Protection of platinum–DNA adduct formation and reversal of cisplatin resistance by anti‐MRP2 hammerhead ribozymes in human cancer cells

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Abstract
Resistance to platinum‐containing antineoplastic drugs is the major limitation in their clinical use. To elucidate the role of the ABC transporter MRP2 in platinum drug resistance, its expression was analyzed in human cisplatin‐resistant cell lines: the ovarian carcinoma line A2780RCIS, the adrenocortical carcinoma line D43/86RCIS and the melanoma line MeWoCIS1. All these cells showed overexpression of MRP2. For reversal of platinum resistance, 2 anti‐MRP2 hammerhead ribozymes were introduced into A2780RCIS cells. Both ribozymes showed gene‐silencing activities and reversed the drug‐resistant phenotype. Moreover, formation of platinum‐induced intrastrand cross‐links was measured in DNA. The level of DNA platination corresponded inversely to the level of MRP2 expression and was accompanied by increased caspase‐3‐dependent apoptosis. Kinetics of formation and elimination of platinum–DNA adducts suggest that the DNA repair capacity was not altered; the decrease in platinum–DNA adduct formation was rather a reflection of the protecting activity of MRP2. In conclusion, functional inhibition of MRP2 might be a promising strategy in the reversal of resistance to platinum‐based anticancer drugs. This was reflected by the specific inhibition of MRP2 by ribozyme technology, indicating that this gene therapeutic approach may be applicable as a specific means to overcome platinum resistance in human neoplasms.