Inhibition of bone resorption by selective inactivators of cysteine proteinases

Inhibition of cysteine protease disturbs the topological relationship between bone resorption and formation in vitro

INTRODUCTION

Osteoporosis is a global health issue. Bisphosphonates that are commonly used to treat osteoporosis suppress both bone resorption and subsequent bone formation. Inhibition of cathepsin K, a cysteine proteinase secreted by osteoclasts, was reported to suppress bone resorption while preserving or increasing bone formation. Analyses of the different effects of antiresorptive reagents such as bisphosphonates and cysteine proteinase inhibitors will contribute to the understanding of the mechanisms underlying bone remodeling.

MATERIALS AND METHODS

Our team has developed an in vitro system in which bone remodeling can be temporally observed at the cellular level by 2-photon microscopy. We used this system in the present study to examine the effects of the cysteine proteinase inhibitor E-64 and those of zoledronic acid on bone remodeling.

RESULTS

In the control group, the amount of the reduction and the increase in the matrix were correlated in each region of interest, indicating the topological and quantitative coordination of bone resorption and formation. Parameters for osteoblasts, osteoclasts, and matrix resorption/formation were also correlated. E-64 disrupted the correlation between resorption and formation by potentially inhibiting the emergence of spherical osteoblasts, which are speculated to be reversal cells in the resorption sites.

CONCLUSION

These new findings help clarify coupling mechanisms and will contribute to the development of new drugs for osteoporosis.

Structure-guided protein engineering of human cathepsin L for efficient collagenolytic activity

Engineering precise substrate specificity of proteases advances the potential to use them in biotechnological and therapeutic applications. Collagen degradation, a physiological process mediated by collagenases, is an integral part of extracellular matrix remodeling and when uncontrolled, implicated in different pathological conditions. Lysosomal cathepsin-K cleaves triple helical collagen fiber, whereas cathepsin-L cannot do so. In this study, we have imparted collagenolytic property to cathepsin-L, by systematically engineering proline-specificity and glycosaminoglycans (GAG)-binding surface in the protease. The proline-specific mutant shows high specificity for prolyl-peptidic substrate but is incapable of cleaving collagen. Engineering a GAG-binding surface on the proline-specific mutant enabled it to degrade type-I collagen in the presence of chondroitin-4-sulfate (C4-S). We also present the crystal structures of proline-specific (1.4 Å) and collagen-specific (1.8 Å) mutants. Finally docking studies with prolyl-peptidic substrate (Ala-Gly-Pro-Arg-Ala) at the active site and a C4-S molecule at the GAG-binding site enable us to identify key structural features responsible for collagenolytic activity of cysteine cathepsins.

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.

Inflammasome Activation by Methamphetamine Potentiates Lipopolysaccharide Stimulation of IL-1β Production in Microglia

Methamphetamine (Meth) is an addictive psychostimulant abused worldwide. Ample evidence indicate that chronic abuse of Meth induces neurotoxicity via microglia-associated neuroinflammation and the activated microglia present in both Meth-administered animals and human abusers. The development of anti-neuroinflammation as a therapeutic strategy against Meth dependence promotes research to identify inflammatory pathways that are specifically tied to Meth-induced neurotoxicity. Currently, the exact mechanisms for Meth-induced microglia activation are largely unknown. NLRP3 is a well-studied cytosolic pattern recognition receptor (PRR), which promotes the assembly of the inflammasome in response to the danger-associated molecular patterns (DAMPs). It is our hypothesis that Meth activates NLRP3 inflammasome in microglia and promotes the processing and release of interleukin (IL)-1β, resulting in neurotoxic activity. To test this hypothesis, we studied the effects of Meth on IL-1β maturation and release from rat cortical microglial cultures. Incubation of microglia with physiologically relevant concentrations of Meth after lipopolysaccharide (LPS) priming produced an enhancement on IL-1β maturation and release. Meth treatment potentiated aggregation of inflammasome adaptor apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), induced activation of the IL-1β converting enzyme caspase-1 and produced lysosomal and mitochondrial impairment. Blockade of capase-1 activity, lysosomal cathepsin B activity or mitochondrial ROS production by their specific inhibitors reversed the effects of Meth, demonstrating an involvement of inflammasome in Meth-induced microglia activation. Taken together, our results suggest that Meth triggers microglial inflammasome activation in a manner dependent on both mitochondrial and lysosomal danger-signaling pathways.

A Track Record on SHOX: From Basic Research to Complex Models and Therapy

SHOX deficiency is the most frequent genetic growth disorder associated with isolated and syndromic forms of short stature. Caused by mutations in the homeobox gene SHOX, its varied clinical manifestations include isolated short stature, Léri-Weill dyschondrosteosis, and Langer mesomelic dysplasia. In addition, SHOX deficiency contributes to the skeletal features in Turner syndrome. Causative SHOX mutations have allowed downstream pathology to be linked to defined molecular lesions. Expression levels of SHOX are tightly regulated, and almost half of the pathogenic mutations have affected enhancers. Clinical severity of SHOX deficiency varies between genders and ranges from normal stature to profound mesomelic skeletal dysplasia. Treatment options for children with SHOX deficiency are available. Two decades of research support the concept of SHOX as a transcription factor that integrates diverse aspects of bone development, growth plate biology, and apoptosis. Due to its absence in mouse, the animal models of choice have become chicken and zebrafish. These models, therefore, together with micromass cultures and primary cell lines, have been used to address SHOX function. Pathway and network analyses have identified interactors, target genes, and regulators. Here, we summarize recent data and give insight into the critical molecular and cellular functions of SHOX in the etiopathogenesis of short stature and limb development.

Ultrasensitive internally quenched substrates of human cathepsin L

Internally quenched cathepsin L (Cat L) substrate ABZ-Bip-Arg-Ala-Gln-Tyr(3-NO2)-NH2 with high specificity constant (kcat/KM=2.6×10(7)M(-1)s(-1)) was synthesized. The resultant compound displayed high selectivity over other members of the cathepsin family (B, S, X, V, C, K, H, F, D, and A). Activity of Cat L at picomolar (pM) concentrations was found using this substrate. Moreover, it was established that the presence of the selective Cat L inhibitor suppressed the proteolysis of the substrate to a non-detectable level. Incubation of the synthesized compound with a cell lysate of healthy and cancer cell lines indicated significant differences in Cat L activity. Based on the obtained results, it is proposed that this substrate could be used for selective monitoring of Cat L activity in biological systems.

The current stage of cathepsin B inhibitors as potential anticancer agents

Cathepsin B is a lysosomal cysteine peptidase, with an important role in the development and progression of cancer. It is involved in the degradation of extracellular matrix proteins, a process promoting invasion and metastasis of tumor cells and tumor angiogenesis. Cathepsin B is unique among cathepsins in possessing both carboxypeptidase and endopeptidase activities. While the former is associated with its physiological role, the latter is involved in pathological degradation of the extracellular matrix. Its activities are regulated by different means, the most important being its endogenous inhibitors, the cystatins. In cancer this peptidase/inhibitor balance is altered, leading to harmful cathepsin B activity. The latter can be prevented by exogenous inhibitors. They differ in modes of inhibition, size, structure, binding affinity, selectivity, toxicity and bioavailability. In this article, we review the properties and function of endogenous and exogenous cathepsin B inhibitors and indicate their application as possible anticancer agents.

SHOX triggers the lysosomal pathway of apoptosis via oxidative stress

The SHOX gene encodes for a transcription factor important for normal bone development. Mutations in the gene are associated with idiopathic short stature and are responsible for the growth failure and skeletal defects found in the majority of patients with Léri-Weill dyschondrosteosis (LWD) and Langer mesomelic dysplasia. SHOX is expressed in growth plate chondrocytes where it is supposed to modulate the proliferation, differentiation and cell death of these cells. Supporting this hypothesis, in vitro studies have shown that SHOX expression induces cell cycle arrest and apoptosis in both transformed and primary cells. In this study, we further characterized the cell death mechanisms triggered by SHOX and compared them with the effects induced by one clinically relevant mutant form of SHOX, detected in LWD patients (SHOX R153L) and a SHOX C-terminally truncated version (L185X). We show that SHOX expression in U2OS osteosarcoma cells leads to oxidative stress that, in turn, induces lysosomal membrane rupture with release of active cathepsin B to the cytosol and subsequent activation of the intrinsic apoptotic pathway characterized by mitochondrial membrane permeabilization and caspase activation. Importantly, cells expressing SHOX R153L or L185X did not display any of these features. Given the fact that many of the events observed in SHOX-expressing cells also characterize the complex cell death process occurring in the growth plate during endochondral ossification, our findings further support the hypothesis that SHOX may play a central role in the regulation of the cell death pathways activated during long bone development.

Cysteine cathepsins: from structure, function and regulation to new frontiers

It is more than 50 years since the lysosome was discovered. Since then its hydrolytic machinery, including proteases and other hydrolases, has been fairly well identified and characterized. Among these are the cysteine cathepsins, members of the family of papain-like cysteine proteases. They have unique reactive-site properties and an uneven tissue-specific expression pattern. In living organisms their activity is a delicate balance of expression, targeting, zymogen activation, inhibition by protein inhibitors and degradation. The specificity of their substrate binding sites, small-molecule inhibitor repertoire and crystal structures are providing new tools for research and development. Their unique reactive-site properties have made it possible to confine the targets simply by the use of appropriate reactive groups. The epoxysuccinyls still dominate the field, but now nitriles seem to be the most appropriate “warhead”. The view of cysteine cathepsins as lysosomal proteases is changing as there is now clear evidence of their localization in other cellular compartments. Besides being involved in protein turnover, they build an important part of the endosomal antigen presentation. Together with the growing number of non-endosomal roles of cysteine cathepsins is growing also the knowledge of their involvement in diseases such as cancer and rheumatoid arthritis, among others. Finally, cysteine cathepsins are important regulators and signaling molecules of an unimaginable number of biological processes. The current challenge is to identify their endogenous substrates, in order to gain an insight into the mechanisms of substrate degradation and processing. In this review, some of the remarkable advances that have taken place in the past decade are presented. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Cathepsin L in metastatic bone disease: therapeutic implications

Cathepsin L is a lysosomal cysteine proteinase primarily devoted to the metabolic turnover of intracellular proteins. However, accumulating evidence suggests that this endopeptidase might also be implicated in the regulation of other important biological functions, including bone resorption in normal and pathological conditions. These findings support the concept that cathepsin L, in concert with other proteolytic enzymes involved in bone remodeling processes, could contribute to facilitate bone metastasis formation. In support of this hypothesis, recent studies indicate that cathepsin L can foster this process by triggering multiple mechanisms which, in part, differ from those of the major cysteine proteinase of osteoclasts, namely cathepsin K. Therefore, cathepsin L can be regarded as an additional target in the treatment of patients with metastatic bone disease. This review discusses the clinical and therapeutic implications related to these findings.

Immune response and expression analysis of cathepsin K in goldfish during Aeromonas hydrophila infection

The innate immunity and expression profiles of cathepsins D were determined in the goldfish (Carassius auratus) tissues after challenge with a fish pathogen Aeromonas hydrophila. The innate immunity of reactive oxygen species (ROS) and reactive nitrogen species (RNS) were determined by peripheral blood leucocytes. Blood and tissue samples of the muscle, gills, liver, kidney, heart, spleen, and intestine were sampled at 1, 3, 6 and 12 h post-infection for cathepsin D expression by semi-quantitative RT-PCR. The ROS and RNS production did not significantly increase at 1 h post-challenged goldfish. However, the ROS and RNS production was significantly increased after 3 h post-challenged fish compared to the control. The cathepsin D expression was found very low in muscle and kidney of the control fish, other tissues was not found the expression. A similar pattern was found in goldfish at 1 h post-challenge with A. hydrophila. However, at 3 h post-challenge goldfish, the cathepsin D expression was high only in the heart. At 6 h post-challenge goldfish, the cathepsin D expression was seen high all the tissues, except in the spleen. However, the expression was decreased at 12 h post-infection samples. This result was suggested that the goldfish infected with A. hydrophila decreased the innate immunity level in peripheral blood and expressed the cathepsin D in tissues.

VEGF-A induces angiogenesis by perturbing the cathepsin-cysteine protease inhibitor balance in venules, causing basement membrane degradation and mother vessel formation

Tumors initiate angiogenesis primarily by secreting vascular endothelial growth factor (VEGF-A(164)). The first new vessels to form are greatly enlarged, pericyte-poor sinusoids, called mother vessels (MV), that originate from preexisting venules. We postulated that the venular enlargement necessary to form MV would require a selective degradation of their basement membranes, rigid structures that resist vascular expansion. To identify the specific proteases responsible for MV formation, we induced angiogenesis in mouse tissues with an adenoviral vector expressing VEGF-A(164) (Ad-VEGF-A(164)) or with VEGF-A-secreting TA3/St mammary tumors. We found that MV formation resulted from greatly increased activity of cathepsins (B>S>L) in venules transitioning into MV, as well as from a reciprocal decrease in the expression of several cysteine protease inhibitors (CPI), stefin A and cystatins B and C, by these same venules. Using a fluorescence probe that selectively binds cellular sites of cathepsin protease activity in vivo, we showed that increased cathepsin activity was localized exclusively to perivenular cells, not to venule endothelial cells. CPI strikingly inhibited angiogenesis in the Matrigel assay, and Ad-VEGF-A(164)-induced angiogenesis was reduced by approximately 50% in cathepsin B-null mice. Thus, VEGF-A, whether expressed by interstitial cells infected with an adenoviral vector or by tumor cells, upsets the normal cathepsin-CPI balance in nearby venules, leading to degradation of their basement membranes, an important first step in angiogenesis.

Regulated proteolysis of nonmuscle myosin IIA stimulates osteoclast fusion

The nonmuscle myosin IIA heavy chain (Myh9) is strongly associated with adhesion structures of osteoclasts. In this study, we demonstrate that during osteoclastogenesis, myosin IIA heavy chain levels are temporarily suppressed, an event that stimulates the onset of cell fusion. This suppression is not mediated by changes in mRNA or translational levels but instead is due to a temporary increase in the rate of myosin IIA degradation. Intracellular activity of cathepsin B is significantly enhanced at the onset of osteoclast precursor fusion, and specific inhibition of its activity prevents myosin IIA degradation. Further, treatment of normal cells with cathepsin B inhibitors during the differentiation process reduces cell fusion and bone resorption capacity, whereas overexpression of cathepsin B enhances fusion. Ongoing suppression of the myosin IIA heavy chain via RNA interference results in formation of large osteoclasts with significantly increased numbers of nuclei, whereas overexpression of myosin IIA results in less osteoclast fusion. Increased multinucleation caused by myosin IIA suppression does not require RANKL. Further, knockdown of myosin IIA enhances cell spreading and lessens motility. These data taken together strongly suggest that base-line expression of nonmuscle myosin IIA inhibits osteoclast precursor fusion and that a temporary, cathepsin B-mediated decrease in myosin IIA levels triggers precursor fusion during osteoclastogenesis.

Bone density, strength, and formation in adult cathepsin K (-/-) mice

Cathepsin K (CatK) is a cysteine protease expressed predominantly in osteoclasts, that plays a prominent role in degrading Type I collagen. Growing CatK null mice have osteopetrosis associated with a reduced ability to degrade bone matrix. Bone strength and histomorphometric endpoints in young adult CatK null mice aged more than 10 weeks have not been studied. The purpose of this paper is to describe bone mass, strength, resorption, and formation in young adult CatK null mice. In male and female wild-type (WT), heterozygous, and homozygous CatK null mice (total N=50) aged 19 weeks, in-life double fluorochrome labeling was performed. Right femurs and lumbar vertebral bodies 1-3 (LV) were evaluated by dual-energy X-ray absorptiometry (DXA) for bone mineral content (BMC) and bone mineral density (BMD). The trabecular region of the femur and the cortical region of the tibia were evaluated by histomorphometry. The left femur and sixth lumbar vertebral body were tested biomechanically. CatK (-/-) mice show higher BMD at the central and distal femur. Central femur ultimate load was positively influenced by genotype, and was positively correlated with both cortical area and BMC. Lumbar vertebral body ultimate load was also positively correlated to BMC. Genotype did not influence the relationship of ultimate load to BMC in either the central femur or vertebral body. CatK (-/-) mice had less lamellar cortical bone than WT mice. Higher bone volume, trabecular thickness, and trabecular number were observed at the distal femur in CatK (-/-) mice. Smaller marrow cavities were also present at the central femur of CatK (-/-) mice. CatK (-/-) mice exhibited greater trabecular mineralizing surface, associated with normal volume-based formation of trabecular bone. Adult CatK (-/-) mice have higher bone mass in both cortical and cancellous regions than WT mice. Though no direct measures of bone resorption rate were made, the higher cortical bone quantity is associated with a smaller marrow cavity and increased retention of non-lamellar bone, signs of decreased endocortical resorption. The relationship of bone strength to BMC does not differ with genotype, indicating the presence of bone tissue of normal quality in the absence of CatK.

Cathepsin B is involved in the trafficking of TNF-alpha-containing vesicles to the plasma membrane in macrophages

TNF-alpha is a potent proinflammatory cytokine, essential for initiating innate immune responses against invading microbes and a key mediator involved in the pathogenesis of acute and chronic inflammatory diseases. To identify molecules involved in the production of TNF-alpha, we used a functional gene identification method using retroviral integration-mediated mutagenesis, followed by LPS-stimulated TNF-alpha production analysis in macrophages. We found that cathepsin B, a lysosomal cysteine proteinase, was required for optimal posttranslational processing of TNF-alpha in response to the bacterial cell wall component LPS. Mouse bone marrow-derived macrophages from cathepsin B-deficient mice and macrophages treated with the cathepsin B-specific chemical inhibitor CA074 methyl ester or small interfering RNA against cathepsin B secreted significantly less TNF-alpha than wild-type or nontreated macrophages. We further showed that the inhibition of cathepsin B caused accumulation of 26-kDa pro-TNF-containing vesicles. Ectopic expression of GFP-conjugated pro-TNF further suggests that pro-TNF failed to reach the plasma membrane without intracellular cathepsin B activity. Altogether, these data suggest that intracellular cathepsin B activity is involved in the TNF-alpha-containing vesicle trafficking to the plasma membrane.

Design, synthesis, and evaluation of inhibitors of cathepsin L: Exploiting a unique thiocarbazate chemotype

Recently, we identified a thiocarbazate that exhibits potent inhibitory activity against human cathepsin L. Since this structure represents a novel chemotype with potential for activity against the entire cysteine protease family, we designed, synthesized, and assayed a series of analogs to probe the mechanism of action, as well as the structural requirements for cathepsin L activity. Molecular docking studies using coordinates of a papain-inhibitor complex as a model for cathepsin L provided useful insights.

The roles of proteolytic enzymes in the development of tumour-induced bone disease in breast and prostate cancer

Tumour-induced bone disease is a common clinical feature of advanced breast and prostate cancer and is associated with considerable morbidity for the affected patients. Our understanding of the molecular mechanisms underlying the development of bone metastases is incomplete, but proteolytic enzymes are implicated in a number of processes involved in both bone metastasis and in normal bone turnover, including matrix degradation, cell migration, angiogenesis, tumour promotion and growth factor activation. Malignant as well as non-malignant cells in the primary and secondary sites express these enzymes, the activity of which may be regulated by soluble factors, cell- or matrix-associated components, as well as a number of cell signalling pathways. A number of secreted and cell surface-associated proteolytic enzymes are implicated in tumour-induced bone disease, including the matrix metalloproteinases, lysosomal cysteine proteinases and plasminogen activators. This review will introduce the role of proteolytic enzymes in normal bone turnover and give an overview of the studies in which their involvement and regulation in the development of bone metastases in breast and prostate cancer has been described. The results from trials involving protease inhibitors in clinical development will also be briefly discussed.

Insulin-like growth factor II receptor-mediated intracellular retention of cathepsin B is essential for transformation of endothelial cells by Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is the pathological agent of Kaposi's sarcoma (KS), a tumor characterized by aberrant proliferation of endothelial-cell-derived spindle cells. Since in many cancers tumorigenesis is associated with an increase in the activity of the cathepsin family, we studied the role of cathepsins in KS using an in vitro model of KSHV-mediated endothelial cell transformation. Small-molecule inhibitors and small interfering RNA (siRNA) targeting CTSB, but not other cathepsins, inhibited KSHV-induced postconfluent proliferation and the formation of spindle cells and foci of dermal microvascular endothelial cells. Interestingly, neither CTSB mRNA nor CTSB protein levels were induced in endothelial cells latently infected with KSHV. Secretion of CTSB was strongly diminished upon KSHV infection. Increased targeting of CTSB to endosomes was caused by the induction by KSHV of the expression of insulin-like growth factor-II receptor (IGF-IIR), a mannose-6-phosphate receptor (M6PR) that binds to cathepsins. Inhibition of IGF-IIR/M6PR expression by siRNA released CTSB for secretion. In contrast to the increased cathepsin secretion observed in most other tumors, viral inhibition of CTSB secretion via induction of an M6PR is crucial for the transformation of endothelial cells.

Profiling signalling pathways of the receptor activator of NF-kappaB ligand-induced osteoclast formation in mouse monocyte cells, RAW264.7

Cell-based signal chemical genomics can profile the signalling pathway for certain cellular events by using a target-known chemical library. To ascertain its usefulness, the receptor activator of NF-kappaB ligand (RANKL)-induced osteoclastogenesis in mouse monocyte/macrophage cells RAW264.7 was used as an in vitro experimental model. Of 180 target-known inhibitors/activators formatted in a 384-well plate, 8 chemicals were shown to inhibit the osteoclast formation, but 4 chemicals enhanced this process. A variety of references support, or possibly lead one to expect the effects of these 12 chemicals on the cellular process of osteoclastogenesis in RAW264.7 cells, but several signalling pathways were newly found in this study; for example, CA-074 Me inhibiting cathepsin B and nitrendipine blocking the calcium channel could have the potential to inhibit the osteoclast formation as well as bone resorption. This is a simple but very fast and powerful method of profiling the signalling pathway of certain cellular events. Signal chemical genomics could provide invaluable information for the exploration of new target signalling processes and further target-based drug discovery strategies.

Calvarial osteoclasts express a higher level of tartrate-resistant acid phosphatase than long bone osteoclasts and activation does not depend on cathepsin K or L activity

Bone resorption by osteoclasts depends on the activity of various proteolytic enzymes, in particular those belonging to the group of cysteine proteinases. Next to these enzymes, tartrate-resistant acid phosphatase (TRAP) is considered to participate in this process. TRAP is synthesized as an inactive proenzyme, and in vitro studies have shown its activation by cysteine proteinases. In the present study, the possible involvement of the latter enzyme class in the in vivo modulation of TRAP was investigated using mice deficient for cathepsin K and/or L and in bones that express a high (long bone) or low (calvaria) level of cysteine proteinase activity. The results demonstrated, in mice lacking cathepsin K but not in those deficient for cathepsin L, significantly higher levels of TRAP activity in long bone. This higher activity was due to a higher number of osteoclasts. Next, we found considerable differences in TRAP activity between calvarial and long bones. Calvarial bones contained a 25-fold higher level of activity than long bones. This difference was seen in all mice, irrespective of genotype. Osteoclasts isolated from the two types of bone revealed that calvarial osteoclasts expressed higher enzyme activity as well as a higher level of mRNA for the enzyme. Analysis of TRAP-deficient mice revealed higher levels of nondigested bone matrix components in and around calvarial osteoclasts than in long bone osteoclasts. Finally, inhibition of cysteine proteinase activity by specific inhibitors resulted in increased TRAP activity. Our data suggest that neither cathepsin K nor L is essential in activating TRAP. The findings also point to functional differences between osteoclasts from different bone sites in terms of participation of TRAP in degradation of bone matrix. We propose that the higher level of TRAP activity in calvarial osteoclasts compared to that in long bone cells may partially compensate for the lower cysteine proteinase activity found in calvarial osteoclasts and TRAP may contribute to the degradation of noncollagenous proteins during the digestion of this type of bone.

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.

Inhibition of cathepsin B reduces beta-amyloid production in regulated secretory vesicles of neuronal chromaffin cells: evidence for cathepsin B as a candidate beta-secretase of Alzheimer's disease

The regulated secretory pathway of neurons is the major source of extracellular A beta that accumulates in Alzheimer's disease (AD). Extracellular A beta secreted from that pathway is generated by beta-secretase processing of amyloid precursor protein (APP). Previously, cysteine protease activity was demonstrated as the major beta-secretase activity in regulated secretory vesicles of neuronal chromaffin cells. In this study, the representative cysteine protease activity in these secretory vesicles was purified and identified as cathepsin B by peptide sequencing. Immunoelectron microscopy demonstrated colocalization of cathepsin B with A beta in these vesicles. The selective cathepsin B inhibitor, CA074, blocked the conversion of endogenous APP to A beta in isolated regulated secretory vesicles. In chromaffin cells, CA074Me (a cell permeable form of CA074) reduced by about 50% the extracellular A beta released by the regulated secretory pathway, but CA074Me had no effect on A beta released by the constitutive pathway. Furthermore, CA074Me inhibited processing of APP into the COOH-terminal beta-secretase-like cleavage product. These results provide evidence for cathepsin B as a candidate beta-secretase in regulated secretory vesicles of neuronal chromaffin cells. These findings implicate cathepsin B as beta-secretase in the regulated secretory pathway of brain neurons, suggesting that inhibitors of cathepsin B may be considered as therapeutic agents to reduce A beta in AD.

Bone microenvironment modulates expression and activity of cathepsin B in prostate cancer

Prostate cancers metastasize to bone leading to osteolysis. Here we assessed proteolysis of DQ-collagen I (a bone matrix protein) and, for comparison, DQ-collagen IV, by living human prostate carcinoma cells in vitro. Both collagens were degraded, and this degradation was reduced by inhibitors of matrix metallo, serine, and cysteine proteases. Because secretion of the cysteine protease cathepsin B is increased in human breast fibroblasts grown on collagen I gels, we analyzed cathepsin B levels and secretion in prostate cells grown on collagen I gels. Levels and secretion were increased only in DU145 cells--cells that expressed the highest baseline levels of cathepsin B. Secretion of cathepsin B was also elevated in DU145 cells grown in vitro on human bone fragments. We further investigated the effect of the bone microenvironment on cathepsin B expression and activity in vivo in a SCID-human model of prostate bone metastasis. High levels of cathepsin B protein and activity were found in DU145, PC3, and LNCaP bone tumors, although the PC3 and LNCaP cells had exhibited low cathepsin B expression in vitro. Our results suggest that tumor-stromal interactions in the context of the bone microenvironment can modulate the expression of the cysteine protease cathepsin B.

Different cysteine proteinases involved in bone resorption and osteoclast formation

Cysteine proteinases, especially cathepsin K, play an important role in osteoclastic degradation of bone matrix proteins and the process can, consequently, be significantly inhibited by cysteine proteinase inhibitors. We have recently reported that cystatin C and other cysteine proteinase inhibitors also reduce osteoclast formation. However, it is not known which cysteine proteinase(s) are involved in osteoclast differentiation. In the present study, we compared the relative potencies of cystatins C and D as inhibitors of bone resorption in cultured mouse calvariae, osteoclastogenesis in mouse bone marrow cultures, and cathepsin K activity. Inhibition of cathepsin K activity was assessed by determining equilibrium constants for inhibitor complexes in fluorogenic substrate assays. The data demonstrate that whereas human cystatins C and D are equipotent as inhibitors of bone resorption, cystatin D is 10-fold less potent as an inhibitor of osteoclastogenesis and 200-fold less potent as an inhibitor of cathepsin K activity. A recombinant human cystatin C variant with Gly substitutions for residues Arg8, Leu9, Val10, and Trp106 did not inhibit bone resorption, had 1,000-fold decreased inhibitory effect on cathepsin K activity compared to wildtype cystatin C, but was equipotent with wildtype cystatin C as an inhibitor of osteoclastogenesis. It is concluded that (i) different cysteine proteinases are likely to be involved in bone resorption and osteoclast formation, (ii) cathepsin K may not be an exclusive target enzyme in any of the two systems, and (iii) the enzyme(s) involved in osteoclastogenesis might not be a typical papain-like cysteine proteinase.

DNA microarray allows molecular profiling of rheumatoid arthritis and identification of pathophysiological targets

This study was undertaken to evaluate the possibility to obtain a molecular signature of rheumatoid arthritis (RA) comparatively osteoarthritis (OA), and to lay the bases to develop new diagnostic tools and identify new targets. Microarray technology was used for such an analysis. The gene expression profiles of synovial tissues from patients with confirmed RA, and patients with OA were established and compared. A set of 63 genes was selected, based, more specifically, on their overexpression or underexpression in RA samples compared to OA. Results for six of these genes have been verified by quantitative PCR using both samples identical to those used in the microarray experiments and entirely separate samples. Expression profile of the 48 known genes allowed the correct classification of additional RA and OA patients. Furthermore, the distinct expression of three of the selected genes was also studied by quantitative RT-PCR in cultured synovial cells. Detailed analysis of the expression profile of the selected genes provided evidence for dysregulated biological pathways, pointed out to chromosomal location and revealed novel genes potentially involved in RA. It is proposed that such an approach allows valuable diagnosis/prognostics tools in RA to be established and potential targets for combating the disease to be identified.

Melanoma heterogeneity: differential, invasive, metastatic properties and profiles of cathepsin B, D and L activities in subclones of the B16F10-NEX2 cell line

Tumour cell lines and in vivo growing tumours are heterogeneous, comprising different cell clones. To understand why some cells primarily invade a tissue, while others are more apt to metastasize, several clones from the established B16F10-Nex2 cell line were isolated and 10 viable cells of each clone were injected intravenously into C57Bl/6 and Balb/c mice. Two cell clones (Nex2B and Nex2D) showed contrasting metastatic abilities. Clone 2D rather than clone 2B colonized the lungs of both mice after intravenous injection. Surprisingly, clone 2B grew more rapidly than 2D after subcutaneous implantation, significantly reducing the survival of injected mice. Clearly, dissociation between subcutaneous growth and metastatic ability was observed in clones from the same tumour cell lineage. Clone Nex2B continuously released proteolytic activity, including cathepsin B, and showed a greater capacity to invade Matrigel than clone Nex2D. Clone Nex2D accumulated cathepsins B, D and L intracellularly and released a moderate proteolytic activity in vitro that was inhibited with the time of incubation. E-64-treated Nex2B cells injected subcutaneously showed a significant delay in tumour development and increased survival of challenged animals. A similar result was obtained on treatment of clone 2B with chagasin, a cysteine proteinase inhibitor from Trypanosoma cruzi, even at 2 microM. Clone Nex2D was less sensitive to pretreatment with inhibitors of cysteine proteases for tumour development in vivo. Our results suggest that, in a tumour cell population, cells dissociate into metastatic and non-metastatic subtypes, and that release or accumulation of cathepsins can be a differential trait of these cells.

Optimized transfection of diced siRNA into mature primary human osteoclasts: Inhibition of cathepsin K mediated bone resorption by siRNA

Osteoclasts are large multinucleated cells responsible for bone resorption. Bone resorption is dependent on the liberation of calcium by acid and protease destruction of the bone matrix by proteinases. The key proteinase produced by the osteoclast is cathepsin K. Targeted knock-down of cathepsin K was performed using small inhibitory RNA (siRNA). siRNA is a method that introduces short double-stranded RNA molecules that instruct the RNA-induced silencing complex (RISC) to degrade mRNA species complementary to the siRNA. Transfection of siRNA by lipid cations allows for short-term inhibition of expression of the targeted gene. We show that transfection of primary human osteoclasts with siRNA to cathepsin K reduces expression by > or = 60% and significantly inhibits bone resorption with a reduction of both resorption pit numbers (P = 0.018) and resorbed area (P = 0.013). We also show that FuGENE 6 is an effective lipid transfection reagent with which to transfect primary human osteoclasts, that does not produce off-target effects.

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

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

A cytochemical assay for osteoclast cathepsin K activity

Cathepsin K is a member of the papain superfamily of cysteine proteases and plays a pivotal role in osteoclast-mediated bone resorption. This enzyme is an excellent target for antiresorptive therapies for osteopenic disorders such as osteoporosis.(1) Although isolated inhibitor studies on purified enzymes is required to discover potent and selective inhibitors of cathepsin K, a quantitative cytochemical assay(2) for cathepsin K would allow inhibitors to be tested on actual osteoclasts within sections of bone. Furthermore cathepsin K activity could be used to identify and analyse osteoclasts at definitive stages of their lifespan. A cytochemical assay is described that localizes osteoclast cathepsin K activity in unfixed, undecalcified cryostat sections of animal and human bone.

Intracellular membrane trafficking in bone resorbing osteoclasts

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

Matrix metalloproteinases (MMP) and cathepsin K contribute differently to osteoclastic activities

The best established proteolytic event of osteoclasts is bone matrix solubilization by the cysteine proteinase cathepsin K. Here, however, we draw the attention on osteoclastic activities depending on matrix metalloproteinases (MMPs). We discuss the observations supporting that MMPs contribute significantly to bone matrix solubilization in specific areas of the skeleton and in some developmental and pathological situations. Our discussion takes into account (1) the characteristics of the bone remodeling persisting in the absence of cathepsin K, (2) the ultrastructure of the resorption zone in response to inactivation of MMPs and of cathepsin K in different bone types, (3) bone resorption levels in MMP knockout mice compared to wild-type mice, (4) the identification of MMPs in osteoclasts and surrounding cells, and (5) the effect of different bone pathologies on the serum concentrations of specific collagen fragments believed to discriminate between cathepsin K and MMP cleavage. Next, we provide evidence that MMPs are very critical for osteoclast migration, thereby controlling also the cell-matrix interactions required for cell attachment/detachment. The evidence supporting this role is based on a model of osteoclast recruitment in primitive long bones, an assay of osteoclast invasion through collagen gel, and the effect of proteinase inhibitors/knockouts in these models. Furthermore, we mention observations indicating a role of MMPs in initiation of bone resorption. Finally, we emphasize the many distinct ways MMPs may alter focally the extracellular environment thereby regulating the osteoclast behavior. Although the understanding of MMPs in osteoclast biology is rapidly expanding, it is suspected that important roles remain to be discovered.

The regulation of bone resorption in tooth formation and eruption processes in mouse alveolar crest devoid of cathepsin k

Osteoclastic bone resorption has recently been implicated in the tooth formation and eruption in alveolar bone. Cathepsin K (CK) is a cysteine proteinase expressed predominantly in osteoclasts and is believed to play a critical role in degradation of bone matrix proteins. Here we present evidence that the alveolar bone resorption is essential for the tooth formation and that eruption proceeds normally in CK-deficient (CK-/-) mice. Radiographic and histological analyses revealed that the alveolar bone from these animals had no significant abnormalities during the tooth development between 5 and 28 days after birth. The tooth crown was normally erupted through the alveolar bone layer at 28 days after birth. The number of tartrate-resistant acid phosphatase-positive multinuclear cells in the alveolar bone around the tooth germ was apparently increased in 5-day-old CK-/- mice compared with age-matched littermates. More important, however, the immunohistochemical localization of matrix metalloproteinase-9 (MMP-9) was clearly increased in the CK-/- osteoclasts. In contrast, no significant difference in the immunoreactivity for cathepsin D was observed between the CK-/- osteoclasts and the wild-type ones. These results indicate that CK-/- osteoclasts are fully differentiated and are capable of degrading the organic phase of alveolar bone during the tooth formation and eruption, which may result from the compensatory action by MMP-9 increasingly expressed in the osteoclasts.

Beta-amyloid peptide in regulated secretory vesicles of chromaffin cells: evidence for multiple cysteine proteolytic activities in distinct pathways for beta-secretase activity in chromaffin vesicles

A key factor in Alzheimer's disease (AD) is the beta-secretase activity that is required for the production of beta-amyloid (Abeta) peptide from its amyloid precursor protein (APP) precursor. In this study, the majority of Abeta secretion from neuronal chromaffin cells was found to occur via the regulated secretory pathway, compared with the constitutive secretory pathway; therefore, beta-secretase activity in the regulated secretory pathway was examined for the production and secretion of Abeta in chromaffin cells obtained from in vivo adrenal medullary tissue. The presence of Abeta(1-40) in APP-containing chromaffin vesicles, which represent regulated secretory vesicles, was demonstrated by radioimmunoassay (RIA) and reverse-phase high-performance liquid chromatography. These vesicles also contain Abeta(1-42), measured by RIA. Significantly, regulated secretion of Abeta(1-40) from chromaffin cells represented the majority of secreted Abeta (> 95% of total secreted Abeta), compared with low levels of constitutively secreted Abeta(1-40). These results indicate the importance of Abeta production and secretion in the regulated secretory pathway as a major source of extracellular Abeta. Beta-secretase activity in isolated chromaffin vesicles was detected with the substrate Z-Val-Lys-Met-/MCA (methylcoumarinamide) that contains the beta-secretase cleavage site. Optimum beta-secretase activity in these vesicles required reducing conditions and acidic pH (pH 5-6), consistent with the in vivo intravesicular environment. Evidence for cysteine protease activity was shown by E64c inhibition of Z-Val-Lys-Met-MCA-cleaving activity, and E64c inhibition of Abeta(1-40) production in isolated chromaffin vesicles. Chromatography resolved the beta-secretase activity into two distinct proteolytic pathways consisting of: (i) direct cleavage of the beta-secretase site at Met-/Asp by two cysteine proteolytic activities represented by peaks Il-A and Il-B, and (ii) an aminopeptidase-dependent pathway represented by peak I cysteine protease activity that cleaves between Lys-/Met, followed by Met-aminopeptidase that would generate the beta-secretase cleavage site. Treatment of chromaffin cells in primary culture with the cysteine protease inhibitor E64d reduced the production of the beta-secretase product, a 12-14 kDa C-terminal APP fragment. In addition, BACE 1 and BACE 2 were detected in chromaffin vesicles; BACE 1 represented a small fraction of total beta-secretase activity in these vesicles. These results illustrate that multiple cysteine proteases, in combination with BACE 1, contribute to beta-secretase activity in the regulated secretory pathway. These results complement earlier findings for BACE 1 as beta3-secretase for Abeta production in the constitutive secretory pathway that provides basal secretion of Abeta into conditioned media. These findings suggest that drug inhibition of several proteases may be required for reducing Abeta levels as a potential therapeutic approach for AD.

Localisation and activity of cathepsins K and B in equine osteoclasts

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

Tissue inhibitor of metalloproteinases 1 and 2 directly stimulate the bone-resorbing activity of isolated mature osteoclasts

Tissue inhibitor metalloproteinases 1 (TIMP-1) and 2 have been reported to inhibit bone resorption. However, here, we report the direct action of both TIMP-1 and TIMP-2 on isolated rabbit mature osteoclasts to stimulate their bone-resorbing activity at significantly lower concentrations (approximately ng/ml) than those (approximately microg/ml) required for the inhibition of bone resorption. The cell population used in this study consisted of a mature osteoclast population with >95% purity. TIMP-1 (approximately 50 ng/ml) and TIMP-2 (approximately 8-10 ng/ml) increased the pit area excavated by the isolated mature osteoclasts. The stimulatory effects of TIMPs were abolished by simultaneous addition of anti-TIMP antibodies. At higher concentrations, the stimulation of bone resorption decreased reversely to the control level. The magnitude of the stimulatory effect of TIMP-2 was more than that of TIMP-1. Metalloproteinase inhibitors such as BE16627B and R94138 could not replace TIMPs with respect to the bone-resorbing activity, suggesting that the osteoclast-stimulating activity of TIMPs was independent of the inhibitory activity on matrix metalloproteinases (MMPs). TIMPs stimulated tyrosine phosphorylation of cellular proteins in the isolated mature osteoclasts. Both herbimycin A, an inhibitor of tyrosine kinases, and PD98059 and U0126, inhibitors of mitogen-activated protein kinase (MAPK), completely blocked the TIMP-induced stimulation of osteoclastic bone-resorbing activity. On the plasma membrane of osteoclasts, some TIMP-2-binding proteins were detected by a cross-linking experiment. These findings show that TIMPs directly stimulate the bone-resorbing activity of isolated mature osteoclasts at their physiological concentrations and that the stimulatory action of TIMPs is likely to be independent of their activities as inhibitors of MMPs.

Calpains play a role in insulin secretion and action

Studies of the genetic basis of type 2 diabetes suggest that variation in the calpain-10 gene affects susceptibility to this common disorder, raising the possibility that calpain-sensitive pathways may play a role in regulating insulin secretion and/or action. Calpains are ubiquitously expressed cysteine proteases that are thought to regulate a variety of normal cellular functions. Here, we report that short-term (4-h) exposure to the cell-permeable calpain inhibitors calpain inhibitor II and E-64-d increases the insulin secretory response to glucose in mouse pancreatic islets. This dose-dependent effect is observed at glucose concentrations above 8 mmol/l. This effect was also seen with other calpain inhibitors with different mechanisms of action but not with cathepsin inhibitors or other protease inhibitors. Enhancement of insulin secretion with short-term exposure to calpain inhibitors is not mediated by increased responses in intracellular Ca2+ or increased glucose metabolism in islets but by accelerated exocytosis of insulin granules. In muscle strips and adipocytes, exposure to both calpain inhibitor II and E-64-d reduced insulin-mediated glucose transport. Incorporation of glucose into glycogen in muscle also was reduced. These results are consistent with a role for calpains in the regulation of insulin secretion and insulin action.

Regulation of cell growth by redox-mediated extracellular proteolysis of platelet-derived growth factor receptor beta

Redox-regulated processes are important elements in various cellular functions. Reducing agents, such as N-acetyl-l-cysteine (NAC), are known to regulate signal transduction and cell growth through their radical scavenging action. However, recent studies have shown that reactive oxygen species are not always involved in ligand-stimulated intracellular signaling. Here, we report a novel mechanism by which NAC blocks platelet-derived growth factor (PDGF)-induced signaling pathways in hepatic stellate cells, a fibrogenic player in the liver. Unlike in vascular smooth muscle cells, we found that reducing agents, including NAC, triggered extracellular proteolysis of PDGF receptor-beta, leading to desensitization of hepatic stellate cells toward PDGF-BB. This effect was mediated by secreted mature cathepsin B. In addition, type II transforming growth factor-beta receptor was also down-regulated. Furthermore, these events seemed to cause a dramatic improvement of rat liver fibrosis. These results indicated that redox processes impact the cell's response to growth factors by regulating the turnover of growth factor receptors and that "redox therapy" is promising for fibrosis-related disease.

Comparison in localization between cystatin C and cathepsin K in osteoclasts and other cells in mouse tibia epiphysis by immunolight and immunoelectron microscopy

We compared the distribution of a cysteine proteinase inhibitor, cystatin C, with that of cathepsin K in osteoclasts of the mouse tibia by immunolight and immunoelectron microscopy. Light microscopically, strong immunoreactivity for cystatin C was found extracellularly along the resorption lacuna and intracellularly in the organelles of osteoclasts. In serial sections, various patterns of cystatin C and cathepsin K localization were seen, specifically: (1) some resorption lacuna were positive for both cystatin C and cathepsin K; (2) others were positive for either cystatin C or cathepsin K, but not both; and (3) some lacuna were negative for both. In osteoclasts, the localization of cystatin C was similar to that of cathepsin K. Furthermore, cystatin C immunoreactivity was detected in preosteoclasts and osteoblasts, whereas cathepsin K was seen only in preosteoclasts. Electron microscopically, cystatin C immunoreactive products were found in the rough endoplasmic reticulum (ER), Golgi apparatus, vesicles, granules, and vacuoles of osteoclasts. These cystatin C-positive vesicles had fused or were in the process of fusion with the ampullar vacuoles (extracellular spaces) containing cystatin C-positive, fragmented, fibril-like structures. The extracellular cystatin C was deposited on and between the cytoplasmic processes of ruffled borders, and on and between type I collagen fibrils. In the basolateral region of osteoclasts, cystatin C-positive vesicles and granules also fused with vacuoles that contained cystatin C-positive or negative fibril-like structures. These results indicate that osteoclasts not only synthesize and secrete cathepsin K from the ruffled border into the bone resorption lacunae, but also a cysteine proteinase inhibitor, cystatin C. Therefore, it is suggested that cystatin C regulates the degradation of bone matrix by cathepsin K, both extracellularly and intracellularly.

Human osteoclast cathepsin K is processed intracellularly prior to attachment and bone resorption

Cathepsin K is a member of the papain superfamily of cysteine proteases and has been proposed to play a pivotal role in osteoclast-mediated bone resorption. We have developed a sensitive cytochemical assay to localize and quantify osteoclast cathepsin K activity in sections of osteoclastoma and human bone. In tissue sections, osteoclasts that are distant from bone express high levels of cathepsin K messenger RNA (mRNA) and protein. However, the majority of the cathepsin K in these cells is in an inactive zymogen form, as assessed using both the cytochemical assay and specific immunostaining. In contrast, osteoclasts that are closer to bone contain high levels of immunoreactive mature cathepsin K that codistributes with enzyme activity in a polarized fashion toward the bone surface. Polarization of active enzyme was clearly evident in osteoclasts in the vicinity of bone. The osteoclasts apposed to the bone surface were almost exclusively expressing the mature form of cathepsin K. These cells showed intense enzyme activity, which was polarized at the ruffled border. These results suggest that the in vivo activation of cathepsin K occurs intracellularly, before secretion into the resorption lacunae and the onset of bone resorption. The processing of procathepsin K to mature cathepsin K occurs as the osteoclast approaches bone, suggesting that local factors may regulate this process.

Quantification of the expression levels of lysosomal cysteine proteinases in purified human osteoclastic cells by competitive RT-PCR

Cathepsin K is a lysosomal cysteine proteinase (LCP) predominantly expressed in osteoclasts. This study was conducted to evaluate the importance of human cathepsin K for osteoclastic bone resorption relative to that of other LCPs. To accomplish this, we quantitatively determined the expression levels of major LCPs (cathepsins B, K, L, and S) in human osteoclastic cells by using competitive RT-PCR. Giant cell tumor of bone (GCT) was used as a source of human osteoclastic cells, since the tissue was shown to contain a large number of cells satisfying the criteria for typical osteoclasts. The involvement of LCPs in the bone-resorption process by the GCT cells was confirmed by showing that trans-epoxysucciny-L-leucylamido-(4-guanidino) butane (E-64), a nonselective cysteine proteinase inhibitor, exerted an inhibitory effect on the pit formation. We isolated osteoclast-like cells (OLCs) positive for tartrate-resistant acid phosphatase (TRAP) and cathepsin K from the GCT tissue to a degree of almost 95% purity. In these cells, the expression of cathepsin K was shown to be approximately 20-, 130-, and 410-fold stronger than that of cathepsins B, L, and S, respectively. A similar result was obtained when human bone marrow cells in culture were used as another source of OLCs. Further, we found that cathepsin K was expressed in OLCs far more strongly than in several human nonosteoclastic cells including osteoblastic cell lines. The abundant and selective expression of cathepsin K in OLCs relative to that of other LCPs suggests that cathepsin K is mainly responsible for osteoclastic degradation of human bone matrix.

Meaningful relationships: the regulation of the Ras/Raf/MEK/ERK pathway by protein interactions

The Ras/Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular-signal-regulated kinase) pathway is at the heart of signalling networks that govern proliferation, differentiation and cell survival. Although the basic regulatory steps have been elucidated, many features of this pathway are only beginning to emerge. This review focuses on the role of protein-protein interactions in the regulation of this pathway, and how they contribute to co-ordinate activation steps, subcellular redistribution, substrate phosphorylation and cross-talk with other signalling pathways.

gp130 Cytokine family and bone cells

Bone tissue is continually being remodelled according to physiological circumstances. Two main cell populations (osteoblasts and osteoclasts) are involved in this process, and cellular activities (including cell differentiation) are modulated by hormones, cytokines and growth factors. Within the last 20 years, many factors involved in bone tissue metabolism have been found to be closely related to the inflammatory process. More recently, a cytokine family sharing a common signal transducer (gp130) had been identified, which appears to be a key factor in bone remodelling. This family includes interleukin 6, interleukin 11, oncostatin M, leukaemia inhibitory factor, ciliary neurotrophic factor and cardiotrophin-1. This paper provides an exhaustive review of recent knowledge on the involvement of gp130 cytokine family in bone cell (osteoblast, osteoclast, etc.) differentiation/activation and in osteoarticular pathologies.

Characterization of mouse cathepsin R, a new member of a family of placentally expressed cysteine proteases

A new mouse cysteine protease, termed cathepsin R, has been identified. The complete nucleotide sequence of this gene was derived from a set of cDNAs generated from 15.5-day mouse placenta. Sequence analysis revealed an open reading frame encoding a 334 amino acid long polypeptide closely related to placentally expressed cathepsins P, Q, and M. RT-PCR analysis indicated that cathepsin R is only expressed in placenta and thus is a new member of the emerging family of cathepsins whose expression is regulated during mouse embryonic development. Modeling and structural analysis suggests that cathepsin R will have a restricted substrate specificity when compared to that of cathepsin L.

A peptidyl derivative structurally based on the inhibitory center of cystatin C inhibits bone resorption in vitro

Human cystatin C is a cysteine proteinase inhibitor belonging to the cystatin superfamily, which previously has been shown to inhibit bone resorption in bone organ culture. The aminoterminal segment, Arg(8)-Leu(9)-Val(10)-Gly(11) (RLVG), of the single polypeptide chain of cystatin C constitutes an essential part of its inhibitory center. In the present study, the effect of benzyloxycarbonyl-Arg(8)-Leu(9)-Val(10)-Gly(11)-diazomethane (Z-RLVG-CHN(2)) on bone resorption in vitro was compared with the effects of cystatin C and calcitonin. Bone resorption was assessed by the release of (45)Ca and (3)H from mouse calvarial bones prelabeled with [(45)Ca]CaCl(2) and [(3)H]-proline, respectively. Z-RLVG-CHN(2) concentration-dependently inhibited the release of (45)Ca and (3)H in bones stimulated by parathyroid hormone (PTH), with half-maximal inhibition obtained at 1 micromol/L. The inhibitory actions of Z-RLVG-CHN(2) and cystatin C were persistent, whereas action induced initially by calcitonin was lost with time. The inhibition caused by Z-RLVG-CHN(2) and cystatin C on PTH-stimulated (45)Ca release was observed after 6 h, whereas inhibition by calcitonin was seen already after 2 h. In contrast, the inhibitory effects of Z-RLVG-CHN(2) and cystatin C, as well as that of calcitonin, on (3)H release was seen already after 2 h. Z-RLVG-CHN(2), in which the reactive carboxyterminal diazomethane was substituted by nonreactive groups [-OH, -NH(2), or -N(CH(3))(2)], resulted in peptidyl derivatives, which, in contrast to Z-RLVG-CHN(2) and cystatin C, inhibited neither cysteine proteinases nor bone resorption. In contrast to wild-type cystatin C, recombinant human cystatin C with Gly substitutions for residues Arg(8), Leu(9), Val(10), and Trp(106), and with low or nonexistent affinity for cysteine proteinases, did not display any inhibitory effect on bone resorption. These data strongly indicate that Z-RLVG-CHN(2) inhibits bone resorption in vitro by a mechanism that seems primarily to be due to an inhibition of bone matrix degradation via cysteine proteinases. The data also corroborate the hypothesis that cystatin C inhibits bone resorption by virtue of its cysteine proteinase inhibitory capacity.

Human growth hormone stimulates proteinase activities of rabbit bone cells via IGF-I

Human growth hormone (hGH) and human insulin-like growth factor-I (hIGF-I) are known to have a marked influence on osteoclastic formation and bone resorption in an unfractionated rabbit bone cell model. This study investigated the effects of both of these factors on the induction of cysteine-proteinases and matrix metalloproteinase-2 (MMP-2) and MMP-9. After 4 days of rabbit bone cell culture, hGH and hIGF-I significantly modulated cathepsin, MMP-9 (latent form) and MMP-2 (active form) activities. Similar studies were performed in the presence of parathyroid hormone (hPTH). hPTH increased MMP-2 and MMP-9 activities whereas it had no effect on the production of cathepsins by bone cells. When neutralizing anti-hIGF-1 antiserum was added to the culture, the stimulatory effects of hGH were totally abolished, indicating that hGH-modulated cathepsin and metalloproteinase activities were partly mediated by local hIGF-I secretion. Cysteine-proteinase activities released by purified osteoclasts were very low and were not modulated by hGH and h-IGF-I. However, hIGF-I but not hGH increased MMP-2 and MMP-9 activities released by purified osteoclasts. It may be concluded that hGH markedly stimulates the expression of proteinases in total rabbit bone cells via local hIGF-I production by stromal cells. Cysteine-proteinase activities are mainly produced by non-osteoclastic cells, while MMP-2 and MMP-9 modulated by hIGF-I are mainly expressed by osteoclastic cells.

Proteinases in bone resorption: obvious and less obvious roles

Bone resorption is critical for the development and the maintenance of the skeleton, and improper regulation of bone resorption leads to pathological situations. Proteinases are necessary for this process. In this review, we show that this need of proteinases is not only because they are required for the solubilization of bone matrix, but also because they are key components of the mechanism that determines where and when bone resorption will be initiated. Moreover, there are indications that proteinases may also determine whether resorption will be followed by bone formation. Some of the proteinases involved in these different steps of the resorption processes were recently identified, as for instance cathepsin K, MMP-9 (gelatinase B), and interstitial collagenase. However, there is also increasing evidence showing that the critical proteinase(s) may vary depending on the bone type or on other factors.