Molecular effects of topoisomerase II inhibitors in AML cell lines: correlation of apoptosis with topoisomerase II activity but not with DNA damage

We examined the cellular effects of topo II inhibitors in two human myeloid cell lines, HL-60 and KG-1 cells, with the purpose of finding molecular ma rkers for the sensitivity of leukemia cells to topo II inhibitors, These ce ll lines are widely used, well characterized and they differ in their sensi tivities to topo II inhibitors. Despite the fact that HL-60 cells are p53-n egative, they are much more sensitive than KG-1 cells. Three different topo II inhibitors with distinct molecular ways of action have been used. Dauno rubicin acid aclarubicin are DNA intercalators that secondarily interact wi th topo II; etoposide, on the other hand, directly binds to the enzyme. In contrast to daunorubicin, which induces protein-associated DNA double-stran d breaks due to the blockage of topo II action, aclarubicin inhibits the ac cess of DNA by topo II. No correlation could be established between the dru g-induced DNA damage and apoptosis. In fact, the amount and pattern of DNA damage examined with the 'comet assay' was characteristic for each drug in both cell lines. The DNA binding of daunorubicin was slightly higher in HL- 60 cells, but there was no notable variance between the cell lines for acla rubicin. The most striking difference could be found for the nuclear topo I I activity, which was about half in KG-I cells and, additionally, less than 1% of the nuclear topo II activity was bound to the DNA in KG-I cells when compared to HL-60 cells. This fraction of topo II interacts with the inhib itors; subsequently these findings might well explain the variance in the c ellular sensitivity. Additional factors are alterations of the apoptotic pa thways, eg loss of p53 in HL-60 cells. Although we found no differences in the quantity of DNA damage between the cell lines after drug treatment, the quality of DNA damage appeared to be distinct for each topo II inhibitor. The morphological appearance of the comet tails after treatment was charact eristic for each drug. Further studies are necessary to decide whether thes e in vitro data are compatible with the clinical situation.