Differential regulation of doxorubicin-induced mitochondrial dysfunction and apoptosis by Bcl-2 in mammary adenocarcinoma (MTLn3) cells

Various anticancer drugs cause mitochondrial perturbations in association with apoptosis. Here we investigated the involvement of caspase- and Bcl-2-dependent pathways in doxorubicin-induced mitochondrial perturbations and apoptosis. For this purpose, we set up a novel three-color flow cytometric assay using rhodamine 123, annexin V-allophycocyanin, and propidium iodide to assess the involvement of the mitochondria in apoptosis caused by doxorubicin in the breast cancer cell line MTLn3. Doxorubicin-induced apoptosis was preceded by up-regulation of CD95 and CD95L and a collapse of mitochondrial membrane potential (Atk) occurring prior to phosphatidylserine externalization. This drop in Deltapsi was independent of caspase activity, since benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone did not inhibit it. Benzyloxycarbonyl-Val-Ala-DLAsp-fluoromethylketone also blocked activation of caspase-8, thus excluding an involvement of the death receptor pathway in Deltapsi dissipation. Furthermore, although overexpression of Bcl-2 in MTLn3 cells inhibited apoptosis, dissipation of Deltapsi was still observed. No decrease in Deltapsi was observed in cells undergoing etoposide-induced apoptosis. Immunofluorescent analysis of Deltapsi and cytochrome c localization on a cell-to-cell basis indicates that the collapse of Deltapsi and cytochrome c release are mutually independent in both normal and Bcl2-overexpressing cells. Together, these data indicate that doxorubicin-induced dissipation of the mitochondrial membrane potential precedes phosphatidylserine externalization and is independent of a caspase- or Bcl2-controlled checkpoint.