Cytotoxicity assay
Samples of 1 × 104 cells/well were plated in 96-well tissue culture plates and allowed to adhere for 24 h. The cells were then pre-incubated for 24 h in medium containing 0, 5, 10, 25 or 50 µM l-buthionine-[S,R]-sulfoximine (BSO) (Sigma), a drug that inhibits glutathione synthesis (31). Later, cells were exposed to 0, 5, 10 or 20 µM CdCl2 in the specified pre-incubation medium for an additional 24 h. Cytotoxicity was determined by the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromid)-based Cell Proliferation Kit I (Roche) according to the manufacturer's instructions.