Epstein-Barr virus EBNA2 blocks Nur77- mediated apoptosis

Application of ATAC-seq in tumor-specific T cell exhaustion

Researches show that chronic viral infection and persistent antigen and/or inflammatory signal exposure in cancer causes the functional status of T cells to be altered, mainly by major changes in the epigenetic and metabolic environment, which then leads to T cell exhaustion. The discovery of the immune checkpoint pathway is an important milestone in understanding and reversing T cell exhaustion. Antibodies targeting these pathways have shown superior ability to reverse T cell exhaustion. However, there are still some limitations in immune checkpoint blocking therapy, such as the short-term nature of therapeutic effects and high individual heterogeneity. Assay for transposase-accessible chromatin with sequencing(ATAC-seq) is a method used to analyze the accessibility of whole-genome chromatin. It uses hyperactive Tn5 transposase to assess chromatin accessibility. Recently, a growing number of studies have reported that ATAC-seq can be used to characterize the dynamic changes of epigenetics in the process of T cell exhaustion. It has been determined that immune checkpoint blocking can only temporarily restore the function of exhausted T cells because of an irreversible change in the epigenetics of exhausted T cells. In this study, we review the latest developments, which provide a clearer molecular understanding of T cell exhaustion, reveal potential new therapeutic targets for persistent viral infection and cancer, and provide new insights for designing effective immunotherapy for treating cancer and chronic infection.

Histone demethylase JMJD3 downregulation protects against aberrant force-induced osteoarthritis through epigenetic control of NR4A1

Osteoarthritis (OA) is a prevalent joint disease with no effective treatment strategies. Aberrant mechanical stimuli was demonstrated to be an essential factor for OA pathogenesis. Although multiple studies have detected potential regulatory mechanisms underlying OA and have concentrated on developing novel treatment strategies, the epigenetic control of OA remains unclear. Histone demethylase JMJD3 has been reported to mediate multiple physiological and pathological processes, including cell differentiation, proliferation, autophagy, and apoptosis. However, the regulation of JMJD3 in aberrant force-related OA and its mediatory effect on disease progression are still unknown. In this work, we confirmed the upregulation of JMJD3 in aberrant force-induced cartilage injury in vitro and in vivo. Functionally, inhibition of JMJD3 by its inhibitor, GSK-J4, or downregulation of JMJD3 by adenovirus infection of sh-JMJD3 could alleviate the aberrant force-induced chondrocyte injury. Mechanistic investigation illustrated that aberrant force induces JMJD3 expression and then demethylates H3K27me3 at the NR4A1 promoter to promote its expression. Further experiments indicated that NR4A1 can regulate chondrocyte apoptosis, cartilage degeneration, extracellular matrix degradation, and inflammatory responses. In vivo, anterior cruciate ligament transection (ACLT) was performed to construct an OA model, and the therapeutic effect of GSK-J4 was validated. More importantly, we adopted a peptide-siRNA nanoplatform to deliver si-JMJD3 into articular cartilage, and the severity of joint degeneration was remarkably mitigated. Taken together, our findings demonstrated that JMJD3 is flow-responsive and epigenetically regulates OA progression. Our work provides evidences for JMJD3 inhibition as an innovative epigenetic therapy approach for joint diseases by utilizing p5RHH-siRNA nanocomplexes.

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.

Epstein-Barr virus nuclear antigen 2 extensively rewires the human chromatin landscape at autoimmune risk loci

The interplay between environmental and genetic factors plays a key role in the development of many autoimmune diseases. In particular, the Epstein-Barr virus (EBV) is an established contributor to multiple sclerosis, lupus, and other disorders. Previously, we showed that the EBV nuclear antigen 2 (EBNA2) transactivating protein occupies up to half of the risk loci for a set of seven autoimmune disorders. To further examine the mechanistic roles played by EBNA2 at these loci on a genome-wide scale, we globally examined gene expression, chromatin accessibility, chromatin looping, and EBNA2 binding in a B cell line that was (1) uninfected, (2) infected with a strain of EBV lacking EBNA2, or (3) infected with a strain that expresses EBNA2. We identified more than 400 EBNA2-dependent differentially expressed human genes and more than 5000 EBNA2 binding events in the human genome. ATAC-seq analysis revealed more than 2000 regions in the human genome with EBNA2-dependent chromatin accessibility, and HiChIP data revealed more than 1700 regions where EBNA2 altered chromatin looping interactions. Autoimmune genetic risk loci were highly enriched at the sites of these EBNA2-dependent chromatin-altering events. We present examples of autoimmune risk genotype-dependent EBNA2 events, nominating genetic risk mechanisms for autoimmune risk loci such as ZMIZ1 Taken together, our results reveal important interactions between host genetic variation and EBNA2-driven disease mechanisms. Further, our study highlights a critical role for EBNA2 in rewiring human gene regulatory programs through rearrangement of the chromatin landscape and nominates these interactions as components of genetic mechanisms that influence the risk of multiple autoimmune diseases.

Targeting Nuclear NOTCH2 by Gliotoxin Recovers a Tumor-Suppressor NOTCH3 Activity in CLL

NOTCH signaling represents a promising therapeutic target in chronic lymphocytic leukemia (CLL). We compared the anti-neoplastic effects of the nuclear NOTCH2 inhibitor gliotoxin and the pan-NOTCH γ-secretase inhibitor RO4929097 in primary CLL cells with special emphasis on the individual roles of the different NOTCH receptors. Gliotoxin rapidly induced apoptosis in all CLL cases tested, whereas RO4929097 exerted a variable and delayed effect on CLL cell viability. Gliotoxin-induced apoptosis was associated with inhibition of the NOTCH2/FCER2 (CD23) axis together with concomitant upregulation of the NOTCH3/NR4A1 axis. In contrast, RO4929097 downregulated the NOTCH3/NR4A1 axis and counteracted the spontaneous and gliotoxin-induced apoptosis. On the cell surface, NOTCH3 and CD23 expression were mutually exclusive, suggesting that downregulation of NOTCH2 signaling is a prerequisite for NOTCH3 expression in CLL cells. ATAC-seq confirmed that gliotoxin targeted the canonical NOTCH signaling, as indicated by the loss of chromatin accessibility at the potential NOTCH/CSL site containing the gene regulatory elements. This was accompanied by a gain in accessibility at the NR4A1, NFκB, and ATF3 motifs close to the genes involved in B-cell activation, differentiation, and apoptosis. In summary, these data show that gliotoxin recovers a non-canonical tumor-suppressing NOTCH3 activity, indicating that nuclear NOTCH2 inhibitors might be beneficial compared to pan-NOTCH inhibitors in the treatment of CLL.

Mechanical Stretching induces the apoptosis of parametrial ligament Fibroblasts via the Actin Cytoskeleton/Nr4a1 signalling pathway

The anatomical positions of pelvic floor organs are maintained mainly by ligaments and muscles. Long-term excessive mechanical tension stimulation of pelvic floor tissue beyond the endurance of ligaments or muscles will lead to the occurrence of pelvic organ prolapse (POP). In addition, cytoskeletal reconstitution is a key process by which cells respond to mechanical stimulation. The aim of the present study was to investigate the protective effect of actin cytoskeleton to resist mechanical stretching (MS)-induced apoptosis in parametrial ligament fibroblasts (PLFs) and the underlying mechanisms. MS provided by a four‑point bending device could significantly induce apoptosis of PLFs from non-POP patients, which exhibited an apoptosis rate close to that of PLFs from POP patients, and the apoptosis rate was higher following latrunculin A (Lat-A, a potent inhibitor of actin) treatment. In addition, Nr4a1 and Bax expression was increased while Bcl-2 and caspase-3 expression was clearly decreased after treatment with MS and Lat-A. However, the apoptosis induced by MS was reduced when the expression of Nr4a1 was downregulated by siRNA. These outcomes reveal a novel mechanism that links the actin cytoskeleton and apoptosis in PLFs by Nr4a1; this mechanism will provide insight into the clinical diagnosis and treatment of POP.

Epstein-Barr Virus and Innate Immunity: Friends or Foes?

Epstein–Barr virus (EBV) successfully persists in the vast majority of adults but causes lymphoid and epithelial malignancies in a small fraction of latently infected individuals. Innate immunity is the first-line antiviral defense, which EBV has to evade in favor of its own replication and infection. EBV uses multiple strategies to perturb innate immune signaling pathways activated by Toll-like, RIG-I-like, NOD-like, and AIM2-like receptors as well as cyclic GMP-AMP synthase. EBV also counteracts interferon production and signaling, including TBK1-IRF3 and JAK-STAT pathways. However, activation of innate immunity also triggers pro-inflammatory response and proteolytic cleavage of caspases, both of which exhibit proviral activity under some circumstances. Pathogenic inflammation also contributes to EBV oncogenesis. EBV activates NFκB signaling and induces pro-inflammatory cytokines. Through differential modulation of the proviral and antiviral roles of caspases and other host factors at different stages of infection, EBV usurps cellular programs for death and inflammation to its own benefits. The outcome of EBV infection is governed by a delicate interplay between innate immunity and EBV. A better understanding of this interplay will instruct prevention and intervention of EBV-associated cancers.

Cytoplasmic location of NR4A1 in aggressive lymphomas is associated with a favourable cancer specific survival

The nuclear orphan receptor NR4A1 functions as tumour suppressor in aggressive lymphomas by pro-apoptotic genomic and non-genomic effects. Here, we immunohistochemically studied the clinico-pathological relevance of NR4A1 protein expression patterns in a cohort of 60 diffuse large B cell lymphoma (DLBCL) patients and non-neoplastic lymph nodes. We observed a significant association between high cytoplasmic NR4A1 and favourable cancer-specific survival and the germinal centre B cell-like subtype, respectively. Moreover, the percentage of lymphoma cells exhibiting cytoplasmic NR4A1 significantly correlated to those showing cleaved caspase 3. Complementary, functional profiling using gene set enrichment of Reactome pathways based on publicly available microarray data was applied to determine pathways potentially implicated in cytoplasmic localization of NR4A1 and validated by means of semi quantitative real-time PCR. The pathway analysis revealed changes in the ERK1/2 pathway, and this was corroborated by the finding that high cytoplasmic NR4A1 was associated with higher expression of ERK1/2 targets in our cohort. These data indicate that high cytoplasmic NR4A1 is associated with a favourable lymphoma-specific survival and highlights the importance of NR4A1 expression patterns as potential prognostic marker for risk assessment in aggressive lymphomas.

Epstein-Barr Virus-Associated Malignancies: Roles of Viral Oncoproteins in Carcinogenesis

The Epstein–Barr virus (EBV) is the first herpesvirus identified to be associated with human cancers known to infect the majority of the world population. EBV-associated malignancies are associated with a latent form of infection, and several of the EBV-encoded latent proteins are known to mediate cellular transformation. These include six nuclear antigens and three latent membrane proteins (LMPs). In lymphoid and epithelial tumors, viral latent gene expressions have distinct pattern. In both primary and metastatic tumors, the constant expression of latent membrane protein 2A (LMP2A) at the RNA level suggests that this protein is the key player in the EBV-associated tumorigenesis. While LMP2A contributing to the malignant transformation possibly by cooperating with the aberrant host genome. This can be done in part by dysregulating signaling pathways at multiple points, notably in the cell cycle and apoptotic pathways. Recent studies also have confirmed that LMP1 and LMP2 contribute to carcinoma progression and that this may reflect the combined effects of these proteins on activation of multiple signaling pathways. This review article aims to investigate the aforementioned EBV-encoded proteins that reveal established roles in tumor formation, with a greater emphasis on the oncogenic LMPs (LMP1 and LMP2A) and their roles in dysregulating signaling pathways. It also aims to provide a quick look on the six members of the EBV nuclear antigens and their roles in dysregulating apoptosis.

Species-specific functions of Epstein-Barr virus nuclear antigen 2 (EBNA2) reveal dual roles for initiation and maintenance of B cell immortalization

Epstein-Barr virus (EBV) and related lymphocryptoviruses (LCV) from non-human primates infect B cells, transform their growth to facilitate life-long viral persistence in the host, and contribute to B cell oncogenesis. Co-evolution of LCV with their primate hosts has led to species-specificity so that LCVs preferentially immortalize B cells from their natural host in vitro. We investigated whether the master regulator of transcription, EBV nuclear antigen 2 (EBNA2), is involved in LCV species-specificity. Using recombinant EBVs, we show that EBNA2 orthologues of LCV isolated from chimpanzees, baboons, cynomolgus or rhesus macaques cannot replace EBV EBNA2 for the immortalization of human B cells. Thus, LCV species-specificity is functionally linked to viral proteins expressed during latent, growth-transforming infection. In addition, we identified three independent domains within EBNA2 that act through species-specific mechanisms. Importantly, the EBNA2 orthologues and species-specific EBNA2 domains separate unique roles for EBNA2 in the initiation of B cell immortalization from those responsible for maintaining the immortalized state. Investigating LCV species-specificity provides a novel approach to identify critical steps underlying EBV-induced B cell growth transformation, persistent infection, and oncogenesis.

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.

c-Myc dysregulation is a co-transforming event for nuclear factor-κB activated B cells

While c-Myc dysregulation is constantly associated with highly proliferating B-cell tumors, nuclear factor (NF)-κB addiction is found in indolent lymphomas as well as diffuse large B-cell lymphomas, either with an activated B-cell like phenotype or associated with the Epstein-Barr virus. We raised the question of the effect of c-Myc in B cells with NF-κB activated by three different inducers: Epstein-Barr virus-latency III program, TLR9 and CD40. Induction of c-Myc overexpression increased proliferation of Epstein-Barr virus-latency III immortalized B cells, an effect that was dependent on NF-κB. Results from transcriptomic signatures and functional studies showed that c-Myc overexpression increased Epstein-Barr virus-latency III-driven proliferation depending on NF-κB. In vitro, induction of c-Myc increased proliferation of B cells with TLR9-dependant activation of MyD88, with decreased apoptosis. In the transgenic λc-Myc mouse model with c-Myc overexpression in B cells, in vivo activation of MyD88 by TLR9 induced splenomegaly related to an increased synthesis phase (S-phase) entry of B cells. Transgenic mice with both continuous CD40 signaling in B cells and the λc-Myc transgene developed very aggressive lymphomas with characteristics of activated diffuse large B-cell lymphomas. The main characteristic gene expression profile signatures of these tumors were those of proliferation and energetic metabolism. These results suggest that c-Myc is an NF-κB co-transforming event in aggressive lymphomas with an activated phenotype, activated B-cell like diffuse large B-cell lymphomas. This would explain why NF-κB is associated with both indolent and aggressive lymphomas, and opens new perspectives on the possibility of combinatory therapies targeting both the c-Myc proliferating program and NF-κB activation pathways in diffuse large B-cell lymphomas.

The Modulation of Apoptotic Pathways by Gammaherpesviruses

Apoptosis or programmed cell death is a tightly regulated process fundamental for cellular development and elimination of damaged or infected cells during the maintenance of cellular homeostasis. It is also an important cellular defense mechanism against viral invasion. In many instances, abnormal regulation of apoptosis has been associated with a number of diseases, including cancer development. Following infection of host cells, persistent and oncogenic viruses such as the members of the Gammaherpesvirus family employ a number of different mechanisms to avoid the host cell’s “burglar” alarm and to alter the extrinsic and intrinsic apoptotic pathways by either deregulating the expressions of cellular signaling genes or by encoding the viral homologs of cellular genes. In this review, we summarize the recent findings on how gammaherpesviruses inhibit cellular apoptosis via virus-encoded proteins by mediating modification of numerous signal transduction pathways. We also list the key viral anti-apoptotic proteins that could be exploited as effective targets for novel antiviral therapies in order to stimulate apoptosis in different types of cancer cells.

Epstein-Barr virus genetic variants are associated with multiple sclerosis

OBJECTIVE

We analyzed the Epstein-Barr nuclear antigen 2 (EBNA2) gene, which contains the most variable region of the viral genome, in persons with multiple sclerosis (MS) and control subjects to verify whether virus genetic variants are involved in disease development.

METHODS

A seminested PCR approach and Sanger sequencing were used to analyze EBNA2 in 53 patients and 38 matched healthy donors (HDs). High-throughput sequencing by Illumina MiSeq was also applied in a subgroup of donors (17 patients and 17 HDs). Patients underwent gadolinium-enhanced MRI and human leucocyte antigen typing.

RESULTS

MS risk significantly correlated with an excess of 1.2 allele (odds ratio [OR] = 5.13; 95% confidence interval [CI] 1.84-14.32; p = 0.016) and underrepresentation of 1.3B allele (OR = 0.23; 95% CI 0.08-0.51; p = 0.0006). We identified new genetic variants, mostly 1.2 allele- and MS-associated (especially amino acid variation at position 245; OR = 9.4; 95% CI 1.19-78.72; p = 0.0123). In all cases, the consensus sequence from deep sequencing confirmed Sanger sequencing (including the cosegregation of newly identified variants with known EBNA2 alleles) and showed that the extent of genotype intraindividual variability was higher than expected: rare EBNA2 variants were detected in all HDs and patients with MS (range 1-17 and 3-19, respectively). EBNA2 variants did not seem to correlate with human leucocyte antigen typing or clinical/MRI features.

CONCLUSIONS

Our study unveils a strong association between Epstein-Barr virus genomic variants and MS, reinforcing the idea that Epstein-Barr virus contributes to disease development.

EBNA2 and Its Coactivator EBNA-LP

While all herpesviruses can switch between lytic and latent life cycle, which are both driven by specific transcription programs, a unique feature of latent EBV infection is the expression of several distinct and well-defined viral latent transcription programs called latency I, II, and III. Growth transformation of B-cells by EBV in vitro is based on the concerted action of Epstein-Barr virus nuclear antigens (EBNAs) and latent membrane proteins(LMPs). EBV growth-transformed B-cells express a viral transcriptional program, termed latency III, which is characterized by the coexpression of EBNA2 and EBNA-LP with EBNA1, EBNA3A, -3B, and -3C as well as LMP1, LMP2A, and LMP2B. The focus of this review will be to discuss the current understanding of how two of these proteins, EBNA2 and EBNA-LP, contribute to EBV-mediated B-cell growth transformation.

Dynamic Epstein-Barr virus gene expression on the path to B-cell transformation

Epstein-Barr virus (EBV) is an oncogenic human herpesvirus in the γ-herpesvirinae subfamily that contains a 170-180kb double-stranded DNA genome. In vivo, EBV commonly infects B and epithelial cells and persists for the life of the host in a latent state in the memory B-cell compartment of the peripheral blood. EBV can be reactivated from its latent state, leading to increased expression of lytic genes that primarily encode for enzymes necessary to replicate the viral genome and structural components of the virion. Lytic cycle proteins also aid in immune evasion, inhibition of apoptosis, and the modulation of other host responses to infection. In vitro, EBV has the potential to infect primary human B cells and induce cellular proliferation to yield effectively immortalized lymphoblastoid cell lines, or LCLs. EBV immortalization of B cells in vitro serves as a model system for studying EBV-mediated lymphomagenesis. While much is known about the steady-state viral gene expression within EBV-immortalized LCLs and other EBV-positive cell lines, relatively little is known about the early events after primary B-cell infection. It was previously thought that upon latent infection, EBV only expressed the well-characterized latency-associated transcripts found in LCLs. However, recent work has characterized the early, but transient, expression of lytic genes necessary for efficient transformation and delayed responses in the known latency genes. This chapter summarizes these recent findings that show how dynamic and controlled expression of multiple EBV genes can control the activation of B cells, entry into the cell cycle, the inhibition of apoptosis, and innate and adaptive immune responses.

Repression of the proapoptotic cellular BIK/NBK gene by Epstein-Barr virus antagonizes transforming growth factor β1-induced B-cell apoptosis

The Epstein-Barr virus (EBV) establishes a lifelong latent infection in humans. EBV infection of primary B cells causes cell activation and proliferation, a process driven by the viral latency III gene expression program, which includes EBV nuclear proteins (EBNAs), latent membrane proteins, and untranslated RNAs, including microRNAs. Some latently infected cells enter the long-lived memory B-cell compartment and express only EBNA1 transiently (Lat I) or no EBV protein at all (Lat 0). Targeting the molecular machinery that controls B-cell fate decisions, including the Bcl-2 family of apoptosis-regulating proteins, is crucial to the EBV cycle of infection. Here, we show that BIK (also known as NBK), which encodes a proapoptotic "sensitizer" protein, is repressed by the EBNA2-driven Lat III program but not the Lat I program. BIK repression occurred soon after infection of primary B cells by EBV but not by a recombinant EBV in which the EBNA2 gene had been knocked out. Ectopic BIK induced apoptosis in Lat III cells by a mechanism dependent on its BH3 domain and the activation of caspases. We show that EBNA2 represses BIK in EBV-negative B-cell lymphoma-derived cell lines and that this host-virus interaction can inhibit the proapoptotic effect of transforming growth factor β1 (TGF-β1), a key physiological mediator of B-cell homeostasis. Reduced levels of TGF-β1-associated regulatory SMAD proteins were bound to the BIK promoter in response to EBV Lat III or ectopic EBNA2. These data are evidence of an additional mechanism used by EBV to promote B-cell survival, namely, the transcriptional repression of the BH3-only sensitizer BIK.

IMPORTANCE

Over 90% of adult humans are infected with the Epstein-Barr virus (EBV). EBV establishes a lifelong silent infection, with its DNA residing in small numbers of blood B cells that are a reservoir from which low-level virus reactivation and shedding in saliva intermittently occur. Importantly, EBV DNA is found in some B-cell-derived tumors in which viral genes play a key role in tumor cell emergence and progression. Here, we report for the first time that EBV can shut off a B-cell gene called BIK. When activated by a molecular signal called transforming growth factor β1 (TGF-β1), BIK plays an important role in killing unwanted B cells, including those infected by viruses. We describe the key EBV-B-cell molecular interactions that lead to BIK shutoff. These findings further our knowledge of how EBV prevents the death of its host cell during infection. They are also relevant to certain posttransplant lymphomas where unregulated cell growth is caused by EBV genes.

Nuclear receptor 4 group A member 1 determines hepatitis C virus entry efficiency through the regulation of cellular receptor and apolipoprotein E expression

Orphan nuclear receptor subfamily 4 group A member 1 (NR4A1) is a transcription factor stimulated by many factors and plays pivotal roles in metabolism, proliferation and apoptosis. In this study, the expression of NR4A1 in Huh7.5.1 cells was significantly upregulated by hepatitis C virus (HCV) infection. The silencing of NR4A1 inhibited the entry of HCV and reduced the specific infectivity of secreted HCV particles but had only minor or no effect on the genome replication and translation, virion assembly and virus release steps of the virus life cycle. Further experiments demonstrated that the silencing of NR4A1 affected virus entry through pan-downregulation of the expression of HCV receptors scavenger receptor BI, occludin, claudin-1 and epidermal growth factor receptor but not CD81. The reduced specific infectivity of HCV in the knockdown cells was due to decreased apolipoprotein E (ApoE) expression. These results explain the delayed spread of HCV in NR4A1 knockdown Huh7.5.1 cells. Thus, NR4A1 plays a role in HCV replication through regulating the expression of HCV receptors and ApoE, and facilitates HCV entry and spread.

NR4A1-mediated apoptosis suppresses lymphomagenesis and is associated with a favorable cancer-specific survival in patients with aggressive B-cell lymphomas

NR4A1 (Nur77) and NR4A3 (Nor-1) function as tumor suppressor genes as demonstrated by the rapid development of acute myeloid leukemia in the NR4A1 and NR4A3 knockout mouse. The aim of our study was to investigate NR4A1 and NR4A3 expression and function in lymphoid malignancies. We found a vastly reduced expression of NR4A1 and NR4A3 in chronic lymphocytic B-cell leukemia (71%), in follicular lymphoma (FL, 70%), and in diffuse large B-cell lymphoma (DLBCL, 74%). In aggressive lymphomas (DLBCL and FL grade 3), low NR4A1 expression was significantly associated with a non-germinal center B-cell subtype and with poor overall survival. To investigate the function of NR4A1 in lymphomas, we overexpressed NR4A1 in several lymphoma cell lines. Overexpression of NR4A1 led to a higher proportion of lymphoma cells undergoing apoptosis. To test the tumor suppressor function of NR4A1 in vivo, the stable lentiviral-transduced SuDHL4 lymphoma cell line harboring an inducible NR4A1 construct was further investigated in xenografts. Induction of NR4A1 abrogated tumor growth in the NSG mice, in contrast to vector controls, which formed massive tumors. Our data suggest that NR4A1 has proapoptotic functions in aggressive lymphoma cells and define NR4A1 as a novel gene with tumor suppressor properties involved in lymphomagenesis.

Dual roles of orphan nuclear receptor TR3/Nur77/NGFI-B in mediating cell survival and apoptosis

As a transcriptional factor, Nur77 has sparked interests across different research fields in recent years. A number of studies have demonstrated the functional complexity of Nur77 in mediating survival/apoptosis in a variety of cells, including tumor cells. Conflicting observations also exist in clinical reports, in that TR3 behaves like an oncogene in tumors of the GI tract, lung, and breast, that is negatively associated with tumor stage and patient prognosis; while functions as a tumor suppressor gene in malignancies of the hematological and lymphatic system, skin, and ovary whose malfunction results in carcinogenesis. This chapter summarizes the apparent opposing effects of Nur77 on cells and explicates the mechanisms that determine the functional preference of Nur77. We conclude that in addition to cell type and agent context, other factors such as cellular localization, signaling pathway, and posttranslational modification also determine the final effects of Nur77 on cells.

The HPV E2-Host Protein-Protein Interactions: A Complex Hijacking of the Cellular Network

Over 100 genotypes of human papillomaviruses (HPVs) have been identified as being responsible for unapparent infections or for lesions ranging from benign skin or genital warts to cancer. The pathogenesis of HPV results from complex relationships between viral and host factors, driven in particular by the interplay between the host proteome and the early viral proteins. The E2 protein regulates the transcription, the replication as well as the mitotic segregation of the viral genome through the recruitment of host cell factors to the HPV regulatory region. It is thereby a pivotal factor for the productive viral life cycle and for viral persistence, a major risk factor for cancer development. In addition, the E2 proteins have been shown to engage numerous interactions through which they play important roles in modulating the host cell. Such E2 activities are probably contributing to create cell conditions appropriate for the successive stages of the viral life cycle, and some of these activities have been demonstrated only for the oncogenic high-risk HPV. The recent mapping of E2-host protein-protein interactions with 12 genotypes representative of HPV diversity has shed some light on the large complexity of the host cell hijacking and on its diversity according to viral genotypes. This article reviews the functions of E2 as they emerge from the E2/host proteome interplay, taking into account the large-scale comparative interactomic study.

Gliotoxin is a potent NOTCH2 transactivation inhibitor and efficiently induces apoptosis in chronic lymphocytic leukaemia (CLL) cells

Chronic lymphocytic leukaemia (CLL) cells express constitutively activated NOTCH2 in a protein kinase C (PKC)- dependent manner. The transcriptional activity of NOTCH2 correlates not only with the expression of its target gene FCER2 (CD23) but is also functionally linked with CLL cell viability. In the majority of CLL cases, DNA-bound NOTCH2 complexes are less sensitive to the γ-secretase inhibitor (GSI) DAPT. Therefore, we searched for compounds that interfere with NOTCH2 signalling at the transcription factor level. Using electrophoretic mobility shift assays (EMSA), we identified the Aspergillum-derived secondary metabolite gliotoxin as a potent NOTCH2 transactivation inhibitor. Gliotoxin completely blocked the formation of DNA-bound NOTCH2 complexes in CLL cells independent of their sensitivity to DAPT. The inhibition of NOTCH2 signalling by gliotoxin was associated with down regulation of CD23 (FCER) expression and induction of apoptosis. Short time exposure of CLL cells indicated that the early apoptotic effect of gliotoxin is independent of proteasome regulated nuclear factor κB activity, and is associated with up regulation of NOTCH3 and NR4A1 expression. Gliotoxin could overcome the supportive effect of primary bone marrow stromal cells in an ex vivo CLL microenvironment model. In conclusion, we identified gliotoxin as a potent NOTCH2 inhibitor with a promising therapeutic potential in CLL.

Analysis of Epstein-Barr virus-regulated host gene expression changes through primary B-cell outgrowth reveals delayed kinetics of latent membrane protein 1-mediated NF-κB activation

Epstein-Barr virus (EBV) is an oncogenic human herpesvirus that dramatically reorganizes host gene expression to immortalize primary B cells. In this study, we analyzed EBV-regulated host gene expression changes following primary B-cell infection, both during initial proliferation and through transformation into lymphoblastoid cell lines (LCLs). While most EBV-regulated mRNAs were changed during the transition from resting, uninfected B cells through initial B-cell proliferation, a substantial number of mRNAs changed uniquely from early proliferation through LCL outgrowth. We identified constitutively and dynamically EBV-regulated biological processes, protein classes, and targets of specific transcription factors. Early after infection, genes associated with proliferation, stress responses, and the p53 pathway were highly enriched. However, the transition from early to long-term outgrowth was characterized by genes involved in the inhibition of apoptosis, the actin cytoskeleton, and NF-κB activity. It was previously thought that the major viral protein responsible for NF-κB activation, latent membrane protein 1 (LMP1), is expressed within 2 days after infection. Our data indicate that while this is true, LCL-level LMP1 expression and NF-κB activity are not evident until 3 weeks after primary B-cell infection. Furthermore, heterologous NF-κB activation during the first week after infection increased the transformation efficiency, while early NF-κB inhibition had no effect on transformation. Rather, inhibition of NF-κB was not toxic to EBV-infected cells until LMP1 levels and NF-κB activity were high. These data collectively highlight the dynamic nature of EBV-regulated host gene expression and support the notion that early EBV-infected proliferating B cells have a fundamentally distinct growth and survival phenotype from that of LCLs.

Latent Epstein-Barr virus can inhibit apoptosis in B cells by blocking the induction of NOXA expression

Latent Epstein-Barr virus (EBV) has been shown to protect Burkitt's lymphoma-derived B cells from apoptosis induced by agents that cause damage to DNA, in the context of mutant p53. This protection requires expression of the latency-associated nuclear proteins EBNA3A and EBNA3C and correlates with their ability to cooperate in the repression of the gene encoding the pro-apoptotic, BH3-only protein BIM. Here we confirm that latent EBV in B cells also inhibits apoptosis induced by two other agents--ionomycin and staurosporine--and show that these act by a distinct pathway that involves a p53-independent increase in expression of another pro-apoptotic, BH3-only protein, NOXA. Analyses employing a variety of B cells infected with naturally occurring EBV or B95.8 EBV-BAC recombinant mutants indicated that the block to NOXA induction does not depend on the well-characterized viral latency-associated genes (EBNAs 1, 2, 3A, 3B, 3C, the LMPs or the EBERs) or expression of BIM. Regulation of NOXA was shown to be at least partly at the level of mRNA and the requirement for NOXA to induce cell death in this context was demonstrated by NOXA-specific shRNA-mediated depletion experiments. Although recombinant EBV with a deletion removing the BHRF1 locus--that encodes the BCL2-homologue BHRF1 and three microRNAs--partially abrogates protection against ionomycin and staurosporine, the deletion has no effect on the EBV-mediated block to NOXA accumulation.

An unhealthy relationship: viral manipulation of the nuclear receptor superfamily

The nuclear receptor (NR) superfamily is a diverse group of over 50 proteins whose function is to regulate the transcription of a vast array of cellular genes. These proteins are able to tune transcription over an extremely dynamic range due to the fact that they may act as either transcriptional activators or repressors depending on promoter context and ligand status. Due to these unique properties, diverse families of viruses have evolved strategies to exploit NRs in order to regulate expression of their own genes and to optimize the cellular milieu to facilitate the viral lifecycle. While the specific NRs targeted by these viruses vary, the strategies used to target them are common. This is accomplished at the cis-level by incorporation of nuclear receptor response elements into the viral genome and at the trans-level by viral proteins that target NRs directly or indirectly to modulate their function. The specific NR(s) targeted by a particular virus are likely to be reflective of the tissue tropism of the virus in question. Thus, the essential role played by NRs in the replication cycles of such diverse viruses underscores the importance of understanding their functions in the context of specific infections. This knowledge will allow appropriate considerations to be made when treating infected individuals with hormone-associated diseases and will potentially assist in the rational design of novel antiviral therapeutics.

The transcription factor NR4A1 (Nur77) controls bone marrow differentiation and the survival of Ly6C- monocytes

The transcription factors that regulate differentiation into the monocyte subset in bone marrow have not yet been identified. Here we found that the orphan nuclear receptor NR4A1 controlled the differentiation of Ly6C- monocytes. Ly6C- monocytes, which function in a surveillance role in circulation, were absent from Nr4a1-/- mice. Normal numbers of myeloid progenitor cells were present in Nr4a1-/- mice, which indicated that the defect occurred during later stages of monocyte development. The defect was cell intrinsic, as wild-type mice that received bone marrow from Nr4a1-/- mice developed fewer patrolling monocytes than did recipients of wild-type bone marrow. The Ly6C- monocytes remaining in the bone marrow of Nr4a1-/- mice were arrested in S phase of the cell cycle and underwent apoptosis. Thus, NR4A1 functions as a master regulator of the differentiation and survival of 'patrolling' Ly6C- monocytes.

Targeting NR4A1 (TR3) in cancer cells and tumors

INTRODUCTION

Nuclear receptor 4A1(NR4A1) (testicular receptor 3 (TR3), nuclear hormone receptor (Nur)77) is a member of the nuclear receptor superfamily of transcription factors and is highly expressed in multiple tumor types. RNA interference studies indicate that NR4A1 exhibits growth-promoting, angiogenic and prosurvival activity in most cancers.

AREAS COVERED

Studies on several apoptosis-inducing agents that activate nuclear export of NR4A1, which subsequently forms a mitochondrial NR4A1-bcl-2 complex that induces the intrinsic pathway for apoptosis are discussed. Cytosporone B and related compounds that induce NR4A1-dependent apoptosis in cancer cells through both modulation of nuclear NR4A1 and nuclear export are discussed. A relatively new class of diindolylmethane analogs (C-DIMs) including 1,1-bis(3'-indolyl)-1-(p-methoxyphenyl)methane (DIM-C-pPhOCH(3)) (NR4A1 activator) and 1,1-bis(3'-indolyl)-1-(p-hydroxyphenyl)methane (DIM-C-pPhOH) (NR4A1 deactivator) are discussed in more detail. These anticancer drugs (C-DIMs) act strictly through nuclear NR4A1 and induce apoptosis in cancer cells and tumors.

EXPERT OPINION

It is clear that NR4A1 plays an important pro-oncogenic role in cancer cells and tumors, and there is increasing evidence that this receptor can be targeted by anticancer drugs that induce cell death via NR4A1-dependent and -independent pathways. Since many of these compounds exhibit relatively low toxicity, they represent an important class of mechanism-based anticancer drugs with excellent potential for clinical applications.

Reduced NR4A gene dosage leads to mixed myelodysplastic/myeloproliferative neoplasms in mice

The NR4A subfamily of nuclear receptors (NR4A1, NR4A2, and NR4A3) function as transcription factors that transduce diverse extracellular signals into altered gene transcription to coordinate apoptosis, proliferation, cell cycle arrest, and DNA repair. We previously discovered that 2 of these receptors, NR4A1 and NR4A3, are potent tumor suppressors of acute myeloid leukemia (AML); they are silenced in human AML, and abrogation of both genes in mice leads to rapid postnatal development of AML. Reduced expression of NR4As is also a common feature of myelodysplastic syndromes (MDSs). Here we show that reduced gene dosage of NR4A1 and NR4A3 in hypoallelic (NR4A1(+/-)NR4A3(-/-) or NR4A1(-/-)NR4A3(+/-)) mice below a critical threshold leads to a chronic myeloid malignancy that closely recapitulates the pathologic features of mixed myelodysplastic/myeloproliferative neoplasms (MDS/MPNs) with progression to AML in rare cases. Enhanced proliferation and excessive apoptosis of hematopoietic stem cells and myeloid progenitors, together with elevated DNA damage, contribute to MDS/MPN disease. We identify the myeloid tumor suppressor genes Egr1 and JunB and the DNA damage checkpoint kinase, polo-like kinase 2 (Plk2) as deregulated genes whose disrupted signaling probably contributes to MDS/MPN. These mice provide a novel model to elucidate the molecular pathogenesis of MDS/MPN and for therapeutic evaluation.

Nuclear proteins acting on mitochondria

An important mechanism in apoptotic regulation is changes in the subcellular distribution of pro- and anti-apoptotic proteins. Among the proteins that change in their localization and may promote apoptosis are nuclear proteins. Several of these nuclear proteins such as p53, Nur77, histone H1.2, and nucleophosmin were reported to accumulate in the cytosol and/or mitochondria and to promote the mitochondrial apoptotic pathway in response to apoptotic stressors. In this review, we will discuss the functions of these and other nuclear proteins in promoting the mitochondrial apoptotic pathway, the mechanisms that regulate their accumulation in the cytosol and/or mitochondria and the potential role of Bax and Bak in this process. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Epstein-Barr virus encoded EBNA-3 binds to vitamin D receptor and blocks activation of its target genes

Epstein-Barr virus (EBV) is a human gamma herpes virus that infects B cells and induces their transformation into immortalized lymphoblasts that can grow as cell lines (LCLs) in vitro. EBNA-3 is a member of the EBNA-3-protein family that can regulate transcription of cellular and viral genes. The identification of EBNA-3 cellular partners and a study of its influence on cellular pathways are important for understanding the transforming action of the virus. In this work, we have identified the vitamin D receptor (VDR) protein as a binding partner of EBNA-3. We found that EBNA3 blocks the activation of VDR-dependent genes and protects LCLs against vitamin-D3-induced growth arrest and/or apoptosis. The presented data shed some light on the anti-apoptotic EBV program and the role of the EBNA-3-VDR interaction in the viral strategy.

Notch signaling maintains proliferation and survival of the HL60 human promyelocytic leukemia cell line and promotes the phosphorylation of the Rb protein

The Notch signaling pathway has been implicated in the development of several leukemia and lymphoma. In order to investigate the relationship between Notch signaling and acute myeloid leukemia (AML), in this study, we expressed a recombinant Notch ligand protein, the DSL domain of the human Jagged1 fused with GST (GST-Jag1). GST-Jag1 could activate Notch signaling in the human promyelocytic leukemia cell line HL60, as shown by a reporter assay and the induced expression of Notch effector gene Hes1 and Hes5. However, GST-Jag1 had no effect on the proliferation and survival of HL60 cells. HL60 cells expressed both Notch ligands and receptors, and had a potential of reciprocal stimulation of Notch signaling between cells. We, therefore, blocked Notch signaling in cultured HL60 cells using a gamma-secretase inhibitor (GSI). We found that GSI inhibited the proliferation of HL60 cells significantly by blocking the cell-cycle progression in the G1 phase. Furthermore, GSI induced remarkably apoptosis of HL60 cells. These changes in GSI-treated HL60 cells correlated with the down-regulation of c-Myc and Bcl2, and the low phosphorylation of the Rb protein. These results suggested that reciprocal Notch signaling might be necessary for the proliferation and survival of AML cells, possibly through the maintenance of the expression of c-Myc and Bcl2, as well as the phosphorylation of the Rb protein.

Minireview: Nuclear receptors, hematopoiesis, and stem cells

Nuclear receptors (NRs) regulate a panoply of biological processes, including the function and development of cells within the hematopoietic and immune system, such as erythrocytes, monocytes, and lymphocytes. Significantly less is known regarding the function of NRs in regulating the fate of hematopoietic stem cells (HSCs), the self-renewing, pluripotent cells that give rise to the entirety of the blood and immune systems throughout the lifetime of an individual. Several recent studies suggest, either directly or indirectly, a role for members of the NR family in regulating the differentiation and self-renewal of HSCs, embryonic stem cells, and induced pluripotent stem cells. Herein, we review in detail the function of specific NRs in controlling HSC and other stem cell fate and propose a framework through which these observations can be translated into therapeutic amplification of HSCs for clinical purposes.

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.

Chromodomain helicase/adenosine triphosphatase DNA binding protein 1-like (CHD1l) gene suppresses the nucleus-to-mitochondria translocation of nur77 to sustain hepatocellular carcinoma cell survival

Amplification of 1q21 has been detected in 58% to 78% of primary hepatocellular carcinoma cases, suggesting that one or more oncogenes within the amplicon play a critical role in the development of this disease. The chromodomain helicase/adenosine triphosphatase DNA binding protein 1-like gene (CHD1L) is a recently identified oncogene localized at 1q21. Our previous studies have demonstrated that CHD1L has strong tumorigenic ability and confers high susceptibility to spontaneous tumors in a CHD1L-transgenic mouse model. In this study, we demonstrate that the antiapoptotic ability of CHD1L is associated with its interaction with Nur77, a critical member of a p53-independent apoptotic pathway. As the first cellular protein identified to bind Nur77, CHD1L is able to inhibit the nucleus-to-mitochondria translocation of Nur77, which is the key step of Nur77-mediated apoptosis, resulting in the hindrance of the release of cytochrome c and the initiation of apoptosis. Knock-down of CHD1L expression by RNA interference could rescue the mitochondrial targeting of Nur77 and the subsequent apoptosis. Further studies found that the C-terminal Macro domain of CHD1L is responsible for the interaction with Nur77, and a CHD1L mutant lacking residues 600-897 failed to interact with Nur77 and prevented Nur77-mediated apoptosis. More importantly, we found that the inhibition of Nur77-mediated apoptosis by endogenous CHD1L is a critical biological cellular process in hepatocarcinogenesis.

CONCLUSION

We demonstrate in this study that overexpression of CHD1L could sustain tumor cell survival by preventing Nur77-mediated apoptosis.

Regulation of transcription by the Epstein-Barr virus nuclear antigen EBNA 2

The EBNA 2 (Epstein-Barr nuclear antigen 2) transcription factor is essential for B-cell transformation by the cancer-associated EBV (Epstein-Barr virus) and for the continuous proliferation of infected cells. EBNA 2 activates transcription from the viral Cp (C promoter) during infection to generate the 120 kb transcript that encodes all nuclear antigens required for immortalization by EBV. EBNA 2 contains an acidic activation domain and can interact with a number of general transcription factors and co-activators. It is now becoming clear, however, that the regulation of transcription elongation in addition to initiation by EBNA 2, at least in part through CDK9 (cyclin-dependent kinase 9)-dependent phosphorylation of the RNA polymerase C-terminal domain, is likely to play a crucial role in the mechanism of action of this key viral protein.

Search for cellular partners of human papillomavirus type 16 E2 protein

Human papillomaviruses (HPVs) are small, double-stranded DNA viruses that infect cutaneous and mucosal epithelia. Type 16 (HPV16) displays tropism to genital epithelia, giving rise to genital warts and cervical intraepithelial neoplasia (CIN), which is a precursor lesion to invasive carcinoma of the cervix. The great majority of human cervical cancers contain integrated HPV DNA where the E2 gene is usually disrupted, suggesting that the loss of the E2 protein is an important step in HPV-induced carcinogenesis. The HPV16 E2 protein is a regulatory protein that seems to be essential for creating favourable conditions for establishment of infection and proper completion of the viral life cycle. Recently, diverse activities of the E2 proteins have been described, but the molecular basis of these processes has not beenfully elucidated. Using a yeast two-hybrid system, we have identified epithelial cellular proteins that bind to the E2 protein of HPV16.

Control of Rta expression critically determines transcription of viral and cellular genes following gammaherpesvirus infection

The replication and transcriptional activator (Rta), encoded by ORF50 of gammaherpesviruses, initiates the lytic cycle of gene expression; therefore understanding the impact of Rta on viral and cellular gene expression is key to elucidating the transcriptional events governing productive infection and reactivation from latency. To this end, the impact of altering Rta transcription on viral and cellular gene expression was studied in the context of a whole virus infection. Recombinant murine gammaherpesvirus (MHV)-68 engineered to overexpress Rta greatly accelerated expression of specific lytic cycle ORFs, but repressed transcription of the major latency gene, ORF73. Increased expression of Rta accelerated the dysregulation in transcription of specific cellular genes when compared with cells infected with wild-type and revertant viruses. A subset of cellular genes was dysregulated only in cells infected with Rta-overexpressing virus, and never in those infected with non-overexpressing viruses. These data highlight the critical role of Rta abundance in governing viral and cellular gene transcription, and demonstrate the importance of understanding how the relative expression of ORF50 during the virus life cycle impacts on these processes.

Abrogation of nuclear receptors Nr4a3 and Nr4a1 leads to development of acute myeloid leukemia

Nur77 (NR4A1) and Nor-1 (NR4A3) are highly homologous orphan nuclear receptors that regulate the transcription of overlapping target genes. The transcriptional activity of both proteins is regulated in a ligand-independent manner by cell- and stimulus-specific gene induction and protein phosphorylation. Nor-1 and Nur77 have been implicated in a variety of cellular processes, including the transduction of hormonal, inflammatory, mitogenic, apoptotic and differentiative signals. Cellular responses to these proteins suggest that they may function as homeostatic regulators of proliferation, apoptosis and differentiation, and thus may regulate cellular susceptibility to tumorigenesis. Their physiological functions, however, remain poorly understood. Here we describe a previously unsuspected function of Nor-1 and Nur77-as critical tumor suppressors of myeloid leukemogenesis. The abrogation of these proteins in mice led to rapidly lethal acute myeloid leukemia (AML), involving abnormal expansion of hematopoietic stem cells (HSCs) and myeloid progenitors, decreased expression of the AP-1 transcription factors JunB and c-Jun and defective extrinsic apoptotic (Fas-L and TRAIL) signaling. We found that downregulation of NR4A3 ( NOR-1 ) and NR4A1 ( NUR77 ) was a common feature in leukemic blasts from human AML patients, irrespective of karyotype. Thus Nor-1 and Nur77 may provide potential targets for therapeutic intervention in AML.

Epstein-Barr virus and virus human protein interaction maps

A comprehensive mapping of interactions among Epstein-Barr virus (EBV) proteins and interactions of EBV proteins with human proteins should provide specific hypotheses and a broad perspective on EBV strategies for replication and persistence. Interactions of EBV proteins with each other and with human proteins were assessed by using a stringent high-throughput yeast two-hybrid system. Overall, 43 interactions between EBV proteins and 173 interactions between EBV and human proteins were identified. EBV-EBV and EBV-human protein interaction, or "interactome" maps provided a framework for hypotheses of protein function. For example, LF2, an EBV protein of unknown function interacted with the EBV immediate early R transactivator (Rta) and was found to inhibit Rta transactivation. From a broader perspective, EBV genes can be divided into two evolutionary classes, "core" genes, which are conserved across all herpesviruses and subfamily specific, or "noncore" genes. Our EBV-EBV interactome map is enriched for interactions among proteins in the same evolutionary class. Furthermore, human proteins targeted by EBV proteins were enriched for highly connected or "hub" proteins and for proteins with relatively short paths to all other proteins in the human interactome network. Targeting of hubs might be an efficient mechanism for EBV reorganization of cellular processes.

Targeting Nur77 translocation

The ultimate growth of a tumour depends on not only the rate of tumour cell proliferation, but also the rate of tumour cell death (apoptosis). Nur77 (also known as TR3 or NGFI-B), an orphan member of the nuclear receptor superfamily, controls both survival and death of cancer cells. A wealth of recent experimental data demonstrates that the Nur77 activities are regulated through its subcellular localisation. In the nucleus, Nur77 functions as an oncogenic survival factor, promoting cancer cell growth. In contrast, it is a potent killer when migrating to mitochondria, where it binds to Bcl-2 and converts its survival phenotype, triggering cytochrome c release and apoptosis. Agents, such as 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (AHPN/CD437), which induce Nur77 migration from the nucleus to mitochondria, effectively induce apoptosis of cancer cells. Moreover, Nur77 translocation is highly controlled by retinoid X receptor (RXR), suggesting a role of RXR ligands in regulating the process. Thus, translocation of Nur77 from the nucleus to mitochondria represents a new paradigm in cancer cell apoptosis, and targeting the Nur77 translocation by AHPN/CD437 or RXR ligands promises to effectively restrict cancer cell growth by simultaneously promoting cancer cell death and suppressing cancer cell proliferation.

Epstein-Barr virus: evasive maneuvers in the development of PTLD

Epstein-Barr virus (EBV) infection is linked to approximately 90% of B-cell lymphomas associated with posttransplant lymphoproliferative disease (PTLD), a serious complication for immunosuppressed transplant recipients. In this paper, we review the myriad ways by which EBV can inadvertently drive the genesis and persistence of B-cell lymphomas, particularly when the antiviral immune response is compromised. Probing the basic mechanisms by which EBV infection proceeds and contributes to malignancy in such cases will hopefully improve our understanding and treatment of PTLD and other EBV-associated malignancies.

Immature CD4+CD8+ thymocytes and mature T cells regulate Nur77 distinctly in response to TCR stimulation

The orphan steroid receptor, Nur77, is thought to be a central participant in events leading to TCR-mediated clonal deletion of immature thymocytes. Interestingly, although both immature and mature murine T cell populations rapidly up-regulate Nur77 after TCR stimulation, immature CD4+CD8+ thymocytes respond by undergoing apoptosis, whereas their mature descendants respond by dividing. To understand these developmental differences in susceptibility to the proapoptotic potential of Nur77, we compared its regulation and compartmentalization and show that mature, but not immature, T cells hyperphosphorylate Nur77 in response to TCR signals. Nur77 resides in the nucleus of immature CD4+CD8+ thymocytes throughout the course of its expression and is not found in either the organellar or cytoplasmic fractions. However, hyperphosphorylation of Nur77 in mature T cells, which is mediated by both the MAPK and PI3K/Akt pathways, shifts its localization from the nucleus to the cytoplasm. The failure of immature CD4+CD8+ thymocytes to hyperphosphorylate Nur77 in response to TCR stimulation may be due in part to decreased Akt activity at this developmental stage.

The CR4 region of EBNA2 confers viability of Epstein-Barr virus-transformed B cells by CBF1-independent signalling

The Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA2) gene product is the key regulator of the latent genes of EBV and essential for EBV-mediated transformation of human primary B cells. Viral mutants were constructed carrying a deletion of the EBNA2 conserved region 4 (CR4). Primary resting B cells infected with the DeltaCR4-EBNA2 mutant virus were dramatically impaired for B cell transformation. Lymphoblastoid cell lines (LCLs) established with this mutant EBV revealed a prolonged population doubling time when cells were cultivated at low cell densities, which are not critical for wild-type-infected cells. Low-level spontaneous cell death occurred when the cells were cultivated at suboptimal cell densities. The phenotype of B cells and LCLs infected with the DeltaCR4-EBNA2 mutant virus indicated that the CR4 region of EBNA2 specifically contributes to the viability of the cells rather than affecting cell division rates.

Three restricted forms of Epstein-Barr virus latency counteracting apoptosis in c-myc-expressing Burkitt lymphoma cells

Epstein-Barr virus (EBV), a human herpesvirus, transforms B cell growth in vitro through expressing six virus-coded Epstein-Barr nuclear antigens (EBNAs) and two latent membrane proteins (LMPs). In many EBV-associated tumors, however, viral antigen expression is more restricted, and the aetiological role of the virus is unclear. For example, endemic Burkitt lymphoma (BL) classically presents as a monoclonal, c-myc-translocation-positive tumor in which every cell carries EBV as an EBNA1-only (Latency I) infection; such homogeneity among EBV-positive cells, and the lack of EBV-negative comparators, hampers attempts to understand EBV's role in BL pathogenesis. Here, we describe an endemic BL that was unusually heterogeneous at the single-cell level and, in early passage culture, yielded a range of cellular clones, all with the same c-myc translocation but differing in EBV status. Rare EBV-negative cells were isolated alongside EBV-positive cells displaying one of three forms of restricted latency: (i) conventional Latency I expressing EBNA1 only from a WT virus genome, (ii) Wp-restricted latency expressing EBNAs 1, 3A, 3B, 3C, and -LP only from an EBNA2-deleted genome, and (iii) a previously undescribed EBNA2(+)/LMP1(-) latency in which all six EBNAs are expressed again in the absence of the LMPs. Interclonal comparisons showed that each form of EBV infection was associated with a specific degree of protection from apoptosis. Our work suggests that EBV acts as an antiapoptotic rather than a growth-promoting agent in BL by selecting among three transcriptional programs, all of which, unlike the full virus growth-transforming program, remain compatible with high c-myc expression.

p53 and Nur77/TR3 - transcription factors that directly target mitochondria for cell death induction

The complex apoptotic functions of the p53 tumor suppressor are central to its antineoplastic activity in vivo. Conversely, p53 function is altered or attenuated in one way or another in the majority of human cancers. Besides its well-understood action as a transcriptional regulator of multiple apoptotic genes, p53 also exerts a direct pro-apoptotic role at the mitochondria by engaging in protein-protein interactions with anti- and pro-apoptotic Bcl2 family members, thereby executing the shortest known circuitry of p53 death signaling. Nur77, also known as TR3 or NGFI-B, is a unique transcription factor belonging to the orphan nuclear receptor superfamily. Even more extreme than p53, Nur77 can exert opposing biological activities of survival and death. Its activities are regulated by subcellular distribution, expression levels, protein modification and heterodimerization with retinoid X receptor. In cancer cells, Nur77 functions in the nucleus as an oncogenic survival factor, but becomes a potent killer when certain death stimuli induce its migration to mitochondria, where it binds to Bcl2 and conformationally converts it to a killer that triggers cytochrome c release and apoptosis. This review focuses on their unexpected transcription-independent pro-death programs at mitochondria and highlights the remarkable mechanistic similarities between them. Moreover, an accumulating body of evidence provides ample rationale to further investigate how these mitochondrial p53 and Nur77 pathways could become exploitable targets for new cancer therapeutics.

Epstein-Barr virus nuclear antigen 2 trans-activates the cellular antiapoptotic bfl-1 gene by a CBF1/RBPJ kappa-dependent pathway

The human herpesvirus Epstein-Barr virus (EBV) establishes latency and promotes the long-term survival of its host B cell by targeting the molecular machinery controlling cell fate decisions. The cellular antiapoptotic bfl-1 gene confers protection from apoptosis under conditions of growth factor deprivation when expressed ectopically in an EBV-negative Burkitt's lymphoma-derived cell line (B. D'Souza, M. Rowe, and D. Walls, J. Virol. 74:6652-6658, 2000), and the EBV latent membrane protein 1 (LMP1) and its cellular functional homologue CD40 can both drive bfl-1 via an NF-kappaB-dependent enhancer element in the bfl-1 promoter (B. N. D'Souza, L. C. Edelstein, P. M. Pegman, S. M. Smith, S. T. Loughran, A. Clarke, A. Mehl, M. Rowe, C. Gélinas, and D. Walls, J. Virol. 78:1800-1816, 2004). Here we show that the EBV nuclear antigen 2 (EBNA2) also upregulates bfl-1. EBNA2 trans-activation of bfl-1 requires CBF1 (or RBP-J kappa), a nuclear component of the Notch signaling pathway, and there is an essential role for a core consensus CBF1-binding site on the bfl-1 promoter. trans-activation is dependent on the EBNA2-CBF1 interaction, is modulated by other EBV gene products known to interact with the CBF1 corepressor complex, and does not involve activation of NF-kappaB. bfl-1 expression is induced and maintained at high levels by the EBV growth program in a lymphoblastoid cell line, and withdrawal of either EBNA2 or LMP1 does not lead to a reduction in bfl-1 mRNA levels in this context, whereas the simultaneous loss of both EBV proteins results in a major decrease in bfl-1 expression. These findings are relevant to our understanding of EBV persistence, its role in malignant disease, and the B-cell developmental process.

Granzyme B Proteolyzes Receptors Important to Proliferation and Survival, Tipping the Balance toward Apoptosis

Granzyme B is critical to the ability of natural killer cells and cytotoxic T lymphocytes to induce efficient cell death of virally infected or tumor cell targets. Although granzyme B can cleave and activate caspases to induce apoptosis, granzyme B can also cause caspase-independent cell death. Thirteen prospective granzyme B substrates were identified from a cDNA expression-cleavage screen, including Hsp70, Notch1, fibroblast growth factor receptor-1 (FGFR1), poly-A-binding protein, cAbl, heterogeneous nuclear ribonucleoprotein H', Br140, and intersectin-1. Validation revealed that Notch1 is a substrate of both granzyme B and caspases, whereas FGFR1 is a caspase-independent substrate of granzyme B. Proteolysis of FGFR1 in prostate cancer cells has functionally relevant consequences that indicate its cleavage may be advantageous for granzyme B to kill prostate cancer cells. Therefore, granzyme B not only activates pro-death functions within a target, but also has a previously unidentified role in inactivating pro-growth signals to cause cell death.

Regulation of Nur77 nuclear export by c-Jun N-terminal kinase and Akt

Proapoptotic nuclear receptor family member Nur77 translocates from the nucleus to the mitochondria, where it interacts with Bcl-2 to trigger apoptosis. Nur77 translocation is induced by certain apoptotic stimuli, including the synthetic retinoid-related 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN)/CD437 class. In this study, we investigated the molecular mechanism by which AHPN/CD437 analog (E)-4-[3-(1-adamantyl)-4-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC) induces Nur77 nuclear export. Our results demonstrate that 3-Cl-AHPC effectively activated Jun N-terminal kinase (JNK), which phosphorylates Nur77. Inhibition of JNK activation by a JNK inhibitor suppressed 3-Cl-AHPC-induced Nur77 nuclear export and apoptosis. In addition, several JNK upstream activators, including the phorbol ester TPA, anisomycin and MAPK kinase kinase-1 (MEKK1), phosphorylated Nur77 and induced its nuclear export. However, Nur77 phosphorylation by JNK, although essential, was not sufficient for inducing Nur77 nuclear export. Induction of Nur77 nuclear export by MEKK1 required a prolonged MEKK1 activation and was attenuated by Akt activation. Expression of constitutively active Akt prevented MEKK1-induced Nur77 nuclear export. Conversely, transfection of dominant-negative Akt or treatment with a phosphatidylinositol 3-kinase (PI3-K) inhibitor accelerated MEKK1-induced Nur77 nuclear export. Furthermore, mutation of an Akt phosphorylation residue Ser351 in Nur77 abolished the effect of Akt or the PI3-K inhibitor. Together, our results demonstrate that both activation of JNK and inhibition of Akt play a role in translocation of Nur77 from the nucleus to the cytoplasm.

Inhibition of apoptosis by Nur77 through NF-kappaB activity modulation

The orphan nuclear receptor Nur77 has been described as a mediator of apoptosis and has also been associated with growth promotion and apoptotic resistance. This study aimed at evaluating the contribution of Nur77 to different apoptotic stimuli. Nur77 overexpression in the fibroblastic cell line HEK293 promoted resistance to programmed cell death induced by death receptor engagement, DNA-damaging agents and endoplasmic reticulum stress. Nur77 overexpression led to enhanced NF-kappaB activity, and DNA-binding inhibitors confirmed the contribution of NF-kappaB to Nur77 antiapoptotic activity. Nur77 overexpression leads to NF-kappaB-dependent induction of the antiapoptotic gene cIAP1. Paradoxically, while dominant-negative Nur77 expression sensitised cells to Fas ligand-induced cell death, it protected cells from endoplasmic reticulum stress apoptosis in a manner similar to wild-type Nur77. These results show that nuclear crosstalk between Nur77 and other transcription factors contribute to cell fate in response to different apoptosis-inducing agents.

HES1 inhibits cycling of hematopoietic progenitor cells via DNA binding

Notch signaling is implicated in stem cell self-renewal, differentiation, and other developmental processes, and the Drosophila hairy and enhancer of split (HES) 1 basic helix-loop-helix protein is a major downstream effector in the Notch pathway. We found that HES1 was expressed at high levels in the hematopoietic stem cell (HSC)-enriched CD34+/[CD38/Lin](- /low) subpopulation but at low levels in more mature progenitor cell populations. When CD34+ cells were cultured for 1 week, the level of HES1 remained high in the CD34+ subset that had remained quiescent during ex vivo culture but was reduced in CD34+ cells that had divided. To investigate the effects of HES1 in human and mouse hematopoietic stem-progenitor cells (HSPCs), we constructed conditional lentiviral vectors (lentivectors) to introduce transgenes encoding either wild-type HES1 or a mutant lacking the DNA-binding domain (BHES1). We found that lentivector-mediated HES1 expression in CD34+ cells inhibited cell cycling in vitro and cell expansion in vivo, associated with upregulation of the cell cycle inhibitor p21(cip1/Waf1) (p21). The HES1 DNA-binding domain was required for these actions. HES1 did not induce programmed cell death or alter differentiation in HSPCs, and while short-term repopulating activity was reduced in HES1-transduced mouse and human cells, long-term reconstituting HSC function was preserved. Our data characterize the complex, cell context-dependent actions of HES1 as a major downstream Notch signaling regulator of HSPC function.