Caspase 3 activation and apoptosis facilitates primary infection of Epstein-Barr virus in HEK293 cells

Abstract

Epstein-Barr virus (EBV) is human herpervirus 4 that has a near 170-kb double-stranded DNA genome. During lytic replication, EBV can expresses over 80 viral genes. Previous research has found out that apoptosis can mediate lytic induction of EBV in gastric carcinoma cells as well as lymphoid cells. Unlike the lytic cycle, only a few viral genes will be expressed in EBV a lifelong latent infection is established. It is also noteworthy that certain latent transcripts of EBV could inhibit apoptosis. Development of cancers are common in EBV-infected people, about 200,000 of them would develop different kinds of cancer each year. In particular, EBV-associated nasopharyngeal carcinoma (NPC) is frequently found in Hong Kong and nearby area. A better understanding of how lytic replication is induced in nasopharyngeal (NP) epithelial cells and whether caspase 3 induced apoptosis are involved in this process provide a deeper insight on EBV-induced NP carcinogenesis. Serendipitously, our previous results found that pre-treatment of HEK293 cells with pro-apoptotic drugs 5-fluorouracil and cisplatin facilitated lytic induction of the M81 strain of EBV, and more cells were infected with EBV. Previous studies have indicated that caspase 3 induced apoptosis is vital for propagation and replication influenza A virus in host cells efficiently. The possibility of a similar mechanism in NP and NPC cells are yet to be investigated. EBV lytic induction by 5-fluorouracil and cisplatin was further assessed. It was found that this induction by 5-fluorouracil and cisplatin was prevented by a broad-spectrum caspase inhibitor, suggesting the requirement of caspase activation in this process. Consistent with the involvement of a common downstream effector, activation of apoptosis through either intrinsic or extrinsic pathways using Bax, FasL or tumor necrosis factor α was capable of facilitating lytic infection of EBV. Thus, our results suggested that apoptosis hold a significant role in promoting EBV lytic induction. Finally, specific inhibitors of caspase 3 were employed to suppress caspase 3 activities and the efficiency of EBV lytic induction was dramatically decreased in their presence. Hence, our findings indicated that caspase 3 activation and apoptosis is indispensable for EBV lytic induction in HEK293 cells. Further investigations are required to elucidate whether proteolytic activation of critical lytic inducer by caspase 3 might be necessary for full activation of EBV lytic cycle. The work provides important new knowledge in EBV biology concerning the initiation and activation of lytic replication. In addition, the knowledge obtained from this study offer a direction for design and development of treatment targeting EBV and NPC.