Molecular pathways: osteoclast-dependent and osteoclast-independent roles of the RANKL/RANK/OPG pathway in tumorigenesis and metastasis

Prostate cancer metastasis: roles of recruitment and reprogramming, cell signal network and three-dimensional growth characteristics

Prostate cancer (PCa) metastasizes to bone and soft tissues, greatly decreasing quality of life, causing bone pain, skeletal complications, and mortality in PCa patients. While new treatment strategies are being developed, the molecular and cellular basis of PCa metastasis and the “cross-talk” between cancer cells and their microenvironment and crucial cell signaling pathways need to be successfully dissected for intervention. In this review, we introduce a new concept of the mechanism of PCa metastasis, the recruitment and reprogramming of bystander and dormant cells (DCs) by a population of metastasis-initiating cells (MICs). We provide evidence that recruited and reprogrammed DCs gain MICs phenotypes and can subsequently metastasize to bone and soft tissues. We show that MICs can also recruit and reprogram circulating tumor cells (CTCs) and this could contribute to cancer cell evolution and the acquisition of therapeutic resistance. We summarize relevant molecular signaling pathways, including androgen receptors (ARs) and their variants and growth factors (GFs) and cytokines that could contribute to the predilection of PCa for homing to bone and soft tissues. To understand the etiology and the biology of PCa and the effectiveness of therapeutic targeting, we briefly summarize the animal and cell models that have been employed. We also report our experience in the use of three-dimensional (3-D) culture and co-culture models to understand cell signaling networks and the use of these attractive tools to conduct drug screening exercises against already-identified molecular targets. Further research into PCa growth and metastasis will improve our ability to target cancer metastasis more effectively and provide better rationales for personalized oncology.

Role of the RANK/RANKL pathway in breast cancer

The discovery of the OPG/RANK/RANKL pathway two decades ago has initiated novel insights into regulation of bone formation. More recently this pathway has been found to be also relevant in osteoclastic-independent mechanisms, mainly in mammary physiology and breast cancer. RANKL/RANK function is essential for epithelial cell proliferation and cellular survival as well as lobulo-alveolar development. The endogenous OPG functions as a soluble decoy receptor, binding the cytokine RANKL to prevent RANKL from activating its receptor RANK. The regulatory function of RANKL is one of the key factors in progesterone-induced proliferation of the breast. Progesterone has a direct action of progesterone on progesterone-receptor (PR) expressing cells but PR-negative cells are affected indirectly through RANKL-induced paracrine actions leading to proliferation of mammary epithelial PR-negative cells. RANK induces epithelial-mesenchymal transition and stemness in human mammary epithelial cells and promotes tumorigenesis and metastasis. Inhibition of the RANK/RANKL pathway using the monoclonal antibody denosumab can neutralize RANKL and inhibiting its interaction with its receptor RANK. Denosumab is currently used to treat osteoporosis and in prevention of skeletal related events in patients suffering from bone metastases due to solid tumors. As preclinical experiments suggest the RANKL/RANK pathway plays an important role in primary breast cancer development. The interference with the RANK/RANKL system could therefore serve as a potential target for prevention and treatment of breast cancer.

Mulberry water extract regulates the osteoblast/osteoclast balance in an ovariectomic rat model

Mulberry has favorable antioxidant ability.

Skeletal metastases from breast cancer: pathogenesis of bone tropism and treatment strategy

Breast cancer (BC) is the most common female cancer worldwide with approximately 10 % of new cases metastatic at diagnosis and 20-50 % of patients with early BC who will eventually develop metastatic disease. Bone is the most frequent site of colonisation and the development of skeletal metastases depends on a complex multistep process, from dissemination and survival of malignant cells into circulation to the actual homing and metastases formation inside bone. Disseminated tumor cells (DTCs) can be detected in bone marrow in approximately 30 % of BC patients, likely reflecting the presence of minimal residual disease that would eventually account for subsequent metastatic disease. Patients with bone marrow DTCs have poorer overall survival compared with patients without them. Although bone-only metastatic disease seems to have a rather indolent behavior compared to visceral disease, bone metastases can cause severe and debilitating effects, including pain, spinal cord compression, hypercalcemia and pathologic fractures. Delivering an appropriate treatment is therefore paramount and ideally it should require interdisciplinary care. Multiple options are currently available, from bisphosphonates to new drugs targeting RANK ligand and radiotherapy. In this review we describe the mechanisms underlying bone colonization and provide an update on existing systemic and locoregional treatments for bone metastases.

Hypercalcemia in metastatic GIST caused by systemic elevated calcitriol: a case report and review of the literature

Background

Hypercalcemia is the most common oncologic metabolic emergency but very rarely observed in patients with gastrointestinal stromal tumour, which is a rare mesenchymal malignancy of the gastrointestinal tract. We describe a case of hypercalcemia caused by elevated levels of activated vitamin D in a patient with gastrointestinal tumour. Prior to this case report, only one paper has reported an association between hypercalcemia, gastrointestinal stromal tumours and elevated levels of vitamin D.

Case presentation

An otherwise healthy 70-year-old Caucasian woman, previously treated for duodenal gastrointestinal stromal tumour, was diagnosed with liver metastasis, and relapse of gastrointestinal stromal tumour was confirmed by biopsy. At presentation, the patient suffered from severe symptoms of hypercalcemia. The most common causes of hypercalcemia, hyperparathyrodism, parathyroid hormone-related peptide secretion from tumour cells, and metastatic bone disease, were all dismissed as the etiology. Analysis of vitamin D subtypes revealed normal levels of both 25-OH Vitamin D2 and 25-OH Vitamin D3, whereas the level of activated vitamin D, 1,25 OH Vitamin D3, also referred to as calcitriol, was elevated.

Conclusion

The fact that plasma calcitriol decreased after initiation of oncological treatment and the finding that hypercalcemia did not recur during treatment support the conclusion that elevated calcitriol was a consequence of the gastrointestinal stromal tumour. We suggest that gastrointestinal stromal tumours should be added to the list of causes of humoral hypercalcemia in malignancy, and propose that gastrointestinal stromal tumour tissue may have high activity of the specific enzyme 1α-hydroxylase, which can lead to increased levels of calcitriol and secondarily hypercalcemia.

RANKL and OPG Polymorphisms Are Associated with Aromatase Inhibitor-Related Musculoskeletal Adverse Events in Chinese Han Breast Cancer Patients

Background

Breast cancer patients treated with aromatase inhibitors (AIs) may experience musculoskeletal adverse events (MS-AEs). Several studies have confirmed that the RANKL/RANK/OPG signaling pathway plays a dominant role in bone health. Therefore, this study aimed to analyze the relationship between the serum levels of RANKL, OPG and their SNPs (single nucleotide polymorphisms) with AI-related MS-AEs.

Methodology and Principal Findings

Patients with early stage, hormone-sensitive breast cancer who were receiving AI therapy were enrolled. We included 208 cases with AI-related MS-AEs and 212 without (controls). The levels of estradiol, bone-turnover markers, multiple inflammatory cytokines, RANKL,OPG and lumbar spine BMD were measured, and questionnaires were completed. We analyzed 29 SNPs of RANKL, RANK and OPG using Sequenom MassARRAY assays and PCR-based TaqMan assays. The levels of bone-turnover markers and RANKL and the ratio of RANKL/OPG were higher in patients with AI-related MS-AEs than controls (all p < 0.05). A genetic assay showed that the RANKL SNP rs7984870 and OPG SNP rs2073618 were associated with AI-related MS-AEs. In patients with AI-related MS-AEs, rs7984870 CC and rs2073618 CC were risk genotypes. Carriers of the rs7984870 CC genotype were more likely to have a higher RANKL level and RANKL/OPG ratio than carriers of the GG genotype, and carriers of the rs2073618 CC genotype were more likely to have a lower OPG level and a higher RANKL/OPG ratio than carriers of the GG genotype (all p < 0.05). Moreover, risk genotypes were associated with higher levels of serum CTX and PINP and a lower lumbar spine BMD (all p < 0.05).

Conclusions and Significance

In conclusion, the RANKL and OPG risk genotypes synergize to negatively impact bone health and predispose breast cancer patients to AI-related MS-AEs.

Expression levels of receptor activator of nuclear factor-κB ligand and osteoprotegerin are associated with middle ear cholesteatoma risk

CONCLUSION

Overexpression of receptor activator of nuclear factor-κB ligand (RANKL) and low expression of osteoprotegerin (OPG) are typical features in middle ear cholesteatoma patients. The altered RANKL/OPG protein ratio suggests that alterations in the RANKL-OPG pathway may be major factors in the pathogenesis of middle ear cholesteatoma.

OBJECTIVE

Our meta-analysis explored the contribution of one important cytokine pathway, the RANKL and OPG pathway, in the development of middle ear cholesteatoma.

METHODS

We screened Embase, the Cochrane Library, CISCOM, CINAHL, Google Scholar, China BioMedicine, China National Knowledge Infrastructure, PubMed, and Web of Science for relevant articles. RANKL expression and ratio of RANKL/OPG were analyzed using Comprehensive Meta-Analysis Version 2 software.

RESULTS

The electronic literature search identified five studies that contained information on the correlation of RANKL and OPG expression with middle ear cholesteatoma. Increased RANKL expression positively correlated with middle ear cholesteatoma, while OPG expression showed an inverse association (p < 0.05). The ratio of RANKL/OPG in middle ear cholesteatoma cases was higher than in healthy controls, indicating that our observations are applicable to each individual case. Subgroup analysis based on country of study revealed that OPG levels decreased in China and Korea, and high RANKL expression was found in Poland, China, and Korea (all p < 0.05).

Osteoprotegerin in breast cancer: beyond bone remodeling

Osteoprotegerin (OPG) is a secreted protein and member of the Tumor Necrosis Factor (TNF) Receptor superfamily. OPG has been well characterized as a regulator of bone metabolism which acts by blocking osteoclast maturation and preventing bone breakdown. Given this role, early studies on OPG in breast cancer focused on the administration of OPG in order to prevent the osteolysis observed with bone metastases. However OPG is also produced by the breast tumor cells themselves. Research focusing on OPG produced by breast tumor cells has revealed actions of OPG which promote tumor progression. In vitro studies into the role of OPG produced by breast tumor cells have demonstrated that OPG can block TNF-related apoptosis inducing ligand (TRAIL)-mediated apoptosis. Furthermore, in vivo studies show that OPG expression by breast tumors can promote tumor growth and metastasis. In addition it has been shown that OPG stimulates endothelial cell survival and tube formation thus it may indirectly promote breast tumor progression through impacting angiogenesis. This article will present a summary of the data concerning the tumor-promoting effects of OPG in breast cancer.

Effect of denosumab on recurrent giant cell reparative granuloma of the lumbar spine

STUDY DESIGN

A case of recurrent giant cell reparative granuloma (GCRG) of the lumbar spine successfully treated with denosumab is reported; a fully human monoclonal antibody against the receptor activator of nuclear factor kappa B (RANK) ligand (RANKL).

OBJECTIVE

To report the first case of recurrent GCRG of the lumbar spine treated with denosumab.

SUMMARY OF BACKGROUND DATA

GCRG is a non-neoplastic osteofibrous lesion usually found in the maxilla and mandible but rarely in the spine. It is clinically distinct from giant cell tumor of bone (GCTB), although common histological characteristics such as the proliferation of spindle-shaped stromal cells and multinucleated giant cells are shared. Denosumab has recently been reported to be effective for unresectable GCTB; however, there is only one report of its effect on GCRG. Moreover, the effect of denosumab on GCRG of the spine is unknown.

METHODS

The clinical course, radiological features, pathology, and treatment outcome of a patient with recurrent GCRG of the lumbar spine treated with denosumab are documented.

RESULTS

Denosumab treatment was used for this patient with unresectable recurrent GCRG of the lumbar spine. Follow-up lumbar radiography showed significant bone formations in the tumor lesion after 3 months of treatment. On follow-up computerized tomography scans of the L2 and L3 vertebral lesions, the replacement of osoteolytic lesions by the formation of cortical-like bone tissue was clearly identified.

CONCLUSION

We report the first case of recurrent GCRG of the spine successfully treated with denosumab. Treatment with denosumab induced significant bone formation in the unresectable lumbar lesion with stable clinical improvement during the 12-month follow-up period without apparent complications. Denosumab shows promise as a new alternative treatment option for osteoclastic giant cell-rich tumors, such as GCRG, especially for unresectable lesions of the spine.

LEVEL OF EVIDENCE

4.

Evaluation of the protective effects of curcuminoid (curcumin and bisdemethoxycurcumin)-loaded liposomes against bone turnover in a cell-based model of osteoarthritis

Curcumin (Cur) and bisdemethoxycurcumin (BDMC), extracted from Curcuma longa, are poorly water-soluble polyphenol compounds that have shown anti-inflammatory potential for the treatment of osteoarthritis. To increase cellular uptake of Cur and BDMC in bone tissue, soybean phosphatidylcholines were used for liposome formulation. In this study, curcuminoid (Cur and BDMC)-loaded liposomes were characterized in terms of particle size, encapsulation efficiency, liposome stability, and cellular uptake. The results show that there is about 70% entrapment efficiency of Cur and BDMC in liposomes and that particle sizes are stable after liposome formation. Both types of liposome can inhibit macrophage inflammation and osteoclast differential activities. In comparison with free drugs (Cur and BDMC), curcuminoid-loaded liposomes were less cytotoxic and expressed high cellular uptake of the drugs. Of note is that Cur-loaded liposomes can prevent liposome-dependent inhibition of osteoblast differentiation and mineralization, but BDMC-loaded liposomes could not. With interleukin (IL)-1β stimulation, curcuminoid-loaded liposomes can successfully downregulate the expression of inflammatory markers on osteoblasts, and show a high osteoprotegerin (OPG)/receptor activator of nuclear factor κB ligand (RANKL) ratio to prevent osteoclastogenesis. In the present study, we demonstrated that Cur and BDMC can be successfully encapsulated in liposomes and can reduce osteoclast activity and maintain osteoblast functions. Therefore, curcuminoid-loaded liposomes may slow osteoarthritis progression.

RANK-mediated signaling network and cancer metastasis

Cancer metastasis is highly inefficient and complex. Common features of metastatic cancer cells have been observed using cancer cell lines and genetically reconstituted mouse and human tumor xenograft models. These include cancer cell interaction with the tumor microenvironment and the ability of cancer cells to sense extracellular stimuli and adapt to adverse growth conditions. This review summarizes the coordinated response of cancer cells to soluble growth factors, such as RANKL, by a unique feed forward mechanism employing coordinated upregulation of RANKL and c-Met with downregulation of androgen receptor. The RANK-mediated signal network was found to drive epithelial to mesenchymal transition in prostate cancer cells, promote osteomimicry and the ability of prostate cancer cells to assume stem cell and neuroendocrine phenotypes, and confer the ability of prostate cancer cells to home to bone. Prostate cancer cells with activated RANK-mediated signal network were observed to recruit and even transform the non-tumorigenic prostate cancer cells to participate in bone and soft tissue colonization. The coordinated regulation of cancer cell invasion and metastasis by the feed forward mechanism involving RANKL, c-Met, transcription factors, and VEGF-neuropilin could offer new therapeutic opportunities to target prostate cancer bone and soft tissue metastases.

Assessment of selected cytokines, proteins, and growth factors in the peritoneal fluid of patients with ovarian cancer and benign gynecological conditions

Objectives

The ovarian tumor microenvironment, ie, the peritoneal fluid, is an intriguing research subject. The goal of this study was to assess the behavior of selected cytokines and growth factors within the peritoneal fluid in pathologies associated with ascites and to assess the relationship between the levels of these substances and select prognostic factors of ovarian cancer.

Methods

A total of 74 patients were enrolled in the study, including 36 patients with ovarian cancer and 38 patients with benign gynecological conditions. Peritoneal fluid collected during surgical procedures was used to assess the levels of interleukin (IL)-6, IL-8, stem cell factor (SCF), dickkopf-1, growth differentiation factor-15 (GDF-15), tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), osteoprotegerin (OPG), osteopontin, osteonectin, and human epididymis protein 4. The median levels of these factors were compared between the two groups, and the levels of selected factors were assessed in the ovarian cancer group with regard to the clinical stage of cancer, tumor differentiation, presence of peritoneal spread and positive peritoneal fluid cytology results. The diagnostic value of the analyzed proteins within the peritoneal fluid was also assessed.

Results

Differences were observed between the patients with ovarian cancer and the patients with benign gynecological conditions associated with ascites with regard to the levels of IL-6, IL-8, GDF-15, SCF, osteopontin, osteonectin, and OPG. There were no differences in dickkopf-1, TRAIL, and human epididymis protein 4 levels between the two study groups. Cancer stage affected only the mean SCF and OPG levels, with lower SCF values and higher OPG values in advanced cancers compared to less-advanced cancers. Tumor differentiation was associated with significantly lower SCF values in the group of poorly differentiated tumors. A significant reduction in SCF values and a significant increase in OPG and IL-6 values were also observed within cancer cell-positive peritoneal fluid. Peritoneal spread was associated with higher levels of TRAIL, osteonectin, and IL-6 in ovarian cancer patients.

Conclusion

On the basis of the conducted studies, it appears that of the studied factors, GDF-15, SCF, and OPG deserve special attention in the context of future research on the tumor microenvironment. With regard to diagnostics, attention should be given primarily to GDF-15, IL-6, and osteonectin.

RANK-ligand (RANKL) expression in young breast cancer patients and during pregnancy

Introduction

RANKL is important in mammary gland development during pregnancy and mediates the initiation and progression of progesterone-induced breast cancer. No clinical data are available on the effect of pregnancy on RANK/RANKL expression in young breast cancer patients.

Methods

We used our previously published dataset of 65 pregnant and 130 matched young breast cancer patients with full clinical, pathological, and survival information. 85% of patients had available transcriptomic data as well. RANK/RANKL expression by immunohistochemistry using H-score on the primary tumor and adjacent normal tissue was performed. We examined the difference in expression of RANK/RANKL between pregnant and non-pregnant patients and their association with clinicopathological features and prognosis. We also evaluated genes and pathways associated with RANK/RANKL expression on primary tumors.

Results

RANKL but not RANK expression was more prevalent in the pregnant group, both on the tumor and adjacent normal tissue, independent of other clinicopathological factors (both P <0.001). 18.7% of pregnant and 5.3% of non-pregnant patients had tumors showing ≥10% of cells with 3+ RANKL expression. RANKL expression was significantly higher in progesterone receptor-positive, and luminal A-like tumors, with negative correlation with Ki-67 (all P <0.001). On the contrary, RANK expression was higher in triple negative tumors (P <0.001). Using false discovery rate <0.05, 151 and 1,207 genes were significantly correlated with tumor-expressed RANKL and RANK expression by immunohistochemistry, respectively. High RANKL expression within primary tumor was associated with pathways related to mammary gland development, bone resorption, T-cell proliferation and regulation of chemotaxis, while RANK expression was associated with immune response and proliferation pathways. At a median follow-up of 65 months, neither RANK nor RANKL expression within tumor was associated with disease free survival in pregnant or non-pregnant group.

Conclusions

Pregnancy increases RANKL expression both in normal breast and primary tumors. These results could guide further development of RANKL-targeted therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-015-0538-7) contains supplementary material, which is available to authorized users.

Human mesenchymal stem cells as delivery of osteoprotegerin gene: homing and therapeutic effect for osteosarcoma

Biological treatments have been studied extensively and previous studies have proved that osteoprotegerin (OPG) can inhibit the development and progress of human osteosarcoma. However, the utility of biologic agents for cancer therapy has a short half-life, which can hardly deliver to and function in tumor sites efficiently. Mesenchymal stem cells (MSCs) have the potential to migrate to tumor sites. In this study, MSCs transfected with adenoviruses carrying the OPG gene (MSCs-OPG) were used via the tail vein to treat athymic nude mice (nu/nu) bearing osteosarcoma. In vivo and ex vivo images were used to validate the MSCs homing to tumors. The therapeutic effect for osteosarcoma was evaluated by observations on growth of tumors and bone destruction. The results showed that infected MSCs-OPG labeled with red fluorescent protein (RFP) can migrate to tumor sites and express OPG protein. The treatment by MSCs-OPG reduced the tumor growth and inhibited bone destruction in vivo. All these indicated that MSCs can deliver OPG to tumor sites, which could be a new direction of biological treatment for human osteosarcoma.

RANK pathway in giant cell tumor of bone: pathogenesis and therapeutic aspects

Giant cell tumor is a relatively uncommon but painful tumor of bone, which can metastasize to the lungs. The RANK pathway is often reported to be involved in the pathogenesis of giant cell tumor of bone (GCTB). This pathway is a key signaling pathway of bone remodeling that plays a critical role in differentiation of precursors into multinucleated osteoclasts, and activation of osteoclasts leading to bone resorption. Dysregulation of RANK ligand (RANKL)-RANK-osteoprotegerin (OPG) signaling cascade induces the imbalance between bone formation and bone resorption, which leads to the changes in bone mass, increases osteoclast-mediated bone destruction, bone metastasis, and the progression of existing skeletal tumors. Recent evidences have shown that targeting the components of RANKL-RANK-OPG signaling pathway is a promising approach in the treatment of GCTB. This review study has focused on the association of RANKL-RANK-OPG pathway in the pathogenesis and progression of GCTB as well as discussed the possible therapeutic strategies by targeting this pathway.

Mycobacterium tuberculosis 10-kDa co-chaperonin regulates the expression levels of receptor activator of nuclear factor-κB ligand and osteoprotegerin in human osteoblasts

The aim of the present study was to investigate the effect of recombinant Mycobacterium tuberculosis (r-Mt) 10-kDa co-chaperonin (cpn10) on the expression of osteoprotegerin (OPG) and receptor activator of nuclear factor-κB ligand (RANKL) in third-generation cultured osteoblasts. The osteoblast-like cultures were isolated from bone fragments taken from patients undergoing surgery. Prior to stimulation with r-Mt cpn10, cells were incubated in serum-free medium for 24 h. r-Mt cpn10 was added into fresh serum-free medium, reaching final concentrations of 0.01–10 μg/ml. The levels of OPG were determined using enzyme-linked immunosorbent assay. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis was performed to determine the levels of RANKL and OPG mRNA. For measurement of the protein levels of OPG and RANKL, a western blotting assay was performed. r-Mt cpn10 downregulated the protein levels of OPG in the third generation cultured osteoblasts at a dose of 10 μg/ml. RT-qPCR revealed that the OPG mRNA level was decreased by 73% after 4 h and by 85.5% after 8 h following incubation with r-Mt cpn10 (10 μg/ml). Western blot analysis demonstrated similar results for the OPG protein level. In the third-generation cultured osteoblasts, the levels of RANKL mRNA and protein were increased by 2.6- and 1-fold, respectively, following incubation with r-Mt cpn10 (10 μg/ml). Furthermore, the RANKL/OPG ratio was markedly increased by r-Mt cpn10 (10 μg/ml) treatment. In conclusion, the results of the current study demonstrated that r-Mt cpn10 decreased the levels of OPG and increased the levels of RANKL in a dose- and time-dependent manner. Notably, the present study indicated that r-Mt cpn10 exerts its effect on osteoblastic cells by increasing the RANKL/OPG ratio.

Metastatic tumors to the jaws and mouth

Metastatic dissemination to the oral cavity is rare and is usually the evidence of a wide spread disease with an average survival rate of 7 months. In almost a quarter of the cases, oral metastasis was found to be the first indication of an occult malignancy at a distant site. Metastatic lesions can be found anywhere in the oral cavity, however, the jaw bones with the molar area is the most frequently involved site. In the oral soft tissues, the gingiva is the most common site, suggesting the possible role of inflammation in the attraction of metastatic deposits. The most common primary malignancies presenting oral metastases were the lung, kidney, liver, and prostate for men, and breast, female genital organs, kidney, and colo-rectum for women. Most patients with jawbone metastasis complain of swelling, pain, and paresthesia. An exophytic lesion is the most common clinical presentation of metastatic lesions in the oral soft tissues. Early lesions, mainly those located in the gingiva, may resemble a hyperplastic or reactive lesion. Once a lesion is recognized as metastasis, the primary tumor site should be identified following clinical, radiological and histopathological investigations. If standardized diagnostic workup fails to detect the site of origin, then the term carcinoma of unknown primary is applied. Personalized medicine tools such as tissue-of-origin assays should be applied, either by immunohistochemical testing or by molecular-profiling methods as these may lead to a more favorable outcome.

Blockade of CD26 signaling inhibits human osteoclast development

Bone remodeling is maintained by the delicate balance between osteoblasts (OBs) and osteoclasts (OCs). However, the role of CD26 in regulating bone remodeling has not yet been characterized. We herein show that CD26 is preferentially expressed on normal human OCs and is intensely expressed on activated human OCs in osteolytic bone alterations. Macrophage-colony stimulating factor (M-CSF) and soluble receptor activator of NF-κB ligand (sRANKL) induced human OC differentiation, in association with CD26 expression on monocyte-macrophage lineage cells. CD26 expression was accompanied by increased phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK), which is crucial for early human OC differentiation. The humanized anti-CD26 monoclonal antibody, huCD26mAb, impaired the formation and function of tartrate-resistant acid phosphatase (TRAP)/CD26 positive multi-nucleated (nuclei > 3) OCs with maturation in the manner of dose-dependency. It was revealed that huCD26mAb inhibits early OC differentiation via the inactivation of MKK3/6, p38 MAPK and subsequent dephosphorylation of microphthalmia-associated transcription factor (mi/Mitf). These inhibitions occur immediately after RANKL binds to RANK on the human OC precursor cells and were demonstrated using the OC functional assays. huCD26mAb subsequently impaired OC maturation and bone resorption by suppressing the expression of TRAP and OC fusion proteins. In addition, p38 MAPK inhibitor also strongly inhibited OC formation and function. Our results suggest that the blockade of CD26 signaling impairs the development of human functional OCs by inhibiting p38 MAPK-mi/Mitf phosphorylation pathway and that targeting human OCs with huCD26mAb may have therapeutic potential for the treatment of osteolytic lesions following metastasis to alleviate bone destruction and reduce total skeletal-related events (SREs).

RANKL inhibition blocks osteolytic lesions and reduces skeletal tumor burden in models of non-small-cell lung cancer bone metastases

INTRODUCTION

Bone metastasis is a serious complication in patients with lung cancer, occurring in up to 40% of patients. Tumor cell-mediated osteolysis occurs ultimately through induction of RANK ligand (RANKL) within the bone stroma although this hypothesis has not been tested extensively in the setting of non-small-cell lung cancer (NSCLC). By using two novel NSCLC bone metastasis mouse models, we examined the effects of RANKL inhibition on osteolysis and tumor progression.

METHODS

We treated mice bearing skeletal NSCLC tumors with osteoprotegerin-Fc (OPG-Fc) to assess whether osteoclast inhibition through RANKL inhibition would affect bone metastases at early or late stages of bone colonization. Progression of skeletal tumor was determined by radiography, longitudinal bioluminescent imaging, and histological analyses.

RESULTS

OPG-Fc reduced development and progression of radiographically evident osteolytic lesions and also significantly reduced skeletal tumor progression in both NSCLC bone metastasis models. In the H1299 human NSCLC bone metastasis model, OPG-Fc plus docetaxel in combination resulted in significantly greater inhibition of skeletal tumor growth compared with either single agent alone. The observed ability of RANKL inhibition to reduce NSCLC osteolytic bone destruction or skeletal tumor burden was associated with decreases in tumor-associated osteoclasts.

CONCLUSIONS

These results demonstrate that RANKL is required for the development of tumor-induced osteolytic bone destruction caused by NSCLC cells in vivo. RANKL inhibition also reduced skeletal tumor burden, presumably through the indirect mechanism of blocking tumor-induced osteoclastogenesis and resultant production of growth factors and calcium from the bone microenvironment. RANKL inhibition also provided an additive benefit to docetaxel treatment by augmenting the reduction of tumor burden.

Direct but not indirect co-culture with osteogenically differentiated human bone marrow stromal cells increases RANKL/OPG ratio in human breast cancer cells generating bone metastases

Background

Bone metastases arise in nearly 70% of patients with advanced breast cancer, but the complex metastatic process has not been completely clarified yet. RANKL/RANK/OPG pathway modifications and the crosstalk between metastatic cells and bone have been indicated as potential drivers of the process. Interactions between tumor and bone cells have been studied in vivo and in vitro, but specific effects of the direct contact between human metastatic cells and human bone cells on RANKL/RANK/OPG pathway have not been investigated.

Findings

We directly co-cultured bone metastatic human breast cancer cells (BOKL) with osteo-differentiated human mesenchymal cells (BMSCs) from 3 different donors. BMSCs and BOKL were then enzymatically separated and FACS sorted. We found a significant increase in the RANKL/OPG ratio as compared to control, which was not observed in BOKL cultured in medium conditioned by BMSCs, neither in BOKL directly cultured with fibroblasts or medium conditioned by fibroblasts. Direct co-culture with osteo-differentiated BMSCs caused BOKL aggregation while proliferation was not affected by co-culture. To more specifically associate RANKL expression to osteogenic differentiation degree of BMSCs, we determined their osteogenic markers expression and matrix calcification relative to osteoblasts and fibroblasts.

Conclusions

In conclusion, our co-culture model allowed to demonstrate for the first time that direct contact but not paracrine interactions between human metastatic breast cancer cells and bone cells has a significant effect on RANKL/OPG expression in bone metastatic cells. Furthermore, only direct contact with the bone microenvironment induced BOKL clustering without however significantly influencing their proliferation and migration.

Electronic supplementary material

The online version of this article (doi:10.1186/1476-4598-13-238) contains supplementary material, which is available to authorized users.

OPG and PgR show similar cohort specific effects as prognostic factors in ER positive breast cancer

The RANK/RANKL/OPG pathway is well known for bone destruction in skeletal metastases but has also been implicated in osteoclast-independent roles in tumorigenesis and de novo metastasis. Experimental data suggest contribution of progesterone to tumorigenesis may be mediated by RANKL. Importantly, modulation of this pathway became possible through the availability of denosumab, an artificial counterpart of OPG, but significant gaps remain in the translation of preclinical findings on the pathway. We analyzed gene expression of RANK, RANKL and OPG from 40 Affymetrix datasets encompassing 4467 primary breast cancers and focused on ER positive disease. We did not observe a significant prognostic value of RANK and RANKL mRNA expression. In contrast, OPG was associated with a better prognosis among 1941 ER positive cancers (HR 0.64, 95% CI 0.53-0.77; P < 0.0001) using a cutoff from its highly bimodal expression. We detected considerable heterogeneity regarding the prognostic value of OPG between different datasets. This heterogeneity could neither be attributed to technical reasons nor to differences in standard clinical parameters or treatments of the cohorts. Interestingly, the prognostic value of the progesterone receptor and of OPG showed similar cohort specific effects. Still both factors were no surrogates for each other but contributed independent prognostic value in multivariate analyses. Thus, both OPG and PgR are independently associated with good prognosis in ER positive breast cancer. However both markers share common cohort specific differences in contrast to proliferation markers as Ki67 which may be based on the underlying biology.

Spanish Menopause Society position statement: Use of denosumab in postmenopausal women

Denosumab is a new drug developed for the treatment of osteoporosis. Moreover, increasing evidences link denosumab with benefits in cancer, an area of interest for those in charge of the postmenopausal health. Denosumab has shown efficacy in the control of bone loss associated with hypogonadic states created by chemotherapy in breast and other cancers. Moreover, some studies reveal efficacy in reducing the progression of metastases. A panel of experts from the Spanish Menopause Society has met to develop usage recommendations based on the best available evidence.

RANK expression as a prognostic and predictive marker in breast cancer

RANK ligand (RANKL) is crucial for the development of mouse mammary glands during pregnancy. RANKL functions as a major paracrine effector of the mitogenic action of progesterone in mammary epithelium via its receptor RANK and has a role in expansion and regenerative potential of mammary stem cells. Pharmacologic inhibition of RANKL attenuates the development of mammary carcinoma and inhibits metastatic progression in multiple mouse models. Primary breast carcinoma samples from the neoadjuvant GeparTrio study were analyzed to correlate the expression of human RANK and RANKL with pathological complete response (pCR), disease-free (DFS), and overall (OS) survival. Pre-treatment FFPE core biopsies (n = 601) were analyzed for percentage and intensity of immunohistochemical RANK and RANKL expression. Antibodies against human RANK (N-1H8; Amgen) and human RANKL (M366; Amgen) were used. RANK protein was expressed in 160 (27 %) patients. Increased RANK expression was observed in 14.5 % of patients and correlated with high tumor grade (p < 0.023) and negative hormone receptor (HR) status (p < 0.001). Patients with high RANK expression showed a higher pCR rate (23.0 % vs. 12.6 %, p = 0.010), shorter DFS (p = 0.038), and OS (p = 0.011). However, prognostic and predictive information was not an independent parameter. Only 6 % of samples expressed RANKL, which was not correlated with any clinical features. Higher RANK expression in the primary tumor is associated with a higher sensitivity to chemotherapy, but also a higher risk of relapse and death. Our study provides a basis for further exploration of the antitumor activity of clinical antibodies against RANKL.

Targeting RANKL in metastasis

Acting through its cognate receptor, receptor activator of nuclear factor-κB (RANK), RANK ligand (RANKL) is an essential mediator of osteoclast function and survival. Preclinical data have now firmly established that blockade of tumor-induced osteoclastogenesis by RANKL inhibition will not only protect against bone destruction but will also inhibit the progression of established bone metastases and delay the formation of de novo bone metastases in cancer models. In patients with bone metastases, skeletal complications are driven by increased osteoclastic activity and may result in pathological fractures, spinal cord compression and the need for radiotherapy to the bone or orthopedic surgery (collectively known as skeletal-related events (SREs)). Denosumab, a fully human monoclonal antibody against RANKL, has been demonstrated to prevent or delay SREs in patients with solid tumors that have metastasized to bone. In addition to its central role in tumor-induced osteolysis, bone destruction and skeletal tumor progression, there is emerging evidence for direct pro-metastatic effects of RANKL, independent of osteoclasts. For example, RANKL also stimulates metastasis via activity on RANK-expressing cancer cells, resulting in increased invasion and migration. Pharmacological inhibition of RANKL may also reduce bone and lung metastasis through blockade of the direct action of RANKL on metastatic cells. This review describes these distinct but potentially overlapping mechanisms by which RANKL may promote metastases.

RANK- and c-Met-mediated signal network promotes prostate cancer metastatic colonization

Prostate cancer (PCa) metastasis to bone is lethal and there is no adequate animal model for studying the mechanisms underlying the metastatic process. Here, we report that receptor activator of NF-κB ligand (RANKL) expressed by PCa cells consistently induced colonization or metastasis to bone in animal models. RANK-mediated signaling established a premetastatic niche through a feed-forward loop, involving the induction of RANKL and c-Met, but repression of androgen receptor (AR) expression and AR signaling pathways. Site-directed mutagenesis and transcription factor (TF) deletion/interference assays identified common TF complexes, c-Myc/Max, and AP4 as critical regulatory nodes. RANKL-RANK signaling activated a number of master regulator TFs that control the epithelial-to-mesenchymal transition (Twist1, Slug, Zeb1, and Zeb2), stem cell properties (Sox2, Myc, Oct3/4, and Nanog), neuroendocrine differentiation (Sox9, HIF1α, and FoxA2), and osteomimicry (c-Myc/Max, Sox2, Sox9, HIF1α, and Runx2). Abrogating RANK or its downstream c-Myc/Max or c-Met signaling network minimized or abolished skeletal metastasis in mice. RANKL-expressing LNCaP cells recruited and induced neighboring non metastatic LNCaP cells to express RANKL, c-Met/activated c-Met, while downregulating AR expression. These initially non-metastatic cells, once retrieved from the tumors, acquired the potential to colonize and grow in bone. These findings identify a novel mechanism of tumor growth in bone that involves tumor cell reprogramming via RANK-RANKL signaling, as well as a form of signal amplification that mediates recruitment and stable transformation of non-metastatic bystander dormant cells.

Differential expression of progesterone receptor, FOXA1, GATA3, and p53 between pre- and postmenopausal women with estrogen receptor-positive breast cancer

Estrogen receptor (ER) is essential for estrogen-dependent growth, and its level of expression is considered a crucial determinant of response to endocrine therapy and prognosis in ER-positive breast cancer. On the other hand, the clinical role of progesterone receptor (PgR) in ER-positive breast cancer remains controversial, although testing of PgR by immunohistochemistry (IHC) has become routine. Recent studies indicated that plasma estradiol levels were related to the expression levels of estrogen-responsive genes in ER-positive breast cancer tissues in both pre- and postmenopausal women. In this study, we analyzed the expression levels of estrogen-responsive genes (PgR and TFF1), a progesterone-responsive gene (RANKL), ER-related genes (FOXA1 and GATA3), HER2, Ki67 and p53 in ER-positive, HER2-negative breast cancer tissues by IHC. Correlations between the expression levels of these molecular markers and clinicopathological factors, including prognosis, were compared between pre- and postmenopausal women. Serum levels of estrone, estradiol, progesterone, and testosterone were also measured. Expression levels of PgR, TFF1, RANKL, and GATA3 were significantly higher in premenopausal women than in postmenopausal women. Serum estradiol levels were positively correlated with Ki67 labeling index (LI) in premenopausal women, but not in postmenopausal women. High expression of FOXA1 and GATA3 was significantly associated with improved disease-free survival in premenopausal women, but not in postmenopausal women, whereas high expression of PgR and low expression of p53 were significantly correlated with the improved disease-free survival in postmenopausal women, but not in premenopausal women. Moreover, the best cutoff points of Ki67 LI for disease-free survival were 30 % for premenopausal women and 14 % for postmenopausal women. Expression levels of ER, TFF1, and RANKL were not associated with the disease-free survival in either pre- or postmenopausal women. Our results suggest that the mechanisms of development and estrogen-dependent growth of ER-positive breast cancer might differ according to menopausal status.

The physiopathological role of IL-33: new highlights in bone biology and a proposed role in periodontal disease

Interleukin-33 (IL-33) is a recently described member of the IL-1 family. IL-33 acts as an alarmin, chemoattractant, and nuclear factor. ST2, a member of the Toll-like receptor/IL-1R superfamily, the receptor of IL-33, triggers a plethora of downstream effectors and leads the activation of NFK-B, leading the expression of several genes. IL-33 and ST2 are expressed in the majority of cell types, and the IL-33/ST2 axis has a role in immune response, bone homeostasis, and osteoclastogenesis. Several studies show opposite roles of IL-33 in osteoclastogenesis and the implication in bone biology. Few works studied the role of IL-33 in periodontal disease, but we hypothesize a possible role of IL-33 in periodontal disease and bone loss.

Betulinic acid, a bioactive pentacyclic triterpenoid, inhibits skeletal-related events induced by breast cancer bone metastases and treatment

Many breast cancer patients experience bone metastases and suffer skeletal complications. The present study provides evidence on the protective and therapeutic potential of betulinic acid on cancer-associated bone diseases. Betulinic acid is a naturally occurring triterpenoid with the beneficial activity to limit the progression and severity of cancer, diabetes, cardiovascular diseases, atherosclerosis, and obesity. We first investigated its effect on breast cancer cells, osteoblastic cells, and osteoclasts in the vicious cycle of osteolytic bone metastasis. Betulinic acid reduced cell viability and the production of parathyroid hormone-related protein (PTHrP), a major osteolytic factor, in MDA-MB-231 human metastatic breast cancer cells stimulated with or without tumor growth factor-β. Betulinic acid blocked an increase in the receptor activator of nuclear factor-kappa B ligand (RANKL)/osteoprotegerin ratio by downregulating RANKL protein expression in PTHrP-treated human osteoblastic cells. In addition, betulinic acid inhibited RANKL-induced osteoclastogenesis in murine bone marrow macrophages and decreased the production of resorbed area in plates with a bone biomimetic synthetic surface by suppressing the secretion of matrix metalloproteinase (MMP)-2, MMP-9, and cathepsin K in RANKL-induced osteoclasts. Furthermore, oral administration of betulinic acid inhibited bone loss in mice intra-tibially inoculated with breast cancer cells and in ovariectomized mice causing estrogen deprivation, as supported by the restored bone morphometric parameters and serum bone turnover markers. Taken together, these findings suggest that betulinic acid may have the potential to prevent bone loss in patients with bone metastases and cancer treatment-induced estrogen deficiency.

Effects of RANKL-Targeted Therapy in Immunity and Cancer

The role of the receptor activator of nuclear factor-κB ligand (RANKL)/RANK system is well characterized within bone, where RANKL/RANK signaling mediates osteoclastogenesis and bone resorption. However, this system has also been shown to influence biologic processes beyond the skeletal system, including in the immune system and in cancer. RANKL/RANK signaling is important in lymph-node development, lymphocyte differentiation, dendritic cell survival, T-cell activation, and tolerance induction. The RANKL/RANK axis may also have direct, osteoclast-independent effects on tumor cells. Indeed, activity of the RANKL/RANK pathway in cancer cells has been correlated with tumor progression and advanced disease. Denosumab, a fully human monoclonal antibody against RANKL, inhibits osteoclastogenesis and is widely used not just for the treatment of osteoporosis, but for the prevention of skeletal-related events from bone metastases in solid malignancies such as breast and prostate cancer. The potential effects of denosumab on the immune system have been largely ignored. Nevertheless, with the emergence of immunotherapies for cancer, denosumab may impact the effectiveness of these therapies, especially if they are given in combination. In this article, we review the role of RANKL/RANK in bone, immunity, and cancer. Examining the potential effects of routine treatment with denosumab beyond the bone represents an important area of investigation.

Stromal cells in tumor microenvironment and breast cancer

Cancer is a systemic disease encompassing multiple components of both tumor cells themselves and host stromal cells. It is now clear that stromal cells in the tumor microenvironment play an important role in cancer development. Molecular events through which reactive stromal cells affect cancer cells can be defined so that biomarkers and therapeutic targets can be identified. Cancer-associated fibroblasts (CAFs) make up the bulk of cancer stroma and affect the tumor microenvironment such that they promote cancer initiation, angiogenesis, invasion, and metastasis. In breast cancer, CAFs not only promote tumor progression but also induce therapeutic resistance. Accordingly, targeting CAFs provides a novel way to control tumors with therapeutic resistance. This review summarizes the current understandings of tumor stroma in breast cancer with a particular emphasis on the role of CAFs and the therapeutic implications of CAFs. In addition, the effects of other stromal components such as endothelial cells, macrophages, and adipocytes in breast cancer are also discussed. Finally, we describe the biologic markers to categorize patients into a specific and confirmed subtype for personalized treatment.

Bone marrow stromal cells derived MCP-1 reverses the inhibitory effects of multiple myeloma cells on osteoclastogenesis by upregulating the RANK expression

Multiple myeloma (MM) cells are responsible for aberrant osteoclast (OC) activation. However, when cocultured monocytes, but not OC precursors, with MM cells, we made a novel observation that MM cells inhibited receptor activator of nuclear factor κB ligand (RANKL)-induced increase of OC differentiation, OC gene expression, signaling pathways and bone resorption activity. Our results showed that MM cells produced multiple inhibitory cytokines of osteoclastogenesis, such as IL-10, which activated STAT3 signaling and induce OC inhibition. However, cocultures of bone marrow stromal cells (BMSCs) reversed MM-induced OC inhibition. We found that MM cells increased production of MCP-1 from BMSCs and BMSC-derived MCP-1 enhanced OC formation. Mechanistic studies showed that IL-10 downregulated RANK expression in monocytes and thus, inhibited RANKL-induced OC formation. In contrast, MCP-1 upregulated RANK expression and thus, enhanced OC formation. Overall, our studies for the first time demonstrated that MM cell have inhibitory effects on osteoclastogenesis by producing inhibitory cytokines. Our results further indicate that activation of osteoclastogenesis in bone marrow requests the crosstalk of MM cells, BMSCs and their produced cytokines. Thus, our studies provide evidences that targeting bone marrow microenvironmental cells and/or cytokines may be a new approach to treating MM bone destruction.

The osteoblastic and osteoclastic interactions in spinal metastases secondary to prostate cancer

Prostate cancer (PC) is one of the most common cancers arising in men and has a high propensity for bone metastasis, particularly to the spine. At this stage, it often causes severe morbidity due to pathological fracture and/or metastatic epidural spinal cord compression which, if untreated, inevitably leads to intractable pain, neurological deficit, and paralysis. Unfortunately, the underlying molecular mechanisms driving growth of secondary PC in the bony vertebral column remain largely unknown. Further investigation is warranted in order to identify therapeutic targets in the future. This review summarizes the current understanding of PC bone metastasis in the spine, highlighting interactions between key tumor and bone-derived factors which influence tumor progression, especially the functional roles of osteoblasts and osteoclasts in the bone microenvironment through their interactions with metastatic PC cells and the critical pathway RANK/RANKL/OPG in bone destruction.

Association between seven common OPG genetic polymorphisms and osteoporosis risk: a meta-analysis

Functional polymorphisms of the osteoprotegerin (OPG) gene are known to be involved in bone mineral density and the development of osteoporosis; however, some conflicting results have been reported. The aim of this meta-analysis is to provide a relatively comprehensive assessment of the relationship between seven common OPG genetic polymorphisms (T149C, A163G, G209A, T245G, T950C, G1181C, and C1217T) and osteoporosis risk. A literature search for eligible studies published before August 1st, 2013 was conducted in PubMed, Embase, Web of Science, Cochrane Library, and CNKI (China National Knowledge Infrastructure) databases. Pooled odds ratios and their corresponding 95% confidence intervals were used to evaluate the strength of the association under fixed- or random-effect models according to a heterogeneity test. All analyses were performed using the STATA software, version 12.0. Fourteen case-control studies with a total of 2383 osteoporosis cases and 2280 healthy controls were included in this meta-analysis. Among the seven polymorphisms, A163G and G1181C revealed significant associations with osteoporosis risk. For A163G (rs3102735), the combined results showed that the G allele of the A163G polymorphism may be associated with an increased risk of osteoporosis. Stratified analyses showed that the magnitude of the effect was similar in Caucasian and postmenopausal woman subgroups. For G1181C (rs2073618), however, we found that individuals with the C allele of the G1181C polymorphism had a decreased risk of osteoporosis, especially in Asian and postmenopausal woman subgroups. In summary, this meta-analysis indicated that the G allele of the OPG A163G polymorphism might increase osteoporosis risk in Caucasians, whereas individuals with the C allele of the G1181C polymorphism had a decreased risk of osteoporosis, especially in Asians. Both of these effects were observed in postmenopausal women. These polymorphisms could probably be used with other genetic markers together to identify individuals at a high risk of osteoporosis.

RANK expression on breast cancer cells promotes skeletal metastasis

RANK ligand (RANKL), acting through its cognate receptor RANK, is a key factor for bone remodeling and metastasis by regulating the differentiation, survival and activation of osteoclasts. RANKL is also crucial for the development of mouse mammary glands during pregnancy and has been recently linked to the etiology of breast cancer via its direct activity on RANK-expressing normal or transformed breast epithelial cells, leading to increased mitogenesis, enhanced regenerative potential of mammary stem cells, and increased invasion and migration. We demonstrate that higher RANK expression in MDA-MB-231 breast cancer cells (MDA-231-RANK cells) is sufficient to confer a significantly greater metastatic growth rate in the bone compared with MDA-MB-231 cells which do not express high levels of RANK. Blockade of osteoclastic bone resorption, achieved with treatment by either RANKL inhibition or zoledronic acid, did reduce skeletal tumor progression of MDA-231-RANK cells suggesting that the vicious cycle contributes to metastatic growth. However, RANKL inhibition reduced skeletal growth of MDA-231-RANK tumors to a significantly greater extent than zoledronic acid, indicating that skeletal growth of RANK-positive tumors is also driven by direct RANKL effects. RANKL stimulated the expression of multiple genes associated with cell invasive behavior, including several matrix metalloproteinases and other genes previously defined as part of a bone metastasis gene signature. These data indicate that RANKL provokes breast cancer bone metastases via two distinct, but potentially overlapping mechanisms: stimulation of tumor-associated osteoclastogenesis and stimulation of RANK-expressing tumor cells.

Osteoprotegerin (OPG) activates integrin, focal adhesion kinase (FAK), and Akt signaling in ovarian cancer cells to attenuate TRAIL-induced apoptosis

BACKGROUND

Resistance to apoptosis is a major problem in ovarian cancer (OC) and correlates with poor prognosis. Osteoprotegerin (OPG) is a soluble secreted factor that acts as a decoy receptor for receptor activator of NF-κB ligand (RANKL) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). OPG has been reported to attenuate TRAIL-induced apoptosis in a variety of cancer cells, including OC cells. OPG-mediated protection against TRAIL has been attributed to its decoy receptor function. However, OPG activates integrin/focal adhesion kinase (FAK) signaling in endothelial cells. In OC cells, activation of integrin/FAK signaling inhibits TRAIL-induced apoptosis. Based on these observations, we hypothesized that OPG could attenuate TRAIL-induced apoptosis in OC cells through integrin/FAK signaling.

METHODS

In vitro experiments including immunoblots, colony formation assays, and apoptosis measurements were used to assess the effect of OPG on TRAIL-induced apoptosis.

RESULTS

Exogenous OPG protected from TRAIL-induced apoptosis in a TRAIL binding-independent manner and OPG protection was αvβ3 and αvβ5 integrin/FAK signaling-dependent. Moreover, OPG-mediated activation of integrin/FAK signaling resulted in the activation of Akt. Inhibition of both integrin/FAK and Akt signaling significantly inhibited OPG-mediated attenuation of TRAIL-induced apoptosis. Although OPG also stimulated ERK1/2 phosphorylation, inhibition of ERK1/2 signaling did not significantly altered OPG protection.

CONCLUSIONS

Our studies provide evidence, for the first time, that OPG can attenuate TRAIL-induced apoptosis in a TRAIL binding-independent manner through the activation of integrin/FAK/Akt signaling in OC cells.

Invasion and metastasis of renal cell carcinoma

Renal cell carcinoma (RCC) represents over 80% of kidney cancer, and about 30% of the patients with RCC develop metastasis after the surgery. Invasion of basement membrane (BM) and extracellular matrix (ECM) is an essential event in tumor invasion and metastasis. Matrix metalloproteinases (MMPs), which digest the main components of BM and ECM, are expressed in RCC. Heparanase, which degrades heparan sulfate proteoglycans, is predominantly expressed in high-grade RCCs with a positive correlation with pathological tumor stage and poor prognosis. Bone metastasis is common among the patients with RCC, and increased osteoclastic activity was observed at metastatic sites. Receptor activator of nuclear factor κB ligand (RANKL), which plays an important role in osteoclastogenesis, is predominantly expressed in high-grade RCC and its expression level is associated with bone metastasis and prognosis. Epithelial-mesenchymal transition (EMT), a switch of epithelial cells to sarcomatoid phenotype, is considered to be critical step during metastasis, and Snail, a major regulator of EMT, is predominantly expressed in high-grade RCC, and high Snail expression is a worse prognostic factor. Accordingly, heparanase, RANKL and Snail may be targets for the development of anti-tumor therapies for RCCs.

Multifunctional properties of RANKL/RANK in cell differentiation, proliferation and metastasis

It is known that there are close relationships between bone destruction and tumor growth in bone metastasis. RANKL is a central factor in bone metastasis, inducing osteoclastogenesis mediated by its receptor RANK. Recent reports demonstrate that RANKL has important roles in organogenesis stimulating proliferation and differentiation of epithelial and stroma cells. RANKL is induced not only by cytokines and hormones but also by UV-irradiation, inflammation and carcinogens. Expression of RANK and RANKL is found in several human cancer cell lines, and RANK signaling stimulates proliferation, migration and epithelial–mesenchymal transition of cancer cells, which may be involved in metastasis via an autocrine/paracrine mechanism. RANKL regulates the number of Tregs that produce RANKL, which may affect cancer metastasis. In this review we discuss the multifunctional roles of RANKL/RANK in osteoclastogenesis, organogenesis, and the metastasis and tumorigenesis of cancer cells.

Role of RANKL/RANK in primary and secondary breast cancer

Bone is one of the most preferential metastatic target sites of breast cancer. Bone possesses unique biological microenvironments in which various growth factors are stored and continuously released through osteoclastic bone resorption, providing fertile soil for circulating breast cancer cells. Bone-disseminated breast cancer cells in turn produce osteotropic cytokines which modulate bone environments. Under the influences of breast cancer-produced cytokines, osteoblasts express elevated levels of Ligand for receptor activator of nuclear factor-κB (RANKL) and stimulate osteoclastogenesis via binding to the receptor receptor activator of nuclear factor-κB (RANK) and activating its downstream signaling pathways in hematopoietic osteoclast precursors, which causes further osteoclastic bone destruction. Establishment of crosstalk with bone microenvironments (so called vicious cycle) is an essential event for metastatic breast cancer cells to develop bone metastasis. RANKL and RANK play a central role in this crosstalk. Moreover, recent studies have demonstrated that RANKL and RANK are involved in tumorigenesis and distant metastasis independent of bone microenvironments. Pharmacological disruption of the RANKL/RANK interplay should be an effective therapeutic intervention for primary breast tumors and bone and non-bone metastasis. In this context, denosumab, which is neutralizing monoclonal antibody against RANKL, is a mechanism-based drug for the treatment of bone metastases and would be beneficial for breast cancer patients with bone metastases and potentially visceral organ metastases.

Targeting RANKL in breast cancer: bone metastasis and beyond

In breast cancer, RANK ligand (RANKL) appears to play an important role in the process of chemotaxis between circulating tumor cells and the bone microenvironment, which enables RANK-expressing breast cancer cells to migrate into the bone. Mounting clinical evidence has further demonstrated that the anti-RANKL monoclonal antibody; denosumab is the most effective approach in the prevention of skeletal-related events. On the other hand, inhibiting RANKL in preclinical models, not only reduced breast cancer formation but also decreased the development of lung metastases, suggesting RANKL as a novel target for breast cancer chemoprevention. In addition, recent data have pointed to a potential role of RANKL in the biology of breast cancer arising at a young age. Hence, RANKL emerges as a key molecule, not only in the field of breast cancer bone metastasis but also in the biology of breast cancer as a whole.

Where Do Bone-Targeted Agents RANK in Breast Cancer Treatment?

Breast cancer cells preferentially metastasise to the skeleton, owing, in part, to the fertile environment provided by bone. Increased bone turnover releases growth factors that promote tumour cell growth. In turn, tumour cells release factors that stimulate further bone turnover, resulting in a vicious cycle of metastasis growth and bone destruction. The RANK-RANK ligand (RANKL) pathway plays a key role in this cycle, and inhibition of RANKL using the fully-human monoclonal antibody denosumab, has demonstrated efficacy in delaying skeletal complications associated with bone metastases in three phase 3 trials. Preclinical studies suggest that the RANKL pathway also plays a role in breast cancer tumourigenesis and migration to bone. In a subgroup analysis of the negative Adjuvant Zoledronic Acid to Reduce Recurrence (AZURE) trial, the bisphosphonate zoledronic acid showed potential for improving survival in patients who were postmenopausal; however, a prospective study in this patient population is required to validate this observation. Ongoing trials are examining whether adjuvant blockade of the RANKL pathway using denosumab can prevent disease recurrence in patients with high-risk breast cancer. These are building on analogous studies that have shown that denosumab improves bone metastasis-free survival in prostate cancer and suggested that it confers an overall survival benefit in non-small-cell lung cancer.

Co-expression of CD44+/RANKL+ tumor cells in the carcinogenesis of oral squamous cell carcinoma

Receptor activator of nuclear factor-kappa (RANK)/receptor activator of nuclear factor-kappa B ligand (RANKL) signaling helps putative cancer stem cells (CSC) to maintain their stemness. Expression of CD44 and RANKL was analyzed in oral squamous cell carcinoma specimen (n = 191). Moreover, RANKL expression was measured in cancer cell lines (BICR3, BICR56) by immunohistochemistry and western blot analysis. Scanned images were digitally analyzed using ImageJ and the immunomembrane plug-in. CD44 and RANKL expression on protein level was correlated with clinical characteristics and impact on survival. RANKL was co-labeled with CD44 in immunohistochemical and immunofluorescence double labeling experiments. Although high CD44+/RANKL+ co-expression was significantly associated with clinicopathological factors and worse survival, multivariate analysis did not demonstrate high CD44+/RANKL+ co-expression as independent prognostic factor. Immunohistochemical and immunofluorescence double labeling experiments revealed RANKL expression by CD44+ cancer cells. RANKL specificity was confirmed by western blot analysis. For the first time, this study provides evidence that RANKL expression in OSCC might be associated with disease recurrence and a cell compartment measured by CD44+/RANKL+ co-expression within the mucosal epithelial basal layer cells.

Adenosine and bone metabolism

Bone is a dynamic organ that undergoes continuous remodeling while maintaining a balance between bone formation and resorption. Osteoblasts, which synthesize and mineralize new bone, and osteoclasts, the cells that resorb bone, act in concert to maintain bone homeostasis. In recent years, there has been increasing appreciation of purinergic regulation of bone metabolism. Adenosine, released locally, mediates its physiologic and pharmacologic actions via interactions with G protein-coupled receptors, and recent work has indicated that these receptors are involved in the regulation of osteoclast differentiation and function, as well as in osteoblast differentiation and bone formation. Moreover, adenosine receptors also regulate chondrocyte and cartilage homeostasis. These recent findings underscore the potential therapeutic importance of adenosine receptors in regulating bone physiology and pathology.

Targeted therapies of metastatic breast cancer: relationships with cancer stem cells

In the last years, many targeted agents have been developed for metastatic breast cancer (MBC) treatment and are being tested in clinical trials. In spite of this, apart from epidermal growth factor receptor 2 (HER2) positive subset, no significant increase in the median overall survival (OS) has been reported. Similarly to conventional chemo- and radiotherapy, the cancer stem cell theory has been evoked to explain the frustrating results often obtained with this emerging category of drugs. This review examines the results in MBC of the approved targeted therapies or those currently under evaluation in experimental studies or in clinical trials, in the light of their relationships with breast CSCs and of the efforts to circumvent the development of resistance. In the next, there is the principal need to investigate if the effects on CSCs may be used to overcome cancer resistance and it will be opportune to consider whether molecular targeted therapies should be used alone or combined with conventional therapy, or with a different target drug specific for CSCs.