Abstract
Novel combinations aiming at maximizing the efficacy of bortezomib are highly valued in the clinic. Therefore the current study investigated the outcomes of combining bortezomib with dipyridamole, a well-known antiplatelet. The co-treatment exerted a synergistic lethality in a panel of human leukemia/lymphoma cell lines of different origin. Mechanistically, dipyridamole did not modulate the proteasome inhibitory activity of bortezomib. However, dipyridamole triggered an endoplasmic reticulum (ER) stress, and co-treatment with bortezomib resulted in higher levels of ER stress than either monotherapies. Relieving ER stress with the protein translation inhibitor, cycloheximide suppressed cell death. Moreover, the enhanced ER stress by the co-treatment was associated with an aggravation of reactive oxygen species (ROS) generation and glutathione (GSH) depletion. Replenishing GSH pools significantly scavenged ROS and rescued the cells. Importantly, the cytotoxicity of the co-treatment was executed mainly via the mitochondrial apoptotic pathway with an efficient suppression of the key anti-apoptotic regulators, Mcl-1, Bcl-xl, Bcl-2 and XIAP, driving the independence of the co-treatment-induced apoptosis of a single apoptotic trigger. Furthermore, the intrinsic potential of bortezomib to inhibit important pro-survival pathways was enhanced by dipyridamole in a GSH/ROS-dependent manner. Interestingly, dipyridamole abrogated JAK2 phosphorylation indirectly and selectively in cancer cells, and the co-treatment-induced cytotoxicity was preserved in a model of stromal-mediated chemoresistance. In nude mice, the antitumor activity of the co-treatment surpassed that of bortezomib monotherapy despite that synergy was lacking. In summary, findings of the present study provided a preclinical rationale which warrants further clinical evaluation of bortezomib/dipyridamole novel combination in hematologic malignancies.