Novel action of paclitaxel against cancer cells: Bystander effect mediated by reactive oxygen species

Generation of reactive oxygen species (ROS) has been observed in cancer cells treated with paclitaxel, but the underlying mechanisms and therapeutic implications remain unclear. In the present study, we showed that paclitaxel promoted ROS generation through enhancing the activity of NADPH oxidase (NOX) associated with plasma membranes. Treatment of breast cancer cells caused an increased translocation of Rac1, a positive regulatory protein of NOX, to the membrane fraction. The paclitaxel-induced ROS generation occurred rapidly within several hours of drug exposure, with O₂^- and H ₂O₂ accumulation mainly outside the cells while the intracellular ROS remained unchanged. Importantly, the increase in extracellular ROS caused lethal damage to the bystander cancer cells not exposed to paclitaxel, as shown by two different methods using coculture systems where the bystander cells were differentiated from the paclitaxel-treated cells by fluorescent or radioactive labeling. This cytotoxic bystander effect was also observed with other microtubule-targeted agents vincristine and taxotere but not with 5-fluorouracil or doxorubicin. This toxic bystander effect was enhanced by CuZnSOD that converts O₂^- to H₂O₂ and was abolished by a catalase that eliminates H₂O₂. Furthermore, paclitaxel was able to induce an almost complete inhibition of proliferation of the bystander cells in the coculture system. Our study revealed a novel mechanism by which paclitaxel induces toxic bystander effect through generation of extracellular H₂O₂ from the membrane-associated NOX. This may contribute to the potent anticancer activity of paclitaxel and provide a novel basis to improve the clinical use of this important drug.