Cell culture
Human NPC cell lines CNE-1 (well differentiation) and CNE-2 (poor differentiation) used in this study were obtained from China Center for Type Culture Collection (Wuhan, China), cultured in RPMI-1640 medium (GIBCO), added with 10% fetal bovine serum (FBS),100 U/ml penicillin and 100 mg/ml streptomycin (Invitrogen, Carlsbad, CA, USA) in 5% CO2 at 37°C.
Establishment of concurrent chemoradioresistant NPC cell lines
When CNE-1 and CNE-2 cells were up to the confluence of 60%, they were cultured in the complete medium containing 0.05ug/ mL cisplatin, and were exposed to 6Gy irradiation using a 6-MV X-ray linear accelerator (ELEKTA, Beijing, China) at 200 cGy/min dose rate. After 24h, the cells were cultured in the fresh complete medium containing 0.05ug/ mL cisplatin (Sigma-Aldrich, Saint Louis, MO, USA). When the cell density reached 80-90%, they were further passaged and cultured. The next chemoradiotherapy was performed when the cell growth was in stable state for a total of 10 times. The CNE-1 and CNE-2 cell lines treated with chemoradiotherapy were named as CNE1R and CNE2R, respectively.
Colony formation assay
Each cell (CNE-1, CNE-2, CNE1R and CNE2R) were respectively inoculated in 6-well plates at a density of 300, 400, 800, 1000, 2000, 4000 cells/well and cultured in 200 μL cell culture medium for 24h. Subsequently, the cells were removed the original medium and cultured in the fresh complete medium containing 0.05ug/ mL cisplatin. At the same time, these cells were respectively treated with gradient doses of irradiation (0 Gy, 2 Gy, 4 Gy, 6 Gy, 8 Gy and 10 Gy) according to the gradient cell numbers under the above radiation condition. After 14 days, the cells were washed, fixed for 30 min using methanol, and stained with 0.1% crystal violet (Sigma, MO, USA) for 10 min. The number of clones >50 was counted under a microscope (DMIRB, Leica, Germany) to calculate the plating efficiency (PE) (%) that is equal to (number of colonies/inoculated cell number) ×100%. Surviving fraction (SF) that is equal to number of colonies/ (number of cells seeded × PE) ×100%. We plotted the cell survival curve using the multi-target single-hit model: SF=1- (1-e-D/D0)N, and the radiobiological parameters D0 (the dose that gave an average of one hit per target), Dq (quasi-threshold dose) and SF2 (the SF following exposure to 2 Gy radiation) were determined using the survival curve. The experiments were repeated in triplicate.
EV isolation, quantitation and characterization
EVs isolation and identification referred to the MISEV2018 guidelines [22]. NPC cells were cultured until 80%-90% confluence was reached, and then changed with serum starvation. After incubation for 48 h, the supernatants were centrifuged at 300 g for 10 min to remove dead cells and cell debris. Subsequently, the supernatants were filtered using a 0.22 μm membrane and concentrated to 1 ml using Amicon Ultra 15 Centrifugal Filter Unit (Millipore, USA). The lipids left was transferred to a fresh microcentrifuge tube. EVs were isolated using the ExoQuick TC kit (System Biosciences, USA) referring to manufacturer's protocol. The isolated EVs were resuspended in PBS. The EVs were fixed using 4% paraformaldehyde and loaded onto a TEM copper grid (Agar Scientific Ltd., Stansted, UK), and then washed with 1% glutaraldehyde, PBS, and distilled water., Next, EVs were incubated with 4% uranyl acetate and observed under TEM (Hitachi H7500 TEM, Tokyo, Japan).
Nanoparticle tracking analysis (NTA) was performed by Nanosight (ZetaView PMX 110, Particle Metrix, Meerbusch, Germany, ZetaView 8.04.02 SP2) to characterize the size distribution and concentration of EVs derived from CNE1, CNE2, CNE1R and CNE2R.
Western Blotting
EV protein quantification was obtained using Pierce BCA Protein Assay Kit (Life Technologies, USA) following the manufacturer’s instructions..The proteins were separated by 10% SDS-PAGE and transferred to nitrocellulose membranes. Membranes were incubated with primary antibodies for mouse anti-CD63 (Cat# ab108950, Abcam, 1:1000) and GAPDH (Proteintech, Chicago, UK; 1:1000) at 4°C overnight, and then washed and incubated with secondary antibody (Santa Cruz, Dallas, USA) for 2 hours. Protein bands were visualized using enhanced chemiluminescence reagent (ECL AdvanceTM; GE Healthcare) and images were obtained via a ChemiDoc MP system (Bio-Rad, Hercules, CA, USA).
EV Labelling and Internalization
EVs were isolated from chemoradioresistant NPC cells (1.5×106 cells), and suspended in 100ul 1×PBS, and then were mixed with 1ul DiI lipophilic dye (Invitrogen, Carlsbad, CA, USA) for 4 min at 25°C. After adding 1ml FBS to terminate the reaction, the 200ul ExoQuick-TC was added into mixture on ice for incubating 30 min. The labelled EVs were reisolated by ultracentrifugation at 14000 rpm at 4°C for 3 min. Recipient CNE1 and CNE2 cells were incubated with 300ul DiI-labelled EVs for 3 h at 37℃ in 5% CO2, fixed using 4% paraformaldehyde at 25°C for 10 min, washed, and incubated with DAPI (Invitrogen, Carlsbad, CA, USA) for 10 minutes at 25°C, and subjected to fluorescence microscope (Nikon, TE300) with 40× magnification (n = 3).
Migration and invasion assays
For the migration assays, 1×105 cells were seeded onto the upper chamber (Cat#353097, FALCON,) added with 500 μl of serum-free DMEM and incubated with or without EVs. Subsequently, the 700 μl DMEM with 10% FBS was supplemented into the lower chambers. After 24 h of incubation, the cells underneath the membrane were fixed using 4% paraformaldehyde and stained with 800μl crystal violet (Cat# C0121; Beyotime, Shanghai, China) for 30 min at room temperature. The number of migrated cells was counted under a microscope. For the invasion assays, the basic steps were the same as above, except that chambers precoated with matrigel (Cat #354480, BioCoat,).
The RNA extraction, cDNA Synthesis Using an RT2 First Strand Kit and human cancer lncRNA PCR array
Total RNA derived from NPC cells was obtained using Trizol reagent (Invitrogen, Carlsbad, CA, USA), and its concentration was measured by Nanodrop 2000 (Tiangen, Beijing, China). RNA were performed with agarose electrophoresis for RNA quality control. The cDNA was synthesized using RT2 first strand kit by reference to the manufacturer’s manual (Cat:330401, Qiagen, Hilden, Germany). The 10 μl reverse transcription mixture was prepared and thoroughly mixed with the genomic elimination mixture (10μl) and performed an incubated at 42℃ for 15 min. After the reaction termination at 95℃ for 3 min, a 91 μl of RNase-free water was added into each reaction mixture and mixed for following assays.
The RT2 SYBR Green Master mix was used for human cancer lncRNA PCR array (Yingbiotech, Shanghai, China). Briefly, a real-time PCR mixture was prepared by adding the 650 μl of 2X PCR master mix, 102µl diluted cDNA, and 548µl RNase free water. The membrane on the PCR Array was carefully, and 10 µl of the PCR mixture was added to each of the wells. Finally, PCR Array was performed in triplicates. GAPDH was used as endogenous control. The 2-∆∆Ct method was applied to calculate the relative lncRNAs expression.
Statistical analysis
All data were expressed as mean ± SD in three duplicate experiments. GraphPad Prism 8 (GraphPad Software, Inc.) was applied for statistical analysis. Student’s T-test was utilized to calculate the differences. The statistically significant differences were considered at p < 0.05.