For MTT assays, cells (2,000 ~ 5,000 cells/well) were subcultured into 96-well plates according to their growth properties. Cell proliferation was assayed at 72 hr after treatment of CHIR-99021 by adding 20 μl of 5 mg/ml 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) solution per 100 μl of growth medium. After incubating for 3-4 h at 37°C, the media were removed and 150 µl/well of MTT solvent (either absolute DMSO or isopropanol containing 4 mM HCl and % Nonidet-40) was added to dissolve the formazan. The absorbance of each well was measured by ELx808 (BioTek, Winooski, VT) or Wallac Victor2 (Perkin-Elmer Life Sciences, Boston, MA) Microplate Reader. Viable cells are presented as percent of control, vehicle-treated cells.
CHO-IR cells expressing human insulin receptor are grown to 80% confluence in Hamm’s F12 medium with 10% fetal bovine serum and without hypoxanthine. Trypsinized cells are seeded in 6-well plates at 1 × 106 cells/well in 2 ml of medium without fetal bovine serum. After 24 h, medium is replaced with 1 ml of serum-free medium containing GSK-3 inhibitor or control (final DMSO concentration <%) for 30 min at 37°C. Cells are lysed and centrifuged 15 min at 4°C/14000g. The activity ratio of GS is calculated as the GS activity in the absence of glucose-6-phosphate divided by the activity in the presence of 5 mmol/l glucose-6-phosphate, using the filter paper assay of Thomas et al.