μ-Calpain regulates caspase-dependent and apoptosis inducing factor-mediated caspase-independent apoptotic pathways in cisplatin-induced apoptosis

Cisplatin, an effective anticancer agent, can induce tumor cell apoptosis via caspase-dependent and-independent pathways. However, the precise mechanism that regulates the pathways remains unclear. In this study, we showed that p-calpain mediated both caspase-dependent and-independent pathways during cisplatin-induced apoptosis in human lung adenocarcinoma cells. After cisplatin treatment, calpain activation, as measured by a fluorescent substrate, was an early event, taking place well before apoptosis inducing factor (AIF) release and caspase-9/-3 activation. Confocal imaging of cells transfected with AIF-GFP demonstrated that AIF release occurred about 9 hr after cisplatin treatment. The increase of p-calpain activity proved to be a crucial event in the apoptotic machinery, as demonstrated by the significant protection of cell death in samples suppressed the endogenous p-calpain expression level, as well as cotreated with the calpain inhibitors, calpeptin and PD150606. Inhibition of p-calpain not only significantly reduced caspase-9/-3 activities but also completely blocked AIF redistribution. Our study also showed that endogenous mitochondrial It-calpain could directly induce the truncation and release of AIF, while caspases and cathepsins were not necessary for this process. In conclusion, the study demonstrated that activation of p-calpain played an essential role in regulating both caspase-dependent and AIF-mediated caspase-independent apoptotic pathways in cisplatin-induced apoptosis. (C) 2009 UICC