The Annexin V-FITC early apoptosis detection kit (Neobioscience, Shenzhen, China) was used to identify the apoptotic cells. Briefly, approximate 105 cells were harvested, washed with cold PBS twice and resuspended with 350 μL 1 × Binding Buffer. Then, 5 μL of the Annexin V-FITC conjugate was added. After 20 minutes' light-prevented incubation at room temperature, cell suspension was diluted to a final volume of 500 μL/assay with ice cold 1 × Binding Buffer. Next, 10 μL of the Propidium Iodide (PI) solution were added to each sample tube, and the samples were analyzed by FACS Canto™II cell analyzer (BD Biosciences, San Jose, CA, USA).