Establishment of DKD mice model. Male C57BL/6 mice (8-week old) were purchased from Slaccas-Shanghai Lab Animal Ltd. (SPF II Certificate; No. SCXK2012-0005) and kept under specific pathogen-free and normal housing conditions in a 12-h light and dark cycle.
After one week of being adapted to the new environment, mice were intraperitoneally injected with STZ (Sigma-Aldrich) in two doses (100 and 150 mg/kg) for two consecutive days (Days 0 and 1; n = 6 mice per group). At Day 8, blood glucose levels were monitored using a glucose meter (Abbott). Mice with blood glucose levels higher than 16.8 mmol/L were defined as hyperglycemic and used for experiments28. At Days 29 and 64, blood glucose levels were measured using assay kits, following manufacturers’ instructions (Fenghui, Shanghai, China). Survival status of the animals in each group was assessed during the whole experiment. The mice were euthanized and the kidneys were collected for further examination at Day 64.
Biochemical assays. Urine was collected for 5 h via a device in a metabolism cage at Days 32 and 50. The concentration of urinary albumin was tested using an enzyme-linked immunosorbent assay (ELISA) kit, and the urinary albumin excretion rate (UAER) was calculated, indicating the total excretion of urinary albumin in 5 h.
At Day 64, blood was collected via the orbital sinus and centrifuged to obtain serum29. High-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), serum creatinine, and blood urine nitrogen (BUN) were measured using assay kits (Fenghui, Shanghai, China).
The left kidney were weighed to calculate kidney index (kidney weight/body weight, mg/g), and then homogenized in RIPA lysis reagent (Beyotime, Shanghai, China)28. TNF-α, IL-6, COI IV and transforming growth factor (TGF-β1) in the renal homogenates were measured using ELISA kits (Boatman, Shanghai, China).
Histopathological examination. At Day 64, the pancreas and right kidney were collected, fixed in 10% formaldehyde, and embedded in paraffin wax. The specimens were processed to obtain 4µm-thick tissue sections using a microtome. The tissue sections were stained with hematoxylin and eosin (HE)29. Kidney sections were also processed for periodic acid-Schiff’s (PAS) and Masson's trichrome (Masson) assays.
Immunohistochemistry examination. The expressions of HMGB1, TLR2, TLR4, F4/80, CD14, collagen IV (Col IV) and fibronectin (FN) in kidney were examined using immunohistochemistry29. The sections were dewaxed, rehydrated, and equilibrated in phosphate-buffered saline (PBS, pH 7.4). After quenching endogenous peroxidase activity with 3% H2O2, and blocked with 5% bovine albumin, the sections were incubated with antibodies recognizing HMGB1 (1:400, Proteintech), TLR2 (1:500, Arigo), TLR4 (1:400, Abcam), F4/80 (1:300, Servicebio), CD14 (1:500, Servicebio), Col IV (1:250, Abcam), and FN (1:250, Abcam) overnight. After incubation with a secondary antibody conjugated with horseshoe peroxidase (HMGB1, TLR2, TLR4, Col IV, FN: HRP; 1:1000, Bioworld; F4/80, CD14: HRP; 1:200, Servicebio) for 1 h at 37℃, signals were detected using an HRP substrate diaminobenzidine. The sections were lightly counterstained with hematoxylin and examined under a light microscope (Leica Inc. Switzerland).
HK-2 cells culture. HK-2 cells were obtained from ATCC (SCSP-511, Manassas, VA) and cultured in DMEM/F12 supplemented with 10% fetal bovine serum (FBS, Gibco, USA), 100 U/mL penicillin and 100 mg/mL streptomycin at 37°C in a humidified atmosphere comprised of 95% air and 5% CO2. The cells were subcultured every 2-3 days. Cell culture medium was changed every 2 days.
Western blotting assay. Kidney specimens were homogenized in RIPA lysis reagent and centrifuged to collect supernatant. HK-2 cells were seeded in 6-well plastic dishes. After cultured for 24 h, Cells were incubated with IL-1β (20 ng/ml) or HMGB1 (400 ng/ml), or treated with C29 (a TLR2 inhibitor, 100 µM) for 24 h. The cells were then harvested and incubated in RIPA lysis reagent and centrifuged. The supernatant was collected and mixed with the same volume of 2× SDS loading buffer. Total protein was measured by the Bradford assay (Beyotime, Shanghai, China).
Proteins in the samples were resolved in 10% SDS polyacrylamide gel by electrophoresis and blotted onto polyvinylidene difluoride (PVDF) immunobilon membranes (Millipore, Bedford, MA, USA). The membranes were blocked with 5% skim milk (in Tris-buffered saline with Tween-20 (TBST) buffer) for 1 h at room temperature and then probed overnight at 4°C with antibodies against HMGB1, TLR2, p-NF-κB, FN, PCNA, GAPDH, and β-actin (all 1:1000), followed by incubated with horseradish peroxidase (HRP)-conjugated IgG (1:2000, Beyotime, Shanghai, China) for 2 h at room temperature. Signals were detected by enhanced electrochemiluminescence (ECL) reagent and captured with a camera-based imaging system (BIO-RAD, Santa Clara, CA, USA).
Human and animal rights. No study involving human participants was contained in our study. All mice received humane care in compliance with the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health. All operations were performed anesthesia under 10% ethyl carbamate, and all efforts were taken to minimize suffering. When carrying out euthanasia in this experiment, mice were anesthetized with 10% ethyl carbamate followed by cervical dislocation.