Microarray data analysis
Two microarray datasets were downloaded from GEO (https://www.ncbi.nlm.nih.gov/gds), a database contains gene expression data. Dataset GSE83270 contains gene profile data from 6 health control tissues and 6 BC patients’ tissues was analyzed at 7th generation of miRCURYTM LNA Array (v.18.0, Exiqon) and submitted by Ya-Wen Wang from Shandong University. Dataset GSE68085 contains gene expression data from 104 BC patients’ tissues and 11 healthy normal tissues was analyzed at GPL10999 platform and submitted by Sambasivarao Damaraju from University of Alberta.
Identification of DEMs
Limma package in R software was employed to identify DEMs between cancer tissues and normal tissues at both datasets. Cut-off values were set as |log2 fold change (FC)| > 2 and P < 0.01. Venn diagram was utilized to identify the overlapping DEMs in these two datasets.
Validation of DEMs expression at ENCORI
The expression of these overlapping DEMs in BC patients was validated using ENCORI, a public accessed platform for studying RNA interactions [14].
miRNA targets prediction
The mRNA targets for DEMs were analyzed at miRTarBase, a database contains experimental validated targets [15]. Only the targets validated by strong methods including luciferase activity reporter assay, western blot, or quantitative real-time PCR (RT-qPCR) were selected for followingly analyses. Based on these results, a miRNA-mRNA regulatory network related to the development of BC was established and visualized at Cytoscape V_3.6.0 software [16].
Protein-protein interaction (PPI) network
PPI network for the above predicted miRNA targets was analyzed at Search Tool for the Retrieval of Interacting Genes (https://string-db.org) and visualized at Cytoscape V_3.6.0 software. Hub genes in this network were analyzed at CytoHubba with the cut-off value of degree above 10.
Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis
Database for annotation, visualization, and integrated discovery (https://david.ncifcrf.gov/) was employed for GO and KEGG analyses of the predicted miRNA targets [17]. P value less than 0.05 was believed as significant enrichment.
Validation of hub gene expression at ENCORI, GEPIA, and UALCAN
The expression level of these 10 identified hub genes in BC was analyzed at both ENCORI, GEPIA, and UALCAN [18,19]. The genes that revealed to be abnormally expressed in these three databases were selected for followingly analyses.
Survival analysis using Kaplan-Meier Plotter
The effects of hub genes on the overall survival of BC patients were analyzed at Kaplan-Meier Plotter [20]. The median value was used as cut-off value to classify patients into high or low expression groups.
Analysis of miRNA and mRNA correlation at ENCORI
Subsequently, hub genes after expression and prognostic value validation were selected to analyze its connection of miRNA at ENCORI. The pairs that inversely correlated were selected for following analyses.
Cell line
Human BC cell lines (MCF7 and MDA-MB-415) and mammary epithelial cell (MCF10A) were incubated in DMEM containing 10 % FBS (Invitrogen, Thermo Fisher Scientific, Inc., Waltham, MA, USA). Cells cultivation atmosphere was maintained at 37 °C containing 5 % of CO2.
RNA extraction and RT-qPCR analysis
Total RNA was extracted from cultured cells with Trizol reagent obtained from Invitrogen (Thermo Fisher Scientific, Inc.). Complementary DNA (cDNA) was synthesized from RNA using miScript Reverse Transcription Kit (Takara, Dalian, Liaoning, China). RT-qPCR was performed at ABI 7500 system (Applied Biosystems, Foster City, CA, USA) using SYBR Green Mix (Takara) to measure the expression level of miR-98-5p or IGF1. The primers used were as follows: hsa-miR-98-5p: forward 5’-TGAGGTAGTAGTTTGTGCTGTT-3’, reverse 5’-GCGAGCACAGAATTAATACGAC-3’; U6 snRNA: forward 5’-CTCGCTTCGGCAGCACA-3’, reverse 5’-AACGCTTCACGAATTTGCGT-3’; IGF1: forward 5'-CACATCATGTCGTCTTCACACC-3', reverse 5'-GGAAGCAACACTCATCCACAATG-3'; GAPDH: forward 5’-CTGGGCTACACTGAGCACC-3’, reverse 5’-AAGTGGTCGTTGAGGGCAATG-3’. 2−ΔΔCt method was utilized to analyze relative expression levels of hsa-miR-98-5p or IGF1 using U6 snRNA or GAPDH as internal control, respectively.
Luciferase activity reporter assay
The 3’-UTR contains the binding site for hsa-miR-98-5p in IGF1 was amplified from genome and cloned into pGL3 vector (Promega, Madison, WI, USA) and named as wt-IGF1. The mutant IGF1 luciferase vector (mt-IGF1) was constructed from wt-IGF1 using site-direct mutagenesis kit (Takara). Cells were co-transfected with wt-IGF1 or mt-IGF1 and hsa-miR-98-5p mimic or negative control miRNA (miR-NC) using Lipofectamine 2000 (Invitrogen). The hsa-miR-98-5p mimic or miR-NC were purchased from RiboBio (Guangzhou, China). After co-transfection for 48 h, relative luciferase activity was measured using dual-luciferase activity reporter system (Promega).
Cell transfection
pcDNA3.1 contains the coding sequence of IGF1 (pIGF1) was designed by GenScript (Nanjing, Jiangsu, China). For miRNAs or siRNAs transfection, Lipofectamine 2000 obtained from Invitrogen was used according to the manufacturer’s protocol.
Cell counting kit-8 assay
Cell Counting Kit-8 (CCK-8) bought from Beyotime (Haimen, Jiangsu, China) was used to measure cell proliferation rate. Briefly, 2,000 cells were cultured into 96-well plates and incubated for indicated time. Then, CCK-8 reagent was added to each well and the detected the optical density value at 450 nm after further incubation for 2 h.
Wound-healing assay
5 × 105 cells were plated in 6-well plates. After wound generation at cell surface, cells were washed with PBS and incubated in serum-free medium. After 48 h, cell images were captured to measure the migration rate.
Transwell invasion assay
Cell invasion ability was detected with Matrigel coated transwell chamber (8-μm pore size). Cells in serum-free medium were plated to upper chamber, while the serum contained medium was filled to lower chamber. Invaded cells were stained by crystal violet and counted under microscope.
Tumor formation in BALB/c nude mice
BALB/c athymic nude mice (female, 4-6 weeks old and 16-20 g) purchased from Beijing Vital River Laboratory Animal Technology Co., Ltd. were used for animal experiments. MCF7 cells stable expressed has-miR-98-5p inhibitor and NC-inhibitor suspended in PBS were inoculated subcutaneously into nude mice (6 per group). Tumor length and width was measured every 5 days. The mice were euthanized via overdose of pentobarbital, and the tumors were weighed. Tumor size was calculated using the formula: (L × W2)/2 (L represents Length; W represents Width). Animal experiments were approved by animal care committee of The First Affiliated Hospital of Jinzhou Medical University.
Statistical analysis
Data obtained in this work were analyzed using GraphPad software (San Diego, CA, USA) and presented as mean ± SD. Differences in groups were analyzed with Student’s t-test (two groups) or ANOVA and post-hoc test (for three groups). P value less than 0.05 was considered as statistically significant.