Epstein-Barr virus essential antigen EBNA3C attenuates H2AX expression

Ubiquitin-Mediated Effects on Oncogenesis during EBV and KSHV Infection

The Herpesviridae include the Epstein-Barr Virus (EBV) and the Kaposi Sarcoma-associated Herpesvirus (KSHV), both of which are oncogenic gamma-herpesviruses. These viruses manipulate host cellular mechanisms, including through ubiquitin-mediated pathways, to promote viral replication and oncogenesis. Ubiquitin, a regulatory protein which tags substrates for degradation or alters their function, is manipulated by both EBV and KSHV to facilitate viral persistence and cancer development. EBV infects approximately 90% of the global population and is implicated in malignancies including Burkitt lymphoma (BL), Hodgkin lymphoma (HL), post-transplant lymphoproliferative disorder (PTLD), and nasopharyngeal carcinoma. EBV latency proteins, notably LMP1 and EBNA3C, use ubiquitin-mediated mechanisms to inhibit apoptosis, promote cell proliferation, and interfere with DNA repair, contributing to tumorigenesis. EBV's lytic proteins, including BZLF1 and BPLF1, further disrupt cellular processes to favor oncogenesis. Similarly, KSHV, a causative agent of Kaposi's Sarcoma and lymphoproliferative disorders, has a latency-associated nuclear antigen (LANA) and other latency proteins that manipulate ubiquitin pathways to degrade tumor suppressors, stabilize oncogenic proteins, and evade immune responses. KSHV's lytic cycle proteins, such as RTA and Orf64, also use ubiquitin-mediated strategies to impair immune functions and promote oncogenesis. This review explores the ubiquitin-mediated interactions of EBV and KSHV proteins, elucidating their roles in viral oncogenesis. Understanding these mechanisms offers insights into the similarities between the viruses, as well as provoking thought about potential therapeutic targets for herpesvirus-associated cancers.

Combination of bortezomib and venetoclax targets the pro-survival function of LMP-1 and EBNA-3C of Epstein-Barr virus in spontaneous lymphoblastoid cell lines

Epstein-Barr virus (EBV) manipulates the ubiquitin-proteasome system and regulators of Bcl-2 family to enable the persistence of the virus and survival of the host cells through the expression of viral proteins in distinct latency patterns. We postulate that the combination of bortezomib (proteasome inhibitor) and venetoclax (Bcl-2 inhibitor) [bort/venetoclax] will cause synergistic killing of post-transplant lymphoproliferative disorder (PTLD) through targeting the pro-survival function of latent viral proteins such as latent membrane protein-1 (LMP-1) and EBV nuclear antigen-3C (EBNA-3C). Bort/venetoclax could synergistically kill spontaneous lymphoblastoid cell lines (sLCLs) derived from patients with PTLD and EBV-associated hemophagocytic lymphohistiocytosis by inducing DNA damage response, apoptosis and G1-S cell cycle arrest in a ROS-dependent manner. Bortezomib potently induced the expression of Noxa, a pro-apoptotic initiator and when combined with venetoclax, inhibited Mcl-1 and Bcl-2 simultaneously. Bortezomib prevented LMP-1 induced proteasomal degradation of IκBα leading to the suppression of the NF-κB signaling pathway. Bortezomib also rescued Bcl-6 from EBNA-3C mediated proteasomal degradation thus maintaining the repression of cyclin D1 and Bcl-2 causing G1-S arrest and apoptosis. Concurrently, venetoclax inhibited Bcl-2 upregulated by either LMP-1 or EBNA-3C. Bort/venetoclax decreased the expression of phosphorylated p65 and Bcl-2 at serine 70 thereby suppressing the NF-κB signaling pathway and promoting apoptosis, respectively. These data corroborated the marked suppression of the growth of xenograft of sLCL in SCID mice (p<0.001). Taken together, the combination of bortezomib and venetoclax targets the pro-survival function of LMP-1 and EBNA-3C of Epstein-Barr virus in spontaneous lymphoblastoid cell lines.

Polymerase theta is a synthetic lethal target for killing Epstein-Barr virus lymphomas

Treatment options for Epstein-Barr virus (EBV)-cancers are limited, underscoring the need for new therapeutic approaches. We have previously shown that EBV-transformed cells and cancers lack homologous recombination (HR) repair, a prominent error-free pathway that repairs double-stranded DNA breaks; instead, EBV-transformed cells demonstrate genome-wide scars of the error-prone microhomology-mediated end joining (MMEJ) repair pathway. This suggests that EBV-cancers are vulnerable to synthetic lethal therapeutic approaches that target MMEJ repair. Indeed, we have previously found that targeting PARP, an enzyme that contributes to MMEJ, results in the death of EBV-lymphoma cells. With the emergence of clinical resistance to PARP inhibitors and the recent discovery of inhibitors of Polymerase theta (POLθ), the polymerase essential for MMEJ, we investigated the role of POLθ in EBV-lymphoma cells. We report that EBV-transformed cell lines, EBV-lymphoma cell lines, and EBV-lymphomas in AIDS patients demonstrate greater abundance of POLθ, driven by the EBV protein EBNA1, compared to EBV-uninfected primary lymphocytes and EBV-negative lymphomas from AIDS patients (a group that also abundantly expresses POLθ). We also find POLθ enriched at cellular DNA replication forks and exposure to the POLθ inhibitor Novobiocin impedes replication fork progress, impairs MMEJ-mediated repair of DNA double-stranded breaks, and kills EBV-lymphoma cells. Notably, cell killing is not due to Novobiocin-induced activation of the lytic/replicative phase of EBV. These findings support a role for POLθ not just in DNA repair but also DNA replication and as a therapeutic target in EBV-lymphomas and potentially other EBV-cancers as EBNA1 is expressed in all EBV-cancers.IMPORTANCEEpstein-Barr virus (EBV) contributes to ~2% of the global cancer burden. With a recent estimate of >200,000 deaths a year, identifying molecular vulnerabilities will be key to the management of these frequently aggressive and treatment-resistant cancers. Building on our earlier work demonstrating reliance of EBV-cancers on microhomology-mediated end-joining repair, we now report that EBV lymphomas and transformed B cell lines abundantly express the MMEJ enzyme POLθ that likely protects cellular replication forks and repairs replication-related cellular DNA breaks. Importantly also, we show that a newly identified POLθ inhibitor kills EBV-cancer cells, revealing a novel strategy to block DNA replication and repair of these aggressive cancers.

Regulation of PKR-dependent RNA translation inhibition by TRIM21 upon virus infection or other stress

The host always employs various ways to defend against viral infection and spread. However, viruses have evolved their own effective strategies, such as inhibition of RNA translation of the antiviral effectors, to destroy the host's defense barriers. Protein synthesis, commonly controlled by the α-subunit of eukaryotic translation initiation factor 2 (eIF2α), is a basic cellular biological process among all species. In response to viral infection, in addition to inducing the transcription of antiviral cytokines by innate immunity, infected cells also inhibit the RNA translation of antiviral factors by activating the protein kinase R (PKR)-eIF2α signaling pathway. Regulation of innate immunity has been well studied; however, regulation of the PKR-eIF2α signaling pathway remains unclear. In this study, we found that the E3 ligase TRIM21 negatively regulates the PKR-eIF2α signaling pathway. Mechanistically, TRIM21 interacts with the PKR phosphatase PP1α and promotes K6-linked polyubiquitination of PP1α. Ubiquitinated PP1α augments its interaction with PKR, causing PKR dephosphorylation and subsequent translational inhibition release. Furthermore, TRIM21 can constitutively restrict viral infection by reversing PKR-dependent translational inhibition of various previously known and unknown antiviral factors. Our study highlights a previously undiscovered role of TRIM21 in regulating translation, which will provide new insights into the host antiviral response and novel targets for the treatment of translation-associated diseases in the clinic.

Tegument proteins of Epstein-Barr virus: Diverse functions, complex networks, and oncogenesis

The tegument is the structure between the envelope and nucleocapsid of herpesvirus particles. Viral (and cellular) proteins accumulate to create the layers of the tegument. Some Epstein-Barr virus (EBV) tegument proteins are conserved widely in Herpesviridae, but others are shared only by members of the gamma-herpesvirus subfamily. As the interface to envelope and nucleocapsid, the tegument functions in virion morphogenesis and budding of the nucleocapsid during progeny production. When a virus particle enters a cell, enzymes such as kinase and deubiquitinase, and transcriptional activators are released from the virion to promote virus infection. Moreover, some EBV tegument proteins are involved in oncogenesis. Here, we summarize the roles of EBV tegument proteins, in comparison to those of other herpesviruses.

p62/SQSTM1 promotes mitophagy and activates the NRF2-mediated anti-oxidant and anti-inflammatory response restraining EBV-driven B lymphocyte proliferation

Reactive oxygen species (ROS) and DNA repair respectively promote and limit oncogenic transformation of B cells driven by Epstein-Barr virus (EBV). We have previously shown that EBV infection reduced autophagy in primary B lymphocytes and enhanced ROS and interleukin 6 (IL-6) release, promoting B cell proliferation and immortalization. In this study, we explored the role of p62/SQSTM1, accumulated as a consequence of autophagy reduction in EBV-infected B lymphocytes, and found that it exerted a growth suppressive effect in these cells. At molecular level, we found that p62 counteracted IL-6 production and ROS increase by interacting with NRF2 and promoting mitophagy. Moreover, p62/NRF2 axis sustained the expression level of H2AX and ataxia-telangiectasia mutated (ATM), whose activation has been shown to have growth-suppressive effects during the first steps of EBV-infection, before latency is established. In conclusion, this study shows for the first time that the accumulation of p62 and the activation of p62/axis counteracted EBV-driven proliferation of primary B lymphocytes.

Herpesvirus Epigenetic Reprogramming and Oncogenesis

Among all of the known biological carcinogens, Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) are two of the classical oncogenic herpesviruses known to induce the oncogenic phenotype. Many studies have revealed important functions related to epigenetic alterations of the EBV and KSHV genomes that mediate oncogenesis, but the detailed mechanisms are not fully understood. It is also challenging to fully describe the critical cellular events that drive oncogenesis as well as a comprehensive map of the molecular contributors. This review introduces the roles of epigenetic modifications of these viral genomes, including DNA methylation, histone modification, chromatin remodeling, and noncoding RNA expression, and elucidates potential strategies utilized for inducing oncogenesis by these human gammaherpesviruses.

Epstein-Barr virus infection modulates blood-brain barrier cells and its co-infection with Plasmodium falciparum induces RBC adhesion

Plasmodium falciparum infection-mediated Epstein-Barr virus (EBV) reactivation is well established in malaria-endemic countries. We hypothesize that, during malaria onset, the reactivated EBV can infect human brain microvascular endothelial cells (HBECs). This may cause severe cerebral manifestations. We infected HBECs with EBV in vitro. The subsequent gene expression pattern of EBV, inflammatory and endothelial markers was analysed using qRT-PCR. Further, a wound-healing assay for cells maintaining blood-brain barrier (BBB) integrity was performed to investigate the effect of EBV-infected HBECs secretions. The RBC adhesion assay was conducted to assess RBC attachment onto HBECs during EBV and P. falciparum mono- and co-infection. Our experiments revealed that EBV infection of HBECs significantly elevated several inflammatory (TNFα, CCL2) and endothelial (integrin β3, PECAM, VEGFA, VWF, claudin-5, cx37) markers. The EBV-infected HBECs secretion significantly reduced migration of HBECs, glial and neuronal cells. Additionally, EBV-P. falciparum co-infection significantly (P < 0.05) enhanced RBC adhesion to HBECs compared to mono-infection scenarios. Conclusively, the EBV infection of HBECs led to endothelial activation and modulated the BBB microenvironment. The EBV-P. falciparum co-infection scenario increased RBC adhesion on ECs which is a hallmark of cerebral malaria. Together with malaria, EBV infection can aid in exacerbation of cerebral malaria pathology.

Histone Loaders CAF1 and HIRA Restrict Epstein-Barr Virus B-Cell Lytic Reactivation

Epstein-Barr virus (EBV) infects 95% of adults worldwide and causes infectious mononucleosis. EBV is associated with endemic Burkitt lymphoma, Hodgkin lymphoma, posttransplant lymphomas, nasopharyngeal and gastric carcinomas. In these cancers and in most infected B-cells, EBV maintains a state of latency, where nearly 80 lytic cycle antigens are epigenetically suppressed. To gain insights into host epigenetic factors necessary for EBV latency, we recently performed a human genome-wide CRISPR screen that identified the chromatin assembly factor CAF1 as a putative Burkitt latency maintenance factor. CAF1 loads histones H3 and H4 onto newly synthesized host DNA, though its roles in EBV genome chromatin assembly are uncharacterized. Here, we found that CAF1 depletion triggered lytic reactivation and virion secretion from Burkitt cells, despite also strongly inducing interferon-stimulated genes. CAF1 perturbation diminished occupancy of histones 3.1 and 3.3 and of repressive histone 3 lysine 9 and 27 trimethyl (H3K9me3 and H3K27me3) marks at multiple viral genome lytic cycle regulatory elements. Suggestive of an early role in establishment of latency, EBV strongly upregulated CAF1 expression in newly infected primary human B-cells prior to the first mitosis, and histone 3.1 and 3.3 were loaded on the EBV genome by this time point. Knockout of CAF1 subunit CHAF1B impaired establishment of latency in newly EBV-infected Burkitt cells. A nonredundant latency maintenance role was also identified for the DNA synthesis-independent histone 3.3 loader histone regulatory homologue A (HIRA). Since EBV latency also requires histone chaperones alpha thalassemia/mental retardation syndrome X-linked chromatin remodeler (ATRX) and death domain-associated protein (DAXX), EBV coopts multiple host histone pathways to maintain latency, and these are potential targets for lytic induction therapeutic approaches.IMPORTANCE Epstein-Barr virus (EBV) was discovered as the first human tumor virus in endemic Burkitt lymphoma, the most common childhood cancer in sub-Saharan Africa. In Burkitt lymphoma and in 200,000 EBV-associated cancers per year, epigenetic mechanisms maintain viral latency, during which lytic cycle factors are silenced. This property complicated EBV's discovery and facilitates tumor immunoevasion. DNA methylation and chromatin-based mechanisms contribute to lytic gene silencing. Here, we identified histone chaperones CAF1 and HIRA, which have key roles in host DNA replication-dependent and replication-independent pathways, respectively, as important for EBV latency. EBV strongly upregulates CAF1 in newly infected B-cells, where viral genomes acquire histone 3.1 and 3.3 variants prior to the first mitosis. Since histone chaperones ATRX and DAXX also function in maintenance of EBV latency, our results suggest that EBV coopts multiple histone pathways to reprogram viral genomes and highlight targets for lytic induction therapeutic strategies.

First Days in the Life of Naive Human B Lymphocytes Infected with Epstein-Barr Virus

Epstein-Barr virus (EBV) infects and activates resting human B lymphocytes, reprograms them, induces their proliferation, and establishes a latent infection in them. In established EBV-infected cell lines, many viral latent genes are expressed. Their roles in supporting the continuous proliferation of EBV-infected B cells in vitro are known, but their functions in the early, prelatent phase of infection have not been investigated systematically. In studies during the first 8 days of infection using derivatives of EBV with mutations in single genes of EBVs, we found only Epstein-Barr nuclear antigen 2 (EBNA2) to be essential for activating naive human B lymphocytes, inducing their growth in cell volume, driving them into rapid cell divisions, and preventing cell death in a subset of infected cells. EBNA-LP, latent membrane protein 2A (LMP2A), and the viral microRNAs have supportive, auxiliary functions, but mutants of LMP1, EBNA3A, EBNA3C, and the noncoding Epstein-Barr virus with small RNA (EBERs) had no discernible phenotype compared with wild-type EBV. B cells infected with a double mutant of EBNA3A and 3C had an unexpected proliferative advantage and did not regulate the DNA damage response (DDR) of the infected host cell in the prelatent phase. Even EBNA1, which has very critical long-term functions in maintaining and replicating the viral genomic DNA in established cell lines, was dispensable for the early activation of infected cells. Our findings document that the virus dose is a decisive parameter and indicate that EBNA2 governs the infected cells initially and implements a strictly controlled temporal program independent of other viral latent genes. It thus appears that EBNA2 is sufficient to control all requirements for clonal cellular expansion and to reprogram human B lymphocytes from energetically quiescent to activated cells.IMPORTANCE The preferred target of Epstein-Barr virus (EBV) is human resting B lymphocytes. We found that their infection induces a well-coordinated, time-driven program that starts with a substantial increase in cell volume, followed by cellular DNA synthesis after 3 days and subsequent rapid rounds of cell divisions on the next day accompanied by some DNA replication stress (DRS). Two to 3 days later, the cells decelerate and turn into stably proliferating lymphoblast cell lines. With the aid of 16 different recombinant EBV strains, we investigated the individual contributions of EBV's multiple latent genes during early B-cell infection and found that many do not exert a detectable phenotype or contribute little to EBV's prelatent phase. The exception is EBNA2 that is essential in governing all aspects of B-cell reprogramming. EBV relies on EBNA2 to turn the infected B lymphocytes into proliferating lymphoblasts preparing the infected host cell for the ensuing stable, latent phase of viral infection. In the early steps of B-cell reprogramming, viral latent genes other than EBNA2 are dispensable, but some, EBNA-LP, for example, support the viral program and presumably stabilize the infected cells once viral latency is established.

Epstein–Barr virus: overcoming the DNA damage response

Infections by DNA viruses including, Epstein–Barr virus (EBV), typically induce cellular DNA damage responses (DDR), in particular double-stranded break signaling. To avoid apoptosis associated with constitutive DDR signaling, downstream steps of this pathway must be inactivated. EBV has developed multiple ways of disabling the DDR using several different viral proteins expressed at various stages of EBV infection. Here the interplay between EBV and host DDRs is discussed at each stage of EBV infection, along with the EBV proteins and miRNAs that are known to interfere with DDR signaling. The newly discovered APOBEC editing of EBV DNA and protection from this mutation is also discussed.

Molecular Biology of EBV in Relationship to HIV/AIDS-Associated Oncogenesis

Herpesvirus-induced disease is one of the most lethal factors which leads to high mortality in HIV/AIDS patients. EBV, also known as human herpesvirus 4, can transform naive B cells into immortalized cells in vitro through the regulation of cell cycle, cell proliferation, and apoptosis. EBV infection is associated with several lymphoma and epithelial cancers in humans, which occurs at a much higher rate in immune deficient individuals than in healthy people, demonstrating that the immune system plays a vital role in inhibiting EBV activities. EBV latency infection proteins can mimic suppression cytokines or upregulate PD-1 on B cells to repress the cytotoxic T cells response. Many malignancies, including Hodgkin Lymphoma and non-Hodgkin's lymphomas occur at a much higher frequency in EBV positive individuals than in EBV negative people during the development of HIV infection. Importantly, understanding EBV pathogenesis at the molecular level will aid the development of novel therapies for EBV-induced diseases in HIV/AIDS patients.

Epstein-Barr Virus MicroRNA miR-BART5-3p Inhibits p53 Expression

Epstein-Barr virus (EBV) is the first human virus found to encode many microRNAs. It is etiologically linked to nasopharyngeal carcinoma and EBV-associated gastric carcinoma. During the latent infection period, there are only a few EBV proteins expressed, whereas EBV microRNAs, such as the BamHI-A region rightward transcript (BART) microRNAs, are highly expressed. However, how these BART miRNAs precisely regulate the tumor growth in nasopharyngeal carcinoma and gastric carcinoma remains obscure. Here, we report that upregulation of EBV-miR-BART5-3p promotes the growth of nasopharyngeal carcinoma and gastric carcinoma cells. BART5-3p directly targets the tumor suppressor gene TP53 on its 3'-untranslated region (3'-UTR) and consequently downregulates CDKN1A, BAX, and FAS expression, leading to acceleration of the cell cycle progress and inhibition of cell apoptosis. BART5-3p contributes to the resistance to chemotherapeutic drugs and ionizing irradiation-induced p53 increase. Moreover, BART5-3p also facilitates degradation of p53 proteins. BART5-3p is the first EBV-microRNA to be identified as inhibiting p53 expression and function, which suggests a novel mechanism underlying the strategies employed by EBV to maintain latent infection and promote the development of EBV-associated carcinomas.IMPORTANCE EBV encodes 44 mature microRNAs, which have been proven to promote EBV-associated diseases by targeting host genes and self-viral genes. In EBV-associated carcinomas, the expression of viral protein is limited but the expression of BART microRNAs is extremely high, suggesting that they could be major factors in the contribution of EBV-associated tumorigenesis. p53 is a critical tumor suppressor. Unlike in most human solid tumors, TP53 mutations are rare in nasopharyngeal carcinoma and EBV-associated gastric carcinoma tissues, suggesting a possibility that some EBV-encoded products suppress the functions of p53. This study provides the first evidence that a BART microRNA can suppress p53 expression by directly targeting its 3'-UTR. This study implies that EBV can use its BART microRNAs to modulate the expression of p53, thus maintaining its latency and contributing to tumorigenesis.

The Cooperative Functions of the EBNA3 Proteins Are Central to EBV Persistence and Latency

The Epstein–Barr nuclear antigen 3 (EBNA3) family of proteins, comprising EBNA3A, EBNA3B, and EBNA3C, play pivotal roles in the asymptomatic persistence and life-long latency of Epstein–Barr virus (EBV) in the worldwide human population. EBNA3-mediated transcriptional reprogramming of numerous host cell genes promotes in vitro B cell transformation and EBV persistence in vivo. Despite structural and sequence similarities, and evidence of substantial cooperative activity between the EBNA3 proteins, they perform quite different, often opposing functions. Both EBNA3A and EBNA3C are involved in the repression of important tumour suppressive pathways and are considered oncogenic. In contrast, EBNA3B exhibits tumour suppressive functions. This review focuses on how the EBNA3 proteins achieve the delicate balance required to support EBV persistence and latency, with emphasis on the contribution of the Allday laboratory to the field of EBNA3 biology.

Activation of ATR-Chk1 pathway facilitates EBV-mediated transformation of primary tonsillar B-cells

Primary infection of the immunocompromised host with the oncovirus Epstein-Barr virus (EBV) that targets mainly B-cells is associated with an increased risk for EBV-associated tumors. The early events subsequent to primary infection with potential for B-cell transformation are poorly studied. Here, we modeled in vitro the primary infection by using B-cells isolated from tonsils, the portal of entry of EBV, since species specificity of EBV hampers modeling in experimental animals. Increasing evidence indicates that the host DNA damage response (DDR) can influence and be influenced by EBV infection. Thus, we inoculated tonsillar B-cells (TBCs) with EBV-B95.8 and investigated cell proliferation and the DDR during the first 96 hours thereafter. We identified for the first time that EBV infection of TBCs induces a period of hyperproliferation 48-96 hours post infection characterized by the activation of ataxia telangiectasia and Rad3-releated (ATR) and checkpoint kinase-1 (Chk1). Whereas inhibition of Chk1 did not affect B-cell transformation, the specific inhibition of ATR robustly decreased the transformation efficiency of EBV. Our results suggest that activation of ATR is key for EBV-induced B-cell transformation. Thus, targeting the interaction between ATR/Chk1 and EBV could offer new options for the treatment of EBV-associated malignancies.

Epstein-Barr Virus: Diseases Linked to Infection and Transformation

Epstein–Barr virus (EBV) was first discovered in 1964, and was the first known human tumor virus now shown to be associated with a vast number of human diseases. Numerous studies have been conducted to understand infection, propagation, and transformation in various cell types linked to human diseases. However, a comprehensive lens through which virus infection, reactivation and transformation of infected host cells can be visualized is yet to be formally established and will need much further investigation. Several human cell types infected by EBV have been linked to associated diseases. However, whether these are a direct result of EBV infection or indirectly due to contributions by additional infectious agents will need to be fully investigated. Therefore, a thorough examination of infection, reactivation, and cell transformation induced by EBV will provide a more detailed view of its contributions that drive pathogenesis. This undoubtedly expand our knowledge of the biology of EBV infection and the signaling activities of targeted cellular factors dysregulated on infection. Furthermore, these insights may lead to identification of therapeutic targets and agents for clinical interventions. Here, we review the spectrum of EBV-associated diseases, the role of the encoded latent antigens, and the switch to latency or lytic replication which occurs in EBV infected cells. Furthermore, we describe the cellular processes and critical factors which contribute to cell transformation. We also describe the fate of B-cells and epithelial cells after EBV infection and the expected consequences which contribute to establishment of viral-associated pathologies.

EBV Nuclear Antigen 3C Mediates Regulation of E2F6 to Inhibit E2F1 Transcription and Promote Cell Proliferation

Epstein–Barr virus (EBV) is considered a ubiquitous herpesvirus with the ability to cause latent infection in humans worldwide. EBV-association is evidently linked to different types of human malignancies, mainly of epithelial and lymphoid origin. Of interest is the EBV nuclear antigen 3C (EBNA3C) which is critical for EBV-mediated immortalization. Recently, EBNA3C was shown to bind the E2F1 transcription regulator. The E2F transcription factors have crucial roles in various cellular functions, including cell cycle, DNA replication, DNA repair, cell mitosis, and cell fate. Specifically, E2F6, one of the unique E2F family members, is known to be a pRb-independent transcription repressor of E2F-target genes. In our current study, we explore the role of EBNA3C in regulating E2F6 activities. We observed that EBNA3C plays an important role in inducing E2F6 expression in LCLs. Our study also shows that EBNA3C physically interacts with E2F6 at its amino and carboxy terminal domains and they form a protein complex in human cells. In addition, EBNA3C stabilizes the E2F6 protein and is co-localized in the nucleus. We also demonstrated that both EBNA3C and E2F6 contribute to reduction in E2F1 transcriptional activity. Moreover, E2F1 forms a protein complex with EBNA3C and E2F6, and EBNA3C competes with E2F1 for E2F6 binding. E2F6 is also recruited by EBNA3C to the E2F1 promoter, which is critical for EBNA3C-mediated cell proliferation. These results demonstrate a critical role for E2F family members in EBV-induced malignancies, and provide new insights for targeting E2F transcription factors in EBV-associated cancers as potential therapeutic intervention strategies.

EBV is associated with a broad range of human cancers. EBV-encoded nuclear antigen 3C (EBNA3C) is one of the essential latent antigens important for deregulating the functions of numerous host transcription factors which play vital roles in B-cell immortalization. The family of E2F transcription factors are involved in diverse cellular functions. More specifically, E2F6 is one of the E2F family members with a unique property of transcriptional repression. Our current study now demonstrates that EBNA3C can enhance E2F6 repressive functions, and is also responsible for increased E2F6 protein expression in EBV-transformed LCLs. EBNA3C directly interacts with E2F6 at its amino and carboxy terminal domains. Additionally, E2F6 was stabilized by EBNA3C and co-localized in nuclear compartments. Our study also demonstrated that EBNA3C and E2F6 expression resulted in decreased transcriptional activity of E2F1, and that EBNA3C, E2F6 and E2F1 can form a protein complex, and EBNA3C competes with E2F1 for E2F6 binding. The recruitment of E2F6 by EBNA3C was also shown to be important for its related cell proliferation. These results showed a crucial role for EBNA3C-mediated deregulation of E2F6 and its impact on the activities of other E2F family members. Our findings also provide new insights for targeting these E2F transcription factors as potential therapeutic intervention strategies in EBV-associated cancers.

Dissecting the contribution of EBNA3C domains important for EBV-induced B-cell growth and proliferation

Epstein-Barr virus (EBV) is an oncogenic gammaherpes virus which is linked to pathogenesis of several human lymphatic malignancies. The EBV essential latent antigen EBNA3C is critical for efficient conversion of primary human B-lymphocytes to lymphoblastic cell lines and for continued LCL growth. EBNA3C, an EBV latent antigen with oncogenic potential can bind and regulate the functions of a wide range of cellular transcription factors. In our current reverse genetics study, we deleted the full length EBNA3C, and independently the RBP-Jκ and Nm23-H1 binding sites within EBNA3C using BACmid recombinant engineering methodology. Our experiments demonstrated that deletion of the EBV EBNA3C open reading frame (ORF) and more specifically the residues 621–675 which binds Nm23H1 and SUMO-1 showed a significant reduction in the ability of the cells to proliferate. Furthermore, they exhibited lower infectivity of human peripheral blood mononuclear cells (PBMCs). We also showed that recombinant EBV with deletions of the EBNA3C ORF, as well as a recombinant with residues 621–675 within EBNA3C ORF deleted had diminished abilities to activate CD40. Our study also revealed that the full length (1–992) and 621–675 aa deletions of EBNA3C when compared to wild type EBV infected PBMCs had differential expression patterns for the phosphorylation of MAP kinases specifically p38, JNK and ERK. Regulation of β-catenin also differed among wild type and EBNA3C deleted mutants. These temporal differences in signaling activities of these recombinant viruses in PBMCs is likely important in defining their functional importance in EBV-mediated B-cell transformation.

Role of EBNA-3 Family Proteins in EBV Associated B-cell Lymphomagenesis

Epstein-Barr virus (EBV) is highly ubiquitous in human population and establishes a lifelong asymptomatic infection within the infected host unless the immune system is compromised. Following initial infection in the oropharyngeal epithelial cells, EBV primarily infects naive B-lymphocytes and develops a number of B-cell lymphomas particularly in immune-deficient individuals. In vitro, EBV can also infect and subsequently transform quiescent B-lymphocytes into continuously proliferating lymphoblastoid cell lines (LCLs) resembling EBV-induced lymphoproliferative disorders in which a subset of latent transcripts are detected. Genetic studies revealed that EBNA-3 family comprising of three adjacent genes in the viral genome-EBNA-3A and -3C, but not -3B, are critical for B-cell transformation. Nevertheless, all three proteins appear to significantly contribute to maintain the overall proliferation and viability of transformed cells, suggesting a critical role in lymphoma development. Apart from functioning as important viral transcriptional regulators, EBNA-3 proteins associate with many cellular proteins in different signaling networks, providing a suitable platform for lifelong survival of the virus and concurrent lymphoma development in the infected host. The chapter describes the function of each these EBV nuclear antigen 3 proteins employed by the virus as a means to understand viral pathogenesis of several EBV-associated B-cell malignancies.

EBNA3C regulates p53 through induction of Aurora kinase B

In multicellular organisms p53 maintains genomic integrity through activation of DNA repair, and apoptosis. EBNA3C can down regulate p53 transcriptional activity. Aurora kinase (AK) B phosphorylates p53, which leads to degradation of p53. Aberrant expression of AK-B is a hallmark of numerous human cancers. Therefore changes in the activities of p53 due to AK-B and EBNA3C expression is important for understanding EBV-mediated cell transformation. Here we show that the activities of p53 and its homolog p73 are dysregulated in EBV infected primary cells which can contribute to increased cell transformation. Further, we showed that the ETS-1 binding site is crucial for EBNA3C-mediated up-regulation of AK-B transcription. Further, we determined the Ser 215 residue of p53 is critical for functional regulation by AK-B and EBNA3C and that the kinase domain of AK-B which includes amino acid residues 106, 111 and 205 was important for p53 regulation. AK-B with a mutation at residue 207 was functionally similar to wild type AK-B in terms of its kinase activities and knockdown of AK-B led to enhanced p73 expression independent of p53. This study explores an additional mechanism by which p53 is regulated by AK-B and EBNA3C contributing to EBV-induced B-cell transformation.

The Role of Gammaherpesviruses in Cancer Pathogenesis

Worldwide, one fifth of cancers in the population are associated with viral infections. Among them, gammaherpesvirus, specifically HHV4 (EBV) and HHV8 (KSHV), are two oncogenic viral agents associated with a large number of human malignancies. In this review, we summarize the current understanding of the molecular mechanisms related to EBV and KSHV infection and their ability to induce cellular transformation. We describe their strategies for manipulating major cellular systems through the utilization of cell cycle, apoptosis, immune modulation, epigenetic modification, and altered signal transduction pathways, including NF-kB, Notch, Wnt, MAPK, TLR, etc. We also discuss the important EBV latent antigens, namely EBNA1, EBNA2, EBNA3’s and LMP’s, which are important for targeting these major cellular pathways. KSHV infection progresses through the engagement of the activities of the major latent proteins LANA, v-FLIP and v-Cyclin, and the lytic replication and transcription activator (RTA). This review is a current, comprehensive approach that describes an in-depth understanding of gammaherpes viral encoded gene manipulation of the host system through targeting important biological processes in viral-associated cancers.

Gammaherpesvirus Infection of Human Neuronal Cells

Gammaherpesviruses human herpesvirus 4 (HHV4) and HHV8 are two prominent members of the herpesvirus family associated with a number of human cancers. HHV4, also known as Epstein-Barr virus (EBV), a ubiquitous gammaherpesvirus prevalent in 90 to 95% of the human population, is clinically associated with various neurological diseases such as primary central nervous system lymphoma, multiple sclerosis, Alzheimer's disease, cerebellar ataxia, and encephalitis. However, the possibility that EBV and Kaposi's sarcoma-associated herpesvirus (KSHV) can directly infect neurons has been largely overlooked. This study has, for the first time, characterized EBV infection in neural cell backgrounds by using the Sh-Sy5y neuroblastoma cell line, teratocarcinoma Ntera2 neurons, and primary human fetal neurons. Furthermore, we also demonstrated KSHV infection of neural Sh-Sy5y cells. These neuronal cells were infected with green fluorescent protein-expressing recombinant EBV or KSHV. Microscopy, genetic analysis, immunofluorescence, and Western blot analyses for specific viral antigens supported and validated the infection of these cells by EBV and KSHV and showed that the infection was efficient and productive. Progeny virus produced from infected neuronal cells efficiently infected fresh neuronal cells, as well as peripheral blood mononuclear cells. Furthermore, acyclovir was effective at inhibiting the production of virus from neuronal cells similar to lymphoblastoid cell lines; this suggests active lytic replication in infected neurons in vitro. These studies represent a potentially new in vitro model of EBV- and KSHV-associated neuronal disease development and pathogenesis.

IMPORTANCE

To date, no in vitro study has demonstrated gammaherpesvirus infection of neuronal cells. Moreover, worldwide clinical findings have linked EBV to neuronal pathologies, including multiple sclerosis, primary central nervous system lymphoma, and Alzheimer's disease. In this study, for the first time, we have successfully demonstrated the in vitro infection of Sh-Sy5y and Ntera2 cells, as well as human primary neurons. We have also determined that the infection is predominately lytic. Additionally, we also report infection of neuronal cells by KSHV in vitro similar to that by EBV. These findings may open new avenues of consideration related to neuronal pathologies and infection with these viruses. Furthermore, their contribution to chronic infection linked to neuronal disease will provide new clues to potential new therapies.

Modulation of DNA damage and repair pathways by human tumour viruses

With between 10% and 15% of human cancers attributable to viral infection, there is great interest, from both a scientific and clinical viewpoint, as to how these pathogens modulate host cell functions. Seven human tumour viruses have been identified as being involved in the development of specific malignancies. It has long been known that the introduction of chromosomal aberrations is a common feature of viral infections. Intensive research over the past two decades has subsequently revealed that viruses specifically interact with cellular mechanisms responsible for the recognition and repair of DNA lesions, collectively known as the DNA damage response (DDR). These interactions can involve activation and deactivation of individual DDR pathways as well as the recruitment of specific proteins to sites of viral replication. Since the DDR has evolved to protect the genome from the accumulation of deleterious mutations, deregulation is inevitably associated with an increased risk of tumour formation. This review summarises the current literature regarding the complex relationship between known human tumour viruses and the DDR and aims to shed light on how these interactions can contribute to genomic instability and ultimately the development of human cancers.

Regulation of DNA Damage Signaling and Cell Death Responses by Epstein-Barr Virus Latent Membrane Protein 1 (LMP1) and LMP2A in Nasopharyngeal Carcinoma Cells

Nasopharyngeal carcinoma (NPC) is closely associated with latent Epstein-Barr virus (EBV) infection. Although EBV infection of preneoplastic epithelial cells is not immortalizing, EBV can modulate oncogenic and cell death mechanisms. The viral latent membrane proteins 1 (LMP1) and LMP2A are consistently expressed in NPC and can cooperate in bitransgenic mice expressed from the keratin-14 promoter to enhance carcinoma development in an epithelial chemical carcinogenesis model. In this study, LMP1 and LMP2A were coexpressed in the EBV-negative NPC cell line HK1 and examined for combined effects in response to genotoxic treatments. In response to DNA damage activation, LMP1 and LMP2A coexpression reduced γH2AX (S139) phosphorylation and caspase cleavage induced by a lower dose (5 μM) of the topoisomerase II inhibitor etoposide. Regulation of γH2AX occurred before the onset of caspase activation without modulation of other DNA damage signaling mediators, including ATM, Chk1, or Chk2, and additionally was suppressed by inducers of DNA single-strand breaks (SSBs) and replication stress. Despite reduced DNA damage repair signaling, LMP1-2A coexpressing cells recovered from cytotoxic doses of etoposide; however, LMP1 expression was sufficient for this effect. LMP1 and LMP2A coexpression did not enhance cell growth, with a moderate increase of cell motility to fibronectin. This study supports that LMP1 and LMP2A jointly regulate DNA repair signaling and cell death activation with no further enhancement in the growth properties of neoplastic cells.

IMPORTANCE

NPC is characterized by clonal EBV infection and accounts for >78,000 annual cancer cases with increased incidence in regions where EBV is endemic, such as southeast Asia. The latent proteins LMP1 and LMP2A coexpressed in NPC can individually enhance growth or survival properties in epithelial cells, but their combined effects and potential regulation of DNA repair and checkpoint mechanisms are relatively undetermined. In this study, LMP1-2A coexpression suppressed activation of the DNA damage response (DDR) protein γH2AX induced by selective genotoxins that promote DNA replication stress or SSBs. Expression of LMP1 was sufficient to recover cells, resulting in outgrowth of LMP1 and LMP1-2A-coexpressing cells and indicating distinct LMP1-dependent effects in the restoration of replicative potential. These findings demonstrate novel properties for LMP1 and LMP2A in the cooperative modulation of DDR and apoptotic signaling pathways, further implicating both proteins in the progression of NPC and epithelial malignancies.

Epstein-Barr virus latent genes

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

The EBNA3 Family: Two Oncoproteins and a Tumour Suppressor that Are Central to the Biology of EBV in B Cells

Epstein-Barr virus nuclear antigens EBNA3A , EBNA3B and EBNA3C are a family of three large latency-associated proteins expressed in B cells induced to proliferate by the virus. Together with the other nuclear antigens (EBNA-LP, EBNA2 and EBNA1), they are expressed from a polycistronic transcription unit that is probably unique to B cells. However, compared with the other EBNAs, hitherto the EBNA3 proteins were relatively neglected and their roles in EBV biology rather poorly understood. In recent years, powerful new technologies have been used to show that these proteins are central to the latency of EBV in B cells, playing major roles in reprogramming the expression of host genes affecting cell proliferation, survival, differentiation and immune surveillance. This indicates that the EBNA3s are critical in EBV persistence in the B cell system and in modulating B cell lymphomagenesis. EBNA3A and EBNA3C are necessary for the efficient proliferation of EBV-infected B cells because they target important tumour suppressor pathways--so operationally they are considered oncoproteins. In contrast, it is emerging that EBNA3B restrains the oncogenic capacity of EBV, so it can be considered a tumour suppressor--to our knowledge the first to be described in a tumour virus. Here, we provide a general overview of the EBNA3 genes and proteins. In particular, we describe recent research that has highlighted the complexity of their functional interactions with each other, with specific sites on the human genome and with the molecular machinery that controls transcription and epigenetic states of diverse host genes.

Kaposi's sarcoma-associated herpesvirus genome programming during the early stages of primary infection of peripheral blood mononuclear cells

The early period of Kaposi’s sarcoma-associated herpesvirus (KSHV) infection involves the dynamic expression of viral genes, which are temporally and epigenetically regulated. KSHV can effectively infect and persist in endothelial as well as human B cells with different gene expression patterns. To understand the temporal epigenetic changes which occur when KSHV infects the lymphocytic compartment, we infected human peripheral blood mononuclear cells (PBMCs) and comprehensively analyzed the changes which occurred at the binding sites of virally encoded lytic as well as latent proteins along with epigenetic modifications across the KSHV genome during early primary infection. Using chromatin immunoprecipitation (ChIP) assays, we showed that the KSHV genome acquires a uniquely distinct histone modification pattern of methylation (H3K4me3, H3K9me3, and H3K27me3) and acetylation (H3Ac) during de novo infection of human PBMCs. This pattern showed that the epigenetic changes were temporally controlled. The binding profiles of KSHV latent protein LANA and the immediate early proteins RTA and K8 showed specific patterns at different times postinfection, which reflects the gene expression program. Further analysis demonstrated that KSHV can concurrently express lytic and latent genes which were associated with histone modifications at these specific regions on the viral genome. We identified three KSHV genes, K3, ORF49, and ORF64, which exhibited different profiles of histone modifications during the early stages of PBMC infection. These studies established a distinct pattern of epigenetic modification which correlates with viral gene expression temporally regulated during the first 7 days of PBMC infection and provides clues to the regulatory program required for successful infection by KSHV of human PBMCs.

Kaposi’s sarcoma-associated herpesvirus (KSHV) has been documented as one of the major contributors to morbidity and mortality in AIDS patients during the AIDS pandemic. During its life cycle, KSHV undergoes latent and lytic replication. Typically, KSHV maintains a stringent preference for latent infection in the infected B cells. However, 1 to 5% of infected cells undergo spontaneous lytic reactivation. KSHV lytic replication and infection of new cells are likely to be critical for maintaining the population of infected cells which drive virus-associated pathogenesis. Here, we explored the temporal changes of crucial histone marks on the KSHV genome during early infection of human primary peripheral blood mononuclear cells (PBMCs), which are a physiologically relevant system for monitoring primary infection. These results showed that KSHV possessed a distinct pattern of epigenetic marks during early infection of PBMCs. Further, KSHV concurrently expressed lytic and latent genes during this early period. These results now provide new evidence which contributes to understanding the molecular mechanism that regulates viral gene expression during early infection.

EBNA3C augments Pim-1 mediated phosphorylation and degradation of p21 to promote B-cell proliferation

Epstein–Barr virus (EBV), a ubiquitous human herpesvirus, can latently infect the human population. EBV is associated with several types of malignancies originating from lymphoid and epithelial cell types. EBV latent antigen 3C (EBNA3C) is essential for EBV-induced immortalization of B-cells. The Moloney murine leukemia provirus integration site (PIM-1), which encodes an oncogenic serine/threonine kinase, is linked to several cellular functions involving cell survival, proliferation, differentiation, and apoptosis. Notably, enhanced expression of Pim-1 kinase is associated with numerous hematological and non-hematological malignancies. A higher expression level of Pim-1 kinase is associated with EBV infection, suggesting a crucial role for Pim-1 in EBV-induced tumorigenesis. We now demonstrate a molecular mechanism which reveals a direct role for EBNA3C in enhancing Pim-1 expression in EBV-infected primary B-cells. We also showed that EBNA3C is physically associated with Pim-1 through its amino-terminal domain, and also forms a molecular complex in B-cells. EBNA3C can stabilize Pim-1 through abrogation of the proteasome/Ubiquitin pathway. Our results demonstrate that EBNA3C enhances Pim-1 mediated phosphorylation of p21 at the Thr¹⁴⁵ residue. EBNA3C also facilitated the nuclear localization of Pim-1, and promoted EBV transformed cell proliferation by altering Pim-1 mediated regulation of the activity of the cell-cycle inhibitor p21/WAF1. Our study demonstrated that EBNA3C significantly induces Pim-1 mediated proteosomal degradation of p21. A significant reduction in cell proliferation of EBV-transformed LCLs was observed upon stable knockdown of Pim-1. This study describes a critical role for the oncoprotein Pim-1 in EBV-mediated oncogenesis, as well as provides novel insights into oncogenic kinase-targeted therapeutic intervention of EBV-associated cancers.

The oncogenic serine/threonine kinase Pim-1 is upregulated in a number of human cancers including lymphomas, gastric, colorectal and prostate carcinomas. EBV nuclear antigen 3C (EBNA3C) is essential for EBV-induced transformation of human primary B-lymphocytes. Our current study revealed that EBNA3C significantly enhances Pim-1 kinase expression at both the transcript and protein levels. EBNA3C also interacts with Pim-1 and can form a complex in EBV-transformed cells. Moreover, EBNA3C increases nuclear localization of Pim-1 and stabilizes Pim-1 protein levels by inhibiting its poly-ubiquitination. Additionally, EBNA3C augments Pim-1 mediated phosphorylation of p21 and its proteosomal degradation. Stable knockdown of Pim-1 using si-RNA showed a significant decrease in proliferation of EBV transformed lymphoblastoid cell lines and subsequent induction of apoptosis by triggering the intrinsic apoptotic pathway. Therefore, our study demonstrated a new mechanism by which the oncogenic Pim-1 kinase targeted by EBV latent antigen 3C can inhibit p21 function, and is therefore a potential therapeutic target for the treatment of EBV-associated malignancies.