DISTINCT PATTERNS OF CELL-CYCLE DISTURBANCE ELICITED BY COMPOUNDS INTERFERING WITH DNA TOPOISOMERASE-I AND TOPOISOMERASE-II ACTIVITY

DNA topoisomerases are enzymes governing the multitude of conformation al changes DNA undergoes during the cell cycle. Several compounds are likely to interfere with specific steps of the catalytic cycle of thes e enzymes. Camptothecin arrests the activity of DNA topoisomerase I by provoking the formation of a single-stranded DNA break with the enzym e molecule covalently attached to the DNA. Exposure to m-AMSA arrests DNA topoisomerase II by the formation of a ternary complex involving t he drug, the enzyme, and DNA carrying a double-stranded break. Netrops in, distamycin A, and berenil inhibit DNA topoisomerase-mediated relax ation of supercoiled DNA by an as-yet unknown mechanism. Here, we anal yze the cell cycle kinetic effects of exposure to camptothecin, m-AMSA , netropsin, distamycin A, and berenil by using continuous bromodeoxyu ridine labeling followed by bivariate Hoechst 33258/ethidium bromide f low cytometry. Camptothecin elicits an accumulation of cells in all co mpartments of the cell cycle, while exposure to m-AMSA leads mainly to retention of cells in the G(0)/G(1) compartment and to accumulation i n the Gz phase. Neither camptothecin nor m-AMSA shows a synergism with bromodeoxyuridine incorporation into the DNA. These results point tow ard 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 emph asis on the G(2) phase. Neither camptothecin nor m-AMSA exhibits a syn ergistic effect in combination with berenil. Hence, at the level of pe rturbed cell cycle kinetics a distinction can be made between compound s provoking an abortive inhibition of the catalytic cycle of DNA topoi somerases (e.g., camptothecin, m-AMSA) and those interfering with the activity of the enzyme by a distinct mechanism. (C) 1995 Academic Pres s, Inc.