Caspase-3 gene activity in radiation-induced apoptosis

Previous experiments indicated that caspase-3 was a mediator of ionizing radiation (IR)-induced apoptosis in U937 cells. In present study, the caspase-3 activity in IR-induced apoptosis was further measured by analyzing the Ac-DEVD-p-nitroanilide cleaving activity, and results showed that caspase-3 activity increased in accordance to the development of apoptosis, and reached peak at 6 h post-irradiation. However, northern blot analysis showed that the expression of caspase-3 was not changed, while Western blot found that the active P20 subunit concentration increased, and from 3 h post-irradiation, active P17 subunit also increased. These findings demonstrate that the increase of caspase-3 activity observed during IR-induced apoptosis depends on the activation of inactive pre-caspase rather than the change in gene expression.