Data collection and screening
Microarray data of GSE62254 were obtained from the Gene Expression Omnibus (www.ncbi.nlm.nih.gov/geo/). GSE62254 data were based on the GPL570 platforms (Affymetrix Human Genome U133 Plus 2.0 Array, 300 GC patients), and 295 samples with both clinical parameters and gene expression data of GC were included in this study. Kaplan Meier-plotter (KMplotter) (http://www.kmplot.com/) was used for external validation.
Survival analysis
Survival analysis was performed by Kaplan-Meier (KM) method and log-rank test. The expression levels of hub-genes were separated according to high and low expression based on the median value. The Cox proportional-hazards regression models was applied to perform uni- and multivariate analyses were performed by. A nomogram was set up by the rms package in R, according to the final multivariate COX regression model. The predictive accuracy was assessed by C-Index. The internal validation of nomogram was measured by a calibration curve.
Chemicals and reagents
DMEM medium, FBS, penicillin/streptomycin, and trypsin were purchased from Invitrogen. Sodium butyrate and Puromycin and were purchased from Sigma. Anti-PolG antibodies (EPR7296) were purchased from Abcam. Antibodies specific to β-actin (A1978) were purchased from Sigma. PKM2 (4053s), pPKM2-Tyr105(3827s), Myc(2276s), Flag(2368s) were purchased from Cell Signaling Technology.
Plasmid construction
A lentiviral PolG shRNA was purchased from Genechem (Shanghai, China). The shRNA sequence targeting the human PolG complementary DNA was 5′-TGTCCAGGGAGAGTTTATA-3′. A scrambled shRNA was included as a negative control (NC). The target sequence was inserted into the GV248 lentiviral vector (Genechem). The expression of the Myc-PolG plasmid was constructed by PCR and subcloned into the Myc-pCMV vector(Clontech). The Flag-PKM2 plasmid was purchased from OriGene.
Cell Culture and Transfection
SGC7901, MGC803 and HEK293T cells were cultured in high-glucose DMEM with 10% FBS and 100 units/ml of penicillin/streptomycin, at 37 oC in a 5% CO2 incubator. Cells were transfection by Lipofectamine 2000 (Thermo Fisher Scientific, 11668019) according to the instructions and harvested 48 h after transfection. 293T cells were used for lentivirus production by transfected with the shRNA-expression vector. The transfected 293T cells supernatant was collected at 24 h, 48 h and 72 h. Following by centrifugation at 1500 g for 30 min, the lentiviral particles were resuspended in PBS and added to the SGC7901 and MGC803 cells for 24 h infection. Stably transfected cell lines were sifted for 5 days in 20 µg/mL puromycin.
Western Blot Analysis
Cells were lysed with an IP lysis buffer supplemented with protease and phosphatase inhibitor cocktails for 30 min on ice. Total protein was harvested by centrifugation at 15,000 rpm for 20 min at 4 °C. Samples were loaded to 10% polyacrylamide gels, separated by SDS-PAGE and transferred to PVDF membranes for 2.5 h at 80-120V. Membranes were incubated through prescribed antibodies at 4˚C overnight after blocked by 5% BSA in TBST for 2 h at room temperature. After 3 times wash with TBST, membranes were incubated with secondary antibody for 2 h at room temperature. At last, bands were analyzed by chemiluminescence detection (Tanon Science & Technology Co., Ltd., Shanghai, China).
Co-immunoprecipitation analysis
SGC7901 cells were dissolved with IP lysis and incubated with antibody for 2 h. Then protein A/G-Sepharose was added and incubate on a mixer overnight at 4 °C. Then use centrifugation for 5 min at 700 g at 4 °C to connect Beads the next day, and cleaned in IP lysis for 3 times (each time for 10 min). The beads were then resuspended with the loading for WB.
Glucose consumption and lactate production analysis
Cells were seeded in six-well culture plates. The medium was changed into phenol red-free DMEM media after 6 h. Then harvest the media after 48 h. Glucose consumption was measured between the media before and after the 48 h incubation period by an assay kit (Sigma, GAHK20). Extracellular lactate levels were normalized to the protein concentration of the samples by a lactate assay kits (Sigma, MAK065).
Cell proliferation assay
Cell counting Kit-8 (CCK8) (Abbkine, KTA1020) was used to evaluate the cell proliferation ability. Cells were seeded into 96-well plates at a density of 5 × 103 cells/well for 6 h, 24 h, 48 h, 72 h. Then the culture medium was replaced with 90 µl of basal DMEM and 10 µl CCK8. After incubation at 37 °C for 3 h, the absorbance was measured using an absorbance reader (TECAN, Switzerland) at 450 nm.
Transwell migration assay
SGC7901 or MGC803 cells (3 × 104) were resuspended with Serum-free DMEM media and seeded into the upper chamber while 10%FBS DMEM with added to the lower chambers (Corning, 3422). After 24 h incubation, invasive cells on the underside were fixed in methanol for 10 min and stained with hematoxylin for 30 min at room temperature. Image was taken by an inverted microscope (Nikon Corp., Tokyo, Japan). Three independent experiments were performed, and five individual fields were counted each for statistical analysis.
In vivo xenograft tumor growth
For the xenograft tumor growth assay, SGC7901 stable cell lines with PolG knockdown were injected subcutaneously into the right flank of 6-week-old male BALB/C nude mice (N = 10). NaB was intraperitoneally injected into mice in the PolG-knockdown group (200 mg/kg) (N = 5). NC group were used for comparative(N = 5). Tumors were cultivated for 14 days. All animal experiments were approved by the Committee of China Medical University.
Statistical analysis
Three independent experiments’ values are expressed as the mean ± standard. Statistical significance was analyzed using a t-test or a one-way analysis of variance. All statistical analysis was performed using SPSS 17.0 and Prism 5.0 software. Values of P < 0.05 were defined as statistically significant.