Abstract 3098: Promotion of in vivo growth of Epstein-Barr virus (EBV)-associated nasopharyngeal carcinoma by miR-BARTs

Epstein-Barr virus (EBV) infection is nearly 100% associated with undifferentiated nasopharyngeal carcinoma (NPC). However, EBV episomes are rapidly lost under in vitro culturing if there is no selective advantage to their retention. The persistence of the viral episomes within the in vivo tumors therefore implies that the virus must be contributing to the growth and/or survival of the tumors. Since most of the growth-promoting EBV latent proteins are immunogenic, their expressions are usually low and heterogeneous within a tumor. Hence, the abundant expression of the non-immunogenic microRNAs derived from EBV-encoded BART transcripts, i.e. miR-BARTs, strongly implicates their pathogenic roles in this cancer. We found that EBV infected NPC cells could develop into bigger tumors in nude mice than the non-infected counterparts. Importantly, the miR-BARTs were all upregulated after the in vivo growth conditions. Nevertheless, EBV-infected and non-infected cells had comparative growth in in vitro culturing, suggesting EBV infection could provide growth advantage to NPC in vivo, but not in vitro. We have recently established a new and representative NPC cell line (NPC43) to study the pathogenesis of EBV in the development of NPC. In consistent will other previous publications, the EBV genomes were lost gradually when growing in vitro, resulting in a cell line with only half of the population is EBV-positive (at PD 50). Interestingly, after this mixture of EBV-positive and EBV-negative cells were grown as tumor in nude mice, the tumor cells were detected to be nearly 100% positive for EBV-infection. This suggests the in vivo growth conditions could select for the EBV-positive NPC cells. miR-BARTs are known to have a predominant role in suppressing cellular apoptosis. We hypothesized that miR-BARTs may help to resist apoptosis under metabolic stress in vivo, when tumor growth outstrips the nutrient supply. In our in vitro experiments, we observed that EBV infection protected the cells from apoptosis if they were grown in culture medium deprived of glucose and amino acids. Moreover, the expression levels of miR-BARTs were also upregulated in this metabolically stressed condition. To further investigate the roles of miR-BARTs, we infected NPC cells (NPC43) and immortalized nasopharyngeal epithelial cells (NP460) with either wild-type M81 EBV or mutant M81 EBV (with BART deleted). We found that NP460-M81WT showed a general trend of higher resistance to starvation-induced cell death than NP460-M81ΔBART. Besides in the spheroid cultures of the pair of NPC43-M81WT and NPC43-M81ΔBART, the WT cells could grow into bigger spheroids, reflecting they could sustain their growth under a gradient of nutrient deficiency which is present in the 3D structure of a spheroid. Taken together, all these suggest miR-BARTs could promote in vivo growth by enhancing survival of the EBV-infected cells. Citation Format: Anna Chi Man Tsang, Shaina Chor Mei Huang, Ming Han Tsai, Henri-Jacques Delecluse, Honglin Chen, George Sai Wah Tsao. Promotion of in vivo growth of Epstein-Barr virus (EBV)-associated nasopharyngeal carcinoma by miR-BARTs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3098.