Selection of human leukemic CEM cells for resistance to the DNA topoisomerase II catalytic inhibitor ICRF-187 results in increased levels of topoisomerase II alpha and altered G(2)/M checkpoint and apoptotic responses

ICRF-187 is a bisdioxopiperazine anticancer drug that inhibits the catalyti c activity of DNA topoisomerase (topo) II without stabilizing DNA-topoII cl eavable complexes. To better understand the mechanisms of action of and res istance to topoII catalytic inhibitors, human leukemic CEM cells were selec ted for resistance to ICRF-187. The clones CEM/ICRF-8 and CEM/ICRF-18 are a pproximately 40- and 69-fold resistant to ICRF-187, and 12- and 67-fold cro ss-resistant to ICRF-193, respectively, but are sensitive to other topoII c atalytic inhibitors (merbarone and aclarubicin), as well as collaterally se nsitive to the DNA-topoII complex-stabilizing drug etoposide (VP-16). Both the number of VP-16-induced DNA-topoII complexes formed and the amount of i n vitro topoII catalytic activity are enhanced in the drug-resistant cells. The ICRF-187-resistant clones contain similar to 5-fold increase in topoII alpha protein levels and similar to 2.2-fold increase in topoII alpha mRNA levels. Furthermore, CEM/ICRF-8 expresses similar to 3.5-fold increase in topoII alpha promoter activity, suggesting that up-regulation of topoII alp ha in this clone occurs at the transcriptional level. Treatment of the drug -resistant or -sensitive cells with equitoxic doses of merbarone or tenipos ide results in a G(2)/M arrest. In marked contrast, when treated with equit oxic ICRF-187 doses, the drug-resistant clones exhibit either a transient a rrest or completely lack the G(2)/M checkpoint compared with the drug-sensi tive cells. This aberrant cell cycle profile is associated with a 48-h dela y in drug-induced apoptotic cell death, as revealed by fluorescent-end labe ling of DNA and poly (ADP-ribose) polymerase cleavage. In summary, resistan ce to ICRF-187 in CEM cells is associated with increased levels of catalyti cally active topoII alpha and altered G(2)/M checkpoint and apoptotic respo nses.