Induction of mitochondrial permeability transition and cytochrome C release in the absence of caspase activation is insufficient for effective apoptosis in human leukemia cells

Induction of mitochondrial permeability transition (MPT) and cytosolic tran slocation of cytochrome C are considered essential components of the apopto tic pathway. Hence, there is the realization that mitochondrial-specific dr ugs could have potential for use as chemotherapeutic agents to trigger apop tosis in tumor cells. Recently, we showed that photoproducts of merocyanine 540 (pMC540) induced tumor cell apoptosis, In this study, we focused on id entifying mitochondrial-specific compounds from pMC540 and studied their ap optotic potential. One purified fraction, C5, induced a drop in mitochondri al transmembrane potential and cytosolic translocation of cytochrome C in H L60 human leukemia cells, Moreover, the addition of C5 to purified rat live r mitochondria induced MPT as indicated by mitochondrial matrix swelling, w hich was completely inhibited by cyclosporin A, an inhibitor of the inner-m embrane pore. Supernatant of C5-treated mitochondria showed a dose-dependen t increase in cytochrome C, which was also inhibited in the presence of cyc losporin A, strongly indicating a direct effect on the inner-membrane pore. Despite the strong mitochondrial reactivity, C5 elicited minimal cytotoxic ity (less than 25%) against HL60 leukemia and M14 melanoma cells because of inefficient caspase activation. However, prior exposure to C5 significantl y enhanced the apoptotic response to etoposide or the CD95 receptor. Thus, we demonstrate that MPT induction and cytochrome C release by the novel com pound C5, in the absence of effective caspase activation, is insufficient f or triggering efficient apoptosis in tumor cells. However, when used in com bination with known apoptosis inducers, such compounds could enhance the se nsitivity of tumor cells to apoptosis. (C) 2000 by The American Society of Hematology.