Expression of cathepsin K messenger RNA in giant cells and their precursors in human osteoarthritic synovial tissues

Cysteine cathepsins: A long and winding road towards clinics

Biomedical research often focuses on properties that differentiate between diseased and healthy tissue; one of the current focuses is elevated expression and altered localisation of proteases. Among these proteases, dysregulation of cysteine cathepsins can frequently be observed in inflammation-associated diseases, which tips the functional balance from normal physiological to pathological manifestations. Their overexpression and secretion regularly exhibit a strong correlation with the development and progression of such diseases, making them attractive pharmacological targets. But beyond their mostly detrimental role in inflammation-associated diseases, cysteine cathepsins are physiologically highly important enzymes involved in various biological processes crucial for maintaining homeostasis and responding to different stimuli. Consequently, several challenges have emerged during the efforts made to translate basic research data into clinical applications. In this review, we present both physiological and pathological roles of cysteine cathepsins and discuss the clinical potential of cysteine cathepsin-targeting strategies for disease management and diagnosis.

Targeting cathepsin K diminishes prostate cancer establishment and growth in murine bone

BACKGROUND

The processes of prostate cancer (PCa) invasion and metastasis are facilitated by proteolytic cascade involving multiple proteases, such as matrix metalloproteinases, serine proteases and cysteine proteases including cathepsin K (CatK). CatK is predominantly secreted by osteoclasts and specifically degrades collagen I leading to bone destruction. PCa and breast cancer preferentially metastasize to the bone. Importantly, CatK expression level is greater in PCa bone metastatic sites compared to primary tumor and normal prostate tissues. However, the underlying mechanism of CatK during PCa metastases into the bone remains to be elucidated. We investigated the functional role of CatK during the PCa establishment and growth process in the murine bone.

METHODS

CatK mRNA expression was validated by RT-PCR, protein expression by immunoblotting in PCa LNCaP, C4-2B, and PC3 cells as well as in PCa tissues. Its protein production was measured using ELISA assay. The effect of both knockdowns via siRNA and CatK inhibitor was compared in regard to PCa cell invasion. We further studied the dose-dependent CatK inhibitor effect on conditioned media-induced bone resorption. In setting up an animal model, C4-2B cells were injected into the tibiae of SCID mice. The animals treated with either vehicle or CatK inhibitor for 8 weeks at the time of tumor cell injection (tumor establishment model; protocol I) or 4 weeks after tumor cell injection (tumor progression model; protocol II) were applied to histological and histomorphometric analyses.

RESULTS

We confirmed CatK expression in PCa LNCaP, C4-2B, and PC3 cells as well as in PCa tissues. Furthermore, we observed the inhibitory effects of a selective CatK inhibitor on PCa cell invasion. The CatK inhibitor dose-dependently inhibited PCa-conditioned media-induced bone resorption. Upon injection of C4-2B cells into the tibiae of SCID mice, the selective CatK inhibitor significantly prevented the tumor establishment in protocol I, and reduced the tumor growth in bone in protocol II. It also decreased serum PSA levels in both animal models. The inhibitory effects of the CatK inhibitor were enhanced in combination with zoledronic acid (ZA).

CONCLUSION

The selective CatK inhibitor may prevent the establishment and progression of PCa in bone, thus making it a novel therapeutic approach for advanced PCa.

Analgesic effects of the cathepsin K inhibitor L-006235 in the monosodium iodoacetate model of osteoarthritis pain

INTRODUCTION

The mounting evidence that osteoclasts play an important role in osteoarthritis (OA) pain lead us to investigate the effects of L-006235, a potent and selective inhibitor of cathepsin K, on pain behaviour and joint pathology in a model of OA pain.

METHODS

Effects of preventative (30 and 100 mg/kg) and therapeutic (100 mg/kg) oral dosing with L-006235 on weight-bearing asymmetry, hind paw withdrawal thresholds, cartilage and bone pathology, synovial inflammation, and drug exposure were studied in the monosodium iodoacetate rat model of OA pain.

RESULTS

Preventative L-006235 inhibited weight-bearing asymmetry from day 14, with this measure nearly abolished by the higher dose. In the same treatment setting, L-006235 prevented lowering of hind paw withdrawal thresholds from day 7. Exposure to L-006235 in plasma was higher for the 100 mg/kg dose, compared with 30 mg/kg. Therapeutic dosing with L-006235 from day 14 significantly inhibited weight-bearing asymmetry, compared with monosodium iodoacetate vehicle rats. Regression analysis revealed a significant interaction coefficient of the effects of L-006235 on weight-bearing asymmetry and synovitis score, but not for cartilage damage nor osteophyte scores.

CONCLUSION

Our novel finding that cathepsin K inhibition is analgesic in a clinically relevant model of OA pain provides new evidence for the therapeutic potential of this target.

Nonclinical and clinical pharmacological characterization of the potent and selective cathepsin K inhibitor MIV-711

BACKGROUND

Cathepsin K is an attractive therapeutic target for diseases in which bone resorption is excessive such as osteoporosis and osteoarthritis (OA). The current paper characterized the pharmacological profile of the potent and selective cathepsin K inhibitor, MIV-711, in vitro and in cynomolgus monkeys, and assessed translation to human based on a single dose clinical study in man.

METHODS

The potency and selectivity of MIV-711 were assessed in vitro using recombinant enzyme assays and differentiated human osteoclasts. MIV-711 was administered to healthy cynomolgus monkeys (3-30 µmol/kg, p.o.). Plasma levels of MIV-711 and the bone resorption biomarker CTX-I were measured after single dose experiments, and urine levels of CTX-I, NTX-I and CTX-II biomarkers were measured after repeat dose experiments. The safety, pharmacokinetics and pharmacodynamics (serum CTX-I) of MIV-711 were assessed in human healthy subjects after single ascending doses from 20 to 600 mg.

RESULTS

MIV-711 was a potent inhibitor of human cathepsin K (K_i: 0.98 nmol/L) with > 1300-fold selectivity towards other human cathepsins. MIV-711 inhibited human osteoclast-mediated bone resorption with an IC₅₀ value of 43 nmol/L. Single oral doses of MIV-711 to monkeys reduced plasma levels of CTX-I in a dose-dependent fashion by up to 57% at trough. The effect on CTX-I was linearly correlated to the plasma exposure of MIV-711, while the efficacy duration outlasted plasma exposure. Repeat oral dosing with MIV-711 also reduced urinary levels of the bone resorption biomarkers CTX-I (by 93%) and NTX-I (by 71%) and the cartilage degradation biomarker CTX-II (by 71%). MIV-711 was safe and well-tolerated when given as single ascending doses to healthy subjects. MIV-711 reduced serum CTX-I levels in a dose-dependent manner by up to 79% at trough. The relationship between MIV-711 exposure and effects on these biomarkers in humans was virtually identical when compared to the corresponding monkey data.

CONCLUSIONS

MIV-711 is a potent and selective cathepsin K inhibitor with dose-dependent effects on biomarkers of bone and cartilage degradation in monkey and human. Taken together, MIV-711 shows promise for the treatment of bone and cartilage related disorders in humans, such as OA. Trial Registration EudraCT number 2011-003024-12, registered on June 22nd 2011.

Anti-CD11b antibody treatment suppresses the osteoclast generation, inflammatory cell infiltration, and autoantibody production in arthritis-prone FcγRIIB-deficient mice

BACKGROUND

Previously we established an arthritis-prone FcγRIIB-deficient mouse strain (designated KO1). Anti-mouse CD11b mAb (5C6) has been reported to inhibit the recruitment of peripheral CD11b⁺ myelomonocytic cells from the blood to the inflammatory site. These cells include neutrophils and monocytes, both of which play important roles in the development of arthritis. Here we treated KO1 mice with 5C6 mAb in order to study its effect on arthritis development.

METHODS

To evaluate the disease-preventive effect of 5C6, 4-month-old preclinical KO1 mice were divided into three groups: the first treated with 5C6 for 6 months, the second treated with normal rat IgG for 6 months, as a control, and the third left untreated. Arthritis severity and immunological abnormalities were compared among the groups, along with transcriptional levels of several important arthritis-related factors in ankle joints, spleen, and peripheral blood cells.

RESULTS

The 5C6 treatment ameliorated arthritis in KO1 mice, showing decreases in inflammatory cell infiltration and osteoclast formation. Analysis of transcriptional levels in ankle joints revealed that compared with the two control groups, the 5C6-treated group showed downregulated expression of RANK, RANKL, MCP-1, RANTES, TNFα, and IL-6, and at the same time showed significantly up-regulated expression of the decoy receptor for RANKL, i.e. osteoprotegerin. In addition, the disease suppression was associated with the lower serum levels of autoantibodies, and the decreased frequencies of activated B cells and plasma cells. The expression levels of B cell activation/differentiation-related cytokines were suppressed in spleen and peripheral leukocytes of the 5C6-treated mice. Intriguingly, while untreated KO1 mice spontaneously developed marked monocytosis, the 5C6-treated mice showed the significantly down-regulated frequency of monocytes.

CONCLUSIONS

The outcome of 5C6 treatment was complex, in which the 5C6-mediated disease-preventive effect is likely due on one hand to the decrease in the recruitment of inflammatory cells and osteoclast precursor monocytes from the periphery into the joints, and on the other hand to the suppression of B cell activation/maturation and of autoantibody production via the suppression of B cell stimulating cytokine production. The lower levels of these cytokines may be the secondary effect of the lower frequency of monocytes, since monocytes/macrophages are the major producers of these cytokines.

Increased expression and activation of cathepsin K in human osteoarthritic cartilage and synovial tissues

Few studies have analyzed Cathepsin K (CatK) expression in human osteoarthritic tissues. We investigated CatK expression and activation in human articular cartilage using clinical specimens. Human osteoarthritic cartilage was obtained during surgery of total hip arthroplasty (n = 10), and control cartilage was from that of femoral head replacement for femoral neck fracture (n = 10). CatB, CatK, CatL, CatS, and Cystatin C (CysC) expressions were evaluated immunohistochemically and by real-time PCR. Intracellular CatK protein was quantified by ELISA. Intracellular CatK activity was also investigated. Osteoarthritis (OA) chondrocytes were strongly stained with CatK, particularly in the superficial layer and more damaged areas. CatB, CatL, CatS, and CysC were weakly stained. CatK mRNA expression was significantly higher in OA group compared to that in control group (p = 0.043), whereas those of CatB, CatL, CatS, and CysC did not differ significantly. Mean CatK concentration (4.83 pmol/g protein) in OA chondrocytes was higher than that (3.91 pmol/g protein) in control chondrocytes (p = 0.001). CatK was enzymatically more activated in OA chondrocytes as compared with control chondrocytes. This study, for the first time, revealed increased CatK expression and activation in human OA cartilage, suggesting possible crucial roles for it in the pathogenesis of osteoarthritic change in articular cartilage.

Osteoarthritic change is delayed in a Ctsk-knockout mouse model of osteoarthritis

OBJECTIVE

Several studies have shown that cathepsin K (CTK) is overexpressed in osteoarthritic (OA) cartilage and subchondral bone. However, it has not been well established whether CTK expression is harmful or beneficial. We undertook this study to investigate the direct involvement of CTK in OA development using Ctsk-knockout (Ctsk(-/-)) mice in a joint instability-induced model of OA.

METHODS

We analyzed the natural course of the phenotype of 25-week-old Ctsk(-/-) mice. OA development was evaluated with a modified Mankin histologic score up to 8 weeks after surgery was performed to destabilize the knee in Ctsk(-/-) and Ctsk(+/+) mice. Histologic analysis was used to evaluate expression of CTK, matrix metalloproteinase 13 (MMP-13), ADAMTS-5, and tartrate-resistant acid phosphatase (TRAP) proteins in chondrocytes, synovial cells, and osteoclasts. Bone architecture was analyzed by histomorphometry.

RESULTS

Bone mineral content and bone volume were higher in Ctsk(-/-) mice at 25 weeks, whereas OA did not develop spontaneously in either Ctsk(-/-) or Ctsk(+/+) mice. In a model of destabilization-induced OA, OA progression was significantly delayed in Ctsk(-/-) mice. CTK was overexpressed in chondrocytes and synovial cells of knee joints developing OA in Ctsk(+/+) mice. MMP-13 and ADAMTS-5 were less strongly expressed in chondrocytes of Ctsk(-/-) mice, and MMP-13 was less strongly expressed in synovial cells. TRAP-positive osteoclasts were overexpressed in Ctsk(-/-) mice.

CONCLUSION

These results indicate that CTK plays crucial direct roles in the early to intermediate stage of OA development. CTK-positive chondrocytes and synovial cells may be a possible target to prevent disease progression in OA.

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.

Inhibition of cathepsin K reduces cartilage degeneration in the anterior cruciate ligament transection rabbit and murine models of osteoarthritis

OBJECTIVE

To investigate the disease modifying effects of cathepsin K (CatK) inhibitor L-006235 compared to alendronate (ALN) in two preclinical models of osteoarthritis (OA).

METHODS

Skeletally mature rabbits underwent sham or anterior cruciate ligament transection (ACLT)-surgery and were treated with L-006235 (L-235, 10 mg/kg or 50 mg/kg, p.o., daily) or ALN (0.6 mg/kg, s.c., weekly) for 8-weeks. ACLT joint instability was also induced in CatK(-/-) versus wild type (wt) mice and treated for 16-weeks. Changes in cartilage degeneration, subchondral bone volume and osteophyte area were determined by histology and μ-CT. Collagen type I helical peptide (HP-I), a bone resorption marker and collagen type II C-telopeptide (CTX-II), a cartilage degradation marker were measured.

RESULTS

L-235 (50 mg/kg) and ALN treatment resulted in significant chondroprotective effects, reducing CTX-II by 60% and the histological Mankin score for cartilage damage by 46% in the ACLT-rabbits. Both doses of L-235 were more potent than ALN in protecting against focal subchondral bone loss, and reducing HP-I by 70% compared to vehicle. L-235 (50 mg/kg) and ALN significantly reduced osteophyte formation in histomorphometric analysis by 55%. The Mankin score in ACLT-CatK(-/-) mice was ~2.5-fold lower than the ACLT-wt mice and was not different from sham-CatK(-/-). Osteophyte development was not different among the groups.

CONCLUSION

Inhibition of CatK provides significant benefits in ACLT-model of OA, including: 1) protection of subchondral bone integrity, 2) protection against cartilage degradation and 3) reduced osteophytosis. Preclinical evidence supports the role of CatK as a potential therapeutic target for the treatment of OA.

Protective effects of a cathepsin K inhibitor, SB-553484, in the canine partial medial meniscectomy model of osteoarthritis

OBJECTIVE

Cathepsin K (cat K), a cysteine protease expressed in osteoclasts, chondrocytes and synovial fibroblasts, degrades several bone and cartilage matrix components suggesting its potential role in osteoarthritis (OA). We investigated the effects of SB-553484, an inhibitor of cat K, on lesion severity and biomarkers of collagen degradation in the canine partial medial meniscectomy model.

METHODS

A partial medial meniscectomy was performed in mature female beagle dogs. Animals were dosed orally with vehicle or SB-553484 at 50mg/kg BID for 28 days. The femorotibial joints were evaluated for gross and microscopic histological changes. Biomarkers of collagen degradation were also analyzed.

RESULTS

In dogs treated with SB-553484, subjective gross and calculated degeneration scores decreased significantly by 29% and 46%, respectively. Histopathologic evaluation demonstrated that the summed tibial degeneration score decreased significantly by 21%. Inhibition of tibial cartilage degeneration was significant in zone 1 (32%) and the depth ratio of any tibial matrix change was decreased significantly by 28%. Urinary biomarkers of bone and cartilage degradation were also significantly reduced.

CONCLUSION

Treatment with SB-553484 resulted in mild to moderate beneficial effects on gross and histopathological parameters. Reduction of biomarkers of collagen type I and II degradation indicated a direct effect of the compound on bone and cartilage. These data suggest that the prevention of cartilage degradation by cat K inhibition may represent a valid strategy for pharmacological intervention in OA and that monitoring collagen degradation biomarkers may provide an indication of the protective effects of inhibition of bone and cartilage degradation.

Bone marrow-derived cathepsin K cleaves SPARC in bone metastasis

Bone metastasis is a hallmark of advanced prostate and breast cancers, yet the critical factors behind attraction of tumors to the skeleton have not been validated. Here, we investigated the involvement of cathepsin K in the progression of prostate tumors in the bone, which occurs both by direct degradation of bone matrix collagen I and by cleavage of other factors in the bone microenvironment. Our results demonstrated that bone marrow-derived cathepsin K is capable of processing and thereby modulating SPARC, a protein implicated in bone metastasis and inflammation. The coincident up-regulation of SPARC and cathepsin K occurred both in vivo in experimental prostate bone tumors, and in vitro in co-cultures of bone marrow stromal cells with PC3 prostate carcinoma cells. PC3-bone marrow stromal cell interaction increased secretion and processing of SPARC, as did co-cultures of bone marrow stromal cells with two other cancer cell lines. In addition, bone marrow stromal cells that were either deficient in cathepsin K or treated with cathepsin K inhibitors had significantly reduced secretion and cleavage of SPARC. Increases in secretion of pro-inflammatory cytokines (ie, interleukin-6, -8) coincident with overexpression of cathepsin K suggest possible mechanisms by which this enzyme contributes to tumor progression in the bone. This is the first study implicating bone marrow cathepsin K in regulation of biological activity of SPARC in bone metastasis.

Cathepsin K regulates adipocyte differentiation: possible involvement of type I collagen degradation

We previously found that cathepsin K (CTSK) played an important role in adipocyte differentiation. However, the underlying molecular mechanism is not clear. Through the time window study, it was observed that CTSK activities were required mainly in the early phases of adipogenic process. At the same time, the expression of type I collagen disappeared. However, type I collagen can still be observed during the whole process when the CTSK inhibitor-E64 was added. The mRNA levels of peroxisome proliferator-activated receptor gamma (PPAR-gamma) and CCAAT/enhancer binding protein alpha (C/EBP-alpha) was also declining. These imply that CTSK may play a role in adipogenesis in early differentiation phases and produce an effect at least partly by degrading type I collagen, which may provides a basis for developing novel therapeutic approaches to treat obesity and the diseases associated with it.

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.

Biochemical properties and regulation of cathepsin K activity

Cysteine cathepsins (11 in humans) are mostly located in the acidic compartments of cells. They have been known for decades to be involved in intracellular protein degradation as housekeeping proteases. However, the discovery of new cathepsins, including cathepsins K, V and F, has provided strong evidence that they also participate in specific biological events. This review focuses on the current knowledge of cathepsin K, the major bone cysteine protease, which is a drug target of clinical interest. Nevertheless, we will not discuss recent developments in cathepsin K inhibitor design since they have been extensively detailed elsewhere. We will cover features of cathepsin K structure, cellular and tissue distribution, substrate specificity, and regulation (pH, propeptide, glycosaminoglycans, oxidants), and its putative roles in physiological or pathophysiological processes. Finally, we will review the kinetic data of its inhibition by natural endogenous inhibitors (stefin B, cystatin C, H- and L-kininogens).

Cathepsin K null mice show reduced adiposity during the rapid accumulation of fat stores

Growing evidences indicate that proteases are implicated in adipogenesis and in the onset of obesity. We previously reported that the cysteine protease cathepsin K (ctsk) is overexpressed in the white adipose tissue (WAT) of obese individuals. We herein characterized the WAT and the metabolic phenotype of ctsk deficient animals (ctsk−/−). When the growth rate of ctsk−/− was compared to that of the wild type animals (WT), we could establish a time window (5–8 weeks of age) within which ctsk−/−display significantly lower body weight and WAT size as compared to WT. Such a difference was not observable in older mice. Upon treatment with high fat diet (HFD) for 12 weeks ctsk−/− gained significantly less weight than WT and showed reduced brown adipose tissue, liver mass and a lower percentage of body fat. Plasma triglycerides, cholesterol and leptin were significantly lower in HFD-fed-ctsk−/− as compared to HFD-fed WT animals. Adipocyte lipolysis rates were increased in both young and HFD-fed-ctsk−/−, as compared to WT. Carnitine palmitoyl transferase-1 activity, was higher in mitochondria isolated from the WAT of HFD treated ctsk−/− as compared to WT. Together, these data indicate that ctsk ablation in mice results in reduced body fat content under conditions requiring a rapid accumulation of fat stores. This observation could be partly explained by an increased release and/or utilization of FFA and by an augmented ratio of lipolysis/lipogenesis. These results also demonstrate that under a HFD, ctsk deficiency confers a partial resistance to the development of dyslipidemia.

The protective effect of licofelone on experimental osteoarthritis is correlated with the downregulation of gene expression and protein synthesis of several major cartilage catabolic factors: MMP-13, cathepsin K and aggrecanases

This study sought to evaluate the levels of mRNA expression and protein synthesis of MMP-13, cathepsin K, aggrecanase-1 (ADAMTS-4), aggrecanase-2 (ADAMTS-5) and 5-lipoxygenase (5-LOX) in cartilage in the experimental anterior cruciate ligament (ACL) dog model of osteoarthritis (OA), and to examine the effects of treatment with licofelone, a 5-lipoxygenase (LOX)/cyclooxygenase (COX) inhibitor, on the levels of these catabolic factors. Sectioning of the ACL of the right knee was performed in three experimental groups: group 1 received no active treatment (placebo group); and groups 2 and 3 received therapeutic concentrations of licofelone (2.5 or 5.0 mg/kg/day orally, respectively) for 8 weeks, beginning the day following surgery. A fourth group consisted of untreated dogs that were used as normal controls. Specimens of cartilage were selected from lesional areas of OA femoral condyles and tibial plateaus, and were processed for real-time quantitative PCR and immunohistochemical analyses. The levels of MMP-13, cathepsin K, ADAMTS-4, ADAMTS-5 and 5-LOX were found to be significantly increased in OA cartilage. Licofelone treatment decreased the levels of both mRNA expression and protein synthesis of the factors studied. Of note was the marked reduction in the level of 5-LOX gene expression. The effects of the drug were about the same at both tested dosages. In vivo treatment with therapeutic dosages of licofelone has been found to reduce the degradation of OA cartilage in experimental OA. This, coupled with the results of the present study, indicates that the effects of licofelone are mediated by the inhibition of the major cartilage catabolic pathways involved in the destruction of cartilage matrix macromolecules. Moreover, our findings also indicate the possible auto-regulation of 5-LOX gene expression by licofelone in OA cartilage.

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.

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.

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.

Involvement of vacuolar H+ -ATPase in incorporation of risedronate into osteoclasts

Although osteoclasts incorporate bisphosphonates during bone resorption, the mechanism of this incorporation by osteoclasts is not known. We previously reported that bisphosphonates disrupt the actin rings (clear zones) formed in normal osteoclasts, but did not disrupt actin rings in osteoclasts derived from osteosclerotic oc/oc mice, which have a defect in the gene encoding vacuolar H(+)-ATPase (V-ATPase). The present study showed that V-ATPase is directly involved in the incorporation of risedronate, a nitrogen containing bisphosphonate, into osteoclasts. Treatment of osteoclasts with risedronate disrupted actin rings and inhibited pit formation by osteoclasts on dentine slices. Bafilomycin A(1), a V-ATPase inhibitor, inhibited the pit-forming activity of osteoclasts but did not disrupt actin rings. Risedronate failed to disrupt actin rings in the presence of bafilomycin A(1). E-64, a lysosomal cysteine proteinase inhibitor, showed no inhibitory effect on the demineralization of dentine by osteoclasts but inhibited the digestion of dentine matrix proteins without disrupting actin rings. Risedronate disrupted actin rings even in the presence of E-64. Treatment of osteoclasts placed on plastic plates with risedronate also disrupted actin rings. Bafilomycin A(1) but not E64 prevented the disruption of actin rings in osteoclasts treated with risedronate on plastic plates. Inhibition of V-ATPase with bafilomycin A(1) also prevented disruption of actin rings by etidronate, a non-nitrogen-containing bisphosphonate. These results suggest that V-ATPase induced acidification beneath the ruffled borders of osteoclasts and subsequent bone demineralization triggers the incorporation of both nitrogen-containing and non-nitrogen-containing bisphosphonates into osteoclasts.

Cathepsin K mRNA and protein expression in prostate cancer progression

Prostate cancer (CaP) is the most commonly diagnosed malignancy in men and is often associated with bone metastases, which cause much of the morbidity associated with CaP. Lesions associated with CaP generally exhibit increased bone formation and resorption. Increased bone resorption may release factors from the extracellular matrix that contribute to tumor growth. Cathepsin K (cat K) is a cysteine protease that exhibits strong degradative activity against the extracellular matrix and is involved in osteoclast-mediated bone destruction. In this study, we analyzed the expression of cat K in CaP cell lines and patient samples. Cat K message was detected in CaP cell lines by reverse transcription-polymerase chain reaction (RT-PCR) and in primary CaP and metastases by in situ hybridization. Immunohistochemistry revealed variable expression of cat K in primary CaP samples, as well as nonosseous metastases, whereas expression in bone metastases was significantly higher than in primary CaP, and normal prostate tissues were negative. Cat K protein was detected in CaP cell lines by Western blotting after immunoprecipitation. Cat K enzymatic activity was also detected in CaP cell lines by a fluorogenic assay and by an assay for degradation of collagen type I. Increased levels of NTx, a marker of bone matrix degradation mediated primarily by cat K, were also detected in sera of patients with CaP bone metastases. We hypothesize that CaP-expressed cat K may contribute to the invasive potential of CaP, while increased expression in bone metastases is consistent with a role in matrix degradation.

Cysteine peptidases of mammals: their biological roles and potential effects in the oral cavity and other tissues in health and disease

Cysteine peptidases (CPs) are phylogenetically ubiquitous enzymes that can be classified into clans of evolutionarily independent proteins based on the structural organization of the active site. In mammals, two of the major clans represented in the genome are: the CA clan, whose members share a structure and evolutionary history with papain; and the CD clan, which includes the legumains and caspases. This review focuses on the properties of these enzymes, with an emphasis on their potential roles in the oral cavity. The human genome encodes at least (but possibly no more than) 11 distinct enzymes, called cathepsins, that are members of the papain family C1A. Ten of these are present in rodents, which also carry additional genes encoding other cathepsins and cathepsin-like proteins. Human cathepsins are best known from the ubiquitously expressed lysosomal cathepsins B, H, and L, and dipeptidyl peptidase I (DPP I), which until recently were considered to mediate primarily "housekeeping" functions in the cell. However, mutations in DPP I have now been shown to underlie Papillon-Lefevre syndrome and pre-pubertal periodontitis. Other cathepsins are involved in tissue-specific functions such as bone remodeling, but relatively little is known about the functions of several recently discovered enzymes. Collectively, CPs participate in multiple host systems that are active in health and in disease. They are involved in tissue remodeling and turnover of the extracellular matrix, immune system function, and modulation and alteration of cell function. Intracellularly, CPs function in diverse processes including normal protein turnover, antigen and proprotein processing, and apoptosis. Extracellularly, they can contribute directly to the degradation of foreign proteins and the extracellular matrix. However, CPs can also participate in proteolytic cascades that amplify the degradative capacity, potentially leading to pathological damage, and facilitating the penetration of tissues by cancer cells. We know relatively little regarding the role of human CPs in the oral cavity in health or disease. Most studies to date have focused on the potential use of the lysosomal enzymes as markers for periodontal disease activity. Human saliva contains high levels of cystatins, which are potent CP inhibitors. Although these proteins are presumed to serve a protective function, their in vivo targets are unknown, and it remains to be discovered whether they serve to control any human CP activity.

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.

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.

Expression of matrix-degrading cysteine proteinase cathepsin K in cholesteatoma

Cholesteatoma is a nonneoplastic lesion of the middle ear space or mastoid that is histologically characterized by a progressive bone erosion of the ossicles and surrounding bone. Several matrix-degrading enzymes have been implicated as mediators of this bone erosion. Because the novel cysteine proteinase cathepsin K has been shown to play a central role in bone resorption, we examined the expression of this enzyme in tissue specimens of cholesteatoma. Tissue specimens of 9 patients with cholesteatoma were obtained during middle-ear surgery. Expression of cathepsin K mRNA was determined by RT-PCR using specific primers. Immunohistochemical analysis of cathepsin K protein expression in tissue sections was performed by using the streptavidin-alkaline phosphatase technique. Expression of both cathepsin K mRNA and protein was detected in areas affected by cholesteatoma, whereas specimens of nonaffected ear cartilage and surrounding tissue were not positive. In addition, cathepsin K was detected in numerous multinucleated giant cells, particularly osteoclasts at the site of bone degradation. In contrast, keratinized squamous epithelium was negative for cathepsin K. These data demonstrate that the matrix-degrading cysteine proteinase cathepsin K may be involved in bone erosion in cholesteatoma. Strong expression of this collagenolytic enzyme in osteoclasts suggests that these cells are mainly involved in cathepsin K-mediated bone destruction.

Acid attack and cathepsin K in bone resorption around total hip replacement prosthesis

Normal bone remodeling and pathological bone destruction have been considered to be osteoclast-driven. Osteoclasts are able to attach to bare bone surface and produce an acidic subcellular space. This leads to acid dissolution of hydroxyapatite, allowing cathepsin K to degrade the organic type I collagen-rich osteoid matrix under the acidic condition prevailing in Howship lacunae. Using a sting pH electrode, the interface membrane around a loosened total hip replacement prosthesis was found to be acidic. Confocal laser scanning disclosed irregular demineralization of the bone surface in contact with the acidic interface. Cathepsin K, an acidic collagenolytic enzyme, was found in interface tissue macrophages/giant cells and pseudosynovial fluid. Tissue extracts contained high levels of cathepsin K messenger RNA (mRNA) and protein. These observations suggest the presence of an acid- and cathepsin K-driven pathological mechanism of bone resorption, mediated not by osteoclasts in subosteoclastic space, but rather by the uncontrolled activity of macrophages in extracellular space.

Cathepsin K--a marker of macrophage differentiation?

Cathepsin K is a cysteine protease with high matrix-degrading activity. Initially, cathepsin K was described as being expressed exclusively by osteoclasts. It was suggested that cathepsin K expression is a specific feature of cells involved in bone remodelling. The aim of this study was to investigate the hypothesis that cathepsin K is expressed not only in bone-resorbing macrophages, but also more generally in specifically differentiated macrophages, such as epithelioid cells and multinucleated giant cells in soft tissues. Specimens obtained from different organs and anatomical locations of patients suffering from sarcoidosis, tuberculosis, granulomas caused by foreign materials, and sarcoid-like lesions were investigated for the expression of cathepsins B, K, and L. Immunohistochemistry and in situ hybridization showed cathepsin K in epithelioid cells and multinucleated giant cells irrespective of the pathological condition and anatomical location, but not in normal resident macrophages. By immunoelectron microscopy, cathepsin K was discovered in cytoplasmic granules of multinucleated giant cells. In contrast, cathepsin B and cathepsin L were expressed ubiquitously in CD68-positive tissue macrophages, epithelioid cells, and multinucleated giant cells. The results demonstrate that cathepsin K, but not cathepsin B or cathepsin L, differentiates specific phenotypes of macrophages independently of the anatomical site. Its enzymatic characteristics, particularly its high matrix-degrading activity, suggest that cathepsin K-positive epithelioid cells and multinucleated giant cells are characterized by an enhanced specific proteolytic capability.

Expression of cysteine proteinases cathepsins B and K and of cysteine proteinase inhibitor cystatin C in giant cell tumor of tendon sheath

The expression of cysteine proteinases cathepsins B and K and of the endogenous inhibitor of cysteine proteinases, cystatin C, was investigated in tissue specimens of patients with giant cell tumor of tendon sheath (GCTTS). Expression of both enzymes was examined by immunohistochemistry in tissue specimens of 14 patients with GCTTS. Applying double-labeling techniques, the coexpression of cathepsin B and its major endogenous inhibitor cystatin C was additionally studied. Cells expressing the respective proteins were further characterized with the macrophage markers HAM56 and anti-CD68 (clone PG-M1). Cathepsin B could be detected in numerous HAM56-positive mononuclear cells (MC), but only in very few giant cells (GC). In contrast, cathepsin K was predominantly identified in GC that were also strongly immunoreactive for cystatin C and CD68. Coexpression of cathepsin B and cystatin C occurred only in a few MC. The strong expression of both cathepsin B and K suggests that in GCTTS, bone erosion might be mediated not only by pressure of the proliferative tissue, but also by matrix-degrading cysteine proteinases. Because previous studies showed that osteoclasts express high levels of CD68, cathepsin K, and cystatin C but not of cathepsin B, our study contributes to the view that GC of GCTTS and osteoclasts are closely associated.

Pro-inflammatory activity of contaminating DNA in hyaluronic acid preparations

Hyaluronic acid (HA), an abundant non-sulfated glycosaminoglycan component of the extracellular matrix, has applications in drug delivery, tissue engineering and as an ingredient in cosmetics. HA preparations containing high-molecular-weight polymers are also used in the treatment of inflammatory disorders such as arthritis and interstitial cystitis. Low-molecular-weight fragments derived from HA have been reported to induce pro-inflammatory cytokines such as IL-12 and TNF-alpha, and could therefore potentially exacerbate existing inflammation. We therefore examined the pro-inflammatory activity of HA preparations, since inflammatory reactions are known to occur following administration of HA. We tested low-molecular-weight fragments obtained from seven different HA preparations, either by sonication (approximately equals 3 x 10(5) Da) or by hyaluronidase digestion (approximately equals 1 x 10(4) Da), for the ability to induce the synthesis of IL-12 and TNF-alpha by human monocytic cells. We found that two of the seven HA preparations tested stimulated the synthesis of IL-12 and TNF-alpha by human monocytic cells. We unexpectedly found that the induction of IL-12 and TNF-alpha by these HA preparations was not due to their degradation to low-molecular-weight fragments, since their native high-molecular-weight forms possessed the same ability to stimulate IL-12 and TNF-alpha synthesis, but was due to the presence of contaminating DNA. Treatment of these two HA preparations with deoxyribonuclease I abrogated or reduced the induction of IL-12 and TNF-alpha. It is clear from this study that HA preparations can induce the synthesis of pro-inflammatory cytokines by monocytes. The ability of HA to act as a pro-inflammatory mediator may not, however, be related to the presence of low-molecular-weight HA fragments, but to the presence of DNA. The presence of pro-inflammatory DNA in HA preparations should be evaluated before its use, not only for the treatment of patients with inflammatory disorders, but also before many other applications.

Effects of intraarticular glucocorticoids on macrophage infiltration and mediators of joint damage in osteoarthritis synovial membranes: findings in a double-blind, placebo-controlled study

OBJECTIVE

To investigate the effects of intraarticular glucocorticoid treatment on macrophage infiltration, the expression of the chemokines monocyte chemoattractant protein 1 (MCP-1) and macrophage inflammatory protein 1alpha (MIP-1alpha), and the expression of matrix metalloproteinases 1 and 3 (MMPs 1 and 3) and their inhibitors, the tissue inhibitors of metalloproteinases 1 and 2 (TIMPs 1 and 2), in osteoarthritis (OA) synovial membranes.

METHODS

Forty patients underwent arthroscopic biopsy before and 1 month after intraarticular injection of glucocorticoids. Twenty-one patients received 120 mg of methylprednisolone acetate (Depo-Medrol; Upjohn, Kalamazoo, MI), and 20 patients received placebo (1 patient received placebo in 1 knee and methylprednisolone acetate in the other). Immunoperoxidase staining for the expression of CD68, MCP-1, MIP-1alpha, MMP-1, MMP-3, TIMP-1, and TIMP-2 was performed, and the immunostaining was quantified by color video image analysis.

RESULTS

CD68, MCP-1, MIP-1alpha, MMP-1, MMP-3, TIMP-1, and TIMP-2 immunostaining was observed in all synovial membranes. Intraarticular glucocorticoid treatment was associated with a small (30%) but statistically significant (P = 0.048) reduction in CD68+ macrophage staining in the synovial lining layer, but there was no change in the CD68 expression in the synovial sublining layer. No significant differences were observed for MCP-1, MIP-1alpha, MMP-1, MMP-3, TIMP-1, and TIMP-2 immunostaining in the synovial lining or sublining layers.

CONCLUSION

Intraarticular glucocorticoids may reduce CD68+ macrophage infiltration into the synovial lining layer, but not the expression of MCP-1, MIP-1alpha, MMP-1, MMP-3, TIMP-1, and TIMP-2 in the synovial membrane, in patients with OA.

Articular cartilage and changes in arthritis: matrix degradation

While many proteases in articular cartilage have been described, current studies indicate that members of two families of metalloproteases - MMPs and the ADAMTSs - are responsible for the degradation of the major components of this tissue. Collagenases (MMPs) make the first cleavage in triple-helical collagen, allowing its further degradation by other proteases. Aggrecanases (ADAMTSs), in conjunction with other MMPs, degrade aggrecan, a component of the proteoglycan aggregate. Antineoepitope antibodies that recognize the cleavage products of collagen and aggrecan generated by these enzymes are now available and are being used to detect the sites of action and to quantitate degradation products.

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.