Distinct patterns of cell cycle disturbance elicited by compounds interfering with DNA topoisomerase I and II activity.

DNA topoisomerases are enzymes governing the multitude of conformational changes DNA undergoes during the cell cycle. Several compounds are likely to interfere with specific steps of the catalytic cycle of these enzymes. Camptothecin arrests the activity of DNA topoisomerase I by provoking the formation of a single-stranded DNA break with the enzyme molecule covalently attached to the DNA. Exposure to m-AMSA arrests DNA topoisomerase II by the formation of a ternary complex involving the drug, the enzyme, and DNA carrying a double-stranded break. Netropsin, distamycin A, and berenil inhibit DNA topoisomerase-mediated relaxation of supercoiled DNA by an as-yet unknown mechanism. Here, we analyze the cell cycle kinetic effects of exposure to camptothecin, m-AMSA, netropsin, distamycin A, and berenil by using continuous bromodeoxyuridine labeling followed by bivariate Hoechst 33258/ethidium bromide flow cytometry. Camptothecin elicits an accumulation of cells in all compartments of the cell cycle, while exposure to m-AMSA leads mainly to retention of cells in the G0/G1 compartment and to accumulation in the G2 phase. Neither camptothecin nor m-AMSA shows a synergism with bromodeoxyuridine incorporation into the DNA. These results point toward distinct functions of the two DNA topoisomerases in the process of cell cycle traverse. The compounds binding to the minor groove of DNA interfere with all phases of the cell cycle, but with a relative emphasis on the G2 phase. Neither camptothecin nor m-AMSA exhibits a synergistic effect in combination with berenil. Hence, at the level of perturbed cell cycle kinetics a distinction can be made between compounds provoking an abortive inhibition of the catalytic cycle of DNA topoisomerases (e.g., camptothecin, m-AMSA) and those interfering with the activity of the enzyme by a distinct mechanism.