Background
Gastric cancer is the most common form of gastrointestinal cancer and a leading cause of cancer-related mortality worldwide [
1]. Despite a marked decrease incidence, especially in mortality rate in many countries, the absolute number of gastric cancer cases and deaths is still a big burden of the local health program [
2]. Although some advancements in diagnostic and therapeutic including surgery, chemotherapy, and radiotherapy, due to the risk of relapse, distant metastasis, and chemoresistance, the 5-year survival rate in GC patients remains larger unsatisfied [
3]. Thus, to explore more molecular biomarkers for predicting responsiveness of treatment, tumor progression, and potential target therapies are becoming more urgent.
Long-non-coding RNA (LncRNAs), which are >200 nt in length, play important roles in diverse biological processes, including cell proliferation, cell apoptosis, cell differentiation, cell invasion, and metastasis by regulating gene expression at the epigenetic, transcriptional, and posttranscriptional levels [
4]. Alterations in long non-coding RNAs (lncRNAs) are associated with human carcinogenesis including gastric cancer. For instance, long non-coding RNA PVT1 indicated a poor prognosis of gastric cancer and promoted cell proliferation through epigenetically regulating p15 and p16 [
5]. Plasma level of H19 was significantly higher in GC patients and could serve as a potential biomarker for diagnosis of GC, in particular for early tumor screening [
6]. MEG3 was decreased in GC patients and MEG3 could up-regulated Bcl-2 via its competing endogenous RNA (ceRNA) activity on miR-181a [
7]. Long non-coding RNA HOXA-AS2 promoted gastric cancer proliferation by epigenetically silencing P21/PLK3/DDIT3 expression [
8]. These studies suggest an important role for MALAT1 in tumor biology.
Increasing evidences have shown that the aberrant expression of MALAT1 played a crucial role in carcinogenesis [
9]. Such as, previous studies suggested that long non-coding RNA MALAT1 promoted tumor growth and metastasis by inducing epithelial-mesenchymal transition in oral squamous cell carcinoma [
10]. MALAT1 increased AKAP-9 expression by promoting SRPK1-catalyzed SRSF1 phosphorylation in colorectal cancer cells [
11]. Wang et al. reported that MALAT1 promoted malignant development of esophageal squamous cell carcinoma by targeting β-catenin via Ezh2 [
12]. In GC, Metastasis-associated long non-coding RNA drove gastric cancer development and promoted peritoneal metastasis and as a novel therapeutic target in patients with gastric neoplasia [
13]. Qi and his assistants also confirmed that MALAT1 bound EZH2, suppressed the tumor suppressor PCDH10, and promotes gastric cellular migration and invasion [
14]. However, the biological role of MALAT1 in GC and the underlying molecular mechanism still remain undefined.
In the study, our results demonstrated MALAT1 was up-regulation in GC tissues and patients with higher MALAT1 expression had a shorter survival and poor prognosis. Further experiments demonstrated that knockdown of MALAT1 inhibited the GC cells proliferation and invasion. Mechanistically, we found that MALAT1 functioned as a molecular sponge for miR-1297, antagonizing its ability to inhibit HMGB2 protein expression. To sum up, our findings revealed that MALAT1 might function as a competing endogenous RNA (ceRNA) sponge for miR-1297 modulating HMGB2 expression. The MALAT1/miR-1297/HMGB2 regulator pathway provided a novel therapeutic method for gastric cancer.
Methods
Patients and tissue samples
Human GC tissues and adjacent non-cancerous tissues were collected from 78 GC patients who underwent surgical resection and obtained from Hebei Cangzhou Central Hospital. The diagnosis of all GC patients was histopathologically confirmed by two professional pathologists. None of the patients had received radiotherapy or chemotherapy before surgery. Written informed consent was obtained from all patients or their families for their related tissues were used. The approval for this study was obtained from Research Ethics Board at Hebei Cangzhou Central Hospital.
Cell culture
The human GC cell lines MKN45, MKN28, BGC-823, and SGC-7901 and an immortalized normal gastric epithelial cell line GES-1 were purchased from the Cell Bank of the Chinese Academy of science. All cells were cultured in RPMI 1640 medium (GIBCO, Carlsbad, CA) with 10% FBS and 5% CO2 at 37 °C.
QRT-PCR analysis
Total RNA from tissues and GC cells was extracted using Trizol reagent (Invitrogen, Carlsbad, USA). The One Step SYBR® Prime Script® PLUS RT-RNA PCR Kit (TaKaRa, Dalian, China) was used for the Real-Time PCR analysis. All experiments were repeated three times. GAPDH or U6 was used as the endogenous control. The relative fold changes in the transcripts were calculated using the 2−ΔΔCt method. The QRT-PCR was performed at 95 °C for 15 s, followed by 40 cycles of 95 °C for 5 s and 60 °C for 34 s at ABI7500 real-time PCR detection system. The primer sequences used were as follows: for GAPDH-forward, 5′-GTCAACGGATTTGGTCTGTATT-3′andGAPDH-reverse, 5′-AGTCTTCTGGGTGGCAGTGAT-3′; MALAT1-forward5′-ATGCGAGTTGTTCTCCGTCT-3′and MALAT1-reverse, 5′-TATCTGCGGTTTCCTCAAGC-3′.
Cell transfection assays
Cell transfections were performed using the Lipofectamine 3000 kit (Invitrogen) according to the manufacturer’s instructions. To knockdown MALAT1, the two siRNA oligonucleotides targeting MALAT1 and negative control were purchased from GenePharma (Shanghai, China) and were transfected into GC cells. The sequences of the targeting MALAT1were si-RNA1 sense-1,5′-CACAGGGAAAGCGAGTGGTTGGTAA-3′antisense-1,5′-TTACCAACCACTCGCTTTCCCTGTG-3′; si-RNA-2, sense-2, 5′-GAGGUGUAAAGGGAUUUAUTT-3′; antisense-2, 5′-AUAAAUCCCUUUACACCUCTT-3′). The working concentration of relative plasimids was 100 nM. MiR-1297 plasmid was transfected into GC cells using Lipofectamine TM 3000 (Invitrogen, Carlsbad, CA, USA).
Cell proliferation and invasion assays
Cell proliferation was measured using the Cell Counting Kit-8 (CCK-8). Briefly, 3000 MKN45 or MKN28 cells/well were seeded into a 96-well plate. At daily intervals of 0, 24, 48, and 72 h, the optical density was measured at 450 nm using a microtiter plate reader (Quant BioTek Instruments). The cell invasion assays was performed according to the previously described using Transwell Chamber Cell Culture (8-μm pore membrane, BD Biosciences) [
15]. The 1 × 10
5 cells GC cells were added to the top chamber of 24-well plates with serum-free medium. The bottom chamber was added medium with 10% FBS. Cells in chamber were fixed with methanol for 20 min and then staining with Crystal violet for 10 min. Invaded cells were finally observed under a microscope and the number was counted. The results represent the average of three replicates.
Western blotting assays
Cells were lysed in RIPA buffer containing fresh protease and phosphatase inhibitor cocktails (Sigma). Protein was separated by 10% SDS–PAGE and was transferred to PVDF membranes. The membranes were blocked in 5% skim milk and incubated with anti-HMGB2 antibody (1:1000 dilution, CST, USA), anti-PCNA antibody (1:3000 dilution, Abcam, CST, USA), anti-Ki67 antibody (1:1000 dilution, CST, USA) and anti-GAPDH antibody (1:2000 dilution, CST, USA) and incubated overnight at 4 °C. GAPDH served as an endogenous control and incubated overnight at 4 °C. Then, Horseradish peroxidase (HRP)-conjugated secondary antibody (1:1000 dilution, Abcam, USA) was incubated at room temperature for 2 h. Blots were developed using enhanced chemiluminescence detection reagents and scanned with a Molecular Imager system (Bio-Rad). Three repeated experiments were performed.
Luciferase reporter assay
PMIRGLO-MALAT1 wild type (WT) or pMIRGLO-MALAT1 mutant type (MUT) reporter plasmid was co-transfected with miR-1297 mimic or NC mimic into MKN45 cells. Reporter plasmid and the internal control renilla luciferase plasmid were carried out with the appropriate plasmids using lipofectamine 3000 (Invitrogen, USA). The relative luciferase activity was normalized to renilla luciferase activity after transfection at 48 h, and luciferase activity was measured using a dual-luciferase reporter gene assay system (Promega, Madison, USA). Moreover, full-length MALAT1 by PCR was constructed and were transfected with a luciferase reporter vector containing perfect miR-1297 binding site (Sensor) wild type (WT) or mutant type (MUT) into theBCG-823 cells that expressed lower MALAT1 levels.
Statistical analyses
All statistical analysis was performed using SPSS 16.0 software (version 16.0; Chicago, IL). Data are presented as mean ± standard deviation. Student’s t tests were used to evaluate significant differences between any two groups of data. One-way analysis of variance (ANOVA) was performed to analyze the differences between groups. Kaplan–Meier method and compared by log-rank test were used to construct survival curves. Pearson correlation analysis was used to detect the significance of association between MALAT1 and miR-1297 expression. The P < 0.05 was considered statistically significant.
Discussion
Recent research has revealed that MALAT1 play pivotal roles in tumorigenesis of cancers including gastric cancer [
16,
18,
19]. MALAT1 was aberrantly highly expressed in GC cell lines and promoted cell proliferation in gastric cancer by recruiting SF2/ASF [
20]. Plasma MALAT1 levels were significantly higher in gastric cancer patients with distant metastasis than patients without distant metastasis and the healthy controls [
21]. In the study, we found the expression level of MALAT1 was significantly higher in gastric cancer tissues compared with adjacent normal tissues. Higher expression of MALAT1 was positively associated with local invasion, lymph node invasion and TNM stage. In addition, overexpression of MALAT1 was associated with short survival time. These results implicated that MALAT1 may play an important role in gastric cancer progression.
Previous reports have revealed that competing endogenous RNA (ceRNA) regulator mechanisms involved in MALAT1. For example, knockdown of long non-coding RNA MALAT1 increases the blood-tumor barrier permeability by up-regulating miR-140 [
22]. MALAT1 induced migration and invasion of human breast cancer cells by competitively binding miR-1 with cdc42 [
23]. Wang et al. reported that long non-coding RNA MALAT1 promoted gallbladder cancer cell proliferation and invasion targeting ANXA2 and KRAS by acting as a molecular sponge to regulate miR-206 [
24]. Another study in lung adenocarcinoma demonstrated that lncRNA MALAT1 exerted oncogenic functions by targeting miR-204 [
25]. It also been verified MALAT1 functioned as a competing endogenous RNA to regulate ZEB2 expression by sponging miR-200s in clear cell kidney carcinoma and promoted cell invasion and metastasis [
26]. According to luciferase reporter assays results, co-transfection of miR-1297 and PMIRGLO- MALAT1-WT significantly decreased the luciferase activity. In vitro studies further confirmed that MALAT1 negatively regulated miR-1297 expression levels, which promoted cell proliferation and cell invasion. Moreover, miR-1297 was up-regulated after MALAT1 silencing, notably, ectopic expression of miR-1297 increased the MALAT1 expression levels. These findings demonstrated that there exist a negative regulation between MALAT1 and miR-1297.
Overexpression of high-mobility group box 2 was associated with tumor aggressiveness and prognosis of hepatocellular carcinoma [
27]. Knockdown of HMGB2 decreased the chemoresistance of gastric cancer cells [
28]. Our results showed that cell proliferation abilities were inhibited and PCNA and Ki-67 expression were reduced after HMGB2 silencing in MKN45 cell. Previous study showed that miR-1297 inducing HCC cell apoptosis and inhibited cell the proliferation and invasion by targeting HMGB2 [
17]. In our study, knockdown of MALAT1 was able to reduce the mRNA and protein levels of HMGA2 and miR-1297 inhibitor could enhanced the HMGB2 expression. What’s more, knockdown of MALAT1 could reduce the mRNA and protein levels of HMGA2 which was dismissed by miR-1297 inhibitor. These data revealed that MALAT1 exerts its biological effects at least in part by modulating HMGA2 expression.
Conclusions
In conclusion, we demonstrated that MALAT1 was significantly overexpressed in gastric cancer tissues and cell lines and revealed a MALAT1/miR-1297/HMGB2 regulator pathway in GC. Thus, our results indicated that intervention of MALAT1/miR-1297/HMGB2 regulator pathway could inhibit gastric cancer progression.
Authors’ contributions
JL and JG designed this study. WT and YL carried out the study and drafted the manuscript. JZ and WT carried out the statistical analysis. JL and JG assisted with the manuscript preparation and data analysis. All authors read and approved the final manuscript.