Cancer Drug Bortezomib, a Proteasomal Inhibitor, Triggers Cytotoxicity in Microvascular Endothelial Cells via Multi-Organelle Stress

Proteasomes maintain cellular homeostasis by degrading abnormal proteins, while cancer cells exploit them for survival, becoming a key chemotherapeutic target. Bortezomib (BTZ), a reversible proteasomal inhibitor, is a front-line treatment for multiple myeloma, mantle cell lymphoma, and non-small cell lung cancer. However, its efficacy is limited by severe side effects, including neurotoxicity and cardiovascular distress, with its toxicity mechanisms largely unexplored. Here, we discover that Bortezomib (BTZ), is cytotoxic to non-cancerous cells distinctly from Carfilzomib (CFZ), the second-line irreversible PI. BTZ or CFZ is administered intravenously, impacting blood vessel (vascular) endothelial cells. We used human pulmonary microvascular endothelial cells (HPMECs) to demonstrate that BTZ but not CFZ elicits endoplasmic reticulum (ER) stress, mitochondrial membrane compromise, mitochondrial reactive oxygen species (ROS) accumulation, and Caspase (CASP)9 activation (mediator of Intrinsic apoptosis) within fifteen hours of treatment. By twenty-four hours, BTZ-treated cells display cleavage of CASP8 (mediator of extrinsic apoptosis), activation of CASP3 (terminal executioner of apoptosis), cell-death and vascular barrier loss. Pan-caspase inhibitor zVAD significantly rescues BTZ-treated cells from cytotoxicity. Both BTZ and CFZ effectively kill MM cells. These findings reveal novel insights into fundamental signaling of regular cells where reversible inhibition of the proteasome dictates a unique cascade of stress distinct from irreversible inhibition. These harmful effects of BTZ emphasize the need to re- evaluate its use as a frontline chemotherapy for MM.