Functional study of epstein-barr virus infection in nasopharyngeal carcinoma

Abstract

The global incidence rate of nasopharyngeal carcinoma (NPC) is low but particularly high in southern China including Hong Kong. Epstein-Barr virus (EBV) has been postulated to play an oncogenic role in the development and progression of NPC. EBV genome can be detected in virtually all biopsies of undifferentiated NPC. This close association of EBV infection with undifferentiated NPC implicates that EBV infection may provide selective growth advantage for NPC cells. The effects of EBV re-infection in an undifferentiated NPC cell line, HONE1, were investigated in details in this study. In vitro growth advantage of EBV infection in NPC cells was not observed. In contrast, EBV-infected NPC exhibited growth advantage in vivo. The EBV-infected HONE1 cells, was shown to grow at a significantly faster rate in nude mice compared to uninfected HONE1 cells. The mechanisms underlying the growth advantage of EBV infection in NPC is unclear. This study has investigated the functional behaviors and gene expression events of EBV-infected NPC cells grown under in vitro and in vivo conditions in an attempt to define the contribution of EBV infection to the pathogenic property of NPC cells in patients.

The limited supply of oxygen (termed as hypoxia) and nutrients are common physiological stresses in human tumour grown in vivo, commonly activate the autophagy machinery in cancer cells. Autophagy is a controlled self-degradation program to protect cells against cell death by isolating damaged organelles and generates energy through specialised biosynthesis processes from t.he degraded macromolecules. The autophagic responses may result in vigorous selection of cells adapted to the harsh conditions of growth in vivo.

The main stem of this project is to elucidate the mechanism and signalling events mediated by EBV infection through which the HONE1 NPC cells may acquire pro-survival ability through autophagy and anti-apoptosis pathways induced by metabolic stresses. In this study, the EBV-infected HONE1 cells was shown to possess enhanced survival with a more active autophagic state upon nutrient starvation in comparison to their control uninfected cells. The cDNA microarray analysis was used to compare the transcriptome profiles of EBV-infected and uninfected HONE1 cells upon nutrient starvation including amino acid, glucose and growth factors to identify altered signalling pathways involved. The Wnt and JAK/STAT pathways along with the expression of multiple metabolism-related genes were observed to be different between the EBV-infected and uninfected HONE1 cells, suggesting their potential contributions to enhance the pro-survival responses in EBV-infected NPC cells.

Besides, LMP1 (Latent Membrane Protein-1) is a potent oncogene encoded by EBV shown to activate the mTOR signalling to mediate many of its downstream events in NPC cells including the upregulation of HIF-1α, an oncoprotein responsible for the oxygen homeostasis in cells (Tsao SW et al., unpublished observations). These findings may provide clues to how EBV infection may support stress adaptation of NPC cells to oxygen and nutrient deprivation when grown in vivo.