Apoptosis induced by the homocamptothecin anticancer drug BN80915 in HL-60cells

The homocamptothecin (hCPT) derivative BN80915 containing a seven-membered lactone ring represents one of the most potent topoisomerase I inhibitors d escribed. This anticancer agent, currently undergoing phase I clinical tria ls, has been shown to produce a greater number of DNA strand breaks than co nventional camptothecins with a six-membered lactone ring. To shed light on the mechanism of action of hCPT at the cellular level, we compared the eff ects of BN80915 and the classic camptothecin SN-38, the active metabolite o f irinotecan, on HL-60 human promyelocytic cancer cells. A variety of bioch emical events, at both the mitochondrial and the nuclear levels, were chara cterized to determine how and to what extent the hCPT derivative can induce apoptotic cell death. The use of cytometry, Western blot analysis, confoca l microscopy, and different colorimetric assays enabled us to demonstrate t hat BN80915 is a potent inducer of apoptosis in HL-60 cells. This induction of apoptosis is associated with cell cycle changes, a marked decrease of i ntracellular pH, activation of caspase-3 and -8, DNA fragmentation, and ext ernalization of phosphatidylserine lipids but no significant changes of the mitochondrial membrane potential or the expression of Bcl-2. The interconn ections between these different events are discussed. Collectively, the res ults indicate that the superior activity expressed at the topoisomerase I l evel leads to a more pronounced induction of apoptosis by BN80915 compared with SN-38. The study identifies and delineates signaling factors involved in BN80915-induced apoptosis in HL-60 cells.