Aplidin™ a new antitumoural drug presently in phase II clinical trials, has shown both in vitro and in vivo activity against human cancer cells. Aplidin™ effectively inhibits cell viability by triggering a canonical apoptotic program resulting in alterations in cell morphology, caspase activation, and chromatin fragmentation. Pro-apoptotic concentrations of Aplidin™ induce early oxidative stress, which results in a rapid and persistent activation of both JNK and p38 MAPK and a biphasic activation of ERK. Inhibition of JNK and p38 MAPK blocks the apoptotic program induced by Aplidin™, demonstrating its central role in the integration of the cellular stress induced by the drug. JNK and p38 MAPK activation results in downstream cytochrome c release and activation of caspases -9 and -3 and PARP cleavage, demonstrating the mediation of the mitochondrial apoptotic pathway in this process. We also demonstrate that protein kinase C delta (PKC-δ) mediates the cytotoxic effect of Aplidin™ and that it is concomitantly processed and activated late in the apoptotic process by a caspase mediated mechanism. Remarkably, cells deficient in PKC-δ show enhanced survival upon drug treatment as compared to its wild type counterpart. PKC-δ thus appears as an important component necessary for full caspase cascade activation and execution of apoptosis, which most probably initiates a positive feedback loop further amplifying the apoptotic process.
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