Cell culture
A Caco2 colorectal cancer cell line was purchased from the Kunming Cell Bank of the Kunming Institute of Zoology, Chinese Academy of Sciences. The Caco2 cell line was derived from a 72-year-old male Caucasian colorectal adenocarcinoma. The 293T cells were provided by the Kunming Institute of Zoology, Chinese Academy of Sciences. The medium required for Caco2 and 293T cells was a DMEM medium ( Dulbecco's Minimum Essential Medium ,Gbico, USA) containing 10% FBS(Fetal Bovine Serum ,Gbico, USA), 100 U/mL penicillin, and 100 mg/mL streptomycin. All cell lines were incubated at 37 °C in a humidified atmosphere with 5% CO2.
Establishment of cell lines
The lentiviral plasmid used for NDRG1 overexpression was GV358-NDRG1, which was purchased from Genechem (Shanghai,China). The plasmid vector used for NDRG1 knockout was pL-CRISPR.EFS.GFP, which was provided by the Kunming Institute of Zoology, Chinese Academy of Sciences. We designed three sgRNAs which were both located in exon 3 of the NDRG1 gene: sgRNA1, sgRNA2,sgRNA3(Table 1), inserting these three sgRNAs into pL -CRISPR.EFS.GFP plasmid, respectively. All constructed plasmids were identified by sequencing. The GV358 and pL-CRISPR.EFS.GFP empty plasmids were used as a control. The plasmid and its corresponding packaging plasmid were co-transfected into 293T cells for virus packaging, and then the cells were infected with virus. Finally, they were sorted by flow cytometry. The monoclonal cells of NDRG1 knockout were isolated by flow cytometry sorting. Then the monoclonal cells were expanded and cultured to extract the DNA; the third exon of NDRG1 was amplified by PCR(Polymerase Chain Reaction). The PCR product was sequenced and identified, the successfully identified cells were subjected to a T-A clone, and T-A clone product was also sent for sequencing. (All the primers used are shown in Table 1.)
Table 1
Names and Primers for the establishment of cell lines.
| Forward Primer | Reverse Primer |
sgRNA1 | 5'ATCCTCACCTACCATGACAT3' | 5' ATGTCATGGTAGGTGAGGAT3' |
sgRNA2 | 5' ACGCTGTGTGGGACTCCCAA3' | 5' TTGGGAGTCCCACACAGCGT3' |
sgRNA3 | 5' GTTCATGCCGATGTCATGGT3' | 5' ACCATGACATCGGCATGAAC3' |
NDRG1 | 5'TTTGGTGCATTTAACAGCGCAGTCT3' | 5'CAGGAAGTCCCAGGCAAAAAGAAAC3' |
RNA extraction and QPCR
The total cellular RNA was extracted (Qiagen miRNeasy Micro Kit, Germany, Co.,Ltd) and inverted into cDNA (Revertaid First Strand Cdna Synthesis Kit, Thermofisher,Co.,Ltd) according to the kit instructions. The ABI qPCR instrument was used to perform an amplification of the reactants. The reaction procedure was: 95 °C for 10 min, 95 °C for 15 s, 60 °C for 1 min, 95 °C for 15 s for 40 cycles, and 60 °C for 1 min. The results were normalized with that of GAPDH, and the relative gene expression levels were calculated by the ΔΔCt method. Three replicate wells were repeated for each sample. (All the primers used are shown in Table 2.)
Table 2
Names and primers for QPCR.
| Forward Primer | Reverse Primer |
GAPDH: | 5'GCACCGTCAAGGCTGAGAAC3' | 5'TGGTGAAGACGCCAGTGGA3' |
NDRG1 | 5'ATGTCTCGGGAGATGCAGGATGTAG3' | 5'CTAGCAGGAGACCTCCATGGACTTG3' |
Western Blotting
High-efficiency RIPA lysate extracted the total cellular protein and quantified protein using the BCA method. 80 µg protein was separated by electrophoresis on a 10%SDS-polyacrylamide gel. The proteins were electrotransferred from the gel to the PVDF membrane. The membrane was blocked with 5% non-fat milk solution for 2 h. The antibody rabbit, anti-human NDRG1 monoclonal antibody (1:500, cat.no. 9485S;CST Co., Ltd), β-tublin (1:5000, cat.no. 6046; Abcam Co., Ltd) was incubated for 1 h, and then incubated at 4 °C overnight. The next day, the corresponding secondary antibody (NDRG1 1:500, β-tublin 1:10000, cat.no. AS014; Abclonal Co., Ltd) was incubated for 1 h at room temperature. The membrane was washed and underwent detection by the enhanced ECL detection system.
Cell counting kit-8 (CCK-8) assay
GV358-control, GV358-NDRG1, pL-control, and pL-NDRG1-knockout cells were seeded in 96-well plates at 2 × 103 cells per well for the CCK-8 cell proliferation assay (CCK8 kit, Beyotime, Co., Ltd), then cultured for 1, 2, 3, and 4 days, respectively. According to the manufacturer's instructions, the cells were incubated with the CCK-8 reagent at 37 °C for 1 hour, with the absorbance of each sample scanned on a microplate reader equipped to read absorbance values at 450 nm.
Assessment of cell cycle by flow cytometric analysis
For cell cycle synchronization, 5 × 105 cells were plated in 6-well plates. After the cells were completely adherent, the cells were cultured in the serum-free medium for 24 h, and then cultured in a serum medium for 24 h. The cells were collected and underwent fixing with pre-cooled 75% ethanol at 4 °C for 14 ~ 24 h. After fixation, PI and RNaseA were added to the stain cells for 30 min in the dark, then detected by flow cytometry.
Assessment of apoptosis by flow cytometric analysis
The cells were trypsinized without EDTA then washed twice with Cell Staining Buffer and resuspended with Annexin V Binding Buffer to adjust the cell concentration to (0.25-1.0) x 107 /mL. 100 µL of the cell suspension was taken into a new 1.5 mL centrifuge tube and 5 µL of APC Annexin V and 10 µL of Propidium Iodide Solution were added, then incubated at room temperature for 15 min in the dark and tested by flow cytometry.
24-transwell for invasion and migration
The invasion experiment was done with the Chamber Matrigel Invasion 24-well DO (cat. no. 354480; Biocoat Co., Ltd) and the migration experiment was done with a Transwell Chamber (cat. no. 3422; Corning Co., Ltd). All experimental steps were carried out according to the instructions. The cells inoculated were 1 × 105, and the lower chamber was added with a DMEM medium containing 20% FBS. After 48 hours, the chambers were fixed in 4% paraformaldehyde, stained with 0.1% crystal violet, and a number that passed through the microscope was observed. Then, the cells stained by crystal violet were eluted with 33% acetic acid, and the absorbance at 570 nm of the eluate was measured by a microplate reader.
Statistical analysis
The data were statistically analyzed using the GraphPad Prism 7.0. The Student’s t test was used for statistical analysis. Data are presented as a means ± standard deviation. P < 0.05 was considered a statistically significant difference.