Vitamin E analogs trigger apoptosis in HER2/erbB2-overexpressing breast cancer cells by signaling via the mitochondrial pathway

α-Tocopheryl succinate (α-TOS) is a redox-silent vitamin E (VE) analog with high pro-apoptotic and anti-neoplastic activity. Here we investigated whether α-TOS and several novel VE analogs kill breast cancer cells over-expressing the anti-apoptotic receptor protein HER2/erbB2. The agents induced apoptosis at comparable levels in both erbB2-low and -high cells. Generation of reactive oxygen species (ROS) preceded mitochondrial destabilization and execution of apoptosis, as evidenced by the anti-apoptotic effects of exogenous superoxide dismutase and mitochondrially targeted coenzyme Q. Dissipation of ΔΨm was followed by cytochrome c and Smac/Diablo re-localization and caspase-dependent cleavage of death substrate. A resistance to apoptosis for the corresponding ρ0 counterparts confirmed a critical dependency for mitochondria during the induction of apoptosis in breast cancer cells mediated by VE analogs and linked apoptosis to generation of radicals as judged by the delayed accumulation of ROS in the cybrid cell types. We conclude that α-TOS causes efficient apoptosis in breast cancer cells independent of their erbB2 status. Since erbB2 is frequently over-expressed in breast cancers and renders the neoplastic disease resistant to established treatment, our findings are of clinical interest.