A unique paclitaxel-mediated modulation of the catalytic activity of topoisomerase II alpha

Paclitaxel (Taxol) is known to act by polymerizing and stabilizing microtub ules. In spite of a known target, the existence of additional targets is su ggested by a poor understanding of the mechanism(s) underlying eventual cel l death by paclitaxel and by the drug's high efficacy, as compared to other spindle poisons. Based on the enhanced sensitivity of a mutant DNA double- strand break repair-deficient Chinese hamster ovary cell line to paclitaxel as well as to various topoisomerase (Topo) II poisons, it was hypothesized that paclitaxel, in addition to having an effect on microtubules, may also alter the activity of Topo II. This study demonstrates the unique, in vitr o effects of paclitaxel on Topo II activity as investigated by monitoring t he decatenation of kinetoplast DNA and relaxation of supercoiled plasmid DN A by Topo II. Unlike classical anti-topoisomerase drugs, low concentrations of paclitaxel (0.02-500 nM) stimulated Topo II catalytic activity, while h igher concentrations over 5 mu M inhibited the activity of Topo II. Further more, these effects of paclitaxel appear to be mediated through a direct in teraction of paclitaxel with Topo II rather than an interaction with DNA or DNA-Topo II complexes. Collectively, the evidence presented suggests the e xistence of an atypical interaction between Topo II and paclitaxel that may disrupt the normal functioning of the enzyme. [(C) 1999 Lippincott William s & Wilkins.].