PTHrP increases RANKL expression by stromal cells from giant cell tumor of bone

The RANK/RANKL/OPG system and tumor bone metastasis: Potential mechanisms and therapeutic strategies

With the markedly increased diagnosis and incidence of cancer in the population, tumor bone metastasis has become a frequent event in tumor patients. Healthy bone integrity is maintained by a delicate balance between bone formation and bone resorption. Unfortunately, many tumors, such as prostate and breast, often metastasize to the bone, and the alterations to the bone homeostasis can particularly favor tumor homing and consequent osteolytic or osteoblastic lesions. Receptor activator of NF-κB ligand (RANKL), its receptor RANK, and osteoprotegerin (OPG) are involved in the regulation of the activation, differentiation, and survival of osteoclasts, which play critical roles in bone metastasis formation. High rates of osteoclastic bone resorption significantly increase fracture risk, cause severe bone pain, and contribute to homing tumor cells in bone and bone marrow. Consequently, suppression of the RANK/RANKL/OPG system and osteoclastic activity can not only ameliorate bone resorption but may also prevent tumor bone metastases. This review summarizes the important role of the RANK/RANKL/OPG system and osteoclasts in bone homeostasis and its effect on tumor bone metastasis and discusses therapeutic strategies based on RANKL inhibition.

Osteoblasts and osteoclasts: an important switch of tumour cell dormancy during bone metastasis

Bone metastasis occurs when tumour cells dissociate from primary tumours, enter the circulation (circulating tumour cells, CTCs), and colonize sites in bone (disseminated tumour cells, DTCs). The bone marrow seems to be a particularly dormancy-inducing environment for DTCs, yet the mechanisms of dormancy initiation, reactivation, and interaction within the bone marrow have to be elucidated. Intriguingly, some evidence has suggested that dormancy is a reversible state that is switched 'on' or 'off' depending on the presence of various bone marrow resident cells, particularly osteoclasts and osteoblasts. It has become clear that these two cells contribute to regulating dormant tumour cells in bone both directly (interaction) and indirectly (secreted factors). The involved mechanisms include TGFβ signalling, the Wnt signalling axis, the Notch2 pathway, etc. There is no detailed review that specifically focuses on ascertaining the dynamic interactions between tumour cell dormancy and bone remodelling. In addition, we highlighted the roles of inflammatory cytokines during this 'cell-to-cell' communication. We also discussed the potential clinical relevance of remodelling the bone marrow niche in controlling dormant tumour cells. Understanding the unique role of osteoclasts and osteoblasts in regulating tumour dormancy in bone marrow will provide new insight into preventing and treating tumour bone metastasis.

Involvement of Met receptor pathway in aggressive behavior of colorectal cancer cells induced by parathyroid hormone-related peptide

BACKGROUND

Parathyroid hormone-related peptide (PTHrP) plays a key role in the development and progression of many tumors. We found that in colorectal cancer (CRC) HCT116 cells, the binding of PTHrP to its receptor PTHR type 1 (PTHR1) activates events associated with an aggressive phenotype. In HCT116 cell xenografts, PTHrP modulates the expression of molecular markers linked to tumor progression. Empirical evidence suggests that the Met receptor is involved in the development and evolution of CRC. Based on these data, we hypothesized that the signaling pathway trigged by PTHrP could be involved in the transactivation of Met and consequently in the aggressive behavior of CRC cells.

AIM

To elucidate the relationship among PTHR1, PTHrP, and Met in CRC models.

METHODS

For in vitro assays, HCT116 and Caco-2 cells derived from human CRC were incubated in the absence or presence of PTHrP (1-34) (10^-8 M). Where indicated, cells were pre-incubated with specific kinase inhibitors or dimethylsulfoxide, the vehicle of the inhibitors. The protein levels were evaluated by Western blot technique. Real-time polymerase chain reaction (RT-qPCR) was carried out to determine the changes in gene expression. Wound healing assay and morphological monitoring were performed to evaluate cell migration and changes related to the epithelial-mesenchymal transition (EMT), respectively. The number of viable HCT116 cells was counted by trypan blue dye exclusion test to evaluate the effects of irinotecan (CPT-11), oxaliplatin (OXA), or doxorubicin (DOXO) with or without PTHrP. For in vivo tests, HCT116 cell xenografts on 6-wk-old male N:NIH (S)_nu mice received daily intratumoral injections of PTHrP (40 μg/kg) in 100 μL phosphate-buffered saline (PBS) or the vehicle (PBS) as a control during 20 d. Humanitarian slaughter was carried out and the tumors were removed, weighed, and fixed in a 4% formaldehyde solution for subsequent treatment by immunoassays. To evaluate the expression of molecular markers in human tumor samples, we studied 23 specimens obtained from CRC patients which were treated at the Hospital Interzonal de Graves y Agudos Dr. José Penna (Bahía Blanca, Buenos Aires, Argentina) and the Hospital Provincial de Neuquén (Neuquén, Neuquén, Argentina) from January 1990 to December 2007. Seven cases with normal colorectal tissues were assigned to the control group. Tumor tissue samples and clinical histories of patients were analyzed. Paraffin-embedded blocks from primary tumors were reviewed by hematoxylin-eosin staining technique; subsequently, representative histological samples were selected from each patient. From each paraffin block, tumor sections were stained for immunohistochemical detection. The statistical significance of differences was analyzed using proper statistical analysis. The results were considered statistically significant at P < 0.05.

RESULTS

By Western blot analysis and using total Met antibody, we found that PTHrP regulated Met expression in HCT116 cells but not in Caco-2 cells. In HCT116 cells, Met protein levels increased at 30 min (P < 0.01) and at 20 h (P < 0.01) whereas the levels diminished at 3 min (P < 0.05), 10 min (P < 0.01), and 1 h to 5 h (P < 0.01) of PTHrP treatment. Using an active Met antibody, we found that where the protein levels of total Met decreased (3 min, 10 min, and 60 min of PTHrP exposure), the status of phosphorylated/activated Met increased (P < 0.01) at the same time, suggesting that Met undergoes proteasomal degradation after its phosphorylation/activation by PTHrP. The increment of its protein level after these decreases (at 30 min and 20 h) suggests a modulation of Met expression by PTHrP in order to improve Met levels and this idea is supported by our observation that the cytokine increased Met mRNA levels at least at 15 min in HCT116 cells as revealed by RT-qPCR analysis (P < 0.05). We then proceeded to evaluate the signaling pathways that mediate the phosphorylation/ activation of Met induced by PTHrP in HCT116 cells. By Western blot technique, we observed that PP1, a specific inhibitor of the activation of the proto-oncogene protein tyrosine kinase Src, blocked the effect of PTHrP on Met phosphorylation (P < 0.05). Furthermore, the selective inhibition of the ERK 1/2 mitogen-activated protein kinase (ERK 1/2 MAPK) using PD98059 and the p38 MAPK using SB203580 diminished the effect of PTHrP on Met phosphorylation/activation (P < 0.05). Using SU11274, the specific inhibitor of Met activation, and trypan blue dye exclusion test, Western blot, wound healing assay, and morphological analysis with a microscope, we observed the reversal of cell events induced by PTHrP such as cell proliferation (P < 0.05), migration (P < 0.05), and the EMT program (P < 0.01) in HCT116 cells. Also, PTHrP favored the chemoresistance to CPT-11 (P < 0.001), OXA (P < 0.01), and DOXO (P < 0.01) through the Met pathway. Taken together, these findings suggest that Met activated by PTHrP participates in events associated with the aggressive phenotype of CRC cells. By immunohistochemical analysis, we found that PTHrP in HCT116 cell xenografts enhanced the protein expression of Met (0.190 ± 0.014) compared to tumors from control mice (0.110 ± 0.012; P < 0.05) and of its own receptor (2.27 ± 0.20) compared to tumors from control mice (1.98 ± 0.14; P < 0.01). Finally, assuming that the changes in the expression of PTHrP and its receptor are directly correlated, we investigated the expression of both Met and PTHR1 in biopsies of CRC patients by immunohistochemical analysis. Comparing histologically differentiated tumors with respect to those less differentiated, we found that the labeling intensity for Met and PTHR1 increased and diminished in a gradual manner, respectively (P < 0.05).

CONCLUSION

PTHrP acts through the Met pathway in CRC cells and regulates Met expression in a CRC animal model. More basic and clinical studies are needed to further evaluate the PTHrP/Met relationship.

Parathyroid hormone-related protein (PTHrP)-dependent modulation of gene expression signatures in cancer cells

PTHrP is encoded by PTHLH gene which can generate by alternative promoter usage and splicing mechanisms at least three mature peptides of 139, 141 and 173 amino acids with distinct carboxy terminus. PTHrP may undergo proteolytic processing into smaller bioactive forms, comprising an amino terminus peptide, which is the mediator of the "classical" PTH-like effect, as well as midregion and carboxy terminus peptides that act as multifaceted critical regulator of proliferation, differentiation and apoptosis via the reprogramming of gene expression in normal and neoplastic cells. Moreover, a nuclear/nucleolar localization signal sequence is present in the [87-107] domain allowing PTHrP nuclear import and "intracrine" effect additional to the autocrine/paracrine one. Within the large number of data available in the literature on PTHrP bioactivities, the goal of this chapter is to pick up selected studies that report the detection of molecular signatures of cancer cell exposure to PTHrP, either as full-length protein or discrete peptides, demonstrated by individual gene or whole genome expression profiling, briefly recapitulating the biological implications associated with the specific gene activation or silencing.

Involvement of parathyroid hormone-related peptide in the aggressive phenotype of colorectal cancer cells

Colorectal cancer (CRC) remains one of the leading causes of mortality from malignant diseases worldwide. In general terms, CRC presents high heterogeneity due to the influence of different genetic and environmental factors; also, the neoplastic cells are strongly influenced by the extracellular matrix and several surrounding cells, known together as the tumor microenvironment (TME). Bidirectional communication takes place between the tumor and the TME through the release of autocrine and paracrine factors. Parathyroid hormone-related peptide (PTHrP) is a cytokine secreted by a wide variety of tissues and is able to regulate several cellular functions both in physiological as well as in pathological processes. It exerts its effects as a paracrine/autocrine factor, although its mode of action is mainly paracrine. It has been shown that this peptide is expressed by several tumors and that the tumor secretion of PTHrP is responsible for the malignant humoral hypercalcemia. Eight years ago, when our research group started studying PTHrP effects in the experimental models derived from intestinal tumors, the literature available at the time addressing the effects of PTHrP on colorectal tumors was limited, and no articles had been published regarding to the paracrine action of PTHrP in CRC cells. Based on this and on our previous findings regarding the role of PTH in CRC cells, our purpose in recent years has been to explore the role of PTHrP in CRC. We analyzed the behavior of CRC cells treated with exogenous PTHrP, focalizing in the study of the following events: Survival, cell cycle progression and proliferation, migration, chemoresistance, tumor-associated angiogenesis, epithelial to mesenchymal transition program and other events also associated with invasion, such us the induction of cancer stem cells features. This work summarizes the major findings obtained by our investigation group using in vitro and in vivo CRC models that evidence the participation of PTHrP in the acquisition of an aggressive phenotype of CRC cells and the molecular mechanisms involved in these processes. Recently, we found that this cytokine induces this malignant behavior not only by its direct action on these intestinal cells but also through its influence on cells derived from TME, promoting a communication between CRC cells and surrounding cells that contributes to the molecular and morphological changes observed in CRC cells. These investigations establish the basis for our next studies in order to address the clinical applicability of our findings. Recognizing the factors and mechanisms that promote invasion in CRC cells, evasion to the cytotoxic effects of current CRC therapies and thus metastasis is decisive for the identification of new markers with the potential to improve early diagnosis and/or to predict prognosis, to predetermine drug resistance and to provide treatment guidelines that include targeted therapies for this disease.

Role of SPARC in the epithelial-mesenchymal transition induced by PTHrP in human colon cancer cells

Parathyroid hormone-related peptide (PTHrP) exerts its effects on cells derived from colorectal cancer (CRC) and tumor microenvironment and is involved in processes requiring the epithelial-mesenchymal transition (EMT). Here, we report that PTHrP modulates factors expression and morphological changes associated with EMT in HCT116 cells from CRC. PTHrP increased the protein expression of SPARC, a factor involved in EMT, in HCT116 cells but not in Caco-2 cells also from CRC but with less aggressiveness. PTHrP also increased SPARC expression and its subsequent release from endothelial HMEC-1 cells. The conditioned media of PTHrP-treated HMEC-1 cells induced early changes related to EMT in HCT116 cells. Moreover, SPARC treatment on HCT116 cells potentiated PTHrP modulation in E-cadherin expression and cell migration. In vivo PTHrP also increased SPARC expression and decreased E-cadherin expression. These results suggest a novel PTHrP action on CRC progression involving the microenvironment in the modulation of events associated with EMT.

Effects of alcohol and nicotine consumption on the development of apical periodontitis in rats: a correlative micro-computed tomographic, histological and immunohistochemical study

AIM

To evaluate the effects of alcohol and nicotine, when used alone or simultaneously, in the development of apical periodontitis induced in rats, using a correlative analytic approach with micro-CT, histological and immunohistochemical analysis.

METHODOLOGY

Twenty-eight male Wistar rats were arranged into four groups: Control, Nicotine, Alcohol and Alcohol + Nicotine. The alcohol groups were exposed to self-administration of a 25% alcohol solution, whilst the other groups drunk only filtered water. The nicotine groups received daily intraperitoneal injections of a solution with 0.19 μL of nicotine per mL, whilst the other groups received saline solution. The pulps of the left mandibular first molars were exposed for 28 days to induce periapical lesions. Throughout the experiment, drug administration was maintained, and the animals had their weight and solid and liquid consumption measured. After euthanasia, the mandibles were removed and the area, volume and major diameter of the periapical lesions were measured using micro-computed tomography images. The samples were submitted to histopathological evaluation and immunohistochemistry for RANKL and PTHrP. Statistical analysis was undertaken with a significance level of 5%. Nonparametric data were analysed using the Kruskal-Wallis test followed by Dunn's test, whilst one-way anova followed by Tukey's test was performed for parametric data.

RESULTS

The alcohol groups had lower solid and liquid consumption and gained less weight when compared to the nonalcohol groups (P < 0.05). The Alcohol + Nicotine group had lesions with significantly larger volume and area when compared to the other groups (P < 0.05), whilst the Alcohol or Nicotine groups had significantly larger lesions than the control group (P < 0.05). There was no significant difference in the largest diameter of the lesions amongst groups (P > 0.05). The experimental groups had greater inflammatory response scores than the control group (P < 0.05), and the representative samples had more pronounced immunoreaction against RANKL and PTHrP antibodies.

CONCLUSIONS

Alcohol and nicotine consumption exacerbated the inflammatory response and the development of periradicular lesions in rats. The association of both substances enhanced their harmful effects.

Short-term RANKL exposure initiates a neoplastic transcriptional program in the basal epithelium of the murine salivary gland

Although salivary gland cancers comprise only ∼3-6% of head and neck cancers, treatment options for patients with advanced-stage disease are limited. Because of their rarity, salivary gland malignancies are understudied compared to other exocrine tissue cancers. The comparative lack of progress in this cancer field is particularly evident when it comes to our incomplete understanding of the key molecular signals that are causal for the development and/or progression of salivary gland cancers. Using a novel conditional transgenic mouse (K5:RANKL), we demonstrate that Receptor Activator of NFkB Ligand (RANKL) targeted to cytokeratin 5-positive basal epithelial cells of the salivary gland causes aggressive tumorigenesis within a short period of RANKL exposure. Genome-wide transcriptomic analysis reveals that RANKL markedly increases the expression levels of numerous gene families involved in cellular proliferation, migration, and intra- and extra-tumoral communication. Importantly, cross-species comparison of the K5:RANKL transcriptomic dataset with The Cancer Genome Atlas cancer signatures reveals the strongest molecular similarity with cancer subtypes of the human head and neck squamous cell carcinoma. These studies not only provide a much needed transcriptomic resource to mine for novel molecular targets for therapy and/or diagnosis but validates the K5:RANKL transgenic as a preclinical model to further investigate the in vivo oncogenic role of RANKL signaling in salivary gland tumorigenesis.

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

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

Bispecific Antibody Binding To RANKL and Osteonectin with Enhanced Localization to the Bone

Therapeutics reducing bone turnover, such as denosumab (Dmab), an anti-RANKL antibody, can provide treatments for patients with bone destruction. However, some patients with osteoporosis or localized primary bone tumors and many patients with various types of bone-metastatic cancer display unsatisfactory responses to Dmab. For achieving greater efficiency of RANKL neutralization in the bone microenvironment by enhancing the distribution of Dmab to the bone, we reengineered Dmab by fusing with single-chain variable fragments of an antibody specific for osteonectin (On), which is abundantly expressed in osseous tissues. The bispecific antibody, Dmab-FvOn, showed a similar activity as Dmab in inhibiting RANKL as examined in an osteoclast differentiation assay. When administered to mice, Dmab-FvOn was found to localize in increased proportions at the endosteum of the bone where osteonectin is abundant. Our study suggests that by linking anti-RANKL with an osteonectin-targeting moiety, a greater proportion of the therapeutic effector can be distributed in the bone. Future studies are needed to investigate whether the bispecific antibody can achieve higher therapeutic efficacy and lower toxicity.

Clival giant cell tumor - A rare case report and review of literature with respect to current line of management

Giant-cell tumor (GCT) involving the skull base is rare. Sphenoid bone is the most commonly involved bone followed by petrous temporal bone. Histopathology and radiological features of these lesions are similar to GCT involving bone elsewhere. Unlike other sites, skull base is not an ideal site for the radical surgery. Hence adjuvant treatment has pivotal role. Radiation therapy with intensity-modulated radiation therapy, stereotactic radiosurgery or chemotherapy with adriamycin are promising as described in some case reports. Bisphosphonates showed good control in local recurrence. In vitro studies with Zolendronate loaded bone cement and phase 2 trials of Denosumab showed hopeful results, may be useful in future.

Giant-cell tumor of bone: treatment options and role of denosumab

Giant-cell tumor of bone is a rare, locally aggressive tumor that typically occurs in the bones of skeletally mature young adults in their second to fourth decades. Traditionally, surgery has been the mainstay of therapy for this disease, but the disease can recur even with optimal procedures. Furthermore, it may occur in locations where a surgical approach would be morbid. The maturation of the understanding of the role of the receptor activator of nuclear factor-κB ligand (RANKL) in the pathophysiology of giant-cell tumor of bone has led to the use of denosumab, a monoclonal antibody against RANKL, in this disease. In 2013, the US Food and Drug Administration approved denosumab for use in patients with recurrent/unresectable/metastatic giant-cell tumor of bone or for patients in whom surgery would be morbid.

MiR-126-5p regulates osteolysis formation and stromal cell proliferation in giant cell tumor through inhibition of PTHrP

Parathyroid hormone-related protein (PTHrP) has been identified to play a crucial role in osteolysis formation and stromal cell (GCTSC) proliferation in giant cell tumor (GCT). MiR-126-5p is an intronic miRNA identified as tumor suppressor in many tumors, but its role in GCT is poorly understood. We found that miR-126-5p was decreased in GCT and could directly regulate PTHrP expression. Furthermore, miR-126-5p could control osteoclast (OC) differentiation, GCTSC proliferation and osteolysis formation in GCT through negative regulation of PTHrP. Thus, these results suggest that miR-126-5p could directly target PTHrP and have a tumor suppressor function in GCT.

Huge giant cell tumor of the sacrum: A case report

The current report describes the case of a 29-year-old female with a sacral giant cell tumor (GCT) during pregnancy. Originally, the patient presented with severe pain in the lumbosacral region, radiating posterolaterally from the lumbar spine into the bilateral thigh and subsequently, into the bilateral crus posterolaterally. Plain X-rays, computed tomography and magnetic resonance imaging showed osteolytic destruction of the sacrococcygeal bones and a huge soft-tissue mass with features of a chordoma. The patient underwent a partial en bloc sacrectomy (partial S1 and completely below) and curettage for tumors located at the sacroiliac joint and underlying left ilium, with bilateral internal iliac arteries ligated to control intraoperative hemorrhage. The patient’s bilateral S2 nerve roots were killed. The diagnosis of conventional GCT was determined based on the histopathological examination of the resected specimen. Urinary and bowel functions were recovered by exercising.

Retrospective analysis of patients with rare-site and metastatic giant cell tumor

A giant cell tumor occurs mainly in the proximal tibia, humerus, distal radius bone and the pelvic bone. It is rarely observed in such sites as the ribs and the temporal bone. The condition is primarily treated with surgical excision and functional reconstruction. The effect of chemotherapy on lung metastases and locally advanced giant cell tumors has remained unknown. We collected and analyzed the data of six patients with rare giant cell tumors located in the head and neck patients. After an average follow-up of 42.6 months after surgery (14 to 90 months), no local recurrence or metastasis was observed. We also collected and analyzed the data of five patients with metastatic giant cell tumors who were undergoing surgery for the primary tumor before; of three patients who had experienced multiple chemotherapy cycles, one had spontaneous regression, and one survived for long timer despite progression. The other two patients had their major metastatic lesions resected by surgery, and presented long-term survival during the follow up. In addition, this study reports one patient with locally advanced giant cell tumor of the rib, who has undergone successful surgical resection following two cycles of chemotherapy with ifosfamide and liposomal doxorubicin. Complete resection of the lesion at the head and neck is the key to relapse-free survival. The prognosis of lung metastases in patients with giant cell tumors is relatively satisfying. Neoadjuvant chemotherapy is also conducive to the surgery for locally advanced lesions and improvement of the quality of life.

Targeting the giant cell tumor stromal cell: functional characterization and a novel therapeutic strategy

Giant cell tumor of bone (GCTB) is a benign, locally destructive neoplasm, with tumors comprised of mesenchymal fibroblast-like stromal cells; monocytic, mononuclear cells of myeloid lineage; and the characteristic osteoclast-like, multinucleated giant cells. Hampering the study of the complex interaction of its constituent cell types is the propensity of longstanding, repeatedly passaged cell cultures to undergo phenotypic alteration and loss of osteoclast-inducing capacities. In this study, we employed a novel, single-step technique to purify freshly harvested stromal cells using a CD14-negative selection column. Using 9 freshly harvested GCTB specimens and the purified stromal cell component, we performed analyses for markers of osteoblast lineage and analyzed the capacity of the stromal cells to undergo osteoblastic differentiation and induce osteoclastogenesis in co-cultures with monocytic cells. Successful purification of the CD14-negative stromal cells was confirmed via flow cytometric analysis and immunocytochemistry. Osteogenic media upregulated the expression of osteocalcin, suggesting an osteoblastic lineage of the GCTB stromal cells. The effects of the Wnt pathway agonist, SB415286, and recombinant human bone morphogenetic protein (BMP)-2 on osteoblastogenesis varied among samples. Notably, osteogenic media and SB415286 reversed the receptor activator of NF-κB ligand (RANKL)/osteoprotegerin (OPG) expression ratio resulting in diminished osteoclastogenic capacity. Recombinant human BMP2 had the opposite effect, resulting in enhanced and sustained support of osteoclastogenesis. Targeting the giant cell tumor stromal cell may be an effective adjunct to existing anti-resorptive strategies.

Giant cell tumor of bone: a basic science perspective

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

Transcriptomic and proteomic analyses in bone tumor cells: Deciphering parathyroid hormone-related protein regulation of the cell cycle and apoptosis

Giant cell tumor of bone (GCT) is an aggressive skeletal tumor characterized by local bone destruction, high recurrence rates, and metastatic potential. Previous works in our laboratory, including functional assays, have shown that neutralization of parathyroid hormone-related protein (PTHrP) in the cell environment inhibits cell proliferation and induces cell death in GCT stromal cells, indicating a role for PTHrP in cell propagation and survival. The objective of this study was to investigate the global gene and protein expression patterns of GCT cells in order to identify the underlying pathways and mechanisms of neoplastic proliferation provided by PTHrP in the bone microenvironment. Primary stromal cell cultures from 10 patients with GCT were used in this study. Cells were exposed to optimized concentrations of either PTHrP peptide or anti-PTHrP neutralizing antiserum and were analyzed with both cDNA microarray and proteomic microarray assays in triplicate. Hierarchical clustering and principal component analyses confirmed that counteraction of PTHrP in GCT stromal cells results in a clear-cut gene expression pattern distinct from all other treatment groups and the control cell line human fetal osteoblast (hFOB). Multiple bioinformatics tools were used to analyze changes in gene/protein expression and identify important gene ontologies and pathways common to this anti-PTHrP-induced regulatory gene network. PTHrP neutralization interferes with multiple cell survival and apoptosis signaling pathways by triggering both death receptors and cell cycle-mediated apoptosis, particularly via the caspase pathway, TRAIL pathway, JAK-STAT signaling pathway, and cyclin E/CDK2-associated G1/S cell cycle progression. These findings indicate that PTHrP neutralization exhibits anticancer potential by regulating cell-cycle progression and apoptosis in bone tumor cells, with the corollary being that PTHrP is a pro-neoplastic factor that can be targeted in the treatment of bone tumors.