The effects of pressure on chondrocyte tumour necrosis factor receptor expression

This work was performed to determine whether one aspect of load, pressure, could alter tumour necrosis factor (TNF) receptor type I (RI) expression on chondrocytes. Encapsulated tsT/AC62, osteoarthritic (OA) or non-arthritic (NA) chondrocytes were centrifuged at speeds representing 5 or 20 MPa, incubated for specific periods, released from alginate and TNFRI and II (TNFRII) expression determined by flow cytometry. Significant (p<0.05, n=4) changes in tsT/AC62 chondrocyte TNFRI expression were apparent 24 hours after application of 20 MPa. Five or 20 MPa increased OA chondrocyte TNFRI expression; chondrocytes from some OA patients were markedly sensitive to 20 MPa. NA chondrocyte TNFRI expression usually decreased in response to 5 and 20 MPa. Significant pressure-induced differences in TNFRI expression between NA and OA groups were apparent at 5, but not 20 MPa. Pressure did not significantly alter TNRFII expression on tsT/AC62, NA or OA chondrocytes. These results suggest a mechanism whereby sensitivity of chondrocytes to the effects of TNFalpha may be increased, in susceptible individuals, in regions of the joint that experience peak loading.

YY1 is a positive regulator of transcription of the Col1a1 gene

Both cell-specific and ubiquitous transcription factors in fibroblasts have been identified as critical for expression of the Col1a1 gene, which encodes the alpha1 chain of type I collagen. Here, we report that Yin Yang 1 (YY1) binds to the Col1a1 promoter immediately upstream of the TATA box, and we examine the functional implications of YY1 binding for regulation of Col1a1 gene expression in BALBc/3T3 fibroblasts. The Col1a1 promoter region spanning base pairs (bp) -56 to -9 bound purified recombinant YY1 and the corresponding binding activity in nuclear extracts was supershifted using a YY1-specific antibody. Mutation of the TATA box to TgTA enhanced YY1 complex formation. Mutation analysis revealed two YY1 core binding sites at -40/-37 bp (YY1A) and, on the reverse strand, at -32/-29 bp (YY1B) immediately adjacent to the TATA box. In transfections using Col1a1-luciferase constructs, mutation of YY1A decreased activity completely (wild-type p350 (p350wt), -222/+113 bp) or partially (p130wt, -84 bp/+13 bp), whereas mutation of YY1B blocked the expression of both promoter constructs. Cotransfection with pCMV-YY1 increased p350wt and p130wt activities by as much as 10-fold, whereas antisense YY1 decreased constitutive expression and blocked the increased activity due to pCMV-YY1 overexpression. The mTgTA constructs were devoid of activity, arguing for a requirement for cognate binding of the TATA box-binding protein (TBP). Electrophoretic mobility shift assays performed under conditions permitting TBP binding showed that recombinant TBP/TFIID and YY1 could bind to the -56/-9 bp fragment and that YY1B was the preferred site for YY1 binding. Our results indicate that YY1 binds to the Col1a1 proximal promoter and functions as a positive regulator of constitutive activity in fibroblasts. Although YY1 is not sufficient for transcriptional initiation, it is a required component of the transcription machinery in this promoter.

Anticytokine therapy for osteoarthritis

Osteoarthritis (OA) is a joint disease that involves degeneration of articular cartilage, weakening of the subchondral bone and limited intra-articular inflammation manifested by synovitis. Since the pathogenesis of OA involves multiple aetiologies, including mechanical, biochemical and genetic factors, it has been difficult to identify unique targets for therapy. Current pharmacological interventions focus primarily on improving symptoms. The rationale for the use of anticytokine therapy in OA is based on evidence from studies in vitro and in vivo that interleukin-1 (IL-1) and tumour necrosis factor (TNF)-alpha are the predominant pro-inflammatory and catabolic cytokines involved in the initiation and progression of articular cartilage destruction. Since the increased levels of catabolic enzymes, prostaglandins, nitric oxide (NO) and other markers in OA fluids and tissues appear to be related to elevated levels of IL-1 and TNF-alpha, therapies that interfere with the expression or actions of these cytokines are most promising. Other cytokines that are anti-inflammatory and are often detected, paradoxically, in OA tissues are also potential therapeutic agents for counteracting the cartilage destruction in OA. Identification of methods for early diagnosis is of key importance, since therapeutic interventions aimed at blocking or reversing structural damage will be more effective when there is the possibility of preserving normal homeostasis. At later stages, cartilage tissue engineering with or without gene therapy will also require anticytokine therapy to block damage to newly repaired cartilage. This review will focus on experimental approaches currently under study that may lead to elucidation of effective strategies for therapy in OA, with special emphasis on anticytokine therapy.

Expression of frizzled genes in mouse costochondral chondrocytes

Protein products of frizzled genes are cell membrane receptors for Wnt proteins that play multiple roles during development. We examined the expression of nine frizzled genes in mouse chondrocytes, and detected transcripts of six of the nine genes. We also cloned the entire cDNA of mouse frizzled-1 and compared its cDNA sequence and the cysteine-rich and transmembrane domains of its translated product to sequences of frizzled-1 from C. elegans, Drosophila, chicken and human. We used the T31 Mouse/Hamster radiation hybrid panel to map the mouse frizzled-1 to mouse chromosome 5 (5 cM from the centromere), and frizzled-9 to mouse chromosome 5 (74 cM from the centromere).

Reversal of autocrine and paracrine effects of interleukin 1 (IL-1) in human arthritis by type II IL-1 decoy receptor. Potential for pharmacological intervention

Interleukin 1 (IL-1), produced by both synovial cells and chondrocytes, plays a pivotal role in the pathogenesis of cartilage destruction in osteoarthritis (OA). We examined the specific expression and function of IL-1 receptor family-related genes in human joint tissues. Gene array analysis of human normal and OA-affected cartilage showed mRNA expression of IL-1 receptor accessory protein (IL-1RAcp) and IL-1 type I receptor (IL-1RI), but not IL-1 antagonist (IL-1ra) and IL-1 type II decoy receptor (IL-1RII). Similarly, human synovial and epithelial cells showed an absence of IL-1RII mRNA. Functional genomic analyses showed that soluble (s) IL-1RII, at picomolar concentrations, but not soluble TNF receptor:Fc, significantly inhibited IL-1beta-induced nitric oxide (NO) and/or prostaglandin E(2) production in chondrocytes, synovial and epithelial cells. In OA-affected cartilage, the IC(50) for inhibition of NO production by sIL-1RII was 2 log orders lower than that for sIL-1RI. Human chondrocytes that overexpressed IL-1RII were resistant to IL-1-induced IL-1beta mRNA accumulation and inhibition of proteoglycan synthesis. In osteoarthritis, deficient expression by chondrocytes of innate regulators or antagonists of IL-1 such as IL-1ra and IL-1RII (soluble or membrane form) may allow the catabolic effects of IL-1 to proceed unopposed. The sensitivity of IL-1 action to inhibition by sIL-1RII has therapeutic implications that could be directed toward correcting this unfavorable tissue(s) dependent imbalance.

Osteoarthritis and cartilage: the role of cytokines

The pathogenesis of osteoarthritis involves multiple etiologies, including mechanical, biochemical, and genetic factors that contribute to the imbalance in the synthesis and destruction of articular cartilage. It is now well documented that interleukin-1 and tumor necrosis factor-alpha are the predominant proinflammatory and catabolic cytokines involved in disease initiation and progression. Other proinflammatory cytokines may amplify or modulate this process, whereas anti-inflammatory cytokines, which are often detected, paradoxically, in osteoarthritis tissues, may counteract the tissue destruction and inflammation. This review focuses on the role of cytokines in the pathogenesis of osteoarthritis with special emphasis on how findings in culture and animal models may be reflected in the human disease process.

Proteoglycan production by immortalized human chondrocyte cell lines cultured under conditions that promote expression of the differentiated phenotype

Large and small proteoglycans are essential components of articular cartilage. How to induce chondrocytes to repair damaged cartilage with normal ratios of matrix components after their loss due to degenerative joint disease has been a major research focus. We have developed immortalized human chondrocyte cell lines for examining the regulation of cartilage-specific matrix gene expression. However, the decreased synthesis and deposition of cartilage matrix associated with a rapid rate of proliferation has presented difficulties for further examination at the protein level. In these studies, proteoglycan synthesis was characterized in two chondrocyte cell lines, T/C-28a2 and tsT/AC62, derived, respectively, from juvenile costal and adult articular cartilage, under culture conditions that either promoted or decreased cell proliferation. Analysis of proteo[36S]glycans by Sepharose CL-4B chromatography and SDS-PAGE showed that the large proteoglycan aggrecan and the small, leucine-rich proteoglycans, decorin and biglycan, were produced under every culture condition studied. In monolayer cultures, a high initial cell density and conditions that promoted proliferation (presence of serum for T/C-28a2 cells or permissive temperature for the temperature-sensitive tsT/AC62 cells) favored cell survival and ratios of proteoglycans expected for differentiated chondrocytes. However, the tsT/AC62 cells produced more proteoglycans at the nonpermissive temperature. Culture of cells suspended in alginate resulted in a significant decrease in proteoglycan production in all culture conditions. While the tsT/AC62 cells continued to produce a larger amount of aggrecan than small proteoglycans, the T/C-28a2 cells lost the ability to produce significant amounts of aggrecan in alginate culture. In addition, our data indicate that immortalized chondrocytes may alter their ability to retain pericellular matrix under changing culture conditions, although the production of the individual matrix components does not change. These findings provide critical information that will assist in the development of a reproducible chondrocyte culture model for the study of regulation of proteoglycan biosynthesis in cartilage.

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

OBJECTIVE

To develop a reproducible immortalized human chondrocyte culture model for studying the regulation of chondrocyte functions relevant to arthritic diseases in adult humans.

METHODS

Primary adult articular chondrocytes were immortalized with a retrovirus expressing a temperature-sensitive mutant of SV40-large T antigen (tsTAg). The established tsT/AC62 chondrocyte cell line was examined in monolayer and alginate culture systems. The levels of messenger RNA (mRNA) encoding cartilage matrix proteins and interleukin-1beta (IL-1beta)-inducible mRNA were analyzed by reverse transcriptase-polymerase chain reaction. Matrix protein synthesis was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 35S-sulfate-labeled proteoglycans and Western blotting of type II collagen and aggrecan. Type II collagen (COL2A1)-luciferase reporter gene expression was analyzed by transient transfection. Phosphorylated stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK), p38 mitogen-activated protein kinase (p38 MAPK), and activating transcription factor 2 (ATF-2) were detected by Western blotting.

RESULTS

The tsT/AC62 cells expressed TAg at the permissive temperature (32degrees C), and the loss of TAg at 37 degrees C and 39 degrees C correlated with decreased cell proliferation. Cells in alginate culture deposited abundant alcian blue-stainable matrix and continued to proliferate at 32 degrees C. Preferential retention of aggrecan was observed in the cell-associated matrix, while biglycan and decorin were secreted into the medium of monolayer and alginate cultures. The levels of COL2A1 and aggrecan mRNA were increased after transfer from monolayer to alginate culture at 32 degrees C. Treatment with IL-1beta decreased COL2A1 and aggrecan mRNA levels and increased the levels of matrix metalloproteinases 1, 3, and 13 mRNA, as well as those of cyclooxygenase 2, type I collagen, and secretory phospholipase A2 type IIA mRNA, but not those of inducible nitric oxide synthase mRNA. IL-1beta also stimulated phosphorylation of p38 MAPK, SAPK/JNK, and ATF-2. The p38 MAPK-selective inhibitor, SB203580, partially reversed IL-1beta-induced inhibition of COL2A1 mRNA levels and COL2A1-luciferase reporter gene expression.

CONCLUSION

The tsT/AC62 cells provide a reproducible model that mimics the adult articular chondrocyte phenotype, particularly in alginate culture, and demonstrates characteristic responses to IL-1beta. These studies also show, for the first time, that p38 MAPK is one of the signals required for IL-1beta-induced inhibition of COL2A1 gene expression. Availability of this model will permit identification of signals that regulate cytokine responses, and will also provide rational strategies for targeting these pathways.

Suppression of type I collagen gene expression by prostaglandins in fibroblasts is mediated at the transcriptional level

BACKGROUND

Tissues undergoing a chronic inflammatory process, such as the synovium in rheumatoid arthritis, are characterized by the infiltration of lymphocytes of different subsets and activation of monocyte/macrophages. Interleukin-1 (IL-1), a monocyte/ macrophage product that stimulates synovial fibroblasts to produce matrix metalloproteinases (MMPs), prostaglandins, and other cytokines, also has profound effects on the synthesis of extracellular matrix components such as type I collagen. In previous studies, we have shown that synovial fibroblasts and chondrocytes isolated from human joint tissues are particularly sensitive to prostaglandins, which modulate the effects of IL-1 on collagen gene expression in an autocrine manner.

MATERIALS AND METHODS

BALBc/3T3 fibroblasts were treated with IL-1 and prostaglandins in the absence and presence of indomethacin to inhibit endogenous prostaglandin biosynthesis. Collagen synthesis was analyzed by SDS-PAGE as [3H]proline-labeled, secreted proteins, and prostaglandin production and cyclic adenosine 3',5'-cyclic monophosphate (camp) content were assayed. The expression of type I collagen gene (Col1a1) promoter-reporter gene constructs was examined in transient transfection experiments, and the binding of nuclear factors to the Col1a1 promoter region spanning -222 bp/+ 116 bp was analyzed by DNase I footprinting and electrophoretic mobility shift (EMSA) assays.

RESULTS

IL-1 increased the synthesis of type I and type III collagens in BALBc/3T3 fibroblasts; greater increases were observed when IL-1-stimulated synthesis of PGE2 was blocked by indomethacin. Transient transfection experiments demonstrated dose-dependent inhibition of the-222 bp Col1a1 promoter by exogenously added prostaglandins with the order of potency of PGF2alpha > PGE2 > PGE1 DNase I footprinting showed increased protection, which extended from the region immediately upstream of the TATA box, owing to the binding of nuclear factors from PGE2- or PGE1-treated BALBc/3T3 cells. EMSA analysis showed zinc-dependent differences in the binding of nuclear factors from untreated and prostaglandin-treated cells to the -84 bp/-29 bp region of the Col1a1 promoter.

CONCLUSIONS

These results show that the inhibition of Col1a1 expression by IL-1 in fibroblasts is mediated by prostaglandins at the transcriptional level and suggest that PGE-responsive factors may interact directly or indirectly with basal regulatory elements in the proximal promoter region of the Col1a1 gene.

Differential effects of IGF-binding proteins, IGFBP-3 and IGFBP-5, on IGF-I action and binding to cell membranes of immortalized human chondrocytes

Insulin-like growth factor-I (IGF-I) is an important anabolic factor for cartilage tissue and its action is, in part, regulated by IGF-binding proteins (IGFBPs). The object of this study was to investigate the effects of IGFBPs on IGF-I action and on binding of IGF-I to cells using a reproducible immortalized human chondrocyte culture model. Treatment of the C-28/I2 cells with IGF-I or des(1-3)IGF-I in serum-free medium stimulated cell proliferation in a dose-dependent manner. However, the effect of des(1-3)IGF-I was more potent, thereby suggesting that endogenously produced IGFBPs inhibited IGF action. The stimulatory effect of IGF-I was inhibited significantly by addition of IGFBP-3 but enhanced slightly by IGFBP-5. However, neither IGFBP-3 nor IGFBP-5 had an effect on basal cell growth. Binding of (125)I-labeled IGF-I to the cells was displaced by both IGFBP-3 and IGFBP-5, although higher concentrations of unlabeled IGFBP-5 were required to displace IGF-I to the same extent as IGFBP-3. Treatment of the cells with IGF-I increased the levels of IGFBP-5 protein measured by Western ligand blotting, and stimulated a corresponding increase in IGFBP-5 mRNA while increasing type II collagen mRNA. Our findings indicate that the balance between IGFBP-3 and IGFBP-5 influences IGF receptor binding and its action on chondrocyte proliferation, and may thereby modulate cartilage metabolism.

The role of cytokines as inflammatory mediators in osteoarthritis: lessons from animal models

Studies in animal models of osteoarthritis (OA) have been used extensively to gain insight into the pathogenesis of OA, but early studies largely ignored inflammation except as a secondary phenomenon. Synovitis has often been noted as a feature in experimental OA, and more recent work has established a central role for inflammatory cytokines as biochemical signals which stimulate chondrocytes to release cartilage-degrading proteinases. Thus, proteinase inhibitors, cytokine antagonists and receptor blocking antibodies, and growth/differentiation factors have been considered as potential therapeutic agents and targets for gene therapy. Although there is some disagreement, it is generally accepted that IL-1 is the pivotal cytokine at early and late stages, while TNF-alpha is involved primarily in the onset of arthritis. Other cytokines released during the inflammatory process in the OA joint may be regulatory (IL-6, IL-8) or inhibitory (IL-4, IL-10, IL-13, IFN-gamma). Furthermore, studies in animal models have illustrated the potentially beneficial effects of anticytokine therapy with monoclonal antibodies or receptor antagonists, although local rather than systemic delivery would be necessary for the largely localized OA in humans. Transgenic or knockout mice have also provided insights into general mechanisms of cytokine-induced cartilage degradation but have not directly addressed OA pathogenesis. Similarly, animals with spontaneous or transgenic modifications in cartilage matrix components, growth/differentiation factors, or developmentally regulated transcription factors have provided information about potential gene defects that predispose to OA without addressing the role of inflammatory mediators in cartilage destruction. Although the multiple etiologies of human OA indicate that it is more complex than any animal model, the use of appropriate, well-defined animal models will establish the feasibility of novel forms of therapy.

Inhibition of caspase-3-like activity prevents apoptosis while retaining functionality of human chondrocytes in vitro

Apoptosis was induced in a human chondrocyte cell line, T/C 28a4, by treatment with various stimuli, including camptothecin, tumor necrosis factor-alpha, staurosporine, okadaic acid, and reduced serum conditions. All stimuli induced a cytosolic DEVDase activity, coincident with apoptosis. Caspase activities in the lysates were characterized and quantitated with peptide cleavage profiles. To confirm that the results were not related to the immortalized nature of the cell line, primary human chondrocytes also were shown to undergo apoptosis under similar conditions, which resulted in increased cytosolic DEVDase activity. There was little or no caspase-1 (interleukin-1beta-converting enzyme) or caspase-8-like activity in the apoptotic cells. In all cases, the irreversible nonselective caspase inhibitor, Z-VAD-FMK, and the caspase-3-selective inhibitor, Ac-DMQD-CHO, inhibited DEVDase activity and apoptosis, whereas the caspase-1-selective inhibitor, Ac-YVAD-CHO, had no effect. Human chondrocytes were stably and transiently transfected with a type-II collagen gene (COL2A1) regulatory sequence driving a luciferase reporter as a specific marker of chondrocyte gene expression. Treatment of the cells with camptothecin or tumor necrosis factor-alpha plus cycloheximide significantly inhibited COL2A1 transcriptional activity. Significantly, cotreatment with Z-VAD-FMK or Ac-DMQD-CHO maintained COL2A1-reporter gene activity, indicating that the prevention of apoptosis by caspase-3 inhibition was sufficient to maintain cell functionality as assessed by the retention of type-II collagen promoter activity.

Integrin expression by primary and immortalized human chondrocytes: evidence of a differential role for alpha1beta1 and alpha2beta1 integrins in mediating chondrocyte adhesion to types II and VI collagen

OBJECTIVE

Chondrocytes have been shown to express beta1-containing integrins both in vitro and in situ, but their role in regulating chondrocyte function is poorly understood. The objective of this study was to determine how the relative expression of different integrins may be modulated in relation to the differentiated state and proliferative capacity of the chondrocyte.

DESIGN

Integrin expression by four different cell lines of human chondrocytes immortalized with Simian virus 40 large T-antigen (SV40-TAg) was studied and compared to primary chondrocytes. Differences in alpha1 and alpha2 integrin subunit expression were utilized to further study the role of these integrins in mediating adhesion to types II and VI collagen.

RESULTS

The overall cell-surface levels of beta1-containing integrins were higher on all four immortalized cell lines which expressed over 10-fold higher levels of alpha2 and alpha3 integrin subunits compared to primary cells. However, primary cells expressed higher levels of the alpha1 integrin subunit which was not expressed by T/C28a4 cells and expressed at variable and lower levels in the other lines. Levels of the alpha3 integrin subunit were significantly greater on the highly proliferative juvenile costal chondrocyte lines (T/C-28a4, C-2812, and C-20a4) compared to primary articular chondrocytes and tsT/AC-62 cells which were derived from adult articular chondrocytes. Expression of alpha5 was similar among primary cells and cell lines except on C-20/A4 cells which had an average of over 4-fold higher levels. None of the primary or immortalized chondrocytes tested expressed significant levels of alpha4. Cell adhesion assays revealed that both alpha1beta1 and alpha2beta1 could serve as chondrocyte adhesion receptors for types II and VI collagen. In cell lines expressing both integrins, alpha1beta1 was the preferential receptor for type VI collagen while alpha2beta1 was the preferential receptor for type II collagen. Rather than inhibiting adhesion, incubation with the alpha3 blocking antibody P1B5 increased adhesion of C-28/12 cells to both fibronectin and type II collagen by 67% and 100% respectively.

CONCLUSIONS

Immortalization with SV40-TAg results in altered integrin expression by chondrocytes. Changes in the relative levels of alpha1, alpha2, and alpha3 subunits may significantly alter the manner in which chondrocytes interact with types II and VI collagen in the extracellular matrix.

Differential mechanisms of inorganic pyrophosphate production by plasma cell membrane glycoprotein-1 and B10 in chondrocytes

OBJECTIVE

Increased nucleoside triphosphate pyrophosphohydrolase (NTPPPH) activity in chondrocytes is associated with cartilage matrix inorganic pyrophosphate (PPi) supersaturation in chondrocalcinosis. This study compared the roles of the transforming growth factor beta (TGFbeta)-inducible plasma cell membrane glycoprotein-1 (PC-1) and the closely related B10 NTPPPH activities in chondrocyte PPi metabolism.

METHODS

NTPPPH expression was studied using reverse transcriptase-polymerase chain reaction and Western blotting. Transmembrane PC-1 (tmPC-1), water-soluble secretory PC-1 (secPC-1), and transmembrane B10 were expressed by adenoviral gene transfer or plasmid transfection, and expression of PPi was assessed in cultured articular chondrocytes and immortalized NTPPPH-deficient costal chondrocytes (TC28 cells).

RESULTS

PC-1 and B10 messenger RNA were demonstrated in articular cartilages in situ, in untreated cultured normal articular chondrocytes, and in TC28 cells. Expression of tmPC-1 and secPC-1, but not B10, rendered the NTPPPH-deficient TC28 cells able to increase expression of extracellular PPi, with or without addition of TGFbeta (10 ng/ml) to the media. More plasma membrane NTPPPH activity was detected in cells transfected with tmPC-1 than in cells transfected with B10. Furthermore, confocal microscopy with immunofluorescent staining of articular chondrocytes confirmed preferential plasma membrane localization of PC-1, relative to B10. Finally, both PC-1 and B10 increased the levels of intracellular PPi, but PC-1 and B10 appeared to act principally in different intracellular compartments (Golgi and post-Golgi versus pre-Golgi, respectively).

CONCLUSION

PC-1 and B10 NTPPPH activities were not redundant in chondrocytes. Although increased PC-1 and B10 expression caused elevations in intracellular PPi, the major effects of PC-1 and B10 were exerted in distinct subcellular compartments. Moreover, PC-1 (transmembrane and secreted), but not B10, increased the levels of extracellular PPi. Differential expression of PC-1 and B10 could modulate cartilage mineralization in degenerative joint diseases.

Human chondrocyte culture models for studying cyclooxygenase expression and prostaglandin regulation of collagen gene expression

Objective Since articular chondrocytes and synovial fibroblasts are particularly responsive to interleukin-1 (IL-1) with respect to stimulation of prostaglandin E(2)(PGE(2)) biosynthesis, we have used them as models to examine feedback modulatory effects of PGE(2), which blocks or attenuates the direct effects of IL-1beta on cell-specific collagen gene expression. Methods Immortalized human chondrocytes were developed for studying responses to cytokines and prostaglandins. Regulatory sequences of the type II collagen gene (COL2A1) in reporter gene constructs were analyzed in transient transfection experiments. Endogenous expression of COL2A1 mRNA, as well as aggrecan, biglycan, and decorin mRNAs, and IL-1-inducible cyclooxygenase (COX-2), phospholipase A2 (PLA2), and inducible nitric oxide synthetase (iNOS) mRNAs were analyzed by RT-PCR. Results Previous work has shown that IL-1beta inhibits, while prostaglandins stimulate COL2A1 expression. In different immortalized chondrocyte cell lines, the ability to respond to IL-1beta with increased levels of COX-2, PLA2, and iNOS mRNAs depends upon expression of the differentiated chrondrocyte phenotype. Conclusions Our studies suggest that some IL-1-induced responses in chondrocytes may require differentiation-specific transcription factors that could serve as therapeutic targets for arthritis.

The EWS/TEC fusion protein encoded by the t(9;22) chromosomal translocation in human chondrosarcomas is a highly potent transcriptional activator

The EWS/TEC gene fusion generated by the t(9;22) chromosomal translocation found in extraskeletal myxoid chondrosarcomas encodes a fusion protein containing the amino-terminal domain of the EWS protein fused to the whole coding sequence of the orphan nuclear receptor TEC. We have compared the DNA-binding and transcriptional activation properties of various TEC isoforms and the corresponding EWS/TEC fusion proteins. Band-shift experiments show that the full-length TEC receptor can efficiently bind the NGFI-B Response Element (NBRE), whereas an isoform lacking the entire carboxyl-terminal domain of the receptor binds much less efficiently the NBRE. Addition of the amino-terminal domain of EWS to either isoforms does not alter significantly their DNA-binding properties to the NBRE. Co-transfection experiments of COS cells and human chondrocytes indicate that whereas TEC moderately activates transcription from a NBRE-containing promoter, the corresponding EWS/TEC fusion protein is a highly potent transcriptional activator of the same promoter, being approximately 270-fold more active than the native receptor. EWS/TEC may thus exert its oncogenic potential in chrondrosarcomas by activating the transcription of target genes involved in cell proliferation.

Parathyroid hormone-related proteins is abundant in osteoarthritic cartilage, and the parathyroid hormone-related protein 1-173 isoform is selectively induced by transforming growth factor beta in articular chondrocytes and suppresses generation of extracellular inorganic pyrophosphate

OBJECTIVE

Parathyroid hormone-related protein (PTHrP) is a major, locally expressed regulator of growth cartilage chondrocyte proliferation, differentiation, synthetic function, and mineralization. Because mechanisms that limit cartilage chondrocytes from maturing and mineralizing are diminished in osteoarthritis (OA), we studied PTHrP expression by articular chondrocytes.

METHODS

PTHrP was studied in normal knee cartilage samples and cultured articular chondrocytes, and in cartilage specimens from knees with advanced OA, obtained at the time of joint replacement.

RESULTS

PTHrP was more abundant in OA than in normal human knee articular cartilage. Both demonstrated PTH/PTHrP receptor expression. PTHrP 1-173, one of three alternatively spliced PTHrP isoforms, was exclusively expressed and induced by transforming growth factor beta in cultured chondrocytes. Chondrocytes mainly used the GC-rich P2 alternative promoter to express PTHrP messenger RNA. Inhibition by PTHrP 1-173, but not by PTHrP 1-146 or PTHrP 1-87, of inorganic pyrophosphate (PPi) elaboration suggested selective functional properties of the 1-173 isoform. Exposure to a neutralizing antibody to PTHrP increased PPi elaboration by articular chondrocytes.

CONCLUSION

Increased expression of PTHrP, including the 1-173 isoform, has the potential to contribute to the pathologic differentiated functions of chondrocytes, including mineralization, in OA.

The role of oncostatin M in animal and human connective tissue collagen turnover and its localization within the rheumatoid joint

OBJECTIVE

To study the interaction of interleukin-1alpha (IL-1alpha) and oncostatin M (OSM) in promoting cartilage collagen destruction.

METHODS

Bovine, porcine, and human cartilage and human chondrocytes were studied in culture. The levels of collagenase (matrix metalloproteinase 1 [MMP-1]) and tissue inhibitor of metalloproteinases 1 (TIMP-1) were measured by bioassay and enzyme-linked immunosorbent assay (ELISA). The levels of OSM in rheumatoid synovial fluid were measured by ELISA.

RESULTS

When combined with OSM, IL-1alpha, IL-1beta, and tumor necrosis factor alpha released proteoglycan and collagen from cartilage. OSM was the only member of the IL-6 family to have this effect. Human tendon also responded to IL-1alpha and OSM. OSM increased the production of MMP-1 and TIMP-1 but when combined with IL-1alpha, synergistically promoted MMP-1 production in human chondrocytes and synovial fibroblasts. High levels of OSM were found in human rheumatoid synovial fluids, and confocal microscopy showed that OSM was produced by macrophages in rheumatoid synovial tissue.

CONCLUSION

These results highlight an important new mechanism by which there is irreversible loss of collagen from cartilage.

Artificial cavitation nuclei significantly enhance acoustically induced cell transfection

The efficiency of ultrasound-mediated gene transfection was enhanced three- to fourfold, compared to previous results, through the use of green fluorescent protein reporter gene, cultured immortalized human chondrocytes and artificial cavitation nuclei in the form of Albunex. Cells were exposed to 1.0-MHz ultrasound transmitted through the bottom of six-well culture plates containing immortalized chondrocytes, media, DNA at a concentration of 40 micrograms/mL and Albunex at 50 x 10(6) bubbles/mL. Transfection efficiency increased linearly with ultrasound exposure pressure with a transfection threshold observed at a spatial average peak positive pressure (SAPP) of 0.12 MPa and reaching about 50% of the living cells when exposed to 0.41 MPa SAPP for 20 s. Adding fresh Albunex at 50 x 10(6) bubbles/mL prior to sequential 1-s, 0.32- or 0.41-MPa exposures increased transfection with each exposure, reaching 43% transfection after four exposures. Efficient in vitro and in vivo transfection now appear possible with these enhancements.

Hsp70 accumulation in chondrocytic cells exposed to high continuous hydrostatic pressure coincides with mRNA stabilization rather than transcriptional activation

In response to various stress stimuli, heat shock genes are induced to express heat shock proteins (Hsps). Previous studies have revealed that expression of heat shock genes is regulated both at transcriptional and posttranscriptional level, and the rapid transcriptional induction of heat shock genes involves activation of the specific transcription factor, heat shock factor 1 (HSF1). Furthermore, the transcriptional induction can vary in intensity and kinetics in a signal- and cell-type-dependent manner. In this study, we demonstrate that mechanical loading in the form of hydrostatic pressure increases heat shock gene expression in human chondrocyte-like cells. The response to continuous high hydrostatic pressure was characterized by elevated mRNA and protein levels of Hsp70, without activation of HSF1 and transcriptional induction of hsp70 gene. The increased expression of Hsp70 was mediated through stabilization of hsp70 mRNA molecules. Interestingly, in contrast to static pressurization, cyclic hydrostatic loading did not result in the induction of heat shock genes. Our findings show that hsp70 gene expression is regulated posttranscriptionally without transcriptional induction in chondrocyte-like cells upon exposure to high continuous hydrostatic pressure. We suggest that the posttranscriptional regulation in the form of hsp70 mRNA stabilization provides an additional mode of heat shock gene regulation that is likely to be of significant importance in certain forms of stress.

NADPH oxidase of chondrocytes contains an isoform of the gp91phox subunit

Previously it has been reported that chondrocytic cells produce oxygen free radicals and express the cytosolic components of NADPH oxidase. Here we report the expression of large subunit of the flavocytochrome of NADPH oxidase in chondrocytes and, further, show that the cDNA sequence contains three single base pair differences compared with the phagocyte gp91phox gene sequence. These base-pair differences may account for the different activity profiles reported between phagocytic and non-phagocytic cells.

Human chondrocyte expression of growth-arrest-specific gene 6 and the tyrosine kinase receptor axl: potential role in autocrine signaling in cartilage

OBJECTIVE

To determine if human articular chondrocytes express the axl tyrosine kinase receptor and its ligand Gas-6, a protein product of growth-arrest-specific gene 6, and to determine if Gas-6 and axl function in the regulation of chondrocyte growth and survival.

METHODS

The presence of Gas-6 and axl was examined in situ in human articular cartilage by immunohistochemistry and in vitro in cell culture studies using primary human chondrocytes and immortalized human chondrocytes. The ability of recombinant Gas-6 to mediate adhesion of chondrocytes and to stimulate chondrocyte axl phosphorylation was determined. Studies of the role of Gas-6 and axl in cell proliferation and survival were also performed.

RESULTS

Both Gas-6 and axl were detected in cartilage by immunohistochemical staining. Gas-6 and axl messenger RNA (mRNA) and protein were also detected in cultures of primary and immortalized human chondrocytes. Compared with cells cultured in medium containing 10% serum, Gas-6 mRNA levels were increased in immortalized chondrocytes cultured in serum-free medium, while axl expression decreased. Chondrocytes attached to Gas-6-coated plastic, and the attachment was blocked by a soluble Ig fusion protein containing the axl extracellular domain. Recombinant human Gas-6 and serum-free conditioned medium from primary and immortalized human chondrocyte cultures stimulated chondrocyte axl tyrosine phosphorylation. A mitogenic effect was noted both when immortalized chondrocytes were stimulated with recombinant Gas-6 or when they were made to overexpress axl by transfection. Addition of recombinant Gas-6 to serum-free medium resulted in increased survival of primary chondrocytes cultured at low density in agarose.

CONCLUSION

These findings present evidence for an autocrine signaling pathway in cartilage involving Gas-6 and the axl tyrosine kinase adhesion receptor. Stimulation of axl by Gas-6 may play an important role in the control of chondrocyte growth and survival.