Abstract 558: Bortezomib induction of the unfolded protein response mediates Epstein-Barr virus lytic activation but is separable from bortezomib-induced cytotoxicity

Abstract

Abstract Epstein-Barr virus (EBV) is associated with many human cancers. Several therapeutic strategies involving the pharmacologic activation of the EBV lytic cycle for tumor killing have been described (1-2). We previously identified bortezomib, a proteasome inhibitor approved for the treatment of multiple myeloma and mantle cell lymphoma, as a potent activator of EBV lytic gene expression. The mechanism of action of bortezomib as either a viral lytic activator or an anticancer drug remains poorly understood. However, induction of the unfolded protein response (UPR) has emerged as a central theme in both areas of investigation (3). In order to further evaluate the role of the UPR in lytic activation, we treated lymphoid and epithelial tumor cell lines with bortezomib and assessed induction of the UPR and EBV lytic gene expression. We found dose-dependent parallel induction of UPR and EBV lytic infection across cell lines. However, an EBV-immortalized lymphoblastoid cell line was relatively resistant to both bortezomib-induced UPR and induction of EBV lytic gene expression, but not resistant to bortezomib-induced cell killing. To further investigate, we assessed induction of the UPR by thapsigargin which leads to depletion of Ca2+ in the ER. Thapsigargin induced the UPR and EBV lytic gene expression across all the cell lines tested. Blocking ER stress and UPR activation, by cycloheximide treatment or by BiP knockdown, diminished EBV lytic gene expression but had no effect on cellular toxicity. Additionally, regulated knockdown of UPR associated transcription factors (XBP1(s), c-Jun) blocked lytic activation by both agents. Taken together, these results suggest that induction of the UPR by bortezomib is tightly linked to EBV lytic activation but may be separable from the cytotoxic effects of bortezomib.