Following the induction of hypoxia, the cells were quickly collected and lysed using RIPA protein extraction reagent (Epizyme, Inc.; Ipsen) supplemented with protease inhibitor and phosphatase inhibitor cocktails (both Epizyme. Inc.; Ipsen). The concentrations of the protein samples were detected using a BCA Protein Assay kit (Thermo Fisher Scientific, Inc.). Protein extracts (15 µg/lane) were separated by 10% SDS-PAGE and transferred to polyvinylidene fluoride membranes. The membranes were blocked for 25 min at room temperature in protein-free blocking buffer (Epizyme, Inc.; Ipsen) and incubated with primary antibodies at 4°C for 12 h. Antibodies targeting CDK2 (cat. no. A0094), cyclin D1 (cat. no. A19038), AKT1 (cat. no. A20799), phosphorylated-AKT (cat. no. AP1172) and β-actin (cat. no. AC026) were used. All primary antibodies were diluted 1:1,000. All the primary antibodies were acquired from ABclonal Biotech Co., Ltd. Membranes were incubated with Anti-rabbit IgG, HRP-linked Antibody (dilution, 1:5,000; cat. no. 7074; Cell Signaling Technology, Inc.) at room temperature for 60 min. Immobilon Western HRP Substrate (WBKLS0050) was purchased from MilliporeSigma. The membrane was exposed to an autoradiography film and autoradiograms were quantified by densitometry using Quantity One software 4.4.6 (Bio-Rad Laboratories, Inc.).