Cell viabilities were determined by 3-(4,5-dimethylthiazol)-2,5-diphenyl tetrazolium bromide (MTT; Sigma) assay. Briefly, 2.5 mg/ml of MTT in phosphate buffered saline was added to wells of 24-well plates and incubated at 37 C for 4 h, followed by theaddition of dimethyl sulfoxide (Duksan, Korea). The absorbance was measured at 570 nm. Absorbance readings were subtracted from the value of blank wells. Cell viabilities were calculated as a percentage of control absorbance.