Introduction
Osteoarthritis (OA), the most prevalent aging-related joint disease, is characterized by degradation of articular cartilage and alterations in other joint tissues. The most important risk factors are aging, obesity, mechanical stress, and inflammation, and these factors impair tissue homeostasis through dysregulation of intracellular signaling mechanisms and extracellular matrix (ECM) remodeling [
1,
2]. The interaction of aging-associated changes in cartilage and molecular mechanisms of OA pathogenesis remains to be elucidated. Oxidative stress has been established as an important factor as it is elevated in joint tissues including articular cartilage in aging and OA [
3‐
5].
Increased oxidative stress results from increased reactive oxygen species (ROS) generation and from reduced antioxidants, and is accompanied by a progressive accumulation of damaged molecules and organelles, leading to activation of catabolic factors such as inflammatory cytokines and extracellular matrix (ECM)-degrading proteases [
1,
2]. Antioxidant enzymes such as Heme oxygenase-1 (HO-1) and superoxide dismutase 2 (SOD2) are an important defense against ROS-mediated damage [
6,
7]. HO-1 promotes iron recycling by degrading heme into ferrous iron, carbon monoxide and biliverdin, and protects cells from various stresses [
6,
8]. Previous studies revealed that induction of HO-1 has beneficial effects in several diseases [
9‐
11]. The expression of HO-1 gene (
Hmox-1) is negatively regulated by BTB and CNC homology 1 (Bach1), a transcriptional repressor, which binds to
Hmox-1 enhancers. Bach1 deficient mice thus have high constitutive HO-1 expression in various tissues under physiological conditions [
12]. These mice have reduced disease severity in several injury models [
13‐
15] and age-related degeneration of the meniscus [
16]. However, potential beneficial effects of HO-1 in OA development have not been determined. The objectives of this study were to investigate the impact of Bach1 deficiency on two different animal models of OA, an aging model, primary OA, and a surgical model, posttraumatic OA.
Methods
Animal models of OA
Bach1
-/- mice on C57BL/6 background were described previously [
12]. Only male mice were used in this study. All animal experiments were performed according to protocols approved by Hiroshima University Animal care and Use Committee. Knee joints were harvested at 6 months (n = 7), 12 months (n = 11), and 22 months (n = 14) to monitor spontaneous age-related OA. Experimental OA was induced in 10 week-old, wild-type mice (n = 13) and Bach1
-/- mice (n = 11) by transection of the medial meniscotibial ligament (MMTL) and the medial collateral ligament (MCL) in the right knees [
17]. Mice were sacrificed 8 weeks after surgery, and the knee joints were collected for histological analysis.
Histological assessments
All knee joints were embedded intact in paraffin after fixation in 4 % Paraformaldehyde Phosphate Buffer Solution and decalcification in K-CX (FALMA, Tokyo, Japan). Knee joints sectioned (4.5 μm) in the sagittal plane through the central weight-bearing region of the medial and lateral femorotibial joint. The sections were stained with Safranin O/fast green and at least two different sections per sample were analyzed microscopically. In this study, we applied multiple separate scoring systems for articular cartilage, meniscus, synovitis, osteophyte formation and subchondral bone thickening. Osteoarthritic damage of articular cartilage was scored using a modified Mankin system [
18,
19]. Meniscus degradation was evaluated using a scoring system [
20] which included the following 6 criteria, meniscus integrity (0 = smooth surface, 1 = irregularity of superficial layer or slight fibrillation, 2 = moderate fibrillation, 3 = severe fraying, tear or disruption), collagen structure (0 = normal, 1 = slight disturbance, 2 = moderate disturbance, 3 = severe disturbance or mucoid substances), cellular abnormalities (0 = normal, 1 = hypercellularity, 2 = cloning tendency, 3 = hypocellularity), stainability of safranin O staining (0 = normal, 1 = slight reduction, 2 = moderate reduction, 3 = severe reduction), calcification and cyst formation (0 = normal, 1 = slight, 2 = moderate, 3 = severe). The maximum possible score per meniscus was 18. Osteophyte formation and subchondral bone thickening were scored on a scale of 0–3, where 0 = normal, 1 = mild, 2 = moderate and 3 = severe changes, and the average scores for tibia and femur were recorded. The severity of synovitis was evaluated according to a previously described histopathological classification system [
21]. The parameters of synovitis included hyperplasia/enlargement of synovial lining layer, degree of inflammatory cell infiltration and activation of resident cells/synovial stroma. All parameters were graded from 0 (absent), 1 (slight), 2 (moderate) to 3 (strongly positive) and summarized ranging from 0–9, where 0–1 corresponds to no synovitis (grade = 0), 2–3 to a slight synovitis (grade 1), 4–6 to moderate synovitis (grade 2), and 7–9 to severe synovitis (grade 3).
Immunohistochemical analysis
Knee joint sections were immunostained with anti-HO-1 antibody (1:75, ab52947, Abcam, Austin, TX, USA) using Vectastain ABC-AP alkaline phosphatase (Vector Laboratories, Burlingame, CA, USA) as described previously [
22]. For anti-microtubule-associated protein 1 light chain 3 (LC3) antibody (1:100, AP1801a, ABGENT, San Diego, CA, USA), anti-manganese superoxide dismutase (MnSOD) antibody (1:100, SPC-117, StressMarq, Victoria, BC, Canada), sections in Immunoactive pH 6.0 (Matsunami Glass, Osaka, Japan) were heated in a microwave oven and kept at 85 °C for 1.5 minutes. Slides were cooled for 20 minutes at room temperature after antigen unmasking. After washing with PBS, 3 % H
2O
2 treated for 10 minutes, sections were blocked with 10 % serum for 20 minutes at room temperature. Antibodies were applied and incubated overnight at 4 °C. After washing with PBS, sections were incubated with biotinylated secondary antibody for 30 minutes at room temperature and then incubated using the peroxidase based Elite ABC system (Vector Laboratories) for 30 minutes. Slides were washed, and sections were incubated with 3,3 -diaminobenzidine (DAB) substrate.
Isolation and culture of mouse articular chondrocytes
Primary articular chondrocytes were dissected from the femoral heads of 1-month-old Bach1-/- and wild-type mice by digestion with 0.3 % collagenase Type 2 (Worthington, Lakewood, NJ, USA) in Dulbecco’s modified Eagle’s medium (DMEM) (Wako, Osaka, Japan) for 2 h. Isolated chondrocytes were cultured in DMEM with 10 % fetal bovine serum.
Transfection of small interfering RNA into mouse articular chondrocytes
Articular chondrocytes from wild-type mice and Bach1-/- mice were seeded at 5 × 104 cells/well on a 24-well plate and were transfected with small interfering RNA (siRNA) for HO-1 using Lipofectamine RNAiMax Reagent (Invitrogen, Carlsbad, CA, USA). The sequences of the siHO-1 were: (sense) 5′- CAACAGUGGCAGUGGGAAUTT -3′ and (antisense) 5′- AUUCCCACUGCCACUGUUGTT-3′ (Hokkaido System Sciences, Hokkaido, Japan). Control siRNAs were also prepared for the control group (siRNA negative control; siNega #1, Invitrogen). At 24 h after transfection, articular chondrocytes were treated with IL-1β (1 ng/ml; PeproTech, Rocky Hill, NJ, USA) for an additional 24 h.
Quantitative real-time polymerase chain reaction (PCR)
Total RNA was extracted from chondrocytes using TRIzol Reagent (Invitrogen). Complementary DNA (cDNA) was synthesized using 500 ng of total RNA with the SuperScript VILO cDNA Synthesis Kit (Invitrogen). A real-time PCR assay was performed using TaqMan Gene Expression Assay probes (Applied Biosystems, Foster City, CA, USA) to amplify the Bach1 (Mm01344527), Hmox-1 (Mm00516005), Col2a1 (Mm01309565_m1), Acan (Mm00545807), Mmp-13 (Mm01168713), Adamts-5 (Mm01344182_m1), and Sod2 (Mm01313000_m1), and Gapdh (Mm99999915_g1) was used as the internal control to normalize the sample differences. Relative expression was calculated using the ΔΔCt values, and results were expressed as 2-ΔΔCt.
DNA microarray analysis
DNA microarray (TORAY, Tokyo, Japan, 3D-Gene, Mouse Oligo chip 24 k) analysis was performed using total RNA from chondrocytes from wild-type mice and Bach1-/- mice.
Immunoblotting assay
For immunoblotting, proteins were extracted from cultured chondrocytes using M-PERTM protein extraction reagent including protease inhibitor cocktail (Thermo Fisher Scientific, Waltham, MA, USA). Anti-HO-1 antibody (diluted 1:2000), anti-LC3 antibody (diluted 1:1000), and anti-MnSOD antibody (diluted 1:1000) were used as primary antibodies. Horseradish peroxidase (HRP)-conjugated goat anti-rabbit immunoglobulin G (IgG) antibody (sc-2030; Santa Cruz Biotechnology, Dallas, TX, USA) and anti-mouse IgG antibody (sc-2005; Santa Cruz Biotechnology) were used as secondary antibody. The signal was detected with chemiluminescence of enhanced immuno-enhancer (Wako, Osaka, Japan) using the ImageQuant LAS 4000 system (GE Healthcare, Uppsala, Sweden).
Apoptosis assay
Articular chondrocytes were seeded at 1.5 × 104 cells/well on 96-well plates and transfected with siHO-1 or siRNA negative control. Articular chondrocytes were treated with tert-butyl hydroperoxide (t-BHP) (200 uM; Wako) for 5 h at 24 h after transfection. Apoptotic chondrocytes were quantitated by counting the numbers of cell nuclei stained with Cell Event Caspase-3/7 Green Detection ReagentTM and NucBlue Live cell stain ReadyProbesTM (Invitrogen) in three random fields on each duplicate well at a magnification of × 10 under a fluorescence microscope (BZ-9000; Keyence, Osaka, Japan).
Statistical analysis
The data were analyzed using the Mann–Whitney U test, Steel or Steel–Dwass to determine statistical differences. Differences were considered statistically significant at P <0.05 (*) and P <0.01 (**).
Discussion
In aging and OA, oxidative stress is elevated in joint tissues including cartilage [
4]. The antioxidant enzyme HO-1 mediates general adaptive responses and provides enhanced resistance to various stresses. HO-1 is negatively regulated by Bach1, a transcriptional repressor. Bach1 is induced by transforming growth factor β (TGF-β), and is inactivated by oxidative stress, binding of heme, and oxidation of cysteine residue [
12,
24‐
26]. HO-1 was markedly decreased in articular cartilage with aging in wild-type mice. These results suggest that maintenance of HO-1 expression has potential to protect against OA development and aging.
In the present study, Bach1
-/-
mice exhibited highly increased HO-1 expression even in articular cartilage of aged mice and reduced severity of age-related OA-like changes. However, Bach1 deficiency and accompanying overexpression of HO-1 did not influence aging and life span [
27]. Inflammation and obesity generally increase with aging. Our study showed that weight of Bach1
-/- mice were lower than wild-type mice at 22 months old. Thus, although the decreased weight of aged Bach1
-/- mice may contribute to joint health through metabolic changes, the level of proinflammatory cytokines in serum was not significant different between wild-type mice and Bach1
-/-
mice at 22 months old (data not shown). Previously, it was reported that inflammatory diseases are less severe in Bach1
-/-
mice, in part through increased HO-1 in macrophages [
28]. In a similar fashion, bone destruction is attenuated in Bach1-deficient mice via altered osteoclastogenesis [
29]. The induction of HO-1 also results in protective effects against inflammatory and degradative responses in OA chondrocytes and OA synoviocytes [
30‐
32]. Thus, the reduced severity of OA-like changes in Bach1
-/-
mice might occur through suppression of inflammation by higher constitutively expressed HO-1 in various cells, including chondrocytes. In aged mice and the surgical OA model, however, the level of proinflammatory cytokines in serum was not significantly different between wild-type mice and Bach1
-/-
mice. In addition, the level of proinflammatory cytokines in serum from Bach1
-/-mice was increased post-surgically compared with pre-surgical levels in the same mice (data not shown). Thus, in spite of increased proinflammatory cytokines in Bach1
-/- mice with surgical OA, the OA-like changes were attenuated, indicating that the reduction of OA-like changes in Bach1
-/-
mice may be not only due to the anti-inflammatory effect of HO-1 upregulation.
SOD2 and autophagy also are key mediators of aging-related diseases and prevent the accumulation of defective mitochondria that produce high levels of ROS in chondrocytes [
23]. SOD2 is the main antioxidant enzyme that scavenges superoxide anions in the inner mitochondrial matrix [
33]. Autophagy has important anti-aging functions and reduces aging-associated cell death, dysfunction, and disease [
34]. Recently, it was reported that SOD2 is reduced during OA development and in aging [
5,
35], and autophagy decreased in articular cartilage of human OA, aging-related and surgically-induced OA in mice [
36]. Autophagy is involved in maintenance of articular cartilage homeostasis, and reduces severity in mouse OA models [
22,
37]. In the present study, Bach1
-/-
mice maintained the expression of SOD2 and LC3 in articular cartilage, as well as HO-1. Although the upregulated LC3 expression in Bach1
-/-
mice was not significantly reduced by HO-1 knockdown, SOD2 was regulated by HO-1. Chondrocyte-specific deletion of SOD2 in human cells and mice accelerates OA-like changes accompanied by oxidative damage and mitochondrial dysfunction [
38]. Thus, maintenance of SOD2 in chondrocytes also may be important for OA prevention. While increased
Mmp-13,
Adamts-5 and apoptosis, key OA-related factors, were suppressed in Bach1
-/-
mouse chondrocytes under stress, only apoptosis was significantly regulated by HO-1. Our findings indicate that the protective effects against OA development in Bach1
-/-
mice appear to be antioxidant activity and cytoprotective effects through HO-1 in chondrocytes and downregulation of ECM-degrading enzymes. Thereby, Bach1 deficiency coordinates maintenance of cartilage homeostasis and joint health.
Our observations suggest that inducers of HO-1 may be effective therapeutic agents for OA prevention. Local gene delivery of HO-1 into the mouse knee joint results in transduction of the joint tissues such as synovium, except for cartilage, however, it does not reduce OA-like changes [
39]. This result suggests that the expression of HO-1 in cartilage is important for the protection and the treatment of OA. Thus, the pharmacological inhibition of Bach1 and induction of HO-1 in cartilage might have potential in the prevention or treatment of OA.
Acknowledgements
This research was supported by MEXT/JPS KAKENHI Grant-in-Aid for Scientific Research (A) Grant Number 21249079 (MO), Young Scientists (A) Grant Number 24689057 (SM), Challenging Exploratory Research Grant Number 25670651 (SM), Kanae Foundation (SM), Fujii Setsuro Memorial Foundation (TT, SM) and NIH grant AG007996 (ML). We thank T Miyata, L Creighton and Y Yoshida for excellent technical support.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
TT conceived of the study, carried out the experimental work using mice, the histological assessment and the statistical analysis, drafted the manuscript and revised the manuscript. SM participated in its design and coordination, interpreted the data, drafted the manuscript and revised the manuscript. HI carried out the gene expression assay, the immunoassays and the apoptosis assay, and helped to revise the manuscript. YH and TN carried out the immunoassays and participated in histological assessment, and helped to revise the manuscript. KI provided Bach1-deficient mice and information about the mouse, and helped to draft the manuscript. MKL interpreted the data, helped to draft the manuscript and revised the manuscript. MO participated in its design, coordination and interpretation of the data, and contributed to drafting and revising the manuscript. All authors made substantive intellectual contributions to the study and read and approved the final manuscript.