Hypoxia induces lytic reactivation of Epstein-Barr virus in stable M81-infected nasopharyngeal epithelial cells

Abstract

Epstein-Barr virus (EBV) is a type of human gamma-herpesvirus, which infects and establishes life-long persistence in over 90% of the world population. Like all other herpesviruses, EBV can infect cells in either latent or lytic forms. Several EBV-encoded oncogenic proteins and non-coding RNAs are expressed in EBV-associated malignancies with latent infection, including Burkitt’s lymphoma, Hodgkin’s disease, gastric cancer and nasopharyngeal carcinoma (NPC). During the pathogenesis of NPC, EBV infection represents an early event. Latent EBV infection is found in every cancer cell of virtually all cases of undifferentiated NPC. To determine the functional role of EBV infection in the malignant transformation of nasopharyngeal epithelial (NPE) cells, representative in vitro and in vivo models should be developed. M81 is a new EBV strain isolated from NPC. As compared to the previously commonly used EBV strains from B-cell lymphoma, M81 was characterized with unusually high tropism for epithelial cells. In this present study, for the first time, we established stable M81-infected NPE cells by cell-free infection or co-culture method. The M81 virus in nasopharyngeal cancerous HONE1 cells could be lytically reactivated with hypoxia treatment, evidenced by the increased expressions of EBV immediate-early lytic genes, BZLF1 and BRLF1, as well as early and late lytic genes, BMRF1 and BLLF1. EBV copy number in HONE1-M81 cells was also enhanced in hypoxia. The underlying mechanism might be due to HIF1α-induced transcription of BZLF1, with hypoxia-responsive element (HRE) identified in BZLF1 promoter. This work was partly supported by General Research Fund (17110315, 17120814, 779713), Theme-Based Research Scheme grant (T12-401/13-R), AoE NPC grant (AoE/M-06/08) and Health and Medical Research Fund (13142201, 13120872).