CHO (Gu et al., 1999 (link); Xi et al., 2003 (link)), 293FT, and GD25 cells were cultured in DMEM complete (Cellgro) supplemented with 10% fetal bovine serum (FBS; Corning), 2 mM l-glutamine (Corning), 100 U/ml penicillin plus 100 μg/ml streptomycin (Corning), 1 mM sodium pyruvate (Gibco), 0.1 mM nonessential amino acids (Gibco), and 10 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer (Gibco). Cells were serum starved in DMEM containing each of the foregoing components except the FBS for at least 4 h before experiments.