Metallothionein gene expression and resistance to cisplatin in human ovarian cancer

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Abstract
Intracellular thiols have been proposed as mediators of resistance to alkylating agents and cisplatin. As metallothionein is the predominant protein thiol, we examined its relationship to cisplatin resistance in human ovarian cancer cell lines. A human ovarian carcinoma cell line, A2780, derived from an untreated patient, was treated with cisplatin in several ways and the induced resistance to cisplatin ranged from 13‐ to 68‐fold. The degree of resistance was dependent upon the method of selection. The drug‐resistant cell lines also developed low levels of cross‐resistance to cadmium. Additional cell lines established from untreated patients or ovarian cancer patients refractory to cisplatin‐ and/or carboplatin‐containing combination chemotherapy were studied. The most cisplatin‐resistant cell lines, OVCAR‐8 and ‐10, were from patients previously treated with intensive chemotherapy. OVCAR‐8 was relatively cross‐resistant to cadmium while OVCAR‐10 appeared relatively sensitive. Cell lines were examined for expression of metallothionein mRNA to evaluate the relationship between cisplatin resistance, cadmium cross‐resistance and metallothionein expression. Only two of the cell lines with in vitro‐induced resistance to cisplatin, 2780E80 and 2780CP70B3, had detectable metallothionein mRNA. The other cell lines selected in vitro for cisplatin resistance, as well as the parental A2780 ovarian cancer cell line, showed no expression at our level of detection. There was variable expression of metallothionein among the OVCAR cell lines. Cell lines from untreated patients, OVCAR‐5 and ‐7, did express metallothionein, while the most cisplatin‐resistant cell lines, OVCAR‐8 and ‐10, did nat. We also examined cisplatin induction of metallothionein mRNA in the cell lines. Only 2780CP70B3 among the cell lines with in vitro‐induced cisplatin resistance showed increased expression after short‐term exposure to cisplatin. OVCAR‐4 also had a slight increase in expression after exposure to cisplatin. Mouse C127 cells transfected with a bovine papilloma virus‐metallothionein gene construct were compared for cisplatin sensitivity to the same cell type transfected with bovine papilloma virus alone. In this model system, metallothionein expression did not influence cisplatin cytotoxicity. On the basis of these studies, we conclude that there is no causal relationship between metallothionein expression and cisplatin resistance.