The role of cell cycle regulation and apoptosis triggering in determining the sensitivity of leukemic cells to topoisomerase I and II inhibitors.

Topoisomerase (topo) inhibitors induce enzyme-linked DNA breaks. Resulting DNA damage can lead to cell cycle arrest and/or cell death by apoptosis. The sensitivity of five human leukemic cell lines to topo I (camptothecin or CPT) and topo II (etoposide or VP-16) inhibitors varied widely (100-fold for CPT and 30-fold for VP-16). Three cell lines were more sensitive (BV173, HL60, U937) and two cell lines were resistant (K562, KCL22) to both drugs. None of these cell lines were selected for drug resistance and overexpressed mdr1 gene. Their sensitivity was not related to their doubling time nor to cell cycle repartition. The initial DNA damage (cleavable complexes) induced by topo I and II inhibitors was measured as DNA-protein crosslinks (DPC) using alkaline elution. Neither DPC level induced by 30-min treatment with CPT or VP-16 nor the levels of topo 1, topo II alpha and topo II beta mRNA were related to sensitivity. Electron microscopy and DNA fragmentation measured by filter elution and agarose gel electrophoresis demonstrated that apoptosis was induced by both drugs in the five cell lines. The kinetics of DNA fragmentation was related to cell sensitivity. At drug concentrations higher than IC50, DNA fragmentation increased very rapidly in the three sensitive, compared with the two resistant, cell lines. Continuous exposure to both drugs induced cell cycle arrest in either G2 or S phase that was related both to cell sensitivity and drug concentration. Comparison between cell lines indicated that the ability of cells to arrest cell cycle in G2 or S phase was related to their drug sensitivity and increased with cell resistance. In a given cell line, cell cycle progression was observed to be progressively inhibited by increasing drug concentrations. Treatment of synchronized cells demonstrated that highly cytotoxic drug concentration induced a complete inhibition of cell cycle progression. Altogether, these data suggest that the ability of leukemic cell lines to regulate cell cycle progression and to trigger apoptosis is more indicative of their sensitivity to topoisomerase poisons than cleavable complexes induced by these drugs.