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.