Fibronectin fragments alter matrix protein synthesis in cartilage tissue cultured in vitro

Therapeutic Potential of Bioactive Compounds in Honey for Treating Osteoarthritis

Dysregulation of joint tissue homeostasis induces articular degenerative changes and musculoskeletal diseases such as osteoarthritis. This pathology represents the first cause of motor disability in individuals over 60 years of age, impacting their quality of life and the costs of health systems. Nowadays, pharmacological treatments for cartilage disease have failed to achieve full tissue regeneration, resulting in a functional loss of the joint; therefore, joint arthroplasty is the gold standard procedure to cure this pathology in severe cases of Osteoarthritis. A different treatment is the use of anti-inflammatory drugs which mitigate pain and inflammation in some degree, but without significant inhibition of disease progression. In this sense, new therapeutic alternatives based on natural compounds have been proposed to delay osteoarthritis progression, particularly those agents that regulate articular homeostasis. Preclinical studies have shown a therapeutic application of honey and its bioactive compounds, ranging from treating wounds, coughs, skin infections, and are also used as a biological stimulant by exerting antioxidant and anti-inflammatory properties. In this article, we reviewed the current medicinal applications of honey with particular emphasis on its use regulating articular homeostasis by inhibiting inflammation and oxidative stress.

Profile of Matrix-Remodeling Proteinases in Osteoarthritis: Impact of Fibronectin

The extracellular matrix (ECM) is a complex and specialized three-dimensional macromolecular network, present in nearly all tissues, that also interacts with cell surface receptors on joint resident cells. Changes in the composition and physical properties of the ECM lead to the development of many diseases, including osteoarthritis (OA). OA is a chronic degenerative rheumatic disease characterized by a progressive loss of synovial joint function as a consequence of the degradation of articular cartilage, also associated with alterations in the synovial membrane and subchondral bone. During OA, ECM-degrading enzymes, including urokinase-type plasminogen activator (uPA), matrix metalloproteinases (MMPs), and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTSs), cleave ECM components, such as fibronectin (Fn), generating fibronectin fragments (Fn-fs) with catabolic properties. In turn, Fn-fs promote activation of these proteinases, establishing a degradative and inflammatory feedback loop. Thus, the aim of this review is to update the contribution of ECM-degrading proteinases to the physiopathology of OA as well as their modulation by Fn-fs.

Quantification of fibronectin as a method to assess ex vivo extracellular matrix remodeling

Altered architecture, composition and quality of the extracellular matrix (ECM) are pathological hallmarks of several inflammatory and fibro-proliferative pathological processes such as osteoarthritis (OA), rheumatoid arthritis (RA), fibrosis and cancer. One of the most important components of the ECM is fibronectin. Fibronectin serves as an adhesion molecule anchoring cells to the underlying basement membrane through direct interaction with integrin receptors. Fibronectin hereby modulates the properties of the ECM and affects cellular processes. Quantification of fibronectin remodeling could therefore be used to assess the changes in the ECM that occur during progression of fibro-proliferative pathologies. Ex vivo models are becoming state-of-the-art tools to study ECM remodeling as the cellular composition and the organization of the ECM are preserved. Ex vivo models may therefore be a valuable tool to study the ECM remodeling that occurs during progression of fibro-proliferative pathologies. The aim of this study was to quantify fibronectin remodeling in ex vivo models of cartilage and cancer. A competitive The enzyme-linked immunosorbent assay (ELISA) against the C-terminus of fibronectin was developed (FBN-C). The assay was evaluated in relation to specificity, technical performance and as a marker for quantification of fibronectin in cartilage and cancer ex vivo models. The ELISA was specific and technically stable. Cleavage of tumor tissue with MMP-2 released significantly higher levels of FBN-C compared to tissue with buffer only and western blot analysis revealed that FBN-C recognizes both full length and degraded fibronectin. When ex vivo cartilage cultures were stimulated with the anabolic factor TGFβ and catabolic factors TNF-α and OSM, significantly higher levels of FBN-C were found in the conditioned media. Lastly, FBN-C was released from a cancer ex vivo model. In conclusion, we were able to quantify fibronectin remodeling in ex vivo models of cartilage and cancer. Quantification of fibronectin remodeling could be a valuable tool to understand ECM remodeling in ex vivo models of fibro-proliferative pathologies.

Calumenin and fibulin-1 on tumor metastasis: Implications for pharmacology

Tumor metastasis is a key cause of cancer mortality, and inhibiting migration of cancer cells is one of the major directions of anti-metastatic drug development. Calumenin and fibulin-1 are two extracellular proteins that synergistically inhibit cell migration and tumor metastasis, and could potentially be served as targets for pharmacological research of anti-metastatic drugs. This review briefly introduces the multi-function of these two proteins, and discusses the mechanism of how they regulate cell migration and tumor metastasis.

Low oxygen tension increased fibronectin fragment induced catabolic activities--response prevented with biomechanical signals

Introduction

The inherent low oxygen tension in normal cartilage has implications on inflammatory conditions associated with osteoarthritis (OA). Biomechanical signals will additionally contribute to changes in tissue remodelling and influence the inflammatory response. In this study, we investigated the combined effects of oxygen tension and fibronectin fragment (FN-f) on the inflammatory response of chondrocytes subjected to biomechanical signals.

Methods

Chondrocytes were cultured under free-swelling conditions at 1%, 5% and 21% oxygen tension or subjected to dynamic compression in an ex vivo 3D/bioreactor model with 29 kDa FN-f, interleukin-1beta (IL-1β) and/or the nitric oxide synthase (NOS) inhibitor for 6 and 48 hours. Markers for catabolic activity (NO, PGE₂), tissue remodelling (GAG, MMPs) and cytokines (IL-1β, IL-6 and TNFα) were quantified by biochemical assay. Aggrecan, collagen type II, iNOS and COX-2 gene expression were examined by real-time quantitative PCR. Two-way ANOVA and a post hoc Bonferroni-corrected t-test were used to analyse data.

Results

Both FN-fs and IL-1β increased NO, PGE₂ and MMP production (all P < 0.001). FN-f was more active than IL-1β with greater levels of NO observed at 5% than 1% or 21% oxygen tension (P < 0.001). Whilst FN-f reduced GAG synthesis at all oxygen tension, the effect of IL-1β was significant at 1% oxygen tension. In unstrained constructs, treatment with FN-f or IL-1β increased iNOS and COX-2 expression and reduced aggrecan and collagen type II (all P < 0.001). In unstrained constructs, FN-f was more effective than IL-1β at 5% oxygen tension and increased production of NO, PGE₂, MMP, IL-1β, IL-6 and TNFα. At 5% and 21% oxygen tension, co-stimulation with compression and the NOS inhibitor abolished fragment or cytokine-induced catabolic activities and restored anabolic response.

Conclusions

The present findings revealed that FN-fs are more potent than IL-1β in exerting catabolic effects dependent on oxygen tension via iNOS and COX-2 upregulation. Stimulation with biomechanical signals abolished catabolic activities in an oxygen-independent manner and NOS inhibitors supported loading-induced recovery resulting in reparative activities. Future investigations will utilize the ex vivo model as a tool to identify key targets and therapeutics for OA treatments.

A Comparative Study of Fibronectin Cleavage by MMP-1, -3, -13, and -14

Fibronectin fragments are important for synovial inflammation and the progression of arthritis, and thus, identifying potential enzymatic pathways that generate these fragments is of vital importance. The objective of this study was to determine the cleavage efficiency of fibronectin by matrix metalloproteinases (MMP-1, MMP-3, MMP-13, and MMP-14). Intact human plasma fibronectin was co-incubated with activated MMPs in neutral or acidic pH for up to 24 hours at 37 °C. The size and distribution of fibronectin fragments were determined by Western blot analysis using antibodies that recognized the N-terminals of fibronectin. All MMPs were able to cleave fibronectin at neutral pH. MMP-13 and -14 had the highest efficiency followed by MMP-3 and -1. MMP-3, -13, and -14 generated 70-kDa fragments, a known pro-inflammatory peptide. Further degradation of fibronectin fragments was only found for MMP-13 and -14, generating 52-kDa, 40-kDa, 32-kDa, and 29-kDa fragments. Fibronectin fragments of similar size were also found in the articular cartilage of femoral condyles of normal bovine knee joints. At acidic pH (5.5), the activities of MMP-1 and -14 were nearly abolished, while MMP-3 had a greater efficiency than MMP-13 even though the activities of both MMPs were significantly reduced. These findings suggest that MMP-13 and -14 may play a significant role in the cleavage of fibronectin and the production of fibronectin fragments in normal and arthritic joints.

Biomechanical modulation of collagen fragment-induced anabolic and catabolic activities in chondrocyte/agarose constructs

INTRODUCTION

The present study examined the effect of collagen fragments on anabolic and catabolic activities by chondrocyte/agarose constructs subjected to dynamic compression.

METHODS

Constructs were cultured under free-swelling conditions or subjected to continuous and intermittent compression regimes, in the presence of the N-terminal (NT) and C-terminal (CT) telopeptides derived from collagen type II and/or 1400 W (inhibits inducible nitric oxide synthase (iNOS)). The anabolic and catabolic activities were compared to the amino-terminal fibronectin fragment (NH2-FN-f) and assessed as follows: nitric oxide (NO) release and sulphated glycosaminoglycan (sGAG) content were quantified using biochemical assays. Tumour necrosis factor-alpha (TNFalpha) and interleukin-1beta (IL-1beta) release were measured by ELISA. Gene expression of matrix metalloproteinase-3 (MMP-3), matrix metalloproteinase-13 (MMP-13), collagen type II and fibronectin were assessed by real-time quantitative polymerase chain reaction (qPCR). Two-way ANOVA and the post hoc Bonferroni-corrected t-test was used to examine data.

RESULTS

The presence of the NT or CT peptides caused a moderate to strong dose-dependent stimulation of NO, TNFalpha and IL-1beta production and inhibition of sGAG content. In some instances, high concentrations of telopeptides were just as potent in stimulating catabolic activities when compared to NH2-FN-f. Depending on the concentration and type of fragment, the increased levels of NO and cytokines were inhibited with 1400 W, resulting in the restoration of sGAG content. Depending on the duration and type of compression regime employed, stimulation with compression or incubation with 1400 W or a combination of both, inhibited telopeptide or NH2-FN-f induced NO release and cytokine production and enhanced sGAG content. All fragments induced MMP-3 and MMP-13 expression in a time-dependent manner. This effect was reversed with compression and/or 1400 W resulting in the restoration of sGAG content and induction of collagen type II and fibronectin expression.

CONCLUSIONS

Collagen fragments containing the N- and C-terminal telopeptides have dose-dependent catabolic activities similar to fibronectin fragments and increase the production of NO, cytokines and MMPs. Catabolic activities were downregulated by dynamic compression or by the presence of the iNOS inhibitor, linking reparative activities by both types of stimuli. Future investigations which examine the signalling cascades of chondrocytes in response to matrix fragments with mechanical influences may provide useful information for early osteoarthritis treatments.

Fibronectin fragments mediate matrix metalloproteinase upregulation and cartilage damage through proline rich tyrosine kinase 2, c-src, NF-kappaB and protein kinase Cdelta

OBJECTIVE

Since fibronectin fragments (Fn-fs) enhance cartilage damage through integrins, the objective was to investigate the role of integrin linked kinases, focal adhesion kinase (FAK) and a soluble form of FAK, proline rich tyrosine kinase 2 (Pyk2) and cellular src kinase (c-src) and the transcription factor, nuclear factor kappaB (NF-kappaB) in cartilage damage.

METHODS

Bovine chondrocytes were cultured with various concentrations of three different Fn-fs, an amino-terminal 29 kDa, a gelatin binding 50 kDa and a central 140-kDa Fn-fs, each with progressively weaker cartilage damaging activity, or with native fibronectin (Fn), and lysates probed for activation of the selected kinases. Confocal microscopy was used to visualize intracellular location of activated kinases and NF-kappaB. Various kinase inhibitors were tested for their effects on Fn-f mediated upregulation of matrix metalloproteinase (MMP)-3 and -13 and cartilage proteoglycan (PG) depletion.

RESULTS

The Fn-fs kinetically enhanced phosphorylation of FAK but did not show a clear dose-response effect. The 29-kDa and 50-kDa Fn-fs enhanced phosphorylation of Pyk2, c-src and NF-kappaB to a much greater extent than the 140-kDa Fn-f and native Fn and did so as a function of dose. The 29-kDa Fn-f enhanced the phosphorylation of nuclear Pyk2 as compared with no treatment or native Fn. Inhibitors of Pyk2, c-src, NF-kappaB and protein kinase Cdelta (PKCdelta) decreased MMP upregulation and decreased Fn-f mediated damage to cartilage.

CONCLUSIONS

These studies enhance our knowledge of crucial factors in Fn-f mediated signaling in MMP upregulation and cartilage damage and because of the potential physiologic relevance of Fn-fs, provide a better knowledge of cartilage degeneration in general.

The cartilage chondrolytic mechanism of fibronectin fragments involves MAP kinases: comparison of three fragments and native fibronectin

OBJECTIVE

To define the role of mitogen activated protein (MAP) kinases in fibronectin fragment (Fn-f) mediated matrix metalloproteinase (MMP) upregulation and damage to bovine cartilage and to compare activities of three Fn-fs with native fibronectin (Fn), which is inactive in terms of cartilage damage.

METHODS

Bovine chondrocytes were cultured with three Fn-fs, an amino-terminal 29-kDa, a gelatin-binding 50-kDa and a central 140-kDa Fn-f or native Fn at concentrations from 0.01 to 1 microM, concentrations lower than those found in osteoarthritis synovial fluids. Lysates were probed for activation of MAP kinases, extracellular signal-regulated kinase 1/2 (ERK1/2), p38 and stress activated protein kinase/c-jun N-terminal kinase (SAPK/JNK). Confocal fluorescent microscopy was used to visualize movement of activated kinases. Kinase inhibitors were tested for their abilities to block Fn-f mediated protein upregulation of MMP-3 and MMP-13 and Fn-f induced depletion of cartilage proteoglycan (PG) from cultured explants.

RESULTS

The 29-kDa, the most potent Fn-f in terms of cartilage damage, enhanced phosphorylation of ERK1/2, p38 and JNK1/2 within a 1-h incubation while the 50 and 140-kDa Fn-fs required up to 4 h for maximal activity and native Fn was only minimally active toward p38 and JNK, but did strongly activate ERK1/2. The activated kinases displayed a distribution toward the nuclear membrane and within the nucleus. MAP kinase inhibitors markedly decreased Fn-f mediated upregulation of MMP-3 or MMP-13 and Fn-f mediated cartilage PG depletion.

CONCLUSIONS

These results suggest that Fn-fs upregulate MMP-3 and MMP-13 in bovine chondrocytes through MAP kinases and that kinase inhibitors afford protection against this degenerative pathway.

Dynamic compression inhibits fibronectin fragment induced iNOS and COX-2 expression in chondrocyte/agarose constructs

Mechanical loading and the fibronectin fragments (FN-fs) are known to stimulate the anabolic and catabolic processes in articular cartilage, possible through pathways mediated by *NO. This study examined the combined effects of dynamic compression and the NH(2)-hep I or COOH-hep II FN-fs on the expression levels of iNOS and COX-2 and production of *NO and PGE(2) release. Both types of fragments induced iNOS and COX-2 expression and stimulated the production of *NO release. This response was inhibited by dynamic compression. Inhibitor experiments indicated that both dynamic compression and the iNOS inhibitor were important in restoring cell proliferation and proteoglycan synthesis in the presence of the FN-fs. This is the first study which demonstrates a downregulation of the FN-f-induced iNOS and COX-2 expression by dynamic compression. The combination of mechanical and pharmacological interventions makes this study a powerful tool to examine further the interactions of biomechanics and cell signalling in osteoarthritis.

Cartilage in normal and osteoarthritis conditions

The preservation of articular cartilage depends on keeping the cartilage architecture intact. Cartilage strength and function depend on both the properties of the tissue and on their structural parameters. The main structural macromolecules are collagen and proteoglycans (aggrecan). During life, cartilage matrix turnover is mediated by a multitude of complex autocrine and paracrine anabolic and catabolic factors. These act on the chondrocytes and can lead to repair, remodeling or catabolic processes like those that occur in osteoarthritis. Osteoarthritis is characterized by degradation and loss of articular cartilage, subchondral bone remodeling, and, at the clinical stage of the disease, inflammation of the synovial membrane. The alterations in osteoarthritic cartilage are numerous and involve morphologic and metabolic changes in chondrocytes, as well as biochemical and structural alterations in the extracellular matrix macromolecules.

Angiogenic response of endothelial cells to heparin-binding domain of fibronectin

Interaction of endothelial cells with cell-binding domain of fibronectin through integrin receptors is important in the process of angiogenesis. The present study was designed to examine the role of heparin-binding domain of fibronectin in angiogenesis using human umbilical vein endothelial cells. Attachment of endothelial cells in vitro to heparin-binding domain of fibronectin was inhibited by heparin. Chick chorioallantoic membrane assay revealed the proangiogenic nature of heparin-binding domain. Analysis by reverse transcription-polymerase chain reaction showed an increase in the expression of vascular endothelial growth factor and its receptor mRNA. Enzyme-linked immunosorbent assay showed a significant increase in the level of vascular endothelial growth factor secreted by cells maintained on heparin-binding domain. Treatment with calphostin C, an inhibitor of protein kinase C, decreased the expression of vascular endothelial growth factor receptor 2. Chick chorioallantoic membrane assay showed that the vascular endothelial growth factor secreted by cells maintained on heparin-binding domain was biologically more active, which appeared to be due to a decrease in its poly-adenosine diphosphate ribosylation. Binding assays showed that heparin-binding domain preferably binds unmodified vascular endothelial growth factor as compared to intact fibronectin. It is concluded that the heparin-binding domain of fibronectin by itself can promote angiogenesis in endothelial cells possibly by interaction with cell surface heparan sulphate proteoglycans involving protein kinase C dependent signaling and making available more active form of vascular endothelial growth factor to the cells.

Cartilage destruction by matrix degradation products

The progressive destruction of articular cartilage is one of the hallmarks of osteoarthritis and rheumatoid arthritis. Cartilage degradation is attributed to different classes of catabolic factors, including proinflammatory cytokines, aggrecanases, matrix metalloproteinases, and nitric oxide. Recently, matrix degradation products generated by excessive proteolysis in arthritis have been found to mediate cartilage destruction. These proteolytic fragments activate chondrocytes and synovial fibroblasts via specific cell surface receptors that can stimulate catabolic intracellular signaling pathways, leading to the induction of such catalysts. This review describes the catabolic activities of matrix degradation products, especially fibronectin fragments, and discusses the pathologic implication in cartilage destruction in osteoarthritis and rheumatoid arthritis. Increased levels of these degradation products, found in diseased joints, may stimulate cartilage breakdown by mechanisms of the kind demonstrated in the review.

Elevated levels and fragmented nature of cellular fibronectin in the plasma of gastrointestinal and head and neck cancer patients

BACKGROUND

Tumor invasion occurs following enzymatic degradation of components of the extracellular matrix. The proteolysis-resistant domains of matrix components are likely to appear in the blood plasma during invasion, and could be used as markers of malignancy. Cellular fibronectin (cFN), a major ECM component, possesses 3 alternately spliced principal protease resistant domains; two of which, extra domain A (EDA) and III connecting segment (IIICS), were selected for this study of the nature of the plasma cFN molecules and its levels in normal subjects (n=51), and patients with gastrointestinal (G-I, n=145) or head and neck (H-N, n=127) cancers.

METHODS

ELISA was used to measure the cFN levels in plasma and Western blotting to analyze its fragmented nature in plasma samples from normal individuals and patients with G-I or H-N cancers.

RESULTS

cFN in blood plasma, as probed by anti-EDA and anti-IIICS antibodies on Western blots, is found to exist entirely in a fragmented form in normal subjects and G-I and H-N cancer patients. The cFN polypeptides in plasma have Mr of 160 and 100. The levels of plasma cFN, determined by ELISA using the 2 antibodies, are found to be increased in G-I and H-N cancers. In a significant number of stomach (43%), gall bladder (35%) and colon (17%) cancer cases an additional anti-EDA-reactive 30 kD peptide is seen in the plasma.

CONCLUSIONS

The mean rise for all sites is statistically significant, and 65% of all patients show cFN levels >80th percentile of normal values. The characterization of the 30 kD peptide showed that it does not contain the IIICS domain and also lacks the central cell- and heparin-binding sites.

The role of human HtrA1 in arthritic disease

Human HtrA1 belongs to a widely conserved family of serine proteases involved in various aspects of protein quality control and cell fate. Although HtrA1 has been implicated in the pathology of several diseases, its precise biological functions remain to be established. Through identification of potential HtrA1 targets, studies presented herein propose that within the context of arthritis pathology HtrA1 contributes to cartilage degradation. Elevated synovial HtrA1 levels were detected in fluids obtained from rheumatoid and osteoarthritis patients, with synovial fibroblasts identified as a major source of secreted HtrA1. Mass spectrometry analysis of potential HtrA1 substrates within synovial fluids identified fibronectin as a candidate target, and treatment of fibronectin with recombinant HtrA1 led to the generation of fibronectin-degradation products that may be involved in cartilage catabolism. Consistently, treatment of synovial fibroblasts with HtrA1 or HtrA1-generated fibronectin fragments resulted in the specific induction of matrix metalloprotease 1 and matrix metalloprotease 3 expression, suggesting that HtrA1 contributes to the destruction of extracellular matrix through both direct and indirect mechanisms.

NF-kappa B mediates the stimulation of cytokine and chemokine expression by human articular chondrocytes in response to fibronectin fragments

Fibronectin fragments (FN-f) that bind to the alpha(5)beta(1) integrin stimulate chondrocyte-mediated cartilage destruction and could play an important role in the progression of arthritis. The objective of this study was to identify potential cytokine mediators of cartilage inflammation and destruction induced by FN-f and to investigate the mechanism of their stimulation. Human articular chondrocytes, isolated from normal ankle cartilage obtained from tissue donors, were treated with a 110-kDa FN-f in serum-free culture, and expression of various cytokine genes was analyzed by cDNA microarray and by a cytokine protein array. Compared with untreated control cultures, stimulation by FN-f resulted in a >2-fold increase in IL-6, IL-8, MCP-1, and growth-related oncogene beta (GRO-beta). Constitutive and FN-f-inducible expression of GRO-alpha and GRO-gamma were also noted by RT-PCR and confirmed by immunoblotting. Previous reports of IL-1beta expression induced by FN-f were also confirmed, while TNF expression was found to be very low. Inhibitor studies revealed that FN-f-induced stimulation of chondrocyte chemokine expression was dependent on NF-kappaB activity, but independent of IL-1 autocrine signaling. The ability of FN-f to stimulate chondrocyte expression of multiple proinflammatory cytokines and chemokines suggests that damage to the cartilage matrix is capable of inducing a proinflammatory state responsible for further progressive matrix destruction, which also includes the chemoattraction of inflammatory cells. Targeting the signaling pathways activated by FN-f may be an effective means of inhibiting production of multiple mediators of cartilage destruction.

Differential effects of fibronectin fragment on proteoglycan metabolism by intervertebral disc cells: a comparison with articular chondrocytes

STUDY DESIGN

This in vitro study used the alginate bead culture system to probe for differences in the effects of fibronectin fragment on cell proliferation and proteoglycan metabolism by different populations of intervertebral disc cells and articular chondrocytes.

OBJECTIVE

To compare the effects of fibronectin fragment on cell proliferation, and proteoglycan synthesis and degradation by cells from the nucleus pulposus, the anulus fibrosus, and articular cartilage.

SUMMARY OF BACKGROUND DATA

In articular cartilage, the administration of fibronectin fragment stimulates cartilage degeneration. Fibronectin fragment levels were increased in human intervertebral discs with increased disc degeneration. Fibronectin fragment injected into the central region of the rabbit intervertebral disc induced a progressive degeneration of that disc.

METHODS

Bovine tails and metacarpophalangeal joints from 14- to 18-month-old animals were used. Alginate beads containing cells isolated from intervertebral discs and articular cartilage were cultured with (1-100 nmol/L) or without (control) fibronectin fragment in the presence of 10% fetal bovine serum. In these cultures, deoxyribonucleic acid and proteoglycan contents, as well as the rate of proteoglycan synthesis were determined. Proteoglycan degradation was measured in cultures with or without 10 nmol/L fibronectin fragment.

RESULTS

In articular chondrocytes, fibronectin fragment strongly suppressed proteoglycan synthesis and stimulated proteoglycan degradation; the total proteoglycan content was diminished in a dose-dependent manner. Compared to articular chondrocytes, nucleus pulposus cells responded to fibronectin fragments in a similar, although less pronounced manner. On the other hand, anulus fibrosus cells treated with fibronectin fragment did not show any significant effects on proteoglycan degradation. A slight but statistically significant up-regulation of proteoglycan synthesis was observed at 10 nmol/L fibronectin fragment in outer anulus fibrosus cells. However, total proteoglycan content was decreased significantly at high concentrations of fibronectin fragment.

CONCLUSIONS

Fibronectin fragment has different effects on cell proliferation, proteoglycan synthesis, degradation, and accumulation by articular chondrocytes and intervertebral disc cells. The different effects of fibronectin fragment in those different cell types suggest metabolic differences between these cells, and may further suggest differences in pathways of fibronectin fragment signaling as well as a differential need of these cells to be involved in tissue remodeling in which both anabolic and catabolic pathways might be altered.

Dynamic compression of chondrocyte-seeded fibrin gels: effects on matrix accumulation and mechanical stiffness

OBJECTIVE

Various strategies have been tested to direct and control matrix synthesis in tissue engineered cartilage, including mechanical stimulation of the construct both before and after implantation. This study examined the effects of oscillatory compression on chondrocytes in a fibrin-based tissue engineered cartilage.

DESIGN

Chondrocyte-seeded fibrin gels were cultured under unconfined mechanical compression for 10 or 20 days (free-swelling, 10% static, or 10+/-4% at 0.1 or 1Hz). During the culture period, accumulation of nitrite, sGAG, and proteolytic enzymes in the culture media were monitored. Following culture, the mechanical stiffness and biochemical content of the gels (DNA, sGAG, and hydroxyproline content and GAG Delta-disaccharide composition) were assessed.

RESULTS

Compared to free-swelling conditions, static compression had little effect on the mechanical stiffness or biochemical content of the gels. Compared to static compression, oscillatory compression produced softer gels, inhibited sGAG and hydroxyproline accumulation in the gels, and stimulated accumulation of nitrite and sGAG in the culture media. Minimal differences were observed in DNA content and Delta-disaccharide composition across treatment conditions.

CONCLUSIONS

In this study, oscillatory compression inhibited formation of cartilage-like tissues by chondrocytes in fibrin gels. These results suggest that the effects of mechanical stimuli on tissue engineered cartilage may vary substantially between different scaffold systems.

Comparison of the catabolic effects of fibronectin fragments in human knee and ankle cartilages

OBJECTIVE

To compare the response of knee and ankle cartilages to fibronectin fragments (Fn-f) in terms of kinetics of matrix proteoglycan (PG) degradation and synthesis, since previous data had shown that knee was more sensitive to Fn-f in terms of steady-state PG content.

DESIGN

Human knee and ankle cartilage explants were treated with the 29kDa Fn-f, and its effects on PG-degradation kinetics, on the half-lives of 35S-sulfate-labeled PG, on PG synthesis suppression and on matrix metalloproteinase -3 (MMP-3) were compared. Cultures were also treated with the interleukin (IL) receptor antagonist protein (IRAP) in order to determine whether IL-1 is involved in the Fn-f effect.

RESULTS

The Fn-f enhanced PG-degradation rates in both human knee and ankle cartilages. Knee cartilage showed a greater effect of Fn-f on half-lives of newly synthesized 35S-labeled PG than ankle. The extent of release of MMP-3 was similar for human ankle and knee cartilages. However, PG synthesis in knee cartilage was sensitive to 10- to 100-fold lower concentrations of Fn-f than was ankle cartilage. IRAP partially reversed Fn-f activity in ankle cartilages.

CONCLUSIONS

The role of Fn-f in proteolysis leading to cartilage damage appears to be minor in human cartilages than had previously been shown for bovine. This decreased proteolysis is true for both knee and ankle. The major difference between human ankle and knee cartilage appears to be greater sensitivity to PG synthesis suppression in knee cartilage. A further indication that IL-1 is involved in the pathway was provided by the partial reversal with IRAP.

Involvement of CD44 in induction of matrix metalloproteinases by a COOH-terminal heparin-binding fragment of fibronectin in human articular cartilage in culture

OBJECTIVE

To investigate the mechanism of induction of matrix metalloproteinases (MMPs) by a 40-kd COOH-terminal heparin-binding fibronectin fragment (HBFN-f) containing III12-14 and IIICS domains in human articular cartilage in culture.

METHODS

Human articular cartilage was removed from macroscopically normal femoral heads and cultured with HBFN-f. MMP secretion into conditioned media was analyzed by immunoblotting (MMPs 1 and 13) and by gelatin zymography (MMPs 2 and 9). Type II collagen cleavage by collagenase was monitored in culture by immunoassay. Involvement of specific peptide-binding domains in HBFN-f and the involvement of CD44 were assessed with synthetic peptides and an anti-CD44 antibody. Immunofluorescence histochemistry was performed using fluorescein isothiocyanate-conjugated anti-CD44 antibody.

RESULTS

HBFN-f stimulated production of MMPs 1, 2, 9, and 13 in association with type II collagen cleavage by collagenase in human articular cartilage. Peptide V (WQPPRARI) of HBFN-f, which can bind cell surface heparan sulfate proteoglycan (HSPG), blocked MMP induction by HBFN-f, while the scrambled peptide V (RPQIPWAR) had no effect. Peptide CS-1 of 25 amino acids in IIICS of HBFN-f caused no significant effect. Treatment of cartilage with anti-CD44 antibody or HSPG resulted in significant inhibition of HBFN-f-stimulated MMP production. Preincubation with peptide V blocked binding of the anti-CD44 antibody to chondrocytes in cartilage.

CONCLUSION

Interaction of the peptide V sequence in HBFN-f with glycosaminoglycans, such as those in CD44, plays an important role in HBFN-f-stimulated MMP production in articular cartilage. Because CD44 is up-regulated in osteoarthritic and rheumatoid arthritic cartilage, the role of the interaction between CD44 and HBFN-f in these pathologies should be of relevance and should be studied further.

High molecular weight hyaluronan promotes repair of IL-1 beta-damaged cartilage explants from both young and old bovines

OBJECTIVE

The addition of exogenous high molecular weight hyaluronic acid (HA) reverses cartilage damage caused by fibronectin fragments (Fn-fs) added to explant cultures of bovine and human cartilage and by Fn-fs in an experimental in vivo model of rabbit knee joint damage. Our objective was to test whether HA was also effective in an IL-1 damage model and whether this repair was stable and occurred in older bovine cartilage.

DESIGN

Bovine cartilage explants from 18-month-old or 6-year-old bovines in 10% serum/Dulbecco's modified Eagle's medium were exposed to Fn-f or to IL-1 and the ability of 1mg/ml HA of 800 kDa to block damage or promote restoration of proteoglycan (PG) after the damage was measured. The damage phase as well as the exposure to HA were varied.

RESULTS

Exposure of exogenous HA decreased Fn-f-mediated damage, but did not decrease IL-1 beta-induced cartilage damage. If explants from 18-month-old bovines were damaged by a 7-day exposure to Fn-f or IL-1 beta and then exposed for 7 days to HA, PG was restored. This reparative activity persisted up to 4 weeks after the removal of HA from the culture medium. The restoration of PG did not occur in 0.1% serum-free cultures, was less when the exposure to the Fn-f was doubled and failed when exposure to IL-1 beta was doubled. In explants from 6-year-old bovines damaged with IL-1 beta for 7 days, HA fully restored PG content to normal levels.

CONCLUSIONS

The reparative activities of HA occur not only in a Fn-f damage model, but also in an IL-1 damage model and occur with older bovine cartilage.

Advances in drug delivery for articular cartilage

The complex structure of articular cartilage, the connective tissue lining diarthrodial joints, enables this tissue to dissipate compressive loads but also appears to hinder its repair ability. At best, both natural and surgical repair attempts replace the highly ordered extracellular matrix of native articular cartilage with fibrous repair tissue of inferior mechanical properties. Numerous bioactive molecules closely regulate the cellular processes in healthy and degenerative articular cartilage. Accordingly, this review outlines the roles of important signaling molecules in cartilage tissue. In addition, drug delivery strategies, aimed at utilizing these bioactive agents to prevent inflammation, to regulate extracellular matrix metabolism, and to control cellular activities, are discussed. As scientists gain further insight into the complex signaling cascades of articular cartilage, continued refinement of drug delivery systems is necessary to develop effective clinical therapies for articular cartilage repair.

Fibronectin fragments active in chondrocytic chondrolysis can be chemically cross-linked to the alpha5 integrin receptor subunit

OBJECTIVE

To determine whether fibronectin fragments (Fn-f) known to enhance cartilage matrix degradation and to alter chondrocyte metabolism, bind on the chondrocyte cell surface close enough to the alpha(5)beta(1) fibronectin (Fn) receptor to be chemically cross-linked to it.

DESIGN

Biotinylated Fn-fs were added to chondrocytes, followed by cross-linking with dithiobissulfosuccinimidyl propionate, and the resultant alpha(5) complexes trapped on to antialpha(5)-agarose. Adherent material was analysed by probing with avidin-HRP. In a more specific approach in which only proximal targets could be cross-linked, photoaffinity labeled Fn-fs or Fn were added to cells, the derivatives activated and the cross-linked material analysed. Interaction of biotinylated Fn-fs and Fn with insolubilized alpha(5)beta(1) receptor was also visualized and quantified.

RESULTS

Biotinylated Fn-fs and Fn, but not a control of BSA, were cross-linked to alpha(5) protein in the presence of the propionate. Photoaffinity label Fn-f and Fn, but not BSA, were cross-linked to alpha(5) protein as well. Interaction was decreased by addition of an excess of unlabeled Fn-f or Fn. Fn-fs bound to alpha(5)beta(1)-agarose, although the affinity was 30-fold weaker and the stoichiometry 20-fold greater when the smallest Fn-f was compared to native Fn.

CONCLUSIONS

These data are consistent with a role for the alpha(5) subunit in Fn-f activities and suggest that the Fn-fs bind proximal or directly to alpha(5) receptors. The weaker, higher stoichiometry interaction of Fn-fs with receptor suggests that fragmentation has allowed de novo interactions not possible in native Fn.

Fibronectin fragments upregulate insulin-like growth factor binding proteins in chondrocytes

Addition of fibronectin fragments (Fn-fs) to cultured cartilage explants has been shown to mediate extensive cartilage matrix degradation followed by anabolic responses.

OBJECTIVE

To determine whether specific Fn-fs regulate cartilage metabolism through a mechanism, in part, involving insulin-like growth factor (IGF) and insulin-like growth factor binding proteins (IGFBPs).

METHODS

Primary bovine articular chondrocyte cultures were treated with Fn-fs. mRNA from the cultures was analysed by Northern blotting. Changes in the levels of IGFBPs in cellular extracts and conditioned media were analysed by Western ligand blotting. Explant cultures of bovine articular cartilage were used to assay release of exogenous IGF-I and IGFBP-2. An analog of IGF-I with altered affinity for IGFBPs was used to assay the effect of IGFBPs on proteoglycan synthesis.

RESULTS

The Fn-fs increased protein levels of IGFBPs-2, -3 and -5 in conditioned media and of IGFBP-2 in cell extracts by as much as nine-fold. Conversely, the protein level of constitutively expressed IGBP-4 was decreased in conditioned medium. Northern blot analysis reflected increased IGFBP-3 mRNA but not decreased IGFBP-4 mRNA. The IGF-I analog was more effective at restoring PG synthesis suppression by Fn-fs than was wild type IGF-I.

CONCLUSIONS

The Fn-fs increased levels of IGFBPs in cultures of bovine articular chondrocytes and elicited release of IGFBP-2 and IGF-I from articular cartilage. The increased level of IGFBPs may trap IGF-I and account in part for the initial suppression of PG synthesis. Induced proteinases may subsequently liberate IGF-I and cause greatly enhanced anabolic processes, contributing to cartilage repair.

Fibronectin in malignancy

Fibronectin (Fn) was the first 'structural' glycoprotein intensively studied as an ubiquitous matrix component of early phylogenetic appearance. Its age-dependent increase in plasma and tissues may be accompanied in pathological states, especially in tumor growth, by its proteolytic breakdown by a number of neutral proteases. It was also shown that several of its proteolytic breakdown products exhibit unexpected and mostly harmful biological activities. The first of these effects was a potentiation of malignant transformation. Some fragments had proteolytic activity, others behaved as proinflammatory agents stimulating IL-1 and collagenolytic MMP up-regulation. This matricryptic potential of Fn was followed by several other examples of proteolytic production of biologically active peptides. The study of solid human tumors showed among the early signs of malignant transformation the fragmentation of pericellular Fn, concommitent with the increase of its production by the peritumoral stroma. These results should encourage further investigations concerning the potential importance of Fn production and breakdown during cancer progression.

Antisense oligonucleotides to the integrin receptor subunit alpha(5) decrease fibronectin fragment mediated cartilage chondrolysis

OBJECTIVE

To investigate involvement of the integrin alpha(5) subunit of the classical fibronectin receptor in cartilage chondrolytic activities of fibronectin fragments (Fn-f).

DESIGN

Bovine chondrocytes and cartilage explants were cultured in the presence of antisense oligonucleotide (ASO), or sense (SO) or scrambled sequence oligonucleotide (SCO) corresponding to the bovine alpha(5) subunit. The effects of the oligonucleotides on mRNA and protein expression of the alpha(5) subunit were analysed by rtPCR and Western blotting, respectively. To test effects on Fn-f activities, three different Fn-f were first added to serum or serum-free cultures, followed by addition of oligonucleotides and the effects on Fn-f mediated proteoglycan (PG) degradation, cartilage PG depletion and PG and general protein synthesis suppression were tested.

RESULTS

The ASO decreased alpha(5) mRNA and protein expression to 69% and 55%, respectively, in monolayer cultures and decreased protein expression 67% in cartilage explants, while SO and SCO were ineffective. The ASO partially reversed the ability of the Fn-fs to suppress PG and general protein synthesis in cartilage explant and high density chondrocyte cultures. Concentrations of ASO from 1 nM to 5 microM effectively suppressed Fn-f activities in particular assays and the effects were reversible, while SO and SCO were not significantly effective. ASO also suppressed, in a dose-dependent and reversible fashion, the ability of the Fn-fs to enhance degradation and release of PG from cartilage explants. The ASO were also effective in suppressing the ability of an antibody to the alpha(5) subunit to enhance PG degradation, but were ineffective in blocking endotoxin or IL-1beta enhanced degradation.

CONCLUSIONS

These data implicate the alpha(5) integrin subunit in Fn-f mediated activities, consistent with a role for the alpha(5)beta(1) integrin in this pathway.

Mechanical compression alters gene expression and extracellular matrix synthesis by chondrocytes cultured in collagen I gels

Articular cartilage responds to its mechanical environment through altered cell metabolism and matrix synthesis. In this study, isolated articular chondrocytes were cultured in collagen type I gels and exposed to uniaxial static compression of 0%, 25%, or 50% of original thickness for 0.5, 4, and 24 h, and to oscillatory (25 +/- 4%, 1 Hz) compression for 24 h. The cellular response was assessed through competitive and real-time RT-PCR to quantify expression of genes for collagen type I, collagen type II, and aggrecan core protein, and through radiolabelled proline and sulfate incorporation to quantify protein and proteoglycan synthesis rates. Static compression for 24 h inhibited expression of collagen I and II mRNAs and inhibited 3H-proline and 35S-sulfate incorporation. The mRNA expression exhibited transient fluctuations at intermediate time points. Oscillatory compression had no effect upon mRNA expression, and 24 h after release from static compression, there was no difference in collagen II or aggrecan mRNA, while there was an inhibition of collagen I. We conclude that the chondrocytes maintained some aspects of their ability to sense and respond to static compression, despite a biochemical and mechanical environment which is different from that in tissue. This suggests that mechanical stimuli may be useful in modulating chondrocyte metabolism in tissue engineering systems using fibrillar protein scaffolds.

A single injection of fibronectin fragments into rabbit knee joints enhances catabolism in the articular cartilage followed by reparative responses but also induces systemic effects in the non-injected knee joints

OBJECTIVE

To investigate effects on cartilage metabolism and degeneration of injection of fibronectin fragments (Fn-fs) into rabbit knee joints.

DESIGN

The knees of adolescent New Zealand white rabbits were intraarticularly injected with rabbit Fn-fs. Cartilage sections from both injected and non-injected joints were treated with Safranin-O, with antibodies to the VDIPEN and NITEGE neoepitopes of degraded aggrecan and to matrix metalloproteinase-3 (MMP-3). Proteoglycan (PG) content of cartilage was measured by a dimethylmethylene blue assay of papain digests. PG synthesis rates were measured by(35)S-sodium sulfate incorporation into explanted cartilage.

RESULTS

In the injected joint cartilage, the Fn-fs bound cells in the upper superficial zone maximally between 6 and 24 h. By day 2, MMP-3 protein was enhanced and cartilage PG content and PG synthesis rates were reduced 40% and 70%, respectively. MMP-3 epitope and VDIPEN and NITEGE neoepitopes were also enhanced. The PG content then increased to supernormal levels from days 14 to 35 and then declined to normal levels by day 70, as did PG synthesis rates. In the non-injected joint cartilage, Fn-fs were not detected. Although MMP-3 expression was enhanced between days 2 and 21 as well as VDIPEN neoepitope, the PG content was never reduced but rather enhanced to supernormal levels from days 21 to 35. This was associated with enhanced PG synthesis by day 7, which decreased to control levels by day 70.

CONCLUSIONS

In this cartilage degeneration model, loss of cartilage PG is followed by supernormal anabolic responses that facilitate PG restoration. Further, the damage causes a systemic effect of enhanced PG synthesis and content in the non-injected joint cartilage.

Cartilage damage by matrix degradation products: fibronectin fragments

Catabolic cytokines play a major role in cartilage degradation not only in rheumatoid arthritis but also in osteoarthritis. Although the major source in rheumatoid arthritis may be mononuclear cells and synovial tissue and the cause of release may be multifactorial, the source of cytokines in osteoarthritis would be mostly from chondrocytes. However, there are few explanations of how upregulation of the cytokines might occur in osteoarthritis. One possibility is that degradation products of the extracellular matrix arising from elevated protease levels, substrate, or both, might regulate cytokine activities. Fragments of the extracellular matrix protein, fibronectin, upregulate cytokine expression and induce the events of suppressed matrix synthesis and upregulation of matrix metalloproteinases, characteristic of osteoarthritis. The catabolic aspects of this system are short term, subsequently serve to enhance anabolic processes above untreated levels, and condition the tissue against additional insult. It will be necessary to determine whether in vivo these degradation products precede cytokine expression and act early and are targets for intervention or instead are a consequence of cytokine damage. Whether they regulate anabolism and catabolism, blocking of their activities may not be ideal.

Histomorphological and proliferative characterization of developing periosteal neochondrocytes in vitro

Periosteal chondrogenesis is relevant to cartilage repair and fracture healing. Cell proliferation is a limiting factor of cartilage production. We used an in vitro organ culture model to test the hypothesis that proliferative activity correlates with cell morphology. One hundred and four periosteal explants from 26 two-month old New Zealand rabbits were cultured for up to 42 days. They were analyzed histomorphologically, and immunohistochemically with proliferative cell nuclear antigen (PCNA). The periosteal neocartilage displayed a consistent zonal pattern of chondrocyte cell shapes. The flat cell zone from day 7 to 21, consisted of uniform-sized small spindle-shaped cells. The round cell zone, which appeared on day 14, consisted of variable-sized round cells averaging 510 +/- 250 microm2 in area. They were subdivided into small round (<510 microm2) and large round cells (>510 microm2). The proliferative index was highest in the small round cell group (32 +/- 6%), intermediate in the flat cell group (27 +/- 6%), and lowest in the large round cell group (20 +/- 7%) (P < 0.001). Furthermore, the proliferative indices in the round cell group were inversely proportional to cell size. Therefore, (1) there is a sequential progression of cell morphology during periosteal chondrogenesis, (2) cell differentiation is arrested prior to terminal differentiation for some cells and not for others, and (3) proliferative activity is strongly related to cell morphology. This organ culture model provides us with opportunities to study the regulation of terminal chondrocyte differentiation and the control of cell proliferation. This will contribute to our understanding of cartilage repair, fracture healing and growth plate physiology.

The proteolytic activity of the recombinant cryptic human fibronectin type IV collagenase from E. coli expression

Human plasma fibronectin (pFN) contains a cryptic metalloprotease present in the collagen-binding domain. The enzyme could be generated and activated in the presence of Ca2+ from the purified 70-kDa pFN fragment produced by cathepsin D digestion. In this work we cloned and expressed the metalloprotease, designated FN type IV collagenase (FnColA), and a truncated variant (FnColB) in E. coli. The recombinant pFN protein fragment was isolated from inclusion bodies, and subjected to folding and autocatalytic degradation in the presence of Ca2+, and yielded an active enzyme capable of digesting gelatin, helical type II and type IV collagen, alpha- and beta-casein, insulin b-chain, and a synthetic Mca-peptide. In contrast, isolated plasma fibronectin, type I collagen, and the DNP-peptide were no substrates. Both catalytically active recombinant pFN fragments were efficiently inhibited by EDTA, and batimastat, and, in contrast to the glycosylated enzyme isolated from plasma fibronectin, were also inhibited by TIMP-2.

An N-terminal peptide from link protein can stimulate biosynthesis of collagen by human articular cartilage

Previous studies have shown that a peptide identical in sequence to the N-terminal of link protein can function as a growth factor and up-regulate proteoglycan synthesis by human articular cartilage in explant culture (L. A. McKenna et al., Arthritis Rheum. 41, 157-162, 1998). The present study has extended these investigations to determine the effects of this peptide on the synthesis of collagen, another essential component of normal cartilage matrix. Explants from normal adult knee cartilage were maintained for periods of up to 8 days in medium with or without serum. Peptides were added during each day of culture. Synthesis of collagen was determined by the incorporation of [3H]proline into hydroxyproline and proteoglycans by incorporation of [35S]sulfate. The type of newly synthesized collagen was measured by SDS-polyacrylamide gel electrophoresis, fluorography, and immunoblotting. The link protein peptide stimulated synthesis of type II collagen in cartilage from a number of different subjects. Maximum up-regulation of synthesis was attained at a concentration of 100 ng/ml, similar to that observed previously for up-regulation of proteoglycan. Synthesis was up-regulated in both the presence and the absence of serum, although the overall rate of synthesis was greater when serum was added. The findings that this link peptide growth factor stimulated synthesis of proteins, including collagen, in a manner analogous to that shown previously for proteoglycans support the hypothesis that this peptide may have an important role in the feedback control of cartilage matrix synthesis.

Fibronectin fragments and their role in inflammatory arthritis

OBJECTIVES

To identify potential immunopathogenic links between fibronectin (Fn) fragmentation and the inflammatory response in chronic joint disease.

METHODS

Scientific papers involving studies of Fn fragments and inflammatory processes important in the pathogenesis of arthritis, including chondrolysis, synoviocyte growth and adhesion, polymorphonuclear leukocyte (PMN) and monocyte function, proteolysis, and immune complex activation were reviewed. In addition, reports identifying Fn fragments in synovial fluid (SF) were assessed.

RESULTS

A series of Fn fragments have been identified in arthritic SF by several investigators. Fn and fragments ranging from 30 to 200 kd are present in elevated concentrations in inflammatory SF. SF Fn fragments display reduced affinity for fibrin and collagen. The 29- and 50-kd amino terminal fragments mediate release of proteoglycan from articular cartilage by RGD-independent mechanisms. Fn fragments can induce fibroblast gene expression of metalloproteinases or can act as proteinases themselves. A 90-kd plasmin generated fragment possesses homology with streptokinase. Fragments mediate PMN chemotaxis and enhance proliferation of CD4+ lymphocytes as well as binding to the C1q component of complement and influencing the behavior of immune complexes.

CONCLUSIONS

Fn fragments can be functionally and biochemically characterized in diseased SF. Modification of fragment formation and inhibition of fragment function may have potential therapeutic value in the interruption of chronic synovial inflammation.

Hyaluronan suppresses fibronectin fragment-mediated damage to human cartilage explant cultures by enhancing proteoglycan synthesis

Hyaluronic acid, recently renamed hyaluronan, has been used as a therapeutic intervention in the treatment of osteoarthritis. We have reported that high-molecular-weight (800 kDa) hyaluronan is effective in blocking the catabolic action of fibronectin fragments in explant cultures of bovine cartilage and in an experimental in vivo model of damage to the rabbit knee joint. The fibronectin fragments induce catabolic cytokines in human cartilage, which, in turn, suppress proteoglycan synthesis and induce matrix metalloproteinases to decrease the proteoglycan content. Since the clinical target of high-molecular-weight hyaluronan is human cartilage, which may differ in certain ways from bovine cartilage, we tested the effect on human knee cartilage. We found that 1 mg/ml hyaluronan completely blocked fibronectin fragment-mediated decreases in proteoglycan content in five of five specimens of cartilage from the human knee. This was associated with binding of exogenous hyaluronan to the superficial surface, suppressed penetration of the fibronectin fragment into the cartilage, decreased expression for the first week in culture of one of the matrix metalloproteinases involved in cartilage degradation, matrix metalloproteinase-3, and proteoglycan synthesis rates that increased to supernormal levels. However, the appearance of the NITEGE and VDIPEN neoepitopes, indices of cartilage degradation, was not blocked but was delayed by 1 week. The addition of hyaluronan to cartilage previously damaged by the fibronectin fragments or to osteoarthritic cartilage fully restored the proteoglycan content to control levels. We conclude that hyaluronan blocked damage at least partly by blocking penetration of the fibronectin fragments and slowing matrix metalloproteinase expression. However, the major effect on blocking damage and promoting repair may be through enhanced proteoglycan synthesis, a mechanism that requires further study. Nonetheless, these data clearly demonstrate that hyaluronan completely protected human cartilage in explant culture and facilitated a full restoration of proteoglycan in damaged cartilage.

The macromolecular characteristics of cartilage proteoglycans do not change when synthesis is up-regulated by link protein peptide

Previous studies have shown that a synthetic, unglycosylated analogue of the N-terminal peptide from link protein can function as a growth factor and up-regulate proteoglycan biosynthesis in explant cultures of normal human articular cartilage from a wide age range of subjects (McKenna et al., Arthritis Rheum. 41 (1998) 157-162). The present work further shows that link peptide increased proteoglycan synthesis by cartilage cultured in both the presence and absence of serum, suggesting that the mechanism of up-regulation may be different from that of insulin-like growth factors. The proteoglycans synthesised during stimulation with link peptide were of normal hydrodynamic size and the ratio of core protein to glycosaminoglycan side chains and the proportions of the large proteoglycan aggrecan to the small proteoglycans, decorin and biglycan, remained constant. Aggrecan molecules were equally capable of forming aggregates as those from control tissues and the relative proportions of decorin and biglycan were unchanged showing that both were co-ordinately up-regulated. These results confirmed that this novel peptide is a potent stimulator of proteoglycan synthesis by articular cartilage and showed that the newly synthesised proteoglycans were of normal composition.

Cyclic compression of articular cartilage explants is associated with progressive consolidation and altered expression pattern of extracellular matrix proteins

In this study, we investigated the biosynthetic response of full thickness, adult bovine articular cartilage explants to 45 h of static and cyclic unconfined compression. The cyclic compression of articular cartilage resulted in a progressive consolidation of the cartilage matrix. The oscillatory loading increased protein synthesis ([35S]methionine incorporation) by as much as 50% above free swelling control values, but had an inhibitory influence on proteoglycan synthesis ([35SO4] incorporation). As expected, static compression was associated with a dose-dependent decrease in biosynthetic activity. ECM oligomeric proteins which were most affected by mechanical loading were fibronectin and cartilage oligomeric matrix protein (COMP). Static compression at all amplitudes caused a significant increase in fibronectin synthesis over free swelling control levels. Cyclic compression of articular cartilage at 0.1 Hz and higher was consistently associated with a dramatic increase in the synthesis of COMP as well as fibronectin. The biosynthetic activity of chondrocytes appears to be sensitive to both the frequency and amplitude of the applied load. The results of this study support the hypothesis that cartilage tissue can remodel its extracellular matrix in response to alterations in functional demand.

Exposure of cartilage to a fibronectin fragment amplifies catabolic processes while also enhancing anabolic processes to limit damage

The addition of fibronectin fragments to cultured cartilage causes an initial suppression of proteoglycan synthesis, induction of matrix metalloproteinases, and resultant decrease in proteoglycan content by about 50% during the first few days in culture. Because the proteoglycan loss appears to be limited, we investigated whether the fibronectin fragments induce anabolic responses that might counter the damage. The effects of various lengths of exposure of cultured cartilage to the fibronectin fragment on proteoglycan content, proteoglycan synthesis rates, stromelysin-1 release, and tumor necrosis factor-alpha, interleukin-1alpha, and interleukin-6 release were investigated. The results showed that about 7 days of exposure of cultured cartilage to the fibronectin fragment was required for maximal cytokine release, proteoglycan depletion, and stromelysin-1 release. However, nearly maximal suppression of proteoglycan synthesis occurred within 1 day of the addition of the fibronectin fragment and, after its removal, the rates increased to supernormal levels. Decreasing exposure to 3 days caused only a small decrease in cartilage proteoglycan content, although stromelysin-1 release still occurred. Decreasing exposure to 1 day caused an immediate increase in proteoglycan synthesis and an increase to supernormal proteoglycan contents. The effect of first treating cartilage with the fibronectin fragment for various periods and then allowing a recovery was to make the cartilage more resistant to secondary exposures. This study shows that cartilage damage can be caused by short exposures to the fibronectin fragment and that exposures either optimal or suboptimal for damage additionally amplify anabolic processes to make the cartilage resistant to further damage and, thus, condition it against pending amplification of damage.

Use of microscopical techniques in the study of human chondrocytes from osteoarthritic cartilage: an overview

Several microscopical techniques, such as high resolution light microscopy, Normaski microscopy, laser confocal and transmission electron microscopy, were used in a correlative morphological study of human osteoarthritic (OA) cartilage. Emphasis was made on the characterization of chondrocytes heterogeneity observed in this tissue. Novel findings were assessed in the morphological and immunocytological study of the chondrocytes organized in aggregates or "clones" typical of this degenerative disease, consisting of the modification of certain elements of the cytoskeleton that influence changes in the cell shape. Also, the presence of cilia and centrioles found in certain cell raised the question if chondrocytes are able to move and regroup as an alternative mechanism to mitosis in the formation of cell clusters or "clones." The presence of two types of secretory chondrocytes was observed and discussed. The use of a correlative approach of several microscopical techniques in a systematic morphological and immunocytological characterization of chondrocyte population within the fibrillated and nonfibrillated human osteoarthritic cartilage gave complementary information that could be important for a better understanding of the histopathogenesis of OA.

Hyaluronic acid suppresses fibronectin fragment mediated cartilage chondrolysis: I. In vitro

A commercial preparation of 800-kDa hyaluronic acid (HA), (ARTZ from Seikagaku, Inc.), has been used as a therapeutic intervention in the treatment of osteoarthritis (OA). We tested the effect of this HA form, HA/800, in an in vitro cartilage chondrolytic system in which a specific amino-terminal 29-kDa fragment of fibronectin (Fn-f) penetrates cartilage tissue to activate chondrocytes to amplify two major chondrolytic activities: suppression of proteoglycan (PG) synthesis and induction of matrix metalloproteinases. We report that HA/800 did not block damage by Fn-f in serum free cartilage cultures. However, HA/800 was effective in blocking the ability of 100 nM Fn-f to cause the degradation and release of half of the total cartilage PG from cartilage in 10% serum/DMEM cultures. While the Fn-f caused a half-time for PG release of 3 days, continuous exposure to 0.1 or 1 mg/ml HA/800 slowed the half-time to 12 days. Further, a single 1 day pre-incubation with 0.1 or 1 mg/ml HA/800 was sufficient to decrease the half-time of 100 nM Fn-f mediated PG depletion to 7 and 12 days, respectively. HA/800 completely blocked the effect of 10 nM Fn-f. Blocking of Fn-f-mediated cartilage PG depletion was associated with a decreased concentration of Fn-f on the superficial cartilage surface and decreased penetration into the cultured cartilage tissue. Further, the two major chondrolytic activities of the Fn-f, suppression of synthesis of PG and enhanced release of stromelysin-1, were suppressed by HA/800. HA/800 also partially restored PG in cartilage first damaged with the Fn-F. We conclude that HA/800 slows Fn-f-mediated cartilage chondrolysis in vitro and has some reparative potential. The damage blocking activity appears to be associated with the ability of HA/800 to block penetration of the Fn-f, rather than with direct effects on cartilage tissue.

Cartilage fibronectin isoforms: in search of functions for a special population of matrix glycoproteins

Fibronectins are a part of the repertoire of matrix molecules produced by the chondrocyte in order to assemble a functional cartilage matrix. They are encoded by a single gene, but significant protein heterogeneity results from alternative RNA splicing. The population of fibronectin isofroms in adult cartilage is significantly different from fibronectins in other tissues and includes relatively high levels (20-30%) of ED-B(+) fibronectins and high levels (50-80%) of the cartilage specific (V + C)- isoform which lacks the V, III-15 and I-10 segments. Less than 4% of the fibronectins in cartilage are ED-A(+). The synthesis and accumulation of cartilage fibronectins are modulated in response to matrix pathology and to biochemical and mechanical mediators. In addition, alternative splicing patterns are altered when chondrocytes are allowed to dedifferentiate in monolayer culture such that the (V + C)- isoform is lost but the ED-A(+) isoform is reexpressed at high levels. Cartilage fibronectins have the potential to participate in cell signalling via integrin mediated pathways and to interact with other cartilage matrix macromolecules. The tissue-specific splicing pattern gives rise to a unique population of fibronectins within the cartilage. Together, this points to a critical role for cartilage fibronectins in chondrocyte cell biology and the organization of a biomechanically sound matrix. However, the precise function (or functions) of the cartilage fibronectins has (or have) not been defined. This minireview examines current information about the structure, synthesis and interactions of cartilage fibronectins. When possible, potential consequences of the inclusion of the ED-B segment or the exclusion of the V, III-15 and I-10 segments are discussed. The goal is to stimulate critical thought and discussion in the field about cartilage fibronectin isoforms, their function(s) in normal cartilage, and their role(s) in the pathogenesis of cartilage diseases.

Fibronectin-fragment-induced cartilage chondrolysis is associated with release of catabolic cytokines

Fibronectin fragments have both catabolic and anabolic activities toward articular cartilage explants in vitro. Whereas a 1 nM concentration of an N-terminal 29 kDa fibronectin fragment (Fn-f) increases the proteoglycan (PG) content of cartilage without induction of matrix metalloproteinases (MMPs), 0.1-1 microM Fn-f temporarily suppresses PG synthesis and enhances MMP release. The higher concentrations cause an initially rapid PG depletion during the first week of culture, followed by much slower PG loss and gradually increasing rates of PG synthesis. To test for the involvement of mediators, human articular cartilage was cultured with Fn-f, and conditioned media were assayed for selected cytokines and factors. With 1 nM Fn-f, the release of the anabolic factors, insulin growth factor-I and transforming growth factor beta1, from cultured cartilage was enhanced by 50-100% during the entire 28-day culture period and this was associated with both supernormal rates of PG synthesis and PG content. However, the higher concentrations of Fn-f additionally enhanced release, by at least 10-fold, of the cytokines, tumour necrosis factor alpha, interleukin-1alpha, interleukin-1beta and interleukin-6 while causing depletion of cartilage PG. Release of tumour necrosis factor alpha, interleukin 1beta and interleukin 1alpha peaked at days 2, 3 and 9 during or slightly after the period of maximal PG depletion and decreased to control levels by days 7, 7 and 21 respectively, whereas release of interleukin 6 was enhanced throughout the culture period. Neutralizing antibodies to the catabolic cytokines reduced Fn-f-mediated MMP-3 release and suppression of PG synthesis. The temporal aspects of this interplay between catabolic and anabolic factors are consistent with the kinetics of Fn-f-mediated cartilage damage and attempted repair and may be relevant to cartilage damage and repair in vivo.

Fibronectin fragment mediated cartilage chondrolysis. I. Suppression by anti-oxidants

Fibronectin fragments damage cartilage in vitro by greatly enhancing metalloproteinases and suppressing proteoglycan (PG) synthesis which results in severe cartilage PG depletion. Since reactive oxygen species (ROS) have been implicated in catabolic cytokine action and preliminary data suggested that catabolic cytokines such as TNF-alpha, IL-1 alpha, IL-1 beta and IL-6 are responsible for fibronectin fragment mediated damage, selected anti-oxidants (AOs) were tested as inhibitors of cytokine. ROS and fibronectin fragment activity. Damage was measured by depletion of cartilage PG during tissue culture. The AO, N-acetylcysteine (NAC), decreased the extent of cartilage PG depletion caused by TNF-alpha and IL-1 alpha and by the ROS, hydrogen peroxide and superoxide anion, confirming that the cytokines operate through ROS and that ROS can initiate cartilage PG depletion. NAC at 0.1 and 1 mM, totally suppressed PG depletion caused by a highly potent amino-terminal 29-kDa fibronectin fragment (Fn-f) for 14 days in culture. NAC at 10 mM totally blocked Fn-f mediated PG depletion for 21 days and increased the cartilage PG content by 30% above normal levels. Glutathione (10 microM) and DMSO (1%) were also totally effective while catalase and superoxide decreased Fn-f mediated damage only during the first week and superoxide dismutase alone caused damage after 1 wk. The AOs caused protection by reducing the major catabolic activities of the Fn-f: enhanced release of stromelysin-1 (MMP-3) and suppression of PG and protein synthesis. NAC also decreased normal rates of PG degradation and increased the half-lives of labeled PG in both control and Fn-f treated cartilage. We conclude that the Fn-f mediates cartilage chondrolysis through ROS, consistent with the involvement of catabolic cytokines in the Fn-f mechanism, and that AOs greatly reduce Fn-f mediated cartilage chondrolysis. In an accompanying manuscript we also report that AOs promote reparative responses in Fn-f and cytokine treated cartilage.

Fibronectin fragment mediated cartilage chondrolysis. II. Reparative effects of anti-oxidants

In an accompanying manuscript, it was shown that the cartilage chondrolytic activities of fibronectin fragments (Fn-f), which are mediated through catabolic cytokines such as TNF-alpha, IL-1 and IL-6, could be suppressed by anti-oxidants (AOs). The AOs neutralized reactive oxygen species (ROS) which are known to mediate catabolic cytokine action. The objective in this work was to test whether AOs would promote restoration of proteoglycan (PG) in Fn-f treated cartilage, since under normal culturing conditions, PG is not restored after removal of the Fn-f. Cartilage was first cultured with an amino-terminal 29-kDa Fn-f to cause loss of about half of the total PG and then treated with NAC (1 and 10 mM) or glutathione (10 microM) or DMSO (0.1 or 1%). Treatment with NAC and glutathione maximally caused restoration of PG within 14 days to normal or supernormal levels, while DMSO was less effective. Catalase, but not superoxide dismutase, enhanced PG content to a small but significant extent. The restoration of PG in Fn-f treated cartilage occurred throughout the full depth of the cartilage slices as shown by histochemical analysis. However, removal of the AO allowed a subsequent decrease in PG content suggesting that the AOs had not blocked cytokine expression but had merely suppressed cytokine activities. Addition of NAC to IL-1 treated cartilage promoted a restoration of PG, while addition to chymopapain or trypsin treated cartilage was not very effective, suggesting that the effect of AOs requires a cytokine driven damage system. We conclude that the AOs promote a restoration of PG in the Fn-f treated cartilage by suppressing the effects of catabolic cytokines. The data suggest a potential for AOs in reversing tissue damage caused by cytokines.

Fibronectin fragments induce the expression of stromelysin-1 mRNA and protein in bovine chondrocytes in monolayer culture

Addition of proteolytically generated fibronectin fragments (Fn-f) to cultured cartilage tissue causes greatly enhanced release of metalloproteinases (MMPs), such as pro-stromelysin-1 (proSln-1), and suppression of proteoglycan (PG) synthesis, through release of catabolic cytokines, while native fibronectin is ineffective. We have investigated whether enhanced release of proSln-1 was due to up-regulation of pro-Sln-1 mRNA. We report the addition of a 29-kDa (amino-terminal heparin-binding Fn-f) or a 140-kDa (central cell-binding Fn-f) to bovine chondrocytes in monolayer culture causes a dose dependent increase in the expression of pro-Sln-1 mRNA and the greatly enhanced release of pro-Sln-1 protein into the culture media. Up to 700 nM pro-Sln-1 was found in the conditioned media and metabolic labeling showed that it constituted a major portion of newly synthesized protein. A potential activator of pro-Sln-1, urokinase (u-PA), was released at elevated levels in the presence of the Fn-f while other activators, tissue plasminogen activator (t-PA) and plasmin activities were not detected. Addition of these activators to conditioned media did not allow conversion of pro-Sln-1 to active Sln-1. However, aminophenyl mercuric acid activated pro-Sln-1 to a 48-kDa Sln-1 form capable of degrading PG when added to cartilage suspensions. Gelatinase A mRNA was also enhanced, suggesting that the Fn-f may induce MMPs in general. However, the major regulator of Sln-1 activity, tissue inhibitor of MMPs form 1 (TIMP-1), was not induced at the gene level. Thus, a major effect of Fn-f on chondrocytes is to up-regulate pro-Sln-1 expression at the gene level, resulting in pro-Sln-1 as a major protein product.

Surface adhesion-mediated regulation of chondrocyte-specific gene expression in the nontransformed RCJ 3.1C5.18 rat chondrocyte cell line

Recent evidence suggests that decreased chondrocyte function in osteoarthritis and other articular disorders may be due to chondrocyte dedifferentiation produced by altered regulatory signals from the cartilage extracellular matrix (ECM). However, there are currently no mammalian chondrocytic cell line systems adapted to the study of this process. We therefore examined the effects of ECM growth conditions on markers of differentiated chondrocytic phenotype expression in the nontransformed rat RCJ 3.1C5.18 (RCJ) chondrocyte cell line, including type II collagen expression, aggrecan production, link protein gene expression, and parathyroid hormone (PTH) receptor number. RCJ cells grown in monolayer on plastic exhibited a dedifferentiated phenotype characterized by flattened cell morphology, with > 80% type I collagen and < 5% type II collagen production, as determined by two-dimensional gel mapping electrophoresis of collagen cyanogen bromide peptides. In addition, aggrecan production was low, and link protein mRNA was not expressed at detectable levels. After transfer to growth under minimal attachment conditions on the surface of a composite type I collagen/agarose (0.15%-0.8%) gel (CAG) for 7 days, RCJ cells developed a rounded, chondrocytic morphology and a pattern of differentiated, chondrocytic gene expression, with 79% type II and 8% type I collagen production. Steady-state type I and type II procollagen mRNA levels were altered in parallel with collagen protein expression. In cells grown on CAG, aggrecan production increased 6-fold, and there was a marked increase in both aggrecan core protein and link protein mRNA levels. In addition, maximal PTH-stimulated cAMP generation increased 15-fold in association with an increased PTH receptor number. Therefore, the RCJ chondrocyte cell line is highly sensitive to ECM regulation of chondrocyte-specific gene expression.

Increased expression of CD44 in bovine articular chondrocytes by catabolic cellular mediators

Bovine articular chondrocytes cultured in alginate beads were used to study the effect of catabolic cellular mediators on CD44 expression. Treatment with either the 29-kDa fragment of fibronectin or interleukin-1 alpha results in a time- and dose-dependent inhibition of proteoglycan synthesis as well as a stimulation in the expression of CD44 mRNA level as determined by semi-quantitative polymerase chain reaction following reverse transcription. No noticeable effect at 6 h was observed. By 24 h, the major CD44 product (CD44H) from fibronectin fragment-treated cultures showed an 8-fold increase; CD44H from interleukin-1 alpha-treated cultures showed a 6-fold increase as compared to control cultures. In addition, a minor band, determined to be an isoform of CD44, was also shown to be up-regulated by both mediators. Stimulation of CD44 mRNA via interleukin-1 was also evident by in situ hybridization studies of bovine as well as human articular cartilage in organ culture. The increased in CD44 mRNA is matched by an increase at the protein level as determined by Western blot analysis. The Western blot reveals a doublet protein band at 80-90 kDa that corresponds to the molecular mass of CD44H. Cultures incubated with fibronectin fragments for 24 h had an 8.0-fold increase in CD44, while a 6.6-fold was observed for interleukin-1 alpha. Fluorescein-conjugated hyaluronan binding and internalization studies indicate that the increase in CD44 protein, induced by interleukin-1 alpha, closely correlates with an increase in functional hyaluronan receptors present at the chondrocyte cell surface. Taken together these results indicate that conditions that up-regulate chondrocyte catabolism also up-regulate the expression of CD44, a cell surface hyaluronan receptor involved in hyaluronan endocytosis.