Abstract
Proteases have received attention as important cellular components responsible for stress response in humans. However, little is known of proteases which play roles in the early steps of X-ray irradiation response. In the present study, we first searched for proteases whose activity is induced in human RSa cells after X-ray irradiation. The activity was identified as fibrinolytic one, using 125I-labeled fibrin as a substrate. Protease samples were prepared by lysation of cells with a buffer containing MEGA-8. RSa cells showed an increased level of protease activity 10min after X-ray (up to 3 Gy) irradiation. We next examined whether this protease inducibility is causally related with X-ray susceptibility of cells. Leupeptin inhibited the protese activity in samples obtained from X-ray-irradiated RSa cells. Furthermore, treatment of RSa cells with the inhibitor before and after X-ray irradiation resulted in an increased susceptibility of the cells to X-ray cell killing. These results suggest that leupeptin-sensitive proteases are involved in the resistance of human cells to X-ray cell killing. [J Radiat Res 44:449 (2003)]
