XIAP inhibitors overcome Bcl-2–mediated resistance to TRAIL-induced apoptosis. (A) Bcl-2 expression was determined in Jurkat leukemia cells transfected with empty vector (Neo) or Bcl-2 by cytoplasmic staining and flow cytometry (dotted thin and thick lines indicate vector control and Bcl-2–overexpressing cells stained with isotype control; thin line, vector control cells stained with Bcl-2 antibody; thick line, Bcl-2–overexpressing cells stained with Bcl-2 antibody). Fluorescence intensity (x-axis) is blotted against counts (y-axis). (B) Jurkat leukemia cells transfected with empty vector (Neo) or Bcl-2 were treated for 72 hours with indicated concentrations of TRAIL in the absence (□) or presence of 10 nM (top and middle panels) or indicated concentrations (bottom panels) of XIAP inhibitors, control compound, or DMSO and 20 μM zVAD.fmk or DMSO. Apoptosis was determined by forwardside scatter analysis and flow cytometry. (C,D) Jurkat leukemia cells transfected with empty vector (Neo) or Bcl-2 were treated for 48 hours with 0.4 ng/mL TRAIL and/or 10 nM XIAP inhibitor 2, control compound, or DMSO. Mitochondrial transmembrane potential (C) and cytochrome c release (D) were assessed by FACS analysis. Mean and SD of 3 experiments each performed in triplicate are shown. For statistical analysis, t test was performed comparing XIAP inhibitor 2 to control compound, *P < .01.