DNA topoisomerase I and II (top1 and top2) are major targets for cancer chemotherapy. Topoisomerase poisons act by stabilizing enzyme-linked DNA breaks in drug-treated cells. The goal of this project is to study drug interactions with cellular DNA and to elucidate the antitumor mechanisms of top1 and top2 poisons, their selectivity for cancer cells, and the function of topoisomerases in normal and cancer cells. We have studied the differential sensitivity and resistance of the human colon cancer cell lines from the NCI Anticancer Drug Screen to identify the parameters that are best correlated with cytotoxicity and that could be used in the clinic to predict and monitor the response to topoisomerase inhibitors. Top1 protein levels were comparable among cell lines and were not predictive, while top1 cleavable complexes were better correlated with cytotoxicity. Studies are in progress to investigate differential DNA repair and cell cycle regulation in these and additional cell lines, including ovarian and breast cancer cells. We and others have described apoptosis as a mode of cell death, especially in leukemia cells exposed to topoisomerase inhibitors. We have studied a panel of leukemia cells with various sensitivities and found that cytotoxicity is correlated with apoptotic ability. HL6O cells rapidly up regulate and down regulate cyclin B/cdc2 kinase activity before undergoing apoptosis after DNA damage. We have also set up an in vitro cell-free system to study the biochemical pathways of apoptosis. It appears that human leukemia HL6O cells induce a protease cascade in response to DNA damage following camptothecin treatment and that the protease(s) activate(s) preexisting nuclear pronuclease(s). New approaches aimed at triggering or suppressing apoptosis may provide new therapeutic strategies and help reduce drug side effects.
