Localization of rat cathepsin K in osteoclasts and resorption pits: inhibition of bone resorption and cathepsin K-activity by peptidyl vinyl sulfones

Effects of antisense mediated inhibition of cathepsin K on human osteoclasts obtained from peripheral blood

Cathepsin K is a cystein protease that displays a proteolytic activity against Type I collagen and is abundantly and selectively expressed in osteoclasts where it plays a critical role in bone degradation. Its direct role in bone tissue has been defined by knock-out mice studies and inhibiting strategies in animals models. However, direct proof of cathepsin K function in human osteoclast model in vitro is lacking. The aim of this study is to analyze cathepsin K expression and localization in human osteoclasts obtained from peripheral blood and to examine cathepsin K function in these cells by antisense oligodeoxynucleotide (AS-ODN) strategy. AS-ODN was added to the culture of osteoclast precursors induced to differentiate by RANKL and M-CSF. AS-ODN treatment produced a significant down-regulation of cathepsin K mRNA (>80%) and protein expression, as verified respectively by Real-time PCR and by immunocytochemistry or Western blot. The cathepsin K inhibition caused an impairment of resorption activity as evaluated by a pit formation assay ( p = 0.045) and by electron microscopy, while the acidification process was unaffected. We demonstrated that antisense strategies against cathepsin K are selectively effective to inhibit resorption activity in human osteoclasts, like in animal models.

Cystatin B as an intracellular modulator of bone resorption

Degradation of organic bone matrix requires proteinase activity. Cathepsin K is a major osteoclast proteinase needed for bone resorption, although osteoclasts also express a variety of other cysteine- and matrix metalloproteinases that are involved in bone remodellation. Cystatin B, an intracellular cysteine proteinase inhibitor, exhibits a lysosomal distribution preferentially in osteoclasts but it's role in osteoclast physiology has remained unknown. The current paper describes a novel regulatory function for cystatin B in bone-resorbing osteoclasts in vitro. Rat osteoclasts were cultured on bovine bone and spleen-derived cystatin B was added to the cultures. Nuclear morphology was evaluated and the number of actively resorbing osteoclasts and resorption pits was counted. Intracellular cathepsin K and tartrate-resistant acid phosphatase (TRACP) activities were monitored using fluorescent enzyme substrates and immunohistology was used to evaluate distribution of cystatin B in rat metaphyseal bone. Microscopical evaluation showed that cystatin B inactivated osteoclasts, thus resulting in impaired bone resorption. Cathepsin K and TRACP positive vesicles disappeared dose-dependently from the cystatin B-treated osteoclasts, indicating a decreased intracellular trafficking of bone degradation products. At the same time, cystatin B protected osteoclasts from experimentally induced apoptosis. These data show for the first time that, in addition to regulating cysteine proteinase activity and promoting cell survival in the nervous system, cystatin B inhibits bone resorption by down-regulating intracellular cathepsin K activity despite increased osteoclast survival.

Substrate profiling of cysteine proteases using a combinatorial peptide library identifies functionally unique specificities

The substrate specificities of papain-like cysteine proteases (clan CA, family C1) papain, bromelain, and human cathepsins L, V, K, S, F, B, and five proteases of parasitic origin were studied using a completely diversified positional scanning synthetic combinatorial library. A bifunctional coumarin fluorophore was used that facilitated synthesis of the library and individual peptide substrates. The library has a total of 160,000 tetrapeptide substrate sequences completely randomizing each of the P1, P2, P3, and P4 positions with 20 amino acids. A microtiter plate assay format permitted a rapid determination of the specificity profile of each enzyme. Individual peptide substrates were then synthesized and tested for a quantitative determination of the specificity of the human cathepsins. Despite the conserved three-dimensional structure and similar substrate specificity of the enzymes studied, distinct amino acid preferences that differentiate each enzyme were identified. The specificities of cathepsins K and S partially match the cleavage site sequences in their physiological substrates. Capitalizing on its unique preference for proline and glycine at the P2 and P3 positions, respectively, selective substrates and a substrate-based inhibitor were developed for cathepsin K. A cluster analysis of the proteases based on the complete specificity profile provided a functional characterization distinct from standard sequence analysis. This approach provides useful information for developing selective chemical probes to study protease-related pathologies and physiologies.

Degradation of the organic phase of bone by osteoclasts: a secondary role for lysosomal acidification

Osteoclasts degrade bone matrix by secretion of hydrochloric acid and proteases. We studied the processes involved in the degradation of the organic matrix of bone in detail and found that lysosomal acidification is involved in this process and that MMPs are capable of degrading the organic matrix in the absence of cathepsin K.

INTRODUCTION

Osteoclasts resorb bone by secretion of acid by the vacuolar H+-adenosine triphosphatase (V-ATPase) and the chloride channel ClC-7, followed by degradation of the matrix, mainly collagen type I, by cathepsin K and possibly by matrix metalloproteinases (MMPs). However, the switch from acidification to proteolysis and the exact roles of both the ion transporters and the proteinases still remain to be studied.

MATERIALS AND METHODS

We isolated CD14+ monocytes from human peripheral blood from either controls or patients with autosomal dominant osteopetrosis type II (ADOII) caused by defective ClC-7 function and cultured them in the presence of RANKL and macrophage-colony stimulating factor (M-CSF) to generate osteoclasts. We decalcified cortical bovine bone slices and studied the osteoclasts with respect to morphology, markers, and degradation of the decalcified matrix in the presence of various inhibitors of osteoclast acidification and proteolysis, using normal calcified bone as a reference.

RESULTS

We found that ADOII osteoclasts not only have reduced resorption of the calcified matrix, but also 40% reduced degradation of the organic phase of bone. We found that both acidification inhibitors and cathepsin K inhibitors reduced degradation of the organic matrix by 40% in normal osteoclasts, but had no effect in the ADOII osteoclasts. Furthermore, we showed that inhibition of MMPs leads to a 70% reduction in the degradation of the organic bone matrix and that MMPs and cathepsin K have additive effects. Finally, we show that osteoclastic MMPs mediate release of the carboxyterminal telopeptide of type I collagen (ICTP) fragment in the absence of cathepsin K activity, and therefore, to some extent, are able to compensate for the loss of cathepsin K activity.

CONCLUSIONS

These data clearly show that osteoclastic acidification of the lysosomes plays a hitherto nonrecognized role in degradation of the organic matrix. Furthermore, these data shed light on the complicated interplay between acidification dependent and independent proteolytic processes, mediated by cathepsin K and the MMPs, respectively.

Serum levels of cathepsin K decrease with age in both women and men

Bone turnover increases with age. In a previous study, we reported on bone metabolism in young and elderly women and men. The aim of the present investigation was to evaluate potential age- and gender-related changes in cathepsin K, a cysteine protease that plays an important role in the degradation of the organic matrix of bone. Twenty-five healthy premenopausal women, 24 young healthy men, 26 elderly women, and 25 elderly men participated in the study. Elderly women and men had significantly lower cathepsin K levels than younger ones. In both men and women, serum levels of cathepsin K were negatively correlated with age. In men there was a statistically significant negative correlation between serum levels of cathepsin K and osteoprotegerin, which inhibits osteoclast differentiation and activation. No association was found between serum levels of cathepsin K and bone-specific alkaline phosphatase, osteocalcin, or 25-hydroxy vitamin D. Thus, the age-related increase in OPG, which markedly inhibits the expression of cathepsin K, may also reduce serum levels of cathepsin K. Despite the age-related increase in bone resorption, this study shows lower cathepsin K values in elderly women and men than in younger subjects. It might be speculated that a different enzyme could compensate for the decline in cathepsin K during old age.

Spontaneous development of synovitis and cartilage degeneration in transgenic mice overexpressing cathepsin K

OBJECTIVE

Several recent studies have demonstrated that cathepsin K, a proteolytic enzyme capable of degrading native fibrillar collagen, is overexpressed in osteoarthritic cartilage and inflamed synovial tissue. However, it is not known whether increased cathepsin K production is a primary or a secondary event in these diseases. The availability of transgenic UTU17 mice, which exhibit constitutive overexpression of the cathepsin K gene, prompted us to study possible arthritic changes in their knee joints.

METHODS

Progression of synovitis and articular cartilage degeneration in the knee joints of UTU17 mice and their nontransgenic littermates was monitored by histologic analyses at 7 and 12 months of age. Distribution of cathepsin K in the knee joints was studied by immunohistochemistry.

RESULTS

At the age of 7 months, UTU17 mice exhibited clear signs of synovitis, with strong immunostaining for cathepsin K in the synovial lining and the stroma, while control knee joints appeared normal. At 12 months, marked synovial thickening and fibrosis and severe degradation of cartilage and subchondral bone were observed in UTU17 mouse knee joints. In areas of cartilage degeneration, both chondrocytes and cells of hypertrophic synovia were positive for cathepsin K. At 12 months, synovia of control mice revealed only a few isolated cathepsin K-positive cells and mild changes in articular cartilage.

CONCLUSION

Our findings demonstrate that overexpression of the cathepsin K gene under its own promoter in transgenic mice makes them susceptible to progressive synovitis, which, upon aging, results in synovial hyperplasia and fibrosis and subsequent destruction of articular cartilage and bone.

Impaired bone resorption in cathepsin K-deficient mice is partially compensated for by enhanced osteoclastogenesis and increased expression of other proteases via an increased RANKL/OPG ratio

Previous reports indicate that mice deficient for cathepsin K (Ctsk), a key protease in osteoclastic bone resorption, develop osteopetrosis due to their inability to properly degrade organic bone matrix. Some features of the phenotype of Ctsk knockout mice, however, suggest the presence of mechanisms by which Ctsk-deficient mice compensate for the lack of cathepsin K. To study these mechanisms in detail, we generated Ctsk-deficient (Ctsk-/-) mice and analyzed them at the age of 2, 7, and 12 months using peripheral quantitative computed tomography, histomorphometry, resorption marker measurements, osteoclast and osteoblast differentiation cultures, and gene expression analyses. The present study verified the previously published osteopetrotic features of Ctsk-deficient mice. However, these changes did not exacerbate during aging indicating the absence of Ctsk to have its most severe effects during the rapid growth period. Resorption markers ICTP and CTX were decreased in the media of Ctsk-/- osteoclasts cultured on bone slices indicating impaired bone resorption. Ctsk-/- mice exhibited several mechanisms attempting to compensate for Ctsk deficiency. The number of osteoclasts in trabecular bone was significantly increased in Ctsk-/- mice compared to controls, as was the number of osteoclast precursors in bone marrow. The mRNA levels for receptor activator of nuclear factor (kappa)B ligand (RANKL) in Ctsk-/- bones were increased resulting in increased RANKL/OPG ratio favoring osteoclastogenesis. In addition, expression of mRNAs of osteoclastic enzymes (MMP-9, TRACP) and for osteoblastic proteases (MMP-13, MMP-14) were increased in Ctsk-/- mice compared to controls. Impaired osteoclastic bone resorption in Ctsk-/- mice results in activation of osteoblastic cells to produce increased amounts of other proteolytic enzymes and RANKL in vivo. We suggest that increased RANKL expression mediates enhanced osteoclastogenesis and increased protease expression by osteoclasts. These observations underline the important role of osteoblastic cells in regulation of osteoclast activity and bone turnover.

Family 2 cystatins inhibit osteoclast-mediated bone resorption in calvarial bone explants

Osteoclastic bone resorption depends on the activity of various proteolytic enzymes, in particular those belonging to the group of cysteine proteinases. Biochemical studies have shown that cystatins, naturally occurring inhibitors of these enzymes, inhibit bone matrix degradation. Since the mechanism by which cystatins exert this inhibitory effect is not completely resolved yet, we studied the effect of cystatins on bone resorption microscopically and by Ca-release measurements. Calvarial bone explants were cultured in the presence or absence of family 2 cystatins and processed for light and electron microscopic analysis, and the culture media were analyzed for calcium release. Both egg white cystatin and human cystatin C decreased calcium release into the medium significantly. Microscopic analyses of the bone explants demonstrated that in the presence of either inhibitor, a high percentage of osteoclasts was associated with demineralized non-degraded bone matrix. Following a 24-h incubation in the presence of cystatin C, 41% of the cells were adjacent to areas of demineralized non-degraded bone matrix, whereas in controls, this was only 6%. If bone explants were cultured with both PTH and cystatin C, 60% of the osteoclasts were associated with demineralized non-degraded bone matrix, compared to 27% for bones treated with PTH only (P < 0.01). Our study provides evidence that cystatins, the naturally occurring inhibitors of cysteine proteinases, reversibly inhibit bone matrix degradation in the resorption lacunae adjacent to osteoclasts. These findings suggest the involvement of cystatins in the modulation of osteoclastic bone degradation.

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.

Cathepsin V, a novel and potent elastolytic activity expressed in activated macrophages

Atherosclerosis is characterized by a thickening and loss of elasticity of the arterial wall. Loss of elasticity has been attributed to the degradation of the arterial elastin matrix. Cathepsins K and S are papain-like cysteine proteases with known elastolytic activities, and both enzymes have been identified in macrophages present in plaque areas of diseased blood vessels. Here we demonstrate that macrophages express a third elastolytic cysteine protease, cathepsin V, which exhibits the most potent elastase activity yet described among human proteases and that cathepsin V is present in atherosclerotic plaque specimens. Approximately 60% of the total elastolytic activity of macrophages can be attributed to cysteine proteases with cathepsins V, K, and S contributing equally. From this 60%, two-thirds occur extracellularly and one-third intracellularly with the latter credited to cathepsin V. Ubiquitously expressed glycosaminoglycans (GAGs) such as chondroitin sulfate specifically inhibit the elastolytic activities of cathepsins V and K via the formation of specific cathepsin-GAG complexes. In contrast, cathepsin S, which does not form complexes with chondroitin sulfate is not inhibited; thus suggesting a specific regulation of elastolytic activities of cathepsins by GAGs. Because the GAG content is reduced in atherosclerotic plaques, an increase of cathepsins V and K activities may accelerate the destruction of the elastin matrix in diseased arteries.

Cathepsin K inhibitor–polymer conjugates: potential drugs for the treatment of osteoporosis and rheumatoid arthritis

The role of the newly discovered cysteine protease, cathepsin K, in osteoporosis and rheumatoid arthritis is reviewed. The current development of cathepsin K inhibitors and their targeted delivery using synthetic polymer carriers are discussed. Future challenges and possible strategies to improve these delivery systems are addressed.

Probing cathepsin K activity with a selective substrate spanning its active site

The limited availability of highly selective cathepsin substrates seriously impairs studies designed to monitor individual cathepsin activities in biological samples. Among mammalian cysteine proteases, cathepsin K has a unique preference for a proline residue at P2, the primary determinant of its substrate specificity. Interestingly, congopain from Trypanosoma congolense also accommodates a proline residue in its S2 subsite. Analysis of a congopain model showed that amino acids forming its S2 subsite are identical with those of cathepsin K, except Leu67 which is replaced by a tyrosine residue in cathepsin K. Furthermore, amino acid residues of the congopain S2' binding pocket, which accepts a proline residue, are strictly identical with those of cathepsin K. Abz-HPGGPQ-EDN2ph [where Abz represents o-aminobenzoic acid and EDN2ph (=EDDnp) represents N -(2,4-dinitrophenyl)-ethylenediamine], a substrate initially developed for trypanosomal enzymes, was efficiently cleaved at the Gly-Gly bond by cathepsin K (kcat/ K(m)=426000 M(-1) x s(-1)). On the other hand, Abz-HPGGPQ-EDN2ph was resistant to hydrolysis by cathepsins B, F, H, L, S and V (20 nM enzyme concentration) and the Y67L (Tyr67-->Leu)/L205A cathepsin K mutant (20 nM), but still acted as a competitive inhibitor. Taken together, the selectivity of Abz-HPGGPQ-EDN2ph to cathepsin K primarily depends on the S2 and S2' subsite specificities of cathepsin K and the ionization state of histidine at P3. Whereas Abz-HPGGPQ-EDN2ph was hydrolysed by wild-type mouse fibroblast lysates, its hydrolysis was completely abolished in the cathepsin K-deficient samples, indicating that Abz-HPGGPQ-EDN2ph can be used to monitor selectively cathepsin K activity in physiological fluids and cell lysates.

Intracellular membrane trafficking in bone resorbing osteoclasts

There is ample evidence now that the two major events in bone resorption, namely dissolution of hydroxyapatite and degradation of the organic matrix, are performed by osteoclasts. The resorption cycle involves several specific cellular activities, where intracellular vesicular trafficking plays a crucial role. Although details of these processes started to open up only recently, it is clear that vesicular trafficking is needed in several specific steps of osteoclast functioning. Several plasma membrane domains are formed during the polarization of the resorbing cells. Multinucleated osteoclasts create a tight sealing to the extracellular matrix as a first indicator of their resorption activity. Initial steps of the sealing zone formation are alpha(v)beta(3)-integrin mediated, but the final molecular interaction(s) between the plasma membrane and mineralized bone matrix is still unknown. A large number of acidic intracellular vesicles then fuse with the bone-facing plasma membrane to form a ruffled border membrane, which is the actual resorbing organelle. The formation of a ruffled border is regulated by a small GTP-binding protein, rab7, which indicates the late endosomal character of the ruffled border membrane. Details of specific membrane transport processes in the osteoclasts, e.g., the formation of the sealing zone and transcytosis of bone degradation products from the resorption lacuna to the functional secretory domain remain to be clarified. It is tempting to speculate that specific features of vesicular trafficking may offer several potential new targets for drug therapy of bone diseases.

S3 to S3' subsite specificity of recombinant human cathepsin K and development of selective internally quenched fluorescent substrates

We have systematically examined the S3 to S3' subsite substrate specificity requirements of cathepsin K using internally quenched fluorescent peptides derived from the lead sequence Abz-KLRFSKQ-EDDnp [where Abz is o -aminobenzoic acid and EDDnp is N -(2,4-dinitrophenyl)ethylenediamine]. We assayed six series of peptides, in which each position except Gln was substituted with various natural amino acids. The results indicated that the S3-S1 subsite requirements are more restricted than those of S1'-S3'. Cathepsin K preferentially accommodates hydrophobic amino acids with aliphatic side chains (Leu, Ile and Val) in the S2 site. Modifications at P1 residues also have a large influence on cathepsin K activity. Positively charged residues (Arg and Lys) represent the best accepted amino acids in this position, although a particular preference for Gly was found as well. Subsite S3 accepted preferentially basic amino acids such as Lys and Arg. A broad range of amino acids was accommodated in the remaining subsites. We further explored the acceptance of a Pro residue in the P2 position by cathepsin K in order to develop specific substrates for the enzyme. Two series of peptides with the general sequences Abz-KXPGSKQ-EDDnp and Abz-KPXGSKQ-EDDnp (where X denotes the position of the amino acid that is altered) were synthesized. The substrates Abz-KPRGSKQ-EDDnp and Abz-KKPGSKQ-EDDnp were cleaved by cathepsin K at the Arg-Gly and Gly-Ser bonds respectively, and have been shown to be specific for cathepsin K when compared with other lysosomal cysteine proteases such as cathepsins L and B and with the aspartyl protease cathepsin D.

Molecular mechanisms underlying osteoclast formation and activation

Osteoporosis is one of the leading causes of morbidity in the elderly and is characterized by a progressive loss of total bone mass and bone density. Bone loss in osteoporosis is due to the persistent excess of osteoclastic bone resorption over osteoblastic bone formation. Receptor activator of NFkappaB ligand (RANKL) critically regulates both osteoclast differentiation and activation. TRAFs appear to be central coupling molecules in the signal transduction pathways that regulate osteoclastogenesis, cathepsin K is the major mediator of osteoclastic bone resorption, and sex steroids and aging also affect osteoclastogenesis and osteoclast activity. However, bone homeostasis depends upon the intimate coupling of bone formation and bone resorption, wherein both osteoclasts and osteoblasts exert vital stimulatory and inhibitory effects upon each other via molecules such as RANKL, TGFbeta, PDGF, BMP2, and Mim-1. This review will highlight some of the major features of the complex circuit of cytokines, growth factors, and hormones that underlies the formation and function of osteoclasts and the dynamic equilibrium that marks the interaction between osteoclasts and osteoblasts.

Cleavage site specificity of cathepsin K toward cartilage proteoglycans and protease complex formation

Cathepsin K is a potent extracellular matrix-degrading protease that requires interactions with soluble glycosaminolycans for its collagenolytic activity in bone and cartilage. The major sources of glycosaminoglycans in cartilage are aggrecan aggregates. Therefore, we investigated whether cathepsin K activity is capable to hydrolyze aggrecan into fragments allowing the formation of glycosaminoglycan-cathepsin K complexes and determined the cleavage site specificity of cathepsin K toward the cartilage-resident link protein and aggrecan. The cleavage site specificity was compared with those of cathepsins S and L. All three cathepsins released glycosaminoglycans from native bovine cartilage at lysosomal pH and to a lesser degree at neutral extracellular pH. Cathepsin-predigested aggrecan complexes and cartilage provided suitable glycosaminoglycan fragments that allowed the formation of collagenolytically active cathepsin K complexes. A detailed analysis of the degradation of aggrecan aggregates revealed two cathepsin K cleavage sites in the link protein and several sites in aggrecan, including one site within the interglobular domain E1. In summary, these results demonstrate that cathepsin K is capable to degrade aggrecan complexes at specific cleavage sites and that cathepsin K activity alone is sufficient to self-provide the glycosaminoglycan fragments required for the formation of its collagenolytically active complex.

A potent small molecule, nonpeptide inhibitor of cathepsin K (SB 331750) prevents bone matrix resorption in the ovariectomized rat

Inhibition of the cyteine proteinase, cathepsin K (E.C. 3.4.22.38) has been postulated as a means to control osteoclast-mediated bone resorption. The preferred animal models for evaluation of antiresorptive activity are in the rat. However, the development of compounds that inhibit rat cathepsin K has proven difficult because the human and rat enzymes differ in key residues in the active site. In this study, a potent, nonpeptide inhibitor of rat cathepsin K (K(i) = 4.7 nmol/L), 5-(2-morpholin-4-yl-ethoxy)-benzofuran-2-carboxylic acid ((S)-3-methyl-1-(3-oxo-1-[2-(3-pyridin-2-yl-phenyl)-ethenoyl]-azepan-4-ylcarbanoyl)-butyl)-amide (SB 331750), is described, which is efficacious in rat models of bone resorption. SB 331750 potently inhibited human cathepsin K activity in vitro (K(i) = 0.0048 nmol/L) and was selective for human cathepsin K vs. cathepsins B (K(i) = 100 nmol/L), L (0.48 nmol/L), or S (K(i) = 14.3 nmol/L). In an in situ enzyme assay, SB 331750 inhibited osteoclast-associated cathepsin activity in tissue sections containing human osteoclasts (IC(50) approximately 60 nmol/L) and this translated into potent inhibition of human osteoclast-mediated bone resorption in vitro (IC(50) approximately 30 nmol/L). In vitro, SB 331750 partially, but dose-dependently, prevented the parathyroid hormone-induced hypercalcemia in an acute rat model of bone resorption. To evaluate the ability of SB 331750 to inhibit bone matrix degradation in vivo, it was administered for 4 weeks at 3, 10, or 30 mg/kg, intraperitoneally (i.p.), u.i.d. in the ovariectomized (ovx) rat. Both 10 and 30 mg/kg doses of compound prevented the ovx-induced elevation in urinary deoxypyridinoline and prevented the ovx-induced increase in percent eroded perimeter. Histological evaluation of the bones from compound-treated animals indicated that SB 331750 retarded bone matrix degradation in vivo at all three doses. The inhibition of bone resorption at the 10 and 30 mg/kg doses resulted in prevention of the ovx-induced reduction in percent trabecular area, trabecular number, and increase in trabecular spacing. These effects on bone resorption were also reflected in inhibition of the ovx-induced loss in trabecular bone volume as assessed using microcomputerized tomography (microCT; approximately 60% at 30 mg/kg). Together, these data indicate that the cathepsin K inhibitor, SB 331750, prevented bone resorption in vivo and this inhibition resulted in prevention of ovariectomy-induced loss in trabecular structure.

Localisation and activity of cathepsins K and B in equine osteoclasts

Cathepsin K and cathepsin B were immunolocalised in equine osteoclasts (OC s) present in ex vivo cartilage/subchondral bone samples. Samples were obtained post mortem from the lateral trochlear ridge (LTR) of six horses and ponies aged between 303 days gestation to 8 months. Strong expression of cathepsin K was detected in OC s, particularly those located at the osteochondral junction, apparently involved in the resorption of calcified cartilage. Cathepsin K expression was also detected in hypertrophic chondrocytes and in the endothelial cells of some blood vessels penetrating the hypertrophic zone of cartilage. By contrast, cathepsin B was either absent or present at very low levels in OC s.Osteoclast-like cells (OCL s) were generated in vitro from bone marrow (BM), obtained from the femurs of one horse and two ponies. High levels of cathepsin K activity but only very low levels of cathepsin B activity were demonstrated in OCL s using fluorogenic substrates for these enzymes. The cathepsin K activity could be blocked by the general cysteine proteinase inhibitor, E-64, but not by the cathepsin B inhibitor, CA-074Me. The cathepsin B activity was completely blocked by both CA-074Me and E-64. Taken together, these results suggest that cathepsin K is more important than cathepsin B in the osteoclastic resorption of bone and calcified cartilage of developing equine long bones. Given the apparent importance of cathepsin K in equine endochondral ossification further investigation into the possibility that abnormal expression of this enzyme is involved in the pathogenesis of equine developmental orthopaedic disease is warranted.

Comparison of cathepsins K and S expression within the rheumatoid and osteoarthritic synovium

OBJECTIVE

To determine and compare the expression of cathepsins K and S proteins in joints with rheumatoid arthritis (RA) and osteoarthritis (OA) and to determine the effect of interleukin-1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF alpha) on the expression of cathepsin K in fibroblast-like synoviocytes.

METHOD

Expression and localization of cathepsins K and S were determined by immunohistochemistry in the synovium of 10 RA- and 8 OA-affected joints. Northern and Western blot analyses were performed to analyze cathepsin K and S expression in primary fibroblast-like synoviocyte cultures from RA and OA patients. The effect of IL-1 beta and TNF alpha on the expression and secretion of cathepsin K in primary cultures of synoviocytes was determined by real-time polymerase chain reaction and Western blot analysis. Staining of in situ activity was used to identify active cathepsin K enzyme in primary synovial fibroblast cultures.

RESULTS

Cathepsin K and S protein expression was identified in the synovium from patients with RA and OA. Cathepsin K protein was localized in synovial fibroblasts, stromal multinucleated giant cells, and, to a lesser degree, in CD68+ macrophage-like synoviocytes. Of note is the expression of cathepsin K in synovial fibroblasts and mononuclear macrophage-like cells at sites of cartilage erosion in RA and in interdigitating cells of lymphocyte-rich areas. In contrast, cathepsin S expression was restricted to CD68+ macrophage-like synoviocytes, interdigitating cells, and endothelial cells of blood vessels. Cathepsin K protein expression in the interstitial areas and perivascular regions of RA-derived synovial specimens was 2-5 times higher than in OA samples (P < 0.001), whereas the expression of cathepsin S did not significantly differ in these diseases. Cathepsin K expression levels in normal synovium were low and restricted to fibroblast-like cells. Of note, cathepsin K also was expressed in repairing fibrocartilage in 1 OA specimen. Primary cell cultures of RA- and OA-derived synovial fibroblasts expressed comparable amounts of cathepsin K at the transcript and protein levels. Both cell cultures secreted mature cathepsin K as well as procathepsin K, and expressed active cathepsin K in cytosolic vesicles. In contrast, neither RA- nor OA-derived fibroblasts expressed detectable levels of cathepsin S. IL-1 beta and TNF alpha stimulated the transcript (7-8-fold) and protein expression (2-fold) of cathepsin K (P < 0.05) in primary synovial fibroblast cultures, without differences in expression between RA- and OA-derived synovial fibroblasts.

CONCLUSION

The presence of cathepsin K polypeptide in synovial fibroblasts and macrophage-like cells in normal, OA, and RA synovia suggests a constitutive expression of this protease and a role in synovial remodeling. The comparable increase in cathepsin K expression after stimulation of RA- and OA-derived synovial fibroblasts with IL-1 beta and TNF alpha further suggests that the expression of cathepsin K is independent of cellular alterations leading to the invasive phenotype of RA-synovial fibroblasts. However, the overexpression of cathepsin K in RA synovia due to an increase in the number of cathepsin K-expressing cells identifies this enzyme as a candidate protease for the pathologic degradation of articular cartilage. Cathepsin S expression in macrophage-like synoviocytes suggests dual activity in antigen presentation and matrix degradation in the inflamed synovia.

Estrogen reduces the depth of resorption pits by disturbing the organic bone matrix degradation activity of mature osteoclasts

Decreased E2 levels after menopause cause bone loss through increased penetrative resorption. The reversal effect of E2 substitution therapy is well documented in vivo, although the detailed mechanism of action is not fully understood. To study the effects of E2 on bone resorption, we developed a novel in vitro bone resorption assay in which degradation of inorganic and organic matrix could be measured separately. E2 treatment significantly decreased the depth of resorption pits, although the area resorbed was not changed. Electron microscopy further revealed that the resorption pits were filled with nondegraded collagen, suggesting that E2 disturbed the organic matrix degradation. Two major groups of proteinases, matrix metalloproteinases (MMPs) and cysteine proteinases, have been suggested to participate in organic matrix degradation by osteoclasts. We show here that MMP-9 released a cross-linked carboxyl-terminal telopeptide of type I collagen from bone collagen, and cathepsin K released another C-terminal fragment, the C-terminal cross-linked peptide of type I collagen. E2 significantly inhibited the release of the C-terminal cross-linked peptide of type I collagen into the culture medium without affecting the release of cross-linked carboxyl-terminal telopeptide of type I collagen in osteoclast cultures. These results suggest that organic matrix degradation is initiated by MMPs and continued by cysteine proteases; the latter event is regulated by E2.

Cathepsin k is a critical protease in synovial fibroblast-mediated collagen degradation

Synovial fibroblasts (SFs) play a critical role in the pathogenesis of rheumatoid arthritis (RA) and are directly involved in joint destruction. Both SF-resident matrix metalloproteases and cathepsins have been implicated in cartilage degradation although their identities and individual contributions remain unclear. The aims of this study were to investigate the expression of cathepsin K in SFs, the correlation between cathepsin K expression and disease severity, and the contribution of cathepsin K to fibroblast-mediated collagen degradation. Immunostaining of joint specimens of 21 patients revealed high expression of cathepsin K in SFs in the synovial lining and the stroma of synovial villi, and to a lesser extent in CD68-positive cells of the synovial lining. Cathepsin K-positive SFs were consistently observed at sites of cartilage and bone degradation. Expression levels of cathepsin K in the sublining and vascularized areas of inflamed synovia showed a highly significant negative correlation with results derived from the Hannover Functional Capacity Questionnaire (r = 0.78, P = 0.003; and r = 0.70, P = 0.012, respectively) as a measure of the severity of RA in individual patients. For comparison, there was no correlation between Hannover Functional Capacity Questionnaire and cathepsin S whose expression is limited to CD-68-positive macrophage-like synoviocytes. The expression of cathepsin K was also demonstrated in primary cell cultures of RA-SFs. Co-cultures of SFs on cartilage disks revealed the ability of fibroblast-like cells to phagocytose collagen fibrils whose intralysosomal hydrolysis was prevented in the presence of a potent cathepsin K inhibitor but not by an inhibitor effective against cathepsins L, B, and S. The selective and critical role of cathepsin K in articular cartilage and subchondral bone erosion was further corroborated by the finding that cathepsin K has a potent aggrecan-degrading activity and that cathepsin K-generated aggrecan cleavage products specifically potentiate the collagenolytic activity of cathepsin K toward type I and II collagens. This study demonstrates for the first time a critical role of cathepsin K in cartilage degradation by SFs in RA that is comparable to its well-known activity in osteoclasts.

OPG and PTH-(1-34) have additive effects on bone density and mechanical strength in osteopenic ovariectomized rats

PTH is a potent bone anabolic factor, and its combination with antiresorptive agents has been proposed as a therapy for osteoporosis. We tested the effects of PTH, alone and in combination with the novel antiresorptive agent OPG, in a rat model of severe osteopenia. Sprague Dawley rats were sham-operated or ovariectomized at 3 months of age. Rats were untreated for 15 months, at which time ovariectomy had caused significant decreases in bone mineral density in the lumbar vertebrae and femur. Rats were then treated for 5.5 months with vehicle (PBS), human PTH-(1-34) (80 microg/kg), rat OPG (10 mg/kg), or OPG plus PTH (all three times per wk, sc). Treatment of ovariectomized rats with OPG or PTH alone increased bone mineral density in the lumbar vertebrae and femur, whereas PTH plus OPG caused significantly greater and more rapid increases than either therapy alone (P < 0.05). OPG significantly reduced osteoclast surface in the lumbar vertebrae and femur (P < 0.05 vs. sham or ovariectomized), but had no effect on osteoblast surface at either site. Ovariectomy significantly decreased the mechanical strength of the lumbar vertebrae and femur. In the lumbar vertebrae, OPG plus PTH was significantly more effective than PTH alone at reversing ovariectomy-induced deficits in stiffness and elastic modulus. These data suggest that OPG plus PTH represent a potentially useful therapeutic option for patients with severe osteoporosis.

Inhibition of osteoclast differentiation and bone resorption by cathepsin K antisense oligonucleotides

We confirmed the expression of cathepsin K, the most abundant and specific cysteine protease found in osteoclasts, at the mRNA level in most of our cases of breast cancer, and even at the protein level in bone metastatic lesions. Therefore, we investigated the functions of cathepsin K in osteoclasts with special attention to bone metastasis from breast cancer. Mouse osteoclast-like cells (OCLs) were established by coculture of mouse bone marrow cells and osteoblastic cells. Rodent cathepsin K antisense (AS) or random control (CL) oligonucleotides were added on day 0, 3, or 6 of culture. Tartrate-resistant acid phosphatase staining confirmed the formation of OCLs after 9 d of incubation. AS treatment significantly reduced both the number of TRAP-positive cells and the percentage of multinuclear cells. For the pit-forming assay, after 9 d of incubation, mature OCLs were collected and incubated on ivory slices with AS or CL for 48 h. The antisense oligonucleotides also inhibited the bone-resorbing activity of OCLs. CL treatment did not affect either the number of TRAP-positive cells or pit formation. Cathepsin K may play important roles in bone resorption as well as in differentiation of osteoclasts. These findings indicate that the inhibition of this enzyme may prevent the development of bone metastasis from breast cancer.

New insights into the regulation of cathepsin K gene expression by osteoprotegerin ligand

Cathepsin K plays a key role in bone resorption. We provide the first evidence that osteoprotegerin ligand (OPGL), a critical pro-resorptive cytokine, acutely stimulates the expression of cathepsin K in osteoclasts. We used in situ RT-PCR and real time quantitative RT-PCR to analyze cathepsin K gene expression. OPGL enhanced cathepsin K mRNA levels in mature osteoclasts isolated from rat neonatal long bones. OPGL together with macrophage colony-stimulating factor (M-CSF) also stimulated cathepsin K gene expression in monocytic cells and multinucleate osteoclasts in bone marrow cultures. Real time quantitative RT-PCR demonstrated high levels of cathepsin K mRNA in bone marrow cultures, paralleling the degree of osteoclastogenesis. We therefore suggest that OPGL enhances bone resorption, at least in part, by inducing cathepsin K gene expression.

Mutations of the C-terminal end of cathepsin K affect proenzyme secretion and intracellular maturation

Transfection of the human cathepsin K cDNA into CHO cells results in the expression of mature catalytically active 27-kDa protein and in cells secreting the 39-kDa proenzyme form. Monensin, which neutralizes the pH of acidic organelles, was found to inhibit intracellular processing of the proenzyme and to stimulate its secretion into the culture medium. Brefeldin A caused alterations in immunofluorescence staining consistent with interference of lysosomal targeting and inhibited both intracellular processing and secretion of cathepsin K. Inhibition of glycosylation by tunicamycin also abolished cathepsin K maturation. Furthermore, the processing of the proenzyme to the mature form was abolished by a single mutation of the terminal Met(329) to Ala. The triple mutation of Ser(325), Pro(327), and Met(329) (all to Ala) inhibited both maturation and secretion, using either transient or stable expression systems. The results indicate that intracellular maturation and secretion of cathepsin K can be affected differentially by various treatments and by mutations of the C-terminal end of the protein. These results are consistent with the involvement of both the secreted proenzyme and the intracellularly processed enzyme in cathepsin K-mediated processes.

Regulation of elastinolytic cysteine proteinase activity in normal and cathepsin K-deficient human macrophages

Human macrophages mediate the dissolution of elastic lamina by mobilizing tissue-destructive cysteine proteinases. While macrophage-mediated elastin degradation has been linked to the expression of cathepsins L and S, these cells also express cathepsin K, a new member of the cysteine proteinase family whose elastinolytic potential exceeds that of all known elastases. To determine the relative role of cathepsin K in elastinolysis, monocytes were differentiated under conditions in which they recapitulated a gene expression profile similar to that observed at sites of tissue damage in vivo. After a 12-d culture period, monocyte-derived macrophages (MDMs) expressed cathepsin K in tandem with cathepsins L and S. Though cysteine proteinases are acidophilic and normally confined to the lysosomal network, MDMs secreted cathepsin K extracellularly in concert with cathepsins L and S. Simultaneously, MDMs increased the expression of vacuolar-type H(+)-ATPase components, acidified the pericellular milieu, and maintained extracellular cathepsin K in an active form. MDMs from a cathepsin K-deficient individual, however, retained the ability to express, process, and secrete cathepsins L and S, and displayed normal elastin-degrading activity. Thus, matrix-destructive MDMs exteriorize a complex mix of proteolytic cysteine proteinases, but maintain full elastinolytic potential in the absence of cathepsin K by mobilizing cathepsins L and S.