Immortalized human adult articular chondrocytes maintain cartilage-specific phenotype and responses to interleukin-1beta

Generation of human immortalized chondrocytes from osteoarthritic and healthy cartilage : a new tool for cartilage pathophysiology studies

AIMS

After a few passages of in vitro culture, primary human articular chondrocytes undergo senescence and loss of their phenotype. Most of the available chondrocyte cell lines have been obtained from cartilage tissues different from diarthrodial joints, and their utility for osteoarthritis (OA) research is reduced. Thus, the goal of this research was the development of immortalized chondrocyte cell lines proceeded from the articular cartilage of patients with and without OA.

METHODS

Using telomerase reverse transcriptase (hTERT) and SV40 large T antigen (SV40LT), we transduced primary OA articular chondrocytes. Proliferative capacity, degree of senescence, and chondrocyte surface antigen expression in transduced chondrocytes were evaluated. In addition, the capacity of transduced chondrocytes to synthesize a tissue similar to cartilage and to respond to interleukin (IL)-1β was assessed.

RESULTS

Coexpression of both transgenes (SV40 and hTERT) were observed in the nuclei of transduced chondrocytes. Generated chondrocyte cell lines showed a high proliferation capacity and less than 2% of senescent cells. These cell lines were able to form 3D aggregates analogous to those generated by primary articular chondrocytes, but were unsuccessful in synthesizing cartilage-like tissue when seeded on type I collagen sponges. However, generated chondrocyte cell lines maintained the potential to respond to IL-1β stimulation.

CONCLUSION

Through SV40LT and hTERT transduction, we successfully immortalized chondrocytes. These immortalized chondrocytes were able to overcome senescence in vitro, but were incapable of synthesizing cartilage-like tissue under the experimental conditions. Nonetheless, these chondrocyte cell lines could be advantageous for OA investigation since, similarly to primary articular chondrocytes, they showed capacity to upregulate inflammatory mediators in response to the IL-1β cytokine.Cite this article: Bone Joint Res 2023;12(1):46-57.

Inhibition of fibroblast activation protein ameliorates cartilage matrix degradation and osteoarthritis progression

Fibroblast activation protein (Fap) is a serine protease that degrades denatured type I collagen, α2-antiplasmin and FGF21. Fap is highly expressed in bone marrow stromal cells and functions as an osteogenic suppressor and can be inhibited by the bone growth factor Osteolectin (Oln). Fap is also expressed in synovial fibroblasts and positively correlated with the severity of rheumatoid arthritis (RA). However, whether Fap plays a critical role in osteoarthritis (OA) remains poorly understood. Here, we found that Fap is significantly elevated in osteoarthritic synovium, while the genetic deletion or pharmacological inhibition of Fap significantly ameliorated posttraumatic OA in mice. Mechanistically, we found that Fap degrades denatured type II collagen (Col II) and Mmp13-cleaved native Col II. Intra-articular injection of rFap significantly accelerated Col II degradation and OA progression. In contrast, Oln is expressed in the superficial layer of articular cartilage and is significantly downregulated in OA. Genetic deletion of Oln significantly exacerbated OA progression, which was partially rescued by Fap deletion or inhibition. Intra-articular injection of rOln significantly ameliorated OA progression. Taken together, these findings identify Fap as a critical pathogenic factor in OA that could be targeted by both synthetic and endogenous inhibitors to ameliorate articular cartilage degradation.

Isoprenaline and salbutamol inhibit pyroptosis and promote mitochondrial biogenesis in arthritic chondrocytes by downregulating β-arrestin and GRK2

Rheumatoid arthritis and osteoarthritis overlap many molecular mechanisms of cartilage destruction. Wear and tear in cartilage is chondrocyte-mediated, where chondrocytes act both as effector and target cells. In current study, role of β2-AR was studied in chondrocytes both in vitro and in vivo. High grade inflammation in vitro and in vivo disease models led to decline in anti-inflammatory β2-AR signaling and use of β2-AR agonist attenuated arthritis symptoms. Detailed analysis in chondrocytes revealed that Isoprenaline (ISO) and Salbutamol (SBT) increased cell viability and relative Bcl-2 expression, meanwhile, decreased proteins levels of TNF-α, IL-6 and IL-8 in arthritic chondrocytes when compared with control, respectively. SBT preserved physiological concentration of antioxidant enzymes (CAT, POD, SOD and GSH) in cartilage homogenates and ISO inhibited IL-1β-mediated genotoxicity in arthritic chondrocytes. Moreover, β2-AR agonist increased mitochondrial biogenesis and proteoglycan biosynthesis by upregulating the gene expression of PGC1-α, NRF2 and COL2A1, Acan, respectively. ISO and SBT inhibited extracellular matrix (ECM) degradation by downregulating the gene expression of MMP1, MMP3, MMP9 and ADAMTS5 in vitro and in vivo study. In mechanism, β2-AR agonists decreased β-arrestin and GRK2 pathway, and as a result mice receiving SBT did not exhibit severe disease. Hence our data suggest β2-AR agonist administered at disease onset can inhibit receptor internalization by downregulating the expression of β-arrestin and GRK2 in chondrocytes.

SAGE: Novel Therapy to Reduce Inflammation in a Naturally Occurring-Dog Model of Periodontal Disease

Objective

Methods

Results

Conclusion

Phenotypic Characterization of Immortalized Chondrocytes from a Desbuquois Dysplasia Type 1 Mouse Model: A Tool for Studying Defects in Glycosaminoglycan Biosynthesis

The complexity of skeletal pathologies makes use of in vivo models essential to elucidate the pathogenesis of the diseases; nevertheless, chondrocyte and osteoblast cell lines provide relevant information on the underlying disease mechanisms. Due to the limitations of primary chondrocytes, immortalized cells represent a unique tool to overcome this problem since they grow very easily for several passages. However, in the immortalization procedure the cells might lose the original phenotype; thus, these cell lines should be deeply characterized before their use. We immortalized primary chondrocytes from a Cant1 knock-out mouse, an animal model of Desbuquois dysplasia type 1, with a plasmid expressing the SV40 large and small T antigen. This cell line, based on morphological and biochemical parameters, showed preservation of the chondrocyte phenotype. In addition reduced proteoglycan synthesis and oversulfation of glycosaminoglycan chains were demonstrated, as already observed in primary chondrocytes from the Cant1 knock-out mouse. In conclusion, immortalized Cant1 knock-out chondrocytes maintained the disease phenotype observed in primary cells validating the in vitro model and providing an additional tool to further study the proteoglycan biosynthesis defect. The same approach might be extended to other cartilage disorders.

A prospect of cell immortalization combined with matrix microenvironmental optimization strategy for tissue engineering and regeneration

Cellular senescence is a major hurdle for primary cell-based tissue engineering and regenerative medicine. Telomere erosion, oxidative stress, the expression of oncogenes and the loss of tumor suppressor genes all may account for the cellular senescence process with the involvement of various signaling pathways. To establish immortalized cell lines for research and clinical use, strategies have been applied including internal genomic or external matrix microenvironment modification. Considering the potential risks of malignant transformation and tumorigenesis of genetic manipulation, environmental modification methods, especially the decellularized cell-deposited extracellular matrix (dECM)-based preconditioning strategy, appear to be promising for tissue engineering-aimed cell immortalization. Due to few review articles focusing on this topic, this review provides a summary of cell senescence and immortalization and discusses advantages and limitations of tissue engineering and regeneration with the use of immortalized cells as well as a potential rejuvenation strategy through combination with the dECM approach.

Interleukin-1β signaling in osteoarthritis - chondrocytes in focus

Osteoarthritis (OA) can be regarded as a chronic, painful and degenerative disease that affects all tissues of a joint and one of the major endpoints being loss of articular cartilage. In most cases, OA is associated with a variable degree of synovial inflammation. A variety of different cell types including chondrocytes, synovial fibroblasts, adipocytes, osteoblasts and osteoclasts as well as stem and immune cells are involved in catabolic and inflammatory processes but also in attempts to counteract the cartilage loss. At the molecular level, these changes are regulated by a complex network of proteolytic enzymes, chemokines and cytokines (for review: [1]). Here, interleukin-1 signaling (IL-1) plays a central role and its effects on the different cell types involved in OA are discussed in this review with a special focus on the chondrocyte.

A Chemically Modified Curcumin (CMC 2.24) Inhibits Nuclear Factor κB Activation and Inflammatory Bone Loss in Murine Models of LPS-Induced Experimental Periodontitis and Diabetes-Associated Natural Periodontitis

The purpose of this study was to assess the effect of a novel chemically modified curcumin (CMC 2.24) on NF-κB and MAPK signaling and inflammatory cytokine production in two experimental models of periodontal disease in rats. Experimental model I: Periodontitis was induced by repeated injections of LPS into the gingiva (3×/week, 3 weeks); control rats received vehicle injections. CMC 2.24, or the vehicle, was administered by daily oral gavage for 4 weeks. Experimental model II: Diabetes was induced in adult male rats by streptozotocin injection; periodontal breakdown then results as a complication of uncontrolled hyperglycemia. Non-diabetic rats served as controls. CMC 2.24, or the vehicle, was administered by oral gavage daily for 3 weeks to the diabetics. Hemimaxillae and gingival tissues were harvested, and bone loss was assessed radiographically. Gingival tissues were pooled according to the experimental conditions and processed for the analysis of matrix metalloproteinases (MMPs) and bone-resorptive cytokines. Activation of p38 MAPK and NF-κB signaling pathways was assessed by western blot. Both LPS and diabetes induced an inflammatory process in the gingival tissues associated with excessive alveolar bone resorption and increased activation of p65 (NF-κB) and p38 MAPK. In both models, the administration of CMC 2.24 produced a marked reduction of inflammatory cytokines and MMPs in the gingival tissues, decreased bone loss, and decreased activation of p65 (NF-κB) and p38 MAPK. Inhibition of these cell signaling pathways by this novel tri-ketonic curcuminoid (natural curcumin is di-ketonic) may play a role in its therapeutic efficacy in locally and systemically associated periodontitis.

Ectopic Telomerase Expression Fails to Maintain Chondrogenic Capacity in Three-Dimensional Cultures of Clinically Relevant Cell Types

The poor healing capacity of cartilage and lack of effective treatment for associated disease and trauma makes it a strong candidate for a regenerative medicine approach. Potential therapies tested to date, although effective, have met with a number of intrinsic difficulties possibly related to limited autologous chondrocyte cell yield and quality of cartilage produced. A potential mechanism to bypass limited cell yields and improve quality of differentiation is to immortalize relevant cell types through the ectopic expression of telomerase. Pellet cultures of human chondrocytes (OK3), bone marrow mesenchymal stem cells (BMA13), and embryonic stem cell (H1 line)-derived cells (1C6) and their human telomerase reverse transcriptase (hTERT) transduced counterparts were maintained for 20 days in standard maintenance medium (MM) or transforming growth factor-β3-supplemented prochondrogenic medium (PChM). Pellets were assessed for volume and density by microcomputed tomography. Quantitative gene expression (COL1A2, COL2A1, COL3A1, COL6A3, COL10A1, ACAN, COMP, SOX9); sulfated glycosaminoglycans (sGAGs), and DNA quantification were performed. Histology and immunohistochemistry were used to determine matrix constituent distribution. Pellet culture in PChM resulted in significantly larger pellets with an overall increased density when compared with MM culture. Gene expression analysis revealed similarities in expression patterns between telomerase-transduced and parental cells in both MM and PChM. Of the three parental cell types examined OK3 and BMA13 produced similar amounts of pellet-associated sGAG in PChM (4.62 ± 1.20 and 4.91 ± 1.37 μg, respectively) with lower amounts in 1C6 (2.89 ± 0.52 μg), corresponding to 3.1, 2.3, and 1.6-fold increases from day 0. In comparison, telomerase-transduced cells all had much lower sGAG with OK3H at 2.74 ± 0.11 μg, BMA13H 1.29 ± 0.34 μg, and 1C6H 0.52 ± 0.01 μg corresponding to 1.2, 0.87, and 0.34-fold changes compared with day 0. Histology of day 20 pellets displayed reduced staining overall for collagens and sGAG in telomerase-transduced cells, most notably with alterations in aggrecan and collagen VI; all cells stained positively for collagen II. We conclude that while telomerase transduction may be an effective technique to extend cellular proliferative capacity, it is not sufficient in isolation to sustain a naive chondrogenic phenotype across multiple cell types.

MicroRNA-29b Contributes to Collagens Imbalance in Human Osteoarthritic and Dedifferentiated Articular Chondrocytes

Objective

Decreased expression of collagen type II in favour of collagen type I or X is one hallmark of chondrocyte phenotype changes in osteoarthritic (OA) cartilage. MicroRNA- (miR-) 29b was previously shown to target collagens in several tissues. We studied whether it could contribute to collagen imbalance in chondrocytes with an impaired phenotype.

Methods

After preliminary microarrays screening, miR-29b levels were measured by RT- quantitative PCR in in vitro models of chondrocyte phenotype changes (IL-1β challenge or serial subculturing) and in chondrocytes from OA and non-OA patients. Potential miR-29b targets identified in silico in 3′-UTRs of collagens mRNAs were tested with luciferase reporter assays. The impact of premiR-29b overexpression in ATDC5 cells was studied on collagen mRNA levels and synthesis (Sirius red staining) during chondrogenesis.

Results

MiR-29b level increased significantly in IL-1β-stimulated and weakly in subcultured chondrocytes. A 5.8-fold increase was observed in chondrocytes from OA versus non-OA patients. Reporter assays showed that miR-29b targeted COL2A1 and COL1A2 3′-UTRs although with a variable recovery upon mutation. In ATDC5 cells overexpressing premiR-29b, collagen production was reduced while mRNA levels increased.

Conclusions

By acting probably as a posttranscriptional regulator with a different efficacy on COL2A1 and COL1A2 expression, miR-29b can contribute to the collagens imbalance associated with an abnormal chondrocyte phenotype.

Applications of Chondrocyte-Based Cartilage Engineering: An Overview

Chondrocytes are the exclusive cells residing in cartilage and maintain the functionality of cartilage tissue. Series of biocomponents such as different growth factors, cytokines, and transcriptional factors regulate the mesenchymal stem cells (MSCs) differentiation to chondrocytes. The number of chondrocytes and dedifferentiation are the key limitations in subsequent clinical application of the chondrocytes. Different culture methods are being developed to overcome such issues. Using tissue engineering and cell based approaches, chondrocytes offer prominent therapeutic option specifically in orthopedics for cartilage repair and to treat ailments such as tracheal defects, facial reconstruction, and urinary incontinence. Matrix-assisted autologous chondrocyte transplantation/implantation is an improved version of traditional autologous chondrocyte transplantation (ACT) method. An increasing number of studies show the clinical significance of this technique for the chondral lesions treatment. Literature survey was carried out to address clinical and functional findings by using various ACT procedures. The current study was conducted to study the pharmacological significance and biomedical application of chondrocytes. Furthermore, it is inferred from the present study that long term follow-up studies are required to evaluate the potential of these methods and specific positive outcomes.

U0126, an Inhibitor of MEK1/2, Increases Tumor Necrosis Factor-α-Induced Apoptosis, but not Interleukin-6 Induced Apoptosis in C-28/I2 Human Chondrocytes

BACKGROUND

Activation of the SAPK/MAPK signaling pathway by pro-inflammatory cytokines is known to induce apoptosis in cultured articular chondrocytes. C-28/I2, an immortalized human juvenile chondrocyte cell line was employed to determine the extent to which recombinant human (rh) forms of the pro-inflammatory cytokines, tumor necrosis factor-α (rhTNF-α,), interleukin-6 (rhIL-6) and oncostatin M (rhOSM) induced apoptosis.

METHODS

The induction of apoptosis in the presence or absence of these cytokines was measured by the DAPI/TUNEL assay, by whether or not pro-caspase-3 was activated and by the extent to which poly-ADP-ribose polymerase (PARP) was degraded.

FINDINGS

Only rhTNF-α, and rhIL-6 significantly increased apoptosis in C-28/I2 chondrocytes, although rhOSM exhibited a strong trend (p=0.067) towards increasing the frequency of apoptotic chondrocytes. The number of apoptotic C28/I2 chondrocytes was significantly increased (p=1.3 × 10-5) by the combination of rhTNF-α and U0126 (10 μM) compared to rhTNF-α alone. However apoptosis was not further increased by combining rhIL-6 with U0126. The LI-COR^® western blot system showed that U0126 (10 μM) inhibited the phosphorylation of extracellular signal-regulated kinase-2 (p-ERK2) by phorbol myristate acetate-treated immortalized myometrial cells, U0126 (10 μM) did not alter total U-ERK2. Western blot analysis also revealed that the increased frequency of apoptotic C-28/I2 chondrocytes induced by rhTNF-α and rhOSM, but not rhIL-6, was associated with PARP degradation. However, none of the cytokines resulted in pro-caspase-3 activation.

CONCLUSION

These results showed that rhTNF-α and rhIL-6 were strong inducers of apoptosis in the immortalized C-28/I2 human chondrocyte cell line. They also suggested that inhibiting ERK2 phosphorylation via U0126-mediated inhibition of MEK1/2 activity, increased rhTNF-α-induced C-28/I2 chondrocyte apoptosis.

New developments in the treatment of osteoarthritis - focus on biologic agents

Osteoarthritis (OA) is one of the most common diseases around the world. Medical, social, and financial consequences oblige clinicians, surgeons, and researchers to focus on finding the best treatment option, to eradicate and stop this degenerative joint disease, in order to avoid surgical options which in many instances are over-indicated. Noninvasive treatments, such as anti-inflammatory drugs, physiotherapy, orthotic devices, dietary supplements, have demonstrated lack of effectiveness. The possibility to perform intra-articular injections with hyaluronic acid, corticosteroids, or the newest but criticized treatment based on platelet-rich plasma (PRP) has changed the management of OA disease. The use of PRP has led to many differences in treatment since there is a lack of consensus about protocols, indications, number of doses, cost-effectiveness, and duration of the treatment. Many publications have suggested efficacy in tendon injuries, but when PRP has been indicated to treat cartilage injuries, things are more inconsistent. Some authors have reported their experience treating OA with PRP, and it seems that, if well indicated, it is an option as a supplementary therapy. Therefore, we need to understand that OA is a mechanical disease which not only produces changes in radiographs, but also affects the quality of life. Pathogenesis of OA has been well explained, providing us new knowledge and future possibilities to improve the clinical approach. From basic science to surgery, there is a great field we all need to contribute to, because the general population is aging and total joint replacements should not be the only solution for OA. So herein is an actual review of the developments for treating OA with biologics, intended to be useful for the population inside orthopedics who could be called bio-orthopedists, since OA is a molecular homeostasis disbalance between catabolism and anabolism triggered by mechanical stress.

Lentivirus-induced knockdown of LRP1 induces osteoarthritic-like effects and increases susceptibility to apoptosis in chondrocytes via the nuclear factor-κB pathway

Low-density lipoprotein receptor-related protein 1 (LRP1) is known to regulate cell survival and inflammation. The present study investigated the involvement of LRP1 in the regulation of tumor necrosis factor (TNF)-α-induced expression of matrix metalloproteinase (MMP)-13. Furthermore, the study aimed to elucidate the mechanisms underlying the effects of LRP1 on TNF-α-induced inflammation and apoptosis of chondrocytes. Lentivirus-mediated RNA interference techniques were used to knockdown the LRP1 gene. Subsequently, the effects of LRP1 on TNF-α-induced MMP-13 expression were determined using quantitative polymerase chain reaction, western blot analysis and ELISA. Furthermore, the TNF-α-induced intracellular pathway was investigated using a nuclear factor (NF)-κB inhibitor (Bay 11-7082). In addition, the effect of LRP1 regulation on growth and apoptosis in chondrocytes was investigated using western blot analysis and a TUNEL assay. LRP1 knockdown was shown to increase TNF-α-induced MMP-13 expression via the activation of the NF-κB (p65) pathway, which reduced the expression of collagen type II and cell viability. In addition, LRP1 inhibited cell apoptosis by increasing the expression of phospho-Akt and B-cell lymphoma 2 (Bcl-2), while suppressing the expression of caspase-3 and Bcl-2-associated X protein. The results of the present study indicated that LRP1 was able to inhibit TNF-α-induced apoptosis and inflammation in chondrocytes. Therefore, LRP1 may be an effective osteoarthritis inhibitor, potentially providing a novel approach for antiarthritic therapeutics.

Immortalised human mesenchymal stem cells undergo chondrogenic differentiation in alginate and PGA/PLLA scaffolds

Adult mesenchymal stem cells (MSCs) are a promising cell source in tissue engineering due to their availability, ease of isolation and high proliferative activity. This study was undertaken to investigate whether immortalised human MSC are able to undergo chondrogenic differentiation when cultured in alginate or in resorbable scaffolds. We directly compared chondrogenesis MSCs with that of human nasoseptal chondrocytes. Two previously established human stem cell lines L87/4 and V54-2 immortalised using the SV40 large T-antigen were either cultured in alginate or in polyglycolic acid/poly-L-lactic acid (PGA/PLLA) (90/10) copolymer scaffolds. TGF-β1 was added for induction of chondrogenesis. Human nasoseptal chondrocytes and human fibroblasts were used as controls. Cultures were analysed for sulfated glycosaminoglycans (alcian blue staining) and for the presence of collagen type I, II and X (immunolabelling). SV40 large T-antigen immortalised human MSCs have the potential to undergo chondrogenic differentiation: After 21 days, cartilage-specific type II collagen was present in alginate and PGA/PLLA scaffolds, independent of the addition of TGF-β1. Collagen type X was present in monolayer cultures as well as in alginate and PGA/PLLA scaffolds. Collagen type I was produced in marginal amounts only. Immortalised human MSCs are a suitable tool to study chondrogenesis in vitro and to screen biomaterials for cartilage tissue engineering applications.

Activation of Indian hedgehog promotes chondrocyte hypertrophy and upregulation of MMP-13 in human osteoarthritic cartilage

OBJECTIVE

The objectives of this study were to (1) determine the correlation between osteoarthritis (OA) and Indian hedgehog (Ihh) expression, and (2) establish the effects of Ihh on expression of markers of chondrocyte hypertrophy and matrix metalloprotease (MMP)-13 in human OA cartilage.

DESIGN

OA cartilage and synovial fluid samples were obtained during total knee arthroplasty. Normal cartilage samples were obtained from intra-articular tumor resections, and normal synovial fluid samples were obtained from healthy volunteers and the contralateral uninjured knee of patients undergoing anterior cruciate ligament reconstruction. OA was graded using the Mankin score. Expression of Ihh in synovial fluid was determined by Western blot. Ihh, type X collagen and MMP-13 mRNA were determined by real time PCR. Protein expression of type X collagen and MMP-13 in cartilage samples was analyzed with immunohistochemistry. Chondrocyte size was measured using image analysis.

RESULTS

Ihh expression was increased 2.6 fold in OA cartilage and 37% in OA synovial fluid when compared to normal control samples. Increased expression of Ihh was associated with the severity of OA and expression of markers of chondrocyte hypertrophy: type X collagen and MMP-13, and chondocyte size. Chondrocytes were more spherical with increasing severity of OA. There was a significant correlation between Mankin score and cell size (r(2) = 0.80) and Ihh intensity (r(2) = 0.89). Exogenous Ihh induced a 6.8 fold increase of type X collagen and 2.8 fold increase of MMP-13 mRNA expression in cultured chondrocytes. Conversely, knockdown of Ihh by siRNA and Hh inhibitor cyclopamine had the opposite effect.

CONCLUSIONS

Ihh expression correlates with OA progression and changes in chondrocyte morphology and gene expression consistent with chondrocyte hypertrophy and cartilage degradation seen in OA cartilage. Thus, Ihh may be a potential therapeutic target to prevent OA progression.

Botanical Extracts from Rosehip (Rosa canina), Willow Bark (Salix alba), and Nettle Leaf (Urtica dioica) Suppress IL-1β-Induced NF-κB Activation in Canine Articular Chondrocytes

The aim of this study was to characterize the anti-inflammatory mode of action of botanical extracts from rosehip (Rosa canina), willow bark (Salix alba), and nettle leaf (Urtica dioica) in an in vitro model of primary canine articular chondrocytes. Methods. The biological effects of the botanical extracts were studied in chondrocytes treated with IL-1β for up to 72 h. Expression of collagen type II, cartilage-specific proteoglycan (CSPG), β1-integrin, SOX-9, COX-2, and MMP-9 and MMP-13 was examined by western blotting. Results. The botanical extracts suppressed IL-1β-induced NF-κB activation by inhibition of IκBα phosphorylation, IκBα degradation, p65 phosphorylation, and p65 nuclear translocation. These events correlated with downregulation of NF-κB targets including COX-2 and MMPs. The extracts also reversed the IL-1β-induced downregulation of collagen type II, CSPG, β1-integrin, and cartilage-specific transcription factor SOX-9 protein expression. In high-density cultures botanical extracts stimulated new cartilage formation even in the presence of IL-1β. Conclusions. Botanical extracts exerted anti-inflammatory and anabolic effects on chondrocytes. The observed reduction of IL-1β-induced NF-κB activation suggests that further studies are warranted to demonstrate the effectiveness of plant extracts in the treatment of OA and other conditions in which NF-κB plays pathophysiological roles.

Human chondrocyte cultures as models of cartilage-specific gene regulation

The human adult articular chondrocyte is a unique cell type that has reached a fully differentiated state as an end point of development. Within the cartilage matrix, chondrocytes are normally quiescent and maintain the matrix constituents in a low-turnover state of equilibrium. Isolated chondrocytes in culture have provided useful models to study cellular responses to alterations in the environment such as those occurring in different forms of arthritis. However, expansion of primary chondrocytes in monolayer culture results in the loss of phenotype, particularly if high cell density is not maintained. This chapter describes strategies for maintaining or restoring differentiated phenotype by culture in suspension, gels, or scaffolds. Techniques for assessing phenotype involving primarily the analysis of synthesis of cartilage-specific matrix proteins as well as the corresponding mRNAs are also described. Approaches for studying gene regulation, including transfection of promoter-driven reporter genes with expression vectors for transcriptional and signaling regulators, chromatin immunoprecipitation, and DNA methylation are also described.

IGF-1 and PDGF-bb suppress IL-1β-induced cartilage degradation through down-regulation of NF-κB signaling: involvement of Src/PI-3K/AKT pathway

Objective

Interleukin-1β (IL-1β) is a pro-inflammatory cytokine that plays a key role in the pathogenesis of osteoarthritis (OA). Growth factors (GFs) capable of antagonizing the catabolic actions of cytokines may have therapeutic potential in the treatment of OA. Herein, we investigated the potential synergistic effects of insulin-like growth factor (IGF-1) and platelet-derived growth factor (PDGF-bb) on different mechanisms participating in IL-1β-induced activation of nuclear transcription factor-κB (NF-κB) and apoptosis in chondrocytes.

Methods

Primary chondrocytes were treated with IL-1β to induce dedifferentiation and co-treated with either IGF-1 or/and PDGF-bb and evaluated by immunoblotting and electron microscopy.

Results

Pretreatment of chondrocytes with IGF-1 or/and PDGF-bb suppressed IL-1β-induced NF-κB activation via inhibition of IκB-α kinase. Inhibition of IκB-α kinase by GFs led to the suppression of IκB-α phosphorylation and degradation, p65 nuclear translocation and NF-κB-regulated gene products involved in inflammation and cartilage degradation (COX-2, MMPs) and apoptosis (caspase-3). GFs or BMS-345541 (specific inhibitor of the IKK) reversed the IL-1β-induced down-regulation of collagen type II, cartilage specific proteoglycans, β1-integrin, Shc, activated MAPKinase, Sox-9 and up-regulation of active caspase-3. Furthermore, the inhibitory effects of IGF-1 or/and PDGF-bb on IL-1β-induced NF-κB activation were sensitive to inhibitors of Src (PP1), PI-3K (wortmannin) and Akt (SH-5), suggesting that the pathway consisting of non-receptor tyrosine kinase (Src), phosphatidylinositol 3-kinase and protein kinase B must be involved in IL-1β signaling.

Conclusion

The results presented suggest that IGF-1 and PDGF-bb are potent inhibitors of IL-1β-mediated activation of NF-κB and apoptosis in chondrocytes, may be mediated in part through suppression of Src/PI-3K/AKT pathway, which may contribute to their anti-inflammatory effects.

Distal interleukin-1β (IL-1β) response element of human matrix metalloproteinase-13 (MMP-13) binds activator protein 1 (AP-1) transcription factors and regulates gene expression

The collagenase matrix metalloproteinase-13 (MMP-13) plays an important role in the destruction of cartilage in arthritic joints. MMP-13 expression is strongly up-regulated in arthritis, largely because of stimulation by inflammatory cytokines such as IL-1β. Treatment of chondrocytes with IL-1β induces transcription of MMP-13 in vitro. IL-1β signaling converges upon the activator protein-1 transcription factors, which have been shown to be required for IL-1β-induced MMP-13 gene expression. Using chromatin immunoprecipitation (ChIP), we detected activator protein-1 binding within an evolutionarily conserved DNA sequence ∼20 kb 5' relative to the MMP-13 transcription start site (TSS). Also using ChIP, we detected histone modifications and binding of RNA polymerase II within this conserved region, all of which are consistent with transcriptional activation. Chromosome conformation capture indicates that chromosome looping brings this region in close proximity with the MMP-13 TSS. Finally, a luciferase reporter construct driven by a component of the conserved region demonstrated an expression pattern similar to that of endogenous MMP-13. These data suggest that a conserved region at 20 kb upstream from the MMP-13 TSS includes a distal transcriptional response element of MMP-13, which contributes to MMP-13 gene expression.

Roles of inflammatory and anabolic cytokines in cartilage metabolism: signals and multiple effectors converge upon MMP-13 regulation in osteoarthritis

Human cartilage is a complex tissue of matrix proteins that vary in amount and orientation from superficial to deep layers and from loaded to unloaded zones. A major challenge to efforts to repair cartilage by stem cell-based and other tissue engineering strategies is the inability of the resident chondrocytes to lay down new matrix with the same structural and resilient properties that it had upon its original formation. This is particularly true of the collagen network, which is susceptible to cleavage once proteoglycans are depleted. Thus, a thorough understanding of the similarities and particularly the marked differences in mechanisms of cartilage remodeling during development, osteoarthritis, and aging may lead to more effective strategies for preventing cartilage damage and promoting repair. To identify and characterize effectors or regulators of cartilage remodeling in these processes, we are using culture models of primary human and mouse chondrocytes and cell lines and mouse genetic models to manipulate gene expression programs leading to matrix remodeling and subsequent chondrocyte hypertrophic differentiation, pivotal processes which both go astray in OA disease. Matrix metalloproteinases (MMP)-13, the major type II collagen-degrading collagenase, is regulated by stress-, inflammation-, and differentiation-induced signals that not only contribute to irreversible joint damage (progression) in OA, but importantly, also to the initiation/onset phase, wherein chondrocytes in articular cartilage leave their natural growth- and differentiation-arrested state. Our work points to common mediators of these processes in human OA cartilage and in early through late stages of OA in surgical and genetic mouse models.

Endoglin differentially regulates TGF-β-induced Smad2/3 and Smad1/5 signalling and its expression correlates with extracellular matrix production and cellular differentiation state in human chondrocytes

OBJECTIVE

Transforming growth factor-β (TGF-β) plays a critical role in cartilage homeostasis and deregulation of its signalling is implicated in osteoarthritis (OA). TGF-β isoforms signal through a pair of transmembrane serine/threonine kinases known as the type I and type II TGF-β receptors. Endoglin is a TGF-β co-receptor that binds TGF-β with high affinity in the presence of the type II TGF-β receptor. We have previously shown that endoglin is expressed in human chondrocytes and that it forms a complex with the TGF-β signalling receptors. However, the functional significance of endoglin expression in chondrocytes is unknown. Our objective was to determine whether endoglin regulates TGF-β/Smad signalling and extracellular matrix (ECM) production in human chondrocytes and whether its expression varies with chondrocyte differentiation state.

METHOD

Endoglin function was determined by overexpression or antisense morpholino/siRNA knockdown of endoglin in human chondrocytes and measuring TGF-β-induced Smad phosphorylation, transcriptional activity and ECM production. Alterations in endoglin expression levels were determined during subculture-induced dedifferentiation of human chondrocytes and in normal vs OA cartilage samples.

RESULTS

Endoglin enhances TGF-β1-induced Smad1/5 phosphorylation and inhibits TGF-β1-induced Smad2 phosphorylation, Smad3-driven transcriptional activity and ECM production in human chondrocytes. In addition, the enhancing effect of endoglin siRNA knockdown on TGF-β1-induced Smad3-driven transcription is reversed by ALK1 overexpression. Furthermore, endoglin levels are increased in chondrocytes following subculture-induced dedifferentiation and in OA cartilage as compared to normal cartilage.

CONCLUSION

Together, our results suggest that endoglin regulates the balance between TGF-β/ALK1/Smad1/5 and ALK5/Smad2/3 signalling and ECM production in human chondrocytes and that endoglin may represent a marker for chondrocyte phenotype.

Curcumin synergizes with resveratrol to stimulate the MAPK signaling pathway in human articular chondrocytes in vitro

The mitogen-activated protein kinase (MAPK) pathway is stimulated in differentiated chondrocytes and is an important signaling cascade for chondrocyte differentiation and survival. Pro-inflammatory cytokines such as interleukin 1β (IL-1β) play important roles in the pathogenesis of osteoarthritis (OA) and rheumatoid arthritis (RA). In this study, we investigated whether curcumin and resveratrol can synergistically inhibit the catabolic effects of IL-1β, specifically the inhibition of the MAPK and subsequent apoptosis in human articular chondrocytes. Chondrocytes were either left untreated or treated with 10 ng/ml IL-1β or 1 μM U0126, a specific inhibitor of MAPK pathway alone for the indicated time periods or pre-treated with 10 μM curcumin, 10 μM resveratrol or 10 μM resveratrol and 10 μM curcumin for 4 h followed by co-treatment with 10 ng/ml IL-1β or 1 μM U0126 and 10 μM resveratrol, 10 μM curcumin or 10 μM resveratrol and 10 μM curcumin for the indicated time periods. Cultures were evaluated by immunoblotting and transmission electron microscopy. Incubation of chondrocytes with IL-1β resulted in induction of apoptosis, downregulation of β1-integrins and the extracellular signal-regulated kinase 1/2 (Erk1/2). Interestingly, U0126 induced apoptosis and blocked the above-mentioned proteins in a similar way to IL-1β. Furthermore, curcumin and resveratrol inhibited IL-1β- or U0126-induced apoptosis and downregulation of β1-integrins and Erk1/2 in human articular chondrocytes. These results suggest that combining these two natural compounds activates MEK/Erk signaling, a pathway that is involved in the maintenance of chondrocyte differentiation and survival.

The inorganic pyrophosphate transporter ANK preserves the differentiated phenotype of articular chondrocyte

The differentiated phenotype of chondrocyte is lost in pathological situations and after interleukin (IL)-1beta challenge. Wnt proteins and the inorganic pyrophosphate (PP(i)) transporter Ank regulate the differentiation process in many cell types. We investigated the possible contribution of Ank and/or PP(i) to the maintenance of the differentiated chondrocyte phenotype with special care to Wnt signaling. Primary articular chondrocytes lost their phenotype upon IL-1beta challenge, with cessation of type II collagen and Sox-9 expression. Ank expression and PP(i) transport were strongly reduced by IL-1beta, whereas Wnt-5a was the only Wnt protein increased. Transient overexpression of Ank counteracted most of IL-1beta effects on Type II collagen, Sox-9, and Wnt-5a expression. When resting chondrocytes were transfected with a siRNA against Ank, this reproduced the phenotype induced by IL-1beta. In both cases, no markers for hypertrophic chondrocytes were detected. The conditioned supernatant from chondrocytes knocked-down for Ank contained Wnt-5a, which activated Tcf/Lef reporter plasmids and promoted translocation of beta-catenin into the nucleus without activating the c-Jun N-terminal kinase (JNK) pathway. Supplementation with PP(i) compensated for most effects of Ank deficiency on Type II collagen, Sox-9, and Wnt-5 expression, both in IL-1beta and Ank knock-down conditions. Phenotype changes induced by IL-1beta were also supported by activation of the JNK pathway, but this latter was not sensitive to PP(i) supplementation. Altogether our data demonstrate that the transport of PP(i) by ANK contributed to the maintenance of the differentiated phenotype of chondrocyte by controlling the canonical Wnt pathway in a Wnt-5a-dependent manner.

Development and characterization of a human articular cartilage-derived chondrocyte cell line that retains chondrocyte phenotype

Chondrocytes, the only cell type present in articular cartilage, regulate tissue homeostasis by a fine balance of metabolism that includes both anabolic and catabolic activities. Therefore, the biology of chondrocytes is critical for understanding cartilage metabolism. One major limitation when studying primary chondrocytes in culture is their loss of phenotype. To overcome this hurdle, limited attempts have been made to develop human chondrocyte cell lines that retain the phenotype for use as a good surrogate model. In this study, we report a novel approach to the establishment and characterization of human articular cartilage-derived chondrocyte cell lines. Adenoviral infection followed by culture of chondrocytes in 3-dimensional matrix within 48 h post-infection maintained the phenotype prior to clonal selection. Cells were then placed in culture either as monolayer, or in 3-dimensional matrix of alginate or agarose. The clones were characterized by their basal gene expression profile of chondrocyte markers. Based on type II collagen expression, 21 clones were analyzed for gene expression following treatment with IL-1 or BMP-7 and compared to similarly stimulated primary chondrocytes. This resulted in selection of two clones that retained the chondrocyte phenotype as evidenced by expression of type II collagen and other extra-cellular matrix molecules. In addition, one clone (AL-4-17) showed similar responses as primary chondrocytes when treated with IL-1 or BMP-7. In summary, this report provides a novel procedure to develop human articular cartilage-derived chondrocyte cell lines, which preserve important characteristics of articular chondrocytes and represent a useful model to study chondrocyte biology.

Reduced limb length and worsened osteoarthritis in adult mice after genetic inhibition of p38 MAP kinase activity in cartilage

OBJECTIVE

MAP kinase p38 is part of an intracellular signaling pathway activated by environmental stress and inflammatory factors. Since in vitro studies show that inhibiting p38 activity leads to a reduction in the release of degenerative metalloproteinase from chondrocytes, we speculated that inactivation of p38 in vivo may be chondroprotective. To test this hypothesis, we examined the morphology of adult mice that express a dominant-negative (DN) p38 MAPK transgene in a cartilage-specific manner.

METHODS

The in vivo effects of the genetic inhibition of p38 MAPK activity in cartilage were investigated in 1-year-old heterozygous DN p38-transgenic mice (n = 10) using morphologic measurements, microfocal computed tomography scanning, biomechanical testing, and histologic analysis. Results were compared with those in wild-type (WT) littermates (n = 9).

RESULTS

Adult DN p38 MAPK+/- -transgenic mice exhibited 50% p38 MAPK activity in articular chondrocytes as compared with WT mice. They were significantly shorter in overall body length as well as in the femur and tibia lengths. There were no differences in bone material or mechanical properties between the transgenic and WT mice. Surprisingly, the transgenic mice had higher grades of osteoarthritis of the knee joint.

CONCLUSION

Genetic inhibition of p38 MAPK activity in cartilage results in shortened limb length and defects in the articular cartilage of the knee joints of adult mice. Our findings demonstrate that chronic life-long reduction of p38 MAPK activity may be harmful to joint health and suggest that the timing of p38 inhibition for chondroprotection in vivo is an important variable that warrants further investigation.

Osteoblasts derived from osteophytes produce interleukin-6, interleukin-8, and matrix metalloproteinase-13 in osteoarthritis

To clarify the significance of the osteophytes that appear during the progression of osteoarthritis (OA), we investigated the expression of inflammatory cytokines and proteases in osteoblasts from osteophytes. We also examined the influence of mechanical stress loading on osteoblasts on the expression of inflammatory cytokines and proteases. Osteoblasts were isolated from osteophytes in 19 patients diagnosed with knee OA and from subchondral bone in 4 patients diagnosed with femoral neck fracture. Messenger RNA expression and protein production of inflammatory cytokines and proteases were analyzed using real-time RT-PCR and ELISA, respectively. To examine the effects of mechanical loading, continuous hydrostatic pressure was applied to the osteoblasts. We determined the mRNA expression and protein production of IL-6, IL-8, and MMP-13, which are involved in the progression of OA, were increased in the osteophytes. Additionally, when OA pathological conditions were simulated by applying a nonphysiological mechanical stress load, the gene expression of IL-6 and IL-8 increased. Our results suggested that nonphysiological mechanical stress may induce the expression of biological factors in the osteophytes and is involved in OA progression. By controlling the expression of these genes in the osteophytes, the progression of cartilage degeneration in OA may be reduced, suggesting a new treatment strategy for OA.

Defining the roles of inflammatory and anabolic cytokines in cartilage metabolism

In osteoarthritis (OA), adult articular chondrocytes undergo phenotypic modulation in response to alterations in the environment owing to mechanical injury and inflammation. These processes not only stimulate the production of enzymes that degrade the cartilage matrix but also inhibit repair. With the use of in vitro and in vivo models, new genes, not known previously to act in cartilage, have been identified and their roles in chondrocyte differentiation during development and in dysregulated chondrocyte function in OA have been examined. These new genes include growth arrest and DNA damage (GADD)45beta and the epithelial-specific ETS (ESE)-1 transcription factor, induced by bone morphogenetic protein (BMP)-2 and inflammatory cytokines, respectively. Both genes are induced by NF-kappaB, suppress COL2A1 and upregulate matrix meatalloproteinase-13 (MMP-13) expression. These genes have also been examined in mouse models of OA, in which discoidin domain receptor 2 is associated with MMP-13-mediated remodelling, in order to understand their roles in physiological cartilage homoeostasis and joint disease.

ALK1 opposes ALK5/Smad3 signaling and expression of extracellular matrix components in human chondrocytes

INTRODUCTION

TGF-beta is a multifunctional regulator of chondrocyte proliferation, differentiation, and extracellular matrix production. Dysregulation of TGF-beta action has been implicated in cartilage diseases such as osteoarthritis. TGF-beta signaling is transduced through a pair of transmembrane serine/threonine kinases, known as the type I (ALK5) and type II receptors. However, recent studies on endothelial cells have identified ALK1 as a second type I TGF-beta receptor and have shown that ALK1 and ALK5 have opposing functions in these cells. Here we examined ALK1 expression and its regulation of TGF-beta signaling and responses in human chondrocytes.

MATERIALS AND METHODS

ALK1 expression in human chondrocytes was examined by RT-PCR and Western blot. The ability of ALK1 to form complexes with other TGF-beta receptors was determined by affinity labeling/immunoprecipitation and by immunoprecipitation followed by Western blot. The effect of ALK1 on TGF-beta1-induced signaling and responses was determined by varying ALK1 expression levels and measuring transcriptional activity using promoter/luciferase assays, Smad1/5 and Smad3 phosphorylation, and expression of type II collagen, PAI-1, and fibronectin.

RESULTS

Our results indicate that ALK1 is expressed in human chondrocytes and that it is a component of the TGF-beta receptor system, associating with ALK5, type II TGF-beta receptor, endoglin, and betaglycan. Furthermore, we show that both ALK1 and ALK5 are needed for TGF-beta-induced phosphorylation of intracellular mediators Smad1/5, whereas only ALK5 is essential for TGF-beta1-induced phosphorylation of Smad3. In addition, our results show that ALK1 inhibits, whereas ALK5 potentiates, TGF-beta-induced Smad3-driven transcriptional activity and the expression of PAI-1, fibronectin, and type II collagen in chondrocytes.

CONCLUSIONS

Our results suggest that ALK1 and ALK5 display opposing functions in human chondrocytes, implicating an essential role for ALK1 in the regulation of TGF-beta signaling and function in these cells.

ESE-1 is a potent repressor of type II collagen gene (COL2A1) transcription in human chondrocytes

The epithelium-specific ETS (ESE)-1 transcription factor is induced in chondrocytes by interleukin-1beta (IL-1beta). We reported previously that early activation of EGR-1 by IL-1beta results in suppression of the proximal COL2A1 promoter activity by displacement of Sp1 from GC boxes. Here we report that ESE-1 is a potent transcriptional suppressor of COL2A1 promoter activity in chondrocytes and accounts for the sustained, NF-kappaB-dependent inhibition by IL-1beta. Of the ETS factors tested, this response was specific to ESE-1, since ESE-3, which was also induced by IL-1beta, suppressed COL2A1 promoter activity only weakly. In contrast, overexpression of ETS-1 increased COL2A1 promoter activity and blocked the inhibition by IL-1beta. These responses to ESE-1 and ETS-1 were confirmed using siRNA-ESE1 and siRNA-ETS1. In transient cotransfections, the inhibitory responses to ESE-1 and IL-1beta colocalized in the -577/-132 bp promoter region, ESE-1 bound specifically to tandem ETS sites at -403/-381 bp, and IL-1-induced binding of ESE-1 to the COL2A1 promoter was confirmed in vivo by ChIP. Our results indicate that ESE-1 serves a potent repressor function by interacting with at least two sites in the COL2A1 promoter. However, the endogenous response may depend upon the balance of other ETS factors such as ETS-1, and other IL-1-induced factors, including EGR-1 at any given time. Intracellular ESE-1 staining in chondrocytes in cartilage from patients with osteoarthritis (OA), but not in normal cartilage, further suggests a fundamental role for ESE-1 in cartilage degeneration and suppression of repair.

Prolactin directly enhances bone turnover by raising osteoblast-expressed receptor activator of nuclear factor kappaB ligand/osteoprotegerin ratio

Hyperprolactinemia leads to high bone turnover as a result of enhanced bone formation and resorption. Although its osteopenic effect has long been explained as hyperprolactinemia-induced hypogonadism, identified prolactin (PRL) receptors in osteoblasts suggested a possible direct action of PRL on bone. In the present study, we found that hyperprolactinemia induced by anterior pituitary transplantation (AP), with or without ovariectomy (Ovx), had no detectable effect on bone mineral density and content measured by dual-energy X-ray absorptiometry (DXA). However, histomorphometric studies revealed increases in the osteoblast and osteoclast surfaces in the AP rats, but a decrease in the osteoblast surface in the AP+Ovx rats. The resorptive activity was predominant since bone volume and trabecular number were decreased, and the trabecular separation was increased in both groups. Estrogen supplement (E2) fully reversed the effect of estrogen depletion in the Ovx but not in the AP+Ovx rats. In contrast to the typical Ovx rats, bone formation and resorption became uncoupled in the AP+Ovx rats. Therefore, hyperprolactinemia was likely to have some estrogen-independent and/or direct actions on bone turnover. Osteoblast-expressed PRL receptor transcripts and proteins shown in the present study confirmed our hypothesis. Furthermore, we demonstrated that the osteoblast-like cells, MG-63, directly exposed to PRL exhibited lower expression of alkaline phosphatase and osteocalcin mRNA, and a decrease in alkaline phosphatase activity. The ratios of receptor activator of nuclear factor kappaB ligand (RANKL) and osteoprotegerin (OPG) proteins were increased, indicating an increase in the osteoclastic bone resorption. The present data thus demonstrated that hyperprolactinemia could act directly on bone to stimulate bone turnover, with more influence on bone resorption than formation. PRL enhanced bone resorption in part by increasing RANKL and decreasing OPG expressions by osteoblasts.

Human immortalized chondrocytes carrying heterozygous FGFR3 mutations: an in vitro model to study chondrodysplasias

Achondroplasia and thanatophoric dysplasia are human chondrodysplasias caused by mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. We have developed an immortalized human chondrocyte culture model to study the regulation of chondrocyte functions. One control and eight mutant chondrocytic lines expressing different FGFR3 heterozygous mutations were obtained. FGFR3 signaling pathways were modified in the mutant lines as revealed by the constitutive activation of the STAT pathway and an increased level of P21(WAF1/CIP1) protein. This model will be useful for the study of FGFR3 function in cartilage studies and future therapeutic approaches in chondrodysplasias.

The post-translational phenotype of collagen synthesized by SAOS-2 osteosarcoma cells

The human osteosarcoma-derived cell line, SAOS-2, exhibits many of the phenotypic characteristics of osteoblasts including the deposition of types I and V collagens in an extracellular matrix. Lesser amounts of collagen XI chains were also detected. The cell layer collagen contains hydroxylysyl pyridinoline cross-links but without the accompanying lysyl pyridinoline typical of human bone collagen. This indicates that the lysine residues at the two helical cross-linking loci are fully hydroxylated. The isoform of lysyl hydroxylase, LH1, known to be required for full hydroxylation at these sites, was shown to be highly expressed by SAOS-2 cells. Our findings provide insight on the mechanism of post-translational overmodification of lysine residues in collagen made by osteosarcoma tumors, and may be relevant for understanding a similar overmodification observed in osteoporotic bone.

Expression and function of TbetaRII-B, a variant of the type II TGF-beta receptor, in human chondrocytes

OBJECTIVE

Transforming growth factor-beta (TGF-beta) has profound effects on chondrocyte proliferation and matrix production, and dysregulation of TGF-beta action has been implicated in osteoarthritis. The mechanisms by which the diverse actions of TGF-beta are regulated in chondrocytes are unclear. Although it is well documented that TGF-beta signaling is transduced by types I and II receptors, other TGF-beta receptors may play critical roles by regulating signaling receptor activity. Our objective was to examine the expression of TbetaRII-B, a splice variant of the type II TGF-beta receptor, and to analyze its role in regulating TGF-beta signaling in human chondrocytes.

METHODS

TbetaRII-B expression was examined in human cartilage tissue specimens, human chondrocyte cell lines C28/I2 and tsT/AC62, and human primary chondrocytes by Western blot and reverse-transcriptase-polymerase chain reaction. Ligand binding and heteromerization of TbetaRII-B with other TGF-beta receptors on the cell surface were analyzed by affinity labeling, immunoprecipitation, and two-dimensional SDS-PAGE. Regulation of TGF-beta responses by TbetaRII-B was determined by examining Smad2 phosphorylation, Smad3-specific signaling, transcriptional activity, and type II collagen levels.

RESULTS

TbetaRII-B is expressed in normal and osteoarthritic human cartilage. Furthermore, it is a dynamic component of the TGF-beta receptor system in human chondrocytes, forming heteromeric complexes with the types I and II TGF-beta receptors, betaglycan and endoglin. Importantly, overexpression of TbetaRII-B leads to enhanced TGF-beta signaling and responses in chondrocytes.

CONCLUSIONS

These results suggest that TbetaRII-B may play a key role in the regulation of TGF-beta action in human chondrocytes.

Suppression of NF-kappaB activation by curcumin leads to inhibition of expression of cyclo-oxygenase-2 and matrix metalloproteinase-9 in human articular chondrocytes: Implications for the treatment of osteoarthritis

Pro-inflammatory cytokines such as interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) play a key role in the pathogenesis of osteoarthritis (OA). Anti-inflammatory agents capable of suppressing the production and catabolic actions of these cytokines may have therapeutic potential in the treatment of OA and a range of other osteoarticular disorders. The purpose of this study was to examine the effects of curcumin (diferuloylmethane), a pharmacologically safe phytochemical agent with potent anti-inflammatory properties on IL-1beta and TNF-alpha signalling pathways in human articular chondrocytes maintained in vitro. The effects of curcumin were studied in cultures of human articular chondrocytes treated with IL-1beta and TNF-alpha for up to 72h. Expression of collagen type II, integrin beta1, cyclo-oxygenase-2 (COX-2) and matrix metalloproteinase-9 (MMP-9) was monitored by western blotting. The effects of curcumin on the expression, phosphorylation and nuclear translocation of protein components of the NF-kappaB system were studied by western blotting and immunofluorescence, respectively. Treatment of chondrocytes with curcumin suppressed IL-1beta-induced NF-kappaB activation via inhibition of IkappaBalpha phosphorylation, IkappaBalpha degradation, p65 phosphorylation and p65 nuclear translocation. Curcumin inhibited the IL-1beta-induced stimulation of up-stream protein kinase B Akt. These events correlated with down-regulation of NF-kappaB targets including COX-2 and MMP-9. Similar results were obtained in chondrocytes stimulated with TNF-alpha. Curcumin also reversed the IL-1beta-induced down-regulation of collagen type II and beta1-integrin receptor expression. These results indicate that curcumin has nutritional potential as a naturally occurring anti-inflammatory agent for treating OA through suppression of NF-kappaB mediated IL-1beta/TNF-alpha catabolic signalling pathways in chondrocytes.

Increased expression of the collagen receptor discoidin domain receptor 2 in articular cartilage as a key event in the pathogenesis of osteoarthritis

OBJECTIVE

To investigate the role of the collagen receptor discoidin domain receptor 2 (DDR-2) in the pathogenesis of osteoarthritis (OA).

METHODS

Histologic and immunohistochemical analyses were performed to characterize femoral head cartilage from 7 patients with OA and 4 patients with fracture, as well as articular cartilage from the knee joints of mice with surgically induced OA. Gene constructs encoding human Raf kinase inhibitor protein (RKIP), DDR-2 lacking the discoidin (DS) domain (DeltaDS-DDR-2) or the protein tyrosine kinase (PTK) core (DeltaPTK-DDR-2), DDR-2 containing a substitution of tyrosine for alanine at position 740 (Y740A), and luciferase driven by the matrix metalloproteinase 13 (MMP-13) promoter were transfected into human chondrocyte cell lines. Activated and neutralized alpha2beta1 integrin polyclonal antibodies, interleukin-1 receptor antagonist, and the chemical inhibitors SB203580, for p38, and SP600125, for JNKs, were used in cell cultures. Real-time polymerase chain reaction was performed to examine MMP-13 and DDR-2 messenger RNA (mRNA).

RESULTS

Increased immunostaining for DDR-2, MMP-13, and MMP-derived type II collagen fragments was detected in cartilage from patients with OA and from mice with surgically induced OA. The discoidin domain and PTK core of DDR-2 were essential for signal transmission and the resulting increased expression of MMP-13 in chondrocytes. Y740A mutation of DDR-2 reduced levels of mRNA for MMP-13 and endogenous DDR-2. The overexpression of RKIP or preincubation with the p38 inhibitor reduced MMP-13 mRNA levels. DDR-2 signaling was independent of the alpha2beta1 integrin and the interleukin-1-induced signaling pathways in chondrocytes.

CONCLUSION

These findings suggest that increased expression of DDR-2, resulting in the elevated expression of MMP-13, may be one of the common events in OA progression.

NAD(P)H oxidase activity of Nox4 in chondrocytes is both inducible and involved in collagenase expression

Reactive oxygen species (ROS) are regulators of redox-sensitive cell signaling pathways. In osteoarthritis, human interleukin-1beta is implicated in cartilage destruction through an ROS-dependent matrix metalloproteinase production. To determine the molecular source of ROS production in the human IL-1beta (hIL-1beta)-sensitive chondrocyte immortalized cell line C-20/A4, transfected cells were constructed that overexpress NAD(P)H oxidases. First, RT-PCR analysis showed that the C-20/A4 cell line expressed Nox2, Nox4, p22( phox ), and p67( phox ), but not p47( phox ). It was found that ROS production by C-20/A4 chondrocytes does not depend on PMA and ionomycin activation. This indicates that Nox2 was not involved in the production of ROS. In C- 20/A4 cells that overexpress Nox4, hIL-1beta stimulated ROS production three times more than the normal production of C-20/A4 cells. Moreover, there was a fourfold increase in the production of collagenase (MMP-1) by chondrocytes that overexpress Nox4. Interestingly, MMP-1 production in cells that overexpress Nox2 was not sensitive to hIL-1beta. These data suggest that under hIL-1beta stimulation, C-20/A4 chondrocytes produce MMP-1 through a Nox4-mediated, ROS-dependent pathway.

The Chondrocyte: Biology and Clinical Application

Chondrocyte is a unique cell type in articular cartilage tissue and is essential for cartilage formation and functionality. It arises from mesenchymal stem cells (MSCs) and is regulated by a series of cytokine and transcription factor interactions, including the transforming growth factor-beta super family, fibroblast growth factors, and insulin-like growth factor-1. To understand the biomechanisms of the chondrocyte differentiation process, various cellular model systems have been employed, such as primary chondrocyte culture, clonal normal cell lines (HCS-2/8, Ch-1, ATDC5, CFK-2, and RCJ3.1C5.18), and transformed clonal cell lines (T/C-28a2, T/C-28a4, C-28/I2, tsT/AC62, and HPV-16 E6/E7). Additionally, cell culture methods, including conventional monolayer culture, three-dimensional scaffold culture, bioreactor culture, pellet culture, and organ culture, have been established to create stable environments for the expansion, phenotypic maintenance, and subsequent biological study of chondrocytes for clinical application. Knowledge gained through these study systems has allowed for the use of chondrocytes in orthopedics for the treatment of cartilage injury and epiphyseal growth plate defects using tissue-engineering approaches. Furthermore, the potential of chondrocyte implantation for facial reconstruction, the treatment of long segmental tracheal defects, and urinary incontinence and vesicoureteral reflux are being investigated. This review summarizes the present study of chondrocyte biology and the potential uses of this cell in orthopedics and other disciplines.

Carboxymethyl-chitosan protects rabbit chondrocytes from interleukin-1beta-induced apoptosis

Chondrocyte apoptosis is important in pathogenesis of osteoarthritis. Chitosan is a non-toxic, biodegradable and biocompatible glycosaminoglycan. In this study, the effects of carboxymethyl-chitosan (CM-chitosan), a soluble derivative of chitosan, on chondrocyte apoptosis were investigated. Primary rabbit chondrocytes were cultured and induced to apoptosis by 10 ng/ml interleukin-1beta (IL-1beta). After treatment with various concentrations of CM-chitosan (50, 100, 200 microg/ml), the apoptotic rate, mitochondrial function, nitric oxide production, and the levels of inducible nitric oxide synthase (iNOS) mRNA and reactive oxygen species in IL-1beta-induced chondrocytes were examined. The results showed that CM-chitosan could inhibit chondrocyte apoptosis in a dose-dependent manner. Furthermore, it could partly restore the levels of mitochondrial membrane potential and ATP, decrease nitric oxide production by down-regulation of iNOS mRNA expression, and scavenge reactive oxygen species in chondrocytes induced by IL-1beta. The results suggested that the inhibitory effects of CM-chitosan on IL-1beta-induced chondrocyte apoptosis were possibly due to the protection of mitochondrial function, the decline in the levels of nitric oxide and reactive oxygen species.

Tissue engineered cartilage using human articular chondrocytes immortalized by HPV-16 E6 and E7 genes

Chondrocytes are useful as a cell culture system for studying arthritic degeneration in tissue engineered cartilage. However, primary chondrocytes have short in vitro lifespan and rapid shift of collagen phenotype. In this study, we used a high dosage of retroviral vector LXSN16E6E7 to transduce human primary chondrocytes and obtained an actively proliferating cell line, designated hPi, which expresses HPV-16 E6/E7 mRNA in early passages. Parental primary chondrocytes cease to grow after five passages, whereas hPi could be propagated beyond 100 passages without requiring additional cell elements in defined medium. After 48 passages, hPi can also give many profiles similar to those of parental primary chondrocyte, including type II collagen in mRNA and protein level, aggrecan in mRNA level, lacunae in type I collagen matrices, and morphology with GAG-specific Alcian blue staining. hPi has shown neoplastic transformation, as examined by NOD-SCID mice tumorigenicity assays for 3 months. Our results indicated that human primary chondrocytes could be immortalized by transduction with HPV-16 E6/E7, preserving stable cartilage-specific differentiation markers. The established chondrocyte cell line could provide a novel model to engineer cartilage in vitro and in vivo for cartilage repair research and clinical application.

A hypothesis for the origin and pathogenesis of rheumatoid diseases

It is well established that a correlation exists between rheumatoid arthritis (RA) and microbial damage. Material analyses have suggested that bacteria may be causative agents. This study was undertaken to further characterize the microbial agent responsible for pathogenesis of RA. In order to investigate whether substances in moist building materials can affect human cartilage, extracts from moist building materials were analysed for microbial components. Exposure of chondrocyte cultures to extracts in vitro showed that they were damaging the cultures. A direct correlation between strength of damage and concentration of MMP3 demonstrated that the effect was dose-dependent. High quantities of LPS were detected in the extracts. Experiments after deactivation with Polymyxin B showed that LPS are the causative agents. The present study leads to the hypothesis that LPS may bind to procollagen, as they bind to scavenger receptors. This procollagen endotoxin complex may block tropocollagen synthesis.

The PLAGL1 gene is down-regulated in human extraskeletal myxoid chondrosarcoma tumors

In approximately 70% of human extraskeletal myxoid chondrosarcoma (EMC) tumors, a t(9;22) chromosome translocation gives rise to a fusion protein, named EWS/NOR1, containing the amino-terminal domain of EWS fused to the complete amino acid sequence of the nuclear receptor NOR1. Several observations suggest that one role of EWS/NOR1 in EMC may be to deregulate the expression of specific genes involved in the tumoral process. In order to identify these genes, we have used a CFK2 chondrogenic cell line over-expressing EWS/NOR1. A differential display analysis has identified the PLAGL1 gene as being down-regulated in the CFK2(EWS/NOR1) cell line compared to native CFK2 cells. RT-PCR analyses show that whereas the PLAGL1 mRNAs encoding the two isoforms of the protein are highly expressed in four human chondrocyte immortalized cell lines and two human chondrocyte primary cultures, they are strongly down-regulated in six EMC tumors. We conclude that down-regulation of PLAGL1 may be a significant contributing factor in the development of EMC tumors.