DNA topoisomerase I and II (top 1 and 2) are major targets for cancer chemotherapy. Topoisomerase poisons act by stabilizing enzyme-linked DNA breaks which can be detected as protein-associated DNA breaks in drug-treated cells. The goal of this project is to elucidate the antitumor mechanisms of top 1 and 2 poisons, their selectivity for cancer cells, and the functions of topoisomerases in normal and cancer cells. We have obtained evidence that top 2 poisons differ from each other by the distribution of cleavage sites in the human c-myc gene in the case of amsacrine and etoposides. Thus, it appears that preferential gene damage by top 2 inhibitors may contribute to their differential anticancer activity. Another new finding is that the same drugs kill different cell lines very differently. Human promyelocytic leukemia HL-60 cells, which are very sensitive to top 2 inhibitors, die by apoptosis within 3 hours (cell shrinkage, chromatin condensation, plasma membrane impermeability, oligonucleosome-size DNA fragmentation), while human colon carcinoma HT-29 cells and Chinese hamster DC3F fibroblasts die by G2 block after 24-48 hours. HL-60 apoptosis differs from other types of apoptosis because it is resistant to modulators of signal transduction (calcium chelators, protein kinase and phosphatase inhibitors). It can only be modulated by agents which act either at the chromatin level (poly[ADP-ribose] inhibitors, zinc, spermine) or which induce monocytoid differentiation (phorbol ester). Interestingly, HL-60 granulocytic differentiation does not appear to inhibit topoisomerase inhibitor-induced apoptosis. DNA breaks induced by caffeine derivatives, and phorbol ester-treatment of mouse epidermal cell lines have also been studied in order to evaluate the involvement of DNA topoisomerases or endonucleases in DNA damage (apoptosis) and repair. We have also studied the effect of transfection of a poly(ADP-ribose) polymerase antisense vector in HeLa cells and demonstrated a role of poly(ADP-ribose) in DNA repair.
