Expression profile of protein tyrosine kinase genes in human osteoarthritis chondrocytes

Cartilage regeneration in zebrafish depends on Nrg1/ErbB signaling pathway

Objective: Cartilage, as the majority of adult mammalian tissues, has limited regeneration capacity. Cartilage degradation consecutive to joint injury or aging then leads to irreversible joint damage and diseases. In contrast, several vertebrate species such as the zebrafish have the remarkable capacity to spontaneously regenerate skeletal structures after severe injuries. The objective of our study was to test the regenerative capacity of Meckel's cartilage (MC) upon mechanical injury in zebrafish and to identify the mechanisms underlying this process. Methods and Results: Cartilage regenerative capacity in zebrafish larvae was investigated after mechanical injuries of the lower jaw MC in TgBAC(col2a1a:mCherry), to visualize the loss and recovery of cartilage. Confocal analysis revealed the formation of new chondrocytes and complete regeneration of MC at 14 days post-injury (dpi) via chondrocyte cell cycle re-entry and proliferation of pre-existing MC chondrocytes near the wound. Through expression analyses, we showed an increase of nrg1 expression in the regenerating lower jaw, which also expresses Nrg1 receptors, ErbB3 and ErbB2. Pharmacological inhibition of the ErbB pathway and specific knockdown of Nrg1 affected MC regeneration indicating the pivotal role of this pathway for cartilage regeneration. Finally, addition of exogenous NRG1 in an in vitro model of osteoarthritic (OA)-like chondrocytes induced by IL1β suggests that Nrg1/ErbB pathway is functional in mammalian chondrocytes and alleviates the increased expression of catabolic markers characteristic of OA-like chondrocytes. Conclusion: Our results show that the Nrg1/ErbB pathway is required for spontaneous cartilage regeneration in zebrafish and is of interest to design new therapeutic approaches to promote cartilage regeneration in mammals.

Tyrosine kinases regulate chondrocyte hypertrophy: promising drug targets for Osteoarthritis

Osteoarthritis (OA) is a major health problem worldwide that affects the joints and causes severe disability. It is characterized by pain and low-grade inflammation. However, the exact pathogenesis remains unknown and the therapeutic options are limited. In OA articular chondrocytes undergo a phenotypic transition becoming hypertrophic, which leads to cartilage damage, aggravating the disease. Therefore, a therapeutic agent inhibiting hypertrophy would be a promising disease-modifying drug. The therapeutic use of tyrosine kinase inhibitors has been mainly focused on oncology, but the Food and Drug Administration (FDA) approval of the Janus kinase inhibitor Tofacitinib in Rheumatoid Arthritis has broadened the applicability of these compounds to other diseases. Interestingly, tyrosine kinases have been associated with chondrocyte hypertrophy. In this review, we discuss the experimental evidence that implicates specific tyrosine kinases in signaling pathways promoting chondrocyte hypertrophy, highlighting their potential as therapeutic targets for OA.

SHP2 inhibition attenuates osteoarthritis by maintaining homeostasis of cartilage metabolism via the DOK1/UPP1/uridine cascade

OBJECTIVE

Protein tyrosine kinases (PTKs) regulate osteoarthritis (OA) progression by activating a series of signal transduction pathways. However, the roles of protein tyrosine phosphatases (PTPs) in OA remain obscure.

METHODS

The expression of 107 PTP genes in human OA cartilage was analyzed based on a single-cell sequencing dataset. The enzyme activity of the PTP SHP2 was detected in primary chondrocytes after interleukin (IL)-1β treatment and in human OA cartilage. Destabilized medial meniscus (DMM) model and IL-1β-stimulated primary mouse chondrocytes were treated with an SHP2 inhibitor and celecoxib (a clinical drug for the treatment of OA). The function of SHP2 in OA pathogenesis was further verified in Aggrecan-Cre^ERT ; SHP2 ^flox/flox mice. The downstream protein expression profile and dephosphorylated substrate of SHP2 were examined by tandem mass tag (TMT) labeling-based global proteomic and stable isotope labeling using amino acids in cell culture (SILAC)-labeled tyrosine phosphoproteomic analysis, respectively.

RESULTS

SHP2 enzyme activity significantly increased in human OA samples with serious articular cartilage injury and in IL-1β-stimulated chondrocytes. Pharmacological inhibition or genetic deletion of SHP2 ameliorated OA progression. SHP2 inhibitors dramatically reduced the expression of cartilage degradation-related genes and simultaneously promoted the expression of cartilage synthesis-related genes. Mechanistically, SHP2 inhibition suppressed the dephosphorylation of DOK1 and subsequently reduced the expression of uridine phosphorylase 1 and increased uridine level, thereby contributing to the homeostasis of cartilage metabolism.

CONCLUSIONS

SHP2 is a novel accelerator of the imbalance in the cartilage homeostasis. Specific inhibition of SHP2 may ameliorate OA by maintaining the anabolic and catabolic balance.

Metabonomic-Transcriptome Integration Analysis on Osteoarthritis and Rheumatoid Arthritis

Purpose

This study is aimed at exploring the potential metabolite/gene biomarkers, as well as the differences between the molecular mechanisms, of osteoarthritis (OA) and rheumatoid arthritis (RA).

Methods

Transcriptome dataset GSE100786 was downloaded to explore the differentially expressed genes (DEGs) between OA samples and RA samples. Meanwhile, metabolomic dataset MTBLS564 was downloaded and preprocessed to obtain metabolites. Then, the principal component analysis (PCA) and linear models were used to reveal DEG-metabolite relations. Finally, metabolic pathway enrichment analysis was performed to investigate the differences between the molecular mechanisms of OA and RA.

Results

A total of 976 DEGs and 171 metabolites were explored between OA samples and RA samples. The PCA and linear module analysis investigated 186 DEG-metabolite interactions including Glycogenin 1- (GYG1-) asparagine_54, hedgehog acyltransferase- (HHAT-) glucose_70, and TNF receptor-associated factor 3- (TRAF3-) acetoacetate_35. Finally, the KEGG pathway analysis showed that these metabolites were mainly enriched in pathways like gap junction, phagosome, NF-kappa B, and IL-17 pathway.

Conclusions

Genes such as HHAT, GYG1, and TRAF3, as well as metabolites including glucose, asparagine, and acetoacetate, might be implicated in the pathogenesis of OA and RA. Metabolites like ethanol and tyrosine might participate differentially in OA and RA progression via the gap junction pathway and phagosome pathway, respectively. TRAF3-acetoacetate interaction may be involved in regulating inflammation in OA and RA by the NF-kappa B and IL-17 pathway.

Integration of Gene Expression Profile Data to Screen and Verify Hub Genes Involved in Osteoarthritis

Osteoarthritis (OA) is one of the most common diseases worldwide, but the pathogenic genes and pathways are largely unclear. The aim of this study was to screen and verify hub genes involved in OA and explore potential molecular mechanisms. The expression profiles of GSE12021 and GSE55235 were downloaded from the Gene Expression Omnibus (GEO) database, which contained 39 samples, including 20 osteoarthritis synovial membranes and 19 matched normal synovial membranes. The raw data were integrated to obtain differentially expressed genes (DEGs) and were deeply analyzed by bioinformatics methods. The Gene Ontology (GO) and pathway enrichment of DEGs were performed by DAVID and Kyoto Encyclopedia of Genes and Genomes (KEGG) online analyses, respectively. The protein-protein interaction (PPI) networks of the DEGs were constructed based on data from the STRING database. The top 10 hub genes VEGFA, IL6, JUN, IL1β, MYC, IL4, PTGS2, ATF3, EGR1, and DUSP1 were identified from the PPI network. Module analysis revealed that OA was associated with significant pathways including TNF signaling pathway, cytokine-cytokine receptor interaction, and osteoclast differentiation. The qRT-PCR result showed that the expression level of IL6, VEGFA, JUN, IL-1β, and ATF3 was significantly increased in OA samples (p < 0.05), and these candidate genes could be used as potential diagnostic biomarkers and therapeutic targets of OA.

Identification of gene expression profiles and key genes in subchondral bone of osteoarthritis using weighted gene coexpression network analysis

Osteoarthritis (OA) significantly influences the quality life of people around the world. It is urgent to find an effective way to understand the genetic etiology of OA. We used weighted gene coexpression network analysis (WGCNA) to explore the key genes involved in the subchondral bone pathological process of OA. Fifty gene expression profiles of GSE51588 were downloaded from the Gene Expression Omnibus database. The OA-associated genes and gene ontologies were acquired from JuniorDoc. Weighted gene coexpression network analysis was used to find disease-related networks based on 21756 gene expression correlation coefficients, hub-genes with the highest connectivity in each module were selected, and the correlation between module eigengene and clinical traits was calculated. The genes in the traits-related gene coexpression modules were subject to functional annotation and pathway enrichment analysis using ClusterProfiler. A total of 73 gene modules were identified, of which, 12 modules were found with high connectivity with clinical traits. Five modules were found with enriched OA-associated genes. Moreover, 310 OA-associated genes were found, and 34 of them were among hub-genes in each module. Consequently, enrichment results indicated some key metabolic pathways, such as extracellular matrix (ECM)-receptor interaction (hsa04512), focal adhesion (hsa04510), the phosphatidylinositol 3'-kinase (PI3K)-Akt signaling pathway (PI3K-AKT) (hsa04151), transforming growth factor beta pathway, and Wnt pathway. We intended to identify some core genes, collagen (COL)6A3, COL6A1, ITGA11, BAMBI, and HCK, which could influence downstream signaling pathways once they were activated. In this study, we identified important genes within key coexpression modules, which associate with a pathological process of subchondral bone in OA. Functional analysis results could provide important information to understand the mechanism of OA.

Negative Regulators of JAK/STAT Signaling in Rheumatoid Arthritis and Osteoarthritis

Elevated levels of pro-inflammatory cytokines are generally thought to be responsible for driving the progression of synovial joint inflammation in rheumatoid arthritis (RA) and osteoarthritis (OA). These cytokines activate several signal transduction pathways, including the Janus kinase/Signal Transducers and Activators of Transcription (JAK/STAT), Stress-Activated/Mitogen-Activated Protein Kinase (SAPK/MAPK) and phosphatidylinositol-3-kinase/Akt/mechanistic target of rapamycin (PI3K/Akt/mTOR) pathways which regulate numerous cellular responses. However, cytokine gene expression, matrix metalloproteinase gene expression and aberrant immune cell and synoviocyte survival via reduced apoptosis are most critical in the context of inflammation characteristic of RA and OA. Negative regulation of JAK/STAT signaling is controlled by Suppressor of Cytokine Signaling (SOCS) proteins. SOCS is produced at lower levels in RA and OA. In addition, gaining further insight into the role played in RA and OA pathology by the inhibitors of the apoptosis protein family, cellular inhibitor of apoptosis protein-1, -2 (c-IAP1, c-IAP2), X (cross)-linked inhibitor of apoptosis protein (XIAP), protein inhibitor of activated STAT (PIAS), and survivin (human) as well as SOCS appears to be a worthy endeavor going forward.

Molecular mechanisms of osteoarthritis using gene microarrays

This study aimed to investigate the molecular mechanisms of osteoarthritis (OA) by microarray analysis. Three gene expression datasets GSE1919, 19664 and 55235 were downloaded from the Gene Expression Omnibus, and data of OA samples and healthy controls were used. After data preprocessing, differential expression analysis between the OA group and controls was performed using LIMMA (Linear Models for Microarray Data) package and genes with |log2FC (fold change)|>1 and P<0.05 were screened as DEGs (differentially expressed genes). The screened DEGs were then subject to functional annotation and pathway enrichment analysis using DAVID (Database for Annotation Visualization and Integrated Discovery). Next, gene-set enrichment analysis was performed using Enrichment map Cytoscape plug-in, followed by detecting sub-networks using clusterONE. Finally, risk subpathways were screened using iSubpathwayMiner package. A total of 141 DEGs were screened, including 52 up-regulated ones and 89 down-regulated ones. These DEGs were enriched in 48 GO terms that were mainly related to locomotory behavior, taxis, adhesion, and 11 pathways that were related to cytokine-cytokine receptor interaction, ECM-receptor interaction, focal adhesion, as well as several signaling pathways. The enrichment map enriched gene-sets mainly related to cell death and apoptosis, and extracellular components. The risk pathways up-regulated DEGs were exclusively related to arachidonic acid metabolism and glycosphingolipid biosynthesis, and the top two risk pathways were tyrosine metabolism for the down-regulated ones. From this study we conclude that genes involved in cell death and apoptosis, as well as cell-extracellular matrix interaction, may be essential for OA pathogenesis by altering multiple signaling pathways.

c-Jun N-terminal kinase - c-Jun pathway transactivates Bim to promote osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disorder. Previous studies have shown abnormally increased apoptosis of chondrocytes in patients and animal models of OA. TNF-α and nitric oxide have been reported to induce chondrocyte ageing; however, the mechanism of chondrocyte apoptosis induced by IL-1β has remained unclear. The aim of this study is to identify the role of the c-Jun N-terminal kinase (JNK) - c-Jun pathway in regulating induction of Bim, and its implication in chondrocyte apoptosis. This study showed that Bim is upregulated in chondrocytes obtained from the articular cartilage of OA patients and in cultured mouse chondrocytes treated with IL-1β. Upregulation of Bim was found to be critical for chondrocyte apoptosis induced by IL-1β, as revealed by the genetic knockdown of Bim, wherein apoptosis was greatly reduced in the chondrocytes. Moreover, activation of the JNK-c-Jun pathway was observed under IL-1β treatment, as indicated by the increased expression levels of c-Jun protein. Suppression of the JNK-c-Jun pathway, using chemical inhibitors and RNA interference, inhibited the Bim upregulation induced by IL-1β. These findings suggest that the JNK-c-Jun pathway is involved in the upregulation of Bim during OA and that the JNK-c-Jun-Bim pathway is vital for chondrocyte apoptosis.

Regulatory role of tyrosine phosphorylation in the swelling-activated chloride current in isolated rabbit articular chondrocytes

Articular chondrocytes are exposed in vivo to the continually changing osmotic environment and thus require volume regulatory mechanisms. The present study was designed to investigate (i) the functional role of the swelling-activated Cl(-) current (I(Cl,swell)) in the regulatory volume decrease (RVD) and (ii) the regulatory role of tyrosine phosphorylation in I(Cl,swell), in isolated rabbit articular chondrocytes. Whole-cell membrane currents were recorded from chondrocytes in isosmotic, hyposmotic and hyperosmotic external solutions under conditions where Na(+), K(+) and Ca(2+) currents were minimized. The cell surface area was also measured using microscope images from a separate set of chondrocytes and was used as an index of cell volume. The isolated chondrocytes exhibited a RVD during sustained exposure to hyposmotic solution, which was mostly inhibited by the I(Cl,swell) blocker 4-(2-butyl-6,7-dichloro-2-cyclopentyl-indan-1-on-5-yl)oxobutyric acid (DCPIB) at 20 microM. Exposure to a hyposmotic solution activated I(Cl,swell), which was also largely inhibited by 20 microM DCPIB. I(Cl,swell) in rabbit articular chondrocytes had a relative taurine permeability (P(tau)/P(Cl)) of 0.21. Activation of I(Cl,swell) was significantly reduced by the protein tyrosine kinase (PTK) inhibitor genistein (30 microM) but was only weakly affected by its inactive analogue daidzein (30 microM). Intracellular application of protein tyrosine phosphatase (PTP) inhibitor sodium orthovanadate (250 and 500 microM) resulted in a gradual activation of a Cl(-) current even in isosmotic solutions. This Cl(-) current was almost completely inhibited by 4,4-diisothiocyanatostilbene-2,2-disulfonate (DIDS, 500 microM) and was also largely suppressed by exposure to hyperosmotic solution, thus indicating a close similarity to I(Cl,swell). Pretreatment of chondrocytes with genistein significantly prevented the activation of the Cl(-) current by sodium orthovanadate, suggesting that the basal activity of endogenous PTK is required for the activation of this Cl(-) current. Our results provide evidence to indicate that activation of I(Cl,swell) is involved in RVD in isolated rabbit articular chondrocytes and is facilitated by tyrosine phosphorylation.

alpha-MSH inhibits TNF-alpha-induced matrix metalloproteinase-13 expression by modulating p38 kinase and nuclear factor kappaB signaling in human chondrosarcoma HTB-94 cells

OBJECTIVE

Proinflammatory cytokine-induced expression of matrix metalloproteinases (MMPs) is a major cause of arthritic cartilage destruction. The neuropeptide, alpha-melanocyte-stimulating hormone (alpha-MSH), has been detected in the synovial fluid of arthritis patients, where it is thought to play an anti-inflammatory role. Here, we examined whether alpha-MSH acts via downregulation of MMP expression, and sought to elucidate the intracellular signal pathways underlying this effect.

DESIGN

Human chondrosarcoma cell line, HTB-94 (SW1353) was pretreated with or without alpha-MSH and then treated with tumor necrosis factor-alpha (TNF-alpha). The effect of alpha-MSH on TNF-alpha-induced MMP-13 expression and mitogen-activated protein kinases' (MAPKs) activation were determined by reverse transcriptase-polymerase chain reaction and Western blot analysis. Additionally, the intracellular signaling of alpha-MSH was investigated using the inhibitors of MAPK and nuclear factor kappaB (NF-kappaB) and plasmids encoding dominant negative (dn) forms of inhibitor kappaB kinase-alpha (IKKalpha) and inhibitor kappaB kinase-beta (IKKbeta).

RESULTS

We found that alpha-MSH pretreatment inhibited TNF-alpha-induced MMP-13 expression and p38 kinase phosphorylation in HTB-94 human chondrosarcoma cells. TNF-alpha-induced MMP-13 expression was not suppressed by extracellular signal-regulated kinase (ERK) inhibitors (PD98059 and U0126) or a c-jun terminal kinase (JNK) inhibitor (SP600125), but was inhibited by inhibitors of p38 kinase (SB203580) and NF-kappaB (SN-50 peptide) and dnIKKalpha and dnIKKbeta.

CONCLUSIONS

Our results suggest that alpha-MSH regulates TNF-alpha-induced MMP-13 expression by decreasing p38 kinase phosphorylation and subsequent NF-kappaB activation in human chondrocytes and may be an effective inhibitor of MMP-13-mediated collagen degradation, providing new potential opportunities for the development of anti-arthritis therapeutics.

Molecular genetic studies of gene identification for osteoporosis: a 2004 update

This review summarizes comprehensively the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of December 2004. It is intended to constitute a sequential update of our previously published review covering the available data up to the end of 2002. Evidence from candidate gene association studies and genome-wide linkage studies in humans, as well as quantitative trait locus mapping animal models are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. An important extension of this update is incorporation of functional genomic studies (including DNA microarrays and proteomics) on osteogenesis and osteoporosis, in light of the rapid advances and the promising prospects of the field. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.

Sensing extracellular matrix: an update on discoidin domain receptor function

Discoidin Domain Receptors (DDRs) have recently emerged as non-integrin-type receptors for collagen. The two mammalian gene products Discoidin Domain Receptor 1 and -2 constitute a subfamily of tyrosine kinase receptors that are selectively expressed in a number of different cell types and organs. Upon collagen activation, DDRs regulate cell adhesion, proliferation and extracellular matrix remodeling. Here we review the various signaling pathways and cellular responses evoked by activated DDRs. Additionally, we give an overview of the more recent advances in understanding the role of DDRs in various human diseases, in particular during tumor progression, atherosclerosis, inflammation and tissue fibrosis. Furthermore, we discuss potential roles of genes homologous to mammalian DDRs identified in flies, worms and sponges. We show that the structural organization of these DDR-related genes is highly conserved throughout evolution suggesting that invertebrate DDRs may also function as receptors for collagen. By highlighting current questions about these unusual collagen receptors, we hope to attract new research on DDRs from a variety of different fields.

Tyrosine 740 phosphorylation of discoidin domain receptor 2 by Src stimulates intramolecular autophosphorylation and Shc signaling complex formation

DDR2 is a receptor tyrosine kinase whose activating ligands are various collagens. DDR2-mediated cellular signaling has been shown to require Src activity. However, the precise mechanism underlying the Src dependence of DDR2 signaling is unknown. Here, using baculoviral co-expression of the DDR2 cytosolic domain and Src, we show that Src targets three tyrosine residues (Tyr-736, Tyr-740, and Tyr-741) in the activation loop of DDR2 for phosphorylation. This phosphorylation by Src stimulates DDR2 cis-autophosphorylation of additional tyrosine residues. In vitro Shc binding assays demonstrate that phosphotyrosines resulting from DDR2 autophosphorylation are involved in Shc binding to the DDR2 cytosolic domain. Mutating tyrosine 740 of DDR2 to phenylalanine stimulates autophosphorylation of DDR2 to an extent similar to that resulting from Src phosphorylation of DDR2. In addition, the DDR2 Y740F mutant protein displays collagen-independent, constitutively activated signaling. These findings suggest that tyrosine 740 inhibits DDR2 autophosphorylation. Collectively, our findings are consistent with the following mechanism for Src-dependent DDR2 activation and signaling: 1) ligand binding promotes phosphorylation of Tyr-740 in the DDR2 activation loop by Src; 2) Tyr-740 phosphorylation stimulates intramolecular autophosphorylation of DDR2; 3) DDR2 autophosphorylation generates cytosolic domain phosphotyrosines that promote the formation of DDR2 cytosolic domain-Shc signaling complexes.

Inhibitory effect of Buthus martensi Karsch extracts on interleukin-1β-induced expression of nitric oxide (NO) synthase and production of NO in human chondrocytes and LPS-induced NO and prostaglandin E2 production in mouse peritoneal macrophages

We examined the effect of Buthus martensi Karsch (BMK) extract on IL-1beta-induced production of nitrogen oxide (NO) in primary human osteoarthritis (OA) chondrocytes. The cells were treated with BMK (10 microg/ml) and IL-1beta (2 ng/ml) for different periods, and inducible nitric oxide synthase (iNOS) mRNA and protein expression were determined by real-time quantitative reverse transcriptase-polymerase chain reaction and Western blotting, respectively. The cytotoxicity of BMK on human OA chondrocytes was very low (IC50 > 250 microg/ml) as measured by the XTT assay method. Production of NO was determined as nitrite in culture supernatant. Human chondrocytes cotreated with BMK produced significantly less NO compared with chondrocytes stimulated with IL-1beta alone. Activation and translocation of and NF-(kappa)B DNA binding activity were determined by Western blotting and specific enzyme-linked immunosorbent assay. The inhibition of NO production correlated with the suppression of induction and expression of nuclear factor-kappaB (NF-(kappa)B) and activation protein-1 (AP-1)-dependent gene. BMK inhibited the activation and translocation of NF-(kappa)B to the nucleus, indicating that BMK inhibits the IL-1beta-induced production of NO in human chondrocytes by interfering with the activation of NF-(kappa)B through a novel mechanism. In addition, BMK reduced prostaglandin E2 (PGE2) production in mouse peritoneal macrophages stimulated with lipopolysaccharide, whereas no influence on the activity of inducible NO synthase (iNOS), cyclooxygenase-2 (COX-2) or cyclooxygenase-1 (COX-1) was observed. Our data, therefore, suggest that BMK may be a therapeutically effective inhibitor of IL-1beta-induced inflammatory effects that are dependent on NF-(kappa)B activation in human OA chondrocytes. The results indicate that BMK exerts anti-inflammatory effects related to the inhibition of neutrophil functions and of NO and PGE2 production, which could be due to a decreased expression of iNOS and COX-2 through the transcription factors NF-(kappa)B and AP-1.

Phenotypic characterization of human chondrocyte cell line C-20/A4: a comparison between monolayer and alginate suspension culture

DNA microarray analysis was used to investigate the molecular phenotype of one of the first human chondrocyte cell lines, C-20/A4, derived from juvenile costal chondrocytes by immortalization with origin-defective simian virus 40 large T antigen. Clontech Human Cancer Arrays 1.2 and quantitative PCR were used to examine gene expression profiles of C-20/A4 cells cultured in the presence of serum in monolayer and alginate beads. In monolayer cultures, genes involved in cell proliferation were strongly upregulated compared to those expressed by human adult articular chondrocytes in primary culture. Of the cell cycle-regulated genes, only two, the CDK regulatory subunit and histone H4, were downregulated after culture in alginate beads, consistent with the ability of these cells to proliferate in suspension culture. In contrast, the expression of several genes that are involved in pericellular matrix formation, including MMP-14, COL6A1, fibronectin, biglycan and decorin, was upregulated when the C-20/A4 cells were transferred to suspension culture in alginate. Also, nexin-1, vimentin, and IGFBP-3, which are known to be expressed by primary chondrocytes, were differentially expressed in our study. Consistent with the proliferative phenotype of this cell line, few genes involved in matrix synthesis and turnover were highly expressed in the presence of serum. These results indicate that immortalized chondrocyte cell lines, rather than substituting for primary chondrocytes, may serve as models for extending findings on chondrocyte function not achievable by the use of primary chondrocytes.

Microarray analysis of gene expression in chondrosarcoma cells treated with bee venom

Bee venom (BV) has a broad array of clinical applications in Korean medicine, including treatment of inflammatory conditions such as arthritis. The final common pathway of many arthropathies is the destruction of articular cartilage and consequent loss of articular function. Chondrocyte dysfunction plays a key role in the pathogenesis of such disorders. To explore the global gene expression profiles in a human chondrocyte-like cell line treated with BV, microarray analysis was performed. The HTB-94 human chondrosarcoma cells were treated with BV, lipopolysaccharide (LPS), or both. Of the 344 genes profiled in this study, with a cut-off level of 4-fold change in the expression, (1) 35 were downregulated following BV treatment, (2) 16 were upregulated and 7 downregulated following LPS treatment, and (3) 32 were downregulated following co-stimulation of BV and LPS. The results of the present study shows that treatment of BV reversed the LPS-induced upregulation of such genes as interleukin-6 (IL-6) receptor, matrix metalloproteinase 15 (MMP-15), tumor necrosis factor (ligand) superfamily-10, caspase-6 and tissue inhibitor of metalloproteinase-1 (TIMP-1). It is thought that microarrays will play an ever-growing role in the advance of our understanding of the pharmacologic actions of BV in the treatment of arthritis.

Protein kinases in chondrocyte signaling and osteoarthritis

Protein kinases, particularly mitogen-activated protein kinases and receptor-tyrosine kinases play crucial roles in mammalian cellular metabolism by regulating intracellular signaling pathways that control proliferation, differentiation, cytokine gene induction and cytokine responsiveness, matrix metalloproteinase gene expression, mechanical transduction, as well as programmed cell death (apoptosis). Many of these pathways are also important components of cartilage homeostasis because alterations in intracellular signaling pathways appear to play a prominent role in chondrocyte dysfunction that is part of osteoarthritis pathogenesis and disease progression. Several mitogen-activated protein kinases and receptor-tyrosine kinases have been characterized as participating in chondrocyte signaling pathways. They are c-Jun-amino-terminal protein kinase, p38 kinase, extracellular signal-regulated protein kinase, and Ror2. Janus kinases and signal transducers and activators of transcription factors (Janus kinase/signal transducers and activators of transcription pathway) are also implicated in modulating the chondrogenic phenotype. Mitogen-activated protein kinase activation is required for their role as phosphorylating enzymes. Activation results from mitogen-activated protein kinase phosphorylation carried out by at least seven upstream kinases known as mitogen-activated protein kinase kinases. Additional upstream kinases (for example, MKKKKs and MKKKs) often require low molecular weight GTP-binding proteins to mediate the mitogen-activated protein-kinase kinases cascade. Identifying the functions of mitogen-activated protein kinases in normal and aging cartilage and the extent to which mitogen-activated protein kinases may be altered in osteoarthritis cartilage and synovium will be critical for developing novel therapies for osteoarthritis management.

Hydrostatic pressure induces apoptosis in human chondrocytes from osteoarthritic cartilage through up-regulation of tumor necrosis factor-alpha, inducible nitric oxide synthase, p53, c-myc, and bax-alpha, and suppression of bcl-2

Hydrostatic pressure (HP) is thought to increase within cartilage extracellular matrix as a consequence of fluid flow inhibition. The biosynthetic response of human articular chondrocytes to HP in vitro varies with the load magnitude, load frequency, as well as duration of loading. We found that continuous cyclic HP (5 MegaPascals (MPa) for 4 h; 1 Hz frequency) induced apoptosis in human chondrocytes derived from osteoarthritic cartilage in vitro as evidenced by reduced chondrocyte viability which was independent of initial cell densities ranging from 8.1 x 10(4) to 1.3 x 10(6) cells ml(-1). HP resulted in internucleosomal DNA fragmentation, activation of caspase-3, and cleavage of poly-ADP-ribose polymerase (PARP). At the molecular level, induction of apoptosis by HP was characterized by up-regulation of p53, c-myc, and bax-alpha after 4 h with concomitant down-regulation of bcl-2 after 2 h at 5 MPa as measured by RT-PCR. In contrast, beta-actin expression was unchanged. Real-time quantitative RT-PCR confirmed a HP-induced (5 MPa) 1.3-2.6 log-fold decrease in bcl-2 mRNA copy number after 2 and 4 h, respectively, and a significant increase (1.9-2.5 log-fold) in tumor necrosis factor-alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS) mRNA copy number after 2 and 4 h, respectively. The up-regulation of p53 and c-myc, and the down-regulation of bcl-2 caused by HP were confirmed at the protein level by Western blotting. These results indicated that HP is a strong inducer of apoptosis in osteoarthritic human chondrocytes in vitro.

Pathophysiological mechanisms in osteoarthritis lead to novel therapeutic strategies

Osteoarthritis (OA) is a debilitating, progressive disease of diarthrodial joints associated with aging. At the molecular level, OA is characterized by an imbalance between anabolic (i.e. extracellular matrix biosynthesis) and catabolic (i.e. extracellular matrix degradation) pathways in which articular cartilage is the principal site of tissue injury responses. The pathophysiology of OA also involves the synovium in that 'nonclassical' inflammatory synovial processes contribute to OA progression. Chondrocytes are critical to the OA process in that the progression of OA can be judged by the vitality of chondrocytes and their ability to resist apoptosis. Growth factors exemplified by insulin-like growth factor-1, its binding proteins and transforming growth factor-beta contribute to anabolic pathways including compensatory biosynthesis of extracellular matrix proteins. Catabolic pathways are altered by cytokine genes such as interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) which are upregulated in OA. In addition, IL-1 and TNF-alpha downregulate extracellular matrix protein biosynthesis while concomitantly upregulating matrix metalloproteinase (MMP) gene expression. When MMPs are activated, cartilage extracellular matrix degradation ensues apparently because levels of endogenous cartilage MMP inhibitors cannot regulate MMP activity. Therapeutic strategies designed to modulate the imbalance between anabolic and catabolic pathways in OA may include neutralizing cytokine activity or MMP gene expression or inhibiting signaling pathways which result in apoptosis dependent on mature caspase activity or mitogen-activated protein kinase (MAPK) activity. MAPK activity appears critical for regulating chondrocyte and synoviocyte apoptosis and MMP genes.

Phenyl N-tert-butylnitrone down-regulates interleukin-1 beta-stimulated matrix metalloproteinase-13 gene expression in human chondrocytes: suppression of c-Jun NH2-terminal kinase, p38-mitogen-activated protein kinase and activating protein-1

Cytokine-mediated induction and overexpression of matrix metalloproteinases (MMPs) is recognized as an important factor in the pathogenesis of arthritis. Interleukin (IL)-1 beta is a proinflammatory cytokine that is known to superinduce the expression and production of MMP-13 in many cell types. Phenyl N-tert-butylnitrone (PBN), a spin trap agent, inhibited the IL-1 beta-induced expression of MMP-13 in human osteoarthritis (OA) chondrocytes. Down-regulation of MMP-13 expression correlated with the inhibition of mitogen-activated protein kinase (MAPK) subgroups c-Jun NH2-terminal kinase (JNK) and p38-MAPK activation, accumulation of phospho-c-jun, and the DNA binding activity of activating protein-1 (AP-1). Results of in vitro kinase assays showed that exogenously added PBN completely blocked the c-Jun phosphorylating activity of JNK. Interestingly, using in vitro kinase assay, we also found that chondrocyte p38-MAPK phosphorylate c-Jun and that PBN was not very effective in inhibiting c-Jun phosphorylating activity of p38-MAPK. In addition, PBN did not block the ATF-2 phosphorylating activity of p38-MAPK and Elk-1 phosphorylating activity of extracellular regulated kinase p44/p42 in vitro, indicating that PBN may act selectively to inhibit the phosphorylation of c-Jun in OA chondrocytes. Together, our results for the first time demonstrate that PBN suppresses the IL-1 beta-stimulated expression of MMP-13 in OA chondrocytes and that this was achieved by inhibiting the activation of JNK and AP-1. These results suggest that use of PBN or compounds derived from it may be of potential benefit in inhibiting signaling events associated with cartilage degradation in arthritis.

Epigallocatechin-3-gallate selectively inhibits interleukin-1beta-induced activation of mitogen activated protein kinase subgroup c-Jun N-terminal kinase in human osteoarthritis chondrocytes

Activation of mitogen activated protein kinases (MAPK) is a critical event in pro-inflammatory cytokine-induced signaling cascade in synoviocytes and chondrocytes that lead to the production of several mediators of cartilage damage in an arthritic joint. Green tea (Camellia sinensis) is a widely consumed beverage and we earlier showed that polyphenols present in green tea (GTP) inhibit the development of inflammation and cartilage damage in an animal model of arthritis. In this study we evaluated the role of epigallocatechin-3-gallate (EGCG), a green tea polyphenol which mimics its anti-inflammatory effects, in modulating the IL-1beta-induced activation of MAPK's in human chondrocytes. We discovered that EGCG inhibited the IL-1beta-induced phosphorylation of c-Jun N-terminal kinase (JNK) isoforms, accumulation of phospho-c-Jun and DNA binding activity of AP-1 in osteoarthritis (OA) chondrocytes. Also IL-1beta, but not EGCG, induced the expression of JNK p46 without modulating the expression of JNK p54 in OA chondrocytes. In immunecomplex kinase assays, EGCG completely blocked the substrate phosphorylating activity of JNK but not of p38-MAPK. EGCG had no inhibitory effect on the activation of extracellular signal-regulated kinase p44/p42 (ERKp44/p42) or p38-MAPK in OA chondrocytes. EGCG or IL-1beta did not alter the total non-phosphorylated levels of either p38-MAPK or ERKp44/p42 in OA chondrocytes. These are novel findings and indicate that EGCG may be of potential benefit in inhibiting IL-1beta-induced catabolic effects in OA chondrocytes that are dependent on JNK activity.

Green tea polyphenol epigallocatechin-3-gallate inhibits the IL-1 beta-induced activity and expression of cyclooxygenase-2 and nitric oxide synthase-2 in human chondrocytes

We have previously shown that green tea polyphenols inhibit the onset and severity of collagen II-induced arthritis in mice. In the present study, we report the pharmacological effects of green tea polyphenol epigallocatechin-3-gallate (EGCG), on interleukin-1 beta (IL-1 beta)-induced expression and activity of cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) in human chondrocytes derived from osteoarthritis (OA) cartilage. Stimulation of human chondrocytes with IL-1 beta (5 ng/ml) for 24 h resulted in significantly enhanced production of nitric oxide (NO) and prostaglandin E(2) (PGE(2)) when compared to untreated controls (p <.001). Pretreament of human chondrocytes with EGCG showed a dose-dependent inhibition in the production of NO and PGE(2) by 48% and 24%, respectively, and correlated with the inhibition of iNOS and COX-2 activities (p <.005). In addition, IL-1 beta-induced expression of iNOS and COX-2 was also markedly inhibited in human chondrocytes pretreated with EGCG (p <.001). Parallel to these findings, EGCG also inhibited the IL-1 beta-induced LDH release in chondrocytes cultures. Overall, the study suggests that EGCG affords protection against IL-1 beta-induced production of catabolic mediators NO and PGE(2) in human chondrocytes by regulating the expression and catalytic activity of their respective enzymes. Furthermore, our results also indicate that ECGC may be of potential therapeutic value for inhibiting cartilage resorption in arthritic joints.

Epigallocatechin-3-gallate inhibits interleukin-1beta-induced expression of nitric oxide synthase and production of nitric oxide in human chondrocytes: suppression of nuclear factor kappaB activation by degradation of the inhibitor of nuclear factor kappaB

OBJECTIVE

The proinflammatory cytokine interleukin-1beta (IL-1beta) induces the production of high levels of nitric oxide (NO) in human chondrocytes. Green tea (Camellia sinensis) polyphenols are potent antiinflammatory agents and have been shown to inhibit NO production in tumor cell lines. In the present study, we examined the effect of epigallocatechin-3-gallate (EGCG), a green tea polyphenol, on IL-1beta-induced production of NO in primary human osteoarthritis (OA) chondrocytes.

METHODS

Human chondrocytes were derived from OA cartilage and were treated with EGCG (100 microM) and IL-1beta (2 ng/ml) for different periods, and inducible nitric oxide synthase (iNOS) messenger RNA and protein expression was determined by real-time quantitative reverse transcriptase-polymerase chain reaction and Western blotting, respectively. Production of NO was determined as nitrite in culture supernatant. Activation and translocation of nuclear factor kappaB (NF-kappaB), levels of inhibitor of nuclear factor kappaB (IkappaB), and NF-kappaB DNA binding activity were determined by Western blotting and a highly sensitive and specific enzyme-linked immunosorbent assay. Activity of IkappaB kinase was determined using in vitro kinase assay.

RESULTS

Human chondrocytes cotreated with EGCG produced significantly less NO compared with chondrocytes stimulated with IL-1beta alone (P < 0.005). The inhibition of NO production correlated with the suppression of induction and expression of NF-kappaB-dependent gene iNOS. EGCG inhibited the activation and translocation of NF-kappaB to the nucleus by suppressing the degradation of its inhibitory protein IkappaBalpha in the cytoplasm.

CONCLUSION

Our results indicate that EGCG inhibits the IL-1beta-induced production of NO in human chondrocytes by interfering with the activation of NF-kappaB through a novel mechanism. Our data further suggest that EGCG may be a therapeutically effective inhibitor of IL-1beta-induced inflammatory effects that are dependent on NF-kappaB activation in human OA chondrocytes.