Apoptosis of articular chondrocytes in rheumatoid arthritis and osteoarthritis: correlation of apoptosis with degree of cartilage destruction and expression of apoptosis-related proteins of p53 and c-myc

Inhibition of the MALT1-LPCAT3 axis protects cartilage degeneration and osteoarthritis

The proinflammatory cytokines and arachidonic acid (AA)-derived eicosanoids play a key role in cartilage degeneration in osteoarthritis (OA). The lysophosphatidylcholine acyltransferase 3 (LPCAT3) preferentially incorporates AA into the membranes. Our recent studies showed that MALT1 [mucosa-associated lymphoid tissue lymphoma translocation protein 1]) plays a crucial role in propagating inflammatory signaling triggered by IL-1β and other inflammatory mediators in endothelial cells. The present study shows that LPCAT3 expression was up-regulated in both human and mice articular cartilage of OA, and correlated with severity of OA. The IL-1β-induces cell death via upregulation of LPCAT3, MMP3, ADAMTS5, and eicosanoids via MALT1. Gene silencing or pharmacological inhibition of LPCAT3 or MALT1 in chondrocytes and human cartilage explants notably suppressed the IL-1β-induced cartilage catabolism through inhibition of expression of MMP3, ADAMTS5, and also secretion of cytokines and eicosanoids. Mechanistically, overexpression of MALT1 in chondrocytes significantly upregulated the expression of LPCAT3 along with MMP3 and ADAMTS5 via c-Myc. Inhibition of c-Myc suppressed the IL-1β-MALT1-dependent upregulation of LPCAT3, MMP3 and ADAMTS5. Consistent with the in vitro data, pharmacological inhibition of MALT1 or gene silencing of LPCAT3 using siRNA-lipid nanoparticles suppressed the synovial articular cartilage erosion, pro-inflammatory cytokines, and eicosanoids such as PGE2, LTB4, and attenuated osteoarthritis induced by the destabilization of the medial meniscus in mice. Overall, our data reveal a previously unrecognized role of the MALT1-LPCAT3 axis in osteoarthritis. Targeting the MALT1-LPCAT3 pathway with MALT1 inhibitors or siRNA-liposomes of LPCAT3 may become an effective strategy to treat OA by suppressing eicosanoids, matrix-degrading enzymes, and proinflammatory cytokines.

Targeting regulated chondrocyte death in osteoarthritis therapy

In vivo articular cartilage degeneration is an essential hallmark of osteoarthritis (OA), involving chondrocyte senescence, extracellular matrix degradation, chondrocyte death, cartilage loss, and bone erosion. Among them, chondrocyte death is one of the major factors leading to cartilage degeneration. Many studies have reported that various cell death modes, including apoptosis, ferroptosis, and autophagy, play a key role in OA chondrocyte death. Currently, there is insufficient understanding of OA pathogenesis, and there remains a lack of treatment methods to prevent OA and inhibit its progression. Studies suggest that OA prevention and treatment are mainly directed to arrest premature or excessive chondrocyte death. In this review, we a) discuss the forms of death of chondrocytes and the associations between them, b) summarize the critical factors in chondrocyte death, c) discuss the vital role of chondrocyte death in OA, d) and, explore new approaches for targeting the regulation of chondrocyte death in OA treatment.

Identification of key genes and small molecule drugs in osteoarthritis by integrated bioinformatics analysis

Background

Osteoarthritis (OA) is a common joint degenerative disease that can affect multiple joints. Genetic events may play an important regulatory role in the early stages of the disease, but the specific mechanisms have not yet been fully elucidated. The main purpose of this study was to screen for disease-causing hub genes and effective small molecule drugs to reveal the pathogenesis of OA and to develop novel drugs for treatment.

Methods

Two gene expression profile datasets, GSE55235 and GSE55457, were integrated and further analyzed. The consistently differentially expressed genes (DEGs) were identified, and functional annotation and pathway analysis of these genes were performed with GO and KEGG. A protein-protein interaction network (PPI) of the DEGs was generated using STRING, and potential small molecule drug screening was performed on the connectivity map (CMap).

Results

A total of 158 consistently differentially expressed genes were identified from the two profile datasets. The functions of these DEGs are mainly related to the TNF signaling pathway, osteoclast differentiation, MAPK signaling pathway and so on. The PPI network contains 127 nodes and 1802 edges, and the ten hub genes were interleukin 6 (IL6), vascular endothelial growth factor A (VEGFA)and so on. 7 small molecule drugs were identified as potential interactors with these hubs.

Conclusions

This study explains the disorder of expression in the pathological process of OA at transcriptome, which will help to understand the pathogenesis of OA.

Role of Sirtuins in the Pathogenesis of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an autoimmune and inflammatory disease leading to joint destruction. The causes of RA are not fully known. Most likely, the development of the disease depends on the coexistence of many factors, such as hereditary factors, immune system defects, gender, infectious agents, nicotine, and stress. Various epigenetic changes have been identified and correlated with the aggressive phenotype of RA, including the involvement of sirtuins, which are enzymes found in all living organisms. Their high content in the human body can slow down the aging processes, reduce cell death, counteract the appearance of inflammation, and regulate metabolic processes. Sirtuins can participate in several steps of RA pathogenesis. This narrative review presents, collects, and discusses the role of all sirtuins (1-7) in the pathogenesis of rheumatoid arthritis.

Long noncoding and micro-RNA expression in a model of articular chondrocyte degeneration induced by stromal cell-derived factor-1

Background

Gene regulatory network analysis has found that long noncoding ribonucleic acids (lncRNAs) are strongly associated with the pathogenesis of osteoarthritis.

Objectives

To determine the differential expression of lncRNAs and microRNAs (miRNAs) in normal chondrocytes and those from a model of articular chondrocyte degeneration.

Methods

Chondrocytes were cultured from cartilage obtained from patients diagnosed with osteoarthritis of the knee. Stromal cell-derived factor-1 (SDF-1) was used to induce their degeneration. Total RNA was extracted, analyzed, amplified, labeled, and hybridized on a chip to determine expression. The set of enriched differentially expressed miRNAs was analyzed by gene ontology and the Kyoto Encyclopedia of Genes and Genomes to describe the functional properties of the key biological processes and pathways. We conducted a bioinformatics analysis using Cytoscape to elucidate the interactions between miRNAs and proteins.

Results

We found that the expression of 186 lncRNAs was significantly different in the model of chondrocyte degeneration, in which 88 lncRNAs were upregulated, and 98 were downregulated. Expression of 684 miRNAs was significantly different. Analysis of the protein-protein interaction (PPI) network indicated that the genes for CXCL10, ISG15, MYC, MX1, OASL, IFIT1, RSAD2, MX2, IFI44L, and BST2 are the top 10 core genes, identifying the most important functional modules to elucidate the differential expression of miRNAs.

Conclusions

These data may provide new insights into the molecular mechanisms of chondrocyte degeneration in osteoarthritis, and the identification of lncRNAs and miRNAs may provide potential targets for the differential diagnosis and therapy of osteoarthritis.

Arthritis and the role of endogenous glucocorticoids

Rheumatoid arthritis and osteoarthritis, the most common forms of arthritis, are chronic, painful, and disabling conditions. Although both diseases differ in etiology, they manifest in progressive joint destruction characterized by pathological changes in the articular cartilage, bone, and synovium. While the potent anti-inflammatory properties of therapeutic (i.e., exogenous) glucocorticoids have been heavily researched and are widely used in clinical practice, the role of endogenous glucocorticoids in arthritis susceptibility and disease progression remains poorly understood. Current evidence from mouse models suggests that local endogenous glucocorticoid signaling is upregulated by the pro-inflammatory microenvironment in rheumatoid arthritis and by aging-related mechanisms in osteoarthritis. Furthermore, these models indicate that endogenous glucocorticoid signaling in macrophages, mast cells, and chondrocytes has anti-inflammatory effects, while signaling in fibroblast-like synoviocytes, myocytes, osteoblasts, and osteocytes has pro-inflammatory actions in rheumatoid arthritis. Conversely, in osteoarthritis, endogenous glucocorticoid signaling in both osteoblasts and chondrocytes has destructive actions. Together these studies provide insights into the role of endogenous glucocorticoids in the pathogenesis of both inflammatory and degenerative joint disease.

Identification of Candidate Genetic Markers and a Novel 4-genes Diagnostic Model in Osteoarthritis through Integrating Multiple Microarray Data

BACKGROUND

Osteoarthritis (OA) is a joint disease that leads to a high disability rate and a low quality of life. With the development of modern molecular biology techniques, some key genes and diagnostic markers have been reported. However, the etiology and pathogenesis of OA are still unknown.

OBJECTIVE

To develop a gene signature in OA.

METHOD

In this study, five microarray data sets were integrated to conduct a comprehensive network and pathway analysis of the biological functions of OA related genes, which can provide valuable information and further explore the etiology and pathogenesis of OA.

RESULTS AND DISCUSSION

Differential expression analysis identified 180 genes with significantly expressed expression in OA. Functional enrichment analysis showed that the up-regulated genes were associated with rheumatoid arthritis (p < 0.01). Down-regulated genes regulate the biological processes of negative regulation of kinase activity and some signaling pathways such as MAPK signaling pathway (p < 0.001) and IL-17 signaling pathway (p < 0.001). In addition, the OA specific protein-protein interaction (PPI) network was constructed based on the differentially expressed genes. The analysis of network topological attributes showed that differentially upregulated VEGFA, MYC, ATF3 and JUN genes were hub genes of the network, which may influence the occurrence and development of OA through regulating cell cycle or apoptosis, and were potential biomarkers of OA. Finally, the support vector machine (SVM) method was used to establish the diagnosis model of OA, which not only had excellent predictive power in internal and external data sets (AUC > 0.9), but also had high predictive performance in different chip platforms (AUC > 0.9) and also had effective ability in blood samples (AUC > 0.8).

CONCLUSION

The 4-genes diagnostic model may be of great help to the early diagnosis and prediction of OA.

Dual Role of Chondrocytes in Rheumatoid Arthritis: The Chicken and the Egg

Rheumatoid arthritis (RA) is one of the inflammatory joint diseases that display features of articular cartilage destruction. The underlying disturbance results from immune dysregulation that directly and indirectly influence chondrocyte physiology. In the last years, significant evidence inferred from studies in vitro and in the animal model offered a more holistic vision of chondrocytes in RA. Chondrocytes, despite being one of injured cells in RA, also undergo molecular alterations to actively participate in inflammation and matrix destruction in the human rheumatoid joint. This review covers current knowledge about the specific cellular and biochemical mechanisms that account for the chondrocyte signatures of RA and its potential applications for diagnosis and prognosis in RA.

Acetazolamide protects rat articular chondrocytes from IL-1β-induced apoptosis by inhibiting the activation of NF-κB signal pathway

Because the excessive apoptosis of articular chondrocytes contributes to extracellular matrix (ECM) loss and cartilage damage in rheumatoid arthritis (RA), inhibiting chondrocyte apoptosis might be a promising strategy for RA. Aquaporin1 (AQP1) is overexpressed in RA cartilage and synovial tissues, and play a vital pathogenic role in RA development. Particularly, we previously reported that acetazolamide (AZ) as an AQP1 inhibitor suppressed secondary inflammation and promoted ECM production in cartilage of adjuvant-induced arthritis rats. Here, we investigated the antiapoptotic effect of AZ on interleukin-1β (IL-1β)-induced apoptosis, a classic in vitro model of chondrocyte apoptosis. AZ treatment could inhibit IL-1β-induced apoptosis, evidenced by increasing cell viability, relieving apoptotic nuclear morphology, decreasing apoptosis rates, and restoring mitochondrial membrane potential. Additionally, AZ reversed IL-1β-induced decrease of Bcl-2 protein and reduced IL-1β-induced increases of Bax and caspase 3 protein, accompanied by inhibiting IκBα degradation and phosphorylation in cytoplasm, reducing NF-κB p65 protein level in nucleus and preventing NF-κB p65 translocation from cytoplasm to nucleus. In conclusion, our findings indicated that AZ could effectively attenuate IL-1β-induced chondrocyte apoptosis mediated by regulating the protein levels of apoptosis-related genes and inhibiting the activation of NF-κB signal pathway, suggesting that AZ might be of potential clinical interest in RA treatment.

Systematic Analysis of Differential Expression Profile in Rheumatoid Arthritis Chondrocytes Using Next-Generation Sequencing and Bioinformatics Approaches

Cartilage destruction in rheumatoid arthritis (RA) occurs primarily in the pannus-cartilage interface. The close contact of the synovium-cartilage interface implicates crosstalk between synovial fibroblasts and chondrocytes. The aim of this study is to explore the differentially expressed genes and novel microRNA regulations potentially implicated in the dysregulated cartilage homeostasis in joint destruction of RA. Total RNAs were extracted from human primary cultured normal and RA chondrocytes for RNA and small RNA expression profiling using next-generation sequencing. Using systematic bioinformatics analyses, we identified 463 differentially expressed genes in RA chondrocytes were enriched in biological functions related to altered cell cycle process, inflammatory response and hypoxic stimulation. Moreover, fibroblast growth factor 9 (FGF9), kynureninase (KYNU), and regulator of cell cycle (RGCC) were among the top dysregulated genes identified to be potentially affected in the RA joint microenvironment, having similar expression patterns observed in arrays of clinical RA synovial tissues from the Gene Expression Omnibus database. Additionally, among the 31 differentially expressed microRNAs and 10 candidate genes with potential microRNA-mRNA interactions in RA chondrocytes, the novel miR-140-3p-FGF9 interaction was validated in different microRNA prediction databases, and proposed to participate in the pathogenesis of joint destruction through dysregulated cell growth in RA. The findings provide new perspectives for target genes in the management of cartilage destruction in RA.

Antagonism of cysteinyl leukotriene receptor 1 (cysLTR1) by montelukast suppresses cell senescence of chondrocytes

Aging is closely associated with osteoarthritis (OA). Although its underlying mechanisms remain unknown, cellular senescence in chondrocytes has become an important therapeutic target for the treatment of OA. Cysteinyl leukotriene receptors (cysLTRs) mediate the pathobiological function of cysteinyl leukotrienes (cysLTs). However, the roles of cysLTRs in the pathogenesis of OA have not been reported before. In the current study, we found that cysLTR1 but not cysLTR2 is expressed in human primary chondrocytes. In addition, stimulation with tumor necrosis factor α (TNF-α) resulted in a significant increase in the expression of cysLTR1. Interestingly, montelukast, a specific cysLTR1 antagonist, attenuated TNF-α-induced up-regulation of the activity of senescence-associated β-galactosidase (SA-β-Gal). In addition, TNF-α led to cell cycle arrest at the G0/G1 phase, which was prevented by treatment with montelukast. Notably, montelukast reduced expression of the senescence markers p53, p21 and PAI-1. In addition, montelukast ameliorated TNF-α-induced K382 acetylation of p53 by promoting the expression of SIRT1. Silencing of SIRT1 using SIRT1 siRNA broke the inhibitory effects of montelukast on K382 acetylation of p53. Importantly, silencing of cysLTR1 reversed the reduction of SIRT1 expression as well as the K382 acetylation of p53. Our findings strongly implicate that cysLTR1 has the capacity to regulate cellular senescence in chondrocytes. It is suggested that montelukast may be a potential therapeutic agent for chondro-protective therapy.

Chondroprotection by urocortin involves blockade of the mechanosensitive ion channel Piezo1

Osteoarthritis (OA) is characterised by progressive destruction of articular cartilage and chondrocyte cell death. Here, we show the expression of the endogenous peptide urocortin1 (Ucn1) and two receptor subtypes, CRF-R1 and CRF-R2, in primary human articular chondrocytes (AC) and demonstrate its role as an autocrine/paracrine pro-survival factor. This effect could only be removed using the CRF-R1 selective antagonist CP-154526, suggesting Ucn1 acts through CRF-R1 when promoting chondrocyte survival. This cell death was characterised by an increase in p53 expression, and cleavage of caspase 9 and 3. Antagonism of CRF-R1 with CP-154526 caused an accumulation of intracellular calcium (Ca2+) over time and cell death. These effects could be prevented with the non-selective cation channel blocker Gadolinium (Gd3+). Therefore, opening of a non-selective cation channel causes cell death and Ucn1 maintains this channel in a closed conformation. This channel was identified to be the mechanosensitive channel Piezo1. We go on to determine that this channel inhibition by Ucn1 is mediated initially by an increase in cyclic adenosine monophosphate (cAMP) and a subsequent inactivation of phospholipase A2 (PLA2), whose metabolites are known to modulate ion channels. Knowledge of these novel pathways may present opportunities for interventions that could abrogate the progression of OA.

Establishment of an in vitro three-dimensional model for cartilage damage in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease that leads to progressive joint destruction. To further understand the process of rheumatoid cartilage damage, an in vitro model consisting of an interactive tri-culture of synovial fibroblasts (SFs), LPS-stimulated macrophages and a primary chondrocyte-based tissue-engineered construct was established. The tissue-engineered construct has a composition similar to that of human cartilage, which is rich in collagen type II and proteoglycans. Data generated from this model revealed that healthy chondrocytes were activated in the presence of SFs and macrophages. The activated chondrocytes subsequently displayed aberrant behaviours as seen in a disease state such as increased apoptosis, decreased gene expression for matrix components such as type II collagen and aggrecan, increased gene expression for tissue-degrading enzymes (MMP-1, -3, -13 and ADAMTS-4, -5), and upregulation of inflammatory mediator gene expression (TNF-α, IL-1β, IL-6 and IKBKB). Additionally, the inclusion of SFs and macrophages in the model enabled both cell types to more closely replicate an in vivo role in mediating cartilage destruction. This is evidenced by extensive matrix loss, detected in the model through immunostaining and biochemical analysis. Subsequent drug treatment with celecoxib has shown that the model was able to respond to the therapeutic effects of this drug by reversing cartilage damage. This study showed that the model was able to recapitulate certain pathological features of an RA cartilage. If properly validated, this model potentially can be used for screening new therapeutic drugs and strategies, thereby contributing to the improvement of anti-rheumatic treatment. Copyright © 2017 John Wiley & Sons, Ltd.

Aberrant methylation patterns affect the molecular pathogenesis of rheumatoid arthritis

This study aims to investigate DNA methylation signatures in fibroblast-like synoviocytes (FLS) from patients with rheumatoid arthritis (RA), and to explore the relationship with transcription factors (TFs) that help to distinguish RA from osteoarthritis (OA). Microarray dataset of GSE46346, including six FLS samples from patients with RA and five FLS samples from patients with OA, was downloaded from the Gene Expression Omnibus database. RA and OA samples were screened for differentially methylated loci (DMLs). The corresponding differentially methylated genes (DMGs) were identified, followed by Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and Gene Ontology (GO) enrichment analysis. A transcriptional regulatory network was built with TFs and their corresponding DMGs. Overall, 280 hypomethylated loci and 561 hypermethylated loci were screened. Genes containing hypermethylated loci were enriched in pathways in cancer, ECM-receptor interaction, focal adhesion and neurotrophin signaling pathways. Genes containing hypomethylated loci were enriched in the neurotrophin signaling pathway. Moreover, we found that CCCTC-binding factor (CTCF), Yin Yang 1 (YY1), v-myc avian myelocytomatosis viral oncogene homolog (c-MYC), and early growth response 1 (EGR1) were important TFs in the transcriptional regulatory network. Therefore, DMGs might participate in the neurotrophin signaling pathway, pathways in cancer, ECM-receptor interaction and focal adhesion pathways in RA. Furthermore, CTCF, c-MYC, YY1, and EGR1 may play important roles in RA through regulating DMGs.

Overexpression of aquaporin 4 in articular chondrocytes exacerbates the severity of adjuvant-induced arthritis in rats: an in vivo and in vitro study

Background

The dysfunction of articular chondrocytes is a crucial step in rheumatoid arthritis (RA) pathogenesis while its molecular mechanisms are not fully known. This study was aimed to investigate the expression of aquaporin 4 (AQP4) in articular chondrocytes of adjuvant-induced arthritis (AIA) rats and its involvement in AIA development.

Methods

Thirty rats were divided into normal and AIA group (n = 15). Rat AIA was induced by intradermal injection of complete Freund’s adjuvant and evaluated by secondary paw swelling and histological assessments on knee joint damage. Localization and protein expression of AQP4 in articular cartilage were examined by immunohistochemistry and western blot. In vitro study, AIA articular chondrocytes were cultured and treated with acetazolamide, an AQPs inhibitor. AQP4 protein level, cell proliferation and mRNA levels of type-II collagen (COII) and aggrecan were measured by western blot, MTT assay and real-time PCR, respectively.

Results

The results of immunohistochemistry and western blot indicated that AQP4 showed higher protein levels in cartilage tissues of AIA rats than that of normal rats. Correlation analysis revealed that AQP4 protein level in cartilage tissues of AIA rats remarkably correlated positively with secondary paw swelling on day 26 after AIA induction as well as pathological scores on joint damage. Additionally, acetazolamide treatment effectively decreased AQP4 protein level, increased cell proliferation and mRNA levels of COII and aggrecan, suggesting AQP4 inhibition by acetazolamide could normalize the dysfunction of AIA articular chondrocytes in vitro.

Conclusions

Our data provide certain experimental evidence that AQP4 over-expression in articular chondrocytes aggravated AIA severity and might be a novel target for RA treatment.

Insights on Molecular Mechanisms of Chondrocytes Death in Osteoarthritis

Osteoarthritis (OA) is a joint pathology characterized by progressive cartilage degradation. Medical care is mainly based on alleviating pain symptoms. Compelling studies report the presence of empty lacunae and hypocellularity in cartilage with aging and OA progression, suggesting that chondrocyte cell death occurs and participates to OA development. However, the relative contribution of apoptosis per se in OA pathogenesis appears complex to evaluate. Indeed, depending on technical approaches, OA stages, cartilage layers, animal models, as well as in vivo or in vitro experiments, the percentage of apoptosis and cell death types can vary. Apoptosis, chondroptosis, necrosis, and autophagic cell death are described in this review. The question of cell death causality in OA progression is also addressed, as well as the molecular pathways leading to cell death in response to the following inducers: Fas, Interleukin-1β (IL-1β), Tumor Necrosis factor-α (TNF-α), leptin, nitric oxide (NO) donors, and mechanical stresses. Furthermore, the protective role of autophagy in chondrocytes is highlighted, as well as its decline during OA progression, enhancing chondrocyte cell death; the transition being mainly controlled by HIF-1α/HIF-2α imbalance. Finally, we have considered whether interfering in chondrocyte apoptosis or promoting autophagy could constitute therapeutic strategies to impede OA progression.

Modulation of Apoptosis and Differentiation by the Treatment of Sulfasalazine in Rabbit Articular Chondrocytes

This study was conducted to examine the cellular regulatory mechanisms of sulfasalazine (SSZ) in rabbit articular chondrocytes treated with sodium nitroprusside (SNP). Cell phenotype was determined, and the MTT assay, Western blot analysis and immunofluorescence staining of type II collagen was performed in control, SNP-treated and SNP plus SSZ (50~200 μg/mL) rabbit articular chondrocytes. Cellular proliferation was decreased significantly in the SNP-treated group compared with that in the control (p < 0.01). SSZ treatment clearly increased the SNP-reduced proliferation levels in a concentration-dependent manner (p < 0.01). SNP treatment induced significant dedifferentiation and inflammation compared with control chondrocytes (p < 0.01). Type II collagen expression levels increased in a concentration-dependent manner in response to SSZ treatment but were unaltered in SNP-treated chondrocytes (p < 0.05 and < 0.01, respectively). Cylooxygenase-2 (COX-2) expression increased in a concentration-dependent manner in response to SSZ treatment but was unaltered in SNP-treated chondrocytes (p < 0.05). Immunofluorescence staining showed that SSZ treatment increased type II collagen expression compared with that in SNP-treated chondrocytes. Furthermore, phosphorylated extracellular regulated kinase (pERK) expression levels were decreased significantly in the SNP-treated group compared with those in control chondrocytes (p < 0.01). Expression levels of pERK increased in a concentration-dependent manner by SSZ but were unaltered in SNP-treated chondrocytes. pp38 kinase expression levels increased in a concentration-dependent manner by SSZ but were unaltered in control chondrocytes (p < 0.01). In summary, SSZ significantly inhibited nitric oxide-induced cell death and dedifferentiation, and regulated extracellular regulated kinases 1 and 2 and p38 kinase in rabbit articular chondrocytes.

Biomarkers of Chondrocyte Apoptosis and Autophagy in Osteoarthritis

Cell death with morphological and molecular features of apoptosis has been detected in osteoarthritic (OA) cartilage, which suggests a key role for chondrocyte death/survival in the pathogenesis of OA. Identification of biomarkers of chondrocyte apoptosis may facilitate the development of novel therapies that may eliminate the cause or, at least, slow down the degenerative processes in OA. The aim of this review was to explore the molecular markers and signals that induce chondrocyte apoptosis in OA. A literature search was conducted in PubMed, Scopus, Web of Science and Google Scholar using the keywords chondrocyte death, apoptosis, osteoarthritis, autophagy and biomarker. Several molecules considered to be markers of chondrocyte apoptosis will be discussed in this brief review. Molecular markers and signalling pathways associated with chondroycte apoptosis may turn out to be therapeutic targets in OA and approaches aimed at neutralizing apoptosis-inducing molecules may at least delay the progression of cartilage degeneration in OA.

Inhibition of hedgehog signal pathway by cyclopamine attenuates inflammation and articular cartilage damage in rats with adjuvant-induced arthritis

OBJECTIVES

We investigated whether inhibition of hedgehog (Hh) signal by cyclopamine attenuated inflammation and cartilage damage in adjuvant-induced arthritis (AIA) rats.

METHODS

Cyclopamine (2.5, 5, 10 mg/kg) was given by intraperitoneal injection once daily from day 12 to 21 after AIA induction. Paw swelling (volume changes), serum pro-inflammatory cytokines levels (ELISA), histological analysis of joint damage (H&E staining), proteoglycans expression (Alcian blue staining), mRNA levels of sonic Hh (Shh), glioma-associated oncogene homologue 1 (Gli1), type II collagen (COII) and aggrecan in cartilage (real-time PCR) and articular chondrocyte apoptosis (terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling) were measured respectively.

KEY FINDINGS

Cyclopamine effectively attenuated inflammation and cartilage damage of AIA rats, as evidenced by reduced paw swelling, serum levels of tumor necrosis factors (TNF)-α, IL-1β, IL-6 and histological scores of joint damage, increased proteoglycans expression and mRNA levels of COII and aggrecan in articular cartilage. Shh or Gli1 mRNA level was correlated negatively with COII and aggrecan mRNA levels, suggesting Hh signal inhibition was associated with promotion of cartilage extracellular matrix production. Furthermore, cyclopamine decreased the number of apoptotic articular chondrocytes of AIA rats, which might be partly related to its mechanisms on relieving cartilage damage.

CONCLUSIONS

Our findings present some experimental evidence that Hh signal inhibition might be of potential clinical interest in rheumatoid arthritis treatment.

Endogenous cannabinoid anandamide impairs cell growth and induces apoptosis in chondrocytes

Endocannabinoids has been described to be involved in articular degenerative disease by modulating nociception and immune system. However, the role of the endocannabinoid anandamide on chondrocyte cell viability is still unclear. Therefore, we decided to study anandamide's effects on chondrocytes viability and to evaluate its interactions with the catabolic factor TNF (tumor necrosis factor). Chondrocyte vitality was evaluated by MTT assay. We investigated LDH release, chromatin condensation, cleavage of focal adhesion kinase (FAK), and caspases-3, 8, and 9 activation. c-MYC mRNA levels were determined by RT-PCR. We studied by Western blot the activation patterns of AKT, AMPK, ERK, p38, and JNK kinases. Finally, we evaluate the effect of anandamide in TNF-induced caspase-3 cleavage. Anandamide decreased chondrocyte vitality independently of its receptors. It induced AMPK activation without LDH release. Anandamide induced chromatin condensation, activation of caspase-3, 8, and 9, and FAK cleavage. Surprisingly, despite anandamide inhibited cell proliferation, it increased c-MYC expression. Moreover anandamide inhibited AKT activation, whilst it induced a sustained activation of ERK, JNK, and p38. Finally, anandamide synergized with TNF-α in the cleavage of caspase-3. In conclusion, our findings suggest that anandamide, alone or in combination with TNF-α, may be a potential destructive agent in cartilage.

Indian hedgehog in synovial fluid is a novel marker for early cartilage lesions in human knee joint

To determine whether there is a correlation between the concentration of Indian hedgehog (Ihh) in synovial fluid (SF) and the severity of cartilage damage in the human knee joints, the knee cartilages from patients were classified using the Outer-bridge scoring system and graded using the Modified Mankin score. Expression of Ihh in cartilage and SF samples were analyzed with immunohistochemistry (IHC), western blot, and enzyme-linked immunosorbent assay (ELISA). Furthermore, we detected and compared Ihh protein levels in rat and mice cartilages between normal control and surgery-induced osteoarthritis (OA) group by IHC and fluorescence molecular tomography in vivo respectively. Ihh expression was increased 5.2-fold in OA cartilage, 3.1-fold in relative normal OA cartilage, and 1.71-fold in OA SF compared to normal control samples. The concentrations of Ihh in cartilage and SF samples was significantly increased in early-stage OA samples when compared to normal samples (r = 0.556; p < 0.001); however, there were no significant differences between normal samples and late-stage OA samples. Up-regulation of Ihh protein was also an early event in the surgery-induced OA models. Increased Ihh is associated with the severity of OA cartilage damage. Elevated Ihh content in human knee joint synovial fluid correlates with early cartilage lesions.

Prolactin promotes cartilage survival and attenuates inflammation in inflammatory arthritis

Chondrocytes are the only cells in cartilage, and their death by apoptosis contributes to cartilage loss in inflammatory joint diseases, such as rheumatoid arthritis (RA). A putative therapeutic intervention for RA is the inhibition of apoptosis-mediated cartilage degradation. The hormone prolactin (PRL) frequently increases in the circulation of patients with RA, but the role of hyperprolactinemia in disease activity is unclear. Here, we demonstrate that PRL inhibits the apoptosis of cultured chondrocytes in response to a mixture of proinflammatory cytokines (TNF-α, IL-1β, and IFN-γ) by preventing the induction of p53 and decreasing the BAX/BCL-2 ratio through a NO-independent, JAK2/STAT3-dependent pathway. Local treatment with PRL or increasing PRL circulating levels also prevented chondrocyte apoptosis evoked by injecting cytokines into the knee joints of rats, whereas the proapoptotic effect of cytokines was enhanced in PRL receptor-null (Prlr(-/-)) mice. Moreover, eliciting hyperprolactinemia in rats before or after inducing the adjuvant model of inflammatory arthritis reduced chondrocyte apoptosis, proinflammatory cytokine expression, pannus formation, bone erosion, joint swelling, and pain. These results reveal the protective effect of PRL against inflammation-induced chondrocyte apoptosis and the therapeutic potential of hyperprolactinemia to reduce permanent joint damage and inflammation in RA.

Increased chondrocyte apoptosis is associated with progression of osteoarthritis in spontaneous Guinea pig models of the disease

Osteoarthritis (OA) is the most common joint disease characterised by degradation of articular cartilage and bone remodelling. For almost a decade chondrocyte apoptosis has been investigated as a possible mechanism of cartilage damage in OA, but its precise role in initiation and/or progression of OA remains to the determined. The aim of this study is to determine the role of chondrocyte apoptosis in spontaneous animal models of OA. Right tibias from six male Dunkin Hartley (DH) and Bristol Strain 2 (BS2) guinea pigs were collected at 10, 16, 24 and 30 weeks of age. Fresh-frozen sections of tibial epiphysis were microscopically scored for OA, and immunostained with caspase-3 and TUNEL for apoptotic chondrocytes. The DH strain had more pronounced cartilage damage than BS2, especially at 30 weeks. At this time point, the apoptotic chondrocytes were largely confined to the deep zone of articular cartilage (AC) with a greater percentage in the medial side of DH than BS2 (DH: 5.7%, 95% CI: 4.2–7.2), BS2: 4.8%, 95% CI: 3.8–5.8), p > 0.05). DH had a significant progression of chondrocyte death between 24 to 30 weeks during which time significant changes were observed in AC fibrillation, proteoglycan depletion and overall microscopic OA score. A strong correlation (p ≤ 0.01) was found between chondrocyte apoptosis and AC fibrillation (r = 0.3), cellularity (r = 0.4) and overall microscopic OA scores (r = 0.4). Overall, the rate of progression in OA and apoptosis over the study period was greater in the DH (versus BS2) and the medial AC (versus lateral). Chondrocyte apoptosis was higher at the later stage of OA development when the cartilage matrix was hypocellular and highly fibrillated, suggesting that chondrocyte apoptosis is a late event in OA.

Differences in Mammalian target of rapamycin gene expression in the peripheral blood and articular cartilages of osteoarthritic patients and disease activity

The gene expression of mTOR, autophagy-related ULK1, caspase 3, CDK-inhibitor p21, and TNF α was measured in the peripheral blood of osteoarthritic (OA) patients at different stages of the disease aiming to establish a gene expression profile that might indicate the activity of the disease and joint destruction. Whole blood of 65 OA outpatients, 27 end-stage OA patients, 27 healthy volunteers, and knee articular cartilages of 28 end-stage OA patients and 26 healthy subjects were examined. OA outpatients were subjected to clinical testing, ultrasonography, and radiographic and WOMAC scoring. Protein levels of p70-S6K, p21, and caspase 3 were quantified by ELISA. Gene expression was measured using real-time RT-PCR. Upregulation of mTOR gene expression was observed in PBMCs of 42 OA outpatients ("High mTOR expression subset") and in PBMCs and articular cartilages of all end-stage OA patients. A positive correlation between mTOR gene expression in PBMCs and cartilage was observed in the end-stage OA patients. 23 OA outpatients in the "Low mTOR expression subset" exhibited significantly lower mTOR gene expression in PBMCs compared to healthy controls. These "Low mTOR" subset subjects experienced significantly more pain upon walking, and standing and increased total joint stiffness versus "High mTOR" subset, while the latter more often exhibited synovitis. The protein concentrations of p70-S6K, p21, and caspase 3 in PBMCs were significantly lower in the "Low" subset versus "High" subset and end-stage subjects. Increases in the expression of mTOR in PBMCs of OA patients are related to disease activity, being associated with synovitis more than with pain.

The identification of CD163 expressing phagocytic chondrocytes in joint cartilage and its novel scavenger role in cartilage degradation

Background

Cartilage degradation is a typical characteristic of arthritis. This study examined whether there was a subset of phagocytic chondrocytes that expressed the specific macrophage marker, CD163, and investigated their role in cartilage degradation.

Methods

Cartilage from the knee and temporomandibular joints of Sprague-Dawley rats was harvested. Cartilage degradation was experimentally-induced in rat temporomandibular joints, using published biomechanical dental methods. The expression levels of CD163 and inflammatory factors within cartilage, and the ability of CD163⁺ chondrocytes to conduct phagocytosis were investigated. Cartilage from the knees of patients with osteoarthritis and normal cartilage from knee amputations was also investigated.

Results

In the experimentally-induced degrading cartilage from temporomandibular joints, phagocytes were capable of engulfing neighboring apoptotic and necrotic cells, and the levels of CD163, TNF-α and MMPs were all increased (P<0.05). However, the levels of ACP-1, NO and ROS, which relate to cellular digestion capability were unchanged (P>0.05). CD163⁺ chondrocytes were found in the cartilage mid-zone of temporomandibular joints and knee from healthy, three-week old rats. Furthermore, an increased number of CD163⁺ chondrocytes with enhanced phagocytic activity were present in Col-II⁺ chondrocytes isolated from the degraded cartilage of temporomandibular joints in the eight-week experimental group compared with their age-matched controls. Increased number with enhanced phagocytic activity of CD163⁺ chondrocytes were also found in isolated Col-II⁺ chondrocytes stimulated with TNF-α (P<0.05). Mid-zone distribution of CD163⁺ cells accompanied with increased expression of CD163 and TNF-α were further confirmed in the isolated Col-II⁺ chondrocytes from the knee cartilage of human patients with osteoarthritis, in contrast to the controls (both P<0.05).

Conclusions

An increased number of CD163⁺ chondrocytes with enhanced phagocytic activity were discovered within degraded joint cartilage, indicating a role in eliminating degraded tissues. Targeting these cells provides a new strategy for the treatment of arthritis.

Inducible hydrogen sulfide synthesis in chondrocytes and mesenchymal progenitor cells: is H2S a novel cytoprotective mediator in the inflamed joint?

Hydrogen sulfide (H₂S) has recently been proposed as an endogenous mediator of inflammation and is present in human synovial fluid. This study determined whether primary human articular chondrocytes (HACs) and mesenchymal progenitor cells (MPCs) could synthesize H₂S in response to pro-inflammatory cytokines relevant to human arthropathies, and to determine the cellular responses to endogenous and pharmacological H₂S. HACs and MPCs were exposed to IL-1β, IL-6, TNF-α and lipopolysaccharide (LPS). The expression and enzymatic activity of the H₂S synthesizing enzymes cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CSE) were determined by Western blot and zinc-trap spectrophotometry, respectively. Cellular oxidative stress was induced by H₂O₂, the peroxynitrite donor SIN-1 and 4-hydroxynonenal (4-HNE). Cell death was assessed by 3-(4,5-dimethyl-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and lactate dehydrogenase (LDH) assays. Mitochondrial membrane potential (DCm) was determined in situ by flow cytometry. Endogenous H₂S synthesis was inhibited by siRNA-mediated knockdown of CSE and CBS and pharmacological inhibitors D,L-propargylglycine and aminoxyacetate, respectively. Exogenous H₂S was generated using GYY4137. Under basal conditions HACs and MPCs expressed CBS and CSE and synthesized H₂S in a CBS-dependent manner, whereas CSE expression and activity was induced by treatment of cells with IL-1β, TNF-α, IL-6 or LPS. Oxidative stress-induced cell death was significantly inhibited by GYY4137 treatment but increased by pharmacological inhibition of H₂S synthesis or by CBS/CSE-siRNA treatment. These data suggest CSE is an inducible source of H₂S in cultured HACs and MPCs. H₂S may represent a novel endogenous mechanism of cytoprotection in the inflamed joint, suggesting a potential opportunity for therapeutic intervention.

Identification of the pathogenic pathways in osteoarthritic hip cartilage: commonality and discord between hip and knee OA

OBJECTIVE

To define for the first time the transcriptomes of normal and end-stage osteoarthritis (OA) hip cartilage.

MATERIALS AND METHODS

RNA was isolated from cartilage within 2h of joint replacement surgery. Gene expression was analyzed using Agilent GeneSpring GX 11 following hybridization to Illumina Human HT-12 V3 microarrays. Real-time reverse-transcription polymerase chain reaction (RT-PCR) was used to validate the expression of six genes identified by microarray as differentially expressed. Gene Set Enrichment Analysis (GSEA) and Ingenuity Pathway Analysis (IPA) were used to investigate enriched functions or canonical pathways amongst differentially expressed genes respectively.

RESULTS

In total we identified 998 differentially expressed genes (fold change ≥ ±1.5, P-value ≤ 0.01) between neck of femur fracture (NOF) (n = 10) and OA hip (n = 9) patient cartilage. These differentially expressed genes were enriched within 71 canonical pathways. A comparison between a comparable knee dataset(20) only identified 229 genes similarly differentially expressed although remarkably 34 canonical pathways overlapped between experiments.

CONCLUSIONS

This study is the first to report a comprehensive gene expression analysis of human hip OA cartilage compared to control (NOF) cartilage at the whole-genome level. Our differential gene expression dataset shows excellent correlation with similar defined studies using comparable tissue but reveals discord between hip and knee OA at the individual gene status but with commonality with regards the molecular pathways involved.

75-kd sirtuin 1 blocks tumor necrosis factor α-mediated apoptosis in human osteoarthritic chondrocytes

OBJECTIVE

Sirtuin 1 (SirT1) has been implicated in the regulation of human cartilage homeostasis and chondrocyte survival. Exposing human osteoarthritic (OA) chondrocytes to tumor necrosis factor α (TNFα) generates a stable and enzymatically inactive 75-kd form of SirT1 (75SirT1) via cathepsin B-mediated cleavage. Because 75SirT1 is resistant to further degradation, we hypothesized that it has a distinct role in OA, and the present study was undertaken to identify this role.

METHODS

The presence of cathepsin B and 75SirT in OA and normal human chondrocytes was analyzed. Confocal imaging of SirT1 was used to monitor its subcellular trafficking following TNFα stimulation. Coimmunofluorescence staining for cathepsin B, mitochondrial cytochrome oxidase subunit IV, and lysosome-associated membrane protein 1 together with SirT1 was performed. Human chondrocytes were tested for apoptosis by fluorescence-activated cell sorter analysis and immunoblotting for caspases 3 and 8. Human chondrocyte mitochondrial extracts were obtained and analyzed for 75SirT1-cytochrome c association.

RESULTS

Confocal imaging and immunoblot analyses following TNFα challenge of human chondrocytes demonstrated that 75SirT1 was exported to the cytoplasm and colocalized with the mitochondrial membrane. Consistent with this, immunoprecipitation and immunoblot analyses revealed that 75SirT1 is enriched in mitochondrial extracts and associates with cytochrome c following TNFα stimulation. Preventing nuclear export of 75SirT1 or reducing levels of full-length SirT1 and 75SirT1 augmented chondrocyte apoptosis in the presence of TNFα. Levels of cathepsin B and 75SirT1 were elevated in OA versus normal chondrocytes. Additional analyses showed that human chondrocytes exposed to OA-derived synovial fluid generated the 75SirT1 fragment.

CONCLUSION

These data suggest that 75SirT1 promotes chondrocyte survival following exposure to proinflammatory cytokines.

Selective COX-2 inhibitor ameliorates osteoarthritis by repressing apoptosis of chondrocyte

BACKGROUND

Celecoxib has a positive effect on human osteoarthritic cartilage, but the mechanisms remain unclear. The aim of this study was to test whether celecoxib could inhibit the apoptosis of chondrocyte and ameliorate type II collagen synthesis to relieve symptoms of OA (osteoarthritis).

MATERIAL/METHODS

130 Wistar rats were randomly divided into 4 groups as celecoxib (CE), ibuprofen (IBP), indomethacin (IN) and normal saline group (NS). The osteoarthritis was induced by the excision of the left Achilles tendon. At the 3th, 6th, 9th month of treatment, the histological structure of articular cartilage was observed using HE staining. Type II collagen was examined using immunohistochemistry. Chondrocyte apoptosis was detected by TUNEL staining, and the change of ultra-microstructure of chondrocyte was examined through a transmission electron microscope.

RESULTS

CE reduced the OA-like histological changes and suppressed chondrocyte apoptosis. However, IN or IBP had deleterious effects on articular cartilage and enhanced the chondrocyte apoptosis. IBP promoted the expression of type II collagen, and IN inhibited its expression, but had no effect in the CE group.

CONCLUSIONS

CE had favorable action on OA progression, and may be the ideal choice in the treatment of chronic destructive joint disease where anti-inflammatory drugs need to be used for a prolonged period.

Increased apoptotic chondrocytes in articular cartilage from adult heterozygous SirT1 mice

OBJECTIVE

A growing body of evidence indicates that the protein deacetylase, SirT1, affects chondrocyte biology and survival. This report aims to evaluate in vivo attributes of SirT1 in cartilage biology of 129/J murine strains.

METHODS

Heterozygous haploinsufficient (SirT1(+/-)) and wild-type (WT; SirT1(+/+)) 129/J mice aged 1 or 9 months were systematically compared for musculoskeletal features, scored for osteoarthritis (OA) severity, and monitored for chondrocyte apoptosis in articular cartilage. Sections of femorotibial joints were stained for type II collagen and aggrecan. Protein extracts from articular chondrocytes were isolated and immunoblotted for SirT1 and active caspase 3.

RESULTS

Phenotypic observations show that, at 1 month of age, SirT1(+/-) mice were smaller than WT and showed a significant decrease in full-length SirT1 (FLSirT1; 110 kDa) protein levels. Levels of FLSirT1 were further decreased in both strains at 9 months. Immunoblot assays for 9-month-old strains revealed the presence of the inactive cleaved SirT1 variant (75 SirT1; 75 kDa) in WT mice, which was undetected in age-matched SirT1(+/-) mice. Nine-month-old SirT1(+/-) mice also showed increased OA and increased levels of apoptosis compared with age-matched WT mice.

CONCLUSION

The data suggest that the presence of 75 SirT1 may prolong viability of articular chondrocytes in adult (9-month-old) mice.

Cilostazol induces cellular senescence and confers resistance to etoposide-induced apoptosis in articular chondrocytes

We recently reported that cilostazol protects chondrocytes against stress-induced apoptosis and prevents cartilage destruction in an osteoarthritis (OA) model. In the present study, we elucidate the mechanism underlying the protective effect induced by cilostazol against stress-induced apoptosis in chondrocytes. Cilostazol significantly reduced the expression of type II collagen and stimulated the accumulation of β-catenin in primary rat articular chondrocytes. Moreover, cilostazol-induced chondrocytes showed induction of senescent phenotypes, such as changes in cell morphology, decrease in cell proliferation and increase in specific senescence-associated β-galactosidase (SA-β-gal) staining. Moreover, dedifferentiated chondrocytes obtained by serial subculture showed cellular senescence that increased with passage number. In addition, the percentage of terminal dUTP nick end-labeling (TUNEL)-positive cells was higher when chondrocytes were treated with cilostazol and the apoptosis inducer etoposide than when the cells were treated with etoposide alone. Our findings suggest that cilostazol induces dedifferentiation and senescence in rat articular chondrocytes and renders them resistant to etoposide-induced apoptosis.

Osteopontin Deficiency Suppresses Tumor Necrosis Factor-α-Induced Apoptosis in Chondrocytes

OBJECTIVE

Apoptosis of chondrocytes in articular cartilage has been observed in rheumatoid arthritis patients. However, molecules involved in such chondrocyte apoptosis in arthritic joints have not been fully understood. We previously observed that apoptosis of chondrocytes is enhanced in a murine arthritis model induced by injection with anti-type II collagen antibodies and lipopolysaccharide (mAbs/LPS), and osteopontin (OPN) deficiency suppresses chondrocyte apoptosis in this arthritis model in vivo. To understand how OPN deficiency renders resistance against chondrocyte apoptosis, we examined the cellular basis for this protection.

DESIGN

Chondrocytes were prepared from wild-type and OPN-deficient mouse ribs, and tumor necrosis factor (TNF)-α-induced cell death was examined based on lactate dehydrogenase (LDH) release assay and TUNEL assay.

RESULTS

TNF-α treatment induced LDH release in wild-type chondrocytes, while OPN deficiency suppressed such LDH release in the cultures of these cells. TNF-α-induced increase in the number of TUNEL-positive cells was observed in wild-type chondrocytes, while OPN deficiency in chondrocytes suppressed the TNF-α induction of TUNEL-positive cells. OPN deficiency suppressed TNF-α-induced increase in caspase-3 activity in chondrocytes in culture. Furthermore, OPN overexpression in chondrocytes enhanced TNF-α-induced apoptosis.

CONCLUSION

These results indicated that the presence of OPN in chondrocytes is involved in the susceptibility of these cells to TNF-α-induced apoptosis.

Chondrocyte apoptosis: a cause or consequence of osteoarthritis?

Osteoarthritis (OA) is a degenerative joint disease characterized by articular cartilage degradation and changes in the subchondral bone. Over the last two decades, there has been increasing evidence showing association between cartilage degradation and chondrocyte death, and different types of cell death in cartilage have been reported, including apoptosis and chondroptosis as well as necrosis, but which of these types of cell death predominate in OA is debatable. There are also some methodological difficulties in detecting the specific form of cell death in articular cartilage. Current 'gold standard' for detecting chondrocyte death is electron microscopy which suggests that the morphological changes of chondrocytes in OA cartilage are attributed to apoptosis and/or chondroptosis. However, the current literature appears to suggest that classic apoptosis plays an important role in OA; but whether chondrocyte apoptosis is a cause or a result of cartilage degeneration in OA is hotly contested. Studies of suitable animal models, especially longitudinal studies, are needed to address the cause-and-effect relationship.

Developmental mechanisms in articular cartilage degradation in osteoarthritis

Osteoarthritis is the most common arthritic condition, which involves progressive degeneration of articular cartilage. The most recent accomplishments have significantly advanced our understanding on the mechanisms of the disease development and progression. The most intriguing is the growing evidence indicating that extracellular matrix destruction in osteoarthritic articular cartilage resembles that in the hypertrophic zone of fetal growth plate during endochondral ossification. This suggests common regulatory mechanisms of matrix degradation in OA and in the development and can provide new approaches for the treatment of the disease by targeting reparation of chondrocyte phenotype.

Prolonged application of high fluid shear to chondrocytes recapitulates gene expression profiles associated with osteoarthritis

Background

Excessive mechanical loading of articular cartilage producing hydrostatic stress, tensile strain and fluid flow leads to irreversible cartilage erosion and osteoarthritic (OA) disease. Since application of high fluid shear to chondrocytes recapitulates some of the earmarks of OA, we aimed to screen the gene expression profiles of shear-activated chondrocytes and assess potential similarities with OA chondrocytes.

Methodology/Principal Findings

Using a cDNA microarray technology, we screened the differentially-regulated genes in human T/C-28a2 chondrocytes subjected to high fluid shear (20 dyn/cm²) for 48 h and 72 h relative to static controls. Confirmation of the expression patterns of select genes was obtained by qRT-PCR. Using significance analysis of microarrays with a 5% false discovery rate, 71 and 60 non-redundant transcripts were identified to be ≥2-fold up-regulated and ≤0.6-fold down-regulated, respectively, in sheared chondrocytes. Published data sets indicate that 42 of these genes, which are related to extracellular matrix/degradation, cell proliferation/differentiation, inflammation and cell survival/death, are differentially-regulated in OA chondrocytes. In view of the pivotal role of cyclooxygenase-2 (COX-2) in the pathogenesis and/or progression of OA in vivo and regulation of shear-induced inflammation and apoptosis in vitro, we identified a collection of genes that are either up- or down-regulated by shear-induced COX-2. COX-2 and L-prostaglandin D synthase (L-PGDS) induce reactive oxygen species production, and negatively regulate genes of the histone and cell cycle families, which may play a critical role in chondrocyte death.

Conclusions/Significance

Prolonged application of high fluid shear stress to chondrocytes recapitulates gene expression profiles associated with osteoarthritis. Our data suggest a potential link between exposure of chondrocytes/cartilage to abnormal mechanical loading and the pathogenesis/progression of OA.

Emerging roles of SUMO modification in arthritis

Dynamic modification involving small ubiquitin-like modifier (SUMO) has emerged as a new mechanism of protein regulation in mammalian biology. Sumoylation is an ATP-dependent, reversible post-translational modification which occurs under both basal and stressful cellular conditions. Sumoylation profoundly influences protein functions and pertinent biological processes. For example, sumoylation modulates multiple components in the NFkappaB pathway and exerts an anti-inflammatory effect. Likewise, sumoylation of peroxisome proliferator-activated receptor gamma (PPARgamma) augments its anti-inflammatory activity. Current evidence suggests a role of sumoylation for resistance to apoptosis in synovial fibroblasts. Dynamic SUMO regulation controls the biological outcomes initiated by various growth factors involved in cartilage homeostasis, including basic fibroblast growth factors (bFGF or FGF-2), transforming growth factor-beta (TGF-beta) and insulin-like growth factor-1 (IGF-1). The impact of these growth factors on cartilage are through sumoylation-dependent control of the transcription factors (e.g., Smad, Elk-1, HIF-1) that are key regulators of matrix components (e.g., aggrecan, collagen) or cartilage-degrading enzymes (e.g., MMPs, aggrecanases). Thus, SUMO modification appears to profoundly affect chondrocyte and synovial fibroblast biology, including cell survival, inflammatory responses, matrix metabolism and hypoxic responses. More recently, evidence suggests that, in addition to their nuclear roles, the SUMO pathways play crucial roles in mitochondrial activity, cellular senescence, and autophagy. With an increasing number of reports linking SUMO to human diseases like arthritis, it is probable that novel and equally important functions of the sumoylation pathway will be elucidated in the near future.

SirT1 enhances survival of human osteoarthritic chondrocytes by repressing protein tyrosine phosphatase 1B and activating the insulin-like growth factor receptor pathway

OBJECTIVE

The protein deacetylase SirT1 inhibits apoptosis in a variety of cell systems by distinct mechanisms, yet its role in chondrocyte death has not been explored. We undertook the present study to assess the role of SirT1 in the survival of osteoarthritic (OA) chondrocytes in humans.

METHODS

SirT1, protein tyrosine phosphatase 1B (PTP1B), and PTP1B mutant expression plasmids as well as SirT1 small interfering RNA (siRNA) and PTP1B siRNA were transfected into primary human chondrocytes. Levels of apoptosis were determined using flow cytometry, and activation of components of the insulin-like growth factor receptor (IGFR)/Akt pathway was assessed using immunoblotting. OA and normal knee cartilage samples were subjected to immunohistochemical analysis.

RESULTS

Expression of SirT1 in chondrocytes led to increased chondrocyte survival in either the presence or the absence of tumor necrosis factor alpha/actinomycin D, while a reduction of SirT1 by siRNA led to increased chondrocyte apoptosis. Expression of SirT1 in chondrocytes led to activation of IGFR and the downstream kinases phosphatidylinositol 3-kinase, phosphoinosite-dependent protein kinase 1, mTOR, and Akt, which in turn phosphorylated MDM2, inhibited p53, and blocked apoptosis. Activation of IGFR occurs at least in part via SirT1-mediated repression of PTP1B. Expression of PTP1B in chondrocytes increased apoptosis and reduced IGFR phosphorylation, while down-regulation of PTP1B by siRNA significantly decreased apoptosis. Examination of cartilage from normal donors and OA patients revealed that PTP1B levels are elevated in OA cartilage in which SirT1 levels are decreased.

CONCLUSION

For the first time, it has been demonstrated that SirT1 is a mediator of human chondrocyte survival via down-regulation of PTP1B, a potent proapoptotic protein that is elevated in OA cartilage.

Joint aging and chondrocyte cell death

Articular cartilage extracellular matrix and cell function change with age and are considered to be the most important factors in the development and progression of osteoarthritis. The multifaceted nature of joint disease indicates that the contribution of cell death can be an important factor at early and late stages of osteoarthritis. Therefore, the pharmacologic inhibition of cell death is likely to be clinically valuable at any stage of the disease. In this article, we will discuss the close association between diverse changes in cartilage aging, how altered conditions influence chondrocyte death, and the implications of preventing cell loss to retard osteoarthritis progression and preserve tissue homeostasis.

Over-expression of c-maf by chondrocytes in osteoarthritis

The c-maf gene expression profile was investigated in normal and osteoarthritic articular cartilage using in situ hybridization, qualitative reverse transcription-polymerase chain reaction (RT-PCR) and quantitative real-time RT-PCR. Osteoarthritic samples were obtained from 10 patients undergoing total knee replacement for severe osteoarthritis of the knee joints, and control samples from 10 trauma patients undergoing amputation. Expression of c-maf was significantly up-regulated in osteoarthritic cartilage compared with normal cartilage. Using in situ hybridization, distribution of a specific c-maf mRNA signal was found in the top zone and a decreased signal was found in the lower middle zone and the deep zone in osteoarthritic cartilage. A prominent c-maf mRNA signal was seen particularly in proliferating 'chondrocyte clusters'. In contrast, in normal cartilage almost no c-maf-positive cells were found. These findings suggest that c-maf may be important in chondrocyte hypertrophy and terminal differentiation, and may be involved in the pathogenesis of osteoarthritis.

Characterization of pro-apoptotic and matrix-degradative gene expression following induction of osteoarthritis in mature and aged rabbits

OBJECTIVE

The genetic and molecular changes leading to the distinctive alterations of aged cartilage and its propensity for developing osteoarthritis (OA) are unknown. We hypothesized that pro-apoptotic and matrix-degradative gene expression in a rabbit model of induced OA using mature and aged animals might elucidate this relationship.

METHODS

Groups of six mature and aged rabbits underwent anterior cruciate ligament transection (ACLT) and were sacrificed 4 weeks after surgery to create an Outerbridge grade II OA. RNA was extracted from the articular cartilage and menisci of the affected knee and was examined with regard to expression of the following genes: Caspase 8, Fas, Fas ligand (Fas-L), p53, aggrecanase, matrix metalloproteinase (MMP)-1, and MMP-3-MMP-13. A second cohort of mature and aged animals was sacrificed with no intervention to the joint and gene expression was assessed in a similar manner.

RESULTS

Fas and Caspase 8 showed significantly increased expression in the cartilage of mature animals with induced OA when compared to unoperated controls while induction of OA in aged rabbits did not significantly increase expression of any of the apoptosis genes. Among unoperated animals, the aged cohort showed significantly increased expression of MMP-1 and aggrecanase in cartilage when compared to mature animals. MMP-13 expression was upregulated in aged cartilage following induction of OA. Although ACLT animals showed gross thinning and irregularities within the meniscus, only the expression of Caspase 8 in the aged rabbits was significantly increased after induction of OA.

CONCLUSIONS

Aging of articular cartilage shares some qualities with the development of OA, as seen in the parallel increases in gene expression of Caspase 8 and Fas. Although this may imply a common mechanism of cartilage degeneration in aging and OA or even a spectrum of disease, both are complex processes requiring further study.

Characterization of mature vs aged rabbit articular cartilage: analysis of cell density, apoptosis-related gene expression and mechanisms controlling chondrocyte apoptosis

OBJECTIVE

The prevalence of osteoarthritis (OA) is increased in aged individuals and a direct correlation between chondrocyte apoptosis and cartilage degradation secondary to OA has been demonstrated. To address the question of whether age predisposes articular cartilage to apoptosis, the objective of the present study was to characterize and compare in aged and mature non-OA rabbit articular cartilage, cell density and expression levels of specific genes associated with apoptosis. Mechanistic studies on the inhibition of induced apoptosis were also carried out.

METHODS

Grade I (non-OA) femoral condyles and tibial plateaus from mature and aged rabbits were taken for assessment of viable cell density (VCD) and mRNA (reverse transcription-polymerase chain reaction) expression levels of the pro-apoptotic genes, Fas, Fas ligand (FasL), caspase-8, inducible nitric oxide synthase (iNOS) and p53. In vitro insulin-like growth factor (IGF-1)-mediated inhibition of nitric oxide (NO)-induced apoptosis was also examined using sodium nitroprusside (SNP) as NO donor.

RESULTS

VCD was decreased 50-70% in aged articular cartilage relative to mature cartilage. mRNA expression levels of Fas, FasL, caspase-8 and p53 were higher in aged cartilage than in mature cartilage. iNOS expression was unchanged. IGF-1-mediated inhibition of NO-induced apoptosis was dose-dependent and reversed with addition of phosphatidylinositol-3 kinase inhibitor.

CONCLUSIONS

This controlled animal model study demonstrates that age predisposes articular cartilage to changes in VCD and expression levels of specific pro-apoptotic genes. It is significant that these findings were demonstrated on cartilage that showed no prior signs of OA; it is also possible that such changes are a prelude to the age-related development of OA.

Osteoarthritis and osteoporosis in postmenopausal women: clinical similarities and differences

Osteoarthritis and osteoporosis are two major health problems affecting more than 60% of post-menopausal women in the United States. The promotion of healthy aging and the prevention and reduction of morbidity and mortality is a main concern for healthcare providers. The similarities and differences in pathophysiology, diagnosis, and treatment for osteoarthritis and osteoporosis are often not clear for clinicians. Some osteoporosis treatments, including bisphosphonates and vitamin D, seem to have a beneficial effect on osteoarthritis as well. A review of these two conditions in terms of bone mineral density, bone turnover, hormonal effects, and treatment options will be discussed.

Peroxynitrite mediates calcium-dependent mitochondrial dysfunction and cell death via activation of calpains

Chondrocyte cell death is a hallmark of inflammatory and degenerative joint diseases such as rheumatoid arthritis (RA) and osteoarthritis (OA), but the molecular and cellular mechanisms involved have yet to be elucidated. Because 3-nitrotyrosine, a marker for reactive nitrogen species such as peroxynitrite, has been observed in OA and RA cartilage and has been associated with chondrocyte cell death, we investigated the mechanisms by which peroxynitrite induces cell death in human articular chondrocytes. The earliest biochemical event observed, subsequent to treatment with either peroxynitrite or the peroxynitrite generator SIN-1, was a rapid rise in intracellular calcium that lead to mitochondrial dysfunction and cell death. Although, chondrocyte death exhibited several classical hallmarks of apoptosis, including annexin V labeling, increased fraction of cells with subG1 DNA content and DNA condensation, we did not find evidence for caspase involvement either by Western blotting, fluorimetric assays, or caspase inhibition. Additionally, peroxynitrite did not inhibit cellular caspase activity. Furthermore, using other established assays of cell viability, including the MTT assay and release of lactate dehydrogenase, we found that the predominant mode of cell death involved calcium-dependent cysteine proteases, otherwise known as calpains. Our data show, for the first time, that peroxynitrite induces mitochondrial dysfunction in cells via a calcium-dependent process that leads to caspase-independent apoptosis mediated by calpains.

Tumour necrosis factor-alpha and apoptosis in the rat temporomandibular joint

The purpose of this investigation was to investigate the roles that tumour necrosis factor-alpha (TNF-alpha) and apoptosis play during acute inflammation of the temporomandibular joint (TMJ). Adult male Sprague-Dawley rats were injected with complete Freund's adjuvant (CFA) into the TMJ or kept as uninjected controls. The TMJ tissues were removed 2 days post-injection to mimic conditions of acute inflammation and analysed for changes in expression of TNF-alpha, the receptor TNF-R1, caspase-3 and -8, and apoptosis. Concentrations of TNF-alpha, TNF-R1, caspase-3 and -8, and apoptosis were significantly elevated in CFA-injected animals compared to uninjected controls. Tissue incubation with TNF-alpha caused a significant increase in caspase-3 and -8. Also, levels of apoptosis were significantly increased during inflammation, which could be inhibited by the addition of either anti-TNF-alpha neutralising antibody or caspase inhibitors. TNF-alpha may play a significant role in the onset of acute CFA-induced TMJ inflammation, and activation of apoptosis signalling pathways may be involved.