Human osteoclastoma-derived stromal cells: correlation of the ability to form mineralized nodules in vitro with formation of bone in vivo

Sclerostin immunohistochemical staining in surgically treated giant cell tumor of bone

BACKGROUND

Giant cell tumor of bone (GCTB) is a destructive lesion with a high potential for recurrence. RANK-ligand targeted therapy has provided promising, yet mixed results. Sclerostin (SOST) inhibition results in a net anabolic response and is currently used in the treatment of osteoporosis. The application to GCTB is unknown.

OBJECTIVES

We sought to determine if GCTB stained for SOST on immunohistochemistry and correlate its expression with predictor variables.

METHODS

All patients at a single institution undergoing surgery for GCTB between 1993 and 2008 with a minimum of 6 months follow-up were included. Primary outcomes included the presence of SOST staining, secondary outcomes included the correlation of patient and tumor-specific predictor variables.

RESULTS

SOST antibody staining of any cell type was present in 47 of 48 cases (97.9%). Positivity of the stromal cells was present in 39 of 48 cases (81.3%) and was associated with radiographic aggressiveness (p = 0.023), symptomatic presentation (p = 0.032), prior surgery (p = 0.005), and patient age (p = 0.034). Positivity of giant cells was present in 41 of 48 cases (85.4%) and was not significant with predictive factors.

CONCLUSIONS

Sclerostin staining in GCTB is a novel finding and warrants further research to define the role of sclerostin as a prognostic factor and therapeutic target.

Serglycin induces osteoclastogenesis and promotes tumor growth in giant cell tumor of bone

Giant cell tumor of bone (GCTB) is an aggressive osteolytic bone tumor characterized by the within-tumor presence of osteoclast-like multinucleated giant cells (MGCs), which are induced by the neoplastic stromal cells and lead to extensive bone destruction. However, the underlying mechanism of the pathological process of osteoclastogenesis in GCTB is poorly understood. Here we show that the proteoglycan Serglycin (SRGN) secreted by neoplastic stromal cells plays a crucial role in the formation of MGCs and tumorigenesis in GCTB. Upregulated SRGN expression and secretion are observed in GCTB tumor cells and patients. Stromal-derived SRGN promotes osteoclast differentiation from monocytes. SRGN knockdown in stromal cells inhibits tumor growth and bone destruction in a patient-derived orthotopic xenograft model of mice. Mechanistically SRGN interacts with CD44 on the cell surface of monocytes and thus activates focal adhesion kinase (FAK), leading to osteoclast differentiation. Importantly, blocking CD44 with a neutralizing antibody reduces the number of MGCs and suppresses tumorigenesis in vivo. Overall, our data reveal a mechanism of MGC induction in GCTB and support CD44-targeting approaches for GCTB treatment.

Histomorphometric Analysis of Pre- and Post-Denosumab-Treated Giant Cell Tumor of Bone

Context. Denosumab is a monoclonal antibody against RANK ligand. Its administration in giant cell tumor of bone (GCTB) cases results in elimination of giant cells and new bone formation. Neoplastic stromal cells of GCTB harbor mutation of histone 3.3 and have pre-osteoblastic properties and thus express SATB2. Objectives. To (1) analyze histological changes in post-denosumab-treated GCTB, (2) analyze expression of H3.3G34W and SATB2 in pre- and post-denosumab-treated samples, and (3) to discuss why changes occur in the expression of not only H3.3G34W but also SATB2. Materials and Methods. Hematoxylin and eosin slides of 19 cases of denosumab-treated GCTB were reviewed. Immunohistochemical stains H3.3G34W and SATB2 were performed. The number of positive mononuclear cells were counted and graded. Results. Complete absence of osteoclast-like giant cells (OCLGCs) was noted in most cases along with a fibro-osseous component merging with peripheral shell of reactive bone. Irregular trabeculae of woven bone and osteoid with focal osteoblastic rimming was seen. Spindle cells were arranged predominantly in fascicular pattern. Morphometric analysis of H3.3G34W showed a mean of 68.8% positive stromal cells in pretreatment and a mean of 26.9% positive stromal cells in posttreated specimens with a statistically significant P value (.001). Mean percentage of SATB2-positive stromal cells in the pre- and posttreatment specimens was 36.46% and 20.8%, respectively. Conclusions. Our study validates that denosumab treatment results in marked reduction of OCLGCs with increased osteoblastic activity. Decreased expression of H3.3G34W in posttreatment may be a result of decreased antigenicity of neoplastic mononuclear cells. No significant change in SATB2 expression was noted.

[Giant cell tumor of bone : Morphology, molecular pathogenesis, and differential diagnosis]

The histological picture of giant cell tumor of bone is characterized by numerous osteoclast-like giant cells. However, these are not the actual tumor cells, but constitute a reactive infiltrate. Rather, the tumor cells are mononuclear mesenchymal cells, which even reveal an osteoblastic line of differentiation. The CD68-positive macrophages form the second group of mononuclear cells. The receptor activator of nuclear factor kappa-B/ligand (RANK/RANKL) system, which belongs to the tumor necrosis factor (TNF) cytokine family, is decisively involved in the activation of the giant cells. It is generally accepted that a RANKL expression of mononuclear stromal cells is responsible for the development and differentiation of osteoclast-like giant cells. Therefore, the RANKL inhibitor denosumab constituted an essential element for giant cell tumor therapy over the last several years, as it blocks the maturation of osteoclasts and thus the osteolytic activity and the spread of tumor. However, with time it became evident that the not risk-free therapy with denosumab may lead to extensive recurrences upon withdrawal, so this therapy is applied with caution today.At the molecular genetic level, the giant cell tumors of bone are characterized by point mutations in the H3F3A gene. The detection of this mutation is used for the diagnostic differentiation from other bone lesions containing giant cells. Giant cell osteosarcomas rarely contain H3F3A mutations. Chondroblastoma is characterized by mutations in the H3F3B gene.

Histone H3.3 mutation in giant cell tumor of bone: an update in pathology

Giant cell tumor of bone (GCTB) is a locally aggressive bone tumor that frequently shows local recurrence and occasionally shows malignant transformation to high-grade sarcoma. Histologically, conventional GCTB is composed mainly of three types of cells: mononuclear neoplastic cells with an osteoblastic precursor phenotype, mononuclear histiocytic cells, and osteoclast-like multinucleated giant cells. These cells interact with each other via the RANKL-RANK axis and other mechanisms for tumor formation. The vast majority of GCTBs were recently revealed to harbor H3F3A p.G34W mutation, and a minor subset have H3F3A p.G34L, p.G34M, p.G34R, or p.G34V mutation. H3.3 G34W mutant-specific immunohistochemistry is a highly sensitive and specific surrogate marker for H3F3A p.G34W mutation in GCTB and thus useful for differential diagnoses of histological mimics. H3.3 mutant-specific immunohistochemistry has also contributed to the understanding of the bone-forming ability of neoplastic cells of GCTB and the remarkable new bone formation after treatment with denosumab, an inhibitor of RANKL. In primary and secondary malignant GCTBs, the H3F3A gene allele can be preserved or lost with malignant transformation.

Intratibial injection of patient-derived tumor cells from giant cell tumor of bone elicits osteolytic reaction in nude mouse

There have been various reports in the literature of an in vivo model for giant cell tumor of bone (GCTB). However, few suitable animal models of GCTB have been established, due to the fact that GCTB contains three histologically different cell types. To the best of our knowledge, injection of patient-derived GCTB cells into bone environment has not been reported until now. In the present study, the biological behavior of GCTB cells in nude mice was investigated through intratibial injection of patient-derived GCTB cells. Patient-derived GCTB cells were obtained from 5 patients who had not undergone chemo- and radiotherapy. Once isolated, the cell suspension was injected into the tibias of nude mice. The growth process was monitored by weekly observation and photographic documentation using X-ray. Four months after injection, nude mice were sacrificed and the injected tibial samples were fixed, and further analyzed using micro-computed tomography (micro-CT), standard histology, tartrate-resistant acid phosphatase (TRAP) staining and mitochondrial immunofluorescence staining. X-ray, micro-CT and standard histology revealed osteolytic destruction in the proximal end of the tibia. TRAP staining identified TRAP-positive, osteoclast-like cells distributed in the bone marrow interface of the lesion area. Anti-human mitochondrial immunofluorescence staining confirmed that the surviving cells in the osteolytic destruction were of human GCTB cell origin. These findings indicate that intratibial injection of patient-derived GCTB cells may elicit osteolytic destruction in nude mice. The results of the current study present a novel animal model for GCTB, opening new perspectives to investigate this disease and develop therapeutic agents.

Diagnostic utility of histone H3.3 G34W, G34R, and G34V mutant-specific antibodies for giant cell tumors of bone

Giant cell tumors of bone (GCTBs) are characterized by mononuclear stromal cells and osteoclast-like giant cells; up to 95% have H3F3A gene mutation. The RANKL inhibitor denosumab, when used for the treatment of GCTB, leads to histological changes such as new bone formation and giant cell depletion. Here we assessed the diagnostic utility of immunohistochemical staining with the antibodies against histone H3.3 G34W, G34R and G34V mutant proteins for GCTB and other histologically similar bone and joint lesions. H3.3 G34W, G34R and G34V expressions were detected in mononuclear stromal cells in 47/51 (92%), 1/51 (2%) and 3/51 (6%) cases of primary GCTBs, respectively, in a mutually exclusive manner. All recurrent/metastatic GCTBs (n=14), post-denosumab GCTBs (n=8) and secondary malignant GCTBs (n=2) were positive for H3.3 G34W. The immunohistochemical results were essentially correlated with the H3F3A genotype determined by mutation analysis. In post-denosumab GCTBs, H3.3 G34W expression was seen in immature bone-forming cells. H3.3 G34W, G34R and G34V were negative in 121/122 cases of non-GCTB, including chondroblastoma, osteosarcoma, primary aneurysmal bone cyst and other giant cell-rich lesions. The exception was a single case of undifferentiated high-grade pleomorphic sarcoma that was positive for H3.3 G34W, suggesting the possibility of sarcomatous overgrowth of primary malignant GCTB. Therefore, H3.3 G34W/R/V mutant-specific antibodies are useful surrogate markers for the H3F3A genotype and helpful for the diagnosis of GCTB and its variants. The expression of H3.3 G34W mutant protein in post-denosumab GCTB suggests that neoplastic stromal cells may play a role in new bone formation.

In Vitro Study of the Effects of Denosumab on Giant Cell Tumor of Bone: Comparison with Zoledronic Acid

Giant cell tumor of bone (GCTB) is a locally aggressive primary bone tumor that contains numerous osteoclasts formed from marrow-derived precursors through receptor activator of nuclear factor κ-B ligand (RANKL), an osteoclast differentiation factor expressed in neoplastic cells of GCTB. Denosumab, a fully human monoclonal antibody targeting RANKL, has recently been used for the treatment of GCTB, and superior treatment effects have been reported. The aim of this work was to elucidate the mechanism of action of denosumab, and the differences between denosumab and zoledronic acid at the level of GCTB cells. We isolated GCTB cells from 3 patients and separated them into osteoclasts, osteoclast precursors and proliferating spindle-shaped stromal cells (the true neoplastic component), and examined the action of denosumab on differentiation, survival and bone resorption activity of osteoclasts. Denosumab and zoledronic acid inhibited osteoclast differentiation from mononuclear cells containing osteoclast precursors. Zoledronic acid inhibited osteoclast survival, whereas an inhibitory effect of denosumab on osteoclast survival was not observed. The inhibitory effect on bone resorption by both agents was confirmed in culture on dentin slices. Furthermore, zoledronic acid showed dose-dependent inhibition of cell growth of neoplastic cells whereas denosumab had no inhibitory effect on these cells. Denosumab has an inhibitory effect on osteoclast differentiation, but no inhibitory effects on survival of osteoclasts or growth of neoplastic cells in GCTBs.

Clinical characteristics and prognoses of six patients with multicentric giant cell tumor of the bone

Multicentric giant cell tumor of the bone (MGCT) is a rare entity whose radiographic, pathological and biological features remain confusing. We retrospectively reviewed six patients (1 male, 5 female; average age, 22.33 years) treated for confirmed MGCT between 2001 and 2015. The patients' clinical information, images from radiographs (n = 14), CT (n = 13), MRI (n = 8), bone scintigraphy (n = 1) and PET-CT (n = 2), as well as histologic features, treatment and prognosis were analyzed. A total of 17 lesions were detected: 4 around the knee joint, 3 in the greater trochanter and head of the femur, 5 in the small bones of the feet, and 2 in flat bones. All these lesions occurred in an ipsilateral extremity. One patient had Paget's disease. On radiographs and CT, 12 lesions exhibited sclerotic margins or patchy sclerosis, 8 showed cortical discontinuity, and 5 showed soft tissue masses. On histopathology, 8 lesions showed signs of sarcomatous transformation and one had transformed into osteosarcoma. Ten lesions in 4 patients were initially treated with surgery, and 3 showed local recurrence. Seven lesions in 3 patients were treated with denosumab. All the patients are currently stable without metastasis. These results suggest MGCT tends to occur in uncommon sites with sclerosis. Because these lesions can be aggressive, patients should be carefully monitored for the recurrence or formation of other lesions, especially in an ipsilateral extremity.

Bortezomib Inhibits Giant Cell Tumor of Bone through Induction of Cell Apoptosis and Inhibition of Osteoclast Recruitment, Giant Cell Formation, and Bone Resorption

Giant cell tumor of bone (GCTB) is a rare and highly osteolytic bone tumor that usually leads to an extensive bone lesion. The purpose of this study was to discover novel therapeutic targets and identify potential agents for treating GCTB. After screening the serum cytokine profiles in 52 GCTB patients and 10 normal individuals using the ELISA assay, we found that NF-κB signaling-related cytokines, including TNFα, MCP-1, IL1α, and IL17A, were significantly increased in GCTB patients. The results were confirmed by IHC that the expression and activity of p65 were significantly increased in GCTB patients. Moreover, all of the NF-κB inhibitors tested suppressed GCTB cell growth, and bortezomib (Velcade), a well-known proteasome inhibitor, was the most potent inhibitor in blocking GCTB cells growth. Our results showed that bortezomib not only induced GCTB neoplastic stromal cell (NSC) apoptosis, but also suppressed GCTB NSC-induced giant cell differentiation, formation, and resorption. Moreover, bortezomib specifically suppressed GCTB NSC-induced preosteoclast recruitment. Furthermore, bortezomib ameliorated GCTB cell-induced bone destruction in vivo As a result, bortezomib suppressed NF-κB-regulated gene expression in GCTB NSC apoptosis, monocyte migration, angiogenesis, and osteoclastogenesis. Particularly, the inhibitory effects of bortezomib were much better than zoledronic acid, a drug currently used in treating GCTB, in our in vitro experimental paradigms. Together, our results demonstrated that NF-κB signaling pathway is highly activated in GCTB, and bortezomib could suppress GCTB and osteolysis in vivo and in vitro, indicating that bortezomib is a potential agent in the treatment of GCTB. Mol Cancer Ther; 15(5); 854-65. ©2016 AACR.

A mouse model of luciferase-transfected stromal cells of giant cell tumor of bone

A major barrier towards the study of the effects of drugs on Giant Cell Tumor of Bone (GCT) has been the lack of an animal model. In this study, we created an animal model in which GCT stromal cells survived and functioned as proliferating neoplastic cells. A proliferative cell line of GCT stromal cells was used to create a stable and luciferase-transduced cell line, Luc-G33. The cell line was characterized and was found that there were no significant differences on cell proliferation rate and recruitment of monocytes when compared with the wild type GCT stromal cells. We delivered the Luc-G33 cells either subcutaneously on the back or to the tibiae of the nude mice. The presence of viable Luc-G33 cells was assessed using real-time live imaging by the IVIS 200 bioluminescent imaging (BLI) system. The tumor cells initially propagated and remained viable on site for 7 weeks in the subcutaneous tumor model. We also tested in vivo antitumor effects of Zoledronate (ZOL) and Geranylgeranyl transferase-I inhibitor (GGTI-298) alone or their combinations in Luc-G33-transplanted nude mice. ZOL alone at 400 µg/kg and the co-treatment of ZOL at 400 µg/kg and GGTI-298 at 1.16 mg/kg reduced tumor cell viability in the model. Furthermore, the anti-tumor effects by ZOL, GGTI-298 and the co-treatment in subcutaneous tumor model were also confirmed by immunohistochemical (IHC) staining. In conclusion, we established a nude mice model of GCT stromal cells which allows non-invasive, real-time assessments of tumor development and testing the in vivo effects of different adjuvants for treating GCT.

Giant cell tumour of bone: new treatments in development

Giant cell tumour of bone (GCTB) is a benign osteolytic tumour with three main cellular components: multinucleated osteoclast-like giant cells, mononuclear spindle-like stromal cells (the main neoplastic components) and mononuclear cells of the monocyte/macrophage lineage. The giant cells overexpress a key mediator in osteoclastogenesis: the RANK receptor, which is stimulated in turn by the cytokine RANKL, which is secreted by the stromal cells. The RANK/RANKL interaction is predominantly responsible for the extensive bone resorption by the tumour. Historically, standard treatment was substantial surgical resection, with or without adjuvant therapy, with recurrence rates of 20–56 %. Studies with denosumab, a monoclonal antibody that specifically binds to RANKL, resulted in dramatic treatment responses, which led to its approval by the United States Food and Drugs Administration (US FDA). Recent advances in the understanding of GCTB pathogenesis are essential to develop new treatments for this locally destructive primary bone tumour.

Ossified soft tissue recurrence of giant cell tumor of the bone: four case reports with follow-up radiographs, CT, ultrasound, and MR images

Giant cell tumor (GCT) of the bone is a benign tumor with a high incidence of recurrence. The majority of recurrence occurs in the bone, typically where curettage was performed previously. Soft tissue recurrence is much less common and often shows ossification at the periphery of the soft tissue mass. We report four cases of ossified soft tissue recurrence of giant cell tumor of the bone after surgery at follow-up examination using plain radiography, ultrasound, CT, and MR imagings. Imaging findings of soft tissue recurrence with peripheral or central ossification were reviewed with pathologic correlation. To the best of our knowledge, this is the first report to describe soft tissue tumor recurrence with ossification illustrated and monitored at various imaging modalities over an extended follow-up period.

The multidisciplinary management of giant cell tumor of bone

Giant cell tumor of bone is a locally aggressive lesion with a predilection for local recurrence, and in a small proportion of patients, metastatic disease can develop. Surgery is the mainstay of management for extremity-based lesions. For tumors located in challenging anatomical locations such as the sacrum and spine however, surgery may be associated with unacceptable functional morbidity. There are limited data regarding other treatment modalities such as radiation therapy, cytotoxic chemotherapy, interferon and bisphosphonates. Serial arterial embolization can be effective in some cases. Recent evidence has demonstrated denosumab to be a promising agent in the treatment of unresectable or metastatic disease.

Myofibroblastic differentiation of stromal cells in giant cell tumor of bone: an immunohistochemical and ultrastructural study

The nature of the mononuclear stromal cells (MSCs) in giant cell tumor of bone (GCTB) has not been thoroughly investigated. The purpose of this study was to evaluate the degree and significance of myofibroblastic differentiation in 18 cases of GCTB by immunohistochemistry (IH) and/or electron microscopy (EM). All immunostained cases were found positive for smooth muscle actin (SMA) and/or muscle specific actin (MSA), most in 1-33% of the MSCs. Ultrastructurally, most MSCs were fibroblasts, and a significant number of cells displayed myofibroblastic differentiation. Myofibroblasts are an important component of MSCs in GCTB. The myofibroblastic population may be responsible in part for the production of matrix metalloproteinases (MMPs), which probably play a role in bone destruction, tumor aggression, and recurrence.

Giant cell tumor of bone: a basic science perspective

Comprehending the pathogenesis of giant cell tumor of bone (GCT) is of critical importance for developing novel targeted treatments for this locally-aggressive primary bone tumor. GCT is characterized by the presence of large multinucleated osteoclast-like giant cells distributed amongst mononuclear spindle-like stromal cells and other monocytes. The giant cells are principally responsible for the extensive bone resorption by the tumor. However, the spindle-like stromal cells chiefly direct the pathology of the tumor by recruiting monocytes and promoting their fusion into giant cells. The stromal cells also enhance the resorptive ability of the giant cells. This review encompasses many of the attributes of GCT, including the process of giant cell formation and the mechanisms of bone resorption. The significance of the receptor activator of nuclear factor-κB ligand (RANKL) in the development of GCT and the importance of proteases, including numerous matrix metalloproteinases, are highlighted. The mesenchymal lineage of the stromal cells and the origin of the hematopoietic monocytes are also discussed. Several aspects of GCT that require further understanding, including the etiology of the tumor, the mechanisms of metastases, and the development of an appropriate animal model, are also considered. By exploring the current status of GCT research, this review accentuates the significant progress made in understanding the biology of the tumor, and discusses important areas for future investigation.

Denosumab induces tumor reduction and bone formation in patients with giant-cell tumor of bone

PURPOSE

Giant-cell tumor of bone (GCTB) is a locally aggressive, benign osteolytic tumor in which bone destruction is mediated by RANK ligand (RANKL). The RANKL inhibitor denosumab is being investigated for treatment of GCTB. We describe histologic analyses of GCTB tumor samples from a phase II study of denosumab in GCTB.

EXPERIMENTAL DESIGN

Adult patients with recurrent or unresectable GCTB received subcutaneous denosumab 120 mg every 4 weeks (with additional doses on days 8 and 15). The primary histologic efficacy endpoint was the proportion of patients who had a 90% or more elimination of giant cells from their tumor. Baseline and on-study specimens were also evaluated for overall tumor morphology and expression of RANK and RANKL.

RESULTS

Baseline tumor samples were typically composed of densely cellular proliferative RANKL-positive tumor stromal cells, RANK-positive rounded mononuclear cells, abundant RANK-positive tumor giant cells, and areas of scant de novo osteoid matrix and woven bone. In on-study samples from 20 of 20 patients (100%), a decrease of 90% or more in tumor giant cells and a reduction in tumor stromal cells were observed. In these analyses, thirteen patients (65%) had an increased proportion of dense fibro-osseous tissue and/or new woven bone, replacing areas of proliferative RANKL-positive stromal cells.

CONCLUSIONS

Denosumab treatment of patients with GCTB significantly reduced or eliminated RANK-positive tumor giant cells. Denosumab also reduced the relative content of proliferative, densely cellular tumor stromal cells, replacing them with nonproliferative, differentiated, densely woven new bone. Denosumab continues to be studied as a potential treatment for GCTB.

A microRNA signature differentiates between giant cell tumor derived neoplastic stromal cells and mesenchymal stem cells

Giant cell tumor (GCT) derived stromal cells (GCTSCs) have been identified as the neoplastic cell population of GCTs. Within these stromal cells a subpopulation has been identified that shares several features with mesenchymal stem cells (MSCs) indicating that these neoplastic cells develop from MSCs. Although spontaneous transformations of MSC have already been observed in vitro and in vivo the underlying molecular mechanisms are poorly understood. As microRNAs are crucially involved in tumorigenesis and the modulation of stem cell fate and behavior, they represent promising candidates for the regulation of this process. Therefore, the aim of this study was the comparative analysis of the microRNA expression profiles of GCTSCs and MSCs in order to identify differentially expressed microRNAs and their target genes. We could identify a microRNA signature consisting of 26 differentially expressed microRNAs that perfectly separates these two cell types. One of the microRNAs with the most pronounced differences in expression levels was miR-224. We could confirm the already known regulation of the apoptosis inhibitor API5 by miR-224 and could further identify three novel miR-224 target genes (SMAD5, SLMAP, H3.3B). The involvement of these genes in the regulation of apoptosis resistance, proliferation, differentiation and the regulation of gene transcription suggests pivotal roles of these genes in the neoplastic transformation of MSCs during GCT development.

Does lactoferrin behave as an immunohistochemical oncofetal marker in bone and cartilage human neoplasms?

By immunohistochemistry, lactoferrin (LF) has been extensively investigated in human neoplastic tissues; moreover, LF is able to promote bone growth in a murine model. Until now, no systematic studies on human osteocartilagineous fetal samples have been performed in comparison to corresponding neoplastic specimens to verify if LF may represent an oncofetal marker in this field of pathology. By a monoclonal antibody (clone 1A1; Biodesign International; w.d. 1:75) the distribution pattern of LF in bones of 25 human fetal tissues (8-34 gestation weeks), 10 adults (47-82 years) and 30 cartilage as well as 27 bone tumours (9-76 years) was analyzed. LF was encountered in 23/57 cases of osteocartilagineous tumors and namely in 10/10 giant cell tumours, 5/7 osteoid osteomas, 3/3 chondroblastomas, 3/3 chondromyxoid fibromas, 1/1 myeloma, 1/1 adamantinoma. No LF immunoexpression was detected in osteosarcomas, chondrosarcomas, ossifying fibromas, osteochondroma and enchondromas. In embryo-fetal tissues, LF immunoreactivity was localized in mesenchymal cells as well as in chondroblasts at the 8th gestational week and in immature osteocytes and osteoblasts up to the 18th gestation week, with a considerable decrease by the 24th week. No LF expression was found in any bone district since the 30th and up to the 34th week of gestation as well as in corresponding adult samples. Our findings indicate a role for LF as a bone growth regulator in the early phases of the human endochondral ossification, although the hypothesis of LF as oncofetal marker appears questionable in bone tumours.

Lactoferrin immuno-expression in human normal and neoplastic bone tissue

Lactoferrin (Lf) expression was investigated by using a Lf monoclonal antibody in 50 formalin-fixed and paraffin-embedded human bone tumours [10 giant cell tumours (GCTs), 7 osteoid osteomas, 6 ossifying fibromas, 19 enchondromas, 2 chondroblastomas, 2 chondrosarcomas, 2 chondroblastic osteosarcomas, 1 myeloma and 1 adamantinoma] as well as in 8 samples of adult and foetal human normal bone specimens. In addition, the immunohistochemical expression of the estrogen receptor (ER), progesterone receptor (PR) and Ki-67 antigen was analysed on parallel sections from the same specimens. Quantification of Lf immunoreactivity was performed by using an Intensity Distribution (ID) score. Lf immuno-expression with a variable ID score was encountered in 19/50 tumours and specifically in 10/10 GCTs, in 5/7 osteoid osteomas, in 2/2 chondroblastomas as well as in the adamantinoma and in the myeloma. With reference to normal bone samples, Lf was expressed by the osteoblasts only in the foetal bone. No immunoreactivity for ER and PR was encountered in all neoplastic samples, and no correlation was found between Lf and sex steroid hormone receptor (ER and PR) immuno-expression. Even more, no association was evidenced between Lf immuno-reactivity and the growth fraction of the tumours, reflected by the Ki-67 labelling index. Lf expression in the osteoblastic lineage of bone-forming tumours, together with its presence in the osteoblasts of foetal bone, requires further investigations, although it cannot be ruled out that Lf might be involved in the bone formation in humans, similarly to what has been demonstrated in other species.

Properties of the stromal cell in giant cell tumor of bone

The histiogenesis and mechanisms of bone destruction in giant cell tumor (GCT) of bone are not well understood. We asked whether the spindle-like stromal cells of GCT of bone exhibit osteoblastic properties, and whether the stromal cells produce active matrix-degrading proteases in vitro. We performed immunohistochemistry on 17 paraffin-embedded archival specimens with a pathologic diagnosis of GCT with monoclonal antibodies for the osteoblastic lineage markers osteopontin, osteonectin, and osteocalcin. The average staining grade for the 17 specimens was highest for osteonectin, followed by osteopontin, and osteocalcin. Primary cell cultures of GCT stromal cells were prepared from two fresh tumor specimens. Western blots were used on the cell lysates and media to detect osteocalcin precursor and the matrix-degrading proteases MMP-2 and MMP-9. We found the stromal cells in culture produce osteocalcin precursor, indicating osteoblastic lineage. The cells also express both the active and inactive isoforms of MMP-2 and MMP-9. Gelatinase assays confirmed the activity of the proteases in vitro. The spindle like stromal cells of GCT have characteristics of osteoblast progenitors and produce active matrix-degrading proteases. These cells may therefore play a central role in bone destruction.

Giant cell tumour of bone: morphological, biological and histogenetical aspects

The giant cell tumour of bone (GCT) is a locally aggressive intraosseous neoplasm of obscure biological behaviour. Although well defined in clinical, radiological and histological terms, detailed information on its biological development is still relatively incomplete. The tumoral tissue consists of three cell types--the neoplastic giant cell tumour stromal cells (GCTSC), representing the proliferative fraction, secondarily recruited mononuclear histiocytic cells (MNHC) and multinuclear giant cells (MNGC). These cellular components interact together with factors that have a role in regulating osteoclast function in normal bone tissue (e.g. RANK, RANKL, OPG, M-CSF). Recent publications suggest that the neoplastic stromal cells express differentiation features of mesenchymal stem cells. Further research of the pathogenesis of GCT as well as the complex interactions of its cellular populations may provide the knowledge necessary for developing approaches for a biological-based therapy of this neoplasm.

Giant cell tumor of bone

Giant cell tumor is a common benign bone tumor that possesses specific features including location at the end of long bone, a strong tendency toward local recurrence, and the rare capacity to metastasize to the lungs. Preferred treatment usually consists of extensive curettage and filling of the cavity with bone graft or cement. Debate still exists about the usefulness of local adjuvant treatment. Functional outcome is usually very good.

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.

Bicyclic peptidomimetic tetrahydrofuro[3,2-b]pyrrol-3-one and hexahydrofuro[3,2-b]pyridine-3-one based scaffolds: synthesis and cysteinyl proteinase inhibition

A stereoselective synthesis of (3aS,6aR)-tetrahydrofuro[3,2-b]pyrrol-3-ones and (3aS,7aR)-hexahydrofuro[3,2-b]pyridine-3-ones has been developed through Fmoc protected scaffolds 12 and 13. A key design element within these novel bicyclic scaffolds, in particular the 5,5-fused system, was the inherent stability of the cis-fused geometry in comparison to that of the corresponding trans-fused. Since the bridgehead stereocentre situated beta to the ketone was of a fixed and stable configuration, the fact that cis ring fusion is both kinetically and thermodynamically stable with respect to trans ring fusion provides chiral stability to the bridgehead stereocentre that is situated alpha to the ketone. To exemplify this principle, building blocks 12 and 13 were designed, prepared and utilised in a solid phase combinatorial synthesis of peptidomimetic inhibitors 10, 45a-e, 11 and 46. Both series were chirally stable with 5,5-series 10 and 45a-e exhibiting potent in vitro activity against a range of CAC1 cysteinyl proteinases. Compound 10, a potent and selective inhibitor of cathepsin K, possessed good primary DMPK properties along with promising activity in an in vitro cell-based human osteoclast assay of bone resorption.

Synthesis and evaluation of cis-hexahydropyrrolo[3,2-b]pyrrol-3-one peptidomimetic inhibitors of CAC1 cysteinyl proteinases

A stereoselective synthesis of functionalised cis-hexahydropyrrolo[3,2-b]pyrrol-3-ones has been developed through Fmoc and Cbz-protected intermediates 5 and 6. Building blocks 5 and 6 were prepared via the intramolecular cyclisation of anti-epoxide 17. The intramolecular reaction occurred exclusively through the anti-epoxide to provide the 5,5-cis-fused bicycle, whereas the syn-epoxide, which theoretically would provide the 5,5-trans-fused bicycle, remained unchanged. These experimental observations are consistent with a key design element that we have introduced within this novel bicyclic ketone scaffold. Our bicyclic design strategy provides chiral stability to the bridgehead stereocentre that is situated alpha to the ketone because the cis-fused geometry is both thermodynamically and kinetically stable. Building blocks 5 and 6 have been utilised in both solid phase and solution phase syntheses of peptidomimetics 22, 36-40, which exhibit potent in vitro inhibition against a range of CAC1 cysteinyl proteinases. Compound 22, a potent and selective inhibitor of human cathepsin K exhibited good primary DMPK properties along with promising activity in an in vitro cell-based human osteoclast assay of bone resorption.

The small leucine-rich proteoglycan biglycan modulates BMP-4-induced osteoblast differentiation

Biglycan (bgn) is a small leucine-rich proteoglycan enriched in extracellular matrices of skeletal tissues. Bgn-deficient mice develop age-related osteopenia with a phenotype that resembles osteoporosis and premature arthritis. In the present study, we have examined the differentiation of bgn-deficient osteoblasts from neonatal murine calvariae and found that the absence of bgn caused less BMP-4 binding, which reduced the sensitivity of osteoblasts to BMP-4 stimulation. The loss of sensitivity resulted in a reduction of Cbfa1 expression, which ultimately led to a defect in the differentiation of osteoblasts. However, the response of bgn-deficient osteoblasts to BMP-4 was completely rescued by reintroduction of biglycan by viral transfection. We propose that biglycan modulates BMP-4-induced signaling to control osteoblast differentiation.

(4-Piperidinylphenyl)aminoethyl amides as a novel class of non-covalent cathepsin K inhibitors

A series of (4-piperidinylphenyl)aminoethyl amides based on dipeptide anilines were synthesized and tested against cathepsin K, cathepsin L and cathepsin B. These new non-covalent inhibitors exhibited single-digit nM inhibition of the cysteine proteases. Compounds 3 and 7 demonstrated potency in both mouse and human osteoclast resorption assays.

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.

A potent small molecule, nonpeptide inhibitor of cathepsin K (SB 331750) prevents bone matrix resorption in the ovariectomized rat

Inhibition of the cyteine proteinase, cathepsin K (E.C. 3.4.22.38) has been postulated as a means to control osteoclast-mediated bone resorption. The preferred animal models for evaluation of antiresorptive activity are in the rat. However, the development of compounds that inhibit rat cathepsin K has proven difficult because the human and rat enzymes differ in key residues in the active site. In this study, a potent, nonpeptide inhibitor of rat cathepsin K (K(i) = 4.7 nmol/L), 5-(2-morpholin-4-yl-ethoxy)-benzofuran-2-carboxylic acid ((S)-3-methyl-1-(3-oxo-1-[2-(3-pyridin-2-yl-phenyl)-ethenoyl]-azepan-4-ylcarbanoyl)-butyl)-amide (SB 331750), is described, which is efficacious in rat models of bone resorption. SB 331750 potently inhibited human cathepsin K activity in vitro (K(i) = 0.0048 nmol/L) and was selective for human cathepsin K vs. cathepsins B (K(i) = 100 nmol/L), L (0.48 nmol/L), or S (K(i) = 14.3 nmol/L). In an in situ enzyme assay, SB 331750 inhibited osteoclast-associated cathepsin activity in tissue sections containing human osteoclasts (IC(50) approximately 60 nmol/L) and this translated into potent inhibition of human osteoclast-mediated bone resorption in vitro (IC(50) approximately 30 nmol/L). In vitro, SB 331750 partially, but dose-dependently, prevented the parathyroid hormone-induced hypercalcemia in an acute rat model of bone resorption. To evaluate the ability of SB 331750 to inhibit bone matrix degradation in vivo, it was administered for 4 weeks at 3, 10, or 30 mg/kg, intraperitoneally (i.p.), u.i.d. in the ovariectomized (ovx) rat. Both 10 and 30 mg/kg doses of compound prevented the ovx-induced elevation in urinary deoxypyridinoline and prevented the ovx-induced increase in percent eroded perimeter. Histological evaluation of the bones from compound-treated animals indicated that SB 331750 retarded bone matrix degradation in vivo at all three doses. The inhibition of bone resorption at the 10 and 30 mg/kg doses resulted in prevention of the ovx-induced reduction in percent trabecular area, trabecular number, and increase in trabecular spacing. These effects on bone resorption were also reflected in inhibition of the ovx-induced loss in trabecular bone volume as assessed using microcomputerized tomography (microCT; approximately 60% at 30 mg/kg). Together, these data indicate that the cathepsin K inhibitor, SB 331750, prevented bone resorption in vivo and this inhibition resulted in prevention of ovariectomy-induced loss in trabecular structure.

Tissue culture models for studies of hormone and vitamin action in bone cells

Osteoporosis is a major health care concern and levies a serious financial burden on the world health care system. For this reason, many physicians and scientists are engaged in research to better understand and treat this disease. To this end, numerous in vitro bone cell models have been developed to explore the cellular and molecular mechanisms of skeletal biology and for the identification and characterization of new drug targets and therapies. In this chapter, we review many of these cellular models as tools to study the hormonal regulation of bone metabolism. In particular, we pay special attention to new human bone cell models, since these have the greatest relevance to osteoporosis research and drug discovery. These new models include (1) the use of peripheral blood mononuclear cells as progenitors of osteoclasts and primary cultures of mesenchymal stem cells as precursors of osteoblasts; (2) the development of conditionally immortalized preosteoclastic and osteoblastic cell lines using temperature-sensitive large T-antigens; and (3) the establishment of the first osteocytic cell lines. Thus, we now have at our disposal many good in vitro models to investigate the regulation of bone resorption and formation by hormones, vitamins and drugs. These models should accelerate our understanding of bone physiology and pathophysiology as well as our ability to develop important new therapies to prevent and treat skeletal diseases.

Potent and selective inhibition of human cathepsin K leads to inhibition of bone resorption in vivo in a nonhuman primate

Cathepsin K is a cysteine protease that plays an essential role in osteoclast-mediated degradation of the organic matrix of bone. Knockout of the enzyme in mice, as well as lack of functional enzyme in the human condition pycnodysostosis, results in osteopetrosis. These results suggests that inhibition of the human enzyme may provide protection from bone loss in states of elevated bone turnover, such as postmenopausal osteoporosis. To test this theory, we have produced a small molecule inhibitor of human cathepsin K, SB-357114, that potently and selectively inhibits this enzyme (Ki = 0.16 nM). This compound potently inhibited cathepsin activity in situ, in human osteoclasts (inhibitor concentration [IC]50 = 70 nM) as well as bone resorption mediated by human osteoclasts in vitro (IC50 = 29 nM). Using SB-357114, we evaluated the effect of inhibition of cathepsin K on bone resorption in vivo using a nonhuman primate model of postmenopausal bone loss in which the active form of cathepsin K is identical to the human orthologue. A gonadotropin-releasing hormone agonist (GnRHa) was used to render cynomolgus monkeys estrogen deficient, which led to an increase in bone turnover. Treatment with SB-357114 (12 mg/kg subcutaneously) resulted in a significant reduction in serum markers of bone resorption relative to untreated controls. The effect was observed 1.5 h after the first dose and was maintained for 24 h. After 5 days of dosing, the reductions in N-terminal telopeptides (NTx) and C-terminal telopeptides (CTx) of type I collagen were 61% and 67%, respectively. A decrease in serum osteocalcin of 22% was also observed. These data show that inhibition of cathepsin K results in a significant reduction of bone resorption in vivo and provide further evidence that this may be a viable approach to the treatment of postmenopausal osteoporosis.

Antagonism of the osteoclast vitronectin receptor with an orally active nonpeptide inhibitor prevents cancellous bone loss in the ovariectomized rat

An orally active, nonpeptide Arg-Gly-Asp (RGD) mimetic alpha(v)beta3 antagonist, (S)-3-Oxo-8-[2-[6-(methylamino)-pyridin-2-yl]-1-ethoxy]-2-(2,2,2-trifluoroethyl)-2,3,4,5-tetrahydro-1H-2-benzazepine-4-acetic acid (compound 1), has been generated, which prevented net bone loss and inhibited cancellous bone turnover in vivo. The compound binds alpha(v)beta3 and the closely related integrin alpha(v)beta5 with low nanomolar affinity but binds only weakly to the related integrins alpha(IIb)beta3, and alpha5beta1. Compound 1 inhibited alpha(v)beta3-mediated cell adhesion with an IC50 = 3 nM. More importantly, the compound inhibited human osteoclast-mediated bone resorption in vitro with an IC50 = 11 nM. In vivo, compound 1 inhibited bone resorption in a dose-dependent fashion, in the acute thyroparathyroidectomized (TPTX) rat model of bone resorption with a circulating EC50 approximately 20 microM. When dosed orally at 30 mg/kg twice a day (b.i.d.) in the chronic ovariectomy (OVX)-induced rat model of osteopenia, compound 1 also prevented bone loss. At doses ranging from 3 to 30 mg/kg b.i.d., compound 1 partially prevented the OVX-induced increase in urinary deoxypyridinoline. In addition, the compound prevented the OVX-induced reduction in cancellous bone volume (BV), trabecular number (Tb.N), and trabecular thickness (Tb.Th), as assessed by quantitative microcomputerized tomography (microCT) and static histomorphometry. Furthermore, both the 10-mg/kg and 30-mg/kg doses of compound prevented the OVX-induced increase in bone turnover, as measured by percent osteoid perimeter (%O.Pm). Together, these data indicate that the alpha(v)beta3 antagonist compound 1 inhibits OVX-induced bone loss. Mechanistically, compound 1 prevents bone loss in vivo by inhibiting osteoclast-mediated bone resorption, ultimately preventing cancellous bone turnover.

Antagonizing the parathyroid calcium receptor stimulates parathyroid hormone secretion and bone formation in osteopenic rats

Parathyroid hormone (PTH) is an effective bone anabolic agent, but it must be administered parenterally. An orally active anabolic agent would provide a valuable alternative for treating osteoporosis. NPS 2143 is a novel, selective antagonist (a "calcilytic") of the parathyroid cell Ca(2+) receptor. Daily oral administration of NPS 2143 to osteopenic ovariectomized (OVX) rats caused a sustained increase in plasma PTH levels, provoking a dramatic increase in bone turnover but no net change in bone mineral density. Concurrent oral administration of NPS 2143 and subcutaneous infusion of 17beta-estradiol also resulted in increased bone turnover. However, the antiresorptive action of estrogen decreased the extent of bone resorption stimulated by the elevated PTH levels, leading to an increase in bone mass compared with OVX controls or to either treatment alone. Despite the sustained stimulation to the parathyroid gland, parathyroid cells did not undergo hyperplasia. These data demonstrate that an increase in endogenous PTH secretion, induced by antagonism of the parathyroid cell Ca(2+) receptor with a small molecule, leads to a dramatic increase in bone turnover, and they suggest a novel approach to the treatment of osteoporosis.

Expression of osteoclast differentiation signals by stromal elements of giant cell tumors

The mechanisms by which primary tumors of the bone cause bone destruction have not been elucidated. Unlike most other lytic bone tumors, osteoclastomas, otherwise known as giant cell tumors (GCT), contain osteoclast-like cells within the tumor stroma. A new member of the TNF-ligand superfamily member, osteoclast differentiation factor (ODF/OPGL/RANKL/TRANCE), was recently identified. ODF was shown to directly stimulate osteoclastogenesis, in the presence of M-CSF. In this study, the expression of ODF was examined in a number of tumor samples associated with bone lysis in vivo. In addition, we investigated expression of the ODF receptor on osteoclast precursors, RANK, as well as the ODF inhibitor osteoprotegerin (OPG), and another TNF-ligand superfamily member, TRAIL, previously shown to abrogate the inhibitory effects of OPG. We report here the novel finding that GCT stromal cells contain abundant ODF mRNA, whereas the giant cell population exclusively expresses RANK mRNA. These results are consistent with the osteoclast-mediated bone destruction by these tumors. We also report the expression of OPG and TRAIL mRNA in GCT samples. A comparison with other lytic and nonlytic tumors of bone showed that GCT express more ODF and TRAIL mRNA relative to OPG mRNA. In addition, GCT were found to express a number of cytokines previously reported to play central roles in osteoclastogenesis, namely, IL-1, -6, -11, -17, as well as TNF-alpha. Importantly, GCT were also found to express high levels of M-CSF mRNA, a cytokine shown to be an essential cofactor of ODF, and a survival factor for mature and developing osteoclasts. Furthermore, expression of these molecules by stromal cells isolated from GCT continued in vitro. Thus GCT constitutively express all of the signals that are currently understood to be necessary for the differentiation of osteoclasts from precursor cells.

Primary lung tumors infiltrated by osteoclast-like giant cells

Six primary lung tumors with numerous multinucleated osteoclast-like giant cells (OLGCs) and no osteogenic component were evaluated histologically and immunohistochemically to examine pulmonary lesions inciting an OLGC response. The patients comprised four women and two men ranging in age from 61 to 80 years (average age, 69 years). The tumors consisted of one adenocarcinoma, two sarcomatoid carcinomas, and three giant cell variants of malignant fibrous histiocytoma. One tumor was endobronchial in location, while five were situated peripherally. Tumor diameter spanned from 1 to 6.5 cm (average, 2.7 cm). In addition to the giant cells, common characteristics included the malignant nature of the neoplasms and, in five of six cases, histologically malignant mesenchyme. This array of cases exemplifies the variability of lung lesions which may elicit an OLGC inflammatory response resulting in areas resembling the giant cell variant of malignant fibrous histiocytoma. The results of this study suggest that OLGCs occur preferentially in malignant rather than benign nonosteogenic lung tumors and that sarcomatoid regions of malignant tumors are more likely to be infiltrated by OLGCs than epithelial regions.

Peptide aldehyde inhibitors of cathepsin K inhibit bone resorption both in vitro and in vivo

We have shown previously that cathepsin K, a recently identified member of the papain superfamily of cysteine proteases, is expressed selectively in osteoclasts and is the predominant cysteine protease in these cells. Based upon its abundant cell type-selective expression, potent endoprotease activity at low pH and cellular localization at the bone interface, cathepsin K has been proposed to play a specialized role in osteoclast-mediated bone resorption. In this study, we evaluated a series of peptide aldehydes and demonstrated that they are potent cathepsin K inhibitors. These compounds inhibited osteoclast-mediated bone resorption in fetal rat long bone (FRLB) organ cultures in vitro in a concentration-dependent manner. Selected compounds were also shown to inhibit bone resorption in a human osteoclast-mediated assay in vitro. Chz-Leu-Leu-Leu-H (in vitro enzyme inhibition Ki,app = 1.4 nM) inhibited parathyroid hormone (PTH)-stimulated resorption in the FRLB assay with an IC-50 of 20 nM and inhibited resorption by isolated human osteoclasts cultured on bovine cortical bone slices with an IC-50 of 100 nM. In the adjuvant-arthritic (AA) rat model, in situ hybridization studies demonstrated high levels of cathepsin K expression in osteoclasts at sites of extensive bone loss in the distal tibia. Cbz-Leu-Leu-Leu-H (30 mg/kg, intraperitoneally) significantly reduced this bone loss, as well as the associated hind paw edema. In the thyroparathyriodectomized rat model, Cbz-Leu-Leu-Leu-H inhibited the increase in blood ionized calcium induced by a 6 h infusion of PTH. These data indicate that inhibitors of cathepsin K are effective at reducing osteoclast-mediated bone resorption and may have therapeutic potential in diseases of excessive bone resorption such as rheumatoid arthritis or osteoporosis.

Purification and characterization of fully functional human osteoclast precursors

The identification and purification of human osteoclast precursors is essential to further our understanding of the mechanisms that control human osteoclast differentiation. Osteoclastoma tissue potentially provides a rich source of human osteoclast precursors, and in previous studies we have demonstrated the existence of a population of mononuclear cells within this tissue that is reactive with osteoclast-selective vitronectin receptor monoclonal antibodies. In this study, mononuclear cells expressing the vitronectin receptor, as defined by their ability to react with a murine monoclonal antibody to the beta 3 chain of the vitronectin receptor (87MEM1), were isolated from collagenase digests of osteoclastoma tissue using a fluorescence activated cell sorter. Based on their fluorescence signal and size, approximately 2-3% of the viable cells (typically 2 x 10(5)) were obtained and prepared for further phenotyping. The isolated cells demonstrated a number of phenotypic characteristics of osteoclasts: positive tartrate-resistant acid phosphatase (TRAP) activity, reactivity with human osteoclast-selective antibodies, expression of calcitonin receptors, cathepsin K (a novel osteoclast-selective cysteine proteinase) mRNA, and osteopontin mRNA and protein. These phenotypic characteristics were also detected in mononuclear cells within cryostat sections of the native osteoclastoma tissue as well as in resorption lacunae of sections of human bone. In contrast, isolated peripheral blood monocytes were negative for TRAP activity and osteopontin expression and, unlike the osteoclastoma-derived cells, demonstrated strong nonspecific esterase activity. Significantly, when the osteoclastoma-derived 87MEM1 positive cells were cocultured on whale dentine for 1-3 weeks with stromal cells, extensive resorption of the dentine surface was observed. This is the first demonstration of the purification of human osteoclast precursors. These cells provide an homogeneous cell population for studying cellular events that occur during human osteoclast differentiation.