In vitro investigations on induction and reversal of cisplatin resistance in a rat ovarian tumor cell line

Resistance of a rat ovarian tumor cell line (O-342/DDP) to cisplatin was induced in vitro by stepwise increase of cisplatin concentrations. Both chemosensitive parental cells (O-342) and resistant O-342/DDP cells grow in a monolayer and enter log-phase growth about 24 h after seeding (cell population doubling time in log-phase growth is about 24 h). O-342/DDP cells show a 33-fold resistance to cisplatin as compared to 0-342 cells (ID50=33 μM in O-342/DDP vs 1 μM in O-342 cells). The intracellular total glutathione (GSH+GSSG) of O-342/DDP cells was twice as high as that of O-342 cells (3.04 vs 1.37 nmol/106 cells), while the intracellular GSSG was increased by 26% in O-342/DDP cells compared to O-342 cells. DNA interstrand crosslinks were found to be 8.5 times higher in O-342 cells than in O-342/DDP cells (204 vs 24 rad equiv.), following cisplatin treatment. DNA single-strand breaks were approximately doubled in the sensitive line as compared to the resistant line following exposure to cisplatin. Chromosome analysis uncovered a change in the karyotype of O-342/DDP cell as compared to O-342 cells. In the sensitive line hyperploid (3n) clones, in the resistant line near-diploid clones predominated. Both DL-buthionine-(S,R)-sulfoximine and 3-aminobenzamide were able to sensitize the resistant line towards cisplatin. Thus, the present results suggest that mechanisms for cisplatin resistance in this tumor line apparently are multifactorial, and include a higher intracellular GSH content and an increased repair activity.