ABSTRACT
110Cisplatin is an effective anticancer drug, whose efficacy is limited by the development of drug resistance in solid tumors. Although the cause for drug resistance is multifactorial, the most common underlying mechanisms include reduced drug uptake/accumulation, lowered bioavailability of the drug, increased efflux, and altered mitochondrial metabolism. These mechanisms can be manipulated by various biochemical/genetic means, to reinforce the therapeutic efficacy of cisplatin. However, this requires greater understanding of the mechanisms of drug resistance and new agents to overcome resistance. Considering the relation between cisplatin and copper in terms of overlapping transport pathways mediated by CTR1, an attempt was made to understand how the intracellular copper depletion affects the cisplatin uptake into the cells. A simple eukaryote Saccharomyces cerevisiae, has been established as a model system for cancer studies, because of the widely conserved family of genes involved in cell cycle progression, proliferation, and apoptosis. In the first investigation, we sought to determine whether copper deprivation affects sensitivity of the yeast to cisplatin. Yeast cultures grown in low copper medium and exposed to bathocuproiene disulfate (BCS) resulted in significant reduction in the intracellular copper, as assessed by atomic absorption spectrophotometry. The low copper medium rendered BY4741 strain hypersensitive to cisplatin (CDDP) as seen from the inhibited growth curves. Yeast grown in low copper medium exhibited ~2.0-fold enhanced cytotoxicity in survival and colony-forming ability compared with copper adequate control cells grown in yeast extract/peptone/dextrose (YPD) rich medium. The effect of copper restriction on CDDP sensitivity appeared to be dependent on the up-regulation of CTR1, facilitating enhanced uptake and accumulation of CDDP. In addition, CDDP further lowered copper deprivation-induced changes in CUP1 metallothionein levels, superoxide dismutase (SOD) activity, and glutathione (GSH) levels, which are the general antioxidant molecules. These changes were associated with increased protein oxidation and lipid peroxidation, indicating that the copper-deficient cells are more prone to oxidative stress in the presence of cisplatin. These results, thus suggest that cisplatin cytotoxicity is potentiated under low copper conditions because of the enhanced uptake and accumulation of cisplatin and also in part because of the lowered antioxidant defense and increased oxidative stress imposed by copper deprivation.
