Small molecule and peptide-mediated inhibition of Epstein-Barr virus nuclear antigen 1 dimerization

Targeting EBV-encoded products: Implications for drug development in EBV-associated diseases

Epstein-Barr virus, a human gamma-herpesvirus, has a close connection to the pathogenesis of cancers and other diseases, which are a burden for public health worldwide. So far, several drugs or biomolecules have been discovered that can target EBV-encoded products for treatment, such as Silvestrol, affinity toxin, roscovitine, H20, H31, curcumin, thymoquinone, and ribosomal protein L22. These drugs activate or inhibit the function of some biomolecules, affecting subsequent signalling pathways by acting on the products of EBV. These drugs usually target LMP1, LMP2; EBNA1, EBNA2, EBNA3; EBER1, EBER2; Bam-HI A rightward transcript and BHRF1. Additionally, some promising findings in the fields of vaccines, immunological, and cellular therapies have been established. In this review, we mainly summarise the function of drugs mentioned above and unique mechanisms, hoping that we can help giving insight to the design of drugs for the treatment of EBV-associated diseases.

Therapeutic approaches to Epstein-Barr virus cancers

Epstein-Barr virus (EBV) establishes a lifelong latent infection that can be a causal agent for a diverse spectrum of cancers and autoimmune disease. A complex and dynamic viral lifecycle evades eradication by the host immune system and confounds antiviral therapeutic strategies. To date, there are no clinically approved vaccines or therapies that selectively target EBV as the underlying cause of EBV-associated disease. Here, we review the challenges and recent advances in the development of EBV-specific therapeutics for treatment of EBV-associated cancers.

EBV-associated diseases: Current therapeutics and emerging technologies

EBV is a prevalent virus, infecting >90% of the world's population. This is an oncogenic virus that causes ~200,000 cancer-related deaths annually. It is, in addition, a significant contributor to the burden of autoimmune diseases. Thus, EBV represents a significant public health burden. Upon infection, EBV remains dormant in host cells for long periods of time. However, the presence or episodic reactivation of the virus increases the risk of transforming healthy cells to malignant cells that routinely escape host immune surveillance or of producing pathogenic autoantibodies. Cancers caused by EBV display distinct molecular behaviors compared to those of the same tissue type that are not caused by EBV, presenting opportunities for targeted treatments. Despite some encouraging results from exploration of vaccines, antiviral agents and immune- and cell-based treatments, the efficacy and safety of most therapeutics remain unclear. Here, we provide an up-to-date review focusing on underlying immune and environmental mechanisms, current therapeutics and vaccines, animal models and emerging technologies to study EBV-associated diseases that may help provide insights for the development of novel effective treatments.

Incorporation of Fmoc-Dab(Mtt)-OH during solid-phase peptide synthesis: a word of caution

As a commercially available and orthogonally protected amino acid building block, Fmoc-Dab(Mtt)-OH showed abnormally poor coupling efficiency during solid-phase peptide synthesis (SPPS).

The Role of EBV-Encoded LMP1 in the NPC Tumor Microenvironment: From Function to Therapy

Nasopharyngeal carcinoma (NPC) is closely associated with Epstein-Barr virus (EBV) infection. It is also characterized by heavy infiltration with non-malignant leucocytes. The EBV-encoded latent membrane protein 1 (LMP1) is believed to play an important role in NPC pathogenesis by virtue of its ability to activate multiple cell signaling pathways which collectively promote cell proliferation and survival, angiogenesis, invasiveness, and aerobic glycolysis. LMP1 also affects cell-cell interactions, antigen presentation, and cytokine and chemokine production. Here, we discuss how LMP1 modulates local immune responses that contribute to the establishment of the NPC tumor microenvironment. We also discuss strategies for targeting the LMP1 protein as a novel therapy for EBV-driven malignancies.

Targeting Epstein-Barr Virus in Nasopharyngeal Carcinoma

Nasopharyngeal carcinoma (NPC) is consistently associated with Epstein-Barr virus (EBV) infection in regions in which it is endemic, including Southern China and Southeast Asia. The high mortality rates of NPC patients with advanced and recurrent disease highlight the urgent need for effective treatments. While recent genomic studies have revealed few druggable targets, the unique interaction between the EBV infection and host cells in NPC strongly implies that targeting EBV may be an efficient approach to cure this virus-associated cancer. Key features of EBV-associated NPC are the persistence of an episomal EBV genome and the requirement for multiple viral latent gene products to enable malignant transformation. Many translational studies have been conducted to exploit these unique features to develop pharmaceutical agents and therapeutic strategies that target EBV latent proteins and induce lytic reactivation in NPC. In particular, inhibitors of the EBV latent protein EBNA1 have been intensively explored, because of this protein's essential roles in maintaining EBV latency and viral genome replication in NPC cells. In addition, recent advances in chemical bioengineering are driving the development of therapeutic agents targeting the critical functional regions of EBNA1. Promising therapeutic effects of the resulting EBNA1-specific inhibitors have been shown in EBV-positive NPC tumors. The efficacy of multiple classes of EBV lytic inducers for NPC cytolytic therapy has also been long investigated. However, the lytic-induction efficiency of these compounds varies among different EBV-positive NPC models in a cell-context-dependent manner. In each tumor, NPC cells can evolve and acquire somatic changes to maintain EBV latency during cancer progression. Unfortunately, the poor understanding of the cellular mechanisms regulating EBV latency-to-lytic switching in NPC cells limits the clinical application of EBV cytolytic treatment. In this review, we discuss the potential approaches for improvement of the above-mentioned EBV-targeting strategies.

Reactivation of Epstein-Barr virus by a dual-responsive fluorescent EBNA1-targeting agent with Zn2+-chelating function

EBNA1 is the only Epstein–Barr virus (EBV) latent protein responsible for viral genome maintenance and is expressed in all EBV-infected cells. Zn²⁺ is essential for oligomerization of the functional EBNA1. We constructed an EBNA1 binding peptide with a Zn²⁺ chelator to create an EBNA1-specific inhibitor (ZRL₅P₄). ZRL₅P₄ by itself is sufficient to reactivate EBV from its latent infection. ZRL₅P₄ is able to emit unique responsive fluorescent signals once it binds with EBNA1 and a Zn²⁺ ion. ZRL₅P₄ can selectively disrupt the EBNA1 oligomerization and cause nasopharyngeal carcinoma (NPC) tumor shrinkage, possibly due to EBV lytic induction. Dicer1 seems essential for this lytic reactivation. As can been seen, EBNA1 is likely to maintain NPC cell survival by suppressing viral reactivation.

Epstein–Barr nuclear antigen 1 (EBNA1) plays a vital role in the maintenance of the viral genome and is the only viral protein expressed in nearly all forms of Epstein–Barr virus (EBV) latency and EBV-associated diseases, including numerous cancer types. To our knowledge, no specific agent against EBV genes or proteins has been established to target EBV lytic reactivation. Here we report an EBNA1- and Zn²⁺-responsive probe (ZRL₅P₄) which alone could reactivate the EBV lytic cycle through specific disruption of EBNA1. We have utilized the Zn²⁺ chelator to further interfere with the higher order of EBNA1 self-association. The bioprobe ZRL₅P₄ can respond independently to its interactions with Zn²⁺ and EBNA1 with different fluorescence changes. It can selectively enter the nuclei of EBV-positive cells and disrupt the oligomerization and oriP-enhanced transactivation of EBNA1. ZRL₅P₄ can also specifically enhance Dicer1 and PML expression, molecular events which had been reported to occur after the depletion of EBNA1 expression. Importantly, we found that treatment with ZRL₅P₄ alone could reactivate EBV lytic induction by expressing the early and late EBV lytic genes/proteins. Lytic induction is likely mediated by disruption of EBNA1 oligomerization and the subsequent change of Dicer1 expression. Our probe ZRL₅P₄ is an EBV protein-specific agent that potently reactivates EBV from latency, leading to the shrinkage of EBV-positive tumors, and our study also suggests the association of EBNA1 oligomerization with the maintenance of EBV latency.

Signatures of B-cell receptor diversity in B lymphocytes following Epstein-Barr virus transformation

Epstein-Barr virus (EBV) is a widespread human virus that establishes latent infection, potentially leading to tumors, hematological disorders, and other severe diseases. EBV infections are associated with diverse symptoms and affect various organs; therefore, early diagnosis and treatment are crucial. B cell receptor (BCR) repertoires of B cell surface immunoglobulins have been widely studied for their association with various infectious diseases. However, the specific genetic changes that modulate the BCR repertoires after an EBV infection are still poorly understood. In this study, we employed high-throughput sequencing (HTS) to investigate the diversity of BCR repertoires in an EBV-transformed lymphoblastic cell line (LCL). Compared with the noninfected control B cell line, the LCL exhibited a decrease in overall BCR diversity but displayed an increase in the expansion of some dominant rearrangements such as IGHV4-31/IGHJ4, IGHV4-59/IGHJ4, IGHV5-51/IGHJ3, and IGHV3-74/IGHJ3. A higher frequency of occurrence of these rearrangement types was confirmed in patients with EBV infection. Interestingly, the IGHV3-74 rearrangement was only detected in EBV-infected children, suggesting that our experimental observations were not coincidental. In addition, we identified a highly dominant consensus motif, CAR(xRx)YGSG(xYx)FD, in complementarity-determining region 3 (CDR3) sequences of the heavy chain in the LCL. Our findings demonstrated the utility of HTS technology for studying the variations in signature motifs of the BCR repertoires after EBV infection. We propose that the analysis of BCR repertoire sequences represents a promising method for diagnosing early EBV infections and developing novel antibody- and vaccine-based therapies against such infections.

EBNA1-targeted inhibitors: Novel approaches for the treatment of Epstein-Barr virus-associated cancers

Epstein-Barr virus (EBV) infects more than 90% of humans worldwide and establishes lifelong latent infection in the hosts. It is closely associated with endemic forms of a wide range of human cancers and directly contributes to the formation of some. Despite its critical role in cancer development, no EBV- or EBV latent protein-targeted therapy is available. The EBV-encoded latent protein, Epstein-Barr nuclear antigen 1 (EBNA1), is expressed in all EBV-associated tumors and acts as the only latent protein in some of these tumors. This versatile protein functions in the maintenance, replication, and segregation of the EBV genome and can therefore serve as an attractive therapeutic target to treat EBV-associated cancers. In the last decades, efforts have been made for designing specific EBNA1 inhibitors to decrease EBNA1 expression or interfere with EBNA1-dependent functions. In this review, we will briefly introduce the salient features of EBNA1, summarize its functional domains, and focus on the recent developments in the identification and design of EBNA1 inhibitors related to various EBNA1 domains as well as discuss their comparative merits.

Responsive upconversion nanoprobe for monitoring and inhibition of EBV-associated cancers via targeting EBNA1

Non-responsive emission enhancement is the disadvantage of upconversion nanomaterials (UCNM) when compared with conventional organic based agents for molecular imaging. We herein show a new strategy by conjugating NaGdF4:Yb3+,Er3+@NaGdF4 (UCNP) with peptides to achieve responsive UC emission enhancement upon binding to a targeted protein - EBNA1. EBNA1 is a well-known viral latent protein for the EBV-associated cancer. Peptide-coating of the functionalized core-shell nanoparticle diminishes upconverted emission intensity drastically. However, the peptide-coated UCNP shows selective and responsive UC emission enhancement via aggregation with the targeted protein. This phenomenon paves a new way for UCNM in molecular imaging.

EBNA1: Oncogenic Activity, Immune Evasion and Biochemical Functions Provide Targets for Novel Therapeutic Strategies against Epstein-Barr Virus- Associated Cancers

The presence of the Epstein-Barr virus (EBV)-encoded nuclear antigen-1 (EBNA1) protein in all EBV-carrying tumours constitutes a marker that distinguishes the virus-associated cancer cells from normal cells and thereby offers opportunities for targeted therapeutic intervention. EBNA1 is essential for viral genome maintenance and also for controlling viral gene expression and without EBNA1, the virus cannot persist. EBNA1 itself has been linked to cell transformation but the underlying mechanism of its oncogenic activity has been unclear. However, recent data are starting to shed light on its growth-promoting pathways, suggesting that targeting EBNA1 can have a direct growth suppressing effect. In order to carry out its tasks, EBNA1 interacts with cellular factors and these interactions are potential therapeutic targets, where the aim would be to cripple the virus and thereby rid the tumour cells of any oncogenic activity related to the virus. Another strategy to target EBNA1 is to interfere with its expression. Controlling the rate of EBNA1 synthesis is critical for the virus to maintain a sufficient level to support viral functions, while at the same time, restricting expression is equally important to prevent the immune system from detecting and destroying EBNA1-positive cells. To achieve this balance EBNA1 has evolved a unique repeat sequence of glycines and alanines that controls its own rate of mRNA translation. As the underlying molecular mechanisms for how this repeat suppresses its own rate of synthesis in cis are starting to be better understood, new therapeutic strategies are emerging that aim to modulate the translation of the EBNA1 mRNA. If translation is induced, it could increase the amount of EBNA1-derived antigenic peptides that are presented to the major histocompatibility (MHC) class I pathway and thus, make EBV-carrying cancers better targets for the immune system. If translation is further suppressed, this would provide another means to cripple the virus.

ERK2 phosphorylation of EBNA1 serine 383 residue is important for EBNA1-dependent transactivation

Functional inhibition of Epstein-Barr virus (EBV)-encoded nuclear antigen 1 (EBNA1) can cause the death of EBV infected cells. In this study, a bioinformatics tool predicted the existence of putative extracellular signal-regulated kinase (ERK) docking and substrate consensus sites on EBNA1, suggesting that ERK2 could bind to and phosphorylate EBNA1. In accordance, ERK2 was found to phosphorylate EBNA1 serine 383 in a reaction suppressed by H20 (a structural congener of the ERK inhibitor), U0126 (an inhibitor of MEK kinase), and mutations at substrate (S383A) or putative ERK docking sites. Wild-type (S383) and phosphomimetic (S383D) EBNA1 demonstrated comparable transactivation function, which was suppressed by H20 or U0126. In contrast, non-phosphorylated EBNA1 mutants displayed significantly impaired transactivation activity. ERK2 knock-down by siRNA, or treatment with U0126 or H20 repressed EBNA1-dependent transactivation. Collectively, these data indicate that blocking ERK2-directed phosphorylation can suppress EBNA1-transactivation function in latent EBV-infected cells, validating ERK2 as a drug target for EBV-associated disorders.

Analysis of Protein Interactions by Surface Plasmon Resonance

Surface plasmon resonance is an optical technique that is utilized for detecting molecular interactions, such as interactions that occur between proteins or other classes of molecules. Binding of a mobile molecule (analyte) to a molecule immobilized on a thin metal film (ligand) changes the refractive index of the film. The angle of extinction of light that is completely reflected after polarized light impinges upon the film, is altered and monitored as a change in detector position for a dip in reflected intensity (the surface plasmon resonance phenomenon). Because the method strictly detects mass, there is no need to label the interacting components, thus eliminating possible changes of their molecular properties. In this chapter, we review essential SPR methodology and present applications to basic science and human disease.

Targeted disruption of EBNA1 in EBV-infected cells attenuated cell growth

Epstein Barr virus (EBV)-encoded nuclear antigen-1 (EBNA1) plays a pivotal in an EBV episome replication and persistence. Despite considerable attempts, there are no EBV drugs or vaccines. We attempted to eradicate EBV episomes by targeting EBNA1 using the transcription activator-like effector nucleases (TALEN) (E1TN). E1TN-mediated transient knockout (KO) of EBNA1 reduced EBNA1 expression, and caused significant loss of EBV genomes and progressive death of EBV-infected cells. Furthermore, when a mixture of EBV-infected Burkitt’s lymphoma (BL) cells and EBV-negative BL cells was targeted by E1TN, EBV-negative cells were counter-selected while most EBV-infected cells died, further substantiating that EBNA1 KO caused selective death of EBV-infected cells. TALEN-mediated transient targeting of EBNA1 attenuated the growth of EBV-infected cells, implicating a possible therapeutic application of E1TN for EBV-associated disorders. [BMB Reports 2016; 49(4): 226-231]

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.

EBNA1-specific luminescent small molecules for the imaging and inhibition of latent EBV-infected tumor cells

An EBNA1-specific small molecule (JLP2) has been synthesised. As a strong binder and dimerization inhibitor of EBNA1 in vitro, JLP2 may be used as a selective luminescent agent for the imaging and inhibition of latent EBV-infected cancer cells.

Assay Development and High-Throughput Screening for Inhibitors of Kaposi's Sarcoma-Associated Herpesvirus N-Terminal Latency-Associated Nuclear Antigen Binding to Nucleosomes

Kaposi's sarcoma-associated herpesvirus (KSHV) has a causative role in several human malignancies, especially in immunocompromised hosts. KSHV latently infects tumor cells and persists as an extrachromosomal episome (plasmid). KSHV latency-associated nuclear antigen (LANA) mediates KSHV episome persistence. LANA binds specific KSHV sequence to replicate viral DNA. In addition, LANA tethers KSHV genomes to mitotic chromosomes to efficiently segregate episomes to daughter nuclei after mitosis. N-terminal LANA (N-LANA) binds histones H2A and H2B to attach to chromosomes. Currently, there are no specific inhibitors of KSHV latent infection. To enable high-throughput screening (HTS) of inhibitors of N-LANA binding to nucleosomes, here we develop, miniaturize, and validate a fluorescence polarization (FP) assay that detects fluorophore-labeled N-LANA peptide binding to nucleosomes. We also miniaturize a counterscreen to identify DNA intercalators that nonspecifically inhibit N-LANA binding to nucleosomes, and also develop an enzyme-linked immunosorbent assay to assess N-LANA binding to nucleosomes in the absence of fluorescence. HTS of libraries containing more than 350,000 compounds identified multiple compounds that inhibited N-LANA binding to nucleosomes. No compounds survived all counterscreens, however. More complex small-molecule libraries will likely be necessary to identify specific inhibitors of N-LANA binding to histones H2A and H2B; these assays should prove useful for future screens.

Small molecule inhibition of Epstein-Barr virus nuclear antigen-1 DNA binding activity interferes with replication and persistence of the viral genome

The replication and persistence of extra chromosomal Epstein-Barr virus (EBV) episome in latently infected cells are primarily dependent on the binding of EBV-encoded nuclear antigen 1 (EBNA1) to the cognate EBV oriP element. In continuation of the previous study, herein we characterized EBNA1 small molecule inhibitors (H20, H31) and their underlying inhibitory mechanisms. In silico docking analyses predicted that H20 fits into a pocket in the EBNA1 DNA binding domain (DBD). However, H20 did not significantly affect EBNA1 binding to its cognate sequence. A limited structure-relationship study of H20 identified a hydrophobic compound H31, as an EBNA1 inhibitor. An in vitro EBNA1 EMSA and in vivo EGFP-EBNA1 confocal microscopy analysis showed that H31 inhibited EBNA1-dependent oriP sequence-specific DNA binding activity, but not sequence-nonspecific chromosomal association. Consistent with this, H31 repressed the EBNA1-dependent transcription, replication, and persistence of an EBV oriP plasmid. Furthermore, H31 induced progressive loss of EBV episome. In addition, H31 selectively retarded the growth of EBV-infected LCL or Burkitt's lymphoma cells. These data indicate that H31 inhibition of EBNA1-dependent DNA binding decreases transcription from and persistence of EBV episome in EBV-infected cells. These new compounds might be useful probes for dissecting EBNA1 functions in vitro and in vivo.