Paclitaxel induces apoptotic cell death in murine cells lacking caspase-10. (A) Caspase-10 is not detected in cells of murine origin. Lysates of MEFs and human Jurkat T cells were immunoblotted with caspase-10–, caspase-8–, and β-actin–specific antibodies. Arrowheads indicate the positions of procaspase-10, procaspase-8, and β-actin. (B) MEFs die by apoptosis in response to paclitaxel. MEFs were incubated for 48 hours with paclitaxel and then analyzed for DNA fragmentation using PI staining of hypodiploid nuclei (top panels), phosphatidylserine exposure using double staining with annexin-V–FITC and PI (middle panels), and loss of mitochondrial membrane potential (ΔΨ_m) using the potentiometric dye DiOC₆ (bottom panels). Representative fluorescence-activated cell sorter (FACS) analyses of untreated MEFs or cells treated with 1 μM paclitaxel are shown (left panels). Hypodiploid nuclei are contained in the marker region M1; normal nuclei with a higher DNA content are shown in M2. The bar diagrams (right panels) summarize the dose response of MEFs treated with the indicated concentrations of paclitaxel and show the mean values (± SD) of hypodiploid nuclei, vital PI-excluding cells with phosphatidylserine exposure, or loss of ΔΨ_m. (C) Paclitaxel treatment induces caspase activation. Immunoblot analysis of cell lysates from MEFs that were either left untreated or incubated for 48 hours with 1 μM paclitaxel. The positions of the procaspases and β-actin are indicated by closed arrowheads. Cleaved active caspase-3 is marked by an open arrowhead.