Role of EBER and BARF1 in nasopharyngeal carcinoma (NPC) tumorigenesis

Treatment of nasopharyngeal carcinoma cells with the histone-deacetylase inhibitor abexinostat: cooperative effects with cis-platin and radiotherapy on patient-derived xenografts

EBV-related nasopharyngeal carcinomas (NPCs) still raise serious therapeutic problems. The therapeutic potential of the histone-deacetylase (HDAC) inhibitor Abexinostat was investigated using 5 preclinical NPC models including 2 patient-derived xenografts (C15 and C17). The cytotoxicity of Abexinostat used either alone or in combination with cis-platin or irradiation was assessed in vitro by MTT and clonogenic assays using 2 EBV-negative (CNE1 and HONE1) and 3 EBV-positive NPC models (C15, C17 and C666-1). Subsequently, the 3 EBV-positive models were used under the form of xenografts to assess the impact of systemic treatments by Abexinostat or combinations of Abexinostat with cis-platin or irradiation. Several cell proteins known to be affected by HDAC inhibitors and the small viral non-coding RNA EBER1 were investigated in the treated tumors. Synergistic cytotoxic effects of Abexinostat combined with cis-platin or irradiation were demonstrated in vitro for each NPC model. When using xenografts, Abexinostat by itself (12.5 mg/kg, BID, 4 days a week for 3 weeks) had significant anti-tumor effects against C17. Cooperative effects with cis-platin (2 mg/kg, IP, at days 3, 10 and 17) and irradiation (1 Gy) were observed for the C15 and C17 xenografts. Simultaneously two types of biological alterations were induced in the tumor tissue, especially in the C17 model: a depletion of the DNA-repair protein RAD51 and a stronger in situ detection of the small viral RNA EBER1. Overall, these results support implementation of phase I/II clinical trials of Abexinostat for the treatment of NPC. A depletion of RAD51 is likely to contribute to the cooperation of Abexinostat with DNA damaging agents. Reduction of RAD51 combined to enhanced detection of EBER 1 might be helpful for early assessment of tumor response.

Modes of infection and oncogenesis by the Epstein-Barr virus

The EBV is a human γ-herpesvirus associated with various neoplasms. It is responsible for causing cancers of B, T, and NK cells as well as cells of epithelial origin. Such diversity in target cells and the complicated steps of oncogenesis are perplexing when we speculate about the mechanisms of action of EBV-positive cancers. Here, we first note three common features that contribute to the development and maintenance of EBV-positive cancers: effects of EBV oncogenes, immunosuppression and evasion/exploitation of the immune system, and genetic and epigenetic predisposition/alteration of the host genome. Then, we demonstrate the mechanisms of oncogenesis and the means by which each EBV-positive cancer develops, with particular focus on the mode of EBV infection. The EBV has two alternative life cycles: lytic and latent. The latter is categorized into four programs (latency types 0-III) in which latent viral genes are expressed differentially depending on the tissue of origin and state of cells. The production of viral latent genes tends to decrease with an increase in time, and, in an approximate manner, the expression levels of viral genes are inversely correlated with the degree of abnormalities in the host genome. Occasional execution of the viral lytic cycle also contributes to oncogenesis. Understanding this life cycle of the EBV and its relevance in oncogenesis may provide valuable clues to the development of effective therapies for the associated cancers.

The labyrinth of interactions of Epstein-Barr virus-encoded small RNAs

Epstein-Barr Virus (EBV) is an oncogenic herpesvirus implicated in the pathogenesis of a number of human malignancies. However, the mechanism by which EBV leads to malignant transformation is not clear. A number of viral latent gene products, including non-protein coding small RNAs, are believed to be involved. Epstein-Barr virus-encoded RNA 1 (EBER1) and EBER2 are two such RNA molecules that are abundantly expressed (up to 10(7) copies) in all EBV-infected cells, but their function remains poorly understood. These polymerase III transcripts have extensive secondary structure and exist as ribonucleoproteins. An accumulating body of evidence suggests that EBERs play an important role, directly or indirectly, in EBV-induced oncogenesis. Here, we summarize the current understanding of the complex interactions of EBERs with various cellular factors and the potential pathways by which these small RNAs are able to influence EBV-infected cells to proliferate and to induce tumorigenesis. The exosome pathway is probably involved in the cellular excretion of EBERs and facilitating some of their biological effects.

Emerging roles of small Epstein-Barr virus derived non-coding RNAs in epithelial malignancy

Latent Epstein-Barr virus (EBV) infection is an etiological factor in the progression of several human epithelial malignancies such as nasopharyngeal carcinoma (NPC) and a subset of gastric carcinoma. Reports have shown that EBV produces several viral oncoproteins, yet their pathological roles in carcinogenesis are not fully elucidated. Studies on the recently discovered of EBV-encoded microRNAs (ebv-miRNAs) showed that these small molecules function as post-transcriptional gene regulators and may play a role in the carcinogenesis process. In NPC and EBV positive gastric carcinoma (EBVaGC), 22 viral miRNAs which are located in the long alternative splicing EBV transcripts, named BamH1 A rightward transcripts (BARTs), are abundantly expressed. The importance of several miR-BARTs in carcinogenesis has recently been demonstrated. These novel findings enhance our understanding of the oncogenic properties of EBV and may lead to a more effective design of therapeutic regimens to combat EBV-associated malignancies. This article will review the pathological roles of miR-BARTs in modulating the expression of cancer-related genes in both host and viral genomes. The expression of other small non-coding RNAs in NPC and the expression pattern of miR-BARTs in rare EBV-associated epithelial cancers will also be discussed.

Toll-like receptor 3 in Epstein-Barr virus-associated nasopharyngeal carcinomas: consistent expression and cytotoxic effects of its synthetic ligand poly(A:U) combined to a Smac-mimetic

Background

Nasopharyngeal carcinomas (NPC) are consistently associated with the Epstein-Barr virus (EBV). Though NPCs are more radiosensitive and chemosensitive than other tumors of the upper aero-digestive tract, many therapeutic challenges remain. In a previous report, we have presented data supporting a possible therapeutic strategy based on artificial TLR3 stimulation combined to the inhibition of the IAP protein family (Inhibitor of Apoptosis Proteins). The present study was designed to progress towards practical applications of this strategy pursuing 2 main objectives: 1) to formally demonstrate expression of the TLR3 protein by malignant NPC cells; 2) to investigate the effect of poly(A:U) as a novel TLR3-agonist more specific than poly(I:C) which was used in our previous study.

Methods

TLR3 expression was investigated in a series of NPC cell lines and clinical specimens by Western blot analysis and immunohistochemistry, respectively. The effects on NPC cells growth of the TLR3 ligand poly(A:U) used either alone or in combination with RMT5265, an IAP inhibitor based on Smac-mimicry, were assessed using MTT assays and clonogenic assays.

Results

TLR3 was detected at a high level in all NPC cell lines and clinical specimens. Low concentrations of poly(A:U) were applied to several types of NPC cells including cells from the C17 xenograft which for the first time have been adapted to permanent propagation in vitro. As a single agent, poly(A:U) had no significant effects on cell growth and cell survival. In contrast, dramatic effects were obtained when it was combined with the IAP inhibitor RMT5265. These effects were obtained using concentrations as low as 0.5 μg/ml (poly(A:U)) and 50 nM (RMT5265).

Conclusion

These data confirm that TLR3 expression is a factor of vulnerability for NPC cells. They suggest that in some specific pathological and pharmacological contexts, it might be worth to use Smac-mimetics at very low doses, allowing a better management of secondary effects. In light of our observations, combined use of both types of compounds should be considered for treatment of nasopharyngeal carcinomas.

Multipronged attenuation of macrophage-colony stimulating factor signaling by Epstein-Barr virus BARF1

The ubiquitous EBV causes infectious mononucleosis and is associated with several types of cancers. The EBV genome encodes an early gene product, BARF1, which contributes to pathogenesis, potentially through growth-altering and immune-modulating activities, but the mechanisms for such activities are poorly understood. We have determined the crystal structure of BARF1 in complex with human macrophage-colony stimulating factor (M-CSF), a hematopoietic cytokine with pleiotropic functions in development and immune response. BARF1 and M-CSF form a high-affinity, stable, ring-like complex in both solution and the crystal, with a BARF1 hexameric ring surrounded by three M-CSF dimers in triangular array. The binding of BARF1 to M-CSF dramatically reduces but does not completely abolish M-CSF binding and signaling through its cognate receptor FMS. A three-pronged down-regulation mechanism is proposed to explain the biological effect of BARF1 on M-CSF:FMS signaling. These prongs entail control of the circulating and effective local M-CSF concentration, perturbation of the receptor-binding surface of M-CSF, and imposition of an unfavorable global orientation of the M-CSF dimer. Each prong may reduce M-CSF:FMS signaling to a limited extent but in combination may alter M-CSF:FMS signaling dramatically. The downregulating mechanism of BARF1 underlines a viral modulation strategy, and provides a basis for understanding EBV pathogenesis.