LINC00665 functions as a competitive endogenous RNA to regulate AGTR1 expression by sponging miR‑34a‑5p in glioma.

Glioma is the most aggressive tumor of the central nervous system. Long non‑coding RNAs (lncRNAs) may be involved in modulating tumor generation. The present study analyzed an lncRNA microarray of glioma and selected long intergenic non‑protein coding RNA 665 (LINC00665) as the research object. The mode of expression and biological function of LINC00665 in glioma were assessed using lncRNA microarray and RT‑qPCR analyses. Gain‑of‑function assays and/or loss‑of‑function assays were implemented to explore the role of LINC00665 in the progression of glioma. Dual‑luciferase reporter and RNA immunoprecipitation assays explored the downstream molecular mechanism of LINC00665. The function of the molecular pathway in progression of glioma was analyzed using rescue assays. High expression of LINC00665 was marked in glioma tissues and cells, which correlated with an unsatisfactory prognosis. Upregulation of LINC00665 significantly promoted the proliferation and invasion of glioma cells. LINC00665 acted as a competing endogenous RNA by sponging miR‑34a‑5p to upregulate angiotensin II receptor type 1 (AGTR1). LINC00665 promoted the progression of glioma by acting as a competitive endogenous RNA to competitively bind to miR‑34a‑5p and mediate AGTR1 expression.

Introduction

Gliomas are the most widely encountered solid tumors of the central nervous system (CNS) (1,2). As reported in 2018, ~100,000 people worldwide are diagnosed as having diffuse gliomas every year (3). Although it comprises <1% of all newly diagnosed cancers, diffuse glioma is associated with substantial mortality (4). Most glioma patients succumb to the disease within 2 years after first diagnosis (5). The capacities to migrate, rapidly diffuse and invade paracancerous tissues, heterogeneity, and incessant proliferation of glioma cells contribute to the overall survival of approximately 15 months for most patients with glioma at the late stage (6–8). Hence, improved understanding of novel mechanisms governing glioma cell growth and metastasis is a key to the exploitation of early diagnostic regimens and personalized treatment.

Long non-coding RNAs (lncRNAs) are ncRNAs at least 200 nucleotides in length (9,10). They have been implicated in diverse epigenetic regulatory processes, including histone modification, chromatin remodeling, RNA alternative splicing, and transcriptional regulation (11–14). Due to their specificity and easy detection, lncRNAs can be used as biomarkers and treatment targets (15–17). For example, Tamang et al confirmed that SNHG12 is a potential therapeutic target and biomarker for human cancer (18). Chen et al reported that lncRNAs can be biomarkers and treatment targets in non-small cell lung cancer (19). The long intergenic non-protein coding RNA 665 (LINC00665) lncRNA promotes impacts in diverse tumors, including gastric cancer (20,21), non-small cell lung cancer (22), lung adenocarcinoma (23) and hepatocellular carcinoma (24). However, the involvement of LINC00665 in the development of glioma is unclear.

In the present study, the high expression of LINC00665 was reported in glioma tissues and cell lines. LINC00665 overexpression (OE) enhanced the proliferative, invasion, and migratory potentials of glioma cells. The findings verified that LINC00665 participated in the development of glioma by competitively binding to miR-34a-5p to mediate the expression of angiotensin II receptor type 1 (AGTR1). The findings offer a new perspective for studying the pathogenesis of glioma.

Materials and methods

Ethical compliance

The Ethics Committee of Wenzhou Hospital Integrated Traditional Chinese and Western Medicine approved the present study. All population-related research complied with the World Medical Association Declaration of Helsinki and all participants provided written informed consent.

Clinical specimens

Forty-eight glioma and paracarcinoma tissues were harvested from patients who had undergone surgical excision at Wenzhou Hospital Integrated Traditional Chinese and Western Medicine from January 2017 to June 2019. The patients had not received chemotherapy or radiotherapy before tissue excision. Prior to RNA extraction, all isolated specimens were rapidly cryopreserved at −80°C. Data concerning the association of LINC00665 expression with clinicopathological features of glioma are provided in Table I.

Table I.

Association of LINC00665 expression with clinicopathological features of glioma.

		Expression of LINC00665
Characteristics	No.	High	Low	P-value
All cases	48	24	24
Age (years)				0.5639
≤48	25	14	11
>48	23	10	13
Sex				0.5612
Male	27	12	15
Female	21	12	9
Clinical stage				0.0189
I–II	21	6	15
III–IV	27	18	9

Total data from 48 tumor tissues of glioma patients were analyzed. For the expression of LINC00665 which was assayed by RT-qPCR, the median expression level was used as the cutoff. Data were analyzed by chi-squared test and Fisher's exact test. The P-value in bold indicates a statistically significant difference.

Cell culture and transfection

Glioma cell lines U87 MG (glioblastoma of unknown origin, ATCC® HTB-14; ATCC), LN229 (ATCC® CRL-2611), A172 (ATCC® CRL-1620), U373 MG (ATCC® HTB-17), U251 (U251 MG; cat. no. YS448C; YaJi Biological), human normal astrocytes NHA (cat. no. YS2144C; YaJi Biological) and 293T cells (cat. no. YS005C; YaJi Biological) were cultured and preserved in DMEM (GIBCO-BRL; Thermo Fisher Scientific, Inc.) supplemented with 100 U/ml penicillin, 10% fetal bovine serum, and 100 mg/ml streptomycin (Beyotime Institute of Biotechnology) in a humidified atmosphere containing 5% CO2 at 37°C. STR profiling analysis was performed for the authentication of cell lines.

As per the guidance of the manufacturer (Shanghai GenePharma Co., Ltd.), LINC00665 overexpression (OE) plasmid/small interfering (si)RNA and microRNA (miR)-34a-5p mimics/inhibitor were used for transfection assays with Lipofectamine 2000 Reagent (Invitrogen; Thermo Fisher Scientific, Inc.). Cells grown to approximately 50–60% confluence in culture dishes were used for transfection. Transfection was performed in serum-free medium for one day.

RNA extraction and reverse transcription-quantitative PCR (RT-qPCR)

Total RNA was extracted from the tissues and cultured cells using TRIzol® (Invitrogen; Thermo Fisher Scientific, Inc.) following the manufacturer's guidelines. Approximately 1 µg of total RNA was reversely transcribed to cDNA using a reverse transcriptase cDNA synthesis kit (Toyobo Co., Ltd.). qPCR was performed using the SYBR Green PCR kit (Roche Diagnostics) by initial denaturation at 94°C for 5 min, followed by 40 cycles including denaturation at 94°C for 30 sec, annealing at 55°C for 30 sec and extension at 72°C for 90 sec. Comparative quantification was assessed using the 2−ΔΔCq method with glyceraldehyde 3-phosphae dehydrogenase (GAPDH) or U6 used as the endogenous control (25). U6 was used for normalization of the miRNA whereas GAPDH was used for the normalization of other genes, such as AGTR1. The PCR primers used are summarized in Table II.

Table II.

Sequences of primers for RT-qPCR and miRNA-related sequences.

Name	Sequence
LINC00665	F: 5′-GGTGCAAAGTGGGAAGTGTG-3′
	R: 5′-CGGTGGACGGATGAGAAACG-3′
miR-34a-5p	F: 5′-ACACTCCAGCTGGGTGTTGGTCGATTCTGT-3′
	R: 5′-CTCAACTGGTGTCGTGGAGTCGGCAATTCAGTTGAGGTGACGGT-3′
AGTR1	F: 5′-ATTTAGCACTGGCTGACTTATGC-3′
	R: 5′-CAGCGGTATTCCATAGCTGTG-3′
U6	F: 5′-GGTCGGGCAGGAAAGAGGGC-3′
	R: 5′-TGGTATCGTGGAAGGACTC-3′
GAPDH	F: 5′-AGTAGAGGCAGGGATGATG-3′
	R: 5′-AGGGGCCATCCACAGTCTTC-3′
si-LINC00665	Sense, 5′-AAUAGCCCAAGACUGAGGACUCACA-3′
	Antisense, 5′-UGUGAGUCCUCAGUCUUGGGCUAUU-3′
miR-34a-5p mimics	Sense, 5′-UGGCAGUGUCUUAGCUGGUUGU-3′
	Antisense, 5′-ACAACCAGCUAAGACACUGCCA-3′
miR-34a-5p inhibitor	Sense, 5′-ACAACCAGCUAAGACACUGCCA-3′

F, forward; R, reverse; AGTR1, angiotensin II receptor type 1.

Cell proliferation assays

Approximately, 1.0×103 transfected U87 MG and U251 cells were cultured in 96-well plates. Cell Counting Kit-8 (CCK-8; 10 µl) reagent (Beyotime Institute of Biotechnology) was added and incubated at 37°C for 1 h. The absorbance at 450 nm was recorded using an Infinite M200 multimode microplate reader (Tecan Group, Ltd.).

After approximately 48 h of transfection, the 5-ethynyl-2´-deoxyuridine (EdU) assay kit provided by Guangzhou Ribo Co., Ltd., was used to examine the proliferation of U87 MG and U251 cells. Specifically, cells were grown in culture medium containing EdU (cat. no. A10044; Invitrogen; Thermo Fisher Scientific, Inc.) solution (1,000:1). At the proliferative stage, the cells were labeled with EdU for 2 h, followed by three rinses with phosphate-buffered saline (PBS; 0.5 g/ml). Subsequently, 4′,6-diamidino-2-phenylindole (DAPI; Invitrogen; Thermo Fisher Scientific, Inc.) was used to stain nuclei of the washed cells for 10 min at room temperature in the dark. The DAPI-stained cells were washed more than twice with PBS. Stained cells were analyzed using the FACSCalibur DxP flow cytometer (BD Biosciences).

Cell migration and invasion assays

Cell migration was examined using a wound healing assay. Cells (5×105) were seeded in a six-well plate and cultured to confluence. When the cells grew to nearly 100% confluency, a 200-µl pipette tip (QIAGEN,) was used to scratch the confluent monolayer of cells. Suspended cells and cell debris were removed by washing three times with PBS. After adding fresh serum-free medium, the plate was incubated for 24 h with 5% CO2 at 37°C for 1 h. The wound was photographed regularly using a computer-assisted microscope (magnification, ×100; Nikon Corporation).

Cell invasion was assessed in a Matrigel assay using a 24-well invasion chamber system from BD Biosciences equipped with polycarbonic membranes (diameter 6.5 mm; pore size 8 µm). Subsequent to incubation at 37°C for 24 h, a fluorescence microscope (magnification, ×200) was used to quantify cells co-cultured with exosomes and invading through the membranes in four fields that were randomly selected. Each assay was repeated at least three times with triplicate samples each time.

Subcellular distribution

The Cytoplasmic and Nuclear RNA Purification Kit (Norgen Biotek Corp.) was used to examine RNA degradation in the cytoplasm or nucleus. U87 MG and U251 cells were lysed on ice for 5 min and then centrifuged at 12,000 × g for 3 min. The supernatant was collected to examine RNAs originating in the cytoplasm, and the nuclear pellet was employed to extract RNAs from the nuclei. Total RNA in each fraction was quantified using RT-qRCR with U6 and GAPDH as internal references for the nucleus and cytoplasm, respectively.

Dual-luciferase reporter gene assay

Wild-type (WT) plasmids LINC00665-WT and AGTR1-WT were constructed, as well as mutant (MUT)-type plasmids LINC00665-MUT and AGTR1-MUT. The putative binding site, WT, and its MUT sequence were subjected to subcloning in a pmirGLO Dual-luciferase vector (Promega Corporation). 293T cells seeded into 24-well plates were co-transfected with 50 nM miR-34a-5p mimics or a negative control and 80 ng wild-type or mutant-type recombinant vectors using Lipofectamine 2000 (Invitrogen; Thermo Fisher Scientific, Inc.). This was followed by the addition of 80 ng of plasmid with 5 ng of pRL-SV40. A Dual-Luciferase Reporter Assay system (Promega Corporation) was utilized to measure the activity of the reporter after 48 h while normalization was in reference to Renilla luciferase activity, according to the manufacturer's protocol.

RNA immunoprecipitation (RIP)

Magna Nuclear RIP™ (Native) Nuclear RNA-Binding Protein Immunoprecipitation Kit (EMD Millipore) was used for the RIP assay, followed by cell lysis in complete RIPA buffer with an RNase inhibitor and protease inhibitor cocktail (all from Beyotime Institute of Biotechnology). The cell extract was subject to incubation with RIP buffer containing magnetic beads conjugated to human anti-AGO2 antibody (cat. no. 03-110; dilution 1:150; Merck KGaA) or IgG control (cat. no. 12-370; dilution 1:150; Merck KGaA) at 4°C overnight. Immunoprecipitated RNA was obtained from protein digestion. Finally, the purified RNA was quantified by RT-qPCR.

Western blotting

Cells were lysed in RIPA buffer containing protease and phosphatase inhibitors (all from CWBio). The concentration of protein was determined using a BCA Protein Assay kit. The same amount of total protein (40 µg protein per lane) was used for 10% SDS-PAGE. The resolved proteins were transferred to polyvinylidene fluoride membranes. The membrane was blocked with 5% BSA (Beyotime Institute of Biotechnology) for 1 h at room temperature, and incubated with antibodies to GAPDH (1:1,000 dilution; product code ab181602; Abcam) and AGTR1 (1:1,000; product code ab124505; Abcam) overnight at 4°C. This was followed by exposure to an appropriate secondary antibody conjugated with horseradish peroxidase at room temperature for 1 h. The secondary antibody used was as follows: HRP-labeled goat anti-rabbit IgG (1:1,000; cat. no. A0208; Beyotime Institute of Biotechnology). Immobilon ECL substrate (EMD Millipore) was used to generate signals, which were detected using the Optimax X-ray Film Processor (Protec GmbH & Co. KG). The protein bands were analyzed using ImageJ software (version 1.48; National Institutes of Health).

Immunohistochemistry

The tissues were embedded with paraffin and cut into 5 µm-thick sections. Tissue sections were dewaxed in xylene and rehydrated in graded alcohol concentrations. Sodium citrate buffer was used for antigen retrieval. The endogenous peroxidase activity of tissues was blocked, and tissues were then incubated with the primary antibody anti-AGTR1 (1:500; product code ab124505) overnight at 4°C, and the secondary antibody anti-rabbit (1:1,000; product code ab97080; both from Abcam). DAB (Vector Laboratories, Inc.) was used to reveal the area targeted by the primary antibodies, and nuclei were counterstained with hematoxylin for 1 min at room temperature. A fluorescence microscope (magnification, ×200) was used to visualize and caprture the images.

Construction of xenograft models

A total of 6, specific pathogen-free 4-week-old mice from Shanghai SLAC Laboratory Animal Co., Ltd. were randomly allocated into two groups, with three mice in each group (weight, 18–20 g). The mice were cultured under standard conditions (24±2°C; 50±10% relative humidity; 12-h light/dark cycles) and with unlimited access to standard rodent maintenance feed (Beijing Keao Xieli Feed Co., Ltd.) and water. Animal health and behavior were monitored every day. U87 MG cells transfected with LINC00665 OE or vector (1×106) were subcutaneously injected into the right flank of the mice. Tumor volumes were determined every 4 days and calculated as (length × width2)/2. Before the surgery, the mice were anaesthetized by intraperitoneal injection of sodium pentobarbital (40 mg/kg) to minimize suffering and distress. The observation days after subcutaneous injection were the specific endpoint. The most frequently selected observation period was 28 days (4 weeks) (26–28). Thus, 28 days after subcutaneous injection, all the six mice were sacrificed by overdose (>120 mg/kg body weight) intraperitoneal injection of pentobarbital, and the tumor tissues were removed. Death was confirmed by complete cessation of a heartbeat and breathing. The mouse experiments were approved by the Animal Care and Use Committee of Wenzhou Medical University. Animal experiments were performed at the specific pathogen-free animal laboratory at Wenzhou Medical University.

Bioinformatics analysis

The association between LINC00665 expression and overall survival of glioma patients was analyzed using TCGA datasets (https://cancergenome.nih.gov/). The samples were divided into two groups based on the expression of LINC00665 and were analyzed using Kaplan-Meier analysis with log-rank testing. The miRNAs containing putative binding sites for LINC00665 were predicted with starBase software 3.0 (http://starbase.sysu.edu.cn/). The potential target genes of miR-34a-5p were also predicted with starBase software 3.0.

Microarray analysis

RNA expression profiling was performed using the Agilent human lncRNA microarray V.2.0 platform (GPL18109; Agilent Technologies, Inc.). Quantile normalization and subsequent data processing were performed using Agilent Gene Spring Software 11.5 (Agilent Technologies, Inc.). Heatmaps representing differentially regulated genes were generated using Cluster software (version 3.0, http://www.clustersoft.com/). The microarray analysis was performed by Beijing Genomics Institute/HuaDa-Shenzhen. The lncRNAs were differentially expressed on the basis of the criteria of log2FC>1 or log2FC<-1, and P<0.05. The heatmap between the glioma tumor tissues and controls (3 vs. 3) was drawn based on the same criteria.

Statistical analyses

GraphPad Prism 6.0 software (GraphPad Software, Inc.) was used for statistical analyses. Experimental results are expressed as the mean ± standard deviation (SD). The statistically significant differences between tumor tissues and adjacent normal tissues were determined using paired Student's t-test. The statistically significant differences between other two groups were determined using Mann-Whitney U-test or unpaired Student's t-test, where appropriate. The comparisons among different groups (multigroup comparisons) were analyzed by one-way ANOVA followed by the post hoc Bonferroni test. Pearson's correlation coefficient was determined to assess associations among LINC00665, miR-34a-5p and AGTR1. Log-rank test and Kaplan-Meier method were used to assess survival rates. Data concerning the association of LINC00665 expression with clinicopathological features of glioma were analyzed by chi-squared test and Fisher's exact test. A P-value <0.05 indicated a statistically significant difference.

Results

LINC00665 expression in glioma tissues

The lncRNA microarray analysis revealed the high expression of LINC00665 in glioma tumor tissues. The lncRNAs were differentially expressed on the basis of the criteria of log2FC>1 or log2FC<-1, and P<0.05. lncRNAs exhibiting different expression, including LINC00665, were identified in glioma tumor tissues and adjacent normal tissues (Fig. 1A). Subsequently, the expression levels of LINC00665 were determined in 48 glioma and 48 paracancerous tissue samples by RT-qPCR analysis. LINC00665 expression was significantly increased in glioma tissues, in contrast to paracancerous tissues (Fig. 1B). Higher LINC00665 expression was observed in the glioma cell lines (U87 MG, LN229, A172, U373 MG, U251) compared with human astrocytes (NHA) (Fig. 1C). The high LINC00665 expression was associated with unsatisfactory overall survival of glioma patients as determined by Kaplan-Meier analysis (P=0.0251, Fig. 1D). The TCGA database also confirmed this result (P=0.0067, Fig. 1E).

Figure 1.

LINC00665 expression in glioma tissues. (A) Heatmap of differentially expressed lncRNAs between glioma tissues and paracarcinoma tissues. LINC00665 expression was increased in glioma tissues. (B) LINC00665 expression was increased in glioma tissue samples. (C) LINC00665 expression was significantly higher in glioma cell lines relative to the NHA cell line. Kaplan-Meier survival curve revealing the overall survival of glioma patients stratified by LINC00665 expression based on (D) our dataset and the (E) TCGA dataset. *P<0.05, **P<0.01 and ***P<0.001 denote statistically significant differences. lncRNAs, long non-coding RNAs; TCGA, The Cancer Genome Atlas.

Functions of LINC00665 in glioma cell lines

To examine the function of LINC00665 in glioma oncogenesis, LINC00665 expression was reduced by transfecting LINC00665 siRNA plasmid into U251 cells, and LINC00665 OE plasmids were used to increase LINC00665 expression in U87 MG cells (Fig. 2A). CCK-8 and EdU assays revealed that reduced expression of LINC00665 decreased glioma cell proliferation, while LINC00665 OE increased proliferation (Fig. 2B and C). Cell migration and invasion assays revealed that, as opposed to LINC00665 downregulation, LINC00665 OE induced migration and invasion of glioma cells (Fig. 2D and E).

Figure 2.

Regulatory effect of LINC00665 on proliferation, migration, and invasion of glioma cells. (A) LINC00665 expression levels in U251 and U87 MG cells were detected by RT-qPCR after transfection with LINC00665 siRNA or LINC00665 OE vector. (B and C) CCK-8 and EdU assays revealed proliferation of U251 cells treated with LINC00665 siRNA and U87 MG cells undergoing LINC00665 OE vector treatment. (D) Wound healing assay demonstrating the migration of glioma cells. (E) Transwell assay demonstrating the invasion of glioma cells. Images were captured by light microscopy (×200). *P<0.05, **P<0.01 and ***P<0.001 denote statistically significant differences. RT-qPCR, reverse transcription-quantitative PCR; LINC00665 OE, LINC00665 overexpression; CCK-8, Cell Counting Kit-8.

LINC00665 is targeted by miR-34a-5p

lncRNA subcellular distribution determines the biological role (29). Glioma cells were separated into the cytoplasm and nuclear fractions to verify the LINC00665 cellular location, with GAPDH and U6 as controls, respectively. RT-qPCR results revealed that LINC00665 was distributed in the cytoplasmic fraction of U251 and U87 MG cells (Fig. 3A). Considering this distribution, it was presumed that LINC00665 functioned as a competitive endogenous RNA (ceRNA) in glioma. Analysis using the starBase bioinformatics prediction database demonstrated that sequences in miR-34a-5p were markedly similar to the LINC00665 3′untranslated region (UTR) (Fig. 3E). RT-qPCR also demonstrated that the expression of miR-34a-5p was associated with a decreasing trend in glioma tissues and cells (Fig. 3B and C). Correlation analysis revealed that miR-34a-5p and LINC00665 expression were inversely associated (Fig. 3D). Next, pGL3-LINC00665-WT and pGL3-LINC00665-MUT were constructed on the basis of binding sequences (Fig. 3E). A significant decrease in the luciferase activity of 293T cells was evident during treatment with LINC00665-WT and miR-34a-5p mimics, however, no change was apparent after treatment with LINC00665-MUT and miR-34a-5p mimics (Fig. 3F). The RIP assay revealed that LINC00665 was enriched in anti-AGO2 antibody. Similar results were revealed for miR-34a-5p (Fig. 3G). The findings indicated that miR-34a-5p probably binds to LINC00665 in vitro.

Figure 3.

LINC00665 directly and mutually acts with miR-34a-5p. (A) Cytoplasmic and nuclear levels of LINC00665 in U251 and U87 MG cells assessed by RT-qPCR. (B) Decreased miR-34a-5p expression in glioma and paracancerous tissue samples. (C) miR-34a-5p expression in glioma cell lines and NHA cells detected by RT-qPCR. (D) Pearson correlation analysis of the correlation of LINC00665 with miR-34a-5p in glioma. (E) Identification of the miR-34a-5p binding site sequence with LINC00665. (F) Dual-Luciferase reporter gene assay for 293T cells. (G) RIP assays for quantifying LINC00665 and miR-34a-5p in U251 and U87 MG cells. *P<0.05, **P<0.01 and ***P<0.001 denote statistically significant differences. RT-qPCR, reverse transcription-quantitative PCR; RIP, RNA immunoprecipitation.

LINC00665 regulates the target gene AGTR1 of miR-34a-5p

To ascertain the possible function of miR-34a-5p in glioma growth, the starBase bioinformatics prediction system was used to screen miR-34a-5p target genes. AGTR1 was identified for subsequent assessment. Subsequent to the establishment of pGL3-AGTR1-WT and pGL3-AGTR1-MUT (Fig. 4A), 293T cells were co-treated with miR-34a-5p mimics/control. Luciferase activity was blocked in the WT reporter group, but not in the MUT reporter group (Fig. 4B). These findings implied that AGTR1 probably is the target gene for miR-34a-5p. The levels of AGTR1 mRNA and protein were significantly increased in glioma tissues (Fig. 4C and D). AGTR1 expression was higher in glioma cell lines than in the NHA cell line (Fig. 4E). Correlation analysis revealed an inverse relationship between miR-34a-5p and AGTR1 expression (Fig. 4F) as well as a positive correlation between AGTR1 and LINC00665 expression (Fig. 4G).

Figure 4.

AGTR1 is a direct target of miR-34a-5p. (A) The possible binding sites in the AGTR1 sequence. (B) Direct target sites verified by the Dual-Luciferase reporter gene assay. (C and D) AGTR1 expression in glioma tissues detected by RT-qPCR and western blotting. (E) AGTR1 expression in glioma cells and NHA cells. (F) Pearson correlation analysis of the correlation of AGTR1 with miR-34a-5p in glioma. (G) Pearson correlation analysis of the correlation of LINC00665 with AGTR1 in glioma. *P<0.05 and ***P<0.001 denote statistically significant differences. AGTR1, angiotensin II receptor type 1; RT-qPCR, reverse transcription-quantitative PCR.

To determine the modulation of LINC00665 on AGTR1 expression by targeting miR-34a-5p, the expression level of AGTR1 in glioma cells was examined after altering LINC00665 or miR-34a-5p expression. The transfection effectiveness of miR-34a-5p mimics/inhibitors was assessed (Fig. 5A). Then, AGTR1 expression was increased by treating U251 cells with miR-34a-5p inhibitors. The increased expression was abrogated by treatment with LINC00665 siRNA (Fig. 5B and C). Furthermore, AGTR1 expression in U87 MG cells treated with miR-34a-5p mimics was impeded, and was reversed by LINC00665 OE treatment (Fig. 5B and D). Subsequently, U251 cells were transfected with LINC00665 OE plasmid/MUT OE plasmid, and AGTR1 expression was examined. RT-qPCR and western blotting revealed that LINC00665 WT OE increased the expression of AGTR1 in glioma cells, while LINC00665 MUT had no influence on AGTR1 expression (Fig. 5E and F). The findings indicated that LINC00665 directly binds to miR-34a-5p to positively modulate AGTR1 expression.

Figure 5.

LINC00665/miR-34a-5p axis is critical for AGTR1 expression. (A and B) miR-34a-5p expression levels in U251 and U87 MG cells were detected by RT-qPCR after transfecting with miR-34a-5p inhibitor/mimics. (C and D) miR-34a-5p inhibitor with or without LINC00665 siRNA were transfected into U251 cells and miR-34a-5p mimics with or without LINC00665 OE vector were transfected into U87 MG cells. AGTR1 expression levels in U251 and U87 MG cells were detected by RT-qPCR and western blot analysis. (E and F) Relative mRNA and protein levels of AGTR1 following transfection with LINC00665-WT/MUT OE plasmid. **P<0.01 and ***P<0.001 denote statistically significant differences. ns, no significant difference. AGTR1, angiotensin II receptor type 1; RT-qPCR, reverse transcription-quantitative PCR; LINC00665-WT, LINC00665 OE, LINC00665 overexpression; LINC00665-WT/MUT, LINC00665 wild-type/mutated.

LINC00665/miR-34a-5p axis regulates the behaviors of glioma cells

CCK-8 and EdU assay results revealed that miR-34a-5p inhibition significantly contributed to the ability of U251 cells to proliferate, in contrast to controls. LINC00665 siRNA partially abrogated this ability (Fig. 6A and C). Additionally, overexpressed miR-34a-5p restricted the proliferation of U87 MG cells, but LINC00665 OE partially reversed this potential (Fig. 6B and D). Moreover, miR-34a-5p-mediated downregulation induced invasion of U251 cells, which was partially reversed by LINC00665 siRNA (Fig. 6E). Overexpressed miR-34a-5p blocked the invasion capability of U87 MG cells, which was partially reversed by LINC00665 OE (Fig. 6F).

Figure 6.

LINC00665 regulates cell function through miR-34a-5p. (A-D) CCK-8 and EdU assays were performed to examine U251 and U87 MG cell proliferation. (E and F) The invasion ability of U251 and U87 MG cells after different treatments. *P<0.05, **P<0.01 and ***P<0.001 denote statistically significant differences.

LINC00665 in U87 MG cells stimulates tumor growth

Nude mice were subcutaneously injected with stably expressed U87 MG cells transfected with vector or LINC00665 OE to assess the function of LINC00665 in glioma in vivo. Upregulation of LINC00665 increased the tumor volume (Fig. 7A and B) and weight (Fig. 7C). Immunohistochemical results demonstrated that mice treated using LINC00665 OE treatment had a higher AGTR1 level (Fig. 7D).

Figure 7.

LINC00665 in U87 MG cells promotes tumor growth in vivo. (A) Typical images of xenografts in nude mice. (B) Tumor volume and (C) weight. (D) Typical images of AGTR1 in immunohistochemical staining analysis. AGTR1, angiotensin II receptor type 1

Discussion

An increasing number of lncRNAs have been implicated as biomarkers for glioma growth. For example, lncRNA PAXIP1-AS1 enhanced cell invasion and blood vessel formation of glioma utilizing transcription factor ETS1 to increase KIF14 expression (30). lncRNA GAS5 inversely regulated miR-18a-5p to modulate glioma cells to proliferate, migrate, and invade (31). Thus, lncRNAs are likely markedly influential in the onset and growth of glioma. Continued examinations of the possible molecular mechanisms and biological functions of lncRNAs in glioma will identify novel molecular targets for disease treatment.

Presently, increased LINC00665 expression was demonstrated in glioma tissues and cells. In addition, decreased LINC00665 expression significantly decreased glioma cell proliferation, migration, and invasion in vitro, indicating that LINC00665 acts as an oncogene to modulate the growth of glioma cells. A tumor xenograft model was used to confirm the role of LINC00665 in glioma. In vivo assays revealed that overexpressing of LINC00665 in U87 MG cells promoted tumor growth. The findings highlight the importance of determining the role of LINC00665 in enhancing the growth of glioma cells to better understand the onset, growth, and migration of glioma.

The cross-regulation between lncRNAs and miRNAs has been demonstrated. lncRNAs may serve as ceRNAs to modulate the expression and functions of miRNAs, and thus have been termed are ‘miRNA sponges’ (32,33). To understand the potential oncogenic mechanisms of LINC00665 in glioma cells, the starBase bioinformatics database was utilized to identify miR-34a-5p as a target of LINC00665. Gao et al revealed that miR-34a-5p suppressed colorectal cancer metastasis and predicted recurrence in patients with stage II/III colorectal cancer (34). Previous studies revealed that miR-34a-5p can suppress tumorigenesis and progression of glioma (35–37). The present results demonstrated that miR-34a-5p was decreased in glioma tissues and cells. Transfection of miR-34a-5p mimics inhibited glioma cell proliferation and invasion, which could be reversed by LINC00665 OE. It can be concluded that both LINC00665 and miR-34a-5p may be involved in the development and progression of glioma.

The RAS component AGTR1 has the potential to stimulate cell growth, migration, or invasion and to promote angiogenesis, inflammation and immunity (38). The present findings affirmed that LINC00665 elevation could increase AGTR1 expression, giving rise to significant proliferation, invasion, and migration of glioma cells. We intend in future studies to investigate other mechanisms that may be related to LINC00665 in strengthening the malignant phenotype of glioma cells.

Nevertheless, the present study has a number of limitations. Firstly, a larger tissue sample size of glioma is required to further explore the clinical value of LINC00665. Secondly, in situ hybridization fluorescence would be valuable to verify the relationship between LINC00665 and miR-34a-5p in future studies. In addition, whether there are other target genes or miRNAs which can interact with LINC00665 requires further exploration.

In conclusion, LINC00665 was increased in human glioma cell lines and tissues, and its decrement in glioma cells impeded proliferation, invasion, and migration of glioma cells. LINC00665 is a ceRNA that modulated AGTR1 expression by sponging miR-34a-5p, thus modulating glioma growth. The present findings could aid in the discovery of new targets for the diagnosis and treatment of glioma.