Diana Chan

Anion-dependent modulations of DNA topoisomerase II-mediated reactions in potassium-containing solutions

DNA binding proteins operate in an intracellular environment of low chloride concentration, yet in vitro assays of the activities of these proteins are often performed in isotonic chloride-containing solutions. Previously, the activity of bacterial DNA-binding proteins was found to be enhanced in potassium-containing solutions in which the anion glutamate (Glu) was substituted for chloride. We have extended this observation to include eukaryotic topoisomerase I and II activities. The concentration ranges over which DNA strand passing activities of these enzymes were observed was broader in KGlu than in KCl. This was also true for the topoisomerase II-mediated DNA strand passage and antineoplastic drug-dependent DNA cleavage produced by nuclear extracts from HL-60 human leukemia cells. The rate of topoisomerase II-mediated DNA strand passage was also dependent on anion moiety and concentration in potassium-containing buffers. Drug-dependent topoisomerase II-mediated DNA cleavage in intact HL-60 cell nuclei was also anion-dependent, suggesting that anion type and concentration may influence topoisomerase II-mediated events in mammalian cells as had been described for other DNA binding proteins in prokaryotic systems. This should be considered in developing biochemical assays of topoisomerase activities to reproduce intracellular conditions.

The effect of staurosporine on drug-induced, topoisomerase II-mediated DNA cleavage in human leukemia cells

SummaryPhorbol-12-myristate 13-acetate (PMA), a stimulator of protein kinase C, dramatically decreased topoisomerase II-reactive drug-induced DNA cleavage in HL-60 human cells. The effect of staurosporine, an inhibitor of protein kinase C, on drug-induced, topoisomerase II-mediated DNA cleavage was quantified in the same cells. Staurosporine decreased the magnitude of 4′-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA)- and etoposide-induced DNA cleavage in a dose-and time-dependent fashion. Measurement of several parameters of cell proliferation revealed no clear and uniform correlation between staurosporines inhibition of these parameters and its effects on drug-induced DNA cleavage. A direct comparison with PMAs effects on drug-induced DNA cleavage showed that whereas PMAs inhibition of etoposide-induced cleavage was much greater than its inhibition ofm-AMSA-induced cleavage, the magnitude of staurosporines effect on the cleavage produced by the two topoisomerase II-reactive drugs was similar. Thus, although PMA stimulates protein kinase C and staurosporine inhibits this enzyme, it is unlikely that the actions of either on topoisomerase II-reactive, drug-induced DNA cleavage are mediated directly via protein kinase C. Furthermore, it is likely that the mechanisms by which PMA and staurosporine inhibit topoisomerase II-reactive drug-induced cleavage are different.

Effect of bryostatin 1 on drug-induced, topoisomerase II-mediated DNA cleavage and topoisomerase 11 gene expression in human leukemia cells

Unlike PMA, bryostatin 1 has been found to have a minimal effect on drug-induced topoisomerase II-mediated DNA cleavage and no effect on topoisomerase II mRNA levels. Furthermore, bryostatin 1 overcame the down-regulatory effects of PMA treatment on (1) drug-induced, topoisomerase II-mediated DNA cleavage, (2) drug-induced cytotoxicity, and (3) topoisomerase II gene expression. Thus, it is unlikely that the effects of phorbol ester treatment on topoisomerase II-mediated events are a direct consequence of protein kinase C activation per se. Rather, the results with bryostatin 1 suggest that the phorbol ester effects are related to more distal effects of phorbol ester treatment that may be related to monocytoid differentiation.