Ovarian cancer cisplatin-resistant cell lines: multiple changes including collateral sensitivity to Taxol

Alteration in apoptosis pathways (in particular mutations of p53 gene) may result in resistance of ovarian carcinoma to cisplatin. However, cisplatin resistance is likely to be multifactorial. An understanding of the molecular alterations associated with the development of resistance may be of considerable relevance in an attempt to optimize the therapeutic approach.

Two cisplatin-resistant sublines (IGROV-1/Pt0.5 and IGROV-1/Pt1), both characterized by mutant p53 (Cancer Res 1996; 56: 556-62), but with different degree of resistance were studied in terms of pattern of cross-resistance, susceptibility to drug-induced apoptosis, expression of gluthathione-dependent system, cellular pharmacokinetics, drug-induced DNA damage. The resistance index (ratio between the IC50 of resistant and sensitive cells) after a 96-hour drug exposure was 10 for IGROV-1/Pt0.5 and 14 for IGROV-1/Pt1 cells.

Resistant cells were cross-resistant to DNA-damaging agents and, interestingly, they had a collateral sensitivity to Taxol. The cellular response to Taxol paralleled the drug ability to induce apoptosis. The intracellular glutathione level was significantly increased in IGROV-1/Pt cells compared to the sensitive counterpart. In contrast, glutathione S-transferase level was consistently reduced in both sublines. gamma-Glutamyl transpeptidase activity, which was lower in resistant than in sensitive cells, was not directly correlated with glutathione level, thus suggesting a complex regulation of cellular glutathione content. In the resistant cells with the highest glutathione content, a reduced level of cisplatin-induced cross-link was found. Analysis of DNA platination revealed a slight decrease of DNA-bound platinum only in IGROV-1/Pt1 cells. Again, this reduction is consistent with a protective role for glutathione. The expression of metallothionein IIa was increased in both resistant variants.

Multiple changes are involved in acquired resistance of ovarian carcinoma cells including reduced susceptibility to apoptosis as consequence of inactivation of p53 and expression of defence mechanisms. The relative contribution is related to the degree of drug resistance. In particular, the glutathione-dependent system could have a role only in the development of a high degree of resistance. Finally, the finding that Taxol was very effective in inducing apoptosis in resistant sublines with p53 mutation supports the expression of an intact p53-independent pathway of apoptosis and suggests the pharmacological interest of Taxol in the treatment of p53-mutated tumors.