Abstract

Resistance to cisplatin, one of the most widely used anticancer chemotherapeutic agents, is a major clinical problem. There is no effective way to predict development of cisplatin resistance in cancers. As determined by reverse transcription-polymerase chain reaction and Western blotting, the expression of γ-catenin, an adherens junction protein, was decreased in KB-CP20 and 7404-CP20 cells compared with parental-sensitive cells. Short-term treatment with cisplatin of the parental cells resulted in proteolysis of γ-catenin as evaluated in membrane pellet preparations, and the extent of cleavage increased as cisplatin concentration was raised from 1 to 5 μg/ml during 1 h of treatment. Uncleaved cytoplasmic γ-catenin increased under the same conditions. These biochemical results were supported by confocal microscopy, which showed a loss of γ-catenin from adherens plaques after cisplatin treatment. Cleavage of γ-catenin was specific to cisplatin treatment in that cleavage did not occur after treatment with doxorubicin and cytosine arabinoside. Pretreatment of KB and 7404 cells with cisplatin for 1 h resulted in reduced uptake of [¹⁴C]carboplatin, suggesting that the biochemical changes induced by cisplatin treatment, including cleavage of γ-catenin, could affect the ability of cells to internalize platinum compounds. Cells transfected with the γ-catenin gene are sensitive to cisplatin compared with cells transfected with a control vector. Our data suggest that proteolysis and altered localization of γ-catenin are early markers for the response of cells to cisplatin, and reduced levels of γ-catenin in resistant cells may indicate an important role for γ-catenin in mediating or modulating the toxicity of cisplatin in cancer cells.