Neutral proteases and cathepsin D in human articular cartilage

The Ins and Outs of Cathepsins: Physiological Function and Role in Disease Management

Cathepsins are the most abundant lysosomal proteases that are mainly found in acidic endo/lysosomal compartments where they play a vital role in intracellular protein degradation, energy metabolism, and immune responses among a host of other functions. The discovery that cathepsins are secreted and remain functionally active outside of the lysosome has caused a paradigm shift. Contemporary research has unraveled many versatile functions of cathepsins in extralysosomal locations including cytosol and extracellular space. Nevertheless, extracellular cathepsins are majorly upregulated in pathological states and are implicated in a wide range of diseases including cancer and cardiovascular diseases. Taking advantage of the differential expression of the cathepsins during pathological conditions, much research is focused on using cathepsins as diagnostic markers and therapeutic targets. A tailored therapeutic approach using selective cathepsin inhibitors is constantly emerging to be safe and efficient. Moreover, recent development of proteomic-based approaches for the identification of novel physiological substrates offers a major opportunity to understand the mechanism of cathepsin action. In this review, we summarize the available evidence regarding the role of cathepsins in health and disease, discuss their potential as biomarkers of disease progression, and shed light on the potential of extracellular cathepsin inhibitors as safe therapeutic tools.

Matrix metalloproteinases and inhibitors in dentistry

OBJECTIVES

Matrix metalloproteinase (MMP) expression has been associated with tissue development, invasive cancer cell behavior, and inflammation. The associations of increased expression of MMPs with diseases have led to intensive research activities to develop MMP inhibitors. Here, the questions are addressed which associations between increased levels of any MMP with dental diseases may be cause or consequence, whether MMP levels may be of diagnostic value and whether and which MMP inhibitors need further investigations for use in dental diseases.

METHODS

To study the role of MMPs and to discriminate between cause or consequence, the literature about measurements of MMPs and about the use of inhibitory drugs and genetic knockout animal models in dentistry was compared.

RESULTS

The only FDA-approved treatment with MMP inhibitors is tetracyclines for periodontitis, whereas a diagnostic test for activated MMP-8 in oral fluids is valued in practical periodontology. The MMP literature in dentistry is artificially skewed to the gelatinases MMP-2 and MMP-9 and to enamelysin, alias MMP-20. The basis for this observation is, respectively, the widely used and sensitive technique of gelatin zymography and enamel proteins as substrates of MMP-20. Studies on additional MMPs are gaining interest in dentistry and MMP inhibitors may provide new applications. In addition, drugs with proven effects for the treatment of dental diseases may be found to act through MMP inhibition.

CONCLUSION AND RELEVANCE

In conclusion, research on MMPs and inhibitors may provide practical applications beyond diagnosis and treatment of periodontitis and will be, directly or indirectly, beneficial for patients with dental or periodontal diseases.

Hybrid 2D/3D-quantitative structure-activity relationship and modeling studies perspectives of pepstatin A analogs as cathepsin D inhibitors

AIM

Cathepsin D, one of the attractive targets in the treatment of breast cancer, has been implicated in HIV neuropathogenesis with potential proteolytic effects on chemokines. Methodology/result: Diverse modeling tools were used to reveal the key structural features affecting the inhibitory activities of 78 pepstatin A analogs. Analyses were performed to investigate the stability, rationality and fluctuation of the analogs. Results showed a clear correlation between the experimental and predicted activities of the analogs as well as the variation in their activities relative to structural modifications.

CONCLUSION

The insight gained from this study offers theoretical references for understanding the mechanism of action of cathepsin D and will aid in the design of more potent and clinically-relevant drugs. Graphical abstract [Formula: see text].

Matrix metalloproteinases, new insights into the understanding of neurodegenerative disorders

Matrix metalloproteinases (MMPs) are a subfamily of zinc-dependent proteases that are responsible for degradation and remodeling of extracellular matrix proteins. The activity of MMPs is tightly regulated at several levels including cleavage of prodomain, allosteric activation, compartmentalization and complex formation with tissue inhibitor of metalloproteinases (TIMPs). In the central nervous system (CNS), MMPs play a wide variety of roles ranging from brain development, synaptic plasticity and repair after injury to the pathogenesis of various brain disorders. Following general discussion on the domain structure and the regulation of activity of MMPs, we emphasize their implication in various brain disorder conditions such as Alzheimer’s disease, multiple sclerosis, ischemia/reperfusion and Parkinson’s disease. We further highlight accumulating evidence that MMPs might be the culprit in Parkinson’s disease (PD). Among them, MMP-3 appears to be involved in a range of pathogenesis processes in PD including neuroinflammation, apoptosis and degradation of α-synuclein and DJ-1. MMP inhibitors could represent potential novel therapeutic strategies for treatments of neurodegenerative diseases.

Quantitative determination and localization of cathepsin D and its inhibitors

A literature survey was performed of the methods of quantitative assessment of the activity and concentration of cathepsin D and its inhibitors. Usefulness of non-modified and modified proteins and synthetic peptides as measurement substrates was evaluated. The survey includes also chemical and immunochemical methods used to determine the distribution of cathepsin D and its inhibitors in cells and tissues.

Matrix metalloproteinases in lung: multiple, multifarious, and multifaceted

The matrix metalloproteinases (MMPs), a family of 25 secreted and cell surface-bound neutral proteinases, process a large array of extracellular and cell surface proteins under normal and pathological conditions. MMPs play critical roles in lung organogenesis, but their expression, for the most part, is downregulated after generation of the alveoli. Our knowledge about the resurgence of the MMPs that occurs in most inflammatory diseases of the lung is rapidly expanding. Although not all members of the MMP family are found within the lung tissue, many are upregulated during the acute and chronic phases of these diseases. Furthermore, potential MMP targets in the lung include all structural proteins in the extracellular matrix (ECM), cell adhesion molecules, growth factors, cytokines, and chemokines. However, what is less known is the role of MMP proteolysis in modulating the function of these substrates in vivo. Because of their multiplicity and substantial substrate overlap, MMPs are thought to have redundant functions. However, as we explore in this review, such redundancy most likely evolved as a necessary compensatory mechanism given the critical regulatory importance of MMPs. While inhibition of MMPs has been proposed as a therapeutic option in a variety of inflammatory lung conditions, a complete understanding of the biology of these complex enzymes is needed before we can reasonably consider them as therapeutic targets.

Matrix metalloproteinase 3 in parturition, premature rupture of the membranes, and microbial invasion of the amniotic cavity

OBJECTIVE

Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases that are expressed in many inflammatory conditions and contribute to connective tissue breakdown. Stromelysin 1 [matrix metalloproteinase 3 (MMP-3)], a novel member of this family, is produced in the context of infection and is able to activate the latent forms of other MMPs. The purpose of this study was to determine if parturition (either term or preterm), premature rupture of the membranes (PROM), and microbial invasion of the amniotic cavity are associated with changes in amniotic fluid concentrations of MMP-3.

STUDY DESIGN

A cross-sectional study was conducted, which included women who underwent transabdominal amniocentesis (n = 365) in the following categories: (1) mid-trimester with a subsequent normal pregnancy outcome (n = 84) and a subsequent fetal loss (n = 10); (2) preterm labor with intact membranes without microbial invasion of the amniotic cavity who delivered at term (n = 36), or prematurely (n = 50), and preterm labor with microbial invasion of the amniotic cavity (n = 25); (3) preterm PROM with (n = 25) and without (n = 26) microbial invasion of the amniotic cavity; (4) term with intact membranes in the absence of microbial invasion of the amniotic cavity, in labor (n = 52) and not in labor (n = 31); and (5) term with PROM in the absence of microbial invasion of the amniotic cavity and not in labor (n = 26). MMP-3 concentrations in amniotic fluid were measured by a sensitive and specific immunoassay that was validated for amniotic fluid. MMP-3 concentrations were normalized using logarithmic transformation for statistical analysis. Parametric statistics were used and a p value < 0.05 was considered statistically significant.

RESULTS

(1) MMP-3 was detected in 99.5% (363/365) of amniotic fluid samples, and its concentration did not change with advancing gestational age. (2) Spontaneous parturition at term and preterm was associated with a significant increase in amniotic fluid MMP-3 concentrations (p = 0.04 and p = 0.002, respectively). (3) Spontaneous rupture of membranes in term and preterm gestations was not associated with significant changes in amniotic fluid MMP-3 concentrations. (4) Intra-amniotic infection was associated with a significant increase in amniotic fluid MMP-3 concentrations in both women with preterm labor and intact membranes (p = 0.03), and women with preterm PROM (p = 0.02). (5) Subsequent fetal loss after genetic amniocentesis was not associated with significant changes in mid-trimester concentrations of amniotic fluid MMP-3.

CONCLUSIONS

(1) MMP-3 is a physiologic constituent of amniotic fluid. (2) MMP-3 may play a role in the mechanisms of human parturition and in the regulation of the host response to intrauterine infection.

MMPs and TIMPs--an historical perspective

There are currently 25 known vertebrate matrix metalloproteinases (MMPs) and 4 tissue inhibitors of metalloproteinases (TIMPs). This article reviews these proteases from an historical perspective in terms of who discovered each protein, when the sequence was established, when action on protein substrates was demonstrated, and what names have been used. A similar approach is taken for the TIMPS, and their multiple functions in addition to protease inhibition are emphasized. MMPs from invertebrates, plants, and bacteria are also discussed. This review is an outgrowth and update of a chapter by the same name originally published in Matrix Metalloproteinase Protocols, pp. 1-23, edited by I. M. Clark and published by Humana Press in 2001.

Localization of cathepsins D, K, and L in degenerated human intervertebral discs

STUDY DESIGN

Localization of cathepsins D, K, and L in degenerated intervertebral discs was examined by immunohistochemistry.

OBJECTIVES

To determine the involvement of cathepsins in the pathomechanism of intervertebral disc degeneration by monitoring the immunolocalization of cathepsins in degenerated intervertebral disc tissue.

SUMMARY OF BACKGROUND DATA

Cathepsins D, K, and L are enzymes that contribute to the matrix destruction seen in the articular cartilage affected by osteoarthritis and rheumatoid arthritis. However, little is known about the contribution of these cathepsins to intervertebral disc degeneration.

METHODS

Paraffin-embedded sections of degenerated intervertebral disc tissue collected at the time of surgery (13 discs from 12 patients) were immunohistochemically stained with antibodies for cathepsins D, K, and L. For further characterization of the stained cells, immunohistochemical detection of CD68 and TRAP staining were performed.

RESULTS

Hematoxylin and eosin staining revealed obvious signs of degeneration in all sections. Cathepsins D and L were immunolocalized in disc fibrochondrocytes at various sites exhibiting degeneration. Cathepsins K were found in tartrate-resistant acid phosphatase-positive multinucleated cells, in particular near the cleft within the cartilaginous endplate. However, few cells were positive for these cathepsins in anulus fibrosus that maintained the lamellar structure of collagen fibers.

CONCLUSIONS

Marked expression of cathepsins D and L was observed at the site of degeneration. Cathepsins D and K localized in tartrate-resistant acid phosphatase-positive multinucleated cells existed at the cleft between the cartilaginous endplate and vertebral body. The site-specific localization of these cathepsins suggests the association of these proteinases with endplate separation and disorganization of the anulus fibrosus in degenerative spinal disorders.

Normal cartilage structure, biochemistry, and metabolism: a review of the literature

PURPOSE

To understand the possible significance of the presence of proteases, cytokines, growth factors, and arachidonic acid metabolites in the osteoarthritic temporomandibular joint (TMJ), a review of the normal physiologic processes and participating factors in the normal TMJ is established, based on knowledge of structure, biochemistry and metabolism of normal cartilage in general.

Proteoglycan synthesis and osteophyte formation in 'metabolically' and 'mechanically' induced murine degenerative joint disease: an in-vivo autoradiographic study

We investigated the in-vivo proteoglycan synthesis in specific areas of murine knee joint articular cartilage after the induction of degenerative joint disease by means of 35S-sulphate autoradiography. Degenerative joint disease was induced either by direct interference with cartilage metabolism (papain and iodoacetate), or by the induction of joint instability (collagenase). Injection of iodoacetate and papain led to inhibition of proteoglycan synthesis mainly in the central parts of the patellae, patellaris femoris and the central part of the medial tibial plateau. Articular cartilage adjacent to the strongly inhibited areas frequently showed a significantly enhanced synthesis of proteoglycans. A strong inhibition of proteoglycan synthesis was observed in the central part of the medial plateau after collagenase injection while other cartilage sites and joint structures such as the capsule and ligaments were stimulated in their proteoglycan synthesis. This study shows that the localization of changes in cartilage metabolism in degenerative joint disease of the knee might be related to differences in the pathogenetic mechanism in different variants of this common joint disorder.

Treatment of proteoglycan aggregates with physeal enzymes reduces their ability to inhibit hydroxyapatite proliferation in a gelatin gel

In vitro, cartilage proteoglycans (PGs) are effective inhibitors of hydroxyapatite formation and growth. Their inhibitory ability decreases with decreasing PG size and charge density. It has been suggested that the enzyme-mediated alteration in the size and conformation of PGs in the growth plate may similarly facilitate the calcification process. In this study, a gelatin gel system was used to monitor hydroxyapatite formation and growth in the presence of proteoglycan aggregates, before and after enzyme treatment. To reproduce the physeal degradation cascade, an enzyme preparation was used that contained all of the growth plate enzymes. At a concentration of 500 micrograms/ml, the untreated proteoglycan aggregates reduced the amount of mineral formed by 30%. When the aggregates were treated with the heat-inactivated enzyme, the same extent of inhibition was found. In contrast, treating the aggregates with the crude growth plate enzyme preparation removed all the inhibitory ability, such that 500 micrograms/ml of proteoglycan preparation yielded 10% more mineral than the controls. Treatment of the aggregates with chondroitinase ABC and trypsin, similarly removed all the inhibitory ability. These data, suggest that enzymatic degradation of proteoglycans may contribute to the regulation of growth plate calcification.

Enzymatic activity and distribution of phospholipase A2 in human cartilage

Extracellular phospholipase A2 (PLA2) with proinflammatory activity has recently been discovered in synovial fluids in inflammatory arthritides. In the search for the sources of synovial fluid PLA2, human synovium and articular cartilage were found to contain large quantities of the enzyme. In rheumatoid arthritis (RA), PLA2 activity in synovium, superficial and deep layers of articular cartilage was 20 +/- 14 (SEM), 168 +/- 62 and 533 +/- 176 nmol/min/mg protein respectively. Corresponding values in osteoarthritis (OA) were 49 +/- 11, 569 +/- 109 and 1709 +/- 243 nmol/min/mg protein, all significantly higher (p less than .01) than in RA. Nasal septal cartilage contained much less PLA2, 19 +/- 5.6. PLA2 in human articular and nasal cartilage has sn-2 specificity, a neutral pH optimum and absolute calcium dependence. High PLA2 concentration in articular cartilage may imply that, at least in part, cartilage is the source of PLA2 in the joint space. Since RA cartilage and synovium have less PLA2 activity than the corresponding OA tissues, additional sources of PLA2 in RA synovial fluids are implicated.

In vitro and in vivo effects of metal chelators on cartilage metabolism

The effect of metallic chelating agents (EDTA, EGTA) on cartilage metabolism was studied both in vitro, on calf cartilage, and in vivo, in rabbits. The question asked was whether it was possible to affect neutral protease activity, and not also inhibit beneficial synthetic systems. In vitro, EDTA suppressed anabolic processes, while EGTA had no effect. However, EDTA in vivo did not suppress glycosaminoglycan or RNA production, but paradoxically stimulated them. At the same time, EDTA inhibited neutral protease activity in normal animals.

Morphometric evaluation of rhesus articular cartilage: correlation of hexosamine and 35SO4 = incorporation

Morphometric evaluation of 20 rhesus articular cartilage samples were correlated with hexosamine and an 18-hour incorporation of 35SO4- as a measure of proteoglycan production on samples from the same joint. Reduced chondrocyte cellularity was the basis of the reduced maintenance of the matrix by the chondrocyte. In the more cellular cartilage, the matrix/lacunae area ratio was less than 25, and the less cellular group had a ratio of greater than 40. An inverse correlation existed between morphometric cartilage matrix/lacunar area ratio and hexosamine content. A significant difference of 35SO4 incorporation was not seen between the three morphometric grades. Morphometric assessment reduces the subjectivity of articular cartilage evaluation.

Drug-induced inhibition of proteoglycanase activity in the Hulth-Telhag model

Evidence has accumulated that there are increased degradative enzymes in osteoarthritis responsible for joint destruction. These enzymes are metal-dependent, and inhibited by EDTA. EDTA was administered intra-articularly to rabbits in an experimental model of osteoarthritis. There was a 25% reduction in neutral proteoglycanase activity and 75% of the animals had a reduction in the severity of the arthritis as measured on a histologic-histochemical grading system. It is suggested that future chemotherapeutic studies on arthritis might focus on enzyme inhibition.

Comparison of urinary glycosaminoglycan excretion in rheumatoid arthritis, osteoarthritis, myocardial infarction, and controls

Urinary glycosaminoglycan (GAG) excretion was measured in 24 patients with active rheumatoid arthritis (RA) before and after treatment with conventional second-line agents. Urinary GAG excretion was also measured in normal controls, patients with osteoarthritis (OA), and patients with acute myocardial infarction (MI). Total GAG excretion was increased in the RA group and fell after second-line therapy (p less than 0.01). More low than high molecular weight GAG was excreted in the active RA group, and this pattern was reversed after treatment. Excretion of total, high and low molecular weight GAG in the OA group did not differ significantly from controls. Total GAG excretion was increased in the MI group when compared with controls (p less than 0.02) and consisted mainly of high molecular weight GAG. The serial measurement of urinary GAG provides a further index of disease activity and may help to monitor response to treatment.

Lipid peroxides in human articular cartilage

The hypothesis that increased generation of lipid peroxides (LP) causes articular cartilage damage in older patients and in those with osteoarthritis was tested by directly measuring LP tissue levels in various layers of human articular cartilage. The LP content was significantly greater in the superficial than in the deeper portion of the cartilage, but lower in cartilage than in liver, kidney, adrenal glands and synovium. When LP were related to the total lipid content of these tissues, a high peroxide per lipid ratio was obtained for articular cartilage. The relevance of these findings to the mechanism of cartilage fibrillation is discussed.

Inhibition of cartilage breakdown by hydrocortisone in a tissue culture model of rheumatoid arthritis

Bovine nasal cartilage discs cocultured with human rheumatoid synovial membrane or synovial-membrane-conditioned media release proteoglycan largely as a result of cartilage breakdown. We assessed the effects of hydrocortisone on proteoglycan distribution between cartilage and culture medium, and on cartilage breakdown expressed as the release of either proteoglycan or 35S-products from prelabelled discs. The presence of synovial membrane inhibited the capacity for net proteoglycan synthesis, preventing its accumulation in cartilage; this was little affected by hydrocortisone. The major response to pharmacological concentrations of hydrocortisone was suppression of both spontaneous and synovial-membrane-induced cartilage breakdown. The autolysis of synovial protein that normally occurred during culture was similarly prevented by comparable doses of corticosteroid. Changes in chromatographic distribution of the 35S-labelled degradation products released from cartilage conformed with a corticosteroid-induced inhibition of endogenous lysosomal or related proteinase activity. Additionally, inhibition of the early events in synovial membrane that are responsible for chondrocyte-mediated breakdown of cartilage may contribute significantly to the overall corticosteroid effect.

Proteoglycan synthesis in chondrocyte cultures from osteoarthrotic and normal human articular cartilage

Chondrocytes derived by outgrowth from normal and osteoarthrotic human femoral head cartilage were grown in high density cultures for five passages. Cultures were analysed for their sulfated macromolecular components in medium, layer-matrix and intracellular compartments. Two fractions were obtained: typical proteoglycans and glycosaminoglycan-peptides (Mr approx. 60 000) which might result from an enzymatic degradation of proteoglycans. Proteoglycans from normal and osteoarthrotic cultures exhibited similar biochemical properties (size, protein:uronic acid ratio, glycosaminoglycan composition). Slightly less proteoglycans were aggregated with hyaluronic acid in osteoarthrotic than in normal cultures. Three populations of proteoglycan subunit were obtained under dissociative conditions (Sepharose CL-2B) in both normal and osteoarthrotic cultures: proteoglycan 1 (Kav=0.04), proteoglycan 2 (Kav=0.26) which were aggregated with hyaluronic acid in associative conditions, and proteoglycan 4 (Kav=0.48). A fourth population, proteoglycan 3 (Kav=0.33, Sepharose 2B and CL-2B) was intracellular in osteoarthrotic cultures. After a 4 day incubation period, about 60% more proteoglycans were found in osteoarthrotic than in normal cultures (medium+52%, layer-matrix + 44% and 17 times the normal value inside the cells). Proteoglycan distribution kinetics in the three compartments showed a higher net accumulation of proteoglycans in osteoarthrotic-derived cultures.

Further characterization of a neutral metalloprotease isolated from human articular cartilage

The neutral metalloprotease extracted from 1,200 gm of human articular cartilage was purified 1,400- to 2,400-fold by diethylaminoethyl- and carboxymethyl-Sephadex chromatography. Disc electrophoresis and an isoelectric focusing method resolved the neutral enzyme activity into 4 bands. All bands had a similar amino acid analysis and a similar molecular weight by sodium dodecylsulfate electrophoresis and gel filtration: 24,000-27,000 daltons. The enzyme degraded proteoglycan subunit and proteoglycan aggregate to products with a sedimentation coefficient of 3S, but at low dilutions the enzyme produced 19.3S fragments. It is postulated that this protease may contribute to the development of osteoarthritis from within the cartilage matrix.

The properties of the neutral proteinase released by primary chondrocyte cultures and its action on proteoglycan aggregate

The enzymatic mechanism of proteoglycan breakdown is of major interest, since it has been proposed that osteoarthritis involves increased proteolytic breakdown of proteoglycans. This paper describes the properties of the proteoglycan-degrading enzymes released into the extracellular milieu by chondrocyte cultures that produce cartilage-specific type II collagen but no detectable type I collagen. Attention has been focused on enzymes active at neutral pH, since the pH of the extracellular matrix is around neutrality. Biogel P-60 chromatography of concentrated culture medium showed a major peak of enzyme activity on proteoglycan monomer entrapped in polyacrylamide beads as well as on native proteoglycan aggregates. The enzyme yields a specific limit digestion peptide from the aggregate of approximately 55,000 daltons (in the presence of SDS). This limit peptide is probably derived from the hyaluronic acid-binding region of proteoglycan. The proteolytic enzyme is latent but can be activated by aminophenylmercuric acetate or trypsin. The molecular weight of both the active and latent forms, determined by gel filtration, is approximately 33,000. The activity is not inhibited by phenylmethylsulfonyl fluoride or pepstatin but is completely inhibited by o-phenanthroline; the activity is restored by Zn or Co ions in the presence of calcium chloride. Removal of calcium by dialysis results in a reversible loss of activity. The release of such a metalloproteinase by chondrocytes into the extracellular milieu, its activity at physiological pH and its ability to degrade native proteoglycans are consistent with a role of the enzyme in proteoglycan metabolism.

Induction of cartilage degradation in experimental arthritis produced by allogeneic and xenogeneic proteoglycan antigens

A single intra-articular injection of proteoglycan (PG) antigens induced sterile inflammatory response and cartilage degradation in preimmunized rabbits and dogs. Both cell-mediated and humoral immune reactions could be detected against PG antigens in these animals, but there was no response against collagen Type II. The lymphocytes isolated from the inflamed synovial layer proved to be predominantly of T cell type. The cartilage degradation was indicated by the accumulation of IgG and complement in the superficial layer of the articular cartilage. By the diminished number of chondrocytes and by the increased binding of specific antibodies in the ground substance. The results suggest that PG antigens trigger local immune reactions which become self-sustaining by enzymatic exposure of antigenic sites. The anti-PG antibodies are cytotoxic to articular chondrocytes and thus can block the continuous neogenesis of matrix components which may lead ultimately to deterioration of the cartilage. This type of experimental arthritis appears to be a model closely related to rheumatoid arthritis.

Mechanisms of action of gold

Since the successful introduction of injectable gold compounds for the treatment of rheumatoid arthritis over 50 years ago, numerous studies on the possible mechanism of action have been performed. This heavy metal has been show to possess a bewildering array of biological effects. Studies using gold performed both in vitro and in vivo can be grouped into anti-microbial, anti-immunologic, anti-inflammatory, anti-enzymatic and other effects. In this survey, we have analyzed these multiple approaches to the study of the mechanism of action of injectable gold preparations and apply the findings to rheumatoid arthritis.

Neutral proteinases from articular chondrocytes in culture. 2. Metal-dependent latent neutral proteoglycanase, and inhibitory activity

Monolayer and spinner cultured rabbit articular chondrocytes released into the medium latent metal-dependent enzyme with activity against bovine proteoglycan. Pretreatment of medium with p-aminophenylmercuric acetate or trypsin followed by soybean trypsin inhibitor significantly increased enzyme activity. The monolayer-cultured chondrocytes released more of this activity than spinner cultures. The neutral proteoglycanase activity increased with medium concentration and incubation time. Like the human cartilage proteoglycanase, its pH optimum on proteoglycan subunit was 7.25. Gel filtration on BioGel P-30 indicated that the proteoglycanase occurred in two molecular weight forms: 20 000--30 000 and 13 000. The latent enzyme was about 30 000--40 000. The metal-chelators, o-phenanthroline (5 mM) and EDTA (10 mM) inhibited the activated proteoglycanase almost completely, but trypsin and chymotrypsin inhibitors had little effect. The cultured chondrocytes also released into the media a heat-labile inhibitor against the proteoglycanase. The inhibitory activity was present in the nonactivated media and eluted on Sephadex G-100 chiefly at a position corresponding to molecular weights of 10 000--13 000.

Metabolism of human femoral head cartilage in osteoarthrosis and subcapital fracture

The cell density and incorporation of 35SO4 and 3H-glycine into human articular cartilage from 8 osteoarthrotic and 7 normal (subcapital fracture) femoral heads were studied. It was found that osteoarthrotic cartilage incorporates on a per cell basis about twice as much 35SO4 and 2--5 times as much 3H-glycine as normal cartilage. There was no relationship between the intensity of incorporation and either the location of the cartilage (weight-bearing versus non weight-bearing areas) in normal cartilage or the degree of damage (normal-like, fibrillated, and ulcerated) in osteoarthrotic articular cartilage. In the latter tissue the increased synthetic capacity of the cells seems to be a diffuse rather than a localised process, for it was also found in cartilage from peripheral osteophytes. Histo-autoradiographic studies showed that the osteoarthrotic chondrocytes are metabolically hyperactive all over the femoral head, including wedge-shaped margins of the zone of exposed bone. These results support the hypothesis that much of the articular cartilage from osteoarthrotic femoral heads is of an immature chondroblastic type. It is suggested that de-novo synthesis of articular cartilage occurs during the process of regional remodelling of the femoral head, which would account for the observed hyperactivity.

Proteolytic enzymes in joint destruction

An increase in both neutral and acid proteolytic activity and proteinase inhibitors is reported in synovial fluid of osteoarthrotic and rheumatoid joints, compared with controls, but in subchondral bone and articular cartilage only Cathepsin D and in synovial tissue only neutral caseinolysis were elevated. Especially high proteolytic activities were found in joint compartments of seronegative rheumatoid and in sera of osteoarthrotic patients. The source of the main part of neutral caseinolytic activity in joint fluid is inflammatory cells, in the case of Cathepsin D the tissues of origin seem to be bone and cartilage.

Proteoglycans in normal and severely osteoarthritic human cartilage

Proteoglycans from osteoarthritic cartilage were compared with those from normal articular cartilage. Normal proteoglycan aggregates are larger in size and more homogeneous than those in osteoarthritis. Proteoglycan monomers from both sources gave two peaks on controlled pore glass-bead chromatography. Although the retarded material from normal cartilage showed an affinity for hyaluronate, the same material from osteoarthritic cartilage did not. The hyaluronate-binding capacity of the material which is partly in the void volume and partly retarded was similar in both types of cartilage. These results suggest that in osteoarthritic cartilage the proteoglycan aggregates are smaller and more heterogeneous and that the chondroitin sulphate side chains are shorter. They also indicate that there are two populations of proteoglycan, one with its hyaluronate-binding-protein region of core protein intact and the other either possessing an inactive binding region or totally lacking it.

Experimental arthritis produced by proteoglycan antigens in rabbits

Spontaneous synovitis developed in the limb joints and rheumatoid factor-like component appeared in the sera of two rabbits from a pool 36 animals in the course of a long-term immunization with bovine nasal cartilage antigens. A single intra-articular injection of proteoglycan antigens regularly provoked a heavy synovitis and cartilage destruction irrespective of whether the booster injections were administered in physiological saline, or in Freund's complete adjuvant. The dose-dependent severity of arthritis suggested that the antibody titre against proteoglycan antigens played an important role in this mechanism. The synovial extract and synovial fluid of knee joints injected with proteoglycan antigens showed an increased enzyme activity concerning the four acid hydrolases (acid phosphatase, cathepsin D, hyaluronidase and beta-glucuronidase). The high activity of lysosomal acid hydrolases which persisted for several months can derange the molecular structure of proteoglycans of cartilage. The degraded proteoglycans may trigger autoimmune reactions, and the process eventually leads to chronic inflammation and joint destruction.

Proteoglycans of the intervertebral disc. Absence of degradation during the isolation of proteoglycans from the intervertebral disc

Proteoglycans extracted with 4M-guanidinium chloride from pig intervetebral discs, and purified by equilibrium density-gradient centrifugation in CsCl, were of smaller hydrodynamic size than those extracted and purified in the same way from the laryngeal cartilage of the same animal. Whether this difference in size arose from degradation during the extraction and purification of the proteoglycans of the disc was investigated. Purified proteoglycans labelled either in the chondroitin sulphate chains or in the core protein were obtained from laryngeal cartilage by short-term organ culture. These labelled proteoglycans were added at the beginning of the extraction of the disc proteoglycans, and labelled cartilage and unlabelled disc proteoglycans were isolated and purified together. There was no appreciable loss of radioactivity after density-gradient centrifugation nor decrease in hydrodynamic size of the labelled cartilage proteoglycans on chromatography on Sepharose 2B, when these were present during the extraction of disc proteoglycans. It is concluded that disc proteoglycans are intrinsically of smaller size than cartilage proteoglycans and this difference in size does not arise from degradation during the extraction.

A tissue-culture model of cartilage breakdown in rheumatoid arthritis. Quantitative aspects of proteoglycan release

1. The destruction of articular cartilage in human rheumatoid and other arthritides is the result of diverse mechanical, inflammatory and local cellular factors. A tissue-culture model for studying cartilage-synovial interactions that may be involved in the final common pathway of joint destruction is described. 2. Matrix breakdown was studied in vitro by using bovine nasal-cartilage discs cultivated in contact with synovium. Synovia were obtained from human and animal sources. Human tissue came from patients with ;classical' rheumatoid arthritis, and animal tissue from rabbits with antigen-induced arthritis. 3. Cartilage discs increased their proteoglycan content 2-3-fold during 8 days in culture. Proteoglycan was also released into culture medium, approx. 70% arising from cartilage breakdown. 4. Synovial explants from human rheumatoid and rabbit antigen-induced arthritis produced equivalent stimulation of proteoglycan release. After an initial lag phase, the breakdown rate rose abruptly to a maximum, resulting in a 2-fold increase of proteoglycan accumulation in culture medium after 8-10 days. 5. High-molecular-weight products shed into culture media were characterized chromatographically and by differential enzymic digestion. Proteoglycan-chondroitin sulphate accounted for 90% of the released polyanion, and its partial degradation in the presence of synovial explants was consistent with limited proteolytic cleavage. 6. Rheumatoid synovium applied to dead cartilage increased the basal rate of proteoglycan release. Living cartilage was capable of more extensive autolysis, even in the absence of synovium. However, optimal proteoglycan release required the interaction of living synovium with live cartilage. These findings support the view that a significant component of cartilage breakdown may be chondrocyte-mediated.

Development and reversal of a proteoglycan aggregation defect in normal canine knee cartilage after immobilization

Healthy adult dogs were studied for a defect in proteoglycan aggregation by immobilizing one limb for varying periods of time. Immobilization for 6 days resulted in a 41% reduction in proteoglycan synthesis by articular cartilage from the restrained knee compared with the contralateral control knee. After 3 weeks of immobilization, proteoglycan aggregation was no longer demonstrable in cartilage from the constrained limb. The aggregation defect was rapidly reversible and aggregates were again normal size 2 weeks after removal of a cast that had been worn for 6 weeks.

Proteoglycan aggregates in adult human costal cartilage

1. A sequential extraction procedure using isotonic KCl and 4.0 M guanidine hydrochloride was used to solubilise proteoglycans from adult human costal cartilage under conditions minimising autolysis. Up to 28% of the total tissue hexuronate was extracted. 2. Proteoglycan fractions were prepared from the extracts by CsCl equilibrium density gradient centrifugation. 3. Each fraction exhibited distinct electrophoretic heterogeneity. 4. Proteoglycans extracted by isotonic solutions are relatively small and may be in vivo degradation products of whole molecules. The major fraction (A1) from high ionic strength extracts has a composition similar to that of A1 proteoglycans from adult human articular cartilage. A2 fractions differ and are highly enriched in protein. 5. A1 and A2 fractions from high ionic strength extracts contain proteoglycan aggregates, but to a much lesser extent than found in other cartilages like bovine nasal or porcine epiglottal cartilage. 6. The aggregates can be dissociated and a 'subunit" proteoglycan isolated by CsCl density gradient centrifugation in 4.0 M guanidine hydrochloride. 'Subunits' can reaggregate partially, when mixed with fractions of lower density from the gradient. 7. Addition of hyaluronic acid to A1GuHCl promotes a large increase in the amount of aggregate present. 8. The hyaluronic acid content of costal cartilage is not deficient compared to that of other hyaline cartilages. 9. Polypeptides with molecular weights suggesting their identity as 'link proteins' are present in A1 and A2 fractions. 10. Additional polypeptides of higher molecular weight than link proteins are also present in A1 and A2 fractions. They may represent the hyaluronic acid binding region of proteoglycans and compete with 'whole' proteoglycans for hyaluronate in the tissue.

Cartilage proteoglycan alterations in an experimentally induced model of rabbit osteoarthritis

Size distribution of cartilage proteoglycans (PG) extracted from control and osteoarthritic rabbit knees after partial meniscectomy was analyzed. In normal control knees, about 30% of PG molecules were in aggregate form and average sedimentation constant was 60S. No aggregates were found in osteoarthritic cartilage, whether ulcer, rim about ulcer, or distant normal-appearing cartilage was examined. Weight average sedimentation constants for PG subunits were similar to controls, 15S. Up to 70% of guanidinium-extractable PG could be extracted from osteoarthritic cartilage by using 0.5M guanidine HC1 (GuHCl); sedimentation characteristics of extracted PG were similar to those using 4.0M GuHCl. Absence of aggregates is consistent with a disorder of link protein, hyaluronic acid, or PG subunit hyaluronic acid binding sites. Absence of subunit degradation was an enexpected finding. The demonstrated ability of 0.5 M GuHCl to extract large amounts of PG from osteoarthritic cartilage will allow study of cartilage proteoglycans in their native nondissociated state.

Extracellular matrix metabolism by chondrocytes. 5. The proteoglycans and glycosaminoglycans synthesized by chondrocytes in high density cultures

Proteoglycans were extracted from the extracellular matrix of cultures of embryonic chick chondrocytes grown at high density and were purified by CsC1 density gradient centrifugation. The chemical, physical and hyaluronate binding properties of the proteoglycans were similar to those observed in proteoglycans from other hyaline cartilages. Proteoglycans in the media were also purified and on analysis showed three populations of proteoglycans to be present. One population had the physical characteristics of a typical proteoglycan subunit and bound hyaluronate, the other two populations were unable to complex with hyaluronate but one had the physical characteristics of the proteoglycan subunit and the other was of smaller molecular weight. The small molecular weight appears to be a product of the enzymatic degradation of the larger molecular weight species.

Correlation of lysozyme activity with proteoglycan biosynthesis in epiphyseal cartilage

Pig epiphyseal cartilage (proximal ulna epiphysis) previously incubated into vitro in the presence of sodium [35S]sulfate or [3H]thymidine was either analyzed by autoradiography or separated into 9 morphologically defined consecutive layers and investigated for 35S-incorporation into the guanidinium chloride-extractable proteoglycans and for lysozyme activity. The lowest 35S incorporation and lysozyme activity were determined in the zone of resting cells, but there is a consecutive increase in the rate of proteoglycan synthesis and lysozyme activity toward the diaphyseal cartilage-bone junction, with the maximum at the lower columnar cell zone and a sharp reduction of both parameters at the hypertrophic zone. The maxima of 35S incorporation and [3H]thymidine incorporation do not coincide. The guanidinium chloride-soluble proteoglycans exhibit macromolecular polydispersity. Fractions excluded from as well as retarded by Sepharose 2B gel could be separated and were detected in all zones. The results indicate a correlation of proteoglycan biosynthesis and lysozyme activity in epiphyseal cartilage.