Inflammation and intracellular metabolism: new targets in OA

SUMMARY

Articular cartilage degeneration is hallmark of osteoarthritis (OA). Low-grade chronic inflammation in the joint can promote OA progression. Emerging evidence indicates that bioenergy sensors couple metabolism with inflammation to switch physiological and clinical phenotypes. Changes in cellular bioenergy metabolism can reprogram inflammatory responses, and inflammation can disturb cellular energy balance and increase cell stress. AMP-activated protein kinase (AMPK) and sirtuin 1 (SIRT1) are two critical bioenergy sensors that regulate energy balance at both cellular and whole-body levels. Dysregulation of AMPK and SIRT1 has been implicated in diverse human diseases and aging. This review reveals recent findings on the role of AMPK and SIRT1 in joint tissue homeostasis and OA, with a focus on how AMPK and SIRT1 in articular chondrocytes modulate intracellular energy metabolism during stress responses (e.g., inflammatory responses) and how these changes dictate specific effector functions, and discusses translational significance of AMPK and SIRT1 as new therapeutic targets for OA.

Introduction

Osteoarthritis (OA) is the most common form of arthritis. Aging, obesity and biomechanical injury increase the risk of developing OA¹. The central characteristic of OA is progressive degeneration of cartilage, which leads to permanent functional joint failure and disability¹. However, OA is a disorder of the whole synovial joint organ¹. Other joint tissues such as synovium and subchondral bone are also affected in OA^1–4. Increasing evidence indicates that low-grade local joint inflammation (synovitis), induced by endogenous molecular products derived from cellular stress and extra-cellular matrix disruption acting through innate inflammatory network, can influence integrity and function of articular cartilage and promote OA progression^5,6. Low-grade systemic inflammation resulted from metabolic disturbance could also contribute to OA progression⁷. Mounting an inflammatory response is an energy-intensive process^8,9. The concept of interplay between cellular bioenergetics/metabolism and inflammation has been emerged^8–10. Pro-inflammatory mediators alter cellular energy metabolism. Disturbances in the maintenance of cellular energy balance trigger cell stress and induce inflammation^8–10. Effective regulation of cellular energy metabolism is critical for tissue homeostasis¹¹. AMP-activated protein kinase (AMPK) and sirtuin 1 (SIRT1) are two critical energy sensors that regulate energy balance at both cellular and whole-body levels¹¹. The aim of this review is to discuss recent advancement in understanding the role of AMPK and SIRT1 in joint tissue homeostasis and OA, and their potential to be used as therapeutic targets in OA.

Function of AMPK and SIRT1 in articular cartilage

Articular cartilage is an avascular and hypoxic connective tissue¹². Chondrocytes embedded in the articular cartilage have to cope with a nutritionally challenging environment in which metabolic demands may change due to alterations in biomechanical demands, local inflammatory mediators, aging and other factors¹². Glucose transport and glycolysis, and less so mitochondrial oxidative phosphorylation (OXPHOS), provide the primary sources of metabolic energy in articular chondrocytes^12,13.

AMPK in chondrocytes

AMPK, a serine/threonine kinase, is a master regulator of cellular energy balance, whose notable conservation in evolution supports its fundamental role in cell biology^14,15. AMPK exists in essentially all eukaryotic cells in the form of heterotrimeric complexes comprising catalytic α-subunits and regulatory β- and γ-subunits. Each subunit has 2–3 isoforms (α1, α2, β1, β2, γ1, γ2, γ3) encoded by different genes^14,15. Importantly, phosphorylation of a conserved threonine within the catalytic domain of both the α1 and α2 subunits (which are 90% homologous in their catalytic cores) is crucial for AMPK activity^14,15. Articular chondrocytes express α1, α2, β1, β2 and γ1 isoforms of AMPK subunits, and α1 appears to be the predominantly expressed and functionally active AMPK α isoform¹⁶. AMPK is activated by metabolic stress (e.g., nutrient deprivation, hypoxia, heat shock, and exercise/muscle contraction)^14,15. Once activated, AMPK responds by phosphorylating multiple downstream targets, which allow inhibition of pathways that consume ATP such as fatty acid, phospholipid, and protein biosynthesis, and activation of pathways that generate ATP such as glucose uptake and fatty acid oxidation^14,15. In this manner, AMPK allows cells to adjust to changes in energy demand^14,15. AMPK can also be activated by pharmacological compounds such as the nucleotide mimetic AICAR (5-aminoimidazole-4-caboxamide 1-b-D-ribofuranoside), and A-769662 which is a selective and direct AMPK activator^14,15.

We discovered that phosphorylation of AMPKα Thr172 (indication of AMPK activity) is constitutively present in normal articular chondrocytes/cartilage, but is decreased in human knee OA chondrocytes/cartilage¹⁶, in mouse knee OA cartilage¹⁷, and in aged mouse knee cartilage¹⁷. In addition, phosphorylation of AMPKα is decreased, correlated with increased catabolic responses in normal chondrocytes challenged with either inflammatory cytokines IL-1β and TNFα or biomechanical injury^16,17. Moreover, AMPK pharmacological activators are able to reverse these effects^16,17. These data suggest that sustained AMPK activity in articular chondrocytes could be critical for cartilage matrix homeostasis.

Regulation of AMPK activity involves phosphorylation by upstream kinases. The liver protein kinase B1 (LKB1) is thought to be the primary upstream kinase that phosphorylates AMPKα Thr172^14,15. This is true in articular chondrocytes, as phosphorylation of AMPKα is nearly completely inhibited in LKB1 knockdown chondrocytes¹⁷. In addition, chondrocyte catabolic responses to IL-1β and TNFα are significantly enhanced in LKB1 knockdown chondrocytes¹⁷. Interestingly, concomitant reduction of phosphorylation of both LKB1 and AMPKα is observed in primary human knee OA chondrocytes, in mouse knee OA cartilage, in aged mouse knee cartilage, and in chondrocytes challenged with mechanical injury¹⁷, indicating that dysregulation of LKB1 in aged and OA cartilage may play a major role in suppression of AMPK activation. AMPK activity, on the other hand, can be negatively regulated through de-phosphorylation by protein phosphatases such as protein phosphatase 2A (PP2A) and PP2Cα^14,15. We found that phosphorylation of AMPKα (Thr172) was increased, while catabolic responses were decreased in the PP2Ca knockdown chondrocytes stimulated with IL-1β and TNFα (unpublished observation). Thus, chondrocytes with reduced AMPK activity are more susceptible to inflammation-induced catabolic responses (Fig. 1).

Fig. 1.

Impaired chondrocyte AMPK and SIRT1 activity disrupts cartilage matrix homeostasis. Aging, inflammation and biomechanical injury in the joint can decrease phosphorylation of AMPKα Thr172 (indicating AMPK activity) and expression of SIRT1 in articular chondrocytes, likely through down-regulation of activity of the major upstream AMPK kinase LKB1 and up-regulation of protein phosphatase PP2Cα, and PTB1B. As a result, the balance between chondrocyte catabolic and anabolic function is disrupted. Overactive chondrocyte catabolic responses ultimately lead to cartilage matrix degradation.

SIRT1 in chondrocytes

SIRT1, another evolutionary conserved energy sensor, is a member of sirtuins that are nicotinamide adenine dinucleotide (NAD⁺)-dependent lysine deacetylases, acting on a wide range of protein substrates^11,18,19. Expression of SIRT1 is decreased in both human and mouse knee OA cartilage, as well as in aged mouse knee cartilages^20–22. Inhibition of SIRT1 in chondrocytes leads to increased apoptosis and enhanced pro-catabolic responses to IL-1β and TNFα^23–25. Moreover, SIRT1 promotes cartilage-specific gene expression²⁶, protects chondrocytes from radiation-induced senescence²⁷, and inhibits apoptosis in chondrocytes^23,28,29. SIRT1 enhances human OA chondrocyte survival by repressing protein tyrosine phosphatase 1B (PTP1B), a potent pro-apoptotic protein²³. Furthermore, adult heterozygous Sirt1 knockout (KO) mice and mice with a Sirt1 point mutation exhibit increased OA progression^29,30, and cartilage-specific Sirt1 KO mice develop accelerated OA progression²². Clearly, SIRT1 has a chondroprotective role.

AMPK and SIRT1 in chondrocyte stress resistance

Activation of AMPK is shown to stimulate the functional activity of SIRT1 by increasing the intracellular concentrations of NAD⁺ in several different cell types^11,31. SIRT1 deacetylates LKB1, which subsequently increase LKB1 activity, leading to AMPK activation^11,31. The same scenario was seen in articular chondrocytes as well (unpublished observation). This positive feedback loop between SIRT1 and AMPK could potentiate the function of AMPK, and effectively control cellular energy balance^11,31. Importantly, AMPK and SIRT1 not only regulate cellular energy metabolism, but also coordinate several housekeeping mechanisms to increase cell stress resistance via certain downstream mediators.

Regulation of mitochondrial biogenesis and function

AMPK phosphorylates PGC-1α (peroxisome proliferator-activated receptor γ co-activator 1α) protein that subsequently allows SIRT1 to deacetylate and activate PGC-1α^11,31. PGC-1α, a transcriptional co-activator, is a master regulator of mitochondrial biogenesis and function^11,31. The primary function of mitochondria is to produce ATP through the process of OXPHOS, transduced by the respiratory complexes (I–IV) and the ATP synthase (complex V)^13,32. Mitochondrial function is known to decline with aging³³. As cells age, the efficacy of the mitochondrial respiratory chain tends to diminish, thus increasing electron leakage that leads to increases in reactive oxygen species (ROS) production and oxidative damage, and reduced ATP generation³³. Mitochondrial function is impaired in OA chondrocytes, reflected by decreased numbers of mitochondria and activity of respiratory complexes I, II and III^13,32,34. Although the majority of the ATP in chondrocytes is made by glycolysis rather than by OXPHOS, ATP levels per chondrocyte are reduced despite glycolysis is increased in OA chondrocytes³⁵, which not only contributes to decreased mitochondrial bioenergetic reserve^36–38, but also adversely affects cellular redox balance^39–42, and chondrocyte homeostatic functions dependent on physiological generation of low levels of ROS^41,42.

Mitochondrial dysfunction is implicated in onset and progression of cartilage degradation. It increases responsiveness of chondrocytes to pro-inflammatory cytokines, leading to increased matrix catabolism^43,44. Mitochondrial biogenesis is important for maintenance of mitochondrial function. We recently found that expression of PGC-1α is decreased in both mouse knee OA cartilage and in aged mouse knee cartilage⁴⁵. In addition, we observed that mitochondrial biogenesis capacity and function are significantly reduced in advanced human knee OA chondrocytes, indicated by deceased mitochondrial DNA content and mitochondrial mass, and reduced oxygen consumption rate and intracellular ATP level, all of which were correlated with concomitant reduction of phosphorylation of AMPKα, expression of SIRT1 and PGC-1α, and increased acetylation of PGC-1α (unpublished observation). Moreover, the established impairments in mitochondrial biogenesis and function in advanced human knee OA chondrocytes can be reversed by either AMPK pharmacologic activation or overexpression of SIRT1 or PGC-1α (unpublished observation).

Inhibition of oxidative stress and inflammatory responses

FOXO3a, a transcription factor that belongs to the forkhead box O (FOXO) family, is another downstream target of AMPK and SIRT1^11,31. As for PGC-1α, AMPK directly phosphorylates FOXO3a, and SIRT1 deacetylates and activates FOXO3a^11,31. PGC-1α and FOXO3a are closely related. FOXO3a is a direct transcriptional regulator of PGC-1α, and PGC-1α itself can augment the transcriptional activity of FOXO3a⁴⁶. Both PGC-1α and FOXO3a have been shown to limit cellular oxidative stress by up-regulating antioxidant enzymes, including manganese superoxide dismutase (MnSOD or SOD2) and catalase^46,47. In this light, reduction of SOD2 expression has been linked to chondrocyte mitochondrial dysfunction⁴⁸ and OA progression⁴⁹. Similar to PGC-1α, expression of FOXO3a is reduced in both mouse knee OA cartilage and in aged mouse knee cartilage⁴⁵. AMPK pharmacologic activation enhances expression of PGC-1α and FOXO3a, as well as SOD2 and catalase, and inhibits oxidative stress in articular chondrocytes⁴⁵.

Mitochondria dysfunction leads to elevated levels of ROS, which promotes cartilage degradation directly by cleaving collagen and aggrecan and indirectly by activating matrix metalloproteinases (MMPs)^50,51. Additionally, ROS act indirectly by modulating redox-sensitive NF-κB and other signaling pathways that increase chondrocyte catabolic activity^41,42,52. NF-κB plays a major role in innate immune responses. The regulation of innate immunity and energy metabolism is connected together through an antagonistic crosstalk between NF-κB and AMPK and SIRT1 signaling pathways^11,53,54. Many studies have demonstrated that activities of AMPK and SIRT1 have anti-inflammatory effects in diverse types of cells and tissues^11,31. Activation of AMPK inhibits NF-κB activation via SIRT1, which deacetylates p65 NF-κB subunit, ultimately primes p65 for proteasome degradation^11,31. Studies have shown that activation of AMPK or SIRT1 inhibits pro-inflammatory responses to IL-1β and TNFα via attenuation of NF-κB activation in articular chondrocytes^{16,24,25,55–57}. In addition, PGC-1α and FOXO3a, at least in part, mediate AMPK to inhibit NF-κB activation and inflamma-tory cytokine-induced catabolic responses in chondrocytes⁴⁵. Inflammatory cytokines have negative impact on both phosphorylation of AMPKα and SIRT1 expression in chondrocytes^16,26. IL-1β and TNFα de-phosphorylate AMPKα partially by inducing increased expression of PP2Cα (unpublished observation). TNFα also reduces SIRT1 activity in chondrocytes by inducing cathepsin B-mediated cleavage of SIRT1⁵⁸. In addition, activation of NF-κB can down-regulate SIRT1 activity through induction of expression of miR-34a, which targets the 3′ UTR of SIRT1 and inhibits the expression of SIRT1⁵⁹. Chondrocytes with reduced activity of AMPK or SIRT1 have increased responsiveness to in-flammatory cytokines^16,25. These findings suggest that chronic low-grade inflammation in OA and aging joint could be associated with dysregulation of AMPK and SIRT1 signaling, which reduces chondrocyte resistance to inflammatory stress and further provokes inflammatory responses.

Modulation of endoplasmic reticulum (ER) stress

The ER is a sub-cellular organelle where all secretory and integral membrane proteins are folded and post-translationally modified. The ER is also a site of calcium storage and lipid biosynthesis. Protein folding in the oxidizing environment of the ER is an energy-requiring process^60,61. Stresses that compromise the ER homeostasis such as perturbations in calcium homeostasis, energy stores, redox state, and metabolic and inflammatory challenges result in the accumulation of misfolded proteins and activation of a stress response termed the unfolded protein response (UPR)^60,61. Several adaptive signaling pathways have evolved to restore an efficient protein-folding environment through the induction of chaperones, degradation of misfolded proteins and attenuation of protein translation^60,61. Inositol-requiring kinase 1 (IRE1), ER eukaryotic translation initiation factor 2 (eIF2a) kinase (PERK), and activating transcription factor 6 (ATF6) are the three branches of UPR signaling cascade, which are triggered by disassociation of the chaperon GRP78 upon ER stress^60,61. When ER stress is too severe or chronic, or the UPR is unable to resolve the protein-folding defects, cells are led to apoptosis through induction of C/EBP homologous protein (CHOP)^60,61.

The UPR is activated in OA articular chondrocytes (Fig. 2), evidenced by increased expression of GRP78 and CHOP, as well as generation of alternatively spliced and transcriptionally activated X-box protein 1 (XBP1) in OA cartilage^62,63. ATF6 upregulates XBP1 expression in OA chondrocytes by promoting direct binding to XBP1 promoter⁶⁴, and increased XBP1s expression accelerates chondrocyte hypertrophy⁶⁵. XBP1 expression is also increased by IL-1β in chondrocytes⁶². Inhibition of XBP1 expression in chondrocytes via siRNA attenuates nitric oxide and MMP-3 release induced by IL-1β⁶². Several factors implicated in OA pathogenesis including biomechanical injury, IL-1β, nitric oxide and advanced glycation end products (AGEs) upregulate expression of GRP78 and CHOP in cultured articular chondrocytes^{62,63,66–69}. CHOP potentiates the capacity of IL-1β to induce catabolic responses, superoxide generation and apoptosis in chondrocytes, and does so by inhibiting AMPK activity⁶². Moreover, CHOP-mediated apoptosis contributes to the progression of cartilage degeneration in mice⁷⁰. Moreover, pharmacologic AMPK activation blunts CHOP expression and catabolic responses induced by IL-1β and biomechanical injury^17,62. These data indicate that AMPK modulates UPR, and activation of AMPK alleviates ER stress in chondrocytes.

Fig. 2.

The UPR in articular chondrocytes in OA. ER stress in OA chondrocytes caused by certain inflammatory mediators, biomechanical injury, nitric oxide and AGEs leads to activation of the UPR signaling cascades triggered by dissociation of the chaperon GRP78 from the ER transmembrane proteins PERK, IRE1α, and ATF6. The active XBP1 spliced form (XBP1s), generated by IRE1a through alternatively splicing mRNA of unspliced XBP1 (XBP1u), promotes chondrocyte hypertrophy and increases catabolic responses. The cleaved active form of ATF6 (p50) can induce XBP1 expression through direct binding to XBP1 promoter. Increased CHOP expression potentiates IL-1β to decrease phosphorylation of AMPKα and induce catabolic responses, superoxide generation and apoptosis.

Regulation of autophagy

Autophagy is a cellular housekeeping and protein quality control mechanism, which can remove damaged or defective proteins and organelles, e.g., damaged mitochondria^11,71. AMPK controls autophagy through mammalian target of rapamycin (mTOR) and Unc-51-like kinase 1 (ULK1) signaling⁷¹. mTOR, a conserved Ser/Thr protein kinase, is a potent inhibitor of autophagy. ULK1 is a critical kinase that governs the cascade of events triggering autophagy. AMPK can inhibit activity of mTOR complex (mTORC1) either by directly phosphorylating Raptor, a regulatory component of mTORC1, or by phosphorylating tuberous sclerosis protein 2 (TSC2), which subsequently suppresses mTOR activity⁷¹. AMPK stimulates autophagy by dissociating mTORC1 from the ULK1 complex via the phosphorylation of the Raptor component, as well as by directly binding to the ULK1 complex and phosphorylating ULK1⁷¹. In addition, AMPK can also enhance the later steps in autophagosome formation through SIRT1 by deacetylating several autophagy-related proteins (e.g., Atg5, Atg7 and Atg8)¹¹. Chondrocyte auto-phagy is known to be a constitutive homeostatic mechanism in articular cartilage⁷², which can be promoted by AMPK signaling^73,74 through mTOR suppression. As for both phosphorylation of AMPKα and SIRT1 expression, chondrocyte autophagy is reduced with a linked increase in apoptosis in human knee OA, mouse knee OA and aged mouse knee cartilages⁷⁵. Suppressed autophagy also is observed in cartilage ex vivo in response to mechanical injury⁷⁶. Inhibition of autophagy in chondrocyte exacerbated IL-1β–induced OA-like gene expression changes and apoptotic signals, while activation of autophagy inhibited them, possibly through modulation of ROS in chondrocytes in vitro⁷⁷. Cartilage-specific genetic deletion of mTOR⁷⁸ and pharmacologic inhibition of mTOR signaling by rapamycin⁷⁹ upregulates autophagy and reduces the severity of experimental OA in vivo.

Function of AMPK and SIRT1 in synovium and bone

The function of bioenergy sensors in synovium is merely studied yet. One study using fibroblast-like synoviocytes (FLS) of rheumatoid arthritis (RA) showed that SIRT1 expression is decreased by HMGB1, but is recovered by pre-treatment with resveratrol, a SIRT1 activator⁸⁰. Interestingly, Cilostazol, a drug used to treat intermittent claudication that has been shown to activate AMPK, prevents HMGB1-induced angiogenesis via SIRT1 in RA FLS in vitro and exhibits anti-angiogenic effect in collagen-induced arthritis (CIA) mouse model in vivo⁸⁰. Simvastatin, a cholesterol-lowering drug that is also known to activate AMPK, inhibits cysteine-rich angiogenic inducer 61 (CYR61) and CCL20 in RA FLS via up-regulation of SIRT1/FOXO3a signaling⁸¹. These results implicate an anti-inflammatory role of synovial activation of AMPK and SIRT1. Whether activation of AMPK and SIRT1 limits synovial inflammation in OA needs to be investigated.

Both AMPK and SIRT1 are involved in regulation of bone metabolism. Activation of AMPK promotes osteoblast differentiation, bone formation and bone mass, and inhibits osteoclast differentiation in vitro^82–91. Mice deficient in AMPKα1 or AMPKα2 exhibit reduced bone mass compared with WT mice⁸³. AMPKα1 deficient mice have an elevated rate of bone remodeling, whereas AMPKα2 deficient mice largely have elevated bone resorption⁸³. Osteoclast- or osteoblast-specific SIRT1 conditional KO mice have decreased bone mass caused by increased resorption and reduced bone formation⁹⁰. Wnt/β-catenin is known to be indispensable for osteoblast generation. SIRT1 is shown to regulate differentiation of mesenchymal stem cells (MSCs) by deacetylating β-catenin⁹². MSCs isolated from MSC-specific SIRT1 KO mice have reduced differentiation towards osteoblasts and chondrocytes in vitro⁹². Activation of AMPK or SIRT1 decreases adipocyte and promotes osteoblast formation during differentiation of MSCs^93–95, suggesting a critical role of AMPK and SIRT1 in regulation of the lineage commitment of MSCs. Osteoprogenitor-specific SIRT1 KO mice display lower cortical thickness in femora and vertebrae because of reduced bone formation at the endocortical surface, associated with decreased Wnt/b-catenin signaling⁸⁸. Decreased expression of SIRT1 is found in human OA subchondral bone osteoblasts⁸⁹. Inhibition of SIRT1 in osteoblasts leads to abnormal sclerostin expression that decreases Wnt/β-catenin activity⁸⁹, which can be inhibited by resveratrol⁸⁹. Obviously, AMPK and SIRT1 play an important role in bone homeostasis. Abnormal AMPK and SIRT1 levels in osteoblasts and osteoclasts may contribute to subchondral bone pathological changes in OA.

Clinical relevance and translation

Dysregulation of AMPK and SIRT1 is linked to diabetes, atherosclerosis, cardiovascular disease, cancer, neurodegenerative diseases^11,33, as well as OA based on recent findings, all of which are age-related diseases. Studies have revealed that responsiveness of AMPK activation declines during the aging process¹¹, and low-grade inflammation present in aging tissues may be at least in part responsible for suppressing AMPK signaling¹¹. The prevalence of OA increases with aging further supports the concept that a dysfunction of AMPK is involved in the disease process.

Nutritional factors can affect AMPK signaling. Caloric restriction is known to stimulate AMPK activity, whereas nutritional overload seems to impair AMPK activity, which can induce insulin resistance in many tissues¹¹. The metabolic disturbance can cause low-grade inflammation leading to development of metabolic syndrome such as obesity and diabetes¹¹, which are often associated with OA⁷. Diet rich in n-3 long chain polyunsaturated fatty acids (PUFAs) is considered as a nutritional tool to prevent insulin resistance associated to type 2 diabetes and obesity⁹⁶. This is probably at least in part owing to the ability of n-3 PUFAs to stimulate AMPK activity^97–100. Exercise is considered as one of the most cost effective approaches to provide health benefits¹⁰¹. Evidence supports benefits of various types of exercise for improving pain and function in knee OA¹⁰². Studies in both animals and humans demonstrate that skeletal muscle contraction and exercise activate AMPK in an intensity and time-dependent manner^101,103, and increased AMPK activation promotes adaptation to muscle endurance exercise^101,103. As such, activation of AMPK provides molecular basis of the benefits of exercise, and supports clinical recommendation of exercise for the management of OA¹⁰².

A number of drugs already in the clinic for arthritis and other conditions (e.g., sodium salicylate, high dose aspirin, methotrexate, metformin), and a variety of natural plant products either present in traditional medicine or derived from food (e.g., berberine, resveratrol, curcumin, quercetin) appealed to have “nutraceutical” properties are able to activate AMPK^104,105. However, the majority of them were in use for their respective purposes before they were known to be activators of AMPK^104,105. Interestingly, a recent study on a randomized placebo-controlled small trial of methotrexate in symptomatic knee OA showed significant improvement in physical function associated with reduced pain and synovitis¹⁰⁶. In addition, a randomized double-blind placebo-controlled small trial of curcuminoid (closely related to curcumin) in treatment of knee OA also showed significant improvements in pain and physical function¹⁰⁷. Moreover, several randomized placebo-controlled trials of avocado–soybean unsaponifiables (ASU) on both knee and hip OA demonstrated significant efficacy in improving the symptoms of OA and reducing progression of joint space narrowing^108–110. ASU is a natural vegetable extract made from avocado and soybean oils used as a dietary supplement. The carotenoids such as β-carotene and lutein present in avocado, and a key isoflavone genistein present in soybean are capable of activating AMPK^111–113. The beneficial effects of the agents tested in the OA clinical trails aforementioned are conceivably in part due to AMPK activation, which warrant further investigation.

Conclusion

Disturbances in the maintenance of energy balance provoke diseases and jeopardize healthy aging¹¹. Reduced capacity of AMPK and SIRT1 activation in articular joint tissues could limit energy availability for cellular maintenance, trigger significant cell stress by inducing mitochondrial dysfunction, oxidative stress and inflammation, ultimately, compromise cell survival and tissue function. These bioenergy sensors can couple metabolism with inflammation to switch physiologic and clinical phenotypes. Given the fact that activation of AMPK and SIRT1 in articular chondrocytes regulates energy balance and coordinates several housekeeping mechanisms to increase cell stress resistance and maintain quality control (Fig. 3), targeted activation of AMPK and SIRT1 appeals to be an attractive therapeutic strategy for OA, which could be potentially achieved by pharmacologics, lifestyle measures (diet and exercise) and nutraceuticals.

Fig. 3.

AMPK and SIRT1 activity in regulation of cartilage homeostasis by increasing chondrocyte stress resistance. Pharmacological activators (e.g., AICAR, A-769662, and metformin), possibly some natural plant products (nutraceuticals), caloric restriction and exercise, activate AMPK in articular chondrocytes. As a result, intracellular NAD⁺ level is increased, leading to activation of SIRT1. Resveratrol, a natural plant product, also activates SIRT1. In turn, SIRT1 activates LKB1, the major upstream kinase of AMPK, through deacetylation of LKB1. Activation of AMPK and SIRT1 coordinates several housekeeping mechanisms to increase chondrocyte stress resistance by increasing mitochondrial biogenesis and function via PGC-1α; inhibiting oxidative stress and via PGC-1α and FOXO3a; attenuating inflammatory responses via inhibition of NF-κB activation, alleviating ER stress via limiting excessive CHOP; and promoting autophagy via suppression of mTOR, ultimately leading to cartilage homeostasis.