Fig. 5. Combined treatment with a low dose of STP and IR specifically activates the cytochrome c-dependent caspase-9 pathway. Cytosolic extracts from untreated cells (CTL) or from cells treated for 6 h with STP (20 nM), IR (5 Gy), or the combination (STP+IR) were probed using (A) monoclonal anti-cytochrome c antibody 7H8.2C12 and (B) a rabbit polyclonal anti-caspase-9 antibody. C, caspase-3-like activity 6 h after combined treatment in cytosolic extracts from cells preincubated with or without the specific caspase-9 inhibitor Z-LEHD-FMK. D, proliferation was assessed 24 h after combined treatment with STP (20 nM) and IR (5 Gy) preincubated with increasing concentrations (0, 50, and 100 µM) of the specific caspase-9 inhibitor Z-LEHD-FMK.