Epstein-Barr virus latent antigen 3C can mediate the degradation of the retinoblastoma protein through an SCF cellular ubiquitin ligase

Epstein-Barr virus nuclear antigen 3C binds to BATF/IRF4 or SPI1/IRF4 composite sites and recruits Sin3A to repress CDKN2A

Epstein-Barr virus nuclear antigen 3C (EBNA3C) repression of CDKN2A p14(ARF) and p16(INK4A) is essential for immortal human B-lymphoblastoid cell line (LCL) growth. EBNA3C ChIP-sequencing identified >13,000 EBNA3C sites in LCL DNA. Most EBNA3C sites were associated with active transcription; 64% were strong H3K4me1- and H3K27ac-marked enhancers and 16% were active promoters marked by H3K4me3 and H3K9ac. Using ENCODE LCL transcription factor ChIP-sequencing data, EBNA3C sites coincided (±250 bp) with RUNX3 (64%), BATF (55%), ATF2 (51%), IRF4 (41%), MEF2A (35%), PAX5 (34%), SPI1 (29%), BCL11a (28%), SP1 (26%), TCF12 (23%), NF-κB (23%), POU2F2 (23%), and RBPJ (16%). EBNA3C sites separated into five distinct clusters: (i) Sin3A, (ii) EBNA2/RBPJ, (iii) SPI1, and (iv) strong or (v) weak BATF/IRF4. EBNA3C signals were positively affected by RUNX3, BATF/IRF4 (AICE) and SPI1/IRF4 (EICE) cooccupancy. Gene set enrichment analyses correlated EBNA3C/Sin3A promoter sites with transcription down-regulation (P < 1.6 × 10(-4)). EBNA3C signals were strongest at BATF/IRF4 and SPI1/IRF4 composite sites. EBNA3C bound strongly to the p14(ARF) promoter through SPI1/IRF4/BATF/RUNX3, establishing RBPJ-, Sin3A-, and REST-mediated repression. EBNA3C immune precipitated with Sin3A and conditional EBNA3C inactivation significantly decreased Sin3A binding at the p14(ARF) promoter (P < 0.05). These data support a model in which EBNA3C binds strongly to BATF/IRF4/SPI1/RUNX3 sites to enhance transcription and recruits RBPJ/Sin3A- and REST/NRSF-repressive complexes to repress p14(ARF) and p16(INK4A) expression.

Evidence for regulation of oxidative stress by latent membrane protein 1 oncoprotein in patients with low-grade leukemic B cell lymphoma with latent Epstein-Barr virus infection

The role of latent Epstein-Barr virus (EBV) infection in the pathogenesis of low-grade B cell non-Hodgkin lymphoma (B-NHL) has not been studied. We therefore investigated the incidence of latent EBV infection in a group of patients with leukemic low-grade B-NHL, as well as the incidence of viral latent membrane protein 1 (LMP1) oncoprotein expression in the same patient group. Furthermore, in an attempt to elucidate the role of this viral oncoprotein in non-EBV-related lymphomas, we correlated the expression of LMP1 with the level of oxidative stress, a parameter related to apoptosis. In the present study we detected lower levels of oxidative stress in the sera of LMP1-positive patients. This possibly implies an anti-apoptotic role of this viral oncoprotein in low-grade B cell lymphomas. However, LMP1 expression status did not affect expression of the major anti-apoptotic gene BCL-2.

Epstein-Barr virus infection correlates with the expression of COX-2, p16(INK4A) and p53 in classic Hodgkin lymphoma

Classic Hodgkin lymphoma (cHL), a germinal-center related B cell neoplasm in almost all cases, is characterized by scarcity of the neoplastic Hodgkin/Reed-Sternberg (H/RS) cells. Epstein-Barr virus (EBV) has been shown to affect cell cycle and regulation of apoptosis. In total, 95 cases of cHL were studied. Five-micrometer sections were prepared and stained with hematoxylin and eosin and immunohistochemical streptavidin-biotin methods for EBV-LMP-1, COX-2, p53, p16, ki-67 and cleaved caspase-3. In-situ hybridization for EBV encoded RNA was used to confirm the detection of EBV in H/RS. There were 49 nodular sclerosis, 32 mixed cellularity, 8 lymphocyte-rich, and 6 lymphocyte-depleted subtypes in this series of cases. EBV, COX-2, p16(INK4A) and p53 were detected in 55% (52/95), 64% (61/95), 62% (59/95), and 65% (62/95) of the cases respectively. EBV was detected in 62% (38/61), 70% (41/59), and 69% (43/62) of COX2, p16 and p53 positive cases respectively. On the other hand, EBV-non-infected cases of cHL are associated with 59% (20/34), 69% (25/36), and 73% (24/33) of COX2, p16 and p53 negative cases respectively. In conclusion, EBV infection is associated with the expression of COX-2, p16(INK4A) and p53. EBV might be the dominant factor in determining the expression of these three proteins.

EBNA3C-mediated regulation of aurora kinase B contributes to Epstein-Barr virus-induced B-cell proliferation through modulation of the activities of the retinoblastoma protein and apoptotic caspases

Epstein-Barr virus (EBV) is an oncogenic gammaherpesvirus that is implicated in several human malignancies, including Burkitt's lymphoma (BL), posttransplant lymphoproliferative disease (PTLD), nasopharyngeal carcinoma (NPC), and AIDS-associated lymphomas. Epstein-Barr nuclear antigen 3C (EBNA3C), one of the essential EBV latent antigens, can induce mammalian cell cycle progression through its interaction with cell cycle regulators. Aurora kinase B (AK-B) is important for cell division, and deregulation of AK-B is associated with aneuploidy, incomplete mitotic exit, and cell death. Our present study shows that EBNA3C contributes to upregulation of AK-B transcript levels by enhancing the activity of its promoter. Further, EBNA3C also increased the stability of the AK-B protein, and the presence of EBNA3C leads to reduced ubiquitination of AK-B. Importantly, EBNA3C in association with wild-type AK-B but not with its kinase-dead mutant led to enhanced cell proliferation, and AK-B knockdown can induce nuclear blebbing and cell death. This phenomenon was rescued in the presence of EBNA3C. Knockdown of AK-B resulted in activation of caspase 3 and caspase 9, along with poly(ADP-ribose) polymerase 1 (PARP1) cleavage, which is known to be an important contributor to apoptotic signaling. Importantly, EBNA3C failed to stabilize the kinase-dead mutant of AK-B compared to wild-type AK-B, which suggests a role for the kinase domain in AK-B stabilization and downstream phosphorylation of the cell cycle regulator retinoblastoma protein (Rb). This study demonstrates the functional relevance of AK-B kinase activity in EBNA3C-regulated B-cell proliferation and apoptosis.

Impact of EBV essential nuclear protein EBNA-3C on B-cell proliferation and apoptosis

For over 40 years, EBV infection has been implicated in the etiology of a variety of lymphoid malignancies with the exceptional ability to drive resting B cells to continuously proliferate by successfully overriding cellular apoptotic stimuli. EBV utilizes the normal physiology of B-cell differentiation to persist within the memory B-cell pool of the immunocompetent host and subsequently establishes a life-long latent infection. During latency, out of a subset of viral genes expressed, EBNA-3C is one of the essential antigens required for in vitro primary B-cell transformation. EBNA-3C acts as a transcriptional coregulator by interacting with various cellular and viral factors. For the last 10 years, we have been actively engaged in discerning the biological significance of these interactions and revealed that EBNA-3C primarily targets two important cellular pathways - cell cycle and apoptosis. This review aims to summarize our current knowledge on EBNA-3C-mediated functions and describe how EBNA-3C seizes these cellular pathways that eventually promote B-cell lymphomagenesis. A scrupulous understanding of the critical relationship between EBNA-3C and these cellular machineries will not only aid in elucidating EBV pathogenesis, but also largely facilitate the development of novel diagnostic, as well as therapeutic, strategies against a vast range of EBV-associated B-cell lymphomas.

Interplay between DNA tumor viruses and the host DNA damage response

Viruses encounter many challenges within host cells in order to replicate their nucleic acid. In the case of DNA viruses, one challenge that must be overcome is recognition of viral DNA structures by the host DNA damage response (DDR) machinery. This is accomplished in elegant and unique ways by different viruses as each has specific needs and sensitivities dependent on its life cycle. In this review, we focus on three DNA tumor viruses and their interactions with the DDR. The viruses Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), and human papillomavirus (HPV) account for nearly all of the virus-associated human cancers worldwide. These viruses have also been excellent models for the study of oncogenic virus-mediated cell transformation. In this review, we will discuss how each of these viruses engage and subvert aspects of the host DDR. The first level of DDR engagement is a result of the genetic linkage between the oncogenic potential of these viruses and their ability to replicate. Namely, the promotion of cells from quiescence into the cell cycle to facilitate virus replication can be sensed through aberrant cellular DNA replication structures which activate the DDR and hinder cell transformation. DNA tumor viruses subvert this growth-suppressive DDR through changes in viral oncoprotein expression which ultimately facilitate virus replication. An additional level of DDR engagement is through direct detection of replicating viral DNA. These interactions parallel those observed in other DNA virus systems in that the need to subvert these intrinsic sensors of aberrant DNA structure in order to replicate must be in place. DNA tumor viruses are no exception. This review will cover the molecular features of DNA tumor virus interactions with the host DDR and the consequences for virus replication.

Posttranslational modifications of the retinoblastoma tumor suppressor protein as determinants of function

The retinoblastoma tumor suppressor protein (pRB) plays an integral role in G1-S checkpoint control and consequently is a frequent target for inactivation in cancer. The RB protein can function as an adaptor, nucleating components such as E2Fs and chromatin regulating enzymes into the same complex. For this reason, pRB's regulation by posttranslational modifications is thought to be critical. pRB is phosphorylated by a number of different kinases such as cyclin dependent kinases (Cdks), p38 MAP kinase, Chk1/2, Abl, and Aurora b. Although phosphorylation of pRB by Cdks has been extensively studied, activities regulated through phosphorylation by other kinases are just starting to be understood. As well as being phosphorylated, pRB is acetylated, methylated, ubiquitylated, and SUMOylated. Acetylation, methylation, and SUMOylation play roles in pRB mediated gene silencing. Ubiquitinylation of pRB promotes its degradation and may be used to regulate apoptosis. Recent proteomic data have revealed that pRB is posttranslationally modified to a much greater extent than previously thought. This new information suggests that many unknown pathways affect pRB regulation. This review focuses on posttranslational modifications of pRB and how they influence its function. The final part of the review summarizes new phosphorylation sites from accumulated proteomic data and discusses the possibilities that might arise from this data.

Epstein - Barr virus transforming protein LMP-1 alters B cells gene expression by promoting accumulation of the oncoprotein ΔNp73α

Many studies have proved that oncogenic viruses develop redundant mechanisms to alter the functions of the tumor suppressor p53. Here we show that Epstein-Barr virus (EBV), via the oncoprotein LMP-1, induces the expression of ΔNp73α, a strong antagonist of p53. This phenomenon is mediated by the LMP-1 dependent activation of c-Jun NH2-terminal kinase 1 (JNK-1) which in turn favours the recruitment of p73 to ΔNp73α promoter. A specific chemical inhibitor of JNK-1 or silencing JNK-1 expression strongly down-regulated ΔNp73α mRNA levels in LMP-1-containing cells. Accordingly, LMP-1 mutants deficient to activate JNK-1 did not induce ΔNp73α accumulation. The recruitment of p73 to the ΔNp73α promoter correlated with the displacement of the histone-lysine N-methyltransferase EZH2 which is part of the transcriptional repressive polycomb 2 complex. Inhibition of ΔNp73α expression in lymphoblastoid cells (LCLs) led to the stimulation of apoptosis and up-regulation of a large number of cellular genes as determined by whole transcriptome shotgun sequencing (RNA-seq). In particular, the expression of genes encoding products known to play anti-proliferative/pro-apoptotic functions, as well as genes known to be deregulated in different B cells malignancy, was altered by ΔNp73α down-regulation. Together, these findings reveal a novel EBV mechanism that appears to play an important role in the transformation of primary B cells.

Approximately 20% of worldwide human cancers have been associated with viral infections. Many oncogenic viruses exert their transforming properties by inactivating the products of tumour suppressor genes. One of the best characterized events induced by ongocenic viruses is the inactivation of the transcriptional factors p53. The mucosal high-risk HPV types, EBV, HTLV-1 and KSHV, via their viral proteins, are able to target p53 by distinct mechanisms. We have recently described a novel p53 inactivation mechanism of some cutaneous beta HPV types which have been suggested to be associated with skin carcinogenesis. Beta HPV38 induces accumulation of the p53 antagonist, ΔNp73α which in turn silences the expression of the p53-regulated genes. Here we report that also EBV, via the oncoprotein LMP-1, induces the expression of ΔNp73α which is dependent on the recruitment of p73 on ΔNp73 promoter and the activation of JNK-1. The recruitment of p73 to the ΔNp73 promoter correlated with the displacement of the histone-lysine N-methyltransferase EZH2 which is part of a transcriptional repressive polycomb 2 complex. We also show that ΔNp73α plays an important role in transformation of primary human B cells and regulates the expression of a large number of cellular genes that encode proteins linked to cancer development, including lymphomagenesis.

Multiple primary enoral soft tissue manifestations of a Hodgkin lymphoma--case report and literature review

BACKGROUND

Hodgkin lymphoma (HL) are lymphoproliferative neoplasms, histologically comprising of mononuclear and multinucleated Hodgkin and Reed Sternberg cells (HRS). About 4 % of all lymphatic malignancies of the head and neck are HL. The typical disease presents itself as a nodal lesion. Extranodal, enoral soft tissue involvement by HL is very rare.

CASE REPORT

A 73-year-old man with a suspect, ulcerating lesion in the left retromolar region of the mandible was assigned to our hospital. Prior anti-inflammatory therapy has been without success. Subsequently, three biopsies were taken which could only show inflammation. Finally, two biopsies from the left retromolar region and the left inner cheek showed HRS cells with positive expressions of CD15 and CD30 corresponding to a Hodgkin lymphoma. No lymphatic node or bone involvement could be detected. The patient was designated to receive radio-chemotherapy, but died 3 weeks after diagnosis of multiple organ failure. In a literature review, together with this report, nine cases were found concerning primary HL of the oral mucosa. Accordingly, this is the first case of primary multiple extranodal HL in the oral mucosa in absence of lymphatic node involvement.

DISCUSSION

Neither clinical features nor radiological appearances of HL presenting as primary enoral lesions are pathognomonic. Especially when only small biopsy specimens are available, histological diagnosis remains challenging, may lead to a delay in therapy and may result in a significant worse prognosis.

Induction of p16(INK4a) is the major barrier to proliferation when Epstein-Barr virus (EBV) transforms primary B cells into lymphoblastoid cell lines

To explore the role of p16^INK4a as an intrinsic barrier to B cell transformation by EBV, we transformed primary B cells from an individual homozygous for a deletion in the CDKN2A locus encoding p16^INK4a and p14^ARF. Using recombinant EBV-BAC viruses expressing conditional EBNA3C (3CHT), we developed a system that allows inactivation of EBNA3C in lymphoblastoid cell lines (LCLs) lacking active p16^INK4a protein but expressing a functional 14^ARF-fusion protein (p14/p16). The INK4a locus is epigenetically repressed by EBNA3C – in cooperation with EBNA3A – despite the absence of functional p16^INK4a. Although inactivation of EBNA3C in LCLs from normal B cells leads to an increase in p16^INK4a and growth arrest, EBNA3C inactivation in the p16^INK4a-null LCLs has no impact on the rate of proliferation, establishing that the repression of INK4a is a major function of EBNA3C in EBV-driven LCL proliferation. This conditional LCL system allowed us to use microarray analysis to identify and confirm genes regulated specifically by EBNA3C, independently of proliferation changes modulated by the p16^INK4a-Rb-E2F axis. Infections of normal primary B cells with recombinant EBV-BAC virus from which EBNA3C is deleted or with 3CHT EBV in the absence of activating ligand 4-hydroxytamoxifen, revealed that EBNA3C is necessary to overcome an EBV-driven increase in p16^INK4a expression and concomitant block to proliferation 2–4 weeks post-infection. If cells are p16^INK4a-null, functional EBNA3C is dispensable for the outgrowth of LCLs.

Epstein-Barr virus (EBV) is a causative agent of several types of B cell lymphoma. In human B cells, EBV reduces protein levels of at least two tumour suppressors that would otherwise be activated in response to over-expressed oncogenes. These proteins are BIM, which induces cell death and p16^INK4a, which prevents cell proliferation. Repression of both is via epigenetic methylation of histones and is dependent on expression of both EBNA3A and EBNA3C – two EBV proteins required for the transformation of normal B cells into lymphoblastoid cell lines (LCLs). In this report we have used EBV with a conditionally active EBNA3C – active only in the presence of 4-hydroxytamoxifen – together with B cells from an individual carrying a homozygous deletion of p16^INK4a to confirm that regulation of p16^INK4a expression is a major function of EBNA3C and demonstrate that if B cells lack p16^INK4a, then EBNA3C is no longer required for EBV-driven proliferation of LCLs. Furthermore we show that early after the infection of normal B cells, EBV induces p16^INK4a accumulation that – if unchecked by EBNA3C (and EBNA3A) – prevents LCL outgrowth. Formal proof that p16^INK4a is the main target of EBNA3C comes with the production of p16-null LCLs that have never expressed functional EBNA3C.

The Role of F-Box Proteins during Viral Infection

The F-box domain is a protein structural motif of about 50 amino acids that mediates protein–protein interactions. The F-box protein is one of the four components of the SCF (SKp1, Cullin, F-box protein) complex, which mediates ubiquitination of proteins targeted for degradation by the proteasome, playing an essential role in many cellular processes. Several discoveries have been made on the use of the ubiquitin–proteasome system by viruses of several families to complete their infection cycle. On the other hand, F-box proteins can be used in the defense response by the host. This review describes the role of F-box proteins and the use of the ubiquitin–proteasome system in virus–host interactions.

Understanding the targeting of the RB family proteins by viral oncoproteins to defeat their oncogenic machinery

The retinoblastoma (RB) family consists of three genes, RB1, RBL1, and RBL2, that code for the pRb, p107, and pRb2/p130 proteins, respectively. All these factors have pivotal roles in controlling fundamental cellular mechanisms such as cell cycle, differentiation and apoptosis. The founder and the most investigated RB family protein is pRb, which is considered to be the paradigm of tumor suppressors. However, p107 and pRb2/p130 clearly display a high degree of structural and functional homology with pRb. Interestingly, these factors were first identified as physical targets of the Adenovirus E1A oncoprotein. Indeed, RB family proteins are the most important and widely investigated targets of small DNA virus oncoproteins, such as Adenovirus E1A, human papillomavirus E7 and Simian virus 40 large T antigen. By interacting with pRb and with other RB family members, these oncoproteins neutralize their growth suppressive properties, thus stimulating proliferation of the infected cells, de-differentiation, and resistance to apoptosis. All these acquired features strongly favor the rise and selection of immortalized and mutation-prone cells, leading to a higher propensity in undergoing transformation. Our present work aims to illustrate and delve into these protein-protein interactions. Considering that these viral oncoproteins are dispensable for normal cellular functions, they can create "oncogene addiction" in the infected/transformed cells. This makes the possibility to dismantle these interactions extremely attractive, thus promoting the development of highly specific smart molecules capable of targeting only the infected/transformed cells that express these viral factors.

Therapeutic targeting of cancer cell cycle using proteasome inhibitors

Proteasomes are multicatalytic protease complexes in the cell, involved in the non-lysosomal recycling of intra-cellular proteins. Proteasomes play a critical role in regulation of cell division in both normal as well as cancer cells. In cancer cells this homeostatic function is deregulated leading to the hyperactivation of the proteasomes. Proteasome inhibitors (PIs) are a class of compounds, which either reversibly or irreversibly block the activity of proteasomes and induce cancer cell death. Interference of PIs with the ubiquitin proteasome pathway (UPP) involved in protein turnover in the cell leads to the accumulation of proteins engaged in cell cycle progression, which ultimately put a halt to cancer cell division and induce apoptosis. Upregulation of many tumor suppressor proteins involved in cell cycle arrest are known to play a role in PI induced cell cycle arrest in a variety of cancer cells. Although many PIs target the proteasomes, not all of them are effective in cancer therapy. Some cancers develop resistance against proteasome inhibition by possibly activating compensatory signaling pathways. However, the details of the activation of these pathways and their contribution to resistance to PI therapy remain obscure. Delineation of these pathways may help in checking resistance against PIs and deducing effective combinational approaches for improved treatment strategies. This review will discuss some of the signaling pathways related to proteasome inhibition and cell division that may help explain the basis of resistance of some cancers to proteasome inhibitors and underline the need for usage of PIs in combination with traditional chemotherapy.

Infection, mutation, and cancer evolution

An understanding of oncogenesis can be fostered by an integration of mechanistic studies with evolutionary considerations, which help explain why these mechanisms occur. This integration emphasizes infections and mutations as joint essential causes for many cancers. It suggests that infections may play a broader causal role in oncogenesis than has been generally appreciated. An evolutionary perspective also suggests that oncogenic viruses will tend to be transmitted by routes that provide infrequent opportunities for transmission, such as transmission by sexual and salivary contact. Such routes increase the intensity of natural selection for persistence within hosts, and molecular mechanisms for persistence often compromise critical barriers to oncogenesis, particularly cell cycle arrest, apoptosis, and a cap on the total number of divisions that a cell can undergo.

E2F1 mediated apoptosis induced by the DNA damage response is blocked by EBV nuclear antigen 3C in lymphoblastoid cells

EBV latent antigen EBNA3C is indispensible for in vitro B-cell immortalization resulting in continuously proliferating lymphoblastoid cell lines (LCLs). EBNA3C was previously shown to target pRb for ubiquitin-proteasome mediated degradation, which facilitates G1 to S transition controlled by the major transcriptional activator E2F1. E2F1 also plays a pivotal role in regulating DNA damage induced apoptosis through both p53-dependent and -independent pathways. In this study, we demonstrate that in response to DNA damage LCLs knocked down for EBNA3C undergo a drastic induction of apoptosis, as a possible consequence of both p53- and E2F1-mediated activities. Importantly, EBNA3C was previously shown to suppress p53-induced apoptosis. Now, we also show that EBNA3C efficiently blocks E2F1-mediated apoptosis, as well as its anti-proliferative effects in a p53-independent manner, in response to DNA damage. The N- and C-terminal domains of EBNA3C form a stable pRb independent complex with the N-terminal DNA-binding region of E2F1 responsible for inducing apoptosis. Mechanistically, we show that EBNA3C represses E2F1 transcriptional activity via blocking its DNA-binding activity at the responsive promoters of p73 and Apaf-1 apoptosis induced genes, and also facilitates E2F1 degradation in an ubiquitin-proteasome dependent fashion. Moreover, in response to DNA damage, E2F1 knockdown LCLs exhibited a significant reduction in apoptosis with higher cell-viability. In the presence of normal mitogenic stimuli the growth rate of LCLs knockdown for E2F1 was markedly impaired; indicating that E2F1 plays a dual role in EBV positive cells and that active engagement of the EBNA3C-E2F1 complex is crucial for inhibition of DNA damage induced E2F1-mediated apoptosis. This study offers novel insights into our current understanding of EBV biology and enhances the potential for development of effective therapies against EBV associated B-cell lymphomas.

Aberrant cellular proliferation due to deregulation of E2F1 transcriptional activity as a result of either genetic or functional alterations of its upstream components is a hallmark of human cancer. Interestingly, E2F1 can also promote cellular apoptosis regardless of p53 status by activating a number of pro-apoptotic genes in response to DNA damage stimuli. Epstein-Barr virus (EBV) encoded essential latent antigen EBNA3C can suppress p53-mediated apoptotic activities. This study now demonstrates that EBNA3C can further impede E2F1 mediated apoptosis by inhibiting its transcriptional ability, as well as by facilitating its degradation in an ubiquitin-proteasome dependent manner. This is the first evidence, which shows through targeting EBNA3C function linked to the E2F1-mediated apoptotic pathway, an additional therapeutic platform could be implemented against EBV-associated human B-cell lymphomas.

Proteasome and cancer

Cancer is one of the most frightful diseases mostly resulting in mortality; it has recently become more possible to overcome with the help of new therapies. In this direction, carcinogenesis is defined as a complicated process that can include several different factors that contribute to its progress. Proteasome is implicated in cancer studies as it is the main degradation system for oxidatively damaged proteins and also for several proteins playing a role in the cell cycle and transcription, which are important for cancer improvement. Because of this crucial role of proteasome in cancer development, myriad in vitro and in vivo studies have focused on the proteasome in different cancer cases. In this chapter, the involvement of proteasome in the degradation of cancer-related proteins is explained with the results of representative studies. Related to these proteins, the use of proteasome inhibitors in cancer treatment is reviewed.

Epstein-Barr virus nuclear antigen 3C stabilizes Gemin3 to block p53-mediated apoptosis

The Epstein-Barr nuclear antigen 3C (EBNA3C), one of the essential latent antigens for Epstein-Barr virus (EBV)-induced immortalization of primary human B lymphocytes in vitro, has been implicated in regulating cell proliferation and anti-apoptosis via interaction with several cellular and viral factors. Gemin3 (also named DDX20 or DP103) is a member of DEAD RNA helicase family which exhibits diverse cellular functions including DNA transcription, recombination and repair, and RNA metabolism. Gemin3 was initially identified as a binding partner to EBNA2 and EBNA3C. However, the mechanism by which EBNA3C regulates Gemin3 function remains unclear. Here, we report that EBNA3C directly interacts with Gemin3 through its C-terminal domains. This interaction results in increased stability of Gemin3 and its accumulation in both B lymphoma cells and EBV transformed lymphoblastoid cell lines (LCLs). Moreover, EBNA3C promotes formation of a complex with p53 and Gemin3 which blocks the DNA-binding affinity of p53. Small hairpin RNA based knockdown of Gemin3 in B lymphoma or LCL cells remarkably attenuates the ability of EBNA3C to inhibit the transcription activity of p53 on its downstream genes p21 and Bax, as well as apoptosis. These findings provide the first evidence that Gemin3 may be a common target of oncogenic viruses for driving cell proliferation and anti-apoptotic activities.

Gemin3 (DDX20 or DP103) is a member of the DEAD-box family of RNA helicases involved in various cellular processes including DNA transcription and RNA processing. The Epstein-Barr virus (EBV) encoded nuclear antigen 3C (EBNA3C) is essential for EBV-induced immortalization of primary human B-lymphocytes in vitro. In this study, we discovered that Gemin3 directly binds to the tumor suppressor p53, and acts as a negative regulator blocking p53 functions. Importantly, EBNA3C induces Gemin3 accumulation and enhances the formation of the complex of Gemin3 and p53 in EBV- transformed primary human B lymphocytes. Remarkably, inhibition of Gemin3 production leads to cell death of B lymphoma cells, particularly EBNA3C positive cells. This is the first evidence which shows that Gemin3 directly impairs p53 function in EBV positive cells, and that Gemin3 could be a potential target for EBV-associated cancer therapy.

Upregulation of the cell-cycle regulator RGC-32 in Epstein-Barr virus-immortalized cells

Epstein-Barr virus (EBV) is implicated in the pathogenesis of multiple human tumours of lymphoid and epithelial origin. The virus infects and immortalizes B cells establishing a persistent latent infection characterized by varying patterns of EBV latent gene expression (latency 0, I, II and III). The CDK1 activator, Response Gene to Complement-32 (RGC-32, C13ORF15), is overexpressed in colon, breast and ovarian cancer tissues and we have detected selective high-level RGC-32 protein expression in EBV-immortalized latency III cells. Significantly, we show that overexpression of RGC-32 in B cells is sufficient to disrupt G2 cell-cycle arrest consistent with activation of CDK1, implicating RGC-32 in the EBV transformation process. Surprisingly, RGC-32 mRNA is expressed at high levels in latency I Burkitt's lymphoma (BL) cells and in some EBV-negative BL cell-lines, although RGC-32 protein expression is not detectable. We show that RGC-32 mRNA expression is elevated in latency I cells due to transcriptional activation by high levels of the differentially expressed RUNX1c transcription factor. We found that proteosomal degradation or blocked cytoplasmic export of the RGC-32 message were not responsible for the lack of RGC-32 protein expression in latency I cells. Significantly, analysis of the ribosomal association of the RGC-32 mRNA in latency I and latency III cells revealed that RGC-32 transcripts were associated with multiple ribosomes in both cell-types implicating post-initiation translational repression mechanisms in the block to RGC-32 protein production in latency I cells. In summary, our results are the first to demonstrate RGC-32 protein upregulation in cells transformed by a human tumour virus and to identify post-initiation translational mechanisms as an expression control point for this key cell-cycle regulator.

Induction of p21-dependent senescence by an NAE inhibitor, MLN4924, as a mechanism of growth suppression

Cullin-RING ubiquitin ligase (CRL), with its founding member of SKP1-Cullins-F-box proteins (SCF) E3 ubiquitin ligase, is the largest family of E3 ligases, which requires cullin neddylation for its activation. Recently, an inhibitor of NEDD8 activating enzyme (NAE), MLN4924, was reported to block cullin neddylation and inactivate CRL/SCF E3, resulting in apoptosis induction and tumor suppression both in vitro and in vivo. We report here that apoptosis is not the sole mechanism by which MLN4924 suppresses tumor cell growth because apoptosis is moderately induced by the drug in some cancer cell lines and drug-induced growth suppression is only partially blocked by a pan-caspase inhibitor, z-VAD. MLN4924 treatment induces the characteristics of senescence phenotypes as evidenced by enlarged and flattened cellular morphology and positive staining of senescence-associated β-Gal. MLN4924-induced senescence is associated with cellular response to DNA damage, triggered by accumulation of DNA-licensing proteins CDT1 and ORC1, as a result of inactivation of CRL/SCF E3s. The senescence occurs in the manner independent of pRB/p16 and p53, but dependent on p21, a known substrate of CRL/SCF E3s and a mediator of senescence, which accumulates on CRL/SCF inactivation by MLN4924. Furthermore, MLN4924-induced senescence is irreversible and coupled with persistent accumulation of p21 and sustained activation of DNA damage response. Our study reveals a novel mechanism of MLN4924 action and showed that MLN4924 could be further developed as an effective anticancer agent by inducing apoptosis and irreversible senescence.

Functional modulation of the metastatic suppressor Nm23-H1 by oncogenic viruses

Evidence over the last two decades from a number of disciplines has solidified some fundamental concepts in metastasis, a major contributor to cancer associated deaths. However, significant advances have been made in controlling this critical cellular process by focusing on targeted therapy. A key set of factors associated with this invasive phenotype is the nm23 family of over twenty metastasis-associated genes. Among the eight known isoforms, Nm23-H1 is the most studied potential anti-metastatic factor associated with human cancers. Importantly, a growing body of work has clearly suggested a critical role for Nm23-H1 in limiting tumor cell motility and progression induced by several tumor viruses, including Epstein-Barr virus (EBV), Kaposi's sarcoma associated herpes virus (KSHV) and human papilloma virus (HPV). A more in depth understanding of the interactions between tumor viruses encoded antigens and Nm23-H1 will facilitate the elucidation of underlying mechanism(s) which contribute to virus-associated cancers. Here, we review recent studies to explore the molecular links between human oncogenic viruses and progression of metastasis, in particular the deregulation of Nm23-H1 mediated suppression.

Viral takeover of the host ubiquitin system

Like the other more well-characterized post-translational modifications (phosphorylation, methylation, acetylation, acylation, etc.), the attachment of the 76 amino acid ubiquitin (Ub) protein to substrates has been shown to govern countless cellular processes. As obligate intracellular parasites, viruses have evolved the capability to commandeer many host processes in order to maximize their own survival, whether it be to increase viral production or to ensure the long-term survival of latently infected host cells. The first evidence that viruses could usurp the Ub system came from the DNA tumor viruses and Adenoviruses, each of which use Ub to dysregulate the host cell cycle (Scheffner et al., 1990; Querido et al., 2001). Today, the list of viruses that utilize Ub includes members from almost every viral class, encompassing both RNA and DNA viruses. Among these, there are examples of Ub usage at every stage of the viral life cycle, involving both ubiquitination and de-ubiquitination. In addition to viruses that merely modify the host Ub system, many of the large DNA viruses encode their own Ub modifying machinery. In this review, we highlight the latest discoveries regarding the myriad ways that viruses utilize Ub to their advantage.

PAX8 promotes tumor cell growth by transcriptionally regulating E2F1 and stabilizing RB protein

The retinoblastoma protein (RB)–E2F1 pathway has a central role in regulating the cell cycle. Several PAX proteins (tissue-specific developmental regulators), including PAX8, interact with the RB protein, and thus regulate the cell cycle directly or indirectly. Here, we report that PAX8 expression is frequent in renal cell carcinoma, bladder, ovarian and thyroid cancer cell lines, and that silencing of PAX8 in cancer cell lines leads to a striking reduction in the expression of E2F1 and its target genes, as well as a proteasome-dependent destabilization of RB protein, with the RB1 mRNA level remaining unaffected. Cancer cells expressing PAX8 undergo a G₁/S arrest and eventually senesce following PAX8 silencing. We demonstrate that PAX8 transcriptionally regulates the E2F1 promoter directly, and E2F1 transcription is enhanced after RB depletion. RB is recruited to the PAX8-binding site, and is involved in PAX8-mediated E2F1 transcription in cancer cells. Therefore, our results suggest that, in cancer, frequent and persistent expression of PAX8 is required for cell growth control through transcriptional activation of E2F1 expression and upregulation of the RB–E2F1 pathway.

Epstein-Barr virus-associated B-cell lymphomas: pathogenesis and clinical outcomes

Epstein-Barr virus (EBV) is a ubiquitous human γ-herpesvirus that establishes a life-long asymptomatic infection in immunocompetent hosts. It is also found to be frequently associated with a broad spectrum of B-cell lymphomas predominantly seen in immunodeficient patients. Despite many resemblances, these EBV-linked lymphoproliferative disorders display heterogeneity at the clinical and the molecular level. Moreover, EBV-associated lymphoproliferative diseases differ in their differential expression patterns of the EBV-encoded latent antigens, which are directly related to their interactions with the host. EBV-driven primary B-cell immortalization is linked to the cooperative functions of these latent proteins, which are critical for perturbing many important cell-signaling pathways maintaining B-cell proliferation. Additionally, it is used as a surrogate model to explore the underlying mechanisms involved in the development of B-cell neoplasms. Recent discoveries have revealed that a number of sophisticated mechanisms are exploited by EBV during cancer progression. This finding will be instrumental in the design of novel approaches for therapeutic interventions against EBV-associated B-cell lymphomas. This review limits the discussion to the biology and pathogenesis of EBV-associated B-cell lymphomas and the related clinical implications.

Epstein-Barr virus nuclear antigen 3C facilitates G1-S transition by stabilizing and enhancing the function of cyclin D1

EBNA3C, one of the Epstein-Barr virus (EBV)-encoded latent antigens, is essential for primary B-cell transformation. Cyclin D1, a key regulator of G1 to S phase progression, is tightly associated and aberrantly expressed in numerous human cancers. Previously, EBNA3C was shown to bind to Cyclin D1 in vitro along with Cyclin A and Cyclin E. In the present study, we provide evidence which demonstrates that EBNA3C forms a complex with Cyclin D1 in human cells. Detailed mapping experiments show that a small N-terminal region which lies between amino acids 130-160 of EBNA3C binds to two different sites of Cyclin D1- the N-terminal pRb binding domain (residues 1-50), and C-terminal domain (residues 171-240), known to regulate Cyclin D1 stability. Cyclin D1 is short-lived and ubiquitin-mediated proteasomal degradation has been targeted as a means of therapeutic intervention. Here, we show that EBNA3C stabilizes Cyclin D1 through inhibition of its poly-ubiquitination, and also increases its nuclear localization by blocking GSK3β activity. We further show that EBNA3C enhances the kinase activity of Cyclin D1/CDK6 which enables subsequent ubiquitination and degradation of pRb. EBNA3C together with Cyclin D1-CDK6 complex also efficiently nullifies the inhibitory effect of pRb on cell growth. Moreover, an sh-RNA based strategy for knock-down of both cyclin D1 and EBNA3C genes in EBV transformed lymphoblastoid cell lines (LCLs) shows a significant reduction in cell-growth. Based on these results, we propose that EBNA3C can stabilize as well as enhance the functional activity of Cyclin D1 thereby facilitating the G1-S transition in EBV transformed lymphoblastoid cell lines.

Epstein-Barr virus nuclear antigens 3C and 3A maintain lymphoblastoid cell growth by repressing p16INK4A and p14ARF expression

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) and EBNA3A are each essential for EBV conversion of primary human B lymphocytes into continuously proliferating lymphoblast cell lines (LCLs) and for maintaining LCL growth. We now find that EBNA3C and EBNA3A's essential roles are to repress p16(INK4A) and p14(ARF). In the absence of EBNA3C or EBNA3A, p16(INK4A) and p14(ARF) expression increased and cell growth ceased. EBNA3C inactivation did not alter p16(INK4A) promoter CpG methylation, but reduced already low H3K27me3, relative to resting B cells, and increased H3K4me3 and H3-acetylation, linking EBNA3C inactivation to histone modifications associated with increased transcription. Importantly, knockdown of p16(INK4A) or p14(ARF) partially rescued LCLs from EBNA3C or EBNA3A inactivation-induced growth arrest and knockdown of both rescued LCL growth, confirming central roles for p16(INK4A) and p14(ARF) in LCL growth arrest following EBNA3C or EBNA3A inactivation. Moreover, blockade of p16(INK4A) and p14(ARF) effects on pRb and p53 by human papilloma virus type 16 E7 and E6 expression, sustained LCL growth after EBNA3C or EBNA3A inactivation. These data indicate that EBNA3C and EBNA3A joint repression of CDKN2A p16(INK4A) and p14(ARF) is essential for LCL growth.

EBNA3C attenuates the function of p53 through interaction with inhibitor of growth family proteins 4 and 5

Epstein-Barr virus (EBV)-encoded EBNA3C is one of the latent proteins essential for the efficient transformation of human primary B lymphocytes into continuously proliferating lymphoblastoid cell lines (LCLs) in vitro through manipulation of a number of major cellular pathways. Although it does not have direct DNA-binding activity, EBNA3C plays a central role in the transcriptional modulation of a wide range of both viral and cellular genes during latent infection. Recently, we showed that EBNA3C can directly bind to the tumor suppressor protein p53 and repress its functions, in part by blocking its transcriptional activity as well as facilitating its degradation through stabilization of its negative regulator, Mdm2. In this study, we further showed that EBNA3C can negatively regulate p53-mediated functions by interacting with its regulatory proteins, the inhibitor of growth family proteins ING4 and ING5, shown to be frequently deregulated in different cancers. Functional mapping revealed that both ING4 and ING5 bound to N-terminal domain residues 129 to 200 of EBNA3C, which was previously demonstrated to associate with p53 and is also essential for LCL growth. In addition, we showed that a conserved domain of either ING4 or ING5 bound to both p53 and EBNA3C in a competitive manner, suggesting a potential role for EBNA3C whereby the ING4 or -5/p53 pathway is modulated in EBV-infected cells. Subsequently, we demonstrated that EBNA3C significantly suppresses both the ING4- and ING5-mediated regulation of p53 transcriptional activity in a dose-dependent manner. A colony formation assay as well as an apoptosis assay showed that EBNA3C nullified the negative regulatory effects on cell proliferation induced by coupled expression of p53 in the presence of either ING4 or ING5 in Saos-2 (p53(-/-)) cells. This report demonstrates a possible role for the candidate tumor suppressor ING genes in the biology of EBV-associated cancers.

Epstein-Barr virus nuclear antigen 3C regulated genes in lymphoblastoid cell lines

EBV nuclear antigen 3C (EBNA3C) is an essential transcription factor for EBV transformed lymphoblast cell line (LCL) growth. To identify EBNA3C-regulated genes in LCLs, microarrays were used to measure RNA abundances in each of three different LCLs that conditionally express EBNA3C fused to a 4-OH-Tamoxifen-dependent estrogen receptor hormone binding domain (EBNA3CHT). At least three RNAs were assayed for each EBNA3CHT LCL under nonpermissive conditions, permissive conditions, and nonpermissive conditions with wild-type EBNA3C transcomplementation. Using a two-way ANOVA model of EBNA3C levels, we identified 550 regulated genes that were at least 1.5-fold up- or down-regulated with false discovery rates < 0.01. EBNA3C-regulated genes overlapped significantly with genes regulated by EBNA2 and EBNA3A consistent with coordinated effects on cell gene transcription. Of the 550 EBNA3C-regulated genes, 106 could be placed in protein networks. A seeded Bayesian network analysis of the 80 most significant EBNA3C-regulated genes suggests that RAC1, LYN, and TNF are upstream of other EBNA3C-regulated genes. Gene set enrichment analysis found enrichment for MAP kinase signaling, cytokine-cytokine receptor interactions, JAK-STAT signaling, and cell adhesion molecules, implicating these pathways in EBNA3C effects on LCL growth or survival. EBNA3C significantly up-regulated the CXCL12 ligand and its CXCR4 receptor and increased LCL migration. CXCL12 up-regulation depended on EBNA3C's interaction with the cell transcription factor, RBPJ, which is essential for LCL growth. EBNA3C also up-regulated MYC 1.3-fold and down-regulated CDKN2A exons 2 and 3, shared by p16 and p14, 1.4-fold, with false discovery rates < 5 × 10(-4).

Development of drugs for Epstein-Barr virus using high-throughput in silico virtual screening

IMPORTANCE OF THE FIELD

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that is causally associated with endemic forms of Burkitt's lymphoma, nasopharyngeal carcinoma and lymphoproliferative disease in immunosuppressed individuals. On a global scale, EBV infects > 90% of the adult population and is responsible for ∼ 1% of all human cancers. To date, there is no efficacious drug or therapy for the treatment of EBV infection and EBV-related diseases.

AREAS COVERED IN THIS REVIEW

In this review, we discuss the existing anti-EBV inhibitors and those under development. We discuss the value of different molecular targets, including EBV lytic DNA replication enzymes as well as proteins that are expressed exclusively during latent infection, such as EBV nuclear antigen 1 (EBNA-1) and latent membrane protein 1. As the atomic structure of the EBNA-1 DNA binding domain has been described, it is an attractive target for in silico methods of drug design and small molecule screening. We discuss the use of computational methods that can greatly facilitate the development of novel inhibitors and how in silico screening methods can be applied to target proteins with known structures, such as EBNA-1, to treat EBV infection and disease.

WHAT THE READER WILL GAIN

The reader is familiarized with the problems in targeting of EBV for inhibition by small molecules and how computational methods can greatly facilitate this process.

TAKE HOME MESSAGE

Despite the impressive efficacy of nucleoside analogs for the treatment of herpesvirus lytic infection, there remain few effective treatments for latent infections. As EBV latent infection persists within and contributes to the formation of EBV-associated cancers, targeting EBV latent proteins is an unmet medical need. High-throughput in silico screening can accelerate the process of drug discovery for novel and selective agents that inhibit EBV latent infection and associated disease.

Tumor viruses and cancer biology: Modulating signaling pathways for therapeutic intervention

Tumor viruses have provided relatively simple genetic systems, which can be manipulated for understanding the molecular mechanisms of the cellular transformation process. A growing body of information in the tumor virology field provides several prospects for rationally targeted therapies. However, further research is needed to better understand the multiple mechanisms utilized by these viruses in cancer progression in order to develop therapeutic strategies. Initially viruses were believed to be associated with cancers as causative agents only in animals. It was almost half a century before the first human tumor virus, Epstein-Barr virus (EBV), was identified in 1964. Subsequently, several human tumor viruses have been identified including Kaposi sarcoma associated herpesvirus (KSHV), human Papillomaviruses (HPV), Hepatitis B virus (HBV), Hepatitis C virus (HCV), Human T lymphotropic virus (HTLV-1) and recently identified Merkel cell Polyomavirus (MCPyV). Tumor viruses are sub-categorized as either DNA viruses, which include EBV, KSHV, HPV, HBV, and MCPyV, or RNA viruses such as HCV and HTLV-1. Tumor-viruses induce oncogenesis through manipulating an array of different cellular pathways. These viruses initiate a series of cellular events, which lead to immortalization and proliferation of the infected cells by disrupting the mitotic checkpoint upon infection of the host cell. This is often accomplished by functional inhibition or proteasomal degradation of many tumor suppressor proteins by virally encoded gene products. The virally infected cells can either be eliminated via cell-mediated apoptosis or persist in a state of chronic infection. Importantly, the chronic persistence of infection by tumor viruses can lead to oncogenesis. This review discusses the major human tumor associated viruses and their ability to modulate numerous cell signaling pathways, which can be targeted for potential therapeutic approaches.

Extensive co-operation between the Epstein-Barr virus EBNA3 proteins in the manipulation of host gene expression and epigenetic chromatin modification

Epstein-Barr virus (EBV) is able to drive the transformation of B-cells, resulting in the generation of lymphoblastoid cell lines (LCLs) in vitro. EBV nuclear proteins EBNA3A and EBNA3C are necessary for efficient transformation, while EBNA3B is dispensable. We describe a transcriptome analysis of BL31 cells infected with a series of EBNA3-knockout EBVs, including one deleted for all three EBNA3 genes. Using Affymetrix Exon 1.0 ST microarrays analysed with the MMBGX algorithm, we have identified over 1000 genes whose regulation by EBV requires one of the EBNA3s. Remarkably, a third of the genes identified require more than one EBNA3 for their regulation, predominantly EBNA3C co-operating with either EBNA3B, EBNA3A or both. The microarray was validated by real-time PCR, while ChIP analysis of a selection of co-operatively repressed promoters indicates a role for polycomb group complexes. Targets include genes involved in apoptosis, cell migration and B-cell differentiation, and show a highly significant but subtle alteration in genes involved in mitosis. In order to assess the relevance of the BL31 system to LCLs, we analysed the transcriptome of a set of EBNA3B knockout (3BKO) LCLs. Around a third of the genes whose expression level in LCLs was altered in the absence of EBNA3B were also altered in 3BKO-BL31 cell lines.

Among these are TERT and TCL1A, implying that EBV-induced changes in the expression of these genes are not required for B-cell transformation. We also identify 26 genes that require both EBNA3A and EBNA3B for their regulation in LCLs. Together, this shows the complexity of the interaction between EBV and its host, whereby multiple EBNA3 proteins co-operate to modulate the behaviour of the host cell.

Interaction of Epstein-Barr virus (EBV) with human B-lymphocytes

Epstein-Barr virus, EBV, and humans have a common history that reaches back to our primate ancestors. The virus co-evolved with man and has established a largely harmless and highly complex co-existence. It is carried as silent infection by almost all human adults. A serendipitous discovery established that it is the causative agent of infectious mononucleosis. Still, EBV became known first in 1964, in a rare, geographically prevalent malignant lymphoma of B-cell origin, Burkitt lymphoma BL. Its association with a malignancy prompted intensive studies and its capacity to immortalize B-lymphocytes in vitro was soon demonstrated. Consequently EBV was classified therefore as a potentially tumorigenic virus. Despite of this property however, the virus carrier state itself does not lead to malignancies because the transformed cells are recognized by the immune response. Consequently the EBV induced proliferation of EBV carrying B-lymphocytes is manifested only under immunosuppressive conditions. The expression of EBV encoded genes is regulated by the cell phenotype. The virus genome can be found in malignancies originating from cell types other than the B-lymphocyte. Even in the EBV infected B-cell, the direct transforming capacity is restricted to a defined window of differentiation. A complex interaction between virally encoded proteins and B-cell specific cellular proteins constitute the proliferation inducing program. In this short review we touch upon aspects which are the subject of our present work. We describe the mechanisms of some of the functional interactions between EBV encoded and cellular proteins that determine the phenotype of latently infected B-cells. The growth promoting EBV encoded genes are not expressed in the virus carrying BL cells. Still, EBV seems to contribute to the etiology of this tumor by modifying events that influence cell survival and proliferation. We describe a possible growth promoting mechanism in the genesis of Burkitt lymphoma that depends on the presence of EBV.

Epigenetic repression of p16(INK4A) by latent Epstein-Barr virus requires the interaction of EBNA3A and EBNA3C with CtBP

As an inhibitor of cyclin-dependent kinases, p16^INK4A is an important tumour suppressor and inducer of cellular senescence that is often inactivated during the development of cancer by promoter DNA methylation. Using newly established lymphoblastoid cell lines (LCLs) expressing a conditional EBNA3C from recombinant EBV, we demonstrate that EBNA3C inactivation initiates chromatin remodelling that resets the epigenetic status of p16^INK4A to permit transcriptional activation: the polycomb-associated repressive H3K27me3 histone modification is substantially reduced, while the activation-related mark H3K4me3 is modestly increased. Activation of EBNA3C reverses the distribution of these epigenetic marks, represses p16^INK4A transcription and allows proliferation. LCLs lacking EBNA3A express relatively high levels of p16^INK4A and have a similar pattern of histone modifications on p16^INK4A as produced by the inactivation of EBNA3C. Since binding to the co-repressor of transcription CtBP has been linked to the oncogenic activity of EBNA3A and EBNA3C, we established LCLs with recombinant viruses encoding EBNA3A- and/or EBNA3C-mutants that no longer bind CtBP. These novel LCLs have revealed that the chromatin remodelling and epigenetic repression of p16^INK4A requires the interaction of both EBNA3A and EBNA3C with CtBP. The repression of p16^INK4A by latent EBV will not only overcome senescence in infected B cells, but may also pave the way for p16^INK4A DNA methylation during B cell lymphomagenesis.

We previously showed that two Epstein-Barr virus latency-associated proteins—EBNA3A and EBNA3C—contribute to enhanced B cell survival by inhibiting the expression of the death-inducing protein BIM. This repression involves remodelling of the BIM gene promoter by polycomb proteins and DNA methylation within an unusually large CpG-island that flanks the transcription initiation site. Here we show that the same two proteins, EBNA3A and EBNA3C, functionally cooperate in the polycomb-mediated chromatin remodelling of another tumour suppressor gene, p16^INK4A, that encodes a cyclin-dependent kinase inhibitor capable of blocking cell proliferation. Both EBV proteins can bind the highly conserved co-repressor of transcription CtBP, and these interactions appear to be required for the efficient repression of p16^INK4A. Thus by utilising the polycomb system to induce the heritable repression of two major tumour suppressor genes—one that induces cell death (BIM) and one that induces growth arrest (p16^INK4A)—EBV profoundly alters latently infected B cells and their progeny, making them significantly more prone to malignant transformation.

Proteasome system of protein degradation and processing

In eukaryotic cells, degradation of most intracellular proteins is realized by proteasomes. The substrates for proteolysis are selected by the fact that the gate to the proteolytic chamber of the proteasome is usually closed, and only proteins carrying a special "label" can get into it. A polyubiquitin chain plays the role of the "label": degradation affects proteins conjugated with a ubiquitin (Ub) chain that consists at minimum of four molecules. Upon entering the proteasome channel, the polypeptide chain of the protein unfolds and stretches along it, being hydrolyzed to short peptides. Ubiquitin per se does not get into the proteasome, but, after destruction of the "labeled" molecule, it is released and labels another molecule. This process has been named "Ub-dependent protein degradation". In this review we systematize current data on the Ub-proteasome system, describe in detail proteasome structure, the ubiquitination system, and the classical ATP/Ub-dependent mechanism of protein degradation, as well as try to focus readers' attention on the existence of alternative mechanisms of proteasomal degradation and processing of proteins. Data on damages of the proteasome system that lead to the development of different diseases are given separately.

Destabilization of Rb by human papillomavirus E7 is cell cycle dependent: E2-25K is involved in the proteolysis

The HPV oncoprotein E7 promotes proteasomal degradation of the tumor suppressor protein Rb. In this study, we analyzed the regulation of E7-induced Rb proteolysis in HPV-containing Caski cervical cancer cells. We show that the Rb proteolysis is cell cycle dependent; in S phase Rb is stable while in post-mitotic early G1 phase cells and in differentiated cells, Rb is unstable. Similarly, the in vivo Rb/E7 interaction is not detected in S-phase cells, but is readily detected in differentiating Caski cells. The ubiquitinating enzymes involved in Rb proteolysis have not been identified. We find that the E3 ligase MDM2 is not involved in the Rb proteolysis in Caski cells. An in vivo analysis using multiple catalytic site mutant dominant negative E2 enzymes show that the C92A E2-25K most effectively blocks E7-induced Rb proteolysis. Taken together, these results show that E7 induces Rb proteolysis in growth-arrested cells and E2-25K is involved in the proteolysis.

Transcriptional downregulation of p27KIP1 through regulation of E2F function during LMP1-mediated transformation

LMP1 induces the phenotypic transformation of fibroblasts and affects regulators of the cell cycle during this process. LMP1 decreases expression of the cyclin-dependent kinase inhibitor p27 and increases the levels and phosphorylation of cyclin-dependent kinase 2 and the retinoblastoma protein. In the present study, the effects of LMP1 on cell cycle progression and the mechanism of p27 downregulation by LMP1 were determined. Although p27 is frequently regulated at the posttranscriptional level during cell cycle progression and in cancer, LMP1 did not decrease ectopically expressed p27. However, LMP1 did decrease p27 RNA levels and inhibited the activity of p27 promoter reporters. The LMP1-regulated promoter element was mapped to a region containing two E2F sites. Electrophoretic mobility shift assays determined that the regulated cis element bound an inhibitory E2F complex containing E2F4 and p130. These findings indicate that LMP1 decreases p27 transcription through effects on E2F family transcription factors. This property likely contributes to the ability of LMP1 to stimulate cell cycle progression.

Epstein-Barr virus nuclear protein 3C domains necessary for lymphoblastoid cell growth: interaction with RBP-Jkappa regulates TCL1

B lymphocytes converted into lymphoblastoid cell lines (LCLs) by an Epstein-Barr virus that expresses a conditional EBNA3C require complementation with EBNA3C for growth under nonpermissive conditions. Complementation with relatively large EBNA3C deletion mutants identified amino acids (aa) 1 to 506 (which includes the RBP-Jkappa/CSL [RBP-Jkappa] binding domain) and 733 to 909 to be essential for LCL growth, aa 728 to 732 and 910 to 992 to be important for full wild-type (wt) growth, and only aa 507 to 727 to be unimportant (S. Maruo, Y. Wu, T. Ito, T. Kanda, E. D. Kieff, and K. Takada, Proc. Natl. Acad. Sci. USA 106:4419-4424, 2009). When mutants with smaller deletions were used, only aa 51 to 400 and 851 to 900 were essential for LCL growth; aa 447 to 544, 701 to 750, 801 to 850, and 901 to 992 were important for full wt growth; and aa 4 to 50, 401 to 450, 550 to 707, and 751 to 800 were unimportant. These data reduce the EBNA3C essential residues from 68% to 40% of the open reading frame. Point mutations confirmed RBP-Jkappa binding to be essential for wt growth and indicated that SUMO and CtBP binding interactions were important only for full wt growth. EBNA3C aa 51 to 150, 249 to 311, and 851 to 900 were necessary for maintaining LCL growth, but not RBP-Jkappa interaction, and likely mediate interactions with other key cell proteins. Moreover, all mutants null for LCL growth had fewer S+G(2)/M-phase cells at 14 days, consistent with EBNA3C interaction with RBP-Jkappa as well as aa 51 to 150, 249 to 311, and 851 to 900 being required to suppress p16(INK4A) (S. Maruo, Y. Wu, S. Ishikawa, T. Kanda, D. Iwakiri, and K. Takada, Proc. Natl. Acad. Sci. USA 103:19500-19505, 2006). We have confirmed that EBNA3C upregulates TCL1 and discovered that EBNA3C upregulates TCL1 through RBP-Jkappa, indicating a central role for EBNA3C interaction with RBP-Jkappa in mediating cell gene transcription.

Comparative analysis of the Epstein-Barr virus encoded nuclear proteins of EBNA-3 family

It is known that the EBNA-3 family proteins (EBNA-3, -4 and -6, alternative nomenclature EBNA-3A, B and C correspondingly) show a limited sequence similarity. We have analyzed EBNA-3 proteins both at the primary sequence and secondary structure levels. EBNA-3 and EBNA-4 were structurally more similar compared to other combinations with EBNA-6. We found "Stonin Homology Domain" profile in EBNA-4 and "Proline Rich Domain" in all EBNA-3 family of proteins. We have also found positive and negative charge clusters in all three proteins and mixed charge clusters in EBNA-3. Charged clusters are believed to play an important role in interactions with DNA or signaling proteins. Additionally, unique primary sequence repeats were found in all three proteins.

Nucleoside diphosphate kinase/Nm23 and Epstein-Barr virus

Nm23-H1 was discovered as the first metastasis suppressor gene about 20 years ago. Since then, extensive work has contributed to understanding its role in various cellular signaling pathways. Its association with a range of human cancers as well as its ability to regulate cell cycle and suppress metastasis has been explored. We have determined that the EBV-encoded nuclear antigens, EBNA3C and EBNA1, required for EBV-mediated lymphoproliferation and for maintenance EBV genome extrachromosomally in dividing mammalian cells, respectively, target and disrupt the physiological role of Nm23-H1 in the context of cell proliferation and cell migration. This review will focus on the interaction of Nm23-H1 with the Epstein-Barr virus nuclear antigens, EBNA3C and EBNA1 and the functional significance of this interaction as it relates to EBV pathogenesis.

Three Epstein-Barr virus latency proteins independently promote genomic instability by inducing DNA damage, inhibiting DNA repair and inactivating cell cycle checkpoints

Epstein-Barr virus (EBV) has been implicated in the pathogenesis of human malignancies, but its contribution to tumorigenesis is not well understood. EBV carriage is associated with increased genomic instability in Burkitt's lymphoma, suggesting that viral products may induce this tumor phenotype. Using a panel of transfected sublines of the B-lymphoma line BJAB expressing the viral genes associated with latent infection, we show that the EBV nuclear antigens, EBNA-1 and EBNA-3C, and the latent membrane protein 1, LMP-1, independently promote genomic instability, as detected by nonclonal chromosomal aberrations, DNA breaks and phosphorylation of histone H2AX. EBNA-1 promotes the generation of DNA damage by inducing reactive oxygen species (ROS), whereas DNA repair is inhibited in LMP-1-expressing cells through downregulation of the DNA damage-sensing kinase, ataxia telangiectasia mutated (ATM), reduction of phosphorylation of its downstream targets Chk2 and inactivation of the G(2) checkpoint. EBNA-3C enhances the propagation of damaged DNA through inactivation of the mitotic spindle checkpoint and transcriptional downregulation of BubR1. Thus, multiple cellular functions involved in the maintenance of genome integrity seem to be independently targeted by EBV, pointing to the induction of genomic instability as a critical event in viral oncogenesis.

Epstein-Barr virus and its role in the pathogenesis of Burkitt's lymphoma: an unresolved issue

For several reasons Burkitt's lymphoma (BL) has become a paradigm in cancer research: for its particular geographical distribution, the presence of Epstein-Barr virus (EBV) in the cases in high incidence areas, and for the activation of the proto-oncogene c-myc by chromosomal translocation in one of the immunoglobulin gene loci. As c-MYC activates both, proliferation and apoptosis, at least two events have to cooperate in lymphomagenesis: activation of c-MYC and a shift in the balance from apoptosis towards survival. Antigenic and/or polyclonal stimulation of the B cell receptor, genetic instability imposed by activation induced deaminase (AID), as well as the viral gene products EBNA1 and several small non-coding non-polyadenylated RNAs are the main factors suspected to play an important role in the pathogenesis of BL. Despite intensive research, the role of the virus has remained largely elusive in the past decades, but the discovery of two viral microRNA clusters that are expressed in EBV associated tumors including BL has raised new hopes and expectations that EBV is going to reveal its mystery. This review focuses on the interplay between cellular and viral factors and puts special emphasis on mouse models and experimental cell culture systems that address these points.

EBV and genomic instability--a new look at the role of the virus in the pathogenesis of Burkitt's lymphoma

Epidemiological and molecular evidence links Epstein-Barr virus (EBV) carriage to the pathogenesis of human malignancies of lymphoid and epithelial cell origin but the mechanisms of viral oncogenesis are poorly understood. Burkitt's lymphoma, a tumor occurring in both EBV-positive and -negative forms, provides a convenient model for analysis of the relative contribution of genetic changes and viral products that are expressed in the malignant cells. Here we review recent findings that highlight several mechanisms by which EBV could play an important role in oncogenesis by promoting genomic instability.

ROC1/RBX1 E3 ubiquitin ligase silencing suppresses tumor cell growth via sequential induction of G2-M arrest, apoptosis, and senescence

Regulator of Cullins-1 (ROC1) or Ring Box Protein-1 (RBX1) is a RING component of SCF (Skp-1, cullins, F-box proteins) E3 ubiquitin ligases, which regulate diverse cellular processes by targeting a variety of substrates for degradation. However, little is known about the role of ROC1 in human cancer. Here, we report that ROC1 is ubiquitously overexpressed in primary human tumor tissues and human cancer cell lines. ROC1 silencing by siRNA significantly inhibited the growth of multiple human cancer cell lines via induction of senescence and apoptosis as well as G(2)-M arrest. Senescence induction is coupled with DNA damage in p53/p21- and p16/pRB-independent manners. Apoptosis is associated with accumulation of Puma and reduction of Bcl-2, Mcl-1, and survivin; and G(2)-M arrest is associated with accumulation of 14-3-3sigma and elimination of cyclin B1 and Cdc2. In U87 glioblastoma cells, these phenotypic changes occur sequentially upon ROC1 silencing, starting with G(2)-M arrest, followed by apoptosis and senescence. Thus, ROC1 silencing triggers multiple death and growth arrest pathways to effectively suppress tumor cell growth, suggesting that ROC1 may serve as a potential anticancer target.

Viral manipulation of DNA repair and cell cycle checkpoints

Recognition and repair of DNA damage is critical for maintaining genomic integrity and suppressing tumorigenesis. In eukaryotic cells, the sensing and repair of DNA damage are coordinated with cell cycle progression and checkpoints, in order to prevent the propagation of damaged DNA. The carefully maintained cellular response to DNA damage is challenged by viruses, which produce a large amount of exogenous DNA during infection. Viruses also express proteins that perturb cellular DNA repair and cell cycle pathways, promoting tumorigenesis in their quest for cellular domination. This review presents an overview of strategies employed by viruses to manipulate DNA damage responses and cell cycle checkpoints as they commandeer the cell to maximize their own viral replication. Studies of viruses have identified key cellular regulators and revealed insights into molecular mechanisms governing DNA repair, cell cycle checkpoints, and transformation.

Epstein-Barr virus nuclear antigen 3C targets p53 and modulates its transcriptional and apoptotic activities

The p53 tumor suppressor gene is one of the most commonly mutated genes in human cancers and the corresponding encoded protein induces apoptosis or cell-cycle arrest at the G1/S checkpoint in response to DNA damage. To date, previous studies have shown that antigens encoded by human tumor viruses such as SV40 large T antigen, adenovirus E1A and HPV E6 interact with p53 and disrupt its functional activity. In a similar fashion, we now show that EBNA3C, one of the EBV latent antigens essential for the B-cell immortalization in vitro, interacts directly with p53. Additionally, we mapped the interaction of EBNA3C with p53 to the C-terminal DNA-binding and the tetramerization domain of p53, and the region of EBNA3C responsible for binding to p53 was mapped to the N-terminal domain of EBNA3C (residues 130-190), previously shown to interact with a number of important cell-cycle components, specifically SCF(Skp2), cyclin A, and cMyc. Furthermore, we demonstrate that EBNA3C substantially represses the transcriptional activity of p53 in luciferase based reporter assays, and rescues apoptosis induced by ectopic p53 expression in SAOS-2 (p53(-/-)) cells. Interestingly, we also show that the DNA-binding ability of p53 is diminished in the presence of EBNA3C. Thus, the interaction between the p53 and EBNA3C provides new insights into the mechanism(s) by which the EBNA3C oncoprotein can alter cellular gene expression in EBV associated human cancers.

Epstein-Barr virus nuclear antigen 3C augments Mdm2-mediated p53 ubiquitination and degradation by deubiquitinating Mdm2

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is one of the essential latent antigens for primary B-cell transformation. Previous studies established that EBNA3C facilitates degradation of several vital cell cycle regulators, including the retinoblastoma (pRb) and p27(KIP) proteins, by recruitment of the SCF(Skp2) E3 ubiquitin ligase complex. EBNA3C was also shown to be ubiquitinated at its N-terminal residues. Furthermore, EBNA3C can bind to and be degraded in vitro by purified 20S proteasomes. Surprisingly, in lymphoblastoid cell lines, EBNA3C is extremely stable, and the mechanism for this stability is unknown. In this report we show that EBNA3C can function as a deubiquitination enzyme capable of deubiquitinating itself in vitro as well as in vivo. Functional mapping using deletion and point mutational analysis showed that both the N- and C-terminal domains of EBNA3C contribute to the deubiquitination activity. We also show that EBNA3C efficiently deubiquitinates Mdm2, an important cellular proto-oncogene, which is known to be overexpressed in several human cancers. The data presented here further demonstrate that the N-terminal domain of EBNA3C can bind to the acidic domain of Mdm2. Additionally, the N-terminal domain of EBNA3C strongly stabilizes Mdm2. Importantly, EBNA3C simultaneously binds to both Mdm2 and p53 and can form a stable ternary complex; however, in the presence of p53 the binding affinity of Mdm2 toward EBNA3C was significantly reduced, suggesting that p53 and Mdm2 might share a common overlapping domain of EBNA3C. We also showed that EBNA3C enhances the intrinsic ubiquitin ligase activity of Mdm2 toward p53, which in turn facilitated p53 ubiquitination and degradation. Thus, manipulation of the oncoprotein Mdm2 by EBNA3C potentially provides a favorable environment for transformation and proliferation of EBV-infected cells.

Epstein-Barr virus nuclear protein EBNA3C residues critical for maintaining lymphoblastoid cell growth

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is essential for efficient conversion of primary human B lymphocytes to lymphoblastoid cell lines (LCLs) and for continued LCL growth. We used a transcomplementation assay in the context of LCLs transformed by an EBV with a conditional EBNA3C to identify the EBNA3C amino acids (aa) necessary for maintaining LCL growth. Surprisingly, we found that most EBNA3C aa were essential for continued LCL growth. Only EBNA3C mutants deleted for residues within aa 507-515, 516-620, 637-675, or 676-727 maintained full LCL growth, and EBNA3C mutants deleted for residues within aa 728-732 or 910-992 maintained slow LCL growth. In contrast, EBNA3C lacking aa 180-231, which mediate RBP-Jkappa association and are necessary for EBNA3C abrogation of EBNA2-induced transcription through RBP-Jkappa, could not support LCL growth. Furthermore, 2 EBNA3C alanine substitution mutants within aa 180-231, which were wild-type (wt) in abrogating EBNA2-mediated transcription through RBP-Jkappa, maintained LCL growth, and 2 alanine substitution mutants within aa 180-231, which were null in abrogating EBNA2-mediated transcription through RBP-Jkappa, did not maintain LCL growth. This indicates that EBNA3C regulation of transcription through RBP-Jkappa is critical to maintaining LCL growth. Several other EBNA3C functions also are critical for LCL growth, because EBNA3C mutants deleted for residues within aa 130-159, 251-506, or 733-909 were wt in abrogating transcription through RBP-Jkappa and expression level, but did not maintain LCL growth.

Regulation of the retinoblastoma proteins by the human herpesviruses

Viruses are obligate intracellular parasites that alter the environment of infected cells in order to replicate more efficiently. One way viruses achieve this is by modulating cell cycle progression. The main regulators of progression out of G0, through G1, and into S phase are the members of the retinoblastoma (Rb) family of tumor suppressors. Rb proteins repress the transcription of genes controlled by the E2F transcription factors. Because the expression of E2F-responsive genes is required for cell cycle progression into the S phase, Rb arrests the cell cycle in G0/G1. A number of viral proteins directly target Rb family members for inactivation, presumably to create an environment more hospitable for viral replication. Such viral proteins include the extensively studied oncoproteins E7 (from human papillomavirus), E1A (from adenovirus), and the large T (tumor) antigen (from simian virus 40). Elucidating how these three viral proteins target and inactivate Rb has proven to be an invaluable approach to augment our understanding of both normal cell cycle progression and carcinogenesis. In addition to these proteins, a number of other virally-encoded inactivators of the Rb family have subsequently been identified including a surprising number encoded by human herpesviruses. Here we review how the human herpesviruses modulate Rb function during infection, introduce the individual viral proteins that directly or indirectly target Rb, and speculate about what roles Rb modulation by these proteins may play in viral replication, pathogenesis, and oncogenesis.