Induction of endoplasmic reticulum stress by bortezomib sensitizes myeloma cells to DR5-mediated cell death

TNF-related apoptosis-including ligand/Apo2 (TRAIL)-mediated immunotherapy is an attractive anti-tumor modality with high tumor specificity. In order to improve its therapeutic efficacy, we further need to implement a novel maneuver for sensitization of malignant cells to TRAIL. Bortezomib (BTZ), a novel anti-myeloma (MM) agent, potently induces endoplasmic reticulum (ER) stress to cause apoptosis. Here, we explored the roles of BTZ in the cytotoxicity of anti-TRAIL receptor agonistic antibodies against MM cells with special reference to ER stress. BTZ enhanced the expression of death receptor 5 (DR5) but not DR4 in MM cells at surface protein as well as mRNA levels. However, the DR5 expression was not affected by BTZ without ER stress induction in MM cells with a point mutation in a BTZ-binding proteasome β5 subunit. Tunicamycin, an ER stress inducer, was able to enhance the DR5 expression even in the BTZ-resistant MM cells, suggesting the role of ER stress in up-regulation of DR5 expression. Interestingly, BTZ facilitated extrinsic caspase-mediated apoptosis by anti-DR5 agonistic antibody in MM cells along with reducing c-FLICE-like interleukin protein, a caspase 8 inhibitor. These results suggest that BTZ enhances DR5 expression and its downstream apoptotic signaling through ER stress to sensitize MM cells to TRAIL-mediated immunotherapy.