BCL-2 PREVENTS TOPOISOMERASE-II INHIBITOR GL331-INDUCED APOPTOSIS IS MEDIATED BY DOWN-REGULATION OF POLY(ADP-RIBOSE)POLYMERASE ACTIVITY

Poly (ADP-ribose) polymerase (PARP), a nuclear enzyme responsible for DNA strand breaks, has been recently suggested to be crucial for apopt osis induced by a number chemotherapeutic drugs, In this study, we dem onstrated that the PARP activity could be evidently elevated with a pe ak at 6 h when HL-60 cells were treated with a new anticancer drug GL3 31, Coincident with the peak of PARP activity, an apparent DNA fragmen tation and apoptotic morphology were observed in cells treated with GL 331. The subsequent apoptotic DNA fragmentation induced by GL331 could be completely blocked by transfecting cells with anti-sense PARP retr oviral vector or by treating cells with PARP inhibitor, 3-aminobenzami de (3-AB). This blocking effect thus suggests that activation of PARP was critically involved in GL331-induced apoptosis, The fact that Bcl- 2 has been found to antagonize cell death induced by a wide variety of agents, accounts for why we examined whether if Bcl-2 could antagoniz e GL331 effects, Interestingly, ectopic overexpression of Bcl-2 in eit her HL-60 or U937 cells caused in resistance towards GL331-elicited DN A fragmentation and cytotoxic effect. Additionally, Bcl-2 also attenua ted the poly(ADP-ribosyl)ation of PARP itself as well as Histone H1 at the early period of drug treatment. However, Bcl-2 did not influence the extent of DNA strand breaks induced by GL331 in either control or Bcl-2-overexpressing cells. In addition, analysis of basal PARP activi ty in control and several Bcl-2 overexpressing clones revealed that Bc l-2 down-regulated PARP activity under the condition without DNA damag es, Above findings suggest that poly(ADP-ribosyl)ation of nuclear targ ets is important for apoptosis induced by DNA-reactive anticancer drug s.