Cathepsin B and cysteine protease inhibitors in human osteoarthritis

Proteomic Analysis of Synovial Fibroblasts and Articular Chondrocytes Co-Cultures Reveals Valuable VIP-Modulated Inflammatory and Degradative Proteins in Osteoarthritis

Osteoarthritis (OA) is the most common musculoskeletal disorder causing a great disability and a reduction in the quality of life. In OA, articular chondrocytes (AC) and synovial fibroblasts (SF) release innate-derived immune mediators that initiate and perpetuate inflammation, inducing cartilage extracellular matrix (ECM) degradation. Given the lack of therapies for the treatment of OA, in this study, we explore biomarkers that enable the development of new therapeutical approaches. We analyze the set of secreted proteins in AC and SF co-cultures by stable isotope labeling with amino acids (SILAC). We describe, for the first time, 115 proteins detected in SF-AC co-cultures stimulated by fibronectin fragments (Fn-fs). We also study the role of the vasoactive intestinal peptide (VIP) in this secretome, providing new proteins involved in the main events of OA, confirmed by ELISA and multiplex analyses. VIP decreases proteins involved in the inflammatory process (CHI3L1, PTX3), complement activation (C1r, C3), and cartilage ECM degradation (DCN, CTSB and MMP2), key events in the initiation and progression of OA. Our results support the anti-inflammatory and anti-catabolic properties of VIP in rheumatic diseases and provide potential new targets for OA treatment.

Progenitor Cells Activated by Platelet Lysate in Human Articular Cartilage as a Tool for Future Cartilage Engineering and Reparative Strategies

Regenerative strategies for human articular cartilage are still challenging despite the presence of resident progenitor cell population. Today, many efforts in the field of regenerative medicine focus on the use of platelet derivatives due to their ability to reactivate endogenous mechanisms supporting tissue repair. While their use in orthopedics continues, mechanisms of action and efficacy need further characterization. We describe that the platelet lysate (PL) is able to activate chondro-progenitor cells in a terminally differentiated cartilage tissue. Primary cultures of human articular chondrocytes (ACs) and cartilage explants were set up from donor hip joint biopsies and were treated in vitro with PL. PL recruited a chondro-progenitors (CPCs)-enriched population from ex vivo cartilage culture, that showed high proliferation rate, clonogenicity and nestin expression. CPCs were positive for in vitro tri-lineage differentiation and formed hyaline cartilage-like tissue in vivo without hypertrophic fate. Moreover, the secretory profile of CPCs was analyzed, together with their migratory capabilities. Some CPC-features were also induced in PL-treated ACs compared to fetal bovine serum (FBS)-control ACs. PL treatment of human articular cartilage activates a stem cell niche responsive to injury. These facts can improve the PL therapeutic efficacy in cartilage applications.

Detecting cathepsin activity in human osteoarthritis via activity-based probes

Introduction

Lysosomal cathepsins have been reported to contribute to Osteoarthritis (OA) pathophysiology due to their increase in pro-inflammatory conditions. Given the causal role of cathepsins in OA, monitoring their specific activity could provide means for assessing OA severity. To this end, we herein sought to assess a cathepsin activity-based probe (ABP), GB123, in vitro and in vivo.

Methods

Protein levels and activity of cathepsins B and S were monitored by immunoblot analysis and GB123 labeling in cultured primary chondrocytes and conditioned media, following stimuli with tumor necrosis factor alpha (TNFα) and/or Interleukin 1 beta (IL-1β). Similarly, cathepsin activity was examined in sections of intact cartilage (IC) and degraded cartilage (DC) regions of OA. Finally, synovial fluid (SF) and serum from donors with no signs of diseases, early OA, late OA and rheumatoid arthritis (RA) patients were analyzed with GB123 to detect distinct activity levels of cathepsin B and S.

Results

Cathepsin activity in cell lysates, conditioned media explants and DC sections showed enhanced enzymatic activity of cathepsins B and S. Further histological analysis revealed that cathepsin activity was found higher in superficial zones of DC than in IC. Examining serum and SF revealed that cathepsin B is significantly elevated with OA severity in serum and SF, yet levels of cathepsin S are more correlated with synovitis and RA.

Conclusions

Based on our data, cathepsin activity monitored by ABPs correlated well with OA severity and joint inflammation, directing towards a novel etiological target for OA, which possesses significant translational potential in developing means for non-invasive detection of early signs of OA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0586-5) contains supplementary material, which is available to authorized users.

Effect of glucosamine and its peptidyl-derivative on the production of extracellular matrix components by human primary chondrocytes

OBJECTIVE

Aim of this study is to investigate the effects of Glucosamine (GlcN) and its peptidyl-derivative, 2-(N-Acetyl)-L-phenylalanylamido-2-deoxy-β-D-glucose (NAPA), on extracellular matrix (ECM) synthesis in human primary chondrocytes (HPCs).

METHODS

Dose-dependent effect of GlcN and NAPA on Glycosaminoglycan (GAG), Collagen type II (Col2) and Small Leucine-Rich Proteoglycans (SLRPs) was examined by incubating HPCs, cultured in micromasses (3D), with various amounts of two molecules, administered as either GlcN alone or NAPA alone or GlcN plus NAPA (G + N). Immunohystochemical and immunofluorescent staining and biochemical analysis were used to determine the impact of the two molecules on ECM production. Gene expression analysis was performed by TaqMan Real-Time Polymerase Chain Reaction (PCR) assays.

RESULTS

The lowest concentration to which GlcN and NAPA were able to affect ECM synthesis was 1 mM. Both molecules administered alone and as G + N stimulated GAGs and SLRPs synthesis at different extent, NAPA and mainly G + N stimulated Col2 production, whereas GlcN was not effective. Both molecules were able to induce Insulin Growth Factor-I (IGF-I) and to stimulate SOX-9, whereas NAPA and G + N were able to up-regulate both Hyaluronic Acid Synthase-2 and Hyaluronic acid. Very interesting is the synergistic effect observed when chondrocyte micromasses were treated with G + N.

CONCLUSIONS

The observed anabolic effects and optimal concentrations of GlcN and NAPA, in addition to beneficial effects on other cellular pathways, previously reported, such as the inhibition of IKKα, could be useful to formulate new cartilage repair strategies.

Quantitative proteomics reveals regulatory differences in the chondrocyte secretome from human medial and lateral femoral condyles in osteoarthritic patients

Background

Osteoarthritis (OA) is a destructive joint disease and there are no known biomarkers available for an early diagnosis. To identify potential disease biomarkers and gain further insight into the disease mechanisms of OA we applied quantitative proteomics with SILAC technology on the secretomes from chondrocytes of OA knees, designated as high Mankin (HM) scored secretome. A quantitative comparison was made between the secretomes of the medial and lateral femur condyle chondrocytes in the same knee since the medial femur condyle is usually more affected in OA than the lateral condyle, which was confirmed by Mankin scoring. The medial/lateral comparison was also made on the secretomes from chondrocytes taken from one individual with no clinically apparent joint-disease, designated as low Mankin (LM) scored secretome.

Results

We identified 825 proteins in the HM secretome and 69 of these showed differential expression when comparing the medial and lateral femoral compartment. The LM scored femoral condyle showed early signs of OA in the medial compartment as assessed by Mankin score. We here report the identification and relative quantification of several proteins of interest for the OA disease mechanism e.g. CYTL1, DMD and STAB1 together with putative early disease markers e.g. TIMP1, PPP2CA and B2M.

Conclusions

The present study reveals differences in protein abundance between medial/lateral femur condyles in OA patients. These regulatory differences expand the knowledge regarding OA disease markers and mechanisms.

Cathepsin B is secreted apically from Xenopus 2F3 cells and cleaves the epithelial sodium channel (ENaC) to increase its activity

The epithelial sodium channel (ENaC) plays an important role in regulating sodium balance, extracellular volume, and blood pressure. Evidence suggests the α and γ subunits of ENaC are cleaved during assembly before they are inserted into the apical membranes of epithelial cells, and maximal activity of ENaC depends on cleavage of the extracellular loops of α and γ subunits. Here, we report that Xenopus 2F3 cells apically express the cysteine protease cathepsin B, as indicated by two-dimensional gel electrophoresis and mass spectrometry analysis. Recombinant GST ENaC α, β, and γ subunit fusion proteins were expressed in Escherichia coli and then purified and recovered from bacterial inclusion bodies. In vitro cleavage studies revealed the full-length ENaC α subunit fusion protein was cleaved by active cathepsin B but not the full-length β or γ subunit fusion proteins. Both single channel patch clamp studies and short circuit current experiments show ENaC activity decreases with the application of a cathepsin B inhibitor directly onto the apical side of 2F3 cells. We suggest a role for the proteolytic cleavage of ENaC by cathepsin B, and we suggest two possible mechanisms by which cathepsin B could regulate ENaC. Cathepsin B may cleave ENaC extracellularly after being secreted or intracellularly, while ENaC is present in the Golgi or in recycling endosomes.

Trichostatin A inhibits expression of cathepsins in experimental osteoarthritis

The aim of this study was to investigate the effects of trichostatin A (TSA) on expression of cathepsins in cartilage in experimental osteoarthritis (OA). OA was induced in 18 rabbits by bilateral anterior cruciate ligament transection (ACLT). Four weeks after surgery, rabbits received intra-articular injection with TSA dissolved in the dimethylsulphoxide (DMSO) in the right knees and DMSO in the left knees once a week for 5 weeks. Rabbits were killed 7 days after the last injection. The knee joints were assessed by morphological and histological examination. Messenger RNA expression of cathepsins K, B, L, S and cystatin C was studied by real-time PCR. TSA inhibited the expression of cathepsins K, B, L, S and cystatin C accompanied with the less degradation in cartilage. The results suggest that TSA exhibits protective effects against cartilage degradation in rabbits with OA and the effects may be associated with the inhibition of cathepsins.

How does heparin prevent the pH inactivation of cathepsin B? Allosteric mechanism elucidated by docking and molecular dynamics

Background

Cathepsin B (catB) is a promising target for anti-cancer drug design due to its implication in several steps of tumorigenesis. catB activity and inhibition are pH-dependent, making it difficult to identify efficient inhibitor candidates for clinical trials. In addition it is known that heparin binding stabilizes the enzyme in alkaline conditions. However, the molecular mechanism of stabilization is not well understood, indicating the need for more detailed structural and dynamic studies in order to clarify the influence of pH and heparin binding on catB stability.

Results

Our pKa calculations of catB titratable residues revealed distinct protonation states under different pH conditions for six key residues, of which four lie in the crucial interdomain interface. This implies changes in the overall charge distribution at the catB surface, as revealed by calculation of the electrostatic potential. We identified two basic surface regions as possible heparin binding sites, which were confirmed by docking calculations. Molecular dynamics (MD) of both apo catB and catB-heparin complexes were performed using protonation states for catB residues corresponding to the relevant acidic or alkaline conditions. The MD of apo catB at pH 5.5 was very stable, and presented the highest number and occupancy of hydrogen bonds within the inter-domain interface. In contrast, under alkaline conditions the enzyme's overall flexibility was increased: interactions between active site residues were lost, helical content decreased, and domain separation was observed as well as high-amplitude motions of the occluding loop – a main target of drug design studies. Essential dynamics analysis revealed that heparin binding modulates large amplitude motions promoting rearrangement of contacts between catB domains, thus favoring the maintenance of helical content as well as active site stability.

Conclusions

The results of our study contribute to unraveling the molecular events involved in catB inactivation in alkaline pH, highlighting the fact that protonation changes of few residues can alter the overall dynamics of an enzyme. Moreover, we propose an allosteric role for heparin in the regulation of catB stability in such a manner that the restriction of enzyme flexibility would allow the establishment of stronger contacts and thus the maintenance of overall structure.

Which elements are involved in reversible and irreversible cartilage degradation in osteoarthritis?

Osteoarthritis (OA) is a disease of the entire joint. Different treatment strategies for OA have been proposed and tested clinically without the desired efficacy. One reason for the scarcity of current chondroprotective agents may be the insufficient understanding of the patho-physiology of the joint and whether the joint damage is reversible or irreversible. In this review, we compile emerging data on cellular and pathological aspects of OA, and ask whether these data could give clue to when cartilage degradation is reversible and whether a point-of-no-return exists. We highlight different stages of OA, and speculate whether different intervention strategies (e.g. DMOAD vs. SMOADs) may only be efficacious at distinct stages of OA.

Variation patterns of two degradation enzyme systems in articular cartilage in different stages of osteoarthritis: regulation by dehydroepiandrosterone

BACKGROUND

Osteoarthritis (OA) is a multifactorial degenerative joint disease in which the cartilaginous matrix of the articular joint is destroyed in a continuous process. We evaluated mRNA levels of cysteine proteinases/cystatin C system and urokinase plasminogen activator/plasminogen activator inhibitor-1 (uPA/PAI-1) system in articular cartilage and regulation by dehydroepiandrosterone (DHEA) in different stages of osteoarthritis (OA).

METHODS

One hundred and eight rabbits underwent anterior cruciate ligament transection (ACLT) in the left knee, 54 received weekly intra-articular injections of DHEA (100 micromol/l) 0.3 ml 3 weeks after transaction as DHEA group. Thirty-six rabbits (18 from 2 groups respectively) were euthanized 6, 9, and 12 weeks after ACLT. All left knee joints were assessed by gross morphology and histology, meantime the gene expression from articular cartilage was analyzed.

RESULTS

Cathepsins and uPA gene increased significantly 6 weeks and reached peak in the 9th week, while declined to extremely low levels 12 weeks after ACLT. Cystatin C decreased accompanied by OA progression, while PAI-1 expressed in the same trend with uPA. Additionally, these 2 enzyme systems were markedly suppressed by DHEA 6 and 9 weeks after ACLT but not in the 12th week.

CONCLUSION

The variation of these 2 enzyme systems was closely related to the progression of OA, and could be regulated by DHEA especially in the early and medium stages of OA.

High abundance synovial fluid proteome: distinct profiles in health and osteoarthritis

The development of increasingly high-throughput and sensitive mass spectroscopy-based proteomic techniques provides new opportunities to examine the physiology and pathophysiology of many biologic fluids and tissues. The purpose of this study was to determine protein expression profiles of high-abundance synovial fluid (SF) proteins in health and in the prevalent joint disease osteoarthritis (OA). A cross-sectional study of 62 patients with early OA (n = 21), patients with late OA (n = 21), and control individuals (n = 20) was conducted. SF proteins were separated by using one-dimensional PAGE, and the in-gel digested proteins were analyzed by electrospray ionization tandem mass spectrometry. A total of 362 spots were examined and 135 high-abundance SF proteins were identified as being expressed across all three study cohorts. A total of 135 SF proteins were identified. Eighteen proteins were found to be significantly differentially expressed between control individuals and OA patients. Two subsets of OA that are not dependent on disease duration were identified using unsupervised analysis of the data. Several novel SF proteins were also identified. Our analyses demonstrate no disease duration-dependent differences in abundant protein composition of SF in OA, and we clearly identified two previously unappreciated yet distinct subsets of protein profiles in this disease cohort. Additionally, our findings reveal novel abundant protein species in healthy SF whose functional contribution to SF physiology was not previously recognized. Finally, our studies identify candidate biomarkers for OA with potential for use as highly sensitive and specific tests for diagnostic purposes or for evaluating therapeutic response.

Effects of intra-articular administration of glucosamine and a peptidyl-glucosamine derivative in a rabbit model of experimental osteoarthritis: a pilot study

The aim of this pilot study was to analyze the effects of glucosamine (GlcN) and its N-acetyl-phenylalanine derivative (NAPA) in Vitamin A model of osteoarthritis (OA) in rabbits. GlcN or NAPA or saline solution was intra-articularly administered in rabbit OA knees. Histological analysis revealed that treatment with GlcN or NAPA was associated with more homogeneous chondrocyte cellularity, absence of fissures and fragmentation and more intense staining of the matrix with Alcian Blue compared to the articular surfaces of the knees treated with saline solution. Comparative in vitro study performed on rabbit primary chondrocytes revealed that GlcN and NAPA were also able to counteract the IL-1beta-upregulation of genes coding for metalloproteases and inflammatory cytokines. Our preliminary in vivo and in vitro studies suggest that GlcN and NAPA could play a disease-modifying protective role in OA by an anti-catabolic effect and an anti-inflammatory activity on chondrocytes.

Up regulation of cathepsin K expression in articular chondrocytes in a transgenic mouse model for osteoarthritis

OBJECTIVES

To study the expression of cysteine proteinases, particularly cathepsin K, and their extracellular inhibitor cystatin C in articular cartilage of transgenic Del1 mice which harbour a short deletion mutation in a type II collagen transgene and are predisposed to early onset osteoarthritis.

METHODS

Northern analysis was used to measure mRNA levels of cathepsins B, H, K, L, and S, and cystatin C in total RNA extracted from knee joints of Del1 mice, using their non-transgenic litter mates as controls. Immunohistochemistry and morphometry was used to study the distribution of cathepsin K and cystatin C in the knee joints.

RESULTS

Up regulation of cathepsin K mRNA expression was seen in the knee joints of transgenic Del1 mice at the onset of cartilage degeneration. Cathepsin K was found near sites of matrix destruction in articular chondrocytes, particularly in clusters of proliferating cells, and in calcified cartilaginous matrix. In intact articular cartilage of control animals, cathepsin K was only seen in a small number of chondrocytes. Upon aging, control animals also developed osteoarthritis, which was accompanied by increased cathepsin K expression. Cystatin C was mostly localised in and around chondrocytes located in calcified cartilage, with no obvious association with the onset of cartilage degeneration.

CONCLUSION

The temporospatial distribution of cathepsin K in osteoarthritic cartilage suggests a role for this enzyme in the pathogenesis of osteoarthritis. Because cathepsin K can digest cartilage matrix components it may contribute to the development of osteoarthritic lesions. These data may provide new clues for the development of treatments aimed at preventing cartilage degeneration.

Molecular regulation of human cathepsin B: implication in pathologies

Cathepsin B is a papain-family cysteine protease that is normally located in lysosomes, where it is involved in the turnover of proteins and plays various roles in maintaining the normal metabolism of cells. This protease has been implicated in pathological conditions, e.g., tumor progression and arthritis. In disease conditions, increases in the expression of cathepsin B occur at both the gene and protein levels. At the gene level, the altered expression results from gene amplification, elevated transcription, use of alternative promoters and alternative splicing. These molecular changes lead to increased cathepsin B protein levels and in turn redistribution, secretion and increased activity. Here we focus on the molecular regulation of cathepsin B and attendant implications for tumor progression and arthritis. The potential of cathepsin B as a therapeutic target is also discussed.

Cathepsin B expression and down-regulation by gene silencing and antisense DNA in human chondrocytes

Cathepsin B, a marker of the dedifferentiated chondrocyte phenotype, contributes to cartilage destruction in osteoarthritis and pathological proteolysis in rheumatoid arthritis and cancer. In search of possible means for neutralizing the action of this enzyme, we compared its expression, biosynthesis and distribution in articular chondrocytes and two lines of immortalized human chondrocytes. Native articular chondrocytes in primary culture and the polyclonal T/C-28a2 chondrocyte cell line were similar with respect to the number of endosomes and lysosomes, the distribution of three alternatively spliced cathepsin B mRNA forms, and the cathepsin B activity. In contrast, the clonal C-28/I2 cell line contained four times higher levels of intracellular cathepsin B activity, slightly higher numbers of endosomes and lysosomes, and uniform distribution of all three cathepsin B transcripts and thus resembled subcultured chondrocytes at an early stage of dedifferentiation. Transfection of T/C-28a2 chondrocytes with double-stranded cathepsin B mRNA resulted in inhibition of cathepsin B biosynthesis by up to 70% due to RNA interference, and single-stranded antisense DNAs of various sizes decreased cathepsin B biosynthesis by up to 78%. An antisense oligonucleotide designed to hybridize to the end of cathepsin B's exons 1 and the beginning of exon 3 was successful in specifically inhibiting the mRNA splice variant lacking exon 2. These results indicate that cathepsin B expression and activity may be targeted for gene silencing by RNA interference and antisense DNA in chondrocytes. Furthermore, the differential expression and distribution of cathepsin B and presence of the necessary molecular apparatus for gene silencing in the immortalized human chondrocyte cell lines indicate that they may serve as a useful model for studying the function of relevant enzymes in cartilage pathologies.

The Novartis-ILAR Rheumatology Prize 2001 Osteoarthritis: from molecule to man

During our careers, we have developed new and innovative concepts pertaining to the pathophysiology of osteoarthritis which have assisted in the development of new therapeutic approaches. Moreover, our laboratory has long sought to develop protective agents for osteoarthritic structural joint tissues. The most significant concepts that have originated from our lab are briefly outlined in this commentary.

Alternative messenger RNA splicing and enzyme forms of cathepsin B in human osteoarthritic cartilage and cultured chondrocytes

OBJECTIVE

In previous studies, we suggested that cathepsin B, which is present at sites of cartilage remodeling in osteoarthritis (OA), may act as an antagonist of cartilage repair, an enhancer of the action of metalloproteinases, and a mediator of cartilage neovascularization and mineralization. Alternative splicing of cathepsin B pre-messenger RNA (pre-mRNA) and/or mRNA overexpression is a plausible regulatory mechanism. In the present study, we investigated the abundance of cathepsin B transcripts and the properties of cathepsin B protein in normal and OA cartilage, osteophytes, and cultured chondrocytes.

METHODS

Cathepsin B mRNA splice variants containing the full-length transcript (CB) and the variants lacking either exon 2 (CB[-2]) or lacking exons 2 and 3 (CB[-2,3]) were measured by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot assays and were localized by in situ RT-PCR. Cathepsin B protein was analyzed by electrophoretic, Western blot, and chromatographic methods.

RESULTS

The relative content of CB, CB(-2), and CB(-2,3) varied considerably in OA cartilage and osteophytes, with less variation in normal cartilage. The mean cathepsin B mRNA level was significantly higher in OA cartilage and osteophytes than in normal cartilage. Normal cultured chondrocytes attained cathepsin B mRNA levels similar to those in OA cartilage. Enzyme overexpression resulted in the secretion of procathepsin B, followed by activation to the proteolytically active form.

CONCLUSION

The high levels of CB and CB(-2) are consistent with an overproduction of secreted procathepsin B in OA. Up-regulation of chondrocyte cathepsin B, which takes place at both the transcriptional and the translational level, suggests a leading role of the enzyme in the progression of OA.

Expression of stromelysin and urokinase type plasminogen activator protein in resection specimens and biopsies at different stages of osteoarthritis of the knee

This study aimed at assessing the possible diagnostic value of cartilage biopsies as a convenient marker for cartilage matrix degradation. We therefore examined cartilage specimens from 56 patients with primary osteoarthritis (OA) of the knee. Resection and biopsy cartilage specimens obtained during joint replacement surgery were used for this study. In addition to histomorphology, immunohistochemistry (ICH) was performed to determine the expression levels and distribution patterns of stromelysin and u-PA protein. The latter data were compared with the degree of histomorphological changes in osteoarthritic cartilage samples, based on a modified version of Mankin's grading score. Compared to the cartilage resection specimens, the biopsies showed comparable expression patterns for both proteinases: the strongest signals were noted in the superficial zone and, as matrix destruction increased, also in the chondrocytes of the transition and deep zones. The strongest signals were ascertained in cell clusters beneath deep matrix fissures. At the immunohistochemical level, we found a direct correlation in the expression of MMP-3 and u-PA between resection specimens and biopsies. Furthermore, in both types of cartilage samples, we noticed a positive relationship between the expression of both proteins and the Mankin score. Analysis of the expression levels revealed significant differences between deep, transition and superficial zones. Histomorphological and immunohistochemical examinations of MMP-3 and u-PA in biopsies of osteoarthritic cartilage turned out to be useful for estimating the pathological changes within osteoarthritic knee joints. Therefore, in future, cartilage biopsies from osteoarthritic knee joints might serve as a diagnostic tool and thus have an influence on further therapeutic strategies.

Immunolocalization of cathepsin B in equine dyschondroplastic articular cartilage

A polyclonal antiserum raised in sheep against human cathepsin B was tested for specificity and cross-reactivity with the horse homologue by SDS-PAGE and Western blotting, prior to being used for immunolocalization of the enzyme in equine articular cartilage. In Western blots, the antiserum recognized the 30 kDa single chain and 25 kDa heavy chain of the mature enzyme in purified bovine cathepsin B, and corresponding bands at 32 and 27 kDa in equine chondrocyte and fibroblast lysates. This antiserum was then used to compare the expression and distribution of cathepsin B in normal and dyschondroplastic cartilage of young horses. In normal articular cartilage (n = 6 animals), significant amounts of enzyme were detected only in hypertrophic chondrocytes in the deep zone. The enzyme was intracellular, located in the lysosomal granules. No extracellular matrix staining was observed. Levels of cathepsin B were increased slightly above normal in the deep zone in age-matched dyschondroplastic cartilage (n = 5 animals). The most striking finding, however, was the abundance of the enzyme in chondrocyte clonal clusters associated with the lesions. Cathepsin B levels were low in chondrocytes isolated from normal cartilage (n = 6), but increased progressively during serial subculture, reaching a maximum at passage 5-6. In contrast, primary cultures of dyschondroplastic chondrocytes (n = 3) expressed abundant cathepsin B.

Detection and preliminary characterization of matrix metalloproteinase activity in temporomandibular joint lavage fluid

In this study, lavage fluid was fractionated from the superior joint space in patients with temporomandibular joint (TMJ) dysfunction. A hide powder azure protease assay was used to assess protease activity in lavage fluid. No correlation between a patient's pain and the level of protease activity was demonstrated. Latent as well as active proteases were detected in the sample lavage fluid. Latent matrix metalloproteinases (MMPs) were activated using trypsin. Stromelysin-1 was detected in an active form in lavage fluid by immunozymography. The presence of high molecular weight species with protease activity was also demonstrated. This study validates the presence of stromelysin-1 as well as other MMPs in TMJ lavage fluid and proposes a mechanism for their physiologic activation.

Characterization of recombinant human cathepsin B expressed at high levels in baculovirus

The lysosomal cysteine protease cathepsin B has been studied intensely for many years because of its unique characteristics and its potential involvement in disease states. A reproducible, high yield expression system for active recombinant protein is key to biochemical and biophysical studies as well as rational drug design. Although several microbial and mammalian expression systems for recombinant human cathepsin B have been described, these have been limited by low or variable yields. Further, in some of these systems hyper-glycosylation of the enzyme near the active site affects its activity. We describe a baculovirus expression system and purification scheme that solve all of these problems. Yields of active, protected enzyme were reproducibly in excess of 25 mg/L. Since this protein was not hyper-glycosylated, it had greater activity than cathepsin B produced in yeast systems as indicated by a threefold increase in Kcat. In addition, the biophysical properties of the baculovirus-expressed cathepsin B, as measured by dynamic light scattering, were more amenable to crystallographic study since the data indicated proteins of more uniform size. Therefore, this system for the production of recombinant human cathepsin B constitutes a major improvement in both quantity and quality over those previously reported. Further, we demonstrate that the manner of expression and purification of this enzyme has profound effects on its kinetic and physical parameters.

Temporal pattern of cysteine endopeptidase (cathepsin B) expression in cartilage and synovium from rabbit knees with experimental osteoarthritis: gene expression in chondrocytes in response to interleukin-1 and matrix depletion

OBJECTIVE

To determine the temporal pattern of expression of cathepsin-B in chondrocytes and synovium in experimental osteoarthritis, and to determine possible mechanisms for upregulation and secretion of cathepsin-B from chondrocytes.

METHODS

Experimental osteoarthritis was induced with partial medial meniscectomy (PM); sham operated (SH) and normal (N) rabbits were used as controls. Cathepsin-B mRNA expression was assessed with northern blotting with a 32P labelled cDNA probe. Cathepsin-B was measured in conditioned media or cell extracts using a fluorogenic substrate Z-Arg-Arg-AMC. Chondrocyte monolayers were used to determine cathepsin-B expression in response to interleukin-1 beta (IL-1 beta). Cartilage explants were used to test the effect of matrix depletion on cathepsin-B release.

RESULTS

Chondrocytes obtained from experimental osteoarthritis knees did not show cathepsin-B mRNA upregulation. However, isolated chondrocytes secreted cathepsin-B into the culture medium. Enzyme release was significantly higher at 8 weeks relative to controls, but not at 12 weeks or 4 weeks. Enzyme released from synovium was significantly higher in PM group compared with SH group at 4 and 8 weeks. IL-1 beta was ineffective in upregulating steady state cathepsin-B mRNA in chondrocytes; however, it upregulated the intracellular enzyme, and this was blocked with cycloheximide. Enzymatic depletion of cartilage matrix after exposure of explants to IL-1 resulted in release of significantly higher amounts of cathepsin-B into the medium by matrix depleted chondrocytes compared with intact explants.

CONCLUSIONS

In experimental osteoarthritis, cathepsin-B is upregulated in synovial tissue during the early degenerative phase. Progression of experimental osteoarthritis is accompanied by upregulation of cathepsin-B in cartilage. Cartilage and synovial cathepsin-B levels decline as experimental osteoarthritis advances to more degenerative states. IL-1 upregulates intracellular cathepsin-B by increasing cathepsin-B protein synthesis; it is not an effective stimulus for enzyme secretion. Depletion of cartilage matrix during progression of experimental osteoarthritis may contribute to secretion of cathepsin-B and perpetuation of cartilage destruction.

Acute-phase proteins in osteoarthritis

The joint destruction of osteoarthritis (OA) comprises loss of articular cartilage resulting from an imbalance of enzyme-catalized cartilage breakdown and regeneration. OA is thought to derive from defective chondrocyte metabolism and thus to inherently lack the large-scale systemic response that is the hallmark of rheumatoid arthritis (RA). Because of the apparent absence of systemic inflammation in OA, acute-phase response proteins have not been as extensively studied in OA as they have been in RA. The diagnosis of OA almost always involves radiographic assessment of joint damage, which is useful only after the disease process has been underway for several months. Radiographic evaluation cannot give a good assessment of current disease activity and is a relatively insensitive indicator of prognosis. Cartilage breakdown products can potentially serve as direct surrogate markers of OA disease activity, but have not been extensively used because of their limited sensitivity and the technical difficulties associated with their measurement. Markers of disease activity in RA are indirect and are derived from the acute-phase response, a cycle of temporal changes in cellular and metabolic function. The early part of the acute-phase response involves the local action and production of cytokines such as interleukin-1 (IL-1), tumor necrosis factor (TNF-alpha) and IL-6. In the late acute-phase response, these cytokines can effect many systemic changes, including increased production of acute-phase proteins (APP). Three valuable surrogate markers of disease activity in RA are provided by the acute-phase response: the time-honored erythrocyte sedimentation rate (ESR) and the newer APPs C-reactive protein (CRP) and serum amyloid A (SAA). As in RA, the joint destruction of OA involves IL-1, TNF-alpha, and IL-6; however, OA can be viewed as an indolent stimulus of the later (systemic) acute-phase response. Recent studies of the acute-phase response in OA suggest that the concentrations of CRP and SAA are elevated in OA, but to a lesser extent than in RA. In the future, long-term monitoring of CRP concentrations in the blood may permit the earlier detection and more effective treatment of OA.

Cathepsin B in osteoarthritis: zonal variation of enzyme activity in human femoral head cartilage

OBJECTIVES

To determine the quantitative topographical distribution of cathepsin B in human femoral head cartilage by measuring the zonal variation of enzyme activity in specimens taken from various anatomical regions of normal and osteoarthritic (OA) tissues, and to correlate this parameter with the severity of the OA lesions.

METHODS

OA articular cartilage was obtained at surgery for total hip replacement and control cartilage obtained at postmortem. Cylinders of full thickness cartilage with underlying bone were retrieved with a biopsy trephine. Sections of cartilage were produced by cryocutting the tissue as slices parallel to the articular surface and assayed for cathepsin B with a specific, highly sensitive fluorogenic substrate. The severity of the OA lesions was graded according to the histopathological-histochemical method of Mankin.

RESULTS

Zonal cathepsin B activity of normal cartilage was uniform and low in all regions of the femoral head. In apparently intact OA cartilage and in severely degraded tissue the zonal distribution and the amounts of enzyme were similar to control values. At sites with active disease, cathepsin B activity was much greater than in controls and its irregular zonal distribution correlated with tissue degeneration, hypercellularity, or cloning of chondrocytes as determined histochemically. Particularly high enzyme levels were observed at sites with regenerating cartilage, where some zonal peaks attained 20-fold activity with respect to controls.

CONCLUSION

Cathepsin B may play a role in sustaining the chronicity of OA, not as an initiator, but rather as a perpetuator of the disease and as an antagonist of regeneration.

Cathepsin B in osteoarthritis: cytochemical and histochemical analysis of human femoral head cartilage

OBJECTIVE

To localise the cysteine endopeptidase cathepsin B in chondrocytes and cartilage from normal and osteoarthritic (OA) human femoral heads in order to provide qualitative information on its cellular expression and distribution at possible sites of action.

METHODS

OA articular cartilage was obtained at surgery for total hip replacement; control cartilage was obtained at postmortem. Chondrocytes were isolated by sequential enzymatic digestion and cathepsin B analysed by immunocytochemistry and activity staining with a fluorogenic substrate. Lysosomes were visualised by fluorescence microscopy after staining of living cells with acridine orange. Using a histochemical reaction, enzyme activity was measured in cryosections of full thickness cartilage.

RESULTS

Chondrocytes from normal cartilage contained very few lysosomes and only a minor cell population was cathepsin B positive. A high proportion of chondrocytes from active OA cartilage contained a large number of lysosomes and an excess of cathepsin B in intracellular organelles; the enzyme was stored in an active form. In this respect, OA chondrocytes closely resembled normal cells that had been phenotypically modulated by serial subcultures. No cathepsin B activity could be detected by histochemistry in either chondrocytes or matrix of normal cartilage. While apparently intact and severely degraded OA cartilage was also cathepsin B negative, tissue at sites of active destruction and, particularly, at repair sites was highly positive.

CONCLUSION

The presence and the particular distribution of active cathepsin B in OA cartilage at 'more involved' sites suggest a pathological role for this enzyme in sustaining and perpetuating cartilage degradation. While other stimuli may also be responsible for cathepsin B expression in OA chondrocytes, the similarity with artificially modulated cells indicates fibroblastic metaplasia as a plausible mechanism.

Cathepsin B and N-acetyl-beta-D-glucosaminidase in human synovial cells in culture: effects of interleukin-1

Human synovial cells were cultured in vitro and tested for the activities of two lysosomal enzymes, cathepsin B and N-acetyl-beta-D-glucosaminidase (NAGA) under various conditions. Unstimulated synovial cells display intracellular and extracellular activities of both enzymes. However, cathepsin B was secreted in a latent pepsin-activatable form, whereas NAGA was secreted in an active form. Most of the cell strains analysed secreted rather limited amounts of the enzymes (less than 25% of total activity); some strains, however were highly secretory, the secreted activity reaching up to 50% of total activity. Cells were then stimulated with human recombinant interleukin-1 alpha (rhIL-1 alpha) or beta. Only the levels of secreted NAGA were clearly increased. Results are to be interpreted in view of the role played by synovial cells and by the lysosomal enzymes they release in inflammatory joint diseases and it would be worthwhile in the future to check for secreted NAGA in various body fluids, such as the synovial fluid of the inflamed joint.

An ultrasensitive chemiluminoenzyme immunoassay for the quantification of human tissue kininogens: application to synovial membrane and cartilage

A sandwich enzyme immunoassay using a chemiluminescent detection has been developed for the quantification of total human (high and low molecular weight) kininogens in tissue extracts. This assay uses monospecific polyclonal IgG labelled with alkaline phosphatase and the commercially available dioxetane derivatives as substrates, for the detection of immune complexes. This method exhibits a sensitivity level of 1 fmol/ml and allows a precise quantification of total kininogens in synovium and cartilage extracts. When characterized by Western blot, the immunoreactive material reveals the presence of both high and low molecular weight kininogens.

Detection of lysosomal cysteine proteinases in microglia: flow cytometric measurement and histochemical localization of cathepsin B and L

The activation and differentiation of microglia is a prominent pathophysiological process in numerous inflammatory and demyelinating diseases of the central nervous system, including Alzheimer's disease and the AIDS encephalopathy. The tissue damage during these diseases has partly been attributed to lipid peroxidating reactive oxygen intermediates for which activated microglia are a major source. The destruction of tissue may also involve the release of proteolytic enzymes, such as the lysosomal cysteine proteinases cathepsin B and L, which are present notably in phagocytic cells. The cathepsins B and L are endopeptidases with a substrate specificity including important proteins, like myelin basic protein, extracellular matrix components, or the class II major histocompatibility complex. Because of this pathophysiological relevance the cathepsins B and L were chosen for histochemical demonstration in isolated and cultured rat microglia and measurement by a new flow cytometric method. Cathepsin B/L activity was measured flow cytometrically in single viable cells by the intracellular cleavage of non-fluorescent (Z-Phe-Arg)2-rhodamine 110 to the green fluorescent monoamide Z-Phe-Arg-rhodamine 110 and rhodamine 110. In microglia we measured a cathepsin B/L activity that was 2.5 times higher than in thioglycolate-elicited, i.e., inflammatory peritoneal rat macrophages. In elicited peritoneal macrophages the formation of fluorescent product was 6.2 times higher than in unstimulated resident peritoneal macrophages, demonstrating that the activation and differentiation of mononuclear phagocytes is accompanied by an increased cathepsin B/L enzyme activity. The subcellular localization of cathepsin B/L activity in plated viable microglia was demonstrated histochemically by the use of Z-Ala-Arg-Arg-4-methoxy-2-naphthylamide. Its blue fluorescent cleavage product 4-methoxy-2-naphthylamide was found in lysosomes. Our study shows that activated microglia are an important potential source of cathepsin B/L. This is particularly interesting as enzymatically active cathepsins have recently been found extracellularly at high levels in the senile plaques of Alzheimer's disease, which are known to contain many activated microglia. The release of proteinases by microglia may play a crucial role in the pathomechanism of tissue-destructing diseases in the brain.

Isolation and characterization of an abundant elastase inhibitor from NaCl extracts of bovine nasal septa and articular cartilage

Extracts of cartilage have been reported to inhibit many serine proteinases and metalloenzymes. Such inhibition may be important in protecting cartilage against degradation by chondrocytic proteinases such as collagenase, stromelysin and by leukocytic proteases, such as elastase. We report here isolation and partial characterization of a 17-kD elastase inhibitor from 0.5 M NaCl extracts of both nasal septum cartilage and articular cartilage, which inhibits elastase and represents 0.08% of the weight of nasal cartilage and 0.002% of the weight of articular cartilage. The protein was highly specific for elastase and did not inhibit cartilage metalloproteinases, suggesting that it may be mainly directed toward protecting cartilage against leukocytic proteases. The inhibitor had a blocked amino-terminus, was high in serine and glycine and lacked carbohydrate. The ease with which the inhibitor was extracted from cartilage suggests that it may function in vivo as a highly abundant elastase inhibitor which is secreted into synovial fluid from cartilage. The inhibitor was shown to be synthesized by bovine articular cartilage in explant culture and nearly all of the metabolically labeled material was secreted into the culture media. The inhibitor cross-reacted with polyclonal antibodies to bovine neck ligament alpha-elastin and antibodies to the inhibitor reacted with bovine neck ligament elastin. The properties of this inhibitor are different than those of any other reported cartilage derived inhibitor.

Are cytokines involved in osteoarthritic pathophysiology?

The putative role and mechanism of action of cytokines in the progression of arthritic diseases such as osteoarthritis (OA) has received particular attention because of the important interaction between articular cartilage and synovium in the pathophysiology of the diseased state. Maintaining matrix homeostasis in the normal adult cartilage phenotype requires normal turnover of matrix components, principally collagen and proteoglycan. Chondrocytes and synovial fibroblasts are targeted, via specific cell-surface receptors, by cytokines like interleukin 1 (IL-1) and tumor necrosis factor alpha (TNF-alpha) to produce matrix proteases and to suppress the synthesis of collagen and proteoglycan. Thus, cytokines not only favor tissue destruction, but also inhibit tissue repair. A structurally heterogeneous group of factors capable of directly antagonizing cytokine action is described, which acts either by blocking cytokine-receptor binding, inhibiting local cytokine synthesis, or complexing the cytokine into an inactive form. Furthermore, many growth factors, such as transforming growth factor-beta (TGF-beta), can counteract the net effect of cytokines by stimulating the synthesis of matrix components or natural inhibitors of cartilage degrading enzymes.

The role of proteinases in cartilage destruction

Most of the organic, extracellular matrix of articular cartilage consists of collagens and proteoglycans. Their degradation is initiated extra- or peri-cellularly by proteinases produced locally by cells in and around the joint. Although enzymes from all four classes of proteinases can degrade the cartilagenous matrix, serine proteinases, particularly plasmin, and various neutral metalloproteinases (NMPs) are likely to be the key enzymes in this process. Much attention has been paid to members of the latter group, which are synthesised both by the resident, mesenchymal cells of the joint and by various types of white blood cells which colonise it during inflammation. NMPs can be conveniently grouped into three classes, the collagenases, the stromelysins and the gelatinases. Two members are known for each class, with the recently identified "pump" (Putative Metalloproteinase) probably constituting a third member of the stromelysin group. Regulation of these enzymes is complex. Cells normally synthesise NMPs at low rates, but their production increases markedly following cellular activation by cytokines or certain other stimuli. Major control points for enzyme synthesis occur at the levels of transcription and the conversion of proenzyme to active enzyme; enzyme activity is further regulated through the action of inhibitors. Alpha-2 macroglobulin is the major systemic inhibitor, while a number of tissue inhibitors act as local regulators. These include at least two TIMPs and several IMPs. Pharmacologic manipulation of NMP activity holds promise as an approach to anti-erosive therapy in arthritis.

Imbalance between the mechanisms of activation and inhibition of metalloproteases in the early lesions of experimental osteoarthritis

Levels of tissue inhibitor of metalloproteases (TIMP) and plasminogen activator (PA)/plasmin were measured and the distribution of PA was studied by immunohistochemical techniques in cartilage and synovium samples from dogs subjected to sectioning of the anterior cruciate ligament of their right knees and sham operation of their left knees (controls). Twenty-three animals were divided into 3 groups and killed at 2, 4, or 8 weeks after surgery. The levels of PA and plasmin were found to be significantly elevated in the osteoarthritic (OA) knee cartilage and synovium at all times after surgery, except for levels of PA in the OA cartilage at 2 weeks. There was a positive correlation between the levels of PA and plasmin in the synovial membrane (r = 0.64, P less than 0.001). In OA knees, the presence of high levels of total and active collagenase was detected in cartilage and in synovium. The levels of these 2 forms of collagenase showed a positive correlation both in cartilage (r = 0.65, P less than 0.001) and in synovium (r = 0.77, P less than 0.001). The levels of TIMP in cartilage from OA and sham operated knees were similar. Although the TIMP level was increased in the OA synovium, it was found only in trace amounts in cartilage. Immunohistochemical studies revealed that both forms of PA, urokinase-type PA and tissue-type PA, and TIMP were present in OA tissues. In the synovium, they were found mainly in monocyte/macrophages, synovial lining cells, and blood vessel cells. In OA cartilage, PA was present only at the superficial level in chondrocytes and in cartilage matrix, whereas TIMP was present in chondrocyte lacunae throughout the full thickness of the cartilage. TIMP was also detected in the superficial level of cartilage from sham operated knees. The results of this study indicate that in OA tissues, there are conditions that favor the synthesis and activation of metalloproteases. PA and plasmin are likely to play an important role in the physiologic activation of metalloproteases, although they are probably not the only system involved in this process. The lack of increased TIMP levels in the OA cartilage, in the presence of increased metalloprotease activity, is also a possible contributing factor in the enzymatic degradation of this tissue.