Three replicate wells were set for each concentration in the experiments. Then, 20 μL of MTT solution (final concentration of 5 mg/mL, freshly diluted in PBS before treatment) was added to each well and incubated for another 4 h. The medium was abandoned, and 150 μL/well DMSO was added. The amount of formazan was determined by measuring the absorbance at 490 nm with a microplate reader (Bio-Rad, USA).