BCL-2 PROTEIN INHIBITS ETOPOSIDE-INDUCED APOPTOSIS THROUGH ITS EFFECTS ON EVENTS SUBSEQUENT TO TOPOISOMERASE II-INDUCED DNA STRAND BREAKS AND THEIR REPAIR

Previous studies have shown that bcl-2 overexpression can inhibit apop tosis induced by DNA-damaging agents widely used in cancer chemotherap y, including X-irradiation, alkylating agents (hydroperoxycyclophospha mide, etc.), and topoisomerase II inhibitors (etoposide, etc.). Howeve r, little is known about the mechanism by which bcl-2 overexpression i nhibits apoptosis triggered by these agents. In this study, we examine d whether bcl-2 overexpression could have effects on etoposide-induced DNA damage and its repair. For these experiments, we developed CH31 c lones (mouse B-cells) stably transfected with human bcl-2 sense plasmi ds and compared these clones with a parental CH31 clone or CH31 clones with antisense plasmids. Overexpression of bcl-2 protein inhibited et oposide-induced apoptosis and cytotoxicity. However, there was no or l ittle difference in the production and repair of DNA-protein cross-lin ks, DNA single-strand breaks, and double-strand breaks among a parenta l CH31 clone and CH31 clones with human bcl-2 sense or antisense plasm ids. These findings indicate that (a) apoptosis or cytotoxicity induce d by etoposide can be separated into early events (formation of double -strand breaks, DNA single-strand breaks, and double-strand breaks) an d later events (secondary DNA fragmentation or cell death) and (b) bcl -2 inhibits apoptosis and cytotoxicity induced by etoposide at some st eps between these events.