INDUCTION OF IN-VITRO NUCLEAR APOPTOSIS ACTIVITY COINCIDES WITH THE PRODUCTION OF 50 KDA CYTOSOLIC PROTEIN

Human monocytic leukemia U937 cells undergo apoptosis when cells are t reated with the anticancer drug etoposide. To study the mechanism of d rug-induced apoptosis, we used an in vitro apoptosis system with cytos ol from etoposide-treated U937 cells. The cytosol from apoptotic U937 cells showed activity to induce morphologic changes and oligonucleosom al DNA fragmentation in isolated nuclei in vitro; both are typical fea tures of apoptosis. We generated monoclonal antibodies to the proteins in the etoposide-treated U937 cytosol. We found that a 50 kDa protein , recognized by SN-1 monoclonal antibody, appeared in the cytosol of U 937 cells, in accordance with its cell-free apoptosis activity. Z-Asp, an inhibitor of interleukin-1 beta converting enzyme (ICE) family pro teases, inhibited the appearance of the 50 kDa protein and the emergen ce of the cell-free apoptosis activity in the etoposide-treated U937 c ytosol. These results indicate that the 50 kDa protein is produced by the activation of ICE family protease during apoptosis and suggest som e roles of the protein in the development of apoptosis.