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

Evidence for reduced bone formation surface relative to bone resorption surface in female femoral fragility fracture patients

Fragility fractures, including neck of femur fractures, result from reductions in the amount, quality and architecture of bone. The aim of this study was to compare the cancellous bone structure, and static indices of bone turnover, in female patients, who had sustained fragility fracture at the femoral neck, with age-matched females without fragility fracture. Bone samples were taken from the intertrochanteric region of the proximal femur of female patients undergoing hip arthroplasty surgery for a subcapital fragility fracture of the femoral neck (#NOF) or from age-matched female control individuals at routine autopsy. Contiguous bone samples were analyzed for undecalcified histomorphometry and for mRNA expression. The histomorphometric data, which were normally distributed, indicated no difference between the mean values for any of the structural parameters in control and fracture samples. In particular, the bone volume (BV/TV) values were not different and did not change significantly with age in these cohorts of individuals aged >65 years. The static indices of bone turnover, eroded surface (ES/BS) and osteoid surface (OS/BS), were positively correlated with age in the >65-year-old control group (p < 0.055 and p < 0.03, respectively). The median values for these indices were not different between the fracture and control groups. However, both the median and the range of OS/BS values were increased for >65-year-old controls compared with a group of younger females aged <65 years, suggesting an increase in bone formation surface in older females in the proximal femur after 65 years of age. When the data were further interrogated, a reduction in the percentage osteoid surface to eroded surface quotient (OS/ES) was found for the fracture group compared with the age-matched control group suggesting a reduced adaptive modeling drift capability in the fracture group. In contiguous bone samples, increased median values for receptor activator of nuclear factor kappa beta (RANK) and interleukin-6 (IL-6) mRNA expression were observed in the fracture group. Study of cultured human osteoblasts showed that recombinant human IL-6 (rhIL-6) inhibited osteoblast differentiation, as measured by an increase in the immature osteoblast marker, STRO-1 and concomitantly decreased expression of the osteoblast maturation marker, alkaline phosphatase. Importantly, cells cultured in the presence of IL-6 showed significantly less mineral deposition in vitro compared with control cultures. These data suggest that perturbations in bone formation surface, relative to resorption surface, are potentially important in producing bone in the proximal femur with increased propensity to fracture.

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.

RANK Expression as a cell surface marker of human osteoclast precursors in peripheral blood, bone marrow, and giant cell tumors of bone

RANK expression in vivo on hematopoietic subsets including pre-osteoclasts, identified by monoclonal antibodies, has not been described. We describe the lineages that express RANK in bone marrow, peripheral blood, and GCTs. We show that CD14(+)RANK(high) cells constitute a circulating pre-osteoclast pool.

INTRODUCTION

The expression of RANK by subsets of hematopoietic cells has not been adequately studied in humans. While attributed to the monocytoid lineage, the phenotype of the pre-osteoclast (pre-OC) with respect to RANK expression in vivo remains unclear. We tested monoclonal antibodies (MAbs) raised against the extracellular domain of recombinant human RANK for reactivity with normal peripheral blood (PB) and bone marrow (BM) mononuclear cells (PBMNCs and BMMNCs, respectively). We also tested reactivity with giant cell tumor cells (GCT), a confirmed source of pre-OC and mature OCs.

MATERIALS AND METHODS

Human PBMNCs, BMMNCs, and GCT cells were analyzed for reactivity with anti-RANK MAbs by flow cytometry in combination with hematopoietic lineage restricted markers. GCTs were also analyzed by immunofluorescence. CD14+ monocytoid cells were sorted by fluorescence-activated cell sorting (FACS) based on their relative RANK expression and cultured under OC-forming conditions.

RESULTS

RANK+ cells were detected similarly by three independent anti-RANK MAbs. One MAb (80736) immunoprecipitated RANK-RANKL complexes from surface-biotinylated GCT lysates. Using dual-color flow cytometry, RANK was detected on CD14+ (monocytoid), CD19+ (B-lymphoid), CD56+ (NK cell), and glycophorin A+ erythroid progenitors. Minor populations of both CD3+ T lymphocytes and BM CD34+ hematopoietic progenitors also expressed cell surface RANK. In GCTs, RANK expression was identified on mononuclear CD45(+)CD14(+)alphaVbeta3(+)c-Fms+ cells, likely to be committed pre-OC, and on multinucleated CD45(+)alphaVbeta3(+)TRACP(+) OCs. Importantly, sorted CD14(+)RANK(high) PBMNCs treated with recombinant RANKL and macrophage-colony stimulating factor (M-CSF) gave rise to approximately twice the number of osteoclasts than RANK(mid) or RANK(low) cells.

CONCLUSIONS

These results suggest that committed monocytoid RANK+ pre-OCs are represented in the marrow and circulate in the periphery, forming a pool of cells capable of responding rapidly to RANKL. The ability to reliably detect committed pre-OC in peripheral blood could have important clinical applications in the management of diseases characterized by abnormal osteoclastic activity.

Role of osteoprotegerin (OPG) in cancer

OPG (osteoprotegerin), a secreted member of the TNF (tumour necrosis factor) receptor superfamily, has a variety of biological functions which include the regulation of bone turnover. OPG is a potent inhibitor of osteoclastic bone resorption and has been investigated as a potential therapeutic for the treatment of both osteoporosis and tumour-induced bone disease. Indeed, in murine models of cancer-induced bone disease, inhibition of osteoclastic activity by OPG was also associated with a reduction in tumour burden. The discovery that OPG can bind to and inhibit the activity of TRAIL (TNF-related apoptosis-inducing ligand) triggered extensive research into the potential role of OPG in the regulation of tumour cell survival. A number of reports from studies using in vitro models have shown that OPG protects tumour cells from the effects of TRAIL, thereby possibly providing tumour cells that produce OPG with a survival advantage. However, the ability of OPG to act as a tumour cell survival factor remains to be verified using appropriate in vivo systems. A third area of interest has been the use of OPG as a prognostic marker in various cancer types, including myeloma, breast and prostate cancer. This review provides an overview of the role of OPG in cancer, both in cancer-induced bone disease and in tumour growth and survival.

Mechanisms of disease: roles of OPG, RANKL and RANK in the pathophysiology of skeletal metastasis

The discovery of osteoprotegerin, receptor activator of nuclear factor kappa B (RANK) and RANK ligand as critical molecular determinants of osteoclastogenesis and regulators of bone resorption, has revolutionized our understanding of the processes of normal and pathological bone biology. Altering the relative biological availabilities of these molecules has direct consequences for the regulation of both bone resorption and bone remodeling. Importantly, recent research suggests a pivotal role for these molecules in mediating cancer-induced bone destruction. This review summarizes the current evidence of osteoprotegerin, RANK ligand and RANK involvement in the pathophysiology of skeletal metastasis, and of therapeutic targeting of this process.

Osteoblast lineage properties in giant cell tumors of bone

BACKGROUND

Giant cell tumors of bone (GCTs), among the most common primary bone tumors, are characterized by the formation of abundant osteoclast-like multinucleated giant cells (MNCs). It is not yet clear about the origin of GCTs and which cells in the lesion are the true neoplastic component. Several recent reports suggested that MNCs are osteoclasts induced by stroma-like tumor cells expressing the ligand for receptor activator of NF-kappaB (RANKL), which is a membrane-bound osteoclast differentiation factor. This hypothesis suggests an osteoblast lineage origin of GCTs, although it has long been speculated about GCTs being of mesenchymal stem cell (MSC) origin.

METHODS

We investigated the expression of osteoblastic differentiation markers in 10 human GCTs by reverse transcription-polymerase chain reaction and immunohistochemistry. We also performed osteoblastic and adipogenic differentiation assays using cultured cells derived from surgically resected lesions to estimate the stem cell-like properties.

RESULTS

GCTs and derived stromal cells expressed many osteoblast lineage marker genes, such as collagen type I, bone sialoprotein, core binding factor a-1, and osteocalcin. Instead of stable expression of mRNA, osteocalcin was not detected among the proteins. The tumor-derived cultures showed osteoblastic but not adipogenic differentiation capability. These findings strongly suggest that GCTs are of osteoblast lineage origin.

CONCLUSIONS

Our results indicated that GCTs expressed many osteoblastic markers and showed properties of pre-osteoblast-like cells rather than those of MSCs. These observations may provide some insight into the mechanisms of disease progression and the origin of GCTs.

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.

Stability of RNA isolated from human trabecular bone at post-mortem and surgery

To determine the reliability of gene expression studies in human post-mortem bone, it is important to evaluate the stability of RNA isolated from such tissues as a function of the post-mortem interval. The stability of total RNA and bone-specific mRNA species was examined in bone samples obtained from routine autopsies and at surgery. The optimal temperature for any storage and transport of the bone before RNA isolation was shown to be 4 degrees C, and RT-PCR analysis is the preferred technique for the analysis of gene expression in post-mortem bone as it tolerates partial RNA degradation. For gene expression studies in bone, post-mortem cases, with a post-mortem interval of less than 48 h, should be selected, and the time that bone is stored after retrieval at autopsy or surgery should be kept to a minimum. Overall, our findings indicate that with appropriate storage and handling, RNA can be reliably isolated from human bone obtained at post-mortem and surgery to study ex vivo the pattern of gene expression in healthy individuals and in patients with musculoskeletal diseases such as osteoporosis and osteoarthritis.

Gene expression in giant-cell tumors

Malignant transformation is thought to be associated with changes in the expression of a number of genes, and this alteration in gene expression is considered critical to the development of the malignant phenotype. In this study, gene expression in 8 samples of giant-cell tumor (GCT) of bone, as well as in bone at the site of osteoarthritis and in a variety of normal tissues, was determined at Gene Logic Inc (Gaithersburg, Md) with the use of Affymetrix GeneChip U_133 arrays containing approximately 40,000 genes/expressed sequence tags (ESTs). Gene-expression analysis was performed with the use of the Gene Logic GeneExpress Software System. Differences in gene expression between GCTs and bone were observed. In addition, genes expressed uniquely in GCTs among these and 519 samples from 20 other tissue types were identified. Some of the genes that were found to be overexpressed in GCTs, such as tartrate-resistant acid phosphatase and the lysosomal H + -transporting ATPase, are also expressed by osteoclasts. Osteoprotegrin ligand (OPGL) was also selectively overexpressed in GCTs. The genes found to be overexpressed in GCTs appear to reflect the genetic profile of osteoclast-lineage cells and also the genetic profile of an osteoclastogenic environment. The genes identified in this study may play a role in the pathogenesis of GCTs, confirm the likely importance of OPGL in GCT pathogenesis, and may indicate other possible targets to which antitumor therapy could be directed.

The molecular triad OPG/RANK/RANKL: involvement in the orchestration of pathophysiological bone remodeling

The past decade has seen an explosion in the field of bone biology. The area of bone biology over this period of time has been marked by a number of key discoveries that have opened up entirely new areas for investigation. The recent identification of the receptor activator of nuclear factor kappaB ligand (RANKL), its cognate receptor RANK, and its decoy receptor osteoprotegerin (OPG) has led to a new molecular perspective on osteoclast biology and bone homeostasis. Specifically, the interaction between RANKL and RANK has been shown to be required for osteoclast differentiation. The third protagonist, OPG, acts as a soluble receptor antagonist for RANKL that prevents it from binding to and activating RANK. Any dysregulation of their respective expression leads to pathological conditions such as bone tumor-associated osteolysis, immune disease, or cardiovascular pathology. In this context, the OPG/RANK/RANKL triad opens novel therapeutic areas in diseases characterized by excessive bone resorption. The present article is an update and extension of an earlier review published by Kwan Tat et al. [Kwan Tat S, Padrines M, Theoleyre S, Heymann D, Fortun Y. IL-6, RANKL, TNF-alpha/IL-1: interrelations in bone resorption pathophysiology. Cytokine Growth Factor Rev 2004;15:49-60].

RANKL/RANK/OPG: new therapeutic targets in bone tumours and associated osteolysis

The emergence of the molecular triad osteoprotegerin (OPG)/Receptor Activator of NF-kB (RANK)/RANK Ligand (RANKL) has helped elucidate a key signalling pathway between stromal cells and osteoclasts. The interaction between RANK and RANKL plays a critical role in promoting osteoclast differentiation and activation leading to bone resorption. OPG is a soluble decoy receptor for RANKL that blocks osteoclast formation by inhibiting RANKL binding to RANK. The OPG/RANK/RANKL system has been shown to be abnormally regulated in several malignant osteolytic pathologies such as multiple myeloma [MM, where enhanced RANKL expression (directly by tumour cells or indirectly by stromal bone cells or T-lymphocytes)] plays an important role in associated bone destruction. By contrast, production of its endogenous counteracting decoy receptor OPG is either inhibited or too low to compensate for the increase in RANKL production. Therefore, targeting the OPG/RANK/RANKL axis may offer a novel therapeutic approach to malignant osteolytic pathologies. In animal models, OPG or soluble RANK was shown both to control hypercalcaemia of malignancy and the establishment and progression of osteolytic metastases caused by various malignant tumours. To this day, only one phase I study has been performed using a recombinant OPG construct that suppressed bone resorption in patients with multiple myeloma or breast carcinoma with radiologically confirmed bone lesions. RANK-Fc also exhibits promising therapeutic effects, as revealed in animal models of prostate cancer and multiple myeloma. If the animal results translate to similar clinical benefits in humans, using RANK-Fc or OPG may yield novel and potent strategies for treating patients with established or imminent malignant bone diseases and where standard therapeutic regimens have failed.

The proliferation and phenotypic expression of human osteoblasts on tantalum metal

Tantalum (Ta) is increasingly used in orthopaedics, although there is a paucity of information on the interaction of human osteoblasts with this material. We investigated the ability of Ta to support the growth and function of normal human osteoblast-like cells (NHBC). Cell responses to polished and textured Ta discs were compared with responses to other common orthopaedic metals, titanium and cobalt-chromium alloy, and tissue culture plastic. No consistent differences, that could be attributed to the different metal substrates or to the surface texture, were found in several measured parameters. Attachment of NHBC to each substrate was similar, as was cell morphology, as determined by confocal microscopy. Cell proliferation was slightly faster on plastic than on Ta at 3 days, but by 7 days neither the absolute cell numbers, nor the number of cell divisions, was different between Ta and the other substrates. No consistent, substrate-dependent differences were seen in the expression of a number of mRNA species corresponding to the pro-osteoclastic or the osteogenic activity of osteoblasts. No substrate-dependent differences were seen in the extent of in vitro mineralisation by NHBC. These results indicate that Ta is a good substrate for the attachment, growth and differentiated function of human osteoblasts.

Bisphosphonates may reduce recurrence in giant cell tumor by inducing apoptosis

Giant cell tumor of bone is an aggressive tumor characterized by extensive bone destruction and high recurrence rates. This tumor consists of stromal cells and hematopoietic cells that interact in an autocrine manner to produce tumoral osteoclastogenesis and bone resorption. This autocrine regulation may be disrupted by novel therapeutic agents. Nonspecific local adjuvant therapies such as phenol or liquid nitrogen have been used in the treatment of giant cell tumor, but specific adjuvant therapies have not been described. The bisphosphonates pamidronate and Zoledronate can induce apoptosis in giant cell tumor culture in a dose-dependent manner. We established giant cell tumor cultures from patients with extensive destruction of bone. One of the four cultures formed osteoclastlike giant cells in vitro after more than six passages without exogenous receptor activator of NF-kappaB ligand or macrophage colony stimulating factor. Annexin V staining, presence of active cleaved form of caspase-3, and disappearance of poly (ADP-ribose) polymerase on Western blotting indicated activation of apoptosis by bisphosphonates in giant cell tumor. These results indicate that topical or systemic use of pamidronate or zoledronate can be a novel adjuvant therapy for giant cell tumor by targeting osteoclastlike giant cells, mononuclear giant cell precursor cells, and the autocrine loop of tumor osteoclastogenesis.

Factors regulating osteoclast formation in human tissues adjacent to peri-implant bone loss: expression of receptor activator NFkappaB, RANK ligand and osteoprotegerin

Aseptic bone loss adjacent to orthopedic joint implants is a common cause of joint implant failure in humans. This study investigates the expression of key regulators of osteoclast formation, receptor activator NFkappaB (RANK), Receptor activator of NFkappaB ligand (RANKL) and osteoprotegerin (OPG), in the peri-implant tissues of patients with osteolysis compared with levels in synovial tissues from osteoarthritic and healthy subjects. Immunohistochemical studies demonstrated that significantly higher levels of RANKL protein (p<0.05) were found in the peri-implant tissues of patients with implant failure than in similar tissues from osteoarthritic and healthy subjects. In contrast, OPG protein levels were similar in all tissues. RANKL, expressed as mRNA and protein, was predominantly associated with cells containing wear particles. Dual labeling studies showed that the cells expressing RANKL protein were macrophages. In situ hybridization studies confirmed that mRNA encoding for these proteins is also expressed by cells in the peri-implant tissues. In addition, RANK mRNA was expressed in cells that contained wear particles. These findings show that abnormally high levels of RANKL are expressed in peri-implant tissues of patients with prosthetic loosening and that these abnormal levels of RANKL may significantly contribute to aseptic implant loosening.

Increased expression of IL-6 and RANK mRNA in human trabecular bone from fragility fracture of the femoral neck

Previous studies have implicated pro-inflammatory cytokines in the bone loss of estrogen deficiency. The aim of this study was to investigate the expression of key regulatory molecules of bone remodeling in the trabecular bone microenvironment in osteoporosis. Bone samples were taken from the intertrochanteric region of the proximal femur of patients undergoing total hip arthroplasty for a subcapital fragility fracture of the femoral neck (#NOF). For comparison, samples were taken from age-matched control individuals at routine autopsy. Expression of RANKL, RANK, osteoprotegerin (OPG), IL-6, IL-11, osteocalcin (OCN), and calcitonin receptor (CTR) messenger RNA (mRNA) species were analyzed and the data were nonparametrically distributed. The median expression of the proresorptive genes, RANK and IL-6, were significantly elevated in the fracture group compared to an age-matched control group (2.2 [1.9-2.9; 25th-75th percentiles] > 1.0 [0.4-2.1], P < 0.03; 3.9 [1.8-6.2] > 0.8 [0.7-1.5], P < 0.002, respectively). In contrast, there were no significant differences in expression of RANKL, OPG, CTR, or OCN mRNA between the #NOF and control groups. The median RANKL/OPG mRNA ratio was significantly greater in hip fracture bone than in bone from controls (4.8 [3.8-7.6] > 3.2 [2.1-4.0], P < 0.05). IL-6 mRNA levels associated strongly with RANKL mRNA levels in the #NOF group (r = 0.77, P < 0.001), but not in the control group. A strong positive association was found between IL-11 mRNA levels and RANKL mRNA levels in the #NOF group (r = 0.81, P < 0.001), consistent with the apparent coordinated regulation of IL-6 and IL-11 in bone samples from the #NOF group (r = 0.93, P < 0.0001). These data suggest a relative increase in the expression of the molecular promoters of osteoclast formation and activity in #NOF bone, which may lead to the imbalance between bone formation and resorption associated with fragility fracture.

IL-6, LIF, and TNF-α regulation of GM-CSF inhibition of osteoclastogenesis in vitro

During pathological bone loss, factors that are both stimulatory and inhibitory for osteoclast differentiation are over-expressed. Despite the presence of inhibitory factors, osteoclast differentiation is significantly enhanced to bring about bone loss. To examine the hypothesis that stimulatory growth factors overcome the effects of inhibitory factors, we have examined the ability of IGF-I, IGF-II, IL-6, LIF, and TNF-alpha to overcome osteoclast differentiation inhibition by GM-CSF in vitro. Osteoclast numbers were significantly elevated by treatment with IGF-I, IGF-II, IL-6, LIF, or TNF-alpha alone whereas GM-CSF treatment of stromal cell and osteoclast co-cultures inhibited osteoclast formation. IL-6, LIF, or TNF-alpha, individually overcame GM-CSF inhibition whereas neither IGF-I nor IGF-II treatment overcame GM-CSF inhibition. Interestingly, GM-CSF addition with either IL-6 or TNF-alpha increased osteoclast numbers beyond that seen with either IL-6 or TNF-alpha alone. Combined treatment with TNF-alpha and IL-6 showed a significant increase in osteoclast numbers with GM-CSF addition. Examination of the impacts of these growth factors individually or in combinations on stromal cell M-CSF, RANKL, and OPG expression revealed a complex pattern involving alterations in the ratio of RANKL to OPG and/or M-CSF expression as candidate mechanisms of action.

Prostate cancer bone metastases promote both osteolytic and osteoblastic activity

Advanced prostate cancer is frequently accompanied by the development of metastasis to bone. In the past, prostate cancer bone metastases were characterized as being osteoblastic (i.e., increasing bone density) based on radiographs. However, emerging evidence suggests that development of prostate cancer bone metastases requires osteoclastic activity in addition to osteoblastic activity. The complexities of how prostate tumor cells influence bone remodeling are just beginning to be elucidated. Prostate cancer cells produce a variety of pro-osteoblastic factors that promote bone mineralization. For example, both bone morphogenetic proteins and endothelin-1 have well recognized pro-osteoblastic activities and are produced by prostate cancer cells. In addition to factors that enhance bone mineralization prostate cancer cells produced factors that promote osteoclast activity. Perhaps the most critical pro-osteoclastogenic factor produced by prostate cancer cells is receptor activator of NFkappaB ligand (RANKL), which has been shown to be required for the development of osteoclasts. Blocking RANKL results in inhibiting prostate cancer-induced osteoclastogenesis and inhibits development and progression of prostate tumor growth in bone. These findings suggest that targeting osteoclast activity may be of therapeutic benefit. However, it remains to be defined how prostate cancer cells synchronize the combination of osteoclastic and osteoblastic activity. We propose that as the bone microenvironment is changed by the developing cancer, this in turn influences the prostate cancer cells' balance between pro-osteoclastic and pro-osteoblastic activity. Accordingly, the determination of how the prostate cancer cells and bone microenvironment crosstalk are important to elucidate how prostate cancer cells modulate bone remodeling.

RANKL/RANK/osteoprotegerin system as novel therapeutic target in the treatment of primary bone tumors and osteolytic metastases

Primary bone tumors and cancers that metastasize to bone require osteoclastic activity to release tumor-supportive growth factors from bone tissue. A number of systemic and locally acting factors are known to influence osteoclast formation, fusion, activation, and survival. Recently, two critical extracellular regulators of osteoclast differentiation and activation have been identified: receptor activator of nuclear factor (NF-kappaB) ligand (RANKL) and osteoprotegerin (OPG). RANKL is a tumor necrosis factor (TNF)-related cytokine that stimulates osteoclast differentiation from hematopoietic precursor cells and activation of mature osteoclasts. RANKL activates its specific receptor, receptor activator of NF- kappaB (RANK), located on osteoclasts, chondrocytes and dendritic cells. Binding of the RANK ligand to its receptor and osteoclastogenesis are prevented by osteoprotegerin, a decoy receptor produced by osteoblasts and marrow stromal cells. The balance between RANKL and OPG is of major importance in bone homeostasis. Disorders of the RANKL/RANK/OPG system have been linked to several human diseases, including primary bone tumors skeletal metastases, and hypercalcemia of malignancy. The discovery and characterization of RANKL, RANK and OPG and subsequent studies have changed the concepts of bone metabolism and may form the basis of innovative therapeutic strategies. Novel treatment strategies for bone tumors are emerging based on blockade of the RANKL/RANK interaction. The advantage of these strategies is their potential to selectively target tumor cells. Combining these new strategies with currently available treatments such as chemotherapy and radiation therapy is under investigation, with promising results.

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.

Expression and localization of extracellular matrix metalloproteinase inducer in giant cell tumor of bone

Matrix metalloproteinases (MMPs) are regarded as a significant regulator in tumor invasion and metastasis. Previous studies have shown that extracellular matrix metalloproteinase inducer (EMMPRIN) in tumor cells induces the synthesis of MMPs. EMMPRIN is abundantly present on the surface of tumor cells and stimulate adjacent stromal cells to synthesize MMPs to induce tumor progression. Giant cell tumor (GCT) of bone is a benign but locally aggressive primary neoplasm of bone. The spindle-shaped mononuclear stromal cells are considered to be the tumor components of GCT, which are capable of inducing osteoclast formation by recruiting the circulating monocyte and macrophage. In this study, we proposed that EMMPRIN is associated with the biological progression and aggressiveness of GCT. We have conducted semi-quantitative RT-PCR to determine the correlation of EMMPRIN expression with the clinical stage of GCT. We have also examined the cellular localization of EMMPRIN in GCT using in-situ hybridization (ISH) and Immunohistochemistry (IH). The results showed that EMMPRIN was present in GCT and its mRNA levels were associated with the clinical stage of GCT. Higher expression level of EMMPRIN was observed in GCT with advanced stage (stage III). There was a great significance (P < 0.05) of EMMPRIN expression between stage I & II and stage III GCTs. Both ISH and IH demonstrated that EMMPRIN is present at the multinuclear osteoclast-like giant cells of GCT, with strong immunostaining on the cell membrane. The stromal-like tumor cells were also positively stained but the intensity was weaker. Interestingly, the production of EMMPRIN in osteoclast-like cells of GCT seems to be regulated by stromal-like tumor cells. Receptor activator of NF-kappaB ligand (RANKL), which has been previously shown to be produced by the stromal-like tumor cells for the recruitment of osteoclast-like giant cells in GCT, enhanced the expression of EMMPRIN mRNA during the differentiation of macrophage-like RAW(264.7) cells into osteoclasts. In short, our studies suggest that EMMPRIN may be an important regulatory factor involved in the biological behaviors of GCT.

Hepatocellular carcinoma with osteoclast-like giant cells: possibility of osteoclastogenesis by hepatocyte-derived cells

Giant cell tumor (GCT) of bone is a primary osteolytic tumor that is characterized by the formation of osteoclast-like giant cells. In addition to GCT of bone, extraskeletal GCT are known to be formed in several soft tissues. Giant cells in GCT of bone were suggested to be identical to osteoclasts, but the characterization of giant cells in extraskeletal GCT remains incomplete. In this study, a case of sarcomatoid hepatocellular carcinoma with osteoclast-like giant cells was analyzed. Immunohistochemistry revealed the expression of almost all markers of osteoclasts: tartrate-resistant acid phosphatase, CD68, CD51, CD54 and matrix metalloprotease-9, in osteoclast-like giant cells in the tumor. In situ hybridization revealed the expression of receptor activator of nuclear factor-kappa B (RANK) in the giant cells and receptor activator of nuclear factor-kappa B ligand (RANKL) in the tumor cells. The hepatic origin of the sarcomatoid hepatocellular carcinoma cells was confirmed by the expression of albumin. This is the first report suggesting that hepatocyte-derived cells possess the potential for osteoclastogenesis. In addition, these findings suggest that osteoclast-like cells in the hepatocellular carcinoma were formed by the same mechanism as osteoclastogenesis in bone.

Multinucleated giant cells in various forms of giant cell containing lesions of the jaws express features of osteoclasts

BACKGROUND

The nature and the mechanism involved in the formation of the multinucleated giant cells (MGCs) in various giant cell-containing lesions of the jaws are not fully understood. The aim of this study is to clarify the osteoclastic features of the MGCs in central giant cell granuloma (CGCG), peripheral giant cell granuloma (PGCG), cherubism, and aneurysmal bone cyst (ABC), and the mechanism underlying the interrelations between cellular components in the formation of the MGCs.

METHODS

Immunohistochemical study with a panel of antibodies including vacuolar H+-ATPase (V-ATPase), carbonic anhydrase II (CA II), Cathepsin K, matrix metalloproteinases-9 (MMP-9), CD68, and proliferating cell nuclear antigen (PCNA), and enzyme histochemical staining for tartarate-resistant acid phosphatase (TRAP) were applied on a total number of 53 cases of giant cell-containing lesions including CGCG (n = 34), PGCG (n = 6), cherubism (n = 7), and ABC (n = 6). In situ hybridization was also carried out to detect the mRNA expression of the receptor activator of NF-kappaB ligand (RANKL), a newly identified cytokine that is shown to be essential in the osteoclastogenesis, its receptor RANK (receptor activator of NF-kappaB ligand), and its decoy receptor OPG (osteoprotegerin) in these four types of lesions.

RESULTS

Immunohistochemical and enzyme histochemical studies showed that both the MGCs and a fraction of mononuclear cells in these lesions were strongly positive for TRAP, V-ATPase, CA II, Cathepsin K, MMP-9, and CD68, while the spindle-shaped mononuclear cells were positive for PCNA. The results with in situ hybridization indicated that RANKL mRNA was mainly expressed in the spindle mononuclear cells while OPG was extensively distributed in both the MGCs and the mononuclear cells. RANK mRNA was expressed in the MGCs and some round mononuclear cells.

CONCLUSIONS

These results suggest that MGCs in the four types of giant cell-containing lesions of the jaws show characteristics of the osteoclast phenotype. The mononuclear stromal cells, which show TRAP positively, may be the precursors of the MGCs. RANKL, OPG, and RANK expressed in these lesions may play important roles in the formation of the MGCs. The similar characteristics and mechanisms in the differentiation of MGCs in these lesions also suggest that there might be a similar kind of pathogenesis involved in the formation of the MGCs in these lesions

RANKL expression is related to the differentiation state of human osteoblasts

Human osteoblast phenotypes that support osteoclast differentiation and bone formation are not well characterized. Osteoblast differentiation markers were examined in relation to RANKL expression. RANKL expression was induced preferentially in immature cells. These results support an important link between diverse osteoblast functions. Cells of the osteoblast lineage support two apparently distinct functions: bone formation and promotion of osteoclast formation. The aim of this study was to examine the relationship between these phenotypes in human osteoblasts (NHBC), in terms of the pre-osteoblast marker, STRO-1, and the mature osteoblast marker, alkaline phosphatase (AP), and the expression of genes involved in osteoclast formation, RANKL and OPG. The osteotropic stimuli, 1alpha,25(OH)2vitamin D3 (vitD3) and dexamethasone, were found to have profound proliferative and phenotypic effects on NHBCs. VitD3 inhibited NHBC proliferation and increased the percentage of cells expressing STRO-1 over an extended culture period, implying that vitD3 promotes and maintains an immature osteogenic phenotype. Concomitantly, RANKL mRNA expression was upregulated and maintained in NHBC in response to vitD3. Dexamethasone progressively promoted the proliferation of AP-expressing cells, resulting in the overall maturation of the cultures. Dexamethasone had little effect on RANKL mRNA expression and downregulated OPG mRNA expression in a donor-dependent manner. Regression analysis showed that RANKL mRNA expression was associated negatively with the percentage of cells expressing AP (p < 0.01) in vitD3- and dexamethasone-treated NHBCs. In contrast, RANKL mRNA expression was associated positively with the percentage of STRO-1+ cells (p < 0.01). In NHBCs sorted by FACS based on STRO-1 expression (STRO-1bright and STRO-1dim populations), it was found that vitD3 upregulated the expression of RANKL mRNA preferentially in STRO-1bright cells. The results suggest that immature osteoblasts respond to osteotropic factors in a potentially pro-osteoclastogenic manner. Additionally, the dual roles of osteoblasts, in supporting osteoclastogenesis or forming bone, may be performed by the same lineage of cells at different stages of their maturation.

Receptor activator of NF-kappaB ligand (RANKL) is expressed in chondroblastoma: possible involvement in osteoclastic giant cell recruitment

AIMS

Chondroblastoma is a rare, locally aggressive bone tumour that causes osteolytic destruction at the epiphyseal end of the affected bone. It is possible that tumour cells may stimulate osteoclastogenesis and osteolytic destruction through the production of receptor activator of NF-kappaB ligand (RANKL), which is a key molecule essential for regulating osteoclast formation and activity. Therefore, the expression of RANKL at both the mRNA and the protein level was investigated in chondroblastoma tumour tissue obtained from patients.

METHODS

The expression of RANKL gene transcripts was analysed by the reverse transcription-polymerase chain reaction (RT-PCR), and the cellular localisation of RANKL mRNA and protein was demonstrated by means of in situ hybridisation and immunohistochemistry.

RESULTS

RT-PCR analysis indicated that RANKL mRNA was present in all chondroblastoma specimens and normal cancellous bone samples, but not in normal articular cartilage and chondrosarcoma tissues. In contrast, gene transcripts of osteoprotegerin (OPG), the decoy receptor of RANKL, were detected in all types of tissues. The chondroid origin of neoplastic mononuclear cells in chondroblastoma was confirmed by positive S-100 immunohistochemical staining. Both RANKL mRNA and protein were exclusively expressed in these neoplastic mononuclear cells.

CONCLUSIONS

These findings suggest that RANKL may be involved in the tumour cell induced recruitment of osteoclast-like cells and consequent osteolytic bone destruction in chondroblastoma.

Inflammatory T cells rapidly induce differentiation of human bone marrow stromal cells into mature osteoblasts

Activated T cells secrete multiple osteoclastogenic cytokines which play a major role in the bone destruction associated with rheumatoid arthritis. While the role of T cells in osteoclastogenesis has received much attention recently, the effect of T cells on osteoblast formation and activity is poorly defined. In this study, we investigated the hypothesis that in chronic inflammation activated T cells contribute to enhanced bone turnover by promoting osteoblastic differentiation. We show that T cells produce soluble factors that induce alkaline phosphatase activity in bone marrow stromal cells and elevated expression of mRNA for Runx2 and osteocalcin. This data indicate that T cell derived factors have the capacity to stimulate the differentiation of bone marrow stromal cells into the osteoblast phenotype. RANKL mRNA was undetectable under any conditions in highly purified bone marrow stromal cells. In contrast, RANKL was constitutively expressed in primary osteoblasts and only moderately up-regulated by activated T cell conditioned medium. Interestingly, both bone marrow stromal cells and osteoblasts expressed mRNA for RANK, which was strongly up-regulated in both cell types by activated T cell conditioned medium. Although, mRNA for the RANKL decoy receptor, osteoprotegerin, was also up-regulated by activated T cell conditioned medium, it's inhibitory effects may be mitigated by a simultaneous rise in the osteoprotegerin competitor TNF-related apoptosis-inducing ligand. Based on our data we propose that during chronic inflammation, T cells regulate bone loss by a dual mechanism involving both direct stimulation of osteoclastogenesis, by production of osteoclastogenic cytokines, and indirectly by induction of osteoblast differentiation and up-regulation of bone turnover via coupling.

Receptor activator for nuclear factor kappaB ligand and osteoprotegerin: regulators of bone physiology and immune responses/potential therapeutic agents and biochemical markers

The discovery of receptor activator for nuclear factor kappaB ligand (RANKL) and osteoprotegerin (OPG) as the fundamental factors in controlling osteoclast formation and activation has led to a greater understanding of bone biology over the past few years. Here we discuss the role of these molecules in immunology and skeletal remodelling and assess their involvement in diseases of bones and joints, including rheumatoid arthritis, Paget's disease, post-menopausal osteoporosis and malignant bone diseases. OPG has been identified as a potential anabolic agent for treating conditions in which there is net bone loss and is currently in Phase I clinical trials. This review examines the current evidence indicating that OPG increases bone mass, and discusses other possible beneficial effects of OPG, such as inhibition of tumour growth and relief from bone cancer pain. OPG can be measured in human serum, and numerous studies have suggested that increased or decreased serum concentrations of this molecule can indicate the existence of remodelling disorders. Here we discuss how abnormal serum OPG concentrations could potentially be used to indicate imbalances of bone resorption and formation. The possible applications of serum OPG concentration as a marker for non-skeletal disease conditions are also considered.

Giant cell tumor of bone

Giant cell tumor of bone is an enigmatic osseous neoplasm that is histologically benign but clinically shows local aggression and metastatic potential. The absence of clinical, radiographic, or pathologic features that are predictive of tumor behavior and patient outcome has resulted in recent attention to the pathobiology of giant cell tumor of bone. In this report, the clinicopathologic features of and current treatment approaches to giant cell tumor of bone are reviewed. Recent investigations of the specific role of the cell populations present in giant cell tumor of bone that influence tumor proliferation, bone resorption, and its clinical behavior are described.

Human osteoblasts are resistant to Apo2L/TRAIL-mediated apoptosis

Apo2 ligand (Apo2L/TRAIL) is a member of the tumor necrosis factor (TNF) cytokine family. Apo2L/TRAIL can selectively induce programmed cell death in transformed cells, although its wide tissue distribution suggests potential physiological roles. We have investigated the expression, in human osteoblast-like cells (NHBC), of Apo2L/TRAIL and the known Apo2L/TRAIL death receptors, DR4 and DR5, and the Apo2L/TRAIL decoy receptors, DcR-1, DcR-2, and osteoprotegerin (OPG). NHBC expressed abundant mRNA corresponding to each of these molecular species. Immunofluorescence staining demonstrated that Apo2L/TRAIL protein was abundant within the cytoplasm of NHBC and OPG was strongly expressed at the cell surface. DR5 and DcR-2 were present in the cell membrane and cytoplasm and DcR-1 was confined to the nucleus. DR4 staining was weak. Neither Apo2L/TRAIL alone, nor in combination with chemotherapeutic agents of clinical relevance to treatment of osteogenic sarcoma, induced cell death in NHBC, as assessed morphologically and by activation of caspase-3. In contrast, the human osteogenic sarcoma cell lines, BTK-143 and G-292, were sensitive to exogenous Apo2L/TRAIL alone, and to the combined effect of Apo2L/TRAIL/cisplatin and Apo2L/TRAIL/doxorubicin treatments, respectively. In NHBC, we observed strong associations between the levels of mRNA corresponding to the pro-apoptotic molecules, Apo2L/TRAIL, DR4, and DR5, and those corresponding to pro-survival molecules, DcR-1, DcR-2, OPG, and FLIP, suggesting that the balance between pro-survival and pro-apoptotic molecules is a mechanism by which NHBC can resist Apo2L/TRAIL-mediated apoptosis. In contrast, osteogenic sarcoma cells had low or absent levels of DcR-1 and DcR-2. These results provide a foundation to explore the role of Apo2L/TRAIL in osteoblast physiology. In addition, they predict that therapeutic use of recombinant Apo2L/TRAIL, in combination with chemotherapeutic agents to treat skeletal malignancies, would have limited toxic effects on normal osteoblastic cells.

Immunohistochemical study of receptor activator of nuclear factor kappa-B ligand (RANK-L) in human osteolytic bone tumors

BACKGROUND AND OBJECTIVES

Osteolytic bone tumors produce intercellular signaling proteins that regulate bone remodeling by altering the rates of osteoclast and osteoblast differentiation and activity. This report examines osteolytic bone tumor expression of receptor activator of nuclear factor B-ligand (RANK-L), a cytokine that is arguably the most critical regulator of osteoclast differentiation and activation.

METHODS

This prospective immunohistochemical study examined RANK-L expression in frozen tissues from sixteen surgical specimens of patients who underwent surgery for the treatment of osteolytic bone tumors between 1999 and 2000.

RESULTS

RANK-L was positive in 13 of the 16 cases. Primary benign bone tumors, primary malignant bone tumors, and metastasis to bone were positive for RANK-L.

CONCLUSIONS

The cells in some, but not all, osteolytic tumors produce the cytokine RANK-L. Further study is necessary to determine in which specific tumors RANK-L is the cytokine responsible for increased osteoclastic activity, and to develop possible therapeutic use of RANK-L antagonists such as osteoprotegerin (OPG).

Receptor activator of nuclear factor-kappaB ligand and osteoprotegerin: potential implications for the pathogenesis and treatment of malignant bone diseases

BACKGROUND

The current review summarizes the roles of the ligand, receptor activator of nuclear factor-kappaB ligand (RANKL), its receptor, receptor activator of nuclear factor-kappaB (RANK), and its decoy receptor, osteoprotegerin (OPG), on osteoclast biology and bone resorption. Furthermore, it highlights the impact of these compounds on the pathogenesis of malignant bone diseases, including tumor metastasis, humoral hypercalcemia of malignancy, and multiple myeloma. Finally, the authors discuss the therapeutic potential of OPG in the management of malignancies involving the skeleton.

METHODS

After its discovery and cloning, the biologic effects of RANKL, RANK, and OPG have been characterized by in vitro experiments and in vivo studies. The generation of knock-out mice and transgenic mice has produced animal models with absent or excessive production of these cytokine components that display opposite abnormal skeletal phenotypes (osteoporosis or osteopetrosis). The potential effect of RANKL and OPG has been assessed by evaluating these compounds in various animal models of metabolic and malignant bone disease and by administering OPG to humans.

RESULTS

Abnormal bone resorption due to local or systemic stimulation of osteoclast differentiation and activation is a hallmark of various benign and malignant bone diseases. RANKL, RANK, and OPG form an essential cytokine system that is capable of regulating all aspects of osteoclast functions, including proliferation, differentiation, fusion, activation, and apoptosis. The balance of bone resorption depends on the local RANKL-to-OPG ratio, which is enhanced in bone metastases and humoral hypercalcemia of malignancy. The exogenous administration of OPG to tumor-bearing animals corrects the increased RANKL-to-OPG ratio, and reverses the skeletal complications of malignancies.

CONCLUSIONS

Abnormalities of the RANKL/OPG system have been implicated in the pathogenesis of various primary and secondary bone malignancies. The systemic administration of OPG appears to be a potent novel therapeutic agent for treatment of these disorders.

The ratio of messenger RNA levels of receptor activator of nuclear factor kappaB ligand to osteoprotegerin correlates with bone remodeling indices in normal human cancellous bone but not in osteoarthritis

The determinants of cancellous bone turnover and trabecular structure are not understood in normal bone or skeletal disease. Bone remodeling is initiated by osteoclastic resorption followed by osteoblastic formation of new bone. Receptor activator of nuclear factor kappaB ligand (RANKL) is a newly described regulator of osteoclast formation and function, the activity of which appears to be a balance between interaction with its receptor RANK and with an antagonist binding protein osteoprotegerin (OPG). Therefore, we have examined the relationship between the expression of RANKL, RANK, and OPG and indices of bone structure and turnover in human cancellous bone from the proximal femur. Bone samples were obtained from individuals with osteoarthritis (OA) at joint replacement surgery and from autopsy controls. Histomorphometric analysis of these samples showed that eroded surface (ES/BS) and osteoid surface (OS/BS) were positively associated in both control (p < 0.001) and OA (p < 0.02), indicating that the processes of bone resorption and bone formation remain coupled in OA, as they are in controls. RANKL, OPG, and RANK messenger RNA (mRNA) were abundant in human cancellous bone, with significant differences between control and OA individuals. In coplotting the molecular and histomorphometric data, strong associations were found between the ratio of RANKL/OPG mRNA and the indices of bone turnover (RANKL/OPG vs. ES/BS: r = 0.93, p < 0.001; RANKL/OPG vs. OS/BS: r = 0.80, p < 0.001). These relationships were not evident in trabecular bone from severe OA, suggesting that bone turnover may be regulated differently in this disease. We propose that the effective concentration of RANKL is related causally to bone turnover.

Osteoprotegerin inhibits osteoclast formation and bone resorbing activity in giant cell tumors of bone

Osteolysis is a common complication of tumors that arise in, or metastasize to, bone. The recent discovery of key regulators of osteoclast formation and activity, including receptor activator of nuclear factor of kappaB ligand (RANKL), RANK, and osteoprotegerin (OPG), may facilitate new treatment regimes for certain tumors associated with excessive bone loss. We recently showed that the stromal cells of osteolytic giant cell tumors (GCT) of bone express high levels of mRNA encoding RANKL, relative to mRNA for the RANKL antagonist, OPG, compared with the expression patterns of other lytic and nonlytic bone tumors. In this study, we found that expression of RANKL and OPG mRNA continued by the stromal element of these tumors in a constitutive manner for at least 9 days in the absence of giant cells. Immunostaining of unfractionated GCT cultured in vitro revealed punctate cytoplasmic/membranous staining for RANKL and both cytoplasmic and extracellular matrix staining for OPG in stromal cells. Giant cells (osteoclasts) were negative for RANKL staining, but stained brightly for cytoplasmic OPG protein. We also investigated the functional relevance of these molecules for GCT osteolysis by adding recombinant OPG and RANKL to cultured GCT cells. We found that OPG treatment potently and dose-dependently inhibited resorption of bone slices by GCT, and could also inhibit the formation of multinucleated osteoclasts from precursors within the GCT. These effects of OPG were reversed by stoichiometric concentrations of exogenous RANKL. These data indicate that both the processes of osteoclast formation and activation in GCT are promoted by RANKL. Therefore, GCT represent a paradigm for the direct stimulation of osteoclast formation and activity by tumor stromal cells, in contrast to the mechanisms described for osteolytic breast tumors and multiple myeloma. The demonstration of these relationships is important in developing approaches to limit tumor-induced osteolysis.