DNA Topoisomerases in Tumors and the Cellular and Molecular Responses to their Inhibitors

W.T. Beck, Y-Y. Mo, T. Khelifa
Cancer Center
University of Illinois at Chicago
Chicago, Illinois 60607-7173

DNA topoisomerases are important cellular enzymes involved in DNA replication, cell division, transcription, and possibly DNA repair In mammalian cells. There are two forms, type II and type I, that are the targets of clinically important anticancer drugs. Tumor Cells frequently express resistance to topo II inhibitors by expressing mutant forms or decreased amounts of the enzyme. However, mutation in "hot spot" regions of the topo lla gene appears to be an infrequent occurence in leukemia, but not lung cancer; although we have recently discovered novel 3' deletions in topo IIa that may be associated with leukemogenesis. We have recently found that down-regulation of topo II in drug-resistant cells is associated with decreased expression of the transcription factor, Sp3. This factor, which usually exhibits inhibitory activity, stimulates topo II promoter activity in our cells, suggesting a basis for the decreased expression of topo II in drug resistance. Studies over the past several years have revealed that inhibition of topoisomerases by anticancer drugs can initiate programmed cell death (apoptosis) pathways, and other studies have shown that the inhibitors themselves appear to act through complex signaling pathways mediated by protooncogenes, tumor suppressors, and proteins involved in the regulation of the cell cycle. We have studied the extent to which these pathways are involved in mediating the cytotoxic events associated with inhibition of the topoisomerases by comparing the effects of different classes of topo II inhibitors - those that stabilize DNA-protein complexes and damage DNA (e g., teniposide), and those that do not, but, rather, inhibit the catalytic activity of the enzyme (e.g., merbarone, ICRF-187) - in cell lines selected for resistance to these agents and their Insensitive parents. Despite their differences in action,these agents all block cells in G2/M, but they activate expression of c-jun and jun kinase (JNK-1) differently in drug-sensitive and -resistant cells. We suggest that activation of different signalling programs is related to differences in mode of inhibition of topo II in drug-sensitive and -resistant cells. Overall, our results show that the actions of inhibitors of DNA topoisomerases in tumor cells are complex.