The human embryonic kidney (HEK) 293T cell line was purchased from the cell bank of the Chinese Academy of Sciences (Shanghai, China). The 293T cells were maintained in Dulbecco's Modified Eagle Medium (DMEM, Hyclone, Logan, UT) supplemented with 10% FBS, 10 mM HEPES, 1% penicillin-streptomycin and 5% CO2 at 37°C.