Introduction
Ovarian cancer is a common tumor and the most lethal malignancy of the female reproductive organs. A combination of carboplatin and paclitaxel has been widely used as chemotherapy for ovarian cancer patients. Although the patients initially respond successfully to paclitaxel-based chemotherapy, in most cases, they eventually become insensitive to the chemotherapy [
1]. Several mechanisms have been demonstrated regarding chemoresistance to paclitaxel, such as over-expression of the multidrug transporter P-glycoprotein [
2], selective expression of beta-tubulin isotypes [
3], down-regulation of bcl-2 [
4], or aberrant cell signaling [
5]. Nevertheless, the overall molecular mechanisms of paclitaxel resistance have yet to be clarified.
MicroRNAs (miRNAs) are a family of short non-coding RNAs that negatively regulate gene expression at the post-transcriptional level. Hundreds of miRNAs have been found in the human genome and play critical roles in regulating cell signaling pathways such as the transforming growth factor-beta, Wnt, Notch and epidermal growth factor pathways by repressing the expression of different mRNAs expression or through co-regulation with transcription factors [
6‐
9]. Consequently, dysfunction of miRNAs and their target genes can lead to a variety of disorders. Therefore studying the role of miRNAs will provide a better understanding of the molecular events involved in diverse biological processes, and contribute to the identification of new targets in tumor prevention and treatment.
MiRNA-149 directly targets the 3’-UTR of MyD88 mRNA and post-transcriptionally regulates MyD88 protein expression. MiRNA-149 may be a key modulator in the TLR/MyD88 signaling pathway in macrophages through negative regulation of MyD88-dependent Toll-like receptor signaling [
10]. In our previous study, the expression of MyD88 was closely associated with paclitaxel resistance in lung cancer A549 cells [
11].
The relationship between miRNA-149 expression and paclitaxel chemoresistance in human ovarian cancer cells remains largely unknown. In this study, we investigated whether miRNA-149 modulates cellular sensitivity to paclitaxel by regulating the expression of MyD88 in ovarian cancer A2780 cells.
Materials and methods
Cell line and maintenance
The A2780 cell line was obtained from the Institute of Cell Biology (Shanghai, China). The cells were maintained in RPMI 1640 medium (Gibco-BRL, Carlsbad, CA, USA) supplemented with 10 % fetal bovine serum (FBS), 100 U/ml penicillin and 100 U/ml streptomycin at 37 °C in a humidified incubator with 5 % CO2. The cells were demonstrated to be free of mycoplasma.
Construction of miRNA-149 inhibitor and MyD88 lentiviral vectors
MiRNA-149 inhibitor (5’-GGGAGUGAAGACACGGAGCCAGA-3’) was inserted into the LV3-pGLV-H1-GFP/puro lentiviral vector, and siRNA (5’-UUCUCCGAACGUGUCACGUdTdT-3’) was used as a negative control. MyD88 whole cDNA synthesized by GenePharma (GenePharma, Shanghai, China) was subcloned into the LV5-pGLV-EF1a-GFP/Puro Lentiviral plasmid vector. Lentiviruses expressing inhibitor against miRNA-149, MyD88, and the controls were produced by co-transfection of 293 T cells using polybrene (GenePharma, Shanghai, China) according to standard protocols. A2780 (5 × 104) cells were infected with lentivirus at a MOI (multiplicity of infection, pfu number/cell) of approximately 100 for 24 h. Cells were then transferred into complete medium.
RNA isolation and reverse transcription polymerase chain reaction (RT-PCR)
Small RNAs were purified from differently treated A2780 cells using an RNA purification kit (TIANGEN Biotech, Beijing, China). Total RNA was extracted with Trizol reagent according to the protocol described by the supplier (TakaRa, Dalian, China). First-strand cDNA was synthesized from 1 μg of total RNA in a 20-μl reaction mixture using the PrimeScript RT reagent kit (TakaRa, Dalian, China). Quantitative real-time PCR-based gene expression analysis was performed on a real-time PCR instrument (7300, Step One Plus, Applied Biosystems, USA) using a standard SYBR-Green PCR kit. The parameters used for all PCR reactions were as follows: One cycle of 95 °C for 2 min, followed by 40 cycles of 95 °C for 15 s, and 60 °C for 30 s. Specific primer sets were used for RT-PCR of the U6 control, miR-149, β-actin control, and MyD88. The relative expression of each target gene was calculated using the 2-ΔΔct method.
Analysis of apoptosis
The percentage of apoptotic cells was quantitated using the Annexin V-PE Kit (Becton-Dickinson) according to the manufacturer's instructions. Stained cells were analyzed by flow cytometry during the first 30 min of staining. ≥10,000 cells were measured using a FACScan instrument (Becton-Dickinson) and the data were analyzed using FlowJo software (Tree Star Inc.).
Cytotoxicity assay
Cell viability was assessed using the Cell Counting Kit 8 (CCK-8) (Dojindo Laboratories, Kumamoto, Japan) assay. 6 × 104 A2780 cells (100 μl) were seeded into 96-well plates. After 24 h of incubation, paclitaxel (Sigma) was added at concentrations of 0 μM, 0.1 μM, 0.2 μM, 0.3 μM, 0.4 μM and 0.5 μM (each concertration were done repeat five experiments); the cells were then incubated for 48 h. After 48 h, 10 μl of CCK-8 solution was added to each well, followed by 4 h of incubation at 37 °C. The OD values were read at dual wave lengths of 450 nm and 630 nm to determine cell viability using a microplate reader (Thermo Fisher Labsystems).
Analysis of cell cycle
Cells (2 × 106) were pelleted by spinning for 5 min at 1000 rpm and 4 °C and resuspended in 1 ml of cold PBS. After fixation by adding 4 ml of absolute ethanol, the cells were centrifuged and resuspended in 1 ml of PBS. Then, 100 μl of 200 μg/ml DNase-free RNaseA was added to the cell suspension and incubated for 30 min at 37 °C. The cells were stained with 100 μl of 1 mg/ml propidium iodide (light sensitive) and incubated for 5–10 min at room temperature before analysis.
Western blot
The protein concentration of each sample was determined by BCA Protein Assay Kit (Beyotime, Jiangsu, China). Equal amounts of protein were loaded and separated discontinuously on 12 % sodium dodecyl sulfate-polyacrylamide gels (SDS-PAGE), and subsequently transferred onto a PVDF membrane (Amersham Pharmacia, UK). The membrane was then incubated in TBST blocking solution (Tris-buffered saline including 0.1 % Tween-20) containing 5 % skim milk for 2 h at room temperature, followed by separate incubation with primary antibodies against Bax, MyD88, Bcl-2 (Cell Signaling, USA) and β-actin (Beyotime, Jiangsu, China) overnight at 4 °C. After washing, the membrane was incubated with HRP-conjugated anti-mouse, anti-rabbit, or anti-goat secondary antibodies for 2 h. After several washes, the immunoblot was detected with enhanced chemi-luminescence reagent (Pierce Biotechnology, USA) according to the manufacturer's instructions.
Immunocytochemistry
Expression of MyD88 protein was determined using immunocytochemistry. In total, 1 × 105 cells were seeded into a Millicell EZ Slide (Millipore, Shanghai, China). After 24 h of incubation, cells were fixed on slides using 4 % paraformaldehyde. The cells were permeabilized for 10 min with 0.1 % Triton X-100 in PBS and blocked with blocking buffer (2 % BSA, 0.1 % Triton X-100) for 30 min at room temperature. After blocking, the cells were washed with PBS and incubated overnight with rabbit anti-human MyD88 antibodies (Sigma, Shanghai, China) at 4 °C. On the following day, the cells were washed three times with PBS, and then incubated with Cy3-labeled goat anti-rabbit IgG (H + L) (Beyotime, China) for 1 h followed by washing with PBS three times.
Cell migration assay
Cell migration were evaluated using the Transwell Permeable Support (Corning) according to the manufacturer’s instructions. Five 200-multiple microscopic fields were randomly selected to calculate the total count of the invaded or migrated cells. All assays were conducted three times.
Statistical analysis
For all analyses, the measurements obtained from the groups were expressed as the means ± SD for all data determined. Statistical analysis was performed using an unpaired Student's t-test followed by Tukey's test. P < 0.05 was considered statistically significant.
Discussion
An increasing number of studies suggests that miRNAs play important roles in diverse biological processes, such as development, cell proliferation, apoptosis, fat metabolism, and oncogenesis [
12]. Over the years, attention has focused on the role of miRNAs in reversing drug resistance and regulating the sensitivity of cancer cells to chemotherapeutants [
13,
14]. Dysregulated expression of various miRNAs has been found in ovarian carcinoma. Dysregulation of the microRNA let-7e and down-regulation of miR-30c, miR-130a, and miR-335 indicate direct involvement of some miRNAs in the development of chemoresistance [
15]. Down-regulation of miR-21 promotes apoptosis and chemosensitivity in ovarian cancer [
16]. MiRNA-370 [
17], miRNA-93 [
18], and miRNA-100 [
19] are related to the chemosensitivity of ovarian cancer.
Aberrant expression of miR-149 has been reported in many cancers including colorectal cancer [
20], nasopharyngeal carcinoma [
21], clear-cell renal cell carcinoma [
22], prostate cancer [
23], and gastric cancer [
24]. Li et al. [
25] found that over-expression of miR-149 inhibited glioblastoma cell proliferation and migration. As a methylation-sensitive miRNA, miR-149 plays an important role as a tumor suppressor in colorectal cancer with prognostic and therapeutic implications [
26]. However, the relationship between miRNA-149 expression and the sensitivity to paclitaxel in ovarian cancer remains unknown. Our results show that down-regulation of miRNA-149 decreases the sensitivity of A2780 cells to paclitaxel treatment.
MyD88 is a key molecular cohesion molecule in the Toll-like receptor (TLR) signaling pathway. Elevated MyD88 expression has been found in parenchymal cells in various types of cancer [
27‐
29]. Expression of MyD88 was elevated in more than 70 % of patients with EOC and has been considered as an indicator of tumor metastasis and paclitaxel chemoresistance, in addition to a factor for significantly poor prognosis [
30,
31]. Overexpression of MyD88 decreases the sensitivity to paclitaxel in ovarian cancer and hepatocellular carcinoma cells [
32,
33].
In the present study, expression of miRNA-149 in A2780 cells was decreased by specific inhibitor using lentiviral vectors. miRNA-149 down-regulation resulted in increased expression of MyD88 in A2780 cells. The results are consistent with previous results that over-expression of miR-149 in RAW264.7 cells was associated with decreased MyD88 at the protein level [
10]. In the presence of paclitaxel, the proliferation of A2780 cells was significantly inhibited in the control group, suggesting that miRNA-149 down-regulation may decrease the sensitivity of A2780 ovarian cancer cells to paclitaxel treatment. Furthermore, the sensitivity to paclitaxel was decreased in MyD88 over-expressing A2780 cells.
Paclitaxel is an effective anti-cancer drug against a variety of cancers. Paclitaxel reduces the dynamicity of the mitotic spindle, causing G2/M cell cycle arrest [
34]. In our study, we analyzed the cell cycle of A2780 cells with miRNA-149 down-regulation after treatment with paclitaxel for 48 h. The G2/M phase transition was decreased significantly in A2780 cells in the transduced group compared to the control group. The reduction in the number of cells at G2 phase and the accumulation of cells at the G0/G1 phase suggest that miRNA-149 is associated with paclitaxel chemosensitivity.
Depending on the cell type and drug, cells undergo apoptosis during mitotic arrest or abnormal mitosis [
35]. Paclitaxel directly induces apoptosis of several types of tumor cells through a variety of mechanisms, such as phosphorylation of Bcl-2, activation of caspase-3 and caspase-9, and the mitogen-activated protein kinase signal transduction pathway [
36‐
38]. In the present study, flow cytometric assays were used to detect the apoptosis of A2780 cells after down-regulation of miRNA-149. We found that down-regulation of miRNA-149 decreased the apoptosis induced by paclitaxel when compared to the control group. Furthermore, we showed that down-regulation of miRNA-149 in A2780 cells enhanced the expression of the anti-apoptotic protein Bcl-2 and inhibited the expression of the pro-apoptotic protein bax, which may have led to paclitaxel resistance. Our previous study demonstrated that [
11] MyD88 was involved in anti-apoptosis and drug resistance through the alteration of bax and bcl-2 levels.
In this study, we have shown that reduced expression of miRNA-149 enhanced the migration of A2780 cells. Chan et al. showed that down-regulation of microRNA-149 suppressed cell migration/invasion and metastasis in breast cancer by targeting migration and invasion-related genes [
39]. Taken together, our findings suggest that miR-149 facilitates the migration of ovarian cancer cells through regulation of MyD88.
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
YZ participated in the design of the study, carried out the molecular genetic studies, and drafted the manuscript. FX participated in the design of the study and performed the statistical analysis. RW involved in revising the manuscript critically. JX carried out the immunoassays. ZN participated in the sequence alignment. JJ participated in cell culture. XK conceived of the study, and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.