Bcl-2 antagonists interact synergistically with bortezomib in DLBCL cells in association with JNK activation and induction of ER stress

Abstract

Mechanisms underlying interactions between the proteasome inhibitor bortezomib and small molecule Bcl-2 antagonists were examined in GC- and ABC-type human DLBCL (diffuse lymphocytic B-cell lymphoma) cells. Concomitant or sequential exposure to non- or minimally toxic concentrations of bortezomib or other proteasome inhibitors and either HA14-1 or gossypol resulted in a striking increase in Bax/Bak conformational change/translocation, cytochrome c release, caspase activation and synergistic induction of apoptosis in both GCand ABC-type cells. These events were associated with a sharp increase in activation of the stress kinase JNK and evidence of ER stress induction (e.g., eIF2α phosphorylation, activation of caspases-2 and -4, and Grp78 upregulation). Pharmacologic or genetic (e.g., shRNA knockdown) interruption of JNK signaling attenuated HA14-1/bortezomib lethality and ER stress induction. Genetic disruption of the ER stress pathway (e.g., in cells expressing caspase-4 shRNA or DN-eIF2α) significantly attenuated lethality. The toxicity of this regimen was independent of ROS generation. Finally, HA14-1 significantly increased bortezomib-mediated JNK activation, ER stress induction, and lethality in bortezomib-resistant cells. Collectively these findings indicate that small molecule Bcl-2 antagonists promote bortezomib-mediated mitochondrial injury and lethality in DLBCL cells in association with enhanced JNK activation and ER stress induction. They also raise the possibility that such a strategy may be effective in different DLBCL sub-types (e.g., GC- or ABC), and in bortezomib-resistant disease. ©2009 Landes Bioscience.