Signal transduction through chondrocyte integrin receptors induces matrix metalloproteinase synthesis and synergizes with interleukin-1

Osteopontin: an intrinsic inhibitor of inflammation in cartilage

OBJECTIVE

To identify extracellular and intraarticular matrix components that are differentially expressed in normal and osteoarthritis (OA)-affected cartilage and to investigate their functions with respect to regulation of mediators of inflammation.

METHODS

Differential-display reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of a pool of messenger RNA (mRNA) from 10 human OA cartilage samples and 5 normal cartilage samples was performed using arbitrary primers. Confirmatory analysis of the up-regulated transcripts of fibronectin (FN) and osteopontin (OPN) was performed by RT-PCR of individual RNA samples from a separate set of donors. The effect of recombinant OPN (or anti-OPN antiserum) on chondrocyte function was examined by analyzing the spontaneous or interleukin-1 (IL-1)-induced release of nitric oxide (NO) and prostaglandin E2 (PGE2) from human OA-affected cartilage under ex vivo conditions.

RESULTS

Up-regulation (300-700%) of FN and OPN mRNA was observed in human OA-affected cartilage as compared with normal cartilage. Functional analysis of the role of OPN in OA cartilage showed that 1) Addition of 1 microg/ml (20 nM) of recombinant OPN to human OA-affected cartilage under ex vivo conditions inhibited spontaneous and IL-1beta-induced NO and PGE2 production, and 2) neutralization of intraarticular OPN with anti-OPN antiserum augmented NO production.

CONCLUSION

The data indicate that one of the functions of intraarticular OPN, which is overexpressed in OA cartilage, is to act as an innate inhibitor of IL-1, NO, and PGE2 production. These findings suggest that the production of pleiotropic mediators of inflammation that influence cartilage homeostasis, such as NO and PGE2, is regulated by the interaction of chondrocytes with differentially expressed proteins within the extracellular matrix.

In vitro evidence for effects of magnesium supplementation on quinolone-treated horse and dog chondrocytes

Quinolones and magnesium deficiency cause similar lesions in joint cartilage of young animals. Chondrocytes cultivated in the presence of quinolones and in Mg-free medium show severe alterations in cytoskeleton and decreased ability to adhere to the culture dish. We investigated whether Mg2+ supplementation can prevent quinolone-mediated effects on chondrocytes in vitro. Chondrocytes cultivated in Dulbecco's modified Eagle's medium/HAM's F-12 medium were treated with ciprofloxacin (80 and 160 microg/ml) and enrofloxacin (100 and 150 microg/ml). Mg2+ was added at a concentration of 0.0612 mg/ml (MgCl) and 0.0488 mg/ml (MgSO4) or a triple dose. In addition, cells were cultivated in Mg-free medium and accordingly treated with Mg2+ supplementation. After 5 days in culture, the number of adherent cells per milliliter was determined. The number of chondrocytes in quinolone-treated groups decreased to 12-36% that of the control group within the culture period. With Mg2+ supplementation, the number of attached cells increased to 40-70% that of control cells. The threefold dose of Mg2+ led to better results than did the single dose. Cell proliferation tested by immunohistochemical staining with Ki67 (clone MIB5) decreased from 70% in control groups to 55%, 48%, and 30% in enrofloxacin-treated groups in a concentration dependent manner (50, 100, and 150 microg/ml). Addition of Mg2+ did not increase the rate of cell proliferation. These results suggest that a great part of quinolone-induced damage is due to magnesium complex formation, as Mg2+ supplementation is able to reduce the effects in vitro. However, quinolone effects on cell proliferation seem to be an independent process that is not influenced by magnesium supplementation.

COX-2, NO, and cartilage damage and repair

The production of nitric oxide (NO) and prostaglandin E2 (PGE(2)) is increased in human osteoarthritis-affected cartilage. These and other inflammatory mediators are spontaneously released by OA cartilage explants ex vivo. The excessive production of nitric oxide inhibits matrix synthesis, and promotes its degradation. Furthermore, by reacting with oxidants such as superoxide anion, nitric oxide promotes cellular injury, and renders the chondrocyte susceptible to cytokine-induced apoptosis. PGE(2) exerts both anabolic and catabolic effects on chondrocytes, depending on the microenvironment and physiological condition. Thus, NO and PGE(2), produced by activated chondrocytes in diseased cartilage, may modulate disease rogression in osteoarthritis, and should therefore be considered potential targets for therapeutic intervention

The effects of high magnitude cyclic tensile load on cartilage matrix metabolism in cultured chondrocytes

Excessive mechanical load is thought to be responsible for the onset of osteoarthrosis (OA), but the mechanisms of cartilage destruction caused by mechanical loads remain unknown. In this study we applied a high magnitude cyclic tensile load to cultured chondrocytes using a Flexercell strain unit, which produces a change in cell morphology from a polygonal to spindle-like shape, and examined the protein level of cartilage matrixes and the gene expression of matrix metalloproteinases (MMPs), tissue inhibitors of matrix metalloproteinases (TIMPs) and proinflammatory cytokines such as IL-1beta and TNF-alpha. Toluidine blue staining, type II collagen immunostaining, and an assay of the incorporation of [35S]sulfate into proteoglycans revealed a decrease in the level of cartilage-specific matrixes in chondrocyte cultures subjected to high magnitude cyclic tensile load. PCR-Southern blot analysis showed that the high magnitude cyclic tensile load increased the mRNA level of MMP-1, MMP-3, MMP-9, IL-1beta, TNF-alpha and TIMP-1 in the cultured chondrocytes, while the mRNA level of MMP-2 and TIMP-2 was unchanged. Moreover, the induction of MMP-1, MMP-3 and MMP-9 mRNA expression was observed in the presence of cycloheximide, an inhibitor of protein synthesis. These findings suggest that excessive mechanical load directly changes the metabolism of cartilage by reducing the matrix components and causing a quantitative imbalance between MMPs and TIMPs.

Expression of osteopontin messenger RNA and protein in rheumatoid arthritis: effects of osteopontin on the release of collagenase 1 from articular chondrocytes and synovial fibroblasts

OBJECTIVE

Osteopontin (OPN) is an extracellular matrix protein that has been implicated in the interactions between tumor cells and host matrix, including those involved in invasion and spread of tumor cells. Because joint destruction in rheumatoid arthritis (RA) is mediated by the invasive growth of synovial tissue through its attachment to cartilage, we examined the expression of OPN in the synovia of patients with RA and the effect of OPN on the production of collagenase 1 in rheumatoid synovial fibroblasts and articular chondrocytes.

METHODS

The expression of OPN messenger RNA (mRNA) and protein in synovia from 10 RA patients was examined by in situ hybridization and immunohistochemistry. Synovial fibroblasts from RA patients and articular chondrocytes from patients without joint disease were cultured in the presence of various concentrations of OPN, and levels of collagenase 1 in the culture supernatants were measured by enzyme-linked immunosorbent assay.

RESULTS

The expression of OPN mRNA and protein was observed in 9 of 10 specimens obtained from patients with RA. OPN was expressed in the synovial lining and sublining layer and at the interface of cartilage and invading synovium. Double labeling revealed that the majority of OPN-expressing cells were positive for the fibroblast-specific enzyme prolyl 4-hydroxylase and negative for the macrophage marker CD68, while only a few, single OPN-expressing cells were positive for CD68 at sites of synovial invasion into cartilage. OPN staining was not observed in lymphocytic infiltrates or leukocyte common antigen (CD45)-positive cells. Three of 3 cultures of human articular chondrocytes secreted detectable basal amounts of collagenase, with a dose-dependent increase upon OPN stimulation, while synovial fibroblast cultures produced much lower levels of collagenase, with only 2 of 4 fibroblast cultures responding in a dose-dependent manner.

CONCLUSION

These findings suggest that OPN produced by synovial fibroblasts in the synovial lining layer and at sites of cartilage invasion not only mediates attachment of these cells to cartilage, but also contributes to matrix degradation in RA by stimulating the secretion of collagenase 1 in articular chondrocytes.

Interleukin-1alpha-dependent regulation of matrix metalloproteinase-9(MMP-9) secretion and activation in the epithelial cells of odontogenic jaw cysts

Interleukin-1alpha (IL-1alpha) and matrix metalloproteinase-9 (MMP-9) are thought to be involved in odontogenic cyst expansion. In this study, we investigated the effects of IL-1alpha on the secretion and activation of MMP-9 in odontogenic jaw cysts. An active form of MMP-9 was present in odontogenic keratocyst (6 of 8 cases) fluids more frequently than dentigerous cyst (3 of 10 cases) and radicular cyst (3 of 10 cases) fluids, although proMMP-9 was present in all cyst fluids. Odontogenic keratocyst fragments in explant culture secreted a larger amount of IL-1alpha than dentigerous cyst and radicular cyst fragments in explant culture, and spontaneously secreted both proMMP-9 and an active form of MMP-9. The fragments of dentigerous cysts and radicular cysts secreted a small amount of proMMP-9, but no active form of MMP-9. Exogenously added recombinant human IL-1alpha (rhlL-1alpha) increased the secretion and activation of proMMP-9 in the fragments of dentigerous cysts and radicular cysts. The epithelial cells isolated from odontogenic keratocysts secreted IL-1alpha and proMMP-9 without stimulation. Under the cultivation on a fibronectin-coated dish, rhIL-1alpha increased the secretion of proMMP-9 from the epithelial cells in a dose-dependent manner. Moreover, rhIL-1alpha induced the secretion of proMMP-3 and plasminogen activator urokinase (u-PA) from the epithelial cells, and converted the secreted proMMP-3 to the active form in the presence of plasminogen. The secreted proMMP-9 was also activated in the presence of rhIL-1alpha and plasminogen. Hence, our results suggest that IL-1alpha may up-regulate not only proMMP-9 secretion but also proMMP-9 activation by inducing proMMP-3 and u-PA production in the cyst epithelial cells by autocrine/paracrine regulatory mechanisms.

Functional genomic analysis in arthritis-affected cartilage: yin-yang regulation of inflammatory mediators by alpha 5 beta 1 and alpha V beta 3 integrins

Osteoarthritis-affected cartilage exhibits enhanced expression of fibronectin (FN) and osteopontin (OPN) mRNA in differential display and bioinformatics screen. Functional genomic analysis shows that the engagement of the integrin receptors alpha 5 beta 1 and alpha v beta 3 of FN and OPN, respectively, have profound effects on chondrocyte functions. Ligation of alpha 5 beta 1 using activating mAb JBS5 (which acts as agonist similar to FN N-terminal fragment) up-regulates the inflammatory mediators such as NO and PGE2 as well as the cytokines, IL-6 and IL-8. Furthermore, up-regulation of these proinflammatory mediators by alpha 5 beta1 integrin ligation is mediated via induction and autocrine production of IL-1 beta, because type II soluble IL-1 decoy receptor inhibits their production. In contrast, alpha v beta 3 complex-specific function-blocking mAb (LM609), which acts as an agonist similar to OPN, attenuates the production of IL-1 beta, NO, and PGE2 (triggered by alpha 5 beta 1, IL-1 beta, IL-18, or IL-1 beta, TNF-alpha, plus LPS) in a dominant negative fashion by osteoarthritis-affected cartilage and activated bovine chondrocytes. These data demonstrate a cross-talk in signaling mechanisms among integrins and show that integrin-mediated "outside in" and "inside out" signaling very likely influences cartilage homeostasis, and its deregulation may play a role in the pathogenesis of osteoarthritis.

Phenotypic stability of articular chondrocytes in vitro: the effects of culture models, bone morphogenetic protein 2, and serum supplementation

Numerous in vitro culture models have been developed for the investigation of chondrocyte and cartilage biology. In this study, we investigated the stability of the chondrocytic phenotype in monolayer, aggregate, pellet, and explant culture models and assessed the effects of recombinant human bone morphogenetic protein 2 (rhBMP-2) and serum supplementation on the phenotype in each model. Phenotypic effects were assessed by analyses of procollagen type II, aggrecan, (V + C)- fibronectin, and procollagen type I messenger RNA expression. In monolayer cultures, we noted a characteristic loss of procollagen type II and induction of procollagen type I expression. The aggregate and pellet culture models supported matrix protein gene expression profiles more reflective of in vivo levels. In explant cultures, expression of matrix protein genes was consistently depressed. Treatment with rhBMP-2 significantly increased the expression of procollagen type II and aggrecan in monolayer cultures; however, other models showed comparatively little response. Similarly, serum supplementation significantly down-regulated procollagen type II and aggrecan expression in monolayer cultures but had less effect on gene expression in the other models. Serum supplementation increased procollagen type I expression in monolayer and aggregate cultures. These results suggest that the influence of exogenous BMP-2 and serum on expression of chondrocyte-specific matrix protein genes is influenced by aspects of substrate attachments, cellular morphology, and/or cytoskeletal organization. Finally, the analyses of fibronectin expression suggest that V and C region alternative splicing in chondrocytes is linked to the establishment of a three-dimensional multicellular complex.

Fc gamma R expression on macrophages is related to severity and chronicity of synovial inflammation and cartilage destruction during experimental immune-complex-mediated arthritis (ICA)

STATEMENT OF FINDINGS: We investigated the role of Fc gamma receptors (Fc gamma Rs) on synovial macrophages in immune-complex-mediated arthritis (ICA). ICA elicited in knee joints of C57BL/6 mice caused a short-lasting, florid inflammation and reversible loss of proteoglycans (PGs), moderate chondrocyte death, and minor erosion of the cartilage. In contrast, when ICA was induced in knee joints of Fc receptor (FcR) gamma-chain(-/-) C57BL/6 mice, which lack functional Fc gamma RI and RIII, inflammation and cartilage destruction were prevented. When ICA was elicited in DBA/1 mice, a very severe, chronic inflammation was observed, and significantly more chondrocyte death and cartilage erosion than in arthritic C57BL/6 mice. The synovial lining and peritoneal macrophages of naïve DBA/1 mice expressed a significantly higher level of Fc gamma Rs than was seen in C57BL/6 mice. Moreover, elevated and prolonged expression of IL-1 was found after stimulation of these cells with immune complexes. Zymosan or streptococcal cell walls caused comparable inflammation and only mild cartilage destruction in all strains. We conclude that Fc gamma R expression on synovial macrophages may be related to the severity of synovial inflammation and cartilage destruction during ICA.

The fibronectin extra domain A activates matrix metalloproteinase gene expression by an interleukin-1-dependent mechanism

The extra domain-A (EDA), present in fibronectin (FN) molecules arising from alternatively spliced transcripts, appears only during specific biological and pathogenic processes. However, its function is poorly understood. To define the physiologic role of this domain in joint connective tissue, the biological effects on rabbit cartilage explants, chondrocytes, and synovial cells were studied. A recombinant EDA protein (rEDA) increased proteoglycan release (3. 6-fold) in cartilage explant cultures and markedly induced production of matrix metalloproteinase (MMP)-1 in chondrocytes. In addition, rEDA induced MMP-1, MMP-3, and MMP-9 in synovial cells. These effects were elicited only by rEDA, while its neighboring type III repeats, III(11) or III(12), scarcely had any such effects. Interestingly, reorganization of F-actin stress fibers accompanied MMP-1 expression in synovial cells treated with rEDA, suggesting alteration of cellular phenotype. Subsequent Northern blotting revealed expression of pro-inflammatory cytokines, including interleukin (IL)-1alpha and IL-1beta, was induced by rEDA prior to MMP-1 expression. Delayed MMP-1 expression suggests that rEDA-induced IL-1s promote MMP-1 expression in an autocrine manner. This hypothesis is supported by the reduction of EDA-induced MMP-1 production by IL-1 receptor antagonist. The effect of EDA on MMP-1 production was reduced by connection with an adjacent type III repeat on either the NH(2) or COOH side of EDA and was abolished by connection on both sides of EDA, suggesting that exposure of either the NH(2) or COOH terminus of EDA domain by proteolytic cleavage releases the inducing activity. In agreement with these results, full-length cellular FN did not induce MMP-1 production. Furthermore, a 160-kDa EDA-positive FN fragment, which was purified from human placental tissue and corresponds to the region from NH(2) terminus through the EDA, induced MMP-1 production. Taken together, these results suggest that the EDA in FN fragments triggers alterations of cell physiology and plays a role in matrix degradation in joint connective tissue.

Up-regulation of MDC15 (metargidin) messenger RNA in human osteoarthritic cartilage

OBJECTIVE

The aim of the study was to investigate the messenger RNA (mRNA) expression of the disintegrin metalloproteinase MDC15 (metargidin, or ADAM-15) in normal and osteoarthritic (OA) articular cartilage.

METHODS

In situ hybridization experiments and reverse transcription-polymerase chain reaction (RT-PCR) were performed on tissue samples of adult normal and OA articular cartilage.

RESULTS

MDC15 mRNA could be detected in normal articular cartilage by RT-PCR using tissue-extracted total RNA as a template. However, the mRNA level remained below the sensitivity of in situ hybridization. In contrast, in situ hybridizations of OA cartilage revealed an intense staining with the MDC15-specific riboprobes. The extension of the analysis to chondrosarcomas showed a strong up-regulation of MDC15 mRNA in these malignant transformed cells.

CONCLUSION

Our results demonstrate a markedly strong up-regulation of MDC15 in adult OA and neoplastic cartilage compared with adult normal articular cartilage, indicating a potential role of the disintegrin metalloproteinase in cartilage remodeling.

Integrins mediate the effects of quinolones and magnesium deficiency on cultured rat chondrocytes

Chondrocyte-matrix interaction is mediated by a series of adhesion molecules. Both alpha and beta integrin subunits are involved and govern crucial functions of cell adhesion and signal transduction. These molecules modulate proliferation and differentiation, thus establishing cartilage integrity. We studied the influence of magnesium deficiency and quinolone antibiotics (which form chelate complexes with divalent cations) on chondrocytes in vitro in order to assess the role of Mg2+ ions in integrin function and to establish cellular changes mediated via integrin signal transduction. Mg2(+)-free medium and quinolone supplementation was found to decrease chondrocyte attachment to collagen type II-coated coverslips. Adhesion and growth of chondrocytes were reduced in the respective medium. Organisation of cytoskeletal fibers (vimentin) was changed and formation of stress fibers (f-actin) was disturbed. Additionally, rates of cell proliferation declined. These results indicate that quinolone-magnesium complex formation is important for chondrotoxicity of these substances. Cell-matrix detachment and morphological alterations described in vitro may explain the lesions observed in articular cartilage after quinolone administration in vivo. The attachment assay described could serve as a simple test to establish the susceptibility of chondrocytes of different species to different quinolones in use or new ones to be introduced.

The activation of ProMMP-2 (gelatinase A) by HT1080 fibrosarcoma cells is promoted by culture on a fibronectin substrate and is concomitant with an increase in processing of MT1-MMP (MMP-14) to a 45 kDa form

We have assessed the effect of fibronectin and laminin-1 on the expression of molecules involved in the activation pathway of MMP-2, a key proteinase in tissue remodelling. HT1080 fibrosarcoma cells cultured on fibronectin were shown to activate endogenous MMP-2, to a level comparable with that elicited by treatment with phorbol ester. In contrast, the MMP-2 expressed by HT1080 cells cultured on laminin-1 was mainly in the pro- (inactive form). Culture of the cells on peptide fragments of fibronectin derived from the central cell binding domain also promoted MMP-2 activation, indicating that signals via fibronectin binding to integrin receptors may be involved. HT1080 cells cultured on immobilised antibodies to the alpha5 and beta1 integrin subunits secreted levels of active MMP-2 similar to those observed for full length fibronectin, whereas cells cultured on an antibody to the alpha6 integrin subunit secreted mainly proMMP-2. The data demonstrate that the activation of MMP-2 by HT1080 cells is regulated by the nature of the extracellular matrix, and that signals via the alpha5beta1 integrin receptor may be involved in the fibronectin induced up-regulation of MMP-2 activation. We then assessed the effect of fibronectin on the components of the putative MT1-MMP/TIMP-2 ‘receptor’ complex implicated in MMP-2 activation. Levels of TIMP-2 protein expressed by HT1080 cells did not vary detectably between cells cultured on fibronectin or laminin-1. However, the expression of MT1-MMP protein was up-regulated when the cells were cultured on fibronectin, which could be attributed to an increase in levels of a truncated 45 kDa form. Parallel studies using gelatin zymography demonstrated that the up-regulation of the production of the 45 kDa band was concomitant with MMP-2 activation. Inhibitor studies revealed that the truncation of MT1-MMP to a 45 kDa form is MMP mediated, although not inhibited by TIMP-1. In vitro, the 45 kDa form could be generated by cleavage of membrane-bound native MT1-MMP with several recombinant MMPs, including both active MT1-MMP and MMP-2. The implication that either MMP-2 or MT1-MMP can process MT1-MMP to 45 kDa, raises the possibility that truncation of MT1-MMP represents a self-regulatory end-point in the activation pathway of MMP-2.

Dedifferentiated chondrocytes cultured in alginate beads: restoration of the differentiated phenotype and of the metabolic responses to interleukin-1beta

Chondrocytes cultivated in monolayer rapidly divide and lose their morphological and biochemical characteristics, whereas they maintain their phenotype for long periods of time when they are cultivated in alginate beads. Because cartilage has a low cellularity and is difficult to obtain in large quantities, the number of available cells often becomes a limiting factor in studies of chondrocyte biology. Therefore, we explored the possibility of restoring the differentiated properties of chondrocytes by cultivating them in alginate beads after two multiplication passages in monolayer. This resulted in the reexpression of the two main markers of differentiated chondrocytes: Aggrecan and type II collagen gene expression was strongly reinduced from day 4 after alginate inclusion and paralleled protein expression. However, 2 weeks were necessary for total suppression of type I and III collagen synthesis, indicators of a modulated phenotype. Interleukin-1beta, a cytokine that is present in the synovial fluid of rheumatoid arthritis patients, induces many metabolic changes on the chondrocyte biology. Compared with cells in primary culture, the production of nitric oxide and 92-kDa gelatinase in response to interleukin-1beta was impaired in cells at passage 2 in monolayer but was fully recovered after their culture in alginate beads for 2 weeks. This suggests that the effects of interleukin-1beta on cartilage depend on the differentiation state of chondrocytes. This makes the culture in alginate beads a relevant model for the study of chondrocyte biology in the presence of interleukin-1beta and other mediators of cartilage destruction in rheumatoid arthritis and osteoarthrosis.

Compressive force promotes sox9, type II collagen and aggrecan and inhibits IL-1beta expression resulting in chondrogenesis in mouse embryonic limb bud mesenchymal cells

The initial modeling and subsequent development of the skeleton is controlled by complex gene-environment interactions. Biomechanical forces may be one of the major epigenetic factors that determine the form and differentiation of skeletal tissues. In order to test the hypothesis that static compressive forces are transduced into molecular signals during early chondrogenesis, we have developed a unique three-dimensional collagen gel cell culture system which is permissive for the proliferation and differentiation of chondrocytes. Mouse embryonic day 10 (E10) limb buds were microdissected and dissociated into cells which were then cultured within a collagen gel matrix and maintained for up to 10 days. Static compressive forces were exerted onto these cultures. The time course for expression pattern and level for cartilage specific markers, type II collagen and aggrecan, and regulators of chondrogenesis, Sox9 and IL-1beta, were analyzed and compared with non-compressed control cultures. Under compressive conditions, histological evaluation showed an apparent acceleration in the rate and extent of chondrogenesis. Quantitatively, there was a significant 2- to 3-fold increase in type II collagen and aggrecan expression beginning at day 5 of culture and the difference was maintained through 10 days of cultures. Compressive force also causes an elevated level of Sox9, a transcriptional activator of type II collagen. In contrast, the expression and accumulation of IL-1beta, a transcriptional repressor of type II collagen was down-regulated. We conclude that static compressive forces promote chondrogenesis in embryonic limb bud mesenchyme, and propose that the signal transduction from a biomechanical stimuli can be mediated by a combination of positive and negative effectors of cartilage specific extracellular matrix macromolecules.

Vascular cell adhesion molecule 1 (CD106) on primary human articular chondrocytes: functional regulation of expression by cytokines and comparison with intercellular adhesion molecule 1 (CD54) and very late activation antigen 2

OBJECTIVE

To investigate the expression of adhesion molecules belonging to the immunoglobulin superfamily on human primary articular chondrocytes and to determine their response pattern to cytokines with respect to the adhesion of lymphocytes.

METHODS

The expression of adhesion molecules was studied by flow cytometry (cultured cells), immunohistochemistry (cartilage), reverse transcription-polymerase chain reaction, and Northern blotting. Adhesion of T cells to chondrocytes was measured using the Jurkat T cell line.

RESULTS

Vascular cell adhesion molecule 1 (VCAM-1) and intercellular adhesion molecule 1 (ICAM-1) were found to be constitutively expressed on large percentages of unstimulated chondrocytes in culture and in cartilage ex vivo. ICAM-2, ICAM-3, and very late activation antigen 4 (VLA-4; alpha4beta1 integrin), the ligand for VCAM-1, were not detected. Interleukin-1beta (IL-1beta) and tumor necrosis factor alpha (TNFalpha) further induced VCAM-1 and ICAM-1 messenger RNA and protein expression. Transforming growth factor beta (TGFbeta) had no effect on ICAM-1 and decreased the expression of VCAM-1. Another adhesion molecule, VLA-2 alpha2beta1 integrin) that was also expressed on unstimulated chondrocytes, was differentially regulated by cytokines. While neither IL-1beta nor TNFalpha had any effect on expression of VLA-2, TGFbeta markedly increased the alpha2 subunit of VLA-2. Adhesion of Jurkat T cells to chondrocytes was further induced by IL-1beta and TNFalpha. Pretreatment of chondrocytes with monoclonal antibodies to VCAM-1 and ICAM-1 inhibited adhesion of T cells to chondrocytes.

CONCLUSION

VCAM-1, ICAM-1, and VLA-2 are constitutively expressed by human articular chondrocytes. Expression is regulated by cytokines. As shown for other chondrocyte genes, IL-1beta/TNFalpha and TGFbeta antagonistically modulate the expression of adhesion molecules. VCAM-1 and ICAM-1 contribute to adhesion of T lymphocytes to chondrocytes, and may thus participate in host defense mechanisms during inflammatory joint conditions such as rheumatoid arthritis and after cartilage transplantation.

Cell-matrix interactions modulate 92-kD gelatinase expression by human bronchial epithelial cells

We have previously reported that primary human bronchial epithelial cells (HBECs) cultured on types I + III collagen were able to differentially regulate the production of major constitutive 92-kD gelatinase, minor 72-kD gelatinase, and their tissue-specific inhibitor, tissue inhibitor of metalloproteinase-1 (TIMP-1) in response to lipopolysaccharide (LPS) or proinflammatory cytokines, suggesting that HBECs may be involved in vivo in the active remodeling of the underlying extracellular matrix (ECM). In this study, we examined the possible effects of specific type IV collagen as compared with types I + III collagen on HBEC behavior and function. We investigated 92-kD gelatinase and TIMP-1 expression with zymography and reverse zymography, respectively, at the protein level, and with quantitative reverse transcription-polymerase chain reaction (RT-PCR) at the mRNA level. Results showed similar morphologic features and identical proliferation rates of HBECs in response to the two matrix substrates. Nevertheless, differences at the protein and mRNA levels between HBEC cultures on type IV collagen and on types I + III collagen included: (1) a lower basal level of 92-kD gelatinase production; (2) less upregulation of 92-kD gelatinase in response to LPS endotoxin or to the proinflammatory cytokines interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha); and (3) loss of activation of the proforms of the 92-kD and 72-kD gelatinases. These findings, together with the maintenance of TIMP-1 expression, strongly suggest that type IV collagen used as a matrix substratum is associated with a homeostatic HBEC phenotype, and limits the ability of HBECs to degrade the matrix. In contrast, types I + III collagen may be associated with a matrix resorption phenotype corresponding to active matrix remodeling and repair. Thus, the ECM underlying HBECs may modulate matrix remodeling by HBECs, particularly in response to inflammatory processes during acute lung injury.

Expression of alpha and beta subunits of the integrin superfamily in articular cartilage from macroscopically normal and osteoarthritic human femoral heads

OBJECTIVE

The objective of this study was to detail the topographical and zonal distribution of alpha and beta subunits of the integrin superfamily in normal and osteoarthritic cartilage.

METHODS

Immunohistochemistry utilising antibodies towards alpha and beta subunits was performed on cryostat sections of human articular cartilage from macroscopically normal (n = 6) and osteoarthritic (n = 6) femoral heads. Samples of articular cartilage were obtained from 12 topographically distinct sites from each femoral head. Each section was divided into zones (superficial, middle, deep) and staining scores were recorded.

RESULTS

Normal cartilage stained for integrin subunits alpha 1, alpha 5, alpha V, beta 1, beta 4, and beta 5, but not for alpha 2, alpha 3, alpha 4, alpha 6, beta 2, beta 3, and beta 6. Intact and non-intact residual cartilage from osteoarthritic femoral heads stained for alpha 1, alpha 2, alpha 5, alpha V, beta 1, beta 4, and beta 5. Staining was occasionally seen for alpha 4 and beta 2, but not for alpha 3, alpha 6, beta 3, and beta 6. There was no topographical variation in the staining for any of the subunits in either normal or osteoarthritic cartilage. The only subunit that displayed a zonal variation was alpha V; staining for this subunit was most pronounced in the superficial zone compared with the middle and deep zones.

CONCLUSION

Chondrocytes in normal and osteoarthritic cartilage express the integrin subunits alpha 1, alpha 5, alpha V, beta 1, beta 4, and beta 5. Chondrocytes in osteoarthritic cartilage, in addition, express the alpha 2, alpha 4, and beta 2 subunits. The alpha v subunit is expressed by more chondrocytes in the superficial zone in comparison with cells in the deeper zones. None of the subunits display topographical variation in expression.

Role of serum amyloid A as an intermediate in the IL-1 and PMA-stimulated signaling pathways regulating expression of rabbit fibroblast collagenase

The matrix metalloproteinase collagenase is expressed by resident tissue cells only when needed for biological remodeling. Exogenous addition of inflammatory and growth-promoting cytokines stimulates collagenase expression in early passage fibroblast cultures. In addition, the signal for collagenase expression in response to phorbol-12 myristate-13 acetate (PMA) or to agents which alter cell shape in early passage fibroblast cultures is routed extracellularly to an autocrine cytokine intermediate, IL-1 alpha. Importantly, fibroblasts, when freshly isolated from the tissue, are not competent for IL-1 alpha gene expression and, therefore, cannot produce collagenase in response to shape change agents. However, they do make a small amount of collagenase in response to PMA via an IL-1-independent pathway that has not been further characterized. In this paper, we investigate the role of a second autocrine, serum amyloid A3 (SAA3), in IL-1-dependent and -independent collagenase gene expression. We demonstrate that SAA3 is required for effective stimulation of collagenase expression by either exogenous or endogenous IL-1. Furthermore, while freshly isolated fibroblasts cannot express IL-1 alpha they can express SAA3, and this autocrine mediator acts independently of IL-1 alpha to control the low level of collagenase expression that can be stimulated by PMA. These results provide further evidence for a newly emerging paradigm of collagenase regulation which emphasizes the requirement for extracellular routing of signals. They also suggest that SAA3 might be utilized independently of IL-1 alpha to control tissue remodeling in vivo.

Restoration of the differentiated functions of serially passaged chondrocytes using staurosporine

Among the various directions explored in order to have a large number of differentiated articular chondrocytes easily available, the restoration of the differentiated properties after cell multiplication in monolayer has been proposed. It has been clearly shown that the synthesis of cartilage proteoglycans and type II collagen synthesis is coincident with the presence of a faint microfibrillar architecture but is absent in chondrocytes showing well-defined actin cables. Staurosporin, mainly described as a protein kinase C inhibitor, has also been shown to rapidly induce the disruption of the actin microfilaments. The purpose of this paper was to investigate whether properties of differentiated chondrocytes were reinitiated upon staurosporin treatment of serially passaged chondrocytes. Results showed, after staurosporine treatment of cells at Passage two for 5 d, complete suppression of type I and type III collagen synthesis and induction of type II collagen synthesis and of Alcian blue stainable matrix. Additionally, we showed that staurosporin restored metabolic responses that chondrocytes in primary culture exhibit upon interleukin-1 beta treatment (decrease of Alcian blue- positive cells, induction of expression of the 92 kDa gelatinase, nitric oxide production). We conclude that staurosporin is a potent redifferentiating agent of articular chondrocytes that have been subcultured up to Passage two for multiplication. Taking into account that the cellularity of cartilage is very low, staurosporine-treated chondrocytes could be useful as an alternative cellular model to evaluate pharmacotoxicological effects of drugs.

Hyperpolarisation of cultured human chondrocytes following cyclical pressure-induced strain: evidence of a role for alpha 5 beta 1 integrin as a chondrocyte mechanoreceptor

Mechanical stimuli influence chondrocyte metabolism, inducing changes in intracellular cyclic adenosine monophosphate and proteoglycan production. We have previously demonstrated that primary monolayer cultures of human chondrocytes have an electrophysiological response after intermittent pressure-induced strain characterised by a membrane hyperpolarisation of approximately 40%. The mechanisms responsible for these changes are not fully understood but potentially involve signalling molecules such as integrins that link extracellular matrix with cytoplasmic components. The results reported in this paper demonstrate that the transduction pathways involved in the hyperpolarisation response of human articular chondrocytes in vitro after cyclical pressure-induced strain involve alpha 5 beta 1 integrin. We have demonstrated, using pharmacological inhibitors of a variety of intracellular signalling pathways, that the actin cytoskeleton, the phospholipase C calmodulin pathway, and both tyrosine protein kinase and protein kinase C activities are important in the transduction of the electrophysiological response. These results suggest that alpha 5 beta 1 is an important chondrocyte mechanoreceptor and a potential regulator of chondrocyte function.

Integrins in morphogenesis and signaling

Integrins are a family of heterodimeric transmembrane receptors that provide a physical and biochemical bridge between components of the extracellular matrix and the intracellular physiological environment. Binding of integrins to their ligands results in the formation of cytoplasmic multi-protein assemblies composed of both cytoskeletal and signaling molecules. The composition and activity of these assemblies is regulated by the nature of integrin-ligand interactions, as well as by intracellular regulators that include tyrosine kinases and phosphatases, PKC, and small GTPases. Integrin-mediated cellular physiological responses include the activation of signal transduction, cytoskeletal rearrangements, and co-regulation of growth factor activities. These responses, combined with integrin-mediated cell adhesion, play a major role in tissue morphogenesis and developmental processes.

Calcitonin inhibits phospholipase A2 and collagenase activity of human osteoarthritic chondrocytes

Calcitonin (CT) is a known potent inhibitor of bone resorption but its effect on cartilage enzymatic degradation has been incompletely studied. Salmon CT, at a concentration of 0, 0.1, 0.25, 0.5, 2.5 and 50 ng/ml, was added at 24 or 72 h to the culture medium of chondrocytes from human osteoarthritic hips and knees. The spontaneous collagenolytic activity, measured using a radiolabeled type II collagen, was inhibited by CT in a dose-dependent manner. However, CT had no effect on the total collagenolytic activity assayed after APMA activation. Stromelysin and plasmin activity, measured by degradation of casein and a synthetic substrate, were also unaffected by CT. Chondrocyte phospholipase A2 activity, assayed using a labeled specific substrate, was decreased by CT. Chondrocyte pre-incubation with CT significantly decreased the cell binding of labeled TNF alpha, but did not affect IL-1 beta cell binding. Attachment of chondrocytes on fibronectin was markedly stimulated by CT, while attachment to type II collagen was not. Significant effects were obtained using at least 2 or 5 ng/ml of CT. CT appears to decrease collagenolytic activity by decreasing its activation and/or increasing its inhibition by tissue inhibitors of metalloproteinases (TIMP). CT might act on osteoarthritic chondrocyte activation via mechanisms such as phospholipase A2 activity, human necrosis factor-alpha or fibronectin receptor expression.

Degradation of type II collagen, but not proteoglycan, correlates with matrix metalloproteinase activity in cartilage explant cultures

OBJECTIVE

To determine the contribution of certain matrix metalloproteinases (MMPs) to the degradation of proteoglycan and type II collagen in cartilage.

METHODS

Bovine nasal and articular cartilage explants were cultured with recombinant human interleukin-1 alpha (IL-1 alpha) for up to 4 weeks. Release of proteoglycan and type II collagen into the medium was determined by colorimetric assay and immunoassay, respectively. The activity of MMPs in the medium was assayed using a quenched fluorescent substrate, as well as with a collagen fibril assay, by zymography, and in Western immunoblots. In some experiments, the effects of specific MMP inhibitors on type II collagen degradation were studied.

RESULTS

In cultures of nasal cartilage with IL-1 alpha, almost all the proteoglycan was released within the first week, whereas there was no detectable release of type II collagen for the first 2 weeks of culture. A rapid period of almost complete dissolution of the collagen occurred in the third or fourth week. MMP activity measured using a quenched fluorescent substrate was negligible during the first 2 weeks of culture but was substantially increased in the third week of culture, at the time of collagen degradation. Similarly, there was a large increase in collagenolytic activity (by collagen fibril assay) and gelatinolytic activity (by zymography) during the third week of culture. Articular cartilage cultured with IL-1 alpha lost proteoglycan progressively during the 4-week period; however, there was no loss of type II collagen from the matrix in that time and no significant increase in MMP activity. The loss of type II collagen from nasal cartilage stimulated with IL-1 alpha was inhibited by BB87, an inhibitor of both collagenases and gelatinases, and by BB3003, a selective inhibitor of gelatinase A. In Western immunoblots, procollagenase and active interstitial collagenase could be readily detected in nasal cartilage cultures. Some procollagenase 3 and active collagenase 3 was also shown to be present.

CONCLUSION

MMP activity correlates with degradation of type II collagen, but not proteoglycan, in cartilage cultures. Interstitial collagenase, collagenase 3, and gelatinases are all likely to contribute to cleavage and removal of collagen from the cartilage matrix. The proteinase(s) responsible for aggrecan breakdown remains unclear.

Integrins can collaborate with growth factors for phosphorylation of receptor tyrosine kinases and MAP kinase activation: roles of integrin aggregation and occupancy of receptors

Integrins mediate cell adhesion, migration, and a variety of signal transduction events. These integrin actions can overlap or even synergize with those of growth factors. We examined for mechanisms of collaboration or synergy between integrins and growth factors involving MAP kinases, which regulate many cellular functions. In cooperation with integrins, the growth factors EGF, PDGF-BB, and basic FGF each produced a marked, transient activation of the ERK (extracellular signal-regulated kinase) class of MAP kinase, but only if the integrins were both aggregated and occupied by ligand. Transmembrane accumulation of total tyrosine-phosphorylated proteins, as well as nonsynergistic MAP kinase activation, could be induced by simple integrin aggregation, whereas enhanced transient accumulation of the EGF-receptor substrate eps8 required integrin aggregation and occupancy, as well as EGF treatment. Each type of growth factor receptor was itself induced to aggregate transiently by integrin ligand-coated beads in a process requiring both aggregation and occupancy of integrin receptors, but not the presence of growth factor ligand. Synergism was also observed between integrins and growth factors for triggering tyrosine phosphorylation of EGF, PDGF, and FGF receptors. This collaborative response also required both integrin aggregation and occupancy. These studies identify mechanisms in the signal transduction response to integrins and growth factors that require various combinations of integrin aggregation and ligands for integrin or growth factor receptors, providing opportunities for collaboration between these major regulatory systems.