VEGF, FLT3 ligand, PlGF and HGF can substitute for M-CSF to induce human osteoclast formation: implications for giant cell tumour pathobiology

Cell communication and relevant signaling pathways in osteogenesis-angiogenesis coupling

Osteogenesis is the process of bone formation mediated by the osteoblasts, participating in various bone-related physiological processes including bone development, bone homeostasis and fracture healing. It exhibits temporal and spatial interconnectivity with angiogenesis, constructed by multiple forms of cell communication occurring between bone and vascular endothelial cells. Molecular regulation among different cell types is crucial for coordinating osteogenesis and angiogenesis to facilitate bone remodeling, fracture healing, and other bone-related processes. The transmission of signaling molecules and the activation of their corresponding signal pathways are indispensable for various forms of cell communication. This communication acts as a "bridge" in coupling osteogenesis to angiogenesis. This article reviews the modes and processes of cell communication in osteogenesis-angiogenesis coupling over the past decade, mainly focusing on interactions among bone-related cells and vascular endothelial cells to provide insights into the mechanism of cell communication of osteogenesis-angiogenesis coupling in different bone-related contexts. Moreover, clinical relevance and applications are also introduced in this review.

Multidisciplinary marvel: surgical management of aggressive giant cell tumor around the knee in pregnancy: a case report

Giant cell tumors (GCTs) are rare, representing 4-5% of all bone tumors. Although uncommon in pregnancy, the literature showed the accelerated progression and recurrence of GCT in pregnancy; however, the tumor growth in pregnancy has not been clarified. We report a case of a 28-year-old woman in her first trimester presenting with an aggressive GCT, significantly affecting the patient's quality of life due to the debilitating nature of her symptoms. Considering the significant functional impairment and aggressive nature of the tumor, a multidisciplinary team opted for surgical intervention, entailing marginal resection and mega-prosthesis implantation. A multidisciplinary approach tailored to the patient's needs enabled successful surgical intervention and positive maternal and fetal outcomes. This case paves the way for the possibility that surgical management of GCTs can be safely performed during pregnancy, highlighting the challenges and critical importance of multidisciplinary care in rare tumor management during pregnancy through timely intervention.

Clinical Outcome of Extended Curettage with Postoperative Denosumab Administration for the Treatment of Campanacci Grade III Giant Cell Tumors of the Extremities

Purpose

To investigate the local recurrence rate, joint preservation status, and functional outcomes after extended curettage and postoperative denosumab treatment for Campanacci Grade III giant cell tumors of the extremities.

Methods

We retrospectively reviewed 23 patients with Campanacci Grade III GCTB of the extremities in our hospital between January 2017 and June 2023 who underwent extended curettage and postoperative denosumab administration alone, without preoperative denosumab treatment. Patients were followed-up for adverse events of denosumab, surgical outcomes, limb function of lesions, and local recurrence following extended curettage with postoperative denosumab.

Results

All incisions healed without deep infections or internal fixation failure. The mean age of the patients at surgery was 36.6 years, and the mean follow-up was 35.8 months (range, 6-72 months). There of the 3 patients experienced a postoperative local recurrence. The recurrence rate was found to be 13.0%. Two patients were treated with repeat intralesional surgery with no additional recurrence two years later, and the other was treated with en bloc resection and reconstruction with a vascularized fibular graft. One patient experienced knee osteoarthritis without oral analgesics. No patient developed pulmonary metastases or malignant transformation of the GCTB. The mean Musculoskeletal Tumor Society functional score at the last follow up was 27.3 30 (range, 25-29). No serious adverse events were observed after the denosumab treatment.

Conclusion

Our observations suggest that extended curettage with postoperative denosumab administration is a reasonable option for treating Campanacci Grade III giant cell tumors of the extremities. Extended curettage with adjuvant denosumab therapy results in beneficial surgical downstaging, including a less morbid surgical procedure or delayed en bloc resection. Resection should be considered when the structural integrity cannot be regained after bone grafting or bone cement filling combined with internal fixation.

Measurable progression of giant cell tumour of bone associated with pregnancy - A tertiary sarcoma centre analysis

Introduction

Giant cell tumour of bone (GCTB) is a benign but locally aggressive bone tumour with a higher predilection for females of reproductive age. GCTB management poses a unique set of challenges during pregnancy due to risks associated with imaging and treatment options. Pregnancy has been implicated in GCTB progression and tumour recurrence, however an exact mechanism has not been established. This study aims to confirm the relationship between the diagnosis and progression of GCTB during pregnancy.

Methods

A 17-year retrospective analysis of our tertiary sarcoma referral centre database was performed to identify the relevant patients. Pregnancy-associated tumours were defined by those already present or diagnosed during pregnancy, and up to 12 months postpartum. Lesion volume was determined by mathematical ellipsoidal modelling technique to simplify the estimation, with cross-sectional measurements obtained from the three standard orthogonal planes on initial and surveillance imaging. Due to logistical challenges, follow-up imaging was performed at either our tertiary sarcoma centre or under guidance at regional imaging centres convenient to the patient.

Results

The diagnosis of GCTB was made in 113 female patients during this 17-year period, of which 20 were associated with pregnancy with a mean age of 28.8 years (range 19-40 years). 12 patients had their primary or recurrent GCTB diagnosed, or known tumour progress during pregnancy, whilst the remaining 8 were diagnosed shortly thereafter to within 12 months postpartum. The most common tumour sites were located around the knee (30 %) and distal radius (25 %). A statistically significant pattern of growth was observed through the surveillance period (p 0.018), within a relatively short mean follow-up period of only 89.8 days (SD 54.5; 13-192 days).

Conclusion

This study demonstrates the significant association that pregnancy has with the growth and progression of both primary and recurrent GCTB. Pregnant patients should be subject to close surveillance well into the postpartum period due to possible accelerated disease progression and potential for disease recurrence.

Hepatocyte growth factor is protective in early stage but bone-destructive in late stage of experimental periodontitis

BACKGROUND AND OBJECTIVE

Clinical studies found high levels of hepatocyte growth factor (HGF) expression in patients with periodontitis. Studies suggest that HGF plays an important role in periodontitis, is involved in inflammation, and modulates alveolar bone integrity in periodontitis. This study aims to investigate the effects and mechanisms of HGF in the progression of experimental periodontitis.

METHODS

We used silk thread ligation to induce periodontitis in HGF-overexpressing transgenic (HGF-Tg) and wild-type C57BL/6J mice. The effects of HGF overexpression on alveolar bone destruction were assessed by microcomputed tomography imaging at baseline and on days 7, 14, 21, and 28. We analyzed the cytokines (IL-6 and TNF-α) and lymphocytes in periodontitis tissues by enzyme-linked immunosorbent assay and flow cytometry. The effects of HGF on alveolar bone destruction were further tested by quantifying the systemic bone metabolism markers CTXI and PINP and by RNA sequencing for the signaling pathways involved in bone destruction. Western blotting and immunohistochemistry were performed to further elucidate the involved signaling pathways.

RESULTS

We found that experimental periodontitis increased HGF production in periodontitis tissues; however, the effects of HGF overexpression were inconsistent with disease progression. In the early stage of periodontitis, periodontal inflammation and alveolar bone destruction were significantly lower in HGF-Tg mice than in wild-type mice. In the late stage, HGF-Tg mice showed higher inflammatory responses and progressively aggravated bone destruction with continued stimulation of inflammation. We identified the IL-17/RANKL/TRAF6 pathway as a signaling pathway involved in the HGF effects on the progression of periodontitis.

CONCLUSION

HGF plays divergent effects in the progression of experimental periodontitis and accelerates osteoclastic activity and bone destruction in the late stage of inflammation.

Giant Cells of Various Lesions Are Characterised by Different Expression Patterns of HLA-Molecules and Molecules Involved in the Cell Cycle, Bone Metabolism, and Lineage Affiliation: An Immunohistochemical Study with a Review of the Literature

Giant cells (GCs) are thought to originate from the fusion of monocytic lineage cells and arise amid multiple backgrounds. To compare GCs of different origins, we immunohistochemically characterised the GCs of reactive and neoplastic lesions (n = 47). We studied the expression of 15 molecules including HLA class II molecules those relevant to the cell cycle, bone metabolism and lineage affiliation. HLA-DR was detectable in the GCs of sarcoidosis, sarcoid-like lesions, tuberculosis, and foreign body granuloma. Cyclin D1 was expressed by the GCs of neoplastic lesions as well as the GCs of bony callus, fibroid epulis, and brown tumours. While cyclin E was detected in the GCs of all lesions, p16 and p21 showed a heterogeneous expression pattern. RANK was expressed by the GCs of all lesions except sarcoid-like lesions and xanthogranuloma. All GCs were RANK-L-negative, and the GCs of all lesions were osteoprotegerin-positive. Osteonectin was limited to the GCs of chondroblastoma. Osteopontin and TRAP were detected in the GCs of all lesions except xanthogranuloma. RUNX2 was heterogeneously expressed in the reactive and neoplastic cohort. The GCs of all lesions except foreign body granuloma expressed CD68, and all GCs were CD163- and langerin-negative. This profiling points to a functional diversity of GCs despite their similar morphology.

Transcriptomic Differences Underlying the Activin-A Induced Large Osteoclast Formation in Both Healthy Control and Fibrodysplasia Ossificans Progressiva Osteoclasts

Fibrodysplasia Ossificans Progressiva (FOP) is a very rare genetic disease characterized by progressive heterotopic ossification (HO) of soft tissues, leading to immobility and premature death. FOP is caused by a mutation in the Activin receptor Type 1 (ACVR1) gene, resulting in altered responsiveness to Activin-A. We recently revealed that Activin-A induces fewer, but larger and more active, osteoclasts regardless of the presence of the mutated ACVR1 receptor. The underlying mechanism of Activin-A-induced changes in osteoclastogenesis at the gene expression level remains unknown. Transcriptomic changes induced by Activin-A during osteoclast formation from healthy controls and patient-derived CD14-positive monocytes were studied using RNA sequencing. CD14-positive monocytes from six FOP patients and six age- and sex-matched healthy controls were differentiated into osteoclasts in the absence or presence of Activin-A. RNA samples were isolated after 14 days of culturing and analyzed by RNA sequencing. Non-supervised principal component analysis (PCA) showed that samples from the same culture conditions (e.g., without or with Activin-A) tended to cluster, indicating that the variability induced by Activin-A treatment was larger than the variability between the control and FOP samples. RNA sequencing analysis revealed 1480 differentially expressed genes induced by Activin-A in healthy control and FOP osteoclasts with p(adj) < 0.01 and a Log2 fold change of ≥±2. Pathway and gene ontology enrichment analysis revealed several significantly enriched pathways for genes upregulated by Activin-A that could be linked to the differentiation or function of osteoclasts, cell fusion or inflammation. Our data showed that Activin-A has a substantial effect on gene expression during osteoclast formation and that this effect occurred regardless of the presence of the mutated ACVR1 receptor causing FOP.

Characterization of the Tumor Microenvironment in Jaw Osteosarcomas, towards Prognostic Markers and New Therapeutic Targets

Background-The purpose of this study was to investigate the bone resorption, as well as the vascular and immune microenvironment, of jaw osteosarcomas (JO) and to correlate these features with patient clinical outcomes. Methods-We studied 50 JO biopsy samples by immunohistochemical analysis of tissue microarrays (TMAs). We investigated the bone remodeling markers RANK/RANKL/OPG, the endothelial glycoprotein CD146, and biomarkers of the immune environment (CD163 and CD68 of macrophages, CD4+ and CD8+ of tumor-infiltrating lymphocytes (TILs), and an immune checkpoint PD-1/PD-L1). The biomarkers were analyzed for their influence on progression (recurrence and metastasis), overall survival (OS), and disease-free survival (DFS). Results-A strong and significant correlation has been found between CD163 staining and lower OS and DFS. The level of CD4+ and CD8+ staining was low and non-significantly associated with survival outcomes. High levels of RANK and RANKL were found in the tumor samples and correlated with lower DFS. Conclusion-Our findings suggest that CD163+ TAMs represent markers of poor prognosis in JO. Targeting TAMs could represent a valuable therapeutic strategy in JO.

Association between Inflammatory Markers and Local Recurrence in Patients with Giant Cell Tumor of Bone: A Preliminary Result

Giant cell tumor of bone (GCTB) has a high local recurrence rate of approximately 20%. Systemic inflammatory markers, such as neutrophil-lymphocyte ratio (NLR), modified Glasgow prognostic score (mGPS), prognostic nutritional index (PNI), lymphocyte-monocyte ratio (LMR), platelet-lymphocyte ratio (PLR), hemoglobin (Hb), alkaline phosphatase (ALP), and lactate dehydrogenase (LDH), have been reported as prognostic markers in patients with malignant tumors. This study aimed to investigate the correlation between these markers and the local recurrence rate of GCTB. In total, 103 patients with GCTB who underwent surgery at the authors' institutions between 1993 and 2021 were included. Thirty patients experienced local recurrence. Univariate and multivariate analysis showed that tumor site, preoperative and postoperative denosumab treatment, and surgery were significantly associated with local recurrence-free survival. LDH was associated with local recurrence-free survival on univariate analysis only. NLR, mGPS, PNI, LMR, and PLR score did not correlate with the local recurrence rate. In conclusion, NLR, mGPS, PNI, LMR, PLR score, Hb, ALP, and LDH levels are not correlated with the local recurrence rate of GCTB. However, due to the small number of patients included in this study, this result should be re-evaluated in a multicenter study with a larger sample size.

The giant cell tumor during pregnancy: A review of literature

BACKGROUND

Giant cell tumors (GTC) of bone are benign, locally aggressive tumors generally occurring in young people with a female predominance during reproductive age. Considering their worsening during pregnancy it has been suggested that pregnancy can accelerate GCT progression or favor recurrence but correlation between tumor growth and pregnancy has not yet been clarified. Aim of this study was to clarify clinical characteristics, timing and type of treatment through a literature review on GTCs occurring during pregnancy.

PATIENTS AND METHODS

An electronic search was performed in December 2020 in PubMed, Scopus, Embase, Medline, Cochrane Register using the keywords "giant cell tumor" AND "pregnancy" looking for papers reporting cases of giant cell tumors of the bone onset or recurred during pregnancy. The electronic search identified 212 papers; sixteen studies were selected, for a total of 32 cases.

RESULTS

The diagnosis was made during pregnancy in 24 cases and after the partum in 8 cases. 27 cases were new diagnoses while 5 cases were recurrences. Pulmonary metastases were reported in 3 patients. The treatment was performed during the pregnancy in 7 out of 32 cases; in the remaining 27 cases treatment was performed after delivery. The hormone receptor status was reported in 14 patients. Data regarding follow-up was reported for 26 out of 32 patients; three patients had local recurrences that were treated with wide resection and amputation in 2 and 1 case, respectively; at the last follow-up all patients were apparently without any evidence of disease except for three patients who had stable lung metastases.

DISCUSSION

In case of GCT during pregnancy, a multidisciplinary approach is necessary to offer the patients the best treatment in terms of mother and child's health. A correct diagnosis is necessary and not confusing tumor symptoms with ones of pregnancy is mandatory in order not to delay the diagnosis and let the tumor progress. Actually, even though pregnancy would seem to promote GCT growth and aggressiveness, the relationship is not clear. More studies are necessary to clarify this interesting aspect.

LEVEL OF EVIDENCE

IV, systematic review.

A Multiparametric Method Based on Clinical and CT-Based Radiomics to Predict the Expression of p53 and VEGF in Patients With Spinal Giant Cell Tumor of Bone

Purpose

This project aimed to assess the significance of vascular endothelial growth factor (VEGF) and p53 for predicting progression-free survival (PFS) in patients with spinal giant cell tumor of bone (GCTB) and to construct models for predicting these two biomarkers based on clinical and computer tomography (CT) radiomics to identify high-risk patients for improving treatment.

Material and Methods

A retrospective study was performed from April 2009 to January 2019. A total of 80 patients with spinal GCTB who underwent surgery in our institution were identified. VEGF and p53 expression and clinical and general imaging information were collected. Multivariate Cox regression models were used to verify the prognostic factors. The radiomics features were extracted from the regions of interest (ROIs) in preoperative CT, and then important features were selected by the SVM to build classification models, evaluated by 10-fold crossvalidation. The clinical variables were processed using the same method to build a conventional model for comparison.

Results

The immunohistochemistry of 80 patients was obtained: 49 with high-VEGF and 31 with low-VEGF, 68 with wild-type p53, and 12 with mutant p53. p53 and VEGF were independent prognostic factors affecting PFS found in multivariate Cox regression analysis. For VEGF, the Spinal Instability Neoplastic Score (SINS) was greater in the high than low groups, p &lt; 0.001. For p53, SINS (p = 0.030) and Enneking stage (p = 0.017) were higher in mutant than wild-type groups. The VEGF radiomics model built using 3 features achieved an area under the curve (AUC) of 0.88, and the p53 radiomics model built using 4 features had an AUC of 0.79. The conventional model built using SINS, and the Enneking stage had a slightly lower AUC of 0.81 for VEGF and 0.72 for p53.

Conclusion

p53 and VEGF are associated with prognosis in patients with spinal GCTB, and the radiomics analysis based on preoperative CT provides a feasible method for the evaluation of these two biomarkers, which may aid in choosing better management strategies.

Bone Turnover Marker (BTM) Changes after Denosumab in Giant Cell Tumors of Bone (GCTB): A Phase II Trial Correlative Study

Background: Giant cell tumors of bone (GCTB) are osteolytic tumors. Denosumab, a RANK-L inhibitor, is approved for GCTB. Data on serum bone turnover marker (sBTM) changes are lacking. We present a phase II correlative study on sBTMs in GCTB patients treated with denosumab. Methods: All GCTB patients receiving denosumab within a multicentre, open-label, phase 2 study were enrolled. Serum levels of carboxyterminal-crosslinked-telopeptide of type I collagen (s-CTX), alkaline phosphatase (ALP), bone-alkaline phosphatase (bALP), parathyroid hormone (sPTH), and osteocalcin (OCN) were prospectively assessed (baseline, T0, 3 months, T1, 6 months, T2). The primary endpoint was assessment of sBTM changes after denosumab; the secondary endpoints were disease-free survival (DFS) and sBTM correlation. Results: In 54 cases, sBTMs decreased during denosumab treatment except for sPTH. With a median follow-up of 59 months, 3-year DFS was 65% (%CI 52−79), with a significantly worse outcome for patients with high (≥500 UI/mL) s-CTX at baseline, as compared to low s-CTX (<500 UI/mL) (3-year DFS for high CTX 45% (95%CI 23−67) vs. 75% (95%CI 59−91) for low s-CTX. Higher median ALP and s-CTX were found for patients with tumor size ≥ 5 cm (p = 0.0512; p = 0.0589). Conclusion: Denosumab induces ALP/OCN and s-CTX reduction. High baseline s-CTX identifies a group of patients at higher risk of progression of the disease.

A Rationale for the Activity of Bone Target Therapy and Tyrosine Kinase Inhibitor Combination in Giant Cell Tumor of Bone and Desmoplastic Fibroma: Translational Evidences

Giant cell tumor of bone (GCTB) and desmoplastic fibroma (DF) are bone sarcomas with intermediate malignant behavior and unpredictable prognosis. These locally aggressive neoplasms exhibit a predilection for the long bone or mandible of young adults, causing a severe bone resorption. In particular, the tumor stromal cells of these lesions are responsible for the recruiting of multinucleated giant cells which ultimately lead to bone disruption. In this regard, the underlying pathological mechanism of osteoclastogenesis processes in GCTB and DF is still poorly understood. Although current therapeutic strategy involves surgery, radiotherapy and chemotherapy, the benefit of the latter is still debated. Thus, in order to shed light on these poorly investigated diseases, we focused on the molecular biology of GCTB and DF. The expression of bone-vicious-cycle- and neoangiogenesis-related genes was investigated. Moreover, combining patient-derived primary cultures with 2D and 3D culture platforms, we investigated the role of denosumab and levantinib in these diseases. The results showed the upregulation of RANK-L, RANK, OPN, CXCR4, RUNX2 and FLT1 and the downregulation of OPG and CXCL12 genes, underlining their involvement and promising role in these neoplasms. Furthermore, in vitro analyses provided evidence for suggesting the combination of denosumab and lenvatinib as a promising therapeutic strategy in GCTB and DF compared to monoregimen chemotherapy. Furthermore, in vivo zebrafish analyses corroborated the obtained data. Finally, the clinical observation of retrospectively enrolled patients confirmed the usefulness of the reported results. In conclusion, here we report for the first time a molecular and pharmacological investigation of GCTB and DF combining the use of translational and clinical data. Taken together, these results represent a starting point for further analyses aimed at improving GCTB and DF management.

The effects of VEGF-centered biomimetic delivery of growth factors on bone regeneration

It is accepted that biomimetic supply of signaling molecules during bone regeneration can provide an appropriate environment for accelerated new bone formation. In this study, we developed a growth factor delivery system based on porous particles and a thermosensitive hydrogel that allowed fast, continuous, and delayed/continuous release of growth factors to mimic their biological production during bone regeneration. It was observed that the Continuous group (continuous release of growth factors) provides a better environment for the osteogenic differentiation of hPDCs than the Biomimetic group (biomimetic release of growth factors), and thus is anticipated to promote bone regeneration. However, contrary to expectation, the Biomimetic group promoted significant new bone formation compared to the Continuous group. From the systematic cell culture experiments, the initial supply of VEGF was considered to have more favorable effects on the osteoclastogenesis than osteogenesis, which may hinder bone regeneration. Our results indicated that the continuous supply of VEGF (in particular, at early stage) from VEGF-loaded biomaterial might not be conducive to new bone formation. Therefore, we suggest that a biomimetic supply of growth factors is a more pivotal parameter for sufficient tissue regeneration. Its use as a molecular delivery system may also serve as a useful tool for the investigation of biological processes and molecules during tissue regeneration processes.

Inhibition of hepatocyte growth factor/c-Met signalling abrogates joint destruction by suppressing monocyte migration in rheumatoid arthritis

OBJECTIVES

To determine the expression of hepatocyte growth factor (HGF) in RA biological fluids, the role of HGF in monocyte migration and the therapeutic effect of the c-Met inhibitor savolitinib in an arthritis model mice.

METHODS

HGF/c-Met expression in serum, SF and synovial tissues (STs) obtained from RA patients and controls, as well as RA fibroblast-like synoviocytes (FLSs), was evaluated by ELISA and immunostaining. To determine the function of HGF in RA SF, we preincubated RA SF with a neutralizing anti-HGF antibody and measured the chemotactic ability of a human acute monocytic leukaemia cell line (THP-1). Additionally, examinations were conducted of SKG mice treated with savolitinib for 4 weeks.

RESULTS

HGF levels in serum from RA patients were significantly higher than those in the controls and were decreased by drug treatment for 24 weeks. Additionally, the HGF level in SF from RA patients was higher than that in SF from OA patients. HGF and c-Met expression was also noted in RA STs. Stimulation of RA FLSs with TNF-α increased HGF/c-Met expression in a concentration-dependent manner, and c-Met signal inhibition suppressed production of fractalkine/CX3CL1 and macrophage inflammatory protein-1α/CCL3. When HGF was removed by immunoprecipitation, migration of THP-1 in RA SF was suppressed. In SKG mice, savolitinib significantly suppressed ankle bone destruction on µCT, with an associated reduction in the number of tartrate-resistant acid phosphatase-positive osteoclasts.

CONCLUSION

HGF produced by inflammation in synovium of RA patients activates monocyte migration to synovium and promotes bone destruction via a chemotactic effect and enhanced chemokine production.

Hepatocyte growth factor overexpression promotes osteoclastogenesis and exacerbates bone loss in CIA mice

Background

Hepatocyte growth factor (HGF) is a multifunctional growth factor that promotes various biological processes. However, the effect of HGF on bone metabolism in rheumatoid arthritis (RA) remains unknown. Here, we investigated the role of HGF in regulating osteoclastogenesis and bone resorption in RA.

Methods

The expression of HGF in RA patients and collagen-induced arthritis (CIA) mice was examined. The role of HGF on osteoclastogenesis was analysed by osteoclastogenesis and bone resorption assays. The effect of HGF inhibition was evaluated in a CIA mice model. The mechanism of HGF in regulating osteoclastogenesis and bone resorption was explored by a series of in vitro studies.

Results

HGF was overexpressed in CIA and RA. HGF stimulated osteoclastogenesis in vitro. SU11274, a selective small molecule blocker of c-Met, impeded the effect of HGF on osteoclastogenesis and bone resorption. HGF regulated osteoclastogenesis by JNK and AKT-GSK-3β-NFATc1 signallings. SU11274 protected CIA mice from pathological bone loss.

Conclusions

These data strongly suggest that the highly expressed HGF in the joint tissues contributes to bone loss in RA. Inhibition of HGF/c-Met could effectively alleviate pathological bone loss and inflammatory symptoms in CIA mice. HGF/c-Met may be used as a new target for the treatment of bone loss in RA.

Distinct roles for the hypoxia-inducible transcription factors HIF-1α and HIF-2α in human osteoclast formation and function

Bone homeostasis is maintained by a balance between osteoblast-mediated bone formation and osteoclast-driven bone resorption. Hypoxia modulates this relationship partially via direct and indirect effects of the hypoxia-inducible factor-1 alpha (HIF-1α) transcription factor on osteoclast formation and bone resorption. Little data is available on the role(s) of the HIF-2α isoform of HIF in osteoclast biology. Here we describe induction of HIF-1α and HIF-2α during the differentiation of human CD14+ monocytes into osteoclasts. Knockdown of HIF-1α did not affect osteoclast differentiation but prevented the increase in bone resorption that occurs under hypoxic conditions. HIF-2α knockdown did not affect bone resorption but moderately inhibited osteoclast formation. Growth of osteoclasts in 3D gels reversed the effect of HIF-2α knockdown; HIF-2α siRNA increasing osteoclast formation in 3D. Glycolysis is the main HIF-regulated pathway that drives bone resorption. HIF knockdown only affected glucose uptake and bone resorption in hypoxic conditions. Inhibition of glycolysis with 2-deoxy-d-glucose (2-DG) reduced osteoclast formation and activity under both basal and hypoxic conditions, emphasising the importance of glycolytic metabolism in osteoclast biology. In summary, HIF-1α and HIF-2α play different but overlapping roles in osteoclast biology, highlighting the importance of the HIF pathway as a potential therapeutic target in osteolytic disease.

The HGF/Met/NF-κB Pathway Regulates RANKL Expression in Osteoblasts and Bone Marrow Stromal Cells

Multiple myeloma (MM)-induced bone disease occurs through hyperactivation of osteoclasts by several factors secreted by MM cells. MM cell-secreted factors induce osteoclast differentiation and activation via direct and indirect actions including enhanced expression of receptor activator of nuclear factor κB ligand (RANKL) in osteoblasts and bone marrow stromal cells (BMSCs). Hepatocyte growth factor (HGF) is elevated in MM patients and is associated with MM-induced bone disease, although the mechanism by which HGF promotes bone disease remains unclear. In the present study, we demonstrated that HGF induces RANKL expression in osteoblasts and BMSCs, and investigated the mechanism of induction. We found that HGF and MM cell supernatants induced RANKL expression in ST2 cells, MC3T3-E1 cells, and mouse BMSCs. In addition, HGF increased phosphorylation of Met and nuclear factor κB (NF-κB) in ST2 cells, MC3T3-E1 cells, or mouse BMSCs. Moreover, Met and NF-κB inhibitors suppressed HGF-induced RANKL expression in ST2 cells, MC3T3-E1 cells, and mouse BMSCs. These results indicated that HGF promotes RANKL expression in osteoblasts and BMSCs via the Met/NF-κB signaling pathway, and Met and NF-κB inhibitors suppressed HGF-induced RANKL expression. Our findings suggest that Met and NF-κB inhibitors are potentially useful in mitigating MM-induced bone disease in patients expressing high levels of HGF.

The extra domain A of fibronectin facilitates osteoclastogenesis in radicular cysts through vascular endothelial growth factor

AIM

To analyse the effects of the alternatively spliced fibronectin (FN) gene and its isoforms on osteoclastogenesis in radicular cysts.

METHODOLOGY

Specimens of radicular cysts were collected surgically from 22 patients whose radiolucent periapical areas were measured on digital panoramic radiographs before surgery. The associations between the radiolucent areas and FN isoforms, vascular endothelial growth factor (VEGF) expression or micro-vessel density, as well as the relationships amongst them, were analysed by immunohistochemical staining using the antibodies IST-9, BC-1, P1F11, VEGF and CD34. Fibroblasts isolated from those specimens were used to induce Trap + MNCs, and the effects of induction were assessed by blocking FN containing extra domain A (EDA + FN), COX-2 or VEGF in vitro. The effects of EDA exon knockout using CRISPR/Cas system were also assessed. Quantitative PCR was used to analyse relative expression of FN isoforms and osteoclastogenic genes. Data were analysed using linear regression, Spearman's rank correlation analysis, chi-square test and Student's t-test; P < 0.05 was considered significant.

RESULTS

Micro-vessel density and EDA + FN staining were positively associated with the size of radiolucent periapical areas (mm² ; P < 0.05), consistent with a positive association between Trap + MNCs and VEGF expression in fibroblasts (P < 0.05). Blocking the interaction between EDA + FN and fibroblasts inhibited Trap + MNC formation. In addition, EDA exon knockout decreased VEGF expression and inhibited Trap + MNC formation to the extent of blocking VEGF by bevacizumab, but osteoclastogenic induction was restored by recombinant VEGF. Using retrospective clinicopathological data, VEGF staining was shown to be positively associated with EDA + FN staining, micro-vessel density and the size of radiolucent areas (P < 0.05).

CONCLUSION

In fibrous capsules of radicular cysts, the alternatively spliced isoform EDA + FN generated by fibroblasts stimulated VEGF expression via an autocrine effect and then facilitated osteoclastogenesis. Both blockage of VEGF and EDA exon knockout could be used to inhibit bone destruction.

Bevacizumab or fibronectin gene editing inhibits the osteoclastogenic effects of fibroblasts derived from human radicular cysts

Fibronectin (FN) is a main component of extracellular matrix (ECM) in most adult tissues. Under pathological conditions, particularly inflammation, wound healing and tumors, an alternatively spliced exon extra domain A (EDA) is included in the FN protein (EDA⁺FN), which facilitates cellular proliferation, motility, and aggressiveness in different lesions. In this study we investigated the effects of EDA⁺FN on bone destruction in human radicular cysts and explored the possibility of editing FN gene or blocking the related paracrine signaling pathway to inhibit the osteoclastogenesis. The specimens of radicular cysts were obtained from 20 patients. We showed that the vessel density was positively associated with both the lesion size (R = 0.49, P = 0.001) and EDA⁺FN staining (R = 0.26, P = 0.022) in the specimens. We isolated fibroblasts from surgical specimens, and used the CRISPR/Cas system to knockout the EDA exon, or used IST-9 antibody and bevacizumab to block EDA⁺FN and VEGF, respectively. Compared to control fibroblasts, the fibroblasts from radicular cysts exhibited significantly more Trap⁺MNCs, the relative expression level of VEGF was positively associated with both the ratio of EDA⁺FN/total FN (R = 0.271, P = 0.019) and with the number of Trap⁺MNCs (R = 0.331, P = 0.008). The knockout of the EDA exon significantly decreased VEGF expression in the fibroblasts derived from radicular cysts, leading to significantly decreased osteoclastogenesis; similar results were observed using bevacizumab to block VEGF, but block of EDA⁺FN with IST-9 antibody had no effect. Furthermore, the inhibitory effects of gene editing on Trap⁺MNC development were restored by exogenous VEGF. These results suggest that EDA⁺FN facilitates osteoclastogenesis in the fibrous capsule of radicular cysts, through a mechanism mediated by VEGF via an autocrine effect on the fibroblasts. Bevacizumab inhibits osteoclastogenesis in radicular cysts as effectively as the exclusion of the EDA exon by gene editing.

Mechanisms of bone destruction in multiple myeloma

Osteolytic bone disease is a frequent complication of multiple myeloma, resulting in skeletal complications that are a significant cause of morbidity and mortality. It is the result of an increased activity of osteoclasts, which is not followed by reactive bone formation by osteoblasts. Recent studies have revealed novel molecules and pathways that are implicated in osteoclast activation and osteoblast inhibition. Among them, the most important include the receptor activator of nuclear factor-kappa B ligand/osteoprotegerin pathway, the macrophage inflammatory proteins and the activin-A that play a crucial role in osteoclast stimulation in myeloma, while the wingless-type (Wnt) signalling inhibitors (sclerostin and dickkopf-1) along with the growth factor independence-1 are considered the most important factors for the osteoblast dysfunction of myeloma patients. Finally, the role of osteocytes, which is the key cell for normal bone remodelling, has also revealed during the last years through their interaction with myeloma cells that leads to their apoptosis and the release of RANKL and sclerostin maintaining bone loss in these patients. This review focuses on the latest available data for the mechanisms of bone destruction in multiple myeloma.

A prospective study on predicting local recurrence of giant cell tumour of bone by evaluating preoperative imaging features of the tumour around the knee joint

PURPOSE

To evaluate the role of medical imaging in predicting local recurrence of giant cell tumour of bone (GCTB) by assessing the preoperative imaging features of GCTB around the knee.

METHODS

Forty-eight consecutive GCTBs in the proximal tibia and distal femur treated with curettage were prospectively enrolled. Patients were grouped in terms of their imaging features on radiography, computed tomography (CT) and magnetic resonance imaging (MRI). All patients were followed up for at least two years after surgery. The association between preoperative imaging features and local recurrence was investigated. Imaging features were retrospectively studied by correlation analysis. The differences between rates were tested by the Chi square and Fisher exact tests; independent factors were determined by multivariate logistic regression analysis.

RESULTS

Cystic change and adjacent soft tissue invasion were associated with a higher rate of local recurrence compared to the negative groups (P < 0.05). Cystic change was identified as an independent risk factor for local recurrence of GCTB (P < 0.05). Expansibility was correlated with the "soap bubble" sign and the fluid-fluid level (P < 0.05); the "soap bubble" sign was correlated with osteosclerosis and the fluid-fluid level (P < 0.05); cortical bone involvement was correlated with adjacent soft tissue invasion (P < 0.05); and cystic change was correlated with the fluid-fluid level (P < 0.05).

CONCLUSION

Cystic change was an independent risk factor for local recurrence of GCTB. Adjacent soft tissue invasion might indirectly relate to local relapse. A cluster of association relationships between imaging features was revealed.

Epigenetic regulation of HGF/Met receptor axis is critical for the outgrowth of bone metastasis from breast carcinoma

Our translational research deals with the influence of microenvironment on the phenotype and colonization of bone metastases from breast carcinoma, and on pre-metastatic niche formation. The aim of the present study was to clarify the origin of hepatocyte growth factor (HGF), ligand of Met receptor, the control of the axis HGF/Met by DNA methylation, and its importance for the nexus supportive cells-metastatic cells and for metastasis outgrowth. In bone metastasis of the 1833-xenograft model, DNA methyltransferase blockade using the chemotherapic drug 5-aza-2′-deoxycytidine (decitabine) strongly reduced the expression of HGF/Met receptor axis and of E-cadherin, with decrease of metastasis wideness and osteolysis, prolonging mice survival. Thus, DNA methylation events acted as commanders of breast carcinoma cells metastatizing to bone influencing the epithelial phenotype. HGF emerged as a bone-marrow stimulus, and the exosomes seemed to furnish HGF to metastatic cells. In fact, decitabine treatment similarly affected some markers of these microvesicles and HGF, indicating that its supply to recipient cells was prevented. Notably, in bone metastasis the hypomethylation of HGF, Met and E-cadherin promoters did not appear responsible for their elevated expression, but we suggest the involvement of hypermethylated regulators and of Wwox oncosuppressor, the latter being affected by decitabine. Wwox expression increased under decitabine strongly localizing in nuclei of bone metastases. We hypothesize a role of Wwox in Met activity since in vitro Wwox overexpression downregulated the level of nuclear-Met protein fragment and Met stability, also under long exposure of 1833 cells to decitabine. HGF enhanced phosphoMet and the activity in nuclei, an effect partially prevented by decitabine. Altogether, the data indicated the importance to target the tumor microenvironment by blocking epigenetic mechanisms, which control critical events for colonization such as HGF/Met axis and Wwox, as therapy of bone metastasis.

Emerging treatment approaches for myeloma-related bone disease

INTRODUCTION

Multiple myeloma is characterized by the presence of osteolytic lesions that leads to devastating skeletal-related events in the majority of patients. Myeloma bone disease is attributed to increased osteoclastic and suppressed osteoblastic activity. Areas covered: Bisphosphonates remain the main treatment option, however they have limitations on their own. Understanding the pathogenesis of myeloma bone disease may provide a roadmap for new therapeutic approaches. The pathway of RANKRANKLOPG pathway has revealed denosumab, a monoclonal antibody targeting RANKL as a novel emerging therapy for myeloma-related bone disease. Furthermore, the Wnt signaling inhibitors dicckopf-1 and sclerostin that are implicated in the pathogenesis of bone destruction of myeloma are now targeted by novel monoclonal antibodies. Activin-A is a TGF-beta superfamily member which increases osteoclast activity and inhibits osteoblast function in myeloma; sotatercept and other molecules targeting activin-A have entered into clinical development. Several other molecules and pathways that play an important role in the pathogenesis of bone destruction in myeloma, such as periostin, adiponectin, Notch and BTK signaling are also targeted in an attempt to develop novel therapies for myeloma-related bone disease. Expert commentary: We summarize the current advances in the biology of myeloma bone disease and the potential therapeutic targets.

Deletion of Opg Leads to Increased Neovascularization and Expression of Inflammatory Cytokines in the Lumbar Intervertebral Disc of Mice

STUDY DESIGN

Neovascularization and expression of inflammatory cytokines were examined in Osteoprotegerin (Opg) knockout (KO) mice that show intervertebral disc (IVD) degeneration.

OBJECTIVE

The aim of this study was to clarify the pathological changes in lumbar IVD degeneration in Opg KO mice.

SUMMARY OF BACKGROUND DATA

Osteoporosis is a controversial risk factor for IVD degeneration. Deletion of Opg resulted in IVD degeneration in mice. Neovascularization and inflammatory cytokines are key factors in IVD degeneration.

METHODS

Opg KO mice and their wild-type (WT) littermates were euthanized. Lumbar IVDs were harvested. Safranin O/Fast Green staining was performed to examine the pathological changes. Microcomputed tomographic (micro-CT) analysis was performed to determine the structural changes at the junction of lumbar IVD cartilage and vertebrae. Tartrate-resistant acid phosphatase (TRAP) staining was performed to evaluate osteoclast formation. Protein expression of vascular endothelial growth factor A (VEGF-A), CD31, VE-cadherin, CD 34, interleukin-1β (IL-1β), and tumor necrosis factors α (TNF-α) were analyzed by immunohistochemistry (IHC) assays. Gene expressions of IL-1β, IL-6, and TNF-α were analyzed by real-time polymerase chain reaction (RT-PCR).

RESULTS

In 12-week-old Opg KO mice, new bone was formed in the endplate cartilage of lumbar IVDs and this became more obvious in 24-week-old Opg KO mice. Three-dimensional (3D) μCT reconstruction analyses showed that the edges of the L4 and L5 vertebrae were rugged with bone marrow cavities in it. Protein expression of VEGF-A, CD31, VE-cadherin, and CD34 was increased in the endplate and growth plate of lumbar IVDs of Opg KO mice. Gene expression of IL-1β, IL-6, and TNF-α as well as protein expression of IL-1β and TNF-α were highly expressed in the lumbar IVDs of Opg KO mice.

CONCLUSION

Deletion of Opg leads to increased neovascularization and expression of inflammatory cytokines in the lumbar disc in Opg KO mice, which may play important roles in IVD degeneration.

LEVEL OF EVIDENCE

N/A.

High Potency VEGFRs/MET/FMS Triple Blockade by TAS-115 Concomitantly Suppresses Tumor Progression and Bone Destruction in Tumor-Induced Bone Disease Model with Lung Carcinoma Cells

Approximately 25-40% of patients with lung cancer show bone metastasis. Bone modifying agents reduce skeletal-related events (SREs), but they do not significantly improve overall survival. Therefore, novel therapeutic approaches are urgently required. In this study, we investigated the anti-tumor effect of TAS-115, a VEGFRs and HGF receptor (MET)-targeted kinase inhibitor, in a tumor-induced bone disease model. A549-Luc-BM1 cells, an osteo-tropic clone of luciferase-transfected A549 human lung adenocarcinoma cells (A549-Luc), produced aggressive bone destruction associated with tumor progression after intra-tibial (IT) implantation into mice. TAS-115 significantly reduced IT tumor growth and bone destruction. Histopathological analysis showed a decrease in tumor vessels after TAS-115 treatment, which might be mediated through VEGFRs inhibition. Furthermore, the number of osteoclasts surrounding the tumor was decreased after TAS-115 treatment. In vitro studies demonstrated that TAS-115 inhibited HGF-, VEGF-, and macrophage-colony stimulating factor (M-CSF)-induced signaling pathways in osteoclasts. Moreover, TAS-115 inhibited Feline McDonough Sarcoma oncogene (FMS) kinase, as well as M-CSF and receptor activator of NF-κB ligand (RANKL)-induced osteoclast differentiation. Thus, VEGFRs/MET/FMS-triple inhibition in osteoclasts might contribute to the potent efficacy of TAS-115. The fact that concomitant dosing of sunitinib (VEGFRs/FMS inhibition) with crizotinib (MET inhibition) exerted comparable inhibitory efficacy for bone destruction to TAS-115 also supports this notion. In conclusion, TAS-115 inhibited tumor growth via VEGFR-kinase blockade, and also suppressed bone destruction possibly through VEGFRs/MET/FMS-kinase inhibition, which resulted in potent efficacy of TAS-115 in an A549-Luc-BM1 bone disease model. Thus, TAS-115 shows promise as a novel therapy for lung cancer patients with bone metastasis.

Myeloma bone disease: Progress in pathogenesis

Myeloma bone disease (MBD) is one of the most serious complications of multiple myeloma (MM) and the most severe cause of MM morbidity. Dysregulation of osteoblast and osteoclast cells plays key roles in MBD. In the bone marrow microenvironment, myeloma cells, osteoblasts, osteoclasts and bone marrow stromal cells can secrete multiple cytokines, categorized as osteoclast cell activating factors (OAFs) and osteoblast cell inactivating factors, which have been discovered to participate in bone metabolism and contribute to the pathogenesis of MBD. Several signaling pathways related to these cytokines were also revealed in the MBD pathogenesis. To better understand the pathogenesis of MBD and therefore the potential therapeutic targets of this disease, we will summarize recent study progress in the factors and underlying signaling pathways involved in the occurrence and development of MBD.

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.

Wear particles derived from metal hip implants induce the generation of multinucleated giant cells in a 3-dimensional peripheral tissue-equivalent model

Multinucleate giant cells (MGCs) are formed by the fusion of 5 to 15 monocytes or macrophages. MGCs can be generated by hip implants at the site where the metal surface of the device is in close contact with tissue. MGCs play a critical role in the inflammatory processes associated with adverse events such as aseptic loosening of the prosthetic joints and bone degeneration process called osteolysis. Upon interaction with metal wear particles, endothelial cells upregulate pro-inflammatory cytokines and other factors that enhance a localized immune response. However, the role of endothelial cells in the generation of MGCs has not been completely investigated. We developed a three-dimensional peripheral tissue-equivalent model (PTE) consisting of collagen gel, supporting a monolayer of endothelial cells and human peripheral blood mononuclear cells (PBMCs) on top, which mimics peripheral tissue under normal physiological conditions. The cultures were incubated for 14 days with Cobalt chromium alloy (CoCr ASTM F75, 1–5 micron) wear particles. PBMC were allowed to transit the endothelium and harvested cells were analyzed for MGC generation via flow cytometry. An increase in forward scatter (cell size) and in the propidium iodide (PI) uptake (DNA intercalating dye) was used to identify MGCs. Our results show that endothelial cells induce the generation of MGCs to a level 4 fold higher in 3-dimentional PTE system as compared to traditional 2-dimensional culture plates. Further characterization of MGCs showed upregulated expression of tartrate resistant alkaline phosphatase (TRAP) and dendritic cell specific transmembrane protein, (DC-STAMP), which are markers of bone degrading cells called osteoclasts. In sum, we have established a robust and relevant model to examine MGC and osteoclast formation in a tissue like environment using flow cytometry and RT-PCR. With endothelial cells help, we observed a consistent generation of metal wear particle- induced MGCs, which heralds metal on metal hip failures.

Alternative pathways of osteoclastogenesis in inflammatory arthritis

Osteoclasts are cells of haematopoietic origin that are uniquely specialized to degrade bone. Under physiological conditions, the osteoclastogenesis pathway depends on macrophage colony-stimulating factor 1 (CSF-1, also known as M-CSF) and receptor activator of nuclear factor κB ligand (RANKL). However, an emerging hypothesis is that alternative pathways of osteoclast generation might be active during inflammatory arthritis. In this Perspectives article, we summarize the physiological pathway of osteoclastogenesis and then focus on experimental findings that support the hypothesis that infiltrating inflammatory cells and the cytokine milieu provide multiple routes to bone destruction. The precise identity of osteoclast precursor(s) is not yet known. We propose that myeloid cell differentiation during inflammation could be an important contributor to the differentiation of osteoclast populations and their associated pathologies. Understanding the dynamics of osteoclast differentiation in inflammatory arthritis is crucial for the development of therapeutic strategies for inflammatory joint disease in children and adults.

Overexpression of hypoxia-inducible factor-1α and vascular endothelial growth factor in sacral giant cell tumors and the correlation with tumor microvessel density

Although classified as benign, giant cell tumors of the bone (GCTB) may be aggressive, recur and even metastasize to the lungs. In addition, the pathogenesis and histogenesis remain unclear; thus, the driving factors behind the strong tumor growth capacity of GCTB require investigation. In the present study, the expression levels of hypoxia-inducible factor (HIF)-1α and vascular endothelial growth factor (VEGF), which are promoted by hypoxic conditions, were determined in 22 sacral GCTB samples using immunohistochemistry and western blot analysis. Furthermore, CD34 expression was analyzed using these methods. The correlation between HIF-1α or VEGF expression and the tumor microvessel density (MVD) was then determined. The results demonstrated that HIF-1α, VEGF and CD34 were overexpressed in the 22 sacral GCTB specimens, and overexpression of HIF-1α and VEGF correlated with the tumor MVD. Thus, the present study has provided novel indicators for the tumor growth capacity of GCTBs.

Osteoclasts: New Insights

Osteoclasts, the bone-resorbing cells, play a pivotal role in skeletal development and adult bone remodeling. They also participate in the pathogenesis of various bone disorders. Osteoclasts differentiate from cells of the monocyte/macrophage lineage upon stimulation of two essential factors, the monocyte/macrophage colony stimulating factor (M-CSF) and receptor activation of NF-κB ligand (RANKL). M-CSF binds to its receptor c-Fms to activate distinct signaling pathways to stimulate the proliferation and survival of osteoclast precursors and the mature cell. RANKL, however, is the primary osteoclast differentiation factor, and promotes osteoclast differentiation mainly through controlling gene expression by activating its receptor, RANK. Osteoclast function depends on polarization of the cell, induced by integrin αvβ3, to form the resorptive machinery characterized by the attachment to the bone matrix and the formation of the bone-apposed ruffled border. Recent studies have provided new insights into the mechanism of osteoclast differentiation and bone resorption. In particular, c-Fms and RANK signaling have been shown to regulate bone resorption by cross-talking with those activated by integrin αvβ3. This review discusses new advances in the understanding of the mechanisms of osteoclast differentiation and function.