Generation of ROSs led to apoptosis in U937 cells. (A) p21-IRS–treated U937 cells produce ROSs. ROS production in U937 cells incubated with medium (gray fill); p21-IRS for 1 hour (dotted line); p21-IRS for 3 hours (solid line); or staurosporine for 4 hours (bold line). (B) The ROS scavenger LNAC rescues cells from p21-IRS–induced apoptosis. U937 cells were preincubated with medium (open bar), or with LNAC at 10 mM (black bar), 15 mM (hatched bar), or 20 mM (stippled bar) before the addition of p21-IRS. (C) The ROS scavenger MnTBAP rescues cells from p21-IRS–induced apoptosis. U937 cells were preincubated with medium (open bar) or with MnTBAP at 50 μM (black bar) or 200 μM (striped bar) before the addition of p21-IRS.