The bortezomib-induced mitochondrial damage is mediated by accumulation of active protein kinase C-delta

Bortezomib (PS-341) is an inhibitor of the S26 proteasome. Bortezomib induces mitochondrial damage but the exact mechanism remains unclear. We studied PKC-delta, a kinase that is regulated by proteasome degradation and translocates to mitochondria in apoptosis, and examined whether PKC-delta could be a potential mediator of bortezomib-induced mitochondrial damage. Co-incubation of bortezomib with a PKC-delta inhibitor, rottlerin, suppressed bortezomib-induced apoptosis in U937 cells. Western analysis of U937 cells treated with bortezomib revealed accumulation of full-length PKC-delta in the first 4 h. By 16 h an active catalytic fragment of PKC-delta accumulated in mitochondria. The cleavage of PKC-delta after bortezomib treatment was mediated by caspases, because a pan-caspase inhibitor BAF prevented the appearance of the active fragment of PKC-delta. These findings indicate that accumulation of the active PKC-delta fragment in mitochondria is responsible for bortezomib-induced mitochondrial damage.