INCREASED EXPRESSION OF DNA TOPOISOMERASE-I GENE AND COLLATERAL SENSITIVITY TO CAMPTOTHECIN IN HUMAN CISPLATIN-RESISTANT BLADDER-CANCER CELLS

We established three cis-diamminedichloroplatinum(II) (cisplatin)-resi stant cell lines, T24/DDP5, T24/DDP7, and T24/DDP10, by the stepwise e xposure of T24 human bladder cancer cells to increasing concentrations of cisplatin. The resistance to cisplatin of T24/DDP5, T24/DDP7, and T24/DDP10 cells was 2.2-, 5.2-, and 8.4-fold that of the parental T24 cells, respectively. The cisplatin-resistant cell lines also showed an increased resistance to vincristine, although their sensitivities to Adriamycin and etoposide resembled that of T24. In contrast, the cispl atin-resistant cells developed a collateral sensitivity to (4s)-4,11-d iethyl-4-hydroxy-9-[(4- piperidinopiperidino)carbonyloxy]dione hydroch loride trithydrate, a camptothecin derivative, and its active metaboli te, 7-ethyl-10-hydroxycamptothecin, that targets DNA topoisomerase I. Both a Northern blot analysis and an immunoblot analysis demonstrated increased cellular levels of DNA topoisomerase I mRNA in the resistant cell lines. However, the expression of DNA topoisomerase II in the th ree resistant cell lines did not differ significantly from that in the T24 cells. No significant differences in the glutathione S-transferas e pi levels were observed, although the intracellular content of gluta thione in the T24/DDP7 cells was slightly but significantly increased. In addition, the intracellular platinum concentration correlated nega tively with the degree of cisplatin resistance and was found to be sig nificantly decreased in T24/DDP10 at an external cisplatin concentrati on of 20 mu g/ml. These results suggest that the increased levels of D NA topoisomerase I mRNA thus play an important role in cisplatin resis tance and produce a collateral sensitivity to ydroxy-9[(4-piperidinopi peridino)carbonyloxy]dione hydrochloride trithydrate and 7-ethyl-10-hy droxycamptothecin in these cisplatin-resistant bladder cancer cell lin es. In addition, the presence of decreased intracellular cisplatin acc umulation may also contribute to the acquisition of resistance to cisp latin in these cell lines.