophagic flux by interacting with BECN1/Beclin 1. In response to the BHRF1-induced mitochondrial fission and macroautophagy/autophagy stimulation, BHRF1 drives mitochondrial network reorganization to form juxtanuclear mitochondrial aggregates known as mito-aggresomes. Mitophagy is a cellular process, which can specifically sequester and degrade mitochondria. Our confocal studies uncovered that numerous mitochondria are present in autophagosomes and acidic compartments using BHRF1-expressing cells. Moreover, mito-aggresome formation allows the induction of mitophagy and the accumulation of PINK1 at the mitochondria. As BHRF1 modulates the mitochondrial fate, we explored the effect of BHRF1 on innate immunity and showed that BHRF1 expression could prevent IFNB induction. Indeed, BHRF1 inhibits the IFNB promoter activation and blocks the nuclear translocation of IRF3 (interferon regulatory factor 3). Thus, we concluded that BHRF1 can counteract innate immunity activation by inducing fission of the mitochondria to facilitate their sequestration in mitophagosomes for degradation.Abbreviations: 3-MA: 3-methyladenine; ACTB: actin beta; BCL2: BCL2 apoptosis regulator; CARD: caspase recruitment domain; CCCP: carbonyl cyanide 3-chlorophenylhydrazone; CI: compaction index; CQ: chloroquine; DAPI: 4',6-diamidino-2-phenylindole, dihydrochloride; DDX58/RIG-I: DExD/H-box helicase 58; DNM1L/Drp1: dynamin 1 like; EBSS: Earle's balanced salt solution; EBV: Epstein-Barr virus; ER: endoplasmic reticulum; EV: empty vector; GFP: green fluorescent protein; HEK: human embryonic kidney; IFN: interferon; IgG: immunoglobulin G; IRF3: interferon regulatory factor 3; LDHA: lactate dehydrogenase A; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MAVS: mitochondrial antiviral signaling protein; MMP: mitochondrial membrane potential; MOM: mitochondrial outer membrane; PINK1: PTEN induced kinase 1; RFP: red fluorescent protein; ROS: reactive oxygen species; SQSTM1/p62: sequestosome 1; STING1: stimulator of interferon response cGAMP interactor 1; TOMM20: translocase of outer mitochondrial membrane 20; VDAC: voltage dependent anion channel.

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.

Epstein-Barr Virus BALF0 and BALF1 Modulate Autophagy

Autophagy is an essential catabolic process that degrades cytoplasmic components within the lysosome, therefore ensuring cell survival and homeostasis. A growing number of viruses, including members of the Herpesviridae family, have been shown to manipulate autophagy to facilitate their persistence or optimize their replication. Previous works showed that the Epstein–Barr virus (EBV), a human transforming gammaherpesvirus, hijacked autophagy during the lytic phase of its cycle, possibly to favor the formation of viral particles. However, the viral proteins that are responsible for an EBV-mediated subversion of the autophagy pathways remain to be characterized. Here we provide the first evidence that the BALF0/1 open reading frame encodes for two conserved proteins of the Bcl-2 family, BALF0 and BALF1, that are expressed during the early phase of the lytic cycle and can modulate autophagy. A putative LC3-interacting region (LIR) has been identified that is required both for BALF1 colocalization with autophagosomes and for its ability to stimulate autophagy.

Conserved Central Intraviral Protein Interactome of the Herpesviridae Family

Protein interactions are major driving forces behind the functional phenotypes of biological processes. As such, evolutionary footprints are reflected in system-level collections of protein-protein interactions (PPIs), i.e., protein interactomes. We conducted a comparative analysis of intraviral protein interactomes for representative species of each of the three subfamilies of herpesviruses (herpes simplex virus 1, human cytomegalovirus, and Epstein-Barr virus), which are highly prevalent etiologic agents of important human diseases. The intraviral interactomes were reconstructed by combining experimentally supported and computationally predicted protein-protein interactions. Using cross-species network comparison, we then identified family-wise conserved interactions and protein complexes, which we defined as a herpesviral "central" intraviral protein interactome. A large number of widely accepted conserved herpesviral protein complexes are present in this central intraviral interactome, encouragingly supporting the biological coherence of our results. Importantly, these protein complexes represent most, if not all, of the essential steps required during a productive life cycle. Hence the central intraviral protein interactome could plausibly represent a minimal infectious interactome of the herpesvirus family across a variety of hosts. Our data, which have been integrated into our herpesvirus interactomics database, HVint2.0, could assist in creating comprehensive system-level computational models of this viral lineage.IMPORTANCE Herpesviruses are an important socioeconomic burden for both humans and livestock. Throughout their long evolutionary history, individual herpesvirus species have developed remarkable host specificity, while collectively the Herpesviridae family has evolved to infect a large variety of eukaryotic hosts. The development of approaches to fight herpesvirus infections has been hampered by the complexity of herpesviruses' genomes, proteomes, and structural features. The data and insights generated by our study add to the understanding of the functional organization of herpesvirus-encoded proteins, specifically of family-wise conserved features defining essential components required for a productive infectious cycle across different hosts, which can contribute toward the conceptualization of antiherpetic infection strategies with an effect on a broader range of target species. All of the generated data have been made freely available through our HVint2.0 database, a dedicated resource of curated herpesvirus interactomics purposely created to promote and assist future studies in the field.

Tiger frog virus ORF104R interacts with cellular VDAC2 to inhibit cell apoptosis

Ranaviruses belong to the family Iridoviridae, and have become a serious threat to both farmed and natural populations of fish and amphibians. Previous reports showed that ranaviruses could encode viral Bcl-2 family-like proteins (vBcl-2), which play a critical role in the regulation of cell apoptosis. However, the mechanism of ranaviruses vBcl-2 interactions with host protein in mediating apoptosis remains unknown. Tiger frog virus (TFV) belonging to the genus Ranavirus has been isolated from infected tadpoles of Rana tigrina rugulosa, and it causes a high mortality rate among tiger frog tadpoles cultured in southern China. This study elucidated the molecular mechanism underlying the interaction of TFV ORF104R with the VDAC2 protein to regulate cell apoptosis. TFV ORF104R is highly similar to other ranaviruses vBcl-2 and host Mcl-1 proteins, indicating that TFV ORF104R is a postulate vBcl-2 protein. Transcription and protein expression levels showed that TFV orf104r was a late viral gene. Western blot results suggested that TFV ORF104R was a viral structural protein. Subcellular localization analysis indicated that TFV ORF104R was predominantly colocalized with the mitochondria. Overexpressed TFV ORF104R could suppress the release of cytochrome C and the activities of caspase-9 and caspase-3. These results indicated that TFV ORF104R might play an important role in anti-apoptosis. Furthermore, the interaction between TFV ORF104R and VDAC2 was detected by co-immunoprecipitation in vitro. The above observations suggest that the molecular mechanism of TFV-regulated anti-apoptosis is through the interaction of TFV ORF104R with the VDAC2 protein. Our study provided a mechanistic basis for the ranaviruses vBcl-2-mediated inhibition of apoptosis and improved the understanding on how TFV subverts host defense mechanisms in vivo.

Interaction of Epstein-Barr virus genes with human gastric carcinoma transcriptome

Gastric carcinoma (GC) is a leading cause of mortality. 10% of GC cases are related with EBV (Epstein-Barr virus) infection. The detailed mechanistic roles EBV genes play and especially the interaction between the viral genes and human genes in GC remain unclear. In this study, raw fastq data from 285 GC samples were downloaded from TCGA (The Cancer Genome Atlas), including 25 EBV positive (EBV+) GC samples and 260 EBV negative (EBV−) GC samples. RNA-seq based expression data were generated for both human genes (among all the samples) and for the EBV genes (among the 25 EBV+ samples). Bioinformatics analyses were performed to identify differentially expressed (DEx) human genes and DEx KEGG pathways in EBV+ vs. EBV− samples and co-expressed human gene modules and hub genes among the DEx genes. Within the EBV+ samples, analyses were conducted to find correlation between EBV gene expression and the human gene expression modules, between EBV gene expression and the human hub genes, and between EBV gene expression and the DEx human pathways. EBV genes LMP-1, BALF1 and BALF2 were found to have significant correlation with human hub genes, CNTD2 and VANGL2. EBV genes BALF4 and BALF5 were found to correlate with human pathways, including Jak-STAT signaling and Phosphatidylinositol Signaling System. Our study has revealed the coordinated expression patterns between EBV and human GC transcriptome and identified several key EBV genes that may play an important role in EBV+ GC pathogenesis through their interactions with human genes and pathways.

Grouper iridovirus GIV66 is a Bcl-2 protein that inhibits apoptosis by exclusively sequestering Bim

Programmed cell death or apoptosis is a critical mechanism for the controlled removal of damaged or infected cells, and proteins of the Bcl-2 family are important arbiters of this process. Viruses have been shown to encode functional and structural homologs of Bcl-2 to counter premature host-cell apoptosis and ensure viral proliferation or survival. Grouper iridovirus (GIV) is a large DNA virus belonging to the Iridoviridae family and harbors GIV66, a putative Bcl-2-like protein and mitochondrially localized apoptosis inhibitor. However, the molecular and structural basis of GIV66-mediated apoptosis inhibition is currently not understood. To gain insight into GIV66's mechanism of action, we systematically evaluated its ability to bind peptides spanning the BH3 domain of pro-apoptotic Bcl-2 family members. Our results revealed that GIV66 harbors an unusually high level of specificity for pro-apoptotic Bcl-2 and displays affinity only for Bcl-2-like 11 (Bcl2L11 or Bim). Using crystal structures of both apo-GIV66 and GIV66 bound to the BH3 domain from Bim, we unexpectedly found that GIV66 forms dimers via an interface that results in occluded access to the canonical Bcl-2 ligand-binding groove, which breaks apart upon Bim binding. This observation suggests that GIV66 dimerization may affect GIV66's ability to bind host pro-death Bcl-2 proteins and enables highly targeted virus-directed suppression of host apoptosis signaling. Our findings provide a mechanistic understanding for the potent anti-apoptotic activity of GIV66 by identifying it as the first single-specificity, pro-survival Bcl-2 protein and identifying a pivotal role of Bim in GIV-mediated inhibition of apoptosis.

Mitochondrial Proteins Coded by Human Tumor Viruses

Viruses must exploit the cellular biosynthetic machinery and evade cellular defense systems to complete their life cycles. Due to their crucial roles in cellular bioenergetics, apoptosis, innate immunity and redox balance, mitochondria are important functional targets of many viruses, including tumor viruses. The present review describes the interactions between mitochondria and proteins coded by the human tumor viruses human T-cell leukemia virus type 1, Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, human hepatitis viruses B and C, and human papillomavirus, and highlights how these interactions contribute to viral replication, persistence and transformation.

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.

The Bcl-2 Family in Host-Virus Interactions

Members of the B cell lymphoma-2 (Bcl-2) family are pivotal arbiters of mitochondrially mediated apoptosis, a process of fundamental importance during tissue development, homeostasis, and disease. At the structural and mechanistic level, the mammalian members of the Bcl-2 family are increasingly well understood, with their interplay ultimately deciding the fate of a cell. Dysregulation of Bcl-2-mediated apoptosis underlies a plethora of diseases, and numerous viruses have acquired homologs of Bcl-2 to subvert host cell apoptosis and autophagy to prevent premature death of an infected cell. Here we review the structural biology, interactions, and mechanisms of action of virus-encoded Bcl-2 proteins, and how they impact on host-virus interactions to ultimately enable successful establishment and propagation of viral infections.

Characterization of the subcellular localization of Epstein-Barr virus encoded proteins in live cells

Epstein-Barr virus (EBV) is the pathogenic factor of numerous human tumors, yet certain of its encoded proteins have not been studied. As a first step for functional identification, we presented the construction of a library of expression constructs for most of the EBV encoded proteins and an explicit subcellular localization map of 81 proteins encoded by EBV in mammalian cells. Viral open reading frames were fused with enhanced yellow fluorescent protein (EYFP) tag in eukaryotic expression plasmid then expressed in COS-7 live cells, and protein localizations were observed by fluorescence microscopy. As results, 34.57% (28 proteins) of all proteins showed pan-nuclear or subnuclear localization, 39.51% (32 proteins) exhibitted pan-cytoplasmic or subcytoplasmic localization, and 25.93% (21 proteins) were found in both the nucleus and cytoplasm. Interestingly, most envelope proteins presented pan-cytoplasmic or membranous localization, and most capsid proteins displayed enriched or complete localization in the nucleus, indicating that the subcellular localization of specific proteins are associated with their roles during viral replication. Taken together, the subcellular localization map of EBV proteins in live cells may lay the foundation for further illustrating the functions of EBV-encoded genes in human diseases especially in its relevant tumors.

Knockout of Epstein-Barr virus BPLF1 retards B-cell transformation and lymphoma formation in humanized mice

BPLF1 of Epstein-Barr virus (EBV) is classified as a late lytic cycle protein but is also found in the viral tegument, suggesting its potential involvement at both initial and late stages of viral infection. BPLF1 possesses both deubiquitinating and deneddylating activity located in its N-terminal domain and is involved in processes that affect viral infectivity, viral DNA replication, DNA repair, and immune evasion. A recently constructed EBV BPLF1-knockout (KO) virus was used in conjunction with a humanized mouse model that can be infected with EBV, enabling the first characterization of BPLF1 function in vivo. Results demonstrate that the BPLF1-knockout virus is approximately 90% less infectious than wild-type (WT) virus. Transformation of human B cells, a hallmark of EBV infection, was delayed and reduced with BPLF1-knockout virus. Humanized mice infected with EBV BPLF1-knockout virus showed less weight loss and survived longer than mice infected with equivalent infectious units of WT virus. Additionally, splenic tumors formed in 100% of mice infected with WT EBV but in only 25% of mice infected with BPLF1-KO virus. Morphological features of spleens containing tumors were similar to those in EBV-induced posttransplant lymphoproliferative disease (PTLD) and were almost identical to cases seen in human diffuse large B-cell lymphoma. The presence of EBV genomes was detected in all mice that developed tumors. The results implicate BPLF1 in human B-cell transformation and tumor formation in humanized mice.

Epstein-Barr virus infects approximately 90% of the world’s population and is the causative agent of infectious mononucleosis. EBV also causes aggressive lymphomas in individuals with acquired and innate immune disorders and is strongly associated with diffuse large B-cell lymphomas, classical Hodgkin lymphoma, Burkitt lymphoma, and nasopharyngeal carcinoma (NPC). Typically, EBV initially infects epithelial cells in the oropharynx, followed by a lifelong persistent latent infection in B-cells, which may develop into lymphomas in immunocompromised individuals. This work is the first of its kind in evaluating the effects of EBV’s BPLF1 in terms of pathogenesis and lymphomagenesis in humanized mice and implicates BPLF1 in B-cell transformation and tumor development. Currently, there is no efficacious treatment for EBV, and therapeutic targeting of BPLF1 may lead to a new path to treatment for immunocompromised individuals or transplant recipients infected with EBV.

BH3 Profiling Reveals Selectivity by Herpesviruses for Specific Bcl-2 Proteins To Mediate Survival of Latently Infected Cells

Herpesviruses, including human cytomegalovirus (HCMV), Epstein-Barr virus (EBV), and Kaposi's sarcoma-associated herpesvirus, establish latency by modulating or mimicking antiapoptotic Bcl-2 proteins to promote survival of carrier cells. BH3 profiling, which assesses the contribution of Bcl-2 proteins towards cellular survival, was able to globally determine the level of dependence on individual cellular and viral Bcl-2 proteins within latently infected cells. Moreover, BH3 profiling predicted the sensitivity of infected cells to small-molecule inhibitors of Bcl-2 proteins.

Disruption of Bcl-2 and Bcl-xL by viral proteins as a possible cause of cancer

The Bcl proteins play a critical role in apoptosis, as mutations in family members interfere with normal programmed cell death. Such events can cause cell transformation, potentially leading to cancer. Recent discoveries indicate that some viral proteins interfere with Bcl proteins either directly or indirectly; however, these data have not been systematically described. Some viruses encode proteins that reprogramme host cellular signalling pathways controlling cell differentiation, proliferation, genomic integrity, cell death, and immune system recognition. This review analyses and summarises the existing data and discusses how viral proteins interfere with normal pro- and anti-apoptotic functions of Bcl-2 and Bcl-xL. Particularly, this article focuses on how viral proteins, such as Herpesviruses, HTLV-1, HPV and HCV, block apoptosis and how accumulation of such interference predisposes cancer development. Finally, we discuss possible ways to prevent and treat cancers using a combination of traditional therapies and antiviral preparations that are effective against these viruses.

Novel gammaherpesvirus functions encoded by bovine herpesvirus 6 (bovine lymphotropic virus)

The genus Macavirus of the subfamily Gammaherpesvirinae includes viruses that infect lymphoid cells of domestic and wild ruminants and swine, causing asymptomatic latent infections in reservoir hosts. Here, we describe the genome of bovine herpesvirus 6 (BoHV-6), a macavirus ubiquitous in healthy cattle populations. The BoHV-6 genome exhibited architecture conserved in macaviruses, including a repetitive H-DNA region and unique 141 kbp L-DNA region predicted to encode 77 genes. BoHV-6 encoded, in variable genomic regions, a novel complement of genes relative to other characterized macaviruses, probably contributing to distinctive aspects of BoHV-6 infection biology and host range. Most notably, BoHV-6 encoded the first herpesviral protein (Bov2.b2) similar to cellular ornithine decarboxylase, an enzyme that catalyses the first and rate-limiting step in the biosynthesis of polyamines. Bov2.b2 conceivably mediates a novel mechanism by which BoHV-6 promotes cell-cycle-dependent viral replication.

Viruses as modulators of mitochondrial functions

Mitochondria are multifunctional organelles with diverse roles including energy production and distribution, apoptosis, eliciting host immune response, and causing diseases and aging. Mitochondria-mediated immune responses might be an evolutionary adaptation by which mitochondria might have prevented the entry of invading microorganisms thus establishing them as an integral part of the cell. This makes them a target for all the invading pathogens including viruses. Viruses either induce or inhibit various mitochondrial processes in a highly specific manner so that they can replicate and produce progeny. Some viruses encode the Bcl2 homologues to counter the proapoptotic functions of the cellular and mitochondrial proteins. Others modulate the permeability transition pore and either prevent or induce the release of the apoptotic proteins from the mitochondria. Viruses like Herpes simplex virus 1 deplete the host mitochondrial DNA and some, like human immunodeficiency virus, hijack the host mitochondrial proteins to function fully inside the host cell. All these processes involve the participation of cellular proteins, mitochondrial proteins, and virus specific proteins. This review will summarize the strategies employed by viruses to utilize cellular mitochondria for successful multiplication and production of progeny virus.

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.

Role of the Crosstalk between Autophagy and Apoptosis in Cancer

Autophagy and apoptosis are catabolic pathways essential for organismal homeostasis. Autophagy is normally a cell-survival pathway involving the degradation and recycling of obsolete, damaged, or harmful macromolecular assemblies; however, excess autophagy has been implicated in type II cell death. Apoptosis is the canonical programmed cell death pathway. Autophagy and apoptosis have now been shown to be interconnected by several molecular nodes of crosstalk, enabling the coordinate regulation of degradation by these pathways. Normally, autophagy and apoptosis are both tumor suppressor pathways. Autophagy fulfils this role as it facilitates the degradation of oncogenic molecules, preventing development of cancers, while apoptosis prevents the survival of cancer cells. Consequently, defective or inadequate levels of either autophagy or apoptosis can lead to cancer. However, autophagy appears to have a dual role in cancer, as it has now been shown that autophagy also facilitates the survival of tumor cells in stress conditions such as hypoxic or low-nutrition environments. Here we review the multiple molecular mechanisms of coordination of autophagy and apoptosis and the role of the proteins involved in this crosstalk in cancer. A comprehensive understanding of the interconnectivity of autophagy and apoptosis is essential for the development of effective cancer therapeutics.

Concurrent targeting of eicosanoid receptor 1/eicosanoid receptor 4 receptors and COX-2 induces synergistic apoptosis in Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus associated non-Hodgkin lymphoma cell lines

The effective antitumorigenic potential of nonsteroidal anti-inflammatory drugs (NSAIDs) and eicosonoid (EP; EP1-4) receptor antagonists prompted us to test their efficacy in Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV) related lymphomas. Our study demonstrated that (1) EP1-4 receptor protein levels vary among the various non-Hodgkin's lymphoma (NHL) cell lines tested (BCBL-1:KSHV+/EBV-;BC-3: KSHV+/EBV-; Akata/EBV+: KSHV-/EBV+; and JSC-1 cells: KSHV+/EBV + cells); (2) 5.0 μM of EP1 antagonist (SC-51322) had a significant antiproliferative effect on BCBL-1, BC-3, Akata/EBV+, and JSC-1 cells; (3) 50.0 μM of EP2 antagonist (AH6809) was required to induce a significant antiproliferative effect on BCBL-1, Akata/EBV+, and JSC-1 cells; (4) 5.0 μM of EP4 antagonist (GW 627368X) had a significant antiproliferative effect on BC-3, Akata/EBV+, and JSC-1 cells; (5) COX-2 selective inhibitor celecoxib (5.0 μM) had significant antiproliferative effects on BCBL-1, BC-3, Akata/EBV+, and JSC-1 cells; and (6) a combination of 1.0 μM each of celecoxib, SC-51322 and GW 627368X could potentiate the proapoptotic properties of celecoxib or vice-versa. Overall, our studies identified the synergistic antiproliferative effect of NSAIDs and EP receptor blockers on KSHV and EBV related B cell malignancies.

Multiple functions of BCL-2 family proteins

BCL-2 family proteins are the regulators of apoptosis, but also have other functions. This family of interacting partners includes inhibitors and inducers of cell death. Together they regulate and mediate the process by which mitochondria contribute to cell death known as the intrinsic apoptosis pathway. This pathway is required for normal embryonic development and for preventing cancer. However, before apoptosis is induced, BCL-2 proteins have critical roles in normal cell physiology related to neuronal activity, autophagy, calcium handling, mitochondrial dynamics and energetics, and other processes of normal healthy cells. The relative importance of these physiological functions compared to their apoptosis functions in overall organismal physiology is difficult to decipher. Apoptotic and noncanonical functions of these proteins may be intertwined to link cell growth to cell death. Disentanglement of these functions may require delineation of biochemical activities inherent to the characteristic three-dimensional shape shared by distantly related viral and cellular BCL-2 family members.

Multipolar functions of BCL-2 proteins link energetics to apoptosis

Classical apoptotic cell death is now sufficiently well understood to be interrogated with mathematical modeling and manipulated with targeted drugs for clinical benefit. However, a biological black hole has emerged with the realization that apoptosis regulators are functionally multipolar. BCL-2 family proteins appear to have much greater effects on cells than can be explained by their known roles in apoptosis. Although these effects may be observable simply because the cell is not dead, the general assumption is that BCL-2 proteins have undiscovered biochemical activities. Conversely, these as yet uncharacterized day-jobs also may underlie their profound effects on cell survival, challenging current assumptions about classical apoptosis. Even their sub-mitochondrial localizations remain controversial. Here we attempt to integrate seemingly conflicting information with the prospect that BCL-2 proteins themselves may be the critical crosstalk between life and death.

A role for Epstein-Barr viral BALF1 in facilitating tumor formation and metastasis potential

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that triggers transformation and tumorigenesis of latently infected B cells in vitro. BALF1, a Bcl-2-like EBV gene expressed in both latent and lytic stages, was recently characterized in EBV-infected cells; however, the role and function of BALF1 has remained elusive. Here, we demonstrate that BALF1 expression alters cellular morphology. Importantly, BALF1 promotes cellular transformation, with tumorigenicity assays showing larger and substantially greater numbers of tumors in BALF1 transfectant-injected mice compared to mice injected with pcDNA control transfectants. In addition, BALF1 expression increases cell survival under low-serum conditions, an effect that is attributable to suppression of apoptosis, not to promotion of cell-cycle progression. Furthermore, BALF1 transfectants exhibit markedly increased tumor metastasis in vitro and in vivo. Taken together, these findings suggest that BALF1 may be a new tumor marker for EBV diagnosis and provide a new direction for research on treatments of EBV-associated tumors.

Bcl-xL regulates mitochondrial energetics by stabilizing the inner membrane potential

To promote cell survival, the antiapoptotic factor Bcl-x_L both inhibits Bax-induced mitochondrial outer membrane permeabilization and stabilizes mitochondrial inner membrane ion flux and thus overall mitochondrial energetic capacity.

Mammalian Bcl-x_L protein localizes to the outer mitochondrial membrane, where it inhibits apoptosis by binding Bax and inhibiting Bax-induced outer membrane permeabilization. Contrary to expectation, we found by electron microscopy and biochemical approaches that endogenous Bcl-x_L also localized to inner mitochondrial cristae. Two-photon microscopy of cultured neurons revealed large fluctuations in inner mitochondrial membrane potential when Bcl-x_L was genetically deleted or pharmacologically inhibited, indicating increased total ion flux into and out of mitochondria. Computational, biochemical, and genetic evidence indicated that Bcl-x_L reduces futile ion flux across the inner mitochondrial membrane to prevent a wasteful drain on cellular resources, thereby preventing an energetic crisis during stress. Given that F₁F_O–ATP synthase directly affects mitochondrial membrane potential and having identified the mitochondrial ATP synthase β subunit in a screen for Bcl-x_L–binding partners, we tested and found that Bcl-x_L failed to protect β subunit–deficient yeast. Thus, by bolstering mitochondrial energetic capacity, Bcl-x_L may contribute importantly to cell survival independently of other Bcl-2 family proteins.

Cell death-independent functions of granzymes: hit viruses where it hurts

Granule exocytosis by cytotoxic lymphocytes is the key mechanism of our immune response to eliminate virus-infected cells. These lytic granules contain the pore-forming protein perforin and a set of five serine proteases called granzymes (GrA, GrB, GrH, GrK, GrM) that display distinct substrate specificities. Granzymes have mostly been studied for their ability to induce cell death. However, viruses have evolved many inhibitors to effectively block apoptosis. Evidence is emerging that granzymes also use noncytotoxic strategies to inhibit viral replication and potential viral reactivation from latency. Granzymes directly cleave viral or host cell proteins that are required in the viral life cycle. Furthermore, granzymes induce a pro-inflammatory cytokine response to create an antiviral environment. In this review, we summarize and discuss these novel strategies by which the immune system counteracts viral infections, and we will address the potential therapeutic applications that could emerge from this intriguing mechanism.

Contribution of Epstein-Barr virus infection to chemoresistance of gastric carcinoma cells to 5-fluorouracil

Although Epstein-Barr virus (EBV) is associated with 6-16% of the gastric carcinoma (GC) cases, the effect of EBV infection on the tumorigenesis process and the responsiveness to chemotherapy remain unclear. We compared chemosensitivity of the EBV-positive GC (AGSEBV) and EBV-negative GC (AGS) cells to 5-fluorouracil (5-FU). Although 5-FU inhibited the growth of both cell lines in a dose- and time-dependent manner, the sensitivity of EBV-positive GC cells to 5-FU was lower than that of EBV-negative GC cells. The cleavage of PARP and caspase-3 was also lower in AGS-EBV cells than in AGS cells following 5-FU treatment. Both the level of Bcl-2 expression and the ratio of Bcl-2/Bax were higher in AGS-EBV than in AGS cells not only at basal state but also following 5-FU treatment. Moreover, p53 and p21 expression was enhanced further by 5-FU in AGS than in AGS-EBV cells. Immunofluorescence assay and Western blot showed that 5-FU induced the expression of EBV-lytic genes including BZLF1, BRLF1, BMRF1 and BHRF1. Our results suggest that latent and lytic EBV infection contributes to the chemoresistance to 5-FU in gastric carcinoma by modulating apoptosis related cellular genes.

Interferon regulatory factor 4 (IRF-4) targets IRF-5 to regulate Epstein-Barr virus transformation

The cellular interferon regulatory factor-4 (IRF-4), which is a member of IRF family, is involved in the development of multiple myeloma and Epstein-Barr virus (EBV)-mediated transformation of B lymphocytes. However, the molecular mechanism of IRF-4 in cellular transformation is unknown. We have found that knockdown of IRF-4 leads to high expression of IRF-5, a pro-apoptotic member in the IRF family. Overexpression of IRF-4 represses IRF-5 expression. Reduction of IRF-4 leads to growth inhibition, and the restoration of IRF-4 by exogenous plasmids correlates with the growth recovery and reduces IRF-5 expression. In addition, IRF-4 negatively regulates IRF-5 promoter reporter activities and binds to IRF-5 promoters in vivo and in vitro. Knockdown of IRF-5 rescues IRF-4 knockdown-mediated growth inhibition, and IRF-5 overexpression alone is sufficient to induce cellular growth inhibition of EBV-transformed cells. Therefore, IRF-5 is one of the targets of IRF-4, and IRF-4 regulates the growth of EBV-transformed cells partially through IRF-5. This work provides insight on how IRFs interact with one another to participate in viral pathogenesis and transformation.

Tegument protein control of latent herpesvirus establishment and animation

Herpesviruses are successful pathogens that infect most vertebrates as well as at least one invertebrate species. Six of the eight human herpesviruses are widely distributed in the population. Herpesviral infections persist for the life of the infected host due in large part to the ability of these viruses to enter a non-productive, latent state in which viral gene expression is limited and immune detection and clearance is avoided. Periodically, the virus will reactivate and enter the lytic cycle, producing progeny virus that can spread within or to new hosts. Latency has been classically divided into establishment, maintenance, and reactivation phases. Here we focus on demonstrated and postulated molecular mechanisms leading to the establishment of latency for representative members of each human herpesvirus family. Maintenance and reactivation are also briefly discussed. In particular, the roles that tegument proteins may play during latency are highlighted. Finally, we introduce the term animation to describe the initiation of lytic phase gene expression from a latent herpesvirus genome, and discuss why this step should be separated, both molecularly and theoretically, from reactivation.

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).

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.

Noncanonical functions of BCL-2 proteins in the nervous system

BCL-2 family proteins form heterodimers or homo-oligomers to inhibit or induce apoptotic cell death, respectively. They often relocalize from the cytoplasm to mitochondria to carry out these functions. The traditional model is that in healthy cells, anti-death family members hold pro-death BCL-2 family members in check. Upon receiving a death stimulus, another set of proteins (BH3-only proteins) inactivate the protective BCL-2 proteins, forcing them to release their pro-death partners that are subsequently triggered to oligomerize and porate the mitochondrial outer membrane leading to cell death. In support of this traditional view, there is a preponderance of supporting evidence derived from the study of events that occur following treatment of cells with a death stimulus. Knockout and mutant mice also exhibit many developmental and treatment-induced phenotypes consistent with this model of antagonism between BCL-2 family proteins. Emphasis is logically placed on those phenotypes that support the model. However, this working model of BCL-2 family interactions has become so engrained that alternative, potentially valid interpretations are sometimes dismissed. Therefore, it is useful to consider the evidence that seems contrary to accepted models. In particular, the analysis of BCL-2 family functions in the nervous system has revealed unexpected outcomes that can serve to further stimulate critical probing of the yet unknown biochemical functions of BCL-2 proteins.

GLTSCR2/PICT-1, a putative tumor suppressor gene product, induces the nucleolar targeting of the Kaposi's sarcoma-associated herpesvirus KS-Bcl-2 protein

KS-Bcl-2, encoded by Kaposi's sarcoma-associated herpesvirus (KSHV), is a structural and functional homologue of the Bcl-2 family of apoptosis regulators. Like several other Bcl-2 family members, KS-Bcl-2 protects cells from apoptosis and autophagy. Using a yeast two-hybrid screen and coimmunoprecipitation assays, we identified a novel KS-Bcl-2-interacting protein, referred to as protein interacting with carboxyl terminus 1 (PICT-1), encoded by a candidate tumor suppressor gene, GLTSCR2. Confocal laser scanning microscopy revealed nucleolar localization of PICT-1, whereas KS-Bcl-2 was located mostly at the mitochondrial membranes with a small fraction in the nucleoli. Ectopic expression of PICT-1 resulted in a large increase in the nucleolar fraction of KS-Bcl-2, and only a minor fraction remained in the cytoplasm. Furthermore, knockdown of endogenous PICT-1 abolished the nucleolar localization of KS-Bcl-2. However, ectopically expressed PICT-1 did not alter the cellular distribution of human Bcl-2. Subsequent analysis mapped the crucial amino acid sequences of both KS-Bcl-2 and PICT-1 required for their interaction and for KS-Bcl-2 targeting to the nucleolus. Functional studies suggest a correlation between nucleolar targeting of KS-Bcl-2 by PICT-1 and reduction of the antiapoptotic activity of KS-Bcl-2. Thus, these studies demonstrate a cellular mechanism to sequester KS-Bcl-2 from the mitochondria and to downregulate its virally encoded antiapoptotic activity. Additional characterization of the interaction of KS-Bcl-2 and PICT-1 is likely to shed light on the functions of both proteins.

Positive selection on apoptosis related genes

Apoptosis is a form of programmed cell death crucial for development, homeostasis, immunity, spermatogenesis, and prevention of cancer. Positive selection acting on mammalian apoptosis related genes targets protein interfaces that interact with pathogens and also elements of signaling complexes. Selection appears primarily to be driven by the immune/defense related function of these genes. Moreover, competitive interactions could be driving positive selection among sperm cells, as well as the need for protection against female anti-sperm immune responses. Trade-offs in fitness are expected out of these selective pressures, which could explain the involvement of these genes in various diseases, including cancer.

How does Epstein-Barr virus (EBV) complement the activation of Myc in the pathogenesis of Burkitt's lymphoma?

A defining characteristic of the aggressive B cell tumour Burkitt's lymphoma (BL) is a reciprocal chromosomal translocation that activates the Myc oncogene by juxtaposing it to one of the immunoglobulin gene loci. The consequences of activating Myc include cell growth and proliferation that can lead to lymphomagenesis; however, as part of a fail-safe mechanism that has evolved in metazoans to reduce the likelihood of neoplastic disease, activated oncogenes such as Myc may also induce cell death by apoptosis and/or an irreversible block to proliferation called senescence. For lymphoma to develop it is necessary that these latter processes are repressed. More than 95% of a subset of BL – known as endemic (e)BL because they are largely restricted to regions of equatorial Africa and similar geographical regions – carry latent Epstein–Barr virus (EBV) in the form of nuclear extra-chromosomal episomes. Although EBV is not generally regarded as a driving force of BL cell proliferation, it plays an important role in the pathogenesis of eBL. Latency-associated EBV gene products can inhibit a variety of pathways that lead to apoptosis and senescence; therefore EBV probably counteracts the proliferation-restricting activities of deregulated Myc and so facilitates the development of BL.

Molecular basis of the regulation of Beclin 1-dependent autophagy by the gamma-herpesvirus 68 Bcl-2 homolog M11

Gamma-herpesviruses (gammaHVs), including important human pathogens such as Epstein Barr virus, Kaposi's sarcoma-associated HV, and the murine gammaHV68, encode homologs of the antiapoptotic, cellular Bcl-2 (cBcl-2) to promote viral replication and pathogenesis. The precise molecular details by which these proteins function in viral infection are poorly understood. Autophagy, a lysosomal degradation pathway, is inhibited by the interaction of cBcl-2s with a key autophagy effector, Beclin 1, and can also be inhibited by gammaHV Bcl-2s. Here we investigate the gammaHV68 M11-Beclin 1 interaction in atomic detail, using biochemical and structural approaches. We show that the Beclin 1 BH3 domain is the primary determinant of binding to M11 and other Bcl-2s, and this domain binds in a hydrophobic groove on M11, reminiscent of the binding of different BH3 domains to other Bcl-2s. Unexpectedly, regions outside of, but contiguous with, the Beclin 1 BH3 domain also contribute to this interaction. We find that M11 binds to Beclin 1 more strongly than do KSHV Bcl-2 or cBcl-2. Further, the differential affinity of M11 for different BH3 domains is caused by subtle, yet significant, variations in the atomic details of each interaction. Consistent with our structural analysis, we find that Beclin 1 residues L116 and F123, and M11 residue pairs G86 + R87 and Y60 + L74, are required for M11 to bind to Beclin 1 and downregulate autophagy. Thus, our results suggest that M11 inhibits autophagy through a mechanism that involves the binding of the Beclin 1 BH3 domain in the M11 hydrophobic surface groove.

Viral control of mitochondrial apoptosis

Throughout the process of pathogen–host co-evolution, viruses have developed a battery of distinct strategies to overcome biochemical and immunological defenses of the host. Thus, viruses have acquired the capacity to subvert host cell apoptosis, control inflammatory responses, and evade immune reactions. Since the elimination of infected cells via programmed cell death is one of the most ancestral defense mechanisms against infection, disabling host cell apoptosis might represent an almost obligate step in the viral life cycle. Conversely, viruses may take advantage of stimulating apoptosis, either to kill uninfected cells from the immune system, or to induce the breakdown of infected cells, thereby favoring viral dissemination. Several viral polypeptides are homologs of host-derived apoptosis-regulatory proteins, such as members of the Bcl-2 family. Moreover, viral factors with no homology to host proteins specifically target key components of the apoptotic machinery. Here, we summarize the current knowledge on the viral modulation of mitochondrial apoptosis, by focusing in particular on the mechanisms by which viral proteins control the host cell death apparatus.