Deregulation of MicroRNA-375 Inhibits Proliferation and Migration in Gastric Cancer in Association With Autophagy-Mediated AKT/mTOR Signaling Pathways.

Gastric cancer is a deadly disease. Some microRNAs are involved in tumor invasion and metastasis. Underexpression of miR-375 has been correlated with tumorigenesis, treatment resistance, and poor prognosis. In this study, we first analyzed the profiles and prognostic values of miR-375 expression in gastric cancer tissues from a public database, and the expression level of miR-375 in gastric cancer samples and gastric cancer cell lines was then analyzed by quantitative real- time polymerase chain reaction. Significant underexpression of miR-375 was seen in all the gastric cancer samples compared to paired paracarcinoma tissues, and the expression level of miR-375 in the gastric cancer cell lines was negatively associated with the cell migration ability. A Cell proliferation (CCK-8) assay was performed to examine cell viability. Overexpression of miR-375 suppressed the proliferation of gastric cancer cells. A Western blot analysis was carried out to test protein expression. Overexpression of miR-375 inhibited autophagy through the AKT/ mammalian target of rapamycin signaling pathway. MiR-375 regulated invasion and migration via AKT/ mammalian target of rapamycin pathway-mediated epithelial-to-mesenchymal transition. Wound healing and migration assays were used to determine the motility of gastric cancer cells. A gastric cancer xenograft nude mouse model was used for an in vivo efficacy evaluation. Overexpression of miR-375 significantly suppressed cell proliferation in the established gastric cancer xenograft nude mouse model. Our results demonstrate that increasing the expression level of miR-375 suppresses proliferation in vitro and in vivo, and they provide a mechanistic and applicable rationale for the future clinical evaluation of miR-375 in gastric cancer treatment. Our findings provide not only new information about the molecular mechanism of microRNAs in regulating invasion and migration in gastric cancer but also a theoretical principle for a potential targeted therapy for gastric cancer.

Introduction

Gastric cancer is one of the most common malignancies worldwide and is a major cause of cancer-related mortality in China.[1] The occurrence of gastric cancer is a complex process of progressive development. Due to the increasing early detection of cancer and the widespread implementation of radical surgery, the overall survival of patients with gastric cancer has improved.[2,3] In gastric cancer, cell invasion into the surrounding tissue is a crucial early step.[4] The mechanisms of gastric cancer cell migration and metastasis are not fully understood; the prognosis of advanced gastric cancer remains poor, and safe and effective adjuvant therapy options are limited. Thus, exploring highly sensitive and low-cost, early-stage diagnostic methods is of great pragmatic value. MicroRNAs (miRNAs) are endogenous small noncoding RNAs that bind 3′-untranslated regions of targeted genes with high fidelity to suppress gene and protein production, thus exerting various regulatory functions on tissue and organ development in both animals and humans.[5-7] MicroRNAs play a crucial role in the regulation of cell growth and differentiation. It has been demonstrated that the aberrant expression of miRNAs is involved in tumorigenesis and tumor development.[8] Some miRNAs are involved in tumor invasion and metastasis. Studies have demonstrated that miR-375 inhibits tumor progression in gastric, prostate, breast, lung, and head and neck cancers.[9-13] To date, a number of miRNAs have been found to be implicated in the progression of gastric cancer metastasis.[14-16] Determining the association between specific dysregulated miRNAs and specific steps of gastric cancer metastasis will provide insight into the potential mechanisms of gastric cancer cell migration, invasion, and metastasis.

In this study, we have examined the association between a specific dysregulated miRNA and specific steps of gastric cancer metastasis, which will provide insight into the potential mechanisms of gastric cancer cell migration, invasion, and metastasis.

Many studies have shown that the overexpression of miR-375 plays a role in the regulation of autophagy[17,18] and in the ability of miR-375 to regulate autophagy through the AKT/mammalian target of rapamycin (mTOR) signaling pathway.[18] It has been confirmed that miR-375 plays a tumor suppressive role, inhibiting proliferation, invasion, and migration and promoting apoptosis via AKT signaling pathways.[13,19] In our study, we found that miR-375 inhibits autophagy through the AKT/mTOR signaling pathway. The miRNAs that inhibit the autophagy of cancer cells might be developed as therapeutics. This approach is clinically challenging because of the complex mechanisms underlying these processes. Therefore, clarifying the molecular mechanisms is an important goal.

Methods

Cell Cultures and Reagents

Gastric cancer cell lines MKN-45 and GT3TKB were purchased from the American Type Culture Collection (ATCC, USA) and provided by Sparklebio (USA). MKN-45 cells were maintained in RPMI medium 1640 (Gibco, Carlsbad, California) supplemented with 10% fetal bovine serum (FBS; Gibco) and 1% penicillin/streptomycin, and GT3TKB cells were maintained in RPMI medium DMEM (Gibco) supplemented with 10% FBS (Gibco) and 1% penicillin/streptomycin and. Cells were incubated in a 5% CO2 humidified incubator at 37°C and collected using 0.05% trypsin EDTA following the specified incubation period. The following primary antibodies were used: E-cadherin (#3195 S), N-cadherin (#13116 S), Vimentin (#5741 S), Snail (#3879 S), AKT (#9272 S), p-AKT (#4060 S), mTOR (#2972 S), p-mTOR (#5536 S), and Slug (#9585 S; purchased from Cell Signaling Technology Inc, USA); GAPDH (#sc-47724) was from Santa Cruz Biotechnology (Shanghai, China). The data were collected from at least 3 independent experiments.

Lentivirus Infection

MiR-375 mimics and inhibitors and miRNA negative control (NC) were synthesized by RiboBio Co, Ltd (Guangzhou, China) (Table 1), and all recombinant lentiviruses were obtained from RiboBio Co, Ltd (Guangzhou, China), and used for hASC infection at an MOI of 80. The packaged lentiviruses used contained miR-375-mimics, NC, and miR-375 inhibitors. Infection was performed by exposing hASCs to dilutions of the viral supernatant in the presence of polybrene (5 mg/mL) and fresh medium for 24 hours, followed by selection with puromycin (Sigma-Aldrich) at 1 mg/mL. Transduction efficiency was evaluated by determining the percentage of GFP-positive cells observed under an inverted fluorescence microscope and reverse-transcriptase polymerase chain reaction (RT-PCR).

Table 1.

The Sequence of miR-375 Mimic and Inhibitor and MicroRNA Negative Control.

	Sequence
miR-375 mimics	5′-TTTGTTCGTTCGGCTCGCCGTGA-3′
miR-375 inhibitors	5′-TCACGCGAGCCGAACGAACAAA-3′
Mimics negative control	5′-TTCTCCGAACGTGTCACGT-3′
Inhibitors negative control	5′-TCGTTCATGAACGAACATT-3′

Quantitative Real-Time RT-PCR

Total RNA was extracted from 30 pairs of frozen tissue samples and gastric cancer cells, MKN-45 and GT3TKB, using TRIzol (Invitrogen, Carlsbad, California), according to the manufacturer’s protocol. Quantitative real-time RT-PCR analysis was performed using an Applied Biosystems 7500 Real-Time PCR System (Foster City, California). The primers used in quantitative real-time PCR analysis as follows: miR-375:(F)5′-CAGGGTCCGAGGTATT-3’ (R)3’-CTGCTTTGTTCGTTCG-5′. The results of U6 qRT-PCR gene expression by 2−ΔΔCt method were used as the control.

Cell Viability Assay

Cell viability was performed with the Cell proliferation assay of CCK-8 (Dojindo, Kumamoto, Japan) following the manufacturer’s instruction. The inhibition rate of cell proliferation was calculated for each well as (A450control cells − A450 treated cells)/A450control cells × 100%. Experiments were performed in triplicate. Cell viability was expressed as mean (standard deviation [SD]) of absorbance from treated cells versus control cells in triplicate.

Wound Healing Assay

Cells were cultured in 6-well plates and allowed to grow to confluence. The cells were then cultured in corresponding medium without FBS for 12 hours and then scratched with a 20-μL pipette tip. The cells were washed 3 times with phosphate-buffered saline. Wound areas were marked and photographed at 0, 24, and 48 hours, respectively. The rate of cell migration was evaluated by both photographing and quantifying the migrated distance of cells moved from the wound edge toward the center. All experiments were repeated 3 times.

Invasion Assay

The capability of cell migration was determined by transwell invasion assay. Transwell invasion assay was performed separately using 24-well transwell inserts with 8 μm pore size (Corning Costar Corp). For transwell invasion assay, 3 × 104 MKN-45 cells and 1.5× 104 GT3TKB cells suspended in 100 μL corresponding culture medium without FBS were loaded into the top chamber of transwell insert with noncoated membrane. The bottom chamber contained 600 mL medium with 20% FBS. Cells were then allowed to migrate at 37°C; cells left on the upper chamber were removed with a cotton swab. The filter was fixed with 95% ethanol for 20 minutes and then stained with 4 g/L Crystal violet for 30 minutes. Cells were photographed in 5 independent 20× magnification fields under inverted microscope and counted. All experiments were independently repeated at least 3 times.

Small-Interfering RNA and GFP-LC3 Interference

The target sequence for ATG7-specific small-interfering RNA (siRNAs) is shown in Table 2, and the control siRNA (no silencing) was synthesized by GenChem Co (Shanghai, China). The plasmid GFP-LC3 was kindly provided by Beth Levine. Transfection was performed following the manufacturer’s protocol.

Table 2.

The Sequence of ATG7-Specific Small-Interfering RNAs.

	Sequence
ATG7-1	5′-GCCGUGGAAUUGAUGGUAUTT-3′
ATG7-2	5′-GAAGCUCCCAAGGACAUUATT-3′
ATG7-3	5′-GGAUCCUGGACUCUCUAAATT-3′

Immunocytochemistry

The cells were plated on sterile coverslips for 24 hours and fixed with 4% paraformaldehyde for 10 minutes at 37°C. After fixation, a permeabilization step was conducted with 0.25% Triton-X 100 for 10 minutes at 4°C, and the cells were subsequently incubated in blocking solution containing 4% bovine serum albumin for 1 hour at 37°C. The nucleus was stained with DAPI (1 mg/mL) for 5 minutes at room temperature. Fluorescence images were then captured by a confocal laser scanning microscope (LSM 700; Carl Zeiss, Oberkochen, Germany).

Western Blot Assays

Cells were harvested in lysis buffer, and tumors were harvested in RIPA buffer. After incubating on ice for 30 minutes, the cells were centrifuged at 12 000g for 15 minutes at 4°C, and the supernatant was collected. Samples were then analyzed by Western blot. Proteins were visualized by incubation with SuperSignal West pico reagents (NCI5079; Thermo), followed by exposure to radiograph film.

Nude Mouse Xenograft Studies

Four-week-old BALB/c (athymic) nude mice were purchased from the Shanghai’ SIPPR-BK Laboratory Animal Co, Ltd. A total of 4 × 106 cells were subcutaneously injected into the right flank of nude mice. Body weights and tumor volumes (V) were measured every 2 days. Tumor volumes were calculated using the formula: V = (length × width2)/2.

Statistical Analysis

All assays were performed in triplicate. Data are expressed as the mean (SD). Statistical analyses were performed using an analysis of variance with SPSS 13.0 software. Statistical significance was set at 2-sided P < .05.

Results

The Clinical Significance of miR-375 in Gastric Cancer From TCGA

TCGA serves as a large repository of high-throughput data regarding DNA, RNA, and protein in diverse human cancers, thus helping to facilitate the comprehensive analysis of the expression of these components in various cancer types.[20,21] The database provides search, browse, and download functions for miRNA pathway data. In our study, we obtained the miR-375 expression profile in various types of human cancer tissues and adjacent normal tissues from a TCGA online data analysis tool (http://bioinfo.life.hust.edu.cn/miR_path/index.html), as shown in Figure 1A to C. We provided a preeminent resource for cancer research by combining the differentially expressed miRNAs/genes with an miRNA regulatory pathway analysis. Reverse-transcriptase polymerase chain reaction was conducted on all 30 pairs of samples to assess the expression levels of miR-375. Significant underexpression of miR-375 (Figure 1D) was seen in all the gastric cancer samples compared to paired paracarcinoma tissues. Consistent with this result, the expression level of miR-375 in gastric cell lines was negatively associated with the cell migration ability. Expression of MiR-375 in MKN-45 cells was lower than that in GT3TKB cells (P < .01; Figure 1E).

Figure 1.

MiR-375 was downregulated in gastric cancer tissues compared to normal tissues and cells with greater migration and invasion abilities. A, Browse of microRNA (miRNA) pathway of tumor types. B, Search for miRNA pathway of tumor types. C, TCGA expression of miRNA or messenger RNA (mRNA) in each tumor type. D, Relative fold-changes between gastric cancer samples. The expression of miR-375 in the 30 pairs of tissues (tumor tissue/normal tissue) samples. Expression of miR-375 in tumor and normal tissues (quantitative reverse transcriptase polymerase chain reaction [qRT-PCR]). E, The expression level of miR-375 in human gastric cancer cell lines with different migration and invasion abilities. The expression level of miR-375 in MKN-45 cells was lower than that of GT3TKB cells. **P < .01. F, The invasion abilities of the in human gastric cancer cell lines were measured with transwell chambers. Photos are representative fields of invasive cells on the membrane. G, Bar graphs represent the average number of cells on the underside of membrane ± standard error (SE). **P < .01. H, The cells migration to the wounded area was photographed by microscopy at 0 and 48 hours postwounding. I, The rate of migration was examined by measuring the distance of cells moved from the wound edge toward the center in 48 hours after scratching ± SE. **P < .01. The data are presented as mean ± SE of at least 3 independent experiments. **P < .01.

Gastric cancer cell lines (GT3TKB, MKN-45) were characterized. As shown in Figure 1F and G, wound healing assays were conducted to investigate migration. As shown in Figure 1H and I, invasion assays were conducted to investigate invasion. The results showed that the migration and invasion abilities of MKN-45 cells were greater than those of GT3TKB cells (P < .01). Our results indicate that miR-375 might have a causal role in gastric cancer metastasis.

Overexpression of miR-375 Suppressed the Proliferation of Gastric Cancer cells

All recombinant lentiviruses were obtained from RiboBio Co, Ltd. The packaged lentiviruses used contained miR-375mimics, NCs, and miR-375 inhibitors. Lentiviral transduction was performed following the manufacturer’s protocol. The transduction efficiency was evaluated by determining the percentage of GFP-positive cells observed under an inverted fluorescence microscope (Figure 2A and C). Reverse-transcriptase polymerase chain reaction was conducted in Eca-109-miR-375, Eca-109-miR-375-mimic, and Eca-109-miR-375-NC cells to assess the expression level of miR-375 (Figure 2B and D). Significant underexpression of miR-375 was seen in GT3TKB-miR-375-inhibitor compared to GT3TKB-LV3-NC (Figure 2B), and significant overexpression of miR-375 was seen in MKN-45-miR-375-mimic compared to MKN-45-LV3-NC (Figure 2D).

Figure 2.

MiR-375 inhibited the cell proliferation. A, Infected cells with lentivirus were detected by inverted microscope in GT3TKB cells. B, The expression level of miR-375 in GT3TKB-miR-375-inhitors and GT3TKB-LV3-NC by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). **P < .01. C, Infected cells with lentivirus were detected by inverted microscope in MKN-45. D, The expression level of miR-375 in MKN-45-miR-375-mimics and MKN-45-LV3-NC by qRT-PCR. **P < .01. E and F, Effects of miR-375 on the proliferation of lentivirus-transduced GT3TKB and MKN45 cells; an CCK-8 assay was performed for 7 days. Absorbance at 490 nm was measured. G, The invasion abilities of lentivirus-transduced GT3TKB cells were measured with transwell chambers. Photos are representative fields of invasive cells on the membrane. H, The invasion abilities of lentivirus-transduced MKN45 cells were measured with transwell chambers. Photos are representative fields of invasive cells on the membrane. I, Bar graphs represent the average number of lentivirus-transduced GT3TKB cells on the underside of membrane ± standard error (SE). **P < .01. J, Bar graphs represent the average number of lentivirus-transduced MKN45 cells on the underside of membrane ± SE. **P < .01. K and L, The cells migration to the wounded area was photographed by microscopy at 0 and 48 hours postwounding. M and N, The rate of migration was examined by measuring the distance of cells moved from the wound edge toward the center in 48 hours after scratching ± SE. **P < .01. The data are presented as mean ± SE of at least 3 independent experiments. **P < .01.

Next, we explored the biological role of miR-375 at the cellular level. To explore the influence of miR-375 expression on cell survival, we used CCK-8 assay to investigate whether miR-375 was involved in the suppression of cell proliferation. As shown in Figure 2E, a significant suppression of cell proliferation was found in GT3TKB-LV3-NC compared with GT3TKB-miR-375-inhitors, and as shown in Figure 2F, a significant suppression of cell proliferation was found in MKN-45-miR-375-mimics compared to MKN-45-LV3-NC. The result suggests that overexpressed miR-375 has a significant inhibiting effect on the proliferation of gastric cancer cells. We explored the effect of miR-375 on the migration and invasion of gastric cancer cells. As shown in Figure 2G–J, the transwell invasion assay showed that the invasion of GT3TKB-miR-375-inhitors cells was much higher than GT3TKB-miR-375-NC and MKN45-miR-375-NC was much higher than MKN45-miR-375-mimics. In consistent with these results, the scratch wound healing assay also revealed that the migration of GT3TKB-miR-375-NC when compared to GT3TKB-miR-375-inhibitors and MKN45-miR-375-mimics was much lower than and MKN45-miR-375-NC toward the wound area was significantly increased (Figure 2K–N). Collectively, these results indicate that overexpression of miR-375 is sufficient to inhibit both the migration and the invasion abilities of gastric cancer cells.

Effect Quantitative of miR-375 on the Expression of Epithelial-to-Mesenchymal Transition-Related Proteins

Quantitative RT-PCR was conducted to investigate the messenger RNA (mRNA) expression levels of the epithelial-to-mesenchymal transition (EMT)-related proteins E-cadherin, N-cadherin, vimentin, Snail, and Slug normalized to GAPDH mRNA expression levels. We detected an associated decrease in E-cadherin expression and an increase in N-cadherin, vimentin, Snail, and Slug expression at the mRNA level in GT3TKB-miR-375-inhibitor compared to their expression levels in GT3TKB-miR-375-NC, as shown in Figure 3A. Furthermore, we observed an associated increase in E-cadherin expression and a decrease in N-cadherin, vimentin, Snail, and Slug expression at the mRNA level in MKN-45-miR-375-mimic compared to their expression levels in MKN45-miR-375-NC, as shown in Figure 3B. These results demonstrated that miR-375 may inhibit EMT in gastric cancer cells. Next, Western blotting was conducted to investigate the expression levels of the EMT-related proteins E-cadherin, N-cadherin, vimentin, Snail, and Slug at the protein level. As shown in Figure 3C, GT3TKB-miR-375-inhibitor demonstrated a decreased expression of E-cadherin and an increased expression of N-cadherin, vimentin, Snail, and Slug compared to GT3TKB-miR-375-NC. As shown in Figure 3D, MKN-45-miR-375-mimic demonstrated an increased expression of E-cadherin and a decreased expression of N-cadherin, vimentin, Snail, and Slug compared to MKN45-miR-375-NC. The results indicated that in gastric cancer cells, E-cadherin, N-cadherin, vimentin, Snail, and Slug were the downstream proteins of miR-375 and that their expression levels were modulated by miR-375. Consistent with the previous results, the overexpression of miR-375 inhibited EMT in gastric cancer cells.

Figure 3.

The effects of miR-375 on EMT markers. A and B, The messenger RNA (mRNA) expression levels of EMT markers (E-cadherin, N-cadherin, Vimentin, Snail, and Slug) were assessed using quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). C and D, The protein levels of EMT markers (E-cadherin, N-cadherin, Vimentin, Snail, and Slug) were assessed using Western blotting.

MiR-375 Inhibits Autophagy Through the AKT/mTOR Signaling Pathway

Autophagy was measured by light microscopic quantitation of cells transfected with GFP-LC3 as described or by Western blot analysis of the levels of LC3.[22] To investigate whether miR-375 inhibits autophagy, we measured the expression levels of LC3, as shown in Figure 4A and B. The intensity of the LC3-II band increased and the conversion of LC3-I into LC3-II was evident, indicating the activation of autophagy. In our study, the results indicate that the overexpression of miR-375 inhibits autophagy. To further confirm these results, we examined LC3 punctate formation under a confocal microscope. LC3 is an autophagosome membrane marker, as demonstrated by accumulation of punctate LC3 in the cytoplasm of cells (shown in Figure 4C–F), suggesting the inhibition of autophagy by the overexpression of miR-375. In our study, siRNAs were designed to reduce ATG7 protein levels, and these siRNAs completely abolished the expression of ATG7 (shown in Figure 4G). ATG7 protein knockdown dramatically counteracted the autophagy inhibited by the overexpression of miR-375 (Figure 4H and I).

Figure 4.

MiR-375 inhibits autophagy through AKT/mammalian target of rapamycin (mTOR) signaling pathway in gastric cancer cells. A and B, The expression of LC3 was measured by Western blot. C and D, Cells were transfected with GFP-LC3 plasmids, then stained with DAPI, and analyzed by fluorescence microscopy. E and F, Statistical analysis of the percentage of LC3 puncta per cell. Columns, mean (n = 3); bars, standard deviation (SD). *P < .01 versus untreated group. LC3 puncta per cell was quantified. G, Cells were transfected with ATG7 small-interfering RNAs (siRNAs). Western blot was used to detect the expression of ATG7. H and I, Cells were transfected with ATG7 siRNAs, and Western blot was conducted using indicated antibodies. J and K, The protein levels of AKT/mTOR signaling pathway (AKT, p-AKT, mTOR, and p-mTOR) were assessed using Western blotting. L and M, Cells were transfected with ATG7 siRNAs. The expression levels of AKT, p-AKT, mTOR, and p-mTOR were analyzed by Western blot.

To investigate the effect of miR-375 on the AKT/mTOR signaling pathway, we examined the protein levels of AKT, p-AKT, mTOR, and p-mTOR by Western blotting. Figure 4J shows that miR-375 inhibitors increased the expression of p-mTOR and p-AKT, and Figure 4K shows that miR-375-mimics decreased the expression of p-mTOR and p-AKT. In our study, we next investigated the role of autophagy in miR-375-mediatedAKT/mTOR signaling pathway induction; siRNAs were designed to reduce the ATG7 protein level. And then, we investigated the expression of AKT, p-AKT, mTOR, and p-mTOR after cells were transfected with ATG7 siRNAs, and the protein levels of AKT, p-AKT, mTOR, p-mTOR were partially observed, as shown in Figure 4L and M. These results indicating that miR-375 inhibits autophagy through AKT/mTOR signaling pathway.

Overexpression of miR-375 Suppressed Tumor Formation By Gastric Cancer Cells in Nude Mice

We then investigated the effect of miR-375 on in vivo gastric cancer tumorigenesis. Gastric cancer-bearing nude mouse models were generated by the subcutaneous inoculation of MKN-45-miR-375-mimic in the treatment group of nude mice and the subcutaneous inoculation of mock-carrier MKN45-miR-375-NC in the control group. The in vivo transplantation study was carried out for 4 weeks. Every 2 days, body weights and tumor volumes (V) were compared between the treatment group and the control group. As shown in Figure 5A to C, the results indicated that the overexpression of miR-375 significantly suppressed the proliferation of gastric cancer cells in vivo. Consistent with in vitro results, expressions levels of p-AKT and p-mTOR were markedly downregulated in MKN-45-miR-375-mimics group xenografts (Figure 5D and E).

Figure 5.

miR-375 suppressed the tumorigenesis in nude mice. A-C, Changes in tumor volume and tumor growth between treatment group and vehicle group of gastric cancer tumor-bearing mice. Data are shown as mean tumor volume ± standard deviation (SD; 8 mice/group). D, The expression level of miR-375 in treatment group and vehicle group by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). **P < .01.

Discussion

MicroRNAs have been demonstrated to play critical roles in cells during the progression of cancers in recent years.[23,24] Studies have shown that miRNAs profoundly affect an array of normal biological processes and that they play important roles in cancer by regulating the expression of various oncogenes and tumor suppressors.[25,26] Some miRNAs are involved in tumor invasion and metastasis. MicroRNAs have been suggested to be posttranscriptional repressors that inhibit target gene expression via translation inhibition or mRNA degradation. Moreover, the aberrant expression of miRNAs is significantly related to gastric cancer tumor stage, size, differentiation, and metastasis. MicroRNAs interrupt cellular signaling pathways and inhibit the activity of tumor suppressor genes in gastric cancer cells. The expression of miR-375 has been found to be significantly downregulated in multiple types of cancers, and miR-375 is believed to act as a cancer suppressor.[12,13,26-30] However, the expression and role of miR-375 in gastric cancer still need to be further studied. By qRT-PCR, the underexpression of miR-375 was demonstrated in gastric cancer tumor tissues compared to normal paracarcinoma tissues. Next, we successfully constructed the miR-375-inhibitor, miR-375-mimic, and miR-375-NClentiviral expression vectors, hence creating the basis for a subsequent study involving the function of miR-375. Cell differentiation is a complex process governed by the interplay of several signaling pathways.[31] We explored the influence of miR-375 expression on cell proliferation. Our results suggest that the overexpression of miR-375 has a significant inhibitory effect on the proliferation of gastric cancer cells. The EMT is a process where epithelial cells lose their characteristics,[32,33] and it plays a critical role in many aspects of cancer behavior, including the phenotypic conversion that is implicated in the initiation of metastasis in gastric cancer progression.[34,35] Furthermore, our results showed that overexpression of miR-375 was sufficient to inhibit both the migration and invasion abilities of gastric cancer cells. Previous studies have found that miR-375 regulates the progression of cancers through signaling pathways related to the EMT process in multiple cancers.[36-38] The EMT is critical in many biological behaviors of cancer, such as migration and invasion. During EMT, the expression of some epithelial cell markers such as E-cadherin decreases, while the expression of vimentin increases. In the present study, we found that miR-375 regulated the EMT process by regulating the expression of E-cadherin, N-cadherin, vimentin, Snail, and Slug in gastric cancer.

The multifaceted miR-375-mediated anticancer effects play a causal but unclear role in mammalian oncogenesis. The AKT/mTOR pathway is major pathway involved in the regulation of cell proliferation and differentiation,[39,40] and it plays a significant role in regulating cell proliferation and differentiation.[41,42] Some antitumor drugs can induce the early autophagy and late apoptosis of tumor cells through the AKT/mTOR signaling pathway.[43,44] In this study, we found that miR-375 inhibits autophagy through the AKT/mTOR signaling pathway.

Studies have demonstrated that miR-375 inhibits tumor progression in gastric, prostate, breast, lung, and head and neck cancers.[9-13] Overexpression of miR-375 significantly suppressed proliferation in an established gastric cancer xenograft nude mouse model. Our findings provide not only new information about the molecular mechanism of miRNAs in regulating invasion and migration in gastric cancer but also a theoretical principle for a potential targeted therapy for gastric cancer.