Whole cell extracts were prepared in 1% SDS solution containing protease inhibitors. Equal amounts of proteins from the cell lysates were separated by SDS-PAGE, followed by immunoblot analyses as previously described (28 (link)). Proteins bound to membrane filters were probed with the specified antibodies and immunoreactive proteins were visualized using chemiluminiscence using either X-ray film or the Bio-Rad Chemidoc MP imaging system. Antibodies using in the immunoblot analyses include the following: P-GCN2 (Abcam, #Ab75836), total GCN2 (Abcam, #ab137543), P-eIF2α (Abcam, #ab32157), total eIF2α (Cell Signaling Technology, #5324S), ATF4 antibody was prepared against the corresponding recombinant human proteins, which were affinity purified (30 (link)), P-mTOR (Cell Signaling Technology, #2971S), total mTOR (Cell Signaling Technology, #2983S), P-S6K1 (Cell Signaling Technology, #9205S), total S6K1 (Cell Signaling Technology, #9202S), P-4EBP1 (Cell Signaling Technology, #2855S), total 4EBP1 (Cell Signaling Technology, #9644S), PRODH (Proteintech, # 22980-1-AP) and β-actin (Sigma, #A5441). Quantification of immunoblots was carried out by measuring relative band intensities measured using ImageJ software, which was normalized as indicated.