Cadmium induces Ca²⁺-dependent necrotic cell death through calpain-triggered mitochondrial depolarization and reactive oxygen species-mediated inhibition of nuclear factor-κB activity

This study investigates the mechanism of cell death induced by cadmium (Cd) in Chinese hamster ovary (CHO) cells. Cells exposed to 4 μM Cd for 24 h did not show signs of apoptosis, such as DNA fragmentation and caspase-3 activation. The pro-apoptotic (Bax) or anti-apoptotic (Bcl-2 and Bcl-xL) protein levels in the Bcl-2 family were not altered. However, an increase in propidium iodide uptake and depletion of ATP, characteristics of necrotic cell death, were observed. Cd treatment increased the intracellular calcium (Ca²⁺) level. Removal of the Ca²⁺ by a chelator, BAPTA-AM, efficiently inhibited Cd-induced necrosis. The increased Ca²⁺ subsequently mediated calpain activation and intracellular ROS production. Calpains then triggered mitochondrial depolarization resulting in cell necrosis. Cyclosporin A, an inhibitor of mitochondrial permeability transition, recovered the membrane potential and reduced the necrotic effect. The generated ROS reduced basal NF-κB activity and led cells to necrosis. An increase of NF-κB activity by its activator, PMA, attenuated Cd-induced necrosis. Calpains and ROS act cooperatively in this process. The calpain inhibitor and the ROS scavenger synergistically inhibited Cd-induced necrosis. Results in this study suggest that Cd stimulates Ca²⁺-dependent necrosis in CHO cells through two separate pathways. It reduces mitochondrial membrane potential by activating calpain and inhibits NF-κB activity by increasing the ROS level.