Clinical samples and cell lines
Tumor specimens were obtained from ESCC patients at the time of surgery and were snap-frozen.
Medical Ethics Committee of the First Affiliated Hospital of Baotou Medical College approved the work. Verbal informed consent was obtained from the patients before the samples were collected and the experiments wit h human tissues conformed to the guidelines set by the Declaration of Helsinki. The cell lines ECa109 and Kyse150 were purchased from China Center for Type Culture Collection (Wuhan, China). Kyse140 cell line was a gift from Dr. Yan Li Sun from Yat-Sen University (Guangdong, China). ECa109, Kyse140, and Kyse150 were cultured in DMEM (Invitrogen, Carlsbad, CA, USA) supplemented with 10% fetal bovine serum (FBS, Gibco) and maintained in a humidified incubator with 5% CO2 at 37°C.
Immunohistochemistry Staining
ESCC tumor tissues were subjected to immunohistochemistry staining (IHC) using anti-KLF5 rabbit polyclonal antibody (ab137676, 1:1000). Staining strictly followed the manufacturer’s protocol. IHC staining intensity was rated as ‘−’ (no staining), ‘+’ (light yellow), ‘+ +’ (light brown), and ‘+ + +’ (brown).
Plasmids, Small Interfering RNA, And Cell Transfection
ECa109, Kyse140, and Kyse150 cells were seeded in 6-well plates and cultured to 70–80% confluency for transfection. The specific small interfering RNA targeting KLF5 (siKLF5), nonspecific control siRNA standard control (siNC), pcDNA-KLF5 plasmid, and blank control vector were transfected into the cells using Lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA) according to the manufacturer’s instructions.
Cell Viability Assays
The indicated number of cells was seeded into a 96-well plate, and the procedure was performed as described previously[17]. After transfected with siKLF5 or pcDNA-KLF5 plasmid in ESCC cells, cell viability was measured using the Cell Counting Kit-8 (CCK-8, Beyotime, China). The optical density of each well was quantified using a microplate reader.
Wound-healing Assay
The transfected ESCC cell suspensions were added into each well of 6-well cultured plates. A small pipette and a ruler were prepared for the vertical lines drawing. When the cells reached 70–80% confluency, they were wounded by scraping with the small pipette. After washing with phosphate-buffered saline (PBS) three times, the floating cells were removed, and 0.2 mL serum-free media was added. The scratched area was detected microscopically at 24 and 48 hours after treatment and then further analyzed using ImageJ software.
Transwell Assays
For the transwell assays, the indicated ESCC cells in 0.1 mL serum-free medium were seeded into the upper chamber (Corning Costar), and 0.5 mL medium supplemented with 10% FBS was added into the lower chamber. After 24 hours, the cells crossed the inserts, were fixed with 4% paraformaldehyde, and stained with crystal violet. After that, the invading cells on the basal side were counted under a Nikon optical microscope.
Western Blotting
KLF5, FGFBP1, and SNAIL2 expression were determined by western blotting analyses, which were performed as previously described[16]. The cells were lysed with RIPA buffer (Beyotime, China) for total protein extraction. Then the protein concentration was calculated using a BCA assay Kit (Beyotime, China). Primary antibodies included in this study were listed as follows: KLF5 (ab137676, 1:1000, Abcam), FGFBP1 (ab215353, 1:500, Abcam), and SNAIL2 (EM1706-65, Cell Signaling Technology). Immunostaining was visualized with an ECL kit (Beyotime, China) and photographed.
Bioinformatics Analysis
Public RNA-sequencing data of ESCC patients from The Cancer Genome Atlas (TCGA, https://portal.gdc.cancer.gov/) and microarray data of ESCC patients from the Gene Expression Omnibus (GEO) (https://www.ncbi.nlm.nih.gov/gds/?term=, GSE16355, and GSE44021) were downloaded. Data of gene expression of ESCC tumors and adjacent noncancerous tissues were retrieved. A web server Gene Expression Profiling Interactive Analysis (GEPIA, http://gepia.cancer-pku.cn/index.html), an R language (https://www.r-project.org/) was used for gene expression analysis based on ESCC datasets from TCGA and GEO, respectively. The correlations between FGFBP1 and KLF5/SNAIL2 were explored using ESCC datasets from TCGA and GEO.
Chromatin Immunoprecipitation-polymerase Chain Reaction (ChIP-PCR)
For this test, a ChIP Kit from Thermo Fisher Scientific was purchased. In order to create DNA-protein cross-linking, treated cells were collected and fixed with 1% formaldehyde. After that, the cells were subjected to ultrasonic processing to create chromatin fragments. To immunoprecipitate the complex, cell lysate was then treated with the KLF5 antibody (rabbit, ab277773, Abcam). ChIP products were quantified using RT-qPCR. The primer sequences of the FGF-BP1 promoter were as follows: FGF-BP1-C1 forward: GCG GGG TGT TTG TGA GGA TA, and reverse: TGA CCA ATT TAC TCA GAG GCT CA; FGF-BP1-C2 forward: GTG TGA CAA TAC TGA GCC TAA TCC, and reverse: GCA ACT GAC TTT TGG GTT TGA GAT; FGF-BP1-C3 forward: AGA GCC TGG TAG AAT TCC CTG AT, and reverse: GGT GAC TGA AGT TGC CGA AGT; FGF-BP1-C4 forward: TCT CCA GAA CCA ATT CCT GCT TT, and reverse TGC CAA ACT ACT GAG CCC ATT.
Dual-luciferase Reporter Assay
FGF-BP1 dual luciferase reporter gene vectors (H_FGF-BP1-WT) and mutant type (MUT) (H_FGF-BP1-MT ) plasmids with KLF5 binding sites (FGF-BP1-C1) were constructed respectively. The H-KLF5 and pcDNA3.1(+) plasmids and the reporter plasmids were then co-transfected into ECa109 cells. Dual-Luciferase® Reporter Assay System (Promega Corporation, Madison, WI) was used to measure the luciferase activity after 48 hours..
Statistical analysis
The measurement data were expressed with mean ± standard deviation (SD). The relative expression level of KLF5 and the number of invaded cells were plotted using GraphPad Prism 5 software (GraphPad Software, San Diego, CA, USA). All statistical analyses were performed in SPSS 20.0 (IBM, Armonk, NY, USA). A statistical difference was determined for P < 0.05.