microRNA-761 regulates glycogen synthase kinase 3β expression and promotes the proliferation and cell cycle of human gastric cancer cells

It has been well documented that aberrant expression of microRNAs (miRs) serves important roles in cancer progression. The present study investigated the roles of miR-761 on gastric cancer (GC) cell proliferation. Reverse transcription polymerase chain reaction indicated that miR-761 was frequently upregulated in GC tissues and cells. Overexpression of miR-761 promoted the cell proliferation, cell colony formation and cell cycle of GC cells. Bioinformatics analysis revealed that miR-761 might target the 3′-untranslated region of glycogen synthase kinase 3β, and was confirmed by luciferase reporter assay and western blot analysis. Taken together, the results of the present study revealed miR-761 as a tumor promoter in GC, and that it could be considered as a novel therapeutic target for patients with GC.


Introduction
Gastric cancer (GC) is the fourth most common lethal neoplasm and the second most common cause of cancer-associated mortality worldwide (1). The primary cause of GC with a poor prognosis is the GC being diagnosed at an advanced stage (2). In order to improve the outcome of GC, it is an urgent requirement to identify genetic events regulating GC cell proliferation.
An increasing number of studies have indicated that microRNAs (miRNAs/miRs) are a class of small, short non-coding RNAs (18-24 nucleotides) that have emerged as important post-transcriptional regulators by targeting the 3'-untranslated region (3'-UTR) of the mRNA of their target genes (3)(4)(5). It was reported that certain miRNAs act as oncogenes or tumor suppressors. miR-761 was reported to promote the progression and metastasis of non-small cell lung cancer by targeting ING4 and TIMP2 (6). miR-761 was revealed to be upregulated and to regulate tumorigenesis in hepatocellular carcinoma by targeting Mitofusin-2 (7). The results of a study undertaken by Shi et al (8) indicated that miR-761 acted as a tumor suppressor that inhibited tumor progression by targeting MSI1 in ovarian carcinoma (8). However, the molecular mechanisms underlying miR-761 in GC remains largely unknown. The results of the present study demonstrated that miR-761 promoted GC cell proliferation via targeting the 3'-UTR of GSK3β. The results provided novel insight into the mechanisms of GC tumor development mediated by miR-761.  RNA extraction and reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Total RNA was extracted from clinical tissues and cells using TRIzol reagent (Invitrogen; Thermo Fisher Scientific, Inc.) according to the manufacturer's protocols. The miRNA Q-PCR Detection kit (GeneCopoeia, Inc.) was used for quantification of miRNA levels according to the manufacturer's protocols. U6 was used as an internal control. The 2 -ΔΔCq method was used to quantify relative RNA expression. All procedures were performed in triplicate (9).

Materials and methods
MTT assays and colony formation. Cell proliferation assays were conducted using MTT assays, SGC-7901 cells (3x10 3 cells/well) were seeded onto 96-well plates with 100 µl DMEM supplemented with 10% FBS. Following incubation of cells for 1, 2, 3, 4, 5 and 6 days, 20 µl 5 mg/ml MTT solution (Sigma-Aldrich; Merck KGaA) was added each well and incubated for 4 h, and then medium was removed and 150 µl DMSO (Sigma-Aldrich; Merck KGaA) was added. Next, the absorbance of each well was measured using a microplate reader set at 490 nm. For the colony formation assay, transfected SGC-7901 cells (1x10 3 cells/well) were added to each well of a 6-well plate and incubated for ~2 weeks until the colony was clearly formed. Next, the cells were fixed with 4% methanol at room temperature for 30 min and stained with 0.5% crystal violet for 10 min at room temperature. Visible colonies were manually counted.
Cell cycle assays by flow cytometry. For analysis of the cell cycle, SGC-7901 cells were harvested after 48 h transfection, prior to being washed with PBS and then fixed in ice-cold 70% ethanol at 4˚C overnight. The next day, the cell were incubated with RNase A at 37˚C for 30 min, and then stained with propidium iodide (PI; Sigma-Aldrich; Merck KGaA) at 4˚C for 30 min in the dark, prior to the cells being analyzed by a flow cytometer using the CellQuest Pro software version 5.1 (BD Biosciences, Franklin Lakes, NJ, USA).
Western blotting. Cells were harvested and lysed with radioimmunoprecipitation assay buffer, and the protein concentration was measured using a bicinchoninic acid assay kit (cat. no. #23227; Thermo Fisher Scientific, Inc.). Equal amounts proteins (50 µg) were separated by 10% SDS-PAGE gels and transferred onto polyvinylidene dif luoride membranes. The membranes were blocked with 5% skimmed milk at room temperature for 1 h and then probed with 1:1,000 diluted anti-GSK3β (cat. no. ab32391), anti-CyclinD1 (cat. no. ab134175) and anti-P27 (cat. no. ab32034) primary antibodies (Abcam, Cambridge, MA, USA) at 4˚C overnight. The membrane was subsequently probed with a horseradish peroxidase-conjugated secondary antibody rabbit anti-mouse IgG (cat. no. P0023D; 1:5,000; Beyotime Institute of Biotechnology, Haimen, China) for 2 h at room temperature. β-actin (dilution 1:5,000; cat. no. ab8227; Abcam) was used as the internal control and was incubated at 4˚C overnight. Signals were visualized using enhanced chemiluminescent substrates (EMD Millipore, Billerica, MA, USA), according to the manufacturer's protocols, and was analyzed using the Quantity One software version 4.6 (Bio-Rad Laboratories, Inc., Hercules, CA, USA).
Statistical analysis. All data are presented as the mean ± standard deviation and all statistical analyses were performed using SPSS 18.0 software (SPSS, Inc., Chicago, IL, USA). All experiments were repeated at least three times independently. Student's t-test was used for pair-wise comparisons and one-way analysis of variance, followed by Tukey's post hoc test was used for multiple comparisons. P<0.05 was considered to indicate a statistically significant difference.

miR-761 was upregulated in human GC tissues and GC cell lines.
Initially, it was revealed that the expression levels of miR-761 were significantly upregulated in GC tissues compared with those in normal gastric mucosal tissues (P<0.05; Fig. 1A). Similarly, significantly higher levels of miR-761 were detected in the human GC SGC-7901, MGC-803, MKN-45 and AGS cell lines compared with those in the human gastric epithelial cells (HGEC; P<0.05; Fig. 1B). Taken together, these results indicated that miR-761 is upregulated in GC primary tumors and cell lines.

miR-761 promoted, while miR-761-in suppressed GC cell proliferation.
To further characterize the functional importance of miR-761 in GC progression, its effect on the proliferation of GC cells was assessed using MTT and colony formation assays. The results of the MTT and colony formation assays indicated that overexpression of miR-761 in SGC-7901 cells significantly promoted cell proliferation and colony formation (P<0.05; Fig. 2A and B). As proliferation is directly associated with the cell cycle, the effect of miR-761 on cell cycle progression was detected in SGC-7901 cells. Compared with the SGC-7901 cells transfected with miR-NC, miR-761 decreased the percentage of G1/G0 phase cells, and increased the percentage of S phase cells (Fig. 2C). However, miR-761-in significantly suppressed the proliferation of GC cells using MTT and colony formation assays (P<0.05; Fig. 3A and B). In addition, flow cytometry revealed that miR-761-in increased the percentage of SGC-7901 cells in the G1/G0 phase, and decreased the percentage of SGC-7901 cells in the S phase (Fig. 3C). Taken together, these results suggested that miR-761 promoted GC growth by regulating the cell cycle.

miR-761 directly targets GSK3β by binding to its 3'-UTR and altering levels of proteins associated with cell proliferation and the cell cycle in GC cells.
To investigate the molecular mechanisms by which miR-761 promoted GC cell proliferation, GSK3β was predicted as a potential target of miR-761 using TargetScan (http://www.targetscan.org/) (Fig. 4A). To confirm potential miR-761 binding sites in the 3'-UTR of GSK3β, luciferase assays were used. As demonstrated in Fig. 4B, the results indicated that compared with the negative control, miR-761 attenuated the luciferase activity of the wild-type GSK3β-3'-UTR, but had no effect on the luciferase activity of the GSK3β-3'-UTR-mut. Furthermore, miR-761-in increased wild-type 3'-UTR GSK3β luciferase activity (Fig. 4C). Additionally, western blot analysis indicated that GSK3β expression was significantly downregulated in SGC-7901 cells following transfection with miR-761 and upregulated in miR-761-in-transfected SGC-7901 cells (P<0.05; Fig. 4C). Cell proliferation and cell cycle regulatory    genes, including Cyclin D1 and P27, were also detected by western blot analysis. The results revealed that Cyclin D1 expression was increased, while P27 expression was decreased in miR-761-overexpressing SGC-7901 cells, while miR-761-in exhibited the opposite effect (Fig. 4B).

Discussion
Numerous studies have demonstrated that miRNAs may act as activators or inhibitors of tumor proliferation (10)(11)(12)(13). An increasing number of studies have revealed that aberrant expression of miRNAs serves essential roles in the carcinogenesis of cancer (14)(15)(16). miR-152 was reported to inhibit gastric cancer cell proliferation and motility by targeting CD151 (17). miR-935 was revealed to promote gastric cancer cell proliferation by regulating SOX7 (18). The results of a study undertaken by Gu et al (19) indicted that miRNA-183 suppressed apoptosis and promoted proliferation and invasion of gastric cancer cells by targeting PDCD4. miR-761 was a newly discovered oncogenic miRNA, which was upregulated and regulates tumorigenesis in hepatocellular carcinoma (7). However, the detailed biological function of miR-761 in GC remains largely unclear.
The results of the present study demonstrated that miR-761 expression levels in GC tissues and cells were significantly upregulated compared with those in normal gastric mucosal tissues and human gastric epithelial cells. Furthermore, ectopic miR-761 expression led to a lower percentage of cells in the G1/G0 phase and a higher percentage of cells in the S phase, and subsequently promoted GC cell proliferation and increased GC cell colony formation, while miR-761-in exhibited the opposite effect. Further experimentation was used to investigate the molecular mechanism of miR-761 in promoting cell proliferation and the cell cycle in GC. Bioinformatics analysis indicated that GSK3β, a crucial modulator of the Wnt/β-catenin signaling pathway (20), was the direct target of miR-761. miR-224 was reported to sustain Wnt/β-catenin signaling and to promote the aggressive phenotype of colorectal cancer by suppressing the expression of GSK3β/SFRP2 (21). Deng et al (22) indicated that miR-519 promoted the progression of colorectal cancer through regulating Orai1 via regulating the Akt/GSK3β signaling pathway. In the present study, the results of the luciferase reporter assay and western blot analysis confirmed that miR-761 suppressed GSK3β expression by targeting the 3'-UTR of GSK3β. Additionally, the cell proliferation and cell cycle regulatory genes, including Cyclin D1 and P27, were detected by western blot analysis. Cyclin D1 expression was increased, while P27 expression was decreased by miR-761 in SGC-7901 GC cells. Taken together, the results of the present study revealed that miR-761 functionally suppressed GSK3β expression, and subsequently regulated cellular proliferation and cell cycle regulators, Cyclin D1 and P27, which suggests that miR-761 is associated with cell proliferation and the cell cycle in GC.
In conclusion, miR-761 was upregulated and functioned as an oncogene in GC. Furthermore, it was revealed that miR-761 promoted cell proliferation and the cell cycle by targeting GSK3β. The present study therefore demonstrated that miR-761 could be considered as a novel therapeutic target for patients with GC.