Sensitivity to rituximab-induced apoptosis is determined at the level of mitochondria. (A) Cell-free activation of caspase-3-like activity (DEVDase) by cytochrome c and dATP (cyto c/dATP) in extracts prepared from rituximab-sensitive and rituximab-resistant B-NHL cell lines is prevented by the caspase inhibitor zVAD-fmk (zVAD). The leukemia cell line K562 served as control (mean values + SD of triplicate experiments). (B) Induction of apoptosis in rituximab-sensitive (SU-DHL-4, Ramos, WSU-NHL) and rituximab-resistant B-NHL cells lines (Sc-1, HT) after incubation with the kinase inhibitor staurosporine (STS, 50 nM) for 48 hours. The fraction of cells with apoptotic DNA fragmentation was quantified flow cytometrically; mean values plus SD of 3 independent experiments are given. (C) Staurosporine (STS) induces the release of cytochrome c from the mitochondria into the cytoplasm in rituximab-sensitive SU-DHL-4 and Ramos B-NHL cells, but not in rituximab-resistant Jeko-1 and HT B-NHL cells (mean values + SD of 3 independent experiments). (D) Immunoblot analyses of the constitutive protein expression of proapoptotic Bax and Bak, and antiapoptotic Bcl-2, Bcl-xL, Mcl-1, and Bfl-1 in the human B-NHL cell lines. Actin served as loading control.