Effect of Dp44mT (1 μM) on the protein levels of active caspase-3, -8, and -9 and the activity of caspase-3, -8, and -9 in cultured M109 cells in the absence and presence of cell-permeable caspase inhibitors. (A) Caspase-3, -8, and -9 levels after Dp44mT treatment at the indicated times as detected by Western blotting (top blots). Anti–β-actin antibody was used to ensure equal protein loading (bottom blot). (B) Densitometric analysis of the expression of caspase-3, -8, and -9 as a function of time normalized to β-actin. (C) Caspase activity induced by Dp44mT (1 μM) at 0 to 48 hours was expressed as a percentage of the 0-hour time value. (D) Cell-permeable inhibitors of caspase-3, -8, or -9 at 1 μM prevented activation of these enzymes when incubated with Dp44mT (1 μM) for 48 hours. Results are mean ± SEM of 3 separate experiments. Horizontal dashed line indicates 100%.