Cisplatin is a widely used chemotherapeutic agent for treatment of ovarian, testicular, lung, and stomach cancers. The initial response to the drug is robust; however, tumor cells commonly develop resistance to cisplatin, which complicates treatment. Recently, overexpression of the Cu-ATPase ATP7B in ovary cells was linked to the increased cellular resistance to cisplatin; and the role for Cu-ATPases in the export of cisplatin from cells was proposed. Our results support functional interactions between cisplatin and ATP7B but argue against the active transport through the copper translocation pathway as a mechanism of drug resistance. In hepatocytes, we observed no correlation between the levels of endogenous ATP7B and the resistance of cells to cisplatin. Unlike copper, cisplatin does not induce trafficking of ATP7B in hepatoma cells, neither does it compete with copper in a transport assay. However, cisplatin binds to ATP7B and stimulates catalytic phosphorylation with EC 50 similar to that of copper. Mutations of the first five N-terminal copper-binding sites of ATP7B do not inhibit the cisplatin-induced phosphorylation of ATP7B. In contrast, the deletion of the first four copper-binding sites abolishes the effect of cisplatin on the ATP7B activity. Thus, cisplatin binding to ATP7B and/or general changes in cellular copper homeostasis are likely contributors to the increased resistance to the drug. The link between changes in copper homeostasis and cisplatin resistance was confirmed by treating the Huh7 cells with copper chelator and increasing their resistance to cisplatin.
Cisplatin, cis-diamminedichloroplatinum (DDP),3 is a common anti-tumor agent that is used to treat many types of cancer. It is especially prescribed for testicular, ovarian, bladder, liver, lung, and stomach cancers (1-5). DDP mediates its cytotoxic effects by binding to DNA, forming the intrastrand crosslinks and thus causing an inhibition of DNA synthesis and repair with eventual cell death (6, 7). The initial tumor response to the treatment with DDP is robust; however, the efficacy of treatment decreases with longer and repetitive therapy cycles. The resistance arises rapidly and is sufficient to cause a failure of DDP therapy (8). The mechanisms by which cells develop resistance to DDP are not fully understood. Detoxification of DDP, enhanced repair and tolerance of DNA adducts, inhibition of apoptosis, impaired uptake and increased efflux of the drug may contribute to the acquired resistance of cells (9 -13).Recently, an unexpected connection was discovered between the resistance of cells to DDP and cellular copper metabolism (14 -16). Either down-regulation of CTR1 (49), a transporter responsible for the uptake of copper, and/or up-regulation of the copper-transporting ATPases (Cu-ATPases) responsible for copper efflux were found to increase cells resistance to DDP, although correlation between the levels of CTR1 and resistance were not always observed (17). The DDP-resistant cells were shown to have a lower copper content (18), and...