Apoptotic response of HL-60 human leukemia cells to the antitumor drug NB-506, a glycosylated indolocarbazole inhibitor of topoisomerase 1

The antitumor drug NB-506 is a glycosylated indolocarbazole derivative targeting topoisomerase I. This DNA-intercalating agent, which is currently undergoing phase I/II clinical trials, was shown to induce apoptosis in HL-60 human leukemia cells. We compared the cellular dysfunctions induced by NB-506 and the reference topoisomerase I poison camptothecin (CPT) at the nuclear, mitochondrial, and cytoplasmic levels. The two drugs NB-506 and CPT were almost equally toxic to HL-60 cells and produced similar cell cycle changes with a considerable increase in the fraction of cells with DNA content less than G1. The sub-G1 fraction, which can be considered as the apoptotic cell population, appeared more rapidly with CPT than with NB-506 but in both cases, the cell cycle perturbation was accompanied by a marked decrease in the mitochondrial transmembrane potential and the intracellular pH. In contrast, no change in the intracellular calcium concentration was detected. Treatment of HL-60 cells with NB-506 resulted in an increase in the activity of the intracellular protease caspase-3, as determined by a DEVD-based colorimetric assay and direct monitoring of poly(ADP-ribose) polymerase (PARP) cleavage by Western blot analysis. The initiator caspase-8 was also stimulated by NB-506 but, as for caspase-3, the extent of the caspase activation was weaker with NB-506 compared to CPT. With both drugs, the protease activation resulted in DNA degradation, as independently confirmed via the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assay and characterization of internucleosomal DNA fragmentation. Collectively, these findings identify some of the molecular events leading to NB-506-induced apoptosis and as such, provide important mechanistic insights into the mode of action of topoisomerase I-targeted indolocarbazole antitumor drugs. (C) 2001 Elsevier Science Inc. All rights reserved.