Spontaneous reduction in Epstein-Barr virus (EBV) DNA copy number in EBV-infected epithelial cell lines

Downregulation of MUS81 expression inhibits cell migration and maintains EBV latent infection through miR-BART9-5p in EBV-associated gastric cancer

Epstein-Barr virus (EBV) infection is associated with the occurrence and development of gastric cancer (GC). Methyl methanesulfonate and ultraviolet-sensitive gene 81 (MUS81) is the catalytic component of a structure-specific endonuclease and plays an important role in chromosomal stability. However, the link between EBV infection and MUS81 remains unclear. In the present study, we found that MUS81 expression was much lower in EBV-associated GC cells than in EBV-negative GC. MUS81 acts as an oncogene in GC by inducing the cell migration and proliferation. Western blot and luciferase reporter assays revealed that miR-BART9-5p directly targeted MUS81 and downregulated its expression. Additionally, overexpression of MUS81 in EBV-positive GC cells inhibited the expression of EBV nuclear antigen 1 (EBNA1). EBNA1 is critical for the pathogenesis of EBV-associated tumors and the maintenance of a stable copy number of the viral genomes. Altogether, these results indicated that the lowering MUS81 expression might be a mechanism by EBV to maintain its latent infection.

CD44+CD24-/low sphere-forming cells of EBV-associated gastric carcinomas show immunosuppressive effects and induce Tregs partially through production of PGE2

EBV-associated gastric carcinoma (EBVaGC) is accompanied by massive lymphocyte infiltration, but therapy resistance and tumor progression still occur in patients with EBVaGC. Cancer stem cells (CSCs) are reported to possess immunomodulatory ability that allows them to resist immune-mediated rejection for many tumor types. However, whether and how CSCs in EBVaGC exhibit immunosuppression has not yet been elucidated. We isolated CSC-like sphere-forming cells (SFCs) from EBVaGC cell line SNU-719 using the cancer sphere method. We validated their CSC-associated properties in the expression of the epithelial-mesenchymal transition (EMT)-related genes, the ability to form colonies, and resistance to chemotherapy drug-induced apoptosis and explored their immunomodulatory ability using the coculture system with PBMC (peripheral blood mononuclear cell). These CSC-like SFCs were CD44⁺CD24^-/low and were more tumorigenic than the parental SNU-719 cells in the xenograft mouse model. Remarkably, in the tumor-PBMC co-culturing experiments, these EBVaGC SFCs demonstrated profound immunosuppression by inhibiting the proliferation of PBMCs and T cell activation as well as inducing the generation of regulatory T cells (Tregs). Furthermore, the induction of Tregs was partially dependent on prostaglandin E2 (PGE2) produced from SFCs. Moreover, the presence of high CD44⁺CD24^-/low cells in tumor tissues predicted a decreased disease-free survival in patients with EBVaGC. Our study collectively confirmed the existence and immune resistance of CSCs in EBVaGC and offers new insights into the development of novel anti-EBVaGC strategies by targeting CSCs.

Characterization of Epstein-Barr virus infection in a human signet ring cell gastric carcinoma cell line, HSC-39

In order to study the mechanism of Epstein-Barr virus (EBV) infection in gastric carcinoma cells, we characterized the EBV infection in signet ring cell line HSC-39, derived from a human gastric carcinoma. HSC-39 cells were highly susceptible to cell-free EBV infection by Akata and P3HR-1 EBV strains. EBV nuclear antigen (EBNA) and EBV-encoded small RNA (EBER) were detected in the infected cells. Akata and P3HR-1 EBV-infected cell clones were isolated by a limiting dilution technique. The Akata and P3HR-1 EBV-infected clones differed from each other in morphology and growth patterns. Akata EBV-infected clones had lower growth rates than did P3HR-1 EBV-infected clones in both liquid and soft agar mediums. Both the infected HSC-39 cells and the clones expressed EBNA1 and EBER, but did not express EBNA2, latent membrane protein (LMP) 1 and LMP2A. The Q promoter (p), but not the Cp/Wp for EBNA transcription, was active in the infected HSC-39 cells and all clones. No lytic infection was observed in either infected parental cells or any clones. Uninfected HSC-39 cells did not express a principal EBV receptor CD21; however, Akata but not P3HR-1 EBV-infected clones expressed low levels of CD21 mRNA. These results demonstrate that the cellular phenotypes of HSC-39 cells are altered by EBV infection in strain-specific manner. We propose the HSC-39 cell line as a model target for the study of the mechanism and significance of EBV infection in gastric carcinoma.

Mechanism for induction of hydroxyurea resistance and loss of latent EBV genome in hydroxyurea-treated Burkitt's lymphoma cell line Raji

Hydroxyurea (HU), as an inhibitor of ribonucleotide reductase (RR) through interaction with the R2 component, has been used in the treatment of malignancies. Recently, therapeutic strategies in Epstein-Barr Virus (EBV)-targeted lymphoma have been reported. In order to study the effect of HU on EBV, infected Burkitt's lymphoma (BL) Raji cells were passaged in medium containing 50 microM HU for more than 2 months. EBV DNA was eliminated in about 40% of the cells in the HU-treated cultures. The cells were cloned from such cultures, and only EBV-positive clones could be isolated in 102 examined clones. No differences were observed in the EBV-latent state, EBV-gene expression, or cell growth between HU-untreated Raji cells and HU-treated clones. However, relative to parental Raji cells, the HU-treated Raji clones were almost eight times resistant to growth inhibition by HU according to the ID50 value, and the expression of the R2 component of RR increased more than two to three times. These results indicate that HU not only efficiently eliminates the EBV genome from Raji cells but also induces HU resistance. HU resistance was accompanied by over-expression of the R2 component of RR. However, the HU-resistant clones were sensitive to gemcitabine, another inhibitor of RR, and this seems highly relevant to chemotherapeutic combination in the use of these drugs.

Down-regulation of spontaneous Epstein-Barr virus reactivation in the P3HR-1 cell line by L-arginine

We tested the hypothesis that inhibition of Epstein-Barr virus (EBV) reactivation is controlled in part by nitric oxide (NO) generated from L-arginine (Arg). The spontaneous reactivation of EBV in the Burkitt's lymphoma (BL) cell line P3HR-1 was inhibited when the cells were cultured in L-Arg-supplemented medium. The expression of EBV early antigen (EA), immediate-early BZLF1 mRNA and the protein ZEBRA, and production of infectious virus were reduced by L-Arg supplementation in a dose-dependent manner. We demonstrated that inducible NO synthase (iNOS) mRNA was constitutively expressed in P3HR-1 cells, as quantitated by the reverse transcription-polymerase chain reaction. L-Arg supplementation enhanced iNOS and NOx expression in the cells. A specific NOS inhibitor, NG-monomethyl-L-Arg enhanced the expression of ZEBRA and early BMRF1 protein EA-D in the cells. L-Arg supplementation also inhibited the spontaneous EBV reactivation in another BL cell line EB1 and a B lymphoblastoid cell line OB. These results indicated that L-Arg induces iNOS and generates NO, which inhibits EBV reactivation in EBV-positive cells.