The cells were seeded in 96-well plates and cultured as mentioned above. Then, 20 μL MTT (5 mg/mL) was added to each well. The plates were incubated for 4 h. DMSO solution (150 μL) was added to resolve the colored material, and the absorbance of each well was measured at 570 nm on a microplate reader (BioRad 680, Hercules, CA, USA).