Autophagy-regulated ROS from xanthine oxidase acts as an early effector for triggering late mitochondria-dependent apoptosis in cathepsin S-targeted tumor cells

Cathepsin S (CTSS), which is highly expressed in various malignant tumor cells, has been proposed to promote tumor progression, migration, and invasion. CTSS inhibition not only blocks tumor cell invasion and endothelial tube formation but also induces cellular cytotoxicity. In our previous studies, we have observed that CTSS inhibition induces autophagy, which is responsible for up-regulating xanthine oxidase for early ROS generation and consequent cell death. However, whether the autophagy-regulated early ROS triggers apoptosis remains unclear. We conducted a long-term follow-up study to investigate the relationship between early autophagy and late mitochondria-dependent apoptosis. We demonstrated that early ROS generation is critical for mitochondria damage and the activation of intrinsic apoptotic pathway. Attenuating the early ROS level diminished later mitochondrial damage and downstream apoptotic signaling. Collectively, mitochondriadependent apoptosis is regulated by autophagy-regulated early ROS, which serves as an early effector that triggers mitochondrial signaling for late apoptosis. The data emphasize the essential role of autophagy-regulated early ROS in triggering late apoptotic signaling.