Study design
HGECs were stimulated by glucose at different times to induce apoptosis. For dose-response studies, hGECs were incubated in different concentrations of glucose (5.5, 15, 25, and 50 mmol/L) for 48h. TdT-mediated dUTP nick end labeling (TUNEL), caspase 9 and MMP were detected. We determined that the lowest concentration of HG-induced apoptosis of hGECs was 25 mmol/L. Then, hGECs were challenged with 25 mmol/L glucose for 12h and 48h, respectively. Cells were divided into the following groups: normal glucose (NG) 12h group (cells were cultured for 12h with 5.5mmol/L glucose); HG 12h group (cells were cultured for 12h with 25 mmol/L glucose); NG 48h group (cells were cultured for 48h with 5.5mmol/L glucose); HG 48h group (cells were cultured for 48h with 25 mmol/L glucose). Then, the autophagy under different conditions was assessed by LC3 Ⅱ and P62. In addition, quantitative real-time polymerase chain reaction (qRT-PCR), western blotting and immunofluorescence staining were performed to evaluate mitophagy. hGECs were transfected with shPINK1 and analyzed by MMP, caspase 9 and annexin V-FITC apoptosis.
Cell culture
The primary hGECs (HUM-iCell-m016) was commercial used and purchased from the company (Cybertron Biotechnology Co., Ltd., Shanghai, China). hGECs were cultured in a complete human epithelial cell media PriMed-iCell-001 (Cybertron Biotechnology Co., Ltd., Shanghai, China) containing 5.5 mmol/L glucose (HyClone Laboratories, Inc. Logan, Utah, USA) supplemented with 10% fetal bovine serum (GIBCO Life Technologies Inc., Grand Island, NY, USA), 1% antibiotics (GIBCO Life Technologies Inc., Grand Island, NY, USA). Cells were cultured in a 37°C incubator with 5% CO2 and 95% air. When the cells reached 80% confluence, they were digested with 0.25% trypsin-EDTA (GIBCO Life Technologies Inc., Grand Island, NY, USA) and used within 3-4 passages for subsequent experiments.
TUNEL assay
Apoptotic cells were detected by a TUNEL detection kit (KeyGen Biotechnology Co., Ltd, Jiangsu, China) according to the manufacturer's protocol [16]. Briefly, cells were fixed with 4% paraformaldehyde for 30min and permeabilized with 1% Triton X-100 for 5 min at room temperature. Cells were cultured in 50 µL TdT enzyme reaction solution for 1h at 37°C. After washing with phosphate-buffered saline, cells were incubated in 50µL of Streptavidin-TRITC working solution at 37°C for 30min. The nuclei were counterstained with 4', 6-diamidino-2-phenylindole (DAPI) at room temperature for 10 min. The whole experiment was carried out in the dark. TUNEL-positive cells were observed and photographed using a laser scanning confocal microscope (LSCM; Olympus Fluoview FV3000, Olympus Corporation, Tokyo, Japan) at excitation/emission wavelengths of 470/550 nm. Five representative fields were selected from each group and the TUNEL-positive cells in each field were calculated by ImageJ software (National Institutes of Health, Bethesda, MD, USA).
Western blotting
Total protein from hGECs treated with different concentrations of glucose was extracted using RIPA lysis buffer with 1% phosphatase inhibitor and 1% PMSF. Then, the protein concentration was measured by the BCA Protein Assay Kit (BOSTER Biological technology Co., Ltd, Wuhan, China) according to the manufacturer’s instructions. Protein expression levels were detected through western blotting as described previously [17]. The following primary antibodies were used: caspase 9 (1:1000 dilution; Cell Signaling Technology Inc., Danvers, MA, USA), LC3 (1:1000 dilution; Cell Signaling Technology Inc., Danvers, MA, USA), PINK1 (1:1000 dilution; Abcam Inc., Cambridge, Massachusetts, USA), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (1:2000 dilution; Bioss Biotechnology Co., Ltd, Beijing, China). Horseradish peroxidase conjugated goat anti-rabbit IgG (1:2000 dilution; BOSTER Biological technology Co., Ltd, Wuhan, China) was used as a secondary antibody. The protein signals were detected by electrogenerated chemiluminescence substrates. The protein gel imaging analysis system (Syngene, United Kingdom) was used to detect grayscale values of bands.
MMP
MMP was measured by a 5,5’,6,6’-tetrachloro-1,1’,3,3’-tetraethylbenzimidazolcarbocyanine iodide (JC-1) kit (Beyotime, Beyotime Biotechnology Co., Ltd., Shanghai, China) according to the method described previously [19]. hGECs were seeded in laser confocal dishes at a density of 8×104 cells per well. Cells were incubated in 1mL JC-1 (10 µm) staining solution for 30 min at 37°C in the dark. Then, the cells were washed twice with JC-1 staining buffer. Cells were observed under LSCM at 530 nm wavelengths for green fluorescence and 590 nm wavelengths for red fluorescence. The ratio of red/green fluorescence intensity was used to measure the depolarization of MMP.
QRT-PCR
Total RNA was isolated from hGECs using TRIzolTM reagent (Invitrogen Corp., Carlsbad, CA) following the manufacturer’s instructions as described previously [19]. RNA concentration was determined with a NanoDrop 2000c spectrophotometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). Precipitated RNA was dissolved in diethylpyrocarbonate water and reverse transcribed into cDNA using PrimeScriptTM RT Master Mix (TaKaRa Biotechnology Co., Ltd., Dalian, China). Primer sequences used for qRT-PCR analysis (LC3 II forward primer 5’-AGCAGCATCCAACCAAAA-3’, reverse primer 5’-CTGTGTCCGTTCACCAACAG-3’; P62 forward primer 5’- TGCCCAGACTACGACTTGTG-3’, reverse primer 5’-GAGAAGCCCTCAGACAGGTG-3’; PINK1 forward primer 5’-GGACGCTGTTCCTCGTTA-3’, reverse primer 5’-ATCTGCGATCACCAGCCA-3’; NIX forward primer 5’-AGTAGCTTATTTGAACTTGAGACCATTG-3’, reverse primer 5’-TGAGGGTTACTGGAATTGGATATGTA-3’; FUNDC1 forward primer 5’-GCAGTAGGTGGTGGCTTTC-3’, reverse primer 5’-TGCTTTGTTCGCTCGTTT; GAPDH forward primer 5’-GCACCGTCAAGGCTGAGAAC-3’, reverse primer 5’-TGGTGAAGACGCCAGTGGA-3’). qRT-PCR was performed on the Applied Biosystems 7500 Fast Real-Time PCR System with SYBR® Premix Ex Taq™ II (TaKaRa Biotechnology Co., Ltd., Dalian, China) according to the manufacturer’s protocol. The obtained CT values were calculated by the 2−ΔΔCt method. The results of relative gene levels were normalized to GAPDH.
Immunofluorescence
The cellular localization of LC3 II and PINK1 was examined by LSCM as described previously [20]. Briefly, hGECs were seeded in laser confocal dishes at a density of 8×104 cells per well. After treatment, cells were fixed for 20 min with 4% precooled paraformaldehyde and washed with phosphate-buffered saline. Then, the cells were permeabilized with 0.2% Triton X-100 for 10 min and blocked with 5% BSA for 30 min at room temperature. hGECs were fixed and incubated with LC3 II (1:100 dilution; Cell Signaling Technology Inc., Danvers, MA, USA) or PINK1 (1:100 dilution; Abcam Inc., Cambridge, Massachusetts, USA), followed by FITC-labeled goat anti-rabbit IgG fluorescent secondary antibody (1:1000 dilution; GenePharma Co., Ltd., Shanghai, China). Then, the cells were counterstained with DAPI (1:1000 dilution; Sigma-Aldrich Trading Co. Ltd., St. Louis, MO, USA) for visualization of the nuclei. Cell images were further observed and photographed using LSCM at excitation/emission wavelengths of 488/530 nm. Fluorescence intensity was calculated using ImageJ software.
Lentivirus transfection of PINK1
PINK1 inhibition by lentivirus-mediated shRNA vector and its corresponding empty vectors were designed and synthesized by GenePharma (GenePharma Co., Ltd., Shanghai, China). hGECs were seeded on six-well plates and allowed to attach overnight. At approximately 60-70% confluence, hGECs were transfected with shPINK1 or shNC. The virus was added with 5 µg/mL polybrene and incubated with cells in 5% CO2 at 37°C. After 18h, the supernatant fluid with the viral particles was removed and replaced with fresh medium. Then, the cells were incubated in 5% CO2 at 37°C for an additional 72h. The fluorescence efficiency of the cells was observed by fluorescence microscopy.
Flow cytometric analysis
Apoptosis in hGECs was assessed by an annexin V-FITC/PI apoptosis analysis kit (KeyGen, KeyGen Biotechnology Co., Ltd., Nanjing, China) according to the manufacturer’s protocol [21]. Briefly, cells were digested with 500 µL trypsin without EDTA and centrifuged for 5 min at 2000 rpm. Then, the cells were resuspended in 500 µL binding buffer stained with 5 µL FITC-conjugated annexin V and 5 µL of propidium iodide for 15 min at room temperature in the dark. Quantitative analysis of apoptosis was detected by flow cytometry (Becton, Dickinson and Company, USA) in 1h at excitation/emission wavelengths of 488/530 nm. The percentage of apoptotic events in a given area was counted as the percentage of total apoptotic cells.
Statistical analysis
All experiments were performed at least in triplicate. Quantitative analysis was expressed as the mean ± standard deviation (SD). One-way analysis of variance and post hoc tests were further compared for each group using SPSS 22.0. The data were considered significant at P < 0.05.