BL-derived cell lines are resistant to etoposide-induced caspase activation. (A) BL and Jurkat cell lines were incubated for 24 hours in the absence (□) or presence (▪) of etoposide, and apoptosis was assessed by flow cytometry. (B) Cells treated for 24 hours with etoposide were harvested for measurement of caspase-3–like enzyme activity according to specific cleavage of the fluorescent DEVD-AMC substrate. (C) Raji and Jurkat cells preincubated in the absence (▪) or presence (▨) of the pan-caspase inhibitor, zVAD-fmk, were treated with etoposide for 6 hours, and caspase activities were determined. □ indicates control cells. Data are depicted as fold increases of AMC release. (D) Cell lysates from BL and Jurkat cell lines treated for 12 hours with etoposide were evaluated for processing of pro–caspase-3 and pro–caspase-9 by using specific antibodies. Results shown in panels A-C are mean values ± SDs (n = 3). Data shown in panel D are representative of 3 independent experiments.