Fig. 2. Apoptosis induction in E1A/ras-transformed MEFs by a combination of low-dose STP and IR. A, caspase 3-like activity was determined 6 h after treatment in cytosolic extracts from control cells (CTL) using Ac-DEVD-pNA as a colorimetric caspase-3 substrate or cells treated with STP, IR, or a combination of both. B, formation of active caspase-3 was determined after the different treatments using an antibody recognizing the cleaved caspase-3 subunit. C, the corresponding samples were probed for endogenous PARP cleavage by Western blot using an anti-PARP antibody.