Monosodium Urate Crystal-Induced Chondrocyte Death via Autophagic Process

Autophagy in Osteoarthritis: A Double-Edged Sword in Cartilage Aging and Mechanical Stress Response: A Systematic Review

Background: Although osteoarthritis (OA) development is epidemiologically multifactorial, a primary underlying mechanism is still under debate. Understanding the pathophysiology of OA remains challenging. Recently, experts have focused on autophagy as a contributor to OA development. Method: To better understand the pathogenesis of OA, we survey the literature on the role of autophagy and the molecular mechanisms of OA development. To identify relevant studies, we used controlled vocabulary and free text keywords to search the MEDLINE, EMBASE, the Cochrane Central Register of Controlled Trials, Web of Science, and SCOPUS database. Thirty-one studies were included for data extraction and systematic review. Among these studies, twenty-five studies investigated the effects of autophagy in aging and OA chondrocytes, six studies examined the effects of autophagy in normal human chondrocytes, and only one study investigated the effects of mechanical stress-induced autophagy on the development of OA in normal chondrocytes. Results: The studies suggest that autophagy activation prevents OA by exerting cell-protective effects in normal human chondrocytes. However, in aging and osteoarthritis (OA) chondrocytes, the role of autophagy is intricate, as certain studies indicate that stimulating autophagy in these cells can have a cytotoxic effect, while others propose that it may have a protective (cytoprotective) effect against damage or degeneration. Conclusions: Mechanical stress-induced autophagy is also thought to be involved in the development of OA, but further research is required to identify the precise mechanism. Thus, autophagy contributions should be interpreted with caution in aging and the types of OA cartilage.

Time- and Concentration-Dependent Stimulation of Oxidative Stress in Chondrocytes by Intracellular Soluble Urate

BACKGROUND

Gout could result in irreversible bone erosion, and chondrocyte might be involved in the process. Increased soluble urate is the early stage of gout and is strongly oxidative.

OBJECTIVE

To explore the effect of intracellular urate on the oxidative status of chondrocytes.

METHODS

A chondrocyte model was used. Serial concentrations of exogenous urate were incubated with chondrocytes for increasing amounts of time. Reactive oxygen species (ROS), oxidant, and anti-oxidant molecules were measured with biochemical assays, rt-PCR, and western blot. A urate transport inhibitor and oxidative inhibitors were used to confirm the effect of exogenous urate.

RESULTS

All concentrations of exogenous urate stimulated the production of ROS in a time- and concentration-dependent manner, as well as oxidant molecules, including hydrogen peroxide (H2O2), nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, nitric oxide (NO) inducible nitric oxide synthase (iNOS), and these effects, could be inhibited by oxidant inhibitors. However, anti-oxidant molecules, including acidic leucine-rich nuclear phosphoprotein-32A (ANP32A), ataxia-telangiectasia mutated (ATM), heme oxygenase-1 (HO-1), and the transcription factor nuclear factor erythroid 2 (NF-E2)-related (Nrf2), was decreased by high concentrations of exogenous urate after prolonged incubation, but not by low to medium concentrations of exogenous urate. By inhibiting soluble urate trafficking, benzbromarone significantly suppressed the effect of urate stimulus on the oxidant and anti-oxidant molecules.

CONCLUSION

Intracellular soluble urate could regulate chondrocyte redox balance in a time and concentration-dependent manner, and would be a target for regulating and protecting chondrocyte function in the early gout stage.

Discovery and Characterization of Moracin C as an Anti-Gouty Arthritis/Hyperuricemia Candidate by Docking-Based Virtual Screening and Pharmacological Evaluation

In the present study, a natural product database of compounds associated with herbs traditionally verified to treat gout/hyperuricemia/arthritis was constructed. 3D-shape and docking-based virtual screening was conducted. To identify potential xanthine oxidase (XOD) inhibitors in the database, eight compounds with commercial availability were identified as high 3D-shape similarity with febuxostat (1), a known XOD inhibitor. Docking was used to further predict the possible interactions between XOD and these compounds. Moracin C (2), moracin D (3), and isoformononetin (8) exhibited higher docking scores and binding energies than other compounds. In vitro, 2 inhibited XOD with an IC50 value of 0.25 ± 0.14 μM, which is similar to that of 1 (0.16 ± 0.08 μM). In a hyperuricemic mouse model, 5-20 mg/kg 2 exhibited satisfying urate-lowering and XOD inhibitory effects. Compound 2 also exhibited antiarthritis activities. In RAW264.7 cells, 2 at 1-10 μM inhibited the expression of IL-1β and TNF-α induced by MSU. In an acute gouty arthritis model in SD rats, 5-20 mg/kg 2 significantly alleviated the toe swelling, inflammatory response, and dysfunction disorder caused by monosodium urate (MSU). Compound 2 inhibited serum IL-1β and TNF-α cytokines as well as reduced the expression of the NLRP3/ASC/caspase-1 inflammasome in joints. In summary, 2 was an effective compound for the treatment of hyperuricemia/gouty arthritis.

The landscape of pathophysiology guided therapeutic strategies for gout treatment

INTRODUCTION

Gout is a common autoinflammatory disease caused by hyperuricemia with acute and/or chronic inflammation as well as tissue damage. Currently, urate-lowering therapy (ULT) and anti-inflammatory therapy are used as first-line strategies for gout treatment. However, traditional drugs for gout treatment exhibit some unexpected side effects and are not suitable for certain patients due to their comorbidity with other chronic disease.

AREAS COVERED

In this review, we described the pathophysiology of hyperuricemia and monosodium urate (MSU) crystal induced inflammatory response during gout development in depth and comprehensively summarized the advances in the investigation of promising ULT drugs as well as anti-inflammatory drugs that might be safer and more effective for gout treatment.

EXPERT OPINION

New drugs that are developed based on these molecular mechanisms exhibited great efficacy on reduction of disease burden both in vitro and in vivo, implying their potential for clinical application. Moreover, hyperthermia also showed regulation effect on MSU crystals formation and the signaling pathways involved in inflammation.

Diallyl trisulfide inhibits monosodium urate-induced NLRP3 inflammasome activation via NOX3/4-dependent mitochondrial oxidative stress in RAW 264.7 and bone marrow-derived macrophages

BACKGROUND

Monosodium urate (MSU) crystals are associated with gouty inflammatory diseases. MSU-associated inflammation is majorly triggered by NOD-like receptor protein 3 (NLRP3) inflammasome that promotes interleukin (IL)-1β secretion. Although diallyl trisulfide (DATS) is well-known polysulfide garlic compounds with anti-inflammatory effects, its action in MSU-induced inflammasome activation has not been known yet.

PURPOSE

The objective of the current study was to investigate anti-inflammasome effects and mechanisms of DATS in RAW 264.7 and bone marrow-derived macrophages (BMDM).

METHODS

The concentrations of IL-1β were analyzed with enzyme-linked immunosorbent assay. The MSU-induced mitochondrial damage and reactive oxygen species (ROS) production were detected by fluorescence microscope and flow cytometry. The protein expressions of NLRP3 signaling molecules, NADPH oxidase (NOX) 3/4 were assessed with Western blotting.

RESULTS

DATS suppressed MSU-induced IL-1β and caspase-1 accompanied by decreased inflammasome complex formation in RAW 264.7 and BMDM. In addition, DATS restored mitochondrial damage. DATS downregulated NOX 3/4 that were upregulated by MSU as predicted by gene microarray and confirmed by Western blotting.

CONCLUSION

This study first reports mechanistic finding that DATS alleviates MSU-induced NLRP3 inflammasome by mediating NOX3/4-dependent mitochondrial ROS production in macrophages in vitro and ex vivo, suggesting DATS could be effective therapeutic candidate for gouty inflammatory condition.

Daurisoline attenuates H2O2-induced chondrocyte autophagy by activating the PI3K/Akt/mTOR signaling pathway

BACKGROUND

Osteoarthritis (OA) is a chronic degenerative joint disease characterized by cartilage degeneration and intra-articular inflammation. Daurisoline (DAS) is an isoquinoline alkaloid isolated from Rhizoma Menispermi, whose antitumor and anti-inflammatory pharmacological effects have been demonstrated, but the effects of DAS on OA have rarely been researched. In this study, we aimed to explore the potential role of DAS in OA and its partial mechanism.

MATERIALS AND METHODS

The cytotoxicity of H₂O₂ and DAS toward chondrocytes was detected by the Cell Counting Kit-8 assay. Safranin O staining was used to detect chondrocyte phenotype changes. Cell apoptosis was measured by both flow cytometry and quantitative analysis of the protein levels of the apoptosis-related factors Bax, Bcl-2 and cleaved caspase-3 by western blot. Western blotting and immunofluorescence were used to assess the expression of the autophagy-related proteins LC3, Beclin-1 and p62. In addition, key signal pathway targets and matrix-degrading indicators were measured by western blot.

RESULTS

Our results indicated that H₂O₂ induced human chondrocyte apoptosis and activated autophagy in a dose-dependent manner. DAS treatment dose-dependently reversed the expression of apoptosis-related proteins (Bax, Bcl-2 and cleaved caspase3) and the apoptosis rate induced by H₂O₂. Western blot and immunofluorescence analyses showed that DAS decreased the H₂O₂-induced upregulation of the autophagy marker Beclin-1 and the LC3 II/LC3 I ratio and upregulated the p62 protein level. Mechanistically, DAS inhibited autophagy through the activation of the classical PI3K/AKT/mTOR signaling pathway and protected chondrocytes from apoptosis. In addition, DAS alleviated the H₂O₂-induced degradation of type II collagen and the high expression of matrix metalloproteinase 3 (MMP3) and MMP13.

CONCLUSION

Our research demonstrated that DAS alleviated chondrocyte autophagy caused by H₂O₂ through activation of the PI3K/AKT/mTOR signaling pathway and protected chondrocytes from apoptosis and matrix degradation. In conclusion, these findings suggest that DAS may serve as a promising therapeutic strategy for OA.

Serum lipid biomarkers and inflammatory cytokines associated with onset and clinical status of patients with early knee osteoarthritis

Introduction

Osteoarthritis (OA) is a common joint condition and one of the greatest causes of disability worldwide. The role of serum lipid and inflammatory biomarkers in the origin and development of the disease is not clear, although it could have important implications for diagnosis and treatment. The primary aim of this study was to evaluate differences of serum lipid and inflammatory biomarkers with knee EOA in comparison with matched controls, in order to determine the role of these factors in the origin of EOA.

Methods

For this proposal, a cross-sectional study with a non-randomized sample was performed. 48 subjects with early osteoarthritis (EOA) and 48 matched controls were selected and serum lipid levels (total cholesterol, LDL, HDL) and inflammatory biomarkers C-reactive protein (CRP), uric acid (UA) were analyzed. In addition, clinical (pain, disability) and functional (gait speed, sit-to-stand) variables were measured to establish their relationship to serum lipid levels and inflammatory biomarkers.

Results

Patients with EOA showed higher levels of total cholesterol LDL, UA, and CRP. Higher levels of total cholesterol, LDL and CRP were correlated with higher levels of pain intensity and higher disability (p < 0.05). In addition, UA and CRP were inversely correlated with gait speed and sit-to-stand tests (r = −0.038 to −0.5, p < 0.05).

Conclusion

These results highlight the relevance of metabolic and proinflammatory aspects in the early stages of knee OA and could be key to developing early diagnoses to prevent the onset and development of the disease.

Downregulation of Sox8 mediates monosodium urate crystal-induced autophagic impairment of cartilage in gout arthritis

The deposition of monosodium urate (MSU) crystals in arthritic joints of gout seriously damages cartilage. This study aimed to investigate whether MSU crystal-induced cartilage impairment was related to autophagic signaling. mRNAs of cartilage from MSU-induced gouty arthritis rat model were sequenced. MSU crystal-treated human chondrocytes were used to evaluate the function of Sox8. The recombinant Sox8 lentiviral vector (lenti-Sox8) was applied to upregulate the expression of Sox8. Transfection of the mRFP-GFP-LC3 plasmid was evaluated by confocal microscopy. The autophagic vacuoles were stained with monodansylcadaverine and examined by flow cytometry. The morphology of autophagosomes was observed by transmission electron microscopy. The ratio of LC3-II/I in the presence or absence of bafilomycin A1 and the expression levels of Beclin1, Sox8, p-PI3K, PI3K, p-AKT, AKT, p-mTOR, and mTOR were detected by Western blot. In vivo, the effect of Sox8 on cartilage of acute gouty model rats was evaluated by safranin-O/fast green staining and Western blot. The expression of Sox8 was significantly downregulated both in vivo and in vitro. In chondrocytes, MSU crystals reduced the expression of Sox8, inhibited the PI3K/AKT/mTOR signaling pathway, and increased the level of autophagy. Overexpression of Sox8 notably inhibited MSU crystal-induced autophagy by rescuing the phosphorylation levels in the PI3K/AKT/mTOR signaling pathway. In vivo, overexpression of Sox8 remarkably alleviated cartilage damage in acute gouty model rats. These results indicate that downregulation of Sox8 plays an important role in MSU-induced chondrocyte autophagy by modulating PI3K/AKT/mTOR signaling, and overexpression of Sox8 may serve as a novel therapy to prevent the impairment of cartilage in gout arthritis.

New insights into the interplay between autophagy and cartilage degeneration in osteoarthritis

Autophagy is an intracellular degradation system that maintains the stable state of cell energy metabolism. Some recent findings have indicated that autophagy dysfunction is an important driving factor for the occurrence and development of osteoarthritis (OA). The decrease of autophagy leads to the accumulation of damaged organelles and macromolecules in chondrocytes, which affects the survival of chondrocytes and ultimately leads to OA. An appropriate level of autophagic activation may be a new method to prevent articular cartilage degeneration in OA. This minireview discussed the mechanism of autophagy and OA, key autophagy targets regulating OA progression, and evaluated therapeutic applications of drugs targeting autophagy in preclinical and clinical research. Some critical issues worth paying attention to were also raised to guide future research efforts.

Evidence for the effect of soluble uric acid in augmenting endoplasmic reticulum stress markers in human peripheral blood mononuclear cells

There is evidence regarding the association of hyperuricemia with inflammatory disorders. Hence, it has been of particular interest to dissect the exact role of alteration in uric acid (UA) levels in the context of inflammation. Recently, the endoplasmic reticulum (ER) stress pathway has come into the forefront as a possible mechanism linking hyperuricemia to inflammation. Here, we intended to examine the role of UA in the presence or absence of a second stimulus, LPS, in human peripheral blood mononuclear cells (PBMCs), and analyzed ROS production as well as expression of ER stress markers: GRP78 and CHOP, and inflammatory cytokines.PBMCs were isolated using Ficoll gradient centrifugation from healthy volunteers. Cell viability was measured by MTT assay. PBMCs were treated with an increasing concentration of soluble UA (0, 5, 12, and 20 mg/dl) for 20 h, followed by the addition of 100 ng/mL of LPS or vehicle for another 4 h. Real time-PCR was performed to investigate the mRNA expression of GRP78, CHOP, TNF-α, IL-1β, and IL-6, and western blot was used to investigate the protein levels of GRP78 and CHOP. Moreover, ELISA was used to evaluate the protein levels of TNF-α, IL-1β, and IL-6. Finally, intracellular ROS production was determined using fluorescent probes (DCFH-DA).High concentrations of UA either alone or combined with LPS increased the protein levels of GRP78 and CHOP. On the other hand, LPS alone increased the protein levels of GRP78 and CHOP. However, there was no significant difference between the mRNA expression of GRP78, CHOP, TNF-α, IL-1β, and IL-6 when PBMCs were treated with UA. High concentrations of UA augmented LPS-stimulated IL-1β transcript and protein levels as well as TNF-α protein levels in PBMC culture. Moreover, high concentrations of UA along with LPS significantly increased intracellular ROS production.It seems that a high concentration of UA not only induces the protein levels of ER stress markers in PBMCs but also augments the impact of LPS on the levels of pro-inflammatory markers and ROS production.

Chlorogenic acid protects human chondrocyte C28/I2 cells from oxidative stress-induced cell death through activation of autophagy

AIMS

The development of osteoarthritis (OA), the most common form of arthritis, is commonly associated with oxidative stress. Indeed, the lack of antioxidant responses largely increases OA incidence. OA is a leading cause of disability in the elderly, which reduces the quality of life and places high socioeconomic burdens on them. Several polyphenolic compounds, including chlorogenic acid (CGA), have shown cytoprotective effects via their antioxidant activity, but the exact mechanism (s) remain elusive. In this study, we demonstrated how CGA protects human chondrocytes against H₂O₂-induced apoptosis.

MATERIALS AND METHODS

The cytoprotective effect by CGA in 500 μM hydrogen peroxide-treated C28/I2 cells was evaluated by cell viability, TUNEL assay, and Western blotting analyses, and autophagy assessment was further performed by AO and MDC staining and tandem mRFP-GFP fluorescence analyses.

KEY FINDINGS

Treatment of CGA to the human chondrocytes under oxidative stress significantly decreased apoptosis markers, such as cleaved caspase 3 and cleaved PARP, and increased anti-apoptotic marker Bcl-xL and the antioxidant response proteins NRF2 and NF-κB. Furthermore, CGA-dependent activation of antioxidant response proteins NRF2 and NF-κB and its protective effects in chondrocytes depended on autophagy. Indeed, CGA treatment and autophagy induction significantly decreased reactive oxygen species (ROS)-induced apoptosis.

SIGNIFICANCE

CGA exhibited the protective effect to human chondrocyte C28/I2 cells against oxidative stress-induced cell death by activating autophagy. These findings indicate that CGA is a potential therapeutic agent for the development of OA drugs.

Serum uric acid and knee osteoarthritis in community residents without gout: a longitudinal study

OBJECTIVES

Emerging evidence suggests a potential link between OA and gout; however, the association between serum uric acid (UA) itself and knee OA remains uncertain due to a lack of longitudinal studies. Here, we investigated the association between serum UA and knee OA according to cartilage status in elderly community residents without gout.

METHODS

In this longitudinal study, participants without a history of gout were recruited from among the Korean cohort of the Hallym Aging Study (n = 296 for radiography study and n = 223 for MRI study). Weight-bearing knee radiographs and 1.5-T MRI scans, along with blood collection for analysis of serum UA, were performed at baseline and after 3 years. The severity and structural progression of knee OA were evaluated using the Kellgren-Lawrence grading system and the Whole-Organ MRI Score (WORMS) cartilage scoring method. Multivariable logistic regression analysis was conducted using generalized estimating equation (GEE) models.

RESULTS

Serum UA levels were not associated with radiographic progression after adjusting for age, sex and BMI. There was no significant association between serum UA and tibiofemoral cartilage loss on MRI. However, baseline serum UA levels were negatively associated with patellofemoral cartilage loss over 3 years (adjusted odd ratio 0.70 per 1 mg/dl increase, 95% CI: 0.49, 0.98).

CONCLUSION

In this population-based cohort, serum UA was not a risk factor for knee OA progression. Further large-scale longitudinal studies in other populations are needed to validate the effects of UA on cartilage damage.

The biology of urate

Urate is the end-product of the purine metabolism in humans. The dominant source of urate is endogenous purines and the remainder comes through diet. Approximately two thirds of urate is eliminated via the kidney with the rest excreted in the feces. While the transporter BCRP, encoded by ABCG2, has been found to play a role in both the gut and kidney, SLC22A12 and SLC2A9 encoding URAT1 and GLUT9, respectively, are the two transporters best characterized. Only 8-12% of the filtered urate is excreted by the kidney. Renal elimination of urate depends substantially on specific transporters, including URAT1, GLUT9 and BCRP. Studies that have assessed the biologic effects of urate have produced highly variable results. Although there is a suggestion that urate may have anti-oxidant properties in some circumstances, the majority of evidence indicates that urate is pro-inflammatory. Hyperuricemia can result in the formation of monosodium urate (MSU) crystals that may be recognized as danger signals by the immune system. This immune response results in the activation of the NLRP3 inflammasome and ultimately in the production and release of interleukin-1β, and IL-18, that mediate both inflammation, pyroptotic cell death, and necroinflammation. It has also been demonstrated that soluble urate mediates effects on the kidney to induce hypertension and can induce long term epigenetic reprogramming in myeloid cells to induce "trained immunity." Together, these sequelae of urate are thought to mediate most of the physiological effects of hyperuricemia and gout, illustrating this biologically active molecule is more than just an "end-product" of purine metabolism.

Pathophysiology of Gout

Multiple interacting checkpoints are involved in the pathophysiology of gout. Hyperuricemia is the key risk factor for gout and is considered a prerequisite for monosodium urate (MSU) crystal formation. Urate underexcretion through renal and gut mechanisms is the major mechanism for hyperuricemia in most people. Multiple genetic, environmental, and metabolic factors are associated with serum urate and alter urate transport or synthesis. Urate supersaturation is the most important factor for MSU crystal formation, and other factors such as temperature, pH, and connective tissue components also play a role. The nucleotide-binding oligomerization domain leucine-rich repeats and pyrin domain-containing protein 3 inflammasome plays a pivotal role in the inflammatory response to MSU crystals, and interleukin 1β is the key cytokine mediating the inflammatory cascade. Variations in the regulatory mechanisms of this inflammatory response may affect an individual's susceptibility to developing gout. Tophus formation is the cardinal feature of advanced gout, and both MSU crystals and the inflammatory tissue component of the tophus contribute to the development of structural joint damage owing to gout. In this article, we review the pathophysiologic mechanisms of hyperuricemia, MSU crystal formation and the associated inflammatory response, tophus formation, and structural joint damage in gout.

Cytoprotective Effects of Delphinidin for Human Chondrocytes against Oxidative Stress through Activation of Autophagy

Antioxidant enzymes are decreased in osteoarthritis (OA) patients, implying the role of oxidative stress in osteoarthritis pathogenesis. The aim of this study was to evaluate the cytoprotective effects of delphinidin, a potent antioxidant, in human chondrocytes and the underlying mechanisms. The cytoprotective mechanism induced by delphinidin against oxidative stress (H₂O₂) in human chondrocytes was investigated. Cell viability and death were evaluated using proapoptotic and antiapoptotic markers such as cleaved caspase-3 (c-caspase-3), cleaved poly(ADP-ribose) polymerase N-acetylcysteine (c-PARP), Bcl-X_L, and transcription factors associated with redox and inflammation regulation, including nuclear factor kappa B (NF-κB) and nuclear factor (erythroid-derived 2)-like 2 (Nrf2). Induction of autophagy was assessed by formation of LC3-II and autophagosome-(LC3 punctate, monodansylcadaverine (MDC) and acridine orange staining) in the presence or absence of an autophagy inhibitor. Treatment with delphinidin itself at concentration below 50 µM for 24 h did not affect viability of chondrocytes. Delphinidin inhibited reactive oxygen species (ROS)-induced apoptosis by significantly decreasing apoptosis markers such as c-caspase-3 and c-PARP while increasing antiapoptotic marker Bcl-X_L and antioxidant response NF-κB and Nrf2 pathways. Delphinidin also activated cytoprotective autophagy to protect chondrocytes during oxidative stresses. Activation of autophagy with autophagy inducer rapamycin also inhibited ROS-induced cell death and decreased proapoptotic proteins but increased antiapoptotic protein Bcl-X_L, NF-κB, and Nrf2. Delphinidin can protect chondrocytes against H₂O₂-induced apoptosis via activation of Nrf2 and NF-κB and protective autophagy. Thus, it can inhibit OA with protection of chondrocytes. Delphinidin can protect chondrocytes against H₂O₂-induced ROS with maintenance of homeostasis and redox. These results suggest that delphinidin could be used to protect chondrocytes against age-related oxidative stress and other oxidative stresses in the treatment of OA. Thus, delphinidin may play a critical role in preventing the development and progression of OA.

Urate and osteoarthritis: Evidence for a reciprocal relationship

Hyperuricemia is a common condition, and in a subset of patients leads to gout, the most common inflammatory arthritis. Osteoarthritis is the most common form of arthritis overall, and gout and osteoarthritis frequently coexist in the same patient. However, the relationship between the two remains poorly defined. More particularly, the impact of osteoarthritis on the development of gout, and the impact of gout on the development of osteoarthritis, remain to be determined. Additionally, whether hyperuricemia mediates osteoarthritis in the absence of gout is uncertain. Here, we review the evidence linking gout and osteoarthritis, with a special focus on the role of hyperuricemia in the presence or absence of gout. Since disease modifying agents are currently available for hyperuricemia and gout but not for osteoarthritis, a contributory role for urate in the pathogenesis of osteoarthritis could have important clinical implications.

Urate transport capacity of glucose transporter 9 and urate transporter 1 in cartilage chondrocytes

Chronic gouty arthritis, caused by a persistent increase in, and the deposition of, soluble uric acid (sUA), can induce pathological chondrocyte destruction; however, the effects of urate transport and intracellular sUA on chondrocyte functionality and viability are yet to be fully determined. Thus, the aim of the present study was to investigate the presence and functionality of a urate transport system in chondrocytes. The expression profiles of two primary urate reabsorptive transporters, glucose transporter 9 (GLUT9) and urate transporter 1 (URAT1), in human articular cartilage and chondrocyte cell lines were examined via western blotting, reverse transcription-quantitative PCR, immunohistochemistry and immunofluorescence. Then, chondrocytes were incubated with exogenous sUA at increasing concentrations. Negative control assays were conducted via the specific knockdown of GLUT9 and URAT1 with lentiviral short hairpin (sh)RNAs, and by pretreatment with benzbromarone, a known inhibitor of the two transporters. Intracellular UA concentrations were measured using colorimetric assays. The expression levels of GLUT9 and URAT1 were determined in cartilage tissues and chondrocyte cell lines. Incubation of chondrocytes with sUA led to a concentration-dependent increase in intracellular urate concentrations, which was inhibited by GLUT9 or URAT1 knockdown, or by benzbromarone pretreatment (27.13±2.70, 44.22±2.34 and 58.46±2.32% reduction, respectively). In particular, benzbromarone further decreased the already-reduced intracellular UA concentrations in HC-shGLUT9 and HC-shURAT1 cells by 46.79±2.46 and 39.79±2.22%, respectively. Cells overexpressing GLUT9 and URAT1 were used as the positive cell control, which showed increased intracellular UA concentrations that could be reversed by treatment with benzbromarone. In conclusion, chondrocytes may possess an active UA transport system. GLUT9 and URAT1 functioned synergistically to transport UA into the chondrocyte cytoplasm, which was inhibited by specific gene knockdowns and drug-induced inhibition. These results may be fundamental in the further investigation of the pathological changes to chondrocytes induced by sUA during gouty arthritis, and identified UA transport processes as potential targets for the early control of chronic gouty arthritis.

The Role of Autophagy in Chondrocyte Metabolism and Osteoarthritis: A Comprehensive Research Review

Chondrocytes are the sole cellular constituents of normal cartilage. The degeneration and apoptosis of these cells are considered the main cause of osteoarthritis (OA). Previous studies have suggested that the enhancement of autophagy in chondrocytes can delay the progression of osteoarthritis by affecting intracellular metabolic activity, i.e., by regulating the metabolism of nutrients, which can delay cell aging and death. In this review, we explored the relationship between autophagy and chondrocyte metabolism and provided new ideas for the prevention and treatment of OA.

Role of the Inflammation-Autophagy-Senescence Integrative Network in Osteoarthritis

Osteoarthritis is the most common musculoskeletal disease causing chronic disability in adults. Studying cartilage aging, chondrocyte senescence, inflammation, and autophagy mechanisms have identified promising targets and pathways with clinical translatability potential. In this review, we highlight the most recent mechanistic and therapeutic preclinical models of aging with particular relevance in the context of articular cartilage and OA. Evidence supporting the role of metabolism, nuclear receptors and transcription factors, cell senescence, and circadian rhythms in the development of musculoskeletal system degeneration assure further translational efforts. This information might be useful not only to propose hypothesis and advanced models to study the molecular mechanisms underlying joint degeneration, but also to translate our knowledge into novel disease-modifying therapies for OA.

Impaired Na+-K+-ATPase signaling in renal proximal tubule contributes to hyperuricemia-induced renal tubular injury

Hyperuricemia contributes to renal inflammation. We aimed to investigate the role of Na⁺–K⁺–ATPase (NKA) in hyperuricemia-induced renal tubular injury. Human primary proximal tubular epithelial cells (PTECs) were incubated with uric acid (UA) at increasing doses or for increasing lengths of time. PTECs were then stimulated by pre-incubation with an NKA α1 expression vector or small interfering RNA before UA (100 μg ml⁻¹, 48 h) stimulation. Hyperuricemic rats were induced by gastric oxonic acid and treated with febuxostat (Feb). ATP levels, the activity of NKA and expression of its α1 subunit, Src, NOD-like receptor pyrin domain-containing protein 3 (NLRP3) and interleukin 1β (IL-1β) were measured both in vitro and in vivo. Beginning at concentrations of 100 μg ml⁻¹, UA started to dose-dependently reduce NKA activity. UA at a concentration of 100 μg ml⁻¹ time-dependently affected the NKA activity, with the maximal increased NKA activity at 24 h, but the activity started to decrease after 48 h. This inhibitory effect of UA on NKA activity at 48 h was in addition to a decrease in NKA α1 expression in the cell membrane, but an increase in lysosomes. This process also involved the subsequent activation of Src kinase and NLRP3, promoting IL-1β processing. In hyperuricemic rats, renal cortex NKA activity and its α1 expression were upregulated at the 7th week and both decreased at the 10th week, accompanied with increased renal cortex expression of Src, NLRP3 and IL-1β. The UA levels were reduced and renal tubular injuries in hyperuricemic rats were alleviated in the Feb group. Our data suggested that the impairment of NKA and its consequent regulation of Src, NLRP3 and IL-1β in the renal proximal tubule contributed to hyperuricemia-induced renal tubular injury.

Soluble uric acid increases PDZK1 and ABCG2 expression in human intestinal cell lines via the TLR4-NLRP3 inflammasome and PI3K/Akt signaling pathway

BACKGROUND

In addition to the kidney, the intestine is one of the most important organs involved in uric acid excretion. However, the mechanism of urate excretion in the intestine remains unclear. Therefore, the relationship between soluble uric acid and the gut excretion in human intestinal cells was explored. The relevant signaling molecules were then also examined.

METHODS

HT-29 and Caco-2 cell lines were stimulated with soluble uric acid. Western blotting and qRT-PCR were used to measure protein and mRNA levels. Subcellular fractionation methods and immunofluorescence were used to quantify the proteins in different subcellular compartments. Flow cytometry experiments examined the function of ATP-binding cassette transporter, subfamily G, member 2 (ABCG2). Small interfering RNA transfection was used to assess the interaction between ABCG2 and PDZ domain-containing 1 (PDZK1).

RESULTS

Soluble uric acid increased the expression of PDZK1 and ABCG2. The stimulation of soluble uric acid also facilitated the translocation of ABCG2 from the intracellular compartment to the plasma membrane and increased its transport activity. Moreover, the upregulation of PDZK1 and ABCG2 by soluble uric acid was partially decreased by either TLR4-NLRP3 inflammasome inhibitors or PI3K/Akt signaling inhibitors. Furthermore, PDZK1 knockdown significantly inhibited the expression and transport activity of ABCG2 regardless of the activation by soluble uric acid, demonstrating a pivotal role for PDZK1 in the regulation of ABCG2.

CONCLUSIONS

These findings suggest that urate upregulates the expression of PDZK1 and ABCG2 for excretion in intestinal cells via activating the TLR4-NLRP3 inflammasome and PI3K/Akt signaling pathway.

Autophagy in osteoarthritis

Osteoarthritis is characterized by continuous degeneration of articular cartilage resulting in disability. The death of chondrocytes and the loss of the extracellular matrix are the central peculiarities in cartilage degeneration during osteoarthritis pathogenesis. Autophagy is an essential cellular homeostasis mechanism whereby cellular organelles and macromolecules are recycled to maintain cellular metabolism. Autophagy is reported to be cytoprotective effects for articular cartilage, and osteoarthritis is associated with decreased autophagy. While autophagy is known to be cytoprotective to chondrocytes, its role may vary with differing stages and models of osteoarthritis. Therefore, more in-depth studies on autophagy are needed to determine its impact on cell survival and death in articular cartilage under various in vitro and in vivo conditions. Application of autophagy on osteoarthritis therapeutics will be possible after a profound understanding is established on the role of autophagy in osteoarthritis pathogenesis.

Gout and Osteoarthritis: Associations, Pathophysiology, and Therapeutic Implications

Osteoarthritis (OA), the most common type of arthritis worldwide, is a degenerative disease of diarthrodial joints resulting in pain, reduced quality of life, and socioeconomic burden. Gout, the most common form of inflammatory arthritis, is a consequence of persistently elevated levels of urate and the formation of proinflammatory monosodium urate crystals in joints. Clinicians have long noted a predilection for both diseases to occur in the same joints. In this review, we provide an overview into research elucidating possible biochemical, mechanical, and immunological relationships between gout and OA. We additionally consider the potential implications of these relationships for OA treatment.