Cell death and apoptosis assays. Cell viability was measured by using a colorimetric method with 3-(4,5-dimethylthiazol-2yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS), according to the manufacturer's instructions (Promega, Madison, WI). For apoptotic death assays, cells were stained with Hoechst stain (H33258), visualized by UV microscopy, and quantitated by counting condensed and fragmented nuclei in five randomly selected areas unless described otherwise. The apoptosis-mediated alteration of membrane phospholipids was monitored by annexin V-fluorescein isothiocyanate and quantified by a fluorescence-activated cell sorter (FACS). Mitochondrial membrane potential (Δψm) was measured with 3,3′-dihexyloxacarbocyanine iodide (DiOC₆) (3), followed by FACS analysis (68). Caspase activity assays were carried out by using the fluorogenic caspase-3 substrates (DEVE-AFC) as described previously (68).