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

The tree shrew as a new animal model for the study of periodontitis

AIM

Periodontitis is an inflammatory, infectious disease of polymicrobial origin that can damage tooth-supporting bone and tissue. Tree shrews, evolutionarily closer to humans than commonly used rodent models, have been increasingly used as biomedical models. However, a tree shrew periodontitis model has not yet been established.

MATERIALS AND METHODS

Periodontitis was induced in male tree shrews/Sprague-Dawley rats by nylon thread ligature placement around the lower first molars. Thereafter, morphometric and histological analyses were performed. The distance from the cemento-enamel junction to the alveolar bone crest was measured using micro-computed tomography. Periodontal pathological tissue damage, inflammation and osteoclastogenesis were assessed using haematoxylin and eosin staining and quantitative immunohistochemistry, respectively.

RESULTS

Post-operatively, gingival swelling, redness and spontaneous bleeding were observed in tree shrews but not in rats. After peaking, bone resorption decreased gradually until plateauing in tree shrews. Contrastingly, rapid and near-complete bone loss was observed in rats. Inflammatory infiltrates were observed 1 week post operation in both models. However, only the tree shrew model transitioned from acute to chronic inflammation.

CONCLUSIONS

Our study revealed that a ligature-induced tree shrew model of periodontitis partly reproduced the pathological features of human periodontitis and provided theoretical support for using tree shrews as a potential model for human periodontitis.

Calcium phosphate-based materials regulate osteoclast-mediated osseointegration

Calcium phosphate-based materials (CaP) have been widely used as bone graft substitutes with a decent osseointegration. However, the mechanism whereby cells function and repair the bone defect in CaP micro-environment is still elusive. The aim of this study is to find the mechanism how osteoclast behaviors mediate bone healing with CaP scaffolds. Recent reports show that behaviors of osteoclast are closely related with osteogenesis, thus we make a hypothesis that active osteoclast behaviors induced by CaP facilitate bone healing. Here, we found a new mechanism that CaP can regulate osteoclast-mediated osseointegration. Calcium phosphate cement (CPC) is selected as a representative CaP. We demonstrate that the osteoclast-mediated osseointegration can be strongly modulated by the stimulation with CaP. An appropriate Ca/P ratio in CaP can effectively promote the RANKL-RANK binding and evoke more activated NF-κB signaling transduction, which results in vigorous osteoclast differentiation. We observe significant improvement of bone healing in vivo, owing to the active coupling effect of osteoclasts. What is more noteworthy is that the phosphate ions released from CaP can be a pivotal role regulating osteoclast activity by changing Ca/P ratio readily in materials. These studies suggest the potential of harnessing osteoclast-mediated osteogenesis in order to develop a materials-manipulated approach for improving osseointegration.

1.

Calcium phosphate-based materials (CaP) can directly participate in bone healing by released ions.

2.

Excessive phosphate ions released from CaP can inhibit the affinity of RANKL and RANK.

3.

Altering Ca/P ratio in CaP can significantly regulate osteoclast differentiation and function through RANKL-RANK dependent NF-κB signaling pathway.

Enantioselective Synthesis of γ-Phenyl-γ-amino Vinyl Phosphonates and Sulfones and Their Application to the Synthesis of Novel Highly Potent Antimalarials

Racemic and enantioselective syntheses of γ-phenyl-γ-amino vinyl phosphonates and sulfones have been achieved using Horner-Wadsworth-Emmons olefination of trityl protected α-phenyl-α-amino aldehydes with tetraethyl methylenediphosphonate and diethyl ((phenylsulfonyl)methyl)phosphonate, respectively, without any racemization. The present strategy has also been successfully applied to the synthesis of peptidyl vinyl phosphonate and peptidyl vinyl sulfone derivatives as potential cysteine protease inhibitors of Chagas disease, K11002, with 100% de. The developed synthetic protocol was further utilized to synthesize hybrid molecules consisting of artemisinin as an inhibitor of major cysteine protease falcipain-2 present in the food vacuole of the malarial parasite. The synthesized artemisinin-dipeptidyl vinyl sulfone hybrid compounds showed effective in vitro inhibition of falcipain-2 and potent parasiticidal efficacies against Plasmodium falciparum in nanomolar ranges. Overall, the developed synthetic protocol could be effectively utilized to design cysteine protease inhibitors not only as novel antimalarial compounds but also to be involved in other life-threatening diseases.

Osteoblast-Osteoclast Coculture Amplifies Inhibitory Effects of FG-4592 on Human Osteoclastogenesis and Reduces Bone Resorption

The link between bone and blood vessels is regulated by hypoxia and the hypoxia‐inducible transcription factor, HIF, which drives both osteogenesis and angiogenesis. The recent clinical approval of PHD enzyme inhibitors, which stabilize HIF protein, introduces the potential for a new clinical strategy to treat osteolytic conditions such as osteoporosis, osteonecrosis, and skeletal fracture and nonunion. However, bone‐resorbing osteoclasts also play a central role in bone remodeling and pathological osteolysis, and HIF promotes osteoclast activation and bone loss in vitro. It is therefore likely that the result of PHD enzyme inhibition in vivo would be mediated by a balance between increased bone formation and increased bone resorption. It is essential that we improve our understanding of the effects of HIF on osteoclast formation and function and consider the potential contribution of inhibitory interactions with other musculoskeletal cells. The PHD enzyme inhibitor FG‐4592 stabilized HIF protein and stimulated osteoclast‐mediated bone resorption, but inhibited differentiation of human CD14+ monocytes into osteoclasts. Formation of osteoclasts in a more physiologically relevant 3D collagen gel did not affect the sensitivity of osteoclastogenesis to FG‐4592, but increased sensitivity to reduced concentrations of RANKL. Coculture with osteoblasts amplified inhibition of osteoclastogenesis by FG‐4592, whether the osteoblasts were proliferating, differentiating, or in the presence of exogenous M‐CSF and RANKL. Osteoblast coculture dampened the ability of high concentrations of FG‐4592 to increase bone resorption. These data provide support for the therapeutic use of PHD enzyme inhibitors to improve bone formation and/or reduce bone loss for the treatment of osteolytic pathologies and indicate that FG‐4592 might act in vivo to inhibit the formation and activity of the osteoclasts that drive osteolysis. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Effect of Nano-Montmorillonite on Osteoblast Differentiation, Mineral Density, and Osteoclast Differentiation in Bone Formation

Calcium-type montmorillonite, a phyllosilicate mineral, has diverse health benefits when introduced into the gastrointestinal tract or applied to the skin. However, the predominant use of this layered material has thus far been in traditional industries, despite its potential application in the pharmaceutical industry. We investigated the effects and mechanism of nano-montmorillonite (NM) on osteoblast and osteoclast differentiation in vivo and in vitro. We examined the osteogenic effects of NM with high calcium content (3.66 wt%) on alkaline phosphatase (ALP) activity, mineralization, bone microarchitecture, and expression level of osteoblast and osteoclast related genes in Ca-deficient ovariectomized (OVX) rats. Micro-computed tomography of OVX rats revealed that NM attenuated the low-Ca-associated changes in trabecular and cortical bone mineral density. It improved ALP activity and mineralization, as well as the expression of osteoblast and osteoclast differentiation associated genes. NM also activated the expression of runt-related transcription factor 2, osteocalcin, bone morphogenetic protein 2, and type 1 collagen via phosphorylated small mothers against decapentaplegic homolog 1/5/8 signaling. Further, NM repressed the expression of receptor activator for cathepsin K, nuclear factor kappa-B ligand and tartrate-resistant acid phosphatase. Therefore, NM inhibits osteoclastogenesis, stimulates osteoblastogenesis, and alleviates osteoporosis.

A Mild Inhibition of Cathepsin K Paradoxically Stimulates the Resorptive Activity of Osteoclasts in Culture

Cathepsin K (CatK) inhibition allows reducing bone resorption with specific advantages compared to the existing anti-osteoporosis drugs. Its clinical use appears even more promising with the recent development of ectosteric inhibitors. A confusing observation, however, is that a low dose of the active site CatK inhibitor odanacatib (ODN) was reported to decrease bone mineral density and increase serum levels of the bone resorption marker carboxy-terminal collagen crosslinks (CTX). The present study provides a possible explanation for this paradox. The resorptive activity of human osteoclasts seeded on bone slices was inhibited when subjected to ODN at doses of 20 nM, but about 100-fold lower doses induced a significant increase in CTX levels and in eroded surface (12 repeats). This low-dose-induced stimulation was prevented by inhibition of non-CatK cysteine proteinases, thereby indicating that the stimulation results from an interplay between CatK and other cysteine proteinases. Effective interplay between these proteinases was also shown in enzymatic assays where the CatK-mediated degradation of collagen was enhanced upon addition of cathepsins B or L. Furthermore, extracts of osteoclasts subjected to a low dose of ODN showed higher levels of cathepsin B compared with extracts of control osteoclasts. In conclusion, the low-dose-induced stimulation of resorption observed in the clinical study can be reproduced in osteoclasts cultured in the absence of any other cell. Our data support an osteoclast-intrinsic mechanism where a mild inhibition of CatK results in increased levels of other proteinases contributing to the collagen degradation process.

An Ectosteric Inhibitor of Cathepsin K Inhibits Bone Resorption in Ovariectomized Mice

The potent cathepsin K (CatK) inhibitor, Tanshinone IIA sulfonic sodium (T06), was tested for its in vitro and in vivo antiresorptive activities. T06 binds in an ectosteric site of CatK remote from its active site and selectively inhibits collagen degradation with an IC₅₀ value of 2.7 ± 0.2 μM (CatK:T06 molar ratio of 1:5). However, it does not suppress fluorogenic peptide cleavage and gelatinolysis at a 2500-fold molar excess. Contrary to active site-directed CatK inhibitors, such as odanacatib, T06 suppresses bone resorption in both human and mouse osteoclasts equally well (IC₅₀ value for human and mouse osteoclasts: 237 ± 60 nM and 245 ± 55 nM, respectively) and its antiresorptive activity is fully reversible in both cell types. Moreover, T06 affects neither the metabolic activity of osteoclasts nor osteoclastogenesis. In in vivo studies, 40 mg T06/kg/d given to 12-week-old ovariectomized (OVX) mice for 3 months reduced plasma CTx-1 by 20% and increased osteoblast numbers and plasma P1NP by ∼28% when compared with the OVX control. μCT analysis of T06-treated OVX mice showed a 35% increase in bone mineral density and other femoral trabecular bone parameters when compared with OVX animals. T06 did not alter the number of osteoclasts, had no estrogenic effect on the uterus, did not change plasma estradiol levels, and did not inhibit fibroblast-mediated TGF-ß1 processing or degradation and cognitive functions in OVX mice. This study indicates that the ectosteric inhibitor, T06, is a selective antiresorptive CatK inhibitor that may overcome the shortcomings of side effect-prone active site-directed drugs, which all failed in clinical trials. © 2017 American Society for Bone and Mineral Research.

Galactosidase Enzyme Fragment Complementation as a High-Throughput Screening Protease Technology

The authors describe a homogeneous, high-throughput screening (HTS) assay for measuring protease activity and detection of inhibitors. The assay comprises a cyclic β-galactosidase (β-gal) enzyme donor peptide (ED) containing a protease-selective cleavage sequence. Alone, the cyclic peptide is inactive, but when linearized following protease cleavage, ED complements with β-gal enzyme acceptor forming active β-gal enzyme. This then catalyzes the formation of either fluorescent or chemiluminescent products, with β-gal turnover providing a highly amplified signal, and thus an assay technology of high sensitivity. To demonstrate the utility of the technology, an EFC assay was developed to measure the activity of 2, caspase 3 and β-secretase. Using a cyclic ED containing the caspase 3 substrate sequence, DEVD, the EFC assay signal was linear with respect to caspase 3 concentration. The assay was very sensitive, being able to detect activity at low picogram amounts of caspase 3. For the β-secretase (BACE) EFC assay, a cyclic ED containing the Swedish mutant cleavage site of amyloid precursor protein (APP), SEVNLDAEFK, was used. In a similar fashion to the caspase 3 assay, the signal induced by BACE activity was linear with respect to enzyme concentration and was highly sensitive, being able to detect nanogram quantities of BACE. The assay was also more sensitive than a commercially available FRET-based assay of BACE activity. It is concluded that the EFC protease assay is a simple, flexible, and sensitive technology for HTS of proteases.

From disease to treatment: from rare skeletal disorders to treatments for osteoporosis

During the past 15 years there has been an expansion of our knowledge of the cellular and molecular mechanisms regulating bone remodeling that identified new signaling pathways fundamental for bone renewal as well as previously unknown interactions between bone cells. Central for these developments have been studies of rare bone disorders. These findings, in turn, have led to new treatment paradigms for osteoporosis some of which are at late stages of clinical development. In this article, we review three rare skeletal disorders with case descriptions, pycnodysostosis and the craniotubular hyperostoses sclerosteosis and van Buchem disease that led to the development of cathepsin K and sclerostin inhibitors, respectively, for the treatment of osteoporosis.

Matrix-metalloproteinase-9 is cleaved and activated by cathepsin K

Background

Matrix-metalloproteinases 9 (MMP-9) belongs to the class of matrix metalloproteinases whose main function is to degrade and remodel the extracellular matrix (ECM). MMP-9 has been shown to be an integral part of many diseases where modulation of the ECM is a key step such as cancer, osteoporosis and fibrosis. MMP-9 is secreted as a latent pro-enzyme that requires activation in the extracellular space. Therefore, identifying physiological and molecular contexts, which can activate MMP-9 is important.

Results

Acidification of osteoclast-conditioned media to pH 5 resulted in a fragment with a size corresponding to active MMP-9. Also, treatment of recombinant proMMP-9 with recombinant cathepsin K (CTSK) at pH 5 yielded a fragment that corresponded to the molecular weight of active MMP-9, and showed MMP-9 activity. This activation was abrogated in the presence of CTSK inhibitor indicating that CTSK was responsible for the activation of pro-MMP-9. Knocking down CTSK in MDA-MB-231 cells also diminished MMP-9 activity compared to wild type control.

Conclusions

Here we provide the first evidence that CTSK can cleave and activate MMP-9 in acidic environments such as seen in tumors and during bone resorption. This finding provides a key link between CTSK expression in tumors and bone and ECM remodeling, through MMP-9 activation. This novel mechanism to activate MMP-9 through extracellular physiological changes elucidated in this study reveals a protease-signaling network involving CTSK and MMP-9 and provides the impetus to explore ECM proteases as physiological markers and pharmacological targets.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-015-1284-8) contains supplementary material, which is available to authorized users.

Inhibition of Osteoclast Differentiation by Ginsenoside Rg3 in RAW264.7 Cells via RANKL, JNK and p38 MAPK Pathways Through a Modulation of Cathepsin K: An In Silico and In Vitro Study

Various studies have demonstrated that overexpression of cathepsin K (Cat-K) causes excessive bone loss, which ultimately leads to a variety of bone diseases including osteoporosis. Therefore, inhibition of Cat-K signifies a potential therapeutic target in osteoporosis treatment. Ginsenoside Rg3 is one of the most promising compound of Panax ginseng Meyer (P. ginseng) with numerous biological activities. Thus, in recent study the inhibitory effect of Rg3 isolated from P. ginseng was investigated in order to impede the osteoclast activity by an in silico approach followed by in vitro study validation using RAW264.7 cells through the investigation of different biological activity prediction such as absorption distribution metabolism and excretion (ADMET) properties against Cat-K protein. The docking results of our study showed that Rg3 is a non-toxic compound and may act as a drug-like molecule. Additionally, the molecular interaction of Rg3 with the active residues of Cat-K markedly describes its inhibitory effects on osteoclastogenesis. Findings of the present study exhibited that Rg3 significantly reduced receptor activator of nuclear factor kappa B ligand (RANKL)-induced tartrate-resistant acid phosphatase (TRAP) activity, pit formation (actin rings), and TRAP-positive multinucleated cells development in RAW264.7 cells. Furthermore, Rg3 dose-dependently reduced the mRNA expression levels of osteoclast-specific markers such as RANK, TRAP, and Cat-K induced by RANKL through the down regulation of p38, extracellular signal-regulated kinase, and c-Jun N-terminal kinase (JNK) pathways. In conclusion, in silico docking study and in vitro validation together suggested that Rg3 inhibits osteoclastogenesis and reduces bone resorption through the inhibition of Cat-K. Therefore, Rg3 might be a useful therapeutic agent for the treatment of osteoporosis and proper bone formation. Copyright © 2015 John Wiley & Sons, Ltd.

New antiresorptive therapies for postmenopausal osteoporosis

Osteoporosis is a systemic skeletal disease whose risk increases with age and it is common among postmenopausal women. Currently, almost all pharmacological agents for osteoporosis target the bone resorption component of bone remodeling activity. Current antiresorptive agents are effective, but the effectiveness of some agents is limited by real or perceived intolerance, longterm adverse events (AEs), coexisting comorbidities, and inadequate long-term adherence. New antiresorptive therapies that may expand options for the prevention and treatment of osteoporosis include denosumab, combination of conjugated estrogen/bazedoxifene and cathepsin K inhibitors. However, the long-term efficacy and AEs of these antiresorptive therapies need to be confirmed in studies with a longer follow-up period.

Osteoprotective effect of cordycepin on estrogen deficiency-induced osteoporosis in vitro and in vivo

The purpose of this study was to verify the effect of cordycepin on ovariectomized osteopenic rats. Fifty Wistar female rats used were divided into 5 groups: (1) sham-operation rats (control), (2) ovariectomized (OVX) rats with osteopenia, and (3) OVX'd rats with osteopenia treated with cordycepin (5 mg, 10 mg, and 20 mg) for 8 weeks. After the rats were treated orally with cordycepin, serum alkaline phosphatase (ALP), tartrate resistant acid phosphatase (TRAP), serum osteocalcin (OC), homocysteine (HCY) , C-terminal crosslinked telopeptides of collagen type I (CTX) level, and oxidative stress were examined, respectively. The femoral neck was used for mechanical compression testing. At the same time, we further investigated the effect of cordycepin in vitro assay. The beneficial effects of cordycepin on improvement of osteoporosis in rats were attributable mainly to decrease ALP activity, TRAP activity, and CTX level. At the same time, cordycepin also increases the OC level in ovariectomized osteopenic rats. The histological examination clearly showed that dietary cordycepin can prevent bone loss caused by estrogen deficiency. These experimental results suggest that complement cordycepin is protective after ovariectomized osteopenic in specific way.

[Cathepsin K antagonists: preclinical and clinical data]

Cathepsin K, a cysteine protease, is an essential enzyme in degradation of collagen type I. Since cathepsin K is relatively specific to osteoclasts, it represents a promising candidate for drug development. In the past decades, efforts have been made in developing highly potent, selective and orally applicable cathepsin K inhibitors. In contrast to balicatib and relacatib, whose drug development programmes were stopped due to cutaneous side-effects related to limited drug specificity, the more specific cathepsin K inhibitors odanacatib (ODN) and ONO-5334 have entered clinical trials. Odanacatib progressively increases bone mineral density (BMD) and decreases bone resorption markers in postmenopausal women with low BMD. Its clinical efficacy and safety was confirmed by several clinical studies but indicates that odanacatib is characterized by a resolution-of-effect with increases in bone resorption and rapid decreases in BMD following treatment discontinuation. A phase III fracture prevention study in postmenopausal women with osteoporosis is currently in the final phase.

Salvia miltiorrhiza: an ancient Chinese herbal medicine as a source for anti-osteoporotic drugs

ETHNOPHARMACOLOGICAL RELEVANCE

Red sage (Salvia miltiorrhiza Bunge), also known as Danshen in Chinese, has been used historically and is currently exploited in combination with other herbs to treat skeletal diseases in traditional Chinese medicine (TCM). With the advance of modern analytical technology, a multitude of bone-targeting, pharmaceutically active, compounds has been isolated and characterized from various sources of TCM including those produced in Salvia miltiorrhiza root. The aim of the review is to provide a comprehensive overview about the historical TCM interpretation of the action of Salvia miltiorrhiza in osteoporosis, its use clinical trials, its main phytochemical constituents, and its action on bone-resorptive and bone formation-stimulating mechanisms in in vitro and in vivo studies.

MATERIALS AND METHODS

Literature sources used were Pubmed, CNKI.net, Cqvip.com, PubChem, and the Web of Science. For the inquiry, keywords such as Salvia, danshen, osteoporosis, bone, osteoclast and osteoblast were used in various combinations. About 130 research papers and reviews were consulted.

RESULTS

In TCM, the anti-osteopororotic effect of Salvia miltiorrhiza is ascribed to its action on liver and blood stasis as main therapeutic targets defining osteoporosis. 36 clinical trials were identified which used Salvia miltiorrhiza in combination with other herbs and components to treat post-menopausal, senile, and secondary osteoporosis. On average the trials were characterized by high efficacy (>80%) and low toxicity problems. However, various limitations such as small patient samples, short treatment duration, frequent lack of detailed numerical data, and no clear endpoints must be taken into consideration. To date, more than 100 individual compounds have been isolated from this plant and tested in various animal models and biochemical assays. Compounds display anti-resorptive and bone formation-stimulating features targeting different pathways in the bone remodeling cycle. Pathways affected include the activation of osteoblasts, the modulation of osteoclastogenesis, and the inhibition of collagen degradation by cathepsin K.

CONCLUSIONS

The inclusion of Salvia miltiorrhiza in more than 30% of all herbal clinical trials successfully targeting osteoporosis has stimulated significant interest in the identification and characterization of individual constituents of this herb. The review highlights the anti-osteoporotic potential of Salvia miltiorrhiza in clinical applications and the potential of the herb to provide potent compounds targeting specific pathways in bone resorption and bone formation.

Inhibition of cathepsin activity in a cell-based assay by a light-activated ruthenium compound

Light-activated inhibition of cathepsin activity was demonstrated in a cell-based assay. Inhibitors of cathepsin K, Cbz-Leu-NHCH2 CN (2) and Cbz-Leu-Ser(OBn)-CN (3), were caged within the complexes cis-[Ru(bpy)2 (2)2 ]Cl2 (4) and cis-[Ru(bpy)2 (3)2 ](BF4 )2 (5) (bpy=2,2'-bipyridine) as 1:1 mixtures of Δ and Λ stereoisomers. Complexes 4 and 5 were characterized by (1) H NMR, IR, and UV/Vis spectroscopies and electrospray mass spectrometry. Photochemical experiments confirm that 4 releases two molecules of 2 upon exposure to visible light for 15 min, whereas release of 3 by 5 requires longer irradiation times. IC50 determinations against purified cathepsin K under light and dark conditions with 4 and 5 confirm that inhibition is enhanced from 35- to 88-fold, respectively, upon irradiation with visible light. No apparent toxicity was observed for 4 in the absence or presence of irradiation in bone marrow macrophage (BMM) or PC3 cells, as determined by MTT assays, at concentrations up to 10 μM. Compound 5 is well tolerated at lower concentrations (<1 μM), but does show growth-inhibitory effects at higher concentrations. Confocal microscopy experiments show that 4 decreases intracellular cathepsin activity in osteoclasts with light activation. These results support the further development of caged nitrile-based inhibitors as chemical tools for investigating spatial aspects of proteolysis within living systems.

New therapeutics for osteoporosis

Two new approaches for the treatment of osteoporosis are summarized, each having arisen out of important new discoveries in bone biology. Odanacatib (ODN) inhibits the enzyme, cathepsin K, that is essential for the resorbing activity of osteoclasts. It is effective in preventing ovariectomy-induced bone loss in preclinical studies, and a phase II clinical study has shown inhibition of resorption sustained over five years. Outcome of a phase III study is awaited. The finding from mouse and human genetics that Wnt signaling is a powerful inducer of bone formation led to developments aimed at enhancing this pathway. Of the several approaches towards this, the most advanced is with a neutralizing antibody against sclerostin, the osteocyte-derived inhibitor of Wnt signaling. Preclinical studies show a powerful bone anabolic effect, and a clinical phase II study shows dose-dependent increases in bone formation and decreases in bone resorption markers.

Implications of osteoblast-osteoclast interactions in the management of osteoporosis by antiresorptive agents denosumab and odanacatib

Antiresorptive agents, used in the treatment of osteoporosis, inhibit either osteoclast formation or function. However, with these approaches, osteoblast activity is also reduced because of the loss of osteoclast-derived coupling factors that serve to stimulate bone formation. This review discusses how osteoclast inhibition influences osteoblast function, comparing the actions of an inhibitor of osteoclast formation [anti-RANKL/Denosumab (DMAB)] with that of a specific inhibitor of osteoclastic cathepsin K activity [Odanacatib (ODN)]. Denosumab rapidly and profoundly, but reversibly, reduces bone formation. In contrast, preclinical studies and clinical trials of ODN showed that bone formation at some skeletal sites was preserved although resorption was reduced. This preservation of bone formation appears to be due to effects of coupling factors, secreted by osteoclasts and released from demineralized bone matrix. This indicates that bone resorptive activities of osteoclasts are separable from their coupling activities.

Thickened skull, scoliosis and other skeletal findings in Unverricht-Lundborg disease link cystatin B function to bone metabolism

PURPOSE

Unverricht-Lundborg disease (EPM1) is a rare type of inherited progressive myoclonic epilepsy resulting from mutations in the cystatin B gene, CSTB, which encodes a cysteine cathepsin inhibitor. Cystatin B, cathepsin K, and altered osteoclast bone resorption activity are interconnected in vitro. This study evaluated the skeletal characteristics of patients with EPM1.

METHODS

Sixty-six genetically verified EPM1 patients and 50 healthy controls underwent head MRI. Skull dimensions and regional calvarial thickness was measured perpendicular to each calvarial bone from T1-weighted 3-dimensional images using multiple planar reconstruction tools. All clinical X-ray files of EPM1 patients were collected and reviewed by an experienced radiologist. A total of 337 X-ray studies were analyzed, and non-traumatic structural anomalies, dysplasias and deformities were registered.

RESULTS

EPM1 patients exhibited significant thickening in all measured cranial bones compared to healthy controls. The mean skull thickness was 10.0±2.0mm in EPM1 patients and 7.6±1.2mm in healthy controls (p<0.001). The difference was evident in all age groups and was not explained by former phenytoin use. Observed abnormalities in other skeletal structures in EPM1 patients included thoracic scoliosis (35% of EPM1 patients) and lumbar spine scoliosis (35%), large paranasal sinuses (27%), accessory ossicles of the foot, and arachnodactyly (18%).

CONCLUSIONS

Skull thickening and an increased prevalence of abnormal findings in skeletal radiographs of patients with EPM1 suggest that this condition is connected to defective cystatin B function. These findings further emphasize the role of cystatin B in bone metabolism in humans.

Potential role of odanacatib in the treatment of osteoporosis

Cathepsin K is a key enzyme involved in the degradation of organic bone matrix by osteoclasts. Inhibition of bone resorption observed in human and animal models deficient for cathepsin K has identified this enzyme as a suitable target for intervention by small molecules with the potential to be used as therapeutic agents in the treatment of osteoporosis. Odanacatib (ODN) is a nonbasic selective cathepsin K inhibitor with good pharmacokinetic parameters such as minimal in vitro metabolism, long half-life, and oral bioavailability. In preclinical studies, ovariectomized monkeys and rabbits treated with ODN showed substantial inhibition of bone resorption markers along with increases in bone mineral density (BMD). Significant differences were observed in the effects of ODN treatment compared with those of other antiresorptive agents such as bisphosphonates and denosumab. ODN displayed compartment-specific effects on trabecular versus cortical bone formation, with treatment resulting in marked increases in periosteal bone formation and cortical thickness in ovariectomized monkeys whereas trabecular bone formation was reduced. Furthermore, osteoclasts remained viable. Phase I and II studies conducted in postmenopausal women showed ODN to be safe and well tolerated. After 5 years, women who received ODN 50 mg weekly continuously from year 1 (n = 13), showed BMD increases from baseline of 11.9% at the lumbar spine, 9.8% at the femoral neck, 10.9% at the hip trochanter, and 8.5% at the total hip. Additionally, these subjects maintained a low level of the urine bone resorption marker N-terminal telopeptide/creatinine (−67.4% from baseline) through 5 years of treatment, while levels of serum bone-specific alkaline phosphatase remained only slightly reduced relative to baseline (−15.3%). In women who were switched from ODN to placebo after 2 years, bone turnover markers were transiently increased and BMD gains reversed after 12 months off medication. Adverse experiences in the ODN-treated group were not significantly different from the placebo group. In conclusion, available data suggests that cathepsin K inhibition could be a promising intervention with which to treat osteoporosis. Ongoing studies are expected to provide information on the long-term efficacy in fracture reduction and safety of prolonged treatment with ODN.

Protease-activated drug development

In this extensive review, we elucidate the importance of proteases and their role in drug development in various diseases with an emphasis on cancer. First, key proteases are introduced along with their function in disease progression. Next, we link these proteases as targets for the development of prodrugs and provide clinical examples of protease-activatable prodrugs. Finally, we provide significant design considerations needed for the development of the next generation protease-targeted and protease-activatable prodrugs.

Fluorescence imaging of osteoclasts using confocal microscopy

In order to understand osteoclast cell biology, it is necessary to culture these cells on a physiological -substrate that they can resorb in vitro, such as bone or dentine. However, this creates problems for analysis by fluorescence microscopy, due to the depth of the sample under investigation. By virtue of its optical sectioning capabilities, confocal microscopy is ideal for analysis of such samples, enabling precise intracellular localisation of proteins in resorbing osteoclasts to be determined. Moreover, by taking a series of images in the axial dimension, it is possible to create axial section views and to reconstruct 3D images of the osteoclasts, enabling the spatial organisation of the structures of interest to be more easily discerned.

Morphea-like skin reactions in patients treated with the cathepsin K inhibitor balicatib

BACKGROUND

In a multicenter clinical trial in North America and Europe that tested the cathepsin K (catK) inhibitor balicatib for the treatment of osteoporosis, several patients developed hardening of the skin.

OBJECTIVE

We sought to characterize these observed adverse events.

METHODS

Patients with skin hardening were examined by a local dermatologist. All of those patients except one had at least one biopsy specimen taken from affected skin, which was read by local and two central dermatopathologists. Workup was directed for consideration of systemic scleroderma.

RESULTS

Nine patients of 709 treated with balicatib developed skin hardening and were given a diagnosis of morphea-like skin changes. No such events were observed in patients taking placebo or the lowest balicatib dose. After discontinuation of balicatib, skin changes resolved completely in 8 and partially in one patient.

LIMITATIONS

Each patient was seen by a different dermatologist in 6 different countries.

CONCLUSIONS

These observations are likely dose-related adverse effects of balicatib. Although catK was originally thought to be expressed only in osteoclasts, it has more recently also been found in lung and dermal fibroblasts and been implicated in the degradation of the extracellular matrix in the lung and the skin. It is therefore plausible that the observed dermal fibrosis in balicatib-treated patients is a result of impaired degradation of extracellular matrix proteins and may represent a class effect of catK inhibitors. We recommend that further exploration of catK inhibition for the treatment of osteoporosis or cancer should include monitoring for similar adverse effects.

Molecular cloning, characterization and expression analysis of cathepsin A gene in Chinese mitten crab, Eriocheir sinensis

Cathepsins, a superfamily of hydrolytic enzymes produced and enclosed within lysosomes, function in immune response in vertebrates; however, their function within the innate immune system of invertebrates remains largely unknown. Therefore, we investigated the immune functionality of cathepsin A (catA) in Chinese mitten crab (Eriocheir sinensis), a commercially important and disease vulnerable aquaculture species. The full length catA cDNA (2200 bp) was cloned via PCR based upon an initial expressed sequence tag (EST) isolated from a hepatopancreatic cDNA library. The catA cDNA contained a 1398 bp open reading frame (ORF) that encoded a putative 465 amino acid (aa) protein. Comparisons with other reported vertebrate cathepsins sequences revealed percent identity range from 48 to 51%. CatA mRNA expression in E. sinensis was (a) tissue-specific, with the highest expression observed in gill and (b) responsive in hemocytes to a Vibrio anguillarum challenge, with peak exposure observed 12 h post-injection. Collectively, data demonstrate the successful isolation of catA from the Chinese mitten crab, and its involvement in the innate immune system of an invertebrate.

Lack of cathepsin activities alter or prevent the development of lung granulomas in a mouse model of sarcoidosis

BACKGROUND

Remodeling of lung tissues during the process of granuloma formation requires significant restructuring of the extra-cellular matrix and cathepsins K, L and S are among the strongest extra-cellular matrix degrading enzymes. Cathepsin K is highly expressed in various pathological granulomatous infiltrates and all three enzymes in their active form are detected in bronchoalveolar lavage fluids from patients with sarcoidosis. Granulomatous inflammation is driven by T-cell response and cathepsins S and L are actively involved in the regulation of antigen presentation and T-cell selection. Here, we show that the disruption of the activities of cathepsins K, L, or S affects the development of lung granulomas in a mouse model of sarcoidosis.

METHODS

Apolipoprotein E-deficient mice lacking cathepsin K or L were fed Paigen diet for 16 weeks and lungs were analyzed and compared with their cathepsin-expressing littermates. The role of cathepsin S in the development of granulomas was evaluated using mice treated for 8 weeks with a potent and selective cathepsin S inhibitor.

RESULTS

When compared to wild-type litters, more cathepsin K-deficient mice had lung granulomas, but individually affected mice developed smaller granulomas that were present in lower numbers. The absence of cathepsin K increased the number of multinucleated giant cells and the collagen content in granulomas. Cathepsin L deficiency resulted in decreased size and number of lung granulomas. Apoe-/- mice treated with a selective cathepsin S inhibitor did not develop lung granulomas and only individual epithelioid cells were observed.

CONCLUSIONS

Cathepsin K deficiency affected mostly the occurrence and composition of lung granulomas, whereas cathepsin L deficiency significantly reduced their number and cathepsin S inhibition prevented the formation of granulomas.

Molecular cloning, characterization and expression analysis of cathepsin C gene involved in the antibacterial response in Chinese mitten crab, Eriocheir sinensis

Cathepsins, a superfamily of hydrolytic enzymes produced and enclosed within lysosomes, function in immune response in vertebrates; however, their function within the innate immune system of invertebrates remains largely unknown. Therefore, we investigated the immune functionality of cathepsin C (catC) in Chinese mitten crab (Eriocheir sinensis), a commercially important and disease vulnerable aquaculture species. The full-length catC cDNA (1481 bp) was cloned via PCR based upon an initial expressed sequence tag (EST) isolated from a hepatopancreatic cDNA library. The catC cDNA contained a 1284 bp open reading frame (ORF) that encoded a putative 427 amino acid (aa) protein. Comparisons with other reported invertebrate and vertebrate cathepsins sequences revealed high percent identity. CatC mRNA expression in E. sinensis was responsive in hemocytes to a Vibrio anguillarum challenge, with peak exposure observed 6 h post-injection. Collectively, data demonstrate the successful isolation of catC from the Chinese mitten crab, and its involvement in the innate immune system of an invertebrate.

ApoE-deficient mice on cholate-containing high-fat diet reveal a pathology similar to lung sarcoidosis

Sarcoidosis is a chronic disease of unknown etiology characterized by the formation of non-necrotizing epithelioid granulomas in various organs, especially in the lungs. The lack of an adequate animal model reflecting the pathogenesis of the human disease is one of the major impediments in studying sarcoidosis. In this report, we describe ApoE-/- mice on a cholate-containing high-fat diet that exhibit granulomatous lung inflammation similar to human sarcoidosis. Histological analysis revealed well-defined and non-necrotizing granulomas in about 40% of mice with the highest number of granulomas after 16 weeks on a cholate-containing high-fat diet. Granulomas contained CD4+ and CD8+ T cells, and the majority of the cells in granulomas showed immunoreactivity for the macrophage marker Mac-3. Cells with morphological features of epithelioid cells expressed angiotensin-converting enzyme, osteopontin, and cathepsin K, all characteristics of epithelioid and giant cells in granulomas of human sarcoidosis. Giant cells and nonspecific inclusions such as Schaumann's bodies and crystalline deposits were also detected in some lungs. Granulomatous inflammation resulted in progressive pulmonary fibrosis. Removal of cholate from the diet prevented the formation of lung granulomas. The observed similarities between the analyzed mouse lung granulomas and granulomas of human sarcoidosis, as well as the chronic disease character leading to fibrosis, suggest that this mouse model might be a useful tool to study sarcoidosis.

Glycosaminoglycan-mediated loss of cathepsin K collagenolytic activity in MPS I contributes to osteoclast and growth plate abnormalities

Mucopolysaccharidoses are a group of lysosomal storage diseases characterized by the build-up of glycosaminoglycans (GAGs) and severe skeletal abnormalities. As GAGs can regulate the collagenolytic activity of the major osteoclastic protease cathepsin K, we investigated the presence and activity of cathepsin K and its co-localization with GAGs in mucopolysaccharidosis (MPS) type I bone. The most dramatic difference between MPS I and wild-type mice was an increase in the amount of cartilage in the growth plates in MPS I bones. Though the number of cathepsin K-expressing osteoclasts was increased in MPS I mice, these mice revealed a significant reduction in cathepsin K-mediated cartilage degradation. As excess heparan and dermatan sulfates inhibit type II collagen degradation by cathepsin K and the spatial overlap between cathepsin K and heparan sulfate strongly increased in MPS I mice, the build up of subepiphyseal cartilage is speculated to be a direct consequence of cathepsin K inhibition by MPS I-associated GAGs. Moreover, isolated MPS I and Ctsk(-/-) osteoclasts displayed fewer actin rings and formed fewer resorption pits on dentine disks, as compared with wild-type cells. These results suggest that the accumulation of GAGs in murine MPS I bone has an inhibitory effect on cathepsin K activity, resulting in impaired osteoclast activity and decreased cartilage resorption, which may contribute to the bone pathology seen in MPS diseases.

Acute hypoxia and osteoclast activity: a balance between enhanced resorption and increased apoptosis

Osteoclasts are the primary mediators of pathological bone resorption in many conditions in which micro-environmental hypoxia is associated with disease progression. However, effects of hypoxia on human osteoclast activity have not been reported. Mature human osteoclasts were differentiated from peripheral blood or obtained from giant cell tumour of bone. Osteoclasts were exposed to a constant hypoxic environment and then assessed for parameters including resorption (toluidine blue staining of dentine slices), membrane integrity (trypan blue exclusion), apoptosis (TUNEL, DAPI), and osteolysis-associated enzyme activity (TRAP, cathepsin K). 24 h exposure to 2% O(2) produced a 2.5-fold increase in resorption associated with increased TRAP and cathepsin K enzyme activity. Hypoxia-Inducible Factor-1alpha (HIF-1alpha) siRNA completely ablated the hypoxic increase in osteoclast resorption. 24 h at 2% O(2) also increased the number of osteoclasts with compromised membrane integrity from 6% to 21%, with no change in the total osteoclast number or the proportion of late-stage apoptotic cells. Transient reoxygenation returned the percentage of trypan blue-positive cells to normoxic levels, suggesting that osteoclasts can recover from the early stages of cell death. Repeated over an extended period, hypoxia/reoxygenation enhanced osteoclast differentiation at this pO(2). These data suggest that in diseased bone, where the pO(2) may fall to <or=2% O(2), a delicate balance between hypoxia-induced osteoclast activation and hypoxia-induced osteoclast apoptosis mediates pathological bone resorption.

Cathepsin K inhibitors for osteoporosis and potential off-target effects

Cathepsin K is a highly potent collagenase and the predominant papain-like cysteine protease expressed in osteoclasts. Cathepsin K deficiencies in humans and mice have underlined the central role of this protease in bone resorption and, thus, have rendered the enzyme as an attractive target for anti-resorptive osteoporosis therapy. In the past decade, a lot of efforts have been made in developing highly potent, selective and orally applicable cathepsin K inhibitors. Some of these inhibitors have passed preclinical studies and are presently in clinical trials at different stages of advancement. The development of the inhibitors and preliminary results of the clinical trials revealed problems and lessons concerning the in situ specificity of the compounds and their tissue targeting. In this review, we briefly summarize the history of cathepsin K research and discuss the current development of cathepsin K inhibitors as novel anti-resorptives for the treatment of osteoporosis. We also discuss potential off-target effects of cathepsin K inhibition and alternative applications of cathepsin K inhibitors in arthritis, atherosclerosis, blood pressure regulation, obesity and cancer.

Overexpression of cathepsin K accelerates the resorption cycle and osteoblast differentiation in vitro

Bone resorption is a multistep process including osteoclast attachment, cytoskeletal reorganization, formation of four distinct plasma membrane domains, and matrix demineralization and degradation followed by cell detachment. The present study describes the intracellular mechanisms by which overexpression of cathepsin K in osteoclasts results in enhanced bone resorption. Osteoclasts and bone marrow-derived osteoclast and osteoblast precursors were isolated from mice homozygous (UTU17(+/+)) and negative for the transgene locus. Cells cultured on bovine cortical bone slices were analyzed by fluorescence and confocal laser scanning microscopy, and bone resorption was studied by measurements of biochemical resorption markers, morphometry, and FESEM. Excessive cathepsin K protein and enzyme activity were microscopically observed in various intracellular vesicles and in the resorption lacunae of cathepsin K-overexpressing osteoclasts. The number of cathepsin K-containing vesicles in UTU17(+/+) osteoclasts was highly increased, and co-localization with markers for the biosynthetic and transcytotic pathways was observed throughout the cytoplasm. As a functional consequence of cathepsin K overexpression, biochemical resorption markers were increased in culture media of UTU17(+/+) osteoclasts. Detailed morphometrical analysis of the erosion in bone slices indicated that the increased biosynthesis of cathepsin K was sufficient to accelerate the osteoclastic bone resorption cycle. Cathepsin K overexpression also enhanced osteogenesis and induced the formation of exceptionally small, actively resorbing osteoclasts from their bone marrow precursors in vitro. The present study describes for the first time how enhancement in one phase of the osteoclastic resorption cycle also stimulates its other phases and further demonstrate that tight control and temporal coupling of mesenchymal and hematopoietic bone cells in this multistep process.

Expression and regulation of cathepsin K in skin fibroblasts

Cathepsin K (catK) is a lysosomal cysteine protease with strong collagenolytic activity well known to mediate bone resorption in osteoclasts. Recently, catK has also been reported to be expressed in other tissues. In the dermis, it is expressed only under certain circumstances such as scarring or inflammation. We therefore investigated the expression and regulation of this protease in dermal fibroblasts using immunoblotting and immunostaining. Cultured skin fibroblasts were found to strongly express catK in lysosomes. Internalization of collagen I and IV to lysosomes of fibroblasts indicates a role of catK in intracellular collagen degradation after endocytosis, a process that is different from the metalloproteinase-mediated collagen degradation in the extracellular space. In fibroblasts, interleukin-1alpha and cellular confluence upregulate catK expression and transforming growth factor-beta1 inhibits confluence-induced catK upregulation in skin fibroblasts. RANKL (ligand of receptor activator of NF-kappaB) did not alter catK expression. These regulators of catK expression are likely to play a role in the as-needed upregulation in certain skin conditions, where the prominent matrix-degrading properties of catK are thought to require tight regulation to maintain the homeostasis of the extracellular matrix.

Cathepsin K activity-dependent regulation of osteoclast actin ring formation and bone resorption

Cathepsin K is responsible for the degradation of type I collagen in osteoclast-mediated bone resorption. Collagen fragments are known to be biologically active in a number of cell types. Here, we investigate their potential to regulate osteoclast activity. Mature murine osteoclasts were seeded on type I collagen for actin ring assays or dentine discs for resorption assays. Cells were treated with cathepsins K-, L-, or MMP-1-predigested type I collagen or soluble bone fragments for 24 h. The presence of actin rings was determined fluorescently by staining for actin. We found that the percentage of osteoclasts displaying actin rings and the area of resorbed dentine decreased significantly on addition of cathepsin K-digested type I collagen or bone fragments, but not with cathepsin L or MMP-1 digests. Counterintuitively, actin ring formation was found to decrease in the presence of the cysteine proteinase inhibitor LHVS and in cathepsin K-deficient osteoclasts. However, cathepsin L deficiency or the general MMP inhibitor GM6001 had no effect on the presence of actin rings. Predigestion of the collagen matrix with cathepsin K, but not by cathepsin L or MMP-1 resulted in an increased actin ring presence in cathepsin K-deficient osteoclasts. These studies suggest that cathepsin K interaction with type I collagen is required for 1) the release of cryptic Arg-Gly-Asp motifs during the initial attachment of osteoclasts and 2) termination of resorption via the creation of autocrine signals originating from type I collagen degradation.

Lysosomal protease expression in mature enamel

The enamel matrix proteins (amelogenin, enamelin and ameloblastin) are degraded by matrix metalloproteinase-20 and kallikrein-4 during enamel development and mature enamel is virtually protein free. The precise mechanism of removal and degradation of the enamel protein cleavage products from the matrix, however, remains poorly understood. It has been proposed that receptor-mediated endocytosis allows for the cleaved proteins to be removed from the matrix during enamel formation and then transported to the lysosome for further degradation. This study aims to identify lysosomal proteases that are present in maturation-stage enamel organ. RNA from first molars of 11-day-old mice was collected and expression was initially assessed by RT-PCR and then quantified by qPCR. The pattern of expression of selected proteases was assessed by immunohistochemical staining of demineralized mouse incisors. With the exception of cathepsin G, all lysosomal proteases assessed were expressed in maturation-stage enamel organ. Identified proteases included cathepsins B, D, F, H, K, L, O, S and Z. Tripeptidyl peptidases I and II as well as dipeptidyl peptidases I, II, III and IV were also found to be expressed. Immunohistochemical staining confirmed that the maturation-stage ameloblasts express cathepsins L and S and tripeptidyl peptidase II. Our results suggest that the ameloblasts are enriched by a large number of lysosomal proteases at maturation that are likely involved in the degradation of the organic matrix.

Diacerein inhibits the synthesis of resorptive enzymes and reduces osteoclastic differentiation/survival in osteoarthritic subchondral bone: a possible mechanism for a protective effect against subchondral bone remodelling

INTRODUCTION

Subchondral bone alterations represent an essential component of osteoarthritis (OA). Modifying the abnormal subchondral bone metabolism may be indicated to treat OA. We investigated the effect of diacerein and rhein on the changes occurring in subchondral bone during OA. To this end, we determined the drugs' effects on metalloprotease-13 (MMP-13) synthesis on subchondral bone and on the osteoblast signalling pathways. In osteoclasts, we studied MMP-13 and cathepsin K production as well as cell differentiation, proliferation, and survival.

METHODS

The effect of diacerein/rhein on the production of subchondral bone MMP-13 was determined by enzyme-linked immunosorbent assay. Signalling pathways were evaluated on osteoblasts by Western blot. Osteoclast experiments were performed using cells from the pre-osteoclastic murine cell line Raw 264.7. Osteoclast MMP-13 and cathepsin K activities were determined by specific bioassays and differentiation of these cells quantified by tartrate-resistant acid phosphatase staining.

RESULTS

Diacerein and rhein reduced, in a dose-dependent manner, the interleukin-1-beta (IL-1beta)-induced MMP-13 production in OA subchondral bone. This effect occurred through the inhibition of ERK1/2 (extracellular signal-regulated kinase-1/2) and p38. In osteoclasts, they significantly reduced the activity of MMP-13 and cathepsin K. Moreover, these drugs effectively blocked the IL-1beta effect on the osteoclast differentiation process and the survival of mature osteoclasts.

CONCLUSION

Altogether, these data suggest that diacerein/rhein could impact the abnormal subchondral bone metabolism in OA by reducing the synthesis of resorptive factors and osteoclast formation.

The cathepsin family and their role in colorectal cancer

Cathepsins are a class of globular proteases, initially described as intracellular peptide hydrolases, although several cathepsins also have extracellular functions. Cathepsins B, C, F, H, L, K, O, S, V, W, and X are cysteine proteases of the papain family, and represent the largest and best-known class of the cathepsins. Cathepsin G is a serine carboxypeptidases, and cathepsins D and E are aspartic proteases. Cathepsins are synthesized as inactive proenzymes and processed to become mature and active enzymes. Endogenous protein inhibitors, such as cystatins and some serpins, inhibit active enzymes. As primarily lysosomal proteases, cathepsins play important roles in proteolysis during physiological processes, as well as in several diseases. On the basis of their ability to degrade extracellular matrix proteins, cathepsins have been implicated to play a role in invasion and metastasis of colorectal cancer. In the present review, the role of cathepsins in the disease process of colorectal cancers and the correlation of cathepsin expression and activity with clinicopathological features is discussed. Furthermore, we give an overview of the recent developments of cathepsins in animal models and in in vitro experiments of colorectal disease, and provide information on inhibitors of cathepsins as possible therapeutics.

Papain-like cysteine proteases

The name "cysteine protease" refers to the protease's nucleophilic cysteine residue that forms a covalent bond with the carbonyl group of the scissile peptide bond in substrates. The papain-like cysteine proteases, classified as the "C1 family" are the most predominant cysteine proteases. These proteases are found in viruses, plants, primitive parasites, invertebrates, and vertebrates alike. Mammalian papain-like cysteine proteases are also known as cathepsins. This unit discusses cathepsins, and their subcellular and tissue localization, catalytic mechanism, and substrate specificity. Several tables illustrate the properties of the various cathepsins.

Cathepsin K in melanoma invasion

Cathepsin K (catK) is a lysosomal cysteine protease with strong collagenolytic activity that mediates bone resorption in osteoclasts. Recently, catK expression has been reported in skin and lung fibroblasts, which suggests a role in maintaining homeostasis of the extracellular matrix outside of bone. Matrix degradation is a pivotal step in tumor invasion and metastasis. As other proteases, in particular matrix metalloproteinases and some cathepsins, but not catK, have been described to mediate melanoma invasion, we studied catK in melanoma. Immunostaining revealed strong catK expression in most primary melanomas and all cutaneous melanoma metastases. Melanocytic nevi also demonstrated catK expression, but it was less intense than in melanomas. Melanoma lines express both the pro- and the active form of catK and internalize extracellular collagen into lysosomes. Inhibition of catK greatly reduced melanoma cell invasion through Matrigel basement membrane matrix and increased detection of internalized collagen. We suggest that catK may play an important role in melanoma invasion and metastasis by mediating intracellular degradation of matrix proteins after phagocytosis. Clinical use of catK inhibitors, a class of medication currently in clinical trials for the treatment of osteoporosis, may be a promising avenue for the treatment of melanoma.

Role of cathepsin K in structural changes in brachiocephalic artery during progression of atherosclerosis in apoE-deficient mice

Cathepsin K, a potent extracellular matrix degrading cysteine protease, has been linked to the pathogenesis of osteoporosis, arthritis, cardiovascular and respiratory diseases. Here, we report the effects of cathepsin K deficiency (ctsK-/-) on atherosclerotic plaque formation in brachiocephalic arteries in an aggressive atherosclerosis model using apoE-deficient mice on cholate-containing high fat diet (HFD). On this diet, apoE-/- mice displayed severe lesions with buried fibrous caps after 8 weeks, whereas the apoE-/-ctsK-/- mice revealed a significantly decreased number of buried fibrous caps accompanied by increased collagen content in plaque areas and fibrous cap thickness. After 16 weeks of HFD, ctsK-/- mice had smaller plaque areas and maintained the structure of the tunica media in terms of their smooth muscle cell content and elastic lamina integrity. Overall macrophage content in the tunica media was lower in ctsK-/- mice but higher in the plaque area after 8 weeks of HFD. Decreased apoptosis rates in atherosclerotic plaques in brachiocephalic arteries of cathepsin K-deficient indicated a lower level of inflammation. In conclusion, cathepsin K deficiency appears to increase lesion stability in brachiocephalic arteries by maintaining the integrity of the tunica media and by decreasing plaque vulnerability to rupture.

Proteolytic processing and polarized secretion of tartrate-resistant acid phosphatase is altered in a subpopulation of metaphyseal osteoclasts in cathepsin K-deficient mice

Tartrate-resistant acid phosphatase (TRAP) is an enzyme highly expressed in osteoclasts and thought to participate in osteoclast-mediated bone turnover. Cathepsin K (Ctsk) is the major collagenolytic cysteine proteinase expressed in osteoclasts and has recently been shown to be able to proteolytically process and activate TRAP in vitro. In this study, 4-week-old Ctsk(-/-) mice were analysed for TRAP expression at the mRNA, protein and enzyme activity levels to delineate a role of cathepsin K in TRAP processing in osteoclasts in vivo. The absence of cathepsin K in osteoclasts was associated with increased expression of TRAP mRNA, monomeric TRAP protein and total TRAP activity. Proteolytic processing of TRAP was not abolished but prematurely arrested at an intermediate stage without changing enzyme activity, a finding confirmed with RANKL-differentiated osteoclast-like cell line RAW264.7 treated with the cysteine proteinase inhibitor E-64. Thus, the increase in total TRAP activity was mainly due to increased cellular content of monomeric TRAP. The increase in monomeric TRAP expression was more pronounced in osteoclasts of the distal compared to the proximal part of the metaphyseal trabecular bone, suggesting a site-dependent role for cathepsin K in TRAP processing. Moreover, intracellular localization of monomeric TRAP was altered in distal metaphyseal osteoclasts from Ctsk(-/-) mice. Additionally, TRAP was secreted into the ruffled border as the processed form in osteoclasts of Ctsk(-/-) mice, unlike in osteoclasts from wild-type mice which secreted TRAP to the resorption lacuna as the monomeric form. The results demonstrate that cathepsin K is not only involved in proteolytic processing but also affects the intracellular trafficking of TRAP, particularly in osteoclasts of the distal metaphysis. However, contribution by other yet unidentified protease(s) to TRAP processing must also be invoked since proteolytic cleavage of TRAP is not abolished in Ctsk(-/-) mice. Importantly, this study highlights functional differences between bone-resorbing clasts within the trabecular metaphyseal bone, suggesting potentially important differences in the regulation of differentiation and activation depending on the precise anatomical localization of the clast population.

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).

The S2 subsites of cathepsins K and L and their contribution to collagen degradation

The exchange of residues 67 and 205 of the S2 pocket of human cysteine cathepsins K and L induces a permutation of their substrate specificity toward fluorogenic peptide substrates. While the cathepsin L-like cathepsin K (Tyr67Leu/Leu205Ala) mutant has a marked preference for Phe, the Leu67Tyr/Ala205Leu cathepsin L variant shows an effective cathepsin K-like preference for Leu and Pro. A similar turnaround of inhibition was observed by using specific inhibitors of cathepsin K [1-(N-Benzyloxycarbonyl-leucyl)-5-(N-Boc-phenylalanyl-leucyl)carbohydrazide] and cathepsin L [N-(4-biphenylacetyl)-S-methylcysteine-(D)-Arg-Phe-beta-phenethylamide]. Molecular modeling studies indicated that mutations alter the character of both S2 and S3 subsites, while docking calculations were consistent with kinetics data. The cathepsin K-like cathepsin L was unable to mimic the collagen-degrading activity of cathepsin K against collagens I and II, DQ-collagens I and IV, and elastin-Congo Red. In summary, double mutations of the S2 pocket of cathepsins K (Y67L/L205A) and L (L67Y/A205L) induce a switch of their enzymatic specificity toward small selective inhibitors and peptidyl substrates, confirming the key role of residues 67 and 205. However, mutations in the S2 subsite pocket of cathepsin L alone without engineering of binding sites to chondroitin sulfate are not sufficient to generate a cathepsin K-like collagenase, emphasizing the pivotal role of the complex formation between glycosaminoglycans and cathepsin K for its unique collagenolytic activity.

A highly potent inhibitor of cathepsin K (relacatib) reduces biomarkers of bone resorption both in vitro and in an acute model of elevated bone turnover in vivo in monkeys

Cathepsin K is an osteoclast-derived cysteine protease that has been implicated as playing a major role in bone resorption. A substantial body of evidence indicates that cathepsin K is critical in osteoclast-mediated bone resorption and suggests that its pharmacological inhibition should result in inhibition of bone resorption in vivo. Here we report the pharmacological characterization of SB-462795 (relacatib) as a potent and orally bioavailable small molecule inhibitor of cathepsin K that inhibits bone resorption both in vitro in human tissue and in vivo in cynomolgus monkeys. SB-462795 is a potent inhibitor of human cathepsins K, L, and V (K(i, app)=41, 68, and 53 pM, respectively) that exhibits 39-300-fold selectivity over other cathepsins. SB-462795 inhibited endogenous cathepsin K in situ in human osteoclasts and human osteoclast-mediated bone resorption with IC50 values of approximately 45 nM and approximately 70 nM, respectively. The anti-resorptive potential of SB-462795 was evaluated in normal as well as medically ovariectomized (Ovx) female cynomolgus monkeys. Serum levels of the C- and N-terminal telopeptides of Type I collagen (CTx and NTx, respectively) and urinary levels of NTx were monitored as biomarkers of bone resorption. Administration of SB-462795 to medically ovariectomized or normal monkeys resulted in an acute reduction in both serum and urinary markers of bone resorption within 1.5 h after dosing, and this effect lasted up to 48 h depending on the dose administered. Our data indicate that SB-462795 potently inhibits human cathepsin K in osteoclasts, resulting in a rapid inhibition of bone resorption both in vitro and in vivo in the monkey. These studies also demonstrate the therapeutic potential of relacatib in the treatment of postmenopausal osteoporosis and serves to model the planned clinical trials in human subjects.

Effects of disease-modifying anti-rheumatic drugs (DMARDs) on the activities of rheumatoid arthritis-associated cathepsins K and S

Rheumatoid arthritis is an inflammatory and disabling joint disease affecting 0.5-1.5% of the population. Although various anti-inflammatory (NSAIDs) and disease-modifying (DMARDs) drugs are in clinical use, their precise mechanisms of action are not always defined. In this report, we discuss the effects of widely used DMARDs such as gold derivatives and chloroquine on cathepsins K and S, which have been implicated as critical mediators of inflammation and joint erosion in rheumatoid arthritis. We demonstrate that clinically potent gold derivatives inhibit cathepsins K and S in in vitro and cell-based assays. An X-ray analysis of the gold thiomalate/cathepsin K complex reveals that the inhibitor is bound to the active-site cysteine residue of the protease. Chloroquine, a lysosomotropic agent of lower clinical potency than gold derivatives, inhibits neutral pH-labile cathepsins intracellularly, but does not affect the neutral pH-stable cathepsin S. The potent inhibition of cathepsins implicated in the pathogenesis of rheumatoid arthritis by gold derivatives may explain the therapeutic efficacy of these drugs.

Osteoclastic bone degradation and the role of different cysteine proteinases and matrix metalloproteinases: differences between calvaria and long bone

Osteoclastic bone degradation involves the activity of cathepsin K. We found that in addition to this enzyme other, yet unknown, cysteine proteinases participate in digestion. The results support the notion that osteoclasts from different bone sites use different enzymes to degrade the collagenous bone matrix.

INTRODUCTION

The osteoclast resorbs bone by lowering the pH in the resorption lacuna, which is followed by secretion of proteolytic enzymes. One of the enzymes taken to be essential in resorption is the cysteine proteinase, cathepsin K. Some immunolabeling and enzyme inhibitor data, however, suggest that other cysteine proteinases and/or proteolytic enzymes belonging to the group of matrix metalloproteinases (MMPs) may participate in the degradation. In this study, we investigated whether, in addition to cathepsin K, other enzymes participate in osteoclastic bone degradation.

MATERIALS AND METHODS

In bones obtained from mice deficient for cathepsin K, B, or L or a combination of K and L, the bone-resorbing activity of osteoclasts was analyzed at the electron microscopic level. In addition, bone explants were cultured in the presence of different selective cysteine proteinase inhibitors and an MMP inhibitor, and the effect on resorption was assessed. Because previous studies showed differences in resorption by calvarial osteoclasts compared with those present in long bones, in all experiments, the two types of bone were compared. Finally, bone extracts were analyzed for the level of activity of cysteine proteinases and the effect of inhibitors hereupon.

RESULTS

The analyses of the cathepsin-deficient bone explants showed that, in addition to cathepsin K, calvarial osteoclasts use other cysteine proteinases to degrade bone matrix. It was also shown that, in the absence of cathepsin K, long bone osteoclasts use MMPs for resorption. Cathepsin L proved to be involved in the MMP-mediated resorption of bone by calvarial osteoclasts; in the absence of this cathepsin, calvarial osteoclasts do not use MMPs for resorption. Selective inhibitors of cathepsin K and other cysteine proteinases showed a stronger effect on calvarial resorption than on long bone resorption.

CONCLUSIONS

Our findings suggest that (1) cathepsin K-deficient long bone osteoclasts compensate the lack of this enzyme by using MMPs in the resorption of bone matrix; (2) cathepsin L is involved in MMP-mediated resorption by calvarial osteoclasts; (3) in addition to cathepsin K, other, yet unknown, cysteine proteinases are likely to participate in skull bone degradation; and finally, (4) the data provide strong additional support for the existence of functionally different bone-site specific osteoclasts.

Role of cathepsin K in the turnover of the dermal extracellular matrix during scar formation

Cathepsins are a group of cysteine proteinases that are involved in various aspects of extracellular matrix turnover. The collagenolytic activity of cathepsin K plays a pivotal role in bone resorption and lung matrix homeostasis, but so far has not been described in skin. To study the role of cathepsin K in the turnover of the cutaneous extracellular matrix, we studied the expression of cathepsin K in human skin and in cultured primary neonatal skin fibroblasts. Normal skin exhibited only low levels or no expression of cathepsin K. In contrast, dermal fibroblasts in surgical scars showed strong cytoplasmic cathepsin K expression. Cathepsin K expression was most prominent in young scars and declined with time. Cultured neonatal primary fibroblasts showed strong cathepsin K staining in the perinuclear endosomal compartment, consistent with intracellular degradation of internalized collagen in lysosomes. Cathepsin K was also found to be strongly expressed in keloids and dermatofibromas, but not in sclerotic areas of morphea. Our data suggest that cathepsin K may play an important role in the homeostasis of the dermal extracellular matrix and the dynamic equilibrium between matrix synthesis and proteolytic degradation, by counteracting deposition of matrix proteins during scar formation with its matrix-degrading activity.

The role of cathepsins in ocular physiology and pathology

Cathepsins are proteases that were originally identified in the lysosome, where they participate in house keeping tasks such as degradation of phagocytosed photoreceptors. More recently, cathepsins have been detected outside of the lysosome, and associated with numerous diseases (keratoconus, retinal detachment, age related macular degeneration, and glaucoma). The most likely mechanism by which cathepsins contribute to ocular pathologies is via degradation of the extracellular matrix, and/or regulation of angiogenesis.