Schematic representation of the mechanism of action of NPI-0052. Inhibition of the chymotrypsin-like, caspase-like, and trypsin-like activities of the 20S proteasome in leukemia cells by NPI-0052 results in activation of caspase-8. This leads to Bid cleavage and drops of mitochondrial membrane potential (ΔΨm). Release of cytochrome c and activation of caspase-9, likely consequences of mitochondrial perturbations, follow. Activation of effector caspase-3 and consequent DNA fragmentation then are observed. Increased levels of intracellular superoxide and peroxide are highest after 14 hours of exposure to NPI-0052. However, caspase-8 inhibitors do not prevent this increase, suggesting that a parallel pathway contributes to cell death by NPI-0052, since the antioxidant NAC prevents the drug's cytotoxicity.