Previous studies have demonstrated that treating cultured cells with cisplatin (CDDP) up-regulated the expression of glutathione (GSH) and its de novo rate-limiting enzyme glutamate-cysteine ligase (GCL), which consists of a catalytic (GCLC) and a modifier (GCLM) subunit. It has also been shown that many CDDP-resistant cell lines exhibit high levels of GCLC/GCLM and GSH. Because the GSH system is the major intracellular regulator of redox conditions that serve as an important detoxification cytoprotector, these results have been taken into consideration that elevated levels of GCL/GSH are responsible for the CDDP resistance. In contrast to this context, we demonstrated here that overexpression of GSH by transfection with an expression plasmid containing the GCLC cDNA conferred sensitization to CDDP through up-regulation of human copper transporter (hCtr) 1, which is also a transporter for CDDP. Depleting GSH levels in these transfected cells reversed CDDP sensitivity with concomitant reduction of hCtr1 expression. Although rates of copper transport were also up-regulated in the transfected cells, these cells exhibited biochemical signature of copper deficiency, suggesting that GSH functions as an intracellular copper-chelator and that overexpression of GSH can alter copper metabolism. More importantly, our results reveal a new role of GSH in the regulation of CDDP sensitivity. Overproduction of GSH depletes the bioavailable copper pool, leading to upregulation of hCtr1 and sensitization of CDDP transport and cell killing. These findings also have important implications in that modulation of the intracellular copper pool may be a novel strategy for improving chemotherapeutic efficacy of platinum-based antitumor agents.
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