The role of SIRT3-mediated mitochondrial homeostasis in osteoarthritis

Abstract

Osteoarthritis is the most common degenerative joint disease and causes major pain and disability in adults. It has been reported that mitochondrial dysfunction in chondrocytes is associated with osteoarthritis. Sirtuins are a family of nicotinamide adenine dinucleotide-dependent histone deacetylases that have the ability to deacetylate protein targets and play an important role in the regulation of cell physiological and pathological processes. Among sirtuin family members, sirtuin 3, which is mainly located in mitochondria, can exert its deacetylation activity to regulate mitochondrial function, regeneration, and dynamics; these processes are presently recognized to maintain redox homeostasis to prevent oxidative stress in cell metabolism. In this review, we provide present opinions on the effect of mitochondrial dysfunction in osteoarthritis. Furthermore, the potential protective mechanism of SIRT3-mediated mitochondrial homeostasis in the progression of osteoarthritis is discussed.

Introduction

Osteoarthritis (OA) is a kind of degenerative joint disease mainly associated with clinical manifestations of joint pain and swelling, limited mobility, stiffness, and joint deformities. With its increasing prevalence due to the aging of the population, OA has become a growing health problem that affects 10–15% of all adults over the age of 60 years, with prevalence higher among women than men [1]. At present, there is no effective treatment for osteoarthritis, and the aetiology of this disease includes age, trauma, obesity, strain, joint congenital anomalies, and other factors. Chondrocyte senescence and apoptosis, extracellular matrix degradation with synovial inflammation, and dysfunction of the subchondral bone are the core pathological changes in osteoarthritis [2–4].

Mitochondria, which are defined as the cellular power factories, play roles in some other essential cellular functions, including cell growth, cycle, death, and differentiation. It is increasingly being realized that mitochondria are involved in the progression of osteoarthritis, which is correlated with oxidative stress and reactive oxygen species (ROS) [5, 6]. Furthermore, mitochondrial dysfunction and metabolic impairment have been identified as hallmarks in the initiation and progression of OA, even though chondrocytes are not enriched in mitochondria [7, 8]. Recent studies have concluded that sirtuin 3 (SIRT3), as a mitochondrial nicotinamide adenine dinucleotide (NAD)-dependent protein deacetylase, can regulate characteristic mitochondrial processes such as the deacetylation of proteins including superoxide dismutase (SOD), which is a master regulator of mitochondrial redox homeostasis for the protection of mitochondrial function [9, 10]. The present review aims to unravel the contribution of mitochondrial dysfunction to OA pathobiology and summarize potential therapeutic strategies involving mitochondrial protection mediated by the deacetylation of SIRT3 during the osteoarthritis process.

The role of mitochondria in osteoarthritis

It is known that mitochondria are complex and dynamic organelles involved in multiple cell physiological processes in addition to energy production. They also mediate several critical biochemical processes, such as apoptosis, redox homeostasis, and proliferation. A number of studies have reported that mitochondrial dysfunction can lead to a wide variety of human diseases, including osteoarthritis [7, 11]. Some recent studies have identified mitochondria-associated processes in OA. Furthermore, some studies have indicated that the core event of mitochondria-promoted osteoarthritis is oxidative stress, which has an effect not only on chondrocytes but also on synoviocytes and cells of subchondral bone [5, 6, 12]. This effect leads to increased cytokine-induced chondrocyte inflammation and matrix catabolism, chondrocyte senescence, increased chondrocyte apoptosis, synovial inflammation, and subchondral bone dysfunction [13–16] (Fig. 1).

Fig. 1 .

Oxidative stress-related mechanisms of osteoarthritis pathogenesis. Various external stimuli, such as IL-1β, TNF-α, and mechanical stress, can cause an imbalance in mitochondrial function of various cells in the knee joint cavity, which results in oxidative stress. Under oxidative stress, chondrocytes undergo senescence and apoptosis; additionally, the ability of matrix synthesis in chondrocytes decreases, and ECM degradation occurs. Oxidative stress causes changes in the functions of osteoblasts and osteoclasts in subchondral bone, which promotes the occurrence and development of osteoarthritis. In addition, synovial inflammation occurs under oxidative stress. ECM extracellular matrix, mtDNA mitochondrial DNA

Mitochondrial respiratory chain and energy production in osteoarthritis

For most human tissue cells, the majority of adenosine triphosphate (ATP) that provides energy for physiological activities is generated by mitochondria. However, chondrocytes mostly rely on glycolysis for the generation of ATP, because metabolism in these cells occurs under low oxygen tension (ranging from 10% at the surface to < 1% in the deep layers) [17]. However, some studies have reported that approximately 25% of ATP production in chondrocytes is attributed to mitochondrial oxidative phosphorylation [18, 19]. At the same time, although chondrocytes do not normally contain abundant mitochondria [8], the mitochondrial respiratory chain (MRC) of cultured human chondrocytes has been proven to exhibit similar sets of enzymes and activity levels when compared to those of the MRC of other mesenchymal cells [20].

An increasing number of studies have shown that the number of mitochondria per cell is decreased in OA chondrocytes and that there is a reduction in ATP levels due to decreased oxidative phosphorylation (OXPHOS) regardless of the increase in glycolysis activity [21, 22]. It has been hypothesized that with the development of OA, chondrocytes have anti-inflammatory reliance on oxidative phosphorylation and a TCA cycle switch to proinflammatory reliance on glycolysis for energy [23–25]. In addition, a more recent study suggested that the reactive oxygen species produced by mitochondrial electron transport (ET) helps to maintain cellular redox balance in favour of glycolysis [26, 27]. The link between glycolysis and ET explains the important role of mitochondria in supporting normal chondrocyte metabolism, which has been previously overlooked. In addition, other studies have revealed that the impaired effect of glycolysis might also cause chondrocyte dysfunction and that, at the same time, restoring glycolysis can reduce the expression of matrix metalloproteinase-13 (MMP-13) caused by external stimuli such as IL-1β and improve chondrocyte viability [28, 29].

The most accepted evaluation standard of mitochondrial function is the analysis of changes in mitochondrial membrane potential (ΔΨm) and respiratory chain enzyme complexes. The mitochondrial respiratory chain (MRC) comprises respiratory complexes I–IV and ATP synthase (complex V). Compared to normal chondrocytes, chondrocytes in OA exhibit an impaired ΔΨm and reduced activity of complexes I–III, indicating that mitochondrial dysfunction is involved in the degeneration of chondrocytes [20, 30]. Furthermore, inflammatory factors, such as tumour necrosis factor-α (TNF-α) and interleukin-1 (IL-1), can damage mitochondrial complex I and the production of ATP, leading to cartilage degradation [31]. A study conducted by Maneiro et al. [20] revealed that the activities of complexes II and III are reduced and that the mitochondrial mass is increased in OA articular chondrocytes compared with cells from normal cartilage [20]. The degradation of the cartilage matrix is a critical process in OA progression, and a study conducted by Cillero-Pastor et al. [32] revealed that MRC dysfunction modulates matrix metalloproteinase (MMP) expression, leading to matrix degradation in human chondrocytes [32].

Reactive oxygen species in osteoarthritis

ROS are free radicals that include oxygen molecules, including superoxide anion (O₂⁻), hydrogen peroxide (H₂O₂), hydroxyl radical (OH⁻), hypochlorite ion (OCl⁻), and nitric oxide (NO). Due to the presence of unpaired electrons, ROS are unstable and short-lived and interact with proteins, lipids, and so on to achieve stability [20]. There are two major processes involved in ROS generation: the mitochondrial pathway and the non-mitochondrial membrane-bound pathway. The non-mitochondrial membrane-bound pathway includes nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and xanthine oxidase (XO). Due to it is avascular and lacks innervations in vivo, some articles consider that chondrocytes produce abnormal levels of ROS through NADPH oxidase, especially in pathological conditions under the stimulation of mechanical stress and inflammatory mediators [12, 33, 34]. Some articles showed that NADPH oxidase expressed in the chondrocytes is the main enzyme that leads to the formation of ROS in the synovial fluid, leading to increased oxidative stress in the joints and mediating the progressive degradation of cartilage [35, 36]. However, oxygen can diffuse into the superficial and middle zones of the articular cartilage regardless of whether chondrocytes exist in an anaerobic environment. And articular chondrocytes possess a certain amount of mitochondria, producing ROS in vitro [18]. Stockwell et al. (1991) reported that the density of mitochondria in chondrocytes is higher in the superficial zones of articular cartilage obtained from dogs than in the deeper zones [37]. Chance and Williams [38] reported that approximately 2–3% of the total O₂ consumed by functional mitochondrial electron transport chains is incompletely reduced to O₂⁻ [38]. Currently, reducing ROS production in the mitochondria represents a novel approach to disease modification in OA [39]. There is an intracellular balance between ROS production and elimination in articular chondrocytes (Fig. 2). A disturbance in the balance between the production of ROS and elimination can result in the disruption of redox signalling and cellular damage, which in turn causes oxidative stress [40]. The levels of ROS are low in normal chondrocytes but become elevated in OA [41, 42]. The reasons for mitochondria-originated ROS production vary and include inflammatory mediators, mechanical stress, and oxygen pressure variations [6, 43]. Mitochondrial ROS production is enhanced by IL-1β. Meanwhile, mitochondrial dysfunction amplifies the responsiveness to cytokine-induced chondrocyte inflammation through ROS production and NF-κB activation [44]. Due to cartilage mechanobiology, mechanically induced mitochondrial ROS are correlated with the magnitude of cartilage deformation, which results in chondrocyte death and matrix degradation [45, 46].

Fig. 2.

Production of reactive oxygen and nitrogen species involved in osteoarthritis pathogenesis. Endogenous ROS/RNS can be produced by NADPH oxidase enzymes, as a byproduct of other biological reactions, such as the mitochondrial electron transport chain, or by metal-catalysed oxidation through xanthine oxidase. The primary types of ROS/RNS produced in cells are superoxide (O₂⁻), nitric oxide (NO), and hydrogen peroxide (H₂O₂). These three molecules can easily react with other substances to form other ROS and RNS species. For example, O₂⁻ can be dismutated by the SOD2 enzyme to form H₂O₂ or react with NO to form peroxynitrite (ONOO⁻). In addition, H₂O₂ rapidly reacts with Cl⁻ to form OCl⁻ and is decomposed through the Fenton reaction, leading to the generation of hydroxyl radicals (OH). The main antioxidant system includes enzymatic and nonenzymatic antioxidants, such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-PX), which can scavenge many forms of ROS, thereby maintaining the intracellular redox milieu. OXPHOS oxidative phosphorylation, NAD nicotinamide adenine dinucleotide, NADPH nicotinamide adenine dinucleotide phosphate, NOS nitric oxide synthase, O₂⁻ superoxide anion, NO nitric oxide, ONOO⁻ peroxynitrite, SOD superoxide dismutase, OH⁻ hydroxyl radical, H₂O₂ hydrogen peroxide, OCl⁻ hypochlorite ion; Cl chlorine, O₂ oxygen, GSH glutathione, G-6-P glucose-6-phosphate, G-6-PDH glucose-6-phosphate dehydrogenase

Under pathological conditions, excessive ROS can act as second messengers, contributing to cartilage degradation by inhibiting matrix synthesis, activating MMPs to degrade matrix components, inducing cytokine production, and inducing chondrocyte apoptosis [13, 47]. For example, some studies have proven that matrix synthesis is suppressed by ROS, which impair mitochondrial ATP formation and oxidative phosphorylation [48, 49]. Furthermore, some studies have indicated that ROS levels regulate chondrocyte proliferation and modulate the initiation of hypertrophic changes in chondrocytes [49, 50] Reed et al. [43] confirmed that mitochondria-originated oxidative stress can regulate the levels of MMPs [43].

On the other hand, the antioxidant system acts as an intracellular mechanism that contributes to the elimination of excessive ROS. This antioxidant system includes enzymatic and nonenzymatic antioxidants, such as SOD, catalase (CAT), and glutathione peroxidase (GSH-PX). SOD converts O₂⁻ into H₂O₂ and O₂. Then, GSH-PX and CAT catalyse H₂O₂ into H₂O and O₂ for final detoxification [51]. Accumulative evidence has shown that antioxidant enzymes are decreased in OA patients, confirming that the antioxidant system plays an important role in OA pathogenesis [42, 52, 53].

In addition, mitochondria are not only producers of ROS but also the target of ROS. The overproduction of ROS can damage mitochondrial DNA (mtDNA) integrity and repair capacity, causing further ROS production, which maintains a vicious cycle that ultimately leads to chondrocyte death [54].

Cartilage extracellular matrix synthesis, catabolism, and calcification

In human cartilage, chondrocytes are embedded within an abundant extracellular matrix (ECM). This matrix has a special structure that supports chondrocytes. Collagen and proteoglycans are the main components of the ECM, which participates in chondrocyte biosynthesis and secretion. Several in vitro studies have proven that the dysfunction of mitochondria induced by MRC inhibitors suppresses the synthesis of proteoglycans and collagen [48, 55]. In addition, this special inhibitor damages the function of complexes III and V in these cells, inducing the production of MMPs and inflammatory factors, such as IL-1, interleukin-6 (IL-6), and prostaglandin E2 (PGE2) [31, 32].

The role of the abnormal mineralization of cartilage in the mitochondria-mediated pathogenesis of OA has received increasing attention. Mitochondria are calcium reservoirs and help to maintain cellular calcium (Ca²⁺) homeostasis [56]. Some studies have indicated that the mitochondria of chondrocytes have the ability to transport Ca²⁺ and are important for the calcification of the ECM, which has been demonstrated in matrix vesicles (MVs) [7, 57]. The direct suppression of mitochondrial respiration promotes MV-mediated mineralization in chondrocytes [48]. Pathologic calcium pyrophosphate dihydrate (CPPD) crystal deposition and hydroxyapatite (HA) crystal deposition are both common in OA [58]. Crystal deposition was identified in half of OA synovial fluid samples [59] and in all OA cartilage samples obtained from knee or hip joint replacement surgery. Changes in cartilage mitochondrial function regulate mineralization, including the regulation of intramitochondrial calcium accumulation and the release of intramitochondrial calcium stores through MV formation. Meanwhile, chondrocyte apoptosis can be associated with the release of apoptotic bodies [60], which may have similar matrix vesicle-like activities [14], contributing to the abnormal calcification of the articular cartilage and providing a sheltered milieu for crystal nucleation [61].

Chondrocyte senescence and apoptosis

Extensive evidence from animal models and in vitro studies has indicated that aging-related and stress-induced oxidative stress is one cause of chondrocyte senescence [62, 63]. Changes in mitochondrial function and metabolism are characteristics of cellular senescence in musculoskeletal aging [64, 65]. Senescent chondrocytes accumulate during aging and show the senescence-associated secretory phenotype (SASP). These senescent chondrocytes overproduce matrix metalloproteinases, such as MMP-3 and MMP-13, and secrete IL-1, IL-6, and TGF-β, all of which can contribute to the progression of OA [63, 66, 67]. An experiment performed by Xu et al. [67] revealed that the injection of senescent cells into the knee region can induce an OA-like state [67]. In addition, another interesting experiment revealed that the selective elimination of senescent cells can attenuate the progression of post-traumatic OA, reduce pain, and increase cartilage development [68]. Another study suggested that the presence of oxidative stress induces telomere genomic instability and chondrocyte senescence in OA cartilage in vitro and that antioxidant therapy can delay the progression of OA [15, 69, 70]. There is evidence that the activity of antioxidants, including the mitochondrial enzyme SOD in human cartilage [71, 72] and catalase in rat cartilage [73], declines in chondrocytes with age. Furthermore, aged chondrocytes are more vulnerable to ROS-induced death, suggesting that chondrocyte loss in aged cartilage may be due to reduced antioxidant activity [73].

OA cartilage contains a large number of apoptotic chondrocytes, and the induction of the mitochondrial apoptosis pathway by inflammatory factors, the overexpression of ROS, and mechanical stress plays a key role in apoptosis [74]. The reduction in ΔΨm is associated with mitochondrial swelling, the rupture of the outer membrane of mitochondria, and the release of pro-apoptotic factors, such as cytochrome c, and pro-caspases, such as caspase-9, from the intermembrane space [75, 76]. Active caspase-9 can lead to chondrocyte apoptosis through the activation of executioner caspases, such as caspase-3 and caspase-7. Caspase-3 is one of the most important execution caspases for the completion of the apoptosis process, and increased expression of caspase-3 has been confirmed in human OA articular cartilage and surgically induced OA models [76–79].

Calcium is an intracellular second messenger responsible for controlling a number of cellular processes, including the apoptosis process. The alteration of intracellular calcium homeostasis is a major upstream contributor to the activation of caspase-9, which may be a stress-induced signalling pathway for articular chondrocyte death. Furthermore, mechanical stimulation has been found to alter calcium homeostasis in most cell types [80, 81], as well as in OA chondrocytes [82, 83]. The excessive accumulation of Ca²⁺ leads to the opening of the mitochondrial permeability transition pore (mPTP) [84], which depolarizes the mitochondrial transmembrane potential and results in the release of calcium and mitochondrial proteins, leading to mitochondrial swelling and cell apoptosis [85]. Chondrocyte death is presently accepted as a hallmark of OA and has also been demonstrated to be a significant factor in the initiation of cartilage degradation [14, 86].

mtDNA mutations and osteoarthritis

Human mitochondrial DNA (mtDNA), which encodes 22 tRNAs, 2 rRNAs, and 13 proteins, is involved in the function of respiration and OXPHOS. Furthermore, mitochondrial gene mutations are known to have significant influences on various pathophysiologic processes and to play a role in OA incidence and progression [87]. The accumulated mtDNA mutations that exist today are high-frequency, continent-specific mtDNA polymorphisms called haplogroups, and the sequential accumulation of mtDNA variants enables humans to adapt to different climates [11]. A study conducted by Rego-Perez et al. [88] demonstrated that mitochondrial DNA haplogroup J is associated with a lower risk of knee osteoarthritis than mitochondrial DNA haplogroup H [88]. Some studies have reported that patients carrying mtDNA haplogroup H have a greater tendency to undergo total joint replacement surgery compared to non-H patients, and the radiographic progression of OA also differs between haplogroup H and non-H patients [89].

The molecular mechanisms underlying different mtDNA haplogroups, including oxygen consumption, ATP production, and the generation of ROS, affect OA. Considering these effects, these haplogroups may be potential complementary candidates for biomarkers of OA.

In addition, mtDNA is considerably more prone to oxidative damage than the nuclear genome. It has been determined that damaged mtDNA accumulates in OA chondrocytes and that OA chondrocytes have a limited mtDNA repair capacity [54]. A study revealed that IL-1β and TNF-α can alter the integrity of mtDNA to induce chondrocyte apoptosis [87]. Furthermore, mtDNA damage upregulates MMP protein levels to cause matrix destruction through oxidative stress [43].

Sirtuins and sirtuin 3

Sirtuins are a highly conserved third class of deacetylase enzymes, and most sirtuin family members possess NAD⁺-dependent protein deacetylase activity. The sirtuin family consists of seven members, SIRT1–SIRT7, which are localized throughout the cell. Each member has a specific cellular localization and downstream target protein. Among these sirtuins, SIRT3, SIRT4, and SIRT5 are located in the mitochondria, highlighting their role in regulating metabolic and respiratory pathways in this organelle. SIRT3 is activated by the cleavage of its N-terminal 142 amino acids by mitochondrial processing peptidase (MPP), and active SIRT3 resides in mitochondria [90]. Some studies have indicated that the activities of various sirtuins decline with age and that reduced sirtuin activity has many detrimental effects on cartilage metabolism and health. Protein acetylation in mitochondria has become a focus, and it was revealed that 63% of mitochondrially localized proteins contain lysine acetylation sites and that oxidative metabolism is inhibited by acetylated central metabolic enzymes [91]. Accumulating evidence has suggested that mitochondrial acetylation is widespread and that the acetylation status of many sites is controlled by the enzymatic activity of the NAD⁺-dependent deacetylase SIRT3 [92–94]. Although SIRT4 and SIRT5 are also localized in mitochondria, SIRT3 still has the most robust deacetylation effects on diverse substrates [95]. Many central enzymes deacetylated by SIRT3, such as isocitrate dehydrogenase (IDH2) and superoxide dismutase 2 (SOD2), have an antioxidant effect, and this effect regulates the mitochondrial MRC to maintain normal oxidative mechanisms [96, 97]. SIRT3 has also been shown to regulate mPTP opening through the deacetylation of CypD, which prevents mitochondrial dysfunction and cardiac hypertrophy during aging [98]. Other studies have shown that the deacetylation of SIRT3 plays a core role in mitochondrial biosensors and mitochondrial dynamics [99, 100].

Regarding the role of the sirtuin family in chondrocyte homeostasis and the pathogenesis of osteoarthritis, a number of previous studies have focused on SIRT1 and SIRT6. SIRT1 plays a role in ECM synthesis and promotes chondrocyte survival through the regulation of mitochondrial effectors, causing delayed chondrocyte senescence and DNA repair under oxidative stress [101, 102]. SIRT6 can prevent DNA damage, telomere dysfunction, and premature senescence in chondrocytes [103]. A study conducted by Wu et al. (2015) revealed that the overexpression of SIRT6 suppresses cellular senescence and NF-κB-mediated inflammatory responses in OA development [104]. However, SIRT3 has recently become the focus of some investigations on the progression of OA due to its functions in metabolic control, oxidative stress reduction, aging, and apoptosis inhibition. A study conducted by Wang et al. [39] revealed that the SIRT3 level is decreased in mouse OA cartilage and that SIRT3 knockdown induces mitochondrial dysfunction in chondrocytes [39]. A more recent study revealed that SIRT3 antagonizes mitochondrial dysfunction in the early stages of OA [105]. The current research progress and the potential mechanism by which SIRT3-mediated mitochondrial homeostasis protects cartilage against the progression of OA are discussed below.

SIRT3 regulates redox homeostasis in chondrocytes

SIRT3 has three main mechanisms for balancing ROS levels in chondrocytes (Fig. 3a). First, SIRT3 can directly activate SOD2 via deacetylation, which induces the clearance of ROS to maintain the intracellular redox milieu. It has been reported that an age-dependent reduction in SIRT3 enhances the acetylated levels of SOD2 and then increases oxidative stress in chondrocytes [106]. An experiment revealed that mechanical loading promotes mitochondrial superoxide generation and selective SOD2 downregulation in chondrocytes in vivo, which promotes OA progression in mice [107]. Furthermore, SIRT3 contributes to glutathione production via the deacetylation of isocitrate dehydrogenase 2 (IDH2), which is a critical molecule for transforming oxidized glutathione (GSSG) to glutathione (GSH) [97, 108]. A study conducted by Chae et al. [109] revealed that the expression levels of IDH2 are decreased in mouse embryonic fibroblasts (MEFs) and the tissues of other aged mice and that the antioxidant level of GSH declines with age, leading to OA [109, 110]. In addition, SIRT3 controls the function of ETC complexes to decrease ROS levels in other fields. Specifically, SIRT3 has been reported to have the ability to deacetylate all ETC complexes, which increases the efficiency of electron transport and maximizes ATP production [111, 112]. In addition, SIRT3 deficiency leads to decreased expression of mitochondrial respiratory complex subunits [105]. Finally, SIRT3 is also known to deacetylate Forkhead box O3a (FOXO3a), which leads to its translocation to the nucleus, and this in turn accounts for the increased transcription of antioxidant enzymes, such as manganese superoxide dismutase (MnSOD/SOD2), and the catalase involved in the clearance of hydrogen and peroxide superoxide [113].

Fig. 3.

Role of SIRT3 in mitochondrial antioxidant stress and mtDNA repair. (A) SIRT3 acts on downstream enzymes through its deacetylation. These enzymes can eliminate and reduce ROS production, thereby maintaining mitochondrial function and exerting antioxidant effects. (B) The deacetylation of SIRT3 can affect mtDNA repair through ACO-2 and OGG1. Otherwise, SIRT3 indirectly promotes mtDNA replication by activating the function of FOXO3a. OXPHOS oxidative phosphorylation, GSH glutathione, IDH2 isocitrate dehydrogenase, SOD2 superoxide dismutase, FOXO3a deacetylate Forkhead box O3a, ACO-2 aconitase-2, OGG1 oxoguanine-DNA glycosylase 1, MtDNA mitochondrial DNA, ROS reactive oxygen species

In conclusion, SIRT3 can rapidly balance ROS levels by regulating protein deacetylation and protect cells from oxidative damage through long-term transcriptional antioxidant programmes through FOXO3a activation. The ability of SIRT3 to regulate ROS is associated with aging, and this phenomenon may be explained by a decline in SIRT3 levels. In summary, the decline in SIRT3 activity and levels may maintain more antioxidant functional proteins in the acetylated state, thereby causing oxidative stress, impairing chondrocyte homeostasis, and leading to chondrocyte apoptosis. This may initiate the progression of OA.

SIRT3 protects mitochondrial structural integrity

Mitochondrial DNA (mtDNA) is more sensitive to oxidative stress than nuclear DNA and plays a role in oxidative stress-induced mitochondrial dysfunction [114]. The depletion of mtDNA is known to suppress ATP synthesis and cause defects in cellular function [115]. Furthermore, mutations and deletions in the mitochondrial genome have been associated with neurodegenerative disorders and other age-related diseases [116]. Some studies have indicated that mtDNA damage and poor mtDNA repair capacity for removing damage caused by oxidative stress may contribute to the pathogenesis of OA [117, 118]. A recent study revealed that OA chondrocytes are prevalent with mitochondrial deletion mutant mtDNA4977 combined with the decreased expression of SIRT3 in vitro [105]. It has been consistently shown that, in human tumour cells, reduced SIRT3 expression leads to mtDNA4977 deletion under oxidative stress [119]. 8-Oxoguanine-DNA glycosylase 1 (OGG1) is a mtDNA repair enzyme that executes the excision of 7,8-dihydro-8-oxoguanine (8-oxoG), which is the most common mutagenic base lesion generated as a result of exposure to ROS (Fig. 3b). The activity of OGG1 is regulated by acetylation status [120]. A study conducted by Hill et al. [121] demonstrated that SIRT3 deficiency greatly increases the acetylation of OGG1 in chondrocytes. SIRT3 can regulate the deacetylation status of OGG1 and stabilize it to repair mtDNA, because acetylated OGG1 is vulnerable to degradation by calpain [121]. The activation of AMPK increases SIRT3 to decrease the acetylation of OGG1 and increase the expression of OGG1, thus maintaining mtDNA content and improving intracellular ATP levels [105]. If 8-oxoG is not promptly repaired, the accumulation of this mutagenic base in mtDNA would result in mitochondrial dysfunction and trigger apoptotic cell death. In addition, SIRT3 deacetylates FOXO3a in the mitochondrial matrix, which allows FOXO3a to bind to mtDNA and promotes the upregulation of all 13 mitochondrial-encoded genes, leading to increased mitochondrial respiration [9]. Mitochondrial aconitase-2 (ACO-2), a TCA cycle enzyme, acts as a biosensor for oxidative stress and stabilizes mtDNA under oxidative stress [122]; meanwhile, OGG1 preserves the activity of ACO-2 to protect it from oxidative degradation [123]. In addition, due to the existence of heteroplasmy in mtDNA, SIRT3-controlled mitochondrial fusion can exchange mitochondrial content to enable the complementation of mtDNA in cells and diminish mtDNA damage. Damaged or depolarized mitochondria that do not fuse can be removed by mitophagy [124].

SIRT3 regulates mitochondrial regeneration and dynamics

Mitochondrial homeostasis is an important process for maintaining mitochondrial function under oxidative stress, which includes mitochondrial regeneration, mitochondrial dynamics, and depolarized mitochondrial clearance, and this complex process is finely regulated by various functional molecules (Fig. 4).

Fig. 4.

Mechanism by which mitochondrial homeostasis prevents oxidative stress. Mitochondrial biogenesis can produce a large number of new mitochondria to participate in cell metabolism. Mitochondrial dynamics, which include fusion and fission, adapt to various stress conditions by altering the morphology of the mitochondrion to meet the energy metabolism and other biological needs of a cell. Mitochondrial fission can remove damaged parts to maintain the mitochondrial membrane potential. Afterwards, dysfunctional mitochondria are cleared by mitophagy to maintain mitochondrial homeostasis. Intracellular mitochondrial homeostasis is critical for preventing oxidative stress caused by defective mitochondria. TFAM mitochondrial transcription factor A, TFB1M mitochondrial transcription factor B1, TFB2M mitochondrial transcription factor B2, NRF nuclear respiratory factor, PGC-1α peroxisome proliferator-activated receptor γ coactivator-1α, Mfn2 Mitofusin2, OPA1 Optic atrophy protein 1, Drp1 dynamin-related protein 1, Fis1 fission 1

AMP-activated protein kinase (AMPK) is a highly conserved master regulator of metabolism that maintains the homeostasis of energy during metabolism at the cellular and physiological levels. This protein is expressed in various cell types and can be activated by various stimuli, including cell stress, exercise, and many substances that affect cell metabolism. AMPK restores energy through some downstream mediators, such as sirtuins [125]. The activated AMPK–SIRT1–PGC-1α pathway increases mitochondrial biogenesis to reduce oxidative stress and procatabolic responses in chondrocytes, thereby limiting OA progression [21]. A study conducted by Chen et al. [105] demonstrated that the knockdown of AMPK expression also leads to decreased SIRT3 activity in chondrocytes, suggesting that AMPK can regulate downstream SIRT3 levels and activity [105]. Meanwhile, this study revealed that young C57BL/6 male mice exhibited mild cartilage degeneration but significant loss of phosphorylation of AMPKα and SIRT3 expression compared to older C57BL/6 male mice, indicating that the loss of AMPK-SIRT3 signalling may be a key contributor to OA development related to aging. The original study revealed that the downregulation of active AMPK is a core event in OA chondrocytes and that the downregulation of active AMPK also mediates a decrease in the level of PGC-1α [113, 126]. PGC-1α is a transcriptional coactivator with nuclear respiratory factors (NRFs) and regulates mitochondrial transcription factor A (TFAM) and transcription factor B (TFB) gene expression [127].

The protein level of TFAM regulates the total copy number of mtDNA and maintains mtDNA completion and stability [128]. TFB proteins, especially TFB1M and TFB2M, enhance mtDNA transcription to strengthen mitochondrial biogenesis. The promoters of TFB1M and TFB2M are regulated by NRFs. However, the effect of NRFs combined the TFB promoter sites are required for PGC-1α family coactivators [127]. Hence, PGC-1α plays an important role in mitochondrial biogenesis, and a study indicated that the content and activity of PGC-1α are decreased in OA chondrocytes [113]. A very recent study reported that PGC-1α can act on the promoter of SIRT3 by recruiting estrogen-related receptor-alpha (ERRa) to promote the expression of SIRT3 [129]. Furthermore, SIRT1 interacts with PGC‐1α to upregulate the transcription of SIRT3 [130], and PGC‐1α also co‐activates NRF2, which is another novel regulator of SIRT3 [131]. At the same time, some studies have shown that SIRT3 can deacetylate liver kinase B1 (LKB1), which can activate the AMPK pathway and increase the level of PGC‐1α [89]. This forms a positive feedback loop, and the activation of AMPK can upregulate PGC-1α expression in response to external stress. Thus, decreased SIRT3 may lead to the dysfunction of mitochondrial biogenesis in OA by reducing the activation of AMPK signalling.

As cells change in response to environmental stress, the structure and function of mitochondria can be altered in response to metabolic changes by controlling the process of morphological changes including fusion and fission, which is defined as mitochondrial dynamics. Optic atrophy protein 1 (OPA1) is a dynamin-related GTPase. It is localized in the intermembrane space and tethered to the inner membrane of mitochondria, and it mediates the function of fusion. SIRT3 also regulates the deacetylation state of OPA1, which directly affects mitochondrial dynamics [132]. The function of mitochondrial fission is correlated with the mitochondrial fission factor, which is called fission 1 (Fis1) [133]. The purpose of fission is to regenerate new mitochondria in growing cells and assist the removal of damaged mitochondria by mitophagy. A study revealed that the depletion of Fis1 can damage chondrocytes and impair lysosomal and peroxisomal function in chondrocytes [134]. A recent study conducted by Zhou et al. [135] revealed that SIRT3 can regulate Fis1 expression via the c-Jun N-terminal kinase (JNK) signalling pathway [135]. Furthermore, mitochondrial dynamics, including fusion and fission, are associated with cell apoptosis. For example, the overexpression of mitochondrial fission molecules, such as dynamin-related protein 1 (Drp1) or Fis1, induces cell apoptosis, while the inhibition Drp1 or Fis1 expression induces an effect that can hinder cell apoptosis [136]. Consistent with this, silencing the expression of mitochondrial fusion proteins, such as Mitofusin1/2 (Mfn1/2) or OPA1, has been proven to contribute to apoptosis. For example, the loss of OPA1 can promote cell apoptosis [136, 137]. Recently, accumulating evidence has demonstrated that changes in mitochondrial dynamics have a great effect on inflammation in a variety of diseases [138, 139]. Meanwhile, inflammatory mediators regulate mitochondrial dynamics [140], but there are a few reports on mitochondrial dynamics in the progression of OA. Hence, further studies in these fields are needed.

SIRT3 regulates mitophagy

Autophagy is a key regulator of cellular homeostatic mechanisms through the removal of damaged organelles and macromolecules, and this process is activated by catabolic and energy stresses [141]. Mitophagy is a kind of selective autophagy through which damaged and depolarized mitochondria are eliminated (Fig. 5). Recent reports have indicated that autophagy activity decreases with age, leading to chondrocyte death and OA. These studies have indicated that the pathological process of OA involves a significant mitochondrial dysfunction induced by the aging-related loss of autophagy in chondrocytes [63, 142, 143]. In addition, the activation of autophagy has an effect on chondrocyte protection, indicating that mitophagy eliminates damaged mitochondria to play a pivotal role in preventing oxidative stress [144, 145]. A recent study revealed that the clearance of dysfunctional mitochondria by Parkin regulates ROS levels and increases the survival of human chondrocytes [146]. Parkin, which is an E3 ubiquitin ligase, can selectively recruit dysfunctional mitochondria with low membrane potential. After recruitment, Parkin can selectively eliminate impaired mitochondria through the engulfment of mitochondria by autophagosomes. A report by Ansari et al. [146] demonstrated that increased expression of Parkin can decrease the production of ROS through the clearance of damaged mitochondria and that the loss of Parkin function can in turn contribute to the pathogenesis of OA [146, 147]. The removal of damaged mitochondria by Parkin is necessary for mitochondrial quality control in chondrocytes [146]. SIRT3 deficiency impairs mitophagy by increasing the acetylation of PTEN-induced kinase (PINK)/Parkin and decreasing Parkin expression via the high acetylation of FOXO3a [148].

Fig. 5.

Mechanism by which SIRT3 regulates mitophagy to maintain mitochondrial homeostasis. OPA1 optic atrophy protein 1, Fis1 fission 1, FOXO1 forkhead box O1, FOXO3a forkhead box O3a, VDAC1 voltage-dependent anion channel 1

FOXO transcriptional factors act a pivotal part in the physiological processes of cells, including metabolism, apoptosis, longevity, and the cell cycle and stress responses. Previous studies have revealed that FOXO transcription factors are dysregulated in aged and OA cartilage [149, 150]. SIRT3 can regulate the deacetylation of FOXO1 and FOXO3a, which have been identified as two important transcriptional factors that are correlated with autophagy‐related genes and are essential for the activation of the autophagy process. It has been reported that SIRT3 promotes autophagy in Ang II-induced myocardial hypertrophy through the deacetylation of FOXO1 and facilitates E3 ubiquitin ligases, inducing the formation of autophagosomes [151]. The same result was reported by Akasaki et al. [149], who proved that the silencing of FOXO1 and FOXO3a can reduce the level of microtubule-associated protein light chain 3 (LC3) in chondrocytes under stimulation with IL-1 [149]. FOXO3a can regulate the transcription of autophagy-associated genes, such as LC3, BNIP3, and sequestosome 1 [152]. Through the activation of downstream signalling proteins, such as Bnip3, Parkin, and sequestosomes, FOXO3a is thought to directly regulate the activation of mitophagy. As reported by Yu et al. [148], in diabetic cardiomyocyte dysfunction, SIRT3 deacetylates FOXO3a to activate mitophagy, and a deficiency in SIRT3 leads to an increase in the acetylation of PTEN-induced kinase (PINK)/Parkin and a decrease in Parkin expression, thus impairing mitochondrial fission and mitophagy [148]. SIRT3 activates hypoxia-induced mitochondrial autophagy by increasing the interaction of VDAC1 with Parkin. Silencing SIRT3 expression inhibits the binding of VDAC1 with Parkin in hypoxic cells and reduces the occurrence of mitophagy [153]. Metformin can reportedly activate SIRT3, which mediates Parkin-dependent mitophagy to protect chondrocytes from oxidative stress through damage to mitochondria induced by IL-1β [154]. Thus, these studies suggest the possible involvement of SIRT3 in the regulation of mitophagy to protect chondrocytes from oxidative stress.

Conclusion and future perspectives

With the pace of population aging, an increasing number of people are suffering from OA. Presently, OA is considered a metabolic disease by some scholars, and mitochondrial dysfunction plays an important role in the development of OA. Furthermore, it is known that mitochondria are “power houses” and are involved in cellular physiological activity, including cell differentiation and apoptosis, intracellular signal transduction, and the regulation of cell growth and the cell cycle. SIRT3, which is an NAD⁺-dependent deacetylase, plays a crucial role in deacetylation in mitochondria. Increasing evidence has shown that SIRT3 is the master regulator of mitochondrial function and can deacetylase a range of targets to maintain mitochondrial function in the antioxidant pathway, energy metabolism, ATP production, and mitochondrial dynamics. Compelling efforts in the search for new therapies to arrest or slow age-associated diseases have led to the demonstration that boosting SIRT3-dependent biological pathways might have an important role in age-associated disease development. For this reason, SIRT3 is a possible target for developing new therapeutic strategies against OA. Given the interest in SIRT3 as a drug target for the treatment of OA, some internal mechanisms need to be further clarified.

Author contributions

The idea for the manuscript came from YH. ZW, LX, KX, and ZC performed the literature search and data analysis, and KX and ZC, JR and LW critically revised the manuscript. All authors read and approved the final version of the manuscript.