INDUCTION OF P53-DEPENDENT AND P53-INDEPENDENT CELLULAR-RESPONSES BY TOPOISOMERASE-1 INHIBITORS

We have previously shown that loss of p53 function in A2780 human ovar ian adenocarcinoma cells confers increased clonogenic resistance to se veral DNA-damaging agents, but not to taxol or camptothecin. We have n ow extended these studies, comparing wild-type p53-expressing A2780 ce lls with isogenic derivatives transfected with a dominant negative mut ant (143; val to ala) p53. We show that, as well as retaining equivale nt clonogenic sensitivity to camptothecin, mutant p53 transfectants of A2780 cells do not acquire significantly increased resistance to the camptothecin analogues topotecan and SN-38, the active metabolite of C PT-11. Compared with vector-alone transfectants they are, however, rel atively (2.2-fold) resistant to GI 147211, a further camptothecin anal ogue undergoing clinical trial. Treatment of A2780 with camptothecin a nd each analogue produces an increase, maximal at 24-48 h after drug e xposure, of cells in the G(2)/M phase of the cell cycle and a decrease in both G(1) and S-phase cells, The G(2) arrest is independent of p53 function for camptothecin and the three analogues. All four compounds can induce apoptosis in A2780, which is reduced in mutant p53 transfe ctants, as measured using the terminal DNA transferase-mediated b-d UT P nick end labelling (TUNEL) assay. Thus, although p53-dependent apopt osis is induced by camptothecin, topotecan and SN-38 in this human ova rian carcinoma cell line, these drugs induce p53-independent death, as measured by clonogenic assay.