Calpain inhibition stimulates caspase-dependent apoptosis induced by taxol in NIH3T3 cells

Taxol is an anticancer drug that triggers apoptosis in a wide spectrum of cancers such as ovarian, breast, lung, head and neck, and bladder carcinoma by both caspase-dependent and -independent apoptosis mechanisms. However, the exact signaling pathways involved in taxol-induced apoptosis strongly depend on the cellular background and they are not completely established yet. In this study we demonstrate that taxol induces caspase-3-independent apoptosis in NIH3T3 cells by a calpain-mediated mechanism. Taxol treatment produced changes in the mitochondrial membrane potential (ΔΨm) which could be responsible of Ca2+ release from the mitochondria and the consequent calpain activation. Interestingly, we show that calpain produced proteolysis of caspase-3 and demonstrate that, accordingly, calpain inhibition increased taxol-induced apoptosis. In addition, we reveal that poly (ADP-ribose) polymerase (PARP) was processed by calpain in taxol-treated cells and by caspase-3 after calpain inhibition. In conclusion, these results demonstrate for the first time that calpain could play an important role modulating taxol-induced apoptosis. Further studies are needed to address the potentiality of inducing apoptosis by a combined use of taxol and calpain inhibitors in cells with increased calpain activity.