Background
Triple-negative breast cancer (TNBC) is generally identified as a malignancy with high aggressive property. Continually, TNBC causes severe health concern among females [
1]. 12 to 17% of breast tumor cases are identified as TNBC, which are featured by negative expression of human epidermal growth factor receptor-2 (HER-2) and hormone receptor (HR) [
2]. Even TNBC only occupies a small percentage of all breast cancer cases, a severe challenge is posed in clinically treating patients with TNBC. Typically, younger populations are susceptible to be diagnosed with TNBC. Moreover, there is a great chance for patients to develop larger tumors, accompanied with the increased possibility of distant aggressiveness and death [
3,
4]. Thus, urgent elucidation of the molecular mechanisms around TNBC progression are needed for the purpose of identifying available therapeutic molecules.
In the studies of tumor biology, long non-coding RNAs (lncRNAs) are emerged as molecular and clinical biomarkers in combating tumor exacerbations [
5,
6]. According to its definition, it refers to transcripts whose length is above 200nt without protein-coding potential [
7]. Initially, the generation of lncRNAs is viewed as useless during the process of transcription [
8]. Recently, the broad regulation of lncRNAs has become more and more evident during a series of cellular biological functions [
9]. To date, massive lncRNAs are corroborated to drive many essential cancerous phenotypes via their interactions with other cellular regulatory molecules [
10]. For instance, lncRNA AFAP1-AS1 induces osteosarcoma tumorigenesis and stimulates epithelial-mesenchymal transition via modulating RhoC/ROCK1/p38MAPK/Twist1 pathway [
11]. It was revealed that lncRNA-CF129 suppresses pancreatic cancer aggravation via the transcription repression of FOXC2 [
12]. Accumulating evidence displayed that lncRNAs residing in nucleus could participate in modulating transcriptional activation, heterochromatin formation, X chromosome inactivation, and maintaining telomeres [
13,
14]. On the other hand, lncRNAs pervasively locating within cytoplasm or shuttling between cytoplasm and nucleus also play their essential parts in mediating the translation or decay of messenger RNAs (mRNAs), and trafficking cytoplasmic protein [
15,
16]. It is well-exemplified that cytoplasmic lncRNAs compete for the binding to miRNAs with mRNAs and therefore lessen miRNA-induced inhibition on targeted mRNAs, giving rise to mRNA increase [
17]. A series of lncRNAs have been underscored. LncRNA SLC25A5-AS1 restores the expression and function of PTEN via acting as a miR-19a-3p in suppressing malignant phenotypes in gastric cancer [
18]. LncRNA SNHG20 facilitates the downstream target ZEB2 and RUNX2 via sponging miR-154 in promoting non-small cell lung cancer progression [
19]. Herein, the expression specificity of LINC01123 (long intergenic non-protein coding RNA 1123) has not been revealed in TNBC. Whether it functions in TNBC or not needs precise investigations.
In addition, the specific expression of lncRNAs is attributed to the transcriptional manipulation of diverse transcriptional factors. Prior studies validated that LINC00460 is a PRDX1-initiated lncRNA in head and neck squamous cell carcinoma [
20]. C8orf76 directly binds to lncRNA DUSP5P1 promoter and induces lncRNA expression in gastric cancer [
21]. However, the transcription of LINC01123 in TNBC remains obscure.
Therefore, our study focused on the biological function of LINC01123 in TNBC and its possible regulatory mechanism, which may enrich the basis for TNBC oncology.
Materials and methods
Cell culture
The cell line of normal mammary epithelial (MCF-10A) and cell lines of human breast cancer (MDA-MB-231, MDA-MB-468, MCF7, HCC1937) were received from the purchasing channel of American Type Culture Collection (Manassas, VA, USA). 10% fetal bovine serum (FBS, Invitrogen, Carlsbad CA, USA) was added into RPMI1640 medium (Invitrogen) for cultivating MDA-MB-468 and HCC1937 cells at 37 °C and in 5% CO2. DMEM medium (Invitrogen) was used to incubate MDA-MB-231 and MCF7 cells with 10% FBS under the temperature of 37 °C and 5% CO2. DMEM/F12 medium (Invitrogen) was used to cultivate MCF10A cells with 5% horse serum, 20 ng/ml EGF, 0.5 mg/ml Hydrocortisone, 100 ng/ml Cholera Toxin, 10 μg/ml Insulin under the temperature of 37 °C and 5% CO2. The use of Mycoplasma Plus PCR Primer Set (Agilent, Santa Clara, CA, USA) was used to detect the cells for mycoplasma contamination and the result was negative.
Cell transfection
The amplified full-length LINC01123 or CMIP sequences were encompassed by the cutting sites of two EcoRI. Then the EcoRI linearized pIRSE2-EGFP vector was used to insert the fragments for constructing LINC01123 expression vector. Lipofectamine 2000 reagent (11668-019, Invitrogen) was utilized to conduct the transfection for 48 h using 10 mM vectors (10 nM) or 50 nM shRNAs with the consistence of 5 × 105 cells. For stable transfection, the shRNAs were inserted into the lentivirus expression vector pCDH-CMV-MCS-EF1-Puro (System Bioscience, Palo Alto, CA, USA). 2 μg/ml of puromycin (Thermo Fisher, Waltham, MA, USA) was then added for screening out the stable cell lines. qRT-PCR was adopted to check the efficiency of transfection. The sequences of indicated shRNAs were presented as follows: sh/NC, 5′-CCGGGATTAGACCTGATAAGAATTATCTCGAGCTAATCTGGACTATTCTTAATATTTTTG-3′, sh/LINC01123#1, 5′-CCGGTCGGAAGCCCCTGTCGCGGTAGCTCGAGAGCCTTCGGGGACAGCGCCATCTTTTTG-3′, sh/LINC01123#2, 5′-CCGGGTGGAGCCAGCAGTCCCCGGCGCTCGAGCACCTCGGTCGTCAGGGGCCGCTTTTTG-3′; sh/NC, 5′-CCGGAAGTTATAGAACAAGAAGTAAACTCGAGTTCAATATCTTGTTCTTCATTTTTTTTG-3′, sh/CMIP#1, 5′-CCGGAGAGACAAACCAAATGGGCAGGCTCGAGTCTCTGTTTGGTTTACCCGTCCTTTTTG-3′, sh/CMIP#2, 5′-CCGGAGAGTCCTGGGTCGCCACCAGCCTCGAGTCTCAGGACCCAGCGGTGGTCGTTTTTG-3′; sh/NC, 5′-CCGGAAGTCAAGTTGATATAAATGTACTCGAGTTCAGTTCAACTATATTTACATTTTTTG-3′, sh/FOXC1#1, 5′-CCGGCGCCCTCTACAAGCTCAGTGTCCTCGAGGCGGGAGATGTTCGAGTCACAGTTTTTG-3′, sh/FOXC1#2, 5′-CCGGTGGGAGTTTCGGCTTGATTTAGCTCGAGACCCTCAAAGCCGAACTAAATCTTTTTG-3′.
qRT-PCR
The use of TRIzol reagent (Invitrogen Life Technologies, Carlsbad, CA, USA) was to separate the total RNA from cultured cells. And then it was estimated by standard denaturing agarose gel electrophoresis and NanoDrop spectrophotometer ND-8000 (NanoDrop Technologies; Thermo Fisher). On the basis of the protocol of manufacturer, PrimeScript™ RT Master Mix (Takara Bio, Otsu, Japan) was used to compose cDNA via reverse transcription. And the total volume was 10 µL. Then, the reaction mix was deposited in a cryogenic environment of − 20 °C for future experiments after DEPC-Treated Water (Ambion®) was used to deliquate the reaction mix. SYBR® Premix Ex Taq™ II (Takara Bio, Otsu, Japan) was used to conduct the quantitative real-time PCR in the PCR reaction mixture of 10 µl (containing 1 μl of cDNA). PCR conditions were comprised of pre-denaturation at 95 °C for 10 min, 40 cycles of denaturation at 95 °C for 15 s, annealing at 60 °C for 1 min and extension at 72 °C for 30 s. And the internal control was GAPDH. The use of ABI 7500 Real-Time PCR system (Applied Biosystems®) was to measure the transcript levels of all lncRNA. And then the fold change (FC) was identified by the 2−ΔΔCt method. The specific PCR primers were listed as follows: miR-663a, Forward Primer, 5′-AGGCGGGGCGCCGCGGGACCGC-3′, Reverse Primer, 5′-CTCAACTGGTGTCGTGGA-3′; LINC01128, Forward Primer, 5′-GCCAGTGGAACATAAACCACC-3′, Reverse Primer, 5′-AGCCTGTCACAAACTGATTCT-3′; LINC01106, Forward Primer, 5′-GGAGCGCGTGCGATAATCT-3′, Reverse Primer, 5′-CTTGGAGTCGGTGAGAAGGC-3′; LINC01123, Forward Primer, 5′-GAACATGTGCTTGGTGTCGT-3′, Reverse Primer, 5′-AGCCACTTGCCTATGCGTG-3′; RUSC1-AS1, Forward Primer, 5′-TAACCCAATGACCCACCCAG-3′, Reverse Primer, 5′-AAAACGGAGCCCAGTTGGAA-3′; CMIP, Forward Primer, 5′-CAGCTCACGATTCCTGGGG-3′, Reverse Primer, 5′-CAGCGGCTTGGGTTACTCA-3′; LSP1, Forward Primer, 5′-GGAGCACCAGAAATGTCAGCA-3′, Reverse Primer, 5′-TCGGTCCTGTCGATGAGTTTG-3′; FOXC1, Forward Primer, 5′-GGCGAGCAGAGCTACTACC-3′, Reverse Primer, 5′-TGCGAGTACACGCTCATGG-3′; GAPDH, Forward Primer, 5′-GGAGCGAGATCCCTCCAAAAT-3′, Reverse Primer, 5′-GGCTGTTGTCATACTTCTCATGG-3′; U6, Forward Primer, 5′-CCAAATCTAGCTGCTGCGGT-3′, Reverse Primer, 5′-AGGTTTGTCGTTCCCGTCTC-3′.
Cell counting kit-8 (CCK-8) assay
The cell samples at the logarithmic growth phase were collected from each group after 48 h of transfection, and then seeded into the 96-well plates with the cell density of 5 × 103 cells in each well. 10 μl of CCK-8 solution (Dojindo Laboratories, Kumamoto, Japan) was added and cultured with cell samples for 2 h for assessing cell viability. At length, the optical density (OD) values at 450 nm were examined at indicated time points by use of spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA).
After indicated transfection, 6-well cell petri dish was utilized to cultivate the cells at the logarithmic growth phase with the consistence of 2 × 104 for 14 days at the temperature of 37 °C in 5% CO2. Then cells were rinsed by the use of PBS and fixed by the cold methanol for 30 min. After that, 0.005% crystal violet was utilized to stain the cells for 30 min. In the end, the quantity of colonies (with more than 50 cells) was manually calculated.
EdU assay
The 12-well plate was adopted to seed the stably transfected cells. When the cells were fully adhered to each other, BeyoClick™ EdU Cell Proliferation Kit (Beyotime, Shanghai, China) was supplemented to every well after it was diluted, following the user guide. The cells were cultivated in an incubator which the temperature was 37 °C and the air contained 5% CO2. The process lasted for 2 h. Then PBS and 4% paraformaldehyde were utilized to rinse and fix the cells separately at RT for 20 min. After that, nuclei were stained by the utilization of DAPI. Finally, a fluorescence microscope (Olympus, Tokyo, Japan) was utilized to obverse the cells.
Apoptosis assay
1 × 106 cells were collected after transfection, and plated into the 6-well plates for cell apoptosis assay. The propidium iodide using an APC Annexin V Apoptosis Detection Kit (BioLegend, San Diego, CA, USA) and fluorescein isothiocyanate-Annexin V were utilized to stain the cells which were gathered before. After they were cultivated in the dark room for 15 min at the temperature of 37 °C, a flow cytometer (BD Biosciences, Franklin Lakes, NJ, USA) was adopted to take the analysis of the situation of cell apoptosis.
TUNEL assays
The TUNEL assay was adopted to detect cell apoptosis via testing the fragmentation of apoptosis-induced DNA. 24-well flat-bottomed plates were utilized to cultivate the HCC1937 and MDA-MB-231 cells in 96-well plates separately and the concentration of each well was 1 × 105 cells. Then 4% (v/v) paraformaldehyde was adopted to fix the cells for half an hour at 4 °C. On the basis of protocols of supplier, cells were then permeabilized with 0.1% Triton-X100. The in situ cell death detection kit (Roche, Basel, Switzerland) was used for the TUNEL staining and then DAPI was used to stain the nuclei for 10 min. The fluorescence microscope (Olympus) was utilized to detect the quantities of TUNEL-positive cells and ImagePro Plus software was adopted to estimate the rate of apoptosis cells.
RIP assay
In accordance of suppliers’ instructions, Magna RIP™ RNA-Binding Protein Immunoprecipitation Kit (Millipore, Bedford, MA, USA) was used to conduct the RIP assay. Simply put, cells were subjected to trypsinization and then washed by the use of ice-cold PBS for two times. After that, RIP lysis buffer which was added with RNase inhibitor and protease inhibitor cocktail was utilized to resuspend the cells. Then the cells were taken to conduct the single freeze–thaw cycle for lysing gently. The 30 μl of magnetic beads were supplemented with antibody for cultivating in the RIP wash buffer at the temperature of 37 °C for half an hour with stirring. The cell lysates were supplemented into the beads for cultivating at 4 °C before the beads were rinsed by the utilization of RIP wash buffer for three times. And the process took one night. Then the proteinase K buffer was utilized to digest and conduct the resuspension to each immunoprecipitant for cultivating at 55 °C. This process lasted for half an hour. With the help of phenol, chloroform, and isoamyl, RNA was separated in line with the protocols of manufacturer and estimated by the adoption of qRT–PCR.
Nuclear and cytoplasmic fractionation
On the basis of protocols of manufacturer, the utilization of the NE-PER™ Nuclear and Cytoplasmic Extraction Reagents was to conduct the nuclear/cytoplasmic isolation, as guided by supplier (Thermo Fisher Scientific). Cell samples were first lysed in cell fractionation buffer, and then centrifuged to collect cell cytoplasmic fraction. After that, cell samples were treated in cell disruption buffer, and then cell nucleus was obtained. Nuclear and cytoplasmic fractions were separated for the extraction of RNA. The utilization of GAPDH and U6 were served as the qRT-PCR markers separately for cytoplasmic and nuclear components.
Western blot
On the basis of instructions of manufacturer, all proteins were abstracted by the utilization of RIPA solution containing RNase inhibitor and protease inhibitor cocktail. The concentration of protein was measured by the adoption of BCA method. Then 20 µg of protein was subjected to SDS-PAGE gel (10%) electrophoresis. Then skimmed milk (5%) was utilized to cultivate the PVDF membranes (Bio-Rad, Inc., Hercules, CA, USA) for blockading after the transfection was completed. Following, the membranes were subjected to overnight incubation with the relevant primary antibodies and GAPDH at the temperature of 4 °C. After that, the membranes were incubated with secondary antibodies for 3 h at room temperature. Then membranes were dropped with ECL luminous liquid (Sigma Aldrich; Merck KGaA, Darmstadt, Germany) for developing the signals. Finally, the densitometry was utilized to detect ECL chromogenic substrate.
RNA pull-down assay
The pBluescript II SK (+) was utilized to clone the cDNA sequence of LINC01123. Biotin-labeled RNAs were subjected to the transcription and purification in vitro. Then the whole cell lysate which was included 1 mg proteins was utilizes to mix the biotinylated RNA and biotinylated RNA was then performed with Streptavidin agarose beads which were rinsed in TBS before. Then they were cultivated for 1 h at the temperature of 37 °C. TBS was used to rinse the beads for 5 min and SDS buffer was used to boil the beads. The RNA–protein mixture was eluted and digested for 2 h. Finally, western blot was applied to estimate the retrieved proteins.
Chromatin immunoprecipitation (ChIP)
Firstly, MDA-MB-231 and HCC1937 cells which were disposed by 200 ng/ml of EGF or not were collected for the ChIP assay and the number of the cells was 3 × 107. 125 mM glycine was used to neutralize the cells for 5 min after 1% formaldehyde had cross-linked the cells at 37 °C for 10 min. Before the cells were put into 0.3 ml of lysis buffer for resuspension and sonication into 200–1000-bp fragments, they were washed cleanly by ice-cold PBS for two times and taken out for putting into 1 ml of ice-cold PBS. When the centrifugation was completed, IP dilution buffer was use to deliquate the supernatants which were gathered before. And then, they were performed with protein A-Sepharose beads for immune clearing at 4 °C. This process took 2 h. In the process of immunoprecipitation, anti-FOXC1 and control IgG (Cat No: 2729S, CST, Danvers, MA, USA) was taken for experiment. After the process was accomplished, they were added the 45 μl protein A-Sepharose for cultivation at least 1 h. qRT-PCR was conducted to analyze DNA in the precipitates after being washed and purified.
Luciferase reporter assays
For the activity experiment of promoter, FuGENE 6 (Roche) was used to transfect 500 ng pGL3.0-basic or pGL3.0- LINC01123 vector (Promega, Madison, WI, USA) plus 5 ng of the Renilla luciferase plasmid as control to the cells. Then the cells were incubated with the inhibitors for 24 h after they were transfected for 48 h, so as to estimate the promoter activity of cells which were treated by inhibitors. With regard to the reporter assay of miRNA Luciferase, a 24-well plate was used to seed 293 cells for 24 h until the transfection was up to 50% confluence. The use of Lipofectamine RNAiMAX (Invitrogen) was to transfect with 30 nM of miR-663a mimics or control mimics (Sigma). After the post-transfection was finished at the deadline of 24 h, FuGENE6 transfection reagent (Roche) was used to transfect 0.125 μg of psiCheck- LINC01123 WT or psiCheck- LINC01123 MUT reporter vector. 48 h later, the reporter vector transfection was completed. Finally, under the help of Fluoroskan Ascent FL fluorometer (Thermo Fisher Scientific, Rockford, IL, USA), the activities were processed with the dual luciferase reporter assay system (Promega).
Statistical analysis
SPSS (SPSS Inc., Chicago, IL, USA) and SAS software were used to take the statistical analysis. All of the results collected from three separately experiments were displayed as mean ± standard deviation (SD). The significance of differences was estimated by the utilization of the Student’s t-test or one-way ANOVA. In addition, P < 0.05 stated clearly the great significance.
Discussion
Aggressiveness of TNBC makes it as a malignancy with higher recurrence together with mortality rate [
2]. Despite the clinical progress in hormone therapy for luminal subtype breast cancer patients and trastuzumab therapy for HER2 subtype treatment, specific feasible treatments for TNBC patients are limited [
26]. At present, a comprehensive portrait of molecular mechanism is in dire need in TNBC oncogenesis. Accumulating works proposed that miRNAs with specific expression is tumor-related. In early works, miR-663a acts as a tumor suppressor in hepatocellular carcinoma [
22], non-small cell lung cancer [
27] and pancreatic cancer [
23]. Meanwhile, it is oncogenic in renal cell carcinoma, which denotes malignant cellular processes and worse prognosis [
28]. In breast cancer, miR-663a is downregulated based on prior research [
24]. Herein, we first portrayed that miR-663a was evidently inhibited in TNBC cell lines. Moreover, overexpression of miR-663a effectively restrained the malignant growth of TNBC cells and conversely resulted in facilitated apoptosis, which provided further evidence for miR-663a participation in TNBC.
Motivated by the lncRNAs-mediated the decline in miRNA availability, we speculated that miR-663a was also under the regulation of a certain lncRNA in TNBC. Through bioinformatics assays, we identified LINC01123 as a possible upstream factor of miR-663a. As annotated before, highly expressed LINC01123 overtly reduces the survival of head neck squamous cell carcinoma [
29]; it also aggravates the uncontrolled proliferation of non-small cell lung cancer cells [
30]. In our work, mechanical data supported that LINC01123 could interact with miR-663a. Besides, downregulation of miR-663a reverse the suppressive proliferation of TNBC cells due to the suppression of LINC01123. Overall, the ceRNA role of LINC01123 was first introduced in TNBC, accompanied with its oncogenic nature.
CMIP is crucial in negatively regulating T-cell signaling pathway [
31]. The participation of CMIP is observed in multiple cellular processes, such as chondrocyte terminal differentiation and cell growth in embryonic lens. Also, CMIP has been reported to have considerable contributions in tumors. For instance, it has been proved to be pro-metastasis and pro-proliferation in gastric cancer [
32] and glioma [
33]. Furthermore, overexpressed CMIP inversely correlated with breast cancer patients’ survival [
34]. In this work, we demonstrated that CMIP was targeted by miR-663a in TNBC. The positive regulation of LINC01123 on CMIP was also confirmed. Functionally, CMIP suppression led to TNBC proliferation retardation and apoptosis enhancement. All findings in this work suggested that LINC01123 exerted its function in TNBC through a CMIP-dependent way via sponging miR-663a. More importantly, we also gave a more detailed description about the oncogenic property of CMIP in TNBC.
For the exploration of LINC01123 upregulation in TNBC, we unveiled that forkhead box C1 (FOXC1) directly amplified the expression of LINC01123 through transcriptional regulation. On a global scale, FOXC1 is identified as a transcriptional amplifier of diverse genes, such as WNT5A [
25] and PTGER3 [
35]. Prior studies summarized that FOXC1 is a tumor facilitator in breast cancer [
36], cervical carcinoma [
37], esophageal cancer [
38] and melanoma [
39]. Of interest, FOXC1 has also been uncovered to be overexpressed in TNBC [
25]. Herein, the promoting effect of FOXC1 on LINC01123 opens up a new page of its activity in TNBC progression.
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