Clinical sample
Tissue samples were collected from 34 RB patients for extirpation operation in Tianjin Medical University Eye Hospital from June 2019 to June 2020, aged 10–37 months (2.28 ± 0.58 years). All RB patients received VCR chemotherapy before operation, but did not receive radiotherapy. According to the recurrence time, they were allocated into two groups: 21 patients in the VCR-sensitive group (within 6 months) and 13 patients in the VCR-resistant group (over 6 months). The normal retinal tissues were collected from 13 patients with eyeball rupture as the control. The tissue samples were washed with phosphate-buffered saline (PBS), frozen overnight in liquid nitrogen, and then stored at -80℃.
Construction, culture and transfection of VCR-resistant RB cell line
RB cell lines (Y79, WERI-Rb-1, SO-Rb50, and SO-Rb70) were obtained from National Infrastructure of Cell Line Resource (www.cellresource), and human retinal astrocyte (HRA) was purchased from ZQXZ Biotech (Shanghai, China). All the cells were cultured in RPMI-1640 medium (Sigma-Aldrich, Merck KGaA, Darmstadt, Germany) containing 10% fetal bovine serum (FBS) and 1% penicillin-streptomycin-glutamine (100X, Gibco, Grand Island, NY, USA) at 37℃ with 5% CO2 [21]. The VCR-resistant RB cell line was constructed by E-test, lasting for 9 months, and the concentration of VCR ranged from 75 µg/mL to 600 µg/mL. Then the cells were maintained for 2 weeks without VCR. The 50% inhibiting concentration (IC50) was determined using cell counting kit-8 (CCK-8) assay [7].
Y79/VCR cells in logarithmic growth phase were seeded into 6-well plates (4 × 105 cells/well). Upon reaching 70–80% confluence, the cells were transfected with mimic-NC, miR-130a-3p mimic, PAX6 overexpression plasmid, and its NC plasmid (Genechem Co., Ltd, Shanghai, China) in line with the instructions of Lipofectamin 2000 (11668-019, Invitrogen, Calsbad, CA, USA). Briefly, 10 µg plasmids (the final concentration was 50 nM) diluted by 250 µL serum-free Opti-MEM were fully mixed with 5 µL Lipofectamin 2000 diluted by 250 µL serum-free Opti-MEM. After standing for 20 min, the mixture was added to the 6-well plates. The transfected cells were cultured under the conditions of 37℃, 5% CO2, and saturation humidity. After 48 h, the medium containing transfection reagent was replaced by RPMI-1640 medium containing 10% FBS for another 24–48 h incubation.
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay
Y79/VCR cells were seeded into 96-well plates (1 × 104 cells/well). After 24, 48, and 72 h-incubation, the cells were supplemented with 20 µL MTT solution (Sigma-Aldrich) and cultured for 4 h in a humidified incubator. After the supernatant was removed, 200 µL dimethyl sulfoxide (DMSO) was added to each well. The absorbance at 490 nm was measured by a microplate reader (Bio-Rad, Hercules, CA, USA) [22].
CCK-8 assay
The transfected Y79/VCR cells were seeded into 96-well plates (1500 cells/well) and cultured for 6 days. The cells were cultured in VCR medium of different concentrations (100 µg/mL, 200 µg/mL, 400 µg/mL, and 600 µg/mL) for 48 h. Then each well was supplemented with 20 mL CCK-8 solution (5 mg/mL) for another 4 h-incubation. The crystal was dissolved by 100 mL DMSO. The absorbance at 570 nm was measured by a microplate reader (Bio-Rad). The IC50 of VCR was calculated, and finally the resistance index (RI) was obtained: RI = IC50 (drug-resistant strain)/IC50 (parental strain) [23].
Reverse transcription quantitative polymerase chain reaction (RT-qPCR)
Total RNA was extracted from tissues and cells using TRIzol (Invitrogen), and the concentration and purity of RNA were detected using ultraviolet spectrophotometer (1011U, NanoDrop Technologies Inc., Wilmington, DE, USA). The cDNA was generated according to the instructions of TaqMan MicroRNA Assays Reverse Transcription primer (4427975, Applied Biosystems, Inc., Carlsbad, CA, USA). The primers of miR-130a-3p and PAX6 were designed and synthesized by Takara (Dalian, China) (Table 1). RT-qPCR was performed on ABI7500 qPCR instrument (ABI, Foster City, CA, USA). The reaction conditions were pre-denaturation at 95℃ for 10 min, denaturation at 95℃ for 10 s, annealing at 60℃ for 20 s, and extension at 72℃ for 34 s, a total of 40 cycles. The relative expression of genes was examined by 2-ΔΔCt method, with GAPDH or U6 as the internal reference. The experiment was repeated 3 times.
Table 1
Primer sequence for RT-qPCR
Name of primer | Sequences |
miR-130a-3p | F: 5'-CGCCGCAGTGCAATGTTAAA-3' |
R: 5'-GTGCAGGGTCCGAGGTATTC-3' |
U6 | F: 5'-GCTTCGGCAGCACATATACT-3' |
R: 5'-GTGCAGGGTCCGAGGTATTC-3' |
PAX6 | F: 5'-AGACACAGCCCTCACAAAC-3' |
R: 5'-ATCATAACTCCGCCCATTC-3' |
GAPDH | F: 5'-CCACCCAGAAGACTGTGGAT-3' |
R: 5'-TTCAGCTCAGGGATGACCTT-3' |
Western blot
The tissues were placed in centrifuge tubes and added with 500 µL radio-immunoprecipitation assay cell lysate (R0020, Solarbio, Beijing, China) (containing 1 mmol/L phenylmethylsulfonyl fluoride). The tissues were homogenized at 1000 g until fully lysis. The homogenate was placed on ice for 30 min and then centrifuged at 4℃ and 12000 g for 4 min, and the supernatant was collected and stored at -80℃. The protein concentration was determined using bicinchoninic acid kit (AR0146, Boster, Wuhan, Hubei, China), and the concentration of each sample was adjusted to 3 µg/µL. The extracted protein was added with the loading buffer and boiled at 95℃ for 10 min. Equal amount of protein on each lane (30 µg) was separated on 10% SDS-polyacrylamide gel and transferred onto polyvinylidene fluoride membranes (P2438, Sigma-Aldrich). The membranes were blocked with 5% bovine serum albumin (10-L16, Biopartner Science & Technology, Beijing, China) for 1 h and cultured with the primary antibody rabbit anti-PAX6 (ab195045, 1:1000, Abcam Inc., Cambridge, MA, USA) at 4℃ overnight. Following tris-buffered saline-tween (TBST) buffer washing (3 times × 5 min), the membranes were cultured with the secondary antibody (ab6721, 1:2000, Abcam) for 1 h. Following TBST washing (3 times × 5 min), the bands were developed with enhanced chemiluminescence reagent. The gray level of each band was analyzed using Image J 1.48 (Media Cybernetics, Bethesda, MA, USA), with GADPH (ab181602, 1:10000, Abcam) as internal reference.
Dual-luciferase assay
The target site sequence (WT) of PAX6 mRNA 3’-UTR region and the sequence after the mutation of WT target site (MUT) were synthesized. The pmiR-RB-REPORT™ plasmid was subjected to restriction enzyme digestion, and then the synthetic target gene fragments WT and MUT were inserted into pmiR-RB-REPORT™, respectively. The constructed WT and MUT vectors were transfected with mimic-NC or miR-130a-3p mimic into 293T cells (CRL-3216, American Type Culture Collection, Manassas, Virginia, USA). The cells were collected and lysed 48 h after transfection. The relative activity of luciferase was detected using dual-luciferase detection kit (RG005, Beyotime, Shanghai, China). The experiment was repeated 3 times.
Xenograft tumor in nude mice
Thirty-two BALB/c nude mice (aged 4–6 weeks and weighing 16-20g) were raised in laminar flow cabinet (specific pathogen grade) and exposed to ultraviolet light at regular intervals. Cages, rubbish, drinking water, and feed were sterilized. The room temperature was maintained at 24℃-26℃, and the relative humidity was maintained at 40%-60%. Y79 or Y79/VCR cells were mixed with PBS respectively to prepare single cell suspension (1 × 106 cells/mL), and then 50 µL cell suspension was subcutaneously injected into the right side of each nude mouse. After 1 week, the mice were randomly allocated into four groups (N = 8): Y79 group, Y79/VCR group, Y79/VCR + agomiR-NC group, and Y79/VCR + agomiR-130a-3p group. After that, agomiR-NC and miR-130a-3p agomiR were injected into the tumor site respectively. All the mice received intraperitoneal injection of VCR (0.5 mg/kg) for 5 weeks, once a week. The tumor growth was measured with vernier caliper every week. The tumor volume was calculated as follows: V = (L × W2)/2. After 5 weeks, the mice were euthanized with 100 mg/kg pentobarbital sodium (P3761, Sigma-Aldrich). The xenograft tumors were resected and weighed.
Immunohistochemistry
The weighed tumor tissues were prepared into paraffin sections, dewaxed, dehydrated with gradient alcohol, and washed with tap water for 2 min. Then the sections were treated with 3% methanol containing H2O2 for 20 min, and washed with distilled water for 2 min and 0.1 M PBS for 3 min. The tissue sections were treated with antigen repair solution and cooled by tap water. The sections were blocked with normal goat serum sealing solution (C-0005, Haoran Bio Technologies Co., Ltd, Shanghai, China) for 20 min, and then incubated with the primary antibody PAX6 (ab195045, 1:500, Abcam) at 4℃ overnight. Following 0.1 M PBS washing (3 times × 5 min), the sections were incubated with the horseradish peroxidase-labeled streptomyces ovalbumin working solution (0343-10000U, Imunbio Biotechnology Co., Ltd, Beijing, China) at 37℃ for 20 min. Afterward, the sections were developed with 2,4-diaminobutyric acid (ST033, Whiga Technology Co., Ltd, Guangzhou, Guangdong, China), counterstained with hematoxylin (PT001, Bogoo Biological Technology Co., Ltd., Shanghai, China) for 1 min, and treated with 1% ammonia. Finally, the sections were dehydrated with gradient alcohol, cleared with xylene, and sealed with neutral resin, followed by observation under the microscope. Five fields were randomly selected from each section, and the number of positive cells in each field was counted [24].
Statistical analysis
Data analysis was introduced using the SPSS 21.0 (IBM Corp., Armonk, NY, USA) and GraphPad Prism 8.0 (GraphPad Software Inc., San Diego, CA, USA). Data are expressed as mean ± standard deviation. The unpaired t test was used for comparison between two groups. One-way analysis of variance (ANOVA) was employed for the comparisons among multiple groups, followed by Tukey's multiple comparisons test. Repeated measure ANOVA was employed for the comparisons between groups at different time points. The p < 0.05 indicated a significant difference.