Inhibition of Knee Osteoarthritis Progression in Mice by Administering SRT2014,an Activator of Silent Information Regulator 2 Ortholog 1

Abstract

Objective

Previous findings suggest that silent information regulator 2 ortholog 1 (SIRT1) plays essential roles in chondrocytes and prevents osteoarthritis (OA) development. The purpose of this study was to investigate the effects of intraperitoneal (i.p.) and intra-articular (i.a.) administration of the SIRT1 activator SRT2104, which has been approved for use in humans.

Design

OA was induced by destabilizing the medial meniscus in the knee joint of 12-week-old CL57BL/6J mice. The mice were divided into 3 groups, that is, the control group, SRT2104 i.p.-injection group, and SRT2104 i.a.-injection group. Tissues were harvested at 4, 8, 12, and 16 weeks postsurgery. OA progression was evaluated using the Osteoarthritis Research Society International (OARSI) score. The production of OA-related proteins in cartilage and synovium was examined by immunohistochemistry.

Results

OARSI scores in the control group were significantly higher at 8 and 12 weeks compared with other 2 groups. Immunohistochemical analysis showed that Sirt1 and type-2 collagen significantly increased, whereas MMP-13, ADAMTS-5, IL-1β, IL-6, cleaved caspase 3, PARP p85, acetylated NF-κB p65, and iNOS decreased significantly in cartilage tissues from the i.p. and i.a, SRT2104 groups. In the synovium, more iNOS-positive M1-like macrophages were observed in the control group than in the i.p. and i.a, SRT2104 groups, whereas more CD206-positive M2-like macrophages were detected in the i.p. and i.a. SRT2104 groups.

Conclusions

Both i.p. and i.a. SRT2104 injection reduced OA progression in the mouse OA model, suggesting that SRT2104 can serve as a new treatment for OA.

Keywords

SRT2104 SIRT1 osteoarthritis intra-articular injection

Introduction

Osteoarthritis (OA) is one of the most common human joint diseases that cause joint pain, deformities, and functional disability. OA is characterized by cartilage erosion and subchondral bone sclerosis. No disease-modifying OA drugs are available.¹ During OA development, several pathological changes occur, including an imbalance between anabolic and catabolic activity of chondrocytes,^2,3 increased production of cartilage-degrading enzymes such as matrix metalloproteinase (MMP) and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS),^4,5 and increased apoptosis.^6,7

Silent information regulator 2 ortholog 1 (SIRT1) is a member of the sirtuin family of nicotinamide adenine dinucleotide (NAD⁺)-dependent deacetylases, which regulates gene-expression levels and protein functions by deacetylating lysine residues in histone and nonhistone proteins.^8-10 SIRT1 is a longevity gene related to many diseases associated with aging¹¹ and that SIRT1 expression in osteoarthritic cartilage decreases in the disease progression of OA¹². In human chondrocytes, SIRT1 promotes cartilage-specific gene expression,¹³ whereas SIRT1 inhibition cause changes in the expression of genes related to OA.^14,15 Furthermore, cartilage-specific Sirt1-deficient mice developed accelerated OA progression under mechanical stress and ageing, suggesting that SIRT1 plays essential roles in chondrocytes and helps prevent against OA development.^16,17

Resveratrol is a natural compound found in various plant sources and red wines, which activates SIRT1. Because of its poor bioavailability, reformulated versions of resveratrol with improved bioavailability have been developed (SRT1720, SRT2104, and SRT2379, among others), the structures of which are unrelated to resveratrol in order to have more potently stimulation for sirtuin activites.¹⁸ We previously reported that intraperitoneal (i.p.) injection of SRT1720, a specific activator of SIRT1, reduced OA progression in mice.¹⁹ Although our previous report suggested that SRT1720 could serve as a new therapeutic for OA, SRT1720 is not clinically applicable for humans. Therefore, other SIRT1 activators should be tested for future clinical applications. SRT2104 is a potent SIRT1 activator that can be used in humans as a potential therapeutic drug for type 2 diabetes and dyslipidemias.²⁰ Recent clinical trials have shown the safety of SRT2104 in humans.^21-23 Thus, our aims were to investigate the effects of SRT2104 on OA development in a murine experimental model of OA model and to test the hypothesis that both i.p. and intra-articular (i.a.) SRT2104 injection can attenuate OA progression.

Materials and Methods

Cell Culture, Cytotoxicity Test, and Real-Time Polymerase Chain Reaction Analysis

SRT2104 was obtained from Shanghai Haoyuan Chemexpress (Shanghai, China), dissolved in dimethyl sulfoxide (DMSO) at 5.04 mg/mL, and stored at −20°C. Primary mouse epiphyseal chondrocytes were obtained from 7-day-old mice as previously described.²⁴ For cytotoxicity testing, passage-0 epiphyseal chondrocytes cells were cultured in monolayers in 96-well plates (5 × 10³/well) for 24 hours. After adding SRT2104 (0, 0.1, 0.5, 1.0, 5.0, 10, or 100 µM, n = 4), the culture plates were transferred to a CO₂ incubator for 24 or 48 hours. Then, 10 µL of Cell Counting Kit-8 solution (Dojindo, Kumamoto, Japan) was added to each well, the plates were incubated for 4 hours, and absorbances were measured at 450 nm (Bio-Rad, Hercules, CA, USA).

For real-time polymerase chain reaction (PCR) analysis, primary mouse epiphyseal chondrocytes were cultured in monolayers in 6-well plates (2 × 10⁵ cells/well) for 48 hours. After reaching 80% confluency, the chondrocytes were incubated for 24 hours with or without 10 ng/mL IL-1β (R&D Systems, Minneapolis, Minnesota). Subsequently, the chondrocytes were incubated for an additional 24 hours with low-glucose Dulbecco’s modified Eagle’s medium (Sigma-Aldrich, St. Louis, MO, USA) containing 1% fetal bovine serum (heat inactivated) (Sigma-Aldrich), with or without 2.0 µM SRT2104. RNA was isolated using an RNeasy Kit (Qiagen, Inc., Valencia, CA, USA), and complementary DNA was synthesized using High-Capacity cDNA Reverse Transcription Kits (Applied Biosystems, Waltham, MA, USA). Real-time PCR analysis was performed using TaqMan assays to examine the expression of Sirt1, Col2a1, Col10a1, Mmp-3, Mmp-13, Aggrecan, Adamts-5 (all from Applied Biosystems) in duplicate for each sample, and the relative gene-expression levels were normalized to glyceraldehyde 3-phosphate dehydrogenase (Gapdh) expression (Applied Biosystems) as a reference control, based on the comparative cycle-threshold method.

Experimental Animals

All procedures were performed following approval from the Institutional Animal Care and Use Committee at Kobe University Graduate School of Medicine (approval number P160606). Mouse was chosen as an experimental animal in this study rather than larger animals such as pig and goat, since experimental OA models have been well established in mice and results can be easily compared with previous studies. Wild-type female C57BL/6J mice (wild-type; Charles River Laboratories Japan, Inc., Yokohama, Japan) were used in this study. Anesthesia was performed via i.p. injection of ketamine (100 mg/kg), and right knee joints were exposed using the medial parapatellar approach. Experimental OA was induced by resecting the medial meniscotibial ligament under a microscope to destabilize the medial meniscus in the knee joint of 12-week-old mice.²⁵ The same littermates were used in each group. The knee joint capsule and skin were closed using 5-0 nylon sutures. Mice were maintained under pathogen-free conditions and allowed free access to food, water, and activity. Mouse body weights were measured at the time of surgery and at the time of sacrifice at 4, 8, 12, or 16 weeks postsurgery.

SRT2104 Treatment

The stock solution of SRT2104 was diluted in phosphate-buffered saline (PBS) to the concentration as described below and prepared for injection. After surgery, mice were divided into 3 groups: control group (10% DMSO in isotonic saline, i.p. injection, 0.2 mL); SRT2104 i.p.-injection group (SRT2104 i.p.; 25 mg/kg, 0.2 mL); and SRT2104 i.a.-injection group (SRT2104 i.a.; 17 ng/kg, 10 µL). I.p. injection was performed twice a week using a 26-gauge needle, and i.a. injection was performed once a week with a 30-gauge needle. The injections were repeated until the mice were sacrificed. The dosage of SRT2104 used for i.p. injection was determined based on previous studies,^19,26 and that of SRT2104 i.a. injection was determined based on the results of cytotoxicity testing.

Histological Analysis

Mice divided into control, SRT2104 i.p., and SRT2104 i.a. groups were injected until euthanization at 4, 8, 12, or 16 weeks postsurgery (n = 5 mice/group for each time point). The entire right knee joints were fixed in 4% paraformaldehyde in 0.1 M PBS overnight at 4°C. Decalcification was performed for 2 weeks with 10% ethylenediaminetetraacetic acid, after which the specimens were embedded in paraffin wax. Each specimen was cut into 6-μm sections along the sagittal plane and stained with safranin O-fast green. Three sections were selected from each medial femoral condyle and medial tibial plateau, and images were obtained at ×40 magnification. The histological OA grade for each field was evaluated using the Osteoarthritis Research Society International (OARSI) cartilage OA histopathology-grading system (scores ranging from 0 to 6).²⁷ OA grading was assessed by a single observer (NM), who was blinded to the study groups.

Immunohistochemistry

Deparaffinized sections were digested with proteinase (Dako Denmark A/S, Glostrup, Denmark) for 10 minutes and treated with 3% hydrogen peroxide (Wako Pure Chemical Industries Ltd., Osaka, Japan) to block endogenous peroxidase activity. After epitope retrieval, the sections were incubated overnight at 4°C with primary antibodies against the following mouse proteins: SIRT1 (1: 50, Millipore, Billerica, MA, USA), MMP-13 (1: 100, Abcam, Cambridge, UK), ADAMTS-5 (1: 100, Santa Cruz Biotechnology Inc., Santa Cruz, CA, USA), cleaved caspase-3 (1: 100, Cell Signaling Technology, Tokyo, Japan), poly(ADP-ribose) polymerase (PARP) p85 (1: 100, Promega, Madison, WI, USA), acetylated NF-κB p65 (1: 100, Sigma-Aldrich), type-II collagen (1:100, Abcam), IL-1β (1: 100, Abcam), inducible nitric oxide synthase (iNOS; 1: 100, Abcam), and CD206 (1: 100, Abcam). The anti-SIRT1 antibody was a mouse monoclonal antibody, and the others were rabbit polyclonal antibodies. Sections were then incubated with peroxidase-labeled anti-rabbit or mouse immunoglobulin (Histofine Simple Stain MAX Po; Nichirei Bioscience, Tokyo, Japan) at room temperature for 30 minutes, and the signals were developed as brown reaction products, using the peroxidase substrate, 3,3′-diaminobenzidine, with methyl green or hematoxylin counterstaining. As negative controls, a non-immune mouse or rabbit IgG (1: 50 dilution) was used instead of the primary antibodies. SIRT1 was evaluated at 4, 8, 12, and 16 weeks postsurgery, and the other proteins were evaluated at 8 weeks. All images were obtained under a Biozero microscope (Keyence Corp., Itasca, OH, USA).

Histological Analysis of Inflammation

Antibodies against IL-1β (1: 100, Abcam), iNOS (1: 100, Abcam), and CD206 (1: 100, Abcam) were used to evaluate the expression of the corresponding proteins in the synovial tissue of the posterior capsule via immunohistochemistry, at 8 weeks postsurgery.

Statistical Analysis

All values are expressed as the mean ± SD. Multiple-comparison testing was performed using the Tukey-Kramer and Bonferroni-Dunn tests to compare differences among groups. All statistical analyses were performed using SPSS statistical software version 22 (IBM Corp., Armonk, NY, USA). P < 0.05 was considered to reflect a statistically significant difference.

Results

Cytotoxicity testing showed that chondrocyte proliferation decreased by 50% after treatment with 100 μM SRT2104, whereas chondrocyte proliferation increased after treatment with <10 μM SRT2104, both at 24 and 48 hours ( Fig. 1 ). Similar stimulatory effects on proliferation were observed with 0.1, 0.5, and 1.0 μM SRT2104. Based on these results and the possibility of systemic side effects, we selected an i.a.-injection dose of 1.0 μM for the in vivo study. In addition, our preliminary in vitro data revealed that treatment with 2.0 µM SRT2104 caused the strongest changes in gene-expression levels, among all concentrations tested. Therefore, 2.0 µM was chosen as the concentration of SRT2104 for the in vitro study.

Figure 1.

Cytotoxicity testing with chondrocytes at 24 and 48 hours (n = 4 independent experiments for each dose). IC₅₀ = 50% inhibitory concentration.

Real-time PCR analysis showed that the expression of Sirt1, Col2a1, Col10a, aggrecan, and Adamts-5 mRNA decreased significantly after IL-1β stimulation and that the Sirt1, Col2a1, and aggrecan mRNA downregulation was reversed by treatment with 2.0 µM SRT2104, when compared with the control ( Fig. 2 ). In contrast, the Mmp-3 and Mmp-13 mRNA-expression levels were significantly increased by stimulation with IL-1β. The upregulated expression of Mmp-13 was significantly decreased by treatment with 2.0 µM SRT2104, compared with the control. Mmp-3 upregulation also tended to be downregulated by treatment with SRT2104, but it did not reach the level of statistical significance.

Figure 2.

Real-time polymerase chain reaction (PCR) analysis of Sirt1, Col2a1, Col10a1, Mmp-3, Mmp-13, Aggrecan, and Adamts-5 mRNA expression in primary mouse chondrocytes. Primary mouse epiphyseal chondrocytes were cultured with 10 ng/mL IL-1β for 24 hours and then stimulated with 2.0 µM SRT2104 for 24 hours (n = 4 independent experiments; *P < 0.05, ^†P < 0.01).

The body weights of mice were measured at the time of destabilization of the medial meniscus surgery and at 4, 8, 12, and 16 weeks postsurgery. No significant difference was observed in the mean body weights between the SRT2104 i.p. and SRT2104 i.a. groups at each time point. The mean body weight of the control group was significantly higher than that of SRT2104 i.a. group at 12 weeks and that of SRT2104 i.p. group at 16 weeks post-surgery (P < 0.05; Table 1 ).

Table 1.

Mean Body Weight (g) of Mice in the Control, SRT2104 i.p., and SRT2104 i.a. Groups at the Indicated Time Points (n = 5 mice/group).

	At Surgery	4 Weeks	8 Weeks	12 Weeks	16 Weeks
Control	21.8 ± 2.6	23.1 ± 1.9	24.9 ± 2.4	27.1 ± 2.9*¹	27.7 ± 2.1*²
SRT2104 i.p.	21.8 ± 1.3	22.4 ± 1.2	23.5 ± 1.5	25.2 ± 1.8	24.5 ± 1.4*²
SRT2104 i.a.	22.5 ± 2.8	22.6 ± 1.8	24.0 ± 1.2	24.6 ± 1.2*¹	25.2 ± 1.4

i.a. = intra-articular; i.p. = intraperitoneal.

Control vs SRT2104 i.a

Control vs SRT2104 i.p.

P < 0.05.

Histological analysis showed that OA developed gradually in all groups. The OARSI scores of the medial femoral condyle and tibial condyle in the control group were significantly higher than those in the SRT2104 i.p. and SRT2104 i.a. groups at 8 and 12 weeks, but not at 4 and 16 weeks ( Fig. 3 ).

Figure 3.

(a) Safranin O-fast green staining of the medial knee joint at 4, 8, 12, and 16 weeks postsurgery. Scale bar = 100 μm. (b) The Osteoarthritis Research Society International (OARSI) scores of the medial femoral and tibial condyle (n = 5 mice/group for each time point; *P < 0.05).

Immunohistochemical analyses revealed that Sirt1-positive chondrocytes were present predominantly in the superficial zone but also showed distribution from the superficial to the deep zone of the cartilage. Significantly more Sirt1-positive chondrocytes were detected in the SRT2014 i.p. and SRT2104 i.a. groups than in the control group at 8 and 12 weeks postsurgery. Sirt1-positive chondrocytes gradually decreased in association with OA progression in the SRT groups, and no significant difference was found among the groups at 16 weeks postsurgery ( Fig. 4 ).

Figure 4.

(a) Immunohistochemistry of Sirt1 in the medial tibial plateau at 4, 8, 12, and 16 weeks postsurgery. Scale bar = 50 μm. (b) The percentage of Sirt1-positive chondrocytes. Three micrographs of the medial tibial plateau were taken under ×40 magnification. The percentage was determined as the positive chondrocytes/total number of chondrocytes at ×100 magnification (n = 5 mice/group for each time point; *P < 0.05).

The abundances of MMP-13-, ADAMTS-5-, cleaved caspase 3-, PARP p85 fragment-, and NF-κB P65-positive chondrocytes significantly decreased in the SRT2104 i.p. and SRT2104 i.a. groups, versus the control group at 8 weeks postsurgery ( Fig. 5 ). However, significant differences were not detected between the SRT2104 i.p. and SRI2104 i.a. groups.

Figure 5.

(a) Immunohistochemistry of type-II collagen, MMP-13, ADAMTS-5, cleaved caspase 3, PARP p85, and acetylated NF-κB P65 in the medial tibial plateau at 8 weeks postsurgery. Scale bar = 50 μm. (b) The percentage of type-II collagen-, MMP-13-, ADAMTS-5-, cleaved caspase 3-, PARP p85 fragment-, and acetylated NF-κB P65-positive cells. Three micrographs of the medial tibial plateau were taken at ×40 magnification. The percentage was determined as the positive cells/total number of cells at ×100 magnification (n = 5 mice/group for each time point; ^†P < 0.01).

Inflammatory cytokines and mediators such as IL-1β-, IL-6-, and iNOS-positive chondrocytes were also significantly decreased in the SRT2104 i.p. and the SRT2104 i.a. groups, compared with the control group ( Fig. 6 ). No statistically significant difference was found between the SRT2104 i.p. and SRI2104 i.a. groups.

Figure 6.

(a) Immunohistochemistry of IL-1β, IL-6, and iNOS in the medial tibial plateau at 8 weeks postsurgery. Scale bar = 50 μm. (b) The percentage of IL-1β-, IL-6-, and iNOS-positive chondrocytes. Three sections were selected for each mouse. The percentage was determined as the positive cells/total number of cells at ×100 magnification (n = 5 mice/group for each time point; ^†P < 0.01).

Macrophage phenotypes was assessed to examine the effects of SRT2104 administration on the synovium. In the posterior capsular synovium, more cells positive for iNOS (an M1-like macrophage marker) were observed in the control group than in the SRT2104 i.p. and SRT2104 i.a. groups, whereas more cells were positive for CD206 (an M2-like macrophage marker) were observed in the SRT2104 i.p. and SRT2104 i.a. groups than in the control group ( Fig. 7 ).

Figure 7.

Immunohistochemistry of IL-1β, iNOS, and CD206 in the synovium of the posterior capsule at 8 weeks postsurgery. Scale bar = 50 μm (n = 5 mice/group for each time point).

Discussion

The main finding of this study was that both i.p. injection and i.a. injection of SRT2104 reduced OA progression in an experimental mouse model of OA. Attenuated OA progression correlated with decreased cartilage-degrading enzymes, apoptosis markers, acetylated NF-κB p65, and inflammatory cytokines, as well as increased type-II collagen in chondrocytes. Moreover, polarization toward M2-like macrophages from M1-like macrophages in the synovium was also associated with OA reduction by SRT2104.

SRT2104 is a synthetic, small-molecule SIRT1 activator that can be used in humans.^20-23,28 Previous reports showed that SRT2104 extended the life span; reduced lipopolysaccharide (LPS)-induced cytokine release; and improved endurance, muscle strength, and locomotor behavior in both humans and experimental animals.^29-31 These findings suggest that SRT2104 exerts beneficial effects on age-related metabolic diseases. Previously, we reported that i.p. injection of SRTI1720, another type of SRT1 activator, delayed OA development in a mouse OA model.¹⁹ Delayed OA progression was associated with decreased levels of MMP-13-, ADAMTS-5-, apoptosis markers, and acetylated NF-κB in p65-positive chondrocytes in the cartilage, as well as reduced synovitis. In addition, SRT1720 partially increased the reduced expression of Col2a1 and aggrecan induced by IL-1β treatment, while reducing the increase in Mmp-13 expression induced by IL-1β. These observations were similar to other results in the current study. Both SRT2104 and SRT1720 could enhance SIRT1 kinase activity by lowering Michaelis constant for peptide substrates, although some off-targets effects were reported.³² In vitro experiments conducted in this study and in our previous study¹⁹ showed that SRTI2104 and SRT1720 increased Sirt1 mRNA expression in vitro. Therefore, the suppressive effects of SRT2104 on OA development may occur through SIRT1 activation and increased SIRT1 expression.

Significantly more IL-1β- and IL-6-positive chondrocytes were detected in the control group than in the SRT2104 i.p. and SRT2104 i.a. groups. IL-6 can mediate HIF-2α-induced degradation of human and mouse cartilage.³³ In addition, elevated levels of inflammatory mediators (including IL-6) have been found in the serum and synovial fluid from patients with OA,^34-36 serving as a prognostic factor of cartilage loss at an early stage of knee OA.³⁷ Increased IL-6 levels were detected not only in patients with OA but also in patients with anterior cruciate ligament injuries and meniscal tears.^34,38,39 These reports indicated that IL-6 can play an important role in OA development. SRT2104 administration reduced LPS-induced elevation of IL-6 and IL-8²⁹ in the serum of human subjects, and improved the conditions associated with inflammatory diseases.^23,40 Therefore, decreased expression of IL-1β and IL-6 in the cartilage following SRT2014 administration might also help delay OA progression.

To examine the mechanisms whereby i.p. and i.a. administrations of SRT2104 reduced OA, we also examined the effects of SRT2104 on the synovium. In the synovium, more cells positive for iNOS (an M1-like macrophage marker) and less cells positive for CD206 (an M2-like macrophage marker) were detected in control mice than in mice treated with SRT2104. Macrophages can be broadly classified into classically activated M1 macrophages and alternatively activated M2 macrophages, depending on the observed phenotypes.⁴¹ M1 macrophages, characterized by iNOS expression, are activated by interferon-γ and LPS or tumor necrosis factor–α (TNF-α), and secrete various pro-inflammatory cytokines and mediators, such as TNF-α, IL-1, IL-6, and IL-12.⁴² In contrast, arginase-positive M2 macrophages have anti-inflammatory activities and contribute to wound-healing.^43,44 Previous reports provided data suggesting the importance of M1/M2 macrophage polarization in OA development.^45-48 In the synovium from patients with OA, an increased ratio of M1 macrophages and activation of mammalian target of rapamycin complex 1 (mTORC1) signaling were detected, when compared with synovial tissue from patients without a history of arthritis. Conditional deletion of tuberous sclerosis complex 1 (TSC1) in myeloid-lineage cells caused accelerated OA progression associated with constitutive mTORC1 activation and increased M1 macrophages, whereas deleting Ras homologue enriched in brain (Rheb) in the myeloid cells led to delayed development of OA associated with increased M2 macrophages in a mouse collagenase-induced OA model.⁴⁷ Data from a recent study showed that treatment with resveratrol reduced joint inflammation in mice with monosodium urate crystal–induced arthritis and decreased iNOS expression in macrophage-like cells.⁴⁹ Taken together with these observations, our results suggest that inhibiting M1 macrophage polarization by SIRT1 activation also contributed to the reduction of OA progression in mice treated with SRT2104.

Several previous reports demonstrated that i.a. injection of resveratrol, rapamycin, and small interfering RNA can effectively attenuate the early phase of OA progression in experimental mouse models of OA^50-53 and that i.p. injection of thalidomide and SRT1720 attenuated OA progression.^19,54 Ghalayani et al. reported that i.a. injection of steroids showed a better therapeutic response than i.p. injection, based on histological assessment of synovitis and vasculitis in a rat model of arthritis with nonweighted joints.⁵⁵ In this study, the effects of SRT2104 on the reduction of OA development were comparable in mice following i.a. or i.p. injection. Of note, SRT2104 was administered to mice in the i.a.-injection group with a lower dose and frequency compared with those in the i.p.-injection group (17 ng/kg vs. 25 mg/kg SRT2104 and once a week versus twice a week in the i.a. and i.p. groups, respectively), suggesting that i.a. administration was more efficient than systemic administration. In previous clinical studies, the tolerability of oral SRT2104 administration was tested in humans, and it was reported that SRT2104 was well tolerated at a dose of 2.0 g/d for 7 to 28 days.^21,30 Administration of 2.0 g SRT2104 to humans with a body weight of 80 kg corresponds to the dose (25 mg/kg) used in SRT2104 i.p. group. Therefore, the dose used in this study appears to be within the tolerable range in humans. However, considering the potential side effects of SRT2104, i.a. injection may be more desirable for clinical applications. Based on our current results, i.a. injection of SRT2104 may be used to reduce progression of OA, possibly in the early phase of OA.

Limitations

This study had some limitations. First, the mean body weight in the control group tended to be higher than that in the other groups, and the differences in the body weights might have affected OA progression. However, these differences were not significant until 8 weeks postsurgery; therefore, the effect might have been negligible. Second, only female mice were used in this study and different results might be obtained with male mice. Third, in this study, the effects of SRT2104 on OA progression were examined in a posttraumatic OA model, and it is unknown whether SRT2104 can attenuate OA progression during aging. Fourth, it was unclear how long SRT2104 administrated by i.a. and i.p. injection remained in the knee joints and affected articular cartilage or synovium tissues. Further studies are needed to investigate the pharmacokinetics, hemodynamics, and systemic effects (such as in internal organs) of SRT2104 in larger animals.

Conclusions

Both i.p. and i.a. injections of SRT2104, a SIRT1 activator, reduced OA progression in a mouse model of OA. Comparable attenuation of OA development was found between mice subjected to i.a. and i.p. injections of SRT2104. Our findings suggest that SRT2104 administration can potentially serve as a new therapeutic approach for OA.

Footnotes

Author Contributions

NM, TM, KN, and RK contributed to the study concept and design. NM, TM, KN and TT contributed to the acquisition, analysis, and interpretation of the data. NM and TM drafted the article and NM, TM, and RK critically revised the article for important intellectual content. All authors gave final approval of the version to be submitted for publication. NM, TM, and RK agreed to be held accountable for all aspects of the work to ensure that questions related to the accuracy or integrity of any part of the work are appropriately addressed.

Acknowledgments and Funding

We appreciate the technical assistance provided by Ms. Kyoko Tanaka, Ms. Minako Nagata, and Ms. Maya Yasuda. The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by a Grant-in-Aid for Scientific Research (grant number 2179139) from the Japan Society for the Promotion of Science.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethical Approval

All procedures were performed following approval from the Institutional Animal Care and Use Committee at Kobe University Graduate School of Medicine (approval number P160606).

Animal Welfare

The present study followed international, national, and/or institutional guidelines for humane animal treatment and complied with relevant legislation.

ORCID iD

Nobuaki Miyaji

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