Oncomine gene expression analysis
We first examined the expression of KIF21B gene in GC by searching microarray gene expression datasets derived from Oncomine database (www.oncomine.com). In brief, the Cancer Type was defined as Gastric Cancer, Data Type was mRNA, and Analysis Type was Cancer versus Normal Analysis. Total three datasets, including Wang Gastric [30], Chen Gastric [31] and DErrico Gastric [32] were included in our analysis. The log-transformed, median-centered and normalized expression values of KIF21B were extracted, analyzed and read from the scatterplot accordingly.
Tissue specimens
Total thirty pairs of fresh tumor tissues and matched adjacent normal stomach mucosa tissues were collected from GC patients who received radical gastrectomy at the Second Hospital, Cheeloo College of Medicine, Shandong University (Shandong Province, China). All patients did not receive radiotherapy and chemotherapy before surgery. In addition, paraffin-embedded specimens of GC were also obtained to evaluate the KIF21B protein expression. Informed consent was obtained from all patients. This study was conducted in accordance with Helsinki Declaration and approved by the Medical Ethics Committee of the Second Hospital, Cheeloo College of Medicine, Shandong University (Shandong Province, China).
Immunohistochemistry
Immunohistochemistry staining was performed to assess the protein expression of KIF21B in tissue samples using the EliVisionTMplus kit (Maixin, Fuzhou, China) according to the instructions provided. Briefly, paraffin-embedded tissues were sliced into 5-μm thick sections. The tissue sections were deparaffinized in xylene and rehydrated in gradient ethanol. After subjected to antigen retrieval by heated citrate buffer, sections were blocked in 3% (v/v) hydrogen peroxide for 30 min and incubated overnight at 4 °C with anti-KIF21B antibody (ab121931; Abcam, Cambridge, UK). Afterwards, the sections were washed with twice with Tris-buffered saline and then incubated with horseradish peroxidase-conjugated secondary antibody for 2 h at room temperature. The immunohistochemistry staining results were evaluated by two independent pathologists according to the multiplication of staining proportion score (0–4: 0, 0-5%; 1, 6–20%; 2, 21–60%; 3, 61–75%; or 4, 76–100%) and staining intensity score (0, no staining; 1, weak; 2, moderate; 3, strong staining) as the final score of KIF21B protein expression. Tissue sections with immunoreactivity score scaling 0-2, 2-4 and over 4 were considered to be weak staining (-+), moderate staining (+) and strong staining (++), respectively.
Cell culture and transfection
Two GC cell lines, including AGS and SNU-5 were purchased from the Cell Bank of China Academy of Sciences (Shanghai, China) and cultured RPMI-1640 medium (Thermo Fisher Scientific, Inc., Waltham, MA, USA) supplemented with 10% fetal bovine serum (FBS, Gibco) at 37 °C in the atmosphere containing 5% CO2. The sequences of small interfering RNA targeting KIF21B (si-KIF21B#1 and si-KIF21B#2), si-NC, miR-132-3p mimics and miR-NC were synthesized by GenePharma Co., Ltd. (Shanghai, China). The pcDNA3.1 containing the open reading frame of KIF21B or empty vector were purchased from GenScript (Nanjing, China). After incubating GC cells in 12-well plates at a density of 8 × 104 cells per well, the cells were transfected with the above oligonucleotides for 48 h using Lipofectamine 2000 (Thermo Fisher Scientific, Inc.).
Reverse transcription quantitative PCR analysis
Total RNA sample was extracted from tissue specimens or cell lines using TRIzol reagent (Invitrogen). Reverse transcription was conducted by One Step PrimeScript miRNA complementary DNA Synthesis Kit (Takara, Dalian, China) for miR-132-3p and a HiFiScript cDNA Synthesis Kit (CWBIO, Beijing, China) for KIF21B. Next, reverse transcription quantitative PCR was performed to determine the expression of miR-132-3p and KIF21B using SYBR Green Human miRNA Assay Kit and a SYBR Premix Ex Taq II kit (Takara, Japan), respectively. The primer sequences were used as follows: miR-132-3p (forward: 5ʹ-GCGCGCGTAACAGTCTACAGG-3ʹ and reverse: 5ʹ-GTCGTATCCAGTGCAGGGTCC-3ʹ); U6 (forward: 5ʹ-CTCGCTTCGGCAGCACATATACT-3ʹ and reverse: 5ʹ-CGCTTCACGAATTTGCGTGT-3ʹ); KIF21B (forward: 5′-CGA GGAGACGGATGAGAACG-3′ and reverse, 5′-CCACCAGGCTCTCTTCACTG-3′); β-actin (forward: 5′-CCCGAGCCGTGTTTCCT-3′ and reverse: 5′-GTCCCAGTTGGT GACGATGC-3′). Relative miR-132-3p and KIF21B mRNA expression were normalized against the endogenous control U6 and β-actin, respectively using the 2−ΔΔCt method [33].
Cell counting kit‑8 (CCK-8) assay
Transfected cells at a density of 4 × 103 cells per well were seeded into 96-well plates and cultured at 37 °C. At 0, 24, 48 and 72 h, respectively, cells in each well were incubated with 10 μl of CCK-8 solution (Solarbio Science & Technology, Beijing, China) for another 2 h at 37 °C. Afterwards, the optical density (OD) value at 450 nm was measured with a microplate reader.
Transwell migration and invasion assay
For migration assay, approximately 4 × 104 transfected cells in 200 µl of serum-free medium were plated into the upper chamber (24-well insert; 8-µm pore size; Corning Costar, Corning, NY, USA), while 600 µl complete medium (with 10% FBS) as chemoattractant was added into the lower chamber. After 24 h incubation, the cells that migrated to the lower chamber were fixed with 4% paraformaldehyde for 30 min and stained with 0.1% crystal violet for 10 min. The number of migrated cells was quantified under a microscope (magnification, ×100) in five random microscope fields. The procedure of invasion assay was similar to the migration assay except for the addition of 50 µl Matrigel on the membranes of transwell inserts and incubation time of 48 h.
Bioinformatic analysis and luciferase reporter assay
The potential miRNAs that target KIF21B were predicted by TargetScan (www.targetscan.org/vert_71/). From all these predictions, we selected miR-132-3p for further analysis. The mRNA 3′-UTR of KIF21B, carrying the predicted binding site or mutant binding site of miR-132-3p, was amplified by PCR and cloned into pmirGLO (Promega, Madison, Wisconsin), which were named as KIF21B wild-type (WT) and mutant (MUT) constructs, respectively. Next, cells were co-transfected with KIF21B WT or KIF21B MUT and miR-132-3p mimics or miR-NC using the transfection reagent Lipofectamine 2000. After 48 h incubation, cells were harvested and relative luciferase activity was measured via a Dual-luciferase reporter assay system (Promega).
Western blot analysis
Total protein sample was extracted with RIPA lysis buffer with protease inhibitor (Solarbio Science & Technology Co., Ltd., Beijing, China) and protein concentration was determined by a BCA kit (Beyotime, Shanghai, China). Then, equal amount of protein sample was separated by 10% SDS-PAGE and transferred onto PVDF membranes (Millipore, Billerica, MA, USA). After blocked with 5% non-fat milk, the membranes were incubated with primary antibodies against KIF21B, Wnt1, c-Myc, β-catenin, PCNA, E-cadherin, N-cadherin and GAPDH (All from Abcam, Cambridge, UK) at 4 °C overnight. Subsequently, the membranes were incubated with horseradish peroxidase-coupled secondary antibody for 2 h at room temperature, followed by detection of protein signals with an enhanced chemiluminescence reagent (EMD Millipore, Billerica, MA, USA).
Statistical analysis
All quantitative data were presented as mean ± SD from three independent experiments. All statistical analyses were performed using GraphPad Prism 6.0 (GraphPad Software, Inc., San Diego, California, USA). Differences between two groups were assessed by paired Student’s t-test. Differences among three groups were evaluated by one-way analysis of variance (ANOVA), followed by Dunnett’s test or Tukey’s test. The correlation between miR-132-3p and KIF21B was evaluated using Pearson's correlation coefficient analysis. The p-value less than 0.05 was considered to be statistically significant.