Neither necrosis nor differentiation was observed at cytotoxic micromolar concentrations of
imperatorin. Further studies showed that the cytochrome c/caspase-9 pathway was responsible for imperatorin-induced
apoptosis; i.e., mitochondrial membrane was depolarized, Bcl-2 was down-regulated, cytochrome c was released from
mitochondria, caspase-9 and caspase-3 were activated, and poly(ADP-ribose) polymerase was cleaved. Furthermore, imperatorin-
induced apoptosis was significantly blocked by Z-VAD-FMK (a broad spectrum caspase inhibitor), Z-LEHDFMK
(a caspase-9 inhibitor) and Ac-DMQD-CHO (a caspase-3 inhibitor), but not by Z-IEDT-FMK (a caspase-8 inhibitor