2.1 Cells and media
CHO-K1 (RIKEN Cell Bank, Tsukuba, Japan) and recombinant CHO cells were cultured in F12 medium (#N6760; Sigma-Aldrich, St Louis, MO, USA) supplemented with 10% fetal calf serum (BioWest, Nuaillé, France) and antibiotics (Penicillin-Streptomycin, #15140122; Invitrogen, Waltham, MA, USA). Cells were cultured in a 5% CO2 incubator at 37°C. For serum-free cultures, CD CHO AGT Medium (#12490025, Invitrogen) containing 4 mM L-glutamine (#074-00522; Fujifilm Wako Pure Chemical Corporation, Osaka, Japan), 0.2% anti-clumping reagent (#0010057AE, Invitrogen), and antibiotics (penicillin-streptomycin) was used. When cells were cultured in serum-containing medium, 100-mm cell culture dishes (BioLite #130182; Thermo Scientific, Waltham, MA, USA) or 24-well tissue culture plates (BioLite #130186, Thermo Scientific) were used. For serum-free suspension culture, bioreactor tubes (#87050; TPP Techno Plastic Products AG, Trasadingen, Switzerland) placed in a shaker (Model #0081704-000; Taitec, Koshigaya, Japan) with a 45° angle were incubated at 37°C in a 5% CO2 incubator.