Linked decreases in liver kinase B1 and AMP-activated protein kinase activity modulate matrix catabolic responses to biomechanical injury in chondrocytes

Chondrocyte Mitochondrial Quality Control: A Novel Insight into Osteoarthritis and Cartilage Regeneration

Significance: Osteoarthritis (OA), one of the most prevalent joint diseases affecting more than 240 million people, strongly influences human health and reduces life quality. This review aims to fill the current research gap regarding the application and potential of mitochondrial quality control (MQC) based therapies in the treatment of OA, thereby providing guidance for future research and clinical practice. Recent Advances: Chondrocytes respond to the inflammatory microenvironment via an array of signaling pathways and thus are critical in cartilage degeneration and OA progression. Mitochondria, as an important metabolic center in chondrocytes, play a vital role in responding to inflammatory stimuli. Multiple MQC mechanisms, including mitochondrial antioxidant defense, mitochondrial protein quality control, mitochondrial DNA repair, mitochondrial dynamics, mitophagy, and mitochondrial biogenesis, sustain mitochondrial homeostasis under pathological conditions. Critical Issues: Despite extensive OA research, effective therapies remain limited. Elucidating MQC mechanisms in disease progression and post-traumatic cartilage repair is crucial. While preclinical studies demonstrate potential, clinical translation requires addressing protocol standardization, patient stratification, and long-term efficacy, as well as safety validation. Future Directions: Future research should focus on developing personalized MQC-based OA therapies guided by biomarker profiling and signaling pathway modulation. However, translational challenges persist, particularly regarding pervasive off-target effects, inadequate OA-specific targeting capacity, interpatient heterogeneity, and reliable evaluation of long-term therapeutic efficacy. Strategic prioritization of OA-specific MQC targets coupled with delivery system optimization may significantly improve both clinical translatability and therapeutic outcomes.

HNF4A functions as a hepatocellular carcinoma oncogene or tumor suppressor depending upon the AMPK pathway activity status

Cancer cells frequently undergo energy metabolic stress induced by the increased dynamics of nutrient supply. Hepatocyte nuclear factor 4A (HNF4A) is a master transcription factor (TF) in hepatocytes that regulates metabolism and differentiation. However, the mechanism underlying how HNF4A functions in cancer progression remains unclear due to conflicting results observed in numerous studies. To address the roles of HNF4A in hepatocellular carcinoma (HCC), we investigated the regulatory functions of HNF4A in HCC cells under different glucose supply conditions. We found that HNF4A exhibited tumor-suppressive effects on the proliferation and migration of HCC cells in glucose-sufficient conditions and tumor-promotive effects on HCC cells in glucose-insufficient conditions. Further investigation revealed that this diverse function of HNF4A was dependent upon the AMPK pathway activity. Similarly, the prognosis predicted by HNF4A was also correlated with whether the AMPKa expression levels were low or high in clinical HCC patients. Multiomics approaches consisting of proteomics and ChIP-seq revealed that key HNF4A target genes, including NEDD4 and RPS6KA2, are involved in the diverse function of HNF4A in HCC in response to the AMPK activity status. Specifically, HNF4A could bind to the promoter region of NEDD4 and RPS6KA2, and upregulating their expression. Our study has demonstrated the relationship between and synergism of AMPK and HNF4A in the progression of HCC under diverse nutrient conditions.

Metabolic Reprogramming in Stromal and Immune Cells in Rheumatoid Arthritis and Osteoarthritis: Therapeutic Possibilities

Metabolic reprogramming of stromal cells, including fibroblast-like synoviocytes (FLS) and chondrocytes, as well as osteoclasts (OCs), are involved in the inflammatory and degenerative processes underlying rheumatoid arthritis (RA) and osteoarthritis (OA). In RA, FLS exhibit mTOR activation, enhanced glycolysis and reduced oxidative phosphorylation, fuelling inflammation, angiogenesis, and cartilage degradation. In OA, chondrocytes undergo metabolic rewiring, characterised by mTOR and NF-κB activation, mitochondrial dysfunction, and increased glycolysis, which promotes matrix metalloproteinase production, extracellular matrix (ECM) degradation, and angiogenesis. Macrophage-derived immunometabolites, including succinate and itaconate further modulate stromal cell function, acting as signalling molecules that modulate inflammatory and catabolic processes. Succinate promotes inflammation whilst itaconate is anti-inflammatory, suppressing inflammatory joint disease in models. Itaconate deficiency also correlates inversely with disease severity in RA in humans. Emerging evidence highlights the potential of targeting metabolic processes as promising therapeutic strategies for connective tissue disorders.

Recent development of mitochondrial metabolism and dysfunction in osteoarthritis

Osteoarthritis is a degenerative joint disorder characterized by cartilage degradation, synovial inflammation, and altered subchondral bone structure. Recent insights have identified mitochondrial dysfunction as a pivotal factor in OA pathogenesis, contributing to chondrocyte apoptosis, oxidative stress, and extracellular matrix degradation. Disruptions in mitochondrial dynamics, including impaired biogenesis, mitophagy, and metabolic shifts from oxidative phosphorylation to glycolysis, exacerbate cartilage damage by promoting the production of reactive oxygen species and matrix-degrading enzymes such as ADAMTS and MMPs. This review explores the molecular mechanisms underlying mitochondrial dysfunction in OA, emphasizing its role in cartilage homeostasis and inflammation. Furthermore, it highlights emerging therapeutic strategies targeting mitochondrial pathways, including antioxidants, mitophagy enhancers, and metabolic modulators, as potential interventions to mitigate disease progression, which offer promising avenues for advancing personalized and disease-modifying treatments in OA.

Yanghe decoction alleviates osteoarthritis by AMPK-SIRT3 positive feedback loop-mediated mitochondrial autophagy

ETHNOPHARMACOLOGICAL RELEVANCE

Yanghe Decoction(YHD) is a traditional Chinese medicine compound known for its efficacy in treating osteoarthritis (OA).

AIM OF THE STUDY

We aimed to explore the underlying mechanisms of YHD in relation to OA.

MATERIALS AND METHODS

UHPLC-MS technology was used to identify the material basis of YHD. In vivo, OA rat model was induced by the modified Hulth method and then treated with YHD at three different doses (0.625, 1.3 and 2.6 g/kg/d). In vitro,YHD-Contained serum was prepared and administrated into rat chondrocytes, followed by simulation of Lipopolysaccharide(LPS). The protective mechanism was determined by observation of morphology, Flow cytometry and Protein level detection.

RESULTS

In vivo, YHD reduced chondrocyte apoptosis and joint inflammation while promoting mitophagy. It also elevated the protein levels of p-AMPK, SIRT3, PINK1, Parkin, and LC3II/I. In vitro, YHD-Contained Serum reduced chondrocyte apoptosis, decreased mitochondrial ROS, enhanced mitochondrial membrane potential, and upregulated the protein expressions of p-AMPK, SIRT3, PINK1, Parkin, and LC3II/I.

CONCLUSION

Through this study, we demonstrated YHD protect chondrocytes against apoptosis, and its underlying mechanisms may involve the regulation of AMPK-SIRT3 positive feedback loop and activation of PINK1/Parkin mediated mitophagy.

Development of novel osteoarthritis therapy by targeting AMPK-β-catenin-Runx2 signaling

Osteoarthritis (OA) is a debilitating chronic joint disease affecting large populations of patients, especially the elderly. The pathological mechanisms of OA are currently unknown. Multiple risk factors are involved in OA development. Among these risk factors, alterations of mechanical loading in the joint leading to changes in biological signaling pathways have been known as a key event in OA development. The importance of AMPK-β-catenin-Runx2 signaling in the initiation and progression of OA has been recognized in recent years. In this review, we discuss the recent progress in understanding the role of this signaling pathway and the underlying interaction mechanisms during OA development. We also discuss the drug development aiming to target this signaling pathway for OA treatment.

Modulating Autophagy in Osteoarthritis: Exploring Emerging Therapeutic Drug Targets

Osteoarthritis (OA) is a degenerative joint disease characterized by the breakdown of cartilage and the subsequent inflammation of joint tissues, leading to pain and reduced mobility. Despite advancements in symptomatic treatments, disease-modifying therapies for OA remain limited. This narrative review examines the dual role of autophagy in OA, emphasizing its protective functions during the early stages and its potential to contribute to cartilage degeneration in later stages. By delving into the molecular pathways that regulate autophagy, this review highlights its intricate interplay with oxidative stress and inflammation, key drivers of OA progression. Emerging therapeutic strategies aimed at modulating autophagy are explored, including pharmacological agents such as AMP kinase activators, and microRNA-based therapies. Preclinical studies reveal encouraging results, demonstrating that enhancing autophagy can reduce inflammation and decelerate cartilage degradation. However, the therapeutic benefits of autophagy modulation depend on precise, stage-specific approaches. Excessive or dysregulated autophagy in advanced OA may lead to chondrocyte apoptosis, exacerbating joint damage. This review underscores the promise of autophagy-based interventions in bridging the gap between experimental research and clinical application. By advancing our understanding of autophagy's role in OA, these findings pave the way for innovative and effective therapies. Nonetheless, further research is essential to optimize these strategies, address potential off-target effects, and develop safe, targeted treatments that improve outcomes for OA patients.

The cereblon-AMPK (AMP-activated protein kinase) axis in chondrocytes regulates the pathogenesis of osteoarthritis

OBJECTIVE

AMP-activated protein kinase (AMPK) dysregulation is implicated in osteoarthritis (OA), but the mechanisms underlying this dysregulation remain unclear. We investigated the role of cereblon, a substrate-recognition protein within the E3-ligase ubiquitin complex, in AMPK dysregulation and OA pathogenesis.

METHODS

Cereblon expression was examined in human (n = 5) and mouse (n = 10) OA cartilage. The role of cereblon was investigated through its adenoviral overexpression (n = 10) or knockout (KO, n = 15) in the destabilization of the medial meniscus (DMM)-operated mice. The therapeutic potentials of the chemical cereblon degrader, TD-165, and the AMPK activator, metformin, were assessed through intra-articular (IA) injection to mice (n = 15).

RESULTS

Immunostaining revealed that cereblon is upregulated in human and mouse OA cartilage. In DMM model mice, cartilage destruction was exacerbated by overexpression of cereblon in mouse joint tissues (OARSI grade; 1.11 [95% CI: 0.50 to 2.75]), but inhibited in global (-2.50 [95% CI: -3.00 to -1.17]) and chondrocyte-specific (-2.17 [95% CI: -3.14 to -1.06]) cereblon KO mice. The inhibitory effects were more pronounced in mice fed a high-fat diet compared to a regular diet. The degradation of cereblon through IA injection of TD-165 inhibited OA cartilage destruction (-2.47 [95% CI: -3.22 to -1.56]). Mechanistically, cereblon exerts its catabolic effects by negatively modulating AMPK activity within chondrocytes. Consistently, activation of AMPK by IA injection of metformin inhibited posttraumatic OA cartilage destruction (-1.20 ([95% CI: -1.89 to -0.45]).

CONCLUSIONS

The cereblon-AMPK axis acts as a catabolic regulator of OA pathogenesis and seems to be a promising therapeutic target in animal models of OA.

β-Hydroxybutyrate enhances chondrocyte mitophagy and reduces cartilage degeneration in osteoarthritis via the HCAR2/AMPK/PINK1/Parkin pathway

Osteoarthritis (OA) is widely recognized as the prevailing joint disease associated with aging. The ketogenic diet (KD) has been postulated to impede the advancement of various inflammatory ailments. β-Hydroxybutyrate (βOHB), a prominent constituent of ketone bodies, has recently been proposed to possess crucial signaling capabilities. In this study, we propose to explore the role and mechanism of βOHB in OA. Tissue staining and inflammatory factor assay were employed to evaluate the impacts of KD and βOHB on OA rats. The oxidative stress conditions in chondrocytes were induced using tert-butyl hydroperoxide (TBHP). The mechanisms were determined using the siRNA of hydroxycarboxylic acid receptor 2 (HCAR2), the antagonist of adenosine monophosphate-activated protein kinase (AMPK), and the inhibitor of mitophagy. The administration of KD demonstrated a reduction in pathological damage to cartilage, as well as a decrease in plasma levels of inflammatory factors. Furthermore, it resulted in an increase in the concentration of βOHB in the blood and synovial fluid. In vitro experiments showed that βOHB facilitated mitophagy and adenosine triphosphate production. Besides, βOHB mitigated chondrocyte senescence, inflammatory factors secretion, extracellular matrix degradation, and apoptosis induced by TBHP. Subsequent investigations indicated that the protective effects of βOHB were no longer observed following the knockdown of HCAR2, the antagonist of AMPK, or the inhibitor of mitophagy. Moreover, in vivo studies suggested that βOHB played a protective role by targeting the HCAR2-AMPK-PINK1 axis. In conclusion, βOHB enhanced chondrocyte mitophagy through the HCAR2/AMPK/PINK1/Parkin pathway, offering a potential therapeutic approach for the treatment of OA.

Mitochondrial quality control dysfunction in osteoarthritis: Mechanisms, therapeutic strategies & future prospects

Osteoarthritis (OA) is a prevalent chronic joint disease characterized by articular cartilage degeneration, pain, and disability. Emerging evidence indicates that mitochondrial quality control dysfunction contributes to OA pathogenesis. Mitochondria are essential organelles to generate cellular energy via oxidative phosphorylation and regulate vital processes. Impaired mitochondria can negatively impact cellular metabolism and result in the generation of harmful reactive oxygen species (ROS). Dysfunction in mitochondrial quality control mechanisms has been increasingly linked to OA onset and progression. This review summarizes current knowledge on the role of mitochondrial quality control disruption in OA, highlighting disturbed mitochondrial dynamics, impaired mitochondrial biogenesis, antioxidant defenses and mitophagy. The review also discusses potential therapeutic strategies targeting mitochondrial Quality Control in OA, offering future perspectives on advancing OA therapeutic strategies.

The involvement of signaling pathways in the pathogenesis of osteoarthritis: An update

Osteoarthritis (OA) is one of the most common disabling pathologies, characterized by joint pain and reduced function, significantly worsening the quality of life. Even if important progresses have been made in OA research, little is yet known about the precise cellular and molecular mechanisms underlying OA. Understanding dysregulated signaling networks and their crosstalk in OA may offer a strong opportunity for the development of combined targeted therapies. Hence, this review highlights the recent findings on the main pathways involved in OA development, including Wnt, Notch, Hedgehog, MAPK, AMPK, and JAK/STAT, providing insights on current targeted therapies in OA patients' management.

The translational potential of this article

The identification of key signaling pathways involved in OA development and the investigation of their signaling crosstalk could pave the way for more effective treatments and improved management of OA patients in the future.

Advances in the study of mitophagy in osteoarthritis

Osteoarthritis (OA), characterized by cartilage degeneration, synovial inflammation, and subchondral bone remodeling, is among the most common musculoskeletal disorders globally in people over 60 years of age. The initiation and progression of OA involves the abnormal metabolism of chondrocytes as an important pathogenic process. Cartilage degeneration features mitochondrial dysfunction as one of the important causative factors of abnormal chondrocyte metabolism. Therefore, maintaining mitochondrial homeostasis is an important strategy to mitigate OA. Mitophagy is a vital process for autophagosomes to target, engulf, and remove damaged and dysfunctional mitochondria, thereby maintaining mitochondrial homeostasis. Cumulative studies have revealed a strong association between mitophagy and OA, suggesting that the regulation of mitophagy may be a novel therapeutic direction for OA. By reviewing the literature on mitophagy and OA published in recent years, this paper elaborates the potential mechanism of mitophagy regulating OA, thus providing a theoretical basis for studies related to mitophagy to develop new treatment options for OA.

Effects of glucose concentration and oxygen partial pressure on the respiratory metabolism of sheep temporomandibular joint disc cells

Temporomandibular joint (TMJ) disc degeneration is a common disease characterized by a decrease in metabolic function. The present study aimed to investigate the pathogenesis of TMJ disc degeneration by analyzing the effects of oxygen and glucose concentrations on metabolism in a simulated TMJ disc cell growth environment. Cell samples were divided into 10 groups and cultured in different nutritional environments, including 21 and 2% O₂ partial pressures and various glucose concentrations (0, 0.5, 3, 5.5 and 22.5 mmol/l). Cell proliferation, extracellular matrix content, mitochondrial function, and cell metabolism were subsequently analyzed. The results demonstrated that hypoxia and a low glucose concentration inhibited cell growth, and low glucose concentration inhibited extracellular matrix synthesis and adenosine 5'-monophosphate-activated protein kinase expression. Hypoxic conditions also induced a compensatory increase in the number of mitochondria, whereas mitochondrial deformation and swelling were observed in the absence of glucose. According to this study, the primary metabolic pathway of TMJ disc cells is glycolysis. It was concluded that hypoxic conditions and normal glucose concentrations are needed for the growth of TMJ disc cells. Glucose is necessary to ensure cell survival, extracellular matrix synthesis and mitochondrial function. Glucose deficiency may be related to disc degeneration, aging and disease mechanisms.

Enpp1 deficiency caused chondrocyte apoptosis by inhibiting AMPK signaling pathway

OBJECTIVE AND BACKGROUND

The deficiency of ectonucleotide pyrophosphatase/phosphodiesterase 1 (Enpp1) causes the phenotype similar to knee osteoarthritis (OA). However, the molecular mechanism is poorly understood.

METHOD

The global deletion of Enpp1 (Enpp1^-/-) mice was created to analyze the role of Enpp1 in the progress of knee OA. The apoptosis, proliferation and chondrogenic differentiation ability of chondrocytes from wild-type (WT) and Enpp1^-/- joints were compared. According to the results of high-throughput quantitative molecular measurements, the proteins of chondrocytes from WT and Enpp1^-/- mice were used to explore the mechanism of Enpp1 deficiency-associated knee OA.

RESULT

In Enpp1^-/- knee joints, we found significant chondrocyte apoptosis and proteomic results showed that abnormal expression of AMP-activated protein kinase (AMPK) signaling pathway may contribute to this phenotype. In primary chondrocyte cultures in vitro, Enpp1 deletion dramatically enhancing chondrocyte apoptosis. Meanwhile, we found Enpp1 deletion inhibits the phosphorylation of AMPK (P-AMPK). We also found that decreased level of P-AMPK and chondrocyte apoptosis, which are caused by Enpp1 deficiency, can be reversed by Acadesine (AICAR), the activator of AMPK.

CONCLUSION

Consequently, Enpp1 deficiency plays an essential role in knee OA by regulating AMPK signaling pathway.

Adenosine A2A receptor activation reduces chondrocyte senescence

Osteoarthritis (OA) pathogenesis is associated with reduced chondrocyte homeostasis and increased levels of cartilage cellular senescence. Chondrosenescence is the development of cartilage senescence that increases with aging joints and disrupts chondrocyte homeostasis and is associated with OA. Adenosine A2A receptor (A2AR) activation in cartilage via intra-articular injection of liposomal A2AR agonist, liposomal-CGS21680, leads to cartilage regeneration in vivo and chondrocyte homeostasis. A2AR knockout mice develop early OA isolated chondrocytes demonstrate upregulated expression of cellular senescence and aging-associated genes. Based on these observations, we hypothesized that A2AR activation would ameliorate cartilage senescence. We found that A2AR stimulation of chondrocytes reduced beta-galactosidase staining and regulated levels and cell localization of common senescence mediators p21 and p16 in vitro in the human TC28a2 chondrocyte cell line. In vivo analysis similarly showed A2AR activation reduced nuclear p21 and p16 in obesity-induced OA mice injected with liposomal-CGS21680 and increased nuclear p21 and p16 in A2AR knockout mouse chondrocytes compared to wild-type mice. A2AR agonism also increased activity of the chondrocyte Sirt1/AMPK energy-sensing pathway by enhancing nuclear Sirt1 localization and upregulating T172-phosphorylated (active) AMPK protein levels. Lastly, A2AR activation in TC28a2 and primary human chondrocytes reduced wild-type p53 and concomitantly increased p53 alternative splicing leading to increase in an anti-senescent p53 variant, Δ133p53α. The results reported here indicate that A2AR signaling promotes chondrocyte homeostasis in vitro and reduces OA cartilage development in vivo by reducing chondrocyte senescence.

Osteoarthritis: pathogenic signaling pathways and therapeutic targets

Osteoarthritis (OA) is a chronic degenerative joint disorder that leads to disability and affects more than 500 million population worldwide. OA was believed to be caused by the wearing and tearing of articular cartilage, but it is now more commonly referred to as a chronic whole-joint disorder that is initiated with biochemical and cellular alterations in the synovial joint tissues, which leads to the histological and structural changes of the joint and ends up with the whole tissue dysfunction. Currently, there is no cure for OA, partly due to a lack of comprehensive understanding of the pathological mechanism of the initiation and progression of the disease. Therefore, a better understanding of pathological signaling pathways and key molecules involved in OA pathogenesis is crucial for therapeutic target design and drug development. In this review, we first summarize the epidemiology of OA, including its prevalence, incidence and burdens, and OA risk factors. We then focus on the roles and regulation of the pathological signaling pathways, such as Wnt/β-catenin, NF-κB, focal adhesion, HIFs, TGFβ/ΒΜP and FGF signaling pathways, and key regulators AMPK, mTOR, and RUNX2 in the onset and development of OA. In addition, the roles of factors associated with OA, including MMPs, ADAMTS/ADAMs, and PRG4, are discussed in detail. Finally, we provide updates on the current clinical therapies and clinical trials of biological treatments and drugs for OA. Research advances in basic knowledge of articular cartilage biology and OA pathogenesis will have a significant impact and translational value in developing OA therapeutic strategies.

Increased Sparc release from subchondral osteoblasts promotes articular chondrocyte degeneration under estrogen withdrawal

OBJECTIVE

The incidence of osteoarthritis (OA) in menopausal women is significantly higher than in same-aged men. Investigating the role of subchondral osteoblasts in estrogen deficiency-induced OA may help elucidate the pathological mechanism, providing new insights for the diagnosis and treatment of menopausal OA.

METHODS

A classical ovariectomy-induced OA (OVX-OA) rat model was utilized to isolate primary articular chondrocytes and subchondral osteoblasts, which were identified and then cocultured in Transwell. The expression of chondrocyte anabolic and catabolic indicators was evaluated. The differentially expressed proteins in the conditioned medium (CM) of osteoblasts were identified by Liquid Chromatograph-Mass Spectrometer (LC-MS/MS). Normal chondrocytes were treated with osteoblast CM, and then RNA sequencing was performed on the treated chondrocytes. KEGG was used to identify significant enrichment of signaling pathways, and Simple Western was used to verify the expression of related proteins in the signaling pathways.

RESULTS

Coculture of OVX-OA subchondral osteoblasts with chondrocytes significantly downregulated the expression of the anabolic indicators and upregulated the expression of the catabolic indicators in chondrocytes. 1,601 proteins were identified in both normal and OVX osteoblast culture supernatants. Protein-protein interaction network analysis revealed that Sparc was one of the hub proteins. The AMPK/Foxo3a signaling pathway of chondrocytes was downregulated by OVX-OA osteoblasts CM. AICAR, the AMPK agonist, partially reversed the catabolic effect of OVX-OA osteoblasts on chondrocytes.

CONCLUSIONS

Sparc secreted by OVX-OA subchondral osteoblasts can downregulate the AMPK/Foxo3a signaling pathway of chondrocytes, thereby promoting chondrocyte degeneration.

Linear ubiquitination of LKB1 activates AMPK pathway to inhibit NLRP3 inflammasome response and reduce chondrocyte pyroptosis in osteoarthritis

Background

Osteoarthritis (OA) is the most common chronic disease. It is characterized by high levels of clinical heterogeneity and low inflammation. Therefore, elucidation of the mechanisms that regulate gene expression is critical for developing effective OA therapies. This study aimed to explore the role of LKB1/AMPK in the progression of OA.

Methods

Anterior cruciate ligament transection (ACLT) was performed on Sprague Dawley (SD) rats right knee to construct OA model, followed by AICAR [AMP-activated protein kinase (AMPK) activator] treatment. The level changes [AMPK, IL-10, IL-13, IL-1β, TNF-α, IL-6, ASC, Caspase-1, Ki67, and hibit Nod-like receptor protein 3 (NLRP3)] and the degree of tissue injury were assessed by western blot, Immunohistochemical (IHC), Enzyme-linked immunosorbent assay (ELISA), Hematoxylin-eosin staining (HE), Immunofluorescence (IF), Terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay, and Safranin O and Fast Green staining (S-O). Human chondrocytes were induced by LPS to construct a cellular inflammatory model, and then transfected with oe-AMPK or oe-HOIL-1-interacting protein (HOIP). Cell viability/apoptotic and intracellular content of AMPK, HOIP, IL-1β, IL-10, IL-13, TNF-α, IL-6, ASC, NLRP3 and Caspase-1 were measured by western blot, ELISA, CCK-8, IF, flow cytometry and TUNEL assays.

Results

After AICAR treatment with OA rats, the expression of p-AMPK, IL-10, IL-13, Ki67 and Bcl-2 increased, the level of NLRP3 inflammasome, TNF-α, IL-6, Bax and Caspase-3 levels were decreased, and tissue damage and apoptosis were significantly alleviated. After transfected with oe-LKB1, chondrocyte activity and LKB1 linear ubiquitination were enhanced, and the level of HOIP, p-AMPK, IL-10 and IL-13 were increased. In contrast, NLRP3 inflammasome (ASC, NLRP3, Caspase-1, IL-1β, and cleaved Caspase-1), TNF-α, and IL-6 levels decreased, apoptosis rate and TUNEL positive rate were attenuated.

Conclusion

LKB1/AMPK pathway significantly ameliorated NLRP3 inflammasome response and chondrocyte injury. Activation of AMPK pathway by linear ubiquitination of LKB1 may be a potential target for OA treatment.

The translational potential of this article

This study highlights the importance of the LKB1/AMPK pathway in NLRP3 inflammatory body response and chondrocyte injury. Activation of LKB1 by modulating linear ubiquitination may be a potential target for OA treatment.

Metformin Prevents or Delays the Development and Progression of Osteoarthritis: New Insight and Mechanism of Action

For over 60 years, metformin has been widely prescribed by physicians to treat type 2 diabetes. Along with more in-depth research on metformin and its molecular mechanism in recent decades, metformin has also been proposed as an effective drug to prevent or delay musculoskeletal disorders, including osteoarthritis (OA). The occurrence and development of OA are deemed to be associated with the impaired mitochondrial functions of articular chondrocytes. Metformin can activate the pathways and expressions of both AMPK and SIRT1 so as to protect the mitochondrial function of chondrocytes, thereby promoting osteoblast production. Moreover, the clinical significance of the metformin combination therapy in preventing OA has also been demonstrated. This review aimed to comprehensively summarize the current research progress on metformin as a proposed drug for OA prevention or treatment.

The effects of metformin in the treatment of osteoarthritis: Current perspectives

Osteoarthritis is a chronic and irreversible disease of the locomotor system which is closely associated with advancing age. Pain and limited mobility frequently affect the quality of life in middle-aged and older adults. With a global population of more than 350 million, osteoarthritis is becoming a health threat alongside cancer and cardiovascular disease. It is challenging to find effective treatments to promote cartilage repair and slow down disease progression. Metformin is the first-line drug for patients with type 2 diabetes, and current perspectives suggest that it cannot only lower glucose but also has anti-inflammatory and anti-aging properties. Experimental studies applying metformin for the treatment of osteoarthritis have received much attention in recent years. In our review, we first presented the history of metformin and the current status of osteoarthritis, followed by a brief review of the mechanism that metformin acts, involving AMPK-dependent and non-dependent pathways. Moreover, we concluded that metformin may be beneficial in the treatment of osteoarthritis by inhibiting inflammation, modulating autophagy, antagonizing oxidative stress, and reducing pain levels. Finally, we analyzed the relevant evidence from animal and human studies. The potential of metformin for the treatment of osteoarthritis deserves to be further explored.

Meta-analysis Integrated With Multi-omics Data Analysis to Elucidate Pathogenic Mechanisms of Age-Related Knee Osteoarthritis in Mice

Increased mechanistic insight into the pathogenesis of knee osteoarthritis (KOA) is needed to develop efficacious disease-modifying treatments. Though age-related pathogenic mechanisms are most relevant to the majority of clinically presenting KOA, the bulk of our mechanistic understanding of KOA has been derived using surgically induced posttraumatic OA (PTOA) models. Here, we took an integrated approach of meta-analysis and multi-omics data analysis to elucidate pathogenic mechanisms of age-related KOA in mice. Protein-level data were integrated with transcriptomic profiling to reveal inflammation, autophagy, and cellular senescence as primary hallmarks of age-related KOA. Importantly, the molecular profiles of cartilage aging were unique from those observed following PTOA, with less than 3% overlap between the 2 models. At the nexus of the 3 aging hallmarks, advanced glycation end product (AGE)/receptor for AGE (RAGE) emerged as the most statistically robust pathway associated with age-related KOA. This pathway was further supported by analysis of mass spectrometry data. Notably, the change in AGE-RAGE signaling over time was exclusively observed in male mice, suggesting sexual dimorphism in the pathogenesis of age-induced KOA in murine models. Collectively, these findings implicate dysregulation of AGE-RAGE signaling as a sex-dependent driver of age-related KOA.

Calcium-binding protein 39 overexpression promotes macrophages from 'M1' into 'M2' phenotype and improves chondrocyte damage in osteoarthritis by activating the AMP-activated protein kinase/sirtuin 1 axis

Osteoarthritis (OA) is a degenerative joint disease that affects cartilage and its peripheral tissues. Up-regulation of Calcium-binding protein 39 (CAB39) has a significant protective effect on osteoblasts, but the role and related molecular mechanisms of CAB39 in OA have not yet been reported. CAB39 overexpression and knockdown models were set up in chondrocytes (ATDC5) and macrophages (RAW264.7). The OA cell model was induced in ATDC5 cells with IL-1β (10 ng/mL). Cell viability was tested by the cell counting kit-8 assay, apoptosis was checked by flow cytometry. Western blot was applied for checking the expression of MMP3, MMP13, Aggrecan, the AMPK/Sirt-1 pathway, apoptosis-related proteins (Bax, Bcl-2, and Caspase-3), and macrophage phenotypic markers (CD86, iNOS, CD206, and Arg1). An OA model was constructed in mice, and CAB39 overexpression plasmids were administered to the knee cavity of the OA model mice. As a result, CAB39 was down-regulated in IL-1β-treated chondrocytes and OA mice. Overexpressing CAB39 enhanced ATDC5 cell viability and choked IL-1β-mediated apoptosis. Overexpression of CAB39 boosted the polarization of macrophages from M1-phenotype into M2 phenotype. In addition, overexpressing CAB39 facilitated the AMPK/Sirt-1 pathway activation, and AMPK inhibitors reversed the protective effect of CAB39 overexpression on chondrocytes. Moreover, CAB39 exhibited anti-inflammatory effects in OA mice by activating the AMPK/Sirt-1 pathway. Collectively, overexpressing CAB39 heightened macrophages’ M2 polarization and declined chondrocyte injury in OA by activating the AMPK/Sirt-1 pathway.Abbreviations AMPK: AMP-activated protein kinaseArg1: arginase 1Bax: Bcl-2-associated X proteinBcl-2: B-cell lymphoma-2CAB39: Calcium-binding protein 39CM: Conditioned mediumDMM: destabilization of the medial meniscusECM: extracellular matrixELISA: enzyme-linked immunosorbent assayFCM: Flow cytometryIL-1β: interleukin-1βIL-4: interleukin-4IL-6: interleukin-6IL-10: interleukin-10IFN – γ: Interferon-gammaIHC: ImmunohistochemistryiNOS: Inducible nitric oxide synthaseLKB1: liver kinase B1MMP3: Matrix metalloproteinase3MMP13:Matrix metalloproteinase13NF-κB: NF-kappaBOA: OsteoarthritisqRT-PCR: Quantitative reverse transcription-polymerase chain reactionRT: room temperatureSirt-1: sirtuin 1STRAD: STE20-related adaptor alphaWB: Western blot

The Function of Metformin in Aging-Related Musculoskeletal Disorders

Metformin is a widely accepted first-line hypoglycemic agent in current clinical practice, and it has been applied to the clinic for more than 60 years. Recently, researchers have identified that metformin not only has an efficient capacity to lower glucose but also exerts anti-aging effects by regulating intracellular signaling molecules. With the accelerating aging process and mankind’s desire for a long and healthy life, studies on aging have witnessed an unprecedented boom. Osteoporosis, sarcopenia, degenerative osteoarthropathy, and frailty are age-related diseases of the musculoskeletal system. The decline in motor function is a problem that many elderly people have to face, and in serious cases, they may even fail to self-care, and their quality of life will be seriously reduced. Therefore, exploring potential treatments to effectively prevent or delay the progression of aging-related diseases is essential to promote healthy aging. In this review, we first briefly describe the origin of metformin and the aging of the movement system, and next review the evidence associated with its ability to extend lifespan. Furthermore, we discuss the mechanisms related to the modulation of aging in the musculoskeletal system by metformin, mainly its contribution to bone homeostasis, muscle aging, and joint degeneration. Finally, we analyze the protective benefits of metformin in aging-related diseases of the musculoskeletal system.

Advanced Glycation End Products Induced Mitochondrial Dysfunction of Chondrocytes through Repression of AMPKα-SIRT1-PGC-1α Pathway

INTRODUCTION

Our previous studies have demonstrated advanced glycation end products (AGEs) was an important mediator in osteoarthritis (OA) which may induce mitochondrial dysfunction. AMP-activated protein kinase (AMPK), sirtuin 1 (SIRT1), and its downstream target peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) are the critical sensors that regulate mitochondrial biogenesis and have been recognized as therapeutic targets in OA. This study was designed to test whether AGEs caused mitochondrial dysfunction through modulation of AMPKα/SIRT1/PGC-1α.

METHODS

We knocked down or overexpressed AMPKα, SIRT1, and PGC-1α by small interfering RNA or plasmid DNA transfection, respectively. Mitochondrial membrane potential (△Ψ) was detected by tetraethylbenzimidazolyl carbocyanine iodide (JC-1) fluorescence probe.

RESULTS

The results showed that AGEs impaired △Ψ, intracellular ATP level, and mitochondrial DNA content, linked to decreased AMPKα, SIRT1, and PGC-1α expression in chondrocyte. AMPKα pharmacologic activation or overexpression of AMPKα, SIRT1, and PGC-1α reversed impairments of mitochondrial biogenesis, oxidative stress, and inflammation in AGEs-induced chondrocytes. However, AMPKα activation using AICAR had decreased capacity to increase each of those same effect readouts in AGEs-treated SIRT1-siRNA or PGC-1α-siRNA chondrocyte.

CONCLUSION

Taken together, AGEs reduced the AMPKα/SIRT1/PGC-1α signaling in chondrocytes, leading to mitochondrial dysfunction as a result of increased oxidative stress, inflammation, and apoptosis. These results indicated that target AMPK may be as a novel therapeutic strategy for AGEs-related OA prevention.

Mitochondrial quality control in cartilage damage and osteoarthritis: new insights and potential therapeutic targets

Osteoarthritis (OA) is a multifactorial arthritic disease of weight-bearing joints concomitant with chronic and intolerable pain, loss of locomotion and impaired quality of life in the elderly population. Although the prevalence of OA increases with age, its specific mechanisms have not been elucidated and effective therapeutic disease-modifying drugs have not been developed. As essential organelles in chondrocytes, mitochondria supply energy and play vital roles in cellular metabolism, proliferation and apoptosis. Mitochondrial quality control (MQC) is the key mechanism to coordinate various mitochondrial biofunctions, primarily through mitochondrial biogenesis, dynamics, autophagy and the newly discovered mitocytosis. An increasing number of studies have revealed that a loss of MQC homeostasis contributes to the cartilage damage during the occurrence and development of OA. Several master MQC-associated signaling pathways and regulators exert chondroprotective roles in OA, while cartilage damage-related molecular mechanisms have been partially identified. In this review, we summarized known mechanisms mediated by dysregulated MQC in the pathogenesis of OA and latent bioactive ingredients and drugs for the prevention and treatment of OA through the maintenance of MQC.

Systemic administration of a pharmacologic iron chelator reduces cartilage lesion development in the Dunkin-Hartley model of primary osteoarthritis

Iron has been emerging as a key contributor to aging-associated, chronic disorders due to the propensity for generating reactive oxygen species. To date, there are a limited number of publications exploring the role of iron in the pathogenesis of primary/age-related osteoarthritis (OA). The objective of this study was to determine whether reduced iron via pharmacologic iron chelation with deferoxamine (DFO) affected the development and/or severity of cartilage lesions in a primary OA model. At 12-weeks-of-age, 15 male Dunkin-Hartley guinea pigs received either 46 mg/kg DFO (n = 8) or vehicle control (n = 7) injected subcutaneously twice daily for five days each week. Movement changes, captured via overhead enclosure monitoring, were also determined. Termination occurred at 30-weeks-of-age. Iron was quantified in serum, urine, liver, and femoral head articular cartilage. Left knees were evaluated for: structural changes using histopathology guidelines; and immunohistochemistry. Gene expression analysis was conducted on right knee articular cartilage. DFO reduced iron levels in femoral head articular cartilage (p = 0.0006) and liver (p = 0.02), and increased iron within urine (p = 0.04) and serum (p = 0.0009). Mobility of control animals declined, while the DFO group maintained activity levels similar to the first month of treatment (p = 0.05). OA-associated cartilage lesions were reduced in knees of DFO animals (p = 0.0001), with chondrocyte hypocellularity a key histologic difference between groups (p < 0.0001). DFO-receiving animals had increased immunostaining for phosphorylated adenosine monophosphate activated protein kinase alpha within knee articular cartilage; lower transcript counts of several proapoptotic genes (p = 0.04-0.0004) and matrix-degrading enzymes (p = 0.02-<0.0001), and increased expression of the anti-apoptotic gene Bcl-2 (p < 0.0001) and a tissue inhibitor of matrix-metalloproteinases (p = 0.03) were also observed. These results suggest that iron chelation delayed the progression of primary OA in an animal model and could hold potential as a translational intervention. These findings provide expanded insight into factors that may contribute to the pathogenesis of primary OA.

Oral administration of berberine limits post-traumatic osteoarthritis development and associated pain via AMP-activated protein kinase (AMPK) in mice

OBJECTIVE

We investigated the effect of berberine, a natural plant product that can activate AMP-activated protein kinase (AMPK), on Osteoarthritis (OA) development and associated pain in mice.

DESIGN

Human primary knee chondrocytes were utilized to investigate how AMPK is activated by berberine. Both global knockout (KO) of AMPKα1 and congenic wild type (WT) mice were subjected to the post-traumatic OA through destabilization of medial meniscus (DMM) surgery. Two weeks after surgery, the mice were randomly divided into two groups with one group receiving berberine chloride daily via drinking water and were sacrificed at 6 and 12 weeks after surgery. OA severity was assessed by histological and histomorphometric analyses of cartilage degradation, synovitis, and osteophyte formation. OA-associated pain behavior was also determined. Immunohistochemistry (IHC) analyses were carried out to examine changes in AMPK signaling.

RESULTS

Berberine induced phosphorylation of AMPKα (Thr172) via liver kinase B1 (LKB1), the major upstream kinase of AMPK, in chondrocytes in vitro. Both WT and AMPKα1KO developed OA and associated pain post DMM surgery. However, treatment with berberine significantly reduced severity of OA and associated pain in WT but not AMPKα1KO mice. IHC analysis of WT DMM knee cartilage further revealed that berberine inhibited concomitant loss of expression and phosphorylation of AMPKα and expression of SIRT1 and SIRT3, suggesting an important role of activation of AMPK signaling in mediating beneficial effect of berberine.

CONCLUSIONS

Berberine acts through AMPK to reduce joint structural damage and pain associated with post-traumatic OA in mice in vivo.

Sanse Powder Essential Oil Nanoemulsion Negatively Regulates TRPA1 by AMPK/mTOR Signaling in Synovitis: Knee Osteoarthritis Rat Model and Fibroblast-Like Synoviocyte Isolates

Synovitis is the primary driving factor for the occurrence and development of knee osteoarthritis (KOA) and fibroblast-like synoviocytes (FLSs) and plays a crucial role during this process. Our previous works revealed that transient receptor potential ankyrin 1 (TRPA1) ion channels mediate the amplification of KOA synovitis. In recent years, essential oils have been proved to have blocking effect on transient receptor potential channels. Meanwhile, the therapeutic effect of Sanse Powder on KOA synovitis has been confirmed in clinical trials and basic studies; although, the mechanism remains unclear. In the present study, Sanse Powder essential oil nanoemulsion (SP-NEs) was prepared, and then chemical composition, physicochemical properties, and stability were investigated. Besides, both in MIA-induced KOA rats and in LPS-stimulated FLSs, we investigated whether SP-NES could alleviate KOA synovitis by interfering with AMP-activated protein kinase- (AMPK-) mammalian target of rapamycin (mTOR), an energy sensing pathway proved to negatively regulate the TRPA1. Our research shows that the top three substances in SP-NEs were tumerone, delta-cadinene, and Ar-tumerone, which accounted for 51.62% of the total, and should be considered as the main pharmacodynamic ingredient. Less inflammatory cell infiltration and type I collagen deposition were found in the synovial tissue of KOA rats treated with SP-NEs, as well as the downregulated expressions of interleukin (IL)-1β, IL-18, and TRPA1. Besides, SP-NEs increased the phosphorylation level of AMPK and decreased the phosphorylation level of mTOR in the KOA model, and SP-NEs also upregulated expressions of peroxisome proliferator-activated receptor-gamma (PPARγ) and PPARγ coactivator-1α and downstream signaling molecules of AMPK-mTOR in vivo and in vitro. To conclude, a kind of Chinese herbal medicine for external use which is effective in treating synovitis of KOA was extracted and prepared into essential oil nanoemulsion with stable properties in the present study. It may alleviate synovitis in experimental KOA through the negative regulation of TRPA1 by AMPK-mTOR signaling.

Rapamycin-induced hyperglycemia is associated with exacerbated age-related osteoarthritis

BACKGROUND

The objective of this study was to determine if mechanistic target of rapamycin (mTOR) inhibition with or without AMP-activated protein kinase (AMPK) activation can protect against primary, age-related OA.

DESIGN

Dunkin-Hartley guinea pigs develop mild primary OA pathology by 5 months of age that progresses to moderate OA by 8 months of age. At 5 months, guinea pigs served as young control (n = 3) or were fed either a control diet (n = 8), a diet enriched with the mTOR-inhibitor rapamycin (Rap, 14 ppm, n = 8), or Rap with the AMPK-activator metformin (Rap+Met, 1000 ppm, n = 8) for 12 weeks. Knee joints were evaluated by OARSI scoring, micro-computed tomography, and immunohistochemistry. Glenohumeral articular cartilage was collected for western blotting.

RESULTS

Rap- and Rap+Met-treated guinea pigs displayed lower body weight than control. Rap and Rap+Met inhibited articular cartilage mTORC1 but not mTORC2 signaling. Rap+Met, but not Rap alone, stimulated AMPK. Despite lower body weight and articular cartilage mTORC1 inhibition, Rap- and Rap+Met-treated guinea pigs had greater OA severity in the medial tibial plateau due to articular cartilage structural damage and/or proteoglycan loss. Rap and Rap+Met increased plasma glucose compared to control. Plasma glucose concentration was positively correlated with proteoglycan loss, suggesting hyperglycemic stress after Rap treatment was related to worsened OA.

CONCLUSIONS

This is the first study to show that Rap induced increase in plasma glucose was associated with greater OA severity. Further, articular cartilage mTORC1 inhibition and bodyweight reduction by dietary Rap and Rap+Met did not appear to protect against primary OA during the prevailing hyperglycemia.

AMPK Signaling in Energy Control, Cartilage Biology, and Osteoarthritis

The adenosine monophosphate (AMP)-activated protein kinase (AMPK) was initially identified as an enzyme acting as an "energy sensor" in maintaining energy homeostasis via serine/threonine phosphorylation when low cellular adenosine triphosphate (ATP) level was sensed. AMPK participates in catabolic and anabolic processes at the molecular and cellular levels and is involved in appetite-regulating circuit in the hypothalamus. AMPK signaling also modulates energy metabolism in organs such as adipose tissue, brain, muscle, and heart, which are highly dependent on energy consumption via adjusting the AMP/ADP:ATP ratio. In clinics, biguanides and thiazolidinediones are prescribed to patients with metabolic disorders through activating AMPK signaling and inhibiting complex I in the mitochondria, leading to a reduction in mitochondrial respiration and elevated ATP production. The role of AMPK in mediating skeletal development and related diseases remains obscure. In this review, in addition to discuss the emerging advances of AMPK studies in energy control, we will also illustrate current discoveries of AMPK in chondrocyte homeostasis, osteoarthritis (OA) development, and the signaling interaction of AMPK with other pathways, such as mTOR (mechanistic target of rapamycin), Wnt, and NF-κB (nuclear factor κB) under OA condition.

Mechanisms linking mitochondrial mechanotransduction and chondrocyte biology in the pathogenesis of osteoarthritis

Mechanical loading is essential for chondrocyte health. Chondrocytes can sense and respond to various extracellular mechanical signals through an integrated set of mechanisms. Recently, it has been found that mitochondria, acting as critical mechanotransducers, are at the intersection between extracellular mechanical signals and chondrocyte biology. Much attention has been focused on identifying how mechanical loading-induced mitochondrial dysfunction contributes to the pathogenesis of osteoarthritis. In contrast, little is known regarding the mechanisms underlying functional alterations in mitochondria induced by mechanical stimulation. In this review, we describe how chondrocytes perceive environmental mechanical signals. We discuss how mechanical load induces mitochondrial functional alterations and highlight the major unanswered questions in this field. We speculate that AMP-activated protein kinase (AMPK), a master regulator of energy homeostasis, may play an important role in coupling force transmission to mitochondrial health and intracellular biological responses.

The role of metabolism in chondrocyte dysfunction and the progression of osteoarthritis

Osteoarthritis (OA) is a degenerative joint disease characterized by low-grade inflammation and high levels of clinical heterogeneity. Aberrant chondrocyte metabolism is a response to changes in the inflammatory microenvironment and may play a key role in cartilage degeneration and OA progression. Under conditions of environmental stress, chondrocytes tend to adapt their metabolism to microenvironmental changes by shifting from one metabolic pathway to another, for example from oxidative phosphorylation to glycolysis. Similar changes occur in other joint cells, including synoviocytes. Switching between these pathways is implicated in metabolic alterations that involve mitochondrial dysfunction, enhanced anaerobic glycolysis, and altered lipid and amino acid metabolism. The shift between oxidative phosphorylation and glycolysis is mainly regulated by the AMP-activated protein kinase (AMPK) and mechanistic target of rapamycin (mTOR) pathways. Chondrocyte metabolic changes are likely to be a feature of different OA phenotypes. Determining the role of chondrocyte metabolism in OA has revealed key features of disease pathogenesis. Future research should place greater emphasis on immunometabolism and altered metabolic pathways as a means to understand the pathophysiology of age-related OA. This knowledge will advance the development of new drugs against therapeutic targets of metabolic significance.

Mechanistic insights into AMPK-SIRT3 positive feedback loop-mediated chondrocyte mitochondrial quality control in osteoarthritis pathogenesis

Osteoarthritis (OA) is a major cause of disability in the elderly population and represents a significant public health problem and socioeconomic burden worldwide. However, no disease-modifying therapeutics are currently available for OA due to an insufficient understanding of the pathogenesis of this disability. As a unique cell type in cartilage, chondrocytes are essential for cartilage homeostasis and play a critical role in OA pathogenesis. Mitochondria are important metabolic centers in chondrocytes and contribute to cell survival, and mitochondrial quality control (MQC) is an emerging mechanism for maintaining cell homeostasis. An increasing number of recent studies have demonstrated that dysregulation of the key processes of chondrocyte MQC, which involve mitochondrial redox, biogenesis, dynamics, and mitophagy, is associated with OA pathogenesis and can be regulated by the chondroprotective molecules 5' adenosine monophosphate-activated protein kinase (AMPK) and sirtuin 3 (SIRT3). Moreover, AMPK and SIRT3 regulate each other, and their expression and activity are always consistent in chondrocytes, which suggests the existence of an AMPK-SIRT3 positive feedback loop (PFL). Although the precise mechanisms are not fully elucidated and need further validation, the current literature indicates that this AMPK-SIRT3 PFL regulates OA development and progression, at least partially by mediating chondrocyte MQC. Therefore, understanding the mechanisms of AMPK-SIRT3 PFL-mediated chondrocyte MQC in OA pathogenesis might yield new ideas and potential targets for subsequent research on the OA pathomechanism and therapeutics.

Osteoarthritis year in review 2020: biology

This year in review about osteoarthritis biology highlights a selection of articles published between the 2019 and 2020 Osteoarthritis Research Society International (OARSI) World Congress meetings, within the field of osteoarthritis biology. Highlights were selected from PubMed searches covering osteoarthritis (OA) cartilage, subchondral bone, synovium and aging. Subsequently, a personal selection was based on new and emerging themes together with common research topics that were studied by multiple groups. Themes discussed include novel insights into the inflammatory changes during OA, with a number of noteworthy publications concerning the role of macrophages in healthy and osteoarthritic joints. Next, the application of mesenchymal stem cells as OA-dampening therapy is discussed, including possible ways to improve their efficacy by pre-treatment. Other significant themes including treatment of OA with metformin, enhancing autophagy to alleviate OA and the involvement of the gastro-intestinal microbiome in development of OA symptoms and structural damage are discussed. An effort was made to connect the seemingly distant topics from which the overarching conclusion can be drawn that over the last year promising breakthroughs have been achieved in further understanding the biology of OA development and that new therapeutic possibilities have been explored.

Metformin Hydrochloride Encapsulation by Alginate Strontium Hydrogel for Cartilage Regeneration by Reliving Cellular Senescence

Cartilage lesion is a common tissue defect and is challenging in clinical practice. Trauma-induced cellular senescence could decrease the chondrocyte capability of maintaining cartilage tissue regeneration. A previous investigation showed that, by controlling the cellular senescence, the cartilage regeneration can be significantly accelerated. Based on this finding, we design a novel hydrogel, Alg/MH-Sr, that combines metformin, an established drug for inhibiting senescence, and strontium, an effective anti-inflammatory material for cartilage tissue engineering. A RT-PCR test suggests the significant inhibitory effect of the hydrogel on senescent, apoptotic, oxidative, and inflammatory genes' expression. Histological examinations demonstrate that the Alg/MH-Sr hydrogel accelerated cartilage repairment, and chondrocyte senescence was significantly inhibited. Our study demonstrates that the Alg/MH-Sr hydrogel is effective for cartilage defect treatment and provides a new clue in accelerating tissue repairment by inhibiting the senescence of cells and tissues.

Adenosine A2A receptor signaling promotes FoxO associated autophagy in chondrocytes

Autophagy, a homeostatic pathway upregulated during cellular stress, is decreased in osteoarthritic chondrocytes and this reduction in autophagy is thought to contribute to the development and progression of osteoarthritis (OA). The adenosine A2A receptor (A2AR) is a potent anti-inflammatory receptor and deficiency of this receptor leads to the development of OA in mice. Moreover, treatment using liposomally conjugated adenosine or a specific A2AR agonist improved joint scores significantly in both rats with post-traumatic OA (PTOA) and mice subjected to a high fat diet obesity induced OA. Importantly, A2AR ligation is beneficial for mitochondrial health and metabolism in vitro in primary and the TC28a2 human cell line. An additional set of metabolic, stress-responsive, and homeostatic mediators include the Forkhead box O transcription factors (FoxOs). Data has shown that mouse FoxO knockouts develop early OA with reduced cartilage autophagy, indicating that FoxO-induced homeostasis is important for articular cartilage. Given the apparent similarities between A2AR and FoxO signaling, we tested the hypothesis that A2AR stimulation improves cartilage function through activation of the FoxO proteins leading to increased autophagy in chondrocytes. We analyzed the signaling pathway in the human TC28a2 cell line and corroborated these findings in vivo in a metabolically relevant obesity-induced OA mouse model. We found that A2AR stimulation increases activation and nuclear localization of FoxO1 and FoxO3, promotes an increase in autophagic flux, improves metabolic function in chondrocytes, and reduces markers of apoptosis in vitro and reduced apoptosis by TUNEL assay in vivo. A2AR ligation additionally enhances in vivo activation of FoxO1 and FoxO3 with evidence of enhanced autophagic flux upon injection of the liposome-associated A2AR agonist in a mouse obesity-induced OA model. These findings offer further evidence that A2AR may be an excellent target for promoting chondrocyte and cartilage homeostasis.

AMPK: implications in osteoarthritis and therapeutic targets

Osteoarthritis (OA) is the most common skeletal disease and the leading cause of pain and disability in the aged population (>65 years). However, the underlying factors involved in OA pathogenesis remain elusive which has resulted in failure to identify disease-modifying OA drugs. Altered metabolism has been shown to be a prominent pathological change in OA. As a critical bioenergy sensor, AMP-activated protein kinase (AMPK) mediates not only energy homeostasis but also redox balance in chondrocytes to counter various cell stress. Dysfunction of AMPK activity has been associated with reduced autophagy, impaired mitochondrial function, excessive reactive oxygen species generation, and inflammation in joint tissue. These abnormalities ultimately trigger articular cartilage degeneration, synovial inflammation, and abnormal subchondral bone remodeling. This review focuses on recent findings describing the central role of AMPK in joint homeostasis and OA development. We also highlight current advances that target AMPK as a novel therapeutic strategy for OA prevention.

Role of mitochondria in mediating chondrocyte response to mechanical stimuli

As the most common form of arthritis, osteoarthritis (OA) has become a major cause of severe joint pain, physical disability, and quality of life impairment in the affected population. To date, precise pathogenesis of OA has not been fully clarified, which leads to significant obstacles in developing efficacious treatments such as failures in finding disease-modifying OA drugs (DMOADs) in the last decades. Given that diarthrodial joints primarily display the weight-bearing and movement-supporting function, it is not surprising that mechanical stress represents one of the major risk factors for OA. However, the inner connection between mechanical stress and OA onset/progression has yet to be explored. Mitochondrion, a widespread organelle involved in complex biological regulation processes such as adenosine triphosphate (ATP) synthesis and cellular metabolism, is believed to have a controlling role in the survival and function implement of chondrocytes, the singular cell type within cartilage. Mitochondrial dysfunction has also been observed in osteoarthritic chondrocytes. In this review, we systemically summarize mitochondrial alterations in chondrocytes during OA progression and discuss our recent progress in understanding the potential role of mitochondria in mediating mechanical stress-associated osteoarthritic alterations of chondrocytes. In particular, we propose the potential signaling pathways that may regulate this process, which provide new views and therapeutic targets for the prevention and treatment of mechanical stress-associated OA.

Metformin inhibits inflammation and bone destruction in collagen-induced arthritis in rats

Background

Metformin (MF) is a widely used biguanide oral hypoglycemic agent, which has obvious anti-inflammatory and immunomodulatory effects. However, the mechanism of MF on rheumatoid arthritis (RA) remains uncertain. In this study, we investigated the therapeutic effects of MF on collagen-induced arthritis (CIA).

Methods

CIA was induced in rats by intradermal injection of a mixture of bovine type II collagen and incomplete Freund's adjuvant (IFA) on day 0 and day 7 through the base of the tail. Intraperitoneal injection of MF (100 mg/kg) was given every 3 days, from day 14 for 3 weeks. The effects of MF on arthritis-induced systemic inflammation and synovitis were studied by pathological analysis of the knee joint and serological examination of peripheral blood in CIA rats. The bone protection effect of MF was studied by microscopic computed tomography (micro-CT) and histological analysis of the knee joint. The effects of MF on chondrocytes in CIA rats were studied by detecting the relevant pro-apoptotic mediators in the chondrocytes.

Results

After administration of MF in CIA rats, systemic inflammation and synovitis caused by arthritis were significantly suppressed. Histomorphometry and micro-CT analysis of the knee joint revealed that MF can protect bone by inhibiting the changes of trabecular bone in CIA rats. Histological analysis of the knee joint found that MF can inhibit osteoclast formation and degradation of the cartilage layer matrix. Detection of the relevant pro-apoptotic mediators in chondrocytes revealed that MF can significantly inhibit the apoptosis of chondrocytes in CIA rats.

Conclusions

Our study showed that MF can inhibit systemic inflammation and synovitis and plays a role in bone protection by inhibiting cartilage layer matrix degradation, osteoclast formation, and chondrocyte apoptosis.

Mitochondria: Potential Targets for Osteoarthritis

Osteoarthritis (OA) is a common and disabling joint disorder that is mainly characterized by cartilage degeneration and narrow joint spaces. The role of mitochondrial dysfunction in promoting the development of OA has gained much attention. Targeting endogenous molecules to improve mitochondrial function is a potential treatment for OA. Moreover, research on exogenous drugs to improve mitochondrial function in OA based on endogenous molecular targets has been accomplished. In addition, stem cells and exosomes have been deeply researched in the context of cartilage regeneration, and these factors both reverse mitochondrial dysfunctions. Thus, we hypothesize that biomedical approaches will be applied to the treatment of OA. Furthermore, we have summarized the global status of mitochondria and osteoarthritis research in the past two decades, which will contribute to the research field and the development of novel treatment strategies for OA.

Serum fatty acid chain length associates with prevalent symptomatic end-stage osteoarthritis, independent of BMI

Higher body mass index (BMI) is associated with osteoarthritis (OA) in both weight-bearing and non-weight-bearing joints, suggesting a link between OA and poor metabolic health beyond mechanical loading. This risk may be influenced by systemic factors accompanying BMI. Fluctuations in concentrations of metabolites may mark or even contribute to development of OA. This study explores the association of metabolites with radiographic knee/hip OA prevalence and progression. A ¹H-NMR-metabolomics assay was performed on plasma samples of 1564 cases for prevalent OA and 2,125 controls collected from the Rotterdam Study, CHECK, GARP/NORREF and LUMC-arthroplasty cohorts. OA prevalence and 5 to 10 year progression was assessed by means of Kellgren-Lawrence (KL) score and the OARSI-atlas. End-stage knee/hip OA (TJA) was defined as indication for arthroplasty surgery. Controls did not have OA at baseline or follow-up. Principal component analysis of 227 metabolites demonstrated 23 factors, of which 19 remained interpretable after quality-control. Associations of factor scores with OA definitions were investigated with logistic regression. Fatty acids chain length (FALen), which was included in two factors which associated with TJA, was individually associated with both overall OA as well as TJA. Increased Fatty Acid chain Length is associated with OA.

Metformin: A Potential Therapeutic Tool for Rheumatologists

Metformin is an oral antihyperglycemic drug widely used to treat type 2 diabetes, acting via indirect activation of 5′ Adenosine Monophosphate-activated Protein Kinase (AMPK). Actually, evidence has accumulated of an intriguing anti-inflammatory activity, mainly mediated by AMPK through a variety of mechanisms such as the inhibition of cytokine-stimulated Nuclear Factor-κB (NF-κB) and the downregulation of the Janus Kinase/Signal Transducer and Activator of Transcription (JAK/STAT) signaling pathways. Moreover, AMPK plays an important role in the modulation of T lymphocytes and other pivotal cells of the innate immune system. The current understanding of these AMPK effects provides a strong rationale for metformin repurposing in the management of autoimmune and inflammatory conditions. Several studies demonstrated metformin’s beneficial effects on both animal and human rheumatologic diseases, especially on rheumatoid arthritis. Unfortunately, even though data are large and remarkable, they almost exclusively come from experimental investigations with only a few from clinical trials. The lack of support from prospective placebo-controlled trials does not allow metformin to enter the therapeutic repertoire of rheumatologists. However, a large proportion of rheumatologic patients can currently benefit from metformin, such as those with concomitant obesity and type 2 diabetes, two conditions strongly associated with rheumatoid arthritis, osteoarthritis, and gout, as well as those with diabetes secondary to steroid therapy.

Berberine and musculoskeletal disorders: The therapeutic potential and underlying molecular mechanisms

BACKGROUND

Musculoskeletal disorders are a group of disorders that affect the joints, bones, and muscles, causing long-term disability. Berberine, an isoquinoline alkaloid, has been previously established to exhibit beneficial properties in preventing various diseases, including musculoskeletal disorders.

PURPOSE

This review article aims to recapitulate the therapeutic potential of berberine and its mechanism of action in treating musculoskeletal disorders.

METHODS

A wide range of literature illustrating the effects of berberine in ameliorating musculoskeletal disorders was retrieved from online electronic databases (PubMed and Medline) and reviewed.

RESULTS

Berberine may potentially retard the progression of osteoporosis, osteoarthritis and rheumatoid arthritis. Limited studies reported the effects of berberine in suppressing the proliferation of osteosarcoma cells. These beneficial properties of berberine are mediated in part through its ability to target multiple signaling pathways, including PKA, p38 MAPK, Wnt/β-catenin, AMPK, RANK/RANKL/OPG, PI3K/Akt, NFAT, NF-κB, Hedgehog, and oxidative stress signaling. In addition, berberine exhibited anti-apoptotic, anti-inflammatory, and immunosuppressive properties.

CONCLUSION

The current evidence indicates that berberine may be effective in preventing musculoskeletal disorders. However, findings from in vitro and in vivo investigations await further validation from human clinical trial.

Metformin limits osteoarthritis development and progression through activation of AMPK signalling

OBJECTIVES

In this study, we aim to determine the effect of metformin on osteoarthritis (OA) development and progression.

METHODS

Destabilisation of the medial meniscus (DMM) surgery was performed in 10-week-old wild type and AMP-activated protein kinase (AMPK)α1 knockout (KO) mice. Metformin (4 mg/day in drinking water) was given, commencing either 2 weeks before or 2 weeks after DMM surgery. Mice were sacrificed 6 and 12 weeks after DMM surgery. OA phenotype was analysed by micro-computerised tomography (μCT), histology and pain-related behaviour tests. AMPKα1 (catalytic alpha subunit of AMPK) expression was examined by immunohistochemistry and immunofluorescence analyses. The OA phenotype was also determined by μCT and MRI in non-human primates.

RESULTS

Metformin upregulated phosphorylated and total AMPK expression in articular cartilage tissue. Mild and more severe cartilage degeneration was observed at 6 and 12 weeks after DMM surgery, evidenced by markedly increased Osteoarthritis Research Society International scores, as well as reduced cartilage areas. The administration of metformin, commencing either before or after DMM surgery, caused significant reduction in cartilage degradation. Prominent synovial hyperplasia and osteophyte formation were observed at both 6 and 12 weeks after DMM surgery; these were significantly inhibited by treatment with metformin either before or after DMM surgery. The protective effects of metformin on OA development were not observed in AMPKα1 KO mice, suggesting that the chondroprotective effect of metformin is mediated by AMPK signalling. In addition, we demonstrated that treatment with metformin could also protect from OA progression in a partial medial meniscectomy animal model in non-human primates.

CONCLUSIONS

The present study suggests that metformin, administered shortly after joint injury, can limit OA development and progression in injury-induced OA animal models.

Exploration of metformin as novel therapy for osteoarthritis: preventing cartilage degeneration and reducing pain behavior

BACKGROUND

Metformin could activate adenosine monophosphate-activated protein kinase (AMPK) which was postulated as a potential therapeutic target for osteoarthritis. This study aimed to examine the effects of metformin on cartilage and pain in osteoarthritis mouse model.

METHODS

Eighty 10-week-old male C57BL/6 mice were randomized to 6 groups: non-operation, sham-operation, destabilization of the medial meniscus (DMM)-operation with intragastric saline/metformin, and DMM-operation with intraarticular saline/metformin. Articular cartilage degeneration was examined by scanning electron microscopy (SEM) and graded using the scoring system recommended by Osteoarthritis Research Society International (OARSI). Mechanical withdrawal threshold and hind paw weight distribution were measured to assess the pain-related behavior. Cell Counting Kit-8 assay, quantificational real-time polymerase chain reaction, and western blot analysis were conducted to examine the anabolic and anti-catabolic effect of metformin and the role of AMPK in mediating its effects on interleukin-1β stimulated primary mice chondrocytes.

RESULTS

Compared with mice receiving intragastric and intraarticular saline, mice in both intragastric and intraarticular metformin displayed attenuated articular cartilage degeneration, indicated by less cartilage damage under SEM and significantly lower OARSI scores. A higher paw withdrawal threshold and a decreased weight-bearing asymmetry were observed in the intragastric and intraarticular metformin mice compared with their corresponding saline groups in DMM model of osteoarthritis. In vitro experiments showed that metformin not only decreased the level of matrix metalloproteinase 13, but also elevated type II collagen production through activating AMPK pathway.

CONCLUSIONS

Metformin attenuates osteoarthritis structural worsening and modulates pain, suggesting its potential for osteoarthritis prevention or treatment.

Metformin attenuates cartilage degeneration in an experimental osteoarthritis model by regulating AMPK/mTOR

Background: It is generally thought that the occurrence and progression of osteoarthritis (OA) results from multiple causes, including degradation and destruction of the cartilage matrix and aging of chondrocytes. Metformin is a first-line drug for the treatment of diabetes, and has great potential for the treatment of other disorders. However, the role of metformin in OA is unknown.

Results: Metformin displayed a protective effect against OA. There were lower OARSI scores and fewer MMP-13-positive cells in DMM mice and cartilage explants after treatment with metformin. In addition, metformin treatment decreased p16^INK4a levels in OA chondrocytes, and enhanced polarization of AMPK and inhibition of mTORC1 in OA mice and chondrocytes in a dose-dependent manner.

Conclusions: Metformin effectively alleviated cartilage degradation and aging through regulation of the AMPK/mTOR signaling pathways, suggesting that it could be an effective treatment for OA.

Methods: The effects of metformin on cartilage degradation and chondrocyte aging was determined in a destabilization of the medial meniscus (DMM)-induced OA mouse model and in IL-1β-treated mouse chondrocytes and cartilage explants. Articular cartilage degeneration was graded using the Osteoarthritis Research Society International (OARSI) criteria. Immunostaining, RT-PCR, and western blot analyses were conducted to detect the relative expressions of protein and RNA.

Estrogen prevents articular cartilage destruction in a mouse model of AMPK deficiency via ERK-mTOR pathway

Background

To investigate the mechanism underlying the chondroprotective effect of estrogen in AMP-activated protein kinase (AMPK) deficiency mice.

Methods

Female cartilage-specific AMPKα double knockout (AMPKα cDKO) mice were generated and subjected to ovariectomy (OVX). The model of osteoarthritis (OA) was induced by destabilization of medial meniscus (DMM). Histopathological changes were evaluated by using OARSI scoring systems. Autophagy changes were analyzed by immunofluorescence staining. Human chondrocytes were subjected to mechanical stress to mimic OA development. and incubated in presence of or absence of 17β-estradiol or/and compound C (AMPK inhibitor) or/and U0126 (ERK inhibitor). The expression levels of ERK1/2 phosphorylation, p70S6K phosphorylation and light chain 3 (LC3) were detected by Western blot.

Results

Compared with in OVX-sham AMPKα cDKO and OVX-sham WT mice, DMM-induced OA is more severe, and significantly low level of LC3 was observed in articular cartilage in OVX AMPK cDKO mice. Both mechanical stress and compound C were shown to induce an increase in phosphorylation of p70S6K, respectively. 17β-estradiol stimulation led to a reduction in the basal level of p70S6K phosphorylation as well as in the compound C or mechanical stress-induced level of p70S6K phosphorylation. 17β-estradiol stimulation not only led to an increase in LC3 conversion but also overrode the inhibitory effect of compound C on LC3 conversion. The effects of 17β-estradiol were abrogated by blocking ERK signaling pathway.

Conclusions

Our findings suggest that estrogen can protect articular cartilage from damage during OA development by promoting chondrocyte autophagy via ERK-mammalian target of rapamycin (mTOR) signaling, and give new insight into the mechanism of the chondroprotective effect of estrogen.

Loganin Attenuates Osteoarthritis in Rats by Inhibiting IL-1β-Induced Catabolism and Apoptosis in Chondrocytes Via Regulation of Phosphatidylinositol 3-Kinases (PI3K)/Akt

Background

Chondrocyte apoptosis and catabolism are 2 major factors that contribute to the progression of osteoarthritis (OA). Loganin, an iridoid glycoside present in several herbs, including Flos lonicerae, Cornus mas L, and Strychnos nux vomica, is a valuable medication with anti-inflammatory and anti-apoptotic effects. Our study examines these effects and explores the potential benefits of loganin in the OA treatment.

Material/Methods

To clarify the roles of loganin in OA and its specific signaling pathway, chondrocytes were administrated with IL-1β and supplemented with or without LY294002 (a classic PI3K/Akt inhibitor). The apoptotic level, catabolic factors (MMP-3 and MMP-13 and ADAMTS-4 and ADAMTS-5), extracellular matrix (ECM) degradation, and activation of the PI3K/Akt pathway were evaluated using western blotting, PCR, and an immunofluorescent assay. The degenerative condition of the cartilage was evaluated using the Safranin O assay in vivo. The expression of cleaved-caspase-3 (C-caspase-3) was measured using immunochemistry.

Results

The data suggested that loganin suppressed the apoptotic level, reduced the release of catabolic enzymes, and decreased the ECM degradation of IL-1β-induced chondrocytes. However, suppressing PI3K/Akt signaling using LY294002 alleviated the therapeutic effects of loganin in chondrocytes. Our in vivo experiment showed that loganin partially attenuated cartilage degradation while inhibiting the apoptotic level.

Conclusions

This work revealed that loganin treatment attenuated IL-1β-treated apoptosis and ECM catabolism in rat chondrocytes via regulation of the PI3K/Akt signaling, revealing that loganin is a potentially useful treatment for OA.

Ageing and Osteoarthritis

The increase in global lifespan has in turn increased the prevalence of osteoarthritis which is now the most common type of arthritis. Cartilage tissue located on articular joints erodes during osteoarthritis which causes pain and may lead to a crippling loss of function in patients. The pathophysiology of osteoarthritis has been understudied and currently no disease modifying treatments exist. The only current end-point treatment remains joint replacement surgery. The primary risk factor for osteoarthritis is age. Clinical and basic research is now focused on understanding the ageing process of cartilage and its role in osteoarthritis. This chapter will outline the physiology of cartilage tissue, the clinical presentation and treatment options for the disease and the cellular ageing processes which are involved in the pathophysiology of the disease.