Induction of apoptosis by chemotherapeutic drugs without generation of reactive oxygen species.

Studies in a variety of cell types have suggested that cancer chemotherapy drugs induce tumor cell apoptosis in part by inducing formation of reactive oxygen species (ROS). Using human B lymphoma cells as the targets, we have found that apoptosis can be induced in the absence of any detectable oxidative stress. Apoptosis was induced with the chemotherapy drugs VP-16 and cisplatin. To determine whether oxidants are formed as part of the drug-induced apoptotic process, intracellular markers of oxidative stress were examined. These included measurement of (1) protein carbonyl groups by Western blot immunoassay, (2) protein methionine sulfoxide residues by amino acid analysis, (3) protein sulfhydryl oxidation by Western blot immunoassay, (4) F2-isoprostanes by GC/MS, and (5) intracellular ROS production using the oxidant-sensitive dyes DCFDA and dihydrorhodamine 123. Apoptosis was quantified using fluorescence microscopy to assess nuclear morphology. The results show that VP-16 and cisplatin induce extensive apoptosis in the absence of any detectable protein or lipid oxidation, measured in both the cytosolic and mitochondrial compartments of the cell. In contrast, H2O2, which kills the cells by nonapoptotic pathways, caused increases in both protein and lipid oxidation. Three different antioxidant compounds (N-acetyl cysteine, Tempol, and MnTBAP) failed to inhibit VP-16-induced apoptosis, while inhibiting H2O2-induced cell death. Only N-acetyl cysteine inhibited cisplatin-induced cell death and this is attributed to its known ability to react directly with and inactivate cisplatin before it enters the cell. The results demonstrate that, at least in B lymphoma cells, chemotherapy-induced apoptosis occurs using a mechanism that does not involve oxidants.