During the process of death receptor-mediated apoptosis, Bid is cleaved by activated caspase-8, and then cleaved Bid conveys apoptotic signals to the mitochondria by activating Bax/Bak. In the present study, we found that D609 (an antitumor drug with multiple activities) blocks Fas-induced apoptosis. D609 did not interfere with activation of caspase-8 and cleavage of Bid, whereas it blocked cytochrome c release from the mitochondria by inhibiting the activation of Bax and Bak. D609 had no protective effect against apoptosis of SKW6.4 cells, which are typical type I cells. Studies using permeabilized cells revealed that in addition to activation of caspase-8, Fas activated a distinct and D609-sensitive signaling pathway that transmitted signal(s) sensitizing the mitochondria to apoptotic stimuli, and that D609 itself promoted mitochondrial resistance to apoptotic stimuli.
