MicroRNAs have been suggested to play a vital role in regulating carcinogenesis, tumor progression and invasion. MiR-335 is involved in suppressing metastasis and invasion in various human cancers. However, the mechanisms responsible for the aberrant expression of miR-335 in gastric cancer (GC) remain unknown.
Methods
Expression of miR-335 in four GC cell lines and 231 GC tissues was determined by real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR). DNA methylation status in the CpG islands upstream of miR-335 in GC cell lines and tissues was determined by methylation-specific PCR and bisulfite sequence-PCR. The effects of the demethylating agent 5-aza-2′-deoxycytidine on cell proliferation, apoptosis, cell cycle, migration, and invasion were investigated in GC cell lines.
Results
Cancer-specific methylation was detected in the upstream CpG-rich regions of miR-335, which dramatically silenced its transcriptional activity in GC cell lines and tissues. Low levels of miR-335 expression and high levels of miR-335 methylation in GC tissues were associated with poor clinical features and prognosis. Restoration of miR-335 expression in GC cells promoted cell apoptosis, inhibited tumor cell migration, invasion, and proliferation, and arrested the cell cycle at G0/G1 phase. Overexpression of miR-335 significantly reduced the activity of a luciferase reporter containing the 3′ untranslated region of V-crk avian sarcoma virus CT10 oncogene homolog-like (CRKL).
Conclusions
MiR-335 functions as a tumor suppressor and may be silenced by promoter hypermethylation. It plays a role in inhibiting tumor cell migration, invasion, and proliferation, arresting the cell cycle at G0/G1 phase, and promoting apoptosis in GC cells through targeting CRKL.
Hinweise
Jia-kui Zhang and Yong-shuang Li contributed equally to the writing of this article
Gastric cancer (GC) is a common cancer and the second leading cause of cancer-related deaths worldwide [1, 2]. Despite considerable improvements in surgical techniques, innovations in clinical diagnostics, and the development of new chemotherapy regimens, the clinical outcome for patients with advanced GC remains poor [3]. A better understanding of the molecular mechanisms underlying GC will therefore improve the prevention, diagnosis, and therapy of this disease.
MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs of 21–25 nucleotides that regulate the expression of target genes in a post-transcriptional manner, leading to mRNA degradation or inhibition of translation through imperfect hybridization to the 3′ untranslated region (3′UTR) of the target mRNAs [4]. miRNAs are promising biomarkers and are involved in regulating diverse biologic processes. They can function as tumor suppressors or oncogenes, depending on the target gene.
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MiR-335 expression was recently shown to be deregulated in several human cancers, and to act as a tumor suppressor in various tumor types [5‐7]. Dysregulation of miRNAs can be caused by epigenetic modifications, including DNA methylation and the covalent modification of histone protein, which do not directly change the DNA sequence. DNA methylation is a heritable and enzyme-induced modification in humans. A high frequency of CpG dinucleotides is often found in the 5′UTR and first exon of most genes, and these dinucleotides control the expression of these genes by modulating their methylation status [8]. Methylation of CpG islands in the gene promoter has been strongly linked to the silencing of tumor-suppressor gene expression in cancer [9‐14]. Several tumor-associated miRNAs have been reported to be silenced by aberrant hypermethylation of their promoter regions in various cancers, including miR-9, miR-137, miR-34b, miR-124a, miR-342, and miR-127 [10, 11, 15, 16]. However, the mechanisms responsible for the downregulation of miR-335 are unclear, and may involve various factors, including genetic and epigenetic modifications. Hypermethylation of the promoter region may also play an important role in silencing miR-335 in GC.
In the present study, we investigated the role of miR-335 methylation and expression in GC cell lines and tumor tissues, the clinicopathological and prognostic values of miR-335 methylation and expression in patients with GC, and the functional impact of ectopic miR-335 expression on GC cell invasion, cell cycle, apoptosis, and proliferation.
Methods
Clinical tissue samples and cell lines
A total of 231 pairs of fresh GC and adjacent non-tumor gastric tissues were obtained from patients with a diagnosis of primary GC who underwent elective surgery at The Cancer Institute of China Medical University (Shenyang, China) between June 2009 and June 2011, after obtaining written informed consent. None of the patients had received chemotherapy or radiotherapy before surgery. The use of tissues for this study was approved by the ethics committee of The Fourth Affiliated Hospital of China Medical University. The median age of the patients (123 men and 108 women) was 61 years (range 26–85 years). Survivors were followed up by phone or mail for a median period of 31 months (range 3–60 months). The study parameters included age, sex, stage, histological grade, macroscopic type, Lauren’s grade, venous invasion, peritoneal dissemination, tumor size, depth of invasion, lymph node metastasis, invasion into lymphatic vessels, and distant metastasis. TNM classification definitions were made according to the American Joint Committee on Cancer TNM staging system (7th edition). Cancer histological grade was assessed according to the criteria set by the World Health Organization. Fresh samples were snap frozen in liquid nitrogen immediately after resection and stored at −80 °C.
The immortalized normal gastric mucosal epithelium cell line (GES-1) and four GC cell lines (SGC-7901, MKN-45, BGC-823, and AGS) were purchased from Shanghai Institutes for Biological Sciences, Chinese Academy of Science (Shanghai, China) in July 2014. All of the cell lines have been tested and authenticated by STR profiling (ATCC) in July 2014. All cells were cultured in RPMI 1640 supplemented with 10% heat-inactivated fetal bovine serum, 100 µg/mL streptomycin, and 100 U/mL penicillin in a humidified cell incubator at 37 °C with an atmosphere of 5% CO2. Cells were treated with 3 µmol/L 5-aza-2′-deoxycytidine (5-aza; Sigma–Aldrich, St Louis, MO, USA) for 72 h to inhibit DNA methyltransferase (DNMT) activity. Cells treated with dimethyl sulfoxide served as a control. An initial report indicated that these conditions could achieve maximal demethylation with minimal toxic effects [17]. SGC-7901 cells were plated 1 day before transfection using Lipofectamine 2000 reagent (Invitrogen, USA) in Opti-MEM (Gibco, USA). A final concentration of 50 nM of RNA mimic or 200 nM of inhibitor and their respective negative control (NCs) were transfected using Lipofectamine 2000 (Invitrogen, USA) according to the manufacturer’s instructions. Cells were harvested for further experiments after 24 or 48 h.
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Extraction of total RNA and real-time polymerase chain reaction
Total RNA was extracted from cell lines using RNAiso Plus (Takara Bio Inc., Shiga, Japan), according to the manufacturer’s protocol. The concentration was determined by ultraviolet–visible spectrophotometry (A260/A280 > 1.9). Quantitative polymerase chain reaction (PCR) was performed using an All-in-One miRNA quantitative RT-PCR Detection Kit (GeneCopoeia, Rockville, MD) according to the manufacturer’s instructions. The primer sequences were as follows: (forward) 5′-TCAAGAGCAATAACGAAAAATGT-3′ (reverse) 5′-GCTGTCAACGATACGCTACGT-3′ (miR-335) and (forward) 5′-CGCTTCGGCAGCACATATAC-3′, 5′-TTCACGAATTTGCGTGTCAT-3′ (U6). U6 mRNA levels were used as an endogenous reference for normalization. Results were normalized to the concentration of U6 RNA and calculated using the \(2^{{ - \Delta \Delta {\text{C}}_{\text{T}} }}\) method, where ∆CT was the difference in threshold cycle (CT) values.
For detection of V-crk avian sarcoma virus CT10 oncogene homolog-like (CRKL) mRNA, first-strand cDNA was synthesized using a High-Capacity cDNA Reverse Transcription Kit (ABI, USA). RT-primers for CRKL mRNA were synthesized as follows: 5′-CATTCCCGGGCGGCTCTCTC-3′ (forward) and 5′-CACGCCTTAGCCCGGCAGAC-3′ (reverse) by Sangon Biotech Company (Shanghai, China). Real-time quantitative PCR (qRT-PCR) was performed according to the TaqMan Gene Expression Assays protocol (ABI, USA). The relative quantification of CRKL mRNA in cell lines was normalized using glyceraldehyde 3-phosphate dehydrogenase (GAPDH) by the \(2^{{ - \Delta \Delta {\text{CT}}}}\) method. The PCR program was designed as follows: 95 °C for 10 min, followed by 35 cycles of 95 °C for 15 s, 60 °C for 30 s, and 72 °C for 45 s. All qRT-PCRs were performed in triplicate.
Luciferase assay
CRKL was predicted as a potential target of miR-335 by bioinformatics analysis (TargetScan, http://www.targetscan.org/). Plasmids containing wild-type and mutant Luc-CRKL 3′UTR were synthesized (GeneCopoeia) and used in luciferase reporter assays. These plasmids contained firefly luciferase and Renilla luciferase as tracking genes. Luciferase activity assays were performed following the manufacturer’s protocols. Briefly, SGC-7901 cells were seeded in six-well plates, cotransfected with miR-335 mimic or NC and lentiviral constructs containing the target gene with wild-type or mutated 3′UTR, using Lipofectamine 2000. Firefly and Renilla luciferase activities were measured 48 h after transfection using a Luc-Pair miR Luciferase Assay Kit (GeneCopoeia) according to the manufacturer’s recommendations. Activities were normalized to Renilla luciferase. Results represent three independent experiments, each performed in triplicate.
Extraction of genomic DNA and bisulfite modification
Genomic DNA was isolated from the cultured cells and specimens using a Universal Genomic DNA Extraction Kit Ver3.0 (Takara Bio Inc.). DNA concentration and purity were controlled by ultraviolet–visible spectrophotometry (1.8 < A260/A280 < 2.0). Genomic DNA (2 µg) was then subjected to bisulfite conversion using an EZ DNA Methylation-Gold Kit (Zymo Research Corporation, Orange, CA, USA). The bisulfite conversion reaction was incubated in a PCR thermocycler at 98 °C for 10 min, followed by 64 °C for 2.5 h, with a final incubation at 4 °C for up to 20 h. The modified DNA samples were dissolved in ddH2O and stored at −80 °C.
DNA methylation bisulfite-modified sequencing
The sequence of miR-335 was searched using the University of California Santa Cruz’s Genome Bioinformatics resource [18]. Scanning for CpG islands in the submitted sequence identified four CpG islands in Methprimer (http://www.urogene.org/index.html). The Berkeley Drosophila Genome Project (BDGP) (http://fruitfy.org:9005/seq_tools/promoter.html) submitted three promoter regions with scores > 0.90: 128–178 base pairs (bp), 935–985 bp, and 1740–1790 bp. We confirmed that the promoter of miR-335 lay within the range of the CpG islands.
Bisulfite-modified sequencing (BSP) primers were designed using Methyl Primer Express® software (v 1.0; Thermo Fisher Scientific. USA): forward 5′-TAAAGGGGGTTTTGTTTTTTTAATT-3′ and reverse 5′-CCCACAAACTACCCACAAAC-3′. The whole process was as follows: DNA methylation bisulfite modification; PCR amplification, electrophoresis, and retrieval; PCR products connected to the pUC18-T vector and transformation; blue/white plaque selection; extraction of plasmids; and finally sequencing of the DNA. The sequencing process was performed by Sangon Biotech (Shanghai, PR China) with genomic DNA from the cell lines and sequencing primers provided by our group.
Methylation-specific PCR
We designed primers for methylation-specific PCR (MSP) using Methyl Primer Express software: methylated forward 5′-GGTTTTAAAAGTCGGTGTTTATTC-3′, reverse 5′-AACTACAACCACTCCGACGTA-3′; and unmethylated forward 5′-GGGTTTTAAAAGTTGGTGTTTATTT-3′, reverse 5′-AACAACTACAACCACTCCAACATA-3′. The amplicon sizes were 125 and 127 bp for methylated and unmethylated products, respectively. We used treated DNA in PCR amplification with TaKaRa Taq Hot Start Version (code number DR007A; Takara Bio Inc.). The PCRs were conducted under the following thermocycling conditions: 5 min at 94 °C, 40 cycles of 30 s at 94 °C, 30 s at 58 °C, 30 s at 72 °C, and a final incubation at 72 °C for 10 min. All PCRs were performed with negative and positive controls using completely methylated and unmethylated human control DNA, respectively, and water. Aliquots of 5 µL of the total 20 µL of the PCR mixture were loaded onto 3% agarose gels, stained with ethidium bromide, and visualized directly under ultraviolet illumination. MSP assays were repeated at least three times for each sample to determine the reproducibility of the results.
Western blot
Cells were lysed using RIPA lysis buffer containing Protease Inhibitor Cocktail (Pierce, USA), and the protein concentration was measured by BCA Protein Assay Kit (Pierce). Proteins were electrophoresed and electrotransferred. The membranes were probed using antibodies against CRKL (1:1000) and GAPDH (1:5000), with horseradish peroxidase-conjugated secondary antibodies. Protein quantities were detected using GAPDH as a loading control.
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Transwell cell migration and Matrigel invasion assays
We determined the invasion ability of 5-aza-treated SGC-7901 and NC cells in vitro by Transwell cell migration and Matrigel invasion assays. Cells were plated in 24-well Transwell plates (8 mm pore size; Corning, NY, USA) to measure their migratory and invasive abilities. For Transwell migration assays, 2.5 × 104 cells were added to the top chamber lined with a non-coated membrane. For invasion assays, chamber inserts were coated with 200 mg/mL Matrigel (BD Biosciences, San Jose, CA, USA), dried overnight under sterile conditions, and 5 × 104 cells were then placed in the top chamber. In both assays, cells were suspended in medium without serum or growth factors, and medium supplemented with serum was used as a chemoattractant in the lower chamber. After 24 h incubation at 37 °C with 5% CO2, the number of cells that had migrated to the basal side of the membrane was quantified by counting 16 independent symmetrical visual fields under a microscope. Cell morphology was observed by staining with hematoxylin–eosin. Assays were conducted in five independent experiments.
Cell proliferation assay and cell cycle analysis
The proliferation abilities of 5-aza-treated SGC-7901, MKN-45 and their NC cells were determined by MTT (3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenylte-trazolium bromide) assay using a Cell Proliferation Kit 1 (Roche Applied Science), according to the manufacturer’s instructions. For cell cycle analysis, cells were treated by ethanol fixation, RNase A, and propidium iodide (PI) staining, and then detected by flow cytometry using a FACSCalibur (Becton–Dickinson, USA). Cell populations in the G0/G1, S, and G2/M phases were quantified by Modfit software (Becton–Dickinson), allowing for cell debris and fixation artifacts.
Cell apoptosis analysis
Cell apoptosis rate was calculated using an Annexin V-FITC Apoptosis Detection Kit I (BD Pharmingen, USA) according to the product instructions. Briefly, cells were washed with phosphate-buffered saline and resuspended in 1× Binding Buffer at a concentration of 1 × 106 cells/mL, followed by the addition of 5 µL fluorescein isothiocyanate (FITC)–Annexin V and 5 µL PI to 100 µL of the cell suspension, incubation for 15 min in the dark, and the addition of 400 µL 1× Binding Buffer. Apoptosis was analyzed by flow cytometry (FACSCalibur, Becton–Dickinson) using CellQuest software (Becton–Dickinson). Apoptotic cells were defined as Annexin V-FITC-positive, PI-negative cells.
Wound-healing assay
5-Aza-treated SGC-7901 and NC cells were grown to confluence. A wound was made through the monolayer using a p20 pipette tip, and the migration rate was calculated according to the equation: percentage wound healing = [(wound length at 0 h) − (wound length at 4, 6, 20, or 27 h)]/(wound length at 0 h) × 100. Two independent experiments were performed.
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Statistical analysis
Statistical analyses were performed using SPSS for Windows statistical software, version 19.0 (SPSS, Chicago, IL, USA). Data are presented as the mean ± standard deviation (SD) from at least three independent experiments. Differences between groups were analyzed using Student’s t tests and nonparametric Mann–Whitney U tests. The relationship between miR-335 expression levels and methylation status in gastric tissues was calculated using χ2 tests (2 × 2 table, Pearson’s analysis). Correlations between methylation status, miR-335 expression levels, and clinicopathologic parameters were analyzed by χ2 with Mann–Whitney U tests for two groups and Kruskal–Wallis tests for three or more groups. Survival curves were plotted using the Kaplan–Meier method and survival differences were assessed using the log-rank test. Univariate and multivariate Cox proportional hazards models were used to assess survival results. Disease-free survival (DFS) was measured from the date of surgery to the time of recurrence, death, or censoring. All P values were two-sided, and differences were considered to be statistically significant when P < 0.05.
Results
Downregulation of miR-335 in human GC cell lines
We examined the expression levels of miR-335 in five human gastric cell lines (SGC-7901, MKN-45, BGC-823, AGS, and GES-1) by qRT-PCR. MiR-335 was downregulated in GC cell lines (SGC-7901, MKN-45, BGC-823, and AGS) compared with normal gastric epithelial cells (GES-1) (MKN-45, 0.154 ± 0.016-fold, P < 0.01; SGC-7901, 0.138 ± 0.013-fold, P < 0.01; BGC-823, 0.432 ± 0.076-fold, P < 0.01; AGS, 0.749 ± 0.072-fold, P = 0.01) (Fig. 1a).
Fig. 1
Expression of miR-335 and CRKL in GC cell lines and tissues. a Analysis of miR-335 expression levels in cell lines by qRT-PCR. MiR-335 expression levels in four GC cell lines were significantly down-regulated compared with levels in normal gastric mucosal epithelial cells (GES-1) (P < 0.01). Each bar represents fold change relative to GES-1 cells calculated by \(2^{{ - \Delta \Delta {\text{CT}}}}\). **P < 0.01 between groups. b Expression levels of MiR-335 in paired GC and adjacent normal tissues. Quantification of miR-335 by qRT-PCR. c Analysis of transcription levels of CRKL in cell lines by qRT-PCR. There was no significant difference in CRKL mRNA levels between the four GC cell lines (SGC-7901, MKN-45, BGC-823, and AGS) and GES-1 cells (P > 0.05). d CRKL protein expression levels in GC cell lines (SGC-7901, MKN-45, BGC-823, and AGS) was significantly up-regulated compared with GES-1 cells, determined by western blot. Numbers above the blot indicate normalized protein amounts relative to the negative control, as determined by densitometry
Fig. 2
Kaplan–Meier plots of disease-free survival in relation to miR-335 expression and methylation levels. a Kaplan–Meier disease-free survival curves in relation to miR-335 expression levels. Disease-free survival of patients with low miR-335 expression (n = 115) was significantly lower than that of patients with high miR-335 expression (n = 116; log-rank test; P = 0.013). b Kaplan–Meier disease-free survival curves in relation to miR-335 methylation levels. Disease-free survival of patients with hypermethylation of miR-335 (n = 127) was significantly lower than that of patients with hypomethylation of miR-335 (n = 104; log-rank test; P = 0.035)
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Relative expression levels of miR-335 in human GC tissues and correlation with clinicopathological characteristics
To determine if miR-335 expression was associated with GC, we compared miR-335 expression levels in a large cohort of 231 primary GC tissues with levels in pair-matched adjacent non-tumor tissues, using qRT-PCR. MiR-335 levels were significantly downregulated in GC tissues compared with paired adjacent non-tumor tissues (P < 0.001; Fig. 1b). We divided the 231 patients with GC into high-miR-335-expression (n = 116) and low-miR-335-expression groups (n = 115), according to miR-335 expression levels above or below the median value, respectively. Low miR-335 expression was significantly associated with tumor size (P = 0.030), venous invasion (P = 0.027), peritoneal dissemination (P = 0.027), lymph node metastasis (P = 0.033), poor pT stage (P = 0.040), poor pN stage (P = 0.038), poor TNM stage (P = 0.005), and invasion into lymphatic vessels (P = 0.041) (Table 1).
Table 1
The correlation between expression levels of miR-335 in GC specimens and GC clinicopathologic features
Low expression (n = 115)
High expression (n = 116)
P
n
%
n
%
Age
0.728
>55
87
76
90
78
≤55
28
24
26
22
Sex
0.641
Male
63
55
60
52
Female
52
45
56
48
Histological grade
0.616
Well
39
34
43
37
Poor
76
66
73
63
Tumor size (cm)
0.030*
≤3.5
24
21
39
34
>3.5
91
79
77
66
Macroscopic type
0.702
Early stage
3
3
3
3
Borrmann I + II
10
9
14
12
Borrmann III + IV
102
88
99
85
Lauren's grade
0.813
Intestinal
36
31
38
33
Diffuse
79
69
78
67
Venous invasion
0.027*
Absent
81
70
96
83
Present
34
30
20
17
Liver metastasis
0.669
Absent
105
91
104
90
Present
10
9
12
10
Peritoneal dissemination
0.027*
Absent
95
83
107
92
Present
20
17
9
8
pT stage
0.040*
T1
1
1
5
4
T2
20
17
24
21
T3
30
26
42
36
T4
64
56
45
39
pN stage
0.038*
N0 + N1
29
25
44
38
N2 + N3
86
75
72
62
Metastasis lymph node
0.033*
Negative
26
23
41
35
Positive
89
77
75
65
Invasion into lymphatic vessels
0.041*
Negative
87
76
100
86
Positive
28
24
16
14
AJCC stage
0.005*
I + II
48
42
70
60
III + IV
67
58
46
40
Based on Pearson χ2 test and Fisher exact test
pT primary tumor, pN primary node, AJCC American Joint Committee on Cancer
* Statistically significant
We also analyzed the association between miR-335 expression and survival duration using Kaplan–Meier analysis with the log-rank test. High miR-335 expression was significantly associated with better DFS (P = 0.013) (Fig. 2a).
Fig. 3
CRKL is a target of miR-335. a MiR-335 target-site residues in the CRKL 3′UTR are highly conserved among different species. b Direct interaction between miR-335 and CRKL was detected by dual-luciferase reporter assay. Over-expression of miR-335 in SGC-7901 cells (SGC-7901/miR-335) significantly decreased the luciferase signal of CRKL/pMIR/WT compared with the negative control (NigmiR), while mutation of the putative miR-335-binding site abolished this suppressive effect. **P < 0.01). c qRT-PCR analysis of miR-335 in SGC-7901 cells transfected with miR-335-mimic or NC miRNA. Relative expression of miR-335 in SGC-7901 cells transfected with miR-335 mimic was significantly higher than in NC cells. ***P < 0.001 between groups. d qRT-PCR analysis of miR-335 in SGC-7901 cells transfected with miR-335 inhibitor or NC. Relative expression of miR-335 in SGC-7901 cells transfected with miR-335 inhibitor was significantly lower than in NC cells. ***P < 0.001 between groups. e Effect of miR-335 on CRKL mRNA levels in SGC-7901 cells detected by qRT-PCR. Over-expressing miR-335 had no significant effect on CRKL mRNA levels in SGC-7901 cells. f Effect of miR-335 on CRKL protein expression in SGC-7901 cells detected by western blot analysis. CRKL protein expression was significantly suppressed by over-expressing miR-335 in SGC-7901 cells
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CRKL levels in GC cells and role as target of miR-335
To identify the potential target genes of miR-335, we integrated bioinformatic algorithms using the publicly available database TargetScan (http://www.targetscan.org). Among a total of 255 genes that were potentially targeted by miR-335 (Fig. 3a), CRKL may contribute to the metastasis, invasion, and proliferation of GC.
We detected CRKL expression in four GC cell lines (SGC-7901, MKN-45, BGC-823, AGS) and one immortalized gastric epithelial cell line (GES-1) by qRT-PCR and western blotting. CRKL mRNA levels were similar in all four GC cell lines and GES-1 cells (P > 0.05) (Fig. 1c). However, western blot analysis indicated that CRKL protein expression levels were increased in GC cell lines compared with GES-1 cells (Fig. 1d), suggesting post-transcriptional regulation of CRKL expression in GC cells.
To validate the miRNA–target interactions, we evaluated CRKL expression in SGC-7901 cells transfected with either miR-335 mimic or NC. Overexpression of miR-335 dramatically suppressed endogenous CRKL at the protein level compared with NC, as demonstrated by western blot and qRT-PCR (Fig. 3c–f). To confirm if CRKL was regulated by miR-335 through direct binding to its 3′UTR, wild-type and mutant CRKL 3′UTRs were constructed and cloned downstream of the luciferase gene in a pGL3-control vector, followed by luciferase assay in SGC-7901 cells. The wild-type 3′UTR reporter construct was associated with significantly lower luciferase activity in cells expressing miR-335 compared with those expressing NC miRNA. In contrast, there was no significant difference in relative luciferase activities of pGL3-CRKL-MUT reporters in SGC-7901 cells transfected with miR-335 or NC miRNA (Fig. 3b). Overall, these results indicated that CRKL was directly targeted by miR-335, and that CRKL expression in GC was negatively regulated by miR-335.
Downregulation of miR-335 by DNA methylation in GC cell lines and tissues
Tumor suppressor miRNAs, including miR-335, have been reported to be silenced by aberrant DNA hypermethylation. We therefore investigated the mechanisms whereby miR-335 functions in GC cells, with an emphasis on epigenetics. We determined if miR-335 expression was silenced by DNA methylation by examining the reactivation of miR-335 expression in GC and NC cells following treatment with the demethylating agent 5-aza. MiR-335 expression levels recovered by 2–3-fold after 5-aza treatment (Fig. 4a). We also detected CRKL expression levels in 5-aza-treated GC and NC cells by qRT-PCR and western blot. CRKL mRNA levels were similar in 5-aza-treated and NC cells (P > 0.05) (Fig. 4b), but CRKL protein levels were significantly reduced after 5-aza treatment, as indicated by western blotting (Fig. 4c). Furthermore, we detected CpG island methylation of miR-335 in GC cell lines by BSP analysis of multiple clones in the cell lines. Bisulfite sequencing in all four GC cell lines confirmed marked methylation of the promoter region of miR-335 (Fig. 4d).
Fig. 4
MiR-335 was down-regulated by DNA methylation in GC cell lines and tissues. a SGC-7901 and MKN-45 cells were cultured in the absence or presence of the demethylating agent 5-aza for 72 h. MiR-335 levels in treated cells were significantly higher than those in control cells. Error bars represent the SD from five replicates. **P < 0.01 between groups. b SGC-7901 and MKN-45 cells were cultured in the absence or presence of 5-aza for 72 h. CRKL mRNA levels were similar in treated and negative control cells (P > 0.05). Error bars represent the SD from five replicates. c CRKL expression levels were significantly reduced after 5-aza treatment, indicated by western blot analysis. d DNA methylation BSP in GC cell lines. Five single clones are represented for each sample. CpG islands are depicted and each CpG site is indicated by a vertical bar. Black and white circles represent hypermethylation and hypomethylation, respectively
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DNA methylation levels in GC cell lines and tissues were analyzed by MSP. Hypermethylation and hypomethylation of PCR products were detected by electrophoresis. All four GC cell lines showed hypermethylation (Fig. 5a). Among the 231 GC tissue samples, 127 samples showed hypermethylation (55%) and 104 samples showed hypomethylation (45%) (Fig. 5b). The MSP results were used to analyze correlations between miR-335 expression and clinicopathologic features in patients with GC.
Fig. 5
MSP in GC cell lines and tissues. Presence of a line under the M or U lanes indicates hypermethylation and hypomethylation sequences, respectively. Normal blood (NB) and in vitro methylated DNA (IVD) were used as positive controls for hypomethylation and hypermethylation, respectively. Abbreviations: M hypermethylation, U hypomethylation, G gastric cancer tissue
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We also analyzed the correlations between miR-335 methylation status and clinicopathologic characteristics. Hypermethylation of the miR-335 gene promoter region was associated with tumor size (P = 0.004), miR-335 expression level (P < 0.001), and invasion into lymphatic vessels (P = 0.001) (Table 2). These results indicated that patients with GC with low miR-335 expression, metastatic lymph nodes, invasion into lymphatic vessels, and large tumor size may have hypermethylation in the miR-335 gene promoter region.
Table 2
Association between the promoter region methylation status of miR-335 and clinicopathologic features in patients with gastric cancer
Hypermethylation (n = 127)
Hypomethylation (n = 104)
P
n
%
n
%
Age
0.830
>55
98
77
79
76
≤55
29
23
25
24
Sex
0.371
Male
71
56
52
50
Female
56
44
52
50
Histological grade
0.765
Well
44
35
38
37
Poor
83
65
66
63
Tumor size (cm)
0.004*
≤3.5
25
20
38
37
>3.5
102
80
66
63
Macroscopic type
0.843
Early stage
4
3
2
2
Borrmann I + II
13
10
11
11
Borrmann III + IV
110
87
91
87
Lauren's grade
0.347
Intestinal
44
35
30
29
Diffuse
83
65
74
71
Venous invasion
0.922
Absent
97
76
80
77
Present
30
24
24
23
Liver metastasis
0.622
Absent
116
91
93
89
Present
11
9
11
11
Peritoneal dissemination
0.223
Absent
108
85
94
90
Present
19
15
10
10
pT stage
0.434
T1
3
2
3
3
T2
23
18
21
20
T3
35
28
37
36
T4
66
52
43
41
pN stage
0.240
N0 + N1
36
28
37
36
N2 + N3
91
72
67
64
Metastasis lymph node
0.058
Negative
30
24
37
35
Positive
97
76
67
65
Invasion into lymphatic vessels
0.001*
Negative
93
73
94
90
Positive
34
27
10
10
AJCC stage
0.197
I + II
60
47
58
56
III + IV
67
53
46
44
MiR-335 level
<0.001*
Low expression
78
61
37
36
High expression
49
39
67
64
Based on Pearson χ2 test and Fisher exact test
pT primary tumor, pN primary node, AJCC American Joint Committee on Cancer
* Statistically significant
We also analyzed the association between miR-335 methylation status and survival duration using Kaplan–Meier analysis with the log-rank test. Patients were divided into hypermethylation (n = 127) and hypomethylation (n = 104) groups. Kaplan–Meier estimation indicated that hypomethylation of miR-335 was significantly associated with better DFS (P = 0.035) (Fig. 2b).
Effects of ectopic miR-335 expression on cell apoptosis, proliferation, migration, and invasion, and cell cycle in SGC-7901 cells
We investigated the contribution of DNA methylation of miR-335 to gastric carcinogenesis by determining the effect of DNA methylation of miR-335 on the phenotype of GC cells. SGC-7901 and MKN-45 cells were treated with 5-aza at a final concentration of 3 µmol/L, and miR-335 expression was analyzed after 72 h. 5-Aza treatment increased miR-335 expression in SGC-7901 and MKN-45 cells, as confirmed by qRT-PCR (Fig. 4a). The effect on CRKL expression was validated by western-blot analysis (Fig. 4c). The migration and invasive capacities of SGC-7901 cells, assessed by Transwell and wound-healing assays, respectively, were significantly reduced following restoration of miR-335 expression (Figs. 6a, b, 7a, b). The proliferation abilities of 5-aza-treated, NC-treated, and parental gastric cancer cells were assessed by MTT assay. Cell proliferation of SGC-7901 and MKN-45 cells was significantly reduced by 5-aza compared with NC and parental cells (Fig. 7c). We also investigated the effect of restoration of miR-335 expression on GC cell cycle. The cell cycle in SGC-7901 cells was significantly arrested at G0/G1 phase when miR-335 expression was restored, as shown by flow cytometry (Fig. 8a, b). The percentage of SGC-7901 cells in G0/G1 phase increased from 62.43 to 81.69% (P < 0.05), S phase cells decreased from 28.83 to 10.62% (P < 0.05), and G2/M phase cells decreased from 8.74 to 7.41% (P < 0.05).
Fig. 6
MiR-335 inhibited GC cell invasion and metastasis in vitro. a Invasion and migration assays for SGC-7901 cells treated with 5-aza or NC. Representative fields of migrating (left) and invading (right) cells on membrane. b Migrating and invading SGC-7901 cells were significantly reduced by 5-aza. Numbers of cells were counted in 16 independent symmetrical visual fields under a microscope (×400 original magnification) from five independent experiments. Results represent the means of the values from five independent experiments. Bars indicate SD. *P < 0.05 between groups
Fig. 7
Impact of miR-335 on GC cell migration, invasion, and proliferation. a, b Wound-healing assay in SGC-7901 cells. Quantitative values indicate the mean distance ± SD between the migration fronts measured at different time-points per three wells, and are representative of three experiments. Cells treated with 5-aza closed the wound significantly slower than untreated cells. *P < 0.05 between groups. c Cell proliferation of 5-aza-treated SGC-7901 and MKN-45 cells was assessed by MTT assay. Values are expressed as the mean ± SD of three experiments. Cell proliferation was significantly lower in SGC-7901 and MKN-45 cells treated with 5-aza compared with untreated and parental cells. *P < 0.05
Fig. 8
Effect of miR-335 on cell cycle distribution and apoptosis in SGC-7901 cells. a Representative histograms depicting cell cycle profiles of 5-aza-treated and untreated SGC-7901 cells. The cell cycle was significantly arrested in G0/G1 phase by up-regulating miR-335 in SGC-7901 cells. b Proportions of cells in various phases of the cell cycle. Results are means of five independent experiments ± SD. *P < 0.05 between groups. c Representative histograms depicting apoptosis of 5-aza-treated and untreated SGC-7901 cells. Representative graph demonstrating increased apoptosis rate in SGC-7901 cells treated with 5-aza compared with negative control cells. d Average apoptotic rate of five independent experiments ± SD are shown. **P < 0.01 between groups
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To clarify the biological functions of miR-335 in GC, we investigated its effect on GC cell apoptosis. The apoptosis rates of NC and 5-aza-treated cells were 9.61 and 16.11%, indicating that restoration of miR-335 expression significantly increased apoptosis of GC cells (P < 0.01).
Discussion
Epigenetic mechanisms are important for regulating cell growth and development in mammals. miRNAs represent a class of small non-coding RNAs that regulate gene expression at the post-transcriptional level, and subsequently control crucial physiological processes. Aberrant expression of miRNAs has recently been reported to have an impact on human carcinogenesis. Expression of miRNAs may be regulated by various processes, including DNA methylation and histone modification, [17, 19, 20] DNA copy abnormalities [21], mutation [22], failure of post-transcriptional regulation [23], regulation by transcription factors [24], and defective miRNA biogenesis pathways [25]. DNA methylation is one of the most important types of epigenetic modifications, and has frequently been reported to regulate gene transcription during tumorigenesis [26]. Promoter hypermethylation is associated with inhibition of gene transcription and function of tumor suppressor genes. Several tumor-suppressive miRNAs are known to be silenced by aberrant DNA methylation of their promoter regions in human cancers, including miR-34, miR-129, miR-137, miR-193a, miR-203, and miR-148a [27‐30]. In this study, we focused on miR-335 silencing by methylation in GC cells.
MiR-335 is transcribed from the genomic region of chromosome 7q32.2 and has been shown to be aberrantly expressed in malignant tumors, and to play crucial roles in tumor initiation and progression [31]. Cao et al. [5] observed that miR-335 expression was reduced in epithelial ovarian cancer tissue samples, especially in omental metastases, and low miR-335 levels emerged as an independent prognostic factor for poor overall and relapse-free survival; Wang et al. [32] also identified miR-335 as a tumor suppressor by targeting the ROCK1 gene and inhibiting osteosarcoma cell migration and invasion. However, the detailed expression pattern and potential role of miR-335 in GC remains unclear. Yan et al. [33] reported that GC patients with high levels of miR-335 had a high frequency of recurrence. In contrast, Xu et al. [34] reported that miR-335 was dramatically downregulated in GC cell lines, and low expression of miR-335 was associated with poor pT stage, poor pN stage, and invasion into lymphatic vessels. Further studies are therefore needed to clarify the biological functions and mechanisms of miR-335 in GC. The current study provides the first investigation focusing on epigenetic modifications of miR-335 in GC.
The results of the current study showed that miR-335 was dramatically downregulated in GC cell lines and tissue specimens, suggesting that low expression of miR-335 was significantly associated with GC. Clinicopathological analysis also found that miR-335 expression was negatively correlated with tumor size, venous invasion, peritoneal dissemination, lymph node metastasis, poor pT stage, poor pN stage, poor TNM stage, and invasion into lymphatic vessels, while low expression of miR-335 was associated with shorter DFS in GC patients.
In the present study, we analyzed the methylation status of miR-335 in GC cell lines using BSP and MSP. Hypermethylation of the promoter region of miR-335 was relatively common in GC cell lines, and was also detected in 55% of 231 GC tissue specimens by MSP. We analyzed the correlation between methylation status and miR-335 expression in GC tissues and showed that downregulated expression of miR-335 was significantly correlated with CpG island DNA hypermethylation in the promoter region. We also showed that miR-335 methylation status was associated with tumor size and invasion into lymphatic vessels in a large sample of patients with GC. These results suggest that promoter hypermethylation might repress miR-335 expression, and that low expression might in turn contribute to metastasis, proliferation, and invasion in GC. Furthermore, we confirmed that miR-335 expression in GC cells was upregulated by the DNA methylation inhibitor 5-aza.
Functional studies demonstrated that restoration of miR-335 reduced cell proliferation, migration, and invasion, arrested the cell cycle, and increased GC cell apoptosis, indicating a potentially suppressive role for miR-335 in GC. We further explored the mechanisms whereby miR-335 regulates tumor invasion, metastasis, proliferation, cell cycle, and apoptosis in GC by identifying miR-335 targets in GC carcinogenesis and progression. We searched for direct target genes by bioinformatics miRNA–mRNA 3′UTR matching, and found that CRKL included a putative miR-335-binding site within its 3′UTR. CRKL acts as a transcriptional activator in Bcr-Abl-expressing cells, and constitutes the major tyrosine phosphorylated protein in chronic myelogenous leukemia [35]. Protein-expression analysis of activated phosphorylated CRKL in normal and malignant tissues indicated that CRKL was associated with malignant transformation, and was selectively up-regulated in numerous epithelial tumors, including gastric, prostate, ovary, and colon cancers [36]. Yeung et al. [37] found that CRKL was over-expressed in rhabdomyosarcoma tissues and cell lines, and was involved in rhabdomyosarcoma tumor growth. Research on non-small cell lung cancer cell lines by Kim et al. [38] showed that CRKL was amplified in diverse oncogenic lung cancer phenotypes, and RNA interference-mediated knock-down of CRKL could significantly suppress the proliferation, survival, and migration of lung cancer cell lines. Natsume et al. [39] recently observed that CRKL protein was amplified in GC, and this report suggests that CRKL may be a critical molecule and a possible therapeutic target involved in the initiation and development of GC.
Our experimental results confirmed that CRKL was a target of miR-335 in GC cells. Overexpression of miR-335 significantly reduced the activity of a luciferase reporter containing the 3′UTR sequence of CRKL, while mutation of the seed region sites in the 3′UTR abolished the regulatory effect of miR-335. Furthermore, restoration of miR-335 down-regulated CRKL protein expression post-transcriptionally. These study thus clearly suggest that miR-335 may directly regulate CRKL expression by inducing mRNA degradation and translational suppression. Collectively, down-regulation of miR-335 may thus contribute to gastric carcinogenesis, at least partly via up-regulation of CRKL.
Conclusion
This study demonstrated for the first time that miR-335 was dramatically down-regulated in GC, and revealed that low expression of miR-335 was associated with tumor size, venous invasion, peritoneal dissemination, lymph node metastasis, poor pT stage, poor pN stage, poor TNM stage, invasion into lymphatic vessels, and shorter DFS. Moreover, restoration of miR-335 suppressed GC cell proliferation, migration, and invasion, arrested the cell cycle, and increased GC cell apoptosis. MiR-335 thus functions as a tumor suppressor in GC. To the best of our knowledge, this study also provides the first evidence for associations between hypermethylation of the miR-335 promoter region and its expression, and between hypermethylation status and shorter DFS, invasion into lymphatic vessels, and large tumor size, which could be predictive factors for tumor prognosis and biological behavior in GC tissues. The effects of miR-335 on GC cell migration, invasion, and proliferation observed in this study may be partly due to its regulation of CRKL. These data warrant further investigations of the miR-335/CRKL axis as a prospective therapeutic target in the treatment of GC. Further large-scale and long-term follow-up studies focusing on the methylation status of miR-335 in GC should be undertaken in the future.
Authors’ contributions
JZ carried out study concepts, study design, data acquisition and interpretation, manuscript preparation and editing, participated in the real-time quantitative reverse transcriptase-PCR, Methylation-specific PCR, bisulfite sequence-PCR (BSP), Luciferase reporter assay and western blot. YL followed up the patients, collected the data, analyzed the data, participated in the in vitro migration and invasion assays, cell cycle analysis, cell apoptosis analysis wound-healing assay. CZ carried out data acquisition, manuscript review, participated in the in vitro migration and invasion assays and cell cycle analysis. DD carried out study concepts, study design, manuscript preparation and editing, manuscript review, participated in the bisulfite sequence-PCR (BSP), Luciferase reporter assay and western blot. All authors read and approved the final manuscript.
Acknowledgements
The authors would like to thank the surgeons and general practitioners for their participation in contact with patients. We also thank Huai-yu Hu (Department of Neuroscience and Physiology, SUNY Upstate Medical University), Zhen Zhang (Department of Biochemistry and Molecular Biology, University of Kansas Medical Center), Xiao-mei Wu (Department of Clinical Epidemiology and Center of Evidence Based Medicine, The First Hospital of China Medical University) and Fang Li–li (Department of Anesthesiology, The Second Affiliated Hospital Zhejiang University School of Medicine.) for their careful reading and kind suggestion.
Competing interests
The authors declare that they have no competing interests.
Availability of data and materials
Please contact author for data requests.
Ethics approval and consent to participate
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The study was conducted with the permission of the Ethics Committee of China Medical University (Shenyang, China). Informed consent was obtained from all individual participants included in the study.
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