lncRNA TTN‑AS1 upregulates RUNX1 to enhance glioma progression via sponging miR‑27b‑3p.

Long non‑coding RNAs (lncRNAs) contribute to the tumorigeneses of numerous types of cancer, including glioma. The present study was designed to unveil a novel lncRNA functioning in glioma and explore the underlying mechanisms. lncRNA titin‑antisense RNA1 (TTN‑AS1), miR‑27b‑3p and Runt‑related transcription factor 1 (RUNX1) expression in glioma tissues and cell lines was estimated by RT‑qPCR. Si‑TTN‑AS1 was transfected into glioma cell lines (U251 and LN229), and CCK‑8 assay, flow cytometry, wound healing and Transwell assays were applied to estimate the function of TTN‑AS1 in glioma cells. miR‑27b‑3p inhibitor was used to explore the mechanisms. The results revealed that TTN‑AS1 was highly expressed in glioma specimens and cell lines. Downregulation of TTN‑AS1 inhibited the proliferation, migration and invasion of the glioma cells, as well as increased the rate of apoptosis. In vivo, the tumor growth was also inhibited by TTN‑AS1 depletion in nude mice. Furthermore, we revealed that TTN‑AS1 exerted oncogenic effects via sponging miR‑27b‑3p and thereby positively regulating RUNX1 expression. In conclusion, the present study supported that TTN‑AS1 acts as an oncogene in glioma by targeting miR‑27b‑3p to release RUNX1. This finding may contribute to gene therapy of glioma.

Introduction

Glioma, derived from glial cells, is the most frequently diagnosed intracranial malignant tumor worldwide (1). It is reported that the incidence of glioma is up to 60% of all brain tumors and accounts for approximately 2% of all human cancers(2). Despite the great efforts in treating glioma, the five-year overall survival rate of glioma patients remains poor (3). Thus, there is an urgent need to discover novel biomarkers and design potential therapeutic strategies for glioma treatment.

Long non-coding RNAs (lncRNAs) are RNAs which are longer than 200 nt and with an incapacity for protein-coding (4). Numerous studies have proved the key roles of lncRNAs in the pathologies of diverse diseases, including cancers (5–7). lncRNAs have been found to function as oncogenes and tumor suppressors in glioma. For instance, small nucleolar RNA host gene 3 (SNHG3) was found to drive the glioma process via modulation of p21 and KLF2 (8). Cancer susceptibility 2 (CASC2) was found to regulate glioma growth and resistance to temozolomide by interfering with PTEN signaling (9). lncRNA activated by transforming growth factor-β (lncRNA-ATB) was found to modulate NF-κB/MAPK to enhance glioma cell metastasis (10). Further in-depth research of lncRNAs is needed to understand the molecular pathology of glioma.

In addition to regulating gene expression directly, lncRNAs usually function as competitive endogenous (ce)RNAs for microRNAs (miRNAs), thus releasing the subsequent target mRNAs post-transcriptionally (11). miRNAs are non-coding RNAs which are conserved and containing approximately 22 nt (12). miRNAs lead to target mRNA degradation or translation blocking by binding to seed sequences of 3′UTRs. Increasing evidence reveals that lncRNAs participate in the tumorigenesis of glioma by acting as ceRNAs. For example, LINC01857 was found to interfere with miR-1281/TRIM65 to enhance glioma growth (13). H19 was found to promote glioma cell metastasis by interacting with miR-140 and thus modulating iASPP (14). LINC00473 was found to impair the miR-637/CDK6 pathway to aggravate glioma (15).

Our research aimed to investigate the expression patterns and biologic effects of TTN-AS1 on glioma, and the underlying mechanisms. We first revealed the upregulation of TTN-AS1 in glioma tissue and cells. Subsequently, TTN-AS1 depletion induced the inhibitory effects on glioma cell growth and metastasis in vivo and in vitro. Finally, we revealed the possible mechanism of the TTN-AS1/miR-27b-3p/RUNX1 pathway.

Materials and methods

Clinical specimens

The study concerning human tissues was authorized by the Ethics Committee of The Second Affiliated Hospital of Zhengzhou University (no. PY-2018092). Written informed consent was provided by all patients enrolled in the present study. A total of 45-paried glioma and normal specimens (collected at a distance from the tumor tissues) were collected from patients who underwent surgery at The Second Affiliated Hospital of Zhengzhou University from March 2017 to December 2018. Clinical features are presented in Table I. Tissue specimens were immediately fresh-frozen for subsequent experiments.

Table I.

Characteristics of the glioma patients (N=45).

		TTN-AS1 expression
Characteristics	N	High (n=30)	Low (n=15)	P-value
Age (years)				0.512
≥50	33	25	8
<50	12	5	7
Tumor size (mm)				0.012[a]
≥4	26	18	8
<4	19	7	12
WHO stage				0.026[a]
III–IV	18	10	8
−II	27	8	19
Lymph-node metastasis				0.018[a]
Yes	21	15	7
No	24	9	15
Histological grade				0.518
Well	26	12	14
Moderately/poorly	19	12	7

P<0.05 represents statistical difference. TTN-AS1, lncRNA titin-antisense RNA1.

Cell culture

The Culture Collection of the Chinese Academy of Sciences (Shanghai, China) provided the human glioma cell lines (U87, A172, LN229 and U251) and normal human astrocytes (NHA). The U87 cell line we used was the U87 MG ATCC version (glioblastoma of unknown origin), and we authenticated the cell line using STR profiling. RPMI-1640 medium (Hyclone; GE Healthcare) containing 10% FBS was utilized to incubate the cells in a humidified incubator at 37°C and 5% CO2.

Cell transfection

Si-TTN-AS1, miR-27b-3p inhibitor, miR-27b-3p mimics and the corresponding negative controls were purchased from Sangon Biotech Co., Ltd. and the sequences are presented in Table II. 3′UTR (untranslated region) of RUNX1 was subcloned into the pGL3-control vector (Promega which contained the luciferase reporter. U251 and LN229 cells were plated in 96-well plates. When cells reached a confluence of 80%, they were transfected with the fragments or plasmids (0.2 µg/well) by Lipo3000 reagent (Invitrogen; Thermo Fisher Scientific, Inc.), according to the protocol. After incubation at 37°C for 24 h, the transfected cells were harvested for subsequent experiments.

Table II.

Sequences of si-TTN-AS1, miR-27b-3p inhibitor, miR-27b-3p mimics and negative controls.

Name	Sequence
si-TTN-AS1	5′-CCAGAGUGAGACACCUCUUTT-3′
si-ctrl	5′-UUCUCCGAACGUGUCACGUTT-3′
miR-27b-3p mimics	5′-CGCCUUGAAUCGCUGACACUU-3′
ctrl mimics	5′-AATTCTCCGAACGTGTCACGT-3′
miR-27b-3p inhibitor	5′-GATCCGAACTTAGCGACTGTGGC-3′
ctrl inhibitor	5′-TCAGTAGTCGGTGTCCTCGAGGA-3′
TTN-AS1, lncRNA titin-antisense RNA1.

RT-PCR

Trizol (Invitrogen; Thermo Fisher Scientific, Inc.) was used to extract total RNA from the cells or tissues following the manufacturer's instructions. SuperScript VILO cDNA Kit (Thermo Fisher Scientific, Inc.) was applied to reversely transcribe RNA into cDNA. qPCR thermocycling conditions were as follows: 95°C for 1 min and 45 cycles of 94°C for 15 sec, 55°C for 20 sec, and 72°C for 30 sec. SYBR Green qPCR Master Mix (MedChenExpress) was used to carry out the quantitative PCR with specific primers (Table III) according to the 2−ΔΔCq method (16).

Table III.

Primers of for RT-qPCR.

Gene	Primers
TTN-AS1
Forward	5′-CGATACCATTGAACACGCTGC-3′
Reverse	5′-GGTTGAGGGTCCCAGTG-3′
miR-27b-3p
Stem-loop	5′-GTCGTATCCAGTGCAGGGTCCGAGGT
RT primer	ATTCGCACTGGATACGACAAGTG-3′
Forward	5′-CGCCTTGAATCGGTG-3′
Reverse	5′-GTGCAGGGTCCGAGGT-3′
RUNX1
Forward	5′-AGGACTTGCACAAGCAGAAC-3′
Reverse	5′-GTTGGCGTACACGGGCGGCT-3′
GAPDH
Forward	5′-AGCCACATCGCTCAGACAC-3′
Reverse	5′-GCCCAATACGACCAAATCC-3′
U6
Forward	5′-GCTTCGGCAGCACATATACTAAAAT-3′
Reverse	5′-CGCTTCACGAATTTGCGTGTCAT-3′

Cell Counting Kit-8 (CCK-8) assay

A 96-well plate was taken to seed glioma cells (2×105 cells/well) (U251 and LN229). After incubation for 24, 48, 72 and 96 h at 37°C, CCK-8 reagent (Tiangen, Hangzhou, China) was added 10 µl/well and cells were incubated for 2 h at 37°C. The absorbance at 450 nm was read by a microplate reader (Thermo Fisher Scientific, Inc.).

Apoptosis assay

The Annexin V-FITC kit (Sigma-Aldrich; Merck KGaA) was used to estimate the apoptotic rate of the cells. In brief, cells (2×105 cells/well) plated in 6-well plates were treated with Binding Buffer containing Annexin-V-FITC and propidium iodide (PI). After incubation in the dark for 15 min, a FACSCalibur flow cytometer (BD Biosciences) was applied to detect the apoptotic state of the cells.

Wound healing assay

Migration was tested by a wound healing assay. Transfected cells were plated in 12-well dishes (5×104 cells/well), and incubated in RPMI-1640 medium (Hyclone; GE Healthcare) without FBS at 37°C, reaching a confluence of 80%. Then the cells were scratched across the surface of the well by a 10-µl pipette. After an incubation at 37°C of 24 h, the scratches were observed.

Transwell assay

Transfected cells (2.5×104 cells) were plated in the upper chamber of Transwell inserts (Corning, Inc.) which was coated with Matrigel (BD Biosciences). After an incubation of 24 h at 37°C, the cells invaded into the bottom chamber which was filled with medium containing 10% FBS. The invaded cells in the lower chamber were treated by methanol and 0.1% crystal violet at room temperature. A light microscope (X7, Nikon) was used to take photos of membrane. Five fields (magnification, ×100) of each sample were photographed randomly from each sample.

Dual-luciferase assay

Plasmids comprising the predicted binding sites identified by TargetScan software (http://www.targetscan.org/vert_71/). Plasmids containing 3′UTR with wild-type sites or mutant sites were purchased from Promega. Plasmids containing the sequences of miR-27b-3p were purchased from Sangon Biotech Co., Ltd. miR-27b-3p mimics were co-transfected with TTN-AS1 wt, TTN-AS1 mt, RUNX1 wt or RUNX1 mut using Lipo3000. After transfection, the cells were incubated at 37°C for 48 h. Dual-Luciferase Reporter Assay System (Promega) was used to measure the luciferase activity. Renilla activity was used as a normalization control.

RNA immunoprecipitation (RIP) assay

EZ-Magna RIP™ RNA-Binding Protein Immunoprecipitation Kit (Labbiotech) was used to carry out the RIP assay. Cells were incubated with RIP buffer containing beads coated with Ago2 antibodies or IgG antibodies (negative control) overnight. Immunoprecipitated complexes were collected for real-time PCR.

Western blot analysis

RIPA reagent buffer (Tiangen) was used to isolate the total protein from cells or tissues, followed by protein concentration determination with BCA protein assay kit (Beijing Solabio Life Sciences Co., Ltd.). SDS-PAGE (10%) was prepared and used to separate the different proteins (30 µg/lane). The separated protein blots were transferred onto PDFV membranes (Millipore). Silk milk (5%) was used to block the membranes at 37°C and then primary antibodies (anti-RUNX1; cat. Ab3692, dilution 1:500; Sigma-Aldrich; Merck KGaA; anti-GAPDH, dilution 1:2,000; KeyGen Biotech Co., Ltd.) were applied for an incubation of 12 h. Membranes were then treated with secondary antibodies (dilution 1:2,000; Keygen, Nanjing). Finally, protein signals were visualized by ECL detection kit (Beyotime Institute of Biotechnology).

Immunohistochemistry (IHC)

Xenograft tumor tissues were fixed with 10% formaldehyde and sectioned into 5-µm-thick slides. Primary antibody against RUNX1 (cat. no. 2883, dilution 1:200; Cell Signaling Technology, Inc.) was used for incubation at 4°C overnight. Thereafter the slides were incubated with HRP-conjugated streptavidin for 1 h at room temperature. DAB chromogen (Promega) was used for visualization. Images were captured by a microscope (X7, Olympus) at ×100 magnification.

Xenograft mouse model

A total of 10 female BALB/c nude mice (6–8 weeks, ~20 g) were purchased from the Shanghai Laboratory Animal Center (Shanghai, China). Mice were housed and maintained under specific pathogen-free conditions at ~20°C, with 20% humidity, a 12 h light:12 h dark cycle, and with commercial rat food and water ad libitum. Mice were divided into 2 groups randomly (5 mice/group) and injected with U251 cells (5×106) which were stably transfected with sh-TTN-AS1 or sh-NC. Tumor volumes were detected every week, according to the formula: V (mm3)=length (mm) × width2 (mm2). Five week later, the mice were anesthetized by intraperitoneal injection of 10% chloral hydrate (400 mg/kg), and then euthanized by cervical dislocation. After death confirmation by cardiac arrest and pupil enlargement, the tumors were removed for tumor weight detection and tumor tissues were collected for subsequent experimentation. The protocol involved in the animal experiments was authorized by The Second Affiliated Hospital of Zhengzhou University.

Statistics analysis

Data analysis was performed by GraphPad Prism 6 (GraphPad Software, Inc). Results were presented as mean ± standard deviation (SD). One-way ANOVA was used to compare differences among multiple groups followed by Bonferroni post hoc test. The paired Student's t- test was used for assessing the TTN-AS1 level in 45 paired glioma and control tissues. The unpaired Student's t-test was applied for two-group comparison of the other assays. Survival analysis was performed using the Kaplan-Meier method with the log-rank test. Correlations were calculated using Spearman's correlation coefficient.

Results

TTN-AS1 expression is elevated in glioma specimens and cell lines

In order to investigate the role of TTN-AS1 in glioma, real-time PCR analysis was used and revealed the significantly high level of TTN-AS1 in human glioma specimens in comparison with the normal tissues (Fig. 1A). High levels of TTN-AS1 were positively correlated with advanced tumor stage of the glioma patients (Fig. 1B and C). RT-qPCR detection of TTN-AS1 revealed significantly higher levels of TTN-AS1 in the glioma cell lines (U87, A172, LN229 and U251), compared with that in normal astrocytes (NHA) (Fig. 1D). Relative TTN-AS1 expressions in glioma specimens were determined and normalized by adjacent normal tissue samples. If the relative level of TTN-AS1was ≥1, it was defined as high expression. If not, it was defined as low expression. Kaplan-Meier analysis demonstrated the survival curve, showing the poorer overall survival of the patients with high TTN-AS1 expression (Fig. 1E).

Figure 1.

lncRNA TTN-AS1 is highly expressed in glioma specimens and cells. (A) TTN-AS1 expression in glioma specimens in comparison with normal controls as detected by qPCR. **P<0.01, as compared with normal tissues. (B) TTN-AS1 expression levels in different tumor stages of glioma patient tissues were assessed by qPCR. (C) TTN-AS1 expression levels in different tumor stages and corresponding nearby normal tissues were measured by qPCR. **P<0.01, as compared with normal tissue. (D) TTN-AS1 expression in glioma cell lines (U87, A172, LN229 and U251) and normal astrocytes (NHA) were estimated by qPCR. **P<0.01, compared with normal astrocytes (NHA). (E) Survival curves of glioma patients with high or low TTN-AS1 levels as analyzed by Kaplan-Meier method. TTN-AS1, long non-coding (lnc)RNA titin-antisense RNA1.

TTN-AS1 knockdown inhibits glioma cell proliferation, migration and invasion

To investigate whether TTN-AS1 participates in tumor genesis of glioma, loss of function assays were applied to assess the cell behavior upon TTN-AS1 depletion. Fig. 2A shows the significant transfection efficiency. CCK-8 assay revealed that TTN-AS1 silencing reduced the proliferation of U251 and LN229 cells (Fig. 2B). Flow cytometry experiments demonstrated that the percentage of apoptotic cells was significantly elevated by TTN-AS1 downregulation (Fig. 2C). Wound healing and Transwell assays demonstrated the significantly decreased abilities of migration (Fig. 2D) and invasion (Fig. 2E) of the U251 and LN229 cell lines following silencing of TTN-AS1. These results led us to propose the oncogenic role of TTN-AS1 in glioma.

Figure 2.

TTN-AS1 accelerates glioma cell viability, migration and invasion, as well as inhibits apoptosis in vitro. (A) Glioma U251 and LN229 cells were transfected with TTN-AS1 knockdown oligonucleotides (si-TTN-AS1-1 and si-TTN-AS1-2) or control (sh-ctrl) and the relative TTN-AS1 level was assessed. (B) CCK-8 assays displayed the viability of glioma cells following silencing. (C) Flow cytometry assays demonstrated the apoptotic state of the glioma cells following silencing of TTN-AS1. (D) Wound healing assays presented the migration ability of the glioma cells following silencing of TTN-AS1. (E) Transwell assays presented the invasion ability of the glioma cells following silencing of TTN-AS1. **P<0.01, compared with the si-ctrl group. TTN-AS1, long non-coding (lnc)RNA titin-antisense RNA1.

miR-27b-3p is sponged by TTN-AS1

Research into the relevant mechanism was further performed. As lncRNAs function by acting as ceRNAs, we first explored the potential target miR-27b-3p by Starbase 3.0 (http://starbase.sysu.edu.cn/) (Fig. 3A) (17). Luciferase reporter activity confirmed the interaction between TTN-AS1 and miR-27b-3p (Fig. 3B). RT-qPCR was performed following si-TTN-AS1 or miR-27b-3p mimic transfection, showing that TTN-AS1 depletion significantly enhanced miR-27b-3p expression when compared to the si-ctrl group (Fig. 3C), whereas miR-27b-3p overexpression significantly reduced TTN-AS1 expression compared with the ctrl mimic group (Fig. 3D). RIP assay revealed that TTN-AS1 and miR-27b-3p were immunoprecipitated in the Ago2 complex (Fig. 3E). In the glioma tissues, miR-27b-3p was downregulated (Fig. 3F), and also had a negative correlation with TTN-AS1 (Fig. 3G).

Figure 3.

TTN-AS1 acts as ceRNA for miR-27b.3p. (A) Schematic diagram of the interacting sites. (B) Wild type (wt)/mutant (mut) of TTN-AS1-AS1 was co-transfected with miR-27b-3p, then luciferase reporter assay was performed. **P<0.01, compared with the ctrl mimic group. (C) miR-27b-3p expression was assessed following si-TTN-AS1 transfection in the U251 and LN229 cells. **P<0.01, compared with the si-ctrl group. (D) TTN-AS1 expression was assessed following miR-27b-3p overexpression. **P<0.01, compared with the ctrl mimic group. (E) RIP experiment followed by qPCR to determine the immunoprecipitation of TTN-AS1 and miR-27b-3p. **P<0.01, compared with IgG. (F) Expression profile of miR-27b-3p in glioma specimens and controls analyzed by qPCR. (G) Correlation between TTN-AS1 and miR-27b-3p estimated by Pearson's analysis. TTN-AS1, long non-coding (lnc)RNA titin-antisense RNA1; ceRNA, competitive endogenous RNA.

miR-27b-3p inhibitor reverses the effects of TTN-AS1 silencing on glioma cells

To ascertain whether miR-27b-3p is involved in the TTN-AS1-regulated glioma cell proliferation, miR-27b-3p inhibitor or control (ctrl inhibitor) was co-transfected with si-TTN-AS1. Fig. 4A shows the transfection efficiency (Fig. 4A) in the U251 and LN229 cell lines. miR-27b-3p inhibitor reversed the reduced viability (Fig. 4B) and increased apoptosis (Fig. 4C) induced by si-TTN-AS1. In addition, the reduced abilities of migration (Fig. 4D) and invasion (Fig. 4E) were reversed by the miR-27b-3p inhibitor. The quantified data are presented in Fig. 4F.

Figure 4.

miR-27b-3p reverses the TTN-AS1 inhibited effects on tumor progression of glioma cells. (A) Glioma cells were transfected with miR-27b-3p knockdown oligonucleotides (miR-27b-3p inhibitor) or control (ctrl inhibitor). Then, miR-27b-3p inhibitor or control was co-transfected with sh-TTN-AS1. **P<0.01, compared with the ctrl inhibitor. (B) CCK-8 assays showing the viability of glioma cells. **P<0.01 vs. si-ctrl+ctrl inhibitor, ##P<0.01 vs. si-TTN-AS1+ ctrl inhibitor. (C) Flow cytometry assays demonstrated the apoptosis state of glioma cells. (D) Wound healing assays presented the migration ability of glioma cells. (E) Transwell assays presented the invasion ability of the glioma cells. (F) Data are presented in bar graphs. **P<0.01 vs. si-ctrl+ctrl inhibitor, ##P<0.01 vs. si-TTN-AS1+ ctrl inhibitor. TTN-AS1, long non-coding (lnc)RNA titin-antisense RNA1.

RUNX1 acts as a downstream target of miR-27b-3p

As miRNAs are well known to exert functions by targeting downstream mRNAs, we investigated the potential target RUNX1 of miR-27b-3p by TargetScan (Fig. 5A). Luciferase reporter assay verified the potential interacting sites (Fig. 5B). Moreover, following transfection of the miR-27b-3p mimic in the U251 and LN229 cell lines, miR-27b-3p overexpression significantly suppressed the expression of RUNX1 both at the mRNA (Fig. 5C) and protein (Fig. 5D) levels. In the glioma tissues, RUNX1 was upregulated when compared with that in the normal tissues (Fig. 5E), which also had a negative correlation with miR-27b-3p (Fig. 5F).

Figure 5.

(A) Schematic diagram of the interacting sites. (B) Wild-type (wt)/mutant (mut) of RUNX1 was co-transfected with miR-27b-3p, and then luciferase reporter assay was performed. **P<0.01, compared with ctrl mimics. (C and D) RUNX1 expression was assessed following miR-27b-3p overexpression in U251 and LN229 cells by qPCR (C) and western blot analysis (D). **P<0.01, compared with ctrl mimics. (E) Expression profiles of RUNX1 in glioma specimens and controls as analyzed by qPCR. **P<0.01, compared with normal tissue. (F) Correlation between RUNX1 and miR-27b-3p estimated by Pearson's analysis. RUNX1, runt-related transcription factor 1.

TTN-AS1 sponges miR-27b-3p to upregulate RUNX1

Thereafter, we observed that silencing of TTN-AS1 could inhibit RUNX1 expression both at the mRNA and protein levels, however these alterations were attenuated by miR-27b-3p inhibitor (Fig. 6A-C). Moreover, in glioma tissues, RUNX1 had a positive correlation with TTN-AS1 (Fig. 6D).

Figure 6.

TTN-AS1 absorbs miR-27b-3p to upregulated RUNX1. miR-27b-3p inhibitor or control was co-transfected with si-TTN-AS1 in the U251 and LN229 cell lines and (A) qPCR was applied to assess the mRNA level of RUNX1, and (B and C) western blot analysis was applied to assess the protein level of RUNX1. **P<0.01 vs. si-ctrl+ctrl inhibitor, ##P<0.01 vs. si-TTN-AS1+ ctrl inhibitor. (D) Correlation between RUNX1 and TTN-AS1 estimated by Pearson's analysis. TTN-AS1, long non-coding (lnc)RNA titin-antisense RNA1; RUNX1, runt-related transcription factor 1.

TTN-AS1 enhances glioma growth in vivo

Nude mice were injected with U251 cells which were stably transfected with sh-TTN-AS1 or sh-ctrl. Thereafter, tumor volumes and weight were analyzed, showing that the tumors of mice with TTN-AS1 knockdown were much smaller and lighter than those derived from the sh-ctrl transfected cells (Fig. 7A and B). Five weeks later, tissues were collected from the sacrificed mice, and miR-27b-3p and RUNX1 expression was estimated. As shown in Fig. 7C, miR-27b-3p was upregulated in the TTN-AS1-silenced cell derived mouse tumor tissues, compared with the sh-ctrl-transfected cell derived mouse tumors. On the contrary, RUNX1 expression was detected by western blot analysis (Fig. 7D) and ICH (Fig. 7E), demonstrating that RUNX1 was decreased in the glioma tissue derived from the TTN-AS1-knockdown cells than that from the sh-ctrl-derived tumor tissue.

Figure 7.

TTN-AS1 accelerates glioma growth in vivo. Nude mice were injected with U251 cells stably transfected with sh-TTN-AS1 or sh-ctrl. (A) Every week, tumor volume was recorded. **P<0.01, compared with the sh-ctrl group. (B) Five weeks later, mice were sacrificed and tumor weights were detected. **P<0.01, compared with the sh-ctrl group. (C) miR-27b-3p expression in tumor tissues was analyzed. **P<0.01, compared with the sh-ctrl group. (D and E) RUNX1 expression in tumor tissues was detected by western blot analysis (D) and immunohistochemistry (IHC) (E). **P<0.01, compared with the sh-ctrl group. TTN-AS1, long non-coding (lnc)RNA titin-antisense RNA1; RUNX1, runt-related transcription factor 1.

Discussion

Increasing evidence has confirmed the biological functions that long non-coding (lnc)RNAs exert in the malignant progression of glioma (18). Numerous lncRNAs are aberrantly expresed in glioma, and affect glioma growth and metastasis (5). In the present study, we focused on lncRNA titin-antisense RNA1 (TTN-AS1). TTN-AS1, located on chromosome 2, was first identified as an oncogene in esophageal cancer (19). In esophageal cancer, TTN-AS1 was expressed higher and upregulated FSCN1 via sequestration of miR-133b, thereby aggravating tumor progression (19). In addition, TTN-AS1 was found to accelerate papillary thyroid cancer cell proliferation by impairing the PTEN/PI3K/AKT pathway (20). Upregulation of TTN-AS1 promoted the malignant progression of osteosarcoma via absorbing miR-376a and modulating dickkopf-1 (21). In the present research, we displayed that TTN-AS1 was overexpressed in human glioma specimens and cells, and was associated with the poor overall survival of glioma patients. In addition, functional experiments were performed using sh-TTN-AS1 transfection, showing that TTN-AS1 silencing obstructed the proliferation, migration and invasion of glioma cells. These findings were consistent with the oncogenic roles of TTN-AS1 reported previously.

The relative mechanisms were further explored. As the lncRNA-miRNA-mRNA axis is the most common mechanism involved in lncRNA regulation, in the present study, we identified the potential target miR-27b-3p using bioinformatic methods. miR-27b-3p was confirmed as a tumor suppressor in several cancer types. miR-27b-3p is important for doxorubicin resistance in anaplastic thyroid cancer by modulating PPARγ expression (22). In endometrial cancer, miR-27b-3p targeted MARCH7 through the Snail pathway (23). miR-27b-3p was found to impair CBLB/GRB2 expression, thus obstructing breast cancer development (24). Moreover, miR-27b-3p was found to exert tumor inhibitory effects in oral cancer (25), hepatocellular carcinoma (26) and colorectal cancer (27). However, the effects of miR-27b-3p on glioma remain obscure. The present study showed that TTN-AS1 directly interacted with miR-27b-3p in the Ago2 complex, and presented a negative correlation in glioma tissues. Additionally, the inhibitory effects induced by TTN-AS1 silencing on glioma cells were reversed partially by miR-27b-3p knockdown. These results are not only in line with the tumor-suppressive role of miR-27b-3p as previously reported, but also revealed that miR-27b-3p participated in glioma development by ceRNA regulation mode.

Thereafter, we sought the potential downstream target of miR-27b-3p. With the help of bioinformatic tools, we focused on RUNX family transcription factor 1 (RUNX1). RUNX1 has been reported to be involved in cancer progression (28). For example, RUNX1 was inhibited by miR-106a-5p, thereby enhancing osteosarcoma tumorigenesis (29). RUNX1 was observed to be increased in renal cell carcinoma and was associated with poor prognosis (30). Keita et al identified RUNX1 as a tumor promotor in ovarian cancer and skin cancer (31). Zhou et al showed that overexpression of RUNX1 elevated epithelial-to-mesenchymal transition in renal carcinoma (32). Similar roles of RUNX1 were observed in endometrial cancer (33) and epithelial cancer (34). Nevertheless, whether RUNX1 is involved in the progression of glioma remains unclear. Herein, we displayed the overexpression of RUNX1 in glioma tissues and found that RUNX1 was positively correlated with TTN-AS1. This finding was in line with the oncogenic role reported previously in a series of cancer types. More importantly, the regulation between TTN-AS1 and RUNX1 was mediated by miR-27b-3p. In another word, we found that TTN-AS1 upregulated RUNX1 via sponging miR-27b-3p, which may be the mechanism of TTN-AS1-regulated glioma development.

In conclusion, we identified the oncogenic role of TTN-AS1 in the malignant progression of glioma in vivo and in vitro, and revealed the mechanism via regulation of the miR-27b-3p/RUNX1 axis. These findings may contribute to the elucidation of glioma pathogenesis and novel clinical treatment strategies.