Niels T. Foged

The collagenolytic activity of cathepsin K is unique among mammalian proteinases.

Type I collagen fibers account for 90% of the organic matrix of bone. The degradation of this collagen is a major event during bone resorption, but its mechanism is unknown. A series of data obtained in biological models strongly suggests that the recently discovered cysteine proteinase cathepsin K plays a key role in bone resorption. Little is known, however, about the actual action of cathepsin K on type I collagen. Here, we show that the activity of cathepsin K alone is sufficient to dissolve completely insoluble collagen of adult human cortical bone. We found that the collagenolytic activity of cathepsin K is directed both outside the helical region of the molecule, i.e. the typical activity of cysteine proteinases, and at various sites inside the helical region, hitherto believed to resist all mammalian proteinases but the collagenases of the matrix metalloproteinase family and the neutrophil elastase. This property of cathepsin K is unique among mammalian proteinases and is reminiscent of bacterial collagenases. It is likely to be responsible for the key role of cathepsin K in bone resorption.

The type I collagen fragments ICTP and CTX reveal distinct enzymatic pathways of bone collagen degradation.

Bone resorption may generate collagen fragments such as ICTP and CTX, which can be quantified in serum and/or urine by using specific immunoassays, and which are used as clinical markers. However, the relative abundance of ICTP and CTX varies according to the type of bone pathology, suggesting that these two fragments are generated through distinct collagenolytic pathways. In this study, we analyzed the release of ICTP and CTX from bone collagen by the proteinases reported to play a role in the solubilization of bone matrix. Cathepsin K released large amounts of CTX, but did not allow a detectable release of ICTP. Conversely, the matrix metalloproteinases (MMPs) MMP‐2, ‐9, ‐13, or ‐14 released ICTP, but did not allow a detectable release of CTX. Next we analyzed the release of ICTP and CTX from bone explants cultured in the presence of well‐established inhibitors of these proteinases and of matrix solubilization. An inhibitor of cysteine proteinases including cathepsin K, inhibited the release of CTX, but not the release of ICTP. MMP inhibitors inhibited the release of ICTP, but also that of CTX, in agreement with the putative role of MMPs in the initiation of bone resorption in addition to matrix solubilization. Similarly the treatment of mice bearing bone metastasis with an MMP inhibitor led to a significant reduction of serum ICTP and CTX, and osteolytic lesions. We conclude that the generation of ICTP and CTX depends on different collagenolytic pathways. This finding may explain why these two markers may discriminate between different bone pathologies.

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.

The Chloride Channel Inhibitor NS3736 Prevents Bone Resorption in Ovariectomized Rats Without Changing Bone Formation

Chloride channel activity is essential for osteoclast function. Consequently, inhibition of the osteoclastic chloride channel should prevent bone resorption. Accordingly, we tested a chloride channel inhibitor on bone turnover and found that it inhibits bone resorption without affecting bone formation. This study indicates that chloride channel inhibitors are highly promising for treatment of osteoporosis.

Transforming Growth Factor-β Controls Human Osteoclastogenesis through the p38 MAPK and Regulation of RANK Expression

Although RANK-L is essential for osteoclast formation, factors such as transforming growth factor-β (TGF-β) are potent modulators of osteoclastogenic stimuli. To systematically investigate the role of TGF-β in human osteoclastogenesis, monocytes were isolated from peripheral blood by three distinct approaches, resulting in either a lymphocyte-rich, a lymphocyte-poor, or a pure osteoclast precursor (CD14-positive) cell population. In each of these osteoclast precursor populations, the effect of TGF-β on proliferation, TRAP activity, and bone resorption was investigated with respect to time and length of exposure. When using the highly pure CD14 osteoclast precursor cell population, the effect of TGF-β was strongly dependent on the stage of osteoclast maturation. When monocytes were exposed to TGF-β during the initial culture period (days 1–7), TRAP activity and bone resorption were increased by 40%, whereas the cell number was reduced by 25%. A similar decrease in cell number was observed when TGF-β was present during the entire culture period (days 1–21), but in direct contrast, TRAP activity, cell fusion, cathepsin K, and matrix metalloproteinase (MMP)-9 expression as well as bone resorption were almost completely abrogated. Moreover, we found that latent TGF-β was strongly activated by incubation with MMP-9 and suggest this to be a highly relevant mechanism for regulating osteoclast activity. To further investigate the molecular mechanism responsible for the divergent effects of continuous versus discontinuous exposure to TGF-β, we examined RANK expression and p38 MAPK activation. We found the TGF-β strongly induced p38 MAPK in monocytes, but not in mature osteoclasts, and that continuous exposure of TGF-β to monocytes down-regulated RANK expression. The current results suggest that TGF-β promotes human osteoclastogenesis in monocytes through stimulation of the p38 MAPK, whereas continuous exposure to TGF-β abrogates osteoclastogenesis through down-regulation of RANK expression and therefore attenuation of RANK-RANK-L signaling.

The Migration of Purified Osteoclasts Through Collagen Is Inhibited by Matrix Metalloproteinase Inhibitors

The most obvious proteolytic event controlled by the osteoclast is bone matrix removal in the resorption compartment. Here, however, we investigated whether matrix metalloproteinase (MMP) activity of the osteoclast might be involved in its migration to its future bone resorption site. We seeded either nonpurified or purified osteoclasts onto either uncoated or collagen‐coated dentine slices and cultured them in the presence or absence of specific MMP inhibitors. When nonpurified osteoclasts were cultured on uncoated dentine, MMP inhibitors did not prevent pit formation, as previously reported. However, when collagen‐coated dentine was used, pit formation was strongly inhibited by MMP inhibitors. The same results were obtained when performing these experiments with purified osteoclasts, thus demonstrating the ability of osteoclasts by themselves to migrate through collagen via an MMP‐dependent pathway. This demonstration was confirmed by using collagen‐coated invasion chambers. In addition, the invasions were not, or only slightly, inhibited by inhibitors of serine proteinases, cysteine proteinases, and carbonic anhydrase, though the latter two are well established bone resorption inhibitors that strongly inhibited pit formation. It is concluded that osteoclasts can migrate through collagen in the absence of other cells and that this migration relies on MMP activity, whereas other enzymes typically required for bone removal in the resorption compartment are not essential for migration. Some of the osteoclast MMPs might thus be relevant to the migratory/invasive activity of the osteoclast, rather than to its bone resorptive activity itself.

Transforming Growth Factor-β-induced Osteoblast Elongation Regulates Osteoclastic Bone Resorption through a p38 Mitogen-activated Protein Kinase- and Matrix Metalloproteinase-dependent Pathway

Transforming growth factor-β (TGF-β) is a powerful modulator of bone metabolism, and both its anabolic and catabolic effects on bone have been described. Here we have tested the hypothesis that TGF-β-induced changes in osteoblast shape promote bone resorption by increasing the surface area of bone that is accessible to osteoclasts. The addition of TGF-β1 to MC3T3-E1 cells resulted in cytoskeletal reorganization, augmented expression of focal adhesion kinase, and cell elongation, accompanied by an increase in the area of cell-free substratum. TGF-β1 also triggered activation of Erk1/2 and p38 mitogen-activated protein (MAP) kinase. The p38 MAP kinase inhibitor PD169316, but not an inhibitor of the Erk1/2 pathway, abrogated the effect of TGF-β1 on cell shape. The matrix metalloproteinase inhibitor GM6001 also interfered with osteoblast elongation. Treatment of MC3T3-E1 cells seeded at confluence onto bone slices to mimic a bone lining cell layer with TGF-β1 also induced cell elongation and increased pit formation by subsequently added osteoclasts. These effects were again blocked by PD169316 and GM6001. We propose that this novel pathway regulating osteoblast morphology plays an important role in the catabolic effects of TGF-β on bone metabolism.

Identification and characterization of the insulin-like growth factor I receptor in mature rabbit osteoclasts

In this study, the insulin‐like growth factor I (IGF‐I) receptor was identified in rabbit osteoclasts at mRNA and protein levels by in situ hybridization and autoradiography, respectively. Using highly purified mature osteoclasts, the IGF‐I receptor was characterized on the molecular level according to its size and its affinity and number per osteoclast by isolation of the receptor–ligand complex and by binding studies, respectively, and on the cellular level according to the response of mature osteoclasts to IGF‐I stimulation. In situ hybridization and autoradiography experiments showed that osteoclasts express IGF‐I receptor mRNA and IGF‐I binding sites. Chemical cross‐linking of125I‐IGF‐I bound to the purified mature osteoclasts and subsequent sodium dodecyl sulfide‐polyacrylamide gel electrophoresis revealed the specific binding of125I‐IGF‐I in complexes with molecular masses of 130 and 230 kD consistent with binding to the IGF‐I receptor. In competition experiments,125I‐IGF‐I binding to mature osteoclasts was dose‐dependently reduced by unlabeled IGF‐I in the picomolar range, whereas 20 nM insulin did not reduce the binding of125I‐IGF‐I binding. The calculated receptor number was 6000 per osteoclast, and the Kd was 0.10 nM. Searching for a role of the IGF‐I receptor in mature osteoclasts, we found no significant influence of IGF‐I on the levels of the proform of matrix metaloproteinase 9 and tartrate‐resistant acid phosphatase. However, the induction of nuclear fragmentation in serum‐depleted cultures of purified mature osteoclasts was dose‐dependently inhibited by IGF‐I in the picomolar range, but not by 1 nM insulin. These data show that functionally active IGF‐I receptor is present in mature osteoclasts.

PEGA supports for combinatorial peptide synthesis and solid-phase enzymatic library assays

Permeable resins cross‐linked with long PEG chains were synthesized for use in solid‐phase enzyme library assays. High molecular weight bis‐amino‐polyethylene glycol (PEG) 4000, 6000, 8000 were synthesized by a three‐step reaction starting from PEG‐bis‐OH. Macromonomers were synthesized by partial or di‐acryloylation of bis‐amino‐PEG derivatives. Bis/mono‐acrylamido–PEG were copolymerized along with acrylamide by inverse suspension copolymerization to yield a less cross‐linked resin (Type I, compounds 6–9). Furthermore, acryloyl–sarcosin ethyl ester was co‐polymerized along with bis‐acrylamido PEG to obtain more crosslinked capacity resin (Type II, compounds 13–19). N,N‐Dimethylacrylamide was used as a co‐monomer in some cases. The polymer was usually obtained in a well‐defined beaded form and was easy to handle under both wet and dry conditions. The supports showed good mechanical properties and were characterized by studying the swelling properties, size distribution of beads, and by estimating the amino group capacity. Depending on the PEG chain length, the monomer composition and the degree of cross‐linking the PEGA supports showed a high degree of swelling in a broad range of solvents, including water, dichloromethane, DMF, acetonitril, THF and toluene; no swelling was observed in diethyl ether. The PEGA resins (Type I) with an amino acid group capacity between 0.07 and 1.0 mmol/g could be obtained by variation of the monomer composition in the polymerization mixture. Fluorescent quenched peptide libraries were synthesized on the new polymer using a multiple column library synthesizer and incubated with the matrix metalloproteinase MMP‐9 after it had been activated by 4‐aminophenyl mercuric acetate resulting in 67/83 kDa active enzyme. The bright beads were separated manually under a fluorescence microscope and sequenced to obtain peptide substrates for MMP‐9. After treatment with ethylene diamine, high‐loaded resins (Type II) have been employed in continuous flow peptide synthesis to yield peptides in excellent yield and purity.

Digital image analysis of membrane connectivity is a robust measure of HER2 immunostains

The purpose of this study was to develop and validate a new software, HER2-CONNECTTM, for digital image analysis of the human epidermal growth factor receptor 2 (HER2) in breast cancer specimens. The software assesses immunohistochemical (IHC) staining reactions of HER2 based on an algorithm evaluating the cell membrane connectivity. The HER2-CONNECTTM algorithm was aligned to match digital image scorings of HER2 performed by 5 experienced assessors in a training set and confirmed in a separate validation set. The training set consisted of 167 breast carcinoma tissue core images in which the assessors individually and blinded outlined regions of interest and gave their HER2 score 0/1+/2+/3+ to the specific tumor region. The validation set consisted of 86 core images where the result of the automated image analysis software was correlated to the scores provided by the 5 assessors. HER2 fluorescence in situ hybridization (FISH) was performed on all cores and used as a reference standard. The overall agreement between the image analysis software and the digital scorings of the 5 assessors was 92.1% (Cohen’s Kappa: 0.859) in the training set and 92.3% (Cohen’s Kappa: 0.864) in the validation set. The image analysis sensitivity was 99.2% and specificity 100% when correlated to FISH. In conclusion, the Visiopharm HER2 IHC algorithm HER2-CONNECTTM can discriminate between amplified and non-amplified cases with high accuracy and diminish the equivocal category and thereby provides a promising supplementary diagnostic tool to increase consistency in HER2 assessment.