Cell lines and cell culture
The normal human epithelial cell line, Hosepic and ovarian cancer cell line, ES-2were obtained from cell bank at the Shanghai Institute of Cell Biology, Shanghai, China, which have been authenticated officially. Hosepic cells were cultured in DMEM (Dulbecco's Modified Eagle Medium) supplemented with 10% fetal bovine serum (FBS), nonessential amino acids, 1% sodium pyruvate (PS) and antibiotics. ES-2 cells were cultured in Mycoy’s media supplemented with 10% fetal bovine serum (FBS), nonessential amino acids, 1% sodium pyruvate (PS), and antibiotics. All cells were cultured in 37 °C incubator with 5% CO2.
Subjects and sample collection
32 plasma samples from advanced-stage OC patients diagnosed were obtained from the tissue bank of Fudan University Shanghai Cancer Center. All samples were stored at − 80 °C prior to use. Informed consent was obtained from all participants when they were asked to collect samples. The survival data were summarized based on the regular follow-ups. This study (SCCIRB-090371-2 in 2015) was approved by the Ethical Committees of Fudan University Shanghai Cancer Center and performed in concordance to the principles of Declaration of Helsinki.
Isolation of EVs
For EVs from cells, cell culture supernatant was collected and transferred to a 15 mL centrifuge tube and spined at 3,000g for 10 min at 4 °C to eliminate cell debris in the sample. Subsequently, the supernatant was added to another tube and centrifuged at 100,000 g at 4°C for 6 hours. The EVs were pelleted at the bottom. 200μL of 1×PBS was added to resuspend the exosome pellet and then transferred to a new 1.5mL tube. The mixture was centrifuged at 12,000g for 2 min at 4°C and the supernatant containing EVs was kept at -80 °C for further use. The protein concentration was measured using BCA assays (Thermo Fisher, USA).
Characterization of EVs
As suggested by the International Society of Extracellular Vesicles (ISEV), a guideline for standardized characterization of particular EVs has been proposed25, leading to further reproducible information through different studies. In this study, the protein markers and morphologies of the isolated samples were evaluated. The proteins in EVs were dissolved in RIPA buffer. The solution was boiled at 95 °C for 5 min and then the mixture was spined at 10,000 g for 5 min to obtain the clean protein solution. The solution was loaded onto an SDS-PAGE gel (5 % stacking gel, 12 % running gel, Bio-Rad). After electrophoresis and transferring, the polyvinylidene fluoride (PVEF) membrane (Bio-Rad, Munich, Germany) was incubated with 0.1% BSA in TBST at room temperature for 1 h. The primary antibody was added onto the membrane and incubated at 4 °C for 12 h. After washing, the secondary antibody was incubated with the membrane at room temperature for another 1 h. The membrane was rinsed for three times. Finally, the membrane was detected under the ChemiDoc XRS imaging system (Bio-Rad, Munich, Germany) using an enhanced chemiluminescence (ECL) system (Thermo Scientific, OR, USA). The concentration and size were measured with nanosight (Malvern, UK) and FEI Tecnai™ T12 electron microscope (Thermo Scientific, OR, USA).
miRNA microarray analyses
miRNAs were extracted by NanoSep 100K (Pall Corporation, USA) and de-salted by pushing through an ultracentrifuge tube (Sartorius Stedim Biotech). miRNA ULSTM Labeling Kit (Kreatech Diagnostics, The Netherlands) was used to make the fluorescent labels. The pre-hybridized Mouse miRNA OneArray® v5 (Phalanx Biotech Group, Hsinchu, Taiwan) was used for fluorescent labels detection. The hybridization was kept for 16 h at 37 ℃ and the non-specific binding was removed by rinsing three times. Then the slide was dried and scanned by an Axon 4000B scanner (Molecular Devices, Sunnyvale, CA, USA). The intensity of Cy5 of every single spot was quantified by GenePix 4.1 software (Molecular Devices). R program (2.12.1) was used for data processing. Data were removed whose flag < 0 within all arrays. The normalization was performed using invariant set normalization method.
Quantification of miRNA levels by RT-qPCR
cDNA was synthesized using PrimeScript RT Reagent Kit with gDNA Eraser (Takara Bio, Shiga, Japan) based on the instructions without further modifications. The quantification process was conducted using TB Green™ Premix Ex Taq™ (Takara Bio, Shiga, Japan) on ABI QuantStudio Dx (Applied Biosystems, USA). Before the amplification and quantification. Even amount of total RNA and cDNA was added in the experiment. The mix was heated at 95 °C for 30 sec, followed by 40 cycles of 95 °C for 3 sec, 60 °C for 30 sec, and 72 °C for 1 sec. The internal standard, U6, was also tested as the reference. The 2–ΔΔCt method was applied for determination of miRNA levels.
Intracellular uptake of EVs
EVs were enriched from patient plasma as described above. The purified EVs were labeled with PKH26 (Thermo Scientific, OR, USA), a red membrane dye, according to the protocol without further modifications, followed by extensive rinsing to eliminate the residue dye. Cells were incubated with PKH-labeled EVs (10μg/ml) for 30 min. Cells were then washed three times with 1 x PBS to remove EVs on the surface. Subsequently, 4% paraformaldehyde was added and kept for another 5 min. The EVs inside the cells were visualized by confocal microscopy (Carl Zeiss Inc, Thornwood, NY, USA).
Preparation of miR-320a mimic/inhibitor loaded EVs
An optimized calcium chloride transfection method has been performed to incorporate the miR-320a mimic or inhibitor into the EVs. In brief, 5 ml of EVs-free media was added in a 60-mm petri dish. 200 pmol miRNA mimic or inhibitor were mixed with 20 μg EVs in 1 x PBS. 0.1 M calcium chloride was added and placed on ice for 30 min. Heat shock was applied at 42 °C for 1 min, and the mixture was kept on ice again for 5 min. For any free miR, 5μg/ml RNase was added and kept for 30 min at 37 °C.
Cell viability assay
To characterize the cell proliferation, the cell counting kit-8 (CCK-8) colorimetric assay (DOJINDO Molecular Technologies Inc, Kumamoto, Japan) was utilized. Both cell lines were inoculated in 96-well plates. After overnight incubation, 200 nM of the miR-320a mimic/miRNA control or miR-320a inhibitor/inhibitor control were added to the cells for transfection. Each experiment was conducted in triplicate. Cell viability was characterized at 48 h by measuring the optical intensity of each well at 450 nm by a plate reader.
Cell migration and invasion assay
Matrigel gel was diluted with serum-free culture medium. 100 μl of diluted gel was added into the upper chamber of the 24-well transwell and then kept at 37°C for 4 h. The cell concentration was adjusted to 5×105 cells/ml and 200 μl cell suspension was added to the upper chamber. As for the lower chamber, 600 μl medium with 5 μg/ml fibronectin was added. For migration assay, the plate was placed at 37°C for 24 hours, while invasion for 48 hours. After incubation, the non-invasive cells were wiped off from the upper chamber. The transwell was fixed with methanol for 5 min and dried. 500 μl of 0.1% crystal violet was added to the 24-well plate, the membrane was immersed in the culture medium for 30 min at 37°C. After being rinsed for three times, the chamber was placed under the microscope for imaging.
Angiogenesis assay
The Matrigel was put in an ice box in a refrigerator at 4°C to slowly melt the glue overnight in prior to the experiment. 10 μl Matrigel was added to ibidi angiogenesis slides and covered with the lid. The ibidi angiogenesis slide was put into the 10 cm petri dish and stood for about 30 minutes for gelation. The concentration of cell suspension was adjusted to 2×105 cells/ml. 50 μl of cell mixture was seeded to each well. The image of angiogenesis was regularly recorded and the length of tube, the area, the number of rings, and the intercepts were analyzed, respectively.
Double luciferase reporter assay
The wild-type and muted binding sites of ZC3H12B 3′UTR were incorporated into the firefly luciferase gene within the psiCHECK-2 vector (Promega), respectively. HEK-293T cells were added into 96-well plates. After incubation for 24 h, 10 ng firefly luciferase reporter plasmid and an equal amount of miR-320a mimic or negative control RNA were added for transfection using Lipofectamine 3000 (Invitrogen, CA, USA). After another 24 h, the luciferase assay kit (Promega, Madison, WI, USA) was used according to the manufacturer’s manual without further modifications.
Statistical analysis
All data are shown as the means ± SEM. One-way analysis of variance (ANOVA) was used for multiple comparisons. Student’s t test was performed for comparisons between two groups. p < 0.05 was considered to indicate statistical significance (*p < 0.05, **p < 0.01, ***p < 0.001). The statistical analysis was performed using GraphPad Prism® (Version 7).