Isolation and culture of hBMSCs
Human bone marrow mesenchymal stem cells (hBMSCs) gifted from the Zhongshan Medical University were isolated from fresh human bone marrow aspirates from patients undergoing fracture management. The patients which incuding two women and a man were aged 23–34 years old. Under sterile conditions, hBMSCs were maintained in alpha minimal essential medium (α-MEM; Sigma-Aldrich CO, St. Louis, MO, USA) and supplemented with 10% fetal bovine serum (FBS, Thermo Fisher scientific, Waltham, MA, USA) and 1% penicillin-streptomycin (Corning,Sigma-Aldrich, St. Louis, MO, USA). The cells were cultured at 37°C in a humidified incubator containing 5% CO2. The medium was changed every 2 days until confluence. Cultures were serially passaged by replating confluent cells at half their confluent density, growing the cells to confluence, and replating again at half their confluent density. Cells were used at passage 3-6.
Cell viability assay
Cell viability was assessed with Cell Counting Kit-8( CCK-8, Beyotime,Nangjing, China)following the manufacturer’s instructions. HBMSCs were seeded in 96-well plates at a density of 5000 cells/well. After 24 hour, they were exposed to 0, 0.625, 1.25, 2.5, 5.0, 10.0, 20.0, 40.0, 80.0 or 160.0 µM CdCl2 (Sigma Aldrich, USA) for 24 hour. Subsequently, a 100 µl volume of CCK-8 solution was added to each, followed by incubation for 2hour at 37°C, 5%CO2. Consequently, absorbance at 450 nm was measured on a microplate reader (Biotek, Winooski, VT, USA).
CdCl2 treatment
For osteogenic differentiation of hBMSCs, CdCl2 (Sigma Aldrich, USA) at concentrations of 0, 2.5, or 5.0 µM was added into the osteogenic induction medium to replace the growth medium.
ALP, Alizarin red S and Oil red O staining
Leukocyte Alkaline Phosphatase Kits (ALP, Sigma Aldrich, USA) was used for ALP staining according to the manufacturer’s instructions. HBMSCs were seeded in 12-well plates at a density of 2×105 cells/well in a growth culture medium. When their confluence reached 80%, hBMSCs were exposed to CdCl2 with osteogenic differentiation for 14 days. After that, cells were fixed with 4% formaldehyde and 5% citrate in acetone at room temperature for 30 seconds. The fixed cells were washed with PBS and incubated with 0.2% naphthol AS-BI and 0.2% diazonium salt at room temperature for another 15 min. After washing the plates with phosphate buffer saline(PBS), images were taken at 10× magnification under an optical microscope (Zeiss, German).
The hBMSCs were seeded into 12-well plates at a density of 2 × 105 cells/well and exposed to CdCl2 with osteogenic differentiation for 14 days. The cells were washed with PBS, fixed with 10% formaldehyde at room temperature for 10 min, and incubated with 40 mM alizarin red S (Sigma Aldrich, USA) solution at room temperature for 20 minutes. After discarding the solutions and washing the plates with PBS for 4 times, images were taken under an optical microscope at 10× magnification (Zeiss, German).
Oil red O powder (Sigma, USA) was used to make oil red O mother liquor, and oil red O dyeing was performed. Take out hBMSCs that have induced adipogenic differentiation from the incubator, discard the old medium, and wash 3 times with PBS, 1 ml each time. Add 3ml 4% paraformaldehyde to the cell culture dish (6cm), return the dish to the incubator, and fix the cells for 30min. Discard the paraformaldehyde, wash the cells 3 times with PBS, and add 3 ml of oil red O application solution (prepared in advance and kept at room temperature for later use). Return the dish to the incubator again and stain for 60 min. Discard the oil red O application solution, wash the cells 3 times with 60% isopropanol, 1 ml each time, and then wash the cells 3 times with sterilized water, 1 ml each time. Finally, 2 ml of 4% paraformaldehyde was added, and the mouth of the dish was sealed with a parafilm, and the image was taken on a light microscope (Nikon, Japan) at 10x magnification.
Protein isolation and western blot
Total cellular proteins were extracted using ice cooled strong RIPA lysis buffer containing 1 mmol/L phenylmethanesulfonyl fluoride and 1 mmol/L phosphatase inhibitor cocktails (all from KeyGEN BitoTECH, Nanjing, CHN), and quantified by bicinchoninic acid protein assay kit (KeyGEN BitoTECH, Nanjing, CHN). Mixtures of cellular proteins and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) loading buffer (5×, PanEra, Guangzhou, CHN) were heated at 100°C for 10 min. Approximately 40 µg of denatured proteins were loaded and separated by SDS-PAGE (12% acrylamide), and then transferred to the polyvinylidene difluoride membranes (0.45 µm, Millipore, Bedford, MA, USA) using a wet-transfer system at 100 V for 50 minutes. After blocking with 5% nonfat milk which was dissolved in Tris-buffered saline-Tween (TBST, 0.1% Tween), membranes were incubated overnight at 4°C with a 1:1000 dilution of anti-glyceraldehyde 3-phosphate dehydrogenase (Cell Signaling Technology, USA) and an antibody for Runx2 (Cell Signaling Technology, USA), Osterix (Abcam, USA), BMP-2/-4/6/7(ABclonal, Wuhan,CHN), SMAD1/4/5, p-Smad1 and p-Samd1/4/9 complex (Cell Signaling Technology, USA). Membranes were thereafter rinsed five times with TBST washing solution, followed by incubating with corresponding horseradish peroxidase co-conjugated secondary antibodies (1:2000 dilution for anti-mouse IgG and 1:5000 dilution for anti-rabbit IgG, all from Santa Cruz, CA, USA) for 1.5 h at room temperature. After washing, strips in membranes were visualized using chemiluminescent peroxidase substrate (Millipore, Bedford, MA, USA) and Tanon-5200 chemical luminescence developing system (Tanon, Shanghai, CHN). GAPDH served as the internal reference. The relative expressions of Runx2, Osx, MP-2/-4/6/7, SMAD1/4/5, p-Smad1 and p-Samd1/4/9 complex proteins in treatment groups were determined by grey value analysis using Image J software (https://imagej.net/) and normalized to the control. ELISA kits were used to detect the concentration of BMP-2(Abcam, Cambridge, UK) and BMP-4(Thermo, Frederick, USA) respectively.
Participant
The target population comprised people, between 40 and 80 years of age, who lived in southern China Guangdong province for 15 years or longer without occupational exposure of cadmium. The population was divided into two groups: namely bone damage and non-bone damage groups depending on DEXA results (T-score) as following: bone damage patients with T-score ≤ − 2.5 and non-bone damage patients with T-score > − 2.5[29]. Both bone damage and non-bone damage groups included 67 subjects (male 28, female 39).
Residents in the area used the cadmium-polluted river water to irrigate their fields and rice is the main food. Participants were excluded who had received any drug known to alter bone metabolism, such as corticosteroids. Written informed consent was obtained from each participant. Variables, such as gender, age, BMI, and smoking status (never/ever) which may act as potential confounders were collected. BMI was calculated as weight in kg divided by squared height in m.
Collection of samples and analytical method
Peripheral blood and first-morning urine samples were collected before breakfast. Blood samples were centrifuged at 1500g for 15 min at 4°C, and serum was separated. Obtained blood and urine samples were subdivided and kept frozen at -80 ℃ until analysis. Those urine samples intended for Cd analysis were acidified and kept at room temperature until analysis. First-morning urine was defined as the first sample collected from an individual at or after 5:00 in the morning. U-Cd concentrations were determined using inductively coupled plasma mass spectrometry (7700 × ICP-MS, Agilent Technologies, USA). Multi-element calibration standards were prepared from an environmental calibration standard (Agilent part number: 5183 − 4688). Then, 10 µg/mL internal standard mix (Agilent part number: 5183 − 4680) was diluted to 1.0 µg /mL with 5% (v/v) HNO3. Further, 10 ng/mL tuning solution (Agilent part number: 5184 − 3566) was diluted to 1 ng/mL with 5% (v/v) HNO3. The inductively coupled plasma mass spectrometry (ICP-MS) was operated in helium collision mode (for interference removal). Percent recovery was between 95% and 105%, and relative standard deviation less than 10%. The commercial QC sample (Seronorm Trace Elements Urine L-2, SERO AS, Norway) was analyzed for every 20 samples to ensure instrument performance. U-Cd concentrations were either adjusted or divided by urine creatinine (µg/g). Urinary levels of creatinine were determined using a microplate reader (BioTek Instruments, Inc., Winooski, VT, USA) at a wavelength of 546nm, which using Creatinine assay kit (sarcosine oxidase, Nan jing Jian Cheng, China). ELISA kits were used to detect human BMP-4 in plasma (Thermo, Frederick, USA) and BMP-2 in serum (Abcam, Cambridge, UK), respectively.
Determination of bone mineral density
Bone mineral density at the forearm was measured using peripheral dual-energy x-ray absorptiometry (pDXA; model EXA-3000, OsteoSys, Korea). The site used was forearm bone density. The apparatus was calibrated before each use, and measurements were performed by an experienced operator. The T-score was derived by comparing the DXA scan result of the subject with that of a healthy adult of the same gender and race. The standard data on Asian adults were used as reference for the highest bone density.
Statistical analysis
Graphpad 7.0 was applied for statistical analyses. Descriptive statistical analyses were performed for characteristics of the study population. Continuous variables were shown as the mean ± standard deviation, and differences between cases and controls were examined using t-tests for normally distributed data or the Mann-Whitney U tests for non-normally distributed data. Categorical variables were presented as numbers and percentages, and intergroup comparisons were analyzed using chi-square tests. Spearman was used to examine the possible relationship between U-Cd, BMP-4 and T value. Mediation analyses were performed to further investigate the influence of U-Cd and BMP-4 on T value.
Data analyses were performed using SPSS25.0 and R(ggplot2, mediation). Two-sided values of P < 0.05 were considered statistically significant.