2.3. Cell culture and collection
The human cervical cancer cell line, HeLa (ATCC, USA), was cultured at 37 °C and 5% CO2 in DMEM with 10% FBS and 1% penicillin/streptomycin. The culture medium was refreshed every 2 - 3 days. Cells were harvested at >80% confluency following treatment with 0.25% trypsin-EDTA solution to obtain a cell suspension. Cells were dispersed in growth medium, and the cell density was measured by an automated cell counter (Cellometer AutoT4, Nexcelom, USA). The defined number of cells was washed three times with ice-cold PBS and centrifuged at 1000 rpm for 10 min at 4 °C. Cells were then placed into 1.5-mL protein LoBind tubes (5 X 105 cells per tube) and stored at −80 °C for future analysis.