High-glucose Dulbecco’s modified Eagle’s medium (H-DMEM) was purchased from Gibco Laboratories (Grand Island, NY, USA). Fetal bovine serum (FBS), bovine serum albumin (BSA), TRIzol, TaqMan qRT-PCR miRNA assay kit, RT-PCR kit, Lipofectamine 2000, miR-22 mimics and an miR-22 inhibitor were purchased from Thermo Fisher Scientific (Waltham, MA, USA). MTT was purchased from Sigma (St. Louis, MO, USA). SYBR-Green qPCR mix was purchased from Toyobo (Osaka, Japan). Mouse anti-ERα monoclonal antibody, mouse anti-GAPDH monoclonal antibody, rabbit anti-mouse secondary antibody and 17β-estradiol (E2) were purchased from Abcam (Cambridge, UK). Propidium iodide (PI) was purchased from Roche Molecular Biochemicals (Indianapolis, IN, USA). A 24-well transwell chamber was obtained from Corning Inc. (Corning, NY, USA). Matrigel was obtained from BD Biosciences (Franklin Lakes, NJ, USA) and matrix metalloproteinase (MMP)-2 and MMP-9 ELISA kits were purchased from R&D Systems (Minneapolis, MN, USA).

The present study was approved by the Ethics Committee of Xinxiang Medical University (Weihui, China). Informed consent was obtained from each patient. In total, 20 fresh-frozen EEC tissues were obtained from patients at the Department of Gynecology and Obstetrics, The First Affiliated Hospital of Xinxiang Medical University (Weihui, Henan, China) from May 2011 to May 2012. In addition, 20 normal endometrial tissues were obtained from patients who underwent hysterectomy to treat myoma. Prior to surgery, no patient had undergone hormone therapy, radiotherapy or chemotherapy. Following surgical removal, all samples were immediately snap-frozen in liquid nitrogen and stored at −80°C until use. ERα expression was confirmed by immunohistochemistry.

Human endometrial cancer RL95-2 and Ishikawa cell lines were purchased from the American Type Culture Collection (ATCC, Manassas, VA, USA). Cells were cultured in H-DMEM medium containing 10% FBS at 37°C with 5% CO₂. All experiments were performed at the third passage.

Total RNA was extracted from tissues and cells using TRIzol. For the detection of miR-22 expression, RNA was synthesized to cDNA using the RT-PCR kit in accordance with the manufacturer’s instructions. A TaqMan qRT-PCR miRNA assay kit was used to perform qPCR according to the manufacturer’s instructions and analyzed with an ABI 7500 Sequence Detection system. U6 was used as an internal control. For detection of mRNA, qPCR analysis was performed using a SYBR-Green qRCR mix and specific primers synthesized by Sangon Biotech Co., Ltd. (Shanghai, China). The following primers were used for the amplification of ERα: ERα forward, 5′-CCCACTCAACAGCGTGTCTC-3′ and reverse, 5′-CGTCGATTATCTGAATTTGGCCT-3′. GAPDH was used as an internal control. GAPDH forward, 5′-ACAACTTTGGTATCGTGGAAGG-3′ and reverse, 5′-GCCATCACGCCACAGTTTC-3′. Independent experiments were repeated three times for each sample and the relative expression levels of genes were analyzed using the 2^−ΔΔCt method.

Tissues or cells were solubilized in cold radioimmunoprecipitation assay lysis buffer [1X phosphate-buffered saline (PBS), 1% Nonidet P-40, 0.5% sodium deoxycholate and 0.1% sodium dodecyl sulfate (SDS); Beyotime Institute of Biotechnology, Shanghai, China.]. Protein (20 μg per lane) was separated with 12% SDS-PAGE. Following that, protein was transferred onto nitrocellulose membranes, which were then blocked in 5% non-fat dried milk in PBS containing with Tween 20 for 3 h, and then incubated overnight with mouse anti-ERα monoclonal antibody (1:200) or mouse anti-GAPDH monoclonal antibody (1:400). Following washing with PBS three times (each for 5 min), the membranes were incubated with rabbit anti-mouse secondary antibody (1:20,000) for 1 h at room temperature. Then, the enhanced chemiluminescence kit (Huyu Group, Co., Shanghai, China) was used to detect the immune complexes. Following that, the membranes were scanned for the relative value of protein expression in gray scale using Image-Pro plus software 6.0 (Media Cybernetics, Inc., Rockville, MD, USA). The relative expression levels of protein were presented as the density ratio versus GAPDH.

Cells were cultured to 70–80% confluence and then resuspended in serum-free H-DMEM at a concentration of 100,000 cells/ml. Six-well plates were used to inoculate with 2 ml suspension for each well and each group set five duplicate wells. miR-22 mimics (chemically synthesized mature microRNAs) or NC mimics (Thermo Fisher Scientific) of 50 pmol were diluted with 0.25 ml serum-free H-DMEM. Lipofectamine 2000 transfection reagent (50 μl) was diluted with 2.5 ml serum-free H-DMEM. Then, the diluted Lipofectamine 2000 transfection reagent was added to the mimics dilution, mixed gently and incubated for 20 min at room temperature. The cell suspension was washed with serum-free H-DMEM two times and then added to the mixture of Lipofectamine 2000 and the mimics above, and then incubated at 37°C and 5% CO₂ for 6 h. Following that, the medium in each well was replaced with the normal serum-containing medium and cultured for 24 h prior to the following experiments.

For all groups, 10,000 cells per well were seeded in a 96-well plate. Following treatment with 10 nM of E2, the plates were incubated for 12, 24, 36 or 48 h at 37°C and 5% CO₂. To assess cell proliferation, an MTT assay was performed according to the manufacturer’s instructions. MTT reagent (50 μl; 5 mg/ml) in PBS was added to each well and incubated for 4 h at 37°C and 5% CO₂. Then, the supernatant was removed and 150 μl of dimethylsulfoxide was added. The absorbance was detected at 570 nm with a Microplate Reader (Bio-Rad, Hercules, CA, USA). Each assay was performed in triplicate wells and repeated three times.

At 48 h following transfection, cells were harvested and fixed in 70% ethanol for 30 min. Then the cells were stained with 25 μg/ml propidium iodide (PI) in PBS containing 0.1% BSA, 0.05% of Triton X-100 and 50 μg/ml of RNaseA for 30 min at room temperature. Following that, the cells were analyzed by a FACScan flow cytometer (Becton-Dickinson, Franklin Lakes, NJ, USA). Experiments were performed three times in triplicate.

The cell invasion assay was performed in a 24-well transwell chamber, which was pre-coated with 100 μg Matrigel^®. Cells in each group were collected and resuspended in serum-free H-DMEM at a concentration of 10,000 cells/ml, respectively. Then, 0.2 ml cell suspension was added into the upper chamber, and the bottom chamber was filled with 0.5 ml H-DMEM containing 10% FBS. Following incubation for 24 h at 37°C and 5% CO₂, a cotton bud was used to remove the cells which had not migrated through the polycarbonate membrane. Then, the cells which had moved through the polycarbonate membrane and adhered to the bottom of it were stained with trypan blue for 15 min, then images were captured (Microscope: CX21BIM-SET5; Olympus, Tokyo, Japan; Camera: DP25; Olympus) and cells were counted.

Cell supernatants in each group were used to determine the secretion of MMP-2 and MMP-9 using ELISA. An MMP-2 and MMP-9 ELISA kit were used and the concentrations of MMP-2 and MMP-9 were calculated according to manufacturer’s instructions. Optical density (OD) values were determined using a microplate reader (PR 3100 TSC; Bio-Rad).

Statistical analysis was performed using SPSS 17.0 statistical software (SPSS, Inc., Chicago, IL, USA). Data are expressed as the mean ± standard deviation. The data were analyzed by one-way analysis of variance. P<0.05 was considered to indicate a statistically significant difference.

Firstly, the miR-22 expression in ERα-positive and ERα-negative human EEC samples as well as normal endometrium tissues were determined, respectively. As shown in Fig. 1A, miR-22 levels were significantly lower in the ERα-positive ECC tissues as compared with those in normal endometrium and ERα-negative EEC tissues. The miR-22 expression levels were then examined in the ERα-positive ECC lines, RL95-2 and Ishikawa. Consistent with the findings above, miR-22 expression was significantly decreased in RL95-2 and Ishikawa cells compared with that in normal endometrium tissues and ERα-negative EEC tissues (Fig. 1B). These findings suggested that miR-22 may be important in ERα-positive EEC.

Since ERα has been demonstrated to be a target of miR-22, RL95-2 and Ishikawa cells were further transfected with miR-22 mimics, in order to study the association between miR-22 and ERα in EEC. As shown in Fig. 2A, following transfection with the miR-22 mimics, the expression levels of miR-22 were significantly increased; however, in the control and NC groups, the levels of miR-22 were not affected, suggesting that the introduction of miR-22 into RL95-2 and Ishikawa cells was successful. The mRNA and protein expression of ERα was determined and it was revealed that following transfection of RL95-2 and Ishikawa cells with miR-22 mimics, the mRNA and protein expression levels of ERα were markedly decreased compared with those in the control and the NC groups (Fig. 2B). These findings suggested that miR-22 had an inhibitory effect on the regulation of ERα expression in ERα-positive EEC cell lines.

Since RL95-2 and Ishikawa cells have been demonstrated to be ER-dependent, E2 was used to stimulate ER-dependent cellular proliferation. As shown in Fig. 3, the cellular proliferation of RL95-2 and Ishikawa cells was significantly increased under the treatment of E2 in a time-dependent manner. However, following the introduction of miR-22 mimics, the proliferative rate of RL95-2 and Ishikawa cells was markedly decreased as compared with that in the E2 group. These findings suggested that miR-22 inhibited E2-induced proliferation of ERα-positive RL95-2 and Ishikawa cells, at least partially via suppressing ERα expression.

Since ER-mediated signaling has been demonstrated to be involved in the regulation of cell cycle progression, the role of miR-22 on the regulation of E2-induced cell cycle progression was further investigated in RL95-2 and Ishikawa cells. As demonstrated in Fig. 4A, E2 significantly promoted cell cycle progression, which was demonstrated by a decrease of cells in G1 phase. However, following the introduction of miR-22 into these cells, the proportion of cells in G1 phase was significantly increased, while the proportion of cells in S phase was decreased, indicating that miR-22 arrested RL95-2 and Ishikawa cells at the G1/S checkpoint and delayed cell cycle entry into S phase.

Notably, cyclin D1 is important in the regulation of the G1/S checkpoint. The aberrant upregulation of cyclin D1 is able to shorten G1 phase and promote cell cycle progression. By contrast, its downregulation may result in a delay of cell cycle progression into S phase. Thus, the mRNA levels of cyclin D1 in each group were examined. As shown in Fig. 4B, E2 significantly upregulated the expression of cyclin D1 in RL95-2 and Ishikawa cells, while the introduction of miR-22 significantly suppressed its expression.

In summary, the findings suggested that miR-22 had an inhibitory effect on E2-stimulated cell cycle progression, at least in part through inducing cyclin D1-mediated cell cycle arrest.

It has been reported that ER-dependent signaling is important in the regulation of invasion of ER-dependent EEC cells. Thus, transwell chambers precoated with Matrigel were used to study the effect of miR-22 on the invasion ability of ERα-positive RL95-2 and Ishikawa cells. As shown in Fig. 5A, E2 significantly promoted the invasion of RL95-2 and Ishikawa cells, which was effectively reversed by the introduction of miR-22. However, non-specific miRNA had no effect. These findings indicated that miR-22 inhibited E2-induced invasion of ERα-positive RL95-2 and Ishikawa cells, possibly via reducing ERα expression.

Further study was performed to examine the molecular mechanisms involved in the miR-22-induced inhibition of ER-dependent invasion of RL95-2 and Ishikawa cells. Since ER-dependent signaling has been demonstrated to regulate the mRNA and protein expression of MMP-2 and MMP-9 (20), which are key enzymes participating in cellular invasion (21), it was investigated whether miR-22 was able to inhibit E2-induced MMP-2 and MMP-9 secretion in RL95-2 and Ishikawa cells. ELISA data demonstrated that the MMP-2 and MMP-9 protein levels were significantly increased by E2. However, overexpression of miR-22 caused a significant reduction in MMP-2 and MMP-9 secretion (Fig. 5B). These findings suggested that miR-22 inhibited the secretion of MMP-2 and MMP-9 in ERα-positive RL95-2 and Ishikawa cells.