Introduction
Osteoarthritis (OA), a degenerative disease affecting the bones, presents clinically as joint pain, restricted movement, and stiffness. Its essential pathological characteristics include the degeneration of articular cartilage (AC), alterations in the subchondral bone, and inflammation of the surrounding soft tissues [
1,
2]. OA demonstrates a significantly heightened prevalence. Specifically, the incidence of OA witnessed a substantial surge of 102% in 2017 when compared to the figures recorded in 1990 [
3]. Joint replacement surgery (JRS) is widely recognized as the singular therapeutic intervention for advanced OA [
4,
5]. Despite this, surgical treatment imposes a serious economic burden on patients and even society, not to mention the limitations of JRS, including persistent pain and limited prostheses [
6]. Joint replacement surgery (JRS) is widely acknowledged as the singular remedy for advanced OA, thus necessitating the exploration of therapeutic targets for OA [
5].
Numerous studies have speculated that inflammation and autophagy are closely linked to OA progression [
7,
8]. Inflammation plays a pivotal role as a significant risk factor in the development of OA. The presence of the inflammatory cytokine interleukin (IL)-1β disrupts the anabolic processes of chondrocytes by suppressing collagen synthesis and oligoglycan production. Additionally, IL-1β stimulates the production of catabolic factors, including matrix metalloproteinases, as well as other inflammatory transmitters such as IL-6, prostaglandin E2, and nitric oxide (NO) [
9‐
11]. Tumor necrosis factor (TNF)-α and IL-6 exert critically in OA progression [
12,
13]. Autophagy occurs when cells phagocytose substances in their cytoplasm and transport them to lysosomes for degradation, meeting their metabolic needs, and renewing certain organelles [
14]. Research has elucidated that the autophagy process in chondrocytes is diminished in both OA patients and mice models of OA. Subsequent to the suppression of autophagy in chondrocytes within animal models, AC undergoes a progressive deterioration and degenerative process. [
15,
16]. Autophagy serves as a protective mechanism to sustain the normal functioning of cartilage, ensuring the maintenance of chondrocytes in a state of optimal health and the preservation of intracellular material and energy metabolism homeostasis. Consequently, the modulation of chondrocyte autophagy is hypothesized to potentially delay or modify the degenerative process of cartilage in OA [
17,
18].
Saikosaponin D (SSD) is a triterpene saponin compound extracted from Radix Bupleuri with multiple pharmacological activities like anti-inflammatory [
19], anti-oxidative stress [
20], anti-tumor [
21,
22], liver cell protection and liver fibrosis repression [
23,
24]. Junsong Jiang et al. [
25] found that SSD is a latent therapeutic drug for OA. SSD produces anti-inflammatory effects and can suppress nuclear factor κB (NF-κB) and mammalian target of rapamycin (mTOR) pathway to stimulate autophagy. There is speculation that SSD may delay OA progression through anti-inflammatory and autophagy modulation. Research was conducted to examine the pharmacological and molecular mechanisms of SSD therapy in OA to offer a clinical basis for treatment with SSD.
Materials and methods
Animals
Sixty healthy and clean male C57BL/6 mice (8–10 weeks of age and 20–23 g) were provided by Beijing Vital River Laboratory Animal Technology Co., Ltd. (Beijing, China). Mice were adaptively fed in a specific pathogen-free laboratory room with regular light exposure and free food and water for a week. All animal procedures were approved by the First Affiliated Hospital of Air Force Military Medical University Ethics Committee on Animal Experiments.
OA model
A mouse OA model of mice was constructed [
26]. Mice were anesthetized by intraperitoneal injection with 2% pentobarbital sodium (2 mL/kg), and the surgical area was routinely disinfected. Para-patella medial parapatellar incision in the right knee was performed, and skin and joint capsule were cut to expose the joint cavity. The patella was retracted laterally, and the knee joint was flexed as much as possible to expose the anterior cruciate ligament (ACL) and anterior angle of the medial meniscus. A cut was made at the anterior angle in order to remove the meniscus from the medial side, which was then resected. The amputation of the ACL was performed under direct view, and a front drawer examination confirmed that it had been completely severed. AC surfaces were protected during operation. The joint cavity was washed with normal saline and the joint capsule and skin were sutured layer by layer.
Animal grouping and treatment
60 C57BL/6 male mice were divided into 6 groups (Sham group, OA group, 0 mg/kg SSD group, 0.5 mg/kg SSD group, 1.0 mg/kg SSD group, and SSD + miR-199-3p Antagomir group) with 10 animals in each group by random number table method. Sham group, joint cavity was exposed in the same way as the OA group, but the cruciate ligament and meniscus were not treated; OA group, OA modeling was performed on mice according to the above method; 0 mg/kg SSD group, mice were intragastrically administered with the same amount of Dimethyl Sulfoxide (DMSO; CS6719; Dingzhou Baikesaisi Biotechnology Co., Ltd, Dingzhou, China) after OA modeling; 0.5 mg/kg SSD group, mice were intragastrically administered with SSD (0.5 mg/kg/d) after OA modeling; 1.0 mg/kg SSD group, mice were intragastrically administered with SSD (1.0 mg/kg/d) after OA modeling; SSD + miR-199-3p Antagomir group, miR-199-3p Antagomir (GenScript, Nanjing, Jiangsu, China) was injected into the knee joint of mice 24 h before OA modeling, and mice were intragastrically administered with SSD (1.0 mg/kg/d) after OA modeling. SSD (purity ≥ 98%; S797046; Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) was dissolved in DMSO.
Tissue specimen treatment
All mice were euthanized eight weeks later. Briefly, mice were sacrificed with the nape facing up. Then, the front legs were immobilized and the skin and soft tissue were removed on the hind leg to make an incision at the knee joint. After exposing the tibial plateau, the surface resembling a regular translucent sphere (articular cartilage) was severed and processed. AC tissues were fixed with 4% paraformaldehyde, decalcified with 10% ethylene diamine tetraacetic acid, dehydrated with conventional gradient alcohol, and permeabilized with xylene. Paraffin-embedded sections were cut into 5 μm slices using a microtome (CUT4060, Leica, Germany). The sections were dewaxed and then subjected to hematoxylin–eosin (HE) staining, TdT-mediated dUTP-biotin nick end-labeling (TUNEL) staining, safranin O-fast green staining, and immunohistochemistry. AC tissues were obtained and preserved in liquid nitrogen for subsequent enzyme-linked immunosorbent assay (ELISA), reverse transcription quantitative polymerase chain reaction (RT-qPCR), and western blot.
HE staining
HE staining was used to evaluate the histopathological condition of AC. Briefly, AC tissue sections were stained with hematoxylin (A600701-0010; Sangon Biotech, Shanghai, China), differentiated with 1% hydrochloric acid alcohol, and treated with 1% ammonia water. AC tissues were counter-stained with 1% Eosin solution (A600440-0025; Sangon Biotech, Shanghai, China). Next, AC tissue sections were dehydrated, cleared (75%, 90% and 95% ethanol, absolute ethanol, xylene × 2 times), dried, and sealed. The morphology and structure of AC were observed under an optical microscope.
Safranin O-fast green staining
Safranin O staining was used to evaluate the damage of AC tissue. In short, AC tissue sections were stained with Weigert 's iron hematoxylin and then continuously incubated with 0.2% fast green solution (C500016-0500; Sangon Biotech, Shanghai, China), 1% ethylic acid solution, and 0.1% safranin O solution (A600815-0025; Sangon Biotech, Shanghai, China). Ultimately, the tissues were dehydrated, cleared, and mounted with neutral balsam. OA cartilage degeneration was assessed by 3 independent researchers according to the Osteoarthritis Research Society International (OARSI) grading method [
27]. The grading ranges from Grade 0 (normal) to Grade 6 as intact cartilage and surface, Grade 0; intact surface, Grade 1; surface incontinuity, Grade 2; vertical fracture, Grade 3; erosion, Grade 4; denudation, Grade 5; and deformation, Grade 6.
TUNEL staining
Chondrocyte apoptosis was detected by TUNEL method. In short, AC tissue sections were treated with 100 µl proteinase K (20 µg/ml; Roche, Shanghai, China) for 20 min at room temperature, and washed with 1 × PBS. Subsequently, chondrocyte apoptosis in the articular cartilage was measured using a cell death detection kit (11684817910; Roche), according to the manufacturer's protocols. Cells with brown nuclei were deemed TUNEL-positive and counted by a microscope in three fields of view/section. A light microscope (BX51; Olympus Corporation, Tokyo, Japan) was used to capture the images (magnification, ×200).
Immunohistochemical staining
LC3-II in AC tissues was analyzed by immunohistochemistry. In short, AC tissue sections were treated with 3% H2O2 to remove endogenous peroxidase and blocked with 5% bovine serum albumin. Afterward, primary antibody LC3-II (1: 200; NB100-2220; Novus Biologicals, CO, USA) was added and then biotinylated Immunoglobulin G (IgG) (1:1000; ab6721; Abcam, Cambridge, UK) and streptavidin–peroxidase (SABC) (SA1027; Wuhan Boster Biological Technology Ltd., Wuhan, China) were incubated. Color development was done with DAB (AR1021; Wuhan Boster Biological Technology Ltd., Wuhan, China). Then, AC tissues were counter-stained with hematoxylin, differentiated with hydrochloric acid alcohol, and rinsed with warm water. Finally, the tissues were dehydrated with gradient alcohol, cleared with xylene, sealed with neutral gum, and observed under a microscope.
Isolation and identification of chondrocytes
AC tissues were obtained from mice in the Sham and OA groups (one mouse from each group) and detached with 2% type II collagenase (LA10274; Biolab-Tech, Hangzhou, China). The detachment solution was centrifuged, and collected cells (about 1 × 106 cells) were kept in Dulbecco’s Modified Eagle Medium (DMEM) (D930057; Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) containing 10% fetal bovine serum (F917980; Shanghai Macklin Biochemical Co., Ltd., Shanghai, China). Then, cells were cultured in a 25 cm2 culture flask, and non-adherent cells were removed. Adherent cells were subcultured until primary cells were combined with slices. Articular chondrocytes at passages 1–3 were collected and identified by toluidine blue staining and type II collagen immunocytochemical staining.
Toluidine blue staining: Chondrocytes at passage 2 were fixed with 40 g/L neutral formaldehyde, stained with 1% toluidine blue (T992695; Shanghai Macklin Biochemical Co., Ltd., Shanghai, China), and observed under an optical microscope.
Immunocytochemical staining of type II collagen: Chondrocytes at passage 2 were fixed in 4% paraformaldehyde, treated with 50 μL 3% H2O2, and incubated with 50 μL Triton X-100 on ice. Then, chondrocytes were treated with 50 μL 5% blocking solution and combined with 50 μL primary antibody Col II polyclonal antibody (AB765P; 1:100; Millipore, MA, USA). PBS was taken as a negative control. Afterward, 50 μL goat anti-rabbit IgG secondary antibody (1:1000; ab6721; Abcam, Cambridge, UK) working solution was added, in combination with 50 μL SABC reagent. After incubation, color development was done with DAB, and chondrocytes were counter-stained with hematoxylin, differentiated with 0.5% hydrochloric acid, dehydrated with gradient ethanol, permeabilized with xylene, and sealed with neutral gum. Cytoplasm staining was observed under a light microscope. Type II collagen-positive staining cytoplasm was light yellow–yellow–brown.
Cell grouping and transfection
In vitro, chondrocytes were divided into 9 groups. (1) Control group, normal chondrocytes isolated from the cartilage tissue of mice in the Sham group were not treated with SSD or transfected; (2) OA group, OA chondrocytes isolated from the cartilage tissue of the OA group mice were not treated with SSD or transfected; (3) 0 μmol/L SSD group, OA chondrocytes were treated with the same amount of 0.1% DMSO; (4) 5 μmol/L SSD group, OA chondrocytes were treated with 5 μmol/L SSD; (5) 10 μmol/L SSD group, OA chondrocytes were treated with 10 μmol/L SSD; (6) SSD (10 μmol/L) + miR-199-3p Inhibitor group: OA chondrocytes were treated with 10 μmol/L SSD and transfected with miR-199-3p Inhibitor (30 μm); (7) Mimic NC group: OA chondrocytes were transfected with mimic NC (30 μm) alone without SSD treatment; (8) miR-199-3p Mimic group: OA chondrocytes were transfected with miR-199-3p Mimic (30 μm) alone without SSD treatment; (9) miR-199-3p Mimic + pcDNA-TCF4 group: OA chondrocytes were transfected with miR-199-3p Mimic (30 μm) + pcDNA-TCF4 (30 μm) alone without SSD treatment.
miR-199-3p Inhibitor, miR-199-3p Mimic, Mimic NC, and pcDNA-TCF4 were synthesized by GenePharma (Shanghai, China). The above oligonucleotides or plasmid vectors were transfected into OA chondrocytes using Lipofectamine™ 2000 (11668030; Thermo Fisher Scientific, Waltham, MA, USA).
3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay
Cell proliferation was detected by MTT assay. In short, a single cell suspension (2 × 105 cells/mL) was seeded in a 96-well plate and cultured at 100 μL per well in serum-free DMEM. MTT (M6494; Thermo Fisher Scientific, Waltham, MA, USA) was added and incubated at 10 μL/well, and then chondrocytes were treated with Formazan dissolving solution at 100 μL/well. Absorbance at 570 nm (A570) was read on a microplate reader.
Annexin V-fluorescein isothiocyanate (FITC)/propidium iodide (PI) staining
Apoptosis was detected by flow cytometry. In short, cells were detached with 0.25% trypsin (25200072; Thermo Fisher Scientific, Waltham, MA, USA) and centrifuged. According to the manufacturer's instructions, cells were treated with 5 μL Annexin V-FITC and 10 μL PI (V13242; Thermo Fisher Scientific, Waltham, MA, USA) and suspended in 400 μL 1 × Binding Buffer. FITC was tested in 30 min with a single emission laser with a wavelength of 480 mm. Data were analyzed by CellQuest software.
ELISA
IL-1β, IL-6, and TNF-α in mouse cartilage tissue and chondrocyte supernatant were assessed in line with the procedures of the IL-1β (P10749), IL-6 (P08505), TNF-α (P06804) ELISA kits (Thermo Fisher Scientific, Waltham, MA, USA).
RT-qPCR
Total RNA was collected with Trizol kit, and reverse transcription of complementary DNA (cDNA) was exerted using RNA reverse transcription kit (RR047A; Takara, Dalian, China). After PCR amplification, the product was verified by agarose gel electrophoresis. Ct was obtained by manually selecting the threshold at the lowest point of parallel rise of each logarithmic magnification curve. Data were analyzed by 2
−ΔΔCt method: ΔΔCt = [Ct (target gene)—Ct (reference gene)] the experiment—[Ct (target gene)—Ct (reference gene)] the control. PCR primers (Table
1) were designed and produced (Takara, Dalian, China).
Table 1
PCR primer sequence
MiR-199-3p | F: 5’-GGCGGACAGTAGTCTGCAC-3′ |
| R: 5′-CCAGTGCAGGGTCCGAGG-3′ |
U6 | F: 5′-CTCGCTTCGGCAGCACA-3′ |
| R: 5′-AACGCTTCACGAATTTGCGT-3′ |
TCF4 | F: 5′-CCTGGCTATGCAGGAATGTT-3′ |
| R: 5′-CAGGAGGCGTACAGGAAGAG-3′ |
ATG1 | F: 5′-GAGCTGCTTCACACTGAGGT-3′ |
| R: 5′-CCCAGCGAGATTCCCTCATC-3′ |
PI3K | F: 5′-AACACAGAAGACCAATACTC-3′ |
| R: 5′-TTCGCCATCTACCACTAC-3′ |
GAPDH | F: 5′-CTGGGCTACACTGAGCACC-3′ |
| R: 5′-AAGTGGTCGTTGAGGGCAATG-3′ |
Western blot
Tissues and cells were added with radio-immunoprecipitation assay cell lysis buffer and protease inhibitor and lysed on ice. After that, proteins were collected after centrifugation and quantified using bicinchoninic acid kit. Then, 80 μL total protein was separated by 12% sulfate–polyacrylamide gel electro-pheresis and electroblotted onto a polyvinylidene fluoride membrane. After treatment with 5% skimmed milk powder, the membrane was incubated with primary antibodies TCF4 (1: 1000), Beclin1 (1: 1000), LC3 (1: 2000), Col2a1 (1: 1000), MMP-13 (1: 1000), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (1:20,000) (Abcam, Cambridge, UK) and with the goat anti-rabbit IgG or goat anti-mouse IgG (1: 2000; ab6721; Abcam, Cambridge, UK). Finally, chemiluminescence luminescence imaging was performed. Gray values were analyzed by Image Lab.
The luciferase activity assay
Binding sites of miR-199-3p and TCF4 3′untranslated region (UTR) were predicted by the bioinformatics software
https://cm.jefferson.edu/rna22. TCF4 3′UTR promoter region sequence containing miR-199-3p binding site was synthesized to construct the TCF4 3′UTR wild-type plasmid (TCF4-WT). Meanwhile, a TCF4 3′UTR mutant plasmid (TCF4-MUT) was constructed by mutating the binding site. TCF4-WT/MUT plasmids were transfected into mouse chondrocytes with Mimic NC and miR-199-3p Mimic, respectively. After 48 h, cells were lysed to determine luciferase activity using a dual luciferase reporter kit (E1910; Promega, Madison, WI, USA).
Statistical analysis
Data analysis was performed with GraphPad Prism 8 (Graphpad, San Diego, CA, USA). Measurement data were represented in the form of mean ± standard deviation (SD). Two-group comparisons were done via t test. Comparisons among the multiple groups were done with one-way analysis of variance (ANOVA) and Tukey’s multiple comparisons test. P < 0.05 was accepted as indicative of significant differences.
Discussion
OA is a degenerative condition influenced by various risk factors. Due to the incomplete understanding of the exact pathogenesis of OA, there is a dearth of effective pharmaceutical interventions and therapies for its management. Consequently, there is a pressing clinical need to investigate and formulate targeted pharmacological remedies for the treatment of OA. In this study, the beneficial therapeutic effects of bioactive SSD derived from Bupleuri on OA were analyzed.
SSD is the monomer with the strongest pharmacological activity extracted from saikosaponins extract. SSD can suppress multiple inflammatory processes. Chen Y et al
. [
30] indicated that SSD suppresses the content of IL-1β in carbon tetrachloride-stimulated hepatitis in mice. Likewise, Yao T et al
. [
31] stated that SSD reduced lipopolysaccharide-stimulated kidney injury by lessening the generation of pro-inflammatory cytokines in kidney tissue. SSD has also been verified to ameliorate lipopolysaccharide-stimulated inflammation-correlated depression-like behaviors via restraining neuroinflammatory response [
32]. It is evident that SSD has a significant therapeutic effect and a wide range of applications in the anti-inflammatory field. Inflammation is a crucial risk factor for OA development, and inflammatory cytokines such as IL-1β, IL-6, and TNF-α have been verified to participate in OA. Therefore, anti-inflammation is a cure method to delay OA [
28]. In this study, in
vivo or in
vitro experiments clarified that SSD effectively suppressed the content of pro-inflammatory factors IL-1β, IL-6, and TNF-α. The anti-inflammatory action of SSD might be ascribed to immediate suppression of these crucial inflammatory cytokines. A report has elucidated that SSD participates in the autophagic death of tumor cells via stimulating autophagosome formation [
33,
34]. Wang B et al
. [
22] found that SSD accelerates autophagy via repressing the phosphorylation of mTOR signaling pathway. The mTOR signaling pathway is involved in chondrocyte autophagy. The critical pathological features of OA cartilage include the activation of mTOR pathway, repression of chondrocyte autophagy, reduced chondrocyte viability, elevated apoptosis, and lessened surviving chondrocyte quantities [
35,
36]. Consequently, activating chondrocyte autophagy may be conducive to alleviating OA. In this study, SSD elevated autophagy proteins Beclin1 and LC3-II/LC3-I ratio and stimulated autophagy in chondrocytes to protect cartilage. In
vivo and in
vitro experiments also suggested that SSD effectively suppressed the apoptosis and inflammation of OA chondrocytes and elevated autophagy.
MiRNA, as a non-coding RNA molecule, is able to mediate cell development with inflammation and autophagy. Several studies have demonstrated the role of non-coding RNA in musculoskeletal diseases [
37‐
39]. Multiple studies have elucidated that miRNAs are aberrant in OA tissues [
40,
41]. The mechanism of action of SSD on OA was further discussed, and it was found that miR-199-3p was abnormal in OA and affected by SSD. Yu Chao et al
. [
42] maintained that miR-199 is silenced in synovia of patients with knee osteoarthritis. Fukuoka M et al
. [
43] stated that miR-199-3p is able to boost muscle regeneration and ameliorate aging muscles and muscular dystrophy. Gu W et al
. [
44] clarified that elevated miR-199-3p stimulates chondrocyte proliferation in KOA. miR-199-3p was downregulated in OA, while augmented miR-199-3p alleviated OA via restraining inflammation and controlling autophagy.
miRNAs frequently mediate target genes via combining with the UTR sequence of mRNA via a completely or incompletely complementary base paired mode, leading to limited translation or mRNA degradation [
45,
46]. TCF4 was aberrantly expressed in OA and modulated by miR-199-3p. TCF4, as a critical risk gene on human chromosome 18, has been repeatedly reported to be elevated in OA [
47] and participate in the occurrence and progression of OA [
48‐
50]. Wang J et al. [
51] maintained that TCF4 exerts a pro-inflammatory action via AMPK/NF-κB pathway. Nevertheless, silenced TCF4 is also deemed to stimulate autophagy [
52]. The study manifested that TCF4 was augmented in OA. Elevated TCF4 was available to partially turn around the influence of miR-199-3p on OA, elucidating that miR-199-3p participated in the occurrence and progression of OA via targeting TCF4.
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