Efficient induction of chromosome-type aberrations by topoisomerase II inhibitors closely associated with stabilization of the cleavable complex in cultured fibroblastic cells.

Eukaryotic topoisomerase II (Topo-II) inhibitors such as etoposide, adriamycin and mitoxantrone, which commonly stabilize the cleavable complex of the enzyme and DNA, have been found to efficiently induce chromosome-type aberrations (mainly breaks and exchanges) in cultured Chinese hamster lung fibroblastic cells (CHL cells). To clarify whether the induction of chromosome-type aberrations is mediated by stabilization of the cleavable complex, the present study investigated (1) the correlation between the induction of chromosome-type aberrations and the amount of cleavable complex formed; and (2) the ATP dependence of the Topo-II inhibitor-induced chromosome-type aberrations due to the ATP requirement of cleavable complex formation by Topo-II. First, in cells treated with the Topo-II inhibitors, (etoposide, adriamycin) and aclarubicin, an antagonist of the inhibitor of cleavable complex formation, the frequency of chromosome-type aberrations decreased dose-dependently with aclarubicin, in contrast to an increase of chromatid-type aberrations. The formation of the cleavable complex was further established by a proteinase K/SDS precipitation assay for cleaved double-strand DNA in a cell-free system and in CHL cells. Results from both experiments showed that aclarubicin caused a dose-dependent suppression of the accumulation of the cleavable complex induced by etoposide, which corresponded particularly well to the reduction of chromosome-type aberrations in etoposide-treated cells. In ATP-depleted cells simultaneously treated with etoposide and dinitrophenol (DNP), chromosome-type aberrations were reduced as compared with DNP-untreated cells, in contrast to an increase of chromatid exchanges in the cells. This means that etoposide-induced chromosome-type aberrations in ATP-depleted cells may be attributable to incompleteness of Topo-II activities to form DNA double-strand breaks. The present findings indicate that the stabilization of the cleavable complex on Topo-II is closely associated with the induction of chromosome-type aberrations.