Background—The importance of free radical homeostasis and apoptosis in normal and diseased hearts and their interrelationships are poorly defined. We tested whether reactive oxygen species can trigger apoptosis in cardiomyocytes, and we explored the underlying pathways.

Methods and Results—A cell culture model of isolated cardiac cells and different reactive oxygen species (ROS)–generating systems were used. Apoptosis became evident when cardiomyocytes were exposed to either H₂O₂ or superoxide anion (O₂⁻). Both H₂O₂- and O₂⁻-induced apoptosis of cardiomyocytes were associated with an increase in p53 protein content, whereas protein levels of Bax and Bcl-2 were unaltered. H₂O₂, but not O₂⁻, induced an increase in the protein content of Bad. Furthermore, H₂O₂ elicited translocation of Bax and Bad from cytosol to mitochondria, where these factors formed heterodimers with Bcl-2, which was followed by the release of cytochrome c, activation of CPP32, and cleavage of poly(ADP-ribose) polymerase. Interestingly, this pathway was not activated by O₂⁻. Instead, O₂⁻ used Mch2α to promote the apoptotic pathway, as revealed by the activation of Mch2α and the cleavage of its substrate, lamin A.

Conclusions—Taken together, these results indicate that ROS may play an important pathophysiological role in cardiac diseases characterized by apoptotic cell death and suggest that different ROS-induced activations of the apoptotic cell death program in cardiomyocytes involve distinct signaling pathways.