Caspase 3 activation and apoptosis facilitates lytic induction of Epstein- Barr virus in epithelial cells

Abstract

Epstein-Barr virus (EBV), also known as human herpervirus 4, has an approximately 170-kb double-stranded DNA genome and expresses more than 80 genes during lytic replication. Apoptosis has previously been shown to mediate lytic induction of EBV in lymphoid and gastric carcinoma cells. In contrast to the lytic cycle, only a dozen or fewer viral genes are expressed when EBV establishes a lifelong latent infection. Notably, some latent transcripts of EBV are capable of preventing apoptosis. Each year, about 200,000 EBV-infected people will develop various types of cancer. Particularly, EBV-associated nasopharyngeal carcinoma (NPC) is prevalent in Hong Kong and adjacent areas. Exactly how lytic replication is induced in nasopharyngeal (NP) epithelial cells and whether caspase 3 activation and apoptosis are involved in this process remain elusive. A better understanding of these questions might shed mechanistic light on EBV-induced NP carcinogenesis. We found serendipitously that pre-treatment of epithelial cells with pro-apoptotic drugs 5-fluorouracil and etoposide facilitated lytic induction of the M81 strain of EBV. The number of EBV-infected cells was much higher. It was previously shown that caspase 3 activation and apoptosis induction are essential for efficient influenza A virus propagation and replication in host cells. Whether a similar mechanism might operate in NP and NPC cells remains to be clarified. EBV lytic induction by different pro-apoptotic agents was further assessed and our results suggested that apoptosis plays a critical role in facilitating EBV lytic induction. Specific inhibitors of caspase 3 was employed to suppress caspase 3 activities and the efficiency of EBV lytic induction was significantly reduced in their presence. Our findings point to an important role of caspase 3 activation and apoptosis in EBV lytic induction in NP epithelial cells. The new knowledge derived might instruct rational design and devlopment of anti-EBV and anti-NPC drugs.