2.1. The isolation of bone marrow macrophages (BMMs)
BMMs were used to obtain the osteoclast progenitor cells. Shortly, BMMs were obtained from 8-10-week-old C57 mice's femurs by flushing the MEM (Gibco, USA) medium's bone marrow cavity. BMMs were cultured in MEM medium with 10% FBS-α-MEM (Gibco, USA). The unattached BMMs were gathered and cultured for three days in the MEM medium with M-CSF (25 ng/mL) and FBS (10%) to settle BMMs
2.2. Osteoclast differentiation
For the osteoclast differentiation, BMMs were collected in α-MEM medium with M-CSF (25 ng/mL), RANKL (100 ng/mL), and FBS (10%). The MC3T3-E1 cells were cultured in the α-MEM medium (Gibco, USA) at 37°C with 5% CO2. For osteoblast differentiation, the cells were plated into the 24-well wells and incubated for 7 days or 21 days in the α-MEM comprising 10% FBS, ascorbic acid (50 µg/ml, Sigma, USA). The osteoblast differentiation was analyzed by ALP staining and alizarin red staining, respectively. The siRNA of Nrf2 (5′-GUAAGAAGCCAGAUGUUAAdUdU-3′) was obtained (RiboBio, China) The transfection in the cells was performed by Liposome 3000 (Invitrogen, USA) according to the manufacturer's instructions.
2.3. CCK-8 assay
The cell viability was analyzed by the CCK-8 assays. About 5×103 BMMs were plated in 96-well paltes and incubated for 12 hours, followed by indicated treatment. The BMMs were incubated with a CCK-8 solution (KeyGEN Biotech, China) and culture for another 2 hours at 37°C. The cell viability was analyzed at 450nm absorbance by applying the ELISA browser (Bio-Tek EL 800, USA).
2.4. Tartrate-resistant acid phosphatase (TRAP) staining
Cells were cleaned using PBS and fixed for 5 minutes based on the paraformaldehyde. Cells were cultured at 37 ̊C for 1 hour away from light in the reaction from Leukocyte Acid Phosphatase Assay Kit (Sigma) according to the manufacturer's guidance. Cells were distilled by water, and TRAP-positive cells comprising five or more nuclear were imagined by microscopy and calculated as mature osteoclasts. The quantification was performed by applying the ImageJ software.
2.5. Bone resorption assay
The bone resorption was analyzed by applying a pit formation assay. BMMs were palted in the 24-well plates. The cells were re-collected every three days with fresh α-MEM medium. After 5 days, the cells were treated with NH4OH (1N, 5 minutes) to eliminate the attached BMMs. The bone resorption area was quantified by utilizing the imageJ software.
2.6. Quantitative reverse transcription-PCR (qRT-PCR)
The total RNAs were extracted by TRIZOL (Invitrogen, USA) from the tissues and cells. The first-strand cDNA was synthesized using Stand cDNA Synthesis Kit (Thermo, USA) as the manufacturer's instruction. The qRT-PCR was carried out by applying SYBR Real-time PCR I kit (Takara, Japan). The standard control for mRNA was GAPDH. Quantitative determination of the RNA levels was conducted by SYBR GreenPremix Ex TaqTM II Kit (TaKaRa, Japan). The experiments were independently repeated at least three times. The primer sequences are as follows:
DC-STAMP Forward: 5'-AAAACCCTTGGGCTGTTCTT-3',
DC-STAMP Reverse: 5'-AATCATGGACGACTCCTTGG-3'
c-Fos Forward: 5'- CGGGTTTCAACGCCGACTA-3'
c-Fos Reverse: 5'- TGGCACTAGAGACGGACAGAT-3'
OC-STAMP Forward: 5'- CTGTAACGAACTACTGACCCAGC-3'
OC-STAMP Reverse: 5'- CCAGGCTTAGGAAGACGAAGA-3'
NFATc1 Forward: 5'-CCGTTGCTTCCAGAAAATAACA-3'
NFATc1 Reverse: 5'-TGTGGGATGTGAACTCGGAA-3'
TRAP Forward: 5'-CTGGAGTGCACGATGCCAGCGACA-3'
TRAP Reverse: 5'-TCCGTGCTCGGCGATGGACCAGA-3'
CTSK Forward: 5'-CTTCCAATACGTGCAGCAGA-3'
CTSK Reverse: 5'-TCTTCAGGGCTTTCTCGTTC-3'
2.7. Western blot analysis
Total proteins were extracted using RIPA buffer (Millipore, USA). Nuclear and cytoplasmic proteins were extracted by applying the Nuclear Extraction Kit (Thermo, USA) and Cytoplasmic Extraction Kit (Thermo, USA), respectively. Protein concentrations were measured by using the BCA Protein Quantification Kit (Abbkine, USA). Same concentration of protein was divided by SDS-PAGE (12% polyacrylamide gels), transferred to PVDF membranes (Millipore, USA) in the subsequent step. The membranes were hindered with 5% milk and hatched overnight at 4°C with the primary antibodies for Nrf2 (1: 1000, CST, USA), HO-1 (1: 1000, Abcam, USA), IκBα (1: 1000, CST, USA), p65 (1: 1000, CST, USA), LaminB (1: 1000, CST, USA), NFATc1 (1: 1000, CST, USA), c-Fos (1: 1000, CST, USA), and β-actin (1: 1000, CST, USA), in which β-actin served as the control. Then, the corresponding second antibodies (1: 1000, Abcam, USA) were used for hatching the membranes 1 hour at room temperature, followed by the visualization by using an Odyssey CLx Infrared Imaging System.
2.8. OVX-induced bone loss mouse model
The C57BL/6 mice (8 weeks, male) were randomly divided in three groups (n = 5), including control group, OVX treatment group, and OVX + Spinosin co-treatment group. In the control group, the mice were injected equal volume water;In the OVX treatment group, the mice were intraperitoneally injected with streptozotocin (STZ, 50 mg/kg, 5 days, sigma, USA); In the OVX + Spinosin co-treatment group, the mice were intraperitoneally injected with OVX and orally administered with Spinosin-treated group (50 mg/kg/day). The mice were sacrificed after 9 day, and the tibias from the mice were subjected in high-resolution micro-CT analysis and Histomorphometric analysis. The levels of TRAcp5B and TNF-α were measure by ELISA assays. The loss of femur bone and osteoclasts was analyzed by Hematoxylin and Eosin (H&E) and TRAP staining. Animal care and method procedure were authorized by the Animal Ethics Committee of Linyi Central Hospital.
2.9. Statistical analysis
Data was presented as mean ± SD, and the statistical analysis was conducted by GraphPad prism 7. The unpaired Student’s t-test was used to compare two groups, and the one-way ANOVA was utilized to compare among multiple groups. P < 0.05 were considered as statistically significant.