The osteoclast functional antigen, implicated in the regulation of bone resorption, is biochemically related to the vitronectin receptor

Toward a Mathematical Modeling of Diseases' Impact on Bone Remodeling: Technical Review

A wide variety of bone diseases have hitherto been discovered, such as osteoporosis, Paget’s disease, osteopetrosis, and metastatic bone disease, which are not well defined in terms of changes in biochemical and mechanobiological regulatory factors. Some of these diseases are secondary to other pathologies, including cancer, or to some clinical treatments. To better understand bone behavior and prevent its deterioration, bone biomechanics have been the subject of mathematical modeling that exponentially increased over the last years. These models are becoming increasingly complex. The current paper provides a timely and critical analysis of previously developed bone remodeling mathematical models, particularly those addressing bone diseases. Besides, mechanistic pharmacokinetic/pharmacodynamic (PK/PD) models, which englobe bone disease and its treatment’s effect on bone health. Therefore, the review starts by presenting bone remodeling cycle and mathematical models describing this process, followed by introducing some bone diseases and discussing models of pathological mechanisms affecting bone, and concludes with exhibiting the available bone treatment procedures considered in the PK/PD models.

Tablysin-15 inhibits osteoclastogenesis and LPS-induced bone loss via attenuating the integrin αvβ3 pathway

Excessive osteoclast leads to the imbalance in bone reconstruction and results in osteolytic diseases, such as osteoporosis and rheumatic arthritis. Integrin α_vβ₃ abundantly expresses on osteoclast and plays a critical role in the formation and function of osteoclast, therefore, blockage of α_vβ₃ has become an attractive therapeutic option for osteolytic diseases. In this study, we find that Tablysin-15, a RGD motif containing disintegrin, concentration-dependently suppresses RANKL-induced osteoclastogenesis, F-actin ring formation and bone resorption without affecting the cell viabilities. Tablysin-15 binds to integrin α_vβ₃ and inhibits the activation of FAK-associated signaling pathways. Tablysin-15 also suppresses the activation of NF-кB, MAPK, and Akt-NFATc1 signaling pathways, which are crucial transcription factors during osteoclast differentiation. Moreover, Tablysin-15 decreases the osteoclastogenesis marker gene expression, including MMP-9, TRAP, CTSK, and c-Src. Finally, Tablysin-15 significantly inhibits LPS-induced bone loss in a mouse model. Taken together, our results indicate that Tablysin-15 significantly suppresses osteoclastogenesis in vitro and in vivo, thus it might be a excellent candidate for treating osteolytic-related diseases.

64Cu-Labeled Phosphonate Cross-Bridged Chelator Conjugates of c(RGDyK) for PET/CT Imaging of Osteolytic Bone Metastases

OBJECTIVE

The goal of this research was to evaluate c(RGDyK) conjugated to phosphonate-based cross-bridged chelators using Cu-free click chemistry in the 4T1 mouse mammary tumor bone metastasis model in comparison with ⁶⁴Cu-CB-TE2A-c(RGDyK), which previously showed selective binding to integrin αvβ3 on osteoclasts.

EXPERIMENTAL

Two phosphonate-based cross-bridged chelators (CB-TE1A1P and CB-TE1K1P) were conjugated to c(RGDyK) through bio-orthogonal strain-promoted alkyne-azide cycloaddition. In vitro and in vivo evaluation of the ⁶⁴Cu-labeled TE1A1P-DBCO-c(RGDyK) (AP-c(RGDyK)), TE1K1P-PEG4-DBCO-c(RGDyK) (KP-c(RGDyK)), and CB-TE2A-c(RGDyK) were compared in the 4T1 mouse model of bone metastasis. The affinities of the unconjugated and chelator-c(RGDyK) analogs for αvβ3 integrin were determined using a competitive-binding assay. For in vivo evaluation, BALB/c mice were injected with 1 × 10⁵ 4T1/Luc cells in the left ventricle. Formation of metastases was monitored by bioluminescence imaging (BLI) followed by small-animal PET/CT 2 h postinjection of radiotracers.

RESULTS

The chelator-peptide conjugates showed similar affinity to integrin αvβ3, in the low nM range. PET imaging demonstrated a higher uptake in bones having metastases for all ⁶⁴Cu-labeled c(RGDyK) analogs compared with bones in nontumor-bearing mice. The correlation between uptake of ⁶⁴Cu-AP-c(RGDyK) and ⁶⁴Cu-KP-c(RGDyK) in bones with metastases based on PET/CT imaging, and osteoclast number based on histomorphometry, was improved over the previously investigated ⁶⁴Cu-CB-TE2A-c(RGDyK).

CONCLUSION

These data suggest that the phosphonate chelator conjugates of c(RDGyK) peptides are promising PET tracers suitable for imaging tumor-associated osteoclasts in bone metastases.

A dual-specific macrophage colony-stimulating factor antagonist of c-FMS and αvβ3 integrin for osteoporosis therapy

There is currently a demand for new highly efficient and specific drugs to treat osteoporosis, a chronic bone disease affecting millions of people worldwide. We have developed a combinatorial strategy for engineering bispecific inhibitors that simultaneously target the unique combination of c-FMS and α_vβ₃ integrin, which act in concert to facilitate bone resorption by osteoclasts. Using functional fluorescence-activated cell sorting (FACS)-based screening assays of random mutagenesis macrophage colony-stimulating factor (M-CSF) libraries against c-FMS and α_vβ₃ integrin, we engineered dual-specific M-CSF mutants with high affinity to both receptors. These bispecific mutants act as functional antagonists of c-FMS and α_vβ₃ integrin activation and hence of osteoclast differentiation in vitro and osteoclast activity in vivo. This study thus introduces a versatile platform for the creation of new-generation therapeutics with high efficacy and specificity for osteoporosis and other bone diseases. It also provides new tools for studying molecular mechanisms and the cell signaling pathways that mediate osteoclast differentiation and function.

Many bone diseases—including osteoporosis, in which the bones become brittle and fragile from loss of tissue—are characterized by excessive and uncontrolled bone resorption by bone-destroying cells known as osteoclasts. Therefore, controlled and specific inhibition of osteoclast activity is a desired outcome in treatments for bone diseases. Osteoclast differentiation and function are coordinated by cell surface receptors, including c-FMS and α_vβ₃ integrin, which cooperate with one another to drive signals that are essential for osteoclast functions. Here, we describe the engineering, characterization, and testing of novel proteins that can target and inhibit both c-FMS and α_vβ₃ integrin at the same time, thereby providing a way of controlling osteoclast function. The study represents the first example of engineering a natural ligand, which acts as a signaling molecule, as a scaffold for binding not only its target protein but also a second target. We show that these engineered proteins inhibit osteoclast activity in a mouse model of osteoporosis. Our study describes potential inhibitors that target all the known functions resulting from c-FMS/integrin α_vβ₃ crosstalk and paves the way to create novel targeting proteins that could be used to treat osteoporosis. It also expands our understanding of the role of the c-FMS/α_vβ₃ integrin pathway in the regulation of osteoclast differentiation and function.

Bone-Targeted Therapies in Cancer-Induced Bone Disease

Cancer-induced bone disease is a major source of morbidity and mortality in cancer patients. Thus, effective bone-targeted therapies are essential to improve disease-free, overall survival and quality of life of cancer patients with bone metastases. Depending of the cancer-type, bone metastases mainly involve the modulation of osteoclast and/or osteoblast activity by tumour cells. To inhibit metastatic bone disease effectively, it is imperative to understand its underlying mechanisms and identify the target cells for therapy. If the aim is to prevent bone metastasis, it is essential to target not only bone metastatic features in the tumour cells, but also tumour-nurturing bone microenvironment properties. The currently available bone-targeted agents mainly affect osteoclasts, inhibiting bone resorption (e.g. bisphosphonates, denosumab). Some agents targeting osteoblasts begin to emerge which target osteoblasts (e.g. romosozumab), activating bone formation. Moreover, certain drugs initially thought to target only osteoclasts are now known to have a dual action (activating osteoblasts and inhibiting osteoclasts, e.g. proteasome inhibitors). This review will focus on the evolution of bone-targeted therapies for the treatment of cancer-induced bone disease, summarizing preclinical and clinical findings obtained with anti-resorptive and bone anabolic therapies.

A dose escalating phase I study of GLPG0187, a broad spectrum integrin receptor antagonist, in adult patients with progressive high-grade glioma and other advanced solid malignancies

BACKGROUND

Integrin signaling is an attractive target for anti-cancer treatment. GLPG0187 is a broad spectrum integrin receptor antagonist (IRA). GLPG0187 inhibited tumor growth and metastasis in mouse models.

METHODS

We aimed to determine the Recommended Phase II Dose (RP2D) and to assess safety and tolerability of continuous i.v. infusion in patients with advanced malignant solid tumors. Anticipated dose levels were 20, 40, 80, 160, 320, and 400 mg/day in a modified 3 + 3 design. Plasma concentrations of GLPG0187 were assessed to characterize the pharmacokinetics (PK). C-terminal telopeptide of type I collagen (CTX) was used as pharmacodynamics marker.

RESULTS

Twenty patients received GLPG0187. No dose limiting toxicities (DLTs) were observed. The highest possible and tested dose was 400 mg/day. Fatigue was the most frequently reported side effect (25%). Recurrent Port-A-Cath-related infections and skin toxicity suggest cutaneous integrin inhibition. No dose-dependent toxicity could be established. PK analysis showed a short average distribution (0.16 h) and elimination (3.8 h) half-life. Continuous infusion resulted in dose proportional PK profiles. We observed decreases in serum CTX levels independent of the dose given, suggesting target engagement at the lowest dose level tested. Single agent treatment did not result in tumor responses.

CONCLUSIONS

GLPG0187 was well tolerated with a dose-proportional PK profile upon continuous infusion. No formal maximal tolerated dose could be established. GLPG0187 showed signs of target engagement with a favourable toxicity profile. However, continuous infusion of GLPG0187 failed to show signs of monotherapy efficacy.

Novel functions for type II procollagen

Cartilage is unique in being established as an avascular tissue during development. Cartilage also has the property of being resistant to tumor invasion with tumors arising on the periphery of cartilage and in bone, but sparing the cartilage. These properties have been investigated for many years beginning in the 1970's. Many anti-angiogenic molecules have been isolated from cartilage in small amounts. Portions of molecules from cartilage also possess anti-angiogenic properties when released from the parent protein by degradative extracellular enzymes. This review highlights a new anti-angiogenic and anti-tumor moiety from cartilage, the NH2-propeptide of type IIB collagen. When released from the procollagen during synthesis, the propeptide has the capacity to act on its own to protect the cartilage by killing of endothelial cell, osteoclasts and tumor cells.

Immunophenotype of Dental Implant-Associated Peripheral Giant Cell Reparative Granuloma in a Representative Case Report

We report the case of a 74-year-old white male patient who had worn an overdenture for the previous 6 years, retained by 4 screwed implants and a bar, who presented with an exophytic multilobed lesion of 2.5 × 2.0 cm on the anterior aspect of 1 implant neck, which was surrounded by pink-reddish tissue. All of the soft tissue around the implant was removed until the periosteum was reached. Histologic examination of the lamina propria revealed a cellular proliferation with imprecise boundaries, dense stromal component composed of spindle- to round-shaped mononucleated cells (fibroblasts and monocytes/macrophages), abundant multinucleated giant cells surrounding microscopic hemorrhagic foci, and deposits of hemosiderin; the diagnosis was peripheral giant-cell reparative granuloma (PGCG). Giant cells share the immunohistochemical expression of monocyte/macrophage markers (CD68, calprotectin [Mc387]) and osteoclastic cell markers (tartrate-resistant acid phosphatase, cathepsin K, and microphthalmia-associated transcription factor). After 6 months of follow-up, no bone resorption or recurrence of implant loss was observed. There have been only 12 case reports on dental implant-associated PGCG. Research results to date indicate that there may be little difference in immunophenotype among the giant cells of PGCG, central giant cell reparative granuloma, and peri-implant osteolysis. In conclusion, the immunohistochemical study confirms an osteoclast like giant cells phenotype differentiation in PGCG.

Novel bioactivity of phosvitin in connective tissue and bone organogenesis revealed by live calvarial bone organ culture models

Egg yolk phosvitin is one of the most highly phosphorylated extracellular matrix proteins known in nature with unique physico-chemical properties deemed to be critical during ex-vivo egg embryo development. We have utilized our unique live mouse calvarial bone organ culture models under conditions which dissociates the two bone remodeling stages, viz., resorption by osteoclasts and formation by osteoblasts, to highlight important and to date unknown critical biological functions of egg phosvitin. In our resorption model live bone cultures were grown in the absence of ascorbate and were stimulated by parathyroid hormone (PTH) to undergo rapid osteoclast formation/differentiation with bone resorption. In this resorption model native phosvitin potently inhibited PTH-induced osteoclastic bone resorption with simultaneous new osteoid/bone formation in the absence of ascorbate (vitamin C). These surprising and critical observations were extended using the bone formation model in the absence of ascorbate and in the presence of phosvitin which supported the above results. The results were corroborated by analyses for calcium release or uptake, tartrate-resistant acid phosphatase activity (marker for osteoclasts), alkaline phosphatase activity (marker for osteoblasts), collagen and hydroxyproline composition, and histological and quantitative histomorphometric evaluations. The data revealed that the discovered bioactivity of phosvitin mirrors that of ascorbate during collagen synthesis and the formation of new osteoid/bone. Complementing those studies use of the synthetic collagen peptide analog and cultured calvarial osteoblasts in conjunction with mass spectrometric analysis provided results that augmented the bone organ culture work and confirmed the capacity of phosvitin to stimulate differentiation of osteoblasts, collagen synthesis, hydroxyproline formation, and biomineralization. There are striking implications and interrelationships of this affect that relates to the evolutionary inactivation of the gene of an enzyme L-gulono-γ-lactone oxidase, which is involved in the final step of ascorbate biosynthesis, in many vertebrate species including passeriform birds, reptiles and teleost fish whose egg yolk contain phosvitin. These represent examples of how developing ex-vivo embryos of such species can achieve connective tissue and skeletal system formation in the absence of ascorbate.

Topographical distribution of phosphorylation sites of phosvitins by mass spectrometry

Phosvitin, derived from the vitellogenin II gene protein, is a highly phosphorylated protein found in egg yolk. A second hypothetical protein has been predicted based on the vitellogenin I gene, but has not been defined at the protein level. Mass spectrometric analysis was used to identify the phosphopeptide sequences and the precise sites of phosphorylation of two phosvitins, phosvitin 1 and phosvitin 2 derived from vitellogenins I and II, respectively. Samples of native phosvitin were subjected to tryptic digestion followed by mass spectrometric analysis: (i) native phosvitin peptides, (ii) after treatment with NaOH, and (iii) after chemical derivatization of P-Ser/P-Thr residues by dithiothreitol under base-catalyzed conditions. A combination of these approaches led to the identification of 68 and 35 phosphopeptides with 89 (81 P-Ser and 8 P-Thr residues) and 62 (57 P-Ser and 5 P-Thr residues) phosphorylation sites of phosvitin 1 and phosvitin 2, respectively. These data for the first time documented on a large scale the major states and sites of phosphorylation of phosvitins with a total of 151 phosphorylation sites. Importantly, the present work also provided the first direct de novo protein amino-acid sequence data for phosvitin 1 protein and evidence for the full expression of vitellogenin I gene.

BIOLOGICAL SIGNIFICANCE

We have for the first time generated a large number of phosphopeptides (~100) and identified 151 phosphorylation sites of phosvitin 1 and phosvitin 2, respectively. Importantly, this study also led to the discovery of a novel phosvitin 1 and provided the first direct de novo protein amino-acid sequence data for the full expression of vitellogenin I gene. There is considerable interest in naturally occurring phosphopeptides/phosphoproteins and their application in biomedical fields and in the food industry because of their molecular characteristics and non-toxic nature, hence, our work opens new avenues to pursue such endeavors. In addition, the results provide important fundamental biologic information relevant to evolutionary developments of vertebrate animals beginning with the earliest fish, reptiles, birds and more contemporary mammals. For instance, the abundance of phosvitins with a unique degree of phosphorylation in the egg yolk of fish, reptiles, and birds suggests potential biological functions of phosvitins which are critical to the development of embryos of these distant vertebrates.

The emerging role of Interleukin 27 in inflammatory arthritis and bone destruction

Although the causes of inflammatory arthritis elude us, aberrant cytokine expression has been linked to joint pathology. Consequently, several approaches in the clinic and/or in clinical trials are targeting cytokines, e.g. tumor necrosis factor (TNF), Interleukin 23 (IL-23) and Interleukin 17 (IL-17), with the goal of antagonizing their respective biologic activity through therapeutic neutralizing antibodies. Such, cytokine signaling-dependent molecular networks orchestrate synovial inflammation on multiple levels including differentiation of myeloid cells to osteoclasts, the central cellular players in arthritis-associated pathologic bone resorption. Hence, understanding of the cellular and molecular mechanisms elicited by synovial cytokine networks that dictate recruitment, differentiation and activation of osteoclast precursors and osteoclasts, respectively, is central to shaping novel therapeutic options for inflammatory arthritis patients. In this article we are discussing the complex signaling interactions involved in the regulation of inflammatory arthritis and it's associated bone loss with a focus on Interleukin 27 (IL-27). The present review will discuss the primary bone-degrading cell, the osteoclast, and on how IL-27, directly or indirectly, modulates osteoclast activity in autoimmune-driven inflammatory joint diseases.

Rheumatoid and pyrophosphate arthritis synovial fibroblasts induce osteoclastogenesis independently of RANKL, TNF and IL-6

Bone destruction is a common feature of inflammatory arthritis and is mediated by osteoclasts, the only specialized cells to carry out bone resorption. Aberrant expression of receptor activator of nuclear factor kappa β ligand (RANKL), an inducer of osteoclast differentiation has been linked with bone pathology and the synovial fibroblast in rheumatoid arthritis (RA). In this manuscript, we challenge the current concept that an increase in RANKL expression governs osteoclastogenesis and bone destruction in autoimmune arthritis. We isolated human fibroblasts from RA, pyrophosphate arthropathy (PPA) and osteoarthritis (OA) patients and analyzed their RANKL/OPG expression profile and the capacity of their secreted factors to induce osteoclastogenesis. We determined a 10-fold increase of RANKL mRNA and protein in fibroblasts isolated from RA relative to PPA and OA patients. Peripheral blood mononuclear cells (PBMC) from healthy volunteers were cultured in the presence of RA, PPA and OA synovial fibroblast conditioned medium. Osteoclast differentiation was assessed by expression of tartrate-resistant acid phosphatase (TRAP), vitronectin receptor (VNR), F-actin ring formation and bone resorption assays. The formation of TRAP(+), VNR(+) multinucleated cells, capable of F-actin ring formation and lacunar resorption in synovial fibroblast conditioned medium cultures occured in the presence of osteoprotegerin (OPG) a RANKL antagonist. Osteoclasts did not form in these cultures in the absence of macrophage colony stimulating factor (M-CSF). Our data suggest that the conditioned medium of pure synovial fibroblast cultures contain inflammatory mediators that can induce osteoclast formation in human PBMC independently of RANKL. Moreover inhibition of the TNF or IL-6 pathway was not sufficient to abolish osteoclastogenic signals derived from arthritic synovial fibroblasts. Collectively, our data clearly show that alternate osteoclastogenic pathways exist in inflammatory arthritis and place the synovial fibroblast as a key regulatory cell in bone and joint destruction, which is a hallmark of autoimmune arthritis.

Hormone-stimulated modulation of endocytic trafficking in osteoclasts

A precise control of vesicular trafficking is crucial not only for osteoclastic bone resorption, but also for the crosstalk between osteoclasts and osteoblasts, which regulates bone homeostasis. In addition to the release of growth factors and modulators, such as glutamate, flux through the intracellular trafficking routes could also provide the osteoclast with a monitoring function of its resorption activity. To establish the signaling pathways regulating trafficking events in resorbing osteoclasts, we used the bone conserving hormone calcitonin, which has the unique property of inducing osteoclast quiescence. Calcitonin acts through the calcitonin receptor and activates multiple signaling pathways. By monitoring trafficking of a fluorescent low molecular weight probe in mature, bone resorbing osteoclasts we show for the first time that calcitonin blocks endocytosis from the ruffled border by phospholipase C (PLC) activation. Furthermore, we identify a requirement for polyunsaturated fatty acids in endocytic trafficking in osteoclasts. Inhibition of PLC prior to calcitonin treatment restores endocytosis to 75% of untreated rates. This effect is independent of protein kinase C activation and can be mimicked by an increase in intracellular calcium. We thus define an essential role for intracellular calcium levels in the maintenance of endocytosis in osteoclasts.

The skeleton: a multi-functional complex organ: the role of key signalling pathways in osteoclast differentiation and in bone resorption

Osteoclasts are the specialised cells that resorb bone matrix and are important both for the growth and shaping of bones throughout development as well as during the process of bone remodelling that occurs throughout life to maintain a healthy skeleton. Osteoclast formation, function and survival are tightly regulated by a network of signalling pathways, many of which have been identified through the study of rare monogenic diseases, knockout mouse models and animal strains carrying naturally occurring mutations in key molecules. In this review, we describe the processes of osteoclast formation, activation and function and discuss the major transcription factors and signalling pathways (including those that control the cytoskeletal rearrangements) that are important at each stage.

Targeting of α(v)-integrins in stem/progenitor cells and supportive microenvironment impairs bone metastasis in human prostate cancer. Neoplasia. 2011;13:516-525. [PMID: 21677875 DOI: 10.1593/neo.11122]

Acquisition of an invasive phenotype by cancer cells is a requirement for bone metastasis. Transformed epithelial cells can switch to a motile, mesenchymal phenotype by epithelial-mesenchymal transition (EMT). Recently, it has been shown that EMT is functionally linked to prostate cancer stem cells, which are not only critically involved in prostate cancer maintenance but also in bone metastasis. We showed that treatment with the non-peptide α(v)-integrin antagonist GLPG0187 dose-dependently increased the E-cadherin/vimentin ratio, rendering the cells a more epithelial, sessile phenotype. In addition, GLPG0187 dose-dependently diminished the size of the aldehyde dehydrogenase high subpopulation of prostate cancer cells, suggesting that α(v)-integrin plays an important role in maintaining the prostate cancer stem/progenitor pool. Our data show that GLPG0187 is a potent inhibitor of osteoclastic bone resorption and angiogenesis in vitro and in vivo. Real-time bioluminescent imaging in preclinical models of prostate cancer demonstrated that blocking α(v)-integrins by GLPG0187 markedly reduced their metastatic tumor growth according to preventive and curative protocols. Bone tumor burden was significantly lower in the preventive protocol. In addition, the number of bone metastases/mouse was significantly inhibited. In the curative protocol, the progression of bone metastases and the formation of new bone metastases during the treatment period was significantly inhibited. In conclusion, we demonstrate that targeting of integrins by GLPG0187 can inhibit the de novo formation and progression of bone metastases in prostate cancer by antitumor (including inhibition of EMT and the size of the prostate cancer stem cell population), antiresorptive, and antiangiogenic mechanisms.

The type II collagen N-propeptide, PIIBNP, inhibits cell survival and bone resorption of osteoclasts via integrin-mediated signaling

OBJECTIVE

Type IIB procollagen is characteristic of cartilage, comprising 50% of the extracellular matrix. The NH(2)-propeptide of type IIB collagen, PIIBNP, can kill tumor cells via binding to integrins α(V)β(3) and α(V)β(5). As osteoclasts rely on α(V)β(3) integrins for function in bone erosion, we sought to determine whether PIIBNP could inhibit osteoclast function.

METHODS

We undertook in vitro and in vivo experiments to evaluate both osteoblast and osteoclast functions in the presence of recombinant PIIBNP. Adhesion of osteoclasts to PIIBNP was analyzed by staining of attached cells with crystal violet. PIIBNP-induced cell death was evaluated by counting Trypan Blue stained cells. The mechanism of cell death was evaluated by DNA fragmentation, TUNEL staining and western blotting to detect cleaved caspases. To determine the role of α(V)β(3) integrin, osteoclasts were pretreated with α(V) or β(3) integrin specific siRNA before the treatment with PIIBNP. To explore PIIBNP function in vivo, a lipopolysaccharide-induced mouse calvaria lysis model was employed.

RESULTS

Osteoclasts adhered to PIIBNP via an RGD-mediated mechanism. When osteoclasts were plated on extracellular matrix proteins, PIIBNP induced apoptosis of osteoclasts via caspase 3/8 activation. Osteoblasts and macrophages were not killed. Reduction of α(V) or β(3) integrin levels on osteoclasts by siRNA reduced cell death in a dose-dependent manner. In vivo, PIIBNP could inhibit bone resorption.

CONCLUSION

We conclude that PIIBNP can inhibit osteoclast survival and bone resorption via signal transduction through the α(V)β(3) integrins. Because of this property and the cell specificity, we propose that PIIBNP may play a role in vivo in protecting cartilage from osteoclast invasion and also could be a new therapeutic strategy for decreasing bone loss.

Regulation of osteoclast function

Osteoclasts are terminally differentiated multinucleated cells that are the principal resorptive cells of bone, playing a central role in the formation of the skeleton and regulation of its mass. The molecular events involved in the differentiation and function of osteoclasts had not been clarified for a long time. Over the past two decades, several novel approaches have been developed and adopted to investigate osteoclast biology. In the present review, we would like to update recent progress in the elucidation of the molecular mechanism of osteoclast activation and function.

Calcium signaling in osteoclasts

It has long been known that many bone diseases, including osteoporosis, involve abnormalities in osteoclastic bone resorption. As a result, there has been intense study of the mechanisms that regulate both the differentiation and bone resorbing function of osteoclast cells. Calcium (Ca(2+)) signaling appears to play a critical role in the differentiation and functions of osteoclasts. Cytoplasmic Ca(2+) oscillations occur during RANKL-mediated osteoclastogenesis. Ca(2+) oscillations provide a digital Ca(2+) signal that induces osteoclasts to up-regulate and autoamplify nuclear factor of activated T cells c1 (NFATc1), a Ca(2+)/calcineurin-dependent master regulator of osteoclastogenesis. Here we review previous studies on Ca(2+) signaling in osteoclasts as well as recent breakthroughs in understanding the basis of RANKL-induced Ca(2+) oscillations, and we discuss possible molecular players in this specialized Ca(2+) response that appears pivotal for normal bone function. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Osteoclast-specific inactivation of the integrin-linked kinase (ILK) inhibits bone resorption

Bone resorption requires the adhesion of osteoclasts to extracellular matrix (ECM) components, a process mediated by the alpha(v)beta(3) integrin. Following engagement with the ECM, integrin receptors signal via multiple downstream effectors, including the integrin-linked kinase (ILK). In order to characterize the physiological role of ILK in bone resorption, we generated mice with an osteoclast-specific Ilk gene ablation by mating mice with a floxed Ilk allele with TRAP-Cre transgenic mice. The TRAP-Cre mice specifically excised floxed alleles in osteoclasts, as revealed by crossing them with the ROSA26R reporter strain. Osteoclast-specific Ilk mutant mice appeared phenotypically normal, but histomorphometric analysis of the proximal tibia revealed an increase in bone volume and trabecular thickness. Osteoclast-specific Ilk ablation was associated with an increase in osteoclastogenesis both in vitro and in vivo. However, the mutant osteoclasts displayed a decrease in resorption activity as assessed by reduced pit formation on dentin slices in vitro and decreased serum concentrations of the C-terminal telopeptide of collagen in vivo. Interestingly, compound heterozygous mice in which one allele of Ilk and one allele of the beta(3) integrin gene were inactivated (ILK(+/-); beta(3) (+/-)) also had increased trabecular thickness, confirming that beta(3) integrin and Ilk form part of the same genetic cascade. Our results show that ILK is important for the function, but not the differentiation, of osteoclasts.

The isolation and characterization of glycosylated phosphoproteins from herring fish bones

Past studies of bone extracellular matrix phosphoproteins such as osteopontin and bone sialoprotein have yielded important biological information regarding their role in calcification and the regulation of cellular activity. Most of these studies have been limited to proteins extracted from mammalian and avian vertebrates and nonvertebrates. The present work describes the isolation and purification of two major highly glycosylated and phosphorylated extracellular matrix proteins of 70 and 22 kDa from herring fish bones. The 70-kDa phosphoprotein has some characteristics of osteopontin with respect to amino acid composition and susceptibility to thrombin cleavage. Unlike osteopontin, however, it was found to contain high levels of sialic acid similar to bone sialoprotein. The 22-kDa protein has very different properties such as very high content of phosphoserine (∼270 Ser(P) residues/1000 amino acid residues), Ala, and Asx residues. The N-terminal amino acid sequence analysis of both the 70-kDa (NPIMA(M)ETTS(M)DSKVNPLL) and the 22-kDa (NQDMAMEASSDPEAA) fish phosphoproteins indicate that these unique amino acid sequences are unlike any published in protein databases. An enzyme-linked immunosorbent assay revealed that the 70-kDa phosphoprotein was present principally in bone and in calcified scales, whereas the 22-kDa phosphoprotein was detected only in bone. Immunohistological analysis revealed diffusely positive immunostaining for both the 70- and 22-kDa phosphoproteins throughout the matrix of the bone. Overall, this work adds additional support to the concept that the mechanism of biological calcification has common evolutionary and fundamental bases throughout vertebrate species.

The antiapoptotic protein Bcl-xL negatively regulates the bone-resorbing activity of osteoclasts in mice

The B cell lymphoma 2 (Bcl-2) family member Bcl-xL has a well-characterized antiapoptotic function in lymphoid cells. However, its functions in other cells--including osteoclasts, which are of hematopoietic origin--and other cellular processes remain unknown. Here we report an unexpected function of Bcl-xL in attenuating the bone-resorbing activity of osteoclasts in mice. To investigate the role of Bcl-xL in osteoclasts, we generated mice with osteoclast-specific conditional deletion of Bcl-x (referred to herein as Bcl-x cKO mice) by mating Bcl-xfl/fl mice with mice in which the gene encoding the Cre recombinase has been knocked into the cathepsin K locus and specifically expressed in mature osteoclasts. Although the Bcl-x cKO mice grew normally with no apparent morphological abnormalities, they developed substantial osteopenia at 1 year of age, which was caused by increased bone resorption. Bcl-x deficiency increased the bone-resorbing activity of osteoclasts despite their high susceptibility to apoptosis, whereas Bcl-xL overexpression produced the opposite effect. In addition, Bcl-x cKO osteoclasts displayed increased c-Src activity, which was linked to increased levels of vitronectin and fibronectin expression. These results suggest that Bcl-xL attenuates osteoclastic bone-resorbing activity through the decreased production of ECM proteins, such as vitronectin and fibronectin, and thus provide evidence for what we believe to be a novel cellular function of Bcl-xL.

Modulation of bone resorption by phosphorylation state of bone sialoprotein

We have determined transmembrane protein tyrosine phosphorylation (outside-in signaling) in cultured osteoclasts and macrophages in response to added native purified bone sialoprotein (nBSP) and its dephosphorylated form (dBSP). There were selective/differential and potent inhibitory effects by dBSP and minimal effect by nBSP on intracellular tyrosine phosphorylation in macrophages and osteoclasts. Further studies on the downstream gene expression effects led to identification of a large number of differentially expressed genes in response to nBSP relative to dBSP in both macrophages and osteoclasts. These studies were extended to a bone resorption model using live mouse neonatal calvarial bone organ cultures stimulated by parathyroid hormone (PTH) to undergo bone resorption. Inclusion of nBSP in such cultures showed no effect on type I collagen telopeptide fragment release, hence overall bone resorption, whereas addition of dBSP abolished the PTH-induced bone resorption. The inhibition of bone resorption by dBSP was shown to be unique since in complementary experiments use of integrin receptor binding ligand, GRGDS peptide, offered only partial reduction on overall bone resorption. Quantitative RANKL analysis indicated that mechanistically the PTH-induced bone resorption was inhibited by dBSP via down-regulation of the osteoblastic RANKL production. This conclusion was supported by the RANKL analysis in cultured MC3T3-E1 osteoblast cells. Overall, these studies provided direct evidence for the involvement of covalently bound phosphates on BSP in receptor mediated "outside-in" signaling via transmembrane tyrosine phosphorylation with concurrent effects on downstream gene expressions. The use of a live bone organ culture system augmented these results with further evidence that links the observed in vivo variable state of phosphorylation with bone remodeling.

Biomimetic hydroxyapatite coating on glass coverslips for the assay of osteoclast activity in vitro

The osteoclast (OC) is the cell type responsible for the resorption of bone. The activity of this cell is important in the aetiology of a large number of skeletal pathologies, and also for the biocompatibility and osseointegration of orthopaedic implant materials. OC mediated acid hydrolysis of calcium phosphate from the bone matrix offers a prime means of studying the biology and activity of this cell type. We have developed a method of coating glass coverslips with a hydroxyapatite (HA)-like mineral, using a biomimetic approach. Hydroxylation followed by formation of a self assembled monolayer (SAM) using the surfactant triethoxysilylpropyl succinic anhydride (TESPSA), allowed biomimetic deposition of HA-like mineral from a simulated body fluid (SBF). The biocompatibility of the TESPSA SAM-HA coated glass coverslips was tested by culturing human mature OC present in samples of giant cell tumour of bone (GCT). Parameters of OC activity were assayed, including F-actin ring formation, release of calcium and formation of osteoclastic resorption pits, confirming that OC were able to attach to and resorb the coated surface. This approach for the preparation of HA coatings on glass coverslips could have wide applicability for the study of osteoclast behaviour in vitro.

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

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

Immune regulation of bone loss by Th17 cells

A significant macrophage and T-cell infiltrate commonly occurs in inflammatory joint conditions such as rheumatoid arthritis that have significant bone destruction. Cytokines produced by activated macrophages and T cells are implicated in arthritis pathogenesis and are involved in osteoclast-mediated bone resorption. The scope of the present review is to analyze current knowledge and to provide a better understanding of how macrophage-derived factors promote the differentiation of a novel T-helper subset (Th17) that promotes osteoclast formation and activation.

Ultrastructural localization of glycoprotein IIIa (GPIIIa, beta 3 integrin) on placental syncytiotrophoblast microvilli: implications for platelet alloimmunization during pregnancy

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia due to anti-human platelet antigen (HPA)-1a more commonly occurs in first pregnancies, unlike hemolytic disease of the newborn. Anti-D is produced after D+ fetomaternal hemorrhage; this usually occurs at parturition. Anti-HPA-1a could develop during pregnancy if maternal immunization is stimulated by HPA-1a expressed not only on platelets but also on other fetal cells.

STUDY DESIGN AND METHODS

An ultrastructural study of fetal placental chorionic villi was undertaken to determine the localization of glycoprotein (GP)IIIa carrying the HPA-1a/1b polymorphism. First trimester and term villi were incubated with a monoclonal antibody (MoAb) to GPIIIa or with positive control MoAbs (anti-placental alkaline phosphatase and ED822 MoAb) to villous syncytiotrophoblast (ST). Binding of MoAbs was detected with a gold-conjugated secondary antibody before processing the tissues and examination of ultrathin sections in an electron microscope.

RESULTS

Gold particles were evident on microvilli on the apical surface of ST when labeled with anti-GPIIIa and the placenta-specific MoAbs but not with an isotype control antibody. Immunolabeling for anti-GPIIIa on first trimester ST was similar to that of term ST.

CONCLUSION

The apical surface of the ST is bathed in maternal blood. During the natural regenerative process of human placenta, senescent parts of the ST are shed into maternal blood during pregnancy. This includes both apoptotic ST nuclei and microparticulate ST debris. The presence of GPIIIa on this circulating ST cellular material could be the source of HPA-1a alloantigen causing primary immunization of susceptible primigravidae early enough for anti-HPA-1a to cause fetal thrombocytopenia during a first pregnancy.

Trabecular bone deterioration in col9a1+/- mice associated with enlarged osteoclasts adhered to collagen IX-deficient bone

INTRODUCTION

Short collagen IX, the exclusive isoform expressed by osteoblasts, is synthesized through alternative transcription of the col9a1 gene. The function of short collagen IX in bone was characterized in col9a1-null mutant mice.

MATERIALS AND METHODS

Trabecular bone morphometry of lumbar bones and tibias was evaluated by muCT and nondecalcified histology. Osteoblastic and osteoclastic activities were evaluated by PCR- and microarray-based gene expression assays and TRACP-5b and C-terminal telopeptide (CTX) assays, as well as in vitro using bone marrow stromal cells and splenocytes. The effect of col9a1(+/-) mutation on osteoclast morphology was evaluated using RAW264.7-derived osteoclastic cells cultured on the mutant or wildtype calvarial bone substrates.

RESULTS

Col9a1 knockout mutation caused little effects on the skeletal development; however, young adult female col9a1(-/-) and col9a1(+/-) mice exhibited significant loss of trabecular bone. The trabecular bone architecture was progressively deteriorated in both male and female heterozygous col9a1(+/-) mice while aging. The aged mutant mice also exhibited signs of thoracic kyphosis and weight loss, resembling the clinical signs of osteoporosis. The col9a1(+/-) osteoblasts synthesized short col9a1 transcripts at decreased rates. Whereas bone formation activities in vitro and in vivo were not affected, the mutant osteoblast expressed the elevated ratio of RANKL/osteoprotegerin. Increased serum TRACP-5b and CTX levels were found in col9a1(+/-) mice, whose bone surface was associated with osteoclastic cells that were abnormally flattened and enlarged. The mutant and wildtype splenocytes underwent similar osteoclastogenesis in vitro; however, RAW264.7-derived osteoclastic cells, when cultured on the col9a1(+/-) calvaria, widely spread over the bone surface and formed large resorption pits. The surface of col9a1(+/-) calvaria was found to lack the typical nanotopography.

CONCLUSIONS

The mineralized bone matrix deficient of short collagen IX may become susceptible to osteoclastic bone resorption, possibly through a novel non-cell-autonomous mechanism. The data suggest the involvement of bone collagen IX in the pathogenesis of osteoporosis.

Effects of Vitaxin, a novel therapeutic in trial for metastatic bone tumors, on osteoclast functions in vitro

The integrin alphavbeta3 mediates cell-matrix interactions. Vitaxin(R), a humanized monoclonal antibody that blocks human and rabbit alphavbeta3 integrins, is in clinical trials for metastatic melanoma and prostate cancer. alphavbeta3 is the predominant integrin on osteoclasts, the cells responsible for bone resorption in health and disease. Here, we report the first investigation of Vitaxin's effects on osteoclast activity. Vitaxin (100-300 ng/ml) decreased total resorption by 50%, but did not alter resorptive activity per osteoclast. Vitaxin (300 ng/ml) decreased osteoclast numbers on plastic by 35% after 48 h. Similarly, attachment after 2 h was reduced by 30% when osteoclasts were incubated with Vitaxin (300 ng/ml) for 25 min prior to plating; however, the rate of fusion of osteoclast precursors in Vitaxin-treated and control groups was equal. Using time-lapse microscopy, we evaluated the effect of Vitaxin on osteoclast morphology and found a significant reduction in osteoclast planar area only when cells were pretreated with macrophage colony stimulating factor (M-CSF). Extracellular Ca(2+) and M-CSF have opposite effects on alphavbeta3 conformation. Elevation of extracellular Ca(2+) eliminated the inhibitory effect of Vitaxin on osteoclast attachment. In contrast, the effect of Vitaxin was enhanced in cells pretreated with M-CSF. This action of M-CSF was suppressed by the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor wortmannin, suggesting that M-CSF increases Vitaxin's inhibitory effect by inside-out activation of alphavbeta3. In conclusion, Vitaxin decreases resorption by impairing osteoclast attachment, without affecting osteoclast formation and multinucleation. Our data also show that Vitaxin's inhibitory effects on osteoclasts can be modulated by factors known to alter the conformation of alphavbeta3.

Identification of angiogenin as the osteoclastic bone resorption-inhibitory factor in bovine milk

We identified, for the first time, the factor responsible for inhibiting osteoclast-mediated bone resorption in the basic protein fraction of bovine milk (milk basic protein, MBP). The protein was purified by a combination of ion and gel column chromatography from MBP, based on its activity to prevent unfractionated rabbit bone cells from forming pits on dentine slices. It was found to have a molecular weight of 15 kDa on SDS-PAGE, and the sequence of the N-terminal 25 amino acid residues was identical to that of bovine angiogenin. The purified bovine angiogenin inhibited the pit-forming activity of both unfractionated bone cells and purified osteoclasts in a dose-dependent manner, and the inhibitory activity was markedly suppressed by treatment with anti-bovine angiogenin antibody. The inhibitory activity was confirmed in mice both in vitro and in vivo. Treatment of osteoclasts with bovine angiogenin resulted in an impairment of the formation F-actin ring and a reduction in the mRNA levels of TRAP and cathepsin K, both known to be essential for the bone resorption activity of osteoclasts. These results suggest that bovine angiogenin is the substance mainly responsible for the inhibitory effect of bovine milk on osteoclast-mediated bone resorption, and that it exerts its activity by acting directly on the osteoclasts.

The matricellular protein CYR61 inhibits osteoclastogenesis by a mechanism independent of alphavbeta3 and alphavbeta5

Cysteine-rich protein 61 (CYR61/CCN1) belongs to the family of CCN matricellular proteins. Most of the known effects of CCN proteins appear to be due to binding to extracellular growth factors or integrins, including alpha(v)beta(3) and alpha(v)beta(5). Although CYR61 can stimulate osteoblast differentiation, until now the effect of CYR61 on osteoclasts was unknown. We demonstrate that recombinant human CYR61 inhibits the formation of multinucleated, alpha(v)beta(3)-positive, or tartrate-resistant acid phosphatase-positive human, mouse, and rabbit osteoclasts in vitro. CYR61 markedly reduced the expression of the osteoclast phenotypic markers tartrate-resistant acid phosphatase, matrix metalloproteinase-9, calcitonin receptor, and cathepsin K. However, CYR61 did not affect the formation of multinucleated osteoclasts when added to osteoclast precursors prior to fusion or affect the number or resorptive activity of osteoclasts cultured on dentine discs, indicating that CYR61 affects early osteoclast precursors but not mature osteoclasts. CYR61 did not affect receptor activator of nuclear factor-kappaB (RANK) ligand-induced phosphorylation of p38 or ERK1/2 in human macrophages and did not affect RANK ligand-induced activation of nuclear factor-kappaB, indicating that CYR61 does not appear to inhibit osteoclastogenesis by affecting RANK signaling. Furthermore, a mutant form of CYR61 defective in binding to alpha(v)beta(3) also inhibited osteoclastogenesis, and CYR61 inhibited osteoclastogenesis similarly in cultures of mouse wild-type or beta(5)(-/-) macrophages. Thus, CYR61 does not appear to inhibit osteoclast formation by interacting with alpha(v)beta(3) or alpha(v)beta(5). These observations demonstrate that CYR61 is a hitherto unrecognized inhibitor of osteoclast formation, although the exact mechanism of inhibition remains to be determined. Given that CYR61 also stimulates osteoblasts, CYR61 could represent an important bifunctional local regulator of bone remodeling.

Bone remodeling

The skeleton is a metabolically active organ that undergoes continuous remodeling throughout life. Bone remodeling involves the removal of mineralized bone by osteoclasts followed by the formation of bone matrix through the osteoblasts that subsequently become mineralized. The remodeling cycle consists of three consecutive phases: resorption, during which osteoclasts digest old bone; reversal, when mononuclear cells appear on the bone surface; and formation, when osteoblasts lay down new bone until the resorbed bone is completely replaced. Bone remodeling serves to adjust bone architecture to meet changing mechanical needs and it helps to repair microdamages in bone matrix preventing the accumulation of old bone. It also plays an important role in maintaining plasma calcium homeostasis. The regulation of bone remodeling is both systemic and local. The major systemic regulators include parathyroid hormone (PTH), calcitriol, and other hormones such as growth hormone, glucocorticoids, thyroid hormones, and sex hormones. Factors such as insulin-like growth factors (IGFs), prostaglandins, tumor growth factor-beta (TGF-beta), bone morphogenetic proteins (BMP), and cytokines are involved as well. As far as local regulation of bone remodeling is concerned, a large number of cytokines and growth factors that affect bone cell functions have been recently identified. Furthermore, through the RANK/receptor activator of NF-kappa B ligand (RANKL)/osteoprotegerin (OPG) system the processes of bone resorption and formation are tightly coupled allowing a wave of bone formation to follow each cycle of bone resorption, thus maintaining skeletal integrity.

Involvement of alpha(v)beta3 integrins in osteoclast function

Integrins are heterodimeric adhesion receptors that mediate cell-matrix interaction. Osteoclast exhibits high expression of the alpha(v)beta(3) integrin, which binds to a variety of extracellular matrix proteins including vitronectin, osteopontin, and bone sialoprotein. Arg-Gly-Asp (RGD)-containing peptides, RGD-mimetics, and blocking antibodies to alpha(v)beta(3) integrin were shown to inhibit bone resorption in vitro and in vivo, suggesting that this integrin may play an important role in regulating osteoclast function. Several lines of evidence have demonstrated that a number of signaling molecules are involved in the alpha(v)beta(3) integrin-dependent signaling pathway, including c-Src, Pyk2, c-Cbl, and p130(Cas). In this article, we review the history of "alpha(v)beta(3) integrin and osteoclasts" and discuss the involvement of alpha(v)beta(3) integrins in osteoclast function at tissue, cellular, and molecular levels. A better understanding of the role of alpha(v)beta(3) integrin in osteoclastic bone resorption would provide opportunities for developing new therapeutics to treat human bone diseases, including rheumatoid arthritis, osteoporosis, and periodontal disease.

Characterization of osteoclasts derived from CD14+ monocytes isolated from peripheral blood

Bone resorption is solely mediated by osteoclasts. Therefore, a pure osteoclast population is of high interest for the investigation of biological aspects of the osteoclasts, such as the direct effect of growth factors and hormones, as well as for testing and characterizing inhibitors of bone resorption. We have established a pure, stable, and reproducible system for purification of human osteoclasts from peripheral blood. We isolated CD14-positive (CD14+) monocytes using anti-CD14-coated beads. After isolation, the monocytes are differentiated into mature osteoclasts by stimulation with macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappaB ligand (RANKL). Osteoclast formation was only observed in the CD14+ population, not in the CD14- population, and only in the presence of both M-CSF and RANKL, confirming that the CD14+ system is a pure population of osteoclast precursors. No expression of osteoclast markers was observed in the absence of RANKL, whereas RANKL dose-dependently induced the expression of cathepsin K, tartrate-resistant acid phosphatase (TRACP), and matrix metallo proteinase (MMP)-9. Furthermore, morphological characterization of the cells demonstrated that actin rings were only formed in the presence of RANKL. Moreover, the osteoclasts were capable of forming acidic resorption lacunae, and inhibitors of lysosomal acidification attenuated this process. Finally, we measured the response to known bone resorption inhibitors, and found that the osteoclasts were sensitive to these and thereby provided a robust and valid method for interpretation of the effect of antiresorptive compounds. In conclusion, we have established a robust assay for developing osteoclasts that can be used to study several biological aspects of the osteoclasts and which in combination with the resorption marker CTX-I provides a useful tool for evaluating osteoclast function in vitro.

RANKL-dependent and RANKL-independent mechanisms of macrophage-osteoclast differentiation in breast cancer

The cellular and humoral mechanisms accounting for tumour osteolysis in metastatic breast cancer are uncertain. Osteoclasts, the specialised multinucleated cells responsible for tumour osteolysis, are derived from monocyte/macrophage precursors. Breast cancer-derived tumour-associated macrophages (TAMs) are capable of osteoclast differentiation but the cellular and humoral mechanisms controlling this activity are uncertain. In this study, TAMs were isolated from primary breast cancers and cultured in the presence and absence of cytokines/growth factors influencing osteoclastogenesis. Extensive TAM-osteoclast differentiation occurred only in the presence of RANKL and M-CSF; this process was inhibited by OPG and RANK:Fc, decoy receptors for RANKL. Breast cancer-derived fibroblasts and human bone stromal cells expressed mRNA for RANKL, OPG and TRAIL, and co-culture of these fibroblasts with human monocytes stimulated osteoclast formation by a RANKL-dependent mechanism. Osteoclast formation and lacunar resorption also occurred by a RANKL-independent mechanism when the conditioned medium from breast cancer cells, MDA-MB-231 and MCF-7, was added (with M-CSF) to monocyte cultures. Our findings indicate that TAMs in breast cancer are capable of osteoclast differentiation and that breast cancer-derived fibroblasts and breast cancer cells contribute to this process by producing soluble factors that influence osteoclast formation by RANKL-dependent and RANKL-independent mechanisms respectively.

Cloning and Characterization of the Annexin II Receptor on Human Marrow Stromal Cells

Annexin II is a heterotetramer, consisting of two 11-kDa (p11) and two 36-kDa (p36) subunits, that is produced by osteoclasts and stimulates osteoclast formation. However, its receptor is unknown. We showed that annexin II binds to normal primary human marrow stromal cells and the Paget's marrow-derived PSV10 stromal cell line to induce osteoclast formation. 125I-Labeled annexin II binding assays with PSV10 cells demonstrated that there was a single class of annexin II receptors with a Kd of 5.79 nm and Bmax of 2.13 x 10(5) receptors/cell. Annexin III or annexin V did not bind this receptor. Using 125I-labeled annexin II binding to screen NIH3T3 transfected with a human marrow cDNA expression library, we identified a putative annexin II receptor clone, which encoded a novel 26-kDa type I membrane receptor protein when expressed in HEK 293 cells. HEK 293 cells transformed with the cloned annexin II receptor cDNA showed a similar binding affinity to annexin II as that observed in PSV10 cells. Chemical cross-linking experiments with biotinylated annexin II and intact PSV10 cells identified a 55-kDa band on Western blot analysis that reacted with both an anti-p11 antibody and streptavidin but not anti-p36 antibody. A rabbit polyclonal antibody raised against the putative recombinant annexin II receptor also recognized the same 26-kDa protein band detected in PSV10 cells. Importantly, the annexin II receptor antibody dose-dependently blocked the stimulatory effects of annexin II on human osteoclast formation, demonstrating that the receptor mediates the effects of annexin II on osteoclast formation.

Osteoclast differentiation and bone resorption in multicentric reticulohistiocytosis

Multicentric reticulohistiocytosis (MR) is a systemic disease of unknown cause characterized by the presence of a heavy macrophage infiltrate in skin and synovial tissues and the development of an erosive polyarthritis. The synovial fluid in MR is known to contain numerous mononuclear cells. In this study, we have determined whether these cells contribute to joint destruction in MR by differentiating them into osteoclasts. Synovial fluid mononuclear cells were isolated from the knee joint of a 44-year-old male with MR. These cells were cultured with various combinations of macrophage-colony stimulating factor, receptor activator for nuclear factor kappaB ligand (RANKL), tumor necrosis factor alpha, interleukin-1alpha, osteoprotegerin, and zoledronate. Osteoclast differentiation was assessed by expression of tartrate-resistant acid phosphatase, vitronectin receptor, and the extent of lacunar resorption. Most MR synovial fluid mononuclear cells expressed a macrophage phenotype (CD14(+), CD68(+), HLA-DR(+), CD11b(+)). Extensive osteoclast formation and lacunar resorption were noted in macrophage-colony stimulating factor/RANKL-treated cell cultures. MR synovial fluid contained increased tumor necrosis factor alpha and decreased osteoprotegerin compared with osteoarthritis synovial fluid. Lacunar resorption was inhibited in cultures containing zoledronate. Pamidronate treatment of the patient also reduced the number of synovial fluid macrophages and resulted in less osteoclast formation and lacunar resorption. MR synovial fluid contains numerous macrophages that are capable of differentiating into osteoclasts by the RANKL signaling pathway. Inhibitors of osteoclast formation and resorption activity may be of use in preventing the severe joint destruction that commonly occurs in MR.

Endostatin inhibits VEGF-A induced osteoclastic bone resorption in vitro

Background

Endostatin is a C-terminal fragment of collagen XVIII which is a component of basement membranes with the structural properties of both collagens and proteoglycans. Endostatin has a major role in angiogenesis which is intimately associated with bone development and remodeling. Signaling between the endothelial cells and the bone cells, for example, may have a role in recruitment of osteoclastic precursor cells. Our study aims at exploring a possibility that endostatin, either as a part of basement membrane or as a soluble molecule, may control osteoclastogenesis and osteoclastic bone resorption in vitro.

Methods

Rat pit formation assay was employed in order to examine the effect of endostatin alone or in combination with vascular endothelial growth factor-A (VEGF-A) on bone resorption in vitro. Effect of these agents on osteoclast differentiation in vitro was also tested. Osteoclastogenesis and the number of osteoclasts were followed by tartrate resistant acid phosphatase (TRACP) staining and resorption was evaluated by measuring the area of excavated pits.

Results

Endostatin inhibited the VEGF-A stimulated osteoclastic bone resorption, whereas endostatin alone had no effect on the basal resorption level in the absence of VEGF-A. In addition, endostatin could inhibit osteoclast differentiation in vitro independent of VEGF-A.

Conclusion

Our in vitro data indicate that collagen XVIII/endostatin can suppress VEGF-A induced osteoclastic bone resorption to the basal level. Osteoclastogenesis is also inhibited by endostatin. The regulatory effect of endostatin, however, is not critical since endostatin alone does not modify the basal bone resorption.

Malignant melanoma and bone resorption

The cellular and humoral mechanisms accounting for osteolysis in skeletal metastases of malignant melanoma are uncertain. Osteoclasts, the specialised multinucleated cells that carry out bone resorption, are derived from monocyte/macrophage precursors. We isolated tumour-associated macrophages (TAMs) from metastatic (lymph node/skin) melanomas and cultured them in the presence and absence of osteoclastogenic cytokines and growth factors. The effect of tumour-derived fibroblasts and melanoma cells on osteoclast formation and resorption was also analysed. Melanoma TAMs (CD14+/CD51-) differentiated into osteoclasts (CD14-/CD51+) in the presence of receptor activator for nuclear factor kappaB ligand (RANKL) and macrophage-colony stimulating factor. Tumour-associated macrophage-osteoclast differentiation also occurred via a RANKL-independent pathway when TAMs were cultured with tumour necrosis factor-alpha and interleukin (IL)-1alpha. RT-PCR showed that fibroblasts isolated from metastatic melanomas expressed RANKL messenger RNA and the conditioned medium of cultured melanoma fibroblasts was found to be capable of inducing osteoclast formation in the absence of RANKL; this effect was inhibited by the addition of osteoprotegerin (OPG). We also found that cultured human SK-Mel-29 melanoma cells produce a soluble factor that induces osteoclast differentiation; this effect was not inhibited by OPG. Our findings indicate that TAMs in metastatic melanomas can differentiate into osteoclasts and that melanoma fibroblasts and melanoma tumour cells can induce osteoclast formation by RANKL-dependent and RANKL-independent mechanisms, respectively.

Stimulation of osteoclast formation by inflammatory synovial fluid

Peri-articular bone resorption is a feature of arthritis due to crystal deposition and rheumatoid disease. Under these conditions, the synovial fluid contains numerous inflammatory cells that produce cytokines and growth factors which promote osteoclast formation. The aim of this study was to determine whether inflammatory synovial fluid stimulates the formation of osteoclasts. Synovial fluid from rheumatoid arthritis (RA), pyrophosphate arthropathy (PPA) and osteoarthritis (OA) patients was added to cultures (n=8) of human peripheral blood mononuclear cells (PBMCs) in the presence and absence of macrophage colony-stimulating factor (M-CSF) and the receptor activator of NF-kappaB ligand (RANKL). Osteoclast formation was assessed by the formation of cells positive for tartrate-resistant acid phosphatase (TRAP) and vitronectin receptor (VNR) and the extent of lacunar resorption. The addition of 10% OA, RA and PPA synovial fluid to PBMC cultures resulted in the formation of numerous multinucleated or mononuclear TRAP(+) and VNR(+) cells which were capable of lacunar resorption. In contrast to PBMC cultures incubated with OA synovial fluid, there was marked stimulation of osteoclast formation and resorption in cultures containing inflammatory RA and PPA synovial fluid which contained high levels of tumour necrosis factor alpha, a factor which is known to stimulate RANKL-induced osteoclast formation.

The role played by cell-substrate interactions in the pathogenesis of osteoclast-mediated peri-implant osteolysis

Prosthetic wear debris-induced peri-implant osteolysis is a major cause of aseptic loosening after total joint replacement. In this condition, wear particles released from the implant components induce a granulomatous inflammatory reaction at the interface between implant and adjacent bone, leading to progressive bone resorption and loss of fixation. The present study was undertaken to characterize definitively the phenotype of osteoclast-like cells associated with regions of peri-implant focal bone resorption and to compare the phenotypic features of these cells with those of mononucleated and multinucleated cells associated with polyethylene wear particles. Peri-implant tissues were obtained from patients undergoing hip revision surgery for aseptic loosening after total joint replacement. Cells were examined for the expression of several markers associated with the osteoclast phenotype using immunohistochemistry, histochemistry, and/or in situ hybridization. CD68 protein, a marker expressed by multiple macrophage lineage cell types, was detected in mononucleated and multinucleated cells associated with polyethylene particles and the bone surface. Cathepsin K and tartrate-resistant acid phosphatase were expressed highly in both mononucleated and multinucleated cells associated with the bone surface. Levels of expression were much lower in cells associated with polyethylene particles. High levels of β₃ integrin protein were detected in cells in contact with bone. Multinucleated cells associated with polyethylene particles exhibited faint positive staining. Calcitonin receptor mRNA expression was detected solely in multinucleated cells present in resorption lacunae on the bone surface and was absent in cells associated with polyethylene particles. Our findings provide further evidence that cells expressing the full repertoire of osteoclast phenotypic markers are involved in the pathogenesis of peri-implant osteolysis after total joint replacement. They also demonstrate that foreign body giant cells, although believed to be phenotypically and functionally distinct from osteoclasts, express many osteoclast-associated genes and gene products. However, the levels and patterns of expression of these genes in the two cell types differ. We speculate that, in addition to the role of cytokines and growth factors, the substrate with which these cells interact plays a critical role in their differential phenotypic and functional properties.

In vitro and in vivo targeting properties of iodine-123- or iodine-131-labeled monoclonal antibody 14C5 in a non-small cell lung cancer and colon carcinoma model

PURPOSE

The monoclonal antibody (mAb) 14C5 is a murine IgG1 directed against a yet undefined molecule involved in cell substrate adhesion found on the surface of malignant breast cancer tissue. mAb 14C5 is able to inhibit cell substrate adhesion and invasion of breast cancer cells in vitro. In normal tissues as well as in the stroma surrounding in situ carcinomas of the breast, no expression of the antigen 14C5 occurs. The aim of this study was to investigate the in vitro and in vivo targeting properties of 123I- and 131I-labeled mAb 14C5 as a novel agent for radioimmunodetection and radioimmunotherapy.

EXPERIMENTAL DESIGN

Internalization of mAb 14C5 was investigated with 125I-labeled mAb 14C5 and by confocal laser scanning microscopy. Biodistribution studies of 131I-labeled mAb 14C5 and planar gamma imaging were done in nude mice bearing an A549 (non-small cell lung carcinoma) or a LoVo (colon carcinoma) tumor.

RESULTS

Internalization studies with both A549 and LoVo cells showed that 125I-labeled mAb 14C5 is slowly internalized with approximately 30% of the initially bound mAb 14C5 internalized after 2 hours at 37 degrees C. Internalization of mAb 14C5 could be visualized with confocal laser scanning microscopy. In vivo, radioisotope uptake peaked at 24 hours for both tumor models (n = 5) with no significant difference in percentage of injected dose/g tissue (A549 10.4 +/- 0.8 and LoVo 9.3 +/- 0.8). Via planar gamma camera imaging, A549 lung tumors as well as LoVo colon tumors could be clearly visualized.

CONCLUSIONS

The in vitro and in vivo targeting properties of 123I- and 131I-labeled mAb 14C5 are promising and could provide a new antibody-based agent for radioimmunodetection and radioimmunotherapy of patients bearing antigen 14C5-expressing tumors.

Synovial fluid macrophages are capable of osteoclast formation and resorption

To determine whether synovial fluid (SF) macrophages isolated from the SF of osteoarthritis (OA), rheumatoid arthritis (RA) and pyrophosphate arthropathy (PPA) joints are capable of osteoclast formation, and to investigate the cellular and humoral factors required for this to occur, SF macrophages (CD14+) were isolated from the knee joint SF from patients with OA, RA and PPA and cultured for up to 14 days with macrophage-colony stimulating factor (M-CSF) and soluble receptor activator for nuclear factor-kappaB ligand (RANKL) or tumour-necrosis factor-alpha (TNFalpha) and interleukin-1alpha (IL-1alpha). Osteoclast differentiation was assessed by expression of tartrate-resistant acid phosphatase (TRAP) and vitronectin receptor (VNR), F-actin ring formation and lacunar resorption. Osteoclast formation and lacunar resorption was seen in RANKL-treated cultures of SF macrophages isolated from OA, RA and PPA joints with the largest amount of resorption noted in RA and PPA SF macrophage cultures. In TNFalpha/IL-1alpha-treated RA and PPA SF macrophage cultures, osteoclasts capable of lacunar resorption were also formed. Lacunar resorption was more extensive in RANKL than TNFalpha/IL-1alpha-treated cultures. These findings indicate that SF macrophages are capable of differentiating into mature osteoclasts capable of lacunar resorption. M-CSF in combination with RANKL or TNFalpha/IL-1alpha was required for osteoclast formation. As inflammatory synovial fluids contain an increase in the number of macrophages and an increase in the amounts of RANKL, TNFalpha and IL-1alpha, these findings suggest that one means whereby bone erosions may form in rheumatoid or crystal arthritis is by differentiation of synovial fluid macrophages into osteoclasts.

The role(s) of Src kinase and Cbl proteins in the regulation of osteoclast differentiation and function

The osteoclast resorbs mineralized bone during bone development, homeostasis, and repair. The deletion of the gene encoding the nonreceptor tyrosine kinase c-Src produces an osteopetrotic skeletal phenotype that is the consequence of the inability of the mature osteoclast to efficiently resorb bone. Src-/- osteoclasts exhibit reduced motility and abnormal organization of the apical secretory domain (the ruffled border) and attachment-related cytoskeletal elements that are necessary for bone resorption. A key function of Src in osteoclasts is to promote the rapid assembly and disassembly of the podosomes, the specialized integrin-based attachment structures of osteoclasts and other highly motile cells. Once recruited to the activated integrins, especially alphavbeta3), by the adhesion tyrosine kinase Pyk2, Src binds and phosphorylates Cbl and Cbl-b, homologous multisite adapter proteins with ubiquitin ligase activity. The Cbl proteins in turn recruit and activate additional signaling effectors, including phosphatidylinositol 3-kinase and dynamin, which play key roles in the development of cell polarity and the regulation of cell attachment and motility. In addition, Src and the Cbl proteins contribute to signaling cascades that are activated by several important receptors, including receptor activator of nuclear factor kappaB and the macrophage colony-stimulating factor receptor, and also downregulate the signaling from many of these receptors.