New model for bone resorption study in vitro: human osteoclast-like cells from giant cell tumors of bone

beta(3) Integrin subunit mediates the bone-resorbing function exerted by cultured myeloma plasma cells

alpha(v)beta(3) integrin was investigated in multiple myeloma in relation to the in vitro osteoclast-like activity of malignant plasma cells. Myeloma cells from patients with skeleton involvement overexpressed alpha(v)beta(3) and produced erosion pits on bone substrates, whereas this effect was not observed by cells from patients with no evidence of bone disease. We therefore explored the alpha(v)beta(3) transcriptional pathway in the bone-resorbing cells. Silencing of beta(3) chain abrogated the ability to produce erosion pits and extracellular signal-regulated kinase 1/2 phosphorylation resulting in the defective function of cFos and nuclear factor activator T cell 1, the terminal effectors of osteoclast activation. A similar defect occurred in constitutively beta(3)-deficient cells from patients with no skeleton disease. Microarray gene analysis of beta(3)(+) myeloma cells showed that several osteoclast-related genes were up-regulated. Their functions include the activation of receptor pathways beta(3) and c-fms that regulate several osteoclast functions. These data emphasize the postulated role of myeloma cells in multiple myeloma bone disease and suggest that their osteoclast-like activity is regulated, at least in vitro, by the beta(3) subunit of the integrin.

Similarities Between Giant Cell Tumor of Bone, Giant Cell Tumor of Tendon Sheath, and Pigmented Villonodular Synovitis Concerning Ultrastructural Cytochemical Features of Multinucleated Giant Cells and Mononuclear Stromal Cells

The authors investigated ultrastructural cytochemical features of multinucleated and mononuclear stromal cells in giant cell tumor of bone (GCTB), giant cell tumor of tendon sheath (GCTTS), and pigmented villonodular synovitis (PVNS). Specimens of each tumor, respectively numbering 4, 4, and 3, were stained for tartrate-resistant acid phosphatase (TRAP) reactions and examined with an electron microscope. In GCTB and GCTTS, multinucleated cells, including some relatively small giant cells, showed TRAP activity and cytoplasmic features characteristic of osteoclasts, and also sometimes abundant rough endoplasmic reticulum and siderosomes. A few giant cells with macrophage-like features and slight TRAP activity were demonstrated in GCCTS and PVNS. In each tumor type, mononuclear cells showing TRAP activity shared cytoplasmic features with osteoclast-like multinucleated giant cells, while some others had macrophage-like features, and still others were poorly differentiated; a few mononuclear cells showed cell-to-cell contact. Ultrastructural similarities of TRAP-positive mononuclear cells in the three tumor types, and those between TRAP-positive multinucleated cells in GCTB and GCTTS, suggest a common cell lineage capable of multinucleated giant cell formation in the 3 tumors, despite differing histogenesis.

Immunohistochemical analyses of beta-catenin and cyclin D1 expression in giant cell tumor of bone (GCTB): a possible role of Wnt pathway in GCTB tumorigenesis

Giant cell tumor of bone (GCTB) is a benign neoplasm but occasionally shows local recurrence, and histologically consists of osteoclast-like giant cells (GC) and stromal mononuclear cells (SC), which are capable of proliferation and osteoblastic differentiation. Activation of Wnt signaling can induce osteoblast differentiation and osteoclastgenesis during bone resorption process. This study analyzed the profiles of beta-catenin and cyclin D1 expression in GCTB to elucidate an involvement of Wnt pathway in tumorigenesis. We performed immunohistochemistry for beta-catenin, cyclin D1, and Ki-67 in 16 GCTB tumors, including 5 recurrent cases that were surgically resected. All 16 cases of GCTB displayed beta-catenin, cyclin D1, and Ki-67 expression. Immunoreactivity for beta-catenin was observed in nuclei of SC and GC. Cyclin D1 immunoreactivity was found mainly in nuclei of GC, while Ki-67 immunoreactivity was restricted to nuclei of SC. The nuclear beta-catenin labeling index (LI) in both SC (60.6 vs. 41.8%, p=0.074) and GC (41.7 vs. 20.1%, p=0.095) was higher in recurrent tumors than in primary tumors in all the 4 cases. However, Ki-67 LI in SC (18.8 vs. 19.9%, p=0.851) and cyclin D1 LI in GC (55.4 vs. 70.1%, p=0.225) were not higher in recurrent tumors than in primary tumors. Our results suggested activation of Wnt/ beta-catenin pathway in GCTB tumorigenesis. Since cyclin D1 in GC was never associated with the expression of the well-known proliferative marker Ki-67, cyclin D1 expression might play a role in GC formation instead of promoting cell proliferation during GCTB tumorigenesis. Importantly, it was suggested that the nuclear beta-catenin staining level might be associated with tumor recurrence in GCTB.

Bone-resorbing cells in multiple myeloma: osteoclasts, myeloma cell polykaryons, or both?

Myeloma bone disease (MBD) leads to progressive destruction of the skeleton and is the most severe cause of morbidity in multiple myeloma. Its pathogenetic mechanisms are not fully understood, though the current evidence points to osteoclast (OC) hyperactivity coupled with defective osteoblast function unable to counteract bone resorption. OCs are generated in bone marrow by myeloid progenitors through increased levels of receptor activator of nuclear factor kappaB ligand and M-CSF, whose intracellular pathways propagate signals that activate sequential transcription factors, resulting in the production of major OC enzymes that drive specific functions such as acidification and degradation of the bone matrix. Osteolytic lesions, however, are not characterized by massive OC content, whereas malignant plasma cells, which are usually present in a high number, may occur as large multinucleated cells. The possibility that myeloma cells fuse and generate polykaryons in vivo is suggested by the in vitro formation of multinuclear cells that express tartrate-resistant acid phosphatase and produce pits and erosive lacunae on experimental osteologic substrates. Further, the detection in vivo of polykaryons with chromosome translocations typical of myeloma cells lends support to the view that myeloma polykaryons may act as functional OCs and participate in the skeletal destruction by resorbing bone.

CD33+ CD14- phenotype is characteristic of multinuclear osteoclast-like cells in giant cell tumor of bone

Giant cell tumor of bone (GCTB) is a benign bone tumor with a shown clinical behavior of local recurrences and rare distant metastases. GCTB is composed of uniformly distributed osteoclastic giant cells, thought to originate from the fusion of monocyte-macrophage lineage cells, in a background consisting of mononuclear rounded cells and spindle-shaped cells. Several reports showed the specific expression of markers, such as CD14 on the mononuclear rounded cell population, however, lacking osteoclastic giant cells. Blood monocytes that were CD14+, CD33+, or CD14+/CD33+ have also been shown to be programmed as pre-osteoclasts. The macrophage marker CD33 is expressed earlier than CD14 in macrophage maturation, whereas CD14 is expressed longer than CD33. The aim of this study was to investigate CD14/CD33 expression profiles in GCTB. Nineteen GCTB tumor samples of 19 patients were studied. Immunofluorescent analyses were performed with monoclonal antibodies against CD14, CD33, RANK, and CD51. To unambiguously further prove the expression of these molecules, quantitative RT-PCR was used with subsequent sequencing of its products. All samples showed similar immunoreactivity profiles. The mononuclear rounded cell population was positive for RANK, CD51, CD14, and CD33. The osteoclastic giant cell population expressed RANK and CD51, as well as CD33, but was consistently negative for CD14 expression. The CD14 and CD33 profiles were confirmed by quantitative RT-PCR. These RT-PCR products were sequence verified. Osteoclasts in GCTB are the result of fusion of CD33-expressing pre-osteoclasts that further fuse with CD14+ mononuclear cells. Although these results reflect a static rather than a dynamic spectrum, we strongly believe that osteoclastogenesis seems not to be the exclusive result of fusion of intratumoral CD14+ mononuclear cells. Moreover, CD33-modulated osteoclastogenesis opens up the possibility for novel therapeutic directions.

Molecular profiling of giant cell tumor of bone and the osteoclastic localization of ligand for receptor activator of nuclear factor kappaB

Giant cell tumor of bone (GCT) is a generally benign, osteolytic neoplasm comprising stromal cells and osteoclast-like giant cells. The osteoclastic cells, which cause bony destruction, are thought to be recruited from normal monocytic pre-osteoclasts by stromal cell expression of the ligand for receptor activator of nuclear factor kappaB (RANKL). This model forms the foundation for clinical trials in GCTs of novel cancer therapeutics targeting RANKL. Using expression profiling, we identified both osteoblast and osteoclast signatures within GCTs, including key regulators of osteoclast differentiation and function such as RANKL, a C-type lectin, osteoprotegerin, and the wnt inhibitor SFRP4. After ex vivo generation of stromal- and osteoclast-enriched cultures, we unexpectedly found that RANKL mRNA and protein were more highly expressed in osteoclasts than in stromal cells, as determined by expression profiling, flow cytometry, immunohistochemistry, and reverse transcriptase-polymerase chain reaction. The expression patterns of molecules implicated in signaling between stromal cells and monocytic osteoclast precursors were analyzed in both primary and fractionated GCTs. Finally, using array-based comparative genomic hybridization, neither GCTs nor the derived stromal cells demonstrated significant genomic gains or losses. These data raise questions regarding the role of RANKL in GCTs that may be relevant to the development of molecularly targeted therapeutics for this disease.

The origin of the neoplastic stromal cell in giant cell tumor of bone

Fibroblastlike stromal cells, which are always present as a component of giant cell tumor of bone (GCT), can be observed in both in vivo and cultured cell samples. Although they are assumed to trigger the cancer process in GCT, the histogenesis of GCT stromal cells is poorly understood. It is known that mesenchymal stem cells (MSCs) can develop to osteoblasts. Evidence has been presented that GCT stromal cells can also develop to osteoblasts. A connection between MSCs and GCT stromal cells was sought by using 2 different laboratory approaches. First, immunohistological analyses revealed that some of the same markers, detected by the SH2, SH3, and SH4 antibodies and the CD166 antigen, were found in GCT stromal cells as in the first developmental stages of osteoblast differentiation from the initial MSCs. These immunohistological findings could be confirmed by reverse transcriptase polymerase chain reaction. Second, cellular differentiation by morphology and lineage-specific staining offered evidence that not only osteoblasts, but also chondroblasts and adipocytes, could be cultured from stromal cells. The presented double approach indicates that GCT stromal cells can originate from MSCs.

Metatarsal giant cell tumors and giant cell reparative granuloma are similar entities

Light and electron microscopic investigations and studies of the resorption ability in vitro of giant cells were done in two patients with giant cell osteolytic lesions of metatarsal bone. Giant cells harvested from both patients were similar in morphologic features and ability to resorb dentin. After other diagnoses of osteolytic lesions of metatarsal bone were ruled out, one lesion was considered a giant cell reparative granuloma and the other a true giant cell tumor of bone. Clinical, radiologic, ultrastructural, functional studies, and data in the literature, indicated that giant cell reparative granuloma only can be differentiated from giant cell tumor by younger age at diagnosis and the occurrence of giant cell clusters. All other features (cortical erosion, high rate of recurrence, hemorrhage areas, predominant intercellular collagenous substance) are characteristic of both lesions. If these two giant cell lesions are different entities, more accurate means are needed to distinguish them.

Decoy oligodeoxynucleotides targeting NF-kappaB transcription factors: induction of apoptosis in human primary osteoclasts

Proteins belonging to the nuclear factor kappaB (NF-kappaB) superfamily are involved in osteoclast formation, playing a very important role for both differentiation of osteoclast precursors and survival of mature osteoclasts. Several drugs used to fight bone loss in a variety of human pathologies, including osteoporosis, act by increasing the frequency of osteoclast apoptosis, since it was demonstrated that small changes in osteoclast apoptosis can result in large changes in bone formation. In this respect, targeting of NF-kappaB transcription factor could be of great interest. Among nonviral gene therapy strategies recently proposed to inhibit or even block NF-kappaB activity, the transcription factor decoy (TFD) should be taken in great consideration. The main issue of the present study was to examine the effects of decoy DNA/DNA molecules targeting NF-kappaB on apoptosis of human osteoclasts (OCs), with the aim to interfere with the pathway regulating osteoclast differentiation and programmed cell death. To this aim, we used a mixture of receptor activator of NF-kappaB ligand (RANKL), macrophage colony-stimulating factor (M-CSF) and parathyroid hormone (PTH) to prepare human OCs from peripheral blood cells. Then, transfection with the decoy molecules targeting NF-kappaB was performed. The results obtained demonstrate that in primary cells expressing typical osteoclast markers such as TRAP and MMP9, NF-kappaB decoy significantly stimulated apoptosis. Inhibition of IL-6 expression and induction of Caspase 3 were found in OCs treated with NF-kappaB DNA/DNA decoys. We consider these data as the basis for setting up experimental conditions allowing nonviral gene therapy of several bone disorders.

Guidelines for using in vitro methods to study the effects of phyto-oestrogens on bone

These guidelines review the relevant literature on the way plant phyto-oestrogens act on bone and the responsiveness of different bone cell systems to phyto-oestrogenic compounds. The primary emphasis is on the experimental conditions used, the markers available for assessing osteoblast and osteoclast function, and their expected sensitivity. Finally, we assess the published results to derive some general recommendations for in vitro experiments in this area of research.

Cyclin alterations in giant cell tumor of bone

Cyclins play an important role in regulating the passage of dividing cells through critical checkpoints in the cell cycle. Because alterations of several cyclins, especially cyclin D1, have been implicated in the development of many human neoplasms, we examined 32 cases of giant cell tumor of long bones for cyclin D1 gene amplification and protein overexpression using differential polymerase chain reaction and immunohistochemistry, respectively. In addition, the expression of cyclin D3, cyclin B1, and the proliferation-associated antigen Ki-67 (MIB-1) was assessed immunohistochemically. Low-level cyclin D1 gene amplification was detected in 61% of giant cell tumor cases. All tumors showed cyclin D1, cyclin D3, cyclin B1, and Ki-67 (MIB-1) staining; however, the distribution was very characteristic. Cyclin D1 protein expression was seen predominantly in the nuclei of the giant cells, with occasional mononuclear cells staining. There was no correlation between cyclin D1 gene amplification and protein overexpression. Cyclin D3 staining showed a similar distribution, with 88% of cases showing protein overexpression. Cyclin D1 and/or D3 staining in the giant cells was never associated with staining for either cyclin B1 or Ki-67 (MIB-1), as the expression of the latter two proteins was restricted to the mononuclear cells. Cyclin B1 overexpression was seen in 44% of cases. Ki-67 (MIB-1) staining was present in all cases, and between 10 to 50% of the mononuclear cells were positive. These results suggest that alterations in cyclin D1 and/or D3 might play a role in the pathogenesis of giant cell tumor of bone.

Localization and possible role of two different alpha v beta 3 integrin conformations in resting and resorbing osteoclasts

Integrins are membrane receptors that mediate interactions between cells and the extracellular matrix. We recently showed that the osteoclast integrinα vβ3 exists in two different conformations,so-called `basal' and `activated', with each exhibiting a distinct function. In this study we demonstrate that, in non-resorbing osteoclasts, the`activated' form of αvβ3 accumulates in the motile areas of the plasma membrane. During bone resorption this conformation is prevalent in the ruffled membrane, whereas the `basal' form ofα vβ3 is also present in the sealing zone. Moreover, hepatocyte growth factor (HGF) and macrophage colony stimulating factor (M-CSF), two molecules involved in osteoclastogenesis and osteoclast survival, modulate αvβ3 conformation in vitro. Preincubation with HGF or M-CSF induces a shift of conformation ofα vβ3 in primary human osteoclasts (OCs) and in the osteoclast-like cell line (GCT 23). Activated integrin promotes osteoclast migration to the αvβ3 ligand osteopontin and enhances bone resorption. Thus, HGF and M-CSF modulate theα vβ3 conformational states required for osteoclast polarization and resorption. The capacity of growth factors to alter the affinity of αvβ3 toward its ligands offers a potential explanation for the diverse responses of osteoclasts to the same ligand.

Gene expression of glucocorticoid receptor alpha and beta in giant cell tumour of bone: evidence of glucocorticoid-stimulated osteoclastogenesis by stromal-like tumour cells

Glucocorticoids have been shown to increase bone resorption in vitro and in vivo, however, the mechanism(s) of this action are not fully understood. Given that human giant cell tumour of bone (GCT) is considered to arise from mesenchymal stromal cells and has the capacity to recruit and harbour macrophages and multinucleated osteoclasts, we have used GCT as a model for studying the effect of glucocorticoids on osteoclast formation. We have demonstrated, by RT-PCR and fluorescence in-situ hybridisation, that both glucocorticoid receptor alpha and beta (GRalpha and GRbeta) gene transcripts were present in the stromal-like tumour cells, macrophage-like cells (putative osteoclast precursors) and multinucleated osteoclast-like cells. Moreover, in the presence of 1,25(OH)(2)D(3), dexamethasone dose-dependently stimulated the formation of osteoclast-like cells from GCT-derived co-culture system of stromal-like tumour cells and macrophage-like cells. The stimulation of osteoclastogenesis by dexamethasone was coincident with the up-regulation of receptor activator of NF-kappaB ligand (RANKL) but down-regulation of osteoprotegerin (OPG) gene expression in stromal-like tumour cells. These data are consistent with the hypothesis that glucocorticoids increase bone resorption by promoting osteoclastogenesis which is at least in part due to the stimulation of RANKL and inhibition of OPG production in bone stromal cells.

Identification of EWS/FLI-1 transcripts in giant-cell tumor of bone

The EWS/FLI-1 fusion gene, resulting from a t(11;22) translocation, plays a key role in the pathogenesis of Ewing's sarcoma. We demonstrate the presence of EWS/FLI-1 hybrid transcripts also in giant-cell tumor, a bone neoplasm featuring intermediate characteristics between benign and malignant lesions. Chimeric products were detected by semi-nested PCR after 2 cycles of amplification in 13/15 cases of giant-cell tumor, and their presence was confirmed by Southern and Western blots and fluorescence in situ hybridization. Moreover, 3/8 primary cultures of giant-cell tumor showed the same type of hybrid transcript observed in the original tumor sample. Sequencing of PCR products confirmed the presence of EWS and FLI-1 sequences in these products. Detection of EWS/FLI-1 fusion transcripts in giant-cell tumor of bone provides a model for the study of the transforming mechanisms of the EWS/FLI-1 fusion gene in mesenchymal tumors.

Electrophysiological characterization of ion channels in osteoclasts isolated from human deciduous teeth

Ion channels contribute to several important processes in osteoclasts, including proton transport and volume regulation. Although ion channels have been described in osteoclasts from several species, little is known about their properties in human osteoclasts. We devised a method for isolation of authentic human osteoclasts from deciduous teeth undergoing root resorption, and characterized currents in these cells using patch-clamp techniques. Three types of K(+) current were identified. Hyperpolarization elicited an inwardly rectifying K(+) current in most osteoclasts, which was inhibited by Ba(2+) in a voltage- and time-dependent manner. Depolarization elicited an outwardly rectifying and tetraethylammonium-sensitive current, consistent with a large-conductance Ca(2+)-dependent K(+) channel. In addition to these basal currents, extracellular adenosine 5'-triphosphate (ATP) elicited a linear current that was identified as a Ca(2+)-dependent K(+) current, based on its reversal potential close to that predicted for K(+), its blockade by quinine, and its activation by Ca(2+) ionophore. Last, an outwardly rectifying current was observed to activate spontaneously or in response to ATP, with properties of a swelling-activated Cl(-) current. This current reversed direction close to the Cl(-) equilibrium potential and was blocked by the anion channel blocker, niflumic acid, identifying it as a Cl(-) current. In summary, we have developed a novel method for isolation of authentic human osteoclasts and have characterized K(+) and Cl(-) currents. Cl(-) current mediates charge compensation during electrogenic H(+) transport, so activation of Cl(-) current may contribute to the stimulatory effects of extracellular ATP on bone resorption.

Breast cancer cell line MDA-231 stimulates osteoclastogenesis and bone resorption in human osteoclasts

Breast cancers commonly cause osteolytic metastases in bone, a process that is dependent upon osteoclast-mediated bone resorption, but the mechanism responsible for tumor-mediated osteoclast activation has not yet been clarified. In the present study we utilized a well-known human breast cancer cell line (MDA-231) in order to assess its capability to influence osteoclastogenesis in human bone marrow cultures and bone resorption in fully differentiated osteoclasts. We demonstrated that conditioned medium (CM) harvested from MDA-231 increased the formation of multinucleated TRAP-positive cells in bone marrow cultures. Bone resorption activity of fully differentiated human osteoclasts and of osteoclast-like cell lines, from giant cell tumors of bone (GCT), was highly increased by the presence of MDA-231 CM. Moreover, while MDA-231 by themselves did not produce IL-6 tumor cell, CM increased the secretion of IL-6 by primary human osteoclasts and GCT cell lines compared to untreated controls. These data suggest that MDA-231 produce osteoclastic activating factor(s) that increase both osteoclast formation in bone marrow culture and bone resorption activity by mature cells. Moreover, breast cancer cells stimulate IL-6 secretion by osteoclasts that is one of the factors known to supports osteoclastogenesis.

Development and characterization of a human in vitro resorption assay: demonstration of utility using novel antiresorptive agents

A human in vitro resorption assay has been developed using osteoclastoma-derived osteoclasts and used to evaluate novel antiresorptive agents including antagonists of the alphavbeta3 integrin, and inhibitors of cathepsin K and the osteoclast ATPase. The potency of novel compounds in the in vitro resorption assay correlates with functional assays for each class of inhibitor: the human alphavbeta3-mediated cell adhesion assay for the vitronectin receptor antagonists (r2 = 0.82), the chick osteoclast vacuolar ATPase enzyme assay for the H+-ATPase inhibitors (r2 = 0.77) and the recombinant human cathepsin K enzyme assay for the cathepsin K inhibitors (r2 = 0.80). Cell suspensions, rich in osteoclasts, are prepared by collagenase digestion of the tumor tissue. These cells can be stored long-term in liquid nitrogen and upon thawing maintain their bone-resorbing phenotype. The cryopreserved cells can be cultured on bovine cortical bone for 24-48 h and resorption can be measured by either confocal microscopy or biochemical assays. The resorptive activity of osteoclasts derived from a number of tumors can be inhibited reproducibly using a number of mechanistically unique antiresorptive compounds. In addition, the measurement of resorption pits by laser confocal microscopy correlates with the release of type I collagen C-telopeptides or N-telopeptides, as measured by enzyme-linked immunosorbent assay. Resorption can be measured reproducibly using a 48-h incubation of osteoclasts on bone slices, or a 24-h incubation with bone particles. This in vitro human osteoclast resorption assay provides a robust system for the evaluation of inhibitors of osteoclastic function that may be developed for the treatment of metabolic bone diseases such as osteoporosis.

A novel calcium sensor stimulating inositol phosphate formation and [Ca2+]i signaling expressed by GCT23 osteoclast-like cells

Osteoclast activity is inhibited by elevated [Ca2+]o; however, the underlying molecular mechanism is unknown. We used the human osteoclast-like cells GCT23 to elucidate their cation-sensing properties. Cells responded to elevated [Ca2+]o with rapid concentration-dependent [Ca2+]i transients (EC50 = 7.8 mm, time to peak 44 +/- 4 sec) that were due to release from intracellular stores, followed by Ca2+ influx across the plasma membrane. Ca2+ store depletion by thapsigargin, endothelin-1, or bradykinin activated calcium entry pathways. Cells responded similarly to Ni2+ and Cd2+ with albeit slower kinetics (EC50 <10 microm and <100 microm, times to peak 140 +/- 25 sec and 150 +/- 24 sec, respectively). The three cations stimulated inositol phosphate production (two-fold, p <.02) similar to bradykinin (2.5-fold, p <. 002), which activates a phospholipase C (PLC)-coupled receptor in GCT23 cells. The cells did not respond to 0.1-1 mM Gd3+ or neomycin B, indicating that the parathyroid calcium receptor (PCaR) is not functionally expressed. In confirmation, PCaR could not be detected by reverse transcriptase polymerase chain reaction in GCT23 cells and in mouse osteoclasts, and the calcimimetic compound NPS R-568 failed to produce the left shift of the concentration-response curve characteristic for PCaR. Our data demonstrate for the first time that cation sensing by osteoclast-like GCT23 cells is mediated by a PLC-coupled receptor that is not identical to PCaR.

Osteoclast markers accumulate on cells developing from human peripheral blood mononuclear precursors

Recent studies show that human osteoclasts develop in vitro from hematopoietic cells; however, special cultures conditions and/or cytokine mobilized peripheral blood are apparently required. Here, we report that cells expressing osteoclast markers differentiate from precursors present in nonmobilized peripheral blood mononuclear cells (PBMC), without the addition of stromal cells, growth factors, cytokines or steroids; and characterize their phenotype. Three days after establishing high-density PBMC cultures (1.5 x 10(6) cells/cm2), in serum-containing medium, small adherent colonies of tartrate resistant acid phosphatase positive (TRAP+) cells emerge, amidst massive monocyte cell death. These adherent cells have an eccentrically placed, round nucleus, and express low levels of TRAP and sodium fluoride-resistant- alpha-naphthyl-acetate-esterase (NaF-R-NSE). Over the next week, this cell population accumulates phenotypic markers of osteoclasts (vitronectin receptor [VR], calcitonin receptor, TRAP, cathepsin K protein, and mRNA) with increased nuclearity, covering the entire surface by 15 days. When cultured on bone, VR+, TRAP+ cells of low multinuclearity appear and cover up to 50% of the surface. Resorption lacunae can be observed by day 22. Although these pits are not nearly as numerous as the cells of preosteoclast phenotype, they do represent the activity of a subset of osteoclast-like cells that has achieved osteoclastic maturity under these culture conditions. Transcripts for osteoprotegerin ligand (OPGL), an osteoclast differentiation factor (also known as RANKL and TRANCE) are expressed, likely by adherent cells. Thus, an adherent population of cells, with preosteoclast/osteoclast phenotypic properties, arises selectively under simple culture conditions from normal PBMC. Further characterization of these cells should identify factors involved in the growth, terminal differentiation and activation of osteoclasts.

Activation of alphav beta3 integrin on human osteoclast-like cells stimulates adhesion and migration in response to osteopontin

Integrins mediate cell adhesion and can induce different cellular responses, including changes in intracellular pH, changes and oscillation in intracellular free calcium, and protein phosphorylation on tyrosine. During bone resorption, the integrin alphav beta3 regulates adhesion of osteoclasts to bone extracellular matrix proteins, such us osteopontin (Opn). Adhesion via alphav beta3 is followed by osteoclast polarization onto the bone surface and by the onset of bone resorption. To characterize these events at the molecular level, we investigated the state of activation of alphav beta3 on the human osteoclast-like cell line GCT23 using the monoclonal antibody AP5 which binds to and can induce, under low calcium conditions, activated alphav beta3. By flow cytometry, approximately 50% of alphav beta3 on the surface of the osteoclast-like cell line GCT23 was reactive with AP5 and was therefore in the activated state. Incubation with AP5 in the presence of low calcium concentrations increased activated alphav beta3 to 90-100%. Activation of alphav beta3 increased the efficiency of GCT23 adhesion to Opn by 2-fold. Furthermore, haptotactic migration on Opn was also enhanced about 40% compared to control. We propose that changes in the activation state of alphav beta3 may be a regulation point for osteoclasts during bone resorption.

Activation of MMP-2 by human GCT23 giant cell tumour cells induced by osteopontin, bone sialoprotein and GRGDSP peptides is RGD and cell shape change dependent

We show that osteopontin (OPN), bone sialoprotein (BSP) and GRGDSP peptides, in solution, induce activation of metalloproteinase-2 (MMP-2) secreted by human GCT23 giant cell tumour cells. Activation of MMP-2 is RGD sequence dependent, possibly involves anti-alphaVbeta3 integrins, is preceded by a change from spread to rounded cell morphology and is mimicked by the actin depolymerising agent cytochalasin B. Cells that had spread on OPN, BSP and GRGDSP substrata failed to activate MMP-2, but subsequent addition of soluble GRGDSP induced rounding and MMP-2 activation. Activation induced by GRGDSP and cytochalasin B was cell mediated, inhibited by EDTA, tissue inhibitor of metalloproteinase-2 (TIMP-2) and carboxyl terminal MMP-2 consistent with a role for membrane type (MT)-MMP but did not involve urokinase, plasmin or thrombin activity. Activation induced by GRGDSP and cytochalasin B, but not cell rounding, was inhibited by herbimycin A, cycloheximide and actinomycin D, suggesting a role for tyrosine kinases, protein and RNA synthesis, but was not associated with changes in mRNA for MT-MMP-1, MMP-1, MMP-2, TIMP-1 or TIMP-2. GRGDSP and cytochalasin B enhanced levels of membrane-associated pro- and active form MMP-1 and MMP-2 but not MT-MMP-1, stimulated cell surface MMP-1 staining and induced that of MT-MMP-1, MMP-2 and TIMP-2. This was consistent with the possible relocation of constitutive MT-MMP-1 to the cell surface as a prerequisite for subsequent cell surface MMP-2/TIMP-2/MT-MMP-1 complex formation and to the potential induction of conditions favourable for reciprocal cell surface MMP-1/MMP-2 activation. Our data provide a novel insight into interactions between RGD containing bone matrices, GCT cells and MMPs of potential relevance to GCT pathology.

In vitro toxicity of N3-methyl-5'-deoxy-5-fluorouridine, a novel metabolite of doxifluridine: a bioanalytical investigation

The cytotoxicity of N3-methyl-5'-deoxy-5-fluorouridine (N3-Me-5'-dFUR), a novel metabolite of the anticancer pro-drug 5'-deoxy-5-fluorouridine (5'-dFUR), has been evaluated by in vitro experiments with cultures of different cancer cell lines. The new metabolic product was found to be non-toxic in all the cell growth experiments performed. The absence of cytotoxicity could be explained by the observation that the metabolite was not recognized as a substrate by thymidine phosphorilase, the enzyme responsible for 5-fluorouracil (5-FU) release from doxifluridine, as ascertained by high-performance liquid chromatography/ultraviolet (HPLC-UV) analysis of the incubation mixture. The biomethylation process leading to N3-Me-5'-dFUR could be considered as a possible detoxification pathway, altering the drug bioavailability, in competition with 5'-dFUR cleavage to the active 5-FU.

Hepatocyte growth factor is a coupling factor for osteoclasts and osteoblasts in vitro

Hepatocyte growth factor (HGF), also known as scatter factor, is a powerful motogen, mitogen, and morphogen produced by cells of mesodermal origin, acting on epithelial and endothelial cells. Its receptor is the tyrosine kinase encoded by the c-MET protooncogene. We show that the HGF receptor is expressed by human primary osteoclasts, by osteoclast-like cell lines, and by osteoblasts. In both cell lineages, HGF stimulation triggers the receptor kinase activity and autophosphorylation. In osteoclasts, HGF receptor activation is followed by increase in intracellular Ca2+ concentration and by activation of the pp60c-Src kinase. HGF induces changes in osteoclast shape and stimulates chemotactic migration and DNA replication. Osteoblasts respond to HGF by entering the cell cycle, as indicated by stimulation of DNA synthesis. Interestingly, osteoclasts were found to synthesize and secrete biologically active HGF. These data strongly suggest the possibility of an autocrine regulation of the osteoclast by HGF and a paracrine regulation of the osteoblast by the HGF produced by the osteoclast.

Human osteoclast-like cells selectively recognize laminin isoforms, an event that induces migration and activates Ca2+ mediated signals

Osteoclast precursors are chemotactically attracted to sites of bone resorption via migration pathways that include transendothelial crossing in blood capillaries. Transendothelial migration involves poorly understood interactions with basal lamina molecules, including laminins. To investigate osteoclast-laminin interactions, we used human osteoclast-like cell lines obtained from giant cell tumors of bone (GCT 23 and GCT 24). These cell lines are a well-characterized model for osteoclast functions, such as bone resorption and the behaviour of osteoclast precursors. Both GCT cell lines adhered to laminin-2 (merosin) coated wells in standard adhesion assays, but failed to adhere to laminin-1 (EHS-laminin). By light microscopy, GCT cells on laminin-2 were partially spread, with a motile morphology. None of the anti-integrin antibodies tested inhibited GCT cells adhesion to laminin-2. Peptides containing the integrin adhesion site RGD or the laminin adhesion sequence IKVAV did not inhibit GCT cell adhesion to laminin-2. By immunofluorescence, beta 1 integrins were organized in focal adhesions. However, in the presence of monensin this reorganization of beta 1 integrins was abolished, indicating that it was probably due to secretion of fibronectin by GCT cells subsequent to adhesion to laminin-2. GCT cells transmigrated through membranes coated with laminin-2, much more efficiently than through membranes coated with collagen. Migration was induced by osteocalcin, as a chemoattractant, in a dose-dependent manner. At low osteocalcin concentrations, transmigration was detectable on laminin-2 but not collagen. In cells loaded with fura-2, a sharp increase in intracellular Ca2+ was detected upon addition of soluble laminin-2, but not laminin-1, due to release from thapsigargin-dependent intracellular stores. In summary, osteoclasts may recognize laminin isoforms differentially. Initial adhesion to laminin-2 appears to be due to integrin-independent mechanisms. Such adhesion, though, may trigger secretion of fibronectin that could then support spreading and efficient chemotactic migration. These mechanisms may play an important role in facilitating chemotactic migration of osteoclast precursors toward the bone surface.

Translocation of protein kinase C isoenzymes by elevated extracellular Ca2+ concentration in cells from a human giant cell tumor of bone

In this study we investigated the protein kinase C isoenzymes expressed by human osteoclast-like cells harvested from a giant cell tumor of bone (GCT23 cells), and by freshly isolated rat osteoclasts. Immunoblotting analysis revealed that the -alpha, -delta, and -epsilon, PKC isoforms, but not the -beta isoenzyme, are expressed by GCT23 cells. Immunofluorescence studies demonstrated that PKC-alpha, -delta, and -epsilon are homogeneously expressed by both mononuclear and multinucleated GCT23 cells, as well as by rat osteoclasts. Similar to authentic osteoclasts, GCT23 cells responded to an increase of extracellular Ca2+ concentration ([Ca2+]o) with a dose-dependent elevation of the cytosolic free Ca2+ concentration ([Ca2+]i). An increase of [Ca2+]o stimulated the translocation of PKC-alpha from the cytosolic to the particulate fraction, suggesting the involvement of this isoenzyme in the signal transduction mechanism prompted by stimulation of the [Ca2+]o sensing. By contrast, PKC-delta was not altered by exposure to elevated [Ca2+]o, whereas PKC-epsilon underwent reciprocal translocation, disappearing from the insoluble fraction and increasing in the cytosol. The effects of PKC on GCT23 cell functions were investigated by treatment with phorbol 12-myristate, 13-acetate (PMA). We observed that activation of PKC by PMA failed to affect adhesion onto the substrate, but down-regulated the [Ca2+]o-induced [Ca2+]i increases. The latter effect was specific, since it was reversed by treatment with the PKC inhibitors staurosporine and chelerythrine.

Expression of two human skeletal calcitonin receptor isoforms cloned from a giant cell tumor of bone. The first intracellular domain modulates ligand binding and signal transduction

Two distinct calcitonin (CT) receptor (CTR)-encoding cDNAs (designated GC-2 and GC-10) were cloned and characterized from giant cell tumor of bone (GCT). Both GC-2 and GC-10 differ structurally from the human ovarian cell CTR (o-hCTR) that we cloned previously, but differ from each other only by the presence (GC-10) or absence (GC-2) of a predicted 16-amino acid insert in the putative first intracellular domain. Expression of all three CTR isoforms in COS cells demonstrated that GC-2 has a lower binding affinity for salmon (s) CT (Kd approximately 15 nM) than GC-10 or o-hCTR (Kd approximately 1.5 nM). Maximal stimulatory concentrations of CT resulted in a mean accumulation of cAMP in GC-2 transfected cells that was greater than eight times higher than in cells transfected with GC-10 after normalizing for the number of receptor-expressing cells. The marked difference in maximal cAMP response was also apparent after normalizing for receptor number. GC-2 also demonstrated a more potent ligand-mediated cAMP response compared with GC-10 for both human (h) and sCT (the EC50 values for GC-2 were approximately 0.2 nM for sCT and approximately 2 nM for hCT; EC50 values for GC-10 were approximately 6 nM for sCT and approximately 25 nM for hCT). Reverse transcriptase PCR of GCT RNA indicated that GC-2 transcripts are more abundant than those encoding for GC-10. In situ hybridization on GCT tissue sections demonstrated CTR mRNA expression in osteoclast-like cells. We localized the human CTR gene to chromosome 7 in band q22. The distinct functional characteristics of GC-2 and GC-10, which differ in structure only in the first intracellular domain, indicate that the first intracellular domain of the CTR plays a previously unidentified role in modulating ligand binding and signal transduction via the G protein/adenylate cyclase system.

Osteocalcin induces chemotaxis, secretion of matrix proteins, and calcium-mediated intracellular signaling in human osteoclast-like cells

Osteocalcin, also called Bone Gla Protein (BGP), is the most abundant of the non-collagenous proteins of bone produced by osteoblasts. It consists of a single chain of 46-50 amino acids, according to the species, and contains three vitamin K-dependent gamma-carboxyglutamic acid residues (GLA), involved in its binding to calcium and hydroxylapatite. Accumulating evidences suggest its involvement in bone remodeling, its physiological role, however, is still unclear. In this study the adhesion properties and the biological effects of osteocalcin on osteoclasts have been analyzed using as an experimental model, human osteoclast-like cells derived from giant cell tumors of bone (GCT). Osteocalcin promoted adhesion and spreading of these cells, triggering the release of bone sialoprotein (BSP), osteopontin (OPN) and fibronectin (FN), that in turn induced the clustering in focal adhesions of beta 1 and beta 3 integrin chains. Spreading was dependent upon the synthesis of these proteins. In fact, when the cells were incubated in the presence of monensin during the adhesion assay, they still adhered but spreading did not occur, focal adhesions disappeared and BSP, OPN, and FN were accumulated in intracellular granules. Furthermore osteocalcin induced chemotaxis in a dose-dependent manner. The action of BGP on osteoclasts was mediated by an intracellular calcium increase due to release from thapsigargin-sensitive stores. These results provide evidences that BGP exerts a role in the resorption process, inducing intracellular signaling, migration and adhesion, followed by synthesis and secretion of endogenous proteins.

Extracellular Ca2+ sensing is modulated by pH in human osteoclast-like cells in vitro

Osteoclasts are polarized cells with a basolateral and an apical membrane exposed to different extracellular Ca2+ ([Ca2+]o) and H+ (pHe) concentrations. Osteoclast bone resorption is inhibited in vitro by increases of [Ca2+]o slightly above physiological levels, detected by a [Ca2+]o sensing causing elevations of the intracellular signal, [Ca2+]i. Nevertheless, during bone resorption the apical membrane is exposed to [Ca2+]o severalfold higher than physiological without apparent inhibition of osteoclast functions. Because pHe facing the apical membrane is acidic, in this single-cell [Ca2+]i and intracellular pH study we addressed the question of whether the responses of human osteoclast-like cells from a giant cell tumor of bone to elevated [Ca2+]o are altered by reducing pHe. We first observed that low pHe stimulated Ca2+ efflux and cell acidification. We then demonstrated that the amplitude of the [Ca2+]o-dependent [Ca2+]i "spikes" is downregulated by low pHe, with approximately 70-fold higher [Ca2+]o required to induce significant responses at pHe 6.0 compared with pHe 7.4. Similar downregulation was observed in authentic freshly isolated rat osteoclasts. Finally, we observed that occupancy of the [Ca2+]o sensing by Ca2+ prompted rapid and transient cell acidification partially counteracted by a Na(+)-dependent amiloride derivative-sensitive H+ transport. These results demonstrate that the cascade of events triggered by activation of the [Ca2+]o sensing is affected by environmental pH and in turn influences cellular H+ transport. Such pH-related features of the [Ca2+] o sensing mechanism might be relevant for the regulation of osteoclast-like function.