Selective inhibition of cysteine proteinases by Z-Phe-AlaCH2F suppresses digestion of collagen by fibroblasts and osteoclasts

Distinctive subdomains in the resorbing surface of osteoclasts

We employed a novel technique to inspect the substrate-apposed surface of activated osteoclasts, the cells that resorb bone, in the scanning electron microscope. The surface revealed unexpected complexity. At the periphery of the cells were circles and crescents of individual or confluent nodules. These corresponded to the podosomes and actin rings that form a 'sealing zone', encircling the resorptive hemivacuole into which protons and enzymes are secreted. Inside these rings and crescents the osteoclast surface was covered with strips and patches of membrane folds, which were flattened against the substrate surface and surrounded by fold-free membrane in which many orifices could be seen. Corresponding regions of folded and fold-free membrane were found by transmission electron microscopy in osteoclasts incubated on bone. We correlated these patterns with the distribution of several proteins crucial to resorption. The strips and patches of membrane folds corresponded in distribution to vacuolar H+-ATPase, and frequently co-localized with F-actin. Cathepsin K localized to F-actin-free foci towards the center of cells with circular actin rings, and at the retreating pole of cells with actin crescents. The chloride/proton antiporter ClC-7 formed a sharply-defined band immediately inside the actin ring, peripheral to vacuolar H+-ATPase. The sealing zone of osteoclasts is permeable to molecules with molecular mass up to 10,000. Therefore, ClC-7 might be distributed at the periphery of the resorptive hemivacuole in order to prevent protons from escaping laterally from the hemivacuole into the sealing zone, where they would dissolve the bone mineral. Since the activation of resorption is attributable to recognition of the αVβ3 ligands bound to bone mineral, such leakage would, by dissolving bone mineral, release the ligands and so terminate resorption. Therefore, ClC-7 might serve not only to provide the counter-ions that enable proton pumping, but also to facilitate resorption by acting as a 'functional sealing zone'.

The resorptive apparatus of osteoclasts supports lysosomotropism and increases potency of basic versus non-basic inhibitors of cathepsin K

In mice and humans, the effect of genetic deficiency of cathepsin K (catK) is impaired bone resorption, or osteopetrosis. Inhibition of catK is therefore a promising strategy for the treatment of osteoporosis. The enzyme acts in an acid environment. This provides a further potential opportunity: if the inhibitor is basic it is more likely to accumulate in membrane-bound acidic compartments (lysosomotropism), so minimizing off-target effects. However, the resorptive hemivacuole is not membrane-bound, and so might not retain lysosomotropic compounds. We therefore elected to determine whether the osteoclastic resorptive apparatus supports such accumulation. First, we attempted to compare the persistence of a lysosomotropic dye in the hemivacuole versus intracellular vesicles. To our surprise the dye could not be detected in the ruffled border region by confocal microscopy. We found that this could be explained by the tight packing of the folds of the ruffled border, and their close apposition to the bone surface. We also found that the dye persisted similarly in resorbing osteoclasts and macrophages, consistent with the notion that resorbing osteoclasts support lysosomotropism. Next, we compared the ability of basic and non-basic inhibitors of catK to suppress bone resorption by human osteoclasts. We found that basic compounds were considerably more potent than non-basic compounds at suppression of osteoclastic resorption than would be anticipated from their potency as enzyme inhibitors. Also consistent with osteoclastic lysosomotropism, basic inhibitors suppressed resorption for substantially longer than a non-basic inhibitor after washout from cell cultures. Furthermore, selectivity of basic inhibitors for inhibition of catK versus other cathepsins persisted: concentrations that inhibited catK in osteoclasts had no detectable effect on cathepsin S (catS) in a cell-based assay. This data is consistent with accumulation and enrichment of such basic inhibitors in the resorptive apparatus of the osteoclast, allowing for prolonged efficacy at the intended site of action. Our results suggest a major advantage for lysosomotropic compounds as inhibitors of bone resorption by osteoclasts in osteoporosis and other diseases caused by excessive osteoclastic activity.

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.

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.

The bone lining cell: its role in cleaning Howship's lacunae and initiating bone formation

In this study we investigated the role of bone lining cells in the coordination of bone resorption and formation. Ultrastructural analysis of mouse long bones and calvariae revealed that bone lining cells enwrap and subsequently digest collagen fibrils protruding from Howship's lacunae that are left by osteoclasts. By using selective proteinase inhibitors we show that this digestion depends on matrix metalloproteinases and, to some extent, on serine proteinases. Autoradiography revealed that after the bone lining cells have finished cleaning, they deposit a thin layer of a collagenous matrix along the Howship's lacuna, in close association with an osteopontin-rich cement line. Collagenous matrix deposition was detected only in completely cleaned pits. In bone from pycnodysostotic patients and cathepsin K-deficient mice, conditions in which osteoclastic bone matrix digestion is greatly inhibited, bone matrix leftovers proved to be degraded by bone lining cells, thus indicating that the bone lining cell "rescues" bone remodeling in these anomalies. We conclude that removal of bone collagen left by osteoclasts in Howship's lacunae is an obligatory step in the link between bone resorption and formation, and that bone lining cells and matrix metalloproteinases are essential in this process.

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.

The history of Z-VAD-FMK, a tool for understanding the significance of caspase inhibition

Dr. Robert Smith is one of the pioneers in histochemistry. One of his most important achievements is the recognition of proteolysis as a major physiological and pathophysiological process. As a consequence, he developed selective fluorogenic and chromogenic substrates and specific inhibitors of proteases that allow the (histochemical) analysis of protease activity. One of the latest successes is the design of Z-VAD-fluoromethylketone (FMK), the specific caspase inhibitor, that is a key compound for studies on apoptosis. Its development was originally meant for therapeutic use but unforeseen cytotoxicity of a metabolic derivative of the FMK compound disabled its potential as a drug. However, as a tool for fundamental research it is a great success. The history of Z-VAD-FMK is an example of the creative brain and the tireless perseverance of Robert Smith for which histochemistry and cytochemistry owes him so much. This history of Z-VAD-FMK is a well-deserved tribute at the occasion of his 70th birthday.

Comparative localization of cathepsin B protein and activity in colorectal cancer

Cathepsin B is a lysosomal cysteine proteinase that may participate in cancer progression. We compared localization of its protein and activity during progression of human colorectal cancer. In adenomas and carcinomas, protein expression and, particularly, activity were elevated compared with those in normal colorectal mucosa. In normal mucosa, cathepsin B protein expression was moderate in stroma and variable in epithelium, whereas activity was mainly present in distinct areas of stroma directly underneath the surface of the colon and in epithelium at the surface of the colon. Stroma in adenomas and carcinomas contained moderate to high protein levels but little activity except for areas of angiogenesis, inflammation, and necrosis, in which activity was high. In adenomas and the majority of well-differentiated carcinomas and moderately differentiated carcinomas, cathepsin B protein and activity were found in granular form in the epithelium, close to the basement membrane. Protein and activity levels were low and diffusely distributed in cancer cells in the remainder of the well-differentiated and moderately differentiated carcinomas and in all poorly differentiated carcinomas. Invasive fronts in most cancers contained moderate protein levels but high activity. We conclude that (a) activity localization is essential to understand the role of cathepsin B in cancer progression, and (b) cathepsin B activity in human colon is associated with invasion of cancer cells, endothelial cells, and inflammatory cells, and in cell death, both apoptotic and necrotic.

Direct comparison of the sensitivity of enzyme histochemical and immunohistochemical methods: cathepsin B expression in human colorectal mucosa

Immunohistochemical localization of the proteinase cathepsin B has been compared directly with localization of cathepsin B activity with a catalytic (enzyme) histochemical method. The 2 approaches demonstrate principally different aspects of an enzyme. The immunohistochemical method localizes the enzyme protein whether it is active or not whereas the catalytic method visualizes the functionally active enzyme only. Sensitivity of both approaches to localize low amounts of enzyme protein or activity has never been compared. In the present study, we show that cathepsin B protein has a wider distribution pattern than cathepsin B activity in human colorectal mucosa, which means that inactive cathepsin B protein is present. With respect to sensitivity of the methods, it is shown that cathepsin B protein could only be demonstrated properly when strong signal amplification was applied by using Nanogold with silver enhancement, whereas activity could be demonstrated with a simple and direct fluorogenic histochemical assay. It is concluded that catalytic histochemical methods are relatively simple methods for the localization of activity of enzymes in tissues and cells and that their sensitivity is high in comparison with immunohistochemical methods.

Signal amplification in immunohistochemistry at the light microscopic level using biotinylated tyramide and nanogold-silver staining

Signal amplification techniques greatly enhance the sensitivity of immunohistochemical (IHC) and in situ hybridization (ISH) methods. In particular, catalyzed signal amplification (CSA) using labeled tyramide or Nanogold-silver staining is an important signal amplification tool. We have applied a combination of both techniques, as has been introduced for ISH, for a further increase in sensitivity of an IHC method to detect cathepsin B. This lysosomal proteinase can also be expressed extracellularly, particularly in relation to cancer metastasis. Higher sensitivity of the IHC method was needed because existing methods failed to demonstrate cathepsin B protein where cathepsin B activity was found with a fluorescence enzyme histochemical method. Combined CSA and Nanogold-silver staining provided the sensitivity that was required. Moreover, this signal amplification method enabled the use of a 10-fold lower concentration of primary antibody (1 microg/ml). Nonspecific background staining was low provided that endogenous biotin, avidin, and peroxidase were completely blocked. The method was reproducible when all steps, and particularly the silver enhancement step, were rigidly controlled. The method resulted in localization patterns of cathepsin B protein that were in agreement with those of cathepsin B activity in serial sections of rat liver containing colon cancer metastases. We concluded that combined application of CSA and Nanogold-silver staining provides high sensitivity for immunohistochemical methods and that activity localization by an enzyme histochemical method is a very attractive alternative to IHC localization of an enzyme because it is at least as sensitive, it is rapid and simple, and it provides direct information on the function of an enzyme.

In vivo inhibition of cysteine proteinases delays the onset of growth of human pancreatic cancer explants

An animal model was used to study the effects of oral treatment with a small molecular selective inhibitor of cysteine proteinases, Z-Phe-Arg-fluoromethylketone (Z-Phe-Arg-FMK) on primary tumour development. Poorly differentiated rapidly growing and moderately differentiated slowly growing human pancreatic tumours were implanted in the neck of nude mice that were orally treated or not with the inhibitor. Growth rates of the tumours were determined during 38 days after implantation. The poorly differentiated tumours were not affected by treatment with the inhibitor. Development of the moderately differentiated tumours was inhibited significantly by Z-Phe-Arg-FMK treatment. Moreover, the amount of stroma was increased and the volume of cancer cells was reduced in the moderately differentiated tumours that had grown in the treated animals. Reduction in size of the tumours was not achieved by reduction in growth rate but in a delay of the onset of growth. It is concluded that cysteine proteinases play a transient role at the start of tumour development only when cancer cells are surrounded by stroma as was the case in the moderately differentiated but not in the poorly differentiated pancreatic tumours. However, this role of cysteine proteinases can easily be taken over by other proteinases.

Detection of collagenase-induced damage of collagen by 9A4, a monoclonal C-terminal neoepitope antibody

To determine whether the collagen network is compromised by collagenase during acute inflammation, a monoclonal antibody (9A4) was developed with specificity for the C-terminal neoepitope sequence generated by collagenase-cleavage of type II collagen (Gly-Pro-Pro-Gly-Pro-Gln-Gly-COOH). 9A4 was shown to detect the collagen collagenase-cleavage neoepitope with a K = 1.7 x 10(-7) M (type II) and K = 2 x 10(-6) M (type I). It does not recognize uncleaved native or denatured collagen. Articular cartilage from control animals is unstained by 9A4. During acute inflammation elicited in hamsters by intra-articular LPS, positive staining for the 9A4 neoepitope indicated the collagen was damaged. Wheel running exercise was used to apply stress to control cartilage and cartilage from animals with damaged collagen. After 6 months of running, the cartilage from normal animals was unaffected. By contrast, in the group with damaged collagen, the cartilage was fibrillated in all animals and in half of those, the cartilage failed and bony eburnation resulted.

Functional heterogeneity of osteoclasts: matrix metalloproteinases participate in osteoclastic resorption of calvarial bone but not in resorption of long bone

Data in the literature suggest that site-specific differences exist in the skeleton with respect to digestion of bone by osteoclasts. Therefore, we investigated whether bone resorption by calvarial osteoclasts (intramembranous bone) differs from resorption by long bone osteoclasts (endochondral bone). The involvement of two major classes of proteolytic enzymes, the cysteine proteinases (CPs) and matrix metalloproteinases (MMPs), was studied by analyzing the effects of selective low molecular weight inhibitors of these enzymes on bone resorption. Mouse tissue explants (calvariae and long bones) as well as rabbit osteoclasts, which had been isolated from both skeletal sites and subsequently seeded on bone slices, were cultured in the presence of inhibitors and resorption was analyzed. The activity of the CP cathepsins B and K and of MMPs was determined biochemically (CPs and MMPs) and enzyme histochemically (CPs) in explants and isolated osteoclasts. We show that osteoclastic resorption of calvarial bone depends on activity of both CPs and MMPs, whereas long bone resorption depends on CPs, but not on the activity of MMPs. Furthermore, significantly higher levels of cathepsin B and cathepsin K activities were expressed by long bone osteoclasts than by calvarial osteoclasts. Resorption of slices of bovine skull or cortical bone by osteoclasts isolated from long bones was not affected by MMP inhibitors, whereas resorption by calvarial osteoclasts was inhibited. Inhibition of CP activity affected the resorption by the two populations of osteoclasts in a similar way. We conclude that this is the first report to show that significant differences exist between osteoclasts of calvariae and long bones with respect to their bone resorbing activities. Resorption by calvarial osteoclasts depends on the activity of CPs and MMPs, whereas resorption by long bone osteoclasts depends primarily on the activity of CPs. We hypothesize that functionally different subpopulations of osteoclasts, such as those described here, originate from different sets of progenitors.

Collagen-phagocytosing ability of periodontal osteoblasts at the bone surface

The collagen-phagocytosing activity of osteoblasts at the alveolar bone-ligament interface of rat mandibular first molars was investigated both histologically and histochemically. Alveolar bones of male Wistar rats (6 months old) were used in this study. Collagen-containing phagosomes appeared in cuboidal osteoblasts aligned on the bone surface. The 5.7% of the osteoblasts exhibiting alkaline phosphatase activity revealed collagen-containing phagosomes, and the collagen fibrils within the phagosomes were at various stages of degradation. In addition, acid phosphatase activity and the immunocytochemical distribution of cathepsin B were found in these collagen-containing phagosomes at similar locations. The presence of both enzymes in the phagosomes suggests that an intracellular degradation of collagen occurs. Therefore, in addition to the osteoblastic functions of synthesizing and secreting bone matrices, osteoblasts are also capable of phagocytosis and the intracellular disintegration of collagen. Our findings suggest that osteoblasts at the alveolar bone-periodontal ligament interface have a collagen-phagocytosing ability and play an important role in the physiological remodeling and metabolic breakdown of collagen fibrils of periodontal ligament without osteoclastic bone remodeling.

Cysteine proteinases and matrix metalloproteinases play distinct roles in the subosteoclastic resorption zone

Digestion of calvarial bone by osteoclasts depends on the activity of cysteine proteinases and matrix metalloproteinases (MMPs). It is unknown, however, whether these enzymes act simultaneously or in a certain (time) sequence. In the present study, this was investigated by culturing mouse calvarial bone explants for various time intervals in the presence or absence of selective low molecular weight inhibitors of cysteine proteinases (E-64, Z-Phe-Tyr(O-t-Bu)CHN2 or CA074[Me]) and MMPs (CI-1, CT1166, or RP59794). The explants were morphometrically analyzed at the electron microscopic level. All proteinase inhibitors induced large areas of nondigested demineralized bone matrix adjacent to the ruffled border of actively resorbing osteoclasts. The appearance of these areas proved to be time dependent. In the presence of the cysteine proteinase inhibitors, a maximal surface area of demineralized bone was seen between 4 and 8 h of culturing, whereas the metalloproteinase inhibitors had their maximal effect at a later time interval (between 16 and 24 h). Because different inhibitors of each of the two classes of proteolytic enzymes had the same effects, our data strongly suggest that cysteine proteinases attack the bone matrix prior to digestion by MMPs. In line with the view that a sequence may exist were differences in the amount of proteoglycans (shown with the selective dye cuprolinic blue) in the subosteoclastic demineralized areas induced by the inhibitors. In the presence of the cysteine proteinase inhibitor, relatively high levels of cuprolinic blue precipitates were found, whereas this was less following inhibition of metalloproteinases. These data suggested that cysteine proteinases are important for digestion of noncollagenous proteins. We propose the following sequence in the digestion of calvarial bone by osteoclasts: after attachment of the cell to the mineralized surface an area with a low pH is created which results in dissolution of the mineral, then cysteine proteinases, active at such a low pH, digest part of the bone matrix, and finally, when the pH has increased somewhat, MMPs exert their activity.

Heterogeneous suppression of experimentally induced colon cancer metastasis in rat liver lobes by inhibition of extracellular cathepsin B

Metastatic rat colon cancer cells but not normal rat hepatocytes showed activity of cathepsin B on their plasma membranes. Activity was visualized in living cells with a new fluorogenic substrate, [Z-Arg]2-cresyl violet, and confocal microscopy. When these cancer cells were injected into the portal vein of rats, the animals developed tumors in the liver in a heterogeneous fashion. Three- to four-fold more tumors were found in the small caudate lobe than in the other three large lobes of the liver. Oral treatment with a selective water-soluble inhibitor of extracellular cathepsin B, Mu-Phe-homoPhe-fluoromethylketone, resulted in 60% reduction of the number of tumors and 80% reduction of the volume of tumors in the three large lobes whereas tumor development was not affected in the small caudate lobe. This study supports the conclusions that (a) extracellular cathepsin B plays a crucial but complex role in liver colonisation by rat colon carcinoma cells in vivo, (b) its selective inhibition suppresses tumor growth heterogeneously in the liver and (c) the caudate lobe of the liver is a relatively large risk factor for tumor development.

Deregulation of collagen phagocytosis in aging human fibroblasts: effects of integrin expression and cell cycle

Intracellular degradation of collagen by phagocytosis in fibroblasts is essential for physiological remodeling of the extracellular matrix in a wide variety of connective tissues but imbalances between degradation and synthesis can lead to loss of tissue collagen. As aging is associated with loss of dermal and periodontal collagen and with increased lysomomal enzyme content in fibroblasts, we examined the regulation of collagen phagocytosis by integrin expression and the cell cycle in an in vitro fibroblast aging model. Two different fibroblast lines (CL1; CL2) at the fourth subculture were passaged up to replicative senescence to model aging processes in vitro. Cells were incubated with collagen-coated or BSA-coated green fluorescent beads for 3 h to assess alpha 2 beta 1-integrin-mediated or nonspecific phagocytosis, respectively. Single-cell suspensions were stained with DAPI and sulforhodamine 101 to separate cycling G1 and noncycling G0 cells. Staining for alpha 2-integrin, bead internalization, and bivariate analyses of DNA/protein content were measured by three-color flow cytometry. Serum deprivation was used to induce increases in the proportion of G0 cells. For G1 cells, the proportion of collagen phagocytic cells was > 50% for all passages and collagen beads were internalized > 5-fold more frequently than BSA beads. In contrast, G0 cells with diploid DNA content but low protein content exhibited greatly reduced phagocytic capacity (< 10% of cells internalized collagen or BSA beads), the number of beads per cells was 4-fold less, and alpha 2 integrin expression was very low compared to G1 cells. The proportion of collagen phagocytic cells and the proportion of alpha 2-integrin-positive cells increased with transit through the cell cycle. At higher passage numbers mean cell volume and cytoplasmic granularity were reduced approximately 30% but at replicative senescence cells with large surface area and subdiploid DNA predominated. The proportion of collagen and BSA phagocytic G1 cells increased 1.5- and 5-fold, respectively, and the number of beads per cell increased < 3-fold. However, surface alpha 2-integrin staining remained unchanged. These data indicate that the collagen and nonspecific internalization pathways were greatly unregulated, independent of cell cycle phase, and that cellular aging in vitro strongly influences the specificity and rate of phagocytic processes in fibroblasts. We suggest that age-related loss of collagen in connective tissues undergoing turnover may be a manifestation of a deregulated increase of collagen phagocytosis in which the net loss of degraded collagen exceeds new synthesis.

Ala-Pro-cresyl violet, a synthetic fluorogenic substrate for the analysis of kinetic parameters of dipeptidyl peptidase IV (CD26) in individual living rat hepatocytes

A new type of fluorogenic substrates for proteases based on the leaving group cresyl violet has been synthesized. Cresyl violet is not fluorescent when amino acids or peptide groups are attached but becomes highly fluorescent after proteolytic liberation. Its fluorescence shows linearity with concentration and barely any fading. The properties of Ala-Pro-cresyl violet as substrate for dipeptidyl peptidase IV (DPPIV) (CD26) for localization and quantification of its activity in individual freshly isolated living rat hepatocytes were investigated using confocal microscopy, image analysis, and flow cytometry. DPPIV activity was localized exclusively in patches at plasma membranes likely being bile canalicular domains. Activity was analyzed quantitatively in individual cells by capturing series of images in time. Production of fluorescence was analyzed on the basis of the series of digital images and it appeared to be nonlinear with time. By calculation of the initial velocity at time zero, activity of DPPIV per individual hepatocyte was calculated. Cresyl violet-dependent fluorescence appeared in a similar way when cells were analyzed by flow cytometry. A dipeptide phosphonate inhibitor inhibited production of fluorescence competitively with a Ki of 7 microM. K(m) values in individual hepatocytes varied in the range of 6-22 microM depending on the individual rat from which the hepatocytes were obtained, whereas the Vmax varied in the range of 4-16 nU. K(m) and Vmax values per individual rat were inversely correlated indicating posttranslational regulation of the kinetic parameters of DPPIV. This relationship was lost when membrane fractions of the same hepatocyte suspensions were analyzed. It is concluded that cresyl violet-based protease substrates are the compounds of choice to localize and quantify protease activity in living cells and tissues.

Ultrastructural localization of cathepsin B in gingival tissue from chronic periodontitis patients

Loss of tooth support during chronic periodontitis is very likely to involve tissue proteases such as cathepsin B. The distribution of this enzyme was, therefore, examined in ultrathin sections of gingival tissue embedded in acrylic resin and labelled with a sheep polyclonal antibody and gold-conjugated secondary antibody. Macrophages and fibroblasts in both inflamed and non-inflamed areas of tissue showed labelling, and this was strongest in lysosomes, corresponding to the normal intracellular location of cathepsin B. However, additional gold particles were found on the surface of these cells. Monocytes in inflamed areas also had surface labelling, some of which was present on microvilli. Labelled collagen fibres adjacent to all three cell types indicated that cathepsin B had been released into the immediate extracellular environment. Plasma membrane cathepsin B has previously been associated with cancers, but enzyme redistribution and release in the gingiva may have been linked to the inflammatory response, since fibroblasts and macrophages in non-inflamed areas showed less labelling of their surface and adjacent collagen. The collagen labelling added to evidence that cathepsin B can function extracellularly as well as intracellularly in connective tissue degradation. This destructive role for the enzyme is supported by our earlier measurements of increased biochemical activity in chronic periodontitis.

Role of integrins in regulation of collagen phagocytosis by human fibroblasts

Phagocytosis of collagen fibrils by fibroblasts is an important pathway for degradation of extracellular matrix in mature connective tissues. To study regulatory mechanisms in phagocytosis, 2-microns fluorescent beads coated with either collagen (COL) or bovine serum albumin (BSA) were incubated with human gingival fibroblasts in vitro. For these studies single cell suspensions were prepared by trypsinization, and bead internalization and collagen receptor expression were assessed by flow cytometry. After 3-h incubations, up to 8-fold more cells internalized COL beads than BSA-coated beads. Increased collagen coating concentration was associated with elevated proportions of cells that internalized COL beads, and was observed also in the presence of competing fibronectin-coated beads. The number of beads per cell and the percent of phagocytic cells increased proportionally with higher bead loadings. At > 4 beads per cell a maximum of approximately 80% of cells were phagocytic. Cells reacted with mAbs against the alpha 1, alpha 2, and alpha 3 integrin subunits were, respectively, 5%, 98% and 93% positively stained above background controls. All cells that internalized COL beads exhibited alpha 2 staining but there were large proportions of phagocytic cells that were not stained for alpha 1. In unfixed cells, bead internalization caused an immediate reduction of surface staining of membrane-bound alpha 2 by approximately 55% which returned to control levels within 3 h, indicating that cell-surface alpha 2 was internalized by phagocytosis. Preincubation of cells with up to 8 COL beads per cell reduced the proportion of phagocytic cells and the number of internalized beads after a second COL bead incubation 4 h later. To assess the relationship between the percent of phagocytic cells and alpha 2 integrin levels, serum starvation and cycloheximide experiments were conducted. Compared to controls, serum starvation for 24 h induced a 3.2-fold increase of cells internalizing COL beads but did not alter alpha 2 staining levels. In contrast, 3 h cycloheximide treatment reduced alpha 2 staining to 60% of control levels and this treatment also inhibited COL bead internalization. GRGDTP peptide as well as mAbs against the alpha 1 and alpha 2 subunits significantly reduced internalization of COL beads by 1.8 to 2.6-fold, whereas GRGESP peptide and alpha 3 mAb exerted no effect. Internalization of BSA beads was not affected by any of these treatments. Collectively, these data indicate that the alpha 2 integrin, along with other, as yet unidentified components, is likely involved in COL bead internalization. The alpha 2 integrin subunit is rapidly recycled or synthesized following a phagocytic load. In contrast, the alpha 1 integrin is not directly required for phagocytosis but may regulate the internalization step.

Phagocytosis and intracellular digestion of collagen, its role in turnover and remodelling

Collagens of most connective tissues are subject to continuous remodelling and turnover, a phenomenon which occurs under both physiological and pathological conditions. Degradation of these proteins involves participation of a variety of proteolytic enzymes including members of the following proteinase classes: matrix metalloproteinases (e.g. collagenase, gelatinase and stromelysin), cysteine proteinases (e.g. cathepsin B and L) and serine proteinases (e.g. plasmin and plasminogen activator). Convincing evidence is available indicating a pivotal role for matrix metalloproteinases, in particular collagenase, in the degradation of collagen under conditions of rapid remodelling, e.g. inflammation and involution of the uterus. Under steady state conditions, such as during turnover of soft connective tissues, involvement of collagenase has yet to be demonstrated. Under these circumstances collagen degradation is likely to take place particularly within the lysosomal apparatus after phagocytosis of the fibrils. We propose that this process involves the following steps: (i) recognition of the fibril by membrane-bound receptors (integrins?), (ii) segregation of the fibril, (iii) partial digestion of the fibril and/or its surrounding non-collagenous proteins by matrix metalloproteinases (possibly gelatinase), and finally (iv) lysosomal digestion by cysteine proteinases, such as cathepsin B and/or L. Modulation of this pathway is carried out under the influence of growth factors and cytokines, including transforming growth factor beta and interleukin 1 alpha.

Effect of periodontal treatment on the protein composition of whole and parotid saliva

Cystatins are physiological inhibitors of cysteine proteinases and widely distributed in human tissues and fluids including saliva. Cystatins S, SA, SN, and D are only found in glandular saliva and tear fluid whereas cystatin C has been detected in all tested biological fluids. Previous studies demonstrated that total cystatin activity and cystatin C concentration were highest in whole and parotid saliva of periodontitis patients compared to healthy subjects suggesting a response of the salivary glands to an inflammatory condition of the oral cavity. Based on these results we studied a possible change in the concentration of cystatin S, cystatin C, albumin, IgA, amylase activity, and cystatin activity in a whole and parotid saliva of 20 periodontitis patients as a consequence of periodontal treatment. Saliva samples were quantified for cystatins S and C, albumin, and IgA by enzyme-linked immunosorbent assay. Amylase was determined in an activity assay and total cystatin activity was measured towards pa-pain. The clinical condition of the subjects improved significantly after 6 months of periodontal therapy whereas biochemical analyses of whole and parotid saliva indicated that significant changes in salivary protein composition occurred only in whole saliva. Total cystatin activity (P < 0.05) and cystatin C concentration (P < 0.05) of whole saliva samples collected after periodontal treatment decreased to normal healthy control values. Further, concentrations of cystatin S were unchanged during the periodontal treatment process. These results suggest that other sources of cystatins than the parotid gland i.e.; other salivary glands or crevicular fluid, are involved in the decrease of total cystatin activity in whole saliva after periodontal treatment.

Protein composition of whole and parotid saliva in healthy and periodontitis subjects. Determination of cystatins, albumin, amylase and IgA

Cystatins are physiological inhibitors of cysteine proteinases and they are widely distributed in human tissues and body fluids including saliva. We previously reported an increased cystatin activity in whole saliva of gingivitis and periodontitis subjects. Based on this result we decided to investigate the type and origin of cystatins involved in this increased cystatin activity by collecting both whole and parotid saliva of 25 healthy and 30 periodontitis subjects. Saliva samples were quantified for cystatins S and C by enzyme-linked immunosorbent assay and cystatin activities were measured toward papain. Besides, three other salivary proteins were determined: the plasma protein albumin, the typical parotid derived amylase and the salivary immunoglobulin IgA. The present investigation shows that levels of total protein and cystatin activity as well as the levels of glandular derived proteins amylase and cystatin C were significantly higher in whole and parotid saliva of subjects with periodontitis than in healthy controls. Cystatin S, the major salivary cystatin, however was higher in the whole saliva of the healthy group. Whole saliva concentrations of albumin and IgA, originating from sources other than the glandular cells, were not different between healthy and periodontitis subjects and were also not correlated with the typical salivary gland proteins. In conclusion, this study provides additional evidence that the human salivary glands may respond to an inflammatory disease of the oral cavity, periodontitis, by enhanced synthesis of some acinar proteins.

Type VI collagen is phagocytosed by fibroblasts and digested in the lysosomal apparatus: involvement of collagenase, serine proteinases and lysosomal enzymes

Type VI collagen is present in most connective tissues, where it is considered to play a crucial role in the attachment of cells to the extracellular matrix and/or in the three-dimensional organization of the collagen meshwork. Although some information is available on its formation, the mechanisms involved in its degradation are not understood. Here, we present evidence for lysosomal digestion of type VI collagen by fibroblasts of periosteal explants. In the lysosomal apparatus of these cells, broad-banded filamentous aggregates characterized by 100-nm periodicity were found, which proved to consist of type VI collagen as indicated by their stainability with anti-type VI collagen antibodies. By interfering with synthesis (ascorbate or alpha, alpha-dipyridyl), intracellular translocation of collagen-containing vesicles (colchicine) as well as phagocytosis (cytochalasin B), it was shown that the intracellular broad-banded type VI collagen represented phagocytosed material. In the presence of acidotropic agents (NH4Cl and methylamine) the amount of intracellular type VI collagen increased significantly (5- to 10-fold), suggesting that a rise of pH in the endosomal/lysosomal apparatus causes inhibition of its degradation. By using a variety of proteinase inhibitors, it was found that inhibition of collagenase (when used in combination with NH4Cl), or inhibition of cysteine proteinases (both with and without NH4Cl), resulted in an increased amount of intracellular type VI collagen, whereas inhibition of serine proteinases significantly lowered the level of intracellular type VI collagen. The data presented are the first to indicate a pathway by which type VI collagen degradation may occur: fibroblasts phagocytose type VI collagen and subsequently digest this collagen in their lysosomal apparatus. Degradation depends on the activity of several enzymes, among them collagenase and serine proteinases, probably exerting their activity in the extracellular space just before the actual internalization. After uptake, digestion involves pH-sensitive lysosomal enzymes, including those belonging to the class of cysteine proteinases.

Cytokines modulate phagocytosis and intracellular digestion of collagen fibrils by fibroblasts in rabbit periosteal explants. Inverse effects on procollagenase production and collagen phagocytosis

Degradation of fibrillar collagen may occur in the extracellular space by enzymes, such as the metalloproteinase collagenase, or in the lysosomal apparatus of fibroblasts following phagocytosis. As the mechanisms involved in the regulation of the latter process are unknown, we investigated possible modulating effects of the cytokines epidermal growth factor (EGF), platelet-derived growth factor (PDGF), interleukin-1 alpha (IL-1 alpha) and transforming growth factor-beta (TGF-beta) on both collagen phagocytosis and the release of collagenase in an in vitro model employing periosteal tissue explants. The data demonstrated that the level of intracellular collagen digestion could be influenced by cytokines: IL-1 alpha inhibited and TGF-beta enhanced phagocytosis of fibrillar collagen by periosteal fibroblasts, whereas the cytokines had an opposite effect on the release of procollagenase. In combination, IL-1 alpha and TGF-beta proved to have an antagonizing effect on either parameter. PDGF and EGF had no effect on phagocytosis or collagenase release. The level of phagocytosed collagen correlated positively with the actual breakdown of collagen as assessed by the release of hydroxyproline but negatively with the level of released procollagenase. Our findings demonstrated that cytokines are able to modulate both the phagocytosis of collagen fibrils by fibroblasts and their subsequent intracellular breakdown, as well as the release of procollagenase, an enzyme considered crucial for extracellular collagenolysis. Moreover, our data show a negative correlation between these two parameters. It is concluded that IL-1 alpha, EGF and TGF-beta may be important in modulating the contribution of the intracellular and extracellular route of collagen breakdown.

Cloning and complete coding sequence of a novel human cathepsin expressed in giant cells of osteoclastomas

A gene encoding a possible novel human cathepsin, a cysteine proteinase that is distinct from previously characterized enzymes, has been identified by differential screening of a human osteoclastoma cDNA library. This molecule, termed cathepsin X, appears to represent the human homolog of the osteoclast-expressed rabbit cathepsin OC-2. Cathepsin X (GenBank accession number U20280) is 93.9% identical to OC-2 at the amino acid level, and is 92% identical at the nucleotide level within the coding region. Cathepsin X is 52.2 and 46.9% identical to cathepsins S and L, respectively, and is therefore clearly distinct from these enzymes. Cathepsin X mRNA was localized to multinucleated giant cells within the osteoclastoma tumor by in situ hybridization. These data strongly support the hypothesis that cathepsin X represents a novel cysteine proteinase which is expressed at high levels in osteoclasts.

Comparative histochemical, biochemical and immunocytochemical studies of cathepsin B in human gingiva

Cathepsin B activity was demonstrated histochemically in unfixed cryostat sections of inflamed human gingiva using the 2-methoxy-4-naphthylamide (MNA) substrates Z-Val-Lys-Lys-Arg-MNA and Z-Ala-Arg-Arg-MNA with a post-azo-coupling technique. Enzyme localisation was confirmed by immunocytochemistry with polyclonal sheep anti-human cathepsin B. In both cases, staining was found in connective tissue fibroblasts and also in cells varying in shape from rounded to more irregular forms. The latter were present both in areas of cellular infiltration and in the oral and pocket epithelium. Examination of adjacent sections with monoclonal antibodies directed against leukocyte differentiation antigens showed that the rounded to irregular cells were CD68 positive macrophages and monocytes. The histochemical staining had the form of fine cytoplasmic particles consistent with the known lysosomal occurrence of cathepsin B. Cells stained by the post-coupling method using the tryptase substrates Z-Ala-Ala-Lys-MNA and D-Val-Leu-Arg-MNA showed a different distribution and morphology, with reaction product confined to mast cell granules. The differences between the cathepsin B and tryptase staining patterns were confirmed by differential extraction from cryostat sections with salt-free and high-salt buffers respectively. Biochemical characterisation of activities in the extracts with the 7-amino-4-trifluoromethyl coumarin (AFC) substrates Z-Val-Lys-Lys-Arg-AFC and Z-Ala-Ala-Lys-AFC and protease inhibitors confirmed the identity of the two enzymes. Selective inhibitors could also be used in histochemical incubations to distinguish between cathepsin B and tryptase staining.

Expression of 92 kD type IV collagenase/gelatinase B in human osteoclasts

The digestion of type I collagen is an essential step in bone resorption. It is well established that osteoclasts solubilize the mineral phase of bone during the resorptive process, but the mechanism by which they degrade type I collagen, the major proteinaceous component of bone, is controversial. Differential screening of a human osteoclastoma cDNA library was performed to characterize genes specifically expressed in osteoclasts. A large number of cDNA clones obtained by this procedure were found to represent 92 kD type IV collagenase (gelatinase B; MMP-9, EC 3.4.24.35), as well as tartrate-resistant acid phosphatase. In situ hybridization localized mRNA for gelatinase B to multinucleated giant cells in human osteoclastomas. Gelatinase B immunoreactivity was demonstrated in giant cells from eight of eight osteoclastomas, osteoclasts in normal bone, and osteoclasts of Paget's disease by use of a polyclonal antiserum raised against a synthetic gelatinase B peptide. In contrast, no immunoreactivity for 72 kD type IV collagenase (gelatinase A; MMP-2, EC 3.4.24.24), which is the product of a separate gene, was detected in osteoclastomas or normal osteoclasts. We propose that the 92 kD type IV collagenase/gelatinase B plays an important role in the resorption of collagen during bone remodeling.

High cathepsin B activity in arthroplasty interface membranes. A histochemical study of 9 loose cemented total hip prostheses

We studied biopsies of interface membranes of 9 aseptically loosened total hip prostheses. The morphologic resemblance of the cement-facing surface of the membranes to synovial tissue of arthritic joints, as noticed by others, was confirmed by histochemical techniques. High cathepsin B activity was found in the bone-facing surface of the membranes. Since this enzyme also plays an important role in tissue destruction of arthritic joints, further similarities in the mechanisms of tissue breakdown in arthritis and aseptic loosening of cemented hip prostheses may be conjectured.

Conversion of skeletal tartrate-sensitive acid phosphatases into tartrate-resistant isoenzymes in vitro

1. Chicken skeletal tartrate-sensitive (TsACP) and -resistant (TrACP) acid phosphatase isoenzymes could be separated from each other by carboxylmethyl-sepharose ion exchange chromatography. 2. Chicken skeletal TsACP showed a gradual time-dependent loss of sensitivity to tartrate inhibition when incubated at room temperature, but not at 4 degrees C. 3. The loss of sensitivity to tartrate inhibition was associated with an activation of the enzyme activity. 4. These changes were accompanied with a shift in the electrophoretic mobility of the enzyme activity from a large molecular sized form to a smaller molecular sized form that resembled the freshly prepared TrACP on the native acidic polyacrylamide electrophoresis gels, and on molecular sieve Superose-12 Fast Protein Liquid Chromatography. 5. Kinetic evaluations of the biochemical properties of the "converted" TsACP activity resembled the TrACP. 6. The apparent "conversion" was not unique to chicken TsACP, since similar "conversion" was observed with partially purified preparations of bovine bone matrix TsACP and of human osteoblastic TsACP. 7. Addition of several serine protease inhibitors did not prevent the "conversion". 8. These findings are consistent with the possibility that skeletal TsACPs are precursors of skeletal TrACPs.

Proteolysis of the major yolk glycoproteins is regulated by acidification of the yolk platelets in sea urchin embryos

The precise function of the yolk platelets of sea urchin embryos during early development is unknown. We have shown previously that the chemical composition of the yolk platelets remains unchanged in terms of phospholipid, triglyceride, hexose, sialic acid, RNA, and total protein content after fertilization and early development. However, the platelet is not entirely static because the major 160-kD yolk glycoprotein YP-160 undergoes limited, step-wise proteolytic cleavage during early development. Based on previous studies by us and others, it has been postulated that yolk platelets become acidified during development, leading to the activation of a cathepsin B-like yolk proteinase that is believed to be responsible for the degradation of the major yolk glycoprotein. To investigate this possibility, we studied the effect of addition of chloroquine, which prevents acidification of lysosomes. Consistent with the postulated requirement for acidification, it was found that chloroquine blocked YP-160 breakdown but had no effect on embryonic development. To directly test the possibility that acidification of the yolk platelets over the course of development temporally correlated with YP-160 proteolysis, we added 3-(2,4-dinitroanilo)-3-amino-N-methyldipropylamine (DAMP) to eggs or embryos. This compound localizes to acidic organelles and can be detected in these organelles by EM. The results of these studies revealed that yolk platelets did, in fact, become transiently acidified during development. This acidification occurred at the same time as yolk protein proteolysis, i.e., at 6 h after fertilization (64-cell stage) in Strongylocentrotus purpuratus and at 48 h after fertilization (late gastrula) in L. pictus. Furthermore, the pH value at the point of maximal acidification of the yolk platelets in vivo was equal to the pH optimum of the enzyme measured in vitro, indicating that this acidification is sufficient to activate the enzyme. For both S. purpuratus and Lytechinus pictus, the observed decrease in the pH was approximately 0.8 U, from 7.0 to 6.2. The trypsin inhibitor benzamidine was found to inhibit the yolk proteinase in vivo. By virtue of the fact that this inhibitor was reversible we established that the activity of the yolk proteinase is developmentally regulated even though the enzyme is present throughout the course of development. These findings indicate that acidification of yolk platelets is a developmentally regulated process that is a prerequisite to initiation of the catabolism of the major yolk glycoprotein.

Phagocytosis of collagen fibrils by periosteal fibroblasts in long bone explants. Effect of concanavalin A

In an attempt to determine whether phagocytosis of collagen by fibroblasts involves binding of the fibril to the plasma membrane, the effect of the lectin concanavalin A (Con A) was studied in an in vitro model system. Metacarpal bone rudiments from 19-day-old mouse fetuses were incubated with varying concentrations of the lectin. Quantitative electron microscopic analysis indicated that Con A caused a dose-related increase in the amount of phagocytosed collagen fibrils in periosteal fibroblasts, suggesting either an enhanced uptake or a decreased intracellular breakdown of fibrils. Since a Con A-inducible increase was not seen in the combined presence of both the lectin and the proteinase inhibitor leupeptin, which is known to inhibit the intracellular digestion of phagocytosed fibrillar collagen, it is unlikely that Con A stimulated phagocytosis. Based on the finding that Con A interfered with the digestion of a synthetic substrate by the collagenolytic lysosomal enzyme cathepsin B it is suggested that the augmentation of intracellular fibrillar collagen under the influence of the lectin was due to a decreased intracellular digestion. Since Con A did not inhibit the uptake of collagen fibrils by the fibroblasts it is concluded that Con A-inhibitable binding sites for collagen molecules are unlikely to be involved in phagocytosis of collagen fibrils by fibroblasts.